processdesign - User contributions [en]

Desalination - Team A

2016-03-12T03:27:11Z

Ezhuang: /* Appendix G. Economic Analysis */

Author: Pear Dhiantravan[2016], Reed Kolbe[2016], Sheridan Lichtor[2016], John Marsiglio[2016], Ellen Zhuang[2016]

Instructors: Fengqi You, David Wegerer

Winter 2016

=Executive Summary=
Seawater desalination is an attractive approach to address the world’s freshwater shortage in light of the increasing global water scarcity. Our team decided to tackle California's growing drought crisis by designing a multistage flash desalination plant in Richmond, CA. Based on market analysis of existing desalination plants and water demands, this is a strategic location for the desalination of 50 million gallons per day of water from the San Francisco Bay.

The MSF process contains 18 flash stages in series. Seawater is taken in from the San Francisco Bay at a rate of 398 million gallons per day and mixed with brine recycled from the system. Within the MSF process, seawater is heated through a series of 18 heat exchangers and a brine heater before enters a series of 18 evaporation chambers. In the evaporation chambers, the seawater flashes and the freshwater vapor is condensed, collected, and sent to a water treatment facility. The brine stream proceeds to the next chamber, and the process continues. Upon exiting the last flashing chamber, the brine stream is split into a recycle brine stream that is mixed with the seawater inlet feed and a waste stream that is diluted with fresh seawater and returned to the sea. Overall, the process yield is 13%. This process was constructed in Aspen HYSYS and optimized around utility costs using the HYSYS Economic Analyzer. Based on duties, temperatures and pressures reported by HYSYS, as well as fundamental mass and energy balances, the equipment were sized appropriately and the economics of the process was analyzed.

The design has a total capital cost of $485.2 MM, an annual operating cost of $144.7 MM ($103.7 MM of which is utilities), and an annual plant revenue of $134.3 MM. Economic analysis reveals that the project has a negative NPV of -$495.7 MM and operates at a loss of $10.4 MM per year. Thus, the proposed design is not profitable. Despite the current projections, it is important to consider the greater implications of running a desalination plant in the Bay Area. The population in the Bay Area is quickly growing while California is experiencing harsher droughts each year. Given the dire situations, the government may support the proposed clean water project with subsidies. Building the facility may be a strategic investment, especially since greater water scarcity is predicted in the future.

Our design meets various environmental, ethical, social and health related constraints. The brine discharged back to the bay is diluted to at most 0.2% above the natural background salinity as required by law so that the plant does not impose harm on any local marine life. The demographic that the plant serves is largely invested in the protection of the environment, which we addressed by reducing energy consumption through direct energy integration with the nearby Chevron plant. Lastly, the product meets the specifications of the California Safe Water Drinking Act by producing water with no impurities.

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=Introduction=
An increasing global water scarcity is fueling initiatives everywhere for clean water treatment, making efficient seawater desalination an attractive aim for chemical plant design. A 2015 market analysis found that the global desalination market earned revenues of $11.66 billion, and this number is expected to reach over $19 billion by 2019. Furthermore, the 17,000 desalination plants currently in operation are expected to double in number by 2020.1

As fresh water sources become increasingly scarce, strict water conservation measures are being observed. California is entering the fourth year of one of its most severe droughts on record. Cities are required to reduce their water usage by 35% to avoid facing fines.2 In lieu of these pressing conditions, California is looking for ways to provide more accessible fresh water to its citizens. This high demand for clean water motivated our choosing Richmond, CA as our plant location. At Richmond, the desalination plant can convert readily available seawater from the San Francisco Bay into fresh water. We propose to desalinate 50 million gallons of water per day to be sent to water treatment plants for potability. This aim is based on the capacity of the desalination plant in Carlsbad, California, which services a similar population.

The salinity of the water in the San Francisco Bay is seasonal. During the dry seasons of summer and fall, salinity is high around 10 PSU (10,000 ppm) because water from the Pacific Ocean flows into the San Francisco Bay. During the wet winter season, fresh water from the rivers flows into the Bay, and salinity drops to around 2 PSU (2,000).3 Designed to meet the “worst case” scenario, our plant will process 398 million gallons of water per day with 10,000 ppm of salt to produce 50 million gallons of distilled water. This gives a yield of 13%.

The desalination plant proposed consists of 18 flash stages in series. This report outlines our investigation of this potential water desalination plant, including design and optimization of the desalination process, an analysis of its energy consumption and environmental impacts, and a study of its economic implications.

=Technical Approach=
For the desalination step of our process, we selected a multistage flash (MSF) distillation process. Not only is MSF distillation a very viable method, but it is also the most common method, currently producing about 60% of the world’s desalinated water. MSF and reverse osmosis (RO) are the two major methods being used in large-scale desalination plants. Both processes require considerable amounts of energy. RO typically has a lower energy demand; however, the high impurity content of the Bay water would frequently necessitate membrane cleaning and/or exchange. Feed going through a RO system requires extensive pretreatment to remove biological organisms and other solids to control the pH and the chemical composition of the water. With the San Francisco Bay as our source of water, MSF is an attractive option because sediment and other large impurities can be separated from the feed before distillation occurs. Additionally, MSF distillation plants can be located near power plants and paired to their waste heat streams to conserve energy. This can reduce energy needs by half or two thirds, making MSF increasingly more practical. Due to the maintenance required in the RO system, the MSF distillation is the better option in terms of operation cost.4

A HYSYS model was built to simulate the MSF distillation section of the process, which spans after seawater pretreatment and before local potable water treatment. The model is used to simulate the distillation steps and to calculate necessary heating, cooling, and input flow rates to produce the required 50 million gallons of distillated water per day. For the simulation, the 398 million gallons per day of seawater feed stream is defined to be composed of 3.31 mol% sodium chloride (NaCl) and 96.69 mol% water. The percentage of NaCl is set to account for other ions and chemicals present after the pretreatment phase. NaCl alone is added to the HYSYS system due to the capabilities of the fluid package used to simulate this process, ElectroNRTL. The feed is set at 22°C, which is the approximate the temperature of the San Francisco Bay year round.

=Process Design=
==Design Overview==
The proposed desalination plant uses MSF distillation to convert the San Francisco Bay water to fresh water. The final process design contains 18 flash stages in series. Overall, 398 million gallons per day of of brackish water is converted to 50 million gallons per day of deionized water, a 13% yield. The production meets the required production rate of 50 million gallons of freshwater per day.

Seawater is taken in from the San Francisco Bay and mixed with the brine recycle stream produced downstream. The feed is then heated through a series of condensers and a heater. Distilled water vapor in each flash stage contacts the tubes carrying the seawater feed and condenses into a liquid stream by exchanging heat with the cooler seawater feed stream. The seawater is sent through a heater to increase its thermal energy before entering the first stage of the MSF. In each flashing stage, some of the water evaporates, leaving the salt behind. The freshwater vapors condense, as described above, and is collected and sent to a water treatment plant. The brine stream is split into the recycle stream and a waste stream, and the waste stream is diluted with fresh seawater before being discharged back into the bay.

A process flow diagram of the process is depicted in Appendix A.

==Pretreatment==
Sediment is removed to prevent solids from plugging the process downstream. Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime, and is controlled for by lowering pH. Acidifying the water will also remove CO2, which is a corrosive gas. To control for oxygen, oxygen scavengers will be included in the system to sequester it. Sodium bisulfate will be used as the scavenger.5

==Condensers (Heat Exchanger Networks)==
In order to conserve and recycle energy, the condensation of the distillate and heating of the seawater feed will be coupled in countercurrent heat exchanger networks. These heat exchanger networks are condensers of the design. The cold feed, which is a mixture of fresh seawater and brine recycle, is fed through multiple heat exchangers in series. Each heat exchanger corresponds to a different evaporation chamber. This process consists of 18 heat exchangers: E-101, E-102, … E-117, and E-118, which correspond to evaporation chambers V-102, V-103, … V-118, and V-119, respectively.

The heat exchangers can be viewed similarly to a shell and tube setup, however there is no actual physical shell. The tubes simply run through the top of the evaporation chambers, where vaporized distilled water in the chamber acts as the shell side fluid. The tube bundles are arranged in long tube configuration and are aligned parallel to the direction of the flashing brine flow in the evaporation chamber. The evaporating brine in an evaporation chamber rise up to the the corresponding heat exchanger and condense on the outside of the tubes, creating collectable pure water. This condensing water is what heats up the seawater in the tubes.

E-101 was used as the basis for the condenser sizing since it has the highest heat requirement, and thus will be the largest heat exchanger. From the HYSYS model, the overall system requires 2.24 x 106 kW of duty. Assuming this energy requirement, 5000 tubes, and 18 m length tubes (the diameter of the evaporation chambers, explained further on the next page), the diameter of each tube was calculated to be 0.4 m. Based on the temperature of the streams and flow, the thickness of the tube is 7 x 10-4 m.

==Brine Heater==
After the seawater exits the final heat exchanger, it enters a brine heater (E-119) where it is brought up to the temperature necessary for the flashing process to begin. This means that the seawater must be overheated compared to stage one, so that the seawater will flash in the evaporation chamber, i.e. release heat and vapor to reach equilibrium with stage conditions. After flowing through the condensers, the seawater enters the brine heater at 96.1 oC and is heated to 120 oC. The overall system requires 2.31 x 106 kW of duty. This is supplied by a total of 4.15 x 106 kg per hour of steam.

==Evaporation Chambers==
When the seawater has been brought to a temperature appropriate for flashing, it enters the first evaporation chamber (V-102). Upon entering the chamber, the water is flashed. Some of the water evaporates, leaving the salt behind. The vapor rises up to the heat exchanger tubes in each evaporation chamber and condenses on the outside into the distilled water product. This condensing water is what heats up the seawater in the tubes discussed above. The seawater that does not evaporate moves to the next stage, which operates at a higher temperature and lower pressure, and the process is repeated. This continues in series through a total of 18 flashing chambers (V-102, V-103, … V-118, and V-119). All of the distillate is collected into one stream and all of the brine is mixed with seawater before being disposed of back in the bay in order to dilute it enough so it does not have an adverse effect on marine life.

Each respective heat exchanger fully condenses the vapor product stream from each stage. However, when all of the individual product streams are consolidated into the final combined product stream, some water exists in the vapor phase due to temperature and pressure changes associated with mixing all of the streams together. Although the vapor product stream from each stage condenses completely across its respective heat exchanger, the total product stream contains some water in the vapor phase due to the temperature change when the mixer sums all the streams. In order to fully condense this stream, which will be sent to local water treatment plans, a heat exchanger (E-120) is put in place to cool down the product using the process feed stream.

The correlations of El-Dessouky and Ettouney were employed in order to size the flash chambers. They specified that conventional MSF plants that operate at a capacity between 27,000 and 36,000 m3/day use flashing stages of the dimensions 18 m diameter and 4 m height.6

==Valves==
The model requires valves in between the evaporation chambers. The seawater streams flow through valves with large pressure drops to flash a portion the liquid into vapor that can then be separated in the vessels. Large pressure drops provide the thermodynamic driving force to vaporize more of the liquid, and are required to achieve high product yield.

==Material of Construction==
Before 1980, the shells and internals of the MSF units were commonly constructed from carbon steel. Seawater, however, corrodes carbon steel. To compensate for this, the thickness of the carbon steel has an additional corrosion allowance, which increases the size and weight of the equipment. After 1980, the stainless steel and duplex stainless steel became increasingly common. Stainless steel metal requires less thickness for equivalent strength and corrosion resistance, which allows for a reduction in the size, weight, and therefore cost of units. Major units in our process will be constructed from stainless steel.6

==Process Alternatives==
In order to ensure that our design is fully optimized, we wanted to consider several alternatives for our process. We looked at alternatives associated with water pretreatment, the distillation process itself and product treatment.

===Water Pretreatment===
Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime. Scaling can be controlled by adjusting pH, temperature, and bicarbonate concentration. We selected acidification to control for process scaling. This is because lowering a process temperature would decrease overall efficiency, and decreasing bicarbonate concentration would be comparatively expensive. Lowering pH would have the added benefit of controlling CO2 concentrations since CO2 is a corrosive gas. To control for corrosion we looked at how to remove oxygen. To target the oxygen we chose an oxygen scavenger. This was chosen over other options such as a deaerator, or a protective coating because a deaerator would have a large capital cost, and the equipment itself would experience corrosion, and a tank coating such as zinc orthophosphate has a mixed effectiveness.7

===MSF: Brine Recycle vs. Once-Through System===
We explored two options for the overall layout of the MSF distillation plant. In the first, the brine passes through the process once; in the second, a portion of the brine is recycled from the last stage to the incoming seawater feed. The largest draw towards implementing a brine recycle is that it would significantly reduce the overall amount of seawater needed to produce the daily target of 50 million gallons per day. This in turn would lower the amount of energy required to run the pump feeding seawater to the process. Another positive is that, with a brine recycle, the amount of water conditioning chemicals that must be added to the seawater feed is reduced. The negative aspects mainly revolve around the fact that the solution throughout the process will be higher in salt concentration. This increase in salinity both raises the likelihood of scaling and corrosion in the plant and raises the amount of seawater that must be added to the final brine to make it safe for discharge back into the environment. Additionally, boiling point of each stage is raised when adding the brine recycle. After weighing both alternatives against one another, we have decided to include a brine recycle in our plant design. Most of the negative aspects of the brine recycle can be combated (e.g. include anti-scaling agents in the feed pretreatment), and the economic benefits of including the brine recycle are very significant.8

===MSF: Continuous vs. Batch===
The distillation step can be done in continuous or batch processes. Batch distillation is more versatile and often used when the product is in small amounts and very high purity. A continuous distillation is more efficient thermodynamically and economically for large amounts of material of constant composition, whereas batch distillation is more effective for small amounts of material of varying compositions. Since the plant is processing large amounts of seawater of relatively constant composition, the distillation system will utilize continuous distillation.9

===MSF: Long Tube vs. Crossed Tube ===
Inside the evaporator stage, the condenser tube bundles may be arranged in either long tube or cross tube design. In the long tube arrangement, the tubes are aligned parallel to the direction of the flashing brine flow. The tubes go from stage to stage without water boxes. In the cross tube arrangement, the tubes are lined perpendicular to the flow of the flashing brine. Water boxes transfer brine between the stages.10 11 While the water boxes in crossed tube bundles allow for more customization, the long tube configuration allows for construction of large unit sizes, as there are manufacturing restrictions with cross tubes. Long tube design can easily accommodate a large number of stages, which lead us to choose long tube for our design.11 12

===MSF: Venting System===
Noncondensable gases exist in the process because of air leakages into stages under vacuum conditions and the release of dissolved gases from the heated brine. These gases are removed by venting the system. An air cooler section concentrates the gas before they are vented, and baffles direct the vapor and condensates out. There are two venting arrangements to consider: series and parallel. The system in series moves gas from one stage directly to the next, and the only loss in the system is vapor in the last stage (since some vapor is extracted with gases). This system is economical, however this configuration increases the amount of noncondensable gases passing from one stage to the next, and disrupting the venting system in one stage will affect the entire system. In parallel venting, gas is released through parallel takeoff pipes at each stage. Although this arrangement allows for venting at any stage to be controlled, the design and maintenance is more complex, and more vapor is lost than in the venting system in series. A third alternative is to use a combination of venting in parallel and in series, which is the common method in industry. For this process, our team chose to do venting in parallel and in series.12

===Additional Energy Source Alternatives===
Energy for the process can also be coupled with renewable energy sources to reduce the desalination plant’s carbon footprint. Energy integration can be direct or indirect. In direct integration, thermal waste heat from industrial sources provide thermal energy or pressure as energy sources. Direct integration is advantageous at the modular level, as it avoids energy losses with electricity conversion. Within indirect integration, there are modular integration and grid/utility-scale integration. Modular integration integrates the plant with wind turbines and other small-scale renewable power generators with mobile deployment. A “hybridization” of renewable power sources with natural gas power generation is also an option, and the coupling of the two provides a stable power source. Indirect renewable energy integration is growing as the cost of renewable energy continues to decreases and provides an economy of scale that direct integration does not, but there are several obstacles towards indirect integration, such as the plant’s finance and engineering the integration of the two systems.13 This distillation plant will be directly integrated with neighboring plants such as Chevron that produce thermal waste heat.

===Product Treatment===
Since the product is distilled water, it needs additional ions to a concentration of around 0.01% before going to the end user. The three alternatives considered were building a water treatment facility on site, simply adding the correct salts to the product stream and then sending the stream for bacterial treatment at a water plant, or simply sending the product distilled water for complete treatment to another facility. Due to the number of nearby water treatment plants and their demand for water, we elected to simply send the product to a nearby water treatment facility.

==Design Constraints and Tradeoffs==
While designing our process, we wanted to make sure that we met several outside constraints, including environmental, political, ethical, social, sustainability, health and safety constraints. First and most importantly, we wanted to ensure that the brine discharged back to the bay would not harm local marine life. Our process was designed around the constraint of diluting the exiting brine to within 0.2% salinity of background seawater. This is not only compliant with California law, but was also verified to be safe for surrounding marine life through our own outside research. We also addressed many of these considerations by deciding to pursue direct energy integration with the nearby Chevron plant in order to reduce environmental impact of our process. Additionally, we ensured that our plant would be located in an area that is densely populated and in great need of fresh drinking water. Finally, we addressed health and safety considerations by making sure we would be able to send our product distilled water to local treatment plants for proper potabilization.

===Design Tradeoffs: Thermodynamics===
Adjusting elements within this process is iterative, and optimization thereof required changing multiple parameters at once. An example of this is in tuning the temperature increase by heater E-100; if the temperature of the stream entering the vessels is too high, the heat exchange between the vapor product and the feed stream will not be realizable because it causes the feed to be too hot to cool and condense the product. Higher temperatures, however, provide greater thermodynamic driving forces for flashing. Setting the entering stream temperature at 120°C provides enough heat for substantial flash at 130 kPa. This is the only heater required until the very last flash stage, which is added to provide more heat for vaporization. This heater (E-105) increases the stream temperature by 15°C, and the vapor product from the last vessel increases the feed stream temperature by almost 30°C. The added energy increases vaporization by 3%, which is a substantial 2.2 million kg/h. The temperature of the streams entering and leaving each stage decreases with each additional flash stage such that the last vessel has the lowest temperature, according to realistic operating guidelines.

===Design Tradeoffs: Energy Consumption, Environmental Impacts, and Product Optimization===
Increases in the pressure drop across any of the valves in this final design causes a temperature cross in the heat exchangers unless a proper increase in pump energy is provided. There is thus a design tradeoff between lowering energy usage and increasing product yield. For example, lowering the operating pressure of the process dictates that the pressure difference used to flash seawater must be smaller, possibly decreasing the amount that is vaporized. The final design aims for a yield of 50 million gallons per day. The 18 stage model was chosen for its low annual utility cost. This is a reflection of environmental mindfulness because energy consumption is an ongoing aspect of the process and should be lowered as much as possible.

==Mass and Energy Balances==
The model of the desalination system assumes that: (1) the properties are uniform in each phase within a stage, (2) heat losses are negligible due to the small surface area to volume ratio for each stage and properly insulated units (heat losses to the surrounding vary from 2 to 5% of the total system energy), (3) the distillate is salt free since the boiling point of water is much lower than that of salt, (4) the subcooling or superheating effects on the system energy balances are negligible, (5) the only contaminant is sodium chloride (there are no non-condensable gases to consider, so no vapors are vented), and (6) the process has 3 evaporation chambers.

The major balances are that the feed is a mixture of the fresh seawater stream and the recycle stream, and that the brine stream exiting the last evaporation chamber is divided into the recycle stream and the waste stream. Within each stage, the change in mass of a phase over time is the balance between the net flow of that phase from the chamber and the rates of evaporation and condensation of water into and out of that phase. The equations governing the mass and energy balances on the system is displayed in Appendix C.

=Optimization=
As initiatives for reduced energy usage and increased environmental sustainability become more prevalent, efforts to minimize the utility cost in plants has increased to accommodate these incentives. Utilities are a continual source of expenditure and consumption, and a well-designed chemical process should seek to minimize this element. The annualized utility cost is a useful parameter to optimize given that its value depends on variations in energy costs; thus, annualized utility costs do not correlate with conventional indices of inflation, such as capital and labor costs. Additionally, utility costs, like other variable costs, can be minimized by improving the design or operational efficiency of the plant.

In optimizing our MSF process, the design variable of focus was the number of stages to use in our process. The performance measure was the annualized utility cost, which was the parameter that was minimized for optimization. The process is constrained by the product specifications, which are kept constant and serve as the standard for comparison between each model. These specifications are to produce 50 million gallons of desalinated liquid water per day.

One feasibility constraint in the process is the size of the heat exchangers, which must provide a large enough area for total condensation of the vapor stream coming out of the flash stages. In order to restrict the heat exchange area needed within each exchanger, the number of stages must be at least 11. Increasing the number of stages decreases the vapor flow rate at each, and decreases the heat exchange area required as well. An 11-stage process allows for appropriate heat exchange between the feed stream and the vapor product, according to the HYSYS economic analyzer. The optimization analysis thus was conducted incorporating only models with 11 or more stages.

The HYSYS economic analyzer was used to systematically optimize the process by finding the model with the lowest annualized utility cost. The performance parameters kept constant across each model were: the inlet composition, pressure, and temperature, the brine outlet temperature, the total product flow rate and composition, and the flash stage schematic. Table 1 displays the annualized utility cost for each model, and illustrates that a clear dip in the utility cost occurs in an 18 stage model. Thus, 18 is the optimal number of stages to use for our MSF distillation given utility costs as the optimized parameter.
[[File:Table_1.PNG|thumb|center|500x300px]]

=Economic Evaluation=
In order to evaluate our process from a fiscal standpoint, we used Aspen Process Economic Analyzer and Aspen cost estimator to determine capital and operating costs for our plant. These costs were calculated based on calculated equipment sizes, as well as energy requirements reported by our HYSYS model. The capital cost was found to be $485.2MM and annual total operating cost was found to be $144.7MM ($103.7MM of which was utilities). Annual plant revenue was estimated based on prices that water districts in the county currently pay for outside water sources, and was found to be $134.3MM.14

The NPV analysis we conducted assumed that the plant would take two years to build, would operate at half capacity during the first year of operation (year 3), and have a 20 year lifetime. We also assumed that our plant would be in operation 350 days per year. A constant discount rate of 12% was assumed to hold throughout the plant’s lifetime. The final NPV of this project is -$495.7MM. This is due to both the large capital cost of the plant and the annual loss of $10.4MM while in full operation. A graph of cumulative cash flow versus time can be found in Appendix E. Tax rates and depreciation do not directly impact the NPV of this project due to operating at a loss, however for future calculations it should be noted that we used a straight line depreciation schedule over a seven year period with a 40% tax rate.

Despite the current projections, it is important to consider the greater implications of running a desalination plant in the Bay Area. The population in the Bay Area is quickly growing, while California is experiencing harsher droughts each year. A consequence of this is the possibility of receiving government subsidies for our clean water product. The similarly scaled desalination plant in Carlsbad, CA operates at a loss as well, but receives significant government subsidies. Building this facility may be a strategic investment, especially since greater water scarcity is predicted in the future.

=Sensitivity Analysis=
We wanted to determine how sensitive NPV of our project is to the cost of capital, price of utilities and price we would be able to sell our clean water product at. We conducted this analysis by varying each of these variables independently between 0.5 times and 1.5 times the original value we calculated. As to be expected, NPV grows more negative with increasing capital costs and utility prices and decreasing water prices. Conversely, NPV becomes less negative as price of water rises and capital and utility costs decrease. A plot summarizing the effects of varying these costs can be found in Appendix E. NPV is most sensitive to the price at which we will be selling our clean water product. It is worth noting that NPV is more sensitive to utilities cost than it is to capital cost, which is why we focused on optimizing the cost of utilities. In future steps, trying to reduce utilities cost even more would be worth looking into to decrease the amount of money being lost on this project.

=Conclusions and Final Recommendation=
The final design achieves the project goal of producing 50 million gallons per day of distilled water at a yield of 13%, comparable to other similarly sized plants. An optimization of the number of MSF stages is realistic, and can be improved upon by taking into account other process parameters.

The proposed design was optimized around utility costs, comparing plants with 11 to 25 stages. The process is constrained by product specifications and by the thermodynamics of the heat exchange between the feed and the product streams. Energy conservation was achieved in using the seawater feed to condense the final product, as well as in all of the heat exchangers increasing the thermal energy of the seawater feed stream before it enters the flash stages.

In order to further optimize this process, other design parameters should be considered such as the brine recycle ratio, the cooler duty of the recycle stream, the pressure drop across each valve at the flash stages, and other economic factors. An optimization around maximizing NPV, for example, might be more comprehensive than that conducted regarding only utility costs. Tuning brine recycle rates could greatly improve process efficiency, and standardizing flash stage pressure drops would make operations more feasible and controlled.

Given our in depth economic analysis, this project would lose an average of $10.4MM per year, and has an NPV of -$495.7MM for a 20 year plant lifetime. From an economic perspective alone it would not be a good project to build. However, other factors need to be considered when building a project with enormous public value. In recent years California droughts have become more severe. Implementing a drought-proof supply of water now could avert a disaster later on. Even without a large disaster, having the desalination plant could also be expected to pay off in the future with the increasing price of water. Considerations such as these were the deciding factors in building the Carlsbad desalination plant. Since there are currently no desalination plants in the Bay area we recommend that preparations should be made to move forward with a desalination plant.

=References=
[1] Lyons J. Environmental Leader. Environmental Leader 2015. Available at: http://www.environmentalleader.com/2015/10/27/desalination-plants-to-double-by-2020. Accessed 2016.

[2] Fitzsimmons E. In California, Cities Braced to Cut Water by 10 to 35%. The New York Times 2015. Available at:
http://www.nytimes.com/2015/04/09/us/in-california-cities-braced-to-cut-water-by-10-to-35.html. Accessed 2016.

[3] Salinity Times Series - USGS Water Quality of SF Bay. Salinity Times Series - USGS Water Quality of SF Bay. Available at:
http://sfbay.wr.usgs.gov/access/wqdata/overview/examp/charts/salin.html. Accessed 2016.

[4] Reverse Osmosis vs. Multi Stage Flash Distillation: A Comparison Between Different Desalination Methods. Brighthub Engineering. Available at:
http://www.brighthubengineering.com/power-plants/29621-comparison-between-the-reverse-osmosis-and-multi-stage-flash-distillation-methods/. Accessed 2016.

[5] Committee on Advancing Desalination Technology, Water Science and Technology Board, Division on Earth and Life Studies, National Research Council. Desalination: A National Perspective. Washington, D.C.:The National Academies Press, 2008.

[6] H. El-Dessouky. Fundamentals of salt water desalination, 1st Ed., Elsevier Science, 2002.

[7] Committee on Advancing Desalination Technology, Water Science and Technology Board, Division on Earth and Life Studies, National Research Council. Desalination: A National Perspective. Washington, D.C.:The National Academies Press, 2008.

[8] Guasp RE, Bello CP, Jara JG, Vega JP, Damien T, Winkel C, Smith HH. Desalination by distillation. Organization of American States website. oas.org. Accessed January 28, 2015.

[9] Batch Distillation. AE 335 Separation Processes website. http://prodpran.che.engr.tu.ac.th/AE335/AE335.html. Accessed January 26, 2016.

[10] Morris R. The development of the multi-stage flash distillation process: A designer's viewpoint. Desalination. 1993;93:57-68.

[11] Darwish MA. Developments in the multi-stage flash desalting system. Desalination. 1995;100:35-64.

[12] Wade N. Distillation plant development and cost update. Desalination. 2001;136:3-12.

[13] Desalination Technologies and the Use of Alternative Energies for Desalination. World Intellectual Property Organization website. http://www.wipo.int/export/sites/www/patentscope/
en/programs/patent_landscapes/documents/patent_landscapes/948-2E-WEB.pdf. Updated November 2011. Accessed January 23, 2016.

[14] California Turns to the Pacific Ocean for Water | MIT Technology Review.MIT Technology Review 2014. Available at:
http://www.technologyreview.com/featuredstory/533446/desalination-out-of-desperation/. Accessed 2016.

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=Appendix A. Process Flow Diagram=
[[File:Appendix 1.PNG]]

'''Figure A1''' Process Flow Diagram of the MSF System

=Appendix B. HYSYS Simulation=
The outputs of the HYSYS model of the process is shown below.

[[File:AppendB.PNG]]

'''Figure B1.''' The HYSYS model of the MSF distillation process.

=Appendix C. Mass and Energy Balances=
Below are equations that describe the mass and energy balances of the system.

In MSF system with brine recycle, the mass flow of the intake seawater I is mixed with the mass flow of the recycle brine R to create the feed seawater stream F. Since this model assumes no non-condensable gases, the feed stream is overall balanced by the brine waste stream W, the recycle brine stream, and the distillate stream D. The brine stream exiting the last evaporation chamber B is divided into the recycle and waste streams. The amount of salt that enters the series of evaporation chambers is the amount of salt that leaves (X is the salt mass fraction).

[[File:Overallmasss.PNG]]

Within each stage, the change in mass of a phase is the mass flows of that phase into and out of the chamber and the rates of evaporation or condensation of the water into and out of that phase.
[[File:Masss2.PNG]]

The energy balances are summarized below.

[[File:Mass3.PNG]]

[[File:Mass4.PNG]]

=Appendix D. Summary of Process Units=

[[File:AppendD1.PNG]]
[[File:AppenD2.PNG]]

=Appendix E. MSF Stage Parameters=
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[[File:Appendix E3.PNG]]

[[File:Appendix E4.PNG]]

=Appendix F. Process Units Calculations=
[[File:AppendF.PNG]]

[[File:AppendF2.PNG]]

[[File:AppendF3.PNG]]

=Appendix G. Economic Analysis=
[[File:Econanalysis.PNG]]

File:Econanalysis.PNG

2016-03-12T03:26:47Z

Ezhuang:

Desalination - Team A

2016-03-12T03:25:01Z

Ezhuang: /* Introduction */

Author: Pear Dhiantravan[2016], Reed Kolbe[2016], Sheridan Lichtor[2016], John Marsiglio[2016], Ellen Zhuang[2016]

Instructors: Fengqi You, David Wegerer

Winter 2016

=Executive Summary=
Seawater desalination is an attractive approach to address the world’s freshwater shortage in light of the increasing global water scarcity. Our team decided to tackle California's growing drought crisis by designing a multistage flash desalination plant in Richmond, CA. Based on market analysis of existing desalination plants and water demands, this is a strategic location for the desalination of 50 million gallons per day of water from the San Francisco Bay.

The MSF process contains 18 flash stages in series. Seawater is taken in from the San Francisco Bay at a rate of 398 million gallons per day and mixed with brine recycled from the system. Within the MSF process, seawater is heated through a series of 18 heat exchangers and a brine heater before enters a series of 18 evaporation chambers. In the evaporation chambers, the seawater flashes and the freshwater vapor is condensed, collected, and sent to a water treatment facility. The brine stream proceeds to the next chamber, and the process continues. Upon exiting the last flashing chamber, the brine stream is split into a recycle brine stream that is mixed with the seawater inlet feed and a waste stream that is diluted with fresh seawater and returned to the sea. Overall, the process yield is 13%. This process was constructed in Aspen HYSYS and optimized around utility costs using the HYSYS Economic Analyzer. Based on duties, temperatures and pressures reported by HYSYS, as well as fundamental mass and energy balances, the equipment were sized appropriately and the economics of the process was analyzed.

The design has a total capital cost of $485.2 MM, an annual operating cost of $144.7 MM ($103.7 MM of which is utilities), and an annual plant revenue of $134.3 MM. Economic analysis reveals that the project has a negative NPV of -$495.7 MM and operates at a loss of $10.4 MM per year. Thus, the proposed design is not profitable. Despite the current projections, it is important to consider the greater implications of running a desalination plant in the Bay Area. The population in the Bay Area is quickly growing while California is experiencing harsher droughts each year. Given the dire situations, the government may support the proposed clean water project with subsidies. Building the facility may be a strategic investment, especially since greater water scarcity is predicted in the future.

Our design meets various environmental, ethical, social and health related constraints. The brine discharged back to the bay is diluted to at most 0.2% above the natural background salinity as required by law so that the plant does not impose harm on any local marine life. The demographic that the plant serves is largely invested in the protection of the environment, which we addressed by reducing energy consumption through direct energy integration with the nearby Chevron plant. Lastly, the product meets the specifications of the California Safe Water Drinking Act by producing water with no impurities.

__TOC__

=Introduction=
An increasing global water scarcity is fueling initiatives everywhere for clean water treatment, making efficient seawater desalination an attractive aim for chemical plant design. A 2015 market analysis found that the global desalination market earned revenues of $11.66 billion, and this number is expected to reach over $19 billion by 2019. Furthermore, the 17,000 desalination plants currently in operation are expected to double in number by 2020.1

As fresh water sources become increasingly scarce, strict water conservation measures are being observed. California is entering the fourth year of one of its most severe droughts on record. Cities are required to reduce their water usage by 35% to avoid facing fines.2 In lieu of these pressing conditions, California is looking for ways to provide more accessible fresh water to its citizens. This high demand for clean water motivated our choosing Richmond, CA as our plant location. At Richmond, the desalination plant can convert readily available seawater from the San Francisco Bay into fresh water. We propose to desalinate 50 million gallons of water per day to be sent to water treatment plants for potability. This aim is based on the capacity of the desalination plant in Carlsbad, California, which services a similar population.

The salinity of the water in the San Francisco Bay is seasonal. During the dry seasons of summer and fall, salinity is high around 10 PSU (10,000 ppm) because water from the Pacific Ocean flows into the San Francisco Bay. During the wet winter season, fresh water from the rivers flows into the Bay, and salinity drops to around 2 PSU (2,000).3 Designed to meet the “worst case” scenario, our plant will process 398 million gallons of water per day with 10,000 ppm of salt to produce 50 million gallons of distilled water. This gives a yield of 13%.

The desalination plant proposed consists of 18 flash stages in series. This report outlines our investigation of this potential water desalination plant, including design and optimization of the desalination process, an analysis of its energy consumption and environmental impacts, and a study of its economic implications.

=Technical Approach=
For the desalination step of our process, we selected a multistage flash (MSF) distillation process. Not only is MSF distillation a very viable method, but it is also the most common method, currently producing about 60% of the world’s desalinated water. MSF and reverse osmosis (RO) are the two major methods being used in large-scale desalination plants. Both processes require considerable amounts of energy. RO typically has a lower energy demand; however, the high impurity content of the Bay water would frequently necessitate membrane cleaning and/or exchange. Feed going through a RO system requires extensive pretreatment to remove biological organisms and other solids to control the pH and the chemical composition of the water. With the San Francisco Bay as our source of water, MSF is an attractive option because sediment and other large impurities can be separated from the feed before distillation occurs. Additionally, MSF distillation plants can be located near power plants and paired to their waste heat streams to conserve energy. This can reduce energy needs by half or two thirds, making MSF increasingly more practical. Due to the maintenance required in the RO system, the MSF distillation is the better option in terms of operation cost.4

A HYSYS model was built to simulate the MSF distillation section of the process, which spans after seawater pretreatment and before local potable water treatment. The model is used to simulate the distillation steps and to calculate necessary heating, cooling, and input flow rates to produce the required 50 million gallons of distillated water per day. For the simulation, the 398 million gallons per day of seawater feed stream is defined to be composed of 3.31 mol% sodium chloride (NaCl) and 96.69 mol% water. The percentage of NaCl is set to account for other ions and chemicals present after the pretreatment phase. NaCl alone is added to the HYSYS system due to the capabilities of the fluid package used to simulate this process, ElectroNRTL. The feed is set at 22°C, which is the approximate the temperature of the San Francisco Bay year round.

=Process Design=
==Design Overview==
The proposed desalination plant uses MSF distillation to convert the San Francisco Bay water to fresh water. The final process design contains 18 flash stages in series. Overall, 398 million gallons per day of of brackish water is converted to 50 million gallons per day of deionized water, a 13% yield. The production meets the required production rate of 50 million gallons of freshwater per day.

Seawater is taken in from the San Francisco Bay and mixed with the brine recycle stream produced downstream. The feed is then heated through a series of condensers and a heater. Distilled water vapor in each flash stage contacts the tubes carrying the seawater feed and condenses into a liquid stream by exchanging heat with the cooler seawater feed stream. The seawater is sent through a heater to increase its thermal energy before entering the first stage of the MSF. In each flashing stage, some of the water evaporates, leaving the salt behind. The freshwater vapors condense, as described above, and is collected and sent to a water treatment plant. The brine stream is split into the recycle stream and a waste stream, and the waste stream is diluted with fresh seawater before being discharged back into the bay.

A process flow diagram of the process is depicted in Appendix A.

==Pretreatment==
Sediment is removed to prevent solids from plugging the process downstream. Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime, and is controlled for by lowering pH. Acidifying the water will also remove CO2, which is a corrosive gas. To control for oxygen, oxygen scavengers will be included in the system to sequester it. Sodium bisulfate will be used as the scavenger.5

==Condensers (Heat Exchanger Networks)==
In order to conserve and recycle energy, the condensation of the distillate and heating of the seawater feed will be coupled in countercurrent heat exchanger networks. These heat exchanger networks are condensers of the design. The cold feed, which is a mixture of fresh seawater and brine recycle, is fed through multiple heat exchangers in series. Each heat exchanger corresponds to a different evaporation chamber. This process consists of 18 heat exchangers: E-101, E-102, … E-117, and E-118, which correspond to evaporation chambers V-102, V-103, … V-118, and V-119, respectively.

The heat exchangers can be viewed similarly to a shell and tube setup, however there is no actual physical shell. The tubes simply run through the top of the evaporation chambers, where vaporized distilled water in the chamber acts as the shell side fluid. The tube bundles are arranged in long tube configuration and are aligned parallel to the direction of the flashing brine flow in the evaporation chamber. The evaporating brine in an evaporation chamber rise up to the the corresponding heat exchanger and condense on the outside of the tubes, creating collectable pure water. This condensing water is what heats up the seawater in the tubes.

E-101 was used as the basis for the condenser sizing since it has the highest heat requirement, and thus will be the largest heat exchanger. From the HYSYS model, the overall system requires 2.24 x 106 kW of duty. Assuming this energy requirement, 5000 tubes, and 18 m length tubes (the diameter of the evaporation chambers, explained further on the next page), the diameter of each tube was calculated to be 0.4 m. Based on the temperature of the streams and flow, the thickness of the tube is 7 x 10-4 m.

==Brine Heater==
After the seawater exits the final heat exchanger, it enters a brine heater (E-119) where it is brought up to the temperature necessary for the flashing process to begin. This means that the seawater must be overheated compared to stage one, so that the seawater will flash in the evaporation chamber, i.e. release heat and vapor to reach equilibrium with stage conditions. After flowing through the condensers, the seawater enters the brine heater at 96.1 oC and is heated to 120 oC. The overall system requires 2.31 x 106 kW of duty. This is supplied by a total of 4.15 x 106 kg per hour of steam.

==Evaporation Chambers==
When the seawater has been brought to a temperature appropriate for flashing, it enters the first evaporation chamber (V-102). Upon entering the chamber, the water is flashed. Some of the water evaporates, leaving the salt behind. The vapor rises up to the heat exchanger tubes in each evaporation chamber and condenses on the outside into the distilled water product. This condensing water is what heats up the seawater in the tubes discussed above. The seawater that does not evaporate moves to the next stage, which operates at a higher temperature and lower pressure, and the process is repeated. This continues in series through a total of 18 flashing chambers (V-102, V-103, … V-118, and V-119). All of the distillate is collected into one stream and all of the brine is mixed with seawater before being disposed of back in the bay in order to dilute it enough so it does not have an adverse effect on marine life.

Each respective heat exchanger fully condenses the vapor product stream from each stage. However, when all of the individual product streams are consolidated into the final combined product stream, some water exists in the vapor phase due to temperature and pressure changes associated with mixing all of the streams together. Although the vapor product stream from each stage condenses completely across its respective heat exchanger, the total product stream contains some water in the vapor phase due to the temperature change when the mixer sums all the streams. In order to fully condense this stream, which will be sent to local water treatment plans, a heat exchanger (E-120) is put in place to cool down the product using the process feed stream.

The correlations of El-Dessouky and Ettouney were employed in order to size the flash chambers. They specified that conventional MSF plants that operate at a capacity between 27,000 and 36,000 m3/day use flashing stages of the dimensions 18 m diameter and 4 m height.6

==Valves==
The model requires valves in between the evaporation chambers. The seawater streams flow through valves with large pressure drops to flash a portion the liquid into vapor that can then be separated in the vessels. Large pressure drops provide the thermodynamic driving force to vaporize more of the liquid, and are required to achieve high product yield.

==Material of Construction==
Before 1980, the shells and internals of the MSF units were commonly constructed from carbon steel. Seawater, however, corrodes carbon steel. To compensate for this, the thickness of the carbon steel has an additional corrosion allowance, which increases the size and weight of the equipment. After 1980, the stainless steel and duplex stainless steel became increasingly common. Stainless steel metal requires less thickness for equivalent strength and corrosion resistance, which allows for a reduction in the size, weight, and therefore cost of units. Major units in our process will be constructed from stainless steel.6

==Process Alternatives==
In order to ensure that our design is fully optimized, we wanted to consider several alternatives for our process. We looked at alternatives associated with water pretreatment, the distillation process itself and product treatment.

===Water Pretreatment===
Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime. Scaling can be controlled by adjusting pH, temperature, and bicarbonate concentration. We selected acidification to control for process scaling. This is because lowering a process temperature would decrease overall efficiency, and decreasing bicarbonate concentration would be comparatively expensive. Lowering pH would have the added benefit of controlling CO2 concentrations since CO2 is a corrosive gas. To control for corrosion we looked at how to remove oxygen. To target the oxygen we chose an oxygen scavenger. This was chosen over other options such as a deaerator, or a protective coating because a deaerator would have a large capital cost, and the equipment itself would experience corrosion, and a tank coating such as zinc orthophosphate has a mixed effectiveness.7

===MSF: Brine Recycle vs. Once-Through System===
We explored two options for the overall layout of the MSF distillation plant. In the first, the brine passes through the process once; in the second, a portion of the brine is recycled from the last stage to the incoming seawater feed. The largest draw towards implementing a brine recycle is that it would significantly reduce the overall amount of seawater needed to produce the daily target of 50 million gallons per day. This in turn would lower the amount of energy required to run the pump feeding seawater to the process. Another positive is that, with a brine recycle, the amount of water conditioning chemicals that must be added to the seawater feed is reduced. The negative aspects mainly revolve around the fact that the solution throughout the process will be higher in salt concentration. This increase in salinity both raises the likelihood of scaling and corrosion in the plant and raises the amount of seawater that must be added to the final brine to make it safe for discharge back into the environment. Additionally, boiling point of each stage is raised when adding the brine recycle. After weighing both alternatives against one another, we have decided to include a brine recycle in our plant design. Most of the negative aspects of the brine recycle can be combated (e.g. include anti-scaling agents in the feed pretreatment), and the economic benefits of including the brine recycle are very significant.8

===MSF: Continuous vs. Batch===
The distillation step can be done in continuous or batch processes. Batch distillation is more versatile and often used when the product is in small amounts and very high purity. A continuous distillation is more efficient thermodynamically and economically for large amounts of material of constant composition, whereas batch distillation is more effective for small amounts of material of varying compositions. Since the plant is processing large amounts of seawater of relatively constant composition, the distillation system will utilize continuous distillation.9

===MSF: Long Tube vs. Crossed Tube ===
Inside the evaporator stage, the condenser tube bundles may be arranged in either long tube or cross tube design. In the long tube arrangement, the tubes are aligned parallel to the direction of the flashing brine flow. The tubes go from stage to stage without water boxes. In the cross tube arrangement, the tubes are lined perpendicular to the flow of the flashing brine. Water boxes transfer brine between the stages.10 11 While the water boxes in crossed tube bundles allow for more customization, the long tube configuration allows for construction of large unit sizes, as there are manufacturing restrictions with cross tubes. Long tube design can easily accommodate a large number of stages, which lead us to choose long tube for our design.11 12

===MSF: Venting System===
Noncondensable gases exist in the process because of air leakages into stages under vacuum conditions and the release of dissolved gases from the heated brine. These gases are removed by venting the system. An air cooler section concentrates the gas before they are vented, and baffles direct the vapor and condensates out. There are two venting arrangements to consider: series and parallel. The system in series moves gas from one stage directly to the next, and the only loss in the system is vapor in the last stage (since some vapor is extracted with gases). This system is economical, however this configuration increases the amount of noncondensable gases passing from one stage to the next, and disrupting the venting system in one stage will affect the entire system. In parallel venting, gas is released through parallel takeoff pipes at each stage. Although this arrangement allows for venting at any stage to be controlled, the design and maintenance is more complex, and more vapor is lost than in the venting system in series. A third alternative is to use a combination of venting in parallel and in series, which is the common method in industry. For this process, our team chose to do venting in parallel and in series.12

===Additional Energy Source Alternatives===
Energy for the process can also be coupled with renewable energy sources to reduce the desalination plant’s carbon footprint. Energy integration can be direct or indirect. In direct integration, thermal waste heat from industrial sources provide thermal energy or pressure as energy sources. Direct integration is advantageous at the modular level, as it avoids energy losses with electricity conversion. Within indirect integration, there are modular integration and grid/utility-scale integration. Modular integration integrates the plant with wind turbines and other small-scale renewable power generators with mobile deployment. A “hybridization” of renewable power sources with natural gas power generation is also an option, and the coupling of the two provides a stable power source. Indirect renewable energy integration is growing as the cost of renewable energy continues to decreases and provides an economy of scale that direct integration does not, but there are several obstacles towards indirect integration, such as the plant’s finance and engineering the integration of the two systems.13 This distillation plant will be directly integrated with neighboring plants such as Chevron that produce thermal waste heat.

===Product Treatment===
Since the product is distilled water, it needs additional ions to a concentration of around 0.01% before going to the end user. The three alternatives considered were building a water treatment facility on site, simply adding the correct salts to the product stream and then sending the stream for bacterial treatment at a water plant, or simply sending the product distilled water for complete treatment to another facility. Due to the number of nearby water treatment plants and their demand for water, we elected to simply send the product to a nearby water treatment facility.

==Design Constraints and Tradeoffs==
While designing our process, we wanted to make sure that we met several outside constraints, including environmental, political, ethical, social, sustainability, health and safety constraints. First and most importantly, we wanted to ensure that the brine discharged back to the bay would not harm local marine life. Our process was designed around the constraint of diluting the exiting brine to within 0.2% salinity of background seawater. This is not only compliant with California law, but was also verified to be safe for surrounding marine life through our own outside research. We also addressed many of these considerations by deciding to pursue direct energy integration with the nearby Chevron plant in order to reduce environmental impact of our process. Additionally, we ensured that our plant would be located in an area that is densely populated and in great need of fresh drinking water. Finally, we addressed health and safety considerations by making sure we would be able to send our product distilled water to local treatment plants for proper potabilization.

===Design Tradeoffs: Thermodynamics===
Adjusting elements within this process is iterative, and optimization thereof required changing multiple parameters at once. An example of this is in tuning the temperature increase by heater E-100; if the temperature of the stream entering the vessels is too high, the heat exchange between the vapor product and the feed stream will not be realizable because it causes the feed to be too hot to cool and condense the product. Higher temperatures, however, provide greater thermodynamic driving forces for flashing. Setting the entering stream temperature at 120°C provides enough heat for substantial flash at 130 kPa. This is the only heater required until the very last flash stage, which is added to provide more heat for vaporization. This heater (E-105) increases the stream temperature by 15°C, and the vapor product from the last vessel increases the feed stream temperature by almost 30°C. The added energy increases vaporization by 3%, which is a substantial 2.2 million kg/h. The temperature of the streams entering and leaving each stage decreases with each additional flash stage such that the last vessel has the lowest temperature, according to realistic operating guidelines.

===Design Tradeoffs: Energy Consumption, Environmental Impacts, and Product Optimization===
Increases in the pressure drop across any of the valves in this final design causes a temperature cross in the heat exchangers unless a proper increase in pump energy is provided. There is thus a design tradeoff between lowering energy usage and increasing product yield. For example, lowering the operating pressure of the process dictates that the pressure difference used to flash seawater must be smaller, possibly decreasing the amount that is vaporized. The final design aims for a yield of 50 million gallons per day. The 18 stage model was chosen for its low annual utility cost. This is a reflection of environmental mindfulness because energy consumption is an ongoing aspect of the process and should be lowered as much as possible.

==Mass and Energy Balances==
The model of the desalination system assumes that: (1) the properties are uniform in each phase within a stage, (2) heat losses are negligible due to the small surface area to volume ratio for each stage and properly insulated units (heat losses to the surrounding vary from 2 to 5% of the total system energy), (3) the distillate is salt free since the boiling point of water is much lower than that of salt, (4) the subcooling or superheating effects on the system energy balances are negligible, (5) the only contaminant is sodium chloride (there are no non-condensable gases to consider, so no vapors are vented), and (6) the process has 3 evaporation chambers.

The major balances are that the feed is a mixture of the fresh seawater stream and the recycle stream, and that the brine stream exiting the last evaporation chamber is divided into the recycle stream and the waste stream. Within each stage, the change in mass of a phase over time is the balance between the net flow of that phase from the chamber and the rates of evaporation and condensation of water into and out of that phase. The equations governing the mass and energy balances on the system is displayed in Appendix C.

=Optimization=
As initiatives for reduced energy usage and increased environmental sustainability become more prevalent, efforts to minimize the utility cost in plants has increased to accommodate these incentives. Utilities are a continual source of expenditure and consumption, and a well-designed chemical process should seek to minimize this element. The annualized utility cost is a useful parameter to optimize given that its value depends on variations in energy costs; thus, annualized utility costs do not correlate with conventional indices of inflation, such as capital and labor costs. Additionally, utility costs, like other variable costs, can be minimized by improving the design or operational efficiency of the plant.

In optimizing our MSF process, the design variable of focus was the number of stages to use in our process. The performance measure was the annualized utility cost, which was the parameter that was minimized for optimization. The process is constrained by the product specifications, which are kept constant and serve as the standard for comparison between each model. These specifications are to produce 50 million gallons of desalinated liquid water per day.

One feasibility constraint in the process is the size of the heat exchangers, which must provide a large enough area for total condensation of the vapor stream coming out of the flash stages. In order to restrict the heat exchange area needed within each exchanger, the number of stages must be at least 11. Increasing the number of stages decreases the vapor flow rate at each, and decreases the heat exchange area required as well. An 11-stage process allows for appropriate heat exchange between the feed stream and the vapor product, according to the HYSYS economic analyzer. The optimization analysis thus was conducted incorporating only models with 11 or more stages.

The HYSYS economic analyzer was used to systematically optimize the process by finding the model with the lowest annualized utility cost. The performance parameters kept constant across each model were: the inlet composition, pressure, and temperature, the brine outlet temperature, the total product flow rate and composition, and the flash stage schematic. Table 1 displays the annualized utility cost for each model, and illustrates that a clear dip in the utility cost occurs in an 18 stage model. Thus, 18 is the optimal number of stages to use for our MSF distillation given utility costs as the optimized parameter.
[[File:Table_1.PNG|thumb|center|500x300px]]

=Economic Evaluation=
In order to evaluate our process from a fiscal standpoint, we used Aspen Process Economic Analyzer and Aspen cost estimator to determine capital and operating costs for our plant. These costs were calculated based on calculated equipment sizes, as well as energy requirements reported by our HYSYS model. The capital cost was found to be $485.2MM and annual total operating cost was found to be $144.7MM ($103.7MM of which was utilities). Annual plant revenue was estimated based on prices that water districts in the county currently pay for outside water sources, and was found to be $134.3MM.14

The NPV analysis we conducted assumed that the plant would take two years to build, would operate at half capacity during the first year of operation (year 3), and have a 20 year lifetime. We also assumed that our plant would be in operation 350 days per year. A constant discount rate of 12% was assumed to hold throughout the plant’s lifetime. The final NPV of this project is -$495.7MM. This is due to both the large capital cost of the plant and the annual loss of $10.4MM while in full operation. A graph of cumulative cash flow versus time can be found in Appendix E. Tax rates and depreciation do not directly impact the NPV of this project due to operating at a loss, however for future calculations it should be noted that we used a straight line depreciation schedule over a seven year period with a 40% tax rate.

Despite the current projections, it is important to consider the greater implications of running a desalination plant in the Bay Area. The population in the Bay Area is quickly growing, while California is experiencing harsher droughts each year. A consequence of this is the possibility of receiving government subsidies for our clean water product. The similarly scaled desalination plant in Carlsbad, CA operates at a loss as well, but receives significant government subsidies. Building this facility may be a strategic investment, especially since greater water scarcity is predicted in the future.

=Sensitivity Analysis=
We wanted to determine how sensitive NPV of our project is to the cost of capital, price of utilities and price we would be able to sell our clean water product at. We conducted this analysis by varying each of these variables independently between 0.5 times and 1.5 times the original value we calculated. As to be expected, NPV grows more negative with increasing capital costs and utility prices and decreasing water prices. Conversely, NPV becomes less negative as price of water rises and capital and utility costs decrease. A plot summarizing the effects of varying these costs can be found in Appendix E. NPV is most sensitive to the price at which we will be selling our clean water product. It is worth noting that NPV is more sensitive to utilities cost than it is to capital cost, which is why we focused on optimizing the cost of utilities. In future steps, trying to reduce utilities cost even more would be worth looking into to decrease the amount of money being lost on this project.

=Conclusions and Final Recommendation=
The final design achieves the project goal of producing 50 million gallons per day of distilled water at a yield of 13%, comparable to other similarly sized plants. An optimization of the number of MSF stages is realistic, and can be improved upon by taking into account other process parameters.

The proposed design was optimized around utility costs, comparing plants with 11 to 25 stages. The process is constrained by product specifications and by the thermodynamics of the heat exchange between the feed and the product streams. Energy conservation was achieved in using the seawater feed to condense the final product, as well as in all of the heat exchangers increasing the thermal energy of the seawater feed stream before it enters the flash stages.

In order to further optimize this process, other design parameters should be considered such as the brine recycle ratio, the cooler duty of the recycle stream, the pressure drop across each valve at the flash stages, and other economic factors. An optimization around maximizing NPV, for example, might be more comprehensive than that conducted regarding only utility costs. Tuning brine recycle rates could greatly improve process efficiency, and standardizing flash stage pressure drops would make operations more feasible and controlled.

Given our in depth economic analysis, this project would lose an average of $10.4MM per year, and has an NPV of -$495.7MM for a 20 year plant lifetime. From an economic perspective alone it would not be a good project to build. However, other factors need to be considered when building a project with enormous public value. In recent years California droughts have become more severe. Implementing a drought-proof supply of water now could avert a disaster later on. Even without a large disaster, having the desalination plant could also be expected to pay off in the future with the increasing price of water. Considerations such as these were the deciding factors in building the Carlsbad desalination plant. Since there are currently no desalination plants in the Bay area we recommend that preparations should be made to move forward with a desalination plant.

=References=
[1] Lyons J. Environmental Leader. Environmental Leader 2015. Available at: http://www.environmentalleader.com/2015/10/27/desalination-plants-to-double-by-2020. Accessed 2016.

[2] Fitzsimmons E. In California, Cities Braced to Cut Water by 10 to 35%. The New York Times 2015. Available at:
http://www.nytimes.com/2015/04/09/us/in-california-cities-braced-to-cut-water-by-10-to-35.html. Accessed 2016.

[3] Salinity Times Series - USGS Water Quality of SF Bay. Salinity Times Series - USGS Water Quality of SF Bay. Available at:
http://sfbay.wr.usgs.gov/access/wqdata/overview/examp/charts/salin.html. Accessed 2016.

[4] Reverse Osmosis vs. Multi Stage Flash Distillation: A Comparison Between Different Desalination Methods. Brighthub Engineering. Available at:
http://www.brighthubengineering.com/power-plants/29621-comparison-between-the-reverse-osmosis-and-multi-stage-flash-distillation-methods/. Accessed 2016.

[5] Committee on Advancing Desalination Technology, Water Science and Technology Board, Division on Earth and Life Studies, National Research Council. Desalination: A National Perspective. Washington, D.C.:The National Academies Press, 2008.

[6] H. El-Dessouky. Fundamentals of salt water desalination, 1st Ed., Elsevier Science, 2002.

[7] Committee on Advancing Desalination Technology, Water Science and Technology Board, Division on Earth and Life Studies, National Research Council. Desalination: A National Perspective. Washington, D.C.:The National Academies Press, 2008.

[8] Guasp RE, Bello CP, Jara JG, Vega JP, Damien T, Winkel C, Smith HH. Desalination by distillation. Organization of American States website. oas.org. Accessed January 28, 2015.

[9] Batch Distillation. AE 335 Separation Processes website. http://prodpran.che.engr.tu.ac.th/AE335/AE335.html. Accessed January 26, 2016.

[10] Morris R. The development of the multi-stage flash distillation process: A designer's viewpoint. Desalination. 1993;93:57-68.

[11] Darwish MA. Developments in the multi-stage flash desalting system. Desalination. 1995;100:35-64.

[12] Wade N. Distillation plant development and cost update. Desalination. 2001;136:3-12.

[13] Desalination Technologies and the Use of Alternative Energies for Desalination. World Intellectual Property Organization website. http://www.wipo.int/export/sites/www/patentscope/
en/programs/patent_landscapes/documents/patent_landscapes/948-2E-WEB.pdf. Updated November 2011. Accessed January 23, 2016.

[14] California Turns to the Pacific Ocean for Water | MIT Technology Review.MIT Technology Review 2014. Available at:
http://www.technologyreview.com/featuredstory/533446/desalination-out-of-desperation/. Accessed 2016.

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=Appendix A. Process Flow Diagram=
[[File:Appendix 1.PNG]]

'''Figure A1''' Process Flow Diagram of the MSF System

=Appendix B. HYSYS Simulation=
The outputs of the HYSYS model of the process is shown below.

[[File:AppendB.PNG]]

'''Figure B1.''' The HYSYS model of the MSF distillation process.

=Appendix C. Mass and Energy Balances=
Below are equations that describe the mass and energy balances of the system.

In MSF system with brine recycle, the mass flow of the intake seawater I is mixed with the mass flow of the recycle brine R to create the feed seawater stream F. Since this model assumes no non-condensable gases, the feed stream is overall balanced by the brine waste stream W, the recycle brine stream, and the distillate stream D. The brine stream exiting the last evaporation chamber B is divided into the recycle and waste streams. The amount of salt that enters the series of evaporation chambers is the amount of salt that leaves (X is the salt mass fraction).

[[File:Overallmasss.PNG]]

Within each stage, the change in mass of a phase is the mass flows of that phase into and out of the chamber and the rates of evaporation or condensation of the water into and out of that phase.
[[File:Masss2.PNG]]

The energy balances are summarized below.

[[File:Mass3.PNG]]

[[File:Mass4.PNG]]

=Appendix D. Summary of Process Units=

[[File:AppendD1.PNG]]
[[File:AppenD2.PNG]]

=Appendix E. MSF Stage Parameters=
[[File:AppendE.PNG]]

[[File:Appendix E3.PNG]]
[[File:Appendix E4.PNG]]

=Appendix F. Process Units Calculations=
[[File:AppendF.PNG]]

=Appendix G. Economic Analysis=

Desalination - Team A

2016-03-12T03:08:47Z

Ezhuang: /* Appendix B. HYSYS Simulation */

Author: Pear Dhiantravan[2016], Reed Kolbe[2016], Sheridan Lichtor[2016], John Marsiglio[2016], Ellen Zhuang[2016]

Instructors: Fengqi You, David Wegerer

Winter 2016

=Executive Summary=
Seawater desalination is an attractive approach to address the world’s freshwater shortage in light of the increasing global water scarcity. Our team decided to tackle California's growing drought crisis by designing a multistage flash desalination plant in Richmond, CA. Based on market analysis of existing desalination plants and water demands, this is a strategic location for the desalination of 50 million gallons per day of water from the San Francisco Bay.

The MSF process contains 18 flash stages in series. Seawater is taken in from the San Francisco Bay at a rate of 398 million gallons per day and mixed with brine recycled from the system. Within the MSF process, seawater is heated through a series of 18 heat exchangers and a brine heater before enters a series of 18 evaporation chambers. In the evaporation chambers, the seawater flashes and the freshwater vapor is condensed, collected, and sent to a water treatment facility. The brine stream proceeds to the next chamber, and the process continues. Upon exiting the last flashing chamber, the brine stream is split into a recycle brine stream that is mixed with the seawater inlet feed and a waste stream that is diluted with fresh seawater and returned to the sea. Overall, the process yield is 13%. This process was constructed in Aspen HYSYS and optimized around utility costs using the HYSYS Economic Analyzer. Based on duties, temperatures and pressures reported by HYSYS, as well as fundamental mass and energy balances, the equipment were sized appropriately and the economics of the process was analyzed.

The design has a total capital cost of $485.2 MM, an annual operating cost of $144.7 MM ($103.7 MM of which is utilities), and an annual plant revenue of $134.3 MM. Economic analysis reveals that the project has a negative NPV of -$495.7 MM and operates at a loss of $10.4 MM per year. Thus, the proposed design is not profitable. Despite the current projections, it is important to consider the greater implications of running a desalination plant in the Bay Area. The population in the Bay Area is quickly growing while California is experiencing harsher droughts each year. Given the dire situations, the government may support the proposed clean water project with subsidies. Building the facility may be a strategic investment, especially since greater water scarcity is predicted in the future.

Our design meets various environmental, ethical, social and health related constraints. The brine discharged back to the bay is diluted to at most 0.2% above the natural background salinity as required by law so that the plant does not impose harm on any local marine life. The demographic that the plant serves is largely invested in the protection of the environment, which we addressed by reducing energy consumption through direct energy integration with the nearby Chevron plant. Lastly, the product meets the specifications of the California Safe Water Drinking Act by producing water with no impurities.

__TOC__

=Introduction=
An increasing global water scarcity is fueling initiatives everywhere for clean water treatment, making efficient seawater desalination an attractive aim for chemical plant design. A 2015 market analysis found that the global desalination market earned revenues of $11.66 billion, and this number is expected to reach over $19 billion by 2019. Furthermore, the 17,000 desalination plants currently in operation are expected to double in number by 2020.1

As fresh water sources become increasingly scarce, strict water conservation measures are being observed. California is entering the fourth year of one of its most severe droughts on record. Cities are required to reduce their water usage by 35% to avoid facing fines.2 In lieu of these pressing conditions, California is looking for ways to provide more accessible fresh water to its citizens. This high demand for clean water motivated our choosing Richmond, CA as our plant location. At Richmond, the desalination plant can convert readily available seawater from the San Francisco Bay into fresh water. We propose to desalinate 50 million gallons of water per day to be sent to water treatment plants for potability. This aim is based on the capacity of the desalination plant in Carlsbad, California, which services a similar population.

The salinity of the water in the San Francisco bay is seasonal. During the dry seasons of summer and fall, salinity is high around 10 PSU (10,000 ppm) because water from the Pacific Ocean flows into the San Francisco Bay. During the wet winter season, fresh water from the rivers flows into the Bay, and salinity drops to around 2 PSU (2,000).3 Designed to meet the “worst case” scenario, our plant will process 398 million gallons of water per day with 10,000 ppm of salt to produce 50 million gallons of water with 1,000 ppm of salt. This gives a yield of 13%.

The desalination plant proposed consists of 18 flash stages in series. This report outlines our investigation of this potential water desalination plant, including design and optimization of the desalination process, an analysis of its energy consumption and environmental impacts, and a study of its economic implications.

=Technical Approach=
For the desalination step of our process, we selected a multistage flash (MSF) distillation process. Not only is MSF distillation a very viable method, but it is also the most common method, currently producing about 60% of the world’s desalinated water. MSF and reverse osmosis (RO) are the two major methods being used in large-scale desalination plants. Both processes require considerable amounts of energy. RO typically has a lower energy demand; however, the high impurity content of the Bay water would frequently necessitate membrane cleaning and/or exchange. Feed going through a RO system requires extensive pretreatment to remove biological organisms and other solids to control the pH and the chemical composition of the water. With the San Francisco Bay as our source of water, MSF is an attractive option because sediment and other large impurities can be separated from the feed before distillation occurs. Additionally, MSF distillation plants can be located near power plants and paired to their waste heat streams to conserve energy. This can reduce energy needs by half or two thirds, making MSF increasingly more practical. Due to the maintenance required in the RO system, the MSF distillation is the better option in terms of operation cost.4

A HYSYS model was built to simulate the MSF distillation section of the process, which spans after seawater pretreatment and before local potable water treatment. The model is used to simulate the distillation steps and to calculate necessary heating, cooling, and input flow rates to produce the required 50 million gallons of distillated water per day. For the simulation, the 398 million gallons per day of seawater feed stream is defined to be composed of 3.31 mol% sodium chloride (NaCl) and 96.69 mol% water. The percentage of NaCl is set to account for other ions and chemicals present after the pretreatment phase. NaCl alone is added to the HYSYS system due to the capabilities of the fluid package used to simulate this process, ElectroNRTL. The feed is set at 22°C, which is the approximate the temperature of the San Francisco Bay year round.

=Process Design=
==Design Overview==
The proposed desalination plant uses MSF distillation to convert the San Francisco Bay water to fresh water. The final process design contains 18 flash stages in series. Overall, 398 million gallons per day of of brackish water is converted to 50 million gallons per day of deionized water, a 13% yield. The production meets the required production rate of 50 million gallons of freshwater per day.

Seawater is taken in from the San Francisco Bay and mixed with the brine recycle stream produced downstream. The feed is then heated through a series of condensers and a heater. Distilled water vapor in each flash stage contacts the tubes carrying the seawater feed and condenses into a liquid stream by exchanging heat with the cooler seawater feed stream. The seawater is sent through a heater to increase its thermal energy before entering the first stage of the MSF. In each flashing stage, some of the water evaporates, leaving the salt behind. The freshwater vapors condense, as described above, and is collected and sent to a water treatment plant. The brine stream is split into the recycle stream and a waste stream, and the waste stream is diluted with fresh seawater before being discharged back into the bay.

A process flow diagram of the process is depicted in Appendix A.

==Pretreatment==
Sediment is removed to prevent solids from plugging the process downstream. Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime, and is controlled for by lowering pH. Acidifying the water will also remove CO2, which is a corrosive gas. To control for oxygen, oxygen scavengers will be included in the system to sequester it. Sodium bisulfate will be used as the scavenger.5

==Condensers (Heat Exchanger Networks)==
In order to conserve and recycle energy, the condensation of the distillate and heating of the seawater feed will be coupled in countercurrent heat exchanger networks. These heat exchanger networks are condensers of the design. The cold feed, which is a mixture of fresh seawater and brine recycle, is fed through multiple heat exchangers in series. Each heat exchanger corresponds to a different evaporation chamber. This process consists of 18 heat exchangers: E-101, E-102, … E-117, and E-118, which correspond to evaporation chambers V-102, V-103, … V-118, and V-119, respectively.

The heat exchangers can be viewed similarly to a shell and tube setup, however there is no actual physical shell. The tubes simply run through the top of the evaporation chambers, where vaporized distilled water in the chamber acts as the shell side fluid. The tube bundles are arranged in long tube configuration and are aligned parallel to the direction of the flashing brine flow in the evaporation chamber. The evaporating brine in an evaporation chamber rise up to the the corresponding heat exchanger and condense on the outside of the tubes, creating collectable pure water. This condensing water is what heats up the seawater in the tubes.

E-101 was used as the basis for the condenser sizing since it has the highest heat requirement, and thus will be the largest heat exchanger. From the HYSYS model, the overall system requires 2.24 x 106 kW of duty. Assuming this energy requirement, 5000 tubes, and 18 m length tubes (the diameter of the evaporation chambers, explained further on the next page), the diameter of each tube was calculated to be 0.4 m. Based on the temperature of the streams and flow, the thickness of the tube is 7 x 10-4 m.

==Brine Heater==
After the seawater exits the final heat exchanger, it enters a brine heater (E-119) where it is brought up to the temperature necessary for the flashing process to begin. This means that the seawater must be overheated compared to stage one, so that the seawater will flash in the evaporation chamber, i.e. release heat and vapor to reach equilibrium with stage conditions. After flowing through the condensers, the seawater enters the brine heater at 96.1 oC and is heated to 120 oC. The overall system requires 2.31 x 106 kW of duty. This is supplied by a total of 4.15 x 106 kg per hour of steam.

==Evaporation Chambers==
When the seawater has been brought to a temperature appropriate for flashing, it enters the first evaporation chamber (V-102). Upon entering the chamber, the water is flashed. Some of the water evaporates, leaving the salt behind. The vapor rises up to the heat exchanger tubes in each evaporation chamber and condenses on the outside into the distilled water product. This condensing water is what heats up the seawater in the tubes discussed above. The seawater that does not evaporate moves to the next stage, which operates at a higher temperature and lower pressure, and the process is repeated. This continues in series through a total of 18 flashing chambers (V-102, V-103, … V-118, and V-119). All of the distillate is collected into one stream and all of the brine is mixed with seawater before being disposed of back in the bay in order to dilute it enough so it does not have an adverse effect on marine life.

Each respective heat exchanger fully condenses the vapor product stream from each stage. However, when all of the individual product streams are consolidated into the final combined product stream, some water exists in the vapor phase due to temperature and pressure changes associated with mixing all of the streams together. Although the vapor product stream from each stage condenses completely across its respective heat exchanger, the total product stream contains some water in the vapor phase due to the temperature change when the mixer sums all the streams. In order to fully condense this stream, which will be sent to local water treatment plans, a heat exchanger (E-120) is put in place to cool down the product using the process feed stream.

The correlations of El-Dessouky and Ettouney were employed in order to size the flash chambers. They specified that conventional MSF plants that operate at a capacity between 27,000 and 36,000 m3/day use flashing stages of the dimensions 18 m diameter and 4 m height.6

==Valves==
The model requires valves in between the evaporation chambers. The seawater streams flow through valves with large pressure drops to flash a portion the liquid into vapor that can then be separated in the vessels. Large pressure drops provide the thermodynamic driving force to vaporize more of the liquid, and are required to achieve high product yield.

==Material of Construction==
Before 1980, the shells and internals of the MSF units were commonly constructed from carbon steel. Seawater, however, corrodes carbon steel. To compensate for this, the thickness of the carbon steel has an additional corrosion allowance, which increases the size and weight of the equipment. After 1980, the stainless steel and duplex stainless steel became increasingly common. Stainless steel metal requires less thickness for equivalent strength and corrosion resistance, which allows for a reduction in the size, weight, and therefore cost of units. Major units in our process will be constructed from stainless steel.6

==Process Alternatives==
In order to ensure that our design is fully optimized, we wanted to consider several alternatives for our process. We looked at alternatives associated with water pretreatment, the distillation process itself and product treatment.

===Water Pretreatment===
Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime. Scaling can be controlled by adjusting pH, temperature, and bicarbonate concentration. We selected acidification to control for process scaling. This is because lowering a process temperature would decrease overall efficiency, and decreasing bicarbonate concentration would be comparatively expensive. Lowering pH would have the added benefit of controlling CO2 concentrations since CO2 is a corrosive gas. To control for corrosion we looked at how to remove oxygen. To target the oxygen we chose an oxygen scavenger. This was chosen over other options such as a deaerator, or a protective coating because a deaerator would have a large capital cost, and the equipment itself would experience corrosion, and a tank coating such as zinc orthophosphate has a mixed effectiveness.7

===MSF: Brine Recycle vs. Once-Through System===
We explored two options for the overall layout of the MSF distillation plant. In the first, the brine passes through the process once; in the second, a portion of the brine is recycled from the last stage to the incoming seawater feed. The largest draw towards implementing a brine recycle is that it would significantly reduce the overall amount of seawater needed to produce the daily target of 50 million gallons per day. This in turn would lower the amount of energy required to run the pump feeding seawater to the process. Another positive is that, with a brine recycle, the amount of water conditioning chemicals that must be added to the seawater feed is reduced. The negative aspects mainly revolve around the fact that the solution throughout the process will be higher in salt concentration. This increase in salinity both raises the likelihood of scaling and corrosion in the plant and raises the amount of seawater that must be added to the final brine to make it safe for discharge back into the environment. Additionally, boiling point of each stage is raised when adding the brine recycle. After weighing both alternatives against one another, we have decided to include a brine recycle in our plant design. Most of the negative aspects of the brine recycle can be combated (e.g. include anti-scaling agents in the feed pretreatment), and the economic benefits of including the brine recycle are very significant.8

===MSF: Continuous vs. Batch===
The distillation step can be done in continuous or batch processes. Batch distillation is more versatile and often used when the product is in small amounts and very high purity. A continuous distillation is more efficient thermodynamically and economically for large amounts of material of constant composition, whereas batch distillation is more effective for small amounts of material of varying compositions. Since the plant is processing large amounts of seawater of relatively constant composition, the distillation system will utilize continuous distillation.9

===MSF: Long Tube vs. Crossed Tube ===
Inside the evaporator stage, the condenser tube bundles may be arranged in either long tube or cross tube design. In the long tube arrangement, the tubes are aligned parallel to the direction of the flashing brine flow. The tubes go from stage to stage without water boxes. In the cross tube arrangement, the tubes are lined perpendicular to the flow of the flashing brine. Water boxes transfer brine between the stages.10 11 While the water boxes in crossed tube bundles allow for more customization, the long tube configuration allows for construction of large unit sizes, as there are manufacturing restrictions with cross tubes. Long tube design can easily accommodate a large number of stages, which lead us to choose long tube for our design.11 12

===MSF: Venting System===
Noncondensable gases exist in the process because of air leakages into stages under vacuum conditions and the release of dissolved gases from the heated brine. These gases are removed by venting the system. An air cooler section concentrates the gas before they are vented, and baffles direct the vapor and condensates out. There are two venting arrangements to consider: series and parallel. The system in series moves gas from one stage directly to the next, and the only loss in the system is vapor in the last stage (since some vapor is extracted with gases). This system is economical, however this configuration increases the amount of noncondensable gases passing from one stage to the next, and disrupting the venting system in one stage will affect the entire system. In parallel venting, gas is released through parallel takeoff pipes at each stage. Although this arrangement allows for venting at any stage to be controlled, the design and maintenance is more complex, and more vapor is lost than in the venting system in series. A third alternative is to use a combination of venting in parallel and in series, which is the common method in industry. For this process, our team chose to do venting in parallel and in series.12

===Additional Energy Source Alternatives===
Energy for the process can also be coupled with renewable energy sources to reduce the desalination plant’s carbon footprint. Energy integration can be direct or indirect. In direct integration, thermal waste heat from industrial sources provide thermal energy or pressure as energy sources. Direct integration is advantageous at the modular level, as it avoids energy losses with electricity conversion. Within indirect integration, there are modular integration and grid/utility-scale integration. Modular integration integrates the plant with wind turbines and other small-scale renewable power generators with mobile deployment. A “hybridization” of renewable power sources with natural gas power generation is also an option, and the coupling of the two provides a stable power source. Indirect renewable energy integration is growing as the cost of renewable energy continues to decreases and provides an economy of scale that direct integration does not, but there are several obstacles towards indirect integration, such as the plant’s finance and engineering the integration of the two systems.13 This distillation plant will be directly integrated with neighboring plants such as Chevron that produce thermal waste heat.

===Product Treatment===
Since the product is distilled water, it needs additional ions to a concentration of around 0.01% before going to the end user. The three alternatives considered were building a water treatment facility on site, simply adding the correct salts to the product stream and then sending the stream for bacterial treatment at a water plant, or simply sending the product distilled water for complete treatment to another facility. Due to the number of nearby water treatment plants and their demand for water, we elected to simply send the product to a nearby water treatment facility.

==Design Constraints and Tradeoffs==
While designing our process, we wanted to make sure that we met several outside constraints, including environmental, political, ethical, social, sustainability, health and safety constraints. First and most importantly, we wanted to ensure that the brine discharged back to the bay would not harm local marine life. Our process was designed around the constraint of diluting the exiting brine to within 0.2% salinity of background seawater. This is not only compliant with California law, but was also verified to be safe for surrounding marine life through our own outside research. We also addressed many of these considerations by deciding to pursue direct energy integration with the nearby Chevron plant in order to reduce environmental impact of our process. Additionally, we ensured that our plant would be located in an area that is densely populated and in great need of fresh drinking water. Finally, we addressed health and safety considerations by making sure we would be able to send our product distilled water to local treatment plants for proper potabilization.

===Design Tradeoffs: Thermodynamics===
Adjusting elements within this process is iterative, and optimization thereof required changing multiple parameters at once. An example of this is in tuning the temperature increase by heater E-100; if the temperature of the stream entering the vessels is too high, the heat exchange between the vapor product and the feed stream will not be realizable because it causes the feed to be too hot to cool and condense the product. Higher temperatures, however, provide greater thermodynamic driving forces for flashing. Setting the entering stream temperature at 120°C provides enough heat for substantial flash at 130 kPa. This is the only heater required until the very last flash stage, which is added to provide more heat for vaporization. This heater (E-105) increases the stream temperature by 15°C, and the vapor product from the last vessel increases the feed stream temperature by almost 30°C. The added energy increases vaporization by 3%, which is a substantial 2.2 million kg/h. The temperature of the streams entering and leaving each stage decreases with each additional flash stage such that the last vessel has the lowest temperature, according to realistic operating guidelines.

===Design Tradeoffs: Energy Consumption, Environmental Impacts, and Product Optimization===
Increases in the pressure drop across any of the valves in this final design causes a temperature cross in the heat exchangers unless a proper increase in pump energy is provided. There is thus a design tradeoff between lowering energy usage and increasing product yield. For example, lowering the operating pressure of the process dictates that the pressure difference used to flash seawater must be smaller, possibly decreasing the amount that is vaporized. The final design aims for a yield of 50 million gallons per day. The 18 stage model was chosen for its low annual utility cost. This is a reflection of environmental mindfulness because energy consumption is an ongoing aspect of the process and should be lowered as much as possible.

==Mass and Energy Balances==
The model of the desalination system assumes that: (1) the properties are uniform in each phase within a stage, (2) heat losses are negligible due to the small surface area to volume ratio for each stage and properly insulated units (heat losses to the surrounding vary from 2 to 5% of the total system energy), (3) the distillate is salt free since the boiling point of water is much lower than that of salt, (4) the subcooling or superheating effects on the system energy balances are negligible, (5) the only contaminant is sodium chloride (there are no non-condensable gases to consider, so no vapors are vented), and (6) the process has 3 evaporation chambers.

The major balances are that the feed is a mixture of the fresh seawater stream and the recycle stream, and that the brine stream exiting the last evaporation chamber is divided into the recycle stream and the waste stream. Within each stage, the change in mass of a phase over time is the balance between the net flow of that phase from the chamber and the rates of evaporation and condensation of water into and out of that phase. The equations governing the mass and energy balances on the system is displayed in Appendix C.

=Optimization=
As initiatives for reduced energy usage and increased environmental sustainability become more prevalent, efforts to minimize the utility cost in plants has increased to accommodate these incentives. Utilities are a continual source of expenditure and consumption, and a well-designed chemical process should seek to minimize this element. The annualized utility cost is a useful parameter to optimize given that its value depends on variations in energy costs; thus, annualized utility costs do not correlate with conventional indices of inflation, such as capital and labor costs. Additionally, utility costs, like other variable costs, can be minimized by improving the design or operational efficiency of the plant.

In optimizing our MSF process, the design variable of focus was the number of stages to use in our process. The performance measure was the annualized utility cost, which was the parameter that was minimized for optimization. The process is constrained by the product specifications, which are kept constant and serve as the standard for comparison between each model. These specifications are to produce 50 million gallons of desalinated liquid water per day.

One feasibility constraint in the process is the size of the heat exchangers, which must provide a large enough area for total condensation of the vapor stream coming out of the flash stages. In order to restrict the heat exchange area needed within each exchanger, the number of stages must be at least 11. Increasing the number of stages decreases the vapor flow rate at each, and decreases the heat exchange area required as well. An 11-stage process allows for appropriate heat exchange between the feed stream and the vapor product, according to the HYSYS economic analyzer. The optimization analysis thus was conducted incorporating only models with 11 or more stages.

The HYSYS economic analyzer was used to systematically optimize the process by finding the model with the lowest annualized utility cost. The performance parameters kept constant across each model were: the inlet composition, pressure, and temperature, the brine outlet temperature, the total product flow rate and composition, and the flash stage schematic. Table 1 displays the annualized utility cost for each model, and illustrates that a clear dip in the utility cost occurs in an 18 stage model. Thus, 18 is the optimal number of stages to use for our MSF distillation given utility costs as the optimized parameter.
[[File:Table_1.PNG|thumb|center|500x300px]]

=Economic Evaluation=
In order to evaluate our process from a fiscal standpoint, we used Aspen Process Economic Analyzer and Aspen cost estimator to determine capital and operating costs for our plant. These costs were calculated based on calculated equipment sizes, as well as energy requirements reported by our HYSYS model. The capital cost was found to be $485.2MM and annual total operating cost was found to be $144.7MM ($103.7MM of which was utilities). Annual plant revenue was estimated based on prices that water districts in the county currently pay for outside water sources, and was found to be $134.3MM.14

The NPV analysis we conducted assumed that the plant would take two years to build, would operate at half capacity during the first year of operation (year 3), and have a 20 year lifetime. We also assumed that our plant would be in operation 350 days per year. A constant discount rate of 12% was assumed to hold throughout the plant’s lifetime. The final NPV of this project is -$495.7MM. This is due to both the large capital cost of the plant and the annual loss of $10.4MM while in full operation. A graph of cumulative cash flow versus time can be found in Appendix E. Tax rates and depreciation do not directly impact the NPV of this project due to operating at a loss, however for future calculations it should be noted that we used a straight line depreciation schedule over a seven year period with a 40% tax rate.

Despite the current projections, it is important to consider the greater implications of running a desalination plant in the Bay Area. The population in the Bay Area is quickly growing, while California is experiencing harsher droughts each year. A consequence of this is the possibility of receiving government subsidies for our clean water product. The similarly scaled desalination plant in Carlsbad, CA operates at a loss as well, but receives significant government subsidies. Building this facility may be a strategic investment, especially since greater water scarcity is predicted in the future.

=Sensitivity Analysis=
We wanted to determine how sensitive NPV of our project is to the cost of capital, price of utilities and price we would be able to sell our clean water product at. We conducted this analysis by varying each of these variables independently between 0.5 times and 1.5 times the original value we calculated. As to be expected, NPV grows more negative with increasing capital costs and utility prices and decreasing water prices. Conversely, NPV becomes less negative as price of water rises and capital and utility costs decrease. A plot summarizing the effects of varying these costs can be found in Appendix E. NPV is most sensitive to the price at which we will be selling our clean water product. It is worth noting that NPV is more sensitive to utilities cost than it is to capital cost, which is why we focused on optimizing the cost of utilities. In future steps, trying to reduce utilities cost even more would be worth looking into to decrease the amount of money being lost on this project.

=Conclusions and Final Recommendation=
The final design achieves the project goal of producing 50 million gallons per day of distilled water at a yield of 13%, comparable to other similarly sized plants. An optimization of the number of MSF stages is realistic, and can be improved upon by taking into account other process parameters.

The proposed design was optimized around utility costs, comparing plants with 11 to 25 stages. The process is constrained by product specifications and by the thermodynamics of the heat exchange between the feed and the product streams. Energy conservation was achieved in using the seawater feed to condense the final product, as well as in all of the heat exchangers increasing the thermal energy of the seawater feed stream before it enters the flash stages.

In order to further optimize this process, other design parameters should be considered such as the brine recycle ratio, the cooler duty of the recycle stream, the pressure drop across each valve at the flash stages, and other economic factors. An optimization around maximizing NPV, for example, might be more comprehensive than that conducted regarding only utility costs. Tuning brine recycle rates could greatly improve process efficiency, and standardizing flash stage pressure drops would make operations more feasible and controlled.

Given our in depth economic analysis, this project would lose an average of $10.4MM per year, and has an NPV of -$495.7MM for a 20 year plant lifetime. From an economic perspective alone it would not be a good project to build. However, other factors need to be considered when building a project with enormous public value. In recent years California droughts have become more severe. Implementing a drought-proof supply of water now could avert a disaster later on. Even without a large disaster, having the desalination plant could also be expected to pay off in the future with the increasing price of water. Considerations such as these were the deciding factors in building the Carlsbad desalination plant. Since there are currently no desalination plants in the Bay area we recommend that preparations should be made to move forward with a desalination plant.

=References=
[1] Lyons J. Environmental Leader. Environmental Leader 2015. Available at: http://www.environmentalleader.com/2015/10/27/desalination-plants-to-double-by-2020. Accessed 2016.

[2] Fitzsimmons E. In California, Cities Braced to Cut Water by 10 to 35%. The New York Times 2015. Available at:
http://www.nytimes.com/2015/04/09/us/in-california-cities-braced-to-cut-water-by-10-to-35.html. Accessed 2016.

[3] Salinity Times Series - USGS Water Quality of SF Bay. Salinity Times Series - USGS Water Quality of SF Bay. Available at:
http://sfbay.wr.usgs.gov/access/wqdata/overview/examp/charts/salin.html. Accessed 2016.

[4] Reverse Osmosis vs. Multi Stage Flash Distillation: A Comparison Between Different Desalination Methods. Brighthub Engineering. Available at:
http://www.brighthubengineering.com/power-plants/29621-comparison-between-the-reverse-osmosis-and-multi-stage-flash-distillation-methods/. Accessed 2016.

[5] Committee on Advancing Desalination Technology, Water Science and Technology Board, Division on Earth and Life Studies, National Research Council. Desalination: A National Perspective. Washington, D.C.:The National Academies Press, 2008.

[6] H. El-Dessouky. Fundamentals of salt water desalination, 1st Ed., Elsevier Science, 2002.

[7] Committee on Advancing Desalination Technology, Water Science and Technology Board, Division on Earth and Life Studies, National Research Council. Desalination: A National Perspective. Washington, D.C.:The National Academies Press, 2008.

[8] Guasp RE, Bello CP, Jara JG, Vega JP, Damien T, Winkel C, Smith HH. Desalination by distillation. Organization of American States website. oas.org. Accessed January 28, 2015.

[9] Batch Distillation. AE 335 Separation Processes website. http://prodpran.che.engr.tu.ac.th/AE335/AE335.html. Accessed January 26, 2016.

[10] Morris R. The development of the multi-stage flash distillation process: A designer's viewpoint. Desalination. 1993;93:57-68.

[11] Darwish MA. Developments in the multi-stage flash desalting system. Desalination. 1995;100:35-64.

[12] Wade N. Distillation plant development and cost update. Desalination. 2001;136:3-12.

[13] Desalination Technologies and the Use of Alternative Energies for Desalination. World Intellectual Property Organization website. http://www.wipo.int/export/sites/www/patentscope/
en/programs/patent_landscapes/documents/patent_landscapes/948-2E-WEB.pdf. Updated November 2011. Accessed January 23, 2016.

[14] California Turns to the Pacific Ocean for Water | MIT Technology Review.MIT Technology Review 2014. Available at:
http://www.technologyreview.com/featuredstory/533446/desalination-out-of-desperation/. Accessed 2016.

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=Appendix A. Process Flow Diagram=
[[File:Appendix 1.PNG]]

'''Figure A1''' Process Flow Diagram of the MSF System

=Appendix B. HYSYS Simulation=
The outputs of the HYSYS model of the process is shown below.

[[File:AppendB.PNG]]

'''Figure B1.''' The HYSYS model of the MSF distillation process.

=Appendix C. Mass and Energy Balances=
Below are equations that describe the mass and energy balances of the system.

In MSF system with brine recycle, the mass flow of the intake seawater I is mixed with the mass flow of the recycle brine R to create the feed seawater stream F. Since this model assumes no non-condensable gases, the feed stream is overall balanced by the brine waste stream W, the recycle brine stream, and the distillate stream D. The brine stream exiting the last evaporation chamber B is divided into the recycle and waste streams. The amount of salt that enters the series of evaporation chambers is the amount of salt that leaves (X is the salt mass fraction).

[[File:Overallmasss.PNG]]

Within each stage, the change in mass of a phase is the mass flows of that phase into and out of the chamber and the rates of evaporation or condensation of the water into and out of that phase.
[[File:Masss2.PNG]]

The energy balances are summarized below.

[[File:Mass3.PNG]]

[[File:Mass4.PNG]]

=Appendix D. Summary of Process Units=
=Appendix E. MSF Stage Parameters=
=Appendix F. Process Units Calculations=
=Appendix G. Economic Analysis=

Desalination - Team A

2016-03-12T03:01:50Z

Ezhuang: /* Appendix C. Mass and Energy Balances */

Author: Pear Dhiantravan[2016], Reed Kolbe[2016], Sheridan Lichtor[2016], John Marsiglio[2016], Ellen Zhuang[2016]

Instructors: Fengqi You, David Wegerer

Winter 2016

=Executive Summary=
Seawater desalination is an attractive approach to address the world’s freshwater shortage in light of the increasing global water scarcity. Our team decided to tackle California's growing drought crisis by designing a multistage flash desalination plant in Richmond, CA. Based on market analysis of existing desalination plants and water demands, this is a strategic location for the desalination of 50 million gallons per day of water from the San Francisco Bay.

The MSF process contains 18 flash stages in series. Seawater is taken in from the San Francisco Bay at a rate of 398 million gallons per day and mixed with brine recycled from the system. Within the MSF process, seawater is heated through a series of 18 heat exchangers and a brine heater before enters a series of 18 evaporation chambers. In the evaporation chambers, the seawater flashes and the freshwater vapor is condensed, collected, and sent to a water treatment facility. The brine stream proceeds to the next chamber, and the process continues. Upon exiting the last flashing chamber, the brine stream is split into a recycle brine stream that is mixed with the seawater inlet feed and a waste stream that is diluted with fresh seawater and returned to the sea. Overall, the process yield is 13%. This process was constructed in Aspen HYSYS and optimized around utility costs using the HYSYS Economic Analyzer. Based on duties, temperatures and pressures reported by HYSYS, as well as fundamental mass and energy balances, the equipment were sized appropriately and the economics of the process was analyzed.

The design has a total capital cost of $485.2 MM, an annual operating cost of $144.7 MM ($103.7 MM of which is utilities), and an annual plant revenue of $134.3 MM. Economic analysis reveals that the project has a negative NPV of -$495.7 MM and operates at a loss of $10.4 MM per year. Thus, the proposed design is not profitable. Despite the current projections, it is important to consider the greater implications of running a desalination plant in the Bay Area. The population in the Bay Area is quickly growing while California is experiencing harsher droughts each year. Given the dire situations, the government may support the proposed clean water project with subsidies. Building the facility may be a strategic investment, especially since greater water scarcity is predicted in the future.

Our design meets various environmental, ethical, social and health related constraints. The brine discharged back to the bay is diluted to at most 0.2% above the natural background salinity as required by law so that the plant does not impose harm on any local marine life. The demographic that the plant serves is largely invested in the protection of the environment, which we addressed by reducing energy consumption through direct energy integration with the nearby Chevron plant. Lastly, the product meets the specifications of the California Safe Water Drinking Act by producing water with no impurities.

__TOC__

=Introduction=
An increasing global water scarcity is fueling initiatives everywhere for clean water treatment, making efficient seawater desalination an attractive aim for chemical plant design. A 2015 market analysis found that the global desalination market earned revenues of $11.66 billion, and this number is expected to reach over $19 billion by 2019. Furthermore, the 17,000 desalination plants currently in operation are expected to double in number by 2020.1

As fresh water sources become increasingly scarce, strict water conservation measures are being observed. California is entering the fourth year of one of its most severe droughts on record. Cities are required to reduce their water usage by 35% to avoid facing fines.2 In lieu of these pressing conditions, California is looking for ways to provide more accessible fresh water to its citizens. This high demand for clean water motivated our choosing Richmond, CA as our plant location. At Richmond, the desalination plant can convert readily available seawater from the San Francisco Bay into fresh water. We propose to desalinate 50 million gallons of water per day to be sent to water treatment plants for potability. This aim is based on the capacity of the desalination plant in Carlsbad, California, which services a similar population.

The salinity of the water in the San Francisco bay is seasonal. During the dry seasons of summer and fall, salinity is high around 10 PSU (10,000 ppm) because water from the Pacific Ocean flows into the San Francisco Bay. During the wet winter season, fresh water from the rivers flows into the Bay, and salinity drops to around 2 PSU (2,000).3 Designed to meet the “worst case” scenario, our plant will process 398 million gallons of water per day with 10,000 ppm of salt to produce 50 million gallons of water with 1,000 ppm of salt. This gives a yield of 13%.

The desalination plant proposed consists of 18 flash stages in series. This report outlines our investigation of this potential water desalination plant, including design and optimization of the desalination process, an analysis of its energy consumption and environmental impacts, and a study of its economic implications.

=Technical Approach=
For the desalination step of our process, we selected a multistage flash (MSF) distillation process. Not only is MSF distillation a very viable method, but it is also the most common method, currently producing about 60% of the world’s desalinated water. MSF and reverse osmosis (RO) are the two major methods being used in large-scale desalination plants. Both processes require considerable amounts of energy. RO typically has a lower energy demand; however, the high impurity content of the Bay water would frequently necessitate membrane cleaning and/or exchange. Feed going through a RO system requires extensive pretreatment to remove biological organisms and other solids to control the pH and the chemical composition of the water. With the San Francisco Bay as our source of water, MSF is an attractive option because sediment and other large impurities can be separated from the feed before distillation occurs. Additionally, MSF distillation plants can be located near power plants and paired to their waste heat streams to conserve energy. This can reduce energy needs by half or two thirds, making MSF increasingly more practical. Due to the maintenance required in the RO system, the MSF distillation is the better option in terms of operation cost.4

A HYSYS model was built to simulate the MSF distillation section of the process, which spans after seawater pretreatment and before local potable water treatment. The model is used to simulate the distillation steps and to calculate necessary heating, cooling, and input flow rates to produce the required 50 million gallons of distillated water per day. For the simulation, the 398 million gallons per day of seawater feed stream is defined to be composed of 3.31 mol% sodium chloride (NaCl) and 96.69 mol% water. The percentage of NaCl is set to account for other ions and chemicals present after the pretreatment phase. NaCl alone is added to the HYSYS system due to the capabilities of the fluid package used to simulate this process, ElectroNRTL. The feed is set at 22°C, which is the approximate the temperature of the San Francisco Bay year round.

=Process Design=
==Design Overview==
The proposed desalination plant uses MSF distillation to convert the San Francisco Bay water to fresh water. The final process design contains 18 flash stages in series. Overall, 398 million gallons per day of of brackish water is converted to 50 million gallons per day of deionized water, a 13% yield. The production meets the required production rate of 50 million gallons of freshwater per day.

Seawater is taken in from the San Francisco Bay and mixed with the brine recycle stream produced downstream. The feed is then heated through a series of condensers and a heater. Distilled water vapor in each flash stage contacts the tubes carrying the seawater feed and condenses into a liquid stream by exchanging heat with the cooler seawater feed stream. The seawater is sent through a heater to increase its thermal energy before entering the first stage of the MSF. In each flashing stage, some of the water evaporates, leaving the salt behind. The freshwater vapors condense, as described above, and is collected and sent to a water treatment plant. The brine stream is split into the recycle stream and a waste stream, and the waste stream is diluted with fresh seawater before being discharged back into the bay.

A process flow diagram of the process is depicted in Appendix A.

==Pretreatment==
Sediment is removed to prevent solids from plugging the process downstream. Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime, and is controlled for by lowering pH. Acidifying the water will also remove CO2, which is a corrosive gas. To control for oxygen, oxygen scavengers will be included in the system to sequester it. Sodium bisulfate will be used as the scavenger.5

==Condensers (Heat Exchanger Networks)==
In order to conserve and recycle energy, the condensation of the distillate and heating of the seawater feed will be coupled in countercurrent heat exchanger networks. These heat exchanger networks are condensers of the design. The cold feed, which is a mixture of fresh seawater and brine recycle, is fed through multiple heat exchangers in series. Each heat exchanger corresponds to a different evaporation chamber. This process consists of 18 heat exchangers: E-101, E-102, … E-117, and E-118, which correspond to evaporation chambers V-102, V-103, … V-118, and V-119, respectively.

The heat exchangers can be viewed similarly to a shell and tube setup, however there is no actual physical shell. The tubes simply run through the top of the evaporation chambers, where vaporized distilled water in the chamber acts as the shell side fluid. The tube bundles are arranged in long tube configuration and are aligned parallel to the direction of the flashing brine flow in the evaporation chamber. The evaporating brine in an evaporation chamber rise up to the the corresponding heat exchanger and condense on the outside of the tubes, creating collectable pure water. This condensing water is what heats up the seawater in the tubes.

E-101 was used as the basis for the condenser sizing since it has the highest heat requirement, and thus will be the largest heat exchanger. From the HYSYS model, the overall system requires 2.24 x 106 kW of duty. Assuming this energy requirement, 5000 tubes, and 18 m length tubes (the diameter of the evaporation chambers, explained further on the next page), the diameter of each tube was calculated to be 0.4 m. Based on the temperature of the streams and flow, the thickness of the tube is 7 x 10-4 m.

==Brine Heater==
After the seawater exits the final heat exchanger, it enters a brine heater (E-119) where it is brought up to the temperature necessary for the flashing process to begin. This means that the seawater must be overheated compared to stage one, so that the seawater will flash in the evaporation chamber, i.e. release heat and vapor to reach equilibrium with stage conditions. After flowing through the condensers, the seawater enters the brine heater at 96.1 oC and is heated to 120 oC. The overall system requires 2.31 x 106 kW of duty. This is supplied by a total of 4.15 x 106 kg per hour of steam.

==Evaporation Chambers==
When the seawater has been brought to a temperature appropriate for flashing, it enters the first evaporation chamber (V-102). Upon entering the chamber, the water is flashed. Some of the water evaporates, leaving the salt behind. The vapor rises up to the heat exchanger tubes in each evaporation chamber and condenses on the outside into the distilled water product. This condensing water is what heats up the seawater in the tubes discussed above. The seawater that does not evaporate moves to the next stage, which operates at a higher temperature and lower pressure, and the process is repeated. This continues in series through a total of 18 flashing chambers (V-102, V-103, … V-118, and V-119). All of the distillate is collected into one stream and all of the brine is mixed with seawater before being disposed of back in the bay in order to dilute it enough so it does not have an adverse effect on marine life.

Each respective heat exchanger fully condenses the vapor product stream from each stage. However, when all of the individual product streams are consolidated into the final combined product stream, some water exists in the vapor phase due to temperature and pressure changes associated with mixing all of the streams together. Although the vapor product stream from each stage condenses completely across its respective heat exchanger, the total product stream contains some water in the vapor phase due to the temperature change when the mixer sums all the streams. In order to fully condense this stream, which will be sent to local water treatment plans, a heat exchanger (E-120) is put in place to cool down the product using the process feed stream.

The correlations of El-Dessouky and Ettouney were employed in order to size the flash chambers. They specified that conventional MSF plants that operate at a capacity between 27,000 and 36,000 m3/day use flashing stages of the dimensions 18 m diameter and 4 m height.6

==Valves==
The model requires valves in between the evaporation chambers. The seawater streams flow through valves with large pressure drops to flash a portion the liquid into vapor that can then be separated in the vessels. Large pressure drops provide the thermodynamic driving force to vaporize more of the liquid, and are required to achieve high product yield.

==Material of Construction==
Before 1980, the shells and internals of the MSF units were commonly constructed from carbon steel. Seawater, however, corrodes carbon steel. To compensate for this, the thickness of the carbon steel has an additional corrosion allowance, which increases the size and weight of the equipment. After 1980, the stainless steel and duplex stainless steel became increasingly common. Stainless steel metal requires less thickness for equivalent strength and corrosion resistance, which allows for a reduction in the size, weight, and therefore cost of units. Major units in our process will be constructed from stainless steel.6

==Process Alternatives==
In order to ensure that our design is fully optimized, we wanted to consider several alternatives for our process. We looked at alternatives associated with water pretreatment, the distillation process itself and product treatment.

===Water Pretreatment===
Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime. Scaling can be controlled by adjusting pH, temperature, and bicarbonate concentration. We selected acidification to control for process scaling. This is because lowering a process temperature would decrease overall efficiency, and decreasing bicarbonate concentration would be comparatively expensive. Lowering pH would have the added benefit of controlling CO2 concentrations since CO2 is a corrosive gas. To control for corrosion we looked at how to remove oxygen. To target the oxygen we chose an oxygen scavenger. This was chosen over other options such as a deaerator, or a protective coating because a deaerator would have a large capital cost, and the equipment itself would experience corrosion, and a tank coating such as zinc orthophosphate has a mixed effectiveness.7

===MSF: Brine Recycle vs. Once-Through System===
We explored two options for the overall layout of the MSF distillation plant. In the first, the brine passes through the process once; in the second, a portion of the brine is recycled from the last stage to the incoming seawater feed. The largest draw towards implementing a brine recycle is that it would significantly reduce the overall amount of seawater needed to produce the daily target of 50 million gallons per day. This in turn would lower the amount of energy required to run the pump feeding seawater to the process. Another positive is that, with a brine recycle, the amount of water conditioning chemicals that must be added to the seawater feed is reduced. The negative aspects mainly revolve around the fact that the solution throughout the process will be higher in salt concentration. This increase in salinity both raises the likelihood of scaling and corrosion in the plant and raises the amount of seawater that must be added to the final brine to make it safe for discharge back into the environment. Additionally, boiling point of each stage is raised when adding the brine recycle. After weighing both alternatives against one another, we have decided to include a brine recycle in our plant design. Most of the negative aspects of the brine recycle can be combated (e.g. include anti-scaling agents in the feed pretreatment), and the economic benefits of including the brine recycle are very significant.8

===MSF: Continuous vs. Batch===
The distillation step can be done in continuous or batch processes. Batch distillation is more versatile and often used when the product is in small amounts and very high purity. A continuous distillation is more efficient thermodynamically and economically for large amounts of material of constant composition, whereas batch distillation is more effective for small amounts of material of varying compositions. Since the plant is processing large amounts of seawater of relatively constant composition, the distillation system will utilize continuous distillation.9

===MSF: Long Tube vs. Crossed Tube ===
Inside the evaporator stage, the condenser tube bundles may be arranged in either long tube or cross tube design. In the long tube arrangement, the tubes are aligned parallel to the direction of the flashing brine flow. The tubes go from stage to stage without water boxes. In the cross tube arrangement, the tubes are lined perpendicular to the flow of the flashing brine. Water boxes transfer brine between the stages.10 11 While the water boxes in crossed tube bundles allow for more customization, the long tube configuration allows for construction of large unit sizes, as there are manufacturing restrictions with cross tubes. Long tube design can easily accommodate a large number of stages, which lead us to choose long tube for our design.11 12

===MSF: Venting System===
Noncondensable gases exist in the process because of air leakages into stages under vacuum conditions and the release of dissolved gases from the heated brine. These gases are removed by venting the system. An air cooler section concentrates the gas before they are vented, and baffles direct the vapor and condensates out. There are two venting arrangements to consider: series and parallel. The system in series moves gas from one stage directly to the next, and the only loss in the system is vapor in the last stage (since some vapor is extracted with gases). This system is economical, however this configuration increases the amount of noncondensable gases passing from one stage to the next, and disrupting the venting system in one stage will affect the entire system. In parallel venting, gas is released through parallel takeoff pipes at each stage. Although this arrangement allows for venting at any stage to be controlled, the design and maintenance is more complex, and more vapor is lost than in the venting system in series. A third alternative is to use a combination of venting in parallel and in series, which is the common method in industry. For this process, our team chose to do venting in parallel and in series.12

===Additional Energy Source Alternatives===
Energy for the process can also be coupled with renewable energy sources to reduce the desalination plant’s carbon footprint. Energy integration can be direct or indirect. In direct integration, thermal waste heat from industrial sources provide thermal energy or pressure as energy sources. Direct integration is advantageous at the modular level, as it avoids energy losses with electricity conversion. Within indirect integration, there are modular integration and grid/utility-scale integration. Modular integration integrates the plant with wind turbines and other small-scale renewable power generators with mobile deployment. A “hybridization” of renewable power sources with natural gas power generation is also an option, and the coupling of the two provides a stable power source. Indirect renewable energy integration is growing as the cost of renewable energy continues to decreases and provides an economy of scale that direct integration does not, but there are several obstacles towards indirect integration, such as the plant’s finance and engineering the integration of the two systems.13 This distillation plant will be directly integrated with neighboring plants such as Chevron that produce thermal waste heat.

===Product Treatment===
Since the product is distilled water, it needs additional ions to a concentration of around 0.01% before going to the end user. The three alternatives considered were building a water treatment facility on site, simply adding the correct salts to the product stream and then sending the stream for bacterial treatment at a water plant, or simply sending the product distilled water for complete treatment to another facility. Due to the number of nearby water treatment plants and their demand for water, we elected to simply send the product to a nearby water treatment facility.

==Design Constraints and Tradeoffs==
While designing our process, we wanted to make sure that we met several outside constraints, including environmental, political, ethical, social, sustainability, health and safety constraints. First and most importantly, we wanted to ensure that the brine discharged back to the bay would not harm local marine life. Our process was designed around the constraint of diluting the exiting brine to within 0.2% salinity of background seawater. This is not only compliant with California law, but was also verified to be safe for surrounding marine life through our own outside research. We also addressed many of these considerations by deciding to pursue direct energy integration with the nearby Chevron plant in order to reduce environmental impact of our process. Additionally, we ensured that our plant would be located in an area that is densely populated and in great need of fresh drinking water. Finally, we addressed health and safety considerations by making sure we would be able to send our product distilled water to local treatment plants for proper potabilization.

===Design Tradeoffs: Thermodynamics===
Adjusting elements within this process is iterative, and optimization thereof required changing multiple parameters at once. An example of this is in tuning the temperature increase by heater E-100; if the temperature of the stream entering the vessels is too high, the heat exchange between the vapor product and the feed stream will not be realizable because it causes the feed to be too hot to cool and condense the product. Higher temperatures, however, provide greater thermodynamic driving forces for flashing. Setting the entering stream temperature at 120°C provides enough heat for substantial flash at 130 kPa. This is the only heater required until the very last flash stage, which is added to provide more heat for vaporization. This heater (E-105) increases the stream temperature by 15°C, and the vapor product from the last vessel increases the feed stream temperature by almost 30°C. The added energy increases vaporization by 3%, which is a substantial 2.2 million kg/h. The temperature of the streams entering and leaving each stage decreases with each additional flash stage such that the last vessel has the lowest temperature, according to realistic operating guidelines.

===Design Tradeoffs: Energy Consumption, Environmental Impacts, and Product Optimization===
Increases in the pressure drop across any of the valves in this final design causes a temperature cross in the heat exchangers unless a proper increase in pump energy is provided. There is thus a design tradeoff between lowering energy usage and increasing product yield. For example, lowering the operating pressure of the process dictates that the pressure difference used to flash seawater must be smaller, possibly decreasing the amount that is vaporized. The final design aims for a yield of 50 million gallons per day. The 18 stage model was chosen for its low annual utility cost. This is a reflection of environmental mindfulness because energy consumption is an ongoing aspect of the process and should be lowered as much as possible.

==Mass and Energy Balances==
The model of the desalination system assumes that: (1) the properties are uniform in each phase within a stage, (2) heat losses are negligible due to the small surface area to volume ratio for each stage and properly insulated units (heat losses to the surrounding vary from 2 to 5% of the total system energy), (3) the distillate is salt free since the boiling point of water is much lower than that of salt, (4) the subcooling or superheating effects on the system energy balances are negligible, (5) the only contaminant is sodium chloride (there are no non-condensable gases to consider, so no vapors are vented), and (6) the process has 3 evaporation chambers.

The major balances are that the feed is a mixture of the fresh seawater stream and the recycle stream, and that the brine stream exiting the last evaporation chamber is divided into the recycle stream and the waste stream. Within each stage, the change in mass of a phase over time is the balance between the net flow of that phase from the chamber and the rates of evaporation and condensation of water into and out of that phase. The equations governing the mass and energy balances on the system is displayed in Appendix C.

=Optimization=
As initiatives for reduced energy usage and increased environmental sustainability become more prevalent, efforts to minimize the utility cost in plants has increased to accommodate these incentives. Utilities are a continual source of expenditure and consumption, and a well-designed chemical process should seek to minimize this element. The annualized utility cost is a useful parameter to optimize given that its value depends on variations in energy costs; thus, annualized utility costs do not correlate with conventional indices of inflation, such as capital and labor costs. Additionally, utility costs, like other variable costs, can be minimized by improving the design or operational efficiency of the plant.

In optimizing our MSF process, the design variable of focus was the number of stages to use in our process. The performance measure was the annualized utility cost, which was the parameter that was minimized for optimization. The process is constrained by the product specifications, which are kept constant and serve as the standard for comparison between each model. These specifications are to produce 50 million gallons of desalinated liquid water per day.

One feasibility constraint in the process is the size of the heat exchangers, which must provide a large enough area for total condensation of the vapor stream coming out of the flash stages. In order to restrict the heat exchange area needed within each exchanger, the number of stages must be at least 11. Increasing the number of stages decreases the vapor flow rate at each, and decreases the heat exchange area required as well. An 11-stage process allows for appropriate heat exchange between the feed stream and the vapor product, according to the HYSYS economic analyzer. The optimization analysis thus was conducted incorporating only models with 11 or more stages.

The HYSYS economic analyzer was used to systematically optimize the process by finding the model with the lowest annualized utility cost. The performance parameters kept constant across each model were: the inlet composition, pressure, and temperature, the brine outlet temperature, the total product flow rate and composition, and the flash stage schematic. Table 1 displays the annualized utility cost for each model, and illustrates that a clear dip in the utility cost occurs in an 18 stage model. Thus, 18 is the optimal number of stages to use for our MSF distillation given utility costs as the optimized parameter.
[[File:Table_1.PNG|thumb|center|500x300px]]

=Economic Evaluation=
In order to evaluate our process from a fiscal standpoint, we used Aspen Process Economic Analyzer and Aspen cost estimator to determine capital and operating costs for our plant. These costs were calculated based on calculated equipment sizes, as well as energy requirements reported by our HYSYS model. The capital cost was found to be $485.2MM and annual total operating cost was found to be $144.7MM ($103.7MM of which was utilities). Annual plant revenue was estimated based on prices that water districts in the county currently pay for outside water sources, and was found to be $134.3MM.14

The NPV analysis we conducted assumed that the plant would take two years to build, would operate at half capacity during the first year of operation (year 3), and have a 20 year lifetime. We also assumed that our plant would be in operation 350 days per year. A constant discount rate of 12% was assumed to hold throughout the plant’s lifetime. The final NPV of this project is -$495.7MM. This is due to both the large capital cost of the plant and the annual loss of $10.4MM while in full operation. A graph of cumulative cash flow versus time can be found in Appendix E. Tax rates and depreciation do not directly impact the NPV of this project due to operating at a loss, however for future calculations it should be noted that we used a straight line depreciation schedule over a seven year period with a 40% tax rate.

Despite the current projections, it is important to consider the greater implications of running a desalination plant in the Bay Area. The population in the Bay Area is quickly growing, while California is experiencing harsher droughts each year. A consequence of this is the possibility of receiving government subsidies for our clean water product. The similarly scaled desalination plant in Carlsbad, CA operates at a loss as well, but receives significant government subsidies. Building this facility may be a strategic investment, especially since greater water scarcity is predicted in the future.

=Sensitivity Analysis=
We wanted to determine how sensitive NPV of our project is to the cost of capital, price of utilities and price we would be able to sell our clean water product at. We conducted this analysis by varying each of these variables independently between 0.5 times and 1.5 times the original value we calculated. As to be expected, NPV grows more negative with increasing capital costs and utility prices and decreasing water prices. Conversely, NPV becomes less negative as price of water rises and capital and utility costs decrease. A plot summarizing the effects of varying these costs can be found in Appendix E. NPV is most sensitive to the price at which we will be selling our clean water product. It is worth noting that NPV is more sensitive to utilities cost than it is to capital cost, which is why we focused on optimizing the cost of utilities. In future steps, trying to reduce utilities cost even more would be worth looking into to decrease the amount of money being lost on this project.

=Conclusions and Final Recommendation=
The final design achieves the project goal of producing 50 million gallons per day of distilled water at a yield of 13%, comparable to other similarly sized plants. An optimization of the number of MSF stages is realistic, and can be improved upon by taking into account other process parameters.

The proposed design was optimized around utility costs, comparing plants with 11 to 25 stages. The process is constrained by product specifications and by the thermodynamics of the heat exchange between the feed and the product streams. Energy conservation was achieved in using the seawater feed to condense the final product, as well as in all of the heat exchangers increasing the thermal energy of the seawater feed stream before it enters the flash stages.

In order to further optimize this process, other design parameters should be considered such as the brine recycle ratio, the cooler duty of the recycle stream, the pressure drop across each valve at the flash stages, and other economic factors. An optimization around maximizing NPV, for example, might be more comprehensive than that conducted regarding only utility costs. Tuning brine recycle rates could greatly improve process efficiency, and standardizing flash stage pressure drops would make operations more feasible and controlled.

Given our in depth economic analysis, this project would lose an average of $10.4MM per year, and has an NPV of -$495.7MM for a 20 year plant lifetime. From an economic perspective alone it would not be a good project to build. However, other factors need to be considered when building a project with enormous public value. In recent years California droughts have become more severe. Implementing a drought-proof supply of water now could avert a disaster later on. Even without a large disaster, having the desalination plant could also be expected to pay off in the future with the increasing price of water. Considerations such as these were the deciding factors in building the Carlsbad desalination plant. Since there are currently no desalination plants in the Bay area we recommend that preparations should be made to move forward with a desalination plant.

=References=
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en/programs/patent_landscapes/documents/patent_landscapes/948-2E-WEB.pdf. Updated November 2011. Accessed January 23, 2016.

[14] California Turns to the Pacific Ocean for Water | MIT Technology Review.MIT Technology Review 2014. Available at:
http://www.technologyreview.com/featuredstory/533446/desalination-out-of-desperation/. Accessed 2016.

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=Appendix A. Process Flow Diagram=
[[File:Appendix 1.PNG]]

'''Figure A1''' Process Flow Diagram of the MSF System

=Appendix B. HYSYS Simulation=
[[File:Appendix B figure B1.PNG]]

=Appendix C. Mass and Energy Balances=
Below are equations that describe the mass and energy balances of the system.

In MSF system with brine recycle, the mass flow of the intake seawater I is mixed with the mass flow of the recycle brine R to create the feed seawater stream F. Since this model assumes no non-condensable gases, the feed stream is overall balanced by the brine waste stream W, the recycle brine stream, and the distillate stream D. The brine stream exiting the last evaporation chamber B is divided into the recycle and waste streams. The amount of salt that enters the series of evaporation chambers is the amount of salt that leaves (X is the salt mass fraction).

=Appendix D. Summary of Process Units=
=Appendix E. MSF Stage Parameters=
=Appendix F. Process Units Calculations=
=Appendix G. Economic Analysis=

Desalination - Team A

2016-03-12T02:59:47Z

Ezhuang: /* References */

Author: Pear Dhiantravan[2016], Reed Kolbe[2016], Sheridan Lichtor[2016], John Marsiglio[2016], Ellen Zhuang[2016]

Instructors: Fengqi You, David Wegerer

Winter 2016

=Executive Summary=
Seawater desalination is an attractive approach to address the world’s freshwater shortage in light of the increasing global water scarcity. Our team decided to tackle California's growing drought crisis by designing a multistage flash desalination plant in Richmond, CA. Based on market analysis of existing desalination plants and water demands, this is a strategic location for the desalination of 50 million gallons per day of water from the San Francisco Bay.

The MSF process contains 18 flash stages in series. Seawater is taken in from the San Francisco Bay at a rate of 398 million gallons per day and mixed with brine recycled from the system. Within the MSF process, seawater is heated through a series of 18 heat exchangers and a brine heater before enters a series of 18 evaporation chambers. In the evaporation chambers, the seawater flashes and the freshwater vapor is condensed, collected, and sent to a water treatment facility. The brine stream proceeds to the next chamber, and the process continues. Upon exiting the last flashing chamber, the brine stream is split into a recycle brine stream that is mixed with the seawater inlet feed and a waste stream that is diluted with fresh seawater and returned to the sea. Overall, the process yield is 13%. This process was constructed in Aspen HYSYS and optimized around utility costs using the HYSYS Economic Analyzer. Based on duties, temperatures and pressures reported by HYSYS, as well as fundamental mass and energy balances, the equipment were sized appropriately and the economics of the process was analyzed.

The design has a total capital cost of $485.2 MM, an annual operating cost of $144.7 MM ($103.7 MM of which is utilities), and an annual plant revenue of $134.3 MM. Economic analysis reveals that the project has a negative NPV of -$495.7 MM and operates at a loss of $10.4 MM per year. Thus, the proposed design is not profitable. Despite the current projections, it is important to consider the greater implications of running a desalination plant in the Bay Area. The population in the Bay Area is quickly growing while California is experiencing harsher droughts each year. Given the dire situations, the government may support the proposed clean water project with subsidies. Building the facility may be a strategic investment, especially since greater water scarcity is predicted in the future.

Our design meets various environmental, ethical, social and health related constraints. The brine discharged back to the bay is diluted to at most 0.2% above the natural background salinity as required by law so that the plant does not impose harm on any local marine life. The demographic that the plant serves is largely invested in the protection of the environment, which we addressed by reducing energy consumption through direct energy integration with the nearby Chevron plant. Lastly, the product meets the specifications of the California Safe Water Drinking Act by producing water with no impurities.

__TOC__

=Introduction=
An increasing global water scarcity is fueling initiatives everywhere for clean water treatment, making efficient seawater desalination an attractive aim for chemical plant design. A 2015 market analysis found that the global desalination market earned revenues of $11.66 billion, and this number is expected to reach over $19 billion by 2019. Furthermore, the 17,000 desalination plants currently in operation are expected to double in number by 2020.1

As fresh water sources become increasingly scarce, strict water conservation measures are being observed. California is entering the fourth year of one of its most severe droughts on record. Cities are required to reduce their water usage by 35% to avoid facing fines.2 In lieu of these pressing conditions, California is looking for ways to provide more accessible fresh water to its citizens. This high demand for clean water motivated our choosing Richmond, CA as our plant location. At Richmond, the desalination plant can convert readily available seawater from the San Francisco Bay into fresh water. We propose to desalinate 50 million gallons of water per day to be sent to water treatment plants for potability. This aim is based on the capacity of the desalination plant in Carlsbad, California, which services a similar population.

The salinity of the water in the San Francisco bay is seasonal. During the dry seasons of summer and fall, salinity is high around 10 PSU (10,000 ppm) because water from the Pacific Ocean flows into the San Francisco Bay. During the wet winter season, fresh water from the rivers flows into the Bay, and salinity drops to around 2 PSU (2,000).3 Designed to meet the “worst case” scenario, our plant will process 398 million gallons of water per day with 10,000 ppm of salt to produce 50 million gallons of water with 1,000 ppm of salt. This gives a yield of 13%.

The desalination plant proposed consists of 18 flash stages in series. This report outlines our investigation of this potential water desalination plant, including design and optimization of the desalination process, an analysis of its energy consumption and environmental impacts, and a study of its economic implications.

=Technical Approach=
For the desalination step of our process, we selected a multistage flash (MSF) distillation process. Not only is MSF distillation a very viable method, but it is also the most common method, currently producing about 60% of the world’s desalinated water. MSF and reverse osmosis (RO) are the two major methods being used in large-scale desalination plants. Both processes require considerable amounts of energy. RO typically has a lower energy demand; however, the high impurity content of the Bay water would frequently necessitate membrane cleaning and/or exchange. Feed going through a RO system requires extensive pretreatment to remove biological organisms and other solids to control the pH and the chemical composition of the water. With the San Francisco Bay as our source of water, MSF is an attractive option because sediment and other large impurities can be separated from the feed before distillation occurs. Additionally, MSF distillation plants can be located near power plants and paired to their waste heat streams to conserve energy. This can reduce energy needs by half or two thirds, making MSF increasingly more practical. Due to the maintenance required in the RO system, the MSF distillation is the better option in terms of operation cost.4

A HYSYS model was built to simulate the MSF distillation section of the process, which spans after seawater pretreatment and before local potable water treatment. The model is used to simulate the distillation steps and to calculate necessary heating, cooling, and input flow rates to produce the required 50 million gallons of distillated water per day. For the simulation, the 398 million gallons per day of seawater feed stream is defined to be composed of 3.31 mol% sodium chloride (NaCl) and 96.69 mol% water. The percentage of NaCl is set to account for other ions and chemicals present after the pretreatment phase. NaCl alone is added to the HYSYS system due to the capabilities of the fluid package used to simulate this process, ElectroNRTL. The feed is set at 22°C, which is the approximate the temperature of the San Francisco Bay year round.

=Process Design=
==Design Overview==
The proposed desalination plant uses MSF distillation to convert the San Francisco Bay water to fresh water. The final process design contains 18 flash stages in series. Overall, 398 million gallons per day of of brackish water is converted to 50 million gallons per day of deionized water, a 13% yield. The production meets the required production rate of 50 million gallons of freshwater per day.

Seawater is taken in from the San Francisco Bay and mixed with the brine recycle stream produced downstream. The feed is then heated through a series of condensers and a heater. Distilled water vapor in each flash stage contacts the tubes carrying the seawater feed and condenses into a liquid stream by exchanging heat with the cooler seawater feed stream. The seawater is sent through a heater to increase its thermal energy before entering the first stage of the MSF. In each flashing stage, some of the water evaporates, leaving the salt behind. The freshwater vapors condense, as described above, and is collected and sent to a water treatment plant. The brine stream is split into the recycle stream and a waste stream, and the waste stream is diluted with fresh seawater before being discharged back into the bay.

A process flow diagram of the process is depicted in Appendix A.

==Pretreatment==
Sediment is removed to prevent solids from plugging the process downstream. Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime, and is controlled for by lowering pH. Acidifying the water will also remove CO2, which is a corrosive gas. To control for oxygen, oxygen scavengers will be included in the system to sequester it. Sodium bisulfate will be used as the scavenger.5

==Condensers (Heat Exchanger Networks)==
In order to conserve and recycle energy, the condensation of the distillate and heating of the seawater feed will be coupled in countercurrent heat exchanger networks. These heat exchanger networks are condensers of the design. The cold feed, which is a mixture of fresh seawater and brine recycle, is fed through multiple heat exchangers in series. Each heat exchanger corresponds to a different evaporation chamber. This process consists of 18 heat exchangers: E-101, E-102, … E-117, and E-118, which correspond to evaporation chambers V-102, V-103, … V-118, and V-119, respectively.

The heat exchangers can be viewed similarly to a shell and tube setup, however there is no actual physical shell. The tubes simply run through the top of the evaporation chambers, where vaporized distilled water in the chamber acts as the shell side fluid. The tube bundles are arranged in long tube configuration and are aligned parallel to the direction of the flashing brine flow in the evaporation chamber. The evaporating brine in an evaporation chamber rise up to the the corresponding heat exchanger and condense on the outside of the tubes, creating collectable pure water. This condensing water is what heats up the seawater in the tubes.

E-101 was used as the basis for the condenser sizing since it has the highest heat requirement, and thus will be the largest heat exchanger. From the HYSYS model, the overall system requires 2.24 x 106 kW of duty. Assuming this energy requirement, 5000 tubes, and 18 m length tubes (the diameter of the evaporation chambers, explained further on the next page), the diameter of each tube was calculated to be 0.4 m. Based on the temperature of the streams and flow, the thickness of the tube is 7 x 10-4 m.

==Brine Heater==
After the seawater exits the final heat exchanger, it enters a brine heater (E-119) where it is brought up to the temperature necessary for the flashing process to begin. This means that the seawater must be overheated compared to stage one, so that the seawater will flash in the evaporation chamber, i.e. release heat and vapor to reach equilibrium with stage conditions. After flowing through the condensers, the seawater enters the brine heater at 96.1 oC and is heated to 120 oC. The overall system requires 2.31 x 106 kW of duty. This is supplied by a total of 4.15 x 106 kg per hour of steam.

==Evaporation Chambers==
When the seawater has been brought to a temperature appropriate for flashing, it enters the first evaporation chamber (V-102). Upon entering the chamber, the water is flashed. Some of the water evaporates, leaving the salt behind. The vapor rises up to the heat exchanger tubes in each evaporation chamber and condenses on the outside into the distilled water product. This condensing water is what heats up the seawater in the tubes discussed above. The seawater that does not evaporate moves to the next stage, which operates at a higher temperature and lower pressure, and the process is repeated. This continues in series through a total of 18 flashing chambers (V-102, V-103, … V-118, and V-119). All of the distillate is collected into one stream and all of the brine is mixed with seawater before being disposed of back in the bay in order to dilute it enough so it does not have an adverse effect on marine life.

Each respective heat exchanger fully condenses the vapor product stream from each stage. However, when all of the individual product streams are consolidated into the final combined product stream, some water exists in the vapor phase due to temperature and pressure changes associated with mixing all of the streams together. Although the vapor product stream from each stage condenses completely across its respective heat exchanger, the total product stream contains some water in the vapor phase due to the temperature change when the mixer sums all the streams. In order to fully condense this stream, which will be sent to local water treatment plans, a heat exchanger (E-120) is put in place to cool down the product using the process feed stream.

The correlations of El-Dessouky and Ettouney were employed in order to size the flash chambers. They specified that conventional MSF plants that operate at a capacity between 27,000 and 36,000 m3/day use flashing stages of the dimensions 18 m diameter and 4 m height.6

==Valves==
The model requires valves in between the evaporation chambers. The seawater streams flow through valves with large pressure drops to flash a portion the liquid into vapor that can then be separated in the vessels. Large pressure drops provide the thermodynamic driving force to vaporize more of the liquid, and are required to achieve high product yield.

==Material of Construction==
Before 1980, the shells and internals of the MSF units were commonly constructed from carbon steel. Seawater, however, corrodes carbon steel. To compensate for this, the thickness of the carbon steel has an additional corrosion allowance, which increases the size and weight of the equipment. After 1980, the stainless steel and duplex stainless steel became increasingly common. Stainless steel metal requires less thickness for equivalent strength and corrosion resistance, which allows for a reduction in the size, weight, and therefore cost of units. Major units in our process will be constructed from stainless steel.6

==Process Alternatives==
In order to ensure that our design is fully optimized, we wanted to consider several alternatives for our process. We looked at alternatives associated with water pretreatment, the distillation process itself and product treatment.

===Water Pretreatment===
Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime. Scaling can be controlled by adjusting pH, temperature, and bicarbonate concentration. We selected acidification to control for process scaling. This is because lowering a process temperature would decrease overall efficiency, and decreasing bicarbonate concentration would be comparatively expensive. Lowering pH would have the added benefit of controlling CO2 concentrations since CO2 is a corrosive gas. To control for corrosion we looked at how to remove oxygen. To target the oxygen we chose an oxygen scavenger. This was chosen over other options such as a deaerator, or a protective coating because a deaerator would have a large capital cost, and the equipment itself would experience corrosion, and a tank coating such as zinc orthophosphate has a mixed effectiveness.7

===MSF: Brine Recycle vs. Once-Through System===
We explored two options for the overall layout of the MSF distillation plant. In the first, the brine passes through the process once; in the second, a portion of the brine is recycled from the last stage to the incoming seawater feed. The largest draw towards implementing a brine recycle is that it would significantly reduce the overall amount of seawater needed to produce the daily target of 50 million gallons per day. This in turn would lower the amount of energy required to run the pump feeding seawater to the process. Another positive is that, with a brine recycle, the amount of water conditioning chemicals that must be added to the seawater feed is reduced. The negative aspects mainly revolve around the fact that the solution throughout the process will be higher in salt concentration. This increase in salinity both raises the likelihood of scaling and corrosion in the plant and raises the amount of seawater that must be added to the final brine to make it safe for discharge back into the environment. Additionally, boiling point of each stage is raised when adding the brine recycle. After weighing both alternatives against one another, we have decided to include a brine recycle in our plant design. Most of the negative aspects of the brine recycle can be combated (e.g. include anti-scaling agents in the feed pretreatment), and the economic benefits of including the brine recycle are very significant.8

===MSF: Continuous vs. Batch===
The distillation step can be done in continuous or batch processes. Batch distillation is more versatile and often used when the product is in small amounts and very high purity. A continuous distillation is more efficient thermodynamically and economically for large amounts of material of constant composition, whereas batch distillation is more effective for small amounts of material of varying compositions. Since the plant is processing large amounts of seawater of relatively constant composition, the distillation system will utilize continuous distillation.9

===MSF: Long Tube vs. Crossed Tube ===
Inside the evaporator stage, the condenser tube bundles may be arranged in either long tube or cross tube design. In the long tube arrangement, the tubes are aligned parallel to the direction of the flashing brine flow. The tubes go from stage to stage without water boxes. In the cross tube arrangement, the tubes are lined perpendicular to the flow of the flashing brine. Water boxes transfer brine between the stages.10 11 While the water boxes in crossed tube bundles allow for more customization, the long tube configuration allows for construction of large unit sizes, as there are manufacturing restrictions with cross tubes. Long tube design can easily accommodate a large number of stages, which lead us to choose long tube for our design.11 12

===MSF: Venting System===
Noncondensable gases exist in the process because of air leakages into stages under vacuum conditions and the release of dissolved gases from the heated brine. These gases are removed by venting the system. An air cooler section concentrates the gas before they are vented, and baffles direct the vapor and condensates out. There are two venting arrangements to consider: series and parallel. The system in series moves gas from one stage directly to the next, and the only loss in the system is vapor in the last stage (since some vapor is extracted with gases). This system is economical, however this configuration increases the amount of noncondensable gases passing from one stage to the next, and disrupting the venting system in one stage will affect the entire system. In parallel venting, gas is released through parallel takeoff pipes at each stage. Although this arrangement allows for venting at any stage to be controlled, the design and maintenance is more complex, and more vapor is lost than in the venting system in series. A third alternative is to use a combination of venting in parallel and in series, which is the common method in industry. For this process, our team chose to do venting in parallel and in series.12

===Additional Energy Source Alternatives===
Energy for the process can also be coupled with renewable energy sources to reduce the desalination plant’s carbon footprint. Energy integration can be direct or indirect. In direct integration, thermal waste heat from industrial sources provide thermal energy or pressure as energy sources. Direct integration is advantageous at the modular level, as it avoids energy losses with electricity conversion. Within indirect integration, there are modular integration and grid/utility-scale integration. Modular integration integrates the plant with wind turbines and other small-scale renewable power generators with mobile deployment. A “hybridization” of renewable power sources with natural gas power generation is also an option, and the coupling of the two provides a stable power source. Indirect renewable energy integration is growing as the cost of renewable energy continues to decreases and provides an economy of scale that direct integration does not, but there are several obstacles towards indirect integration, such as the plant’s finance and engineering the integration of the two systems.13 This distillation plant will be directly integrated with neighboring plants such as Chevron that produce thermal waste heat.

===Product Treatment===
Since the product is distilled water, it needs additional ions to a concentration of around 0.01% before going to the end user. The three alternatives considered were building a water treatment facility on site, simply adding the correct salts to the product stream and then sending the stream for bacterial treatment at a water plant, or simply sending the product distilled water for complete treatment to another facility. Due to the number of nearby water treatment plants and their demand for water, we elected to simply send the product to a nearby water treatment facility.

==Design Constraints and Tradeoffs==
While designing our process, we wanted to make sure that we met several outside constraints, including environmental, political, ethical, social, sustainability, health and safety constraints. First and most importantly, we wanted to ensure that the brine discharged back to the bay would not harm local marine life. Our process was designed around the constraint of diluting the exiting brine to within 0.2% salinity of background seawater. This is not only compliant with California law, but was also verified to be safe for surrounding marine life through our own outside research. We also addressed many of these considerations by deciding to pursue direct energy integration with the nearby Chevron plant in order to reduce environmental impact of our process. Additionally, we ensured that our plant would be located in an area that is densely populated and in great need of fresh drinking water. Finally, we addressed health and safety considerations by making sure we would be able to send our product distilled water to local treatment plants for proper potabilization.

===Design Tradeoffs: Thermodynamics===
Adjusting elements within this process is iterative, and optimization thereof required changing multiple parameters at once. An example of this is in tuning the temperature increase by heater E-100; if the temperature of the stream entering the vessels is too high, the heat exchange between the vapor product and the feed stream will not be realizable because it causes the feed to be too hot to cool and condense the product. Higher temperatures, however, provide greater thermodynamic driving forces for flashing. Setting the entering stream temperature at 120°C provides enough heat for substantial flash at 130 kPa. This is the only heater required until the very last flash stage, which is added to provide more heat for vaporization. This heater (E-105) increases the stream temperature by 15°C, and the vapor product from the last vessel increases the feed stream temperature by almost 30°C. The added energy increases vaporization by 3%, which is a substantial 2.2 million kg/h. The temperature of the streams entering and leaving each stage decreases with each additional flash stage such that the last vessel has the lowest temperature, according to realistic operating guidelines.

===Design Tradeoffs: Energy Consumption, Environmental Impacts, and Product Optimization===
Increases in the pressure drop across any of the valves in this final design causes a temperature cross in the heat exchangers unless a proper increase in pump energy is provided. There is thus a design tradeoff between lowering energy usage and increasing product yield. For example, lowering the operating pressure of the process dictates that the pressure difference used to flash seawater must be smaller, possibly decreasing the amount that is vaporized. The final design aims for a yield of 50 million gallons per day. The 18 stage model was chosen for its low annual utility cost. This is a reflection of environmental mindfulness because energy consumption is an ongoing aspect of the process and should be lowered as much as possible.

==Mass and Energy Balances==
The model of the desalination system assumes that: (1) the properties are uniform in each phase within a stage, (2) heat losses are negligible due to the small surface area to volume ratio for each stage and properly insulated units (heat losses to the surrounding vary from 2 to 5% of the total system energy), (3) the distillate is salt free since the boiling point of water is much lower than that of salt, (4) the subcooling or superheating effects on the system energy balances are negligible, (5) the only contaminant is sodium chloride (there are no non-condensable gases to consider, so no vapors are vented), and (6) the process has 3 evaporation chambers.

The major balances are that the feed is a mixture of the fresh seawater stream and the recycle stream, and that the brine stream exiting the last evaporation chamber is divided into the recycle stream and the waste stream. Within each stage, the change in mass of a phase over time is the balance between the net flow of that phase from the chamber and the rates of evaporation and condensation of water into and out of that phase. The equations governing the mass and energy balances on the system is displayed in Appendix C.

=Optimization=
As initiatives for reduced energy usage and increased environmental sustainability become more prevalent, efforts to minimize the utility cost in plants has increased to accommodate these incentives. Utilities are a continual source of expenditure and consumption, and a well-designed chemical process should seek to minimize this element. The annualized utility cost is a useful parameter to optimize given that its value depends on variations in energy costs; thus, annualized utility costs do not correlate with conventional indices of inflation, such as capital and labor costs. Additionally, utility costs, like other variable costs, can be minimized by improving the design or operational efficiency of the plant.

In optimizing our MSF process, the design variable of focus was the number of stages to use in our process. The performance measure was the annualized utility cost, which was the parameter that was minimized for optimization. The process is constrained by the product specifications, which are kept constant and serve as the standard for comparison between each model. These specifications are to produce 50 million gallons of desalinated liquid water per day.

One feasibility constraint in the process is the size of the heat exchangers, which must provide a large enough area for total condensation of the vapor stream coming out of the flash stages. In order to restrict the heat exchange area needed within each exchanger, the number of stages must be at least 11. Increasing the number of stages decreases the vapor flow rate at each, and decreases the heat exchange area required as well. An 11-stage process allows for appropriate heat exchange between the feed stream and the vapor product, according to the HYSYS economic analyzer. The optimization analysis thus was conducted incorporating only models with 11 or more stages.

The HYSYS economic analyzer was used to systematically optimize the process by finding the model with the lowest annualized utility cost. The performance parameters kept constant across each model were: the inlet composition, pressure, and temperature, the brine outlet temperature, the total product flow rate and composition, and the flash stage schematic. Table 1 displays the annualized utility cost for each model, and illustrates that a clear dip in the utility cost occurs in an 18 stage model. Thus, 18 is the optimal number of stages to use for our MSF distillation given utility costs as the optimized parameter.
[[File:Table_1.PNG|thumb|center|500x300px]]

=Economic Evaluation=
In order to evaluate our process from a fiscal standpoint, we used Aspen Process Economic Analyzer and Aspen cost estimator to determine capital and operating costs for our plant. These costs were calculated based on calculated equipment sizes, as well as energy requirements reported by our HYSYS model. The capital cost was found to be $485.2MM and annual total operating cost was found to be $144.7MM ($103.7MM of which was utilities). Annual plant revenue was estimated based on prices that water districts in the county currently pay for outside water sources, and was found to be $134.3MM.14

The NPV analysis we conducted assumed that the plant would take two years to build, would operate at half capacity during the first year of operation (year 3), and have a 20 year lifetime. We also assumed that our plant would be in operation 350 days per year. A constant discount rate of 12% was assumed to hold throughout the plant’s lifetime. The final NPV of this project is -$495.7MM. This is due to both the large capital cost of the plant and the annual loss of $10.4MM while in full operation. A graph of cumulative cash flow versus time can be found in Appendix E. Tax rates and depreciation do not directly impact the NPV of this project due to operating at a loss, however for future calculations it should be noted that we used a straight line depreciation schedule over a seven year period with a 40% tax rate.

Despite the current projections, it is important to consider the greater implications of running a desalination plant in the Bay Area. The population in the Bay Area is quickly growing, while California is experiencing harsher droughts each year. A consequence of this is the possibility of receiving government subsidies for our clean water product. The similarly scaled desalination plant in Carlsbad, CA operates at a loss as well, but receives significant government subsidies. Building this facility may be a strategic investment, especially since greater water scarcity is predicted in the future.

=Sensitivity Analysis=
We wanted to determine how sensitive NPV of our project is to the cost of capital, price of utilities and price we would be able to sell our clean water product at. We conducted this analysis by varying each of these variables independently between 0.5 times and 1.5 times the original value we calculated. As to be expected, NPV grows more negative with increasing capital costs and utility prices and decreasing water prices. Conversely, NPV becomes less negative as price of water rises and capital and utility costs decrease. A plot summarizing the effects of varying these costs can be found in Appendix E. NPV is most sensitive to the price at which we will be selling our clean water product. It is worth noting that NPV is more sensitive to utilities cost than it is to capital cost, which is why we focused on optimizing the cost of utilities. In future steps, trying to reduce utilities cost even more would be worth looking into to decrease the amount of money being lost on this project.

=Conclusions and Final Recommendation=
The final design achieves the project goal of producing 50 million gallons per day of distilled water at a yield of 13%, comparable to other similarly sized plants. An optimization of the number of MSF stages is realistic, and can be improved upon by taking into account other process parameters.

The proposed design was optimized around utility costs, comparing plants with 11 to 25 stages. The process is constrained by product specifications and by the thermodynamics of the heat exchange between the feed and the product streams. Energy conservation was achieved in using the seawater feed to condense the final product, as well as in all of the heat exchangers increasing the thermal energy of the seawater feed stream before it enters the flash stages.

In order to further optimize this process, other design parameters should be considered such as the brine recycle ratio, the cooler duty of the recycle stream, the pressure drop across each valve at the flash stages, and other economic factors. An optimization around maximizing NPV, for example, might be more comprehensive than that conducted regarding only utility costs. Tuning brine recycle rates could greatly improve process efficiency, and standardizing flash stage pressure drops would make operations more feasible and controlled.

Given our in depth economic analysis, this project would lose an average of $10.4MM per year, and has an NPV of -$495.7MM for a 20 year plant lifetime. From an economic perspective alone it would not be a good project to build. However, other factors need to be considered when building a project with enormous public value. In recent years California droughts have become more severe. Implementing a drought-proof supply of water now could avert a disaster later on. Even without a large disaster, having the desalination plant could also be expected to pay off in the future with the increasing price of water. Considerations such as these were the deciding factors in building the Carlsbad desalination plant. Since there are currently no desalination plants in the Bay area we recommend that preparations should be made to move forward with a desalination plant.

=References=
[1] Lyons J. Environmental Leader. Environmental Leader 2015. Available at: http://www.environmentalleader.com/2015/10/27/desalination-plants-to-double-by-2020. Accessed 2016.

[2] Fitzsimmons E. In California, Cities Braced to Cut Water by 10 to 35%. The New York Times 2015. Available at:
http://www.nytimes.com/2015/04/09/us/in-california-cities-braced-to-cut-water-by-10-to-35.html. Accessed 2016.

[3] Salinity Times Series - USGS Water Quality of SF Bay. Salinity Times Series - USGS Water Quality of SF Bay. Available at:
http://sfbay.wr.usgs.gov/access/wqdata/overview/examp/charts/salin.html. Accessed 2016.

[4] Reverse Osmosis vs. Multi Stage Flash Distillation: A Comparison Between Different Desalination Methods. Brighthub Engineering. Available at:
http://www.brighthubengineering.com/power-plants/29621-comparison-between-the-reverse-osmosis-and-multi-stage-flash-distillation-methods/. Accessed 2016.

[5] Committee on Advancing Desalination Technology, Water Science and Technology Board, Division on Earth and Life Studies, National Research Council. Desalination: A National Perspective. Washington, D.C.:The National Academies Press, 2008.

[6] H. El-Dessouky. Fundamentals of salt water desalination, 1st Ed., Elsevier Science, 2002.

[7] Committee on Advancing Desalination Technology, Water Science and Technology Board, Division on Earth and Life Studies, National Research Council. Desalination: A National Perspective. Washington, D.C.:The National Academies Press, 2008.

[8] Guasp RE, Bello CP, Jara JG, Vega JP, Damien T, Winkel C, Smith HH. Desalination by distillation. Organization of American States website. oas.org. Accessed January 28, 2015.

[9] Batch Distillation. AE 335 Separation Processes website. http://prodpran.che.engr.tu.ac.th/AE335/AE335.html. Accessed January 26, 2016.

[10] Morris R. The development of the multi-stage flash distillation process: A designer's viewpoint. Desalination. 1993;93:57-68.

[11] Darwish MA. Developments in the multi-stage flash desalting system. Desalination. 1995;100:35-64.

[12] Wade N. Distillation plant development and cost update. Desalination. 2001;136:3-12.

[13] Desalination Technologies and the Use of Alternative Energies for Desalination. World Intellectual Property Organization website. http://www.wipo.int/export/sites/www/patentscope/
en/programs/patent_landscapes/documents/patent_landscapes/948-2E-WEB.pdf. Updated November 2011. Accessed January 23, 2016.

[14] California Turns to the Pacific Ocean for Water | MIT Technology Review.MIT Technology Review 2014. Available at:
http://www.technologyreview.com/featuredstory/533446/desalination-out-of-desperation/. Accessed 2016.

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=Appendix A. Process Flow Diagram=
[[File:Appendix 1.PNG]]

'''Figure A1''' Process Flow Diagram of the MSF System

=Appendix B. HYSYS Simulation=
[[File:Appendix B figure B1.PNG]]

=Appendix C. Mass and Energy Balances=
=Appendix D. Summary of Process Units=
=Appendix E. MSF Stage Parameters=
=Appendix F. Process Units Calculations=
=Appendix G. Economic Analysis=

Desalination - Team A

2016-03-12T02:59:37Z

Ezhuang: /* Conclusions and Final Recommendation */

Author: Pear Dhiantravan[2016], Reed Kolbe[2016], Sheridan Lichtor[2016], John Marsiglio[2016], Ellen Zhuang[2016]

Instructors: Fengqi You, David Wegerer

Winter 2016

=Executive Summary=
Seawater desalination is an attractive approach to address the world’s freshwater shortage in light of the increasing global water scarcity. Our team decided to tackle California's growing drought crisis by designing a multistage flash desalination plant in Richmond, CA. Based on market analysis of existing desalination plants and water demands, this is a strategic location for the desalination of 50 million gallons per day of water from the San Francisco Bay.

The MSF process contains 18 flash stages in series. Seawater is taken in from the San Francisco Bay at a rate of 398 million gallons per day and mixed with brine recycled from the system. Within the MSF process, seawater is heated through a series of 18 heat exchangers and a brine heater before enters a series of 18 evaporation chambers. In the evaporation chambers, the seawater flashes and the freshwater vapor is condensed, collected, and sent to a water treatment facility. The brine stream proceeds to the next chamber, and the process continues. Upon exiting the last flashing chamber, the brine stream is split into a recycle brine stream that is mixed with the seawater inlet feed and a waste stream that is diluted with fresh seawater and returned to the sea. Overall, the process yield is 13%. This process was constructed in Aspen HYSYS and optimized around utility costs using the HYSYS Economic Analyzer. Based on duties, temperatures and pressures reported by HYSYS, as well as fundamental mass and energy balances, the equipment were sized appropriately and the economics of the process was analyzed.

The design has a total capital cost of $485.2 MM, an annual operating cost of $144.7 MM ($103.7 MM of which is utilities), and an annual plant revenue of $134.3 MM. Economic analysis reveals that the project has a negative NPV of -$495.7 MM and operates at a loss of $10.4 MM per year. Thus, the proposed design is not profitable. Despite the current projections, it is important to consider the greater implications of running a desalination plant in the Bay Area. The population in the Bay Area is quickly growing while California is experiencing harsher droughts each year. Given the dire situations, the government may support the proposed clean water project with subsidies. Building the facility may be a strategic investment, especially since greater water scarcity is predicted in the future.

Our design meets various environmental, ethical, social and health related constraints. The brine discharged back to the bay is diluted to at most 0.2% above the natural background salinity as required by law so that the plant does not impose harm on any local marine life. The demographic that the plant serves is largely invested in the protection of the environment, which we addressed by reducing energy consumption through direct energy integration with the nearby Chevron plant. Lastly, the product meets the specifications of the California Safe Water Drinking Act by producing water with no impurities.

__TOC__

=Introduction=
An increasing global water scarcity is fueling initiatives everywhere for clean water treatment, making efficient seawater desalination an attractive aim for chemical plant design. A 2015 market analysis found that the global desalination market earned revenues of $11.66 billion, and this number is expected to reach over $19 billion by 2019. Furthermore, the 17,000 desalination plants currently in operation are expected to double in number by 2020.1

As fresh water sources become increasingly scarce, strict water conservation measures are being observed. California is entering the fourth year of one of its most severe droughts on record. Cities are required to reduce their water usage by 35% to avoid facing fines.2 In lieu of these pressing conditions, California is looking for ways to provide more accessible fresh water to its citizens. This high demand for clean water motivated our choosing Richmond, CA as our plant location. At Richmond, the desalination plant can convert readily available seawater from the San Francisco Bay into fresh water. We propose to desalinate 50 million gallons of water per day to be sent to water treatment plants for potability. This aim is based on the capacity of the desalination plant in Carlsbad, California, which services a similar population.

The salinity of the water in the San Francisco bay is seasonal. During the dry seasons of summer and fall, salinity is high around 10 PSU (10,000 ppm) because water from the Pacific Ocean flows into the San Francisco Bay. During the wet winter season, fresh water from the rivers flows into the Bay, and salinity drops to around 2 PSU (2,000).3 Designed to meet the “worst case” scenario, our plant will process 398 million gallons of water per day with 10,000 ppm of salt to produce 50 million gallons of water with 1,000 ppm of salt. This gives a yield of 13%.

The desalination plant proposed consists of 18 flash stages in series. This report outlines our investigation of this potential water desalination plant, including design and optimization of the desalination process, an analysis of its energy consumption and environmental impacts, and a study of its economic implications.

=Technical Approach=
For the desalination step of our process, we selected a multistage flash (MSF) distillation process. Not only is MSF distillation a very viable method, but it is also the most common method, currently producing about 60% of the world’s desalinated water. MSF and reverse osmosis (RO) are the two major methods being used in large-scale desalination plants. Both processes require considerable amounts of energy. RO typically has a lower energy demand; however, the high impurity content of the Bay water would frequently necessitate membrane cleaning and/or exchange. Feed going through a RO system requires extensive pretreatment to remove biological organisms and other solids to control the pH and the chemical composition of the water. With the San Francisco Bay as our source of water, MSF is an attractive option because sediment and other large impurities can be separated from the feed before distillation occurs. Additionally, MSF distillation plants can be located near power plants and paired to their waste heat streams to conserve energy. This can reduce energy needs by half or two thirds, making MSF increasingly more practical. Due to the maintenance required in the RO system, the MSF distillation is the better option in terms of operation cost.4

A HYSYS model was built to simulate the MSF distillation section of the process, which spans after seawater pretreatment and before local potable water treatment. The model is used to simulate the distillation steps and to calculate necessary heating, cooling, and input flow rates to produce the required 50 million gallons of distillated water per day. For the simulation, the 398 million gallons per day of seawater feed stream is defined to be composed of 3.31 mol% sodium chloride (NaCl) and 96.69 mol% water. The percentage of NaCl is set to account for other ions and chemicals present after the pretreatment phase. NaCl alone is added to the HYSYS system due to the capabilities of the fluid package used to simulate this process, ElectroNRTL. The feed is set at 22°C, which is the approximate the temperature of the San Francisco Bay year round.

=Process Design=
==Design Overview==
The proposed desalination plant uses MSF distillation to convert the San Francisco Bay water to fresh water. The final process design contains 18 flash stages in series. Overall, 398 million gallons per day of of brackish water is converted to 50 million gallons per day of deionized water, a 13% yield. The production meets the required production rate of 50 million gallons of freshwater per day.

Seawater is taken in from the San Francisco Bay and mixed with the brine recycle stream produced downstream. The feed is then heated through a series of condensers and a heater. Distilled water vapor in each flash stage contacts the tubes carrying the seawater feed and condenses into a liquid stream by exchanging heat with the cooler seawater feed stream. The seawater is sent through a heater to increase its thermal energy before entering the first stage of the MSF. In each flashing stage, some of the water evaporates, leaving the salt behind. The freshwater vapors condense, as described above, and is collected and sent to a water treatment plant. The brine stream is split into the recycle stream and a waste stream, and the waste stream is diluted with fresh seawater before being discharged back into the bay.

A process flow diagram of the process is depicted in Appendix A.

==Pretreatment==
Sediment is removed to prevent solids from plugging the process downstream. Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime, and is controlled for by lowering pH. Acidifying the water will also remove CO2, which is a corrosive gas. To control for oxygen, oxygen scavengers will be included in the system to sequester it. Sodium bisulfate will be used as the scavenger.5

==Condensers (Heat Exchanger Networks)==
In order to conserve and recycle energy, the condensation of the distillate and heating of the seawater feed will be coupled in countercurrent heat exchanger networks. These heat exchanger networks are condensers of the design. The cold feed, which is a mixture of fresh seawater and brine recycle, is fed through multiple heat exchangers in series. Each heat exchanger corresponds to a different evaporation chamber. This process consists of 18 heat exchangers: E-101, E-102, … E-117, and E-118, which correspond to evaporation chambers V-102, V-103, … V-118, and V-119, respectively.

The heat exchangers can be viewed similarly to a shell and tube setup, however there is no actual physical shell. The tubes simply run through the top of the evaporation chambers, where vaporized distilled water in the chamber acts as the shell side fluid. The tube bundles are arranged in long tube configuration and are aligned parallel to the direction of the flashing brine flow in the evaporation chamber. The evaporating brine in an evaporation chamber rise up to the the corresponding heat exchanger and condense on the outside of the tubes, creating collectable pure water. This condensing water is what heats up the seawater in the tubes.

E-101 was used as the basis for the condenser sizing since it has the highest heat requirement, and thus will be the largest heat exchanger. From the HYSYS model, the overall system requires 2.24 x 106 kW of duty. Assuming this energy requirement, 5000 tubes, and 18 m length tubes (the diameter of the evaporation chambers, explained further on the next page), the diameter of each tube was calculated to be 0.4 m. Based on the temperature of the streams and flow, the thickness of the tube is 7 x 10-4 m.

==Brine Heater==
After the seawater exits the final heat exchanger, it enters a brine heater (E-119) where it is brought up to the temperature necessary for the flashing process to begin. This means that the seawater must be overheated compared to stage one, so that the seawater will flash in the evaporation chamber, i.e. release heat and vapor to reach equilibrium with stage conditions. After flowing through the condensers, the seawater enters the brine heater at 96.1 oC and is heated to 120 oC. The overall system requires 2.31 x 106 kW of duty. This is supplied by a total of 4.15 x 106 kg per hour of steam.

==Evaporation Chambers==
When the seawater has been brought to a temperature appropriate for flashing, it enters the first evaporation chamber (V-102). Upon entering the chamber, the water is flashed. Some of the water evaporates, leaving the salt behind. The vapor rises up to the heat exchanger tubes in each evaporation chamber and condenses on the outside into the distilled water product. This condensing water is what heats up the seawater in the tubes discussed above. The seawater that does not evaporate moves to the next stage, which operates at a higher temperature and lower pressure, and the process is repeated. This continues in series through a total of 18 flashing chambers (V-102, V-103, … V-118, and V-119). All of the distillate is collected into one stream and all of the brine is mixed with seawater before being disposed of back in the bay in order to dilute it enough so it does not have an adverse effect on marine life.

Each respective heat exchanger fully condenses the vapor product stream from each stage. However, when all of the individual product streams are consolidated into the final combined product stream, some water exists in the vapor phase due to temperature and pressure changes associated with mixing all of the streams together. Although the vapor product stream from each stage condenses completely across its respective heat exchanger, the total product stream contains some water in the vapor phase due to the temperature change when the mixer sums all the streams. In order to fully condense this stream, which will be sent to local water treatment plans, a heat exchanger (E-120) is put in place to cool down the product using the process feed stream.

The correlations of El-Dessouky and Ettouney were employed in order to size the flash chambers. They specified that conventional MSF plants that operate at a capacity between 27,000 and 36,000 m3/day use flashing stages of the dimensions 18 m diameter and 4 m height.6

==Valves==
The model requires valves in between the evaporation chambers. The seawater streams flow through valves with large pressure drops to flash a portion the liquid into vapor that can then be separated in the vessels. Large pressure drops provide the thermodynamic driving force to vaporize more of the liquid, and are required to achieve high product yield.

==Material of Construction==
Before 1980, the shells and internals of the MSF units were commonly constructed from carbon steel. Seawater, however, corrodes carbon steel. To compensate for this, the thickness of the carbon steel has an additional corrosion allowance, which increases the size and weight of the equipment. After 1980, the stainless steel and duplex stainless steel became increasingly common. Stainless steel metal requires less thickness for equivalent strength and corrosion resistance, which allows for a reduction in the size, weight, and therefore cost of units. Major units in our process will be constructed from stainless steel.6

==Process Alternatives==
In order to ensure that our design is fully optimized, we wanted to consider several alternatives for our process. We looked at alternatives associated with water pretreatment, the distillation process itself and product treatment.

===Water Pretreatment===
Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime. Scaling can be controlled by adjusting pH, temperature, and bicarbonate concentration. We selected acidification to control for process scaling. This is because lowering a process temperature would decrease overall efficiency, and decreasing bicarbonate concentration would be comparatively expensive. Lowering pH would have the added benefit of controlling CO2 concentrations since CO2 is a corrosive gas. To control for corrosion we looked at how to remove oxygen. To target the oxygen we chose an oxygen scavenger. This was chosen over other options such as a deaerator, or a protective coating because a deaerator would have a large capital cost, and the equipment itself would experience corrosion, and a tank coating such as zinc orthophosphate has a mixed effectiveness.7

===MSF: Brine Recycle vs. Once-Through System===
We explored two options for the overall layout of the MSF distillation plant. In the first, the brine passes through the process once; in the second, a portion of the brine is recycled from the last stage to the incoming seawater feed. The largest draw towards implementing a brine recycle is that it would significantly reduce the overall amount of seawater needed to produce the daily target of 50 million gallons per day. This in turn would lower the amount of energy required to run the pump feeding seawater to the process. Another positive is that, with a brine recycle, the amount of water conditioning chemicals that must be added to the seawater feed is reduced. The negative aspects mainly revolve around the fact that the solution throughout the process will be higher in salt concentration. This increase in salinity both raises the likelihood of scaling and corrosion in the plant and raises the amount of seawater that must be added to the final brine to make it safe for discharge back into the environment. Additionally, boiling point of each stage is raised when adding the brine recycle. After weighing both alternatives against one another, we have decided to include a brine recycle in our plant design. Most of the negative aspects of the brine recycle can be combated (e.g. include anti-scaling agents in the feed pretreatment), and the economic benefits of including the brine recycle are very significant.8

===MSF: Continuous vs. Batch===
The distillation step can be done in continuous or batch processes. Batch distillation is more versatile and often used when the product is in small amounts and very high purity. A continuous distillation is more efficient thermodynamically and economically for large amounts of material of constant composition, whereas batch distillation is more effective for small amounts of material of varying compositions. Since the plant is processing large amounts of seawater of relatively constant composition, the distillation system will utilize continuous distillation.9

===MSF: Long Tube vs. Crossed Tube ===
Inside the evaporator stage, the condenser tube bundles may be arranged in either long tube or cross tube design. In the long tube arrangement, the tubes are aligned parallel to the direction of the flashing brine flow. The tubes go from stage to stage without water boxes. In the cross tube arrangement, the tubes are lined perpendicular to the flow of the flashing brine. Water boxes transfer brine between the stages.10 11 While the water boxes in crossed tube bundles allow for more customization, the long tube configuration allows for construction of large unit sizes, as there are manufacturing restrictions with cross tubes. Long tube design can easily accommodate a large number of stages, which lead us to choose long tube for our design.11 12

===MSF: Venting System===
Noncondensable gases exist in the process because of air leakages into stages under vacuum conditions and the release of dissolved gases from the heated brine. These gases are removed by venting the system. An air cooler section concentrates the gas before they are vented, and baffles direct the vapor and condensates out. There are two venting arrangements to consider: series and parallel. The system in series moves gas from one stage directly to the next, and the only loss in the system is vapor in the last stage (since some vapor is extracted with gases). This system is economical, however this configuration increases the amount of noncondensable gases passing from one stage to the next, and disrupting the venting system in one stage will affect the entire system. In parallel venting, gas is released through parallel takeoff pipes at each stage. Although this arrangement allows for venting at any stage to be controlled, the design and maintenance is more complex, and more vapor is lost than in the venting system in series. A third alternative is to use a combination of venting in parallel and in series, which is the common method in industry. For this process, our team chose to do venting in parallel and in series.12

===Additional Energy Source Alternatives===
Energy for the process can also be coupled with renewable energy sources to reduce the desalination plant’s carbon footprint. Energy integration can be direct or indirect. In direct integration, thermal waste heat from industrial sources provide thermal energy or pressure as energy sources. Direct integration is advantageous at the modular level, as it avoids energy losses with electricity conversion. Within indirect integration, there are modular integration and grid/utility-scale integration. Modular integration integrates the plant with wind turbines and other small-scale renewable power generators with mobile deployment. A “hybridization” of renewable power sources with natural gas power generation is also an option, and the coupling of the two provides a stable power source. Indirect renewable energy integration is growing as the cost of renewable energy continues to decreases and provides an economy of scale that direct integration does not, but there are several obstacles towards indirect integration, such as the plant’s finance and engineering the integration of the two systems.13 This distillation plant will be directly integrated with neighboring plants such as Chevron that produce thermal waste heat.

===Product Treatment===
Since the product is distilled water, it needs additional ions to a concentration of around 0.01% before going to the end user. The three alternatives considered were building a water treatment facility on site, simply adding the correct salts to the product stream and then sending the stream for bacterial treatment at a water plant, or simply sending the product distilled water for complete treatment to another facility. Due to the number of nearby water treatment plants and their demand for water, we elected to simply send the product to a nearby water treatment facility.

==Design Constraints and Tradeoffs==
While designing our process, we wanted to make sure that we met several outside constraints, including environmental, political, ethical, social, sustainability, health and safety constraints. First and most importantly, we wanted to ensure that the brine discharged back to the bay would not harm local marine life. Our process was designed around the constraint of diluting the exiting brine to within 0.2% salinity of background seawater. This is not only compliant with California law, but was also verified to be safe for surrounding marine life through our own outside research. We also addressed many of these considerations by deciding to pursue direct energy integration with the nearby Chevron plant in order to reduce environmental impact of our process. Additionally, we ensured that our plant would be located in an area that is densely populated and in great need of fresh drinking water. Finally, we addressed health and safety considerations by making sure we would be able to send our product distilled water to local treatment plants for proper potabilization.

===Design Tradeoffs: Thermodynamics===
Adjusting elements within this process is iterative, and optimization thereof required changing multiple parameters at once. An example of this is in tuning the temperature increase by heater E-100; if the temperature of the stream entering the vessels is too high, the heat exchange between the vapor product and the feed stream will not be realizable because it causes the feed to be too hot to cool and condense the product. Higher temperatures, however, provide greater thermodynamic driving forces for flashing. Setting the entering stream temperature at 120°C provides enough heat for substantial flash at 130 kPa. This is the only heater required until the very last flash stage, which is added to provide more heat for vaporization. This heater (E-105) increases the stream temperature by 15°C, and the vapor product from the last vessel increases the feed stream temperature by almost 30°C. The added energy increases vaporization by 3%, which is a substantial 2.2 million kg/h. The temperature of the streams entering and leaving each stage decreases with each additional flash stage such that the last vessel has the lowest temperature, according to realistic operating guidelines.

===Design Tradeoffs: Energy Consumption, Environmental Impacts, and Product Optimization===
Increases in the pressure drop across any of the valves in this final design causes a temperature cross in the heat exchangers unless a proper increase in pump energy is provided. There is thus a design tradeoff between lowering energy usage and increasing product yield. For example, lowering the operating pressure of the process dictates that the pressure difference used to flash seawater must be smaller, possibly decreasing the amount that is vaporized. The final design aims for a yield of 50 million gallons per day. The 18 stage model was chosen for its low annual utility cost. This is a reflection of environmental mindfulness because energy consumption is an ongoing aspect of the process and should be lowered as much as possible.

==Mass and Energy Balances==
The model of the desalination system assumes that: (1) the properties are uniform in each phase within a stage, (2) heat losses are negligible due to the small surface area to volume ratio for each stage and properly insulated units (heat losses to the surrounding vary from 2 to 5% of the total system energy), (3) the distillate is salt free since the boiling point of water is much lower than that of salt, (4) the subcooling or superheating effects on the system energy balances are negligible, (5) the only contaminant is sodium chloride (there are no non-condensable gases to consider, so no vapors are vented), and (6) the process has 3 evaporation chambers.

The major balances are that the feed is a mixture of the fresh seawater stream and the recycle stream, and that the brine stream exiting the last evaporation chamber is divided into the recycle stream and the waste stream. Within each stage, the change in mass of a phase over time is the balance between the net flow of that phase from the chamber and the rates of evaporation and condensation of water into and out of that phase. The equations governing the mass and energy balances on the system is displayed in Appendix C.

=Optimization=
As initiatives for reduced energy usage and increased environmental sustainability become more prevalent, efforts to minimize the utility cost in plants has increased to accommodate these incentives. Utilities are a continual source of expenditure and consumption, and a well-designed chemical process should seek to minimize this element. The annualized utility cost is a useful parameter to optimize given that its value depends on variations in energy costs; thus, annualized utility costs do not correlate with conventional indices of inflation, such as capital and labor costs. Additionally, utility costs, like other variable costs, can be minimized by improving the design or operational efficiency of the plant.

In optimizing our MSF process, the design variable of focus was the number of stages to use in our process. The performance measure was the annualized utility cost, which was the parameter that was minimized for optimization. The process is constrained by the product specifications, which are kept constant and serve as the standard for comparison between each model. These specifications are to produce 50 million gallons of desalinated liquid water per day.

One feasibility constraint in the process is the size of the heat exchangers, which must provide a large enough area for total condensation of the vapor stream coming out of the flash stages. In order to restrict the heat exchange area needed within each exchanger, the number of stages must be at least 11. Increasing the number of stages decreases the vapor flow rate at each, and decreases the heat exchange area required as well. An 11-stage process allows for appropriate heat exchange between the feed stream and the vapor product, according to the HYSYS economic analyzer. The optimization analysis thus was conducted incorporating only models with 11 or more stages.

The HYSYS economic analyzer was used to systematically optimize the process by finding the model with the lowest annualized utility cost. The performance parameters kept constant across each model were: the inlet composition, pressure, and temperature, the brine outlet temperature, the total product flow rate and composition, and the flash stage schematic. Table 1 displays the annualized utility cost for each model, and illustrates that a clear dip in the utility cost occurs in an 18 stage model. Thus, 18 is the optimal number of stages to use for our MSF distillation given utility costs as the optimized parameter.
[[File:Table_1.PNG|thumb|center|500x300px]]

=Economic Evaluation=
In order to evaluate our process from a fiscal standpoint, we used Aspen Process Economic Analyzer and Aspen cost estimator to determine capital and operating costs for our plant. These costs were calculated based on calculated equipment sizes, as well as energy requirements reported by our HYSYS model. The capital cost was found to be $485.2MM and annual total operating cost was found to be $144.7MM ($103.7MM of which was utilities). Annual plant revenue was estimated based on prices that water districts in the county currently pay for outside water sources, and was found to be $134.3MM.14

The NPV analysis we conducted assumed that the plant would take two years to build, would operate at half capacity during the first year of operation (year 3), and have a 20 year lifetime. We also assumed that our plant would be in operation 350 days per year. A constant discount rate of 12% was assumed to hold throughout the plant’s lifetime. The final NPV of this project is -$495.7MM. This is due to both the large capital cost of the plant and the annual loss of $10.4MM while in full operation. A graph of cumulative cash flow versus time can be found in Appendix E. Tax rates and depreciation do not directly impact the NPV of this project due to operating at a loss, however for future calculations it should be noted that we used a straight line depreciation schedule over a seven year period with a 40% tax rate.

Despite the current projections, it is important to consider the greater implications of running a desalination plant in the Bay Area. The population in the Bay Area is quickly growing, while California is experiencing harsher droughts each year. A consequence of this is the possibility of receiving government subsidies for our clean water product. The similarly scaled desalination plant in Carlsbad, CA operates at a loss as well, but receives significant government subsidies. Building this facility may be a strategic investment, especially since greater water scarcity is predicted in the future.

=Sensitivity Analysis=
We wanted to determine how sensitive NPV of our project is to the cost of capital, price of utilities and price we would be able to sell our clean water product at. We conducted this analysis by varying each of these variables independently between 0.5 times and 1.5 times the original value we calculated. As to be expected, NPV grows more negative with increasing capital costs and utility prices and decreasing water prices. Conversely, NPV becomes less negative as price of water rises and capital and utility costs decrease. A plot summarizing the effects of varying these costs can be found in Appendix E. NPV is most sensitive to the price at which we will be selling our clean water product. It is worth noting that NPV is more sensitive to utilities cost than it is to capital cost, which is why we focused on optimizing the cost of utilities. In future steps, trying to reduce utilities cost even more would be worth looking into to decrease the amount of money being lost on this project.

=Conclusions and Final Recommendation=
The final design achieves the project goal of producing 50 million gallons per day of distilled water at a yield of 13%, comparable to other similarly sized plants. An optimization of the number of MSF stages is realistic, and can be improved upon by taking into account other process parameters.

The proposed design was optimized around utility costs, comparing plants with 11 to 25 stages. The process is constrained by product specifications and by the thermodynamics of the heat exchange between the feed and the product streams. Energy conservation was achieved in using the seawater feed to condense the final product, as well as in all of the heat exchangers increasing the thermal energy of the seawater feed stream before it enters the flash stages.

In order to further optimize this process, other design parameters should be considered such as the brine recycle ratio, the cooler duty of the recycle stream, the pressure drop across each valve at the flash stages, and other economic factors. An optimization around maximizing NPV, for example, might be more comprehensive than that conducted regarding only utility costs. Tuning brine recycle rates could greatly improve process efficiency, and standardizing flash stage pressure drops would make operations more feasible and controlled.

Given our in depth economic analysis, this project would lose an average of $10.4MM per year, and has an NPV of -$495.7MM for a 20 year plant lifetime. From an economic perspective alone it would not be a good project to build. However, other factors need to be considered when building a project with enormous public value. In recent years California droughts have become more severe. Implementing a drought-proof supply of water now could avert a disaster later on. Even without a large disaster, having the desalination plant could also be expected to pay off in the future with the increasing price of water. Considerations such as these were the deciding factors in building the Carlsbad desalination plant. Since there are currently no desalination plants in the Bay area we recommend that preparations should be made to move forward with a desalination plant.

=References=
References
[1] Lyons J. Environmental Leader. Environmental Leader 2015. Available at: http://www.environmentalleader.com/2015/10/27/desalination-plants-to-double-by-2020. Accessed 2016.

[2] Fitzsimmons E. In California, Cities Braced to Cut Water by 10 to 35%. The New York Times 2015. Available at:
http://www.nytimes.com/2015/04/09/us/in-california-cities-braced-to-cut-water-by-10-to-35.html. Accessed 2016.

[3] Salinity Times Series - USGS Water Quality of SF Bay. Salinity Times Series - USGS Water Quality of SF Bay. Available at:
http://sfbay.wr.usgs.gov/access/wqdata/overview/examp/charts/salin.html. Accessed 2016.

[4] Reverse Osmosis vs. Multi Stage Flash Distillation: A Comparison Between Different Desalination Methods. Brighthub Engineering. Available at:
http://www.brighthubengineering.com/power-plants/29621-comparison-between-the-reverse-osmosis-and-multi-stage-flash-distillation-methods/. Accessed 2016.

[5] Committee on Advancing Desalination Technology, Water Science and Technology Board, Division on Earth and Life Studies, National Research Council. Desalination: A National Perspective. Washington, D.C.:The National Academies Press, 2008.

[6] H. El-Dessouky. Fundamentals of salt water desalination, 1st Ed., Elsevier Science, 2002.

[7] Committee on Advancing Desalination Technology, Water Science and Technology Board, Division on Earth and Life Studies, National Research Council. Desalination: A National Perspective. Washington, D.C.:The National Academies Press, 2008.

[8] Guasp RE, Bello CP, Jara JG, Vega JP, Damien T, Winkel C, Smith HH. Desalination by distillation. Organization of American States website. oas.org. Accessed January 28, 2015.

[9] Batch Distillation. AE 335 Separation Processes website. http://prodpran.che.engr.tu.ac.th/AE335/AE335.html. Accessed January 26, 2016.

[10] Morris R. The development of the multi-stage flash distillation process: A designer's viewpoint. Desalination. 1993;93:57-68.

[11] Darwish MA. Developments in the multi-stage flash desalting system. Desalination. 1995;100:35-64.

[12] Wade N. Distillation plant development and cost update. Desalination. 2001;136:3-12.

[13] Desalination Technologies and the Use of Alternative Energies for Desalination. World Intellectual Property Organization website. http://www.wipo.int/export/sites/www/patentscope/
en/programs/patent_landscapes/documents/patent_landscapes/948-2E-WEB.pdf. Updated November 2011. Accessed January 23, 2016.

[14] California Turns to the Pacific Ocean for Water | MIT Technology Review.MIT Technology Review 2014. Available at:
http://www.technologyreview.com/featuredstory/533446/desalination-out-of-desperation/. Accessed 2016.

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=Appendix A. Process Flow Diagram=
[[File:Appendix 1.PNG]]

'''Figure A1''' Process Flow Diagram of the MSF System

=Appendix B. HYSYS Simulation=
=Appendix C. Mass and Energy Balances=
=Appendix D. Summary of Process Units=
=Appendix E. MSF Stage Parameters=
=Appendix F. Process Units Calculations=
=Appendix G. Economic Analysis=

Desalination - Team A

2016-03-12T02:57:56Z

Ezhuang: /* Appendix A. Process Flow Diagram */

Author: Pear Dhiantravan[2016], Reed Kolbe[2016], Sheridan Lichtor[2016], John Marsiglio[2016], Ellen Zhuang[2016]

Instructors: Fengqi You, David Wegerer

Winter 2016

Seawater desalination is an attractive approach to address the world’s freshwater shortage in light of the increasing global water scarcity. Our team decided to tackle California's growing drought crisis by designing a multistage flash desalination plant in Richmond, CA. Based on market analysis of existing desalination plants and water demands, this is a strategic location for the desalination of 50 million gallons per day of water from the San Francisco Bay.

The MSF process contains 18 flash stages in series. Seawater is taken in from the San Francisco Bay at a rate of 398 million gallons per day and mixed with brine recycled from the system. Within the MSF process, seawater is heated through a series of 18 heat exchangers and a brine heater before enters a series of 18 evaporation chambers. In the evaporation chambers, the seawater flashes and the freshwater vapor is condensed, collected, and sent to a water treatment facility. The brine stream proceeds to the next chamber, and the process continues. Upon exiting the last flashing chamber, the brine stream is split into a recycle brine stream that is mixed with the seawater inlet feed and a waste stream that is diluted with fresh seawater and returned to the sea. Overall, the process yield is 13%. This process was constructed in Aspen HYSYS and optimized around utility costs using the HYSYS Economic Analyzer. Based on duties, temperatures and pressures reported by HYSYS, as well as fundamental mass and energy balances, the equipment were sized appropriately and the economics of the process was analyzed.

The design has a total capital cost of $485.2 MM, an annual operating cost of $144.7 MM ($103.7 MM of which is utilities), and an annual plant revenue of $134.3 MM. Economic analysis reveals that the project has a negative NPV of -$495.7 MM and operates at a loss of $10.4 MM per year. Thus, the proposed design is not profitable. Despite the current projections, it is important to consider the greater implications of running a desalination plant in the Bay Area. The population in the Bay Area is quickly growing while California is experiencing harsher droughts each year. Given the dire situations, the government may support the proposed clean water project with subsidies. Building the facility may be a strategic investment, especially since greater water scarcity is predicted in the future.

Our design meets various environmental, ethical, social and health related constraints. The brine discharged back to the bay is diluted to at most 0.2% above the natural background salinity as required by law so that the plant does not impose harm on any local marine life. The demographic that the plant serves is largely invested in the protection of the environment, which we addressed by reducing energy consumption through direct energy integration with the nearby Chevron plant. Lastly, the product meets the specifications of the California Safe Water Drinking Act by producing water with no impurities.

=Introduction=
An increasing global water scarcity is fueling initiatives everywhere for clean water treatment, making efficient seawater desalination an attractive aim for chemical plant design. A 2015 market analysis found that the global desalination market earned revenues of $11.66 billion, and this number is expected to reach over $19 billion by 2019. Furthermore, the 17,000 desalination plants currently in operation are expected to double in number by 2020.1

As fresh water sources become increasingly scarce, strict water conservation measures are being observed. California is entering the fourth year of one of its most severe droughts on record. Cities are required to reduce their water usage by 35% to avoid facing fines.2 In lieu of these pressing conditions, California is looking for ways to provide more accessible fresh water to its citizens. This high demand for clean water motivated our choosing Richmond, CA as our plant location. At Richmond, the desalination plant can convert readily available seawater from the San Francisco Bay into fresh water. We propose to desalinate 50 million gallons of water per day to be sent to water treatment plants for potability. This aim is based on the capacity of the desalination plant in Carlsbad, California, which services a similar population.

The salinity of the water in the San Francisco bay is seasonal. During the dry seasons of summer and fall, salinity is high around 10 PSU (10,000 ppm) because water from the Pacific Ocean flows into the San Francisco Bay. During the wet winter season, fresh water from the rivers flows into the Bay, and salinity drops to around 2 PSU (2,000).3 Designed to meet the “worst case” scenario, our plant will process 398 million gallons of water per day with 10,000 ppm of salt to produce 50 million gallons of water with 1,000 ppm of salt. This gives a yield of 13%.

The desalination plant proposed consists of 18 flash stages in series. This report outlines our investigation of this potential water desalination plant, including design and optimization of the desalination process, an analysis of its energy consumption and environmental impacts, and a study of its economic implications.

=Technical Approach=
For the desalination step of our process, we selected a multistage flash (MSF) distillation process. Not only is MSF distillation a very viable method, but it is also the most common method, currently producing about 60% of the world’s desalinated water. MSF and reverse osmosis (RO) are the two major methods being used in large-scale desalination plants. Both processes require considerable amounts of energy. RO typically has a lower energy demand; however, the high impurity content of the Bay water would frequently necessitate membrane cleaning and/or exchange. Feed going through a RO system requires extensive pretreatment to remove biological organisms and other solids to control the pH and the chemical composition of the water. With the San Francisco Bay as our source of water, MSF is an attractive option because sediment and other large impurities can be separated from the feed before distillation occurs. Additionally, MSF distillation plants can be located near power plants and paired to their waste heat streams to conserve energy. This can reduce energy needs by half or two thirds, making MSF increasingly more practical. Due to the maintenance required in the RO system, the MSF distillation is the better option in terms of operation cost.4

A HYSYS model was built to simulate the MSF distillation section of the process, which spans after seawater pretreatment and before local potable water treatment. The model is used to simulate the distillation steps and to calculate necessary heating, cooling, and input flow rates to produce the required 50 million gallons of distillated water per day. For the simulation, the 398 million gallons per day of seawater feed stream is defined to be composed of 3.31 mol% sodium chloride (NaCl) and 96.69 mol% water. The percentage of NaCl is set to account for other ions and chemicals present after the pretreatment phase. NaCl alone is added to the HYSYS system due to the capabilities of the fluid package used to simulate this process, ElectroNRTL. The feed is set at 22°C, which is the approximate the temperature of the San Francisco Bay year round.

=Process Design=
==Design Overview==
The proposed desalination plant uses MSF distillation to convert the San Francisco Bay water to fresh water. The final process design contains 18 flash stages in series. Overall, 398 million gallons per day of of brackish water is converted to 50 million gallons per day of deionized water, a 13% yield. The production meets the required production rate of 50 million gallons of freshwater per day.

Seawater is taken in from the San Francisco Bay and mixed with the brine recycle stream produced downstream. The feed is then heated through a series of condensers and a heater. Distilled water vapor in each flash stage contacts the tubes carrying the seawater feed and condenses into a liquid stream by exchanging heat with the cooler seawater feed stream. The seawater is sent through a heater to increase its thermal energy before entering the first stage of the MSF. In each flashing stage, some of the water evaporates, leaving the salt behind. The freshwater vapors condense, as described above, and is collected and sent to a water treatment plant. The brine stream is split into the recycle stream and a waste stream, and the waste stream is diluted with fresh seawater before being discharged back into the bay.

A process flow diagram of the process is depicted in Appendix A.

==Pretreatment==
Sediment is removed to prevent solids from plugging the process downstream. Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime, and is controlled for by lowering pH. Acidifying the water will also remove CO2, which is a corrosive gas. To control for oxygen, oxygen scavengers will be included in the system to sequester it. Sodium bisulfate will be used as the scavenger.5

==Condensers (Heat Exchanger Networks)==
In order to conserve and recycle energy, the condensation of the distillate and heating of the seawater feed will be coupled in countercurrent heat exchanger networks. These heat exchanger networks are condensers of the design. The cold feed, which is a mixture of fresh seawater and brine recycle, is fed through multiple heat exchangers in series. Each heat exchanger corresponds to a different evaporation chamber. This process consists of 18 heat exchangers: E-101, E-102, … E-117, and E-118, which correspond to evaporation chambers V-102, V-103, … V-118, and V-119, respectively.

The heat exchangers can be viewed similarly to a shell and tube setup, however there is no actual physical shell. The tubes simply run through the top of the evaporation chambers, where vaporized distilled water in the chamber acts as the shell side fluid. The tube bundles are arranged in long tube configuration and are aligned parallel to the direction of the flashing brine flow in the evaporation chamber. The evaporating brine in an evaporation chamber rise up to the the corresponding heat exchanger and condense on the outside of the tubes, creating collectable pure water. This condensing water is what heats up the seawater in the tubes.

E-101 was used as the basis for the condenser sizing since it has the highest heat requirement, and thus will be the largest heat exchanger. From the HYSYS model, the overall system requires 2.24 x 106 kW of duty. Assuming this energy requirement, 5000 tubes, and 18 m length tubes (the diameter of the evaporation chambers, explained further on the next page), the diameter of each tube was calculated to be 0.4 m. Based on the temperature of the streams and flow, the thickness of the tube is 7 x 10-4 m.

==Brine Heater==
After the seawater exits the final heat exchanger, it enters a brine heater (E-119) where it is brought up to the temperature necessary for the flashing process to begin. This means that the seawater must be overheated compared to stage one, so that the seawater will flash in the evaporation chamber, i.e. release heat and vapor to reach equilibrium with stage conditions. After flowing through the condensers, the seawater enters the brine heater at 96.1 oC and is heated to 120 oC. The overall system requires 2.31 x 106 kW of duty. This is supplied by a total of 4.15 x 106 kg per hour of steam.

==Evaporation Chambers==
When the seawater has been brought to a temperature appropriate for flashing, it enters the first evaporation chamber (V-102). Upon entering the chamber, the water is flashed. Some of the water evaporates, leaving the salt behind. The vapor rises up to the heat exchanger tubes in each evaporation chamber and condenses on the outside into the distilled water product. This condensing water is what heats up the seawater in the tubes discussed above. The seawater that does not evaporate moves to the next stage, which operates at a higher temperature and lower pressure, and the process is repeated. This continues in series through a total of 18 flashing chambers (V-102, V-103, … V-118, and V-119). All of the distillate is collected into one stream and all of the brine is mixed with seawater before being disposed of back in the bay in order to dilute it enough so it does not have an adverse effect on marine life.

Each respective heat exchanger fully condenses the vapor product stream from each stage. However, when all of the individual product streams are consolidated into the final combined product stream, some water exists in the vapor phase due to temperature and pressure changes associated with mixing all of the streams together. Although the vapor product stream from each stage condenses completely across its respective heat exchanger, the total product stream contains some water in the vapor phase due to the temperature change when the mixer sums all the streams. In order to fully condense this stream, which will be sent to local water treatment plans, a heat exchanger (E-120) is put in place to cool down the product using the process feed stream.

The correlations of El-Dessouky and Ettouney were employed in order to size the flash chambers. They specified that conventional MSF plants that operate at a capacity between 27,000 and 36,000 m3/day use flashing stages of the dimensions 18 m diameter and 4 m height.6

==Valves==
The model requires valves in between the evaporation chambers. The seawater streams flow through valves with large pressure drops to flash a portion the liquid into vapor that can then be separated in the vessels. Large pressure drops provide the thermodynamic driving force to vaporize more of the liquid, and are required to achieve high product yield.

==Material of Construction==
Before 1980, the shells and internals of the MSF units were commonly constructed from carbon steel. Seawater, however, corrodes carbon steel. To compensate for this, the thickness of the carbon steel has an additional corrosion allowance, which increases the size and weight of the equipment. After 1980, the stainless steel and duplex stainless steel became increasingly common. Stainless steel metal requires less thickness for equivalent strength and corrosion resistance, which allows for a reduction in the size, weight, and therefore cost of units. Major units in our process will be constructed from stainless steel.6

==Process Alternatives==
In order to ensure that our design is fully optimized, we wanted to consider several alternatives for our process. We looked at alternatives associated with water pretreatment, the distillation process itself and product treatment.

===Water Pretreatment===
Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime. Scaling can be controlled by adjusting pH, temperature, and bicarbonate concentration. We selected acidification to control for process scaling. This is because lowering a process temperature would decrease overall efficiency, and decreasing bicarbonate concentration would be comparatively expensive. Lowering pH would have the added benefit of controlling CO2 concentrations since CO2 is a corrosive gas. To control for corrosion we looked at how to remove oxygen. To target the oxygen we chose an oxygen scavenger. This was chosen over other options such as a deaerator, or a protective coating because a deaerator would have a large capital cost, and the equipment itself would experience corrosion, and a tank coating such as zinc orthophosphate has a mixed effectiveness.7

===MSF: Brine Recycle vs. Once-Through System===
We explored two options for the overall layout of the MSF distillation plant. In the first, the brine passes through the process once; in the second, a portion of the brine is recycled from the last stage to the incoming seawater feed. The largest draw towards implementing a brine recycle is that it would significantly reduce the overall amount of seawater needed to produce the daily target of 50 million gallons per day. This in turn would lower the amount of energy required to run the pump feeding seawater to the process. Another positive is that, with a brine recycle, the amount of water conditioning chemicals that must be added to the seawater feed is reduced. The negative aspects mainly revolve around the fact that the solution throughout the process will be higher in salt concentration. This increase in salinity both raises the likelihood of scaling and corrosion in the plant and raises the amount of seawater that must be added to the final brine to make it safe for discharge back into the environment. Additionally, boiling point of each stage is raised when adding the brine recycle. After weighing both alternatives against one another, we have decided to include a brine recycle in our plant design. Most of the negative aspects of the brine recycle can be combated (e.g. include anti-scaling agents in the feed pretreatment), and the economic benefits of including the brine recycle are very significant.8

===MSF: Continuous vs. Batch===
The distillation step can be done in continuous or batch processes. Batch distillation is more versatile and often used when the product is in small amounts and very high purity. A continuous distillation is more efficient thermodynamically and economically for large amounts of material of constant composition, whereas batch distillation is more effective for small amounts of material of varying compositions. Since the plant is processing large amounts of seawater of relatively constant composition, the distillation system will utilize continuous distillation.9

===MSF: Long Tube vs. Crossed Tube ===
Inside the evaporator stage, the condenser tube bundles may be arranged in either long tube or cross tube design. In the long tube arrangement, the tubes are aligned parallel to the direction of the flashing brine flow. The tubes go from stage to stage without water boxes. In the cross tube arrangement, the tubes are lined perpendicular to the flow of the flashing brine. Water boxes transfer brine between the stages.10 11 While the water boxes in crossed tube bundles allow for more customization, the long tube configuration allows for construction of large unit sizes, as there are manufacturing restrictions with cross tubes. Long tube design can easily accommodate a large number of stages, which lead us to choose long tube for our design.11 12

===MSF: Venting System===
Noncondensable gases exist in the process because of air leakages into stages under vacuum conditions and the release of dissolved gases from the heated brine. These gases are removed by venting the system. An air cooler section concentrates the gas before they are vented, and baffles direct the vapor and condensates out. There are two venting arrangements to consider: series and parallel. The system in series moves gas from one stage directly to the next, and the only loss in the system is vapor in the last stage (since some vapor is extracted with gases). This system is economical, however this configuration increases the amount of noncondensable gases passing from one stage to the next, and disrupting the venting system in one stage will affect the entire system. In parallel venting, gas is released through parallel takeoff pipes at each stage. Although this arrangement allows for venting at any stage to be controlled, the design and maintenance is more complex, and more vapor is lost than in the venting system in series. A third alternative is to use a combination of venting in parallel and in series, which is the common method in industry. For this process, our team chose to do venting in parallel and in series.12

===Additional Energy Source Alternatives===
Energy for the process can also be coupled with renewable energy sources to reduce the desalination plant’s carbon footprint. Energy integration can be direct or indirect. In direct integration, thermal waste heat from industrial sources provide thermal energy or pressure as energy sources. Direct integration is advantageous at the modular level, as it avoids energy losses with electricity conversion. Within indirect integration, there are modular integration and grid/utility-scale integration. Modular integration integrates the plant with wind turbines and other small-scale renewable power generators with mobile deployment. A “hybridization” of renewable power sources with natural gas power generation is also an option, and the coupling of the two provides a stable power source. Indirect renewable energy integration is growing as the cost of renewable energy continues to decreases and provides an economy of scale that direct integration does not, but there are several obstacles towards indirect integration, such as the plant’s finance and engineering the integration of the two systems.13 This distillation plant will be directly integrated with neighboring plants such as Chevron that produce thermal waste heat.

===Product Treatment===
Since the product is distilled water, it needs additional ions to a concentration of around 0.01% before going to the end user. The three alternatives considered were building a water treatment facility on site, simply adding the correct salts to the product stream and then sending the stream for bacterial treatment at a water plant, or simply sending the product distilled water for complete treatment to another facility. Due to the number of nearby water treatment plants and their demand for water, we elected to simply send the product to a nearby water treatment facility.

==Design Constraints and Tradeoffs==
While designing our process, we wanted to make sure that we met several outside constraints, including environmental, political, ethical, social, sustainability, health and safety constraints. First and most importantly, we wanted to ensure that the brine discharged back to the bay would not harm local marine life. Our process was designed around the constraint of diluting the exiting brine to within 0.2% salinity of background seawater. This is not only compliant with California law, but was also verified to be safe for surrounding marine life through our own outside research. We also addressed many of these considerations by deciding to pursue direct energy integration with the nearby Chevron plant in order to reduce environmental impact of our process. Additionally, we ensured that our plant would be located in an area that is densely populated and in great need of fresh drinking water. Finally, we addressed health and safety considerations by making sure we would be able to send our product distilled water to local treatment plants for proper potabilization.

===Design Tradeoffs: Thermodynamics===
Adjusting elements within this process is iterative, and optimization thereof required changing multiple parameters at once. An example of this is in tuning the temperature increase by heater E-100; if the temperature of the stream entering the vessels is too high, the heat exchange between the vapor product and the feed stream will not be realizable because it causes the feed to be too hot to cool and condense the product. Higher temperatures, however, provide greater thermodynamic driving forces for flashing. Setting the entering stream temperature at 120°C provides enough heat for substantial flash at 130 kPa. This is the only heater required until the very last flash stage, which is added to provide more heat for vaporization. This heater (E-105) increases the stream temperature by 15°C, and the vapor product from the last vessel increases the feed stream temperature by almost 30°C. The added energy increases vaporization by 3%, which is a substantial 2.2 million kg/h. The temperature of the streams entering and leaving each stage decreases with each additional flash stage such that the last vessel has the lowest temperature, according to realistic operating guidelines.

===Design Tradeoffs: Energy Consumption, Environmental Impacts, and Product Optimization===
Increases in the pressure drop across any of the valves in this final design causes a temperature cross in the heat exchangers unless a proper increase in pump energy is provided. There is thus a design tradeoff between lowering energy usage and increasing product yield. For example, lowering the operating pressure of the process dictates that the pressure difference used to flash seawater must be smaller, possibly decreasing the amount that is vaporized. The final design aims for a yield of 50 million gallons per day. The 18 stage model was chosen for its low annual utility cost. This is a reflection of environmental mindfulness because energy consumption is an ongoing aspect of the process and should be lowered as much as possible.

==Mass and Energy Balances==
The model of the desalination system assumes that: (1) the properties are uniform in each phase within a stage, (2) heat losses are negligible due to the small surface area to volume ratio for each stage and properly insulated units (heat losses to the surrounding vary from 2 to 5% of the total system energy), (3) the distillate is salt free since the boiling point of water is much lower than that of salt, (4) the subcooling or superheating effects on the system energy balances are negligible, (5) the only contaminant is sodium chloride (there are no non-condensable gases to consider, so no vapors are vented), and (6) the process has 3 evaporation chambers.

The major balances are that the feed is a mixture of the fresh seawater stream and the recycle stream, and that the brine stream exiting the last evaporation chamber is divided into the recycle stream and the waste stream. Within each stage, the change in mass of a phase over time is the balance between the net flow of that phase from the chamber and the rates of evaporation and condensation of water into and out of that phase. The equations governing the mass and energy balances on the system is displayed in Appendix C.

=Optimization=
As initiatives for reduced energy usage and increased environmental sustainability become more prevalent, efforts to minimize the utility cost in plants has increased to accommodate these incentives. Utilities are a continual source of expenditure and consumption, and a well-designed chemical process should seek to minimize this element. The annualized utility cost is a useful parameter to optimize given that its value depends on variations in energy costs; thus, annualized utility costs do not correlate with conventional indices of inflation, such as capital and labor costs. Additionally, utility costs, like other variable costs, can be minimized by improving the design or operational efficiency of the plant.

In optimizing our MSF process, the design variable of focus was the number of stages to use in our process. The performance measure was the annualized utility cost, which was the parameter that was minimized for optimization. The process is constrained by the product specifications, which are kept constant and serve as the standard for comparison between each model. These specifications are to produce 50 million gallons of desalinated liquid water per day.

One feasibility constraint in the process is the size of the heat exchangers, which must provide a large enough area for total condensation of the vapor stream coming out of the flash stages. In order to restrict the heat exchange area needed within each exchanger, the number of stages must be at least 11. Increasing the number of stages decreases the vapor flow rate at each, and decreases the heat exchange area required as well. An 11-stage process allows for appropriate heat exchange between the feed stream and the vapor product, according to the HYSYS economic analyzer. The optimization analysis thus was conducted incorporating only models with 11 or more stages.

The HYSYS economic analyzer was used to systematically optimize the process by finding the model with the lowest annualized utility cost. The performance parameters kept constant across each model were: the inlet composition, pressure, and temperature, the brine outlet temperature, the total product flow rate and composition, and the flash stage schematic. Table 1 displays the annualized utility cost for each model, and illustrates that a clear dip in the utility cost occurs in an 18 stage model. Thus, 18 is the optimal number of stages to use for our MSF distillation given utility costs as the optimized parameter.
[[File:Table_1.PNG|thumb|center|500x300px]]

=Economic Evaluation=
In order to evaluate our process from a fiscal standpoint, we used Aspen Process Economic Analyzer and Aspen cost estimator to determine capital and operating costs for our plant. These costs were calculated based on calculated equipment sizes, as well as energy requirements reported by our HYSYS model. The capital cost was found to be $485.2MM and annual total operating cost was found to be $144.7MM ($103.7MM of which was utilities). Annual plant revenue was estimated based on prices that water districts in the county currently pay for outside water sources, and was found to be $134.3MM.14

The NPV analysis we conducted assumed that the plant would take two years to build, would operate at half capacity during the first year of operation (year 3), and have a 20 year lifetime. We also assumed that our plant would be in operation 350 days per year. A constant discount rate of 12% was assumed to hold throughout the plant’s lifetime. The final NPV of this project is -$495.7MM. This is due to both the large capital cost of the plant and the annual loss of $10.4MM while in full operation. A graph of cumulative cash flow versus time can be found in Appendix E. Tax rates and depreciation do not directly impact the NPV of this project due to operating at a loss, however for future calculations it should be noted that we used a straight line depreciation schedule over a seven year period with a 40% tax rate.

Despite the current projections, it is important to consider the greater implications of running a desalination plant in the Bay Area. The population in the Bay Area is quickly growing, while California is experiencing harsher droughts each year. A consequence of this is the possibility of receiving government subsidies for our clean water product. The similarly scaled desalination plant in Carlsbad, CA operates at a loss as well, but receives significant government subsidies. Building this facility may be a strategic investment, especially since greater water scarcity is predicted in the future.

=Sensitivity Analysis=
We wanted to determine how sensitive NPV of our project is to the cost of capital, price of utilities and price we would be able to sell our clean water product at. We conducted this analysis by varying each of these variables independently between 0.5 times and 1.5 times the original value we calculated. As to be expected, NPV grows more negative with increasing capital costs and utility prices and decreasing water prices. Conversely, NPV becomes less negative as price of water rises and capital and utility costs decrease. A plot summarizing the effects of varying these costs can be found in Appendix E. NPV is most sensitive to the price at which we will be selling our clean water product. It is worth noting that NPV is more sensitive to utilities cost than it is to capital cost, which is why we focused on optimizing the cost of utilities. In future steps, trying to reduce utilities cost even more would be worth looking into to decrease the amount of money being lost on this project.

=Conclusions and Final Recommendation=
The final design achieves the project goal of producing 50 million gallons per day of distilled water at a yield of 13%, comparable to other similarly sized plants. An optimization of the number of MSF stages is realistic, and can be improved upon by taking into account other process parameters.

The proposed design was optimized around utility costs, comparing plants with 11 to 25 stages. The process is constrained by product specifications and by the thermodynamics of the heat exchange between the feed and the product streams. Energy conservation was achieved in using the seawater feed to condense the final product, as well as in all of the heat exchangers increasing the thermal energy of the seawater feed stream before it enters the flash stages.

In order to further optimize this process, other design parameters should be considered such as the brine recycle ratio, the cooler duty of the recycle stream, the pressure drop across each valve at the flash stages, and other economic factors. An optimization around maximizing NPV, for example, might be more comprehensive than that conducted regarding only utility costs. Tuning brine recycle rates could greatly improve process efficiency, and standardizing flash stage pressure drops would make operations more feasible and controlled.

Given our in depth economic analysis, this project would lose an average of $10.4MM per year, and has an NPV of -$495.7MM for a 20 year plant lifetime. From an economic perspective alone it would not be a good project to build. However, other factors need to be considered when building a project with enormous public value. In recent years California droughts have become more severe. Implementing a drought-proof supply of water now could avert a disaster later on. Even without a large disaster, having the desalination plant could also be expected to pay off in the future with the increasing price of water. Considerations such as these were the deciding factors in building the Carlsbad desalination plant. Since there are currently no desalination plants in the Bay area we recommend that preparations should be made to move forward with a desalination plant.

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=Appendix A. Process Flow Diagram=
[[File:Appendix 1.PNG]]

'''Figure A1''' Process Flow Diagram of the MSF System

=Appendix B. HYSYS Simulation=
=Appendix C. Mass and Energy Balances=
=Appendix D. Summary of Process Units=
=Appendix E. MSF Stage Parameters=
=Appendix F. Process Units Calculations=
=Appendix G. Economic Analysis=

Desalination - Team A

2016-03-12T02:57:43Z

Ezhuang: /* Appendix A. Process Flow Diagram */

Author: Pear Dhiantravan[2016], Reed Kolbe[2016], Sheridan Lichtor[2016], John Marsiglio[2016], Ellen Zhuang[2016]

Instructors: Fengqi You, David Wegerer

Winter 2016

Seawater desalination is an attractive approach to address the world’s freshwater shortage in light of the increasing global water scarcity. Our team decided to tackle California's growing drought crisis by designing a multistage flash desalination plant in Richmond, CA. Based on market analysis of existing desalination plants and water demands, this is a strategic location for the desalination of 50 million gallons per day of water from the San Francisco Bay.

The MSF process contains 18 flash stages in series. Seawater is taken in from the San Francisco Bay at a rate of 398 million gallons per day and mixed with brine recycled from the system. Within the MSF process, seawater is heated through a series of 18 heat exchangers and a brine heater before enters a series of 18 evaporation chambers. In the evaporation chambers, the seawater flashes and the freshwater vapor is condensed, collected, and sent to a water treatment facility. The brine stream proceeds to the next chamber, and the process continues. Upon exiting the last flashing chamber, the brine stream is split into a recycle brine stream that is mixed with the seawater inlet feed and a waste stream that is diluted with fresh seawater and returned to the sea. Overall, the process yield is 13%. This process was constructed in Aspen HYSYS and optimized around utility costs using the HYSYS Economic Analyzer. Based on duties, temperatures and pressures reported by HYSYS, as well as fundamental mass and energy balances, the equipment were sized appropriately and the economics of the process was analyzed.

The design has a total capital cost of $485.2 MM, an annual operating cost of $144.7 MM ($103.7 MM of which is utilities), and an annual plant revenue of $134.3 MM. Economic analysis reveals that the project has a negative NPV of -$495.7 MM and operates at a loss of $10.4 MM per year. Thus, the proposed design is not profitable. Despite the current projections, it is important to consider the greater implications of running a desalination plant in the Bay Area. The population in the Bay Area is quickly growing while California is experiencing harsher droughts each year. Given the dire situations, the government may support the proposed clean water project with subsidies. Building the facility may be a strategic investment, especially since greater water scarcity is predicted in the future.

Our design meets various environmental, ethical, social and health related constraints. The brine discharged back to the bay is diluted to at most 0.2% above the natural background salinity as required by law so that the plant does not impose harm on any local marine life. The demographic that the plant serves is largely invested in the protection of the environment, which we addressed by reducing energy consumption through direct energy integration with the nearby Chevron plant. Lastly, the product meets the specifications of the California Safe Water Drinking Act by producing water with no impurities.

=Introduction=
An increasing global water scarcity is fueling initiatives everywhere for clean water treatment, making efficient seawater desalination an attractive aim for chemical plant design. A 2015 market analysis found that the global desalination market earned revenues of $11.66 billion, and this number is expected to reach over $19 billion by 2019. Furthermore, the 17,000 desalination plants currently in operation are expected to double in number by 2020.1

As fresh water sources become increasingly scarce, strict water conservation measures are being observed. California is entering the fourth year of one of its most severe droughts on record. Cities are required to reduce their water usage by 35% to avoid facing fines.2 In lieu of these pressing conditions, California is looking for ways to provide more accessible fresh water to its citizens. This high demand for clean water motivated our choosing Richmond, CA as our plant location. At Richmond, the desalination plant can convert readily available seawater from the San Francisco Bay into fresh water. We propose to desalinate 50 million gallons of water per day to be sent to water treatment plants for potability. This aim is based on the capacity of the desalination plant in Carlsbad, California, which services a similar population.

The salinity of the water in the San Francisco bay is seasonal. During the dry seasons of summer and fall, salinity is high around 10 PSU (10,000 ppm) because water from the Pacific Ocean flows into the San Francisco Bay. During the wet winter season, fresh water from the rivers flows into the Bay, and salinity drops to around 2 PSU (2,000).3 Designed to meet the “worst case” scenario, our plant will process 398 million gallons of water per day with 10,000 ppm of salt to produce 50 million gallons of water with 1,000 ppm of salt. This gives a yield of 13%.

The desalination plant proposed consists of 18 flash stages in series. This report outlines our investigation of this potential water desalination plant, including design and optimization of the desalination process, an analysis of its energy consumption and environmental impacts, and a study of its economic implications.

=Technical Approach=
For the desalination step of our process, we selected a multistage flash (MSF) distillation process. Not only is MSF distillation a very viable method, but it is also the most common method, currently producing about 60% of the world’s desalinated water. MSF and reverse osmosis (RO) are the two major methods being used in large-scale desalination plants. Both processes require considerable amounts of energy. RO typically has a lower energy demand; however, the high impurity content of the Bay water would frequently necessitate membrane cleaning and/or exchange. Feed going through a RO system requires extensive pretreatment to remove biological organisms and other solids to control the pH and the chemical composition of the water. With the San Francisco Bay as our source of water, MSF is an attractive option because sediment and other large impurities can be separated from the feed before distillation occurs. Additionally, MSF distillation plants can be located near power plants and paired to their waste heat streams to conserve energy. This can reduce energy needs by half or two thirds, making MSF increasingly more practical. Due to the maintenance required in the RO system, the MSF distillation is the better option in terms of operation cost.4

A HYSYS model was built to simulate the MSF distillation section of the process, which spans after seawater pretreatment and before local potable water treatment. The model is used to simulate the distillation steps and to calculate necessary heating, cooling, and input flow rates to produce the required 50 million gallons of distillated water per day. For the simulation, the 398 million gallons per day of seawater feed stream is defined to be composed of 3.31 mol% sodium chloride (NaCl) and 96.69 mol% water. The percentage of NaCl is set to account for other ions and chemicals present after the pretreatment phase. NaCl alone is added to the HYSYS system due to the capabilities of the fluid package used to simulate this process, ElectroNRTL. The feed is set at 22°C, which is the approximate the temperature of the San Francisco Bay year round.

=Process Design=
==Design Overview==
The proposed desalination plant uses MSF distillation to convert the San Francisco Bay water to fresh water. The final process design contains 18 flash stages in series. Overall, 398 million gallons per day of of brackish water is converted to 50 million gallons per day of deionized water, a 13% yield. The production meets the required production rate of 50 million gallons of freshwater per day.

Seawater is taken in from the San Francisco Bay and mixed with the brine recycle stream produced downstream. The feed is then heated through a series of condensers and a heater. Distilled water vapor in each flash stage contacts the tubes carrying the seawater feed and condenses into a liquid stream by exchanging heat with the cooler seawater feed stream. The seawater is sent through a heater to increase its thermal energy before entering the first stage of the MSF. In each flashing stage, some of the water evaporates, leaving the salt behind. The freshwater vapors condense, as described above, and is collected and sent to a water treatment plant. The brine stream is split into the recycle stream and a waste stream, and the waste stream is diluted with fresh seawater before being discharged back into the bay.

A process flow diagram of the process is depicted in Appendix A.

==Pretreatment==
Sediment is removed to prevent solids from plugging the process downstream. Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime, and is controlled for by lowering pH. Acidifying the water will also remove CO2, which is a corrosive gas. To control for oxygen, oxygen scavengers will be included in the system to sequester it. Sodium bisulfate will be used as the scavenger.5

==Condensers (Heat Exchanger Networks)==
In order to conserve and recycle energy, the condensation of the distillate and heating of the seawater feed will be coupled in countercurrent heat exchanger networks. These heat exchanger networks are condensers of the design. The cold feed, which is a mixture of fresh seawater and brine recycle, is fed through multiple heat exchangers in series. Each heat exchanger corresponds to a different evaporation chamber. This process consists of 18 heat exchangers: E-101, E-102, … E-117, and E-118, which correspond to evaporation chambers V-102, V-103, … V-118, and V-119, respectively.

The heat exchangers can be viewed similarly to a shell and tube setup, however there is no actual physical shell. The tubes simply run through the top of the evaporation chambers, where vaporized distilled water in the chamber acts as the shell side fluid. The tube bundles are arranged in long tube configuration and are aligned parallel to the direction of the flashing brine flow in the evaporation chamber. The evaporating brine in an evaporation chamber rise up to the the corresponding heat exchanger and condense on the outside of the tubes, creating collectable pure water. This condensing water is what heats up the seawater in the tubes.

E-101 was used as the basis for the condenser sizing since it has the highest heat requirement, and thus will be the largest heat exchanger. From the HYSYS model, the overall system requires 2.24 x 106 kW of duty. Assuming this energy requirement, 5000 tubes, and 18 m length tubes (the diameter of the evaporation chambers, explained further on the next page), the diameter of each tube was calculated to be 0.4 m. Based on the temperature of the streams and flow, the thickness of the tube is 7 x 10-4 m.

==Brine Heater==
After the seawater exits the final heat exchanger, it enters a brine heater (E-119) where it is brought up to the temperature necessary for the flashing process to begin. This means that the seawater must be overheated compared to stage one, so that the seawater will flash in the evaporation chamber, i.e. release heat and vapor to reach equilibrium with stage conditions. After flowing through the condensers, the seawater enters the brine heater at 96.1 oC and is heated to 120 oC. The overall system requires 2.31 x 106 kW of duty. This is supplied by a total of 4.15 x 106 kg per hour of steam.

==Evaporation Chambers==
When the seawater has been brought to a temperature appropriate for flashing, it enters the first evaporation chamber (V-102). Upon entering the chamber, the water is flashed. Some of the water evaporates, leaving the salt behind. The vapor rises up to the heat exchanger tubes in each evaporation chamber and condenses on the outside into the distilled water product. This condensing water is what heats up the seawater in the tubes discussed above. The seawater that does not evaporate moves to the next stage, which operates at a higher temperature and lower pressure, and the process is repeated. This continues in series through a total of 18 flashing chambers (V-102, V-103, … V-118, and V-119). All of the distillate is collected into one stream and all of the brine is mixed with seawater before being disposed of back in the bay in order to dilute it enough so it does not have an adverse effect on marine life.

Each respective heat exchanger fully condenses the vapor product stream from each stage. However, when all of the individual product streams are consolidated into the final combined product stream, some water exists in the vapor phase due to temperature and pressure changes associated with mixing all of the streams together. Although the vapor product stream from each stage condenses completely across its respective heat exchanger, the total product stream contains some water in the vapor phase due to the temperature change when the mixer sums all the streams. In order to fully condense this stream, which will be sent to local water treatment plans, a heat exchanger (E-120) is put in place to cool down the product using the process feed stream.

The correlations of El-Dessouky and Ettouney were employed in order to size the flash chambers. They specified that conventional MSF plants that operate at a capacity between 27,000 and 36,000 m3/day use flashing stages of the dimensions 18 m diameter and 4 m height.6

==Valves==
The model requires valves in between the evaporation chambers. The seawater streams flow through valves with large pressure drops to flash a portion the liquid into vapor that can then be separated in the vessels. Large pressure drops provide the thermodynamic driving force to vaporize more of the liquid, and are required to achieve high product yield.

==Material of Construction==
Before 1980, the shells and internals of the MSF units were commonly constructed from carbon steel. Seawater, however, corrodes carbon steel. To compensate for this, the thickness of the carbon steel has an additional corrosion allowance, which increases the size and weight of the equipment. After 1980, the stainless steel and duplex stainless steel became increasingly common. Stainless steel metal requires less thickness for equivalent strength and corrosion resistance, which allows for a reduction in the size, weight, and therefore cost of units. Major units in our process will be constructed from stainless steel.6

==Process Alternatives==
In order to ensure that our design is fully optimized, we wanted to consider several alternatives for our process. We looked at alternatives associated with water pretreatment, the distillation process itself and product treatment.

===Water Pretreatment===
Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime. Scaling can be controlled by adjusting pH, temperature, and bicarbonate concentration. We selected acidification to control for process scaling. This is because lowering a process temperature would decrease overall efficiency, and decreasing bicarbonate concentration would be comparatively expensive. Lowering pH would have the added benefit of controlling CO2 concentrations since CO2 is a corrosive gas. To control for corrosion we looked at how to remove oxygen. To target the oxygen we chose an oxygen scavenger. This was chosen over other options such as a deaerator, or a protective coating because a deaerator would have a large capital cost, and the equipment itself would experience corrosion, and a tank coating such as zinc orthophosphate has a mixed effectiveness.7

===MSF: Brine Recycle vs. Once-Through System===
We explored two options for the overall layout of the MSF distillation plant. In the first, the brine passes through the process once; in the second, a portion of the brine is recycled from the last stage to the incoming seawater feed. The largest draw towards implementing a brine recycle is that it would significantly reduce the overall amount of seawater needed to produce the daily target of 50 million gallons per day. This in turn would lower the amount of energy required to run the pump feeding seawater to the process. Another positive is that, with a brine recycle, the amount of water conditioning chemicals that must be added to the seawater feed is reduced. The negative aspects mainly revolve around the fact that the solution throughout the process will be higher in salt concentration. This increase in salinity both raises the likelihood of scaling and corrosion in the plant and raises the amount of seawater that must be added to the final brine to make it safe for discharge back into the environment. Additionally, boiling point of each stage is raised when adding the brine recycle. After weighing both alternatives against one another, we have decided to include a brine recycle in our plant design. Most of the negative aspects of the brine recycle can be combated (e.g. include anti-scaling agents in the feed pretreatment), and the economic benefits of including the brine recycle are very significant.8

===MSF: Continuous vs. Batch===
The distillation step can be done in continuous or batch processes. Batch distillation is more versatile and often used when the product is in small amounts and very high purity. A continuous distillation is more efficient thermodynamically and economically for large amounts of material of constant composition, whereas batch distillation is more effective for small amounts of material of varying compositions. Since the plant is processing large amounts of seawater of relatively constant composition, the distillation system will utilize continuous distillation.9

===MSF: Long Tube vs. Crossed Tube ===
Inside the evaporator stage, the condenser tube bundles may be arranged in either long tube or cross tube design. In the long tube arrangement, the tubes are aligned parallel to the direction of the flashing brine flow. The tubes go from stage to stage without water boxes. In the cross tube arrangement, the tubes are lined perpendicular to the flow of the flashing brine. Water boxes transfer brine between the stages.10 11 While the water boxes in crossed tube bundles allow for more customization, the long tube configuration allows for construction of large unit sizes, as there are manufacturing restrictions with cross tubes. Long tube design can easily accommodate a large number of stages, which lead us to choose long tube for our design.11 12

===MSF: Venting System===
Noncondensable gases exist in the process because of air leakages into stages under vacuum conditions and the release of dissolved gases from the heated brine. These gases are removed by venting the system. An air cooler section concentrates the gas before they are vented, and baffles direct the vapor and condensates out. There are two venting arrangements to consider: series and parallel. The system in series moves gas from one stage directly to the next, and the only loss in the system is vapor in the last stage (since some vapor is extracted with gases). This system is economical, however this configuration increases the amount of noncondensable gases passing from one stage to the next, and disrupting the venting system in one stage will affect the entire system. In parallel venting, gas is released through parallel takeoff pipes at each stage. Although this arrangement allows for venting at any stage to be controlled, the design and maintenance is more complex, and more vapor is lost than in the venting system in series. A third alternative is to use a combination of venting in parallel and in series, which is the common method in industry. For this process, our team chose to do venting in parallel and in series.12

===Additional Energy Source Alternatives===
Energy for the process can also be coupled with renewable energy sources to reduce the desalination plant’s carbon footprint. Energy integration can be direct or indirect. In direct integration, thermal waste heat from industrial sources provide thermal energy or pressure as energy sources. Direct integration is advantageous at the modular level, as it avoids energy losses with electricity conversion. Within indirect integration, there are modular integration and grid/utility-scale integration. Modular integration integrates the plant with wind turbines and other small-scale renewable power generators with mobile deployment. A “hybridization” of renewable power sources with natural gas power generation is also an option, and the coupling of the two provides a stable power source. Indirect renewable energy integration is growing as the cost of renewable energy continues to decreases and provides an economy of scale that direct integration does not, but there are several obstacles towards indirect integration, such as the plant’s finance and engineering the integration of the two systems.13 This distillation plant will be directly integrated with neighboring plants such as Chevron that produce thermal waste heat.

===Product Treatment===
Since the product is distilled water, it needs additional ions to a concentration of around 0.01% before going to the end user. The three alternatives considered were building a water treatment facility on site, simply adding the correct salts to the product stream and then sending the stream for bacterial treatment at a water plant, or simply sending the product distilled water for complete treatment to another facility. Due to the number of nearby water treatment plants and their demand for water, we elected to simply send the product to a nearby water treatment facility.

==Design Constraints and Tradeoffs==
While designing our process, we wanted to make sure that we met several outside constraints, including environmental, political, ethical, social, sustainability, health and safety constraints. First and most importantly, we wanted to ensure that the brine discharged back to the bay would not harm local marine life. Our process was designed around the constraint of diluting the exiting brine to within 0.2% salinity of background seawater. This is not only compliant with California law, but was also verified to be safe for surrounding marine life through our own outside research. We also addressed many of these considerations by deciding to pursue direct energy integration with the nearby Chevron plant in order to reduce environmental impact of our process. Additionally, we ensured that our plant would be located in an area that is densely populated and in great need of fresh drinking water. Finally, we addressed health and safety considerations by making sure we would be able to send our product distilled water to local treatment plants for proper potabilization.

===Design Tradeoffs: Thermodynamics===
Adjusting elements within this process is iterative, and optimization thereof required changing multiple parameters at once. An example of this is in tuning the temperature increase by heater E-100; if the temperature of the stream entering the vessels is too high, the heat exchange between the vapor product and the feed stream will not be realizable because it causes the feed to be too hot to cool and condense the product. Higher temperatures, however, provide greater thermodynamic driving forces for flashing. Setting the entering stream temperature at 120°C provides enough heat for substantial flash at 130 kPa. This is the only heater required until the very last flash stage, which is added to provide more heat for vaporization. This heater (E-105) increases the stream temperature by 15°C, and the vapor product from the last vessel increases the feed stream temperature by almost 30°C. The added energy increases vaporization by 3%, which is a substantial 2.2 million kg/h. The temperature of the streams entering and leaving each stage decreases with each additional flash stage such that the last vessel has the lowest temperature, according to realistic operating guidelines.

===Design Tradeoffs: Energy Consumption, Environmental Impacts, and Product Optimization===
Increases in the pressure drop across any of the valves in this final design causes a temperature cross in the heat exchangers unless a proper increase in pump energy is provided. There is thus a design tradeoff between lowering energy usage and increasing product yield. For example, lowering the operating pressure of the process dictates that the pressure difference used to flash seawater must be smaller, possibly decreasing the amount that is vaporized. The final design aims for a yield of 50 million gallons per day. The 18 stage model was chosen for its low annual utility cost. This is a reflection of environmental mindfulness because energy consumption is an ongoing aspect of the process and should be lowered as much as possible.

==Mass and Energy Balances==
The model of the desalination system assumes that: (1) the properties are uniform in each phase within a stage, (2) heat losses are negligible due to the small surface area to volume ratio for each stage and properly insulated units (heat losses to the surrounding vary from 2 to 5% of the total system energy), (3) the distillate is salt free since the boiling point of water is much lower than that of salt, (4) the subcooling or superheating effects on the system energy balances are negligible, (5) the only contaminant is sodium chloride (there are no non-condensable gases to consider, so no vapors are vented), and (6) the process has 3 evaporation chambers.

The major balances are that the feed is a mixture of the fresh seawater stream and the recycle stream, and that the brine stream exiting the last evaporation chamber is divided into the recycle stream and the waste stream. Within each stage, the change in mass of a phase over time is the balance between the net flow of that phase from the chamber and the rates of evaporation and condensation of water into and out of that phase. The equations governing the mass and energy balances on the system is displayed in Appendix C.

=Optimization=
As initiatives for reduced energy usage and increased environmental sustainability become more prevalent, efforts to minimize the utility cost in plants has increased to accommodate these incentives. Utilities are a continual source of expenditure and consumption, and a well-designed chemical process should seek to minimize this element. The annualized utility cost is a useful parameter to optimize given that its value depends on variations in energy costs; thus, annualized utility costs do not correlate with conventional indices of inflation, such as capital and labor costs. Additionally, utility costs, like other variable costs, can be minimized by improving the design or operational efficiency of the plant.

In optimizing our MSF process, the design variable of focus was the number of stages to use in our process. The performance measure was the annualized utility cost, which was the parameter that was minimized for optimization. The process is constrained by the product specifications, which are kept constant and serve as the standard for comparison between each model. These specifications are to produce 50 million gallons of desalinated liquid water per day.

One feasibility constraint in the process is the size of the heat exchangers, which must provide a large enough area for total condensation of the vapor stream coming out of the flash stages. In order to restrict the heat exchange area needed within each exchanger, the number of stages must be at least 11. Increasing the number of stages decreases the vapor flow rate at each, and decreases the heat exchange area required as well. An 11-stage process allows for appropriate heat exchange between the feed stream and the vapor product, according to the HYSYS economic analyzer. The optimization analysis thus was conducted incorporating only models with 11 or more stages.

The HYSYS economic analyzer was used to systematically optimize the process by finding the model with the lowest annualized utility cost. The performance parameters kept constant across each model were: the inlet composition, pressure, and temperature, the brine outlet temperature, the total product flow rate and composition, and the flash stage schematic. Table 1 displays the annualized utility cost for each model, and illustrates that a clear dip in the utility cost occurs in an 18 stage model. Thus, 18 is the optimal number of stages to use for our MSF distillation given utility costs as the optimized parameter.
[[File:Table_1.PNG|thumb|center|500x300px]]

=Economic Evaluation=
In order to evaluate our process from a fiscal standpoint, we used Aspen Process Economic Analyzer and Aspen cost estimator to determine capital and operating costs for our plant. These costs were calculated based on calculated equipment sizes, as well as energy requirements reported by our HYSYS model. The capital cost was found to be $485.2MM and annual total operating cost was found to be $144.7MM ($103.7MM of which was utilities). Annual plant revenue was estimated based on prices that water districts in the county currently pay for outside water sources, and was found to be $134.3MM.14

The NPV analysis we conducted assumed that the plant would take two years to build, would operate at half capacity during the first year of operation (year 3), and have a 20 year lifetime. We also assumed that our plant would be in operation 350 days per year. A constant discount rate of 12% was assumed to hold throughout the plant’s lifetime. The final NPV of this project is -$495.7MM. This is due to both the large capital cost of the plant and the annual loss of $10.4MM while in full operation. A graph of cumulative cash flow versus time can be found in Appendix E. Tax rates and depreciation do not directly impact the NPV of this project due to operating at a loss, however for future calculations it should be noted that we used a straight line depreciation schedule over a seven year period with a 40% tax rate.

Despite the current projections, it is important to consider the greater implications of running a desalination plant in the Bay Area. The population in the Bay Area is quickly growing, while California is experiencing harsher droughts each year. A consequence of this is the possibility of receiving government subsidies for our clean water product. The similarly scaled desalination plant in Carlsbad, CA operates at a loss as well, but receives significant government subsidies. Building this facility may be a strategic investment, especially since greater water scarcity is predicted in the future.

=Sensitivity Analysis=
We wanted to determine how sensitive NPV of our project is to the cost of capital, price of utilities and price we would be able to sell our clean water product at. We conducted this analysis by varying each of these variables independently between 0.5 times and 1.5 times the original value we calculated. As to be expected, NPV grows more negative with increasing capital costs and utility prices and decreasing water prices. Conversely, NPV becomes less negative as price of water rises and capital and utility costs decrease. A plot summarizing the effects of varying these costs can be found in Appendix E. NPV is most sensitive to the price at which we will be selling our clean water product. It is worth noting that NPV is more sensitive to utilities cost than it is to capital cost, which is why we focused on optimizing the cost of utilities. In future steps, trying to reduce utilities cost even more would be worth looking into to decrease the amount of money being lost on this project.

=Conclusions and Final Recommendation=
The final design achieves the project goal of producing 50 million gallons per day of distilled water at a yield of 13%, comparable to other similarly sized plants. An optimization of the number of MSF stages is realistic, and can be improved upon by taking into account other process parameters.

The proposed design was optimized around utility costs, comparing plants with 11 to 25 stages. The process is constrained by product specifications and by the thermodynamics of the heat exchange between the feed and the product streams. Energy conservation was achieved in using the seawater feed to condense the final product, as well as in all of the heat exchangers increasing the thermal energy of the seawater feed stream before it enters the flash stages.

In order to further optimize this process, other design parameters should be considered such as the brine recycle ratio, the cooler duty of the recycle stream, the pressure drop across each valve at the flash stages, and other economic factors. An optimization around maximizing NPV, for example, might be more comprehensive than that conducted regarding only utility costs. Tuning brine recycle rates could greatly improve process efficiency, and standardizing flash stage pressure drops would make operations more feasible and controlled.

Given our in depth economic analysis, this project would lose an average of $10.4MM per year, and has an NPV of -$495.7MM for a 20 year plant lifetime. From an economic perspective alone it would not be a good project to build. However, other factors need to be considered when building a project with enormous public value. In recent years California droughts have become more severe. Implementing a drought-proof supply of water now could avert a disaster later on. Even without a large disaster, having the desalination plant could also be expected to pay off in the future with the increasing price of water. Considerations such as these were the deciding factors in building the Carlsbad desalination plant. Since there are currently no desalination plants in the Bay area we recommend that preparations should be made to move forward with a desalination plant.

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=Appendix A. Process Flow Diagram=
[[File:Appendix 1.PNG|900x600px]]

'''Figure A1''' Process Flow Diagram of the MSF System

=Appendix B. HYSYS Simulation=
=Appendix C. Mass and Energy Balances=
=Appendix D. Summary of Process Units=
=Appendix E. MSF Stage Parameters=
=Appendix F. Process Units Calculations=
=Appendix G. Economic Analysis=

Desalination - Team A

2016-03-12T02:57:30Z

Ezhuang: /* Appendix A. Process Flow Diagram */

Author: Pear Dhiantravan[2016], Reed Kolbe[2016], Sheridan Lichtor[2016], John Marsiglio[2016], Ellen Zhuang[2016]

Instructors: Fengqi You, David Wegerer

Winter 2016

Seawater desalination is an attractive approach to address the world’s freshwater shortage in light of the increasing global water scarcity. Our team decided to tackle California's growing drought crisis by designing a multistage flash desalination plant in Richmond, CA. Based on market analysis of existing desalination plants and water demands, this is a strategic location for the desalination of 50 million gallons per day of water from the San Francisco Bay.

The MSF process contains 18 flash stages in series. Seawater is taken in from the San Francisco Bay at a rate of 398 million gallons per day and mixed with brine recycled from the system. Within the MSF process, seawater is heated through a series of 18 heat exchangers and a brine heater before enters a series of 18 evaporation chambers. In the evaporation chambers, the seawater flashes and the freshwater vapor is condensed, collected, and sent to a water treatment facility. The brine stream proceeds to the next chamber, and the process continues. Upon exiting the last flashing chamber, the brine stream is split into a recycle brine stream that is mixed with the seawater inlet feed and a waste stream that is diluted with fresh seawater and returned to the sea. Overall, the process yield is 13%. This process was constructed in Aspen HYSYS and optimized around utility costs using the HYSYS Economic Analyzer. Based on duties, temperatures and pressures reported by HYSYS, as well as fundamental mass and energy balances, the equipment were sized appropriately and the economics of the process was analyzed.

The design has a total capital cost of $485.2 MM, an annual operating cost of $144.7 MM ($103.7 MM of which is utilities), and an annual plant revenue of $134.3 MM. Economic analysis reveals that the project has a negative NPV of -$495.7 MM and operates at a loss of $10.4 MM per year. Thus, the proposed design is not profitable. Despite the current projections, it is important to consider the greater implications of running a desalination plant in the Bay Area. The population in the Bay Area is quickly growing while California is experiencing harsher droughts each year. Given the dire situations, the government may support the proposed clean water project with subsidies. Building the facility may be a strategic investment, especially since greater water scarcity is predicted in the future.

Our design meets various environmental, ethical, social and health related constraints. The brine discharged back to the bay is diluted to at most 0.2% above the natural background salinity as required by law so that the plant does not impose harm on any local marine life. The demographic that the plant serves is largely invested in the protection of the environment, which we addressed by reducing energy consumption through direct energy integration with the nearby Chevron plant. Lastly, the product meets the specifications of the California Safe Water Drinking Act by producing water with no impurities.

=Introduction=
An increasing global water scarcity is fueling initiatives everywhere for clean water treatment, making efficient seawater desalination an attractive aim for chemical plant design. A 2015 market analysis found that the global desalination market earned revenues of $11.66 billion, and this number is expected to reach over $19 billion by 2019. Furthermore, the 17,000 desalination plants currently in operation are expected to double in number by 2020.1

As fresh water sources become increasingly scarce, strict water conservation measures are being observed. California is entering the fourth year of one of its most severe droughts on record. Cities are required to reduce their water usage by 35% to avoid facing fines.2 In lieu of these pressing conditions, California is looking for ways to provide more accessible fresh water to its citizens. This high demand for clean water motivated our choosing Richmond, CA as our plant location. At Richmond, the desalination plant can convert readily available seawater from the San Francisco Bay into fresh water. We propose to desalinate 50 million gallons of water per day to be sent to water treatment plants for potability. This aim is based on the capacity of the desalination plant in Carlsbad, California, which services a similar population.

The salinity of the water in the San Francisco bay is seasonal. During the dry seasons of summer and fall, salinity is high around 10 PSU (10,000 ppm) because water from the Pacific Ocean flows into the San Francisco Bay. During the wet winter season, fresh water from the rivers flows into the Bay, and salinity drops to around 2 PSU (2,000).3 Designed to meet the “worst case” scenario, our plant will process 398 million gallons of water per day with 10,000 ppm of salt to produce 50 million gallons of water with 1,000 ppm of salt. This gives a yield of 13%.

The desalination plant proposed consists of 18 flash stages in series. This report outlines our investigation of this potential water desalination plant, including design and optimization of the desalination process, an analysis of its energy consumption and environmental impacts, and a study of its economic implications.

=Technical Approach=
For the desalination step of our process, we selected a multistage flash (MSF) distillation process. Not only is MSF distillation a very viable method, but it is also the most common method, currently producing about 60% of the world’s desalinated water. MSF and reverse osmosis (RO) are the two major methods being used in large-scale desalination plants. Both processes require considerable amounts of energy. RO typically has a lower energy demand; however, the high impurity content of the Bay water would frequently necessitate membrane cleaning and/or exchange. Feed going through a RO system requires extensive pretreatment to remove biological organisms and other solids to control the pH and the chemical composition of the water. With the San Francisco Bay as our source of water, MSF is an attractive option because sediment and other large impurities can be separated from the feed before distillation occurs. Additionally, MSF distillation plants can be located near power plants and paired to their waste heat streams to conserve energy. This can reduce energy needs by half or two thirds, making MSF increasingly more practical. Due to the maintenance required in the RO system, the MSF distillation is the better option in terms of operation cost.4

A HYSYS model was built to simulate the MSF distillation section of the process, which spans after seawater pretreatment and before local potable water treatment. The model is used to simulate the distillation steps and to calculate necessary heating, cooling, and input flow rates to produce the required 50 million gallons of distillated water per day. For the simulation, the 398 million gallons per day of seawater feed stream is defined to be composed of 3.31 mol% sodium chloride (NaCl) and 96.69 mol% water. The percentage of NaCl is set to account for other ions and chemicals present after the pretreatment phase. NaCl alone is added to the HYSYS system due to the capabilities of the fluid package used to simulate this process, ElectroNRTL. The feed is set at 22°C, which is the approximate the temperature of the San Francisco Bay year round.

=Process Design=
==Design Overview==
The proposed desalination plant uses MSF distillation to convert the San Francisco Bay water to fresh water. The final process design contains 18 flash stages in series. Overall, 398 million gallons per day of of brackish water is converted to 50 million gallons per day of deionized water, a 13% yield. The production meets the required production rate of 50 million gallons of freshwater per day.

Seawater is taken in from the San Francisco Bay and mixed with the brine recycle stream produced downstream. The feed is then heated through a series of condensers and a heater. Distilled water vapor in each flash stage contacts the tubes carrying the seawater feed and condenses into a liquid stream by exchanging heat with the cooler seawater feed stream. The seawater is sent through a heater to increase its thermal energy before entering the first stage of the MSF. In each flashing stage, some of the water evaporates, leaving the salt behind. The freshwater vapors condense, as described above, and is collected and sent to a water treatment plant. The brine stream is split into the recycle stream and a waste stream, and the waste stream is diluted with fresh seawater before being discharged back into the bay.

A process flow diagram of the process is depicted in Appendix A.

==Pretreatment==
Sediment is removed to prevent solids from plugging the process downstream. Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime, and is controlled for by lowering pH. Acidifying the water will also remove CO2, which is a corrosive gas. To control for oxygen, oxygen scavengers will be included in the system to sequester it. Sodium bisulfate will be used as the scavenger.5

==Condensers (Heat Exchanger Networks)==
In order to conserve and recycle energy, the condensation of the distillate and heating of the seawater feed will be coupled in countercurrent heat exchanger networks. These heat exchanger networks are condensers of the design. The cold feed, which is a mixture of fresh seawater and brine recycle, is fed through multiple heat exchangers in series. Each heat exchanger corresponds to a different evaporation chamber. This process consists of 18 heat exchangers: E-101, E-102, … E-117, and E-118, which correspond to evaporation chambers V-102, V-103, … V-118, and V-119, respectively.

The heat exchangers can be viewed similarly to a shell and tube setup, however there is no actual physical shell. The tubes simply run through the top of the evaporation chambers, where vaporized distilled water in the chamber acts as the shell side fluid. The tube bundles are arranged in long tube configuration and are aligned parallel to the direction of the flashing brine flow in the evaporation chamber. The evaporating brine in an evaporation chamber rise up to the the corresponding heat exchanger and condense on the outside of the tubes, creating collectable pure water. This condensing water is what heats up the seawater in the tubes.

E-101 was used as the basis for the condenser sizing since it has the highest heat requirement, and thus will be the largest heat exchanger. From the HYSYS model, the overall system requires 2.24 x 106 kW of duty. Assuming this energy requirement, 5000 tubes, and 18 m length tubes (the diameter of the evaporation chambers, explained further on the next page), the diameter of each tube was calculated to be 0.4 m. Based on the temperature of the streams and flow, the thickness of the tube is 7 x 10-4 m.

==Brine Heater==
After the seawater exits the final heat exchanger, it enters a brine heater (E-119) where it is brought up to the temperature necessary for the flashing process to begin. This means that the seawater must be overheated compared to stage one, so that the seawater will flash in the evaporation chamber, i.e. release heat and vapor to reach equilibrium with stage conditions. After flowing through the condensers, the seawater enters the brine heater at 96.1 oC and is heated to 120 oC. The overall system requires 2.31 x 106 kW of duty. This is supplied by a total of 4.15 x 106 kg per hour of steam.

==Evaporation Chambers==
When the seawater has been brought to a temperature appropriate for flashing, it enters the first evaporation chamber (V-102). Upon entering the chamber, the water is flashed. Some of the water evaporates, leaving the salt behind. The vapor rises up to the heat exchanger tubes in each evaporation chamber and condenses on the outside into the distilled water product. This condensing water is what heats up the seawater in the tubes discussed above. The seawater that does not evaporate moves to the next stage, which operates at a higher temperature and lower pressure, and the process is repeated. This continues in series through a total of 18 flashing chambers (V-102, V-103, … V-118, and V-119). All of the distillate is collected into one stream and all of the brine is mixed with seawater before being disposed of back in the bay in order to dilute it enough so it does not have an adverse effect on marine life.

Each respective heat exchanger fully condenses the vapor product stream from each stage. However, when all of the individual product streams are consolidated into the final combined product stream, some water exists in the vapor phase due to temperature and pressure changes associated with mixing all of the streams together. Although the vapor product stream from each stage condenses completely across its respective heat exchanger, the total product stream contains some water in the vapor phase due to the temperature change when the mixer sums all the streams. In order to fully condense this stream, which will be sent to local water treatment plans, a heat exchanger (E-120) is put in place to cool down the product using the process feed stream.

The correlations of El-Dessouky and Ettouney were employed in order to size the flash chambers. They specified that conventional MSF plants that operate at a capacity between 27,000 and 36,000 m3/day use flashing stages of the dimensions 18 m diameter and 4 m height.6

==Valves==
The model requires valves in between the evaporation chambers. The seawater streams flow through valves with large pressure drops to flash a portion the liquid into vapor that can then be separated in the vessels. Large pressure drops provide the thermodynamic driving force to vaporize more of the liquid, and are required to achieve high product yield.

==Material of Construction==
Before 1980, the shells and internals of the MSF units were commonly constructed from carbon steel. Seawater, however, corrodes carbon steel. To compensate for this, the thickness of the carbon steel has an additional corrosion allowance, which increases the size and weight of the equipment. After 1980, the stainless steel and duplex stainless steel became increasingly common. Stainless steel metal requires less thickness for equivalent strength and corrosion resistance, which allows for a reduction in the size, weight, and therefore cost of units. Major units in our process will be constructed from stainless steel.6

==Process Alternatives==
In order to ensure that our design is fully optimized, we wanted to consider several alternatives for our process. We looked at alternatives associated with water pretreatment, the distillation process itself and product treatment.

===Water Pretreatment===
Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime. Scaling can be controlled by adjusting pH, temperature, and bicarbonate concentration. We selected acidification to control for process scaling. This is because lowering a process temperature would decrease overall efficiency, and decreasing bicarbonate concentration would be comparatively expensive. Lowering pH would have the added benefit of controlling CO2 concentrations since CO2 is a corrosive gas. To control for corrosion we looked at how to remove oxygen. To target the oxygen we chose an oxygen scavenger. This was chosen over other options such as a deaerator, or a protective coating because a deaerator would have a large capital cost, and the equipment itself would experience corrosion, and a tank coating such as zinc orthophosphate has a mixed effectiveness.7

===MSF: Brine Recycle vs. Once-Through System===
We explored two options for the overall layout of the MSF distillation plant. In the first, the brine passes through the process once; in the second, a portion of the brine is recycled from the last stage to the incoming seawater feed. The largest draw towards implementing a brine recycle is that it would significantly reduce the overall amount of seawater needed to produce the daily target of 50 million gallons per day. This in turn would lower the amount of energy required to run the pump feeding seawater to the process. Another positive is that, with a brine recycle, the amount of water conditioning chemicals that must be added to the seawater feed is reduced. The negative aspects mainly revolve around the fact that the solution throughout the process will be higher in salt concentration. This increase in salinity both raises the likelihood of scaling and corrosion in the plant and raises the amount of seawater that must be added to the final brine to make it safe for discharge back into the environment. Additionally, boiling point of each stage is raised when adding the brine recycle. After weighing both alternatives against one another, we have decided to include a brine recycle in our plant design. Most of the negative aspects of the brine recycle can be combated (e.g. include anti-scaling agents in the feed pretreatment), and the economic benefits of including the brine recycle are very significant.8

===MSF: Continuous vs. Batch===
The distillation step can be done in continuous or batch processes. Batch distillation is more versatile and often used when the product is in small amounts and very high purity. A continuous distillation is more efficient thermodynamically and economically for large amounts of material of constant composition, whereas batch distillation is more effective for small amounts of material of varying compositions. Since the plant is processing large amounts of seawater of relatively constant composition, the distillation system will utilize continuous distillation.9

===MSF: Long Tube vs. Crossed Tube ===
Inside the evaporator stage, the condenser tube bundles may be arranged in either long tube or cross tube design. In the long tube arrangement, the tubes are aligned parallel to the direction of the flashing brine flow. The tubes go from stage to stage without water boxes. In the cross tube arrangement, the tubes are lined perpendicular to the flow of the flashing brine. Water boxes transfer brine between the stages.10 11 While the water boxes in crossed tube bundles allow for more customization, the long tube configuration allows for construction of large unit sizes, as there are manufacturing restrictions with cross tubes. Long tube design can easily accommodate a large number of stages, which lead us to choose long tube for our design.11 12

===MSF: Venting System===
Noncondensable gases exist in the process because of air leakages into stages under vacuum conditions and the release of dissolved gases from the heated brine. These gases are removed by venting the system. An air cooler section concentrates the gas before they are vented, and baffles direct the vapor and condensates out. There are two venting arrangements to consider: series and parallel. The system in series moves gas from one stage directly to the next, and the only loss in the system is vapor in the last stage (since some vapor is extracted with gases). This system is economical, however this configuration increases the amount of noncondensable gases passing from one stage to the next, and disrupting the venting system in one stage will affect the entire system. In parallel venting, gas is released through parallel takeoff pipes at each stage. Although this arrangement allows for venting at any stage to be controlled, the design and maintenance is more complex, and more vapor is lost than in the venting system in series. A third alternative is to use a combination of venting in parallel and in series, which is the common method in industry. For this process, our team chose to do venting in parallel and in series.12

===Additional Energy Source Alternatives===
Energy for the process can also be coupled with renewable energy sources to reduce the desalination plant’s carbon footprint. Energy integration can be direct or indirect. In direct integration, thermal waste heat from industrial sources provide thermal energy or pressure as energy sources. Direct integration is advantageous at the modular level, as it avoids energy losses with electricity conversion. Within indirect integration, there are modular integration and grid/utility-scale integration. Modular integration integrates the plant with wind turbines and other small-scale renewable power generators with mobile deployment. A “hybridization” of renewable power sources with natural gas power generation is also an option, and the coupling of the two provides a stable power source. Indirect renewable energy integration is growing as the cost of renewable energy continues to decreases and provides an economy of scale that direct integration does not, but there are several obstacles towards indirect integration, such as the plant’s finance and engineering the integration of the two systems.13 This distillation plant will be directly integrated with neighboring plants such as Chevron that produce thermal waste heat.

===Product Treatment===
Since the product is distilled water, it needs additional ions to a concentration of around 0.01% before going to the end user. The three alternatives considered were building a water treatment facility on site, simply adding the correct salts to the product stream and then sending the stream for bacterial treatment at a water plant, or simply sending the product distilled water for complete treatment to another facility. Due to the number of nearby water treatment plants and their demand for water, we elected to simply send the product to a nearby water treatment facility.

==Design Constraints and Tradeoffs==
While designing our process, we wanted to make sure that we met several outside constraints, including environmental, political, ethical, social, sustainability, health and safety constraints. First and most importantly, we wanted to ensure that the brine discharged back to the bay would not harm local marine life. Our process was designed around the constraint of diluting the exiting brine to within 0.2% salinity of background seawater. This is not only compliant with California law, but was also verified to be safe for surrounding marine life through our own outside research. We also addressed many of these considerations by deciding to pursue direct energy integration with the nearby Chevron plant in order to reduce environmental impact of our process. Additionally, we ensured that our plant would be located in an area that is densely populated and in great need of fresh drinking water. Finally, we addressed health and safety considerations by making sure we would be able to send our product distilled water to local treatment plants for proper potabilization.

===Design Tradeoffs: Thermodynamics===
Adjusting elements within this process is iterative, and optimization thereof required changing multiple parameters at once. An example of this is in tuning the temperature increase by heater E-100; if the temperature of the stream entering the vessels is too high, the heat exchange between the vapor product and the feed stream will not be realizable because it causes the feed to be too hot to cool and condense the product. Higher temperatures, however, provide greater thermodynamic driving forces for flashing. Setting the entering stream temperature at 120°C provides enough heat for substantial flash at 130 kPa. This is the only heater required until the very last flash stage, which is added to provide more heat for vaporization. This heater (E-105) increases the stream temperature by 15°C, and the vapor product from the last vessel increases the feed stream temperature by almost 30°C. The added energy increases vaporization by 3%, which is a substantial 2.2 million kg/h. The temperature of the streams entering and leaving each stage decreases with each additional flash stage such that the last vessel has the lowest temperature, according to realistic operating guidelines.

===Design Tradeoffs: Energy Consumption, Environmental Impacts, and Product Optimization===
Increases in the pressure drop across any of the valves in this final design causes a temperature cross in the heat exchangers unless a proper increase in pump energy is provided. There is thus a design tradeoff between lowering energy usage and increasing product yield. For example, lowering the operating pressure of the process dictates that the pressure difference used to flash seawater must be smaller, possibly decreasing the amount that is vaporized. The final design aims for a yield of 50 million gallons per day. The 18 stage model was chosen for its low annual utility cost. This is a reflection of environmental mindfulness because energy consumption is an ongoing aspect of the process and should be lowered as much as possible.

==Mass and Energy Balances==
The model of the desalination system assumes that: (1) the properties are uniform in each phase within a stage, (2) heat losses are negligible due to the small surface area to volume ratio for each stage and properly insulated units (heat losses to the surrounding vary from 2 to 5% of the total system energy), (3) the distillate is salt free since the boiling point of water is much lower than that of salt, (4) the subcooling or superheating effects on the system energy balances are negligible, (5) the only contaminant is sodium chloride (there are no non-condensable gases to consider, so no vapors are vented), and (6) the process has 3 evaporation chambers.

The major balances are that the feed is a mixture of the fresh seawater stream and the recycle stream, and that the brine stream exiting the last evaporation chamber is divided into the recycle stream and the waste stream. Within each stage, the change in mass of a phase over time is the balance between the net flow of that phase from the chamber and the rates of evaporation and condensation of water into and out of that phase. The equations governing the mass and energy balances on the system is displayed in Appendix C.

=Optimization=
As initiatives for reduced energy usage and increased environmental sustainability become more prevalent, efforts to minimize the utility cost in plants has increased to accommodate these incentives. Utilities are a continual source of expenditure and consumption, and a well-designed chemical process should seek to minimize this element. The annualized utility cost is a useful parameter to optimize given that its value depends on variations in energy costs; thus, annualized utility costs do not correlate with conventional indices of inflation, such as capital and labor costs. Additionally, utility costs, like other variable costs, can be minimized by improving the design or operational efficiency of the plant.

In optimizing our MSF process, the design variable of focus was the number of stages to use in our process. The performance measure was the annualized utility cost, which was the parameter that was minimized for optimization. The process is constrained by the product specifications, which are kept constant and serve as the standard for comparison between each model. These specifications are to produce 50 million gallons of desalinated liquid water per day.

One feasibility constraint in the process is the size of the heat exchangers, which must provide a large enough area for total condensation of the vapor stream coming out of the flash stages. In order to restrict the heat exchange area needed within each exchanger, the number of stages must be at least 11. Increasing the number of stages decreases the vapor flow rate at each, and decreases the heat exchange area required as well. An 11-stage process allows for appropriate heat exchange between the feed stream and the vapor product, according to the HYSYS economic analyzer. The optimization analysis thus was conducted incorporating only models with 11 or more stages.

The HYSYS economic analyzer was used to systematically optimize the process by finding the model with the lowest annualized utility cost. The performance parameters kept constant across each model were: the inlet composition, pressure, and temperature, the brine outlet temperature, the total product flow rate and composition, and the flash stage schematic. Table 1 displays the annualized utility cost for each model, and illustrates that a clear dip in the utility cost occurs in an 18 stage model. Thus, 18 is the optimal number of stages to use for our MSF distillation given utility costs as the optimized parameter.
[[File:Table_1.PNG|thumb|center|500x300px]]

=Economic Evaluation=
In order to evaluate our process from a fiscal standpoint, we used Aspen Process Economic Analyzer and Aspen cost estimator to determine capital and operating costs for our plant. These costs were calculated based on calculated equipment sizes, as well as energy requirements reported by our HYSYS model. The capital cost was found to be $485.2MM and annual total operating cost was found to be $144.7MM ($103.7MM of which was utilities). Annual plant revenue was estimated based on prices that water districts in the county currently pay for outside water sources, and was found to be $134.3MM.14

The NPV analysis we conducted assumed that the plant would take two years to build, would operate at half capacity during the first year of operation (year 3), and have a 20 year lifetime. We also assumed that our plant would be in operation 350 days per year. A constant discount rate of 12% was assumed to hold throughout the plant’s lifetime. The final NPV of this project is -$495.7MM. This is due to both the large capital cost of the plant and the annual loss of $10.4MM while in full operation. A graph of cumulative cash flow versus time can be found in Appendix E. Tax rates and depreciation do not directly impact the NPV of this project due to operating at a loss, however for future calculations it should be noted that we used a straight line depreciation schedule over a seven year period with a 40% tax rate.

Despite the current projections, it is important to consider the greater implications of running a desalination plant in the Bay Area. The population in the Bay Area is quickly growing, while California is experiencing harsher droughts each year. A consequence of this is the possibility of receiving government subsidies for our clean water product. The similarly scaled desalination plant in Carlsbad, CA operates at a loss as well, but receives significant government subsidies. Building this facility may be a strategic investment, especially since greater water scarcity is predicted in the future.

=Sensitivity Analysis=
We wanted to determine how sensitive NPV of our project is to the cost of capital, price of utilities and price we would be able to sell our clean water product at. We conducted this analysis by varying each of these variables independently between 0.5 times and 1.5 times the original value we calculated. As to be expected, NPV grows more negative with increasing capital costs and utility prices and decreasing water prices. Conversely, NPV becomes less negative as price of water rises and capital and utility costs decrease. A plot summarizing the effects of varying these costs can be found in Appendix E. NPV is most sensitive to the price at which we will be selling our clean water product. It is worth noting that NPV is more sensitive to utilities cost than it is to capital cost, which is why we focused on optimizing the cost of utilities. In future steps, trying to reduce utilities cost even more would be worth looking into to decrease the amount of money being lost on this project.

=Conclusions and Final Recommendation=
The final design achieves the project goal of producing 50 million gallons per day of distilled water at a yield of 13%, comparable to other similarly sized plants. An optimization of the number of MSF stages is realistic, and can be improved upon by taking into account other process parameters.

The proposed design was optimized around utility costs, comparing plants with 11 to 25 stages. The process is constrained by product specifications and by the thermodynamics of the heat exchange between the feed and the product streams. Energy conservation was achieved in using the seawater feed to condense the final product, as well as in all of the heat exchangers increasing the thermal energy of the seawater feed stream before it enters the flash stages.

In order to further optimize this process, other design parameters should be considered such as the brine recycle ratio, the cooler duty of the recycle stream, the pressure drop across each valve at the flash stages, and other economic factors. An optimization around maximizing NPV, for example, might be more comprehensive than that conducted regarding only utility costs. Tuning brine recycle rates could greatly improve process efficiency, and standardizing flash stage pressure drops would make operations more feasible and controlled.

Given our in depth economic analysis, this project would lose an average of $10.4MM per year, and has an NPV of -$495.7MM for a 20 year plant lifetime. From an economic perspective alone it would not be a good project to build. However, other factors need to be considered when building a project with enormous public value. In recent years California droughts have become more severe. Implementing a drought-proof supply of water now could avert a disaster later on. Even without a large disaster, having the desalination plant could also be expected to pay off in the future with the increasing price of water. Considerations such as these were the deciding factors in building the Carlsbad desalination plant. Since there are currently no desalination plants in the Bay area we recommend that preparations should be made to move forward with a desalination plant.

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=Appendix A. Process Flow Diagram=
[[File:Appendix 1.PNG|center|900x600px]]

'''Figure A1''' Process Flow Diagram of the MSF System

=Appendix B. HYSYS Simulation=
=Appendix C. Mass and Energy Balances=
=Appendix D. Summary of Process Units=
=Appendix E. MSF Stage Parameters=
=Appendix F. Process Units Calculations=
=Appendix G. Economic Analysis=

Desalination - Team A

2016-03-12T02:57:13Z

Ezhuang: /* Appendix A. Process Flow Diagram */

Author: Pear Dhiantravan[2016], Reed Kolbe[2016], Sheridan Lichtor[2016], John Marsiglio[2016], Ellen Zhuang[2016]

Instructors: Fengqi You, David Wegerer

Winter 2016

Seawater desalination is an attractive approach to address the world’s freshwater shortage in light of the increasing global water scarcity. Our team decided to tackle California's growing drought crisis by designing a multistage flash desalination plant in Richmond, CA. Based on market analysis of existing desalination plants and water demands, this is a strategic location for the desalination of 50 million gallons per day of water from the San Francisco Bay.

The MSF process contains 18 flash stages in series. Seawater is taken in from the San Francisco Bay at a rate of 398 million gallons per day and mixed with brine recycled from the system. Within the MSF process, seawater is heated through a series of 18 heat exchangers and a brine heater before enters a series of 18 evaporation chambers. In the evaporation chambers, the seawater flashes and the freshwater vapor is condensed, collected, and sent to a water treatment facility. The brine stream proceeds to the next chamber, and the process continues. Upon exiting the last flashing chamber, the brine stream is split into a recycle brine stream that is mixed with the seawater inlet feed and a waste stream that is diluted with fresh seawater and returned to the sea. Overall, the process yield is 13%. This process was constructed in Aspen HYSYS and optimized around utility costs using the HYSYS Economic Analyzer. Based on duties, temperatures and pressures reported by HYSYS, as well as fundamental mass and energy balances, the equipment were sized appropriately and the economics of the process was analyzed.

The design has a total capital cost of $485.2 MM, an annual operating cost of $144.7 MM ($103.7 MM of which is utilities), and an annual plant revenue of $134.3 MM. Economic analysis reveals that the project has a negative NPV of -$495.7 MM and operates at a loss of $10.4 MM per year. Thus, the proposed design is not profitable. Despite the current projections, it is important to consider the greater implications of running a desalination plant in the Bay Area. The population in the Bay Area is quickly growing while California is experiencing harsher droughts each year. Given the dire situations, the government may support the proposed clean water project with subsidies. Building the facility may be a strategic investment, especially since greater water scarcity is predicted in the future.

Our design meets various environmental, ethical, social and health related constraints. The brine discharged back to the bay is diluted to at most 0.2% above the natural background salinity as required by law so that the plant does not impose harm on any local marine life. The demographic that the plant serves is largely invested in the protection of the environment, which we addressed by reducing energy consumption through direct energy integration with the nearby Chevron plant. Lastly, the product meets the specifications of the California Safe Water Drinking Act by producing water with no impurities.

=Introduction=
An increasing global water scarcity is fueling initiatives everywhere for clean water treatment, making efficient seawater desalination an attractive aim for chemical plant design. A 2015 market analysis found that the global desalination market earned revenues of $11.66 billion, and this number is expected to reach over $19 billion by 2019. Furthermore, the 17,000 desalination plants currently in operation are expected to double in number by 2020.1

As fresh water sources become increasingly scarce, strict water conservation measures are being observed. California is entering the fourth year of one of its most severe droughts on record. Cities are required to reduce their water usage by 35% to avoid facing fines.2 In lieu of these pressing conditions, California is looking for ways to provide more accessible fresh water to its citizens. This high demand for clean water motivated our choosing Richmond, CA as our plant location. At Richmond, the desalination plant can convert readily available seawater from the San Francisco Bay into fresh water. We propose to desalinate 50 million gallons of water per day to be sent to water treatment plants for potability. This aim is based on the capacity of the desalination plant in Carlsbad, California, which services a similar population.

The salinity of the water in the San Francisco bay is seasonal. During the dry seasons of summer and fall, salinity is high around 10 PSU (10,000 ppm) because water from the Pacific Ocean flows into the San Francisco Bay. During the wet winter season, fresh water from the rivers flows into the Bay, and salinity drops to around 2 PSU (2,000).3 Designed to meet the “worst case” scenario, our plant will process 398 million gallons of water per day with 10,000 ppm of salt to produce 50 million gallons of water with 1,000 ppm of salt. This gives a yield of 13%.

The desalination plant proposed consists of 18 flash stages in series. This report outlines our investigation of this potential water desalination plant, including design and optimization of the desalination process, an analysis of its energy consumption and environmental impacts, and a study of its economic implications.

=Technical Approach=
For the desalination step of our process, we selected a multistage flash (MSF) distillation process. Not only is MSF distillation a very viable method, but it is also the most common method, currently producing about 60% of the world’s desalinated water. MSF and reverse osmosis (RO) are the two major methods being used in large-scale desalination plants. Both processes require considerable amounts of energy. RO typically has a lower energy demand; however, the high impurity content of the Bay water would frequently necessitate membrane cleaning and/or exchange. Feed going through a RO system requires extensive pretreatment to remove biological organisms and other solids to control the pH and the chemical composition of the water. With the San Francisco Bay as our source of water, MSF is an attractive option because sediment and other large impurities can be separated from the feed before distillation occurs. Additionally, MSF distillation plants can be located near power plants and paired to their waste heat streams to conserve energy. This can reduce energy needs by half or two thirds, making MSF increasingly more practical. Due to the maintenance required in the RO system, the MSF distillation is the better option in terms of operation cost.4

A HYSYS model was built to simulate the MSF distillation section of the process, which spans after seawater pretreatment and before local potable water treatment. The model is used to simulate the distillation steps and to calculate necessary heating, cooling, and input flow rates to produce the required 50 million gallons of distillated water per day. For the simulation, the 398 million gallons per day of seawater feed stream is defined to be composed of 3.31 mol% sodium chloride (NaCl) and 96.69 mol% water. The percentage of NaCl is set to account for other ions and chemicals present after the pretreatment phase. NaCl alone is added to the HYSYS system due to the capabilities of the fluid package used to simulate this process, ElectroNRTL. The feed is set at 22°C, which is the approximate the temperature of the San Francisco Bay year round.

=Process Design=
==Design Overview==
The proposed desalination plant uses MSF distillation to convert the San Francisco Bay water to fresh water. The final process design contains 18 flash stages in series. Overall, 398 million gallons per day of of brackish water is converted to 50 million gallons per day of deionized water, a 13% yield. The production meets the required production rate of 50 million gallons of freshwater per day.

Seawater is taken in from the San Francisco Bay and mixed with the brine recycle stream produced downstream. The feed is then heated through a series of condensers and a heater. Distilled water vapor in each flash stage contacts the tubes carrying the seawater feed and condenses into a liquid stream by exchanging heat with the cooler seawater feed stream. The seawater is sent through a heater to increase its thermal energy before entering the first stage of the MSF. In each flashing stage, some of the water evaporates, leaving the salt behind. The freshwater vapors condense, as described above, and is collected and sent to a water treatment plant. The brine stream is split into the recycle stream and a waste stream, and the waste stream is diluted with fresh seawater before being discharged back into the bay.

A process flow diagram of the process is depicted in Appendix A.

==Pretreatment==
Sediment is removed to prevent solids from plugging the process downstream. Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime, and is controlled for by lowering pH. Acidifying the water will also remove CO2, which is a corrosive gas. To control for oxygen, oxygen scavengers will be included in the system to sequester it. Sodium bisulfate will be used as the scavenger.5

==Condensers (Heat Exchanger Networks)==
In order to conserve and recycle energy, the condensation of the distillate and heating of the seawater feed will be coupled in countercurrent heat exchanger networks. These heat exchanger networks are condensers of the design. The cold feed, which is a mixture of fresh seawater and brine recycle, is fed through multiple heat exchangers in series. Each heat exchanger corresponds to a different evaporation chamber. This process consists of 18 heat exchangers: E-101, E-102, … E-117, and E-118, which correspond to evaporation chambers V-102, V-103, … V-118, and V-119, respectively.

The heat exchangers can be viewed similarly to a shell and tube setup, however there is no actual physical shell. The tubes simply run through the top of the evaporation chambers, where vaporized distilled water in the chamber acts as the shell side fluid. The tube bundles are arranged in long tube configuration and are aligned parallel to the direction of the flashing brine flow in the evaporation chamber. The evaporating brine in an evaporation chamber rise up to the the corresponding heat exchanger and condense on the outside of the tubes, creating collectable pure water. This condensing water is what heats up the seawater in the tubes.

E-101 was used as the basis for the condenser sizing since it has the highest heat requirement, and thus will be the largest heat exchanger. From the HYSYS model, the overall system requires 2.24 x 106 kW of duty. Assuming this energy requirement, 5000 tubes, and 18 m length tubes (the diameter of the evaporation chambers, explained further on the next page), the diameter of each tube was calculated to be 0.4 m. Based on the temperature of the streams and flow, the thickness of the tube is 7 x 10-4 m.

==Brine Heater==
After the seawater exits the final heat exchanger, it enters a brine heater (E-119) where it is brought up to the temperature necessary for the flashing process to begin. This means that the seawater must be overheated compared to stage one, so that the seawater will flash in the evaporation chamber, i.e. release heat and vapor to reach equilibrium with stage conditions. After flowing through the condensers, the seawater enters the brine heater at 96.1 oC and is heated to 120 oC. The overall system requires 2.31 x 106 kW of duty. This is supplied by a total of 4.15 x 106 kg per hour of steam.

==Evaporation Chambers==
When the seawater has been brought to a temperature appropriate for flashing, it enters the first evaporation chamber (V-102). Upon entering the chamber, the water is flashed. Some of the water evaporates, leaving the salt behind. The vapor rises up to the heat exchanger tubes in each evaporation chamber and condenses on the outside into the distilled water product. This condensing water is what heats up the seawater in the tubes discussed above. The seawater that does not evaporate moves to the next stage, which operates at a higher temperature and lower pressure, and the process is repeated. This continues in series through a total of 18 flashing chambers (V-102, V-103, … V-118, and V-119). All of the distillate is collected into one stream and all of the brine is mixed with seawater before being disposed of back in the bay in order to dilute it enough so it does not have an adverse effect on marine life.

Each respective heat exchanger fully condenses the vapor product stream from each stage. However, when all of the individual product streams are consolidated into the final combined product stream, some water exists in the vapor phase due to temperature and pressure changes associated with mixing all of the streams together. Although the vapor product stream from each stage condenses completely across its respective heat exchanger, the total product stream contains some water in the vapor phase due to the temperature change when the mixer sums all the streams. In order to fully condense this stream, which will be sent to local water treatment plans, a heat exchanger (E-120) is put in place to cool down the product using the process feed stream.

The correlations of El-Dessouky and Ettouney were employed in order to size the flash chambers. They specified that conventional MSF plants that operate at a capacity between 27,000 and 36,000 m3/day use flashing stages of the dimensions 18 m diameter and 4 m height.6

==Valves==
The model requires valves in between the evaporation chambers. The seawater streams flow through valves with large pressure drops to flash a portion the liquid into vapor that can then be separated in the vessels. Large pressure drops provide the thermodynamic driving force to vaporize more of the liquid, and are required to achieve high product yield.

==Material of Construction==
Before 1980, the shells and internals of the MSF units were commonly constructed from carbon steel. Seawater, however, corrodes carbon steel. To compensate for this, the thickness of the carbon steel has an additional corrosion allowance, which increases the size and weight of the equipment. After 1980, the stainless steel and duplex stainless steel became increasingly common. Stainless steel metal requires less thickness for equivalent strength and corrosion resistance, which allows for a reduction in the size, weight, and therefore cost of units. Major units in our process will be constructed from stainless steel.6

==Process Alternatives==
In order to ensure that our design is fully optimized, we wanted to consider several alternatives for our process. We looked at alternatives associated with water pretreatment, the distillation process itself and product treatment.

===Water Pretreatment===
Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime. Scaling can be controlled by adjusting pH, temperature, and bicarbonate concentration. We selected acidification to control for process scaling. This is because lowering a process temperature would decrease overall efficiency, and decreasing bicarbonate concentration would be comparatively expensive. Lowering pH would have the added benefit of controlling CO2 concentrations since CO2 is a corrosive gas. To control for corrosion we looked at how to remove oxygen. To target the oxygen we chose an oxygen scavenger. This was chosen over other options such as a deaerator, or a protective coating because a deaerator would have a large capital cost, and the equipment itself would experience corrosion, and a tank coating such as zinc orthophosphate has a mixed effectiveness.7

===MSF: Brine Recycle vs. Once-Through System===
We explored two options for the overall layout of the MSF distillation plant. In the first, the brine passes through the process once; in the second, a portion of the brine is recycled from the last stage to the incoming seawater feed. The largest draw towards implementing a brine recycle is that it would significantly reduce the overall amount of seawater needed to produce the daily target of 50 million gallons per day. This in turn would lower the amount of energy required to run the pump feeding seawater to the process. Another positive is that, with a brine recycle, the amount of water conditioning chemicals that must be added to the seawater feed is reduced. The negative aspects mainly revolve around the fact that the solution throughout the process will be higher in salt concentration. This increase in salinity both raises the likelihood of scaling and corrosion in the plant and raises the amount of seawater that must be added to the final brine to make it safe for discharge back into the environment. Additionally, boiling point of each stage is raised when adding the brine recycle. After weighing both alternatives against one another, we have decided to include a brine recycle in our plant design. Most of the negative aspects of the brine recycle can be combated (e.g. include anti-scaling agents in the feed pretreatment), and the economic benefits of including the brine recycle are very significant.8

===MSF: Continuous vs. Batch===
The distillation step can be done in continuous or batch processes. Batch distillation is more versatile and often used when the product is in small amounts and very high purity. A continuous distillation is more efficient thermodynamically and economically for large amounts of material of constant composition, whereas batch distillation is more effective for small amounts of material of varying compositions. Since the plant is processing large amounts of seawater of relatively constant composition, the distillation system will utilize continuous distillation.9

===MSF: Long Tube vs. Crossed Tube ===
Inside the evaporator stage, the condenser tube bundles may be arranged in either long tube or cross tube design. In the long tube arrangement, the tubes are aligned parallel to the direction of the flashing brine flow. The tubes go from stage to stage without water boxes. In the cross tube arrangement, the tubes are lined perpendicular to the flow of the flashing brine. Water boxes transfer brine between the stages.10 11 While the water boxes in crossed tube bundles allow for more customization, the long tube configuration allows for construction of large unit sizes, as there are manufacturing restrictions with cross tubes. Long tube design can easily accommodate a large number of stages, which lead us to choose long tube for our design.11 12

===MSF: Venting System===
Noncondensable gases exist in the process because of air leakages into stages under vacuum conditions and the release of dissolved gases from the heated brine. These gases are removed by venting the system. An air cooler section concentrates the gas before they are vented, and baffles direct the vapor and condensates out. There are two venting arrangements to consider: series and parallel. The system in series moves gas from one stage directly to the next, and the only loss in the system is vapor in the last stage (since some vapor is extracted with gases). This system is economical, however this configuration increases the amount of noncondensable gases passing from one stage to the next, and disrupting the venting system in one stage will affect the entire system. In parallel venting, gas is released through parallel takeoff pipes at each stage. Although this arrangement allows for venting at any stage to be controlled, the design and maintenance is more complex, and more vapor is lost than in the venting system in series. A third alternative is to use a combination of venting in parallel and in series, which is the common method in industry. For this process, our team chose to do venting in parallel and in series.12

===Additional Energy Source Alternatives===
Energy for the process can also be coupled with renewable energy sources to reduce the desalination plant’s carbon footprint. Energy integration can be direct or indirect. In direct integration, thermal waste heat from industrial sources provide thermal energy or pressure as energy sources. Direct integration is advantageous at the modular level, as it avoids energy losses with electricity conversion. Within indirect integration, there are modular integration and grid/utility-scale integration. Modular integration integrates the plant with wind turbines and other small-scale renewable power generators with mobile deployment. A “hybridization” of renewable power sources with natural gas power generation is also an option, and the coupling of the two provides a stable power source. Indirect renewable energy integration is growing as the cost of renewable energy continues to decreases and provides an economy of scale that direct integration does not, but there are several obstacles towards indirect integration, such as the plant’s finance and engineering the integration of the two systems.13 This distillation plant will be directly integrated with neighboring plants such as Chevron that produce thermal waste heat.

===Product Treatment===
Since the product is distilled water, it needs additional ions to a concentration of around 0.01% before going to the end user. The three alternatives considered were building a water treatment facility on site, simply adding the correct salts to the product stream and then sending the stream for bacterial treatment at a water plant, or simply sending the product distilled water for complete treatment to another facility. Due to the number of nearby water treatment plants and their demand for water, we elected to simply send the product to a nearby water treatment facility.

==Design Constraints and Tradeoffs==
While designing our process, we wanted to make sure that we met several outside constraints, including environmental, political, ethical, social, sustainability, health and safety constraints. First and most importantly, we wanted to ensure that the brine discharged back to the bay would not harm local marine life. Our process was designed around the constraint of diluting the exiting brine to within 0.2% salinity of background seawater. This is not only compliant with California law, but was also verified to be safe for surrounding marine life through our own outside research. We also addressed many of these considerations by deciding to pursue direct energy integration with the nearby Chevron plant in order to reduce environmental impact of our process. Additionally, we ensured that our plant would be located in an area that is densely populated and in great need of fresh drinking water. Finally, we addressed health and safety considerations by making sure we would be able to send our product distilled water to local treatment plants for proper potabilization.

===Design Tradeoffs: Thermodynamics===
Adjusting elements within this process is iterative, and optimization thereof required changing multiple parameters at once. An example of this is in tuning the temperature increase by heater E-100; if the temperature of the stream entering the vessels is too high, the heat exchange between the vapor product and the feed stream will not be realizable because it causes the feed to be too hot to cool and condense the product. Higher temperatures, however, provide greater thermodynamic driving forces for flashing. Setting the entering stream temperature at 120°C provides enough heat for substantial flash at 130 kPa. This is the only heater required until the very last flash stage, which is added to provide more heat for vaporization. This heater (E-105) increases the stream temperature by 15°C, and the vapor product from the last vessel increases the feed stream temperature by almost 30°C. The added energy increases vaporization by 3%, which is a substantial 2.2 million kg/h. The temperature of the streams entering and leaving each stage decreases with each additional flash stage such that the last vessel has the lowest temperature, according to realistic operating guidelines.

===Design Tradeoffs: Energy Consumption, Environmental Impacts, and Product Optimization===
Increases in the pressure drop across any of the valves in this final design causes a temperature cross in the heat exchangers unless a proper increase in pump energy is provided. There is thus a design tradeoff between lowering energy usage and increasing product yield. For example, lowering the operating pressure of the process dictates that the pressure difference used to flash seawater must be smaller, possibly decreasing the amount that is vaporized. The final design aims for a yield of 50 million gallons per day. The 18 stage model was chosen for its low annual utility cost. This is a reflection of environmental mindfulness because energy consumption is an ongoing aspect of the process and should be lowered as much as possible.

==Mass and Energy Balances==
The model of the desalination system assumes that: (1) the properties are uniform in each phase within a stage, (2) heat losses are negligible due to the small surface area to volume ratio for each stage and properly insulated units (heat losses to the surrounding vary from 2 to 5% of the total system energy), (3) the distillate is salt free since the boiling point of water is much lower than that of salt, (4) the subcooling or superheating effects on the system energy balances are negligible, (5) the only contaminant is sodium chloride (there are no non-condensable gases to consider, so no vapors are vented), and (6) the process has 3 evaporation chambers.

The major balances are that the feed is a mixture of the fresh seawater stream and the recycle stream, and that the brine stream exiting the last evaporation chamber is divided into the recycle stream and the waste stream. Within each stage, the change in mass of a phase over time is the balance between the net flow of that phase from the chamber and the rates of evaporation and condensation of water into and out of that phase. The equations governing the mass and energy balances on the system is displayed in Appendix C.

=Optimization=
As initiatives for reduced energy usage and increased environmental sustainability become more prevalent, efforts to minimize the utility cost in plants has increased to accommodate these incentives. Utilities are a continual source of expenditure and consumption, and a well-designed chemical process should seek to minimize this element. The annualized utility cost is a useful parameter to optimize given that its value depends on variations in energy costs; thus, annualized utility costs do not correlate with conventional indices of inflation, such as capital and labor costs. Additionally, utility costs, like other variable costs, can be minimized by improving the design or operational efficiency of the plant.

In optimizing our MSF process, the design variable of focus was the number of stages to use in our process. The performance measure was the annualized utility cost, which was the parameter that was minimized for optimization. The process is constrained by the product specifications, which are kept constant and serve as the standard for comparison between each model. These specifications are to produce 50 million gallons of desalinated liquid water per day.

One feasibility constraint in the process is the size of the heat exchangers, which must provide a large enough area for total condensation of the vapor stream coming out of the flash stages. In order to restrict the heat exchange area needed within each exchanger, the number of stages must be at least 11. Increasing the number of stages decreases the vapor flow rate at each, and decreases the heat exchange area required as well. An 11-stage process allows for appropriate heat exchange between the feed stream and the vapor product, according to the HYSYS economic analyzer. The optimization analysis thus was conducted incorporating only models with 11 or more stages.

The HYSYS economic analyzer was used to systematically optimize the process by finding the model with the lowest annualized utility cost. The performance parameters kept constant across each model were: the inlet composition, pressure, and temperature, the brine outlet temperature, the total product flow rate and composition, and the flash stage schematic. Table 1 displays the annualized utility cost for each model, and illustrates that a clear dip in the utility cost occurs in an 18 stage model. Thus, 18 is the optimal number of stages to use for our MSF distillation given utility costs as the optimized parameter.
[[File:Table_1.PNG|thumb|center|500x300px]]

=Economic Evaluation=
In order to evaluate our process from a fiscal standpoint, we used Aspen Process Economic Analyzer and Aspen cost estimator to determine capital and operating costs for our plant. These costs were calculated based on calculated equipment sizes, as well as energy requirements reported by our HYSYS model. The capital cost was found to be $485.2MM and annual total operating cost was found to be $144.7MM ($103.7MM of which was utilities). Annual plant revenue was estimated based on prices that water districts in the county currently pay for outside water sources, and was found to be $134.3MM.14

The NPV analysis we conducted assumed that the plant would take two years to build, would operate at half capacity during the first year of operation (year 3), and have a 20 year lifetime. We also assumed that our plant would be in operation 350 days per year. A constant discount rate of 12% was assumed to hold throughout the plant’s lifetime. The final NPV of this project is -$495.7MM. This is due to both the large capital cost of the plant and the annual loss of $10.4MM while in full operation. A graph of cumulative cash flow versus time can be found in Appendix E. Tax rates and depreciation do not directly impact the NPV of this project due to operating at a loss, however for future calculations it should be noted that we used a straight line depreciation schedule over a seven year period with a 40% tax rate.

Despite the current projections, it is important to consider the greater implications of running a desalination plant in the Bay Area. The population in the Bay Area is quickly growing, while California is experiencing harsher droughts each year. A consequence of this is the possibility of receiving government subsidies for our clean water product. The similarly scaled desalination plant in Carlsbad, CA operates at a loss as well, but receives significant government subsidies. Building this facility may be a strategic investment, especially since greater water scarcity is predicted in the future.

=Sensitivity Analysis=
We wanted to determine how sensitive NPV of our project is to the cost of capital, price of utilities and price we would be able to sell our clean water product at. We conducted this analysis by varying each of these variables independently between 0.5 times and 1.5 times the original value we calculated. As to be expected, NPV grows more negative with increasing capital costs and utility prices and decreasing water prices. Conversely, NPV becomes less negative as price of water rises and capital and utility costs decrease. A plot summarizing the effects of varying these costs can be found in Appendix E. NPV is most sensitive to the price at which we will be selling our clean water product. It is worth noting that NPV is more sensitive to utilities cost than it is to capital cost, which is why we focused on optimizing the cost of utilities. In future steps, trying to reduce utilities cost even more would be worth looking into to decrease the amount of money being lost on this project.

=Conclusions and Final Recommendation=
The final design achieves the project goal of producing 50 million gallons per day of distilled water at a yield of 13%, comparable to other similarly sized plants. An optimization of the number of MSF stages is realistic, and can be improved upon by taking into account other process parameters.

The proposed design was optimized around utility costs, comparing plants with 11 to 25 stages. The process is constrained by product specifications and by the thermodynamics of the heat exchange between the feed and the product streams. Energy conservation was achieved in using the seawater feed to condense the final product, as well as in all of the heat exchangers increasing the thermal energy of the seawater feed stream before it enters the flash stages.

In order to further optimize this process, other design parameters should be considered such as the brine recycle ratio, the cooler duty of the recycle stream, the pressure drop across each valve at the flash stages, and other economic factors. An optimization around maximizing NPV, for example, might be more comprehensive than that conducted regarding only utility costs. Tuning brine recycle rates could greatly improve process efficiency, and standardizing flash stage pressure drops would make operations more feasible and controlled.

Given our in depth economic analysis, this project would lose an average of $10.4MM per year, and has an NPV of -$495.7MM for a 20 year plant lifetime. From an economic perspective alone it would not be a good project to build. However, other factors need to be considered when building a project with enormous public value. In recent years California droughts have become more severe. Implementing a drought-proof supply of water now could avert a disaster later on. Even without a large disaster, having the desalination plant could also be expected to pay off in the future with the increasing price of water. Considerations such as these were the deciding factors in building the Carlsbad desalination plant. Since there are currently no desalination plants in the Bay area we recommend that preparations should be made to move forward with a desalination plant.

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=Appendix A. Process Flow Diagram=
[[File:Appendix 1.PNG|left|900x600px]]
'''Figure A1''' Process Flow Diagram of the MSF System

=Appendix B. HYSYS Simulation=
=Appendix C. Mass and Energy Balances=
=Appendix D. Summary of Process Units=
=Appendix E. MSF Stage Parameters=
=Appendix F. Process Units Calculations=
=Appendix G. Economic Analysis=

Desalination - Team A

2016-03-12T02:55:34Z

Ezhuang: /* Evaporation Chambers */

Author: Pear Dhiantravan[2016], Reed Kolbe[2016], Sheridan Lichtor[2016], John Marsiglio[2016], Ellen Zhuang[2016]

Instructors: Fengqi You, David Wegerer

Winter 2016

Seawater desalination is an attractive approach to address the world’s freshwater shortage in light of the increasing global water scarcity. Our team decided to tackle California's growing drought crisis by designing a multistage flash desalination plant in Richmond, CA. Based on market analysis of existing desalination plants and water demands, this is a strategic location for the desalination of 50 million gallons per day of water from the San Francisco Bay.

The MSF process contains 18 flash stages in series. Seawater is taken in from the San Francisco Bay at a rate of 398 million gallons per day and mixed with brine recycled from the system. Within the MSF process, seawater is heated through a series of 18 heat exchangers and a brine heater before enters a series of 18 evaporation chambers. In the evaporation chambers, the seawater flashes and the freshwater vapor is condensed, collected, and sent to a water treatment facility. The brine stream proceeds to the next chamber, and the process continues. Upon exiting the last flashing chamber, the brine stream is split into a recycle brine stream that is mixed with the seawater inlet feed and a waste stream that is diluted with fresh seawater and returned to the sea. Overall, the process yield is 13%. This process was constructed in Aspen HYSYS and optimized around utility costs using the HYSYS Economic Analyzer. Based on duties, temperatures and pressures reported by HYSYS, as well as fundamental mass and energy balances, the equipment were sized appropriately and the economics of the process was analyzed.

The design has a total capital cost of $485.2 MM, an annual operating cost of $144.7 MM ($103.7 MM of which is utilities), and an annual plant revenue of $134.3 MM. Economic analysis reveals that the project has a negative NPV of -$495.7 MM and operates at a loss of $10.4 MM per year. Thus, the proposed design is not profitable. Despite the current projections, it is important to consider the greater implications of running a desalination plant in the Bay Area. The population in the Bay Area is quickly growing while California is experiencing harsher droughts each year. Given the dire situations, the government may support the proposed clean water project with subsidies. Building the facility may be a strategic investment, especially since greater water scarcity is predicted in the future.

Our design meets various environmental, ethical, social and health related constraints. The brine discharged back to the bay is diluted to at most 0.2% above the natural background salinity as required by law so that the plant does not impose harm on any local marine life. The demographic that the plant serves is largely invested in the protection of the environment, which we addressed by reducing energy consumption through direct energy integration with the nearby Chevron plant. Lastly, the product meets the specifications of the California Safe Water Drinking Act by producing water with no impurities.

=Introduction=
An increasing global water scarcity is fueling initiatives everywhere for clean water treatment, making efficient seawater desalination an attractive aim for chemical plant design. A 2015 market analysis found that the global desalination market earned revenues of $11.66 billion, and this number is expected to reach over $19 billion by 2019. Furthermore, the 17,000 desalination plants currently in operation are expected to double in number by 2020.1

As fresh water sources become increasingly scarce, strict water conservation measures are being observed. California is entering the fourth year of one of its most severe droughts on record. Cities are required to reduce their water usage by 35% to avoid facing fines.2 In lieu of these pressing conditions, California is looking for ways to provide more accessible fresh water to its citizens. This high demand for clean water motivated our choosing Richmond, CA as our plant location. At Richmond, the desalination plant can convert readily available seawater from the San Francisco Bay into fresh water. We propose to desalinate 50 million gallons of water per day to be sent to water treatment plants for potability. This aim is based on the capacity of the desalination plant in Carlsbad, California, which services a similar population.

The salinity of the water in the San Francisco bay is seasonal. During the dry seasons of summer and fall, salinity is high around 10 PSU (10,000 ppm) because water from the Pacific Ocean flows into the San Francisco Bay. During the wet winter season, fresh water from the rivers flows into the Bay, and salinity drops to around 2 PSU (2,000).3 Designed to meet the “worst case” scenario, our plant will process 398 million gallons of water per day with 10,000 ppm of salt to produce 50 million gallons of water with 1,000 ppm of salt. This gives a yield of 13%.

The desalination plant proposed consists of 18 flash stages in series. This report outlines our investigation of this potential water desalination plant, including design and optimization of the desalination process, an analysis of its energy consumption and environmental impacts, and a study of its economic implications.

=Technical Approach=
For the desalination step of our process, we selected a multistage flash (MSF) distillation process. Not only is MSF distillation a very viable method, but it is also the most common method, currently producing about 60% of the world’s desalinated water. MSF and reverse osmosis (RO) are the two major methods being used in large-scale desalination plants. Both processes require considerable amounts of energy. RO typically has a lower energy demand; however, the high impurity content of the Bay water would frequently necessitate membrane cleaning and/or exchange. Feed going through a RO system requires extensive pretreatment to remove biological organisms and other solids to control the pH and the chemical composition of the water. With the San Francisco Bay as our source of water, MSF is an attractive option because sediment and other large impurities can be separated from the feed before distillation occurs. Additionally, MSF distillation plants can be located near power plants and paired to their waste heat streams to conserve energy. This can reduce energy needs by half or two thirds, making MSF increasingly more practical. Due to the maintenance required in the RO system, the MSF distillation is the better option in terms of operation cost.4

A HYSYS model was built to simulate the MSF distillation section of the process, which spans after seawater pretreatment and before local potable water treatment. The model is used to simulate the distillation steps and to calculate necessary heating, cooling, and input flow rates to produce the required 50 million gallons of distillated water per day. For the simulation, the 398 million gallons per day of seawater feed stream is defined to be composed of 3.31 mol% sodium chloride (NaCl) and 96.69 mol% water. The percentage of NaCl is set to account for other ions and chemicals present after the pretreatment phase. NaCl alone is added to the HYSYS system due to the capabilities of the fluid package used to simulate this process, ElectroNRTL. The feed is set at 22°C, which is the approximate the temperature of the San Francisco Bay year round.

=Process Design=
==Design Overview==
The proposed desalination plant uses MSF distillation to convert the San Francisco Bay water to fresh water. The final process design contains 18 flash stages in series. Overall, 398 million gallons per day of of brackish water is converted to 50 million gallons per day of deionized water, a 13% yield. The production meets the required production rate of 50 million gallons of freshwater per day.

Seawater is taken in from the San Francisco Bay and mixed with the brine recycle stream produced downstream. The feed is then heated through a series of condensers and a heater. Distilled water vapor in each flash stage contacts the tubes carrying the seawater feed and condenses into a liquid stream by exchanging heat with the cooler seawater feed stream. The seawater is sent through a heater to increase its thermal energy before entering the first stage of the MSF. In each flashing stage, some of the water evaporates, leaving the salt behind. The freshwater vapors condense, as described above, and is collected and sent to a water treatment plant. The brine stream is split into the recycle stream and a waste stream, and the waste stream is diluted with fresh seawater before being discharged back into the bay.

A process flow diagram of the process is depicted in Appendix A.

==Pretreatment==
Sediment is removed to prevent solids from plugging the process downstream. Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime, and is controlled for by lowering pH. Acidifying the water will also remove CO2, which is a corrosive gas. To control for oxygen, oxygen scavengers will be included in the system to sequester it. Sodium bisulfate will be used as the scavenger.5

==Condensers (Heat Exchanger Networks)==
In order to conserve and recycle energy, the condensation of the distillate and heating of the seawater feed will be coupled in countercurrent heat exchanger networks. These heat exchanger networks are condensers of the design. The cold feed, which is a mixture of fresh seawater and brine recycle, is fed through multiple heat exchangers in series. Each heat exchanger corresponds to a different evaporation chamber. This process consists of 18 heat exchangers: E-101, E-102, … E-117, and E-118, which correspond to evaporation chambers V-102, V-103, … V-118, and V-119, respectively.

The heat exchangers can be viewed similarly to a shell and tube setup, however there is no actual physical shell. The tubes simply run through the top of the evaporation chambers, where vaporized distilled water in the chamber acts as the shell side fluid. The tube bundles are arranged in long tube configuration and are aligned parallel to the direction of the flashing brine flow in the evaporation chamber. The evaporating brine in an evaporation chamber rise up to the the corresponding heat exchanger and condense on the outside of the tubes, creating collectable pure water. This condensing water is what heats up the seawater in the tubes.

E-101 was used as the basis for the condenser sizing since it has the highest heat requirement, and thus will be the largest heat exchanger. From the HYSYS model, the overall system requires 2.24 x 106 kW of duty. Assuming this energy requirement, 5000 tubes, and 18 m length tubes (the diameter of the evaporation chambers, explained further on the next page), the diameter of each tube was calculated to be 0.4 m. Based on the temperature of the streams and flow, the thickness of the tube is 7 x 10-4 m.

==Brine Heater==
After the seawater exits the final heat exchanger, it enters a brine heater (E-119) where it is brought up to the temperature necessary for the flashing process to begin. This means that the seawater must be overheated compared to stage one, so that the seawater will flash in the evaporation chamber, i.e. release heat and vapor to reach equilibrium with stage conditions. After flowing through the condensers, the seawater enters the brine heater at 96.1 oC and is heated to 120 oC. The overall system requires 2.31 x 106 kW of duty. This is supplied by a total of 4.15 x 106 kg per hour of steam.

==Evaporation Chambers==
When the seawater has been brought to a temperature appropriate for flashing, it enters the first evaporation chamber (V-102). Upon entering the chamber, the water is flashed. Some of the water evaporates, leaving the salt behind. The vapor rises up to the heat exchanger tubes in each evaporation chamber and condenses on the outside into the distilled water product. This condensing water is what heats up the seawater in the tubes discussed above. The seawater that does not evaporate moves to the next stage, which operates at a higher temperature and lower pressure, and the process is repeated. This continues in series through a total of 18 flashing chambers (V-102, V-103, … V-118, and V-119). All of the distillate is collected into one stream and all of the brine is mixed with seawater before being disposed of back in the bay in order to dilute it enough so it does not have an adverse effect on marine life.

Each respective heat exchanger fully condenses the vapor product stream from each stage. However, when all of the individual product streams are consolidated into the final combined product stream, some water exists in the vapor phase due to temperature and pressure changes associated with mixing all of the streams together. Although the vapor product stream from each stage condenses completely across its respective heat exchanger, the total product stream contains some water in the vapor phase due to the temperature change when the mixer sums all the streams. In order to fully condense this stream, which will be sent to local water treatment plans, a heat exchanger (E-120) is put in place to cool down the product using the process feed stream.

The correlations of El-Dessouky and Ettouney were employed in order to size the flash chambers. They specified that conventional MSF plants that operate at a capacity between 27,000 and 36,000 m3/day use flashing stages of the dimensions 18 m diameter and 4 m height.6

==Valves==
The model requires valves in between the evaporation chambers. The seawater streams flow through valves with large pressure drops to flash a portion the liquid into vapor that can then be separated in the vessels. Large pressure drops provide the thermodynamic driving force to vaporize more of the liquid, and are required to achieve high product yield.

==Material of Construction==
Before 1980, the shells and internals of the MSF units were commonly constructed from carbon steel. Seawater, however, corrodes carbon steel. To compensate for this, the thickness of the carbon steel has an additional corrosion allowance, which increases the size and weight of the equipment. After 1980, the stainless steel and duplex stainless steel became increasingly common. Stainless steel metal requires less thickness for equivalent strength and corrosion resistance, which allows for a reduction in the size, weight, and therefore cost of units. Major units in our process will be constructed from stainless steel.6

==Process Alternatives==
In order to ensure that our design is fully optimized, we wanted to consider several alternatives for our process. We looked at alternatives associated with water pretreatment, the distillation process itself and product treatment.

===Water Pretreatment===
Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime. Scaling can be controlled by adjusting pH, temperature, and bicarbonate concentration. We selected acidification to control for process scaling. This is because lowering a process temperature would decrease overall efficiency, and decreasing bicarbonate concentration would be comparatively expensive. Lowering pH would have the added benefit of controlling CO2 concentrations since CO2 is a corrosive gas. To control for corrosion we looked at how to remove oxygen. To target the oxygen we chose an oxygen scavenger. This was chosen over other options such as a deaerator, or a protective coating because a deaerator would have a large capital cost, and the equipment itself would experience corrosion, and a tank coating such as zinc orthophosphate has a mixed effectiveness.7

===MSF: Brine Recycle vs. Once-Through System===
We explored two options for the overall layout of the MSF distillation plant. In the first, the brine passes through the process once; in the second, a portion of the brine is recycled from the last stage to the incoming seawater feed. The largest draw towards implementing a brine recycle is that it would significantly reduce the overall amount of seawater needed to produce the daily target of 50 million gallons per day. This in turn would lower the amount of energy required to run the pump feeding seawater to the process. Another positive is that, with a brine recycle, the amount of water conditioning chemicals that must be added to the seawater feed is reduced. The negative aspects mainly revolve around the fact that the solution throughout the process will be higher in salt concentration. This increase in salinity both raises the likelihood of scaling and corrosion in the plant and raises the amount of seawater that must be added to the final brine to make it safe for discharge back into the environment. Additionally, boiling point of each stage is raised when adding the brine recycle. After weighing both alternatives against one another, we have decided to include a brine recycle in our plant design. Most of the negative aspects of the brine recycle can be combated (e.g. include anti-scaling agents in the feed pretreatment), and the economic benefits of including the brine recycle are very significant.8

===MSF: Continuous vs. Batch===
The distillation step can be done in continuous or batch processes. Batch distillation is more versatile and often used when the product is in small amounts and very high purity. A continuous distillation is more efficient thermodynamically and economically for large amounts of material of constant composition, whereas batch distillation is more effective for small amounts of material of varying compositions. Since the plant is processing large amounts of seawater of relatively constant composition, the distillation system will utilize continuous distillation.9

===MSF: Long Tube vs. Crossed Tube ===
Inside the evaporator stage, the condenser tube bundles may be arranged in either long tube or cross tube design. In the long tube arrangement, the tubes are aligned parallel to the direction of the flashing brine flow. The tubes go from stage to stage without water boxes. In the cross tube arrangement, the tubes are lined perpendicular to the flow of the flashing brine. Water boxes transfer brine between the stages.10 11 While the water boxes in crossed tube bundles allow for more customization, the long tube configuration allows for construction of large unit sizes, as there are manufacturing restrictions with cross tubes. Long tube design can easily accommodate a large number of stages, which lead us to choose long tube for our design.11 12

===MSF: Venting System===
Noncondensable gases exist in the process because of air leakages into stages under vacuum conditions and the release of dissolved gases from the heated brine. These gases are removed by venting the system. An air cooler section concentrates the gas before they are vented, and baffles direct the vapor and condensates out. There are two venting arrangements to consider: series and parallel. The system in series moves gas from one stage directly to the next, and the only loss in the system is vapor in the last stage (since some vapor is extracted with gases). This system is economical, however this configuration increases the amount of noncondensable gases passing from one stage to the next, and disrupting the venting system in one stage will affect the entire system. In parallel venting, gas is released through parallel takeoff pipes at each stage. Although this arrangement allows for venting at any stage to be controlled, the design and maintenance is more complex, and more vapor is lost than in the venting system in series. A third alternative is to use a combination of venting in parallel and in series, which is the common method in industry. For this process, our team chose to do venting in parallel and in series.12

===Additional Energy Source Alternatives===
Energy for the process can also be coupled with renewable energy sources to reduce the desalination plant’s carbon footprint. Energy integration can be direct or indirect. In direct integration, thermal waste heat from industrial sources provide thermal energy or pressure as energy sources. Direct integration is advantageous at the modular level, as it avoids energy losses with electricity conversion. Within indirect integration, there are modular integration and grid/utility-scale integration. Modular integration integrates the plant with wind turbines and other small-scale renewable power generators with mobile deployment. A “hybridization” of renewable power sources with natural gas power generation is also an option, and the coupling of the two provides a stable power source. Indirect renewable energy integration is growing as the cost of renewable energy continues to decreases and provides an economy of scale that direct integration does not, but there are several obstacles towards indirect integration, such as the plant’s finance and engineering the integration of the two systems.13 This distillation plant will be directly integrated with neighboring plants such as Chevron that produce thermal waste heat.

===Product Treatment===
Since the product is distilled water, it needs additional ions to a concentration of around 0.01% before going to the end user. The three alternatives considered were building a water treatment facility on site, simply adding the correct salts to the product stream and then sending the stream for bacterial treatment at a water plant, or simply sending the product distilled water for complete treatment to another facility. Due to the number of nearby water treatment plants and their demand for water, we elected to simply send the product to a nearby water treatment facility.

==Design Constraints and Tradeoffs==
While designing our process, we wanted to make sure that we met several outside constraints, including environmental, political, ethical, social, sustainability, health and safety constraints. First and most importantly, we wanted to ensure that the brine discharged back to the bay would not harm local marine life. Our process was designed around the constraint of diluting the exiting brine to within 0.2% salinity of background seawater. This is not only compliant with California law, but was also verified to be safe for surrounding marine life through our own outside research. We also addressed many of these considerations by deciding to pursue direct energy integration with the nearby Chevron plant in order to reduce environmental impact of our process. Additionally, we ensured that our plant would be located in an area that is densely populated and in great need of fresh drinking water. Finally, we addressed health and safety considerations by making sure we would be able to send our product distilled water to local treatment plants for proper potabilization.

===Design Tradeoffs: Thermodynamics===
Adjusting elements within this process is iterative, and optimization thereof required changing multiple parameters at once. An example of this is in tuning the temperature increase by heater E-100; if the temperature of the stream entering the vessels is too high, the heat exchange between the vapor product and the feed stream will not be realizable because it causes the feed to be too hot to cool and condense the product. Higher temperatures, however, provide greater thermodynamic driving forces for flashing. Setting the entering stream temperature at 120°C provides enough heat for substantial flash at 130 kPa. This is the only heater required until the very last flash stage, which is added to provide more heat for vaporization. This heater (E-105) increases the stream temperature by 15°C, and the vapor product from the last vessel increases the feed stream temperature by almost 30°C. The added energy increases vaporization by 3%, which is a substantial 2.2 million kg/h. The temperature of the streams entering and leaving each stage decreases with each additional flash stage such that the last vessel has the lowest temperature, according to realistic operating guidelines.

===Design Tradeoffs: Energy Consumption, Environmental Impacts, and Product Optimization===
Increases in the pressure drop across any of the valves in this final design causes a temperature cross in the heat exchangers unless a proper increase in pump energy is provided. There is thus a design tradeoff between lowering energy usage and increasing product yield. For example, lowering the operating pressure of the process dictates that the pressure difference used to flash seawater must be smaller, possibly decreasing the amount that is vaporized. The final design aims for a yield of 50 million gallons per day. The 18 stage model was chosen for its low annual utility cost. This is a reflection of environmental mindfulness because energy consumption is an ongoing aspect of the process and should be lowered as much as possible.

==Mass and Energy Balances==
The model of the desalination system assumes that: (1) the properties are uniform in each phase within a stage, (2) heat losses are negligible due to the small surface area to volume ratio for each stage and properly insulated units (heat losses to the surrounding vary from 2 to 5% of the total system energy), (3) the distillate is salt free since the boiling point of water is much lower than that of salt, (4) the subcooling or superheating effects on the system energy balances are negligible, (5) the only contaminant is sodium chloride (there are no non-condensable gases to consider, so no vapors are vented), and (6) the process has 3 evaporation chambers.

The major balances are that the feed is a mixture of the fresh seawater stream and the recycle stream, and that the brine stream exiting the last evaporation chamber is divided into the recycle stream and the waste stream. Within each stage, the change in mass of a phase over time is the balance between the net flow of that phase from the chamber and the rates of evaporation and condensation of water into and out of that phase. The equations governing the mass and energy balances on the system is displayed in Appendix C.

=Optimization=
As initiatives for reduced energy usage and increased environmental sustainability become more prevalent, efforts to minimize the utility cost in plants has increased to accommodate these incentives. Utilities are a continual source of expenditure and consumption, and a well-designed chemical process should seek to minimize this element. The annualized utility cost is a useful parameter to optimize given that its value depends on variations in energy costs; thus, annualized utility costs do not correlate with conventional indices of inflation, such as capital and labor costs. Additionally, utility costs, like other variable costs, can be minimized by improving the design or operational efficiency of the plant.

In optimizing our MSF process, the design variable of focus was the number of stages to use in our process. The performance measure was the annualized utility cost, which was the parameter that was minimized for optimization. The process is constrained by the product specifications, which are kept constant and serve as the standard for comparison between each model. These specifications are to produce 50 million gallons of desalinated liquid water per day.

One feasibility constraint in the process is the size of the heat exchangers, which must provide a large enough area for total condensation of the vapor stream coming out of the flash stages. In order to restrict the heat exchange area needed within each exchanger, the number of stages must be at least 11. Increasing the number of stages decreases the vapor flow rate at each, and decreases the heat exchange area required as well. An 11-stage process allows for appropriate heat exchange between the feed stream and the vapor product, according to the HYSYS economic analyzer. The optimization analysis thus was conducted incorporating only models with 11 or more stages.

The HYSYS economic analyzer was used to systematically optimize the process by finding the model with the lowest annualized utility cost. The performance parameters kept constant across each model were: the inlet composition, pressure, and temperature, the brine outlet temperature, the total product flow rate and composition, and the flash stage schematic. Table 1 displays the annualized utility cost for each model, and illustrates that a clear dip in the utility cost occurs in an 18 stage model. Thus, 18 is the optimal number of stages to use for our MSF distillation given utility costs as the optimized parameter.
[[File:Table_1.PNG|thumb|center|500x300px]]

=Economic Evaluation=
In order to evaluate our process from a fiscal standpoint, we used Aspen Process Economic Analyzer and Aspen cost estimator to determine capital and operating costs for our plant. These costs were calculated based on calculated equipment sizes, as well as energy requirements reported by our HYSYS model. The capital cost was found to be $485.2MM and annual total operating cost was found to be $144.7MM ($103.7MM of which was utilities). Annual plant revenue was estimated based on prices that water districts in the county currently pay for outside water sources, and was found to be $134.3MM.14

The NPV analysis we conducted assumed that the plant would take two years to build, would operate at half capacity during the first year of operation (year 3), and have a 20 year lifetime. We also assumed that our plant would be in operation 350 days per year. A constant discount rate of 12% was assumed to hold throughout the plant’s lifetime. The final NPV of this project is -$495.7MM. This is due to both the large capital cost of the plant and the annual loss of $10.4MM while in full operation. A graph of cumulative cash flow versus time can be found in Appendix E. Tax rates and depreciation do not directly impact the NPV of this project due to operating at a loss, however for future calculations it should be noted that we used a straight line depreciation schedule over a seven year period with a 40% tax rate.

Despite the current projections, it is important to consider the greater implications of running a desalination plant in the Bay Area. The population in the Bay Area is quickly growing, while California is experiencing harsher droughts each year. A consequence of this is the possibility of receiving government subsidies for our clean water product. The similarly scaled desalination plant in Carlsbad, CA operates at a loss as well, but receives significant government subsidies. Building this facility may be a strategic investment, especially since greater water scarcity is predicted in the future.

=Sensitivity Analysis=
We wanted to determine how sensitive NPV of our project is to the cost of capital, price of utilities and price we would be able to sell our clean water product at. We conducted this analysis by varying each of these variables independently between 0.5 times and 1.5 times the original value we calculated. As to be expected, NPV grows more negative with increasing capital costs and utility prices and decreasing water prices. Conversely, NPV becomes less negative as price of water rises and capital and utility costs decrease. A plot summarizing the effects of varying these costs can be found in Appendix E. NPV is most sensitive to the price at which we will be selling our clean water product. It is worth noting that NPV is more sensitive to utilities cost than it is to capital cost, which is why we focused on optimizing the cost of utilities. In future steps, trying to reduce utilities cost even more would be worth looking into to decrease the amount of money being lost on this project.

=Conclusions and Final Recommendation=
The final design achieves the project goal of producing 50 million gallons per day of distilled water at a yield of 13%, comparable to other similarly sized plants. An optimization of the number of MSF stages is realistic, and can be improved upon by taking into account other process parameters.

The proposed design was optimized around utility costs, comparing plants with 11 to 25 stages. The process is constrained by product specifications and by the thermodynamics of the heat exchange between the feed and the product streams. Energy conservation was achieved in using the seawater feed to condense the final product, as well as in all of the heat exchangers increasing the thermal energy of the seawater feed stream before it enters the flash stages.

In order to further optimize this process, other design parameters should be considered such as the brine recycle ratio, the cooler duty of the recycle stream, the pressure drop across each valve at the flash stages, and other economic factors. An optimization around maximizing NPV, for example, might be more comprehensive than that conducted regarding only utility costs. Tuning brine recycle rates could greatly improve process efficiency, and standardizing flash stage pressure drops would make operations more feasible and controlled.

Given our in depth economic analysis, this project would lose an average of $10.4MM per year, and has an NPV of -$495.7MM for a 20 year plant lifetime. From an economic perspective alone it would not be a good project to build. However, other factors need to be considered when building a project with enormous public value. In recent years California droughts have become more severe. Implementing a drought-proof supply of water now could avert a disaster later on. Even without a large disaster, having the desalination plant could also be expected to pay off in the future with the increasing price of water. Considerations such as these were the deciding factors in building the Carlsbad desalination plant. Since there are currently no desalination plants in the Bay area we recommend that preparations should be made to move forward with a desalination plant.

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=Appendix A. Process Flow Diagram=
[[File:appendix1.PNG|thumb|center|'''Figure A1''' Process Flow Diagram of the MSF System|500x300px]]
=Appendix B. HYSYS Simulation=
=Appendix C. Mass and Energy Balances=
=Appendix D. Summary of Process Units=
=Appendix E. MSF Stage Parameters=
=Appendix F. Process Units Calculations=
=Appendix G. Economic Analysis=

Desalination - Team A

2016-03-12T02:54:33Z

Ezhuang: /* Evaporation Chambers */

Author: Pear Dhiantravan[2016], Reed Kolbe[2016], Sheridan Lichtor[2016], John Marsiglio[2016], Ellen Zhuang[2016]

Instructors: Fengqi You, David Wegerer

Winter 2016

Seawater desalination is an attractive approach to address the world’s freshwater shortage in light of the increasing global water scarcity. Our team decided to tackle California's growing drought crisis by designing a multistage flash desalination plant in Richmond, CA. Based on market analysis of existing desalination plants and water demands, this is a strategic location for the desalination of 50 million gallons per day of water from the San Francisco Bay.

The MSF process contains 18 flash stages in series. Seawater is taken in from the San Francisco Bay at a rate of 398 million gallons per day and mixed with brine recycled from the system. Within the MSF process, seawater is heated through a series of 18 heat exchangers and a brine heater before enters a series of 18 evaporation chambers. In the evaporation chambers, the seawater flashes and the freshwater vapor is condensed, collected, and sent to a water treatment facility. The brine stream proceeds to the next chamber, and the process continues. Upon exiting the last flashing chamber, the brine stream is split into a recycle brine stream that is mixed with the seawater inlet feed and a waste stream that is diluted with fresh seawater and returned to the sea. Overall, the process yield is 13%. This process was constructed in Aspen HYSYS and optimized around utility costs using the HYSYS Economic Analyzer. Based on duties, temperatures and pressures reported by HYSYS, as well as fundamental mass and energy balances, the equipment were sized appropriately and the economics of the process was analyzed.

The design has a total capital cost of $485.2 MM, an annual operating cost of $144.7 MM ($103.7 MM of which is utilities), and an annual plant revenue of $134.3 MM. Economic analysis reveals that the project has a negative NPV of -$495.7 MM and operates at a loss of $10.4 MM per year. Thus, the proposed design is not profitable. Despite the current projections, it is important to consider the greater implications of running a desalination plant in the Bay Area. The population in the Bay Area is quickly growing while California is experiencing harsher droughts each year. Given the dire situations, the government may support the proposed clean water project with subsidies. Building the facility may be a strategic investment, especially since greater water scarcity is predicted in the future.

Our design meets various environmental, ethical, social and health related constraints. The brine discharged back to the bay is diluted to at most 0.2% above the natural background salinity as required by law so that the plant does not impose harm on any local marine life. The demographic that the plant serves is largely invested in the protection of the environment, which we addressed by reducing energy consumption through direct energy integration with the nearby Chevron plant. Lastly, the product meets the specifications of the California Safe Water Drinking Act by producing water with no impurities.

=Introduction=
An increasing global water scarcity is fueling initiatives everywhere for clean water treatment, making efficient seawater desalination an attractive aim for chemical plant design. A 2015 market analysis found that the global desalination market earned revenues of $11.66 billion, and this number is expected to reach over $19 billion by 2019. Furthermore, the 17,000 desalination plants currently in operation are expected to double in number by 2020.1

As fresh water sources become increasingly scarce, strict water conservation measures are being observed. California is entering the fourth year of one of its most severe droughts on record. Cities are required to reduce their water usage by 35% to avoid facing fines.2 In lieu of these pressing conditions, California is looking for ways to provide more accessible fresh water to its citizens. This high demand for clean water motivated our choosing Richmond, CA as our plant location. At Richmond, the desalination plant can convert readily available seawater from the San Francisco Bay into fresh water. We propose to desalinate 50 million gallons of water per day to be sent to water treatment plants for potability. This aim is based on the capacity of the desalination plant in Carlsbad, California, which services a similar population.

The salinity of the water in the San Francisco bay is seasonal. During the dry seasons of summer and fall, salinity is high around 10 PSU (10,000 ppm) because water from the Pacific Ocean flows into the San Francisco Bay. During the wet winter season, fresh water from the rivers flows into the Bay, and salinity drops to around 2 PSU (2,000).3 Designed to meet the “worst case” scenario, our plant will process 398 million gallons of water per day with 10,000 ppm of salt to produce 50 million gallons of water with 1,000 ppm of salt. This gives a yield of 13%.

The desalination plant proposed consists of 18 flash stages in series. This report outlines our investigation of this potential water desalination plant, including design and optimization of the desalination process, an analysis of its energy consumption and environmental impacts, and a study of its economic implications.

=Technical Approach=
For the desalination step of our process, we selected a multistage flash (MSF) distillation process. Not only is MSF distillation a very viable method, but it is also the most common method, currently producing about 60% of the world’s desalinated water. MSF and reverse osmosis (RO) are the two major methods being used in large-scale desalination plants. Both processes require considerable amounts of energy. RO typically has a lower energy demand; however, the high impurity content of the Bay water would frequently necessitate membrane cleaning and/or exchange. Feed going through a RO system requires extensive pretreatment to remove biological organisms and other solids to control the pH and the chemical composition of the water. With the San Francisco Bay as our source of water, MSF is an attractive option because sediment and other large impurities can be separated from the feed before distillation occurs. Additionally, MSF distillation plants can be located near power plants and paired to their waste heat streams to conserve energy. This can reduce energy needs by half or two thirds, making MSF increasingly more practical. Due to the maintenance required in the RO system, the MSF distillation is the better option in terms of operation cost.4

A HYSYS model was built to simulate the MSF distillation section of the process, which spans after seawater pretreatment and before local potable water treatment. The model is used to simulate the distillation steps and to calculate necessary heating, cooling, and input flow rates to produce the required 50 million gallons of distillated water per day. For the simulation, the 398 million gallons per day of seawater feed stream is defined to be composed of 3.31 mol% sodium chloride (NaCl) and 96.69 mol% water. The percentage of NaCl is set to account for other ions and chemicals present after the pretreatment phase. NaCl alone is added to the HYSYS system due to the capabilities of the fluid package used to simulate this process, ElectroNRTL. The feed is set at 22°C, which is the approximate the temperature of the San Francisco Bay year round.

=Process Design=
==Design Overview==
The proposed desalination plant uses MSF distillation to convert the San Francisco Bay water to fresh water. The final process design contains 18 flash stages in series. Overall, 398 million gallons per day of of brackish water is converted to 50 million gallons per day of deionized water, a 13% yield. The production meets the required production rate of 50 million gallons of freshwater per day.

Seawater is taken in from the San Francisco Bay and mixed with the brine recycle stream produced downstream. The feed is then heated through a series of condensers and a heater. Distilled water vapor in each flash stage contacts the tubes carrying the seawater feed and condenses into a liquid stream by exchanging heat with the cooler seawater feed stream. The seawater is sent through a heater to increase its thermal energy before entering the first stage of the MSF. In each flashing stage, some of the water evaporates, leaving the salt behind. The freshwater vapors condense, as described above, and is collected and sent to a water treatment plant. The brine stream is split into the recycle stream and a waste stream, and the waste stream is diluted with fresh seawater before being discharged back into the bay.

A process flow diagram of the process is depicted in Appendix A.

==Pretreatment==
Sediment is removed to prevent solids from plugging the process downstream. Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime, and is controlled for by lowering pH. Acidifying the water will also remove CO2, which is a corrosive gas. To control for oxygen, oxygen scavengers will be included in the system to sequester it. Sodium bisulfate will be used as the scavenger.5

==Condensers (Heat Exchanger Networks)==
In order to conserve and recycle energy, the condensation of the distillate and heating of the seawater feed will be coupled in countercurrent heat exchanger networks. These heat exchanger networks are condensers of the design. The cold feed, which is a mixture of fresh seawater and brine recycle, is fed through multiple heat exchangers in series. Each heat exchanger corresponds to a different evaporation chamber. This process consists of 18 heat exchangers: E-101, E-102, … E-117, and E-118, which correspond to evaporation chambers V-102, V-103, … V-118, and V-119, respectively.

The heat exchangers can be viewed similarly to a shell and tube setup, however there is no actual physical shell. The tubes simply run through the top of the evaporation chambers, where vaporized distilled water in the chamber acts as the shell side fluid. The tube bundles are arranged in long tube configuration and are aligned parallel to the direction of the flashing brine flow in the evaporation chamber. The evaporating brine in an evaporation chamber rise up to the the corresponding heat exchanger and condense on the outside of the tubes, creating collectable pure water. This condensing water is what heats up the seawater in the tubes.

E-101 was used as the basis for the condenser sizing since it has the highest heat requirement, and thus will be the largest heat exchanger. From the HYSYS model, the overall system requires 2.24 x 106 kW of duty. Assuming this energy requirement, 5000 tubes, and 18 m length tubes (the diameter of the evaporation chambers, explained further on the next page), the diameter of each tube was calculated to be 0.4 m. Based on the temperature of the streams and flow, the thickness of the tube is 7 x 10-4 m.

==Brine Heater==
After the seawater exits the final heat exchanger, it enters a brine heater (E-119) where it is brought up to the temperature necessary for the flashing process to begin. This means that the seawater must be overheated compared to stage one, so that the seawater will flash in the evaporation chamber, i.e. release heat and vapor to reach equilibrium with stage conditions. After flowing through the condensers, the seawater enters the brine heater at 96.1 oC and is heated to 120 oC. The overall system requires 2.31 x 106 kW of duty. This is supplied by a total of 4.15 x 106 kg per hour of steam.

==Evaporation Chambers==
When the seawater has been brought to a temperature appropriate for flashing, it enters the first evaporation chamber (V-102). Upon entering the chamber, the water is flashed. Some of the water evaporates, leaving the salt behind. The vapor rises up to the heat exchanger tubes in each evaporation chamber and condenses on the outside into the distilled water product. This condensing water is what heats up the seawater in the tubes discussed above. The seawater that does not evaporate moves to the next stage, which operates at a higher temperature and lower pressure, and the process is repeated. This continues in series through a total of 18 flashing chambers (V-102, V-103, … V-118, and V-119). All of the distillate is collected into one stream and all of the brine is mixed with seawater before being disposed of back in the bay in order to dilute it enough so it does not have an adverse effect on marine life.

Each respective heat exchanger fully condenses the vapor product stream from each stage. However, when all of the individual product streams are consolidated into the final combined product stream, some water exists in the vapor phase due to temperature and pressure changes associated with mixing all of the streams together. Although the vapor product stream from each stage condenses completely across its respective heat exchanger, the total product stream contains some water in the vapor phase due to the temperature change when the mixer sums all the streams. In order to fully condense this stream, which will be sent to local water treatment plans, a heat exchanger (E-120) is put in place to cool down the product using the process feed stream.

The correlations of El-Dessouky and Ettouney were employed in order to size the flash chambers. They specified that conventional MSF plants that operate at a capacity between 27,000 and 36,000 m3/day use flashing stages of the dimensions 18 x 4 meters.6

==Valves==
The model requires valves in between the evaporation chambers. The seawater streams flow through valves with large pressure drops to flash a portion the liquid into vapor that can then be separated in the vessels. Large pressure drops provide the thermodynamic driving force to vaporize more of the liquid, and are required to achieve high product yield.

==Material of Construction==
Before 1980, the shells and internals of the MSF units were commonly constructed from carbon steel. Seawater, however, corrodes carbon steel. To compensate for this, the thickness of the carbon steel has an additional corrosion allowance, which increases the size and weight of the equipment. After 1980, the stainless steel and duplex stainless steel became increasingly common. Stainless steel metal requires less thickness for equivalent strength and corrosion resistance, which allows for a reduction in the size, weight, and therefore cost of units. Major units in our process will be constructed from stainless steel.6

==Process Alternatives==
In order to ensure that our design is fully optimized, we wanted to consider several alternatives for our process. We looked at alternatives associated with water pretreatment, the distillation process itself and product treatment.

===Water Pretreatment===
Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime. Scaling can be controlled by adjusting pH, temperature, and bicarbonate concentration. We selected acidification to control for process scaling. This is because lowering a process temperature would decrease overall efficiency, and decreasing bicarbonate concentration would be comparatively expensive. Lowering pH would have the added benefit of controlling CO2 concentrations since CO2 is a corrosive gas. To control for corrosion we looked at how to remove oxygen. To target the oxygen we chose an oxygen scavenger. This was chosen over other options such as a deaerator, or a protective coating because a deaerator would have a large capital cost, and the equipment itself would experience corrosion, and a tank coating such as zinc orthophosphate has a mixed effectiveness.7

===MSF: Brine Recycle vs. Once-Through System===
We explored two options for the overall layout of the MSF distillation plant. In the first, the brine passes through the process once; in the second, a portion of the brine is recycled from the last stage to the incoming seawater feed. The largest draw towards implementing a brine recycle is that it would significantly reduce the overall amount of seawater needed to produce the daily target of 50 million gallons per day. This in turn would lower the amount of energy required to run the pump feeding seawater to the process. Another positive is that, with a brine recycle, the amount of water conditioning chemicals that must be added to the seawater feed is reduced. The negative aspects mainly revolve around the fact that the solution throughout the process will be higher in salt concentration. This increase in salinity both raises the likelihood of scaling and corrosion in the plant and raises the amount of seawater that must be added to the final brine to make it safe for discharge back into the environment. Additionally, boiling point of each stage is raised when adding the brine recycle. After weighing both alternatives against one another, we have decided to include a brine recycle in our plant design. Most of the negative aspects of the brine recycle can be combated (e.g. include anti-scaling agents in the feed pretreatment), and the economic benefits of including the brine recycle are very significant.8

===MSF: Continuous vs. Batch===
The distillation step can be done in continuous or batch processes. Batch distillation is more versatile and often used when the product is in small amounts and very high purity. A continuous distillation is more efficient thermodynamically and economically for large amounts of material of constant composition, whereas batch distillation is more effective for small amounts of material of varying compositions. Since the plant is processing large amounts of seawater of relatively constant composition, the distillation system will utilize continuous distillation.9

===MSF: Long Tube vs. Crossed Tube ===
Inside the evaporator stage, the condenser tube bundles may be arranged in either long tube or cross tube design. In the long tube arrangement, the tubes are aligned parallel to the direction of the flashing brine flow. The tubes go from stage to stage without water boxes. In the cross tube arrangement, the tubes are lined perpendicular to the flow of the flashing brine. Water boxes transfer brine between the stages.10 11 While the water boxes in crossed tube bundles allow for more customization, the long tube configuration allows for construction of large unit sizes, as there are manufacturing restrictions with cross tubes. Long tube design can easily accommodate a large number of stages, which lead us to choose long tube for our design.11 12

===MSF: Venting System===
Noncondensable gases exist in the process because of air leakages into stages under vacuum conditions and the release of dissolved gases from the heated brine. These gases are removed by venting the system. An air cooler section concentrates the gas before they are vented, and baffles direct the vapor and condensates out. There are two venting arrangements to consider: series and parallel. The system in series moves gas from one stage directly to the next, and the only loss in the system is vapor in the last stage (since some vapor is extracted with gases). This system is economical, however this configuration increases the amount of noncondensable gases passing from one stage to the next, and disrupting the venting system in one stage will affect the entire system. In parallel venting, gas is released through parallel takeoff pipes at each stage. Although this arrangement allows for venting at any stage to be controlled, the design and maintenance is more complex, and more vapor is lost than in the venting system in series. A third alternative is to use a combination of venting in parallel and in series, which is the common method in industry. For this process, our team chose to do venting in parallel and in series.12

===Additional Energy Source Alternatives===
Energy for the process can also be coupled with renewable energy sources to reduce the desalination plant’s carbon footprint. Energy integration can be direct or indirect. In direct integration, thermal waste heat from industrial sources provide thermal energy or pressure as energy sources. Direct integration is advantageous at the modular level, as it avoids energy losses with electricity conversion. Within indirect integration, there are modular integration and grid/utility-scale integration. Modular integration integrates the plant with wind turbines and other small-scale renewable power generators with mobile deployment. A “hybridization” of renewable power sources with natural gas power generation is also an option, and the coupling of the two provides a stable power source. Indirect renewable energy integration is growing as the cost of renewable energy continues to decreases and provides an economy of scale that direct integration does not, but there are several obstacles towards indirect integration, such as the plant’s finance and engineering the integration of the two systems.13 This distillation plant will be directly integrated with neighboring plants such as Chevron that produce thermal waste heat.

===Product Treatment===
Since the product is distilled water, it needs additional ions to a concentration of around 0.01% before going to the end user. The three alternatives considered were building a water treatment facility on site, simply adding the correct salts to the product stream and then sending the stream for bacterial treatment at a water plant, or simply sending the product distilled water for complete treatment to another facility. Due to the number of nearby water treatment plants and their demand for water, we elected to simply send the product to a nearby water treatment facility.

==Design Constraints and Tradeoffs==
While designing our process, we wanted to make sure that we met several outside constraints, including environmental, political, ethical, social, sustainability, health and safety constraints. First and most importantly, we wanted to ensure that the brine discharged back to the bay would not harm local marine life. Our process was designed around the constraint of diluting the exiting brine to within 0.2% salinity of background seawater. This is not only compliant with California law, but was also verified to be safe for surrounding marine life through our own outside research. We also addressed many of these considerations by deciding to pursue direct energy integration with the nearby Chevron plant in order to reduce environmental impact of our process. Additionally, we ensured that our plant would be located in an area that is densely populated and in great need of fresh drinking water. Finally, we addressed health and safety considerations by making sure we would be able to send our product distilled water to local treatment plants for proper potabilization.

===Design Tradeoffs: Thermodynamics===
Adjusting elements within this process is iterative, and optimization thereof required changing multiple parameters at once. An example of this is in tuning the temperature increase by heater E-100; if the temperature of the stream entering the vessels is too high, the heat exchange between the vapor product and the feed stream will not be realizable because it causes the feed to be too hot to cool and condense the product. Higher temperatures, however, provide greater thermodynamic driving forces for flashing. Setting the entering stream temperature at 120°C provides enough heat for substantial flash at 130 kPa. This is the only heater required until the very last flash stage, which is added to provide more heat for vaporization. This heater (E-105) increases the stream temperature by 15°C, and the vapor product from the last vessel increases the feed stream temperature by almost 30°C. The added energy increases vaporization by 3%, which is a substantial 2.2 million kg/h. The temperature of the streams entering and leaving each stage decreases with each additional flash stage such that the last vessel has the lowest temperature, according to realistic operating guidelines.

===Design Tradeoffs: Energy Consumption, Environmental Impacts, and Product Optimization===
Increases in the pressure drop across any of the valves in this final design causes a temperature cross in the heat exchangers unless a proper increase in pump energy is provided. There is thus a design tradeoff between lowering energy usage and increasing product yield. For example, lowering the operating pressure of the process dictates that the pressure difference used to flash seawater must be smaller, possibly decreasing the amount that is vaporized. The final design aims for a yield of 50 million gallons per day. The 18 stage model was chosen for its low annual utility cost. This is a reflection of environmental mindfulness because energy consumption is an ongoing aspect of the process and should be lowered as much as possible.

==Mass and Energy Balances==
The model of the desalination system assumes that: (1) the properties are uniform in each phase within a stage, (2) heat losses are negligible due to the small surface area to volume ratio for each stage and properly insulated units (heat losses to the surrounding vary from 2 to 5% of the total system energy), (3) the distillate is salt free since the boiling point of water is much lower than that of salt, (4) the subcooling or superheating effects on the system energy balances are negligible, (5) the only contaminant is sodium chloride (there are no non-condensable gases to consider, so no vapors are vented), and (6) the process has 3 evaporation chambers.

The major balances are that the feed is a mixture of the fresh seawater stream and the recycle stream, and that the brine stream exiting the last evaporation chamber is divided into the recycle stream and the waste stream. Within each stage, the change in mass of a phase over time is the balance between the net flow of that phase from the chamber and the rates of evaporation and condensation of water into and out of that phase. The equations governing the mass and energy balances on the system is displayed in Appendix C.

=Optimization=
As initiatives for reduced energy usage and increased environmental sustainability become more prevalent, efforts to minimize the utility cost in plants has increased to accommodate these incentives. Utilities are a continual source of expenditure and consumption, and a well-designed chemical process should seek to minimize this element. The annualized utility cost is a useful parameter to optimize given that its value depends on variations in energy costs; thus, annualized utility costs do not correlate with conventional indices of inflation, such as capital and labor costs. Additionally, utility costs, like other variable costs, can be minimized by improving the design or operational efficiency of the plant.

In optimizing our MSF process, the design variable of focus was the number of stages to use in our process. The performance measure was the annualized utility cost, which was the parameter that was minimized for optimization. The process is constrained by the product specifications, which are kept constant and serve as the standard for comparison between each model. These specifications are to produce 50 million gallons of desalinated liquid water per day.

One feasibility constraint in the process is the size of the heat exchangers, which must provide a large enough area for total condensation of the vapor stream coming out of the flash stages. In order to restrict the heat exchange area needed within each exchanger, the number of stages must be at least 11. Increasing the number of stages decreases the vapor flow rate at each, and decreases the heat exchange area required as well. An 11-stage process allows for appropriate heat exchange between the feed stream and the vapor product, according to the HYSYS economic analyzer. The optimization analysis thus was conducted incorporating only models with 11 or more stages.

The HYSYS economic analyzer was used to systematically optimize the process by finding the model with the lowest annualized utility cost. The performance parameters kept constant across each model were: the inlet composition, pressure, and temperature, the brine outlet temperature, the total product flow rate and composition, and the flash stage schematic. Table 1 displays the annualized utility cost for each model, and illustrates that a clear dip in the utility cost occurs in an 18 stage model. Thus, 18 is the optimal number of stages to use for our MSF distillation given utility costs as the optimized parameter.
[[File:Table_1.PNG|thumb|center|500x300px]]

=Economic Evaluation=
In order to evaluate our process from a fiscal standpoint, we used Aspen Process Economic Analyzer and Aspen cost estimator to determine capital and operating costs for our plant. These costs were calculated based on calculated equipment sizes, as well as energy requirements reported by our HYSYS model. The capital cost was found to be $485.2MM and annual total operating cost was found to be $144.7MM ($103.7MM of which was utilities). Annual plant revenue was estimated based on prices that water districts in the county currently pay for outside water sources, and was found to be $134.3MM.14

The NPV analysis we conducted assumed that the plant would take two years to build, would operate at half capacity during the first year of operation (year 3), and have a 20 year lifetime. We also assumed that our plant would be in operation 350 days per year. A constant discount rate of 12% was assumed to hold throughout the plant’s lifetime. The final NPV of this project is -$495.7MM. This is due to both the large capital cost of the plant and the annual loss of $10.4MM while in full operation. A graph of cumulative cash flow versus time can be found in Appendix E. Tax rates and depreciation do not directly impact the NPV of this project due to operating at a loss, however for future calculations it should be noted that we used a straight line depreciation schedule over a seven year period with a 40% tax rate.

Despite the current projections, it is important to consider the greater implications of running a desalination plant in the Bay Area. The population in the Bay Area is quickly growing, while California is experiencing harsher droughts each year. A consequence of this is the possibility of receiving government subsidies for our clean water product. The similarly scaled desalination plant in Carlsbad, CA operates at a loss as well, but receives significant government subsidies. Building this facility may be a strategic investment, especially since greater water scarcity is predicted in the future.

=Sensitivity Analysis=
We wanted to determine how sensitive NPV of our project is to the cost of capital, price of utilities and price we would be able to sell our clean water product at. We conducted this analysis by varying each of these variables independently between 0.5 times and 1.5 times the original value we calculated. As to be expected, NPV grows more negative with increasing capital costs and utility prices and decreasing water prices. Conversely, NPV becomes less negative as price of water rises and capital and utility costs decrease. A plot summarizing the effects of varying these costs can be found in Appendix E. NPV is most sensitive to the price at which we will be selling our clean water product. It is worth noting that NPV is more sensitive to utilities cost than it is to capital cost, which is why we focused on optimizing the cost of utilities. In future steps, trying to reduce utilities cost even more would be worth looking into to decrease the amount of money being lost on this project.

=Conclusions and Final Recommendation=
The final design achieves the project goal of producing 50 million gallons per day of distilled water at a yield of 13%, comparable to other similarly sized plants. An optimization of the number of MSF stages is realistic, and can be improved upon by taking into account other process parameters.

The proposed design was optimized around utility costs, comparing plants with 11 to 25 stages. The process is constrained by product specifications and by the thermodynamics of the heat exchange between the feed and the product streams. Energy conservation was achieved in using the seawater feed to condense the final product, as well as in all of the heat exchangers increasing the thermal energy of the seawater feed stream before it enters the flash stages.

In order to further optimize this process, other design parameters should be considered such as the brine recycle ratio, the cooler duty of the recycle stream, the pressure drop across each valve at the flash stages, and other economic factors. An optimization around maximizing NPV, for example, might be more comprehensive than that conducted regarding only utility costs. Tuning brine recycle rates could greatly improve process efficiency, and standardizing flash stage pressure drops would make operations more feasible and controlled.

Given our in depth economic analysis, this project would lose an average of $10.4MM per year, and has an NPV of -$495.7MM for a 20 year plant lifetime. From an economic perspective alone it would not be a good project to build. However, other factors need to be considered when building a project with enormous public value. In recent years California droughts have become more severe. Implementing a drought-proof supply of water now could avert a disaster later on. Even without a large disaster, having the desalination plant could also be expected to pay off in the future with the increasing price of water. Considerations such as these were the deciding factors in building the Carlsbad desalination plant. Since there are currently no desalination plants in the Bay area we recommend that preparations should be made to move forward with a desalination plant.

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=Appendix A. Process Flow Diagram=
=Appendix B. HYSYS Simulation=
=Appendix C. Mass and Energy Balances=
=Appendix D. Summary of Process Units=
=Appendix E. MSF Stage Parameters=
=Appendix F. Process Units Calculations=
=Appendix G. Economic Analysis=

Desalination - Team A

2016-03-12T02:53:23Z

Ezhuang: /* Conclusions and Final Recommendation */

Author: Pear Dhiantravan[2016], Reed Kolbe[2016], Sheridan Lichtor[2016], John Marsiglio[2016], Ellen Zhuang[2016]

Instructors: Fengqi You, David Wegerer

Winter 2016

Seawater desalination is an attractive approach to address the world’s freshwater shortage in light of the increasing global water scarcity. Our team decided to tackle California's growing drought crisis by designing a multistage flash desalination plant in Richmond, CA. Based on market analysis of existing desalination plants and water demands, this is a strategic location for the desalination of 50 million gallons per day of water from the San Francisco Bay.

The MSF process contains 18 flash stages in series. Seawater is taken in from the San Francisco Bay at a rate of 398 million gallons per day and mixed with brine recycled from the system. Within the MSF process, seawater is heated through a series of 18 heat exchangers and a brine heater before enters a series of 18 evaporation chambers. In the evaporation chambers, the seawater flashes and the freshwater vapor is condensed, collected, and sent to a water treatment facility. The brine stream proceeds to the next chamber, and the process continues. Upon exiting the last flashing chamber, the brine stream is split into a recycle brine stream that is mixed with the seawater inlet feed and a waste stream that is diluted with fresh seawater and returned to the sea. Overall, the process yield is 13%. This process was constructed in Aspen HYSYS and optimized around utility costs using the HYSYS Economic Analyzer. Based on duties, temperatures and pressures reported by HYSYS, as well as fundamental mass and energy balances, the equipment were sized appropriately and the economics of the process was analyzed.

The design has a total capital cost of $485.2 MM, an annual operating cost of $144.7 MM ($103.7 MM of which is utilities), and an annual plant revenue of $134.3 MM. Economic analysis reveals that the project has a negative NPV of -$495.7 MM and operates at a loss of $10.4 MM per year. Thus, the proposed design is not profitable. Despite the current projections, it is important to consider the greater implications of running a desalination plant in the Bay Area. The population in the Bay Area is quickly growing while California is experiencing harsher droughts each year. Given the dire situations, the government may support the proposed clean water project with subsidies. Building the facility may be a strategic investment, especially since greater water scarcity is predicted in the future.

Our design meets various environmental, ethical, social and health related constraints. The brine discharged back to the bay is diluted to at most 0.2% above the natural background salinity as required by law so that the plant does not impose harm on any local marine life. The demographic that the plant serves is largely invested in the protection of the environment, which we addressed by reducing energy consumption through direct energy integration with the nearby Chevron plant. Lastly, the product meets the specifications of the California Safe Water Drinking Act by producing water with no impurities.

=Introduction=
An increasing global water scarcity is fueling initiatives everywhere for clean water treatment, making efficient seawater desalination an attractive aim for chemical plant design. A 2015 market analysis found that the global desalination market earned revenues of $11.66 billion, and this number is expected to reach over $19 billion by 2019. Furthermore, the 17,000 desalination plants currently in operation are expected to double in number by 2020.1

As fresh water sources become increasingly scarce, strict water conservation measures are being observed. California is entering the fourth year of one of its most severe droughts on record. Cities are required to reduce their water usage by 35% to avoid facing fines.2 In lieu of these pressing conditions, California is looking for ways to provide more accessible fresh water to its citizens. This high demand for clean water motivated our choosing Richmond, CA as our plant location. At Richmond, the desalination plant can convert readily available seawater from the San Francisco Bay into fresh water. We propose to desalinate 50 million gallons of water per day to be sent to water treatment plants for potability. This aim is based on the capacity of the desalination plant in Carlsbad, California, which services a similar population.

The salinity of the water in the San Francisco bay is seasonal. During the dry seasons of summer and fall, salinity is high around 10 PSU (10,000 ppm) because water from the Pacific Ocean flows into the San Francisco Bay. During the wet winter season, fresh water from the rivers flows into the Bay, and salinity drops to around 2 PSU (2,000).3 Designed to meet the “worst case” scenario, our plant will process 398 million gallons of water per day with 10,000 ppm of salt to produce 50 million gallons of water with 1,000 ppm of salt. This gives a yield of 13%.

The desalination plant proposed consists of 18 flash stages in series. This report outlines our investigation of this potential water desalination plant, including design and optimization of the desalination process, an analysis of its energy consumption and environmental impacts, and a study of its economic implications.

=Technical Approach=
For the desalination step of our process, we selected a multistage flash (MSF) distillation process. Not only is MSF distillation a very viable method, but it is also the most common method, currently producing about 60% of the world’s desalinated water. MSF and reverse osmosis (RO) are the two major methods being used in large-scale desalination plants. Both processes require considerable amounts of energy. RO typically has a lower energy demand; however, the high impurity content of the Bay water would frequently necessitate membrane cleaning and/or exchange. Feed going through a RO system requires extensive pretreatment to remove biological organisms and other solids to control the pH and the chemical composition of the water. With the San Francisco Bay as our source of water, MSF is an attractive option because sediment and other large impurities can be separated from the feed before distillation occurs. Additionally, MSF distillation plants can be located near power plants and paired to their waste heat streams to conserve energy. This can reduce energy needs by half or two thirds, making MSF increasingly more practical. Due to the maintenance required in the RO system, the MSF distillation is the better option in terms of operation cost.4

A HYSYS model was built to simulate the MSF distillation section of the process, which spans after seawater pretreatment and before local potable water treatment. The model is used to simulate the distillation steps and to calculate necessary heating, cooling, and input flow rates to produce the required 50 million gallons of distillated water per day. For the simulation, the 398 million gallons per day of seawater feed stream is defined to be composed of 3.31 mol% sodium chloride (NaCl) and 96.69 mol% water. The percentage of NaCl is set to account for other ions and chemicals present after the pretreatment phase. NaCl alone is added to the HYSYS system due to the capabilities of the fluid package used to simulate this process, ElectroNRTL. The feed is set at 22°C, which is the approximate the temperature of the San Francisco Bay year round.

=Process Design=
==Design Overview==
The proposed desalination plant uses MSF distillation to convert the San Francisco Bay water to fresh water. The final process design contains 18 flash stages in series. Overall, 398 million gallons per day of of brackish water is converted to 50 million gallons per day of deionized water, a 13% yield. The production meets the required production rate of 50 million gallons of freshwater per day.

Seawater is taken in from the San Francisco Bay and mixed with the brine recycle stream produced downstream. The feed is then heated through a series of condensers and a heater. Distilled water vapor in each flash stage contacts the tubes carrying the seawater feed and condenses into a liquid stream by exchanging heat with the cooler seawater feed stream. The seawater is sent through a heater to increase its thermal energy before entering the first stage of the MSF. In each flashing stage, some of the water evaporates, leaving the salt behind. The freshwater vapors condense, as described above, and is collected and sent to a water treatment plant. The brine stream is split into the recycle stream and a waste stream, and the waste stream is diluted with fresh seawater before being discharged back into the bay.

A process flow diagram of the process is depicted in Appendix A.

==Pretreatment==
Sediment is removed to prevent solids from plugging the process downstream. Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime, and is controlled for by lowering pH. Acidifying the water will also remove CO2, which is a corrosive gas. To control for oxygen, oxygen scavengers will be included in the system to sequester it. Sodium bisulfate will be used as the scavenger.5

==Condensers (Heat Exchanger Networks)==
In order to conserve and recycle energy, the condensation of the distillate and heating of the seawater feed will be coupled in countercurrent heat exchanger networks. These heat exchanger networks are condensers of the design. The cold feed, which is a mixture of fresh seawater and brine recycle, is fed through multiple heat exchangers in series. Each heat exchanger corresponds to a different evaporation chamber. This process consists of 18 heat exchangers: E-101, E-102, … E-117, and E-118, which correspond to evaporation chambers V-102, V-103, … V-118, and V-119, respectively.

The heat exchangers can be viewed similarly to a shell and tube setup, however there is no actual physical shell. The tubes simply run through the top of the evaporation chambers, where vaporized distilled water in the chamber acts as the shell side fluid. The tube bundles are arranged in long tube configuration and are aligned parallel to the direction of the flashing brine flow in the evaporation chamber. The evaporating brine in an evaporation chamber rise up to the the corresponding heat exchanger and condense on the outside of the tubes, creating collectable pure water. This condensing water is what heats up the seawater in the tubes.

E-101 was used as the basis for the condenser sizing since it has the highest heat requirement, and thus will be the largest heat exchanger. From the HYSYS model, the overall system requires 2.24 x 106 kW of duty. Assuming this energy requirement, 5000 tubes, and 18 m length tubes (the diameter of the evaporation chambers, explained further on the next page), the diameter of each tube was calculated to be 0.4 m. Based on the temperature of the streams and flow, the thickness of the tube is 7 x 10-4 m.

==Brine Heater==
After the seawater exits the final heat exchanger, it enters a brine heater (E-119) where it is brought up to the temperature necessary for the flashing process to begin. This means that the seawater must be overheated compared to stage one, so that the seawater will flash in the evaporation chamber, i.e. release heat and vapor to reach equilibrium with stage conditions. After flowing through the condensers, the seawater enters the brine heater at 96.1 oC and is heated to 120 oC. The overall system requires 2.31 x 106 kW of duty. This is supplied by a total of 4.15 x 106 kg per hour of steam.

==Evaporation Chambers==
When the seawater has been brought to a temperature appropriate for flashing, it enters the first evaporation chamber (V-102). Upon entering the chamber, the water is flashed. Some of the water evaporates, leaving the salt behind. The vapor rises up to the heat exchanger tubes in each evaporation chamber and condenses on the outside into the distilled water product. This condensing water is what heats up the seawater in the tubes discussed above. The seawater that does not evaporate moves to the next stage, which operates at a higher temperature and lower pressure, and the process is repeated. This continues in series through a total of 18 flashing chambers (V-102, V-103, … V-118, and V-119). All of the distillate is collected into one stream and all of the brine is mixed with seawater before being disposed of back in the bay in order to dilute it enough so it does not have an adverse effect on marine life.

Each respective heat exchanger fully condenses the vapor product stream from each stage. However, when all of the individual product streams are consolidated into the final combined product stream, some water exists in the vapor phase due to temperature and pressure changes associated with mixing all of the streams together. Although the vapor product stream from each stage condenses completely across its respective heat exchanger, the total product stream contains some water in the vapor phase due to the temperature change when the mixer sums all the streams. In order to fully condense this stream, which will be sent to local water treatment plans, a heat exchanger is put in place to cool down the product using the process feed stream.

The correlations of El-Dessouky and Ettouney were employed in order to size the flash chambers. They specified that conventional MSF plants that operate at a capacity between 27,000 and 36,000 m3/day use flashing stages of the dimensions 18 x 4 meters.6

==Valves==
The model requires valves in between the evaporation chambers. The seawater streams flow through valves with large pressure drops to flash a portion the liquid into vapor that can then be separated in the vessels. Large pressure drops provide the thermodynamic driving force to vaporize more of the liquid, and are required to achieve high product yield.

==Material of Construction==
Before 1980, the shells and internals of the MSF units were commonly constructed from carbon steel. Seawater, however, corrodes carbon steel. To compensate for this, the thickness of the carbon steel has an additional corrosion allowance, which increases the size and weight of the equipment. After 1980, the stainless steel and duplex stainless steel became increasingly common. Stainless steel metal requires less thickness for equivalent strength and corrosion resistance, which allows for a reduction in the size, weight, and therefore cost of units. Major units in our process will be constructed from stainless steel.6

==Process Alternatives==
In order to ensure that our design is fully optimized, we wanted to consider several alternatives for our process. We looked at alternatives associated with water pretreatment, the distillation process itself and product treatment.

===Water Pretreatment===
Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime. Scaling can be controlled by adjusting pH, temperature, and bicarbonate concentration. We selected acidification to control for process scaling. This is because lowering a process temperature would decrease overall efficiency, and decreasing bicarbonate concentration would be comparatively expensive. Lowering pH would have the added benefit of controlling CO2 concentrations since CO2 is a corrosive gas. To control for corrosion we looked at how to remove oxygen. To target the oxygen we chose an oxygen scavenger. This was chosen over other options such as a deaerator, or a protective coating because a deaerator would have a large capital cost, and the equipment itself would experience corrosion, and a tank coating such as zinc orthophosphate has a mixed effectiveness.7

===MSF: Brine Recycle vs. Once-Through System===
We explored two options for the overall layout of the MSF distillation plant. In the first, the brine passes through the process once; in the second, a portion of the brine is recycled from the last stage to the incoming seawater feed. The largest draw towards implementing a brine recycle is that it would significantly reduce the overall amount of seawater needed to produce the daily target of 50 million gallons per day. This in turn would lower the amount of energy required to run the pump feeding seawater to the process. Another positive is that, with a brine recycle, the amount of water conditioning chemicals that must be added to the seawater feed is reduced. The negative aspects mainly revolve around the fact that the solution throughout the process will be higher in salt concentration. This increase in salinity both raises the likelihood of scaling and corrosion in the plant and raises the amount of seawater that must be added to the final brine to make it safe for discharge back into the environment. Additionally, boiling point of each stage is raised when adding the brine recycle. After weighing both alternatives against one another, we have decided to include a brine recycle in our plant design. Most of the negative aspects of the brine recycle can be combated (e.g. include anti-scaling agents in the feed pretreatment), and the economic benefits of including the brine recycle are very significant.8

===MSF: Continuous vs. Batch===
The distillation step can be done in continuous or batch processes. Batch distillation is more versatile and often used when the product is in small amounts and very high purity. A continuous distillation is more efficient thermodynamically and economically for large amounts of material of constant composition, whereas batch distillation is more effective for small amounts of material of varying compositions. Since the plant is processing large amounts of seawater of relatively constant composition, the distillation system will utilize continuous distillation.9

===MSF: Long Tube vs. Crossed Tube ===
Inside the evaporator stage, the condenser tube bundles may be arranged in either long tube or cross tube design. In the long tube arrangement, the tubes are aligned parallel to the direction of the flashing brine flow. The tubes go from stage to stage without water boxes. In the cross tube arrangement, the tubes are lined perpendicular to the flow of the flashing brine. Water boxes transfer brine between the stages.10 11 While the water boxes in crossed tube bundles allow for more customization, the long tube configuration allows for construction of large unit sizes, as there are manufacturing restrictions with cross tubes. Long tube design can easily accommodate a large number of stages, which lead us to choose long tube for our design.11 12

===MSF: Venting System===
Noncondensable gases exist in the process because of air leakages into stages under vacuum conditions and the release of dissolved gases from the heated brine. These gases are removed by venting the system. An air cooler section concentrates the gas before they are vented, and baffles direct the vapor and condensates out. There are two venting arrangements to consider: series and parallel. The system in series moves gas from one stage directly to the next, and the only loss in the system is vapor in the last stage (since some vapor is extracted with gases). This system is economical, however this configuration increases the amount of noncondensable gases passing from one stage to the next, and disrupting the venting system in one stage will affect the entire system. In parallel venting, gas is released through parallel takeoff pipes at each stage. Although this arrangement allows for venting at any stage to be controlled, the design and maintenance is more complex, and more vapor is lost than in the venting system in series. A third alternative is to use a combination of venting in parallel and in series, which is the common method in industry. For this process, our team chose to do venting in parallel and in series.12

===Additional Energy Source Alternatives===
Energy for the process can also be coupled with renewable energy sources to reduce the desalination plant’s carbon footprint. Energy integration can be direct or indirect. In direct integration, thermal waste heat from industrial sources provide thermal energy or pressure as energy sources. Direct integration is advantageous at the modular level, as it avoids energy losses with electricity conversion. Within indirect integration, there are modular integration and grid/utility-scale integration. Modular integration integrates the plant with wind turbines and other small-scale renewable power generators with mobile deployment. A “hybridization” of renewable power sources with natural gas power generation is also an option, and the coupling of the two provides a stable power source. Indirect renewable energy integration is growing as the cost of renewable energy continues to decreases and provides an economy of scale that direct integration does not, but there are several obstacles towards indirect integration, such as the plant’s finance and engineering the integration of the two systems.13 This distillation plant will be directly integrated with neighboring plants such as Chevron that produce thermal waste heat.

===Product Treatment===
Since the product is distilled water, it needs additional ions to a concentration of around 0.01% before going to the end user. The three alternatives considered were building a water treatment facility on site, simply adding the correct salts to the product stream and then sending the stream for bacterial treatment at a water plant, or simply sending the product distilled water for complete treatment to another facility. Due to the number of nearby water treatment plants and their demand for water, we elected to simply send the product to a nearby water treatment facility.

==Design Constraints and Tradeoffs==
While designing our process, we wanted to make sure that we met several outside constraints, including environmental, political, ethical, social, sustainability, health and safety constraints. First and most importantly, we wanted to ensure that the brine discharged back to the bay would not harm local marine life. Our process was designed around the constraint of diluting the exiting brine to within 0.2% salinity of background seawater. This is not only compliant with California law, but was also verified to be safe for surrounding marine life through our own outside research. We also addressed many of these considerations by deciding to pursue direct energy integration with the nearby Chevron plant in order to reduce environmental impact of our process. Additionally, we ensured that our plant would be located in an area that is densely populated and in great need of fresh drinking water. Finally, we addressed health and safety considerations by making sure we would be able to send our product distilled water to local treatment plants for proper potabilization.

===Design Tradeoffs: Thermodynamics===
Adjusting elements within this process is iterative, and optimization thereof required changing multiple parameters at once. An example of this is in tuning the temperature increase by heater E-100; if the temperature of the stream entering the vessels is too high, the heat exchange between the vapor product and the feed stream will not be realizable because it causes the feed to be too hot to cool and condense the product. Higher temperatures, however, provide greater thermodynamic driving forces for flashing. Setting the entering stream temperature at 120°C provides enough heat for substantial flash at 130 kPa. This is the only heater required until the very last flash stage, which is added to provide more heat for vaporization. This heater (E-105) increases the stream temperature by 15°C, and the vapor product from the last vessel increases the feed stream temperature by almost 30°C. The added energy increases vaporization by 3%, which is a substantial 2.2 million kg/h. The temperature of the streams entering and leaving each stage decreases with each additional flash stage such that the last vessel has the lowest temperature, according to realistic operating guidelines.

===Design Tradeoffs: Energy Consumption, Environmental Impacts, and Product Optimization===
Increases in the pressure drop across any of the valves in this final design causes a temperature cross in the heat exchangers unless a proper increase in pump energy is provided. There is thus a design tradeoff between lowering energy usage and increasing product yield. For example, lowering the operating pressure of the process dictates that the pressure difference used to flash seawater must be smaller, possibly decreasing the amount that is vaporized. The final design aims for a yield of 50 million gallons per day. The 18 stage model was chosen for its low annual utility cost. This is a reflection of environmental mindfulness because energy consumption is an ongoing aspect of the process and should be lowered as much as possible.

==Mass and Energy Balances==
The model of the desalination system assumes that: (1) the properties are uniform in each phase within a stage, (2) heat losses are negligible due to the small surface area to volume ratio for each stage and properly insulated units (heat losses to the surrounding vary from 2 to 5% of the total system energy), (3) the distillate is salt free since the boiling point of water is much lower than that of salt, (4) the subcooling or superheating effects on the system energy balances are negligible, (5) the only contaminant is sodium chloride (there are no non-condensable gases to consider, so no vapors are vented), and (6) the process has 3 evaporation chambers.

The major balances are that the feed is a mixture of the fresh seawater stream and the recycle stream, and that the brine stream exiting the last evaporation chamber is divided into the recycle stream and the waste stream. Within each stage, the change in mass of a phase over time is the balance between the net flow of that phase from the chamber and the rates of evaporation and condensation of water into and out of that phase. The equations governing the mass and energy balances on the system is displayed in Appendix C.

=Optimization=
As initiatives for reduced energy usage and increased environmental sustainability become more prevalent, efforts to minimize the utility cost in plants has increased to accommodate these incentives. Utilities are a continual source of expenditure and consumption, and a well-designed chemical process should seek to minimize this element. The annualized utility cost is a useful parameter to optimize given that its value depends on variations in energy costs; thus, annualized utility costs do not correlate with conventional indices of inflation, such as capital and labor costs. Additionally, utility costs, like other variable costs, can be minimized by improving the design or operational efficiency of the plant.

In optimizing our MSF process, the design variable of focus was the number of stages to use in our process. The performance measure was the annualized utility cost, which was the parameter that was minimized for optimization. The process is constrained by the product specifications, which are kept constant and serve as the standard for comparison between each model. These specifications are to produce 50 million gallons of desalinated liquid water per day.

One feasibility constraint in the process is the size of the heat exchangers, which must provide a large enough area for total condensation of the vapor stream coming out of the flash stages. In order to restrict the heat exchange area needed within each exchanger, the number of stages must be at least 11. Increasing the number of stages decreases the vapor flow rate at each, and decreases the heat exchange area required as well. An 11-stage process allows for appropriate heat exchange between the feed stream and the vapor product, according to the HYSYS economic analyzer. The optimization analysis thus was conducted incorporating only models with 11 or more stages.

The HYSYS economic analyzer was used to systematically optimize the process by finding the model with the lowest annualized utility cost. The performance parameters kept constant across each model were: the inlet composition, pressure, and temperature, the brine outlet temperature, the total product flow rate and composition, and the flash stage schematic. Table 1 displays the annualized utility cost for each model, and illustrates that a clear dip in the utility cost occurs in an 18 stage model. Thus, 18 is the optimal number of stages to use for our MSF distillation given utility costs as the optimized parameter.
[[File:Table_1.PNG|thumb|center|500x300px]]

=Economic Evaluation=
In order to evaluate our process from a fiscal standpoint, we used Aspen Process Economic Analyzer and Aspen cost estimator to determine capital and operating costs for our plant. These costs were calculated based on calculated equipment sizes, as well as energy requirements reported by our HYSYS model. The capital cost was found to be $485.2MM and annual total operating cost was found to be $144.7MM ($103.7MM of which was utilities). Annual plant revenue was estimated based on prices that water districts in the county currently pay for outside water sources, and was found to be $134.3MM.14

The NPV analysis we conducted assumed that the plant would take two years to build, would operate at half capacity during the first year of operation (year 3), and have a 20 year lifetime. We also assumed that our plant would be in operation 350 days per year. A constant discount rate of 12% was assumed to hold throughout the plant’s lifetime. The final NPV of this project is -$495.7MM. This is due to both the large capital cost of the plant and the annual loss of $10.4MM while in full operation. A graph of cumulative cash flow versus time can be found in Appendix E. Tax rates and depreciation do not directly impact the NPV of this project due to operating at a loss, however for future calculations it should be noted that we used a straight line depreciation schedule over a seven year period with a 40% tax rate.

Despite the current projections, it is important to consider the greater implications of running a desalination plant in the Bay Area. The population in the Bay Area is quickly growing, while California is experiencing harsher droughts each year. A consequence of this is the possibility of receiving government subsidies for our clean water product. The similarly scaled desalination plant in Carlsbad, CA operates at a loss as well, but receives significant government subsidies. Building this facility may be a strategic investment, especially since greater water scarcity is predicted in the future.

=Sensitivity Analysis=
We wanted to determine how sensitive NPV of our project is to the cost of capital, price of utilities and price we would be able to sell our clean water product at. We conducted this analysis by varying each of these variables independently between 0.5 times and 1.5 times the original value we calculated. As to be expected, NPV grows more negative with increasing capital costs and utility prices and decreasing water prices. Conversely, NPV becomes less negative as price of water rises and capital and utility costs decrease. A plot summarizing the effects of varying these costs can be found in Appendix E. NPV is most sensitive to the price at which we will be selling our clean water product. It is worth noting that NPV is more sensitive to utilities cost than it is to capital cost, which is why we focused on optimizing the cost of utilities. In future steps, trying to reduce utilities cost even more would be worth looking into to decrease the amount of money being lost on this project.

=Conclusions and Final Recommendation=
The final design achieves the project goal of producing 50 million gallons per day of distilled water at a yield of 13%, comparable to other similarly sized plants. An optimization of the number of MSF stages is realistic, and can be improved upon by taking into account other process parameters.

The proposed design was optimized around utility costs, comparing plants with 11 to 25 stages. The process is constrained by product specifications and by the thermodynamics of the heat exchange between the feed and the product streams. Energy conservation was achieved in using the seawater feed to condense the final product, as well as in all of the heat exchangers increasing the thermal energy of the seawater feed stream before it enters the flash stages.

In order to further optimize this process, other design parameters should be considered such as the brine recycle ratio, the cooler duty of the recycle stream, the pressure drop across each valve at the flash stages, and other economic factors. An optimization around maximizing NPV, for example, might be more comprehensive than that conducted regarding only utility costs. Tuning brine recycle rates could greatly improve process efficiency, and standardizing flash stage pressure drops would make operations more feasible and controlled.

Given our in depth economic analysis, this project would lose an average of $10.4MM per year, and has an NPV of -$495.7MM for a 20 year plant lifetime. From an economic perspective alone it would not be a good project to build. However, other factors need to be considered when building a project with enormous public value. In recent years California droughts have become more severe. Implementing a drought-proof supply of water now could avert a disaster later on. Even without a large disaster, having the desalination plant could also be expected to pay off in the future with the increasing price of water. Considerations such as these were the deciding factors in building the Carlsbad desalination plant. Since there are currently no desalination plants in the Bay area we recommend that preparations should be made to move forward with a desalination plant.

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=Appendix A. Process Flow Diagram=
=Appendix B. HYSYS Simulation=
=Appendix C. Mass and Energy Balances=
=Appendix D. Summary of Process Units=
=Appendix E. MSF Stage Parameters=
=Appendix F. Process Units Calculations=
=Appendix G. Economic Analysis=

Desalination - Team A

2016-03-12T02:53:09Z

Ezhuang: /* Conclusions and Final Recommendation */

Author: Pear Dhiantravan[2016], Reed Kolbe[2016], Sheridan Lichtor[2016], John Marsiglio[2016], Ellen Zhuang[2016]

Instructors: Fengqi You, David Wegerer

Winter 2016

Seawater desalination is an attractive approach to address the world’s freshwater shortage in light of the increasing global water scarcity. Our team decided to tackle California's growing drought crisis by designing a multistage flash desalination plant in Richmond, CA. Based on market analysis of existing desalination plants and water demands, this is a strategic location for the desalination of 50 million gallons per day of water from the San Francisco Bay.

The MSF process contains 18 flash stages in series. Seawater is taken in from the San Francisco Bay at a rate of 398 million gallons per day and mixed with brine recycled from the system. Within the MSF process, seawater is heated through a series of 18 heat exchangers and a brine heater before enters a series of 18 evaporation chambers. In the evaporation chambers, the seawater flashes and the freshwater vapor is condensed, collected, and sent to a water treatment facility. The brine stream proceeds to the next chamber, and the process continues. Upon exiting the last flashing chamber, the brine stream is split into a recycle brine stream that is mixed with the seawater inlet feed and a waste stream that is diluted with fresh seawater and returned to the sea. Overall, the process yield is 13%. This process was constructed in Aspen HYSYS and optimized around utility costs using the HYSYS Economic Analyzer. Based on duties, temperatures and pressures reported by HYSYS, as well as fundamental mass and energy balances, the equipment were sized appropriately and the economics of the process was analyzed.

The design has a total capital cost of $485.2 MM, an annual operating cost of $144.7 MM ($103.7 MM of which is utilities), and an annual plant revenue of $134.3 MM. Economic analysis reveals that the project has a negative NPV of -$495.7 MM and operates at a loss of $10.4 MM per year. Thus, the proposed design is not profitable. Despite the current projections, it is important to consider the greater implications of running a desalination plant in the Bay Area. The population in the Bay Area is quickly growing while California is experiencing harsher droughts each year. Given the dire situations, the government may support the proposed clean water project with subsidies. Building the facility may be a strategic investment, especially since greater water scarcity is predicted in the future.

Our design meets various environmental, ethical, social and health related constraints. The brine discharged back to the bay is diluted to at most 0.2% above the natural background salinity as required by law so that the plant does not impose harm on any local marine life. The demographic that the plant serves is largely invested in the protection of the environment, which we addressed by reducing energy consumption through direct energy integration with the nearby Chevron plant. Lastly, the product meets the specifications of the California Safe Water Drinking Act by producing water with no impurities.

=Introduction=
An increasing global water scarcity is fueling initiatives everywhere for clean water treatment, making efficient seawater desalination an attractive aim for chemical plant design. A 2015 market analysis found that the global desalination market earned revenues of $11.66 billion, and this number is expected to reach over $19 billion by 2019. Furthermore, the 17,000 desalination plants currently in operation are expected to double in number by 2020.1

As fresh water sources become increasingly scarce, strict water conservation measures are being observed. California is entering the fourth year of one of its most severe droughts on record. Cities are required to reduce their water usage by 35% to avoid facing fines.2 In lieu of these pressing conditions, California is looking for ways to provide more accessible fresh water to its citizens. This high demand for clean water motivated our choosing Richmond, CA as our plant location. At Richmond, the desalination plant can convert readily available seawater from the San Francisco Bay into fresh water. We propose to desalinate 50 million gallons of water per day to be sent to water treatment plants for potability. This aim is based on the capacity of the desalination plant in Carlsbad, California, which services a similar population.

The salinity of the water in the San Francisco bay is seasonal. During the dry seasons of summer and fall, salinity is high around 10 PSU (10,000 ppm) because water from the Pacific Ocean flows into the San Francisco Bay. During the wet winter season, fresh water from the rivers flows into the Bay, and salinity drops to around 2 PSU (2,000).3 Designed to meet the “worst case” scenario, our plant will process 398 million gallons of water per day with 10,000 ppm of salt to produce 50 million gallons of water with 1,000 ppm of salt. This gives a yield of 13%.

The desalination plant proposed consists of 18 flash stages in series. This report outlines our investigation of this potential water desalination plant, including design and optimization of the desalination process, an analysis of its energy consumption and environmental impacts, and a study of its economic implications.

=Technical Approach=
For the desalination step of our process, we selected a multistage flash (MSF) distillation process. Not only is MSF distillation a very viable method, but it is also the most common method, currently producing about 60% of the world’s desalinated water. MSF and reverse osmosis (RO) are the two major methods being used in large-scale desalination plants. Both processes require considerable amounts of energy. RO typically has a lower energy demand; however, the high impurity content of the Bay water would frequently necessitate membrane cleaning and/or exchange. Feed going through a RO system requires extensive pretreatment to remove biological organisms and other solids to control the pH and the chemical composition of the water. With the San Francisco Bay as our source of water, MSF is an attractive option because sediment and other large impurities can be separated from the feed before distillation occurs. Additionally, MSF distillation plants can be located near power plants and paired to their waste heat streams to conserve energy. This can reduce energy needs by half or two thirds, making MSF increasingly more practical. Due to the maintenance required in the RO system, the MSF distillation is the better option in terms of operation cost.4

A HYSYS model was built to simulate the MSF distillation section of the process, which spans after seawater pretreatment and before local potable water treatment. The model is used to simulate the distillation steps and to calculate necessary heating, cooling, and input flow rates to produce the required 50 million gallons of distillated water per day. For the simulation, the 398 million gallons per day of seawater feed stream is defined to be composed of 3.31 mol% sodium chloride (NaCl) and 96.69 mol% water. The percentage of NaCl is set to account for other ions and chemicals present after the pretreatment phase. NaCl alone is added to the HYSYS system due to the capabilities of the fluid package used to simulate this process, ElectroNRTL. The feed is set at 22°C, which is the approximate the temperature of the San Francisco Bay year round.

=Process Design=
==Design Overview==
The proposed desalination plant uses MSF distillation to convert the San Francisco Bay water to fresh water. The final process design contains 18 flash stages in series. Overall, 398 million gallons per day of of brackish water is converted to 50 million gallons per day of deionized water, a 13% yield. The production meets the required production rate of 50 million gallons of freshwater per day.

Seawater is taken in from the San Francisco Bay and mixed with the brine recycle stream produced downstream. The feed is then heated through a series of condensers and a heater. Distilled water vapor in each flash stage contacts the tubes carrying the seawater feed and condenses into a liquid stream by exchanging heat with the cooler seawater feed stream. The seawater is sent through a heater to increase its thermal energy before entering the first stage of the MSF. In each flashing stage, some of the water evaporates, leaving the salt behind. The freshwater vapors condense, as described above, and is collected and sent to a water treatment plant. The brine stream is split into the recycle stream and a waste stream, and the waste stream is diluted with fresh seawater before being discharged back into the bay.

A process flow diagram of the process is depicted in Appendix A.

==Pretreatment==
Sediment is removed to prevent solids from plugging the process downstream. Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime, and is controlled for by lowering pH. Acidifying the water will also remove CO2, which is a corrosive gas. To control for oxygen, oxygen scavengers will be included in the system to sequester it. Sodium bisulfate will be used as the scavenger.5

==Condensers (Heat Exchanger Networks)==
In order to conserve and recycle energy, the condensation of the distillate and heating of the seawater feed will be coupled in countercurrent heat exchanger networks. These heat exchanger networks are condensers of the design. The cold feed, which is a mixture of fresh seawater and brine recycle, is fed through multiple heat exchangers in series. Each heat exchanger corresponds to a different evaporation chamber. This process consists of 18 heat exchangers: E-101, E-102, … E-117, and E-118, which correspond to evaporation chambers V-102, V-103, … V-118, and V-119, respectively.

The heat exchangers can be viewed similarly to a shell and tube setup, however there is no actual physical shell. The tubes simply run through the top of the evaporation chambers, where vaporized distilled water in the chamber acts as the shell side fluid. The tube bundles are arranged in long tube configuration and are aligned parallel to the direction of the flashing brine flow in the evaporation chamber. The evaporating brine in an evaporation chamber rise up to the the corresponding heat exchanger and condense on the outside of the tubes, creating collectable pure water. This condensing water is what heats up the seawater in the tubes.

E-101 was used as the basis for the condenser sizing since it has the highest heat requirement, and thus will be the largest heat exchanger. From the HYSYS model, the overall system requires 2.24 x 106 kW of duty. Assuming this energy requirement, 5000 tubes, and 18 m length tubes (the diameter of the evaporation chambers, explained further on the next page), the diameter of each tube was calculated to be 0.4 m. Based on the temperature of the streams and flow, the thickness of the tube is 7 x 10-4 m.

==Brine Heater==
After the seawater exits the final heat exchanger, it enters a brine heater (E-119) where it is brought up to the temperature necessary for the flashing process to begin. This means that the seawater must be overheated compared to stage one, so that the seawater will flash in the evaporation chamber, i.e. release heat and vapor to reach equilibrium with stage conditions. After flowing through the condensers, the seawater enters the brine heater at 96.1 oC and is heated to 120 oC. The overall system requires 2.31 x 106 kW of duty. This is supplied by a total of 4.15 x 106 kg per hour of steam.

==Evaporation Chambers==
When the seawater has been brought to a temperature appropriate for flashing, it enters the first evaporation chamber (V-102). Upon entering the chamber, the water is flashed. Some of the water evaporates, leaving the salt behind. The vapor rises up to the heat exchanger tubes in each evaporation chamber and condenses on the outside into the distilled water product. This condensing water is what heats up the seawater in the tubes discussed above. The seawater that does not evaporate moves to the next stage, which operates at a higher temperature and lower pressure, and the process is repeated. This continues in series through a total of 18 flashing chambers (V-102, V-103, … V-118, and V-119). All of the distillate is collected into one stream and all of the brine is mixed with seawater before being disposed of back in the bay in order to dilute it enough so it does not have an adverse effect on marine life.

Each respective heat exchanger fully condenses the vapor product stream from each stage. However, when all of the individual product streams are consolidated into the final combined product stream, some water exists in the vapor phase due to temperature and pressure changes associated with mixing all of the streams together. Although the vapor product stream from each stage condenses completely across its respective heat exchanger, the total product stream contains some water in the vapor phase due to the temperature change when the mixer sums all the streams. In order to fully condense this stream, which will be sent to local water treatment plans, a heat exchanger is put in place to cool down the product using the process feed stream.

The correlations of El-Dessouky and Ettouney were employed in order to size the flash chambers. They specified that conventional MSF plants that operate at a capacity between 27,000 and 36,000 m3/day use flashing stages of the dimensions 18 x 4 meters.6

==Valves==
The model requires valves in between the evaporation chambers. The seawater streams flow through valves with large pressure drops to flash a portion the liquid into vapor that can then be separated in the vessels. Large pressure drops provide the thermodynamic driving force to vaporize more of the liquid, and are required to achieve high product yield.

==Material of Construction==
Before 1980, the shells and internals of the MSF units were commonly constructed from carbon steel. Seawater, however, corrodes carbon steel. To compensate for this, the thickness of the carbon steel has an additional corrosion allowance, which increases the size and weight of the equipment. After 1980, the stainless steel and duplex stainless steel became increasingly common. Stainless steel metal requires less thickness for equivalent strength and corrosion resistance, which allows for a reduction in the size, weight, and therefore cost of units. Major units in our process will be constructed from stainless steel.6

==Process Alternatives==
In order to ensure that our design is fully optimized, we wanted to consider several alternatives for our process. We looked at alternatives associated with water pretreatment, the distillation process itself and product treatment.

===Water Pretreatment===
Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime. Scaling can be controlled by adjusting pH, temperature, and bicarbonate concentration. We selected acidification to control for process scaling. This is because lowering a process temperature would decrease overall efficiency, and decreasing bicarbonate concentration would be comparatively expensive. Lowering pH would have the added benefit of controlling CO2 concentrations since CO2 is a corrosive gas. To control for corrosion we looked at how to remove oxygen. To target the oxygen we chose an oxygen scavenger. This was chosen over other options such as a deaerator, or a protective coating because a deaerator would have a large capital cost, and the equipment itself would experience corrosion, and a tank coating such as zinc orthophosphate has a mixed effectiveness.7

===MSF: Brine Recycle vs. Once-Through System===
We explored two options for the overall layout of the MSF distillation plant. In the first, the brine passes through the process once; in the second, a portion of the brine is recycled from the last stage to the incoming seawater feed. The largest draw towards implementing a brine recycle is that it would significantly reduce the overall amount of seawater needed to produce the daily target of 50 million gallons per day. This in turn would lower the amount of energy required to run the pump feeding seawater to the process. Another positive is that, with a brine recycle, the amount of water conditioning chemicals that must be added to the seawater feed is reduced. The negative aspects mainly revolve around the fact that the solution throughout the process will be higher in salt concentration. This increase in salinity both raises the likelihood of scaling and corrosion in the plant and raises the amount of seawater that must be added to the final brine to make it safe for discharge back into the environment. Additionally, boiling point of each stage is raised when adding the brine recycle. After weighing both alternatives against one another, we have decided to include a brine recycle in our plant design. Most of the negative aspects of the brine recycle can be combated (e.g. include anti-scaling agents in the feed pretreatment), and the economic benefits of including the brine recycle are very significant.8

===MSF: Continuous vs. Batch===
The distillation step can be done in continuous or batch processes. Batch distillation is more versatile and often used when the product is in small amounts and very high purity. A continuous distillation is more efficient thermodynamically and economically for large amounts of material of constant composition, whereas batch distillation is more effective for small amounts of material of varying compositions. Since the plant is processing large amounts of seawater of relatively constant composition, the distillation system will utilize continuous distillation.9

===MSF: Long Tube vs. Crossed Tube ===
Inside the evaporator stage, the condenser tube bundles may be arranged in either long tube or cross tube design. In the long tube arrangement, the tubes are aligned parallel to the direction of the flashing brine flow. The tubes go from stage to stage without water boxes. In the cross tube arrangement, the tubes are lined perpendicular to the flow of the flashing brine. Water boxes transfer brine between the stages.10 11 While the water boxes in crossed tube bundles allow for more customization, the long tube configuration allows for construction of large unit sizes, as there are manufacturing restrictions with cross tubes. Long tube design can easily accommodate a large number of stages, which lead us to choose long tube for our design.11 12

===MSF: Venting System===
Noncondensable gases exist in the process because of air leakages into stages under vacuum conditions and the release of dissolved gases from the heated brine. These gases are removed by venting the system. An air cooler section concentrates the gas before they are vented, and baffles direct the vapor and condensates out. There are two venting arrangements to consider: series and parallel. The system in series moves gas from one stage directly to the next, and the only loss in the system is vapor in the last stage (since some vapor is extracted with gases). This system is economical, however this configuration increases the amount of noncondensable gases passing from one stage to the next, and disrupting the venting system in one stage will affect the entire system. In parallel venting, gas is released through parallel takeoff pipes at each stage. Although this arrangement allows for venting at any stage to be controlled, the design and maintenance is more complex, and more vapor is lost than in the venting system in series. A third alternative is to use a combination of venting in parallel and in series, which is the common method in industry. For this process, our team chose to do venting in parallel and in series.12

===Additional Energy Source Alternatives===
Energy for the process can also be coupled with renewable energy sources to reduce the desalination plant’s carbon footprint. Energy integration can be direct or indirect. In direct integration, thermal waste heat from industrial sources provide thermal energy or pressure as energy sources. Direct integration is advantageous at the modular level, as it avoids energy losses with electricity conversion. Within indirect integration, there are modular integration and grid/utility-scale integration. Modular integration integrates the plant with wind turbines and other small-scale renewable power generators with mobile deployment. A “hybridization” of renewable power sources with natural gas power generation is also an option, and the coupling of the two provides a stable power source. Indirect renewable energy integration is growing as the cost of renewable energy continues to decreases and provides an economy of scale that direct integration does not, but there are several obstacles towards indirect integration, such as the plant’s finance and engineering the integration of the two systems.13 This distillation plant will be directly integrated with neighboring plants such as Chevron that produce thermal waste heat.

===Product Treatment===
Since the product is distilled water, it needs additional ions to a concentration of around 0.01% before going to the end user. The three alternatives considered were building a water treatment facility on site, simply adding the correct salts to the product stream and then sending the stream for bacterial treatment at a water plant, or simply sending the product distilled water for complete treatment to another facility. Due to the number of nearby water treatment plants and their demand for water, we elected to simply send the product to a nearby water treatment facility.

==Design Constraints and Tradeoffs==
While designing our process, we wanted to make sure that we met several outside constraints, including environmental, political, ethical, social, sustainability, health and safety constraints. First and most importantly, we wanted to ensure that the brine discharged back to the bay would not harm local marine life. Our process was designed around the constraint of diluting the exiting brine to within 0.2% salinity of background seawater. This is not only compliant with California law, but was also verified to be safe for surrounding marine life through our own outside research. We also addressed many of these considerations by deciding to pursue direct energy integration with the nearby Chevron plant in order to reduce environmental impact of our process. Additionally, we ensured that our plant would be located in an area that is densely populated and in great need of fresh drinking water. Finally, we addressed health and safety considerations by making sure we would be able to send our product distilled water to local treatment plants for proper potabilization.

===Design Tradeoffs: Thermodynamics===
Adjusting elements within this process is iterative, and optimization thereof required changing multiple parameters at once. An example of this is in tuning the temperature increase by heater E-100; if the temperature of the stream entering the vessels is too high, the heat exchange between the vapor product and the feed stream will not be realizable because it causes the feed to be too hot to cool and condense the product. Higher temperatures, however, provide greater thermodynamic driving forces for flashing. Setting the entering stream temperature at 120°C provides enough heat for substantial flash at 130 kPa. This is the only heater required until the very last flash stage, which is added to provide more heat for vaporization. This heater (E-105) increases the stream temperature by 15°C, and the vapor product from the last vessel increases the feed stream temperature by almost 30°C. The added energy increases vaporization by 3%, which is a substantial 2.2 million kg/h. The temperature of the streams entering and leaving each stage decreases with each additional flash stage such that the last vessel has the lowest temperature, according to realistic operating guidelines.

===Design Tradeoffs: Energy Consumption, Environmental Impacts, and Product Optimization===
Increases in the pressure drop across any of the valves in this final design causes a temperature cross in the heat exchangers unless a proper increase in pump energy is provided. There is thus a design tradeoff between lowering energy usage and increasing product yield. For example, lowering the operating pressure of the process dictates that the pressure difference used to flash seawater must be smaller, possibly decreasing the amount that is vaporized. The final design aims for a yield of 50 million gallons per day. The 18 stage model was chosen for its low annual utility cost. This is a reflection of environmental mindfulness because energy consumption is an ongoing aspect of the process and should be lowered as much as possible.

==Mass and Energy Balances==
The model of the desalination system assumes that: (1) the properties are uniform in each phase within a stage, (2) heat losses are negligible due to the small surface area to volume ratio for each stage and properly insulated units (heat losses to the surrounding vary from 2 to 5% of the total system energy), (3) the distillate is salt free since the boiling point of water is much lower than that of salt, (4) the subcooling or superheating effects on the system energy balances are negligible, (5) the only contaminant is sodium chloride (there are no non-condensable gases to consider, so no vapors are vented), and (6) the process has 3 evaporation chambers.

The major balances are that the feed is a mixture of the fresh seawater stream and the recycle stream, and that the brine stream exiting the last evaporation chamber is divided into the recycle stream and the waste stream. Within each stage, the change in mass of a phase over time is the balance between the net flow of that phase from the chamber and the rates of evaporation and condensation of water into and out of that phase. The equations governing the mass and energy balances on the system is displayed in Appendix C.

=Optimization=
As initiatives for reduced energy usage and increased environmental sustainability become more prevalent, efforts to minimize the utility cost in plants has increased to accommodate these incentives. Utilities are a continual source of expenditure and consumption, and a well-designed chemical process should seek to minimize this element. The annualized utility cost is a useful parameter to optimize given that its value depends on variations in energy costs; thus, annualized utility costs do not correlate with conventional indices of inflation, such as capital and labor costs. Additionally, utility costs, like other variable costs, can be minimized by improving the design or operational efficiency of the plant.

In optimizing our MSF process, the design variable of focus was the number of stages to use in our process. The performance measure was the annualized utility cost, which was the parameter that was minimized for optimization. The process is constrained by the product specifications, which are kept constant and serve as the standard for comparison between each model. These specifications are to produce 50 million gallons of desalinated liquid water per day.

One feasibility constraint in the process is the size of the heat exchangers, which must provide a large enough area for total condensation of the vapor stream coming out of the flash stages. In order to restrict the heat exchange area needed within each exchanger, the number of stages must be at least 11. Increasing the number of stages decreases the vapor flow rate at each, and decreases the heat exchange area required as well. An 11-stage process allows for appropriate heat exchange between the feed stream and the vapor product, according to the HYSYS economic analyzer. The optimization analysis thus was conducted incorporating only models with 11 or more stages.

The HYSYS economic analyzer was used to systematically optimize the process by finding the model with the lowest annualized utility cost. The performance parameters kept constant across each model were: the inlet composition, pressure, and temperature, the brine outlet temperature, the total product flow rate and composition, and the flash stage schematic. Table 1 displays the annualized utility cost for each model, and illustrates that a clear dip in the utility cost occurs in an 18 stage model. Thus, 18 is the optimal number of stages to use for our MSF distillation given utility costs as the optimized parameter.
[[File:Table_1.PNG|thumb|center|500x300px]]

=Economic Evaluation=
In order to evaluate our process from a fiscal standpoint, we used Aspen Process Economic Analyzer and Aspen cost estimator to determine capital and operating costs for our plant. These costs were calculated based on calculated equipment sizes, as well as energy requirements reported by our HYSYS model. The capital cost was found to be $485.2MM and annual total operating cost was found to be $144.7MM ($103.7MM of which was utilities). Annual plant revenue was estimated based on prices that water districts in the county currently pay for outside water sources, and was found to be $134.3MM.14

The NPV analysis we conducted assumed that the plant would take two years to build, would operate at half capacity during the first year of operation (year 3), and have a 20 year lifetime. We also assumed that our plant would be in operation 350 days per year. A constant discount rate of 12% was assumed to hold throughout the plant’s lifetime. The final NPV of this project is -$495.7MM. This is due to both the large capital cost of the plant and the annual loss of $10.4MM while in full operation. A graph of cumulative cash flow versus time can be found in Appendix E. Tax rates and depreciation do not directly impact the NPV of this project due to operating at a loss, however for future calculations it should be noted that we used a straight line depreciation schedule over a seven year period with a 40% tax rate.

Despite the current projections, it is important to consider the greater implications of running a desalination plant in the Bay Area. The population in the Bay Area is quickly growing, while California is experiencing harsher droughts each year. A consequence of this is the possibility of receiving government subsidies for our clean water product. The similarly scaled desalination plant in Carlsbad, CA operates at a loss as well, but receives significant government subsidies. Building this facility may be a strategic investment, especially since greater water scarcity is predicted in the future.

=Sensitivity Analysis=
We wanted to determine how sensitive NPV of our project is to the cost of capital, price of utilities and price we would be able to sell our clean water product at. We conducted this analysis by varying each of these variables independently between 0.5 times and 1.5 times the original value we calculated. As to be expected, NPV grows more negative with increasing capital costs and utility prices and decreasing water prices. Conversely, NPV becomes less negative as price of water rises and capital and utility costs decrease. A plot summarizing the effects of varying these costs can be found in Appendix E. NPV is most sensitive to the price at which we will be selling our clean water product. It is worth noting that NPV is more sensitive to utilities cost than it is to capital cost, which is why we focused on optimizing the cost of utilities. In future steps, trying to reduce utilities cost even more would be worth looking into to decrease the amount of money being lost on this project.

=Conclusions and Final Recommendation=
The final design achieves the project goal of producing 50 million gallons per day of distilled water at a yield of 13%, comparable to other similarly sized plants. An optimization of the number of MSF stages is realistic, and can be improved upon by taking into account other process parameters.

The proposed design was optimized around utility costs, comparing plants with 11 to 25 stages. The process is constrained by product specifications and by the thermodynamics of the heat exchange between the feed and the product streams. Energy conservation was achieved in using the seawater feed to condense the final product, as well as in all of the heat exchangers increasing the thermal energy of the seawater feed stream before it enters the flash stages.

In order to further optimize this process, other design parameters should be considered such as the brine recycle ratio, the cooler duty of the recycle stream, the pressure drop across each valve at the flash stages, and other economic factors. An optimization around maximizing NPV, for example, might be more comprehensive than that conducted regarding only utility costs. Tuning brine recycle rates could greatly improve process efficiency, and standardizing flash stage pressure drops would make operations more feasible and controlled.

Given our in depth economic analysis, this project would lose an average of $10.4MM per year, and has an NPV of -$495.7MM for a 20 year plant lifetime. From an economic perspective alone it would not be a good project to build. However, other factors need to be considered when building a project with enormous public value. In recent years California droughts have become more severe. Implementing a drought-proof supply of water now could avert a disaster later on. Even without a large disaster, having the desalination plant could also be expected to pay off in the future with the increasing price of water. Considerations such as these were the deciding factors in building the Carlsbad desalination plant. Since there are currently no desalination plants in the Bay area we recommend that preparations should be made to move forward with a desalination plant.

----

=Appendix A. Process Flow Diagram=
=Appendix B. HYSYS Simulation=
=Appendix C. Mass and Energy Balances=
=Appendix D. Summary of Process Units=
=Appendix E. MSF Stage Parameters=
=Appendix F. Process Units Calculations=
=Appendix G. Economic Analysis=

Desalination - Team A

2016-03-12T02:50:55Z

Ezhuang:

Author: Pear Dhiantravan[2016], Reed Kolbe[2016], Sheridan Lichtor[2016], John Marsiglio[2016], Ellen Zhuang[2016]

Instructors: Fengqi You, David Wegerer

Winter 2016

Seawater desalination is an attractive approach to address the world’s freshwater shortage in light of the increasing global water scarcity. Our team decided to tackle California's growing drought crisis by designing a multistage flash desalination plant in Richmond, CA. Based on market analysis of existing desalination plants and water demands, this is a strategic location for the desalination of 50 million gallons per day of water from the San Francisco Bay.

The MSF process contains 18 flash stages in series. Seawater is taken in from the San Francisco Bay at a rate of 398 million gallons per day and mixed with brine recycled from the system. Within the MSF process, seawater is heated through a series of 18 heat exchangers and a brine heater before enters a series of 18 evaporation chambers. In the evaporation chambers, the seawater flashes and the freshwater vapor is condensed, collected, and sent to a water treatment facility. The brine stream proceeds to the next chamber, and the process continues. Upon exiting the last flashing chamber, the brine stream is split into a recycle brine stream that is mixed with the seawater inlet feed and a waste stream that is diluted with fresh seawater and returned to the sea. Overall, the process yield is 13%. This process was constructed in Aspen HYSYS and optimized around utility costs using the HYSYS Economic Analyzer. Based on duties, temperatures and pressures reported by HYSYS, as well as fundamental mass and energy balances, the equipment were sized appropriately and the economics of the process was analyzed.

The design has a total capital cost of $485.2 MM, an annual operating cost of $144.7 MM ($103.7 MM of which is utilities), and an annual plant revenue of $134.3 MM. Economic analysis reveals that the project has a negative NPV of -$495.7 MM and operates at a loss of $10.4 MM per year. Thus, the proposed design is not profitable. Despite the current projections, it is important to consider the greater implications of running a desalination plant in the Bay Area. The population in the Bay Area is quickly growing while California is experiencing harsher droughts each year. Given the dire situations, the government may support the proposed clean water project with subsidies. Building the facility may be a strategic investment, especially since greater water scarcity is predicted in the future.

Our design meets various environmental, ethical, social and health related constraints. The brine discharged back to the bay is diluted to at most 0.2% above the natural background salinity as required by law so that the plant does not impose harm on any local marine life. The demographic that the plant serves is largely invested in the protection of the environment, which we addressed by reducing energy consumption through direct energy integration with the nearby Chevron plant. Lastly, the product meets the specifications of the California Safe Water Drinking Act by producing water with no impurities.

=Introduction=
An increasing global water scarcity is fueling initiatives everywhere for clean water treatment, making efficient seawater desalination an attractive aim for chemical plant design. A 2015 market analysis found that the global desalination market earned revenues of $11.66 billion, and this number is expected to reach over $19 billion by 2019. Furthermore, the 17,000 desalination plants currently in operation are expected to double in number by 2020.1

As fresh water sources become increasingly scarce, strict water conservation measures are being observed. California is entering the fourth year of one of its most severe droughts on record. Cities are required to reduce their water usage by 35% to avoid facing fines.2 In lieu of these pressing conditions, California is looking for ways to provide more accessible fresh water to its citizens. This high demand for clean water motivated our choosing Richmond, CA as our plant location. At Richmond, the desalination plant can convert readily available seawater from the San Francisco Bay into fresh water. We propose to desalinate 50 million gallons of water per day to be sent to water treatment plants for potability. This aim is based on the capacity of the desalination plant in Carlsbad, California, which services a similar population.

The salinity of the water in the San Francisco bay is seasonal. During the dry seasons of summer and fall, salinity is high around 10 PSU (10,000 ppm) because water from the Pacific Ocean flows into the San Francisco Bay. During the wet winter season, fresh water from the rivers flows into the Bay, and salinity drops to around 2 PSU (2,000).3 Designed to meet the “worst case” scenario, our plant will process 398 million gallons of water per day with 10,000 ppm of salt to produce 50 million gallons of water with 1,000 ppm of salt. This gives a yield of 13%.

The desalination plant proposed consists of 18 flash stages in series. This report outlines our investigation of this potential water desalination plant, including design and optimization of the desalination process, an analysis of its energy consumption and environmental impacts, and a study of its economic implications.

=Technical Approach=
For the desalination step of our process, we selected a multistage flash (MSF) distillation process. Not only is MSF distillation a very viable method, but it is also the most common method, currently producing about 60% of the world’s desalinated water. MSF and reverse osmosis (RO) are the two major methods being used in large-scale desalination plants. Both processes require considerable amounts of energy. RO typically has a lower energy demand; however, the high impurity content of the Bay water would frequently necessitate membrane cleaning and/or exchange. Feed going through a RO system requires extensive pretreatment to remove biological organisms and other solids to control the pH and the chemical composition of the water. With the San Francisco Bay as our source of water, MSF is an attractive option because sediment and other large impurities can be separated from the feed before distillation occurs. Additionally, MSF distillation plants can be located near power plants and paired to their waste heat streams to conserve energy. This can reduce energy needs by half or two thirds, making MSF increasingly more practical. Due to the maintenance required in the RO system, the MSF distillation is the better option in terms of operation cost.4

A HYSYS model was built to simulate the MSF distillation section of the process, which spans after seawater pretreatment and before local potable water treatment. The model is used to simulate the distillation steps and to calculate necessary heating, cooling, and input flow rates to produce the required 50 million gallons of distillated water per day. For the simulation, the 398 million gallons per day of seawater feed stream is defined to be composed of 3.31 mol% sodium chloride (NaCl) and 96.69 mol% water. The percentage of NaCl is set to account for other ions and chemicals present after the pretreatment phase. NaCl alone is added to the HYSYS system due to the capabilities of the fluid package used to simulate this process, ElectroNRTL. The feed is set at 22°C, which is the approximate the temperature of the San Francisco Bay year round.

=Process Design=
==Design Overview==
The proposed desalination plant uses MSF distillation to convert the San Francisco Bay water to fresh water. The final process design contains 18 flash stages in series. Overall, 398 million gallons per day of of brackish water is converted to 50 million gallons per day of deionized water, a 13% yield. The production meets the required production rate of 50 million gallons of freshwater per day.

Seawater is taken in from the San Francisco Bay and mixed with the brine recycle stream produced downstream. The feed is then heated through a series of condensers and a heater. Distilled water vapor in each flash stage contacts the tubes carrying the seawater feed and condenses into a liquid stream by exchanging heat with the cooler seawater feed stream. The seawater is sent through a heater to increase its thermal energy before entering the first stage of the MSF. In each flashing stage, some of the water evaporates, leaving the salt behind. The freshwater vapors condense, as described above, and is collected and sent to a water treatment plant. The brine stream is split into the recycle stream and a waste stream, and the waste stream is diluted with fresh seawater before being discharged back into the bay.

A process flow diagram of the process is depicted in Appendix A.

==Pretreatment==
Sediment is removed to prevent solids from plugging the process downstream. Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime, and is controlled for by lowering pH. Acidifying the water will also remove CO2, which is a corrosive gas. To control for oxygen, oxygen scavengers will be included in the system to sequester it. Sodium bisulfate will be used as the scavenger.5

==Condensers (Heat Exchanger Networks)==
In order to conserve and recycle energy, the condensation of the distillate and heating of the seawater feed will be coupled in countercurrent heat exchanger networks. These heat exchanger networks are condensers of the design. The cold feed, which is a mixture of fresh seawater and brine recycle, is fed through multiple heat exchangers in series. Each heat exchanger corresponds to a different evaporation chamber. This process consists of 18 heat exchangers: E-101, E-102, … E-117, and E-118, which correspond to evaporation chambers V-102, V-103, … V-118, and V-119, respectively.

The heat exchangers can be viewed similarly to a shell and tube setup, however there is no actual physical shell. The tubes simply run through the top of the evaporation chambers, where vaporized distilled water in the chamber acts as the shell side fluid. The tube bundles are arranged in long tube configuration and are aligned parallel to the direction of the flashing brine flow in the evaporation chamber. The evaporating brine in an evaporation chamber rise up to the the corresponding heat exchanger and condense on the outside of the tubes, creating collectable pure water. This condensing water is what heats up the seawater in the tubes.

E-101 was used as the basis for the condenser sizing since it has the highest heat requirement, and thus will be the largest heat exchanger. From the HYSYS model, the overall system requires 2.24 x 106 kW of duty. Assuming this energy requirement, 5000 tubes, and 18 m length tubes (the diameter of the evaporation chambers, explained further on the next page), the diameter of each tube was calculated to be 0.4 m. Based on the temperature of the streams and flow, the thickness of the tube is 7 x 10-4 m.

==Brine Heater==
After the seawater exits the final heat exchanger, it enters a brine heater (E-119) where it is brought up to the temperature necessary for the flashing process to begin. This means that the seawater must be overheated compared to stage one, so that the seawater will flash in the evaporation chamber, i.e. release heat and vapor to reach equilibrium with stage conditions. After flowing through the condensers, the seawater enters the brine heater at 96.1 oC and is heated to 120 oC. The overall system requires 2.31 x 106 kW of duty. This is supplied by a total of 4.15 x 106 kg per hour of steam.

==Evaporation Chambers==
When the seawater has been brought to a temperature appropriate for flashing, it enters the first evaporation chamber (V-102). Upon entering the chamber, the water is flashed. Some of the water evaporates, leaving the salt behind. The vapor rises up to the heat exchanger tubes in each evaporation chamber and condenses on the outside into the distilled water product. This condensing water is what heats up the seawater in the tubes discussed above. The seawater that does not evaporate moves to the next stage, which operates at a higher temperature and lower pressure, and the process is repeated. This continues in series through a total of 18 flashing chambers (V-102, V-103, … V-118, and V-119). All of the distillate is collected into one stream and all of the brine is mixed with seawater before being disposed of back in the bay in order to dilute it enough so it does not have an adverse effect on marine life.

Each respective heat exchanger fully condenses the vapor product stream from each stage. However, when all of the individual product streams are consolidated into the final combined product stream, some water exists in the vapor phase due to temperature and pressure changes associated with mixing all of the streams together. Although the vapor product stream from each stage condenses completely across its respective heat exchanger, the total product stream contains some water in the vapor phase due to the temperature change when the mixer sums all the streams. In order to fully condense this stream, which will be sent to local water treatment plans, a heat exchanger is put in place to cool down the product using the process feed stream.

The correlations of El-Dessouky and Ettouney were employed in order to size the flash chambers. They specified that conventional MSF plants that operate at a capacity between 27,000 and 36,000 m3/day use flashing stages of the dimensions 18 x 4 meters.6

==Valves==
The model requires valves in between the evaporation chambers. The seawater streams flow through valves with large pressure drops to flash a portion the liquid into vapor that can then be separated in the vessels. Large pressure drops provide the thermodynamic driving force to vaporize more of the liquid, and are required to achieve high product yield.

==Material of Construction==
Before 1980, the shells and internals of the MSF units were commonly constructed from carbon steel. Seawater, however, corrodes carbon steel. To compensate for this, the thickness of the carbon steel has an additional corrosion allowance, which increases the size and weight of the equipment. After 1980, the stainless steel and duplex stainless steel became increasingly common. Stainless steel metal requires less thickness for equivalent strength and corrosion resistance, which allows for a reduction in the size, weight, and therefore cost of units. Major units in our process will be constructed from stainless steel.6

==Process Alternatives==
In order to ensure that our design is fully optimized, we wanted to consider several alternatives for our process. We looked at alternatives associated with water pretreatment, the distillation process itself and product treatment.

===Water Pretreatment===
Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime. Scaling can be controlled by adjusting pH, temperature, and bicarbonate concentration. We selected acidification to control for process scaling. This is because lowering a process temperature would decrease overall efficiency, and decreasing bicarbonate concentration would be comparatively expensive. Lowering pH would have the added benefit of controlling CO2 concentrations since CO2 is a corrosive gas. To control for corrosion we looked at how to remove oxygen. To target the oxygen we chose an oxygen scavenger. This was chosen over other options such as a deaerator, or a protective coating because a deaerator would have a large capital cost, and the equipment itself would experience corrosion, and a tank coating such as zinc orthophosphate has a mixed effectiveness.7

===MSF: Brine Recycle vs. Once-Through System===
We explored two options for the overall layout of the MSF distillation plant. In the first, the brine passes through the process once; in the second, a portion of the brine is recycled from the last stage to the incoming seawater feed. The largest draw towards implementing a brine recycle is that it would significantly reduce the overall amount of seawater needed to produce the daily target of 50 million gallons per day. This in turn would lower the amount of energy required to run the pump feeding seawater to the process. Another positive is that, with a brine recycle, the amount of water conditioning chemicals that must be added to the seawater feed is reduced. The negative aspects mainly revolve around the fact that the solution throughout the process will be higher in salt concentration. This increase in salinity both raises the likelihood of scaling and corrosion in the plant and raises the amount of seawater that must be added to the final brine to make it safe for discharge back into the environment. Additionally, boiling point of each stage is raised when adding the brine recycle. After weighing both alternatives against one another, we have decided to include a brine recycle in our plant design. Most of the negative aspects of the brine recycle can be combated (e.g. include anti-scaling agents in the feed pretreatment), and the economic benefits of including the brine recycle are very significant.8

===MSF: Continuous vs. Batch===
The distillation step can be done in continuous or batch processes. Batch distillation is more versatile and often used when the product is in small amounts and very high purity. A continuous distillation is more efficient thermodynamically and economically for large amounts of material of constant composition, whereas batch distillation is more effective for small amounts of material of varying compositions. Since the plant is processing large amounts of seawater of relatively constant composition, the distillation system will utilize continuous distillation.9

===MSF: Long Tube vs. Crossed Tube ===
Inside the evaporator stage, the condenser tube bundles may be arranged in either long tube or cross tube design. In the long tube arrangement, the tubes are aligned parallel to the direction of the flashing brine flow. The tubes go from stage to stage without water boxes. In the cross tube arrangement, the tubes are lined perpendicular to the flow of the flashing brine. Water boxes transfer brine between the stages.10 11 While the water boxes in crossed tube bundles allow for more customization, the long tube configuration allows for construction of large unit sizes, as there are manufacturing restrictions with cross tubes. Long tube design can easily accommodate a large number of stages, which lead us to choose long tube for our design.11 12

===MSF: Venting System===
Noncondensable gases exist in the process because of air leakages into stages under vacuum conditions and the release of dissolved gases from the heated brine. These gases are removed by venting the system. An air cooler section concentrates the gas before they are vented, and baffles direct the vapor and condensates out. There are two venting arrangements to consider: series and parallel. The system in series moves gas from one stage directly to the next, and the only loss in the system is vapor in the last stage (since some vapor is extracted with gases). This system is economical, however this configuration increases the amount of noncondensable gases passing from one stage to the next, and disrupting the venting system in one stage will affect the entire system. In parallel venting, gas is released through parallel takeoff pipes at each stage. Although this arrangement allows for venting at any stage to be controlled, the design and maintenance is more complex, and more vapor is lost than in the venting system in series. A third alternative is to use a combination of venting in parallel and in series, which is the common method in industry. For this process, our team chose to do venting in parallel and in series.12

===Additional Energy Source Alternatives===
Energy for the process can also be coupled with renewable energy sources to reduce the desalination plant’s carbon footprint. Energy integration can be direct or indirect. In direct integration, thermal waste heat from industrial sources provide thermal energy or pressure as energy sources. Direct integration is advantageous at the modular level, as it avoids energy losses with electricity conversion. Within indirect integration, there are modular integration and grid/utility-scale integration. Modular integration integrates the plant with wind turbines and other small-scale renewable power generators with mobile deployment. A “hybridization” of renewable power sources with natural gas power generation is also an option, and the coupling of the two provides a stable power source. Indirect renewable energy integration is growing as the cost of renewable energy continues to decreases and provides an economy of scale that direct integration does not, but there are several obstacles towards indirect integration, such as the plant’s finance and engineering the integration of the two systems.13 This distillation plant will be directly integrated with neighboring plants such as Chevron that produce thermal waste heat.

===Product Treatment===
Since the product is distilled water, it needs additional ions to a concentration of around 0.01% before going to the end user. The three alternatives considered were building a water treatment facility on site, simply adding the correct salts to the product stream and then sending the stream for bacterial treatment at a water plant, or simply sending the product distilled water for complete treatment to another facility. Due to the number of nearby water treatment plants and their demand for water, we elected to simply send the product to a nearby water treatment facility.

==Design Constraints and Tradeoffs==
While designing our process, we wanted to make sure that we met several outside constraints, including environmental, political, ethical, social, sustainability, health and safety constraints. First and most importantly, we wanted to ensure that the brine discharged back to the bay would not harm local marine life. Our process was designed around the constraint of diluting the exiting brine to within 0.2% salinity of background seawater. This is not only compliant with California law, but was also verified to be safe for surrounding marine life through our own outside research. We also addressed many of these considerations by deciding to pursue direct energy integration with the nearby Chevron plant in order to reduce environmental impact of our process. Additionally, we ensured that our plant would be located in an area that is densely populated and in great need of fresh drinking water. Finally, we addressed health and safety considerations by making sure we would be able to send our product distilled water to local treatment plants for proper potabilization.

===Design Tradeoffs: Thermodynamics===
Adjusting elements within this process is iterative, and optimization thereof required changing multiple parameters at once. An example of this is in tuning the temperature increase by heater E-100; if the temperature of the stream entering the vessels is too high, the heat exchange between the vapor product and the feed stream will not be realizable because it causes the feed to be too hot to cool and condense the product. Higher temperatures, however, provide greater thermodynamic driving forces for flashing. Setting the entering stream temperature at 120°C provides enough heat for substantial flash at 130 kPa. This is the only heater required until the very last flash stage, which is added to provide more heat for vaporization. This heater (E-105) increases the stream temperature by 15°C, and the vapor product from the last vessel increases the feed stream temperature by almost 30°C. The added energy increases vaporization by 3%, which is a substantial 2.2 million kg/h. The temperature of the streams entering and leaving each stage decreases with each additional flash stage such that the last vessel has the lowest temperature, according to realistic operating guidelines.

===Design Tradeoffs: Energy Consumption, Environmental Impacts, and Product Optimization===
Increases in the pressure drop across any of the valves in this final design causes a temperature cross in the heat exchangers unless a proper increase in pump energy is provided. There is thus a design tradeoff between lowering energy usage and increasing product yield. For example, lowering the operating pressure of the process dictates that the pressure difference used to flash seawater must be smaller, possibly decreasing the amount that is vaporized. The final design aims for a yield of 50 million gallons per day. The 18 stage model was chosen for its low annual utility cost. This is a reflection of environmental mindfulness because energy consumption is an ongoing aspect of the process and should be lowered as much as possible.

==Mass and Energy Balances==
The model of the desalination system assumes that: (1) the properties are uniform in each phase within a stage, (2) heat losses are negligible due to the small surface area to volume ratio for each stage and properly insulated units (heat losses to the surrounding vary from 2 to 5% of the total system energy), (3) the distillate is salt free since the boiling point of water is much lower than that of salt, (4) the subcooling or superheating effects on the system energy balances are negligible, (5) the only contaminant is sodium chloride (there are no non-condensable gases to consider, so no vapors are vented), and (6) the process has 3 evaporation chambers.

The major balances are that the feed is a mixture of the fresh seawater stream and the recycle stream, and that the brine stream exiting the last evaporation chamber is divided into the recycle stream and the waste stream. Within each stage, the change in mass of a phase over time is the balance between the net flow of that phase from the chamber and the rates of evaporation and condensation of water into and out of that phase. The equations governing the mass and energy balances on the system is displayed in Appendix C.

=Optimization=
As initiatives for reduced energy usage and increased environmental sustainability become more prevalent, efforts to minimize the utility cost in plants has increased to accommodate these incentives. Utilities are a continual source of expenditure and consumption, and a well-designed chemical process should seek to minimize this element. The annualized utility cost is a useful parameter to optimize given that its value depends on variations in energy costs; thus, annualized utility costs do not correlate with conventional indices of inflation, such as capital and labor costs. Additionally, utility costs, like other variable costs, can be minimized by improving the design or operational efficiency of the plant.

In optimizing our MSF process, the design variable of focus was the number of stages to use in our process. The performance measure was the annualized utility cost, which was the parameter that was minimized for optimization. The process is constrained by the product specifications, which are kept constant and serve as the standard for comparison between each model. These specifications are to produce 50 million gallons of desalinated liquid water per day.

One feasibility constraint in the process is the size of the heat exchangers, which must provide a large enough area for total condensation of the vapor stream coming out of the flash stages. In order to restrict the heat exchange area needed within each exchanger, the number of stages must be at least 11. Increasing the number of stages decreases the vapor flow rate at each, and decreases the heat exchange area required as well. An 11-stage process allows for appropriate heat exchange between the feed stream and the vapor product, according to the HYSYS economic analyzer. The optimization analysis thus was conducted incorporating only models with 11 or more stages.

The HYSYS economic analyzer was used to systematically optimize the process by finding the model with the lowest annualized utility cost. The performance parameters kept constant across each model were: the inlet composition, pressure, and temperature, the brine outlet temperature, the total product flow rate and composition, and the flash stage schematic. Table 1 displays the annualized utility cost for each model, and illustrates that a clear dip in the utility cost occurs in an 18 stage model. Thus, 18 is the optimal number of stages to use for our MSF distillation given utility costs as the optimized parameter.

=Economic Evaluation=
In order to evaluate our process from a fiscal standpoint, we used Aspen Process Economic Analyzer and Aspen cost estimator to determine capital and operating costs for our plant. These costs were calculated based on calculated equipment sizes, as well as energy requirements reported by our HYSYS model. The capital cost was found to be $485.2MM and annual total operating cost was found to be $144.7MM ($103.7MM of which was utilities). Annual plant revenue was estimated based on prices that water districts in the county currently pay for outside water sources, and was found to be $134.3MM.14

The NPV analysis we conducted assumed that the plant would take two years to build, would operate at half capacity during the first year of operation (year 3), and have a 20 year lifetime. We also assumed that our plant would be in operation 350 days per year. A constant discount rate of 12% was assumed to hold throughout the plant’s lifetime. The final NPV of this project is -$495.7MM. This is due to both the large capital cost of the plant and the annual loss of $10.4MM while in full operation. A graph of cumulative cash flow versus time can be found in Appendix E. Tax rates and depreciation do not directly impact the NPV of this project due to operating at a loss, however for future calculations it should be noted that we used a straight line depreciation schedule over a seven year period with a 40% tax rate.

Despite the current projections, it is important to consider the greater implications of running a desalination plant in the Bay Area. The population in the Bay Area is quickly growing, while California is experiencing harsher droughts each year. A consequence of this is the possibility of receiving government subsidies for our clean water product. The similarly scaled desalination plant in Carlsbad, CA operates at a loss as well, but receives significant government subsidies. Building this facility may be a strategic investment, especially since greater water scarcity is predicted in the future.

=Sensitivity Analysis=
We wanted to determine how sensitive NPV of our project is to the cost of capital, price of utilities and price we would be able to sell our clean water product at. We conducted this analysis by varying each of these variables independently between 0.5 times and 1.5 times the original value we calculated. As to be expected, NPV grows more negative with increasing capital costs and utility prices and decreasing water prices. Conversely, NPV becomes less negative as price of water rises and capital and utility costs decrease. A plot summarizing the effects of varying these costs can be found in Appendix E. NPV is most sensitive to the price at which we will be selling our clean water product. It is worth noting that NPV is more sensitive to utilities cost than it is to capital cost, which is why we focused on optimizing the cost of utilities. In future steps, trying to reduce utilities cost even more would be worth looking into to decrease the amount of money being lost on this project.

=Conclusions and Final Recommendation=
The final design achieves the project goal of producing 50 million gallons per day of distilled water at a yield of 13%, comparable to other similarly sized plants. An optimization of the number of MSF stages is realistic, and can be improved upon by taking into account other process parameters.

The proposed design was optimized around utility costs, comparing plants with 11 to 25 stages. The process is constrained by product specifications and by the thermodynamics of the heat exchange between the feed and the product streams. Energy conservation was achieved in using the seawater feed to condense the final product, as well as in all of the heat exchangers increasing the thermal energy of the seawater feed stream before it enters the flash stages.

In order to further optimize this process, other design parameters should be considered such as the brine recycle ratio, the cooler duty of the recycle stream, the pressure drop across each valve at the flash stages, and other economic factors. An optimization around maximizing NPV, for example, might be more comprehensive than that conducted regarding only utility costs. Tuning brine recycle rates could greatly improve process efficiency, and standardizing flash stage pressure drops would make operations more feasible and controlled.

Given our in depth economic analysis, this project would lose an average of $10.4MM per year, and has an NPV of -$495.7MM for a 20 year plant lifetime. From an economic perspective alone it would not be a good project to build. However, other factors need to be considered when building a project with enormous public value. In recent years California droughts have become more severe. Implementing a drought-proof supply of water now could avert a disaster later on. Even without a large disaster, having the desalination plant could also be expected to pay off in the future with the increasing price of water. Considerations such as these were the deciding factors in building the Carlsbad desalination plant. Since there are currently no desalination plants in the Bay area we recommend that preparations should be made to move forward with a desalination plant.

Desalination - Team A

2016-03-12T02:50:02Z

Ezhuang: /* Design Tradeoffs: Energy Consumption, Environmental Impacts, and Product Optimization */

Author: Pear Dhiantravan[2016], Reed Kolbe[2016], Sheridan Lichtor[2016], John Marsiglio[2016], Ellen Zhuang[2016]

Instructors: Fengqi You, David Wegerer

Winter 2016

=Introduction=
An increasing global water scarcity is fueling initiatives everywhere for clean water treatment, making efficient seawater desalination an attractive aim for chemical plant design. A 2015 market analysis found that the global desalination market earned revenues of $11.66 billion, and this number is expected to reach over $19 billion by 2019. Furthermore, the 17,000 desalination plants currently in operation are expected to double in number by 2020.1

As fresh water sources become increasingly scarce, strict water conservation measures are being observed. California is entering the fourth year of one of its most severe droughts on record. Cities are required to reduce their water usage by 35% to avoid facing fines.2 In lieu of these pressing conditions, California is looking for ways to provide more accessible fresh water to its citizens. This high demand for clean water motivated our choosing Richmond, CA as our plant location. At Richmond, the desalination plant can convert readily available seawater from the San Francisco Bay into fresh water. We propose to desalinate 50 million gallons of water per day to be sent to water treatment plants for potability. This aim is based on the capacity of the desalination plant in Carlsbad, California, which services a similar population.

The salinity of the water in the San Francisco bay is seasonal. During the dry seasons of summer and fall, salinity is high around 10 PSU (10,000 ppm) because water from the Pacific Ocean flows into the San Francisco Bay. During the wet winter season, fresh water from the rivers flows into the Bay, and salinity drops to around 2 PSU (2,000).3 Designed to meet the “worst case” scenario, our plant will process 398 million gallons of water per day with 10,000 ppm of salt to produce 50 million gallons of water with 1,000 ppm of salt. This gives a yield of 13%.

The desalination plant proposed consists of 18 flash stages in series. This report outlines our investigation of this potential water desalination plant, including design and optimization of the desalination process, an analysis of its energy consumption and environmental impacts, and a study of its economic implications.

=Technical Approach=
For the desalination step of our process, we selected a multistage flash (MSF) distillation process. Not only is MSF distillation a very viable method, but it is also the most common method, currently producing about 60% of the world’s desalinated water. MSF and reverse osmosis (RO) are the two major methods being used in large-scale desalination plants. Both processes require considerable amounts of energy. RO typically has a lower energy demand; however, the high impurity content of the Bay water would frequently necessitate membrane cleaning and/or exchange. Feed going through a RO system requires extensive pretreatment to remove biological organisms and other solids to control the pH and the chemical composition of the water. With the San Francisco Bay as our source of water, MSF is an attractive option because sediment and other large impurities can be separated from the feed before distillation occurs. Additionally, MSF distillation plants can be located near power plants and paired to their waste heat streams to conserve energy. This can reduce energy needs by half or two thirds, making MSF increasingly more practical. Due to the maintenance required in the RO system, the MSF distillation is the better option in terms of operation cost.4

A HYSYS model was built to simulate the MSF distillation section of the process, which spans after seawater pretreatment and before local potable water treatment. The model is used to simulate the distillation steps and to calculate necessary heating, cooling, and input flow rates to produce the required 50 million gallons of distillated water per day. For the simulation, the 398 million gallons per day of seawater feed stream is defined to be composed of 3.31 mol% sodium chloride (NaCl) and 96.69 mol% water. The percentage of NaCl is set to account for other ions and chemicals present after the pretreatment phase. NaCl alone is added to the HYSYS system due to the capabilities of the fluid package used to simulate this process, ElectroNRTL. The feed is set at 22°C, which is the approximate the temperature of the San Francisco Bay year round.

=Process Design=
==Design Overview==
The proposed desalination plant uses MSF distillation to convert the San Francisco Bay water to fresh water. The final process design contains 18 flash stages in series. Overall, 398 million gallons per day of of brackish water is converted to 50 million gallons per day of deionized water, a 13% yield. The production meets the required production rate of 50 million gallons of freshwater per day.

Seawater is taken in from the San Francisco Bay and mixed with the brine recycle stream produced downstream. The feed is then heated through a series of condensers and a heater. Distilled water vapor in each flash stage contacts the tubes carrying the seawater feed and condenses into a liquid stream by exchanging heat with the cooler seawater feed stream. The seawater is sent through a heater to increase its thermal energy before entering the first stage of the MSF. In each flashing stage, some of the water evaporates, leaving the salt behind. The freshwater vapors condense, as described above, and is collected and sent to a water treatment plant. The brine stream is split into the recycle stream and a waste stream, and the waste stream is diluted with fresh seawater before being discharged back into the bay.

A process flow diagram of the process is depicted in Appendix A.

==Pretreatment==
Sediment is removed to prevent solids from plugging the process downstream. Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime, and is controlled for by lowering pH. Acidifying the water will also remove CO2, which is a corrosive gas. To control for oxygen, oxygen scavengers will be included in the system to sequester it. Sodium bisulfate will be used as the scavenger.5

==Condensers (Heat Exchanger Networks)==
In order to conserve and recycle energy, the condensation of the distillate and heating of the seawater feed will be coupled in countercurrent heat exchanger networks. These heat exchanger networks are condensers of the design. The cold feed, which is a mixture of fresh seawater and brine recycle, is fed through multiple heat exchangers in series. Each heat exchanger corresponds to a different evaporation chamber. This process consists of 18 heat exchangers: E-101, E-102, … E-117, and E-118, which correspond to evaporation chambers V-102, V-103, … V-118, and V-119, respectively.

The heat exchangers can be viewed similarly to a shell and tube setup, however there is no actual physical shell. The tubes simply run through the top of the evaporation chambers, where vaporized distilled water in the chamber acts as the shell side fluid. The tube bundles are arranged in long tube configuration and are aligned parallel to the direction of the flashing brine flow in the evaporation chamber. The evaporating brine in an evaporation chamber rise up to the the corresponding heat exchanger and condense on the outside of the tubes, creating collectable pure water. This condensing water is what heats up the seawater in the tubes.

E-101 was used as the basis for the condenser sizing since it has the highest heat requirement, and thus will be the largest heat exchanger. From the HYSYS model, the overall system requires 2.24 x 106 kW of duty. Assuming this energy requirement, 5000 tubes, and 18 m length tubes (the diameter of the evaporation chambers, explained further on the next page), the diameter of each tube was calculated to be 0.4 m. Based on the temperature of the streams and flow, the thickness of the tube is 7 x 10-4 m.

==Brine Heater==
After the seawater exits the final heat exchanger, it enters a brine heater (E-119) where it is brought up to the temperature necessary for the flashing process to begin. This means that the seawater must be overheated compared to stage one, so that the seawater will flash in the evaporation chamber, i.e. release heat and vapor to reach equilibrium with stage conditions. After flowing through the condensers, the seawater enters the brine heater at 96.1 oC and is heated to 120 oC. The overall system requires 2.31 x 106 kW of duty. This is supplied by a total of 4.15 x 106 kg per hour of steam.

==Evaporation Chambers==
When the seawater has been brought to a temperature appropriate for flashing, it enters the first evaporation chamber (V-102). Upon entering the chamber, the water is flashed. Some of the water evaporates, leaving the salt behind. The vapor rises up to the heat exchanger tubes in each evaporation chamber and condenses on the outside into the distilled water product. This condensing water is what heats up the seawater in the tubes discussed above. The seawater that does not evaporate moves to the next stage, which operates at a higher temperature and lower pressure, and the process is repeated. This continues in series through a total of 18 flashing chambers (V-102, V-103, … V-118, and V-119). All of the distillate is collected into one stream and all of the brine is mixed with seawater before being disposed of back in the bay in order to dilute it enough so it does not have an adverse effect on marine life.

Each respective heat exchanger fully condenses the vapor product stream from each stage. However, when all of the individual product streams are consolidated into the final combined product stream, some water exists in the vapor phase due to temperature and pressure changes associated with mixing all of the streams together. Although the vapor product stream from each stage condenses completely across its respective heat exchanger, the total product stream contains some water in the vapor phase due to the temperature change when the mixer sums all the streams. In order to fully condense this stream, which will be sent to local water treatment plans, a heat exchanger is put in place to cool down the product using the process feed stream.

The correlations of El-Dessouky and Ettouney were employed in order to size the flash chambers. They specified that conventional MSF plants that operate at a capacity between 27,000 and 36,000 m3/day use flashing stages of the dimensions 18 x 4 meters.6

==Valves==
The model requires valves in between the evaporation chambers. The seawater streams flow through valves with large pressure drops to flash a portion the liquid into vapor that can then be separated in the vessels. Large pressure drops provide the thermodynamic driving force to vaporize more of the liquid, and are required to achieve high product yield.

==Material of Construction==
Before 1980, the shells and internals of the MSF units were commonly constructed from carbon steel. Seawater, however, corrodes carbon steel. To compensate for this, the thickness of the carbon steel has an additional corrosion allowance, which increases the size and weight of the equipment. After 1980, the stainless steel and duplex stainless steel became increasingly common. Stainless steel metal requires less thickness for equivalent strength and corrosion resistance, which allows for a reduction in the size, weight, and therefore cost of units. Major units in our process will be constructed from stainless steel.6

==Process Alternatives==
In order to ensure that our design is fully optimized, we wanted to consider several alternatives for our process. We looked at alternatives associated with water pretreatment, the distillation process itself and product treatment.

===Water Pretreatment===
Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime. Scaling can be controlled by adjusting pH, temperature, and bicarbonate concentration. We selected acidification to control for process scaling. This is because lowering a process temperature would decrease overall efficiency, and decreasing bicarbonate concentration would be comparatively expensive. Lowering pH would have the added benefit of controlling CO2 concentrations since CO2 is a corrosive gas. To control for corrosion we looked at how to remove oxygen. To target the oxygen we chose an oxygen scavenger. This was chosen over other options such as a deaerator, or a protective coating because a deaerator would have a large capital cost, and the equipment itself would experience corrosion, and a tank coating such as zinc orthophosphate has a mixed effectiveness.7

===MSF: Brine Recycle vs. Once-Through System===
We explored two options for the overall layout of the MSF distillation plant. In the first, the brine passes through the process once; in the second, a portion of the brine is recycled from the last stage to the incoming seawater feed. The largest draw towards implementing a brine recycle is that it would significantly reduce the overall amount of seawater needed to produce the daily target of 50 million gallons per day. This in turn would lower the amount of energy required to run the pump feeding seawater to the process. Another positive is that, with a brine recycle, the amount of water conditioning chemicals that must be added to the seawater feed is reduced. The negative aspects mainly revolve around the fact that the solution throughout the process will be higher in salt concentration. This increase in salinity both raises the likelihood of scaling and corrosion in the plant and raises the amount of seawater that must be added to the final brine to make it safe for discharge back into the environment. Additionally, boiling point of each stage is raised when adding the brine recycle. After weighing both alternatives against one another, we have decided to include a brine recycle in our plant design. Most of the negative aspects of the brine recycle can be combated (e.g. include anti-scaling agents in the feed pretreatment), and the economic benefits of including the brine recycle are very significant.8

===MSF: Continuous vs. Batch===
The distillation step can be done in continuous or batch processes. Batch distillation is more versatile and often used when the product is in small amounts and very high purity. A continuous distillation is more efficient thermodynamically and economically for large amounts of material of constant composition, whereas batch distillation is more effective for small amounts of material of varying compositions. Since the plant is processing large amounts of seawater of relatively constant composition, the distillation system will utilize continuous distillation.9

===MSF: Long Tube vs. Crossed Tube ===
Inside the evaporator stage, the condenser tube bundles may be arranged in either long tube or cross tube design. In the long tube arrangement, the tubes are aligned parallel to the direction of the flashing brine flow. The tubes go from stage to stage without water boxes. In the cross tube arrangement, the tubes are lined perpendicular to the flow of the flashing brine. Water boxes transfer brine between the stages.10 11 While the water boxes in crossed tube bundles allow for more customization, the long tube configuration allows for construction of large unit sizes, as there are manufacturing restrictions with cross tubes. Long tube design can easily accommodate a large number of stages, which lead us to choose long tube for our design.11 12

===MSF: Venting System===
Noncondensable gases exist in the process because of air leakages into stages under vacuum conditions and the release of dissolved gases from the heated brine. These gases are removed by venting the system. An air cooler section concentrates the gas before they are vented, and baffles direct the vapor and condensates out. There are two venting arrangements to consider: series and parallel. The system in series moves gas from one stage directly to the next, and the only loss in the system is vapor in the last stage (since some vapor is extracted with gases). This system is economical, however this configuration increases the amount of noncondensable gases passing from one stage to the next, and disrupting the venting system in one stage will affect the entire system. In parallel venting, gas is released through parallel takeoff pipes at each stage. Although this arrangement allows for venting at any stage to be controlled, the design and maintenance is more complex, and more vapor is lost than in the venting system in series. A third alternative is to use a combination of venting in parallel and in series, which is the common method in industry. For this process, our team chose to do venting in parallel and in series.12

===Additional Energy Source Alternatives===
Energy for the process can also be coupled with renewable energy sources to reduce the desalination plant’s carbon footprint. Energy integration can be direct or indirect. In direct integration, thermal waste heat from industrial sources provide thermal energy or pressure as energy sources. Direct integration is advantageous at the modular level, as it avoids energy losses with electricity conversion. Within indirect integration, there are modular integration and grid/utility-scale integration. Modular integration integrates the plant with wind turbines and other small-scale renewable power generators with mobile deployment. A “hybridization” of renewable power sources with natural gas power generation is also an option, and the coupling of the two provides a stable power source. Indirect renewable energy integration is growing as the cost of renewable energy continues to decreases and provides an economy of scale that direct integration does not, but there are several obstacles towards indirect integration, such as the plant’s finance and engineering the integration of the two systems.13 This distillation plant will be directly integrated with neighboring plants such as Chevron that produce thermal waste heat.

===Product Treatment===
Since the product is distilled water, it needs additional ions to a concentration of around 0.01% before going to the end user. The three alternatives considered were building a water treatment facility on site, simply adding the correct salts to the product stream and then sending the stream for bacterial treatment at a water plant, or simply sending the product distilled water for complete treatment to another facility. Due to the number of nearby water treatment plants and their demand for water, we elected to simply send the product to a nearby water treatment facility.

==Design Constraints and Tradeoffs==
While designing our process, we wanted to make sure that we met several outside constraints, including environmental, political, ethical, social, sustainability, health and safety constraints. First and most importantly, we wanted to ensure that the brine discharged back to the bay would not harm local marine life. Our process was designed around the constraint of diluting the exiting brine to within 0.2% salinity of background seawater. This is not only compliant with California law, but was also verified to be safe for surrounding marine life through our own outside research. We also addressed many of these considerations by deciding to pursue direct energy integration with the nearby Chevron plant in order to reduce environmental impact of our process. Additionally, we ensured that our plant would be located in an area that is densely populated and in great need of fresh drinking water. Finally, we addressed health and safety considerations by making sure we would be able to send our product distilled water to local treatment plants for proper potabilization.

===Design Tradeoffs: Thermodynamics===
Adjusting elements within this process is iterative, and optimization thereof required changing multiple parameters at once. An example of this is in tuning the temperature increase by heater E-100; if the temperature of the stream entering the vessels is too high, the heat exchange between the vapor product and the feed stream will not be realizable because it causes the feed to be too hot to cool and condense the product. Higher temperatures, however, provide greater thermodynamic driving forces for flashing. Setting the entering stream temperature at 120°C provides enough heat for substantial flash at 130 kPa. This is the only heater required until the very last flash stage, which is added to provide more heat for vaporization. This heater (E-105) increases the stream temperature by 15°C, and the vapor product from the last vessel increases the feed stream temperature by almost 30°C. The added energy increases vaporization by 3%, which is a substantial 2.2 million kg/h. The temperature of the streams entering and leaving each stage decreases with each additional flash stage such that the last vessel has the lowest temperature, according to realistic operating guidelines.

===Design Tradeoffs: Energy Consumption, Environmental Impacts, and Product Optimization===
Increases in the pressure drop across any of the valves in this final design causes a temperature cross in the heat exchangers unless a proper increase in pump energy is provided. There is thus a design tradeoff between lowering energy usage and increasing product yield. For example, lowering the operating pressure of the process dictates that the pressure difference used to flash seawater must be smaller, possibly decreasing the amount that is vaporized. The final design aims for a yield of 50 million gallons per day. The 18 stage model was chosen for its low annual utility cost. This is a reflection of environmental mindfulness because energy consumption is an ongoing aspect of the process and should be lowered as much as possible.

==Mass and Energy Balances==
The model of the desalination system assumes that: (1) the properties are uniform in each phase within a stage, (2) heat losses are negligible due to the small surface area to volume ratio for each stage and properly insulated units (heat losses to the surrounding vary from 2 to 5% of the total system energy), (3) the distillate is salt free since the boiling point of water is much lower than that of salt, (4) the subcooling or superheating effects on the system energy balances are negligible, (5) the only contaminant is sodium chloride (there are no non-condensable gases to consider, so no vapors are vented), and (6) the process has 3 evaporation chambers.

The major balances are that the feed is a mixture of the fresh seawater stream and the recycle stream, and that the brine stream exiting the last evaporation chamber is divided into the recycle stream and the waste stream. Within each stage, the change in mass of a phase over time is the balance between the net flow of that phase from the chamber and the rates of evaporation and condensation of water into and out of that phase. The equations governing the mass and energy balances on the system is displayed in Appendix C.

=Optimization=
As initiatives for reduced energy usage and increased environmental sustainability become more prevalent, efforts to minimize the utility cost in plants has increased to accommodate these incentives. Utilities are a continual source of expenditure and consumption, and a well-designed chemical process should seek to minimize this element. The annualized utility cost is a useful parameter to optimize given that its value depends on variations in energy costs; thus, annualized utility costs do not correlate with conventional indices of inflation, such as capital and labor costs. Additionally, utility costs, like other variable costs, can be minimized by improving the design or operational efficiency of the plant.

In optimizing our MSF process, the design variable of focus was the number of stages to use in our process. The performance measure was the annualized utility cost, which was the parameter that was minimized for optimization. The process is constrained by the product specifications, which are kept constant and serve as the standard for comparison between each model. These specifications are to produce 50 million gallons of desalinated liquid water per day.

One feasibility constraint in the process is the size of the heat exchangers, which must provide a large enough area for total condensation of the vapor stream coming out of the flash stages. In order to restrict the heat exchange area needed within each exchanger, the number of stages must be at least 11. Increasing the number of stages decreases the vapor flow rate at each, and decreases the heat exchange area required as well. An 11-stage process allows for appropriate heat exchange between the feed stream and the vapor product, according to the HYSYS economic analyzer. The optimization analysis thus was conducted incorporating only models with 11 or more stages.

The HYSYS economic analyzer was used to systematically optimize the process by finding the model with the lowest annualized utility cost. The performance parameters kept constant across each model were: the inlet composition, pressure, and temperature, the brine outlet temperature, the total product flow rate and composition, and the flash stage schematic. Table 1 displays the annualized utility cost for each model, and illustrates that a clear dip in the utility cost occurs in an 18 stage model. Thus, 18 is the optimal number of stages to use for our MSF distillation given utility costs as the optimized parameter.

=Economic Evaluation=
In order to evaluate our process from a fiscal standpoint, we used Aspen Process Economic Analyzer and Aspen cost estimator to determine capital and operating costs for our plant. These costs were calculated based on calculated equipment sizes, as well as energy requirements reported by our HYSYS model. The capital cost was found to be $485.2MM and annual total operating cost was found to be $144.7MM ($103.7MM of which was utilities). Annual plant revenue was estimated based on prices that water districts in the county currently pay for outside water sources, and was found to be $134.3MM.14

The NPV analysis we conducted assumed that the plant would take two years to build, would operate at half capacity during the first year of operation (year 3), and have a 20 year lifetime. We also assumed that our plant would be in operation 350 days per year. A constant discount rate of 12% was assumed to hold throughout the plant’s lifetime. The final NPV of this project is -$495.7MM. This is due to both the large capital cost of the plant and the annual loss of $10.4MM while in full operation. A graph of cumulative cash flow versus time can be found in Appendix E. Tax rates and depreciation do not directly impact the NPV of this project due to operating at a loss, however for future calculations it should be noted that we used a straight line depreciation schedule over a seven year period with a 40% tax rate.

Despite the current projections, it is important to consider the greater implications of running a desalination plant in the Bay Area. The population in the Bay Area is quickly growing, while California is experiencing harsher droughts each year. A consequence of this is the possibility of receiving government subsidies for our clean water product. The similarly scaled desalination plant in Carlsbad, CA operates at a loss as well, but receives significant government subsidies. Building this facility may be a strategic investment, especially since greater water scarcity is predicted in the future.

=Sensitivity Analysis=
We wanted to determine how sensitive NPV of our project is to the cost of capital, price of utilities and price we would be able to sell our clean water product at. We conducted this analysis by varying each of these variables independently between 0.5 times and 1.5 times the original value we calculated. As to be expected, NPV grows more negative with increasing capital costs and utility prices and decreasing water prices. Conversely, NPV becomes less negative as price of water rises and capital and utility costs decrease. A plot summarizing the effects of varying these costs can be found in Appendix E. NPV is most sensitive to the price at which we will be selling our clean water product. It is worth noting that NPV is more sensitive to utilities cost than it is to capital cost, which is why we focused on optimizing the cost of utilities. In future steps, trying to reduce utilities cost even more would be worth looking into to decrease the amount of money being lost on this project.

=Conclusions and Final Recommendation=
The final design achieves the project goal of producing 50 million gallons per day of distilled water at a yield of 13%, comparable to other similarly sized plants. An optimization of the number of MSF stages is realistic, and can be improved upon by taking into account other process parameters.

The proposed design was optimized around utility costs, comparing plants with 11 to 25 stages. The process is constrained by product specifications and by the thermodynamics of the heat exchange between the feed and the product streams. Energy conservation was achieved in using the seawater feed to condense the final product, as well as in all of the heat exchangers increasing the thermal energy of the seawater feed stream before it enters the flash stages.

In order to further optimize this process, other design parameters should be considered such as the brine recycle ratio, the cooler duty of the recycle stream, the pressure drop across each valve at the flash stages, and other economic factors. An optimization around maximizing NPV, for example, might be more comprehensive than that conducted regarding only utility costs. Tuning brine recycle rates could greatly improve process efficiency, and standardizing flash stage pressure drops would make operations more feasible and controlled.

Given our in depth economic analysis, this project would lose an average of $10.4MM per year, and has an NPV of -$495.7MM for a 20 year plant lifetime. From an economic perspective alone it would not be a good project to build. However, other factors need to be considered when building a project with enormous public value. In recent years California droughts have become more severe. Implementing a drought-proof supply of water now could avert a disaster later on. Even without a large disaster, having the desalination plant could also be expected to pay off in the future with the increasing price of water. Considerations such as these were the deciding factors in building the Carlsbad desalination plant. Since there are currently no desalination plants in the Bay area we recommend that preparations should be made to move forward with a desalination plant.

Desalination - Team A

2016-03-12T02:48:33Z

Ezhuang: /* Design Constraints and Tradeoffs */

Author: Pear Dhiantravan[2016], Reed Kolbe[2016], Sheridan Lichtor[2016], John Marsiglio[2016], Ellen Zhuang[2016]

Instructors: Fengqi You, David Wegerer

Winter 2016

=Introduction=
An increasing global water scarcity is fueling initiatives everywhere for clean water treatment, making efficient seawater desalination an attractive aim for chemical plant design. A 2015 market analysis found that the global desalination market earned revenues of $11.66 billion, and this number is expected to reach over $19 billion by 2019. Furthermore, the 17,000 desalination plants currently in operation are expected to double in number by 2020.1

As fresh water sources become increasingly scarce, strict water conservation measures are being observed. California is entering the fourth year of one of its most severe droughts on record. Cities are required to reduce their water usage by 35% to avoid facing fines.2 In lieu of these pressing conditions, California is looking for ways to provide more accessible fresh water to its citizens. This high demand for clean water motivated our choosing Richmond, CA as our plant location. At Richmond, the desalination plant can convert readily available seawater from the San Francisco Bay into fresh water. We propose to desalinate 50 million gallons of water per day to be sent to water treatment plants for potability. This aim is based on the capacity of the desalination plant in Carlsbad, California, which services a similar population.

The salinity of the water in the San Francisco bay is seasonal. During the dry seasons of summer and fall, salinity is high around 10 PSU (10,000 ppm) because water from the Pacific Ocean flows into the San Francisco Bay. During the wet winter season, fresh water from the rivers flows into the Bay, and salinity drops to around 2 PSU (2,000).3 Designed to meet the “worst case” scenario, our plant will process 398 million gallons of water per day with 10,000 ppm of salt to produce 50 million gallons of water with 1,000 ppm of salt. This gives a yield of 13%.

The desalination plant proposed consists of 18 flash stages in series. This report outlines our investigation of this potential water desalination plant, including design and optimization of the desalination process, an analysis of its energy consumption and environmental impacts, and a study of its economic implications.

=Technical Approach=
For the desalination step of our process, we selected a multistage flash (MSF) distillation process. Not only is MSF distillation a very viable method, but it is also the most common method, currently producing about 60% of the world’s desalinated water. MSF and reverse osmosis (RO) are the two major methods being used in large-scale desalination plants. Both processes require considerable amounts of energy. RO typically has a lower energy demand; however, the high impurity content of the Bay water would frequently necessitate membrane cleaning and/or exchange. Feed going through a RO system requires extensive pretreatment to remove biological organisms and other solids to control the pH and the chemical composition of the water. With the San Francisco Bay as our source of water, MSF is an attractive option because sediment and other large impurities can be separated from the feed before distillation occurs. Additionally, MSF distillation plants can be located near power plants and paired to their waste heat streams to conserve energy. This can reduce energy needs by half or two thirds, making MSF increasingly more practical. Due to the maintenance required in the RO system, the MSF distillation is the better option in terms of operation cost.4

A HYSYS model was built to simulate the MSF distillation section of the process, which spans after seawater pretreatment and before local potable water treatment. The model is used to simulate the distillation steps and to calculate necessary heating, cooling, and input flow rates to produce the required 50 million gallons of distillated water per day. For the simulation, the 398 million gallons per day of seawater feed stream is defined to be composed of 3.31 mol% sodium chloride (NaCl) and 96.69 mol% water. The percentage of NaCl is set to account for other ions and chemicals present after the pretreatment phase. NaCl alone is added to the HYSYS system due to the capabilities of the fluid package used to simulate this process, ElectroNRTL. The feed is set at 22°C, which is the approximate the temperature of the San Francisco Bay year round.

=Process Design=
==Design Overview==
The proposed desalination plant uses MSF distillation to convert the San Francisco Bay water to fresh water. The final process design contains 18 flash stages in series. Overall, 398 million gallons per day of of brackish water is converted to 50 million gallons per day of deionized water, a 13% yield. The production meets the required production rate of 50 million gallons of freshwater per day.

Seawater is taken in from the San Francisco Bay and mixed with the brine recycle stream produced downstream. The feed is then heated through a series of condensers and a heater. Distilled water vapor in each flash stage contacts the tubes carrying the seawater feed and condenses into a liquid stream by exchanging heat with the cooler seawater feed stream. The seawater is sent through a heater to increase its thermal energy before entering the first stage of the MSF. In each flashing stage, some of the water evaporates, leaving the salt behind. The freshwater vapors condense, as described above, and is collected and sent to a water treatment plant. The brine stream is split into the recycle stream and a waste stream, and the waste stream is diluted with fresh seawater before being discharged back into the bay.

A process flow diagram of the process is depicted in Appendix A.

==Pretreatment==
Sediment is removed to prevent solids from plugging the process downstream. Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime, and is controlled for by lowering pH. Acidifying the water will also remove CO2, which is a corrosive gas. To control for oxygen, oxygen scavengers will be included in the system to sequester it. Sodium bisulfate will be used as the scavenger.5

==Condensers (Heat Exchanger Networks)==
In order to conserve and recycle energy, the condensation of the distillate and heating of the seawater feed will be coupled in countercurrent heat exchanger networks. These heat exchanger networks are condensers of the design. The cold feed, which is a mixture of fresh seawater and brine recycle, is fed through multiple heat exchangers in series. Each heat exchanger corresponds to a different evaporation chamber. This process consists of 18 heat exchangers: E-101, E-102, … E-117, and E-118, which correspond to evaporation chambers V-102, V-103, … V-118, and V-119, respectively.

The heat exchangers can be viewed similarly to a shell and tube setup, however there is no actual physical shell. The tubes simply run through the top of the evaporation chambers, where vaporized distilled water in the chamber acts as the shell side fluid. The tube bundles are arranged in long tube configuration and are aligned parallel to the direction of the flashing brine flow in the evaporation chamber. The evaporating brine in an evaporation chamber rise up to the the corresponding heat exchanger and condense on the outside of the tubes, creating collectable pure water. This condensing water is what heats up the seawater in the tubes.

E-101 was used as the basis for the condenser sizing since it has the highest heat requirement, and thus will be the largest heat exchanger. From the HYSYS model, the overall system requires 2.24 x 106 kW of duty. Assuming this energy requirement, 5000 tubes, and 18 m length tubes (the diameter of the evaporation chambers, explained further on the next page), the diameter of each tube was calculated to be 0.4 m. Based on the temperature of the streams and flow, the thickness of the tube is 7 x 10-4 m.

==Brine Heater==
After the seawater exits the final heat exchanger, it enters a brine heater (E-119) where it is brought up to the temperature necessary for the flashing process to begin. This means that the seawater must be overheated compared to stage one, so that the seawater will flash in the evaporation chamber, i.e. release heat and vapor to reach equilibrium with stage conditions. After flowing through the condensers, the seawater enters the brine heater at 96.1 oC and is heated to 120 oC. The overall system requires 2.31 x 106 kW of duty. This is supplied by a total of 4.15 x 106 kg per hour of steam.

==Evaporation Chambers==
When the seawater has been brought to a temperature appropriate for flashing, it enters the first evaporation chamber (V-102). Upon entering the chamber, the water is flashed. Some of the water evaporates, leaving the salt behind. The vapor rises up to the heat exchanger tubes in each evaporation chamber and condenses on the outside into the distilled water product. This condensing water is what heats up the seawater in the tubes discussed above. The seawater that does not evaporate moves to the next stage, which operates at a higher temperature and lower pressure, and the process is repeated. This continues in series through a total of 18 flashing chambers (V-102, V-103, … V-118, and V-119). All of the distillate is collected into one stream and all of the brine is mixed with seawater before being disposed of back in the bay in order to dilute it enough so it does not have an adverse effect on marine life.

Each respective heat exchanger fully condenses the vapor product stream from each stage. However, when all of the individual product streams are consolidated into the final combined product stream, some water exists in the vapor phase due to temperature and pressure changes associated with mixing all of the streams together. Although the vapor product stream from each stage condenses completely across its respective heat exchanger, the total product stream contains some water in the vapor phase due to the temperature change when the mixer sums all the streams. In order to fully condense this stream, which will be sent to local water treatment plans, a heat exchanger is put in place to cool down the product using the process feed stream.

The correlations of El-Dessouky and Ettouney were employed in order to size the flash chambers. They specified that conventional MSF plants that operate at a capacity between 27,000 and 36,000 m3/day use flashing stages of the dimensions 18 x 4 meters.6

==Valves==
The model requires valves in between the evaporation chambers. The seawater streams flow through valves with large pressure drops to flash a portion the liquid into vapor that can then be separated in the vessels. Large pressure drops provide the thermodynamic driving force to vaporize more of the liquid, and are required to achieve high product yield.

==Material of Construction==
Before 1980, the shells and internals of the MSF units were commonly constructed from carbon steel. Seawater, however, corrodes carbon steel. To compensate for this, the thickness of the carbon steel has an additional corrosion allowance, which increases the size and weight of the equipment. After 1980, the stainless steel and duplex stainless steel became increasingly common. Stainless steel metal requires less thickness for equivalent strength and corrosion resistance, which allows for a reduction in the size, weight, and therefore cost of units. Major units in our process will be constructed from stainless steel.6

==Process Alternatives==
In order to ensure that our design is fully optimized, we wanted to consider several alternatives for our process. We looked at alternatives associated with water pretreatment, the distillation process itself and product treatment.

===Water Pretreatment===
Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime. Scaling can be controlled by adjusting pH, temperature, and bicarbonate concentration. We selected acidification to control for process scaling. This is because lowering a process temperature would decrease overall efficiency, and decreasing bicarbonate concentration would be comparatively expensive. Lowering pH would have the added benefit of controlling CO2 concentrations since CO2 is a corrosive gas. To control for corrosion we looked at how to remove oxygen. To target the oxygen we chose an oxygen scavenger. This was chosen over other options such as a deaerator, or a protective coating because a deaerator would have a large capital cost, and the equipment itself would experience corrosion, and a tank coating such as zinc orthophosphate has a mixed effectiveness.7

===MSF: Brine Recycle vs. Once-Through System===
We explored two options for the overall layout of the MSF distillation plant. In the first, the brine passes through the process once; in the second, a portion of the brine is recycled from the last stage to the incoming seawater feed. The largest draw towards implementing a brine recycle is that it would significantly reduce the overall amount of seawater needed to produce the daily target of 50 million gallons per day. This in turn would lower the amount of energy required to run the pump feeding seawater to the process. Another positive is that, with a brine recycle, the amount of water conditioning chemicals that must be added to the seawater feed is reduced. The negative aspects mainly revolve around the fact that the solution throughout the process will be higher in salt concentration. This increase in salinity both raises the likelihood of scaling and corrosion in the plant and raises the amount of seawater that must be added to the final brine to make it safe for discharge back into the environment. Additionally, boiling point of each stage is raised when adding the brine recycle. After weighing both alternatives against one another, we have decided to include a brine recycle in our plant design. Most of the negative aspects of the brine recycle can be combated (e.g. include anti-scaling agents in the feed pretreatment), and the economic benefits of including the brine recycle are very significant.8

===MSF: Continuous vs. Batch===
The distillation step can be done in continuous or batch processes. Batch distillation is more versatile and often used when the product is in small amounts and very high purity. A continuous distillation is more efficient thermodynamically and economically for large amounts of material of constant composition, whereas batch distillation is more effective for small amounts of material of varying compositions. Since the plant is processing large amounts of seawater of relatively constant composition, the distillation system will utilize continuous distillation.9

===MSF: Long Tube vs. Crossed Tube ===
Inside the evaporator stage, the condenser tube bundles may be arranged in either long tube or cross tube design. In the long tube arrangement, the tubes are aligned parallel to the direction of the flashing brine flow. The tubes go from stage to stage without water boxes. In the cross tube arrangement, the tubes are lined perpendicular to the flow of the flashing brine. Water boxes transfer brine between the stages.10 11 While the water boxes in crossed tube bundles allow for more customization, the long tube configuration allows for construction of large unit sizes, as there are manufacturing restrictions with cross tubes. Long tube design can easily accommodate a large number of stages, which lead us to choose long tube for our design.11 12

===MSF: Venting System===
Noncondensable gases exist in the process because of air leakages into stages under vacuum conditions and the release of dissolved gases from the heated brine. These gases are removed by venting the system. An air cooler section concentrates the gas before they are vented, and baffles direct the vapor and condensates out. There are two venting arrangements to consider: series and parallel. The system in series moves gas from one stage directly to the next, and the only loss in the system is vapor in the last stage (since some vapor is extracted with gases). This system is economical, however this configuration increases the amount of noncondensable gases passing from one stage to the next, and disrupting the venting system in one stage will affect the entire system. In parallel venting, gas is released through parallel takeoff pipes at each stage. Although this arrangement allows for venting at any stage to be controlled, the design and maintenance is more complex, and more vapor is lost than in the venting system in series. A third alternative is to use a combination of venting in parallel and in series, which is the common method in industry. For this process, our team chose to do venting in parallel and in series.12

===Additional Energy Source Alternatives===
Energy for the process can also be coupled with renewable energy sources to reduce the desalination plant’s carbon footprint. Energy integration can be direct or indirect. In direct integration, thermal waste heat from industrial sources provide thermal energy or pressure as energy sources. Direct integration is advantageous at the modular level, as it avoids energy losses with electricity conversion. Within indirect integration, there are modular integration and grid/utility-scale integration. Modular integration integrates the plant with wind turbines and other small-scale renewable power generators with mobile deployment. A “hybridization” of renewable power sources with natural gas power generation is also an option, and the coupling of the two provides a stable power source. Indirect renewable energy integration is growing as the cost of renewable energy continues to decreases and provides an economy of scale that direct integration does not, but there are several obstacles towards indirect integration, such as the plant’s finance and engineering the integration of the two systems.13 This distillation plant will be directly integrated with neighboring plants such as Chevron that produce thermal waste heat.

===Product Treatment===
Since the product is distilled water, it needs additional ions to a concentration of around 0.01% before going to the end user. The three alternatives considered were building a water treatment facility on site, simply adding the correct salts to the product stream and then sending the stream for bacterial treatment at a water plant, or simply sending the product distilled water for complete treatment to another facility. Due to the number of nearby water treatment plants and their demand for water, we elected to simply send the product to a nearby water treatment facility.

==Design Constraints and Tradeoffs==
While designing our process, we wanted to make sure that we met several outside constraints, including environmental, political, ethical, social, sustainability, health and safety constraints. First and most importantly, we wanted to ensure that the brine discharged back to the bay would not harm local marine life. Our process was designed around the constraint of diluting the exiting brine to within 0.2% salinity of background seawater. This is not only compliant with California law, but was also verified to be safe for surrounding marine life through our own outside research. We also addressed many of these considerations by deciding to pursue direct energy integration with the nearby Chevron plant in order to reduce environmental impact of our process. Additionally, we ensured that our plant would be located in an area that is densely populated and in great need of fresh drinking water. Finally, we addressed health and safety considerations by making sure we would be able to send our product distilled water to local treatment plants for proper potabilization.

===Design Tradeoffs: Thermodynamics===
Adjusting elements within this process is iterative, and optimization thereof required changing multiple parameters at once. An example of this is in tuning the temperature increase by heater E-100; if the temperature of the stream entering the vessels is too high, the heat exchange between the vapor product and the feed stream will not be realizable because it causes the feed to be too hot to cool and condense the product. Higher temperatures, however, provide greater thermodynamic driving forces for flashing. Setting the entering stream temperature at 120°C provides enough heat for substantial flash at 130 kPa. This is the only heater required until the very last flash stage, which is added to provide more heat for vaporization. This heater (E-105) increases the stream temperature by 15°C, and the vapor product from the last vessel increases the feed stream temperature by almost 30°C. The added energy increases vaporization by 3%, which is a substantial 2.2 million kg/h. The temperature of the streams entering and leaving each stage decreases with each additional flash stage such that the last vessel has the lowest temperature, according to realistic operating guidelines.

===Design Tradeoffs: Energy Consumption, Environmental Impacts, and Product Optimization===
Increases in the pressure drop across any of the valves in this final design causes a temperature cross in the heat exchangers unless a proper increase in pump energy is provided. There is thus a design tradeoff between lowering energy usage and increasing product yield. For example, lowering the operating pressure of the process dictates that the pressure difference used to flash seawater must be smaller, possibly decreasing the amount that is vaporized. The final design aims for a yield of 50 million gallons per day. The 18 stage model was chosen for its low annual utility cost. This is a reflection of environmental mindfulness because energy consumption is an ongoing aspect of the process and should be lowered as much as possible.

Desalination - Team A

2016-03-12T02:47:54Z

Ezhuang: /* Product Treatment */

Author: Pear Dhiantravan[2016], Reed Kolbe[2016], Sheridan Lichtor[2016], John Marsiglio[2016], Ellen Zhuang[2016]

Instructors: Fengqi You, David Wegerer

Winter 2016

=Introduction=
An increasing global water scarcity is fueling initiatives everywhere for clean water treatment, making efficient seawater desalination an attractive aim for chemical plant design. A 2015 market analysis found that the global desalination market earned revenues of $11.66 billion, and this number is expected to reach over $19 billion by 2019. Furthermore, the 17,000 desalination plants currently in operation are expected to double in number by 2020.1

As fresh water sources become increasingly scarce, strict water conservation measures are being observed. California is entering the fourth year of one of its most severe droughts on record. Cities are required to reduce their water usage by 35% to avoid facing fines.2 In lieu of these pressing conditions, California is looking for ways to provide more accessible fresh water to its citizens. This high demand for clean water motivated our choosing Richmond, CA as our plant location. At Richmond, the desalination plant can convert readily available seawater from the San Francisco Bay into fresh water. We propose to desalinate 50 million gallons of water per day to be sent to water treatment plants for potability. This aim is based on the capacity of the desalination plant in Carlsbad, California, which services a similar population.

The salinity of the water in the San Francisco bay is seasonal. During the dry seasons of summer and fall, salinity is high around 10 PSU (10,000 ppm) because water from the Pacific Ocean flows into the San Francisco Bay. During the wet winter season, fresh water from the rivers flows into the Bay, and salinity drops to around 2 PSU (2,000).3 Designed to meet the “worst case” scenario, our plant will process 398 million gallons of water per day with 10,000 ppm of salt to produce 50 million gallons of water with 1,000 ppm of salt. This gives a yield of 13%.

The desalination plant proposed consists of 18 flash stages in series. This report outlines our investigation of this potential water desalination plant, including design and optimization of the desalination process, an analysis of its energy consumption and environmental impacts, and a study of its economic implications.

=Technical Approach=
For the desalination step of our process, we selected a multistage flash (MSF) distillation process. Not only is MSF distillation a very viable method, but it is also the most common method, currently producing about 60% of the world’s desalinated water. MSF and reverse osmosis (RO) are the two major methods being used in large-scale desalination plants. Both processes require considerable amounts of energy. RO typically has a lower energy demand; however, the high impurity content of the Bay water would frequently necessitate membrane cleaning and/or exchange. Feed going through a RO system requires extensive pretreatment to remove biological organisms and other solids to control the pH and the chemical composition of the water. With the San Francisco Bay as our source of water, MSF is an attractive option because sediment and other large impurities can be separated from the feed before distillation occurs. Additionally, MSF distillation plants can be located near power plants and paired to their waste heat streams to conserve energy. This can reduce energy needs by half or two thirds, making MSF increasingly more practical. Due to the maintenance required in the RO system, the MSF distillation is the better option in terms of operation cost.4

A HYSYS model was built to simulate the MSF distillation section of the process, which spans after seawater pretreatment and before local potable water treatment. The model is used to simulate the distillation steps and to calculate necessary heating, cooling, and input flow rates to produce the required 50 million gallons of distillated water per day. For the simulation, the 398 million gallons per day of seawater feed stream is defined to be composed of 3.31 mol% sodium chloride (NaCl) and 96.69 mol% water. The percentage of NaCl is set to account for other ions and chemicals present after the pretreatment phase. NaCl alone is added to the HYSYS system due to the capabilities of the fluid package used to simulate this process, ElectroNRTL. The feed is set at 22°C, which is the approximate the temperature of the San Francisco Bay year round.

=Process Design=
==Design Overview==
The proposed desalination plant uses MSF distillation to convert the San Francisco Bay water to fresh water. The final process design contains 18 flash stages in series. Overall, 398 million gallons per day of of brackish water is converted to 50 million gallons per day of deionized water, a 13% yield. The production meets the required production rate of 50 million gallons of freshwater per day.

Seawater is taken in from the San Francisco Bay and mixed with the brine recycle stream produced downstream. The feed is then heated through a series of condensers and a heater. Distilled water vapor in each flash stage contacts the tubes carrying the seawater feed and condenses into a liquid stream by exchanging heat with the cooler seawater feed stream. The seawater is sent through a heater to increase its thermal energy before entering the first stage of the MSF. In each flashing stage, some of the water evaporates, leaving the salt behind. The freshwater vapors condense, as described above, and is collected and sent to a water treatment plant. The brine stream is split into the recycle stream and a waste stream, and the waste stream is diluted with fresh seawater before being discharged back into the bay.

A process flow diagram of the process is depicted in Appendix A.

==Pretreatment==
Sediment is removed to prevent solids from plugging the process downstream. Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime, and is controlled for by lowering pH. Acidifying the water will also remove CO2, which is a corrosive gas. To control for oxygen, oxygen scavengers will be included in the system to sequester it. Sodium bisulfate will be used as the scavenger.5

==Condensers (Heat Exchanger Networks)==
In order to conserve and recycle energy, the condensation of the distillate and heating of the seawater feed will be coupled in countercurrent heat exchanger networks. These heat exchanger networks are condensers of the design. The cold feed, which is a mixture of fresh seawater and brine recycle, is fed through multiple heat exchangers in series. Each heat exchanger corresponds to a different evaporation chamber. This process consists of 18 heat exchangers: E-101, E-102, … E-117, and E-118, which correspond to evaporation chambers V-102, V-103, … V-118, and V-119, respectively.

The heat exchangers can be viewed similarly to a shell and tube setup, however there is no actual physical shell. The tubes simply run through the top of the evaporation chambers, where vaporized distilled water in the chamber acts as the shell side fluid. The tube bundles are arranged in long tube configuration and are aligned parallel to the direction of the flashing brine flow in the evaporation chamber. The evaporating brine in an evaporation chamber rise up to the the corresponding heat exchanger and condense on the outside of the tubes, creating collectable pure water. This condensing water is what heats up the seawater in the tubes.

E-101 was used as the basis for the condenser sizing since it has the highest heat requirement, and thus will be the largest heat exchanger. From the HYSYS model, the overall system requires 2.24 x 106 kW of duty. Assuming this energy requirement, 5000 tubes, and 18 m length tubes (the diameter of the evaporation chambers, explained further on the next page), the diameter of each tube was calculated to be 0.4 m. Based on the temperature of the streams and flow, the thickness of the tube is 7 x 10-4 m.

==Brine Heater==
After the seawater exits the final heat exchanger, it enters a brine heater (E-119) where it is brought up to the temperature necessary for the flashing process to begin. This means that the seawater must be overheated compared to stage one, so that the seawater will flash in the evaporation chamber, i.e. release heat and vapor to reach equilibrium with stage conditions. After flowing through the condensers, the seawater enters the brine heater at 96.1 oC and is heated to 120 oC. The overall system requires 2.31 x 106 kW of duty. This is supplied by a total of 4.15 x 106 kg per hour of steam.

==Evaporation Chambers==
When the seawater has been brought to a temperature appropriate for flashing, it enters the first evaporation chamber (V-102). Upon entering the chamber, the water is flashed. Some of the water evaporates, leaving the salt behind. The vapor rises up to the heat exchanger tubes in each evaporation chamber and condenses on the outside into the distilled water product. This condensing water is what heats up the seawater in the tubes discussed above. The seawater that does not evaporate moves to the next stage, which operates at a higher temperature and lower pressure, and the process is repeated. This continues in series through a total of 18 flashing chambers (V-102, V-103, … V-118, and V-119). All of the distillate is collected into one stream and all of the brine is mixed with seawater before being disposed of back in the bay in order to dilute it enough so it does not have an adverse effect on marine life.

Each respective heat exchanger fully condenses the vapor product stream from each stage. However, when all of the individual product streams are consolidated into the final combined product stream, some water exists in the vapor phase due to temperature and pressure changes associated with mixing all of the streams together. Although the vapor product stream from each stage condenses completely across its respective heat exchanger, the total product stream contains some water in the vapor phase due to the temperature change when the mixer sums all the streams. In order to fully condense this stream, which will be sent to local water treatment plans, a heat exchanger is put in place to cool down the product using the process feed stream.

The correlations of El-Dessouky and Ettouney were employed in order to size the flash chambers. They specified that conventional MSF plants that operate at a capacity between 27,000 and 36,000 m3/day use flashing stages of the dimensions 18 x 4 meters.6

==Valves==
The model requires valves in between the evaporation chambers. The seawater streams flow through valves with large pressure drops to flash a portion the liquid into vapor that can then be separated in the vessels. Large pressure drops provide the thermodynamic driving force to vaporize more of the liquid, and are required to achieve high product yield.

==Material of Construction==
Before 1980, the shells and internals of the MSF units were commonly constructed from carbon steel. Seawater, however, corrodes carbon steel. To compensate for this, the thickness of the carbon steel has an additional corrosion allowance, which increases the size and weight of the equipment. After 1980, the stainless steel and duplex stainless steel became increasingly common. Stainless steel metal requires less thickness for equivalent strength and corrosion resistance, which allows for a reduction in the size, weight, and therefore cost of units. Major units in our process will be constructed from stainless steel.6

==Process Alternatives==
In order to ensure that our design is fully optimized, we wanted to consider several alternatives for our process. We looked at alternatives associated with water pretreatment, the distillation process itself and product treatment.

===Water Pretreatment===
Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime. Scaling can be controlled by adjusting pH, temperature, and bicarbonate concentration. We selected acidification to control for process scaling. This is because lowering a process temperature would decrease overall efficiency, and decreasing bicarbonate concentration would be comparatively expensive. Lowering pH would have the added benefit of controlling CO2 concentrations since CO2 is a corrosive gas. To control for corrosion we looked at how to remove oxygen. To target the oxygen we chose an oxygen scavenger. This was chosen over other options such as a deaerator, or a protective coating because a deaerator would have a large capital cost, and the equipment itself would experience corrosion, and a tank coating such as zinc orthophosphate has a mixed effectiveness.7

===MSF: Brine Recycle vs. Once-Through System===
We explored two options for the overall layout of the MSF distillation plant. In the first, the brine passes through the process once; in the second, a portion of the brine is recycled from the last stage to the incoming seawater feed. The largest draw towards implementing a brine recycle is that it would significantly reduce the overall amount of seawater needed to produce the daily target of 50 million gallons per day. This in turn would lower the amount of energy required to run the pump feeding seawater to the process. Another positive is that, with a brine recycle, the amount of water conditioning chemicals that must be added to the seawater feed is reduced. The negative aspects mainly revolve around the fact that the solution throughout the process will be higher in salt concentration. This increase in salinity both raises the likelihood of scaling and corrosion in the plant and raises the amount of seawater that must be added to the final brine to make it safe for discharge back into the environment. Additionally, boiling point of each stage is raised when adding the brine recycle. After weighing both alternatives against one another, we have decided to include a brine recycle in our plant design. Most of the negative aspects of the brine recycle can be combated (e.g. include anti-scaling agents in the feed pretreatment), and the economic benefits of including the brine recycle are very significant.8

===MSF: Continuous vs. Batch===
The distillation step can be done in continuous or batch processes. Batch distillation is more versatile and often used when the product is in small amounts and very high purity. A continuous distillation is more efficient thermodynamically and economically for large amounts of material of constant composition, whereas batch distillation is more effective for small amounts of material of varying compositions. Since the plant is processing large amounts of seawater of relatively constant composition, the distillation system will utilize continuous distillation.9

===MSF: Long Tube vs. Crossed Tube ===
Inside the evaporator stage, the condenser tube bundles may be arranged in either long tube or cross tube design. In the long tube arrangement, the tubes are aligned parallel to the direction of the flashing brine flow. The tubes go from stage to stage without water boxes. In the cross tube arrangement, the tubes are lined perpendicular to the flow of the flashing brine. Water boxes transfer brine between the stages.10 11 While the water boxes in crossed tube bundles allow for more customization, the long tube configuration allows for construction of large unit sizes, as there are manufacturing restrictions with cross tubes. Long tube design can easily accommodate a large number of stages, which lead us to choose long tube for our design.11 12

===MSF: Venting System===
Noncondensable gases exist in the process because of air leakages into stages under vacuum conditions and the release of dissolved gases from the heated brine. These gases are removed by venting the system. An air cooler section concentrates the gas before they are vented, and baffles direct the vapor and condensates out. There are two venting arrangements to consider: series and parallel. The system in series moves gas from one stage directly to the next, and the only loss in the system is vapor in the last stage (since some vapor is extracted with gases). This system is economical, however this configuration increases the amount of noncondensable gases passing from one stage to the next, and disrupting the venting system in one stage will affect the entire system. In parallel venting, gas is released through parallel takeoff pipes at each stage. Although this arrangement allows for venting at any stage to be controlled, the design and maintenance is more complex, and more vapor is lost than in the venting system in series. A third alternative is to use a combination of venting in parallel and in series, which is the common method in industry. For this process, our team chose to do venting in parallel and in series.12

===Additional Energy Source Alternatives===
Energy for the process can also be coupled with renewable energy sources to reduce the desalination plant’s carbon footprint. Energy integration can be direct or indirect. In direct integration, thermal waste heat from industrial sources provide thermal energy or pressure as energy sources. Direct integration is advantageous at the modular level, as it avoids energy losses with electricity conversion. Within indirect integration, there are modular integration and grid/utility-scale integration. Modular integration integrates the plant with wind turbines and other small-scale renewable power generators with mobile deployment. A “hybridization” of renewable power sources with natural gas power generation is also an option, and the coupling of the two provides a stable power source. Indirect renewable energy integration is growing as the cost of renewable energy continues to decreases and provides an economy of scale that direct integration does not, but there are several obstacles towards indirect integration, such as the plant’s finance and engineering the integration of the two systems.13 This distillation plant will be directly integrated with neighboring plants such as Chevron that produce thermal waste heat.

===Product Treatment===
Since the product is distilled water, it needs additional ions to a concentration of around 0.01% before going to the end user. The three alternatives considered were building a water treatment facility on site, simply adding the correct salts to the product stream and then sending the stream for bacterial treatment at a water plant, or simply sending the product distilled water for complete treatment to another facility. Due to the number of nearby water treatment plants and their demand for water, we elected to simply send the product to a nearby water treatment facility.

==Design Constraints and Tradeoffs==
While designing our process, we wanted to make sure that we met several outside constraints, including environmental, political, ethical, social, sustainability, health and safety constraints. First and most importantly, we wanted to ensure that the brine discharged back to the bay would not harm local marine life. Our process was designed around the constraint of diluting the exiting brine to within 0.2% salinity of background seawater. This is not only compliant with California law, but was also verified to be safe for surrounding marine life through our own outside research. We also addressed many of these considerations by deciding to pursue direct energy integration with the nearby Chevron plant in order to reduce environmental impact of our process. Additionally, we ensured that our plant would be located in an area that is densely populated and in great need of fresh drinking water. Finally, we addressed health and safety considerations by making sure we would be able to send our product distilled water to local treatment plants for proper potabilization.

Desalination - Team A

2016-03-12T02:47:29Z

Ezhuang: /* MSF: Continuous vs. Batch */

Author: Pear Dhiantravan[2016], Reed Kolbe[2016], Sheridan Lichtor[2016], John Marsiglio[2016], Ellen Zhuang[2016]

Instructors: Fengqi You, David Wegerer

Winter 2016

=Introduction=
An increasing global water scarcity is fueling initiatives everywhere for clean water treatment, making efficient seawater desalination an attractive aim for chemical plant design. A 2015 market analysis found that the global desalination market earned revenues of $11.66 billion, and this number is expected to reach over $19 billion by 2019. Furthermore, the 17,000 desalination plants currently in operation are expected to double in number by 2020.1

As fresh water sources become increasingly scarce, strict water conservation measures are being observed. California is entering the fourth year of one of its most severe droughts on record. Cities are required to reduce their water usage by 35% to avoid facing fines.2 In lieu of these pressing conditions, California is looking for ways to provide more accessible fresh water to its citizens. This high demand for clean water motivated our choosing Richmond, CA as our plant location. At Richmond, the desalination plant can convert readily available seawater from the San Francisco Bay into fresh water. We propose to desalinate 50 million gallons of water per day to be sent to water treatment plants for potability. This aim is based on the capacity of the desalination plant in Carlsbad, California, which services a similar population.

The salinity of the water in the San Francisco bay is seasonal. During the dry seasons of summer and fall, salinity is high around 10 PSU (10,000 ppm) because water from the Pacific Ocean flows into the San Francisco Bay. During the wet winter season, fresh water from the rivers flows into the Bay, and salinity drops to around 2 PSU (2,000).3 Designed to meet the “worst case” scenario, our plant will process 398 million gallons of water per day with 10,000 ppm of salt to produce 50 million gallons of water with 1,000 ppm of salt. This gives a yield of 13%.

The desalination plant proposed consists of 18 flash stages in series. This report outlines our investigation of this potential water desalination plant, including design and optimization of the desalination process, an analysis of its energy consumption and environmental impacts, and a study of its economic implications.

=Technical Approach=
For the desalination step of our process, we selected a multistage flash (MSF) distillation process. Not only is MSF distillation a very viable method, but it is also the most common method, currently producing about 60% of the world’s desalinated water. MSF and reverse osmosis (RO) are the two major methods being used in large-scale desalination plants. Both processes require considerable amounts of energy. RO typically has a lower energy demand; however, the high impurity content of the Bay water would frequently necessitate membrane cleaning and/or exchange. Feed going through a RO system requires extensive pretreatment to remove biological organisms and other solids to control the pH and the chemical composition of the water. With the San Francisco Bay as our source of water, MSF is an attractive option because sediment and other large impurities can be separated from the feed before distillation occurs. Additionally, MSF distillation plants can be located near power plants and paired to their waste heat streams to conserve energy. This can reduce energy needs by half or two thirds, making MSF increasingly more practical. Due to the maintenance required in the RO system, the MSF distillation is the better option in terms of operation cost.4

A HYSYS model was built to simulate the MSF distillation section of the process, which spans after seawater pretreatment and before local potable water treatment. The model is used to simulate the distillation steps and to calculate necessary heating, cooling, and input flow rates to produce the required 50 million gallons of distillated water per day. For the simulation, the 398 million gallons per day of seawater feed stream is defined to be composed of 3.31 mol% sodium chloride (NaCl) and 96.69 mol% water. The percentage of NaCl is set to account for other ions and chemicals present after the pretreatment phase. NaCl alone is added to the HYSYS system due to the capabilities of the fluid package used to simulate this process, ElectroNRTL. The feed is set at 22°C, which is the approximate the temperature of the San Francisco Bay year round.

=Process Design=
==Design Overview==
The proposed desalination plant uses MSF distillation to convert the San Francisco Bay water to fresh water. The final process design contains 18 flash stages in series. Overall, 398 million gallons per day of of brackish water is converted to 50 million gallons per day of deionized water, a 13% yield. The production meets the required production rate of 50 million gallons of freshwater per day.

Seawater is taken in from the San Francisco Bay and mixed with the brine recycle stream produced downstream. The feed is then heated through a series of condensers and a heater. Distilled water vapor in each flash stage contacts the tubes carrying the seawater feed and condenses into a liquid stream by exchanging heat with the cooler seawater feed stream. The seawater is sent through a heater to increase its thermal energy before entering the first stage of the MSF. In each flashing stage, some of the water evaporates, leaving the salt behind. The freshwater vapors condense, as described above, and is collected and sent to a water treatment plant. The brine stream is split into the recycle stream and a waste stream, and the waste stream is diluted with fresh seawater before being discharged back into the bay.

A process flow diagram of the process is depicted in Appendix A.

==Pretreatment==
Sediment is removed to prevent solids from plugging the process downstream. Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime, and is controlled for by lowering pH. Acidifying the water will also remove CO2, which is a corrosive gas. To control for oxygen, oxygen scavengers will be included in the system to sequester it. Sodium bisulfate will be used as the scavenger.5

==Condensers (Heat Exchanger Networks)==
In order to conserve and recycle energy, the condensation of the distillate and heating of the seawater feed will be coupled in countercurrent heat exchanger networks. These heat exchanger networks are condensers of the design. The cold feed, which is a mixture of fresh seawater and brine recycle, is fed through multiple heat exchangers in series. Each heat exchanger corresponds to a different evaporation chamber. This process consists of 18 heat exchangers: E-101, E-102, … E-117, and E-118, which correspond to evaporation chambers V-102, V-103, … V-118, and V-119, respectively.

The heat exchangers can be viewed similarly to a shell and tube setup, however there is no actual physical shell. The tubes simply run through the top of the evaporation chambers, where vaporized distilled water in the chamber acts as the shell side fluid. The tube bundles are arranged in long tube configuration and are aligned parallel to the direction of the flashing brine flow in the evaporation chamber. The evaporating brine in an evaporation chamber rise up to the the corresponding heat exchanger and condense on the outside of the tubes, creating collectable pure water. This condensing water is what heats up the seawater in the tubes.

E-101 was used as the basis for the condenser sizing since it has the highest heat requirement, and thus will be the largest heat exchanger. From the HYSYS model, the overall system requires 2.24 x 106 kW of duty. Assuming this energy requirement, 5000 tubes, and 18 m length tubes (the diameter of the evaporation chambers, explained further on the next page), the diameter of each tube was calculated to be 0.4 m. Based on the temperature of the streams and flow, the thickness of the tube is 7 x 10-4 m.

==Brine Heater==
After the seawater exits the final heat exchanger, it enters a brine heater (E-119) where it is brought up to the temperature necessary for the flashing process to begin. This means that the seawater must be overheated compared to stage one, so that the seawater will flash in the evaporation chamber, i.e. release heat and vapor to reach equilibrium with stage conditions. After flowing through the condensers, the seawater enters the brine heater at 96.1 oC and is heated to 120 oC. The overall system requires 2.31 x 106 kW of duty. This is supplied by a total of 4.15 x 106 kg per hour of steam.

==Evaporation Chambers==
When the seawater has been brought to a temperature appropriate for flashing, it enters the first evaporation chamber (V-102). Upon entering the chamber, the water is flashed. Some of the water evaporates, leaving the salt behind. The vapor rises up to the heat exchanger tubes in each evaporation chamber and condenses on the outside into the distilled water product. This condensing water is what heats up the seawater in the tubes discussed above. The seawater that does not evaporate moves to the next stage, which operates at a higher temperature and lower pressure, and the process is repeated. This continues in series through a total of 18 flashing chambers (V-102, V-103, … V-118, and V-119). All of the distillate is collected into one stream and all of the brine is mixed with seawater before being disposed of back in the bay in order to dilute it enough so it does not have an adverse effect on marine life.

Each respective heat exchanger fully condenses the vapor product stream from each stage. However, when all of the individual product streams are consolidated into the final combined product stream, some water exists in the vapor phase due to temperature and pressure changes associated with mixing all of the streams together. Although the vapor product stream from each stage condenses completely across its respective heat exchanger, the total product stream contains some water in the vapor phase due to the temperature change when the mixer sums all the streams. In order to fully condense this stream, which will be sent to local water treatment plans, a heat exchanger is put in place to cool down the product using the process feed stream.

The correlations of El-Dessouky and Ettouney were employed in order to size the flash chambers. They specified that conventional MSF plants that operate at a capacity between 27,000 and 36,000 m3/day use flashing stages of the dimensions 18 x 4 meters.6

==Valves==
The model requires valves in between the evaporation chambers. The seawater streams flow through valves with large pressure drops to flash a portion the liquid into vapor that can then be separated in the vessels. Large pressure drops provide the thermodynamic driving force to vaporize more of the liquid, and are required to achieve high product yield.

==Material of Construction==
Before 1980, the shells and internals of the MSF units were commonly constructed from carbon steel. Seawater, however, corrodes carbon steel. To compensate for this, the thickness of the carbon steel has an additional corrosion allowance, which increases the size and weight of the equipment. After 1980, the stainless steel and duplex stainless steel became increasingly common. Stainless steel metal requires less thickness for equivalent strength and corrosion resistance, which allows for a reduction in the size, weight, and therefore cost of units. Major units in our process will be constructed from stainless steel.6

==Process Alternatives==
In order to ensure that our design is fully optimized, we wanted to consider several alternatives for our process. We looked at alternatives associated with water pretreatment, the distillation process itself and product treatment.

===Water Pretreatment===
Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime. Scaling can be controlled by adjusting pH, temperature, and bicarbonate concentration. We selected acidification to control for process scaling. This is because lowering a process temperature would decrease overall efficiency, and decreasing bicarbonate concentration would be comparatively expensive. Lowering pH would have the added benefit of controlling CO2 concentrations since CO2 is a corrosive gas. To control for corrosion we looked at how to remove oxygen. To target the oxygen we chose an oxygen scavenger. This was chosen over other options such as a deaerator, or a protective coating because a deaerator would have a large capital cost, and the equipment itself would experience corrosion, and a tank coating such as zinc orthophosphate has a mixed effectiveness.7

===MSF: Brine Recycle vs. Once-Through System===
We explored two options for the overall layout of the MSF distillation plant. In the first, the brine passes through the process once; in the second, a portion of the brine is recycled from the last stage to the incoming seawater feed. The largest draw towards implementing a brine recycle is that it would significantly reduce the overall amount of seawater needed to produce the daily target of 50 million gallons per day. This in turn would lower the amount of energy required to run the pump feeding seawater to the process. Another positive is that, with a brine recycle, the amount of water conditioning chemicals that must be added to the seawater feed is reduced. The negative aspects mainly revolve around the fact that the solution throughout the process will be higher in salt concentration. This increase in salinity both raises the likelihood of scaling and corrosion in the plant and raises the amount of seawater that must be added to the final brine to make it safe for discharge back into the environment. Additionally, boiling point of each stage is raised when adding the brine recycle. After weighing both alternatives against one another, we have decided to include a brine recycle in our plant design. Most of the negative aspects of the brine recycle can be combated (e.g. include anti-scaling agents in the feed pretreatment), and the economic benefits of including the brine recycle are very significant.8

===MSF: Continuous vs. Batch===
The distillation step can be done in continuous or batch processes. Batch distillation is more versatile and often used when the product is in small amounts and very high purity. A continuous distillation is more efficient thermodynamically and economically for large amounts of material of constant composition, whereas batch distillation is more effective for small amounts of material of varying compositions. Since the plant is processing large amounts of seawater of relatively constant composition, the distillation system will utilize continuous distillation.9

===MSF: Long Tube vs. Crossed Tube ===
Inside the evaporator stage, the condenser tube bundles may be arranged in either long tube or cross tube design. In the long tube arrangement, the tubes are aligned parallel to the direction of the flashing brine flow. The tubes go from stage to stage without water boxes. In the cross tube arrangement, the tubes are lined perpendicular to the flow of the flashing brine. Water boxes transfer brine between the stages.10 11 While the water boxes in crossed tube bundles allow for more customization, the long tube configuration allows for construction of large unit sizes, as there are manufacturing restrictions with cross tubes. Long tube design can easily accommodate a large number of stages, which lead us to choose long tube for our design.11 12

===MSF: Venting System===
Noncondensable gases exist in the process because of air leakages into stages under vacuum conditions and the release of dissolved gases from the heated brine. These gases are removed by venting the system. An air cooler section concentrates the gas before they are vented, and baffles direct the vapor and condensates out. There are two venting arrangements to consider: series and parallel. The system in series moves gas from one stage directly to the next, and the only loss in the system is vapor in the last stage (since some vapor is extracted with gases). This system is economical, however this configuration increases the amount of noncondensable gases passing from one stage to the next, and disrupting the venting system in one stage will affect the entire system. In parallel venting, gas is released through parallel takeoff pipes at each stage. Although this arrangement allows for venting at any stage to be controlled, the design and maintenance is more complex, and more vapor is lost than in the venting system in series. A third alternative is to use a combination of venting in parallel and in series, which is the common method in industry. For this process, our team chose to do venting in parallel and in series.12

===Additional Energy Source Alternatives===
Energy for the process can also be coupled with renewable energy sources to reduce the desalination plant’s carbon footprint. Energy integration can be direct or indirect. In direct integration, thermal waste heat from industrial sources provide thermal energy or pressure as energy sources. Direct integration is advantageous at the modular level, as it avoids energy losses with electricity conversion. Within indirect integration, there are modular integration and grid/utility-scale integration. Modular integration integrates the plant with wind turbines and other small-scale renewable power generators with mobile deployment. A “hybridization” of renewable power sources with natural gas power generation is also an option, and the coupling of the two provides a stable power source. Indirect renewable energy integration is growing as the cost of renewable energy continues to decreases and provides an economy of scale that direct integration does not, but there are several obstacles towards indirect integration, such as the plant’s finance and engineering the integration of the two systems.13 This distillation plant will be directly integrated with neighboring plants such as Chevron that produce thermal waste heat.

===Product Treatment===
Since the product is distilled water, it needs additional ions to a concentration of around 0.01% before going to the end user. The three alternatives considered were building a water treatment facility on site, simply adding the correct salts to the product stream and then sending the stream for bacterial treatment at a water plant, or simply sending the product distilled water for complete treatment to another facility. Due to the number of nearby water treatment plants and their demand for water, we elected to simply send the product to a nearby water treatment facility.

Desalination - Team A

2016-03-12T02:45:48Z

Ezhuang: /* Water Pretreatment */

Author: Pear Dhiantravan[2016], Reed Kolbe[2016], Sheridan Lichtor[2016], John Marsiglio[2016], Ellen Zhuang[2016]

Instructors: Fengqi You, David Wegerer

Winter 2016

=Introduction=
An increasing global water scarcity is fueling initiatives everywhere for clean water treatment, making efficient seawater desalination an attractive aim for chemical plant design. A 2015 market analysis found that the global desalination market earned revenues of $11.66 billion, and this number is expected to reach over $19 billion by 2019. Furthermore, the 17,000 desalination plants currently in operation are expected to double in number by 2020.1

As fresh water sources become increasingly scarce, strict water conservation measures are being observed. California is entering the fourth year of one of its most severe droughts on record. Cities are required to reduce their water usage by 35% to avoid facing fines.2 In lieu of these pressing conditions, California is looking for ways to provide more accessible fresh water to its citizens. This high demand for clean water motivated our choosing Richmond, CA as our plant location. At Richmond, the desalination plant can convert readily available seawater from the San Francisco Bay into fresh water. We propose to desalinate 50 million gallons of water per day to be sent to water treatment plants for potability. This aim is based on the capacity of the desalination plant in Carlsbad, California, which services a similar population.

The salinity of the water in the San Francisco bay is seasonal. During the dry seasons of summer and fall, salinity is high around 10 PSU (10,000 ppm) because water from the Pacific Ocean flows into the San Francisco Bay. During the wet winter season, fresh water from the rivers flows into the Bay, and salinity drops to around 2 PSU (2,000).3 Designed to meet the “worst case” scenario, our plant will process 398 million gallons of water per day with 10,000 ppm of salt to produce 50 million gallons of water with 1,000 ppm of salt. This gives a yield of 13%.

The desalination plant proposed consists of 18 flash stages in series. This report outlines our investigation of this potential water desalination plant, including design and optimization of the desalination process, an analysis of its energy consumption and environmental impacts, and a study of its economic implications.

=Technical Approach=
For the desalination step of our process, we selected a multistage flash (MSF) distillation process. Not only is MSF distillation a very viable method, but it is also the most common method, currently producing about 60% of the world’s desalinated water. MSF and reverse osmosis (RO) are the two major methods being used in large-scale desalination plants. Both processes require considerable amounts of energy. RO typically has a lower energy demand; however, the high impurity content of the Bay water would frequently necessitate membrane cleaning and/or exchange. Feed going through a RO system requires extensive pretreatment to remove biological organisms and other solids to control the pH and the chemical composition of the water. With the San Francisco Bay as our source of water, MSF is an attractive option because sediment and other large impurities can be separated from the feed before distillation occurs. Additionally, MSF distillation plants can be located near power plants and paired to their waste heat streams to conserve energy. This can reduce energy needs by half or two thirds, making MSF increasingly more practical. Due to the maintenance required in the RO system, the MSF distillation is the better option in terms of operation cost.4

A HYSYS model was built to simulate the MSF distillation section of the process, which spans after seawater pretreatment and before local potable water treatment. The model is used to simulate the distillation steps and to calculate necessary heating, cooling, and input flow rates to produce the required 50 million gallons of distillated water per day. For the simulation, the 398 million gallons per day of seawater feed stream is defined to be composed of 3.31 mol% sodium chloride (NaCl) and 96.69 mol% water. The percentage of NaCl is set to account for other ions and chemicals present after the pretreatment phase. NaCl alone is added to the HYSYS system due to the capabilities of the fluid package used to simulate this process, ElectroNRTL. The feed is set at 22°C, which is the approximate the temperature of the San Francisco Bay year round.

=Process Design=
==Design Overview==
The proposed desalination plant uses MSF distillation to convert the San Francisco Bay water to fresh water. The final process design contains 18 flash stages in series. Overall, 398 million gallons per day of of brackish water is converted to 50 million gallons per day of deionized water, a 13% yield. The production meets the required production rate of 50 million gallons of freshwater per day.

Seawater is taken in from the San Francisco Bay and mixed with the brine recycle stream produced downstream. The feed is then heated through a series of condensers and a heater. Distilled water vapor in each flash stage contacts the tubes carrying the seawater feed and condenses into a liquid stream by exchanging heat with the cooler seawater feed stream. The seawater is sent through a heater to increase its thermal energy before entering the first stage of the MSF. In each flashing stage, some of the water evaporates, leaving the salt behind. The freshwater vapors condense, as described above, and is collected and sent to a water treatment plant. The brine stream is split into the recycle stream and a waste stream, and the waste stream is diluted with fresh seawater before being discharged back into the bay.

A process flow diagram of the process is depicted in Appendix A.

==Pretreatment==
Sediment is removed to prevent solids from plugging the process downstream. Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime, and is controlled for by lowering pH. Acidifying the water will also remove CO2, which is a corrosive gas. To control for oxygen, oxygen scavengers will be included in the system to sequester it. Sodium bisulfate will be used as the scavenger.5

==Condensers (Heat Exchanger Networks)==
In order to conserve and recycle energy, the condensation of the distillate and heating of the seawater feed will be coupled in countercurrent heat exchanger networks. These heat exchanger networks are condensers of the design. The cold feed, which is a mixture of fresh seawater and brine recycle, is fed through multiple heat exchangers in series. Each heat exchanger corresponds to a different evaporation chamber. This process consists of 18 heat exchangers: E-101, E-102, … E-117, and E-118, which correspond to evaporation chambers V-102, V-103, … V-118, and V-119, respectively.

The heat exchangers can be viewed similarly to a shell and tube setup, however there is no actual physical shell. The tubes simply run through the top of the evaporation chambers, where vaporized distilled water in the chamber acts as the shell side fluid. The tube bundles are arranged in long tube configuration and are aligned parallel to the direction of the flashing brine flow in the evaporation chamber. The evaporating brine in an evaporation chamber rise up to the the corresponding heat exchanger and condense on the outside of the tubes, creating collectable pure water. This condensing water is what heats up the seawater in the tubes.

E-101 was used as the basis for the condenser sizing since it has the highest heat requirement, and thus will be the largest heat exchanger. From the HYSYS model, the overall system requires 2.24 x 106 kW of duty. Assuming this energy requirement, 5000 tubes, and 18 m length tubes (the diameter of the evaporation chambers, explained further on the next page), the diameter of each tube was calculated to be 0.4 m. Based on the temperature of the streams and flow, the thickness of the tube is 7 x 10-4 m.

==Brine Heater==
After the seawater exits the final heat exchanger, it enters a brine heater (E-119) where it is brought up to the temperature necessary for the flashing process to begin. This means that the seawater must be overheated compared to stage one, so that the seawater will flash in the evaporation chamber, i.e. release heat and vapor to reach equilibrium with stage conditions. After flowing through the condensers, the seawater enters the brine heater at 96.1 oC and is heated to 120 oC. The overall system requires 2.31 x 106 kW of duty. This is supplied by a total of 4.15 x 106 kg per hour of steam.

==Evaporation Chambers==
When the seawater has been brought to a temperature appropriate for flashing, it enters the first evaporation chamber (V-102). Upon entering the chamber, the water is flashed. Some of the water evaporates, leaving the salt behind. The vapor rises up to the heat exchanger tubes in each evaporation chamber and condenses on the outside into the distilled water product. This condensing water is what heats up the seawater in the tubes discussed above. The seawater that does not evaporate moves to the next stage, which operates at a higher temperature and lower pressure, and the process is repeated. This continues in series through a total of 18 flashing chambers (V-102, V-103, … V-118, and V-119). All of the distillate is collected into one stream and all of the brine is mixed with seawater before being disposed of back in the bay in order to dilute it enough so it does not have an adverse effect on marine life.

Each respective heat exchanger fully condenses the vapor product stream from each stage. However, when all of the individual product streams are consolidated into the final combined product stream, some water exists in the vapor phase due to temperature and pressure changes associated with mixing all of the streams together. Although the vapor product stream from each stage condenses completely across its respective heat exchanger, the total product stream contains some water in the vapor phase due to the temperature change when the mixer sums all the streams. In order to fully condense this stream, which will be sent to local water treatment plans, a heat exchanger is put in place to cool down the product using the process feed stream.

The correlations of El-Dessouky and Ettouney were employed in order to size the flash chambers. They specified that conventional MSF plants that operate at a capacity between 27,000 and 36,000 m3/day use flashing stages of the dimensions 18 x 4 meters.6

==Valves==
The model requires valves in between the evaporation chambers. The seawater streams flow through valves with large pressure drops to flash a portion the liquid into vapor that can then be separated in the vessels. Large pressure drops provide the thermodynamic driving force to vaporize more of the liquid, and are required to achieve high product yield.

==Material of Construction==
Before 1980, the shells and internals of the MSF units were commonly constructed from carbon steel. Seawater, however, corrodes carbon steel. To compensate for this, the thickness of the carbon steel has an additional corrosion allowance, which increases the size and weight of the equipment. After 1980, the stainless steel and duplex stainless steel became increasingly common. Stainless steel metal requires less thickness for equivalent strength and corrosion resistance, which allows for a reduction in the size, weight, and therefore cost of units. Major units in our process will be constructed from stainless steel.6

==Process Alternatives==
In order to ensure that our design is fully optimized, we wanted to consider several alternatives for our process. We looked at alternatives associated with water pretreatment, the distillation process itself and product treatment.

===Water Pretreatment===
Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime. Scaling can be controlled by adjusting pH, temperature, and bicarbonate concentration. We selected acidification to control for process scaling. This is because lowering a process temperature would decrease overall efficiency, and decreasing bicarbonate concentration would be comparatively expensive. Lowering pH would have the added benefit of controlling CO2 concentrations since CO2 is a corrosive gas. To control for corrosion we looked at how to remove oxygen. To target the oxygen we chose an oxygen scavenger. This was chosen over other options such as a deaerator, or a protective coating because a deaerator would have a large capital cost, and the equipment itself would experience corrosion, and a tank coating such as zinc orthophosphate has a mixed effectiveness.7

===MSF: Brine Recycle vs. Once-Through System===
We explored two options for the overall layout of the MSF distillation plant. In the first, the brine passes through the process once; in the second, a portion of the brine is recycled from the last stage to the incoming seawater feed. The largest draw towards implementing a brine recycle is that it would significantly reduce the overall amount of seawater needed to produce the daily target of 50 million gallons per day. This in turn would lower the amount of energy required to run the pump feeding seawater to the process. Another positive is that, with a brine recycle, the amount of water conditioning chemicals that must be added to the seawater feed is reduced. The negative aspects mainly revolve around the fact that the solution throughout the process will be higher in salt concentration. This increase in salinity both raises the likelihood of scaling and corrosion in the plant and raises the amount of seawater that must be added to the final brine to make it safe for discharge back into the environment. Additionally, boiling point of each stage is raised when adding the brine recycle. After weighing both alternatives against one another, we have decided to include a brine recycle in our plant design. Most of the negative aspects of the brine recycle can be combated (e.g. include anti-scaling agents in the feed pretreatment), and the economic benefits of including the brine recycle are very significant.8

===MSF: Continuous vs. Batch===
The distillation step can be done in continuous or batch processes. Batch distillation is more versatile and often used when the product is in small amounts and very high purity. A continuous distillation is more efficient thermodynamically and economically for large amounts of material of constant composition, whereas batch distillation is more effective for small amounts of material of varying compositions. Since the plant is processing large amounts of seawater of relatively constant composition, the distillation system will utilize continuous distillation.9

Desalination - Team A

2016-03-12T02:44:57Z

Ezhuang: /* Process Alternatives */

Author: Pear Dhiantravan[2016], Reed Kolbe[2016], Sheridan Lichtor[2016], John Marsiglio[2016], Ellen Zhuang[2016]

Instructors: Fengqi You, David Wegerer

Winter 2016

=Introduction=
An increasing global water scarcity is fueling initiatives everywhere for clean water treatment, making efficient seawater desalination an attractive aim for chemical plant design. A 2015 market analysis found that the global desalination market earned revenues of $11.66 billion, and this number is expected to reach over $19 billion by 2019. Furthermore, the 17,000 desalination plants currently in operation are expected to double in number by 2020.1

As fresh water sources become increasingly scarce, strict water conservation measures are being observed. California is entering the fourth year of one of its most severe droughts on record. Cities are required to reduce their water usage by 35% to avoid facing fines.2 In lieu of these pressing conditions, California is looking for ways to provide more accessible fresh water to its citizens. This high demand for clean water motivated our choosing Richmond, CA as our plant location. At Richmond, the desalination plant can convert readily available seawater from the San Francisco Bay into fresh water. We propose to desalinate 50 million gallons of water per day to be sent to water treatment plants for potability. This aim is based on the capacity of the desalination plant in Carlsbad, California, which services a similar population.

The salinity of the water in the San Francisco bay is seasonal. During the dry seasons of summer and fall, salinity is high around 10 PSU (10,000 ppm) because water from the Pacific Ocean flows into the San Francisco Bay. During the wet winter season, fresh water from the rivers flows into the Bay, and salinity drops to around 2 PSU (2,000).3 Designed to meet the “worst case” scenario, our plant will process 398 million gallons of water per day with 10,000 ppm of salt to produce 50 million gallons of water with 1,000 ppm of salt. This gives a yield of 13%.

The desalination plant proposed consists of 18 flash stages in series. This report outlines our investigation of this potential water desalination plant, including design and optimization of the desalination process, an analysis of its energy consumption and environmental impacts, and a study of its economic implications.

=Technical Approach=
For the desalination step of our process, we selected a multistage flash (MSF) distillation process. Not only is MSF distillation a very viable method, but it is also the most common method, currently producing about 60% of the world’s desalinated water. MSF and reverse osmosis (RO) are the two major methods being used in large-scale desalination plants. Both processes require considerable amounts of energy. RO typically has a lower energy demand; however, the high impurity content of the Bay water would frequently necessitate membrane cleaning and/or exchange. Feed going through a RO system requires extensive pretreatment to remove biological organisms and other solids to control the pH and the chemical composition of the water. With the San Francisco Bay as our source of water, MSF is an attractive option because sediment and other large impurities can be separated from the feed before distillation occurs. Additionally, MSF distillation plants can be located near power plants and paired to their waste heat streams to conserve energy. This can reduce energy needs by half or two thirds, making MSF increasingly more practical. Due to the maintenance required in the RO system, the MSF distillation is the better option in terms of operation cost.4

A HYSYS model was built to simulate the MSF distillation section of the process, which spans after seawater pretreatment and before local potable water treatment. The model is used to simulate the distillation steps and to calculate necessary heating, cooling, and input flow rates to produce the required 50 million gallons of distillated water per day. For the simulation, the 398 million gallons per day of seawater feed stream is defined to be composed of 3.31 mol% sodium chloride (NaCl) and 96.69 mol% water. The percentage of NaCl is set to account for other ions and chemicals present after the pretreatment phase. NaCl alone is added to the HYSYS system due to the capabilities of the fluid package used to simulate this process, ElectroNRTL. The feed is set at 22°C, which is the approximate the temperature of the San Francisco Bay year round.

=Process Design=
==Design Overview==
The proposed desalination plant uses MSF distillation to convert the San Francisco Bay water to fresh water. The final process design contains 18 flash stages in series. Overall, 398 million gallons per day of of brackish water is converted to 50 million gallons per day of deionized water, a 13% yield. The production meets the required production rate of 50 million gallons of freshwater per day.

Seawater is taken in from the San Francisco Bay and mixed with the brine recycle stream produced downstream. The feed is then heated through a series of condensers and a heater. Distilled water vapor in each flash stage contacts the tubes carrying the seawater feed and condenses into a liquid stream by exchanging heat with the cooler seawater feed stream. The seawater is sent through a heater to increase its thermal energy before entering the first stage of the MSF. In each flashing stage, some of the water evaporates, leaving the salt behind. The freshwater vapors condense, as described above, and is collected and sent to a water treatment plant. The brine stream is split into the recycle stream and a waste stream, and the waste stream is diluted with fresh seawater before being discharged back into the bay.

A process flow diagram of the process is depicted in Appendix A.

==Pretreatment==
Sediment is removed to prevent solids from plugging the process downstream. Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime, and is controlled for by lowering pH. Acidifying the water will also remove CO2, which is a corrosive gas. To control for oxygen, oxygen scavengers will be included in the system to sequester it. Sodium bisulfate will be used as the scavenger.5

==Condensers (Heat Exchanger Networks)==
In order to conserve and recycle energy, the condensation of the distillate and heating of the seawater feed will be coupled in countercurrent heat exchanger networks. These heat exchanger networks are condensers of the design. The cold feed, which is a mixture of fresh seawater and brine recycle, is fed through multiple heat exchangers in series. Each heat exchanger corresponds to a different evaporation chamber. This process consists of 18 heat exchangers: E-101, E-102, … E-117, and E-118, which correspond to evaporation chambers V-102, V-103, … V-118, and V-119, respectively.

The heat exchangers can be viewed similarly to a shell and tube setup, however there is no actual physical shell. The tubes simply run through the top of the evaporation chambers, where vaporized distilled water in the chamber acts as the shell side fluid. The tube bundles are arranged in long tube configuration and are aligned parallel to the direction of the flashing brine flow in the evaporation chamber. The evaporating brine in an evaporation chamber rise up to the the corresponding heat exchanger and condense on the outside of the tubes, creating collectable pure water. This condensing water is what heats up the seawater in the tubes.

E-101 was used as the basis for the condenser sizing since it has the highest heat requirement, and thus will be the largest heat exchanger. From the HYSYS model, the overall system requires 2.24 x 106 kW of duty. Assuming this energy requirement, 5000 tubes, and 18 m length tubes (the diameter of the evaporation chambers, explained further on the next page), the diameter of each tube was calculated to be 0.4 m. Based on the temperature of the streams and flow, the thickness of the tube is 7 x 10-4 m.

==Brine Heater==
After the seawater exits the final heat exchanger, it enters a brine heater (E-119) where it is brought up to the temperature necessary for the flashing process to begin. This means that the seawater must be overheated compared to stage one, so that the seawater will flash in the evaporation chamber, i.e. release heat and vapor to reach equilibrium with stage conditions. After flowing through the condensers, the seawater enters the brine heater at 96.1 oC and is heated to 120 oC. The overall system requires 2.31 x 106 kW of duty. This is supplied by a total of 4.15 x 106 kg per hour of steam.

==Evaporation Chambers==
When the seawater has been brought to a temperature appropriate for flashing, it enters the first evaporation chamber (V-102). Upon entering the chamber, the water is flashed. Some of the water evaporates, leaving the salt behind. The vapor rises up to the heat exchanger tubes in each evaporation chamber and condenses on the outside into the distilled water product. This condensing water is what heats up the seawater in the tubes discussed above. The seawater that does not evaporate moves to the next stage, which operates at a higher temperature and lower pressure, and the process is repeated. This continues in series through a total of 18 flashing chambers (V-102, V-103, … V-118, and V-119). All of the distillate is collected into one stream and all of the brine is mixed with seawater before being disposed of back in the bay in order to dilute it enough so it does not have an adverse effect on marine life.

Each respective heat exchanger fully condenses the vapor product stream from each stage. However, when all of the individual product streams are consolidated into the final combined product stream, some water exists in the vapor phase due to temperature and pressure changes associated with mixing all of the streams together. Although the vapor product stream from each stage condenses completely across its respective heat exchanger, the total product stream contains some water in the vapor phase due to the temperature change when the mixer sums all the streams. In order to fully condense this stream, which will be sent to local water treatment plans, a heat exchanger is put in place to cool down the product using the process feed stream.

The correlations of El-Dessouky and Ettouney were employed in order to size the flash chambers. They specified that conventional MSF plants that operate at a capacity between 27,000 and 36,000 m3/day use flashing stages of the dimensions 18 x 4 meters.6

==Valves==
The model requires valves in between the evaporation chambers. The seawater streams flow through valves with large pressure drops to flash a portion the liquid into vapor that can then be separated in the vessels. Large pressure drops provide the thermodynamic driving force to vaporize more of the liquid, and are required to achieve high product yield.

==Material of Construction==
Before 1980, the shells and internals of the MSF units were commonly constructed from carbon steel. Seawater, however, corrodes carbon steel. To compensate for this, the thickness of the carbon steel has an additional corrosion allowance, which increases the size and weight of the equipment. After 1980, the stainless steel and duplex stainless steel became increasingly common. Stainless steel metal requires less thickness for equivalent strength and corrosion resistance, which allows for a reduction in the size, weight, and therefore cost of units. Major units in our process will be constructed from stainless steel.6

==Process Alternatives==
In order to ensure that our design is fully optimized, we wanted to consider several alternatives for our process. We looked at alternatives associated with water pretreatment, the distillation process itself and product treatment.

===Water Pretreatment===
Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime. Scaling can be controlled by adjusting pH, temperature, and bicarbonate concentration. We selected acidification to control for process scaling. This is because lowering a process temperature would decrease overall efficiency, and decreasing bicarbonate concentration would be comparatively expensive. Lowering pH would have the added benefit of controlling CO2 concentrations since CO2 is a corrosive gas. To control for corrosion we looked at how to remove oxygen. To target the oxygen we chose an oxygen scavenger. This was chosen over other options such as a deaerator, or a protective coating because a deaerator would have a large capital cost, and the equipment itself would experience corrosion, and a tank coating such as zinc orthophosphate has a mixed effectiveness.7

Desalination - Team A

2016-03-12T02:44:20Z

Ezhuang: /* Brine Heater */

Author: Pear Dhiantravan[2016], Reed Kolbe[2016], Sheridan Lichtor[2016], John Marsiglio[2016], Ellen Zhuang[2016]

Instructors: Fengqi You, David Wegerer

Winter 2016

=Introduction=
An increasing global water scarcity is fueling initiatives everywhere for clean water treatment, making efficient seawater desalination an attractive aim for chemical plant design. A 2015 market analysis found that the global desalination market earned revenues of $11.66 billion, and this number is expected to reach over $19 billion by 2019. Furthermore, the 17,000 desalination plants currently in operation are expected to double in number by 2020.1

As fresh water sources become increasingly scarce, strict water conservation measures are being observed. California is entering the fourth year of one of its most severe droughts on record. Cities are required to reduce their water usage by 35% to avoid facing fines.2 In lieu of these pressing conditions, California is looking for ways to provide more accessible fresh water to its citizens. This high demand for clean water motivated our choosing Richmond, CA as our plant location. At Richmond, the desalination plant can convert readily available seawater from the San Francisco Bay into fresh water. We propose to desalinate 50 million gallons of water per day to be sent to water treatment plants for potability. This aim is based on the capacity of the desalination plant in Carlsbad, California, which services a similar population.

The salinity of the water in the San Francisco bay is seasonal. During the dry seasons of summer and fall, salinity is high around 10 PSU (10,000 ppm) because water from the Pacific Ocean flows into the San Francisco Bay. During the wet winter season, fresh water from the rivers flows into the Bay, and salinity drops to around 2 PSU (2,000).3 Designed to meet the “worst case” scenario, our plant will process 398 million gallons of water per day with 10,000 ppm of salt to produce 50 million gallons of water with 1,000 ppm of salt. This gives a yield of 13%.

The desalination plant proposed consists of 18 flash stages in series. This report outlines our investigation of this potential water desalination plant, including design and optimization of the desalination process, an analysis of its energy consumption and environmental impacts, and a study of its economic implications.

=Technical Approach=
For the desalination step of our process, we selected a multistage flash (MSF) distillation process. Not only is MSF distillation a very viable method, but it is also the most common method, currently producing about 60% of the world’s desalinated water. MSF and reverse osmosis (RO) are the two major methods being used in large-scale desalination plants. Both processes require considerable amounts of energy. RO typically has a lower energy demand; however, the high impurity content of the Bay water would frequently necessitate membrane cleaning and/or exchange. Feed going through a RO system requires extensive pretreatment to remove biological organisms and other solids to control the pH and the chemical composition of the water. With the San Francisco Bay as our source of water, MSF is an attractive option because sediment and other large impurities can be separated from the feed before distillation occurs. Additionally, MSF distillation plants can be located near power plants and paired to their waste heat streams to conserve energy. This can reduce energy needs by half or two thirds, making MSF increasingly more practical. Due to the maintenance required in the RO system, the MSF distillation is the better option in terms of operation cost.4

A HYSYS model was built to simulate the MSF distillation section of the process, which spans after seawater pretreatment and before local potable water treatment. The model is used to simulate the distillation steps and to calculate necessary heating, cooling, and input flow rates to produce the required 50 million gallons of distillated water per day. For the simulation, the 398 million gallons per day of seawater feed stream is defined to be composed of 3.31 mol% sodium chloride (NaCl) and 96.69 mol% water. The percentage of NaCl is set to account for other ions and chemicals present after the pretreatment phase. NaCl alone is added to the HYSYS system due to the capabilities of the fluid package used to simulate this process, ElectroNRTL. The feed is set at 22°C, which is the approximate the temperature of the San Francisco Bay year round.

=Process Design=
==Design Overview==
The proposed desalination plant uses MSF distillation to convert the San Francisco Bay water to fresh water. The final process design contains 18 flash stages in series. Overall, 398 million gallons per day of of brackish water is converted to 50 million gallons per day of deionized water, a 13% yield. The production meets the required production rate of 50 million gallons of freshwater per day.

Seawater is taken in from the San Francisco Bay and mixed with the brine recycle stream produced downstream. The feed is then heated through a series of condensers and a heater. Distilled water vapor in each flash stage contacts the tubes carrying the seawater feed and condenses into a liquid stream by exchanging heat with the cooler seawater feed stream. The seawater is sent through a heater to increase its thermal energy before entering the first stage of the MSF. In each flashing stage, some of the water evaporates, leaving the salt behind. The freshwater vapors condense, as described above, and is collected and sent to a water treatment plant. The brine stream is split into the recycle stream and a waste stream, and the waste stream is diluted with fresh seawater before being discharged back into the bay.

A process flow diagram of the process is depicted in Appendix A.

==Pretreatment==
Sediment is removed to prevent solids from plugging the process downstream. Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime, and is controlled for by lowering pH. Acidifying the water will also remove CO2, which is a corrosive gas. To control for oxygen, oxygen scavengers will be included in the system to sequester it. Sodium bisulfate will be used as the scavenger.5

==Condensers (Heat Exchanger Networks)==
In order to conserve and recycle energy, the condensation of the distillate and heating of the seawater feed will be coupled in countercurrent heat exchanger networks. These heat exchanger networks are condensers of the design. The cold feed, which is a mixture of fresh seawater and brine recycle, is fed through multiple heat exchangers in series. Each heat exchanger corresponds to a different evaporation chamber. This process consists of 18 heat exchangers: E-101, E-102, … E-117, and E-118, which correspond to evaporation chambers V-102, V-103, … V-118, and V-119, respectively.

The heat exchangers can be viewed similarly to a shell and tube setup, however there is no actual physical shell. The tubes simply run through the top of the evaporation chambers, where vaporized distilled water in the chamber acts as the shell side fluid. The tube bundles are arranged in long tube configuration and are aligned parallel to the direction of the flashing brine flow in the evaporation chamber. The evaporating brine in an evaporation chamber rise up to the the corresponding heat exchanger and condense on the outside of the tubes, creating collectable pure water. This condensing water is what heats up the seawater in the tubes.

E-101 was used as the basis for the condenser sizing since it has the highest heat requirement, and thus will be the largest heat exchanger. From the HYSYS model, the overall system requires 2.24 x 106 kW of duty. Assuming this energy requirement, 5000 tubes, and 18 m length tubes (the diameter of the evaporation chambers, explained further on the next page), the diameter of each tube was calculated to be 0.4 m. Based on the temperature of the streams and flow, the thickness of the tube is 7 x 10-4 m.

==Brine Heater==
After the seawater exits the final heat exchanger, it enters a brine heater (E-119) where it is brought up to the temperature necessary for the flashing process to begin. This means that the seawater must be overheated compared to stage one, so that the seawater will flash in the evaporation chamber, i.e. release heat and vapor to reach equilibrium with stage conditions. After flowing through the condensers, the seawater enters the brine heater at 96.1 oC and is heated to 120 oC. The overall system requires 2.31 x 106 kW of duty. This is supplied by a total of 4.15 x 106 kg per hour of steam.

==Evaporation Chambers==
When the seawater has been brought to a temperature appropriate for flashing, it enters the first evaporation chamber (V-102). Upon entering the chamber, the water is flashed. Some of the water evaporates, leaving the salt behind. The vapor rises up to the heat exchanger tubes in each evaporation chamber and condenses on the outside into the distilled water product. This condensing water is what heats up the seawater in the tubes discussed above. The seawater that does not evaporate moves to the next stage, which operates at a higher temperature and lower pressure, and the process is repeated. This continues in series through a total of 18 flashing chambers (V-102, V-103, … V-118, and V-119). All of the distillate is collected into one stream and all of the brine is mixed with seawater before being disposed of back in the bay in order to dilute it enough so it does not have an adverse effect on marine life.

Each respective heat exchanger fully condenses the vapor product stream from each stage. However, when all of the individual product streams are consolidated into the final combined product stream, some water exists in the vapor phase due to temperature and pressure changes associated with mixing all of the streams together. Although the vapor product stream from each stage condenses completely across its respective heat exchanger, the total product stream contains some water in the vapor phase due to the temperature change when the mixer sums all the streams. In order to fully condense this stream, which will be sent to local water treatment plans, a heat exchanger is put in place to cool down the product using the process feed stream.

The correlations of El-Dessouky and Ettouney were employed in order to size the flash chambers. They specified that conventional MSF plants that operate at a capacity between 27,000 and 36,000 m3/day use flashing stages of the dimensions 18 x 4 meters.6

==Valves==
The model requires valves in between the evaporation chambers. The seawater streams flow through valves with large pressure drops to flash a portion the liquid into vapor that can then be separated in the vessels. Large pressure drops provide the thermodynamic driving force to vaporize more of the liquid, and are required to achieve high product yield.

==Material of Construction==
Before 1980, the shells and internals of the MSF units were commonly constructed from carbon steel. Seawater, however, corrodes carbon steel. To compensate for this, the thickness of the carbon steel has an additional corrosion allowance, which increases the size and weight of the equipment. After 1980, the stainless steel and duplex stainless steel became increasingly common. Stainless steel metal requires less thickness for equivalent strength and corrosion resistance, which allows for a reduction in the size, weight, and therefore cost of units. Major units in our process will be constructed from stainless steel.6

==Process Alternatives==
In order to ensure that our design is fully optimized, we wanted to consider several alternatives for our process. We looked at alternatives associated with water pretreatment, the distillation process itself and product treatment.

Desalination - Team A

2016-03-12T02:43:07Z

Ezhuang:

Author: Pear Dhiantravan[2016], Reed Kolbe[2016], Sheridan Lichtor[2016], John Marsiglio[2016], Ellen Zhuang[2016]

Instructors: Fengqi You, David Wegerer

Winter 2016

=Introduction=
An increasing global water scarcity is fueling initiatives everywhere for clean water treatment, making efficient seawater desalination an attractive aim for chemical plant design. A 2015 market analysis found that the global desalination market earned revenues of $11.66 billion, and this number is expected to reach over $19 billion by 2019. Furthermore, the 17,000 desalination plants currently in operation are expected to double in number by 2020.1

As fresh water sources become increasingly scarce, strict water conservation measures are being observed. California is entering the fourth year of one of its most severe droughts on record. Cities are required to reduce their water usage by 35% to avoid facing fines.2 In lieu of these pressing conditions, California is looking for ways to provide more accessible fresh water to its citizens. This high demand for clean water motivated our choosing Richmond, CA as our plant location. At Richmond, the desalination plant can convert readily available seawater from the San Francisco Bay into fresh water. We propose to desalinate 50 million gallons of water per day to be sent to water treatment plants for potability. This aim is based on the capacity of the desalination plant in Carlsbad, California, which services a similar population.

The salinity of the water in the San Francisco bay is seasonal. During the dry seasons of summer and fall, salinity is high around 10 PSU (10,000 ppm) because water from the Pacific Ocean flows into the San Francisco Bay. During the wet winter season, fresh water from the rivers flows into the Bay, and salinity drops to around 2 PSU (2,000).3 Designed to meet the “worst case” scenario, our plant will process 398 million gallons of water per day with 10,000 ppm of salt to produce 50 million gallons of water with 1,000 ppm of salt. This gives a yield of 13%.

The desalination plant proposed consists of 18 flash stages in series. This report outlines our investigation of this potential water desalination plant, including design and optimization of the desalination process, an analysis of its energy consumption and environmental impacts, and a study of its economic implications.

=Technical Approach=
For the desalination step of our process, we selected a multistage flash (MSF) distillation process. Not only is MSF distillation a very viable method, but it is also the most common method, currently producing about 60% of the world’s desalinated water. MSF and reverse osmosis (RO) are the two major methods being used in large-scale desalination plants. Both processes require considerable amounts of energy. RO typically has a lower energy demand; however, the high impurity content of the Bay water would frequently necessitate membrane cleaning and/or exchange. Feed going through a RO system requires extensive pretreatment to remove biological organisms and other solids to control the pH and the chemical composition of the water. With the San Francisco Bay as our source of water, MSF is an attractive option because sediment and other large impurities can be separated from the feed before distillation occurs. Additionally, MSF distillation plants can be located near power plants and paired to their waste heat streams to conserve energy. This can reduce energy needs by half or two thirds, making MSF increasingly more practical. Due to the maintenance required in the RO system, the MSF distillation is the better option in terms of operation cost.4

A HYSYS model was built to simulate the MSF distillation section of the process, which spans after seawater pretreatment and before local potable water treatment. The model is used to simulate the distillation steps and to calculate necessary heating, cooling, and input flow rates to produce the required 50 million gallons of distillated water per day. For the simulation, the 398 million gallons per day of seawater feed stream is defined to be composed of 3.31 mol% sodium chloride (NaCl) and 96.69 mol% water. The percentage of NaCl is set to account for other ions and chemicals present after the pretreatment phase. NaCl alone is added to the HYSYS system due to the capabilities of the fluid package used to simulate this process, ElectroNRTL. The feed is set at 22°C, which is the approximate the temperature of the San Francisco Bay year round.

=Process Design=
==Design Overview==
The proposed desalination plant uses MSF distillation to convert the San Francisco Bay water to fresh water. The final process design contains 18 flash stages in series. Overall, 398 million gallons per day of of brackish water is converted to 50 million gallons per day of deionized water, a 13% yield. The production meets the required production rate of 50 million gallons of freshwater per day.

Seawater is taken in from the San Francisco Bay and mixed with the brine recycle stream produced downstream. The feed is then heated through a series of condensers and a heater. Distilled water vapor in each flash stage contacts the tubes carrying the seawater feed and condenses into a liquid stream by exchanging heat with the cooler seawater feed stream. The seawater is sent through a heater to increase its thermal energy before entering the first stage of the MSF. In each flashing stage, some of the water evaporates, leaving the salt behind. The freshwater vapors condense, as described above, and is collected and sent to a water treatment plant. The brine stream is split into the recycle stream and a waste stream, and the waste stream is diluted with fresh seawater before being discharged back into the bay.

A process flow diagram of the process is depicted in Appendix A.

==Pretreatment==
Sediment is removed to prevent solids from plugging the process downstream. Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime, and is controlled for by lowering pH. Acidifying the water will also remove CO2, which is a corrosive gas. To control for oxygen, oxygen scavengers will be included in the system to sequester it. Sodium bisulfate will be used as the scavenger.5

==Condensers (Heat Exchanger Networks)==
In order to conserve and recycle energy, the condensation of the distillate and heating of the seawater feed will be coupled in countercurrent heat exchanger networks. These heat exchanger networks are condensers of the design. The cold feed, which is a mixture of fresh seawater and brine recycle, is fed through multiple heat exchangers in series. Each heat exchanger corresponds to a different evaporation chamber. This process consists of 18 heat exchangers: E-101, E-102, … E-117, and E-118, which correspond to evaporation chambers V-102, V-103, … V-118, and V-119, respectively.

The heat exchangers can be viewed similarly to a shell and tube setup, however there is no actual physical shell. The tubes simply run through the top of the evaporation chambers, where vaporized distilled water in the chamber acts as the shell side fluid. The tube bundles are arranged in long tube configuration and are aligned parallel to the direction of the flashing brine flow in the evaporation chamber. The evaporating brine in an evaporation chamber rise up to the the corresponding heat exchanger and condense on the outside of the tubes, creating collectable pure water. This condensing water is what heats up the seawater in the tubes.

E-101 was used as the basis for the condenser sizing since it has the highest heat requirement, and thus will be the largest heat exchanger. From the HYSYS model, the overall system requires 2.24 x 106 kW of duty. Assuming this energy requirement, 5000 tubes, and 18 m length tubes (the diameter of the evaporation chambers, explained further on the next page), the diameter of each tube was calculated to be 0.4 m. Based on the temperature of the streams and flow, the thickness of the tube is 7 x 10-4 m.

==Brine Heater==
After the seawater exits the final heat exchanger, it enters a brine heater (E-119) where it is brought up to the temperature necessary for the flashing process to begin. This means that the seawater must be overheated compared to stage one, so that the seawater will flash in the evaporation chamber, i.e. release heat and vapor to reach equilibrium with stage conditions. After flowing through the condensers, the seawater enters the brine heater at 96.1 oC and is heated to 120 oC. The overall system requires 2.31 x 106 kW of duty. This is supplied by a total of 4.15 x 106 kg per hour of steam.

Desalination - Team A

2016-03-12T02:42:03Z

Ezhuang:

Author: Pear Dhiantravan[2016], Reed Kolbe[2016], Sheridan Lichtor[2016], John Marsiglio[2016], Ellen Zhuang[2016]

Instructors: Fengqi You, David Wegerer

Winter 2016

=Introduction=
An increasing global water scarcity is fueling initiatives everywhere for clean water treatment, making efficient seawater desalination an attractive aim for chemical plant design. A 2015 market analysis found that the global desalination market earned revenues of $11.66 billion, and this number is expected to reach over $19 billion by 2019. Furthermore, the 17,000 desalination plants currently in operation are expected to double in number by 2020.1

As fresh water sources become increasingly scarce, strict water conservation measures are being observed. California is entering the fourth year of one of its most severe droughts on record. Cities are required to reduce their water usage by 35% to avoid facing fines.2 In lieu of these pressing conditions, California is looking for ways to provide more accessible fresh water to its citizens. This high demand for clean water motivated our choosing Richmond, CA as our plant location. At Richmond, the desalination plant can convert readily available seawater from the San Francisco Bay into fresh water. We propose to desalinate 50 million gallons of water per day to be sent to water treatment plants for potability. This aim is based on the capacity of the desalination plant in Carlsbad, California, which services a similar population.

The salinity of the water in the San Francisco bay is seasonal. During the dry seasons of summer and fall, salinity is high around 10 PSU (10,000 ppm) because water from the Pacific Ocean flows into the San Francisco Bay. During the wet winter season, fresh water from the rivers flows into the Bay, and salinity drops to around 2 PSU (2,000).3 Designed to meet the “worst case” scenario, our plant will process 398 million gallons of water per day with 10,000 ppm of salt to produce 50 million gallons of water with 1,000 ppm of salt. This gives a yield of 13%.

The desalination plant proposed consists of 18 flash stages in series. This report outlines our investigation of this potential water desalination plant, including design and optimization of the desalination process, an analysis of its energy consumption and environmental impacts, and a study of its economic implications.

=Technical Approach=
For the desalination step of our process, we selected a multistage flash (MSF) distillation process. Not only is MSF distillation a very viable method, but it is also the most common method, currently producing about 60% of the world’s desalinated water. MSF and reverse osmosis (RO) are the two major methods being used in large-scale desalination plants. Both processes require considerable amounts of energy. RO typically has a lower energy demand; however, the high impurity content of the Bay water would frequently necessitate membrane cleaning and/or exchange. Feed going through a RO system requires extensive pretreatment to remove biological organisms and other solids to control the pH and the chemical composition of the water. With the San Francisco Bay as our source of water, MSF is an attractive option because sediment and other large impurities can be separated from the feed before distillation occurs. Additionally, MSF distillation plants can be located near power plants and paired to their waste heat streams to conserve energy. This can reduce energy needs by half or two thirds, making MSF increasingly more practical. Due to the maintenance required in the RO system, the MSF distillation is the better option in terms of operation cost.4

A HYSYS model was built to simulate the MSF distillation section of the process, which spans after seawater pretreatment and before local potable water treatment. The model is used to simulate the distillation steps and to calculate necessary heating, cooling, and input flow rates to produce the required 50 million gallons of distillated water per day. For the simulation, the 398 million gallons per day of seawater feed stream is defined to be composed of 3.31 mol% sodium chloride (NaCl) and 96.69 mol% water. The percentage of NaCl is set to account for other ions and chemicals present after the pretreatment phase. NaCl alone is added to the HYSYS system due to the capabilities of the fluid package used to simulate this process, ElectroNRTL. The feed is set at 22°C, which is the approximate the temperature of the San Francisco Bay year round.

=Process Design=
==Design Overview==
The proposed desalination plant uses MSF distillation to convert the San Francisco Bay water to fresh water. The final process design contains 18 flash stages in series. Overall, 398 million gallons per day of of brackish water is converted to 50 million gallons per day of deionized water, a 13% yield. The production meets the required production rate of 50 million gallons of freshwater per day.

Seawater is taken in from the San Francisco Bay and mixed with the brine recycle stream produced downstream. The feed is then heated through a series of condensers and a heater. Distilled water vapor in each flash stage contacts the tubes carrying the seawater feed and condenses into a liquid stream by exchanging heat with the cooler seawater feed stream. The seawater is sent through a heater to increase its thermal energy before entering the first stage of the MSF. In each flashing stage, some of the water evaporates, leaving the salt behind. The freshwater vapors condense, as described above, and is collected and sent to a water treatment plant. The brine stream is split into the recycle stream and a waste stream, and the waste stream is diluted with fresh seawater before being discharged back into the bay.

A process flow diagram of the process is depicted in Appendix A.

==Pretreatment==
Sediment is removed to prevent solids from plugging the process downstream. Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime, and is controlled for by lowering pH. Acidifying the water will also remove CO2, which is a corrosive gas. To control for oxygen, oxygen scavengers will be included in the system to sequester it. Sodium bisulfate will be used as the scavenger.5

==Condensers (Heat Exchanger Networks)==
In order to conserve and recycle energy, the condensation of the distillate and heating of the seawater feed will be coupled in countercurrent heat exchanger networks. These heat exchanger networks are condensers of the design. The cold feed, which is a mixture of fresh seawater and brine recycle, is fed through multiple heat exchangers in series. Each heat exchanger corresponds to a different evaporation chamber. This process consists of 18 heat exchangers: E-101, E-102, … E-117, and E-118, which correspond to evaporation chambers V-102, V-103, … V-118, and V-119, respectively.

The heat exchangers can be viewed similarly to a shell and tube setup, however there is no actual physical shell. The tubes simply run through the top of the evaporation chambers, where vaporized distilled water in the chamber acts as the shell side fluid. The tube bundles are arranged in long tube configuration and are aligned parallel to the direction of the flashing brine flow in the evaporation chamber. The evaporating brine in an evaporation chamber rise up to the the corresponding heat exchanger and condense on the outside of the tubes, creating collectable pure water. This condensing water is what heats up the seawater in the tubes.

E-101 was used as the basis for the condenser sizing since it has the highest heat requirement, and thus will be the largest heat exchanger. From the HYSYS model, the overall system requires 2.24 x 106 kW of duty. Assuming this energy requirement, 5000 tubes, and 18 m length tubes (the diameter of the evaporation chambers, explained further on the next page), the diameter of each tube was calculated to be 0.4 m. Based on the temperature of the streams and flow, the thickness of the tube is 7 x 10-4 m.

Desalination - Team A

2016-03-12T02:40:44Z

Ezhuang:

Author: Pear Dhiantravan[2016], Reed Kolbe[2016], Sheridan Lichtor[2016], John Marsiglio[2016], Ellen Zhuang[2016]

Instructors: Fengqi You, David Wegerer

Winter 2016

=Introduction=
An increasing global water scarcity is fueling initiatives everywhere for clean water treatment, making efficient seawater desalination an attractive aim for chemical plant design. A 2015 market analysis found that the global desalination market earned revenues of $11.66 billion, and this number is expected to reach over $19 billion by 2019. Furthermore, the 17,000 desalination plants currently in operation are expected to double in number by 2020.1

As fresh water sources become increasingly scarce, strict water conservation measures are being observed. California is entering the fourth year of one of its most severe droughts on record. Cities are required to reduce their water usage by 35% to avoid facing fines.2 In lieu of these pressing conditions, California is looking for ways to provide more accessible fresh water to its citizens. This high demand for clean water motivated our choosing Richmond, CA as our plant location. At Richmond, the desalination plant can convert readily available seawater from the San Francisco Bay into fresh water. We propose to desalinate 50 million gallons of water per day to be sent to water treatment plants for potability. This aim is based on the capacity of the desalination plant in Carlsbad, California, which services a similar population.

The salinity of the water in the San Francisco bay is seasonal. During the dry seasons of summer and fall, salinity is high around 10 PSU (10,000 ppm) because water from the Pacific Ocean flows into the San Francisco Bay. During the wet winter season, fresh water from the rivers flows into the Bay, and salinity drops to around 2 PSU (2,000).3 Designed to meet the “worst case” scenario, our plant will process 398 million gallons of water per day with 10,000 ppm of salt to produce 50 million gallons of water with 1,000 ppm of salt. This gives a yield of 13%.

The desalination plant proposed consists of 18 flash stages in series. This report outlines our investigation of this potential water desalination plant, including design and optimization of the desalination process, an analysis of its energy consumption and environmental impacts, and a study of its economic implications.

=Technical Approach=
For the desalination step of our process, we selected a multistage flash (MSF) distillation process. Not only is MSF distillation a very viable method, but it is also the most common method, currently producing about 60% of the world’s desalinated water. MSF and reverse osmosis (RO) are the two major methods being used in large-scale desalination plants. Both processes require considerable amounts of energy. RO typically has a lower energy demand; however, the high impurity content of the Bay water would frequently necessitate membrane cleaning and/or exchange. Feed going through a RO system requires extensive pretreatment to remove biological organisms and other solids to control the pH and the chemical composition of the water. With the San Francisco Bay as our source of water, MSF is an attractive option because sediment and other large impurities can be separated from the feed before distillation occurs. Additionally, MSF distillation plants can be located near power plants and paired to their waste heat streams to conserve energy. This can reduce energy needs by half or two thirds, making MSF increasingly more practical. Due to the maintenance required in the RO system, the MSF distillation is the better option in terms of operation cost.4

A HYSYS model was built to simulate the MSF distillation section of the process, which spans after seawater pretreatment and before local potable water treatment. The model is used to simulate the distillation steps and to calculate necessary heating, cooling, and input flow rates to produce the required 50 million gallons of distillated water per day. For the simulation, the 398 million gallons per day of seawater feed stream is defined to be composed of 3.31 mol% sodium chloride (NaCl) and 96.69 mol% water. The percentage of NaCl is set to account for other ions and chemicals present after the pretreatment phase. NaCl alone is added to the HYSYS system due to the capabilities of the fluid package used to simulate this process, ElectroNRTL. The feed is set at 22°C, which is the approximate the temperature of the San Francisco Bay year round.

=Process Design=
==Design Overview==
The proposed desalination plant uses MSF distillation to convert the San Francisco Bay water to fresh water. The final process design contains 18 flash stages in series. Overall, 398 million gallons per day of of brackish water is converted to 50 million gallons per day of deionized water, a 13% yield. The production meets the required production rate of 50 million gallons of freshwater per day.

Seawater is taken in from the San Francisco Bay and mixed with the brine recycle stream produced downstream. The feed is then heated through a series of condensers and a heater. Distilled water vapor in each flash stage contacts the tubes carrying the seawater feed and condenses into a liquid stream by exchanging heat with the cooler seawater feed stream. The seawater is sent through a heater to increase its thermal energy before entering the first stage of the MSF. In each flashing stage, some of the water evaporates, leaving the salt behind. The freshwater vapors condense, as described above, and is collected and sent to a water treatment plant. The brine stream is split into the recycle stream and a waste stream, and the waste stream is diluted with fresh seawater before being discharged back into the bay.

A process flow diagram of the process is depicted in Appendix A.

==Pretreatment==
Sediment is removed to prevent solids from plugging the process downstream. Scaling of the process units leads to decreased efficiency and more frequent unplanned downtime, and is controlled for by lowering pH. Acidifying the water will also remove CO2, which is a corrosive gas. To control for oxygen, oxygen scavengers will be included in the system to sequester it. Sodium bisulfate will be used as the scavenger.5

Desalination - Team A

2016-03-12T02:39:43Z

Ezhuang:

Author: Pear Dhiantravan[2016], Reed Kolbe[2016], Sheridan Lichtor[2016], John Marsiglio[2016], Ellen Zhuang[2016]

Instructors: Fengqi You, David Wegerer

Winter 2016

=Introduction=
An increasing global water scarcity is fueling initiatives everywhere for clean water treatment, making efficient seawater desalination an attractive aim for chemical plant design. A 2015 market analysis found that the global desalination market earned revenues of $11.66 billion, and this number is expected to reach over $19 billion by 2019. Furthermore, the 17,000 desalination plants currently in operation are expected to double in number by 2020.1

As fresh water sources become increasingly scarce, strict water conservation measures are being observed. California is entering the fourth year of one of its most severe droughts on record. Cities are required to reduce their water usage by 35% to avoid facing fines.2 In lieu of these pressing conditions, California is looking for ways to provide more accessible fresh water to its citizens. This high demand for clean water motivated our choosing Richmond, CA as our plant location. At Richmond, the desalination plant can convert readily available seawater from the San Francisco Bay into fresh water. We propose to desalinate 50 million gallons of water per day to be sent to water treatment plants for potability. This aim is based on the capacity of the desalination plant in Carlsbad, California, which services a similar population.

The salinity of the water in the San Francisco bay is seasonal. During the dry seasons of summer and fall, salinity is high around 10 PSU (10,000 ppm) because water from the Pacific Ocean flows into the San Francisco Bay. During the wet winter season, fresh water from the rivers flows into the Bay, and salinity drops to around 2 PSU (2,000).3 Designed to meet the “worst case” scenario, our plant will process 398 million gallons of water per day with 10,000 ppm of salt to produce 50 million gallons of water with 1,000 ppm of salt. This gives a yield of 13%.

The desalination plant proposed consists of 18 flash stages in series. This report outlines our investigation of this potential water desalination plant, including design and optimization of the desalination process, an analysis of its energy consumption and environmental impacts, and a study of its economic implications.

=Technical Approach=
For the desalination step of our process, we selected a multistage flash (MSF) distillation process. Not only is MSF distillation a very viable method, but it is also the most common method, currently producing about 60% of the world’s desalinated water. MSF and reverse osmosis (RO) are the two major methods being used in large-scale desalination plants. Both processes require considerable amounts of energy. RO typically has a lower energy demand; however, the high impurity content of the Bay water would frequently necessitate membrane cleaning and/or exchange. Feed going through a RO system requires extensive pretreatment to remove biological organisms and other solids to control the pH and the chemical composition of the water. With the San Francisco Bay as our source of water, MSF is an attractive option because sediment and other large impurities can be separated from the feed before distillation occurs. Additionally, MSF distillation plants can be located near power plants and paired to their waste heat streams to conserve energy. This can reduce energy needs by half or two thirds, making MSF increasingly more practical. Due to the maintenance required in the RO system, the MSF distillation is the better option in terms of operation cost.4

A HYSYS model was built to simulate the MSF distillation section of the process, which spans after seawater pretreatment and before local potable water treatment. The model is used to simulate the distillation steps and to calculate necessary heating, cooling, and input flow rates to produce the required 50 million gallons of distillated water per day. For the simulation, the 398 million gallons per day of seawater feed stream is defined to be composed of 3.31 mol% sodium chloride (NaCl) and 96.69 mol% water. The percentage of NaCl is set to account for other ions and chemicals present after the pretreatment phase. NaCl alone is added to the HYSYS system due to the capabilities of the fluid package used to simulate this process, ElectroNRTL. The feed is set at 22°C, which is the approximate the temperature of the San Francisco Bay year round.

Desalination - Team A

2016-03-12T02:39:00Z

Ezhuang: /* Introduction */

Author: Pear Dhiantravan[2016], Reed Kolbe[2016], Sheridan Lichtor[2016], John Marsiglio[2016], Ellen Zhuang[2016]

Instructors: Fengqi You, David Wegerer

Winter 2016

=Introduction=
An increasing global water scarcity is fueling initiatives everywhere for clean water treatment, making efficient seawater desalination an attractive aim for chemical plant design. A 2015 market analysis found that the global desalination market earned revenues of $11.66 billion, and this number is expected to reach over $19 billion by 2019. Furthermore, the 17,000 desalination plants currently in operation are expected to double in number by 2020.1

As fresh water sources become increasingly scarce, strict water conservation measures are being observed. California is entering the fourth year of one of its most severe droughts on record. Cities are required to reduce their water usage by 35% to avoid facing fines.2 In lieu of these pressing conditions, California is looking for ways to provide more accessible fresh water to its citizens. This high demand for clean water motivated our choosing Richmond, CA as our plant location. At Richmond, the desalination plant can convert readily available seawater from the San Francisco Bay into fresh water. We propose to desalinate 50 million gallons of water per day to be sent to water treatment plants for potability. This aim is based on the capacity of the desalination plant in Carlsbad, California, which services a similar population.

The salinity of the water in the San Francisco bay is seasonal. During the dry seasons of summer and fall, salinity is high around 10 PSU (10,000 ppm) because water from the Pacific Ocean flows into the San Francisco Bay. During the wet winter season, fresh water from the rivers flows into the Bay, and salinity drops to around 2 PSU (2,000).3 Designed to meet the “worst case” scenario, our plant will process 398 million gallons of water per day with 10,000 ppm of salt to produce 50 million gallons of water with 1,000 ppm of salt. This gives a yield of 13%.

The desalination plant proposed consists of 18 flash stages in series. This report outlines our investigation of this potential water desalination plant, including design and optimization of the desalination process, an analysis of its energy consumption and environmental impacts, and a study of its economic implications.

Desalination - Team A

2016-03-12T02:37:53Z

Ezhuang:

Desalination - Team A

2016-03-12T02:37:45Z

Ezhuang:

Desalination - Team A

2016-03-12T02:35:56Z

Ezhuang: Created page with "=Introduction="

=Introduction=

Process location and layout decisions

2016-02-22T02:44:41Z

Ezhuang: /* Conclusion */

Author: Ellen Zhuang [2016]

Stewards: Daniel Garcia, and Fengqi You

=Introduction=
Location is one of the first decisions in the design of a new chemical plant. It impacts profitability and scope for future expansion. If the project is a new facility, a suitable site must be found and an optimal layout of the site and process units must be planned. If the project adds to an existing site, the impact of the new addition on the existing plant must be considered. The plant also needs to accommodate for the nearby infrastructure, the services that it requires, and its environmental impacts.

=Location Selection=
===Location Decision Factors===
Initially, economists viewed the plant location decision as a cost-minimization problem. The optimal location was one where the transportation costs of raw material to the plant and product to the market was minimized. Between the early 1900s and post World War II period, most industries were sensitive to the cost of transportation. As transportation costs became less of an obstacle, the approach to the problem developed with considerations of trade-offs between transportation and factors such as wages, energy, local regulations, etc. If lower wages could balance the higher transportation costs of building the plant in a low-wage area, the low-wage location may be more desirable.

Although location is a long-term investment, a firm does not decide on a location with the sole objective of maximizing its profits or minimizing its costs. Managers may choose a “safer” location that is more likely to produce higher profits rather than a riskier location that yields the maximum investment return. Personal factors of the people involved are also influences. For example, new businesses tend to locate where the founders live.2

In the current age, a plant's site is chosen based on several factors. These include:1,2,3,8

'''1. Raw material supply:''' The source and price of raw materials is one of the most important factors that determine the location of a plant. Facilities that produce chemicals in bulk are usually located close to the source of raw material if the costs of shipping the product is less than the costs of shipping the feed. For example, ethylene production is growing in the Middle East since cheap ethane from natural gas is readily available. Oil refineries tend to be located near areas with high population and crude oil supplies since it is expensive to transport the oil.

'''2. Location with respect to market:''' If the plant produces high-volume and low-cost products, such as cement and fertilizer, it may be better to situate the plant closer to the primary market since transportation cost is a large fraction of the sales price. If the product is low-volume and high-cost, like pharmaceuticals, then the benefits of being closer to the primary market may not be there.

'''3. Transport facilities:''' Facilities should be close to at least two major forms of transportation, whether that be road, rail, waterway, and/or seaport. For example, paper manufacturing plants in the US use various types of pulp that are delivered by truck or by train from various places in North and South America, so paper facilities need to be close to rail and major roads. Transportation by road is common for local distribution from central warehouses, while transportation by rail is more widespread for long distance transport of bulk chemicals. Pipeline is used to ship industrial gases and bulk fuels. Air freight can be efficient for shipment of personnel and essential units and supplies and for small volume products that have high value, such as pharmaceuticals. Of course, products that are delivered by air must meet aviation regulations.

'''4. Availability of labor:''' Skilled workers are usually brought to the plant from outside the area. There should be a local pool of unskilled labor that can be trained to operate the plant, and of skilled craft workers to maintain the process units. Local labor laws, trade union customs, restrictive practices for recruitment and training should also be taken into consideration. A 10% increase in unionization of a state's labor force is projected to reduce the number of expanding facilities by 30 to 45%.

'''5. Availability of utilities:''' Processes that require a substantial amount of cooling water is usually located near water sources, such as rivers or wells. Cooling water may be directly taken from the water source, or may be stored in cooling towers. Those that need large quantities of power, such as electrochemical ones, are typically close to cheap power sources.

'''6. Availability of suitable land:''' The ideal land is flat, well-drained, with suitable load-bearing characteristics. Further considerations have to be made if the land is reclaimed land near the ocean in earthquake zones. Property tax is also a factor when choosing a site since property taxes vary area to area. Under a third of plants that relocate move to regions with lower property taxes, which is the proportion that would be expected if companies move to a new location regardless of property tax. High property taxes is not as significant as other factors such as labor supply and land costs.

'''7. Environmental impact:''' Depending on the location, it may be more difficult and costly to dispose of wastes. During the project design phase, experts are typically consulted to learn more about an area's local regulations. More details about environmental regulations are found below.

'''8. Local community considerations:''' State and local planners are typically motivated by the desire to create jobs and improve the tax base. Introduction of facilities to an area is usually viewed as the most direct way to stimulate the area's economy. However, recent studies have found that communities with high-growth are already characterized by the fast growth of businesses that are already there. It is rare for a plant to completely close in one area and relocate to another, and if plants do relocate, the majority is over short distances and often within the same community. Therefore, local policymakers favor the expansion of existing plants. The opening of a new plant at a location should impost no additional risk to the local residents. For example, they should be downwind of the residential areas. Local communities also need to be able to accommodate the plant personnelles. For example, traffic, housing, and facilities must be able to accommodate the influx of workers. Additional factors are property taxes and water consumption.

'''9. Climate:''' The climate of the area may affect processes and costs. For example, plants in cold areas need more insulation and special heating. Facilities in earthquake areas need to be seismically sound. Plants in areas with high ambient humidity will usually use air cooling instead of water cooling. More detailed information about the effects of weather can be found in the [[Site condition and design]] page.

'''10. Political and strategic considerations:''' Government sometimes gives capital grants, tax concessions, and other incentives to encourage plants to be built in specific areas. Physical assistance such as roads, water, and other public infrastructure are more popular than financial assistance. Companies can also globalize and take advantage of areas with preferential tariff agreements. The tax policy of an area is inversely related to growth. High personal income sometimes hinder employment growth. Personal income does not affect the cash flow of the company, but it reduces the after-tax income of its managers, and thus high personal income tax can be classified as a personal region. High state corporate taxes has also and detrimental effects on growth, but that is not always the case. Corporate tax is more important to firms with high capital expenditures.

===Analysis of the Importance of Factors===
In 1963, Morgan surveyed 17 companies and found that on average the most important factors on industrial location decisions are market, labor, and raw material. Taxes and financial incentives were of little significance. More recently, Fortune magazine surveyed among the 1000 largest US companies and found that the most important factors were market and labor. The survey also found that personal preferences of a firm's executive, tax, and central cities have great influence over the site of corporate headquarters. Surveys of interstate locational decisions found that the factors that played key roles for new firms were access to customers and the growing market, labor force, transportation, personal reasons of management, and availability of capital. The cost of land had the least important influence. For firms that expanded across states, the influential factors were labor costs and labor supply. Companies seeking expansion usually have made their production more routine, so quick access to suppliers is less of a concern. Cost minimization becomes more relevant to existing corporations.2

===Site Selection Process===
The decision for the location of a facility is part of a larger corporate planning process. Usually, a corporate planning office or a division of the company initiates the site selection process by forecasting future capacity requirements. If capacity shortages are in the forecasts, the managers may choose to outsource, increase price to reduce the demand, expand existing sites, or open a new facility. If the managers decide on a new facility, the site selection team enters the project.

The structure of the site selection team depends on the firm's organization. In companies with centralized staff, the site selection team generally consists of representatives from relevant areas, such as engineering, real estate, and transportation. In companies with strong divisions, the locational decisions may be done at the divisional level, with the corporate office supervising the process.

The site selection team determines what characteristics are important for the new location by considering how the new facility will fit in the company's overall strategy, if the company wants to target new markets, if the corporation wants to divide or integrate its functions, or how the company wants to be seen by the public. Next, potential locations are listed and studied against the desired characteristics.

Locational decisions are typically made sequentially. The first step is at the state or regional level. Then, the team studies specific communities and sites. Different locational factors are important to different stages. When selecting a general region, the site selection team focuses on factors with interregional variations such as labor, tax policies, climate, and market locations. At the more focused stage, details like inexpensive land, access to to major roads, and good schools are important. Consultants are often hired to do site analyses.

Once site options are narrowed down, the company discusses potential problems and incentives with local public officials. Construction costs are estimated, and a feasibility analysis is done to show that the project has a high rate of return.2

===Biofuel Supply Chain Example===
Biofuels is a popular potential alternative to fossil fuels. Various biomass resources, including food crops, non-food crops, and agricultural residues, are converted into biofuels. The two most common biofuels currently are ethanol and biodiesel. The supply chain for biofuel is a network consisting of several nodes: biomass cultivation sites, biofuel production plants, and demand centers. The locations of these facilities, and the location of the demand centers with respect to the biofuel demand impact transportation costs, production resource, demand, etc., and thus affects and sets constraints to the cost minimization of the supply chain. The optimal supply chain design here minimizes the total cost, which is a sum of the total investment cost, production cost, transportation cost, and outsourcing and import cost. The total transportation cost captures the transportation of biomass and biofuel between areas and the local bio mass transfer. Location of the facilities also sets constraints on the regional demand of biofuel and availability of local biomass. Sustainability constraints ensure that plants do not negatively affect the food production, sustainably use the land, and don't compete with other industries that use biomass.4

[[File:biofuelsupplychain.png|400px|thumb|center|'''Figure 1.''' The biofuels supply chain consists of three notes: biomass cultivation sites, biofuel production plants, and biofuel demand centers.1]]

[[File:Biofueloptimal.png|300px|thumb|center|'''Figure 2.''' An example of a Great Britain biofuel supply chain design that minimizes cost. The plants are near areas with high raw material supply. The locations of the plants with respect to the demand centers are also optimized. The main modes of transportation are road, rail, and ship.4]]

=Site Layout=
The process units and buildings need to be arranged in such a way that allows for the most economical flow of materials and people. Dangerous processes need to be a safe distance from other buildings, and the layout should be planned to allow for future expansion.

Process units are usually laid out first in an arrangement that allows for smooth flow of materials between the process steps. The distance between equipment is usually at least 30 m. Next, the location of the principal ancillary buildings are sited as to minimize the time that it takes the workers to travel between buildings. Administrative offices and laboratories are located away from hazardous processes. Control rooms are next to the processing equipment. Utility buildings are located as to minimize piping between the process units. Storage is placed between the loading and unloading facilities and next to the process units that they serve. Tanks containing hazardous material are placed at least 70 m from the plant. An example of a typical site plan is shown below.1

[[File:Site Location.png|400px|thumb|center|'''Figure 3.''' A typical site layout.1]]

===Site Layout Factors===
The main factors that are considered when planning the layout of the plant are listed below.1

'''1. Economic considerations (construction and operating costs):'''
Construction costs can be minimized by arranging process units and buildings that minimize pipes between equipment, the amount of structural steel work, etc. However, this layout may conflict with the layout that gives the optimal operation and maintenance.

'''2. Process requirements:''' Examples of process considerations that must be taken into account is the elevation of the base of columns to give enough net positive suction head to a pump.

'''3. Operation convenience:''' Process units that are attended to frequently should be placed with convenient access. Valves, heads, and sample heads should be placed where operators can easily access. If the plant anticipates replacement of equipment, space must be allowed for removal and installation.

'''4. Maintenance convenience:''' Equipment that requires maintenance should be in a location with easy access, and should have sufficient space for the maintenance tasks. For example, shell-and-tube exchangers need space so that tube bundles can be removed for cleaning and repair.

'''5. Future expansion:''' The layout should be planned to conveniently allow for future expansion of processes. Pipe racks should have space for future piping, and pipes should be oversized to allow for more flow in the future.

'''6. Modular Construction:''' Modular construction is where sections of the plant is constructed outside of the plant, and then transported to the site by road or sea. Advantages include improved quality control, reduced construction costs, less requirements for skilled labor on site. Tradeoffs are more flanged connections and possible problems with onsite assembly.

'''7. Safety:''' Escape routes for workers need to be in place at each level in process buildings. Blast walls must isolate equipment that pose hazards to confine potential explosions.

First, a conceptual flowsheet for the process is developed. The types of equipment and their connections with each other is described in a process flow diagram (PFD). Before the PFD is translated into detailed piping and instrumentation diagrams (P&ID) the layout of the process units must be planned.3,5 Scale drawings are made to show the relationships between storage space and process equipment based on the flow of materials and people, and on future expansion. Three-dimensional visualization are the layouts are then carried out with cardboard cutouts of the equipment outlines or rectangular and cylindrical blocks. When a layout of the major process units has been decided, drawings of the plan and elevation are made, and design of the structural steelwork and foundations are done. Computer-aided design has also become increasingly popular.1,3,7

=Environmental Considerations=
Laws that protect the environment restrict the waste that plants can emit in order to preserve air, land, and water quality. States, provinces, and municipals usually have additional laws to national ones. In the US, environmental policies are moving towards more uniform and strict regulations. Businesses that produce more pollution are more likely to move to regions where local regulations are more lax, and industries in areas with more stringent policies will decline. The tighter national regulations are implemented to prevent this competition between regions. For example, amendments to the Clean Air Act and the Environmental Protection Agencies in the 1970s and 1980s reduced differences in regions' air pollution controls. Congress voted on the PSD (prevention of significant deterioration) to "level the playing field" across regions.1

Some of the main environmental legislations in North America are:

*'''The National Environmental Policy Act of 1969 (NEPA)''' - Signed by President Nixon, NEPA was one of the first laws that provide a national framework for the protection of the environment. It requires that all executive federal agencies consider the environmental impact of proposed actions. Government agencies must prepare environmental assessments and environmental impact statements that describe potential environmental effects. NEPA is implemented when buildings, airports, military complexes, parkland purchases, and highways are proposed.6

*'''The Clean Air Act (CAA, 1970)''' - The CAA was implemented to control air quality on a national level. The law was amended in 1990 to address issues such as acid rain, ozone depletion, and toxic air pollution, and to increase enforcement authority. Under the National Ambient Air Quality Standards (NAAQS), the law sets limits to allowable ambient levels of seven contaminants: ozone, carbon monoxide, lead, nitrogen dioxide, sulfur dioxide, PM10, and PM2.5. The CAA also requires that the EPA set US National Emissions Standards for Hazardous Air Pollutants (NESHAP), which regulates the emission of 189 hazardous air pollutants. The Maximum Available Control Technology (MACT) was implemented to limit the emission of volatile organics, sulfur, and other inorganic compounds from oil refineries. The CAA has had significant impact on the level of air pollutants that it regulates. Particularly in the Midwest and the Northwest, water acidity has been reduced. On the other hand, reduction of ozone damage has not been as successful since nitrogen oxide, which is linked to ozone formation, is not as tightly regulated.

*'''The Federal Water Pollution Control Act (The Clean Water Act, 1972)''' - The Clean Water Act was initially passed to clean water used for swimming and boating, and to protect wildlife. Amendments were later added to address water quality and toxic compound emissions. Under the CWA, the EPA can set water quality standards and monitor the industries' discharge into waters.

*'''The Safe Drinking Water Act (SDWA, 1974)''' - This act allows the EPA to set standards to potential drinking water. Public water systems must operate according to these standards.

*'''The Resource Conservation and Recovery Act (RCRA, 1976)''' - The RCRA protects current and future groundwater facilities from contamination. Firms that produce waste must have a "cradle to grave" approach to managing waste, meaning that the waste producer is legally responsible for the waste from the moment it is produced until it is finally disposed.

*'''The Comprehensive Environmental Response, Compensation and Liability Act (CERCLA or Superfund, 1980)''' - This legislation creates a tax on chemical and petroleum industries in order to cover the remediation of hazardous waste sites that are uncontrolled or abandoned. The CERCLA empowers EPA to remediate waste sites by short-term removal of material or long-term remediation actions, depending on the risk that the waste imposes on life.

*'''The Pollution Prevention Act (PPA, 1990)''' - This act promotes minimization of waste and pollution at their source. It encourages more efficient processing and raw materials and recycling through grants, technical assistant, and information.

*'''The Oil Pollution Act of 1990 (OPA, 1990)''' - This act was passed after the Exxon Valdez oil spill in the Prince William Sound in Alaska, creating a tax on oil to cover the cost of responding to spills when the corporation that creates the spill does not respond.

*'''The Department of the Environment Act (E-10)''' - The Department of Environment Act created the Department of Environment and charges the Minister of the Environment with creating programs that protect the environment, assessing environmental impacts of federal program, and reporting information to the public in Canada.

*'''The Canadian Environmental Protection Act (CEPA, C-15.31, 1999)''' - CEPA empowers Environment Canada to control toxic substances, reduce pollution, and reduce bioaccumulating toxic substances. Canada will use CEPA to implement regulations that helps it meet its commitments to the Kyoto Protocol.

*'''The Canada Water Act (C-11)''' - This law allows the Minister of the Environment to manage water resources in Canada. This includes research, implementing conservation programs, utilization of water resources, and setting water quality standards.

Wastes comprise mainly of by-products, unused reactants, and off-specification product produced by misoperation. Leaking seals and flanges, and spills and discharges also emit waste. Material is also discharged in emergency situations. Instead of considering how to treat or manage waste, designers should start by tackling the source and find ways to minimize the production of waste.

The process of designing waste management systems are: (1) source reduction, (2) waste stream recycle, (3) waste treatment to reduce environmental impact, and (4) disposal that is legally sound. Source reduction can be achieved by reducing the concentration of impurities in the feed, protecting catalysts and adsorbents from contaminants, eliminating the use of extraneous materials, increasing recovery from separation, and improving the quality of the fuel by switching to cleaner-burning fuel. Unused feed can be recycled, and off-specification products can be reprocessed. In integral processes, the waste of one process is used as the feed for another. By-products can also be sold to another company for use as raw material. Tighter control systems, alarm, and interlocks can reduce misoperation of process unit.1

Further information about environmental considerations can be found in the [[Environmental concerns]] page.

=Conclusion=
The location and layout of a plant can greatly impact its economic and operational success. Objectives such as cost minimization, efficient material distribution, room for expansion, and safety of the plant operators and local community play important roles in the site decision. The major factors affecting this decision are market and labor. National and regional environmental legislations impact the processes, and thus make some areas more desirable than others. Least important are land costs and financial incentives. When planning to build a new facility or expanding an existing site, companies utilize their site selection team to assemble a list of desirable locational characteristics and determine how the new facility will fit in with the overall company. Then with the help of consultants, a location is narrowed down and further analyses are done to justify the continuation of the project. The location should be obtained before design of the details of the process. Within the site itself, process units are arranged to allow for the most economical flow of people and material, optimal safety, and room for future expansion.

=References=
[1] G.P. Towler, R. Sinnott, ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design'', Elsevier, 2012.

[2] J.P. Blair, R. Premus. Major Factors in Industrial Location: A Review. Economic Development Quarterly. 1987;1(1):72-85.

[3] M.S. Peters, K.D. Timmerhaus, ''Plant Design and Economics for Chemical Engineers'', 5th Ed., McGraw-Hill: New York, 2003.

[4] O. Akgul, N. Shah, L.G. Papageorgiou. Economic optimisation of a UK advanced biofuel supply chain. Biomass and Bioenergy. 2012;41:57-72.

[5] L.T. Biegler, I.E. Grossmann, A.W. Westerberg, ''Systematic Methods of Chemical Process Design'', Prentice-Hall: Upper Saddle River, 1997.

[6] Summary of the National Environmental Policy Act website. http://www.epa.gov/laws-regulations/summary-national-environmental-policy-act. August 2015. Accessed February 2016.

[7] R.T. Turton, R.C. Bailie, W.B. Whiting, J.A. Shaeiwitz, ''Analysis, Synthesis, and Design of Chemical Processes'', Prentice Hall: Upper Saddle River, 2003.

[8] R.W. Schmenner, Look Beyond the Obvious in Plant Location website. https://hbr.org/1979/01/look-beyond-the-obvious-in-plant-location. January 1979. Accessed February 2016.

Process location and layout decisions

2016-02-22T02:43:23Z

Ezhuang: /* Environmental Considerations */

Author: Ellen Zhuang [2016]

Stewards: Daniel Garcia, and Fengqi You

=Introduction=
Location is one of the first decisions in the design of a new chemical plant. It impacts profitability and scope for future expansion. If the project is a new facility, a suitable site must be found and an optimal layout of the site and process units must be planned. If the project adds to an existing site, the impact of the new addition on the existing plant must be considered. The plant also needs to accommodate for the nearby infrastructure, the services that it requires, and its environmental impacts.

=Location Selection=
===Location Decision Factors===
Initially, economists viewed the plant location decision as a cost-minimization problem. The optimal location was one where the transportation costs of raw material to the plant and product to the market was minimized. Between the early 1900s and post World War II period, most industries were sensitive to the cost of transportation. As transportation costs became less of an obstacle, the approach to the problem developed with considerations of trade-offs between transportation and factors such as wages, energy, local regulations, etc. If lower wages could balance the higher transportation costs of building the plant in a low-wage area, the low-wage location may be more desirable.

Although location is a long-term investment, a firm does not decide on a location with the sole objective of maximizing its profits or minimizing its costs. Managers may choose a “safer” location that is more likely to produce higher profits rather than a riskier location that yields the maximum investment return. Personal factors of the people involved are also influences. For example, new businesses tend to locate where the founders live.2

In the current age, a plant's site is chosen based on several factors. These include:1,2,3,8

'''1. Raw material supply:''' The source and price of raw materials is one of the most important factors that determine the location of a plant. Facilities that produce chemicals in bulk are usually located close to the source of raw material if the costs of shipping the product is less than the costs of shipping the feed. For example, ethylene production is growing in the Middle East since cheap ethane from natural gas is readily available. Oil refineries tend to be located near areas with high population and crude oil supplies since it is expensive to transport the oil.

'''2. Location with respect to market:''' If the plant produces high-volume and low-cost products, such as cement and fertilizer, it may be better to situate the plant closer to the primary market since transportation cost is a large fraction of the sales price. If the product is low-volume and high-cost, like pharmaceuticals, then the benefits of being closer to the primary market may not be there.

'''3. Transport facilities:''' Facilities should be close to at least two major forms of transportation, whether that be road, rail, waterway, and/or seaport. For example, paper manufacturing plants in the US use various types of pulp that are delivered by truck or by train from various places in North and South America, so paper facilities need to be close to rail and major roads. Transportation by road is common for local distribution from central warehouses, while transportation by rail is more widespread for long distance transport of bulk chemicals. Pipeline is used to ship industrial gases and bulk fuels. Air freight can be efficient for shipment of personnel and essential units and supplies and for small volume products that have high value, such as pharmaceuticals. Of course, products that are delivered by air must meet aviation regulations.

'''4. Availability of labor:''' Skilled workers are usually brought to the plant from outside the area. There should be a local pool of unskilled labor that can be trained to operate the plant, and of skilled craft workers to maintain the process units. Local labor laws, trade union customs, restrictive practices for recruitment and training should also be taken into consideration. A 10% increase in unionization of a state's labor force is projected to reduce the number of expanding facilities by 30 to 45%.

'''5. Availability of utilities:''' Processes that require a substantial amount of cooling water is usually located near water sources, such as rivers or wells. Cooling water may be directly taken from the water source, or may be stored in cooling towers. Those that need large quantities of power, such as electrochemical ones, are typically close to cheap power sources.

'''6. Availability of suitable land:''' The ideal land is flat, well-drained, with suitable load-bearing characteristics. Further considerations have to be made if the land is reclaimed land near the ocean in earthquake zones. Property tax is also a factor when choosing a site since property taxes vary area to area. Under a third of plants that relocate move to regions with lower property taxes, which is the proportion that would be expected if companies move to a new location regardless of property tax. High property taxes is not as significant as other factors such as labor supply and land costs.

'''7. Environmental impact:''' Depending on the location, it may be more difficult and costly to dispose of wastes. During the project design phase, experts are typically consulted to learn more about an area's local regulations. More details about environmental regulations are found below.

'''8. Local community considerations:''' State and local planners are typically motivated by the desire to create jobs and improve the tax base. Introduction of facilities to an area is usually viewed as the most direct way to stimulate the area's economy. However, recent studies have found that communities with high-growth are already characterized by the fast growth of businesses that are already there. It is rare for a plant to completely close in one area and relocate to another, and if plants do relocate, the majority is over short distances and often within the same community. Therefore, local policymakers favor the expansion of existing plants. The opening of a new plant at a location should impost no additional risk to the local residents. For example, they should be downwind of the residential areas. Local communities also need to be able to accommodate the plant personnelles. For example, traffic, housing, and facilities must be able to accommodate the influx of workers. Additional factors are property taxes and water consumption.

'''9. Climate:''' The climate of the area may affect processes and costs. For example, plants in cold areas need more insulation and special heating. Facilities in earthquake areas need to be seismically sound. Plants in areas with high ambient humidity will usually use air cooling instead of water cooling. More detailed information about the effects of weather can be found in the [[Site condition and design]] page.

'''10. Political and strategic considerations:''' Government sometimes gives capital grants, tax concessions, and other incentives to encourage plants to be built in specific areas. Physical assistance such as roads, water, and other public infrastructure are more popular than financial assistance. Companies can also globalize and take advantage of areas with preferential tariff agreements. The tax policy of an area is inversely related to growth. High personal income sometimes hinder employment growth. Personal income does not affect the cash flow of the company, but it reduces the after-tax income of its managers, and thus high personal income tax can be classified as a personal region. High state corporate taxes has also and detrimental effects on growth, but that is not always the case. Corporate tax is more important to firms with high capital expenditures.

===Analysis of the Importance of Factors===
In 1963, Morgan surveyed 17 companies and found that on average the most important factors on industrial location decisions are market, labor, and raw material. Taxes and financial incentives were of little significance. More recently, Fortune magazine surveyed among the 1000 largest US companies and found that the most important factors were market and labor. The survey also found that personal preferences of a firm's executive, tax, and central cities have great influence over the site of corporate headquarters. Surveys of interstate locational decisions found that the factors that played key roles for new firms were access to customers and the growing market, labor force, transportation, personal reasons of management, and availability of capital. The cost of land had the least important influence. For firms that expanded across states, the influential factors were labor costs and labor supply. Companies seeking expansion usually have made their production more routine, so quick access to suppliers is less of a concern. Cost minimization becomes more relevant to existing corporations.2

===Site Selection Process===
The decision for the location of a facility is part of a larger corporate planning process. Usually, a corporate planning office or a division of the company initiates the site selection process by forecasting future capacity requirements. If capacity shortages are in the forecasts, the managers may choose to outsource, increase price to reduce the demand, expand existing sites, or open a new facility. If the managers decide on a new facility, the site selection team enters the project.

The structure of the site selection team depends on the firm's organization. In companies with centralized staff, the site selection team generally consists of representatives from relevant areas, such as engineering, real estate, and transportation. In companies with strong divisions, the locational decisions may be done at the divisional level, with the corporate office supervising the process.

The site selection team determines what characteristics are important for the new location by considering how the new facility will fit in the company's overall strategy, if the company wants to target new markets, if the corporation wants to divide or integrate its functions, or how the company wants to be seen by the public. Next, potential locations are listed and studied against the desired characteristics.

Locational decisions are typically made sequentially. The first step is at the state or regional level. Then, the team studies specific communities and sites. Different locational factors are important to different stages. When selecting a general region, the site selection team focuses on factors with interregional variations such as labor, tax policies, climate, and market locations. At the more focused stage, details like inexpensive land, access to to major roads, and good schools are important. Consultants are often hired to do site analyses.

Once site options are narrowed down, the company discusses potential problems and incentives with local public officials. Construction costs are estimated, and a feasibility analysis is done to show that the project has a high rate of return.2

===Biofuel Supply Chain Example===
Biofuels is a popular potential alternative to fossil fuels. Various biomass resources, including food crops, non-food crops, and agricultural residues, are converted into biofuels. The two most common biofuels currently are ethanol and biodiesel. The supply chain for biofuel is a network consisting of several nodes: biomass cultivation sites, biofuel production plants, and demand centers. The locations of these facilities, and the location of the demand centers with respect to the biofuel demand impact transportation costs, production resource, demand, etc., and thus affects and sets constraints to the cost minimization of the supply chain. The optimal supply chain design here minimizes the total cost, which is a sum of the total investment cost, production cost, transportation cost, and outsourcing and import cost. The total transportation cost captures the transportation of biomass and biofuel between areas and the local bio mass transfer. Location of the facilities also sets constraints on the regional demand of biofuel and availability of local biomass. Sustainability constraints ensure that plants do not negatively affect the food production, sustainably use the land, and don't compete with other industries that use biomass.4

[[File:biofuelsupplychain.png|400px|thumb|center|'''Figure 1.''' The biofuels supply chain consists of three notes: biomass cultivation sites, biofuel production plants, and biofuel demand centers.1]]

[[File:Biofueloptimal.png|300px|thumb|center|'''Figure 2.''' An example of a Great Britain biofuel supply chain design that minimizes cost. The plants are near areas with high raw material supply. The locations of the plants with respect to the demand centers are also optimized. The main modes of transportation are road, rail, and ship.4]]

=Site Layout=
The process units and buildings need to be arranged in such a way that allows for the most economical flow of materials and people. Dangerous processes need to be a safe distance from other buildings, and the layout should be planned to allow for future expansion.

Process units are usually laid out first in an arrangement that allows for smooth flow of materials between the process steps. The distance between equipment is usually at least 30 m. Next, the location of the principal ancillary buildings are sited as to minimize the time that it takes the workers to travel between buildings. Administrative offices and laboratories are located away from hazardous processes. Control rooms are next to the processing equipment. Utility buildings are located as to minimize piping between the process units. Storage is placed between the loading and unloading facilities and next to the process units that they serve. Tanks containing hazardous material are placed at least 70 m from the plant. An example of a typical site plan is shown below.1

[[File:Site Location.png|400px|thumb|center|'''Figure 3.''' A typical site layout.1]]

===Site Layout Factors===
The main factors that are considered when planning the layout of the plant are listed below.1

'''1. Economic considerations (construction and operating costs):'''
Construction costs can be minimized by arranging process units and buildings that minimize pipes between equipment, the amount of structural steel work, etc. However, this layout may conflict with the layout that gives the optimal operation and maintenance.

'''2. Process requirements:''' Examples of process considerations that must be taken into account is the elevation of the base of columns to give enough net positive suction head to a pump.

'''3. Operation convenience:''' Process units that are attended to frequently should be placed with convenient access. Valves, heads, and sample heads should be placed where operators can easily access. If the plant anticipates replacement of equipment, space must be allowed for removal and installation.

'''4. Maintenance convenience:''' Equipment that requires maintenance should be in a location with easy access, and should have sufficient space for the maintenance tasks. For example, shell-and-tube exchangers need space so that tube bundles can be removed for cleaning and repair.

'''5. Future expansion:''' The layout should be planned to conveniently allow for future expansion of processes. Pipe racks should have space for future piping, and pipes should be oversized to allow for more flow in the future.

'''6. Modular Construction:''' Modular construction is where sections of the plant is constructed outside of the plant, and then transported to the site by road or sea. Advantages include improved quality control, reduced construction costs, less requirements for skilled labor on site. Tradeoffs are more flanged connections and possible problems with onsite assembly.

'''7. Safety:''' Escape routes for workers need to be in place at each level in process buildings. Blast walls must isolate equipment that pose hazards to confine potential explosions.

First, a conceptual flowsheet for the process is developed. The types of equipment and their connections with each other is described in a process flow diagram (PFD). Before the PFD is translated into detailed piping and instrumentation diagrams (P&ID) the layout of the process units must be planned.3,5 Scale drawings are made to show the relationships between storage space and process equipment based on the flow of materials and people, and on future expansion. Three-dimensional visualization are the layouts are then carried out with cardboard cutouts of the equipment outlines or rectangular and cylindrical blocks. When a layout of the major process units has been decided, drawings of the plan and elevation are made, and design of the structural steelwork and foundations are done. Computer-aided design has also become increasingly popular.1,3,7

=Environmental Considerations=
Laws that protect the environment restrict the waste that plants can emit in order to preserve air, land, and water quality. States, provinces, and municipals usually have additional laws to national ones. In the US, environmental policies are moving towards more uniform and strict regulations. Businesses that produce more pollution are more likely to move to regions where local regulations are more lax, and industries in areas with more stringent policies will decline. The tighter national regulations are implemented to prevent this competition between regions. For example, amendments to the Clean Air Act and the Environmental Protection Agencies in the 1970s and 1980s reduced differences in regions' air pollution controls. Congress voted on the PSD (prevention of significant deterioration) to "level the playing field" across regions.1

Some of the main environmental legislations in North America are:

*'''The National Environmental Policy Act of 1969 (NEPA)''' - Signed by President Nixon, NEPA was one of the first laws that provide a national framework for the protection of the environment. It requires that all executive federal agencies consider the environmental impact of proposed actions. Government agencies must prepare environmental assessments and environmental impact statements that describe potential environmental effects. NEPA is implemented when buildings, airports, military complexes, parkland purchases, and highways are proposed.6

*'''The Clean Air Act (CAA, 1970)''' - The CAA was implemented to control air quality on a national level. The law was amended in 1990 to address issues such as acid rain, ozone depletion, and toxic air pollution, and to increase enforcement authority. Under the National Ambient Air Quality Standards (NAAQS), the law sets limits to allowable ambient levels of seven contaminants: ozone, carbon monoxide, lead, nitrogen dioxide, sulfur dioxide, PM10, and PM2.5. The CAA also requires that the EPA set US National Emissions Standards for Hazardous Air Pollutants (NESHAP), which regulates the emission of 189 hazardous air pollutants. The Maximum Available Control Technology (MACT) was implemented to limit the emission of volatile organics, sulfur, and other inorganic compounds from oil refineries. The CAA has had significant impact on the level of air pollutants that it regulates. Particularly in the Midwest and the Northwest, water acidity has been reduced. On the other hand, reduction of ozone damage has not been as successful since nitrogen oxide, which is linked to ozone formation, is not as tightly regulated.

*'''The Federal Water Pollution Control Act (The Clean Water Act, 1972)''' - The Clean Water Act was initially passed to clean water used for swimming and boating, and to protect wildlife. Amendments were later added to address water quality and toxic compound emissions. Under the CWA, the EPA can set water quality standards and monitor the industries' discharge into waters.

*'''The Safe Drinking Water Act (SDWA, 1974)''' - This act allows the EPA to set standards to potential drinking water. Public water systems must operate according to these standards.

*'''The Resource Conservation and Recovery Act (RCRA, 1976)''' - The RCRA protects current and future groundwater facilities from contamination. Firms that produce waste must have a "cradle to grave" approach to managing waste, meaning that the waste producer is legally responsible for the waste from the moment it is produced until it is finally disposed.

*'''The Comprehensive Environmental Response, Compensation and Liability Act (CERCLA or Superfund, 1980)''' - This legislation creates a tax on chemical and petroleum industries in order to cover the remediation of hazardous waste sites that are uncontrolled or abandoned. The CERCLA empowers EPA to remediate waste sites by short-term removal of material or long-term remediation actions, depending on the risk that the waste imposes on life.

*'''The Pollution Prevention Act (PPA, 1990)''' - This act promotes minimization of waste and pollution at their source. It encourages more efficient processing and raw materials and recycling through grants, technical assistant, and information.

*'''The Oil Pollution Act of 1990 (OPA, 1990)''' - This act was passed after the Exxon Valdez oil spill in the Prince William Sound in Alaska, creating a tax on oil to cover the cost of responding to spills when the corporation that creates the spill does not respond.

*'''The Department of the Environment Act (E-10)''' - The Department of Environment Act created the Department of Environment and charges the Minister of the Environment with creating programs that protect the environment, assessing environmental impacts of federal program, and reporting information to the public in Canada.

*'''The Canadian Environmental Protection Act (CEPA, C-15.31, 1999)''' - CEPA empowers Environment Canada to control toxic substances, reduce pollution, and reduce bioaccumulating toxic substances. Canada will use CEPA to implement regulations that helps it meet its commitments to the Kyoto Protocol.

*'''The Canada Water Act (C-11)''' - This law allows the Minister of the Environment to manage water resources in Canada. This includes research, implementing conservation programs, utilization of water resources, and setting water quality standards.

Wastes comprise mainly of by-products, unused reactants, and off-specification product produced by misoperation. Leaking seals and flanges, and spills and discharges also emit waste. Material is also discharged in emergency situations. Instead of considering how to treat or manage waste, designers should start by tackling the source and find ways to minimize the production of waste.

The process of designing waste management systems are: (1) source reduction, (2) waste stream recycle, (3) waste treatment to reduce environmental impact, and (4) disposal that is legally sound. Source reduction can be achieved by reducing the concentration of impurities in the feed, protecting catalysts and adsorbents from contaminants, eliminating the use of extraneous materials, increasing recovery from separation, and improving the quality of the fuel by switching to cleaner-burning fuel. Unused feed can be recycled, and off-specification products can be reprocessed. In integral processes, the waste of one process is used as the feed for another. By-products can also be sold to another company for use as raw material. Tighter control systems, alarm, and interlocks can reduce misoperation of process unit.1

Further information about environmental considerations can be found in the [[Environmental concerns]] page.

=Conclusion=
The location and layout of a plant can greatly impact its economic and operational success. Objectives such as cost minimization, distribution, room for expansion, and safety of the plant operators and local community play important roles in the site decision. The major factors affecting this decision are market and labor. National and regional environmental legislations impact the processes, and thus make some areas more desirable than others. Least important are land costs and financial incentives. When planning to build a new facility or expanding an existing site, companies utilize their site selection team to assemble a list of desirable locational characteristics and determine how the new facility will fit in with the overall company. Then with the help of consultants, a location is narrowed down and further analyses are done to justify the continuation of the project. The location should be obtained before design of the details of the process. Within the site itself, process units are arranged to allow for the most economical flow of people and material, optimal safety, and room for future expansion.

=References=
[1] G.P. Towler, R. Sinnott, ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design'', Elsevier, 2012.

[2] J.P. Blair, R. Premus. Major Factors in Industrial Location: A Review. Economic Development Quarterly. 1987;1(1):72-85.

[3] M.S. Peters, K.D. Timmerhaus, ''Plant Design and Economics for Chemical Engineers'', 5th Ed., McGraw-Hill: New York, 2003.

[4] O. Akgul, N. Shah, L.G. Papageorgiou. Economic optimisation of a UK advanced biofuel supply chain. Biomass and Bioenergy. 2012;41:57-72.

[5] L.T. Biegler, I.E. Grossmann, A.W. Westerberg, ''Systematic Methods of Chemical Process Design'', Prentice-Hall: Upper Saddle River, 1997.

[6] Summary of the National Environmental Policy Act website. http://www.epa.gov/laws-regulations/summary-national-environmental-policy-act. August 2015. Accessed February 2016.

[7] R.T. Turton, R.C. Bailie, W.B. Whiting, J.A. Shaeiwitz, ''Analysis, Synthesis, and Design of Chemical Processes'', Prentice Hall: Upper Saddle River, 2003.

[8] R.W. Schmenner, Look Beyond the Obvious in Plant Location website. https://hbr.org/1979/01/look-beyond-the-obvious-in-plant-location. January 1979. Accessed February 2016.

Process location and layout decisions

2016-02-22T02:42:11Z

Ezhuang: /* Site Layout Factors */

Author: Ellen Zhuang [2016]

Stewards: Daniel Garcia, and Fengqi You

=Introduction=
Location is one of the first decisions in the design of a new chemical plant. It impacts profitability and scope for future expansion. If the project is a new facility, a suitable site must be found and an optimal layout of the site and process units must be planned. If the project adds to an existing site, the impact of the new addition on the existing plant must be considered. The plant also needs to accommodate for the nearby infrastructure, the services that it requires, and its environmental impacts.

=Location Selection=
===Location Decision Factors===
Initially, economists viewed the plant location decision as a cost-minimization problem. The optimal location was one where the transportation costs of raw material to the plant and product to the market was minimized. Between the early 1900s and post World War II period, most industries were sensitive to the cost of transportation. As transportation costs became less of an obstacle, the approach to the problem developed with considerations of trade-offs between transportation and factors such as wages, energy, local regulations, etc. If lower wages could balance the higher transportation costs of building the plant in a low-wage area, the low-wage location may be more desirable.

Although location is a long-term investment, a firm does not decide on a location with the sole objective of maximizing its profits or minimizing its costs. Managers may choose a “safer” location that is more likely to produce higher profits rather than a riskier location that yields the maximum investment return. Personal factors of the people involved are also influences. For example, new businesses tend to locate where the founders live.2

In the current age, a plant's site is chosen based on several factors. These include:1,2,3,8

'''1. Raw material supply:''' The source and price of raw materials is one of the most important factors that determine the location of a plant. Facilities that produce chemicals in bulk are usually located close to the source of raw material if the costs of shipping the product is less than the costs of shipping the feed. For example, ethylene production is growing in the Middle East since cheap ethane from natural gas is readily available. Oil refineries tend to be located near areas with high population and crude oil supplies since it is expensive to transport the oil.

'''2. Location with respect to market:''' If the plant produces high-volume and low-cost products, such as cement and fertilizer, it may be better to situate the plant closer to the primary market since transportation cost is a large fraction of the sales price. If the product is low-volume and high-cost, like pharmaceuticals, then the benefits of being closer to the primary market may not be there.

'''3. Transport facilities:''' Facilities should be close to at least two major forms of transportation, whether that be road, rail, waterway, and/or seaport. For example, paper manufacturing plants in the US use various types of pulp that are delivered by truck or by train from various places in North and South America, so paper facilities need to be close to rail and major roads. Transportation by road is common for local distribution from central warehouses, while transportation by rail is more widespread for long distance transport of bulk chemicals. Pipeline is used to ship industrial gases and bulk fuels. Air freight can be efficient for shipment of personnel and essential units and supplies and for small volume products that have high value, such as pharmaceuticals. Of course, products that are delivered by air must meet aviation regulations.

'''4. Availability of labor:''' Skilled workers are usually brought to the plant from outside the area. There should be a local pool of unskilled labor that can be trained to operate the plant, and of skilled craft workers to maintain the process units. Local labor laws, trade union customs, restrictive practices for recruitment and training should also be taken into consideration. A 10% increase in unionization of a state's labor force is projected to reduce the number of expanding facilities by 30 to 45%.

'''5. Availability of utilities:''' Processes that require a substantial amount of cooling water is usually located near water sources, such as rivers or wells. Cooling water may be directly taken from the water source, or may be stored in cooling towers. Those that need large quantities of power, such as electrochemical ones, are typically close to cheap power sources.

'''6. Availability of suitable land:''' The ideal land is flat, well-drained, with suitable load-bearing characteristics. Further considerations have to be made if the land is reclaimed land near the ocean in earthquake zones. Property tax is also a factor when choosing a site since property taxes vary area to area. Under a third of plants that relocate move to regions with lower property taxes, which is the proportion that would be expected if companies move to a new location regardless of property tax. High property taxes is not as significant as other factors such as labor supply and land costs.

'''7. Environmental impact:''' Depending on the location, it may be more difficult and costly to dispose of wastes. During the project design phase, experts are typically consulted to learn more about an area's local regulations. More details about environmental regulations are found below.

'''8. Local community considerations:''' State and local planners are typically motivated by the desire to create jobs and improve the tax base. Introduction of facilities to an area is usually viewed as the most direct way to stimulate the area's economy. However, recent studies have found that communities with high-growth are already characterized by the fast growth of businesses that are already there. It is rare for a plant to completely close in one area and relocate to another, and if plants do relocate, the majority is over short distances and often within the same community. Therefore, local policymakers favor the expansion of existing plants. The opening of a new plant at a location should impost no additional risk to the local residents. For example, they should be downwind of the residential areas. Local communities also need to be able to accommodate the plant personnelles. For example, traffic, housing, and facilities must be able to accommodate the influx of workers. Additional factors are property taxes and water consumption.

'''9. Climate:''' The climate of the area may affect processes and costs. For example, plants in cold areas need more insulation and special heating. Facilities in earthquake areas need to be seismically sound. Plants in areas with high ambient humidity will usually use air cooling instead of water cooling. More detailed information about the effects of weather can be found in the [[Site condition and design]] page.

'''10. Political and strategic considerations:''' Government sometimes gives capital grants, tax concessions, and other incentives to encourage plants to be built in specific areas. Physical assistance such as roads, water, and other public infrastructure are more popular than financial assistance. Companies can also globalize and take advantage of areas with preferential tariff agreements. The tax policy of an area is inversely related to growth. High personal income sometimes hinder employment growth. Personal income does not affect the cash flow of the company, but it reduces the after-tax income of its managers, and thus high personal income tax can be classified as a personal region. High state corporate taxes has also and detrimental effects on growth, but that is not always the case. Corporate tax is more important to firms with high capital expenditures.

===Analysis of the Importance of Factors===
In 1963, Morgan surveyed 17 companies and found that on average the most important factors on industrial location decisions are market, labor, and raw material. Taxes and financial incentives were of little significance. More recently, Fortune magazine surveyed among the 1000 largest US companies and found that the most important factors were market and labor. The survey also found that personal preferences of a firm's executive, tax, and central cities have great influence over the site of corporate headquarters. Surveys of interstate locational decisions found that the factors that played key roles for new firms were access to customers and the growing market, labor force, transportation, personal reasons of management, and availability of capital. The cost of land had the least important influence. For firms that expanded across states, the influential factors were labor costs and labor supply. Companies seeking expansion usually have made their production more routine, so quick access to suppliers is less of a concern. Cost minimization becomes more relevant to existing corporations.2

===Site Selection Process===
The decision for the location of a facility is part of a larger corporate planning process. Usually, a corporate planning office or a division of the company initiates the site selection process by forecasting future capacity requirements. If capacity shortages are in the forecasts, the managers may choose to outsource, increase price to reduce the demand, expand existing sites, or open a new facility. If the managers decide on a new facility, the site selection team enters the project.

The structure of the site selection team depends on the firm's organization. In companies with centralized staff, the site selection team generally consists of representatives from relevant areas, such as engineering, real estate, and transportation. In companies with strong divisions, the locational decisions may be done at the divisional level, with the corporate office supervising the process.

The site selection team determines what characteristics are important for the new location by considering how the new facility will fit in the company's overall strategy, if the company wants to target new markets, if the corporation wants to divide or integrate its functions, or how the company wants to be seen by the public. Next, potential locations are listed and studied against the desired characteristics.

Locational decisions are typically made sequentially. The first step is at the state or regional level. Then, the team studies specific communities and sites. Different locational factors are important to different stages. When selecting a general region, the site selection team focuses on factors with interregional variations such as labor, tax policies, climate, and market locations. At the more focused stage, details like inexpensive land, access to to major roads, and good schools are important. Consultants are often hired to do site analyses.

Once site options are narrowed down, the company discusses potential problems and incentives with local public officials. Construction costs are estimated, and a feasibility analysis is done to show that the project has a high rate of return.2

===Biofuel Supply Chain Example===
Biofuels is a popular potential alternative to fossil fuels. Various biomass resources, including food crops, non-food crops, and agricultural residues, are converted into biofuels. The two most common biofuels currently are ethanol and biodiesel. The supply chain for biofuel is a network consisting of several nodes: biomass cultivation sites, biofuel production plants, and demand centers. The locations of these facilities, and the location of the demand centers with respect to the biofuel demand impact transportation costs, production resource, demand, etc., and thus affects and sets constraints to the cost minimization of the supply chain. The optimal supply chain design here minimizes the total cost, which is a sum of the total investment cost, production cost, transportation cost, and outsourcing and import cost. The total transportation cost captures the transportation of biomass and biofuel between areas and the local bio mass transfer. Location of the facilities also sets constraints on the regional demand of biofuel and availability of local biomass. Sustainability constraints ensure that plants do not negatively affect the food production, sustainably use the land, and don't compete with other industries that use biomass.4

[[File:biofuelsupplychain.png|400px|thumb|center|'''Figure 1.''' The biofuels supply chain consists of three notes: biomass cultivation sites, biofuel production plants, and biofuel demand centers.1]]

[[File:Biofueloptimal.png|300px|thumb|center|'''Figure 2.''' An example of a Great Britain biofuel supply chain design that minimizes cost. The plants are near areas with high raw material supply. The locations of the plants with respect to the demand centers are also optimized. The main modes of transportation are road, rail, and ship.4]]

=Site Layout=
The process units and buildings need to be arranged in such a way that allows for the most economical flow of materials and people. Dangerous processes need to be a safe distance from other buildings, and the layout should be planned to allow for future expansion.

Process units are usually laid out first in an arrangement that allows for smooth flow of materials between the process steps. The distance between equipment is usually at least 30 m. Next, the location of the principal ancillary buildings are sited as to minimize the time that it takes the workers to travel between buildings. Administrative offices and laboratories are located away from hazardous processes. Control rooms are next to the processing equipment. Utility buildings are located as to minimize piping between the process units. Storage is placed between the loading and unloading facilities and next to the process units that they serve. Tanks containing hazardous material are placed at least 70 m from the plant. An example of a typical site plan is shown below.1

[[File:Site Location.png|400px|thumb|center|'''Figure 3.''' A typical site layout.1]]

===Site Layout Factors===
The main factors that are considered when planning the layout of the plant are listed below.1

'''1. Economic considerations (construction and operating costs):'''
Construction costs can be minimized by arranging process units and buildings that minimize pipes between equipment, the amount of structural steel work, etc. However, this layout may conflict with the layout that gives the optimal operation and maintenance.

'''2. Process requirements:''' Examples of process considerations that must be taken into account is the elevation of the base of columns to give enough net positive suction head to a pump.

'''3. Operation convenience:''' Process units that are attended to frequently should be placed with convenient access. Valves, heads, and sample heads should be placed where operators can easily access. If the plant anticipates replacement of equipment, space must be allowed for removal and installation.

'''4. Maintenance convenience:''' Equipment that requires maintenance should be in a location with easy access, and should have sufficient space for the maintenance tasks. For example, shell-and-tube exchangers need space so that tube bundles can be removed for cleaning and repair.

'''5. Future expansion:''' The layout should be planned to conveniently allow for future expansion of processes. Pipe racks should have space for future piping, and pipes should be oversized to allow for more flow in the future.

'''6. Modular Construction:''' Modular construction is where sections of the plant is constructed outside of the plant, and then transported to the site by road or sea. Advantages include improved quality control, reduced construction costs, less requirements for skilled labor on site. Tradeoffs are more flanged connections and possible problems with onsite assembly.

'''7. Safety:''' Escape routes for workers need to be in place at each level in process buildings. Blast walls must isolate equipment that pose hazards to confine potential explosions.

First, a conceptual flowsheet for the process is developed. The types of equipment and their connections with each other is described in a process flow diagram (PFD). Before the PFD is translated into detailed piping and instrumentation diagrams (P&ID) the layout of the process units must be planned.3,5 Scale drawings are made to show the relationships between storage space and process equipment based on the flow of materials and people, and on future expansion. Three-dimensional visualization are the layouts are then carried out with cardboard cutouts of the equipment outlines or rectangular and cylindrical blocks. When a layout of the major process units has been decided, drawings of the plan and elevation are made, and design of the structural steelwork and foundations are done. Computer-aided design has also become increasingly popular.1,3,7

=Environmental Considerations=
Laws that protect the environment restrict the waste that plants can emit in order to preserve air, land, and water quality. States, provinces, and municipals usually have additional laws to national ones. In the US, environmental policies are moving towards more uniform and strict regulations. Businesses that produce more pollution are more likely to move to regions where local regulations are more lax, and industries in areas with more stringent policies will decline. The tighter national regulations are implemented to prevent this competition between regions. For example, amendments to the Clean Air Act and the Environmental Protection Agencies in the 1970s and 1980s reduced differences in regions' air pollution controls. Congress voted on the PSD (prevention of significant deterioration) to "level the playing field" across regions.1

Some of the main environmental legislations in North America are:

*'''The National Environmental Policy Act of 1969 (NEPA)''' - Signed by President Nixon, NEPA was one of the first laws that provide a national framework for the protection of the environment. It requires that all executive federal agencies consider the environmental impact of proposed actions. Government agencies must prepare environmental assessments and environmental impact statements that describe potential environmental effects. NEPA is implemented when buildings, airports, military complexes, parkland purchases, and highways are proposed.6

*'''The Clean Air Act (CAA, 1970)''' - The CAA was implemented to control air quality on a national level. The law was amended in 1990 to address issues such as acid rain, ozone depletion, and toxic air pollution, and to increase enforcement authority. Under the National Ambient Air Quality Standards (NAAQS), the law sets limits to allowable ambient levels of seven contaminants: ozone, carbon monoxide, lead, nitrogen dioxide, sulfur dioxide, PM10, and PM2.5. The CAA also requires that the EPA set US National Emissions Standards for Hazardous Air Pollutants (NESHAP), which regulates the emission of 189 hazardous air pollutants. The Maximum Available Control Technology (MACT) was implemented to limit the emission of volatile organics, sulfur, and other inorganic compounds from oil refineries. The CAA has had significant impact on the level of air pollutants that it regulates. Particularly in the Midwest and the Northwest, water acidity has been reduced. On the other hand, reduction of ozone damage has not been as successful since nitrogen oxide, which is linked to ozone formation, is not as tightly regulated.

*'''The Federal Water Pollution Control Act (The Clean Water Act, 1972)''' - The Clean Water Act was initially passed to clean water used for swimming and boating, and to protect wildlife. Amendments were later added to address water quality and toxic compound emissions. Under the CWA, the EPA can set water quality standards and monitor the industries' discharge into waters.

*'''The Safe Drinking Water Act (SDWA, 1974)''' - This act allows the EPA to set standards to potential drinking water. Public water systems must operate according to these standards.

*'''The Resource Conservation and Recovery Act (RCRA, 1976)''' - The RCRA protects current and future groundwater facilities from contamination. Firms that produce waste must have a "cradle to grave" approach to managing waste, meaning that the waste producer is legally responsible for the waste from the moment it is produced until it is finally disposed.

*'''The Comprehensive Environmental Response, Compensation and Liability Act (CERCLA or Superfund, 1980)''' - This legislation creates a tax on chemical and petroleum industries in order to cover the remediation of hazardous waste sites that are uncontrolled or abandoned. The CERCLA empowers EPA to remediate waste sites by short-term removal of material or long-term remediation actions, depending on the risk that the waste imposes on life.

*The Superfund Amendments and Reauthorization Act (SARA, 1986)
*'''The Pollution Prevention Act (PPA, 1990)''' - This act promotes minimization of waste and pollution at their source. It encourages more efficient processing and raw materials and recycling through grants, technical assistant, and information.

*'''The Oil Pollution Act of 1990 (OPA, 1990)''' - This act was passed after the Exxon Valdez oil spill in the Prince William Sound in Alaska, creating a tax on oil to cover the cost of responding to spills when the corporation that creates the spill does not respond.

*'''The Department of the Environment Act (E-10)''' - The Department of Environment Act created the Department of Environment and charges the Minister of the Environment with creating programs that protect the environment, assessing environmental impacts of federal program, and reporting information to the public in Canada.

*'''The Canadian Environmental Protection Act (CEPA, C-15.31, 1999)''' - CEPA empowers Environment Canada to control toxic substances, reduce pollution, and reduce bioaccumulating toxic substances. Canada will use CEPA to implement regulations that helps it meet its commitments to the Kyoto Protocol.

*'''The Canada Water Act (C-11)''' - This law allows the Minister of the Environment to manage water resources in Canada. This includes research, implementing conservation programs, utilization of water resources, and setting water quality standards.

Wastes comprise mainly of by-products, unused reactants, and off-specification product produced by misoperation. Leaking seals and flanges, and spills and discharges also emit waste. Material is also discharged in emergency situations. Instead of considering how to treat or manage waste, designers should start by tackling the source and find ways to minimize the production of waste.

The process of designing waste management systems are: (1) source reduction, (2) waste stream recycle, (3) waste treatment to reduce environmental impact, and (4) disposal that is legally sound. Source reduction can be achieved by reducing the concentration of impurities in the feed, protecting catalysts and adsorbents from contaminants, eliminating the use of extraneous materials, increasing recovery from separation, and improving the quality of the fuel by switching to cleaner-burning fuel. Unused feed can be recycled, and off-specification products can be reprocessed. In integral processes, the waste of one process is used as the feed for another. By-products can also be sold to another company for use as raw material. Tighter control systems, alarm, and interlocks can reduce misoperation of process unit.1

Further information about environmental considerations can be found in the [[Environmental concerns]] page.

=Conclusion=
The location and layout of a plant can greatly impact its economic and operational success. Objectives such as cost minimization, distribution, room for expansion, and safety of the plant operators and local community play important roles in the site decision. The major factors affecting this decision are market and labor. National and regional environmental legislations impact the processes, and thus make some areas more desirable than others. Least important are land costs and financial incentives. When planning to build a new facility or expanding an existing site, companies utilize their site selection team to assemble a list of desirable locational characteristics and determine how the new facility will fit in with the overall company. Then with the help of consultants, a location is narrowed down and further analyses are done to justify the continuation of the project. The location should be obtained before design of the details of the process. Within the site itself, process units are arranged to allow for the most economical flow of people and material, optimal safety, and room for future expansion.

=References=
[1] G.P. Towler, R. Sinnott, ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design'', Elsevier, 2012.

[2] J.P. Blair, R. Premus. Major Factors in Industrial Location: A Review. Economic Development Quarterly. 1987;1(1):72-85.

[3] M.S. Peters, K.D. Timmerhaus, ''Plant Design and Economics for Chemical Engineers'', 5th Ed., McGraw-Hill: New York, 2003.

[4] O. Akgul, N. Shah, L.G. Papageorgiou. Economic optimisation of a UK advanced biofuel supply chain. Biomass and Bioenergy. 2012;41:57-72.

[5] L.T. Biegler, I.E. Grossmann, A.W. Westerberg, ''Systematic Methods of Chemical Process Design'', Prentice-Hall: Upper Saddle River, 1997.

[6] Summary of the National Environmental Policy Act website. http://www.epa.gov/laws-regulations/summary-national-environmental-policy-act. August 2015. Accessed February 2016.

[7] R.T. Turton, R.C. Bailie, W.B. Whiting, J.A. Shaeiwitz, ''Analysis, Synthesis, and Design of Chemical Processes'', Prentice Hall: Upper Saddle River, 2003.

[8] R.W. Schmenner, Look Beyond the Obvious in Plant Location website. https://hbr.org/1979/01/look-beyond-the-obvious-in-plant-location. January 1979. Accessed February 2016.

Process location and layout decisions

2016-02-22T02:40:52Z

Ezhuang: /* Site Layout */

Author: Ellen Zhuang [2016]

Stewards: Daniel Garcia, and Fengqi You

=Introduction=
Location is one of the first decisions in the design of a new chemical plant. It impacts profitability and scope for future expansion. If the project is a new facility, a suitable site must be found and an optimal layout of the site and process units must be planned. If the project adds to an existing site, the impact of the new addition on the existing plant must be considered. The plant also needs to accommodate for the nearby infrastructure, the services that it requires, and its environmental impacts.

=Location Selection=
===Location Decision Factors===
Initially, economists viewed the plant location decision as a cost-minimization problem. The optimal location was one where the transportation costs of raw material to the plant and product to the market was minimized. Between the early 1900s and post World War II period, most industries were sensitive to the cost of transportation. As transportation costs became less of an obstacle, the approach to the problem developed with considerations of trade-offs between transportation and factors such as wages, energy, local regulations, etc. If lower wages could balance the higher transportation costs of building the plant in a low-wage area, the low-wage location may be more desirable.

Although location is a long-term investment, a firm does not decide on a location with the sole objective of maximizing its profits or minimizing its costs. Managers may choose a “safer” location that is more likely to produce higher profits rather than a riskier location that yields the maximum investment return. Personal factors of the people involved are also influences. For example, new businesses tend to locate where the founders live.2

In the current age, a plant's site is chosen based on several factors. These include:1,2,3,8

'''1. Raw material supply:''' The source and price of raw materials is one of the most important factors that determine the location of a plant. Facilities that produce chemicals in bulk are usually located close to the source of raw material if the costs of shipping the product is less than the costs of shipping the feed. For example, ethylene production is growing in the Middle East since cheap ethane from natural gas is readily available. Oil refineries tend to be located near areas with high population and crude oil supplies since it is expensive to transport the oil.

'''2. Location with respect to market:''' If the plant produces high-volume and low-cost products, such as cement and fertilizer, it may be better to situate the plant closer to the primary market since transportation cost is a large fraction of the sales price. If the product is low-volume and high-cost, like pharmaceuticals, then the benefits of being closer to the primary market may not be there.

'''3. Transport facilities:''' Facilities should be close to at least two major forms of transportation, whether that be road, rail, waterway, and/or seaport. For example, paper manufacturing plants in the US use various types of pulp that are delivered by truck or by train from various places in North and South America, so paper facilities need to be close to rail and major roads. Transportation by road is common for local distribution from central warehouses, while transportation by rail is more widespread for long distance transport of bulk chemicals. Pipeline is used to ship industrial gases and bulk fuels. Air freight can be efficient for shipment of personnel and essential units and supplies and for small volume products that have high value, such as pharmaceuticals. Of course, products that are delivered by air must meet aviation regulations.

'''4. Availability of labor:''' Skilled workers are usually brought to the plant from outside the area. There should be a local pool of unskilled labor that can be trained to operate the plant, and of skilled craft workers to maintain the process units. Local labor laws, trade union customs, restrictive practices for recruitment and training should also be taken into consideration. A 10% increase in unionization of a state's labor force is projected to reduce the number of expanding facilities by 30 to 45%.

'''5. Availability of utilities:''' Processes that require a substantial amount of cooling water is usually located near water sources, such as rivers or wells. Cooling water may be directly taken from the water source, or may be stored in cooling towers. Those that need large quantities of power, such as electrochemical ones, are typically close to cheap power sources.

'''6. Availability of suitable land:''' The ideal land is flat, well-drained, with suitable load-bearing characteristics. Further considerations have to be made if the land is reclaimed land near the ocean in earthquake zones. Property tax is also a factor when choosing a site since property taxes vary area to area. Under a third of plants that relocate move to regions with lower property taxes, which is the proportion that would be expected if companies move to a new location regardless of property tax. High property taxes is not as significant as other factors such as labor supply and land costs.

'''7. Environmental impact:''' Depending on the location, it may be more difficult and costly to dispose of wastes. During the project design phase, experts are typically consulted to learn more about an area's local regulations. More details about environmental regulations are found below.

'''8. Local community considerations:''' State and local planners are typically motivated by the desire to create jobs and improve the tax base. Introduction of facilities to an area is usually viewed as the most direct way to stimulate the area's economy. However, recent studies have found that communities with high-growth are already characterized by the fast growth of businesses that are already there. It is rare for a plant to completely close in one area and relocate to another, and if plants do relocate, the majority is over short distances and often within the same community. Therefore, local policymakers favor the expansion of existing plants. The opening of a new plant at a location should impost no additional risk to the local residents. For example, they should be downwind of the residential areas. Local communities also need to be able to accommodate the plant personnelles. For example, traffic, housing, and facilities must be able to accommodate the influx of workers. Additional factors are property taxes and water consumption.

'''9. Climate:''' The climate of the area may affect processes and costs. For example, plants in cold areas need more insulation and special heating. Facilities in earthquake areas need to be seismically sound. Plants in areas with high ambient humidity will usually use air cooling instead of water cooling. More detailed information about the effects of weather can be found in the [[Site condition and design]] page.

'''10. Political and strategic considerations:''' Government sometimes gives capital grants, tax concessions, and other incentives to encourage plants to be built in specific areas. Physical assistance such as roads, water, and other public infrastructure are more popular than financial assistance. Companies can also globalize and take advantage of areas with preferential tariff agreements. The tax policy of an area is inversely related to growth. High personal income sometimes hinder employment growth. Personal income does not affect the cash flow of the company, but it reduces the after-tax income of its managers, and thus high personal income tax can be classified as a personal region. High state corporate taxes has also and detrimental effects on growth, but that is not always the case. Corporate tax is more important to firms with high capital expenditures.

===Analysis of the Importance of Factors===
In 1963, Morgan surveyed 17 companies and found that on average the most important factors on industrial location decisions are market, labor, and raw material. Taxes and financial incentives were of little significance. More recently, Fortune magazine surveyed among the 1000 largest US companies and found that the most important factors were market and labor. The survey also found that personal preferences of a firm's executive, tax, and central cities have great influence over the site of corporate headquarters. Surveys of interstate locational decisions found that the factors that played key roles for new firms were access to customers and the growing market, labor force, transportation, personal reasons of management, and availability of capital. The cost of land had the least important influence. For firms that expanded across states, the influential factors were labor costs and labor supply. Companies seeking expansion usually have made their production more routine, so quick access to suppliers is less of a concern. Cost minimization becomes more relevant to existing corporations.2

===Site Selection Process===
The decision for the location of a facility is part of a larger corporate planning process. Usually, a corporate planning office or a division of the company initiates the site selection process by forecasting future capacity requirements. If capacity shortages are in the forecasts, the managers may choose to outsource, increase price to reduce the demand, expand existing sites, or open a new facility. If the managers decide on a new facility, the site selection team enters the project.

The structure of the site selection team depends on the firm's organization. In companies with centralized staff, the site selection team generally consists of representatives from relevant areas, such as engineering, real estate, and transportation. In companies with strong divisions, the locational decisions may be done at the divisional level, with the corporate office supervising the process.

The site selection team determines what characteristics are important for the new location by considering how the new facility will fit in the company's overall strategy, if the company wants to target new markets, if the corporation wants to divide or integrate its functions, or how the company wants to be seen by the public. Next, potential locations are listed and studied against the desired characteristics.

Locational decisions are typically made sequentially. The first step is at the state or regional level. Then, the team studies specific communities and sites. Different locational factors are important to different stages. When selecting a general region, the site selection team focuses on factors with interregional variations such as labor, tax policies, climate, and market locations. At the more focused stage, details like inexpensive land, access to to major roads, and good schools are important. Consultants are often hired to do site analyses.

Once site options are narrowed down, the company discusses potential problems and incentives with local public officials. Construction costs are estimated, and a feasibility analysis is done to show that the project has a high rate of return.2

===Biofuel Supply Chain Example===
Biofuels is a popular potential alternative to fossil fuels. Various biomass resources, including food crops, non-food crops, and agricultural residues, are converted into biofuels. The two most common biofuels currently are ethanol and biodiesel. The supply chain for biofuel is a network consisting of several nodes: biomass cultivation sites, biofuel production plants, and demand centers. The locations of these facilities, and the location of the demand centers with respect to the biofuel demand impact transportation costs, production resource, demand, etc., and thus affects and sets constraints to the cost minimization of the supply chain. The optimal supply chain design here minimizes the total cost, which is a sum of the total investment cost, production cost, transportation cost, and outsourcing and import cost. The total transportation cost captures the transportation of biomass and biofuel between areas and the local bio mass transfer. Location of the facilities also sets constraints on the regional demand of biofuel and availability of local biomass. Sustainability constraints ensure that plants do not negatively affect the food production, sustainably use the land, and don't compete with other industries that use biomass.4

[[File:biofuelsupplychain.png|400px|thumb|center|'''Figure 1.''' The biofuels supply chain consists of three notes: biomass cultivation sites, biofuel production plants, and biofuel demand centers.1]]

[[File:Biofueloptimal.png|300px|thumb|center|'''Figure 2.''' An example of a Great Britain biofuel supply chain design that minimizes cost. The plants are near areas with high raw material supply. The locations of the plants with respect to the demand centers are also optimized. The main modes of transportation are road, rail, and ship.4]]

=Site Layout=
The process units and buildings need to be arranged in such a way that allows for the most economical flow of materials and people. Dangerous processes need to be a safe distance from other buildings, and the layout should be planned to allow for future expansion.

Process units are usually laid out first in an arrangement that allows for smooth flow of materials between the process steps. The distance between equipment is usually at least 30 m. Next, the location of the principal ancillary buildings are sited as to minimize the time that it takes the workers to travel between buildings. Administrative offices and laboratories are located away from hazardous processes. Control rooms are next to the processing equipment. Utility buildings are located as to minimize piping between the process units. Storage is placed between the loading and unloading facilities and next to the process units that they serve. Tanks containing hazardous material are placed at least 70 m from the plant. An example of a typical site plan is shown below.1

[[File:Site Location.png|400px|thumb|center|'''Figure 3.''' A typical site layout.1]]

===Site Layout Factors===
The main factors that are considered when planning the layout of the plant are listed below.1

'''1. Economic considerations (construction and operating costs):'''
Construction costs can be minimized by arranging process units and buildings that minimize pipes between equipment, the amount of structural steel work, etc. However, this layout may conflict with the layout that gives the optimal operation and maintenance.

'''2. Process requirements:''' Examples of process considerations that must be taken into account is the elevation of the base of columns to give enough net positive suction head to a pump.

'''3. Operation convenience:''' Process units that are attended to frequently should be placed with convenient access. Valves, heads, and sample heads should be placed where operators can easily access. If the plant anticipates replacement of equipment, space must be allowed for removal and installation.

'''4. Maintenance convenience:''' Equipment that requires maintenance should be in a location with easy access, and should have sufficient space for the maintenance tasks. For example, shell-and-tube exchangers need space so that tube bundles can be removed for cleaning and repair.

'''5. Future expansion:''' The layout should be planned to conveniently allow for future expansion of processes. Pipe racks should have space for future piping, and pipes should be oversized to allow for more flow in the future.

'''6. Modular Construction:''' Modular construction is where sections of the plant is constructed outside of the plant, and then transported to the site by road or sea. Advantages include improved quality control, reduced construction costs, less requirements for skilled labor on site. Tradeoffs are more flanged connections and possible problems with onsite assembly.

'''7. Safety:''' Escape routes for workers need to be in place at each level in process buildings. Blast walls must isolate equipment that pose hazards to confine potential explosions.

First, a conceptual flowsheet for the process is developed. The types of equipment and their connections with each other is described in a process flow diagram (PFD). Before the PFD is translated into detailed piping and instrumentation diagrams (P&ID) the layout of the process units must be planned.2,3 Scale drawings are made to show the relationships between storage space and process equipment based on the flow of materials and people, and on future expansion. Three-dimensional visualization are the layouts are then carried out with cardboard cutouts of the equipment outlines or rectangular and cylindrical blocks. When a layout of the major process units has been decided, drawings of the plan and elevation are made, and design of the structural steelwork and foundations are done. Computer-aided design has also become increasingly popular.1,3,7

=Environmental Considerations=
Laws that protect the environment restrict the waste that plants can emit in order to preserve air, land, and water quality. States, provinces, and municipals usually have additional laws to national ones. In the US, environmental policies are moving towards more uniform and strict regulations. Businesses that produce more pollution are more likely to move to regions where local regulations are more lax, and industries in areas with more stringent policies will decline. The tighter national regulations are implemented to prevent this competition between regions. For example, amendments to the Clean Air Act and the Environmental Protection Agencies in the 1970s and 1980s reduced differences in regions' air pollution controls. Congress voted on the PSD (prevention of significant deterioration) to "level the playing field" across regions.1

Some of the main environmental legislations in North America are:

*'''The National Environmental Policy Act of 1969 (NEPA)''' - Signed by President Nixon, NEPA was one of the first laws that provide a national framework for the protection of the environment. It requires that all executive federal agencies consider the environmental impact of proposed actions. Government agencies must prepare environmental assessments and environmental impact statements that describe potential environmental effects. NEPA is implemented when buildings, airports, military complexes, parkland purchases, and highways are proposed.6

*'''The Clean Air Act (CAA, 1970)''' - The CAA was implemented to control air quality on a national level. The law was amended in 1990 to address issues such as acid rain, ozone depletion, and toxic air pollution, and to increase enforcement authority. Under the National Ambient Air Quality Standards (NAAQS), the law sets limits to allowable ambient levels of seven contaminants: ozone, carbon monoxide, lead, nitrogen dioxide, sulfur dioxide, PM10, and PM2.5. The CAA also requires that the EPA set US National Emissions Standards for Hazardous Air Pollutants (NESHAP), which regulates the emission of 189 hazardous air pollutants. The Maximum Available Control Technology (MACT) was implemented to limit the emission of volatile organics, sulfur, and other inorganic compounds from oil refineries. The CAA has had significant impact on the level of air pollutants that it regulates. Particularly in the Midwest and the Northwest, water acidity has been reduced. On the other hand, reduction of ozone damage has not been as successful since nitrogen oxide, which is linked to ozone formation, is not as tightly regulated.

*'''The Federal Water Pollution Control Act (The Clean Water Act, 1972)''' - The Clean Water Act was initially passed to clean water used for swimming and boating, and to protect wildlife. Amendments were later added to address water quality and toxic compound emissions. Under the CWA, the EPA can set water quality standards and monitor the industries' discharge into waters.

*'''The Safe Drinking Water Act (SDWA, 1974)''' - This act allows the EPA to set standards to potential drinking water. Public water systems must operate according to these standards.

*'''The Resource Conservation and Recovery Act (RCRA, 1976)''' - The RCRA protects current and future groundwater facilities from contamination. Firms that produce waste must have a "cradle to grave" approach to managing waste, meaning that the waste producer is legally responsible for the waste from the moment it is produced until it is finally disposed.

*'''The Comprehensive Environmental Response, Compensation and Liability Act (CERCLA or Superfund, 1980)''' - This legislation creates a tax on chemical and petroleum industries in order to cover the remediation of hazardous waste sites that are uncontrolled or abandoned. The CERCLA empowers EPA to remediate waste sites by short-term removal of material or long-term remediation actions, depending on the risk that the waste imposes on life.

*The Superfund Amendments and Reauthorization Act (SARA, 1986)
*'''The Pollution Prevention Act (PPA, 1990)''' - This act promotes minimization of waste and pollution at their source. It encourages more efficient processing and raw materials and recycling through grants, technical assistant, and information.

*'''The Oil Pollution Act of 1990 (OPA, 1990)''' - This act was passed after the Exxon Valdez oil spill in the Prince William Sound in Alaska, creating a tax on oil to cover the cost of responding to spills when the corporation that creates the spill does not respond.

*'''The Department of the Environment Act (E-10)''' - The Department of Environment Act created the Department of Environment and charges the Minister of the Environment with creating programs that protect the environment, assessing environmental impacts of federal program, and reporting information to the public in Canada.

*'''The Canadian Environmental Protection Act (CEPA, C-15.31, 1999)''' - CEPA empowers Environment Canada to control toxic substances, reduce pollution, and reduce bioaccumulating toxic substances. Canada will use CEPA to implement regulations that helps it meet its commitments to the Kyoto Protocol.

*'''The Canada Water Act (C-11)''' - This law allows the Minister of the Environment to manage water resources in Canada. This includes research, implementing conservation programs, utilization of water resources, and setting water quality standards.

Wastes comprise mainly of by-products, unused reactants, and off-specification product produced by misoperation. Leaking seals and flanges, and spills and discharges also emit waste. Material is also discharged in emergency situations. Instead of considering how to treat or manage waste, designers should start by tackling the source and find ways to minimize the production of waste.

The process of designing waste management systems are: (1) source reduction, (2) waste stream recycle, (3) waste treatment to reduce environmental impact, and (4) disposal that is legally sound. Source reduction can be achieved by reducing the concentration of impurities in the feed, protecting catalysts and adsorbents from contaminants, eliminating the use of extraneous materials, increasing recovery from separation, and improving the quality of the fuel by switching to cleaner-burning fuel. Unused feed can be recycled, and off-specification products can be reprocessed. In integral processes, the waste of one process is used as the feed for another. By-products can also be sold to another company for use as raw material. Tighter control systems, alarm, and interlocks can reduce misoperation of process unit.1

Further information about environmental considerations can be found in the [[Environmental concerns]] page.

=Conclusion=
The location and layout of a plant can greatly impact its economic and operational success. Objectives such as cost minimization, distribution, room for expansion, and safety of the plant operators and local community play important roles in the site decision. The major factors affecting this decision are market and labor. National and regional environmental legislations impact the processes, and thus make some areas more desirable than others. Least important are land costs and financial incentives. When planning to build a new facility or expanding an existing site, companies utilize their site selection team to assemble a list of desirable locational characteristics and determine how the new facility will fit in with the overall company. Then with the help of consultants, a location is narrowed down and further analyses are done to justify the continuation of the project. The location should be obtained before design of the details of the process. Within the site itself, process units are arranged to allow for the most economical flow of people and material, optimal safety, and room for future expansion.

=References=
[1] G.P. Towler, R. Sinnott, ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design'', Elsevier, 2012.

[2] J.P. Blair, R. Premus. Major Factors in Industrial Location: A Review. Economic Development Quarterly. 1987;1(1):72-85.

[3] M.S. Peters, K.D. Timmerhaus, ''Plant Design and Economics for Chemical Engineers'', 5th Ed., McGraw-Hill: New York, 2003.

[4] O. Akgul, N. Shah, L.G. Papageorgiou. Economic optimisation of a UK advanced biofuel supply chain. Biomass and Bioenergy. 2012;41:57-72.

[5] L.T. Biegler, I.E. Grossmann, A.W. Westerberg, ''Systematic Methods of Chemical Process Design'', Prentice-Hall: Upper Saddle River, 1997.

[6] Summary of the National Environmental Policy Act website. http://www.epa.gov/laws-regulations/summary-national-environmental-policy-act. August 2015. Accessed February 2016.

[7] R.T. Turton, R.C. Bailie, W.B. Whiting, J.A. Shaeiwitz, ''Analysis, Synthesis, and Design of Chemical Processes'', Prentice Hall: Upper Saddle River, 2003.

[8] R.W. Schmenner, Look Beyond the Obvious in Plant Location website. https://hbr.org/1979/01/look-beyond-the-obvious-in-plant-location. January 1979. Accessed February 2016.

Process location and layout decisions

2016-02-22T02:40:30Z

Ezhuang: /* Biofuel Supply Chain Example */

Author: Ellen Zhuang [2016]

Stewards: Daniel Garcia, and Fengqi You

=Introduction=
Location is one of the first decisions in the design of a new chemical plant. It impacts profitability and scope for future expansion. If the project is a new facility, a suitable site must be found and an optimal layout of the site and process units must be planned. If the project adds to an existing site, the impact of the new addition on the existing plant must be considered. The plant also needs to accommodate for the nearby infrastructure, the services that it requires, and its environmental impacts.

=Location Selection=
===Location Decision Factors===
Initially, economists viewed the plant location decision as a cost-minimization problem. The optimal location was one where the transportation costs of raw material to the plant and product to the market was minimized. Between the early 1900s and post World War II period, most industries were sensitive to the cost of transportation. As transportation costs became less of an obstacle, the approach to the problem developed with considerations of trade-offs between transportation and factors such as wages, energy, local regulations, etc. If lower wages could balance the higher transportation costs of building the plant in a low-wage area, the low-wage location may be more desirable.

Although location is a long-term investment, a firm does not decide on a location with the sole objective of maximizing its profits or minimizing its costs. Managers may choose a “safer” location that is more likely to produce higher profits rather than a riskier location that yields the maximum investment return. Personal factors of the people involved are also influences. For example, new businesses tend to locate where the founders live.2

In the current age, a plant's site is chosen based on several factors. These include:1,2,3,8

'''1. Raw material supply:''' The source and price of raw materials is one of the most important factors that determine the location of a plant. Facilities that produce chemicals in bulk are usually located close to the source of raw material if the costs of shipping the product is less than the costs of shipping the feed. For example, ethylene production is growing in the Middle East since cheap ethane from natural gas is readily available. Oil refineries tend to be located near areas with high population and crude oil supplies since it is expensive to transport the oil.

'''2. Location with respect to market:''' If the plant produces high-volume and low-cost products, such as cement and fertilizer, it may be better to situate the plant closer to the primary market since transportation cost is a large fraction of the sales price. If the product is low-volume and high-cost, like pharmaceuticals, then the benefits of being closer to the primary market may not be there.

'''3. Transport facilities:''' Facilities should be close to at least two major forms of transportation, whether that be road, rail, waterway, and/or seaport. For example, paper manufacturing plants in the US use various types of pulp that are delivered by truck or by train from various places in North and South America, so paper facilities need to be close to rail and major roads. Transportation by road is common for local distribution from central warehouses, while transportation by rail is more widespread for long distance transport of bulk chemicals. Pipeline is used to ship industrial gases and bulk fuels. Air freight can be efficient for shipment of personnel and essential units and supplies and for small volume products that have high value, such as pharmaceuticals. Of course, products that are delivered by air must meet aviation regulations.

'''4. Availability of labor:''' Skilled workers are usually brought to the plant from outside the area. There should be a local pool of unskilled labor that can be trained to operate the plant, and of skilled craft workers to maintain the process units. Local labor laws, trade union customs, restrictive practices for recruitment and training should also be taken into consideration. A 10% increase in unionization of a state's labor force is projected to reduce the number of expanding facilities by 30 to 45%.

'''5. Availability of utilities:''' Processes that require a substantial amount of cooling water is usually located near water sources, such as rivers or wells. Cooling water may be directly taken from the water source, or may be stored in cooling towers. Those that need large quantities of power, such as electrochemical ones, are typically close to cheap power sources.

'''6. Availability of suitable land:''' The ideal land is flat, well-drained, with suitable load-bearing characteristics. Further considerations have to be made if the land is reclaimed land near the ocean in earthquake zones. Property tax is also a factor when choosing a site since property taxes vary area to area. Under a third of plants that relocate move to regions with lower property taxes, which is the proportion that would be expected if companies move to a new location regardless of property tax. High property taxes is not as significant as other factors such as labor supply and land costs.

'''7. Environmental impact:''' Depending on the location, it may be more difficult and costly to dispose of wastes. During the project design phase, experts are typically consulted to learn more about an area's local regulations. More details about environmental regulations are found below.

'''8. Local community considerations:''' State and local planners are typically motivated by the desire to create jobs and improve the tax base. Introduction of facilities to an area is usually viewed as the most direct way to stimulate the area's economy. However, recent studies have found that communities with high-growth are already characterized by the fast growth of businesses that are already there. It is rare for a plant to completely close in one area and relocate to another, and if plants do relocate, the majority is over short distances and often within the same community. Therefore, local policymakers favor the expansion of existing plants. The opening of a new plant at a location should impost no additional risk to the local residents. For example, they should be downwind of the residential areas. Local communities also need to be able to accommodate the plant personnelles. For example, traffic, housing, and facilities must be able to accommodate the influx of workers. Additional factors are property taxes and water consumption.

'''9. Climate:''' The climate of the area may affect processes and costs. For example, plants in cold areas need more insulation and special heating. Facilities in earthquake areas need to be seismically sound. Plants in areas with high ambient humidity will usually use air cooling instead of water cooling. More detailed information about the effects of weather can be found in the [[Site condition and design]] page.

'''10. Political and strategic considerations:''' Government sometimes gives capital grants, tax concessions, and other incentives to encourage plants to be built in specific areas. Physical assistance such as roads, water, and other public infrastructure are more popular than financial assistance. Companies can also globalize and take advantage of areas with preferential tariff agreements. The tax policy of an area is inversely related to growth. High personal income sometimes hinder employment growth. Personal income does not affect the cash flow of the company, but it reduces the after-tax income of its managers, and thus high personal income tax can be classified as a personal region. High state corporate taxes has also and detrimental effects on growth, but that is not always the case. Corporate tax is more important to firms with high capital expenditures.

===Analysis of the Importance of Factors===
In 1963, Morgan surveyed 17 companies and found that on average the most important factors on industrial location decisions are market, labor, and raw material. Taxes and financial incentives were of little significance. More recently, Fortune magazine surveyed among the 1000 largest US companies and found that the most important factors were market and labor. The survey also found that personal preferences of a firm's executive, tax, and central cities have great influence over the site of corporate headquarters. Surveys of interstate locational decisions found that the factors that played key roles for new firms were access to customers and the growing market, labor force, transportation, personal reasons of management, and availability of capital. The cost of land had the least important influence. For firms that expanded across states, the influential factors were labor costs and labor supply. Companies seeking expansion usually have made their production more routine, so quick access to suppliers is less of a concern. Cost minimization becomes more relevant to existing corporations.2

===Site Selection Process===
The decision for the location of a facility is part of a larger corporate planning process. Usually, a corporate planning office or a division of the company initiates the site selection process by forecasting future capacity requirements. If capacity shortages are in the forecasts, the managers may choose to outsource, increase price to reduce the demand, expand existing sites, or open a new facility. If the managers decide on a new facility, the site selection team enters the project.

The structure of the site selection team depends on the firm's organization. In companies with centralized staff, the site selection team generally consists of representatives from relevant areas, such as engineering, real estate, and transportation. In companies with strong divisions, the locational decisions may be done at the divisional level, with the corporate office supervising the process.

The site selection team determines what characteristics are important for the new location by considering how the new facility will fit in the company's overall strategy, if the company wants to target new markets, if the corporation wants to divide or integrate its functions, or how the company wants to be seen by the public. Next, potential locations are listed and studied against the desired characteristics.

Locational decisions are typically made sequentially. The first step is at the state or regional level. Then, the team studies specific communities and sites. Different locational factors are important to different stages. When selecting a general region, the site selection team focuses on factors with interregional variations such as labor, tax policies, climate, and market locations. At the more focused stage, details like inexpensive land, access to to major roads, and good schools are important. Consultants are often hired to do site analyses.

Once site options are narrowed down, the company discusses potential problems and incentives with local public officials. Construction costs are estimated, and a feasibility analysis is done to show that the project has a high rate of return.2

===Biofuel Supply Chain Example===
Biofuels is a popular potential alternative to fossil fuels. Various biomass resources, including food crops, non-food crops, and agricultural residues, are converted into biofuels. The two most common biofuels currently are ethanol and biodiesel. The supply chain for biofuel is a network consisting of several nodes: biomass cultivation sites, biofuel production plants, and demand centers. The locations of these facilities, and the location of the demand centers with respect to the biofuel demand impact transportation costs, production resource, demand, etc., and thus affects and sets constraints to the cost minimization of the supply chain. The optimal supply chain design here minimizes the total cost, which is a sum of the total investment cost, production cost, transportation cost, and outsourcing and import cost. The total transportation cost captures the transportation of biomass and biofuel between areas and the local bio mass transfer. Location of the facilities also sets constraints on the regional demand of biofuel and availability of local biomass. Sustainability constraints ensure that plants do not negatively affect the food production, sustainably use the land, and don't compete with other industries that use biomass.4

[[File:biofuelsupplychain.png|400px|thumb|center|'''Figure 1.''' The biofuels supply chain consists of three notes: biomass cultivation sites, biofuel production plants, and biofuel demand centers.1]]

[[File:Biofueloptimal.png|300px|thumb|center|'''Figure 2.''' An example of a Great Britain biofuel supply chain design that minimizes cost. The plants are near areas with high raw material supply. The locations of the plants with respect to the demand centers are also optimized. The main modes of transportation are road, rail, and ship.4]]

=Site Layout=
The process units and buildings need to be arranged in such a way that allows for the most economical flow of materials and people. Furthermore, dangerous processes need to be a safe distance from other buildings, and the layout should be planned to allow for future expansion.

Process units are usually laid out first in an arrangement that allows for smooth flow of materials between the process steps. The distance between equipment is usually at least 30 m. Next, the location of the principal ancillary buildings are sited as to minimize the time that it takes the workers to travel between buildings. Administrative offices and laboratories are located away from hazardous processes. Control rooms are next to the processing equipment. Utility buildings are located as to minimize piping between the process units. Storage is placed between the loading and unloading facilities and next to the process units that they serve. Tanks containing hazardous material are placed at least 70 m from the plant. An example of a typical site plan is shown below.1

[[File:Site Location.png|400px|thumb|center|'''Figure 3.''' A typical site layout.1]]

===Site Layout Factors===
The main factors that are considered when planning the layout of the plant are listed below.1

'''1. Economic considerations (construction and operating costs):'''
Construction costs can be minimized by arranging process units and buildings that minimize pipes between equipment, the amount of structural steel work, etc. However, this layout may conflict with the layout that gives the optimal operation and maintenance.

'''2. Process requirements:''' Examples of process considerations that must be taken into account is the elevation of the base of columns to give enough net positive suction head to a pump.

'''3. Operation convenience:''' Process units that are attended to frequently should be placed with convenient access. Valves, heads, and sample heads should be placed where operators can easily access. If the plant anticipates replacement of equipment, space must be allowed for removal and installation.

'''4. Maintenance convenience:''' Equipment that requires maintenance should be in a location with easy access, and should have sufficient space for the maintenance tasks. For example, shell-and-tube exchangers need space so that tube bundles can be removed for cleaning and repair.

'''5. Future expansion:''' The layout should be planned to conveniently allow for future expansion of processes. Pipe racks should have space for future piping, and pipes should be oversized to allow for more flow in the future.

'''6. Modular Construction:''' Modular construction is where sections of the plant is constructed outside of the plant, and then transported to the site by road or sea. Advantages include improved quality control, reduced construction costs, less requirements for skilled labor on site. Tradeoffs are more flanged connections and possible problems with onsite assembly.

'''7. Safety:''' Escape routes for workers need to be in place at each level in process buildings. Blast walls must isolate equipment that pose hazards to confine potential explosions.

First, a conceptual flowsheet for the process is developed. The types of equipment and their connections with each other is described in a process flow diagram (PFD). Before the PFD is translated into detailed piping and instrumentation diagrams (P&ID) the layout of the process units must be planned.2,3 Scale drawings are made to show the relationships between storage space and process equipment based on the flow of materials and people, and on future expansion. Three-dimensional visualization are the layouts are then carried out with cardboard cutouts of the equipment outlines or rectangular and cylindrical blocks. When a layout of the major process units has been decided, drawings of the plan and elevation are made, and design of the structural steelwork and foundations are done. Computer-aided design has also become increasingly popular.1,3,7

=Environmental Considerations=
Laws that protect the environment restrict the waste that plants can emit in order to preserve air, land, and water quality. States, provinces, and municipals usually have additional laws to national ones. In the US, environmental policies are moving towards more uniform and strict regulations. Businesses that produce more pollution are more likely to move to regions where local regulations are more lax, and industries in areas with more stringent policies will decline. The tighter national regulations are implemented to prevent this competition between regions. For example, amendments to the Clean Air Act and the Environmental Protection Agencies in the 1970s and 1980s reduced differences in regions' air pollution controls. Congress voted on the PSD (prevention of significant deterioration) to "level the playing field" across regions.1

Some of the main environmental legislations in North America are:

*'''The National Environmental Policy Act of 1969 (NEPA)''' - Signed by President Nixon, NEPA was one of the first laws that provide a national framework for the protection of the environment. It requires that all executive federal agencies consider the environmental impact of proposed actions. Government agencies must prepare environmental assessments and environmental impact statements that describe potential environmental effects. NEPA is implemented when buildings, airports, military complexes, parkland purchases, and highways are proposed.6

*'''The Clean Air Act (CAA, 1970)''' - The CAA was implemented to control air quality on a national level. The law was amended in 1990 to address issues such as acid rain, ozone depletion, and toxic air pollution, and to increase enforcement authority. Under the National Ambient Air Quality Standards (NAAQS), the law sets limits to allowable ambient levels of seven contaminants: ozone, carbon monoxide, lead, nitrogen dioxide, sulfur dioxide, PM10, and PM2.5. The CAA also requires that the EPA set US National Emissions Standards for Hazardous Air Pollutants (NESHAP), which regulates the emission of 189 hazardous air pollutants. The Maximum Available Control Technology (MACT) was implemented to limit the emission of volatile organics, sulfur, and other inorganic compounds from oil refineries. The CAA has had significant impact on the level of air pollutants that it regulates. Particularly in the Midwest and the Northwest, water acidity has been reduced. On the other hand, reduction of ozone damage has not been as successful since nitrogen oxide, which is linked to ozone formation, is not as tightly regulated.

*'''The Federal Water Pollution Control Act (The Clean Water Act, 1972)''' - The Clean Water Act was initially passed to clean water used for swimming and boating, and to protect wildlife. Amendments were later added to address water quality and toxic compound emissions. Under the CWA, the EPA can set water quality standards and monitor the industries' discharge into waters.

*'''The Safe Drinking Water Act (SDWA, 1974)''' - This act allows the EPA to set standards to potential drinking water. Public water systems must operate according to these standards.

*'''The Resource Conservation and Recovery Act (RCRA, 1976)''' - The RCRA protects current and future groundwater facilities from contamination. Firms that produce waste must have a "cradle to grave" approach to managing waste, meaning that the waste producer is legally responsible for the waste from the moment it is produced until it is finally disposed.

*'''The Comprehensive Environmental Response, Compensation and Liability Act (CERCLA or Superfund, 1980)''' - This legislation creates a tax on chemical and petroleum industries in order to cover the remediation of hazardous waste sites that are uncontrolled or abandoned. The CERCLA empowers EPA to remediate waste sites by short-term removal of material or long-term remediation actions, depending on the risk that the waste imposes on life.

*The Superfund Amendments and Reauthorization Act (SARA, 1986)
*'''The Pollution Prevention Act (PPA, 1990)''' - This act promotes minimization of waste and pollution at their source. It encourages more efficient processing and raw materials and recycling through grants, technical assistant, and information.

*'''The Oil Pollution Act of 1990 (OPA, 1990)''' - This act was passed after the Exxon Valdez oil spill in the Prince William Sound in Alaska, creating a tax on oil to cover the cost of responding to spills when the corporation that creates the spill does not respond.

*'''The Department of the Environment Act (E-10)''' - The Department of Environment Act created the Department of Environment and charges the Minister of the Environment with creating programs that protect the environment, assessing environmental impacts of federal program, and reporting information to the public in Canada.

*'''The Canadian Environmental Protection Act (CEPA, C-15.31, 1999)''' - CEPA empowers Environment Canada to control toxic substances, reduce pollution, and reduce bioaccumulating toxic substances. Canada will use CEPA to implement regulations that helps it meet its commitments to the Kyoto Protocol.

*'''The Canada Water Act (C-11)''' - This law allows the Minister of the Environment to manage water resources in Canada. This includes research, implementing conservation programs, utilization of water resources, and setting water quality standards.

Wastes comprise mainly of by-products, unused reactants, and off-specification product produced by misoperation. Leaking seals and flanges, and spills and discharges also emit waste. Material is also discharged in emergency situations. Instead of considering how to treat or manage waste, designers should start by tackling the source and find ways to minimize the production of waste.

The process of designing waste management systems are: (1) source reduction, (2) waste stream recycle, (3) waste treatment to reduce environmental impact, and (4) disposal that is legally sound. Source reduction can be achieved by reducing the concentration of impurities in the feed, protecting catalysts and adsorbents from contaminants, eliminating the use of extraneous materials, increasing recovery from separation, and improving the quality of the fuel by switching to cleaner-burning fuel. Unused feed can be recycled, and off-specification products can be reprocessed. In integral processes, the waste of one process is used as the feed for another. By-products can also be sold to another company for use as raw material. Tighter control systems, alarm, and interlocks can reduce misoperation of process unit.1

Further information about environmental considerations can be found in the [[Environmental concerns]] page.

=Conclusion=
The location and layout of a plant can greatly impact its economic and operational success. Objectives such as cost minimization, distribution, room for expansion, and safety of the plant operators and local community play important roles in the site decision. The major factors affecting this decision are market and labor. National and regional environmental legislations impact the processes, and thus make some areas more desirable than others. Least important are land costs and financial incentives. When planning to build a new facility or expanding an existing site, companies utilize their site selection team to assemble a list of desirable locational characteristics and determine how the new facility will fit in with the overall company. Then with the help of consultants, a location is narrowed down and further analyses are done to justify the continuation of the project. The location should be obtained before design of the details of the process. Within the site itself, process units are arranged to allow for the most economical flow of people and material, optimal safety, and room for future expansion.

=References=
[1] G.P. Towler, R. Sinnott, ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design'', Elsevier, 2012.

[2] J.P. Blair, R. Premus. Major Factors in Industrial Location: A Review. Economic Development Quarterly. 1987;1(1):72-85.

[3] M.S. Peters, K.D. Timmerhaus, ''Plant Design and Economics for Chemical Engineers'', 5th Ed., McGraw-Hill: New York, 2003.

[4] O. Akgul, N. Shah, L.G. Papageorgiou. Economic optimisation of a UK advanced biofuel supply chain. Biomass and Bioenergy. 2012;41:57-72.

[5] L.T. Biegler, I.E. Grossmann, A.W. Westerberg, ''Systematic Methods of Chemical Process Design'', Prentice-Hall: Upper Saddle River, 1997.

[6] Summary of the National Environmental Policy Act website. http://www.epa.gov/laws-regulations/summary-national-environmental-policy-act. August 2015. Accessed February 2016.

[7] R.T. Turton, R.C. Bailie, W.B. Whiting, J.A. Shaeiwitz, ''Analysis, Synthesis, and Design of Chemical Processes'', Prentice Hall: Upper Saddle River, 2003.

[8] R.W. Schmenner, Look Beyond the Obvious in Plant Location website. https://hbr.org/1979/01/look-beyond-the-obvious-in-plant-location. January 1979. Accessed February 2016.

Process location and layout decisions

2016-02-22T02:38:45Z

Ezhuang: /* Biofuel Supply Chain Example */

Author: Ellen Zhuang [2016]

Stewards: Daniel Garcia, and Fengqi You

=Introduction=
Location is one of the first decisions in the design of a new chemical plant. It impacts profitability and scope for future expansion. If the project is a new facility, a suitable site must be found and an optimal layout of the site and process units must be planned. If the project adds to an existing site, the impact of the new addition on the existing plant must be considered. The plant also needs to accommodate for the nearby infrastructure, the services that it requires, and its environmental impacts.

=Location Selection=
===Location Decision Factors===
Initially, economists viewed the plant location decision as a cost-minimization problem. The optimal location was one where the transportation costs of raw material to the plant and product to the market was minimized. Between the early 1900s and post World War II period, most industries were sensitive to the cost of transportation. As transportation costs became less of an obstacle, the approach to the problem developed with considerations of trade-offs between transportation and factors such as wages, energy, local regulations, etc. If lower wages could balance the higher transportation costs of building the plant in a low-wage area, the low-wage location may be more desirable.

Although location is a long-term investment, a firm does not decide on a location with the sole objective of maximizing its profits or minimizing its costs. Managers may choose a “safer” location that is more likely to produce higher profits rather than a riskier location that yields the maximum investment return. Personal factors of the people involved are also influences. For example, new businesses tend to locate where the founders live.2

In the current age, a plant's site is chosen based on several factors. These include:1,2,3,8

'''1. Raw material supply:''' The source and price of raw materials is one of the most important factors that determine the location of a plant. Facilities that produce chemicals in bulk are usually located close to the source of raw material if the costs of shipping the product is less than the costs of shipping the feed. For example, ethylene production is growing in the Middle East since cheap ethane from natural gas is readily available. Oil refineries tend to be located near areas with high population and crude oil supplies since it is expensive to transport the oil.

'''2. Location with respect to market:''' If the plant produces high-volume and low-cost products, such as cement and fertilizer, it may be better to situate the plant closer to the primary market since transportation cost is a large fraction of the sales price. If the product is low-volume and high-cost, like pharmaceuticals, then the benefits of being closer to the primary market may not be there.

'''3. Transport facilities:''' Facilities should be close to at least two major forms of transportation, whether that be road, rail, waterway, and/or seaport. For example, paper manufacturing plants in the US use various types of pulp that are delivered by truck or by train from various places in North and South America, so paper facilities need to be close to rail and major roads. Transportation by road is common for local distribution from central warehouses, while transportation by rail is more widespread for long distance transport of bulk chemicals. Pipeline is used to ship industrial gases and bulk fuels. Air freight can be efficient for shipment of personnel and essential units and supplies and for small volume products that have high value, such as pharmaceuticals. Of course, products that are delivered by air must meet aviation regulations.

'''4. Availability of labor:''' Skilled workers are usually brought to the plant from outside the area. There should be a local pool of unskilled labor that can be trained to operate the plant, and of skilled craft workers to maintain the process units. Local labor laws, trade union customs, restrictive practices for recruitment and training should also be taken into consideration. A 10% increase in unionization of a state's labor force is projected to reduce the number of expanding facilities by 30 to 45%.

'''5. Availability of utilities:''' Processes that require a substantial amount of cooling water is usually located near water sources, such as rivers or wells. Cooling water may be directly taken from the water source, or may be stored in cooling towers. Those that need large quantities of power, such as electrochemical ones, are typically close to cheap power sources.

'''6. Availability of suitable land:''' The ideal land is flat, well-drained, with suitable load-bearing characteristics. Further considerations have to be made if the land is reclaimed land near the ocean in earthquake zones. Property tax is also a factor when choosing a site since property taxes vary area to area. Under a third of plants that relocate move to regions with lower property taxes, which is the proportion that would be expected if companies move to a new location regardless of property tax. High property taxes is not as significant as other factors such as labor supply and land costs.

'''7. Environmental impact:''' Depending on the location, it may be more difficult and costly to dispose of wastes. During the project design phase, experts are typically consulted to learn more about an area's local regulations. More details about environmental regulations are found below.

'''8. Local community considerations:''' State and local planners are typically motivated by the desire to create jobs and improve the tax base. Introduction of facilities to an area is usually viewed as the most direct way to stimulate the area's economy. However, recent studies have found that communities with high-growth are already characterized by the fast growth of businesses that are already there. It is rare for a plant to completely close in one area and relocate to another, and if plants do relocate, the majority is over short distances and often within the same community. Therefore, local policymakers favor the expansion of existing plants. The opening of a new plant at a location should impost no additional risk to the local residents. For example, they should be downwind of the residential areas. Local communities also need to be able to accommodate the plant personnelles. For example, traffic, housing, and facilities must be able to accommodate the influx of workers. Additional factors are property taxes and water consumption.

'''9. Climate:''' The climate of the area may affect processes and costs. For example, plants in cold areas need more insulation and special heating. Facilities in earthquake areas need to be seismically sound. Plants in areas with high ambient humidity will usually use air cooling instead of water cooling. More detailed information about the effects of weather can be found in the [[Site condition and design]] page.

'''10. Political and strategic considerations:''' Government sometimes gives capital grants, tax concessions, and other incentives to encourage plants to be built in specific areas. Physical assistance such as roads, water, and other public infrastructure are more popular than financial assistance. Companies can also globalize and take advantage of areas with preferential tariff agreements. The tax policy of an area is inversely related to growth. High personal income sometimes hinder employment growth. Personal income does not affect the cash flow of the company, but it reduces the after-tax income of its managers, and thus high personal income tax can be classified as a personal region. High state corporate taxes has also and detrimental effects on growth, but that is not always the case. Corporate tax is more important to firms with high capital expenditures.

===Analysis of the Importance of Factors===
In 1963, Morgan surveyed 17 companies and found that on average the most important factors on industrial location decisions are market, labor, and raw material. Taxes and financial incentives were of little significance. More recently, Fortune magazine surveyed among the 1000 largest US companies and found that the most important factors were market and labor. The survey also found that personal preferences of a firm's executive, tax, and central cities have great influence over the site of corporate headquarters. Surveys of interstate locational decisions found that the factors that played key roles for new firms were access to customers and the growing market, labor force, transportation, personal reasons of management, and availability of capital. The cost of land had the least important influence. For firms that expanded across states, the influential factors were labor costs and labor supply. Companies seeking expansion usually have made their production more routine, so quick access to suppliers is less of a concern. Cost minimization becomes more relevant to existing corporations.2

===Site Selection Process===
The decision for the location of a facility is part of a larger corporate planning process. Usually, a corporate planning office or a division of the company initiates the site selection process by forecasting future capacity requirements. If capacity shortages are in the forecasts, the managers may choose to outsource, increase price to reduce the demand, expand existing sites, or open a new facility. If the managers decide on a new facility, the site selection team enters the project.

The structure of the site selection team depends on the firm's organization. In companies with centralized staff, the site selection team generally consists of representatives from relevant areas, such as engineering, real estate, and transportation. In companies with strong divisions, the locational decisions may be done at the divisional level, with the corporate office supervising the process.

The site selection team determines what characteristics are important for the new location by considering how the new facility will fit in the company's overall strategy, if the company wants to target new markets, if the corporation wants to divide or integrate its functions, or how the company wants to be seen by the public. Next, potential locations are listed and studied against the desired characteristics.

Locational decisions are typically made sequentially. The first step is at the state or regional level. Then, the team studies specific communities and sites. Different locational factors are important to different stages. When selecting a general region, the site selection team focuses on factors with interregional variations such as labor, tax policies, climate, and market locations. At the more focused stage, details like inexpensive land, access to to major roads, and good schools are important. Consultants are often hired to do site analyses.

Once site options are narrowed down, the company discusses potential problems and incentives with local public officials. Construction costs are estimated, and a feasibility analysis is done to show that the project has a high rate of return.2

===Biofuel Supply Chain Example===
Biofuels is a popular potential alternative to fossil fuels. Various biomass resources, including food crops, non-food crops, and agricultural residues, are converted into biofuels. The two most common biofuels currently are ethanol and biodiesel. The supply chain for biofuel is a network consisting of several nodes: biomass cultivation sites, biofuel production plants, and demand centers. The locations of these facilities, and the location of the demand centers with respect to the biofuel demand impact transportation costs, production resource, demand, etc., and thus affects and sets constraints to the cost minimization of the supply chain. The optimal supply chain design here minimizes the total cost, which is a sum of the total investment cost, production cost, transportation cost, and outsourcing and import cost. The total transportation cost captures the transportation of biomass and biofuel between areas and the local bio mass transfer. Location of the facilities also sets constraints on the regional demand of biofuel and availability of local biomass. Sustainability constraints ensure that plants do not negatively affect the food production, sustainably use the land, and don't compete with other industries that use biomass.4

[[File:biofuelsupplychain.png|400px|thumb|center|'''Figure 1.''' The biofuels supply chain consists of three notes: biomass cultivation sites, biofuel production plants, and biofuel demand centers.1]]

[[File:Biofueloptimal.png|300px|thumb|center|'''Figure 2.''' A Great Britain biofuel supply chain design that minimizes cost. The plants are near areas with high wheat cultivation and wheat straw collection rates. The location of the plants with respect to the demand centers is also optimized. The main modes of transportation are road, rail, and ship.4]]

=Site Layout=
The process units and buildings need to be arranged in such a way that allows for the most economical flow of materials and people. Furthermore, dangerous processes need to be a safe distance from other buildings, and the layout should be planned to allow for future expansion.

Process units are usually laid out first in an arrangement that allows for smooth flow of materials between the process steps. The distance between equipment is usually at least 30 m. Next, the location of the principal ancillary buildings are sited as to minimize the time that it takes the workers to travel between buildings. Administrative offices and laboratories are located away from hazardous processes. Control rooms are next to the processing equipment. Utility buildings are located as to minimize piping between the process units. Storage is placed between the loading and unloading facilities and next to the process units that they serve. Tanks containing hazardous material are placed at least 70 m from the plant. An example of a typical site plan is shown below.1

[[File:Site Location.png|400px|thumb|center|'''Figure 3.''' A typical site layout.1]]

===Site Layout Factors===
The main factors that are considered when planning the layout of the plant are listed below.1

'''1. Economic considerations (construction and operating costs):'''
Construction costs can be minimized by arranging process units and buildings that minimize pipes between equipment, the amount of structural steel work, etc. However, this layout may conflict with the layout that gives the optimal operation and maintenance.

'''2. Process requirements:''' Examples of process considerations that must be taken into account is the elevation of the base of columns to give enough net positive suction head to a pump.

'''3. Operation convenience:''' Process units that are attended to frequently should be placed with convenient access. Valves, heads, and sample heads should be placed where operators can easily access. If the plant anticipates replacement of equipment, space must be allowed for removal and installation.

'''4. Maintenance convenience:''' Equipment that requires maintenance should be in a location with easy access, and should have sufficient space for the maintenance tasks. For example, shell-and-tube exchangers need space so that tube bundles can be removed for cleaning and repair.

'''5. Future expansion:''' The layout should be planned to conveniently allow for future expansion of processes. Pipe racks should have space for future piping, and pipes should be oversized to allow for more flow in the future.

'''6. Modular Construction:''' Modular construction is where sections of the plant is constructed outside of the plant, and then transported to the site by road or sea. Advantages include improved quality control, reduced construction costs, less requirements for skilled labor on site. Tradeoffs are more flanged connections and possible problems with onsite assembly.

'''7. Safety:''' Escape routes for workers need to be in place at each level in process buildings. Blast walls must isolate equipment that pose hazards to confine potential explosions.

First, a conceptual flowsheet for the process is developed. The types of equipment and their connections with each other is described in a process flow diagram (PFD). Before the PFD is translated into detailed piping and instrumentation diagrams (P&ID) the layout of the process units must be planned.2,3 Scale drawings are made to show the relationships between storage space and process equipment based on the flow of materials and people, and on future expansion. Three-dimensional visualization are the layouts are then carried out with cardboard cutouts of the equipment outlines or rectangular and cylindrical blocks. When a layout of the major process units has been decided, drawings of the plan and elevation are made, and design of the structural steelwork and foundations are done. Computer-aided design has also become increasingly popular.1,3,7

=Environmental Considerations=
Laws that protect the environment restrict the waste that plants can emit in order to preserve air, land, and water quality. States, provinces, and municipals usually have additional laws to national ones. In the US, environmental policies are moving towards more uniform and strict regulations. Businesses that produce more pollution are more likely to move to regions where local regulations are more lax, and industries in areas with more stringent policies will decline. The tighter national regulations are implemented to prevent this competition between regions. For example, amendments to the Clean Air Act and the Environmental Protection Agencies in the 1970s and 1980s reduced differences in regions' air pollution controls. Congress voted on the PSD (prevention of significant deterioration) to "level the playing field" across regions.1

Some of the main environmental legislations in North America are:

*'''The National Environmental Policy Act of 1969 (NEPA)''' - Signed by President Nixon, NEPA was one of the first laws that provide a national framework for the protection of the environment. It requires that all executive federal agencies consider the environmental impact of proposed actions. Government agencies must prepare environmental assessments and environmental impact statements that describe potential environmental effects. NEPA is implemented when buildings, airports, military complexes, parkland purchases, and highways are proposed.6

*'''The Clean Air Act (CAA, 1970)''' - The CAA was implemented to control air quality on a national level. The law was amended in 1990 to address issues such as acid rain, ozone depletion, and toxic air pollution, and to increase enforcement authority. Under the National Ambient Air Quality Standards (NAAQS), the law sets limits to allowable ambient levels of seven contaminants: ozone, carbon monoxide, lead, nitrogen dioxide, sulfur dioxide, PM10, and PM2.5. The CAA also requires that the EPA set US National Emissions Standards for Hazardous Air Pollutants (NESHAP), which regulates the emission of 189 hazardous air pollutants. The Maximum Available Control Technology (MACT) was implemented to limit the emission of volatile organics, sulfur, and other inorganic compounds from oil refineries. The CAA has had significant impact on the level of air pollutants that it regulates. Particularly in the Midwest and the Northwest, water acidity has been reduced. On the other hand, reduction of ozone damage has not been as successful since nitrogen oxide, which is linked to ozone formation, is not as tightly regulated.

*'''The Federal Water Pollution Control Act (The Clean Water Act, 1972)''' - The Clean Water Act was initially passed to clean water used for swimming and boating, and to protect wildlife. Amendments were later added to address water quality and toxic compound emissions. Under the CWA, the EPA can set water quality standards and monitor the industries' discharge into waters.

*'''The Safe Drinking Water Act (SDWA, 1974)''' - This act allows the EPA to set standards to potential drinking water. Public water systems must operate according to these standards.

*'''The Resource Conservation and Recovery Act (RCRA, 1976)''' - The RCRA protects current and future groundwater facilities from contamination. Firms that produce waste must have a "cradle to grave" approach to managing waste, meaning that the waste producer is legally responsible for the waste from the moment it is produced until it is finally disposed.

*'''The Comprehensive Environmental Response, Compensation and Liability Act (CERCLA or Superfund, 1980)''' - This legislation creates a tax on chemical and petroleum industries in order to cover the remediation of hazardous waste sites that are uncontrolled or abandoned. The CERCLA empowers EPA to remediate waste sites by short-term removal of material or long-term remediation actions, depending on the risk that the waste imposes on life.

*The Superfund Amendments and Reauthorization Act (SARA, 1986)
*'''The Pollution Prevention Act (PPA, 1990)''' - This act promotes minimization of waste and pollution at their source. It encourages more efficient processing and raw materials and recycling through grants, technical assistant, and information.

*'''The Oil Pollution Act of 1990 (OPA, 1990)''' - This act was passed after the Exxon Valdez oil spill in the Prince William Sound in Alaska, creating a tax on oil to cover the cost of responding to spills when the corporation that creates the spill does not respond.

*'''The Department of the Environment Act (E-10)''' - The Department of Environment Act created the Department of Environment and charges the Minister of the Environment with creating programs that protect the environment, assessing environmental impacts of federal program, and reporting information to the public in Canada.

*'''The Canadian Environmental Protection Act (CEPA, C-15.31, 1999)''' - CEPA empowers Environment Canada to control toxic substances, reduce pollution, and reduce bioaccumulating toxic substances. Canada will use CEPA to implement regulations that helps it meet its commitments to the Kyoto Protocol.

*'''The Canada Water Act (C-11)''' - This law allows the Minister of the Environment to manage water resources in Canada. This includes research, implementing conservation programs, utilization of water resources, and setting water quality standards.

Wastes comprise mainly of by-products, unused reactants, and off-specification product produced by misoperation. Leaking seals and flanges, and spills and discharges also emit waste. Material is also discharged in emergency situations. Instead of considering how to treat or manage waste, designers should start by tackling the source and find ways to minimize the production of waste.

The process of designing waste management systems are: (1) source reduction, (2) waste stream recycle, (3) waste treatment to reduce environmental impact, and (4) disposal that is legally sound. Source reduction can be achieved by reducing the concentration of impurities in the feed, protecting catalysts and adsorbents from contaminants, eliminating the use of extraneous materials, increasing recovery from separation, and improving the quality of the fuel by switching to cleaner-burning fuel. Unused feed can be recycled, and off-specification products can be reprocessed. In integral processes, the waste of one process is used as the feed for another. By-products can also be sold to another company for use as raw material. Tighter control systems, alarm, and interlocks can reduce misoperation of process unit.1

Further information about environmental considerations can be found in the [[Environmental concerns]] page.

=Conclusion=
The location and layout of a plant can greatly impact its economic and operational success. Objectives such as cost minimization, distribution, room for expansion, and safety of the plant operators and local community play important roles in the site decision. The major factors affecting this decision are market and labor. National and regional environmental legislations impact the processes, and thus make some areas more desirable than others. Least important are land costs and financial incentives. When planning to build a new facility or expanding an existing site, companies utilize their site selection team to assemble a list of desirable locational characteristics and determine how the new facility will fit in with the overall company. Then with the help of consultants, a location is narrowed down and further analyses are done to justify the continuation of the project. The location should be obtained before design of the details of the process. Within the site itself, process units are arranged to allow for the most economical flow of people and material, optimal safety, and room for future expansion.

=References=
[1] G.P. Towler, R. Sinnott, ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design'', Elsevier, 2012.

[2] J.P. Blair, R. Premus. Major Factors in Industrial Location: A Review. Economic Development Quarterly. 1987;1(1):72-85.

[3] M.S. Peters, K.D. Timmerhaus, ''Plant Design and Economics for Chemical Engineers'', 5th Ed., McGraw-Hill: New York, 2003.

[4] O. Akgul, N. Shah, L.G. Papageorgiou. Economic optimisation of a UK advanced biofuel supply chain. Biomass and Bioenergy. 2012;41:57-72.

[5] L.T. Biegler, I.E. Grossmann, A.W. Westerberg, ''Systematic Methods of Chemical Process Design'', Prentice-Hall: Upper Saddle River, 1997.

[6] Summary of the National Environmental Policy Act website. http://www.epa.gov/laws-regulations/summary-national-environmental-policy-act. August 2015. Accessed February 2016.

[7] R.T. Turton, R.C. Bailie, W.B. Whiting, J.A. Shaeiwitz, ''Analysis, Synthesis, and Design of Chemical Processes'', Prentice Hall: Upper Saddle River, 2003.

[8] R.W. Schmenner, Look Beyond the Obvious in Plant Location website. https://hbr.org/1979/01/look-beyond-the-obvious-in-plant-location. January 1979. Accessed February 2016.

Process location and layout decisions

2016-02-22T02:34:06Z

Ezhuang: /* Biofuel Supply Chain Example */

Author: Ellen Zhuang [2016]

Stewards: Daniel Garcia, and Fengqi You

=Introduction=
Location is one of the first decisions in the design of a new chemical plant. It impacts profitability and scope for future expansion. If the project is a new facility, a suitable site must be found and an optimal layout of the site and process units must be planned. If the project adds to an existing site, the impact of the new addition on the existing plant must be considered. The plant also needs to accommodate for the nearby infrastructure, the services that it requires, and its environmental impacts.

=Location Selection=
===Location Decision Factors===
Initially, economists viewed the plant location decision as a cost-minimization problem. The optimal location was one where the transportation costs of raw material to the plant and product to the market was minimized. Between the early 1900s and post World War II period, most industries were sensitive to the cost of transportation. As transportation costs became less of an obstacle, the approach to the problem developed with considerations of trade-offs between transportation and factors such as wages, energy, local regulations, etc. If lower wages could balance the higher transportation costs of building the plant in a low-wage area, the low-wage location may be more desirable.

Although location is a long-term investment, a firm does not decide on a location with the sole objective of maximizing its profits or minimizing its costs. Managers may choose a “safer” location that is more likely to produce higher profits rather than a riskier location that yields the maximum investment return. Personal factors of the people involved are also influences. For example, new businesses tend to locate where the founders live.2

In the current age, a plant's site is chosen based on several factors. These include:1,2,3,8

'''1. Raw material supply:''' The source and price of raw materials is one of the most important factors that determine the location of a plant. Facilities that produce chemicals in bulk are usually located close to the source of raw material if the costs of shipping the product is less than the costs of shipping the feed. For example, ethylene production is growing in the Middle East since cheap ethane from natural gas is readily available. Oil refineries tend to be located near areas with high population and crude oil supplies since it is expensive to transport the oil.

'''2. Location with respect to market:''' If the plant produces high-volume and low-cost products, such as cement and fertilizer, it may be better to situate the plant closer to the primary market since transportation cost is a large fraction of the sales price. If the product is low-volume and high-cost, like pharmaceuticals, then the benefits of being closer to the primary market may not be there.

'''3. Transport facilities:''' Facilities should be close to at least two major forms of transportation, whether that be road, rail, waterway, and/or seaport. For example, paper manufacturing plants in the US use various types of pulp that are delivered by truck or by train from various places in North and South America, so paper facilities need to be close to rail and major roads. Transportation by road is common for local distribution from central warehouses, while transportation by rail is more widespread for long distance transport of bulk chemicals. Pipeline is used to ship industrial gases and bulk fuels. Air freight can be efficient for shipment of personnel and essential units and supplies and for small volume products that have high value, such as pharmaceuticals. Of course, products that are delivered by air must meet aviation regulations.

'''4. Availability of labor:''' Skilled workers are usually brought to the plant from outside the area. There should be a local pool of unskilled labor that can be trained to operate the plant, and of skilled craft workers to maintain the process units. Local labor laws, trade union customs, restrictive practices for recruitment and training should also be taken into consideration. A 10% increase in unionization of a state's labor force is projected to reduce the number of expanding facilities by 30 to 45%.

'''5. Availability of utilities:''' Processes that require a substantial amount of cooling water is usually located near water sources, such as rivers or wells. Cooling water may be directly taken from the water source, or may be stored in cooling towers. Those that need large quantities of power, such as electrochemical ones, are typically close to cheap power sources.

'''6. Availability of suitable land:''' The ideal land is flat, well-drained, with suitable load-bearing characteristics. Further considerations have to be made if the land is reclaimed land near the ocean in earthquake zones. Property tax is also a factor when choosing a site since property taxes vary area to area. Under a third of plants that relocate move to regions with lower property taxes, which is the proportion that would be expected if companies move to a new location regardless of property tax. High property taxes is not as significant as other factors such as labor supply and land costs.

'''7. Environmental impact:''' Depending on the location, it may be more difficult and costly to dispose of wastes. During the project design phase, experts are typically consulted to learn more about an area's local regulations. More details about environmental regulations are found below.

'''8. Local community considerations:''' State and local planners are typically motivated by the desire to create jobs and improve the tax base. Introduction of facilities to an area is usually viewed as the most direct way to stimulate the area's economy. However, recent studies have found that communities with high-growth are already characterized by the fast growth of businesses that are already there. It is rare for a plant to completely close in one area and relocate to another, and if plants do relocate, the majority is over short distances and often within the same community. Therefore, local policymakers favor the expansion of existing plants. The opening of a new plant at a location should impost no additional risk to the local residents. For example, they should be downwind of the residential areas. Local communities also need to be able to accommodate the plant personnelles. For example, traffic, housing, and facilities must be able to accommodate the influx of workers. Additional factors are property taxes and water consumption.

'''9. Climate:''' The climate of the area may affect processes and costs. For example, plants in cold areas need more insulation and special heating. Facilities in earthquake areas need to be seismically sound. Plants in areas with high ambient humidity will usually use air cooling instead of water cooling. More detailed information about the effects of weather can be found in the [[Site condition and design]] page.

'''10. Political and strategic considerations:''' Government sometimes gives capital grants, tax concessions, and other incentives to encourage plants to be built in specific areas. Physical assistance such as roads, water, and other public infrastructure are more popular than financial assistance. Companies can also globalize and take advantage of areas with preferential tariff agreements. The tax policy of an area is inversely related to growth. High personal income sometimes hinder employment growth. Personal income does not affect the cash flow of the company, but it reduces the after-tax income of its managers, and thus high personal income tax can be classified as a personal region. High state corporate taxes has also and detrimental effects on growth, but that is not always the case. Corporate tax is more important to firms with high capital expenditures.

===Analysis of the Importance of Factors===
In 1963, Morgan surveyed 17 companies and found that on average the most important factors on industrial location decisions are market, labor, and raw material. Taxes and financial incentives were of little significance. More recently, Fortune magazine surveyed among the 1000 largest US companies and found that the most important factors were market and labor. The survey also found that personal preferences of a firm's executive, tax, and central cities have great influence over the site of corporate headquarters. Surveys of interstate locational decisions found that the factors that played key roles for new firms were access to customers and the growing market, labor force, transportation, personal reasons of management, and availability of capital. The cost of land had the least important influence. For firms that expanded across states, the influential factors were labor costs and labor supply. Companies seeking expansion usually have made their production more routine, so quick access to suppliers is less of a concern. Cost minimization becomes more relevant to existing corporations.2

===Site Selection Process===
The decision for the location of a facility is part of a larger corporate planning process. Usually, a corporate planning office or a division of the company initiates the site selection process by forecasting future capacity requirements. If capacity shortages are in the forecasts, the managers may choose to outsource, increase price to reduce the demand, expand existing sites, or open a new facility. If the managers decide on a new facility, the site selection team enters the project.

The structure of the site selection team depends on the firm's organization. In companies with centralized staff, the site selection team generally consists of representatives from relevant areas, such as engineering, real estate, and transportation. In companies with strong divisions, the locational decisions may be done at the divisional level, with the corporate office supervising the process.

The site selection team determines what characteristics are important for the new location by considering how the new facility will fit in the company's overall strategy, if the company wants to target new markets, if the corporation wants to divide or integrate its functions, or how the company wants to be seen by the public. Next, potential locations are listed and studied against the desired characteristics.

Locational decisions are typically made sequentially. The first step is at the state or regional level. Then, the team studies specific communities and sites. Different locational factors are important to different stages. When selecting a general region, the site selection team focuses on factors with interregional variations such as labor, tax policies, climate, and market locations. At the more focused stage, details like inexpensive land, access to to major roads, and good schools are important. Consultants are often hired to do site analyses.

Once site options are narrowed down, the company discusses potential problems and incentives with local public officials. Construction costs are estimated, and a feasibility analysis is done to show that the project has a high rate of return.2

===Biofuel Supply Chain Example===
Biofuels is a popular potential alternative to fossil fuels. Various biomass resources, including food crops, non-food crops, and agricultural residues, are converted into biofuels. The two most common biofuels currently are ethanol and biodiesel. The supply chain for biofuel is a network consisting of several nodes: biomass cultivation sites, biofuel production plants, and demand centers. The locations of these facilities, and the location of the demand centers with respect to the biofuel demand impact transportation costs, production resource, demand, etc., and thus affects and sets constraints to the cost minimization of the supply chain. The optimal supply chain design here minimizes the total cost, which is a sum of the total investment cost, production cost, transportation cost, and outsourcing and import cost. The total transportation cost captures the transportation of biomass and biofuel between areas and the local bio mass transfer.4

[[File:biofuelsupplychain.png|400px|thumb|center|'''Figure 1.''' The biofuels supply chain consists of three notes: biomass cultivation sites, biofuel production plants, and biofuel demand centers.1]]

[[File:Biofueloptimal.png|300px|thumb|center|'''Figure 2.''' A Great Britain biofuel supply chain design that minimizes cost. The plants are near areas with high wheat cultivation and wheat straw collection rates. The location of the plants with respect to the demand centers is also optimized. The main modes of transportation are road, rail, and ship.4]]

=Site Layout=
The process units and buildings need to be arranged in such a way that allows for the most economical flow of materials and people. Furthermore, dangerous processes need to be a safe distance from other buildings, and the layout should be planned to allow for future expansion.

Process units are usually laid out first in an arrangement that allows for smooth flow of materials between the process steps. The distance between equipment is usually at least 30 m. Next, the location of the principal ancillary buildings are sited as to minimize the time that it takes the workers to travel between buildings. Administrative offices and laboratories are located away from hazardous processes. Control rooms are next to the processing equipment. Utility buildings are located as to minimize piping between the process units. Storage is placed between the loading and unloading facilities and next to the process units that they serve. Tanks containing hazardous material are placed at least 70 m from the plant. An example of a typical site plan is shown below.1

[[File:Site Location.png|400px|thumb|center|'''Figure 3.''' A typical site layout.1]]

===Site Layout Factors===
The main factors that are considered when planning the layout of the plant are listed below.1

'''1. Economic considerations (construction and operating costs):'''
Construction costs can be minimized by arranging process units and buildings that minimize pipes between equipment, the amount of structural steel work, etc. However, this layout may conflict with the layout that gives the optimal operation and maintenance.

'''2. Process requirements:''' Examples of process considerations that must be taken into account is the elevation of the base of columns to give enough net positive suction head to a pump.

'''3. Operation convenience:''' Process units that are attended to frequently should be placed with convenient access. Valves, heads, and sample heads should be placed where operators can easily access. If the plant anticipates replacement of equipment, space must be allowed for removal and installation.

'''4. Maintenance convenience:''' Equipment that requires maintenance should be in a location with easy access, and should have sufficient space for the maintenance tasks. For example, shell-and-tube exchangers need space so that tube bundles can be removed for cleaning and repair.

'''5. Future expansion:''' The layout should be planned to conveniently allow for future expansion of processes. Pipe racks should have space for future piping, and pipes should be oversized to allow for more flow in the future.

'''6. Modular Construction:''' Modular construction is where sections of the plant is constructed outside of the plant, and then transported to the site by road or sea. Advantages include improved quality control, reduced construction costs, less requirements for skilled labor on site. Tradeoffs are more flanged connections and possible problems with onsite assembly.

'''7. Safety:''' Escape routes for workers need to be in place at each level in process buildings. Blast walls must isolate equipment that pose hazards to confine potential explosions.

First, a conceptual flowsheet for the process is developed. The types of equipment and their connections with each other is described in a process flow diagram (PFD). Before the PFD is translated into detailed piping and instrumentation diagrams (P&ID) the layout of the process units must be planned.2,3 Scale drawings are made to show the relationships between storage space and process equipment based on the flow of materials and people, and on future expansion. Three-dimensional visualization are the layouts are then carried out with cardboard cutouts of the equipment outlines or rectangular and cylindrical blocks. When a layout of the major process units has been decided, drawings of the plan and elevation are made, and design of the structural steelwork and foundations are done. Computer-aided design has also become increasingly popular.1,3,7

=Environmental Considerations=
Laws that protect the environment restrict the waste that plants can emit in order to preserve air, land, and water quality. States, provinces, and municipals usually have additional laws to national ones. In the US, environmental policies are moving towards more uniform and strict regulations. Businesses that produce more pollution are more likely to move to regions where local regulations are more lax, and industries in areas with more stringent policies will decline. The tighter national regulations are implemented to prevent this competition between regions. For example, amendments to the Clean Air Act and the Environmental Protection Agencies in the 1970s and 1980s reduced differences in regions' air pollution controls. Congress voted on the PSD (prevention of significant deterioration) to "level the playing field" across regions.1

Some of the main environmental legislations in North America are:

*'''The National Environmental Policy Act of 1969 (NEPA)''' - Signed by President Nixon, NEPA was one of the first laws that provide a national framework for the protection of the environment. It requires that all executive federal agencies consider the environmental impact of proposed actions. Government agencies must prepare environmental assessments and environmental impact statements that describe potential environmental effects. NEPA is implemented when buildings, airports, military complexes, parkland purchases, and highways are proposed.6

*'''The Clean Air Act (CAA, 1970)''' - The CAA was implemented to control air quality on a national level. The law was amended in 1990 to address issues such as acid rain, ozone depletion, and toxic air pollution, and to increase enforcement authority. Under the National Ambient Air Quality Standards (NAAQS), the law sets limits to allowable ambient levels of seven contaminants: ozone, carbon monoxide, lead, nitrogen dioxide, sulfur dioxide, PM10, and PM2.5. The CAA also requires that the EPA set US National Emissions Standards for Hazardous Air Pollutants (NESHAP), which regulates the emission of 189 hazardous air pollutants. The Maximum Available Control Technology (MACT) was implemented to limit the emission of volatile organics, sulfur, and other inorganic compounds from oil refineries. The CAA has had significant impact on the level of air pollutants that it regulates. Particularly in the Midwest and the Northwest, water acidity has been reduced. On the other hand, reduction of ozone damage has not been as successful since nitrogen oxide, which is linked to ozone formation, is not as tightly regulated.

*'''The Federal Water Pollution Control Act (The Clean Water Act, 1972)''' - The Clean Water Act was initially passed to clean water used for swimming and boating, and to protect wildlife. Amendments were later added to address water quality and toxic compound emissions. Under the CWA, the EPA can set water quality standards and monitor the industries' discharge into waters.

*'''The Safe Drinking Water Act (SDWA, 1974)''' - This act allows the EPA to set standards to potential drinking water. Public water systems must operate according to these standards.

*'''The Resource Conservation and Recovery Act (RCRA, 1976)''' - The RCRA protects current and future groundwater facilities from contamination. Firms that produce waste must have a "cradle to grave" approach to managing waste, meaning that the waste producer is legally responsible for the waste from the moment it is produced until it is finally disposed.

*'''The Comprehensive Environmental Response, Compensation and Liability Act (CERCLA or Superfund, 1980)''' - This legislation creates a tax on chemical and petroleum industries in order to cover the remediation of hazardous waste sites that are uncontrolled or abandoned. The CERCLA empowers EPA to remediate waste sites by short-term removal of material or long-term remediation actions, depending on the risk that the waste imposes on life.

*The Superfund Amendments and Reauthorization Act (SARA, 1986)
*'''The Pollution Prevention Act (PPA, 1990)''' - This act promotes minimization of waste and pollution at their source. It encourages more efficient processing and raw materials and recycling through grants, technical assistant, and information.

*'''The Oil Pollution Act of 1990 (OPA, 1990)''' - This act was passed after the Exxon Valdez oil spill in the Prince William Sound in Alaska, creating a tax on oil to cover the cost of responding to spills when the corporation that creates the spill does not respond.

*'''The Department of the Environment Act (E-10)''' - The Department of Environment Act created the Department of Environment and charges the Minister of the Environment with creating programs that protect the environment, assessing environmental impacts of federal program, and reporting information to the public in Canada.

*'''The Canadian Environmental Protection Act (CEPA, C-15.31, 1999)''' - CEPA empowers Environment Canada to control toxic substances, reduce pollution, and reduce bioaccumulating toxic substances. Canada will use CEPA to implement regulations that helps it meet its commitments to the Kyoto Protocol.

*'''The Canada Water Act (C-11)''' - This law allows the Minister of the Environment to manage water resources in Canada. This includes research, implementing conservation programs, utilization of water resources, and setting water quality standards.

Wastes comprise mainly of by-products, unused reactants, and off-specification product produced by misoperation. Leaking seals and flanges, and spills and discharges also emit waste. Material is also discharged in emergency situations. Instead of considering how to treat or manage waste, designers should start by tackling the source and find ways to minimize the production of waste.

The process of designing waste management systems are: (1) source reduction, (2) waste stream recycle, (3) waste treatment to reduce environmental impact, and (4) disposal that is legally sound. Source reduction can be achieved by reducing the concentration of impurities in the feed, protecting catalysts and adsorbents from contaminants, eliminating the use of extraneous materials, increasing recovery from separation, and improving the quality of the fuel by switching to cleaner-burning fuel. Unused feed can be recycled, and off-specification products can be reprocessed. In integral processes, the waste of one process is used as the feed for another. By-products can also be sold to another company for use as raw material. Tighter control systems, alarm, and interlocks can reduce misoperation of process unit.1

Further information about environmental considerations can be found in the [[Environmental concerns]] page.

=Conclusion=
The location and layout of a plant can greatly impact its economic and operational success. Objectives such as cost minimization, distribution, room for expansion, and safety of the plant operators and local community play important roles in the site decision. The major factors affecting this decision are market and labor. National and regional environmental legislations impact the processes, and thus make some areas more desirable than others. Least important are land costs and financial incentives. When planning to build a new facility or expanding an existing site, companies utilize their site selection team to assemble a list of desirable locational characteristics and determine how the new facility will fit in with the overall company. Then with the help of consultants, a location is narrowed down and further analyses are done to justify the continuation of the project. The location should be obtained before design of the details of the process. Within the site itself, process units are arranged to allow for the most economical flow of people and material, optimal safety, and room for future expansion.

=References=
[1] G.P. Towler, R. Sinnott, ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design'', Elsevier, 2012.

[2] J.P. Blair, R. Premus. Major Factors in Industrial Location: A Review. Economic Development Quarterly. 1987;1(1):72-85.

[3] M.S. Peters, K.D. Timmerhaus, ''Plant Design and Economics for Chemical Engineers'', 5th Ed., McGraw-Hill: New York, 2003.

[4] O. Akgul, N. Shah, L.G. Papageorgiou. Economic optimisation of a UK advanced biofuel supply chain. Biomass and Bioenergy. 2012;41:57-72.

[5] L.T. Biegler, I.E. Grossmann, A.W. Westerberg, ''Systematic Methods of Chemical Process Design'', Prentice-Hall: Upper Saddle River, 1997.

[6] Summary of the National Environmental Policy Act website. http://www.epa.gov/laws-regulations/summary-national-environmental-policy-act. August 2015. Accessed February 2016.

[7] R.T. Turton, R.C. Bailie, W.B. Whiting, J.A. Shaeiwitz, ''Analysis, Synthesis, and Design of Chemical Processes'', Prentice Hall: Upper Saddle River, 2003.

[8] R.W. Schmenner, Look Beyond the Obvious in Plant Location website. https://hbr.org/1979/01/look-beyond-the-obvious-in-plant-location. January 1979. Accessed February 2016.

Process location and layout decisions

2016-02-22T02:31:15Z

Ezhuang: /* Location Decision Factors */

Author: Ellen Zhuang [2016]

Stewards: Daniel Garcia, and Fengqi You

=Introduction=
Location is one of the first decisions in the design of a new chemical plant. It impacts profitability and scope for future expansion. If the project is a new facility, a suitable site must be found and an optimal layout of the site and process units must be planned. If the project adds to an existing site, the impact of the new addition on the existing plant must be considered. The plant also needs to accommodate for the nearby infrastructure, the services that it requires, and its environmental impacts.

=Location Selection=
===Location Decision Factors===
Initially, economists viewed the plant location decision as a cost-minimization problem. The optimal location was one where the transportation costs of raw material to the plant and product to the market was minimized. Between the early 1900s and post World War II period, most industries were sensitive to the cost of transportation. As transportation costs became less of an obstacle, the approach to the problem developed with considerations of trade-offs between transportation and factors such as wages, energy, local regulations, etc. If lower wages could balance the higher transportation costs of building the plant in a low-wage area, the low-wage location may be more desirable.

Although location is a long-term investment, a firm does not decide on a location with the sole objective of maximizing its profits or minimizing its costs. Managers may choose a “safer” location that is more likely to produce higher profits rather than a riskier location that yields the maximum investment return. Personal factors of the people involved are also influences. For example, new businesses tend to locate where the founders live.2

In the current age, a plant's site is chosen based on several factors. These include:1,2,3,8

'''1. Raw material supply:''' The source and price of raw materials is one of the most important factors that determine the location of a plant. Facilities that produce chemicals in bulk are usually located close to the source of raw material if the costs of shipping the product is less than the costs of shipping the feed. For example, ethylene production is growing in the Middle East since cheap ethane from natural gas is readily available. Oil refineries tend to be located near areas with high population and crude oil supplies since it is expensive to transport the oil.

'''2. Location with respect to market:''' If the plant produces high-volume and low-cost products, such as cement and fertilizer, it may be better to situate the plant closer to the primary market since transportation cost is a large fraction of the sales price. If the product is low-volume and high-cost, like pharmaceuticals, then the benefits of being closer to the primary market may not be there.

'''3. Transport facilities:''' Facilities should be close to at least two major forms of transportation, whether that be road, rail, waterway, and/or seaport. For example, paper manufacturing plants in the US use various types of pulp that are delivered by truck or by train from various places in North and South America, so paper facilities need to be close to rail and major roads. Transportation by road is common for local distribution from central warehouses, while transportation by rail is more widespread for long distance transport of bulk chemicals. Pipeline is used to ship industrial gases and bulk fuels. Air freight can be efficient for shipment of personnel and essential units and supplies and for small volume products that have high value, such as pharmaceuticals. Of course, products that are delivered by air must meet aviation regulations.

'''4. Availability of labor:''' Skilled workers are usually brought to the plant from outside the area. There should be a local pool of unskilled labor that can be trained to operate the plant, and of skilled craft workers to maintain the process units. Local labor laws, trade union customs, restrictive practices for recruitment and training should also be taken into consideration. A 10% increase in unionization of a state's labor force is projected to reduce the number of expanding facilities by 30 to 45%.

'''5. Availability of utilities:''' Processes that require a substantial amount of cooling water is usually located near water sources, such as rivers or wells. Cooling water may be directly taken from the water source, or may be stored in cooling towers. Those that need large quantities of power, such as electrochemical ones, are typically close to cheap power sources.

'''6. Availability of suitable land:''' The ideal land is flat, well-drained, with suitable load-bearing characteristics. Further considerations have to be made if the land is reclaimed land near the ocean in earthquake zones. Property tax is also a factor when choosing a site since property taxes vary area to area. Under a third of plants that relocate move to regions with lower property taxes, which is the proportion that would be expected if companies move to a new location regardless of property tax. High property taxes is not as significant as other factors such as labor supply and land costs.

'''7. Environmental impact:''' Depending on the location, it may be more difficult and costly to dispose of wastes. During the project design phase, experts are typically consulted to learn more about an area's local regulations. More details about environmental regulations are found below.

'''8. Local community considerations:''' State and local planners are typically motivated by the desire to create jobs and improve the tax base. Introduction of facilities to an area is usually viewed as the most direct way to stimulate the area's economy. However, recent studies have found that communities with high-growth are already characterized by the fast growth of businesses that are already there. It is rare for a plant to completely close in one area and relocate to another, and if plants do relocate, the majority is over short distances and often within the same community. Therefore, local policymakers favor the expansion of existing plants. The opening of a new plant at a location should impost no additional risk to the local residents. For example, they should be downwind of the residential areas. Local communities also need to be able to accommodate the plant personnelles. For example, traffic, housing, and facilities must be able to accommodate the influx of workers. Additional factors are property taxes and water consumption.

'''9. Climate:''' The climate of the area may affect processes and costs. For example, plants in cold areas need more insulation and special heating. Facilities in earthquake areas need to be seismically sound. Plants in areas with high ambient humidity will usually use air cooling instead of water cooling. More detailed information about the effects of weather can be found in the [[Site condition and design]] page.

'''10. Political and strategic considerations:''' Government sometimes gives capital grants, tax concessions, and other incentives to encourage plants to be built in specific areas. Physical assistance such as roads, water, and other public infrastructure are more popular than financial assistance. Companies can also globalize and take advantage of areas with preferential tariff agreements. The tax policy of an area is inversely related to growth. High personal income sometimes hinder employment growth. Personal income does not affect the cash flow of the company, but it reduces the after-tax income of its managers, and thus high personal income tax can be classified as a personal region. High state corporate taxes has also and detrimental effects on growth, but that is not always the case. Corporate tax is more important to firms with high capital expenditures.

===Analysis of the Importance of Factors===
In 1963, Morgan surveyed 17 companies and found that on average the most important factors on industrial location decisions are market, labor, and raw material. Taxes and financial incentives were of little significance. More recently, Fortune magazine surveyed among the 1000 largest US companies and found that the most important factors were market and labor. The survey also found that personal preferences of a firm's executive, tax, and central cities have great influence over the site of corporate headquarters. Surveys of interstate locational decisions found that the factors that played key roles for new firms were access to customers and the growing market, labor force, transportation, personal reasons of management, and availability of capital. The cost of land had the least important influence. For firms that expanded across states, the influential factors were labor costs and labor supply. Companies seeking expansion usually have made their production more routine, so quick access to suppliers is less of a concern. Cost minimization becomes more relevant to existing corporations.2

===Site Selection Process===
The decision for the location of a facility is part of a larger corporate planning process. Usually, a corporate planning office or a division of the company initiates the site selection process by forecasting future capacity requirements. If capacity shortages are in the forecasts, the managers may choose to outsource, increase price to reduce the demand, expand existing sites, or open a new facility. If the managers decide on a new facility, the site selection team enters the project.

The structure of the site selection team depends on the firm's organization. In companies with centralized staff, the site selection team generally consists of representatives from relevant areas, such as engineering, real estate, and transportation. In companies with strong divisions, the locational decisions may be done at the divisional level, with the corporate office supervising the process.

The site selection team determines what characteristics are important for the new location by considering how the new facility will fit in the company's overall strategy, if the company wants to target new markets, if the corporation wants to divide or integrate its functions, or how the company wants to be seen by the public. Next, potential locations are listed and studied against the desired characteristics.

Locational decisions are typically made sequentially. The first step is at the state or regional level. Then, the team studies specific communities and sites. Different locational factors are important to different stages. When selecting a general region, the site selection team focuses on factors with interregional variations such as labor, tax policies, climate, and market locations. At the more focused stage, details like inexpensive land, access to to major roads, and good schools are important. Consultants are often hired to do site analyses.

Once site options are narrowed down, the company discusses potential problems and incentives with local public officials. Construction costs are estimated, and a feasibility analysis is done to show that the project has a high rate of return.2

===Biofuel Supply Chain Example===
Biofuels is a popular potential alternative to fossil fuels. Various biomass resources, including food crops, non-food crops, and agricultural residues, are converted into biofuels. The two most common biofuels currently are ethanol and biodiesel. The supply chain for biofuel is a network consisting of several nodes: biomass cultivation sites, biofuel production plants, and demand centers. The locations of these facilities, and the location of the demand centers with respect to the biofuel demand impact transportation costs, production resource, demand, etc., and thus affects and sets constraints to the cost minimization of the supply chain. The optimal supply chain design minimize the total cost, which is a sum of the total investment cost, production cost, transportation cost, and outsourcing and import cost. The total transportation cost captures the transportation of biomass and biofuel between areas and the local bio mass transfer.4

[[File:biofuelsupplychain.png|400px|thumb|center|'''Figure 1.''' The biofuels supply chain consists of three notes: biomass cultivation sites, biofuel production plants, and biofuel demand centers.1]]

[[File:Biofueloptimal.png|300px|thumb|center|'''Figure 2.''' A Great Britain biofuel supply chain design that minimizes cost. The plants are near areas with high wheat cultivation and wheat straw collection rates. The location of the plants with respect to the demand centers is also optimized. The main modes of transportation are road, rail, and ship.4]]

=Site Layout=
The process units and buildings need to be arranged in such a way that allows for the most economical flow of materials and people. Furthermore, dangerous processes need to be a safe distance from other buildings, and the layout should be planned to allow for future expansion.

Process units are usually laid out first in an arrangement that allows for smooth flow of materials between the process steps. The distance between equipment is usually at least 30 m. Next, the location of the principal ancillary buildings are sited as to minimize the time that it takes the workers to travel between buildings. Administrative offices and laboratories are located away from hazardous processes. Control rooms are next to the processing equipment. Utility buildings are located as to minimize piping between the process units. Storage is placed between the loading and unloading facilities and next to the process units that they serve. Tanks containing hazardous material are placed at least 70 m from the plant. An example of a typical site plan is shown below.1

[[File:Site Location.png|400px|thumb|center|'''Figure 3.''' A typical site layout.1]]

===Site Layout Factors===
The main factors that are considered when planning the layout of the plant are listed below.1

'''1. Economic considerations (construction and operating costs):'''
Construction costs can be minimized by arranging process units and buildings that minimize pipes between equipment, the amount of structural steel work, etc. However, this layout may conflict with the layout that gives the optimal operation and maintenance.

'''2. Process requirements:''' Examples of process considerations that must be taken into account is the elevation of the base of columns to give enough net positive suction head to a pump.

'''3. Operation convenience:''' Process units that are attended to frequently should be placed with convenient access. Valves, heads, and sample heads should be placed where operators can easily access. If the plant anticipates replacement of equipment, space must be allowed for removal and installation.

'''4. Maintenance convenience:''' Equipment that requires maintenance should be in a location with easy access, and should have sufficient space for the maintenance tasks. For example, shell-and-tube exchangers need space so that tube bundles can be removed for cleaning and repair.

'''5. Future expansion:''' The layout should be planned to conveniently allow for future expansion of processes. Pipe racks should have space for future piping, and pipes should be oversized to allow for more flow in the future.

'''6. Modular Construction:''' Modular construction is where sections of the plant is constructed outside of the plant, and then transported to the site by road or sea. Advantages include improved quality control, reduced construction costs, less requirements for skilled labor on site. Tradeoffs are more flanged connections and possible problems with onsite assembly.

'''7. Safety:''' Escape routes for workers need to be in place at each level in process buildings. Blast walls must isolate equipment that pose hazards to confine potential explosions.

First, a conceptual flowsheet for the process is developed. The types of equipment and their connections with each other is described in a process flow diagram (PFD). Before the PFD is translated into detailed piping and instrumentation diagrams (P&ID) the layout of the process units must be planned.2,3 Scale drawings are made to show the relationships between storage space and process equipment based on the flow of materials and people, and on future expansion. Three-dimensional visualization are the layouts are then carried out with cardboard cutouts of the equipment outlines or rectangular and cylindrical blocks. When a layout of the major process units has been decided, drawings of the plan and elevation are made, and design of the structural steelwork and foundations are done. Computer-aided design has also become increasingly popular.1,3,7

=Environmental Considerations=
Laws that protect the environment restrict the waste that plants can emit in order to preserve air, land, and water quality. States, provinces, and municipals usually have additional laws to national ones. In the US, environmental policies are moving towards more uniform and strict regulations. Businesses that produce more pollution are more likely to move to regions where local regulations are more lax, and industries in areas with more stringent policies will decline. The tighter national regulations are implemented to prevent this competition between regions. For example, amendments to the Clean Air Act and the Environmental Protection Agencies in the 1970s and 1980s reduced differences in regions' air pollution controls. Congress voted on the PSD (prevention of significant deterioration) to "level the playing field" across regions.1

Some of the main environmental legislations in North America are:

*'''The National Environmental Policy Act of 1969 (NEPA)''' - Signed by President Nixon, NEPA was one of the first laws that provide a national framework for the protection of the environment. It requires that all executive federal agencies consider the environmental impact of proposed actions. Government agencies must prepare environmental assessments and environmental impact statements that describe potential environmental effects. NEPA is implemented when buildings, airports, military complexes, parkland purchases, and highways are proposed.6

*'''The Clean Air Act (CAA, 1970)''' - The CAA was implemented to control air quality on a national level. The law was amended in 1990 to address issues such as acid rain, ozone depletion, and toxic air pollution, and to increase enforcement authority. Under the National Ambient Air Quality Standards (NAAQS), the law sets limits to allowable ambient levels of seven contaminants: ozone, carbon monoxide, lead, nitrogen dioxide, sulfur dioxide, PM10, and PM2.5. The CAA also requires that the EPA set US National Emissions Standards for Hazardous Air Pollutants (NESHAP), which regulates the emission of 189 hazardous air pollutants. The Maximum Available Control Technology (MACT) was implemented to limit the emission of volatile organics, sulfur, and other inorganic compounds from oil refineries. The CAA has had significant impact on the level of air pollutants that it regulates. Particularly in the Midwest and the Northwest, water acidity has been reduced. On the other hand, reduction of ozone damage has not been as successful since nitrogen oxide, which is linked to ozone formation, is not as tightly regulated.

*'''The Federal Water Pollution Control Act (The Clean Water Act, 1972)''' - The Clean Water Act was initially passed to clean water used for swimming and boating, and to protect wildlife. Amendments were later added to address water quality and toxic compound emissions. Under the CWA, the EPA can set water quality standards and monitor the industries' discharge into waters.

*'''The Safe Drinking Water Act (SDWA, 1974)''' - This act allows the EPA to set standards to potential drinking water. Public water systems must operate according to these standards.

*'''The Resource Conservation and Recovery Act (RCRA, 1976)''' - The RCRA protects current and future groundwater facilities from contamination. Firms that produce waste must have a "cradle to grave" approach to managing waste, meaning that the waste producer is legally responsible for the waste from the moment it is produced until it is finally disposed.

*'''The Comprehensive Environmental Response, Compensation and Liability Act (CERCLA or Superfund, 1980)''' - This legislation creates a tax on chemical and petroleum industries in order to cover the remediation of hazardous waste sites that are uncontrolled or abandoned. The CERCLA empowers EPA to remediate waste sites by short-term removal of material or long-term remediation actions, depending on the risk that the waste imposes on life.

*The Superfund Amendments and Reauthorization Act (SARA, 1986)
*'''The Pollution Prevention Act (PPA, 1990)''' - This act promotes minimization of waste and pollution at their source. It encourages more efficient processing and raw materials and recycling through grants, technical assistant, and information.

*'''The Oil Pollution Act of 1990 (OPA, 1990)''' - This act was passed after the Exxon Valdez oil spill in the Prince William Sound in Alaska, creating a tax on oil to cover the cost of responding to spills when the corporation that creates the spill does not respond.

*'''The Department of the Environment Act (E-10)''' - The Department of Environment Act created the Department of Environment and charges the Minister of the Environment with creating programs that protect the environment, assessing environmental impacts of federal program, and reporting information to the public in Canada.

*'''The Canadian Environmental Protection Act (CEPA, C-15.31, 1999)''' - CEPA empowers Environment Canada to control toxic substances, reduce pollution, and reduce bioaccumulating toxic substances. Canada will use CEPA to implement regulations that helps it meet its commitments to the Kyoto Protocol.

*'''The Canada Water Act (C-11)''' - This law allows the Minister of the Environment to manage water resources in Canada. This includes research, implementing conservation programs, utilization of water resources, and setting water quality standards.

Wastes comprise mainly of by-products, unused reactants, and off-specification product produced by misoperation. Leaking seals and flanges, and spills and discharges also emit waste. Material is also discharged in emergency situations. Instead of considering how to treat or manage waste, designers should start by tackling the source and find ways to minimize the production of waste.

The process of designing waste management systems are: (1) source reduction, (2) waste stream recycle, (3) waste treatment to reduce environmental impact, and (4) disposal that is legally sound. Source reduction can be achieved by reducing the concentration of impurities in the feed, protecting catalysts and adsorbents from contaminants, eliminating the use of extraneous materials, increasing recovery from separation, and improving the quality of the fuel by switching to cleaner-burning fuel. Unused feed can be recycled, and off-specification products can be reprocessed. In integral processes, the waste of one process is used as the feed for another. By-products can also be sold to another company for use as raw material. Tighter control systems, alarm, and interlocks can reduce misoperation of process unit.1

Further information about environmental considerations can be found in the [[Environmental concerns]] page.

=Conclusion=
The location and layout of a plant can greatly impact its economic and operational success. Objectives such as cost minimization, distribution, room for expansion, and safety of the plant operators and local community play important roles in the site decision. The major factors affecting this decision are market and labor. National and regional environmental legislations impact the processes, and thus make some areas more desirable than others. Least important are land costs and financial incentives. When planning to build a new facility or expanding an existing site, companies utilize their site selection team to assemble a list of desirable locational characteristics and determine how the new facility will fit in with the overall company. Then with the help of consultants, a location is narrowed down and further analyses are done to justify the continuation of the project. The location should be obtained before design of the details of the process. Within the site itself, process units are arranged to allow for the most economical flow of people and material, optimal safety, and room for future expansion.

=References=
[1] G.P. Towler, R. Sinnott, ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design'', Elsevier, 2012.

[2] J.P. Blair, R. Premus. Major Factors in Industrial Location: A Review. Economic Development Quarterly. 1987;1(1):72-85.

[3] M.S. Peters, K.D. Timmerhaus, ''Plant Design and Economics for Chemical Engineers'', 5th Ed., McGraw-Hill: New York, 2003.

[4] O. Akgul, N. Shah, L.G. Papageorgiou. Economic optimisation of a UK advanced biofuel supply chain. Biomass and Bioenergy. 2012;41:57-72.

[5] L.T. Biegler, I.E. Grossmann, A.W. Westerberg, ''Systematic Methods of Chemical Process Design'', Prentice-Hall: Upper Saddle River, 1997.

[6] Summary of the National Environmental Policy Act website. http://www.epa.gov/laws-regulations/summary-national-environmental-policy-act. August 2015. Accessed February 2016.

[7] R.T. Turton, R.C. Bailie, W.B. Whiting, J.A. Shaeiwitz, ''Analysis, Synthesis, and Design of Chemical Processes'', Prentice Hall: Upper Saddle River, 2003.

[8] R.W. Schmenner, Look Beyond the Obvious in Plant Location website. https://hbr.org/1979/01/look-beyond-the-obvious-in-plant-location. January 1979. Accessed February 2016.

Process location and layout decisions

2016-02-22T02:27:39Z

Ezhuang: /* Location Decision Factors */

Author: Ellen Zhuang [2016]

Stewards: Daniel Garcia, and Fengqi You

=Introduction=
Location is one of the first decisions in the design of a new chemical plant. It impacts profitability and scope for future expansion. If the project is a new facility, a suitable site must be found and an optimal layout of the site and process units must be planned. If the project adds to an existing site, the impact of the new addition on the existing plant must be considered. The plant also needs to accommodate for the nearby infrastructure, the services that it requires, and its environmental impacts.

=Location Selection=
===Location Decision Factors===
Initially, economists viewed the plant location decision as a cost-minimization problem. The optimal location was one where the transportation costs of raw material to the plant and product to the market was minimized. Between the early 1900s and post World War II period, most industries were sensitive to the cost of transportation. As transportation costs became less of an obstacle, the approach to the problem developed with considerations of trade-offs between transportation and factors such as wages, energy, local regulations, etc. If lower wages could balance the higher transportation costs of building the plant in a low-wage area, the low-wage location may be more desirable.

Although location is a long-term investment, a firm does not decide on a location with the sole objective of maximizing its profits or minimizing its costs. Managers may choose a “safer” location that is more likely to produce higher profits rather than a riskier location that yields the maximum investment return. Personal factors of the people involved are also influences. For example, new businesses tend to locate where the founders live.2

In the current age, a plant's site is chosen based on several factors. These include:1,2,3,8

'''1. Raw material supply:''' The source and price of raw materials is one of the most important factors that determine the location of a plant. Facilities that produce chemicals in bulk are usually located close to the source of raw material if the costs of shipping the product is less than the costs of shipping the feed. For example, ethylene production is growing in the Middle East since cheap ethane from natural gas is readily available. Oil refineries tend to be located near areas with high population and crude oil supplies since it is expensive to transport the oil.

'''2. Location with respect to market:''' If the plant produces high-volume and low-cost products, such as cement and fertilizer, it may be better to situate the plant closer to the primary market since transportation cost is a large fraction of the sales price. If the product is low-volume and high-cost, like pharmaceuticals, then the benefits of being closer to the primary market may not be there.

'''3. Transport facilities:''' Facilities should be close to at least two major forms of transportation, whether that be road, rail, waterway, and/or seaport. For example, paper manufacturing plants in the US use various types of pulp that are delivered by truck or by train from various places in North and South America, so paper facilities need to be close to rail and major roads. Transportation by road is common for local distribution from central warehouses, while transportation by rail is more widespread for long distance transport of bulk chemicals. Pipeline is used to ship industrial gases and bulk fuels. Air freight can be efficient for shipment of personnel and essential units and supplies and for small volume products that have high value, such as pharmaceuticals. Of course, products that are delivered by air must meet aviation regulations.

'''4. Availability of labor:''' Skilled workers are usually brought to the plant from outside the area. There should be a local pool of unskilled labor that can be trained to operate the plant, and of skilled craft workers to maintain the process units. Local labor laws, trade union customs, restrictive practices for recruitment and training should also be taken into consideration. A 10% increase in unionization of a state's labor force is projected to reduce the number of expanding facilities by 30 to 45%.

'''5. Availability of utilities:''' Processes that require a substantial amount of cooling water is usually located near water sources, such as rivers or wells. Cooling water may be directly taken from the water source, or may be stored in cooling towers. Those that need large quantities of power, such as electrochemical ones, are typically close to cheap power sources.

'''6. Availability of suitable land:''' The ideal land is flat, well-drained, with suitable load-bearing characteristics. Further considerations have to be made if the land is reclaimed land near the ocean in earthquake zones. Property tax is also a factor when choosing a site since property taxes vary area to area. Under a third of plants that relocate move to regions with lower property taxes, which is the proportion that would be expected if companies move to a new location regardless of property tax. High property taxes is not as significant as other factors such as labor supply and land costs.

'''7. Environmental impact:''' Depending on the location, it may be more difficult and costly to dispose of wastes. During the project design phase, experts are typically consulted to learn more about an area's local regulations. More details about environmental regulations are found below.

'''8. Local community considerations:''' State and local planners are typically motivated by the desire to create jobs and improve the tax base. Introduction of facilities to an area is usually viewed as the most direct way to stimulate the area's economy. However, recent studies have found that communities with high-growth are already characterized by the fast growth of businesses that are already there. It is rare for a plant to completely close in one area and relocate to another, and if plants do relocate, the majority is over short distances and often within the same community. Therefore, local policymakers favor the expansion of existing plants. The opening of a new plant at a location should impost no additional risk to the local residents. For example, they should be downwind of the residential areas. Local communities also need to be able to accommodate the plant personnelles. For example, traffic, housing, and facilities must be able to accommodate the influx of workers. Additional factors are property taxes and water consumption.

'''9. Climate:''' The climate of the area may affect processes and costs. For example, plants in cold areas need more insulation and special heating. Facilities in earthquake areas need to be seismically sound. Plants in areas with high ambient humidity will usually use air cooling instead of water cooling.

'''10. Political and strategic considerations:''' Government sometimes gives capital grants, tax concessions, and other incentives to encourage plants to be built in specific areas. Physical assistance such as roads, water, and other public infrastructure are more popular than financial assistance. Companies can also globalize and take advantage of areas with preferential tariff agreements. The tax policy of an area is inversely related to growth. High personal income sometimes hinder employment growth. Personal income does not affect the cash flow of the company, but it reduces the after-tax income of its managers, and thus high personal income tax can be classified as a personal region. High state corporate taxes has also and detrimental effects on growth, but that is not always the case. Corporate tax is more important to firms with high capital expenditures.

===Analysis of the Importance of Factors===
In 1963, Morgan surveyed 17 companies and found that on average the most important factors on industrial location decisions are market, labor, and raw material. Taxes and financial incentives were of little significance. More recently, Fortune magazine surveyed among the 1000 largest US companies and found that the most important factors were market and labor. The survey also found that personal preferences of a firm's executive, tax, and central cities have great influence over the site of corporate headquarters. Surveys of interstate locational decisions found that the factors that played key roles for new firms were access to customers and the growing market, labor force, transportation, personal reasons of management, and availability of capital. The cost of land had the least important influence. For firms that expanded across states, the influential factors were labor costs and labor supply. Companies seeking expansion usually have made their production more routine, so quick access to suppliers is less of a concern. Cost minimization becomes more relevant to existing corporations.2

===Site Selection Process===
The decision for the location of a facility is part of a larger corporate planning process. Usually, a corporate planning office or a division of the company initiates the site selection process by forecasting future capacity requirements. If capacity shortages are in the forecasts, the managers may choose to outsource, increase price to reduce the demand, expand existing sites, or open a new facility. If the managers decide on a new facility, the site selection team enters the project.

The structure of the site selection team depends on the firm's organization. In companies with centralized staff, the site selection team generally consists of representatives from relevant areas, such as engineering, real estate, and transportation. In companies with strong divisions, the locational decisions may be done at the divisional level, with the corporate office supervising the process.

The site selection team determines what characteristics are important for the new location by considering how the new facility will fit in the company's overall strategy, if the company wants to target new markets, if the corporation wants to divide or integrate its functions, or how the company wants to be seen by the public. Next, potential locations are listed and studied against the desired characteristics.

Locational decisions are typically made sequentially. The first step is at the state or regional level. Then, the team studies specific communities and sites. Different locational factors are important to different stages. When selecting a general region, the site selection team focuses on factors with interregional variations such as labor, tax policies, climate, and market locations. At the more focused stage, details like inexpensive land, access to to major roads, and good schools are important. Consultants are often hired to do site analyses.

Once site options are narrowed down, the company discusses potential problems and incentives with local public officials. Construction costs are estimated, and a feasibility analysis is done to show that the project has a high rate of return.2

===Biofuel Supply Chain Example===
Biofuels is a popular potential alternative to fossil fuels. Various biomass resources, including food crops, non-food crops, and agricultural residues, are converted into biofuels. The two most common biofuels currently are ethanol and biodiesel. The supply chain for biofuel is a network consisting of several nodes: biomass cultivation sites, biofuel production plants, and demand centers. The locations of these facilities, and the location of the demand centers with respect to the biofuel demand impact transportation costs, production resource, demand, etc., and thus affects and sets constraints to the cost minimization of the supply chain. The optimal supply chain design minimize the total cost, which is a sum of the total investment cost, production cost, transportation cost, and outsourcing and import cost. The total transportation cost captures the transportation of biomass and biofuel between areas and the local bio mass transfer.4

[[File:biofuelsupplychain.png|400px|thumb|center|'''Figure 1.''' The biofuels supply chain consists of three notes: biomass cultivation sites, biofuel production plants, and biofuel demand centers.1]]

[[File:Biofueloptimal.png|300px|thumb|center|'''Figure 2.''' A Great Britain biofuel supply chain design that minimizes cost. The plants are near areas with high wheat cultivation and wheat straw collection rates. The location of the plants with respect to the demand centers is also optimized. The main modes of transportation are road, rail, and ship.4]]

=Site Layout=
The process units and buildings need to be arranged in such a way that allows for the most economical flow of materials and people. Furthermore, dangerous processes need to be a safe distance from other buildings, and the layout should be planned to allow for future expansion.

Process units are usually laid out first in an arrangement that allows for smooth flow of materials between the process steps. The distance between equipment is usually at least 30 m. Next, the location of the principal ancillary buildings are sited as to minimize the time that it takes the workers to travel between buildings. Administrative offices and laboratories are located away from hazardous processes. Control rooms are next to the processing equipment. Utility buildings are located as to minimize piping between the process units. Storage is placed between the loading and unloading facilities and next to the process units that they serve. Tanks containing hazardous material are placed at least 70 m from the plant. An example of a typical site plan is shown below.1

[[File:Site Location.png|400px|thumb|center|'''Figure 3.''' A typical site layout.1]]

===Site Layout Factors===
The main factors that are considered when planning the layout of the plant are listed below.1

'''1. Economic considerations (construction and operating costs):'''
Construction costs can be minimized by arranging process units and buildings that minimize pipes between equipment, the amount of structural steel work, etc. However, this layout may conflict with the layout that gives the optimal operation and maintenance.

'''2. Process requirements:''' Examples of process considerations that must be taken into account is the elevation of the base of columns to give enough net positive suction head to a pump.

'''3. Operation convenience:''' Process units that are attended to frequently should be placed with convenient access. Valves, heads, and sample heads should be placed where operators can easily access. If the plant anticipates replacement of equipment, space must be allowed for removal and installation.

'''4. Maintenance convenience:''' Equipment that requires maintenance should be in a location with easy access, and should have sufficient space for the maintenance tasks. For example, shell-and-tube exchangers need space so that tube bundles can be removed for cleaning and repair.

'''5. Future expansion:''' The layout should be planned to conveniently allow for future expansion of processes. Pipe racks should have space for future piping, and pipes should be oversized to allow for more flow in the future.

'''6. Modular Construction:''' Modular construction is where sections of the plant is constructed outside of the plant, and then transported to the site by road or sea. Advantages include improved quality control, reduced construction costs, less requirements for skilled labor on site. Tradeoffs are more flanged connections and possible problems with onsite assembly.

'''7. Safety:''' Escape routes for workers need to be in place at each level in process buildings. Blast walls must isolate equipment that pose hazards to confine potential explosions.

First, a conceptual flowsheet for the process is developed. The types of equipment and their connections with each other is described in a process flow diagram (PFD). Before the PFD is translated into detailed piping and instrumentation diagrams (P&ID) the layout of the process units must be planned.2,3 Scale drawings are made to show the relationships between storage space and process equipment based on the flow of materials and people, and on future expansion. Three-dimensional visualization are the layouts are then carried out with cardboard cutouts of the equipment outlines or rectangular and cylindrical blocks. When a layout of the major process units has been decided, drawings of the plan and elevation are made, and design of the structural steelwork and foundations are done. Computer-aided design has also become increasingly popular.1,3,7

=Environmental Considerations=
Laws that protect the environment restrict the waste that plants can emit in order to preserve air, land, and water quality. States, provinces, and municipals usually have additional laws to national ones. In the US, environmental policies are moving towards more uniform and strict regulations. Businesses that produce more pollution are more likely to move to regions where local regulations are more lax, and industries in areas with more stringent policies will decline. The tighter national regulations are implemented to prevent this competition between regions. For example, amendments to the Clean Air Act and the Environmental Protection Agencies in the 1970s and 1980s reduced differences in regions' air pollution controls. Congress voted on the PSD (prevention of significant deterioration) to "level the playing field" across regions.1

Some of the main environmental legislations in North America are:

*'''The National Environmental Policy Act of 1969 (NEPA)''' - Signed by President Nixon, NEPA was one of the first laws that provide a national framework for the protection of the environment. It requires that all executive federal agencies consider the environmental impact of proposed actions. Government agencies must prepare environmental assessments and environmental impact statements that describe potential environmental effects. NEPA is implemented when buildings, airports, military complexes, parkland purchases, and highways are proposed.6

*'''The Clean Air Act (CAA, 1970)''' - The CAA was implemented to control air quality on a national level. The law was amended in 1990 to address issues such as acid rain, ozone depletion, and toxic air pollution, and to increase enforcement authority. Under the National Ambient Air Quality Standards (NAAQS), the law sets limits to allowable ambient levels of seven contaminants: ozone, carbon monoxide, lead, nitrogen dioxide, sulfur dioxide, PM10, and PM2.5. The CAA also requires that the EPA set US National Emissions Standards for Hazardous Air Pollutants (NESHAP), which regulates the emission of 189 hazardous air pollutants. The Maximum Available Control Technology (MACT) was implemented to limit the emission of volatile organics, sulfur, and other inorganic compounds from oil refineries. The CAA has had significant impact on the level of air pollutants that it regulates. Particularly in the Midwest and the Northwest, water acidity has been reduced. On the other hand, reduction of ozone damage has not been as successful since nitrogen oxide, which is linked to ozone formation, is not as tightly regulated.

*'''The Federal Water Pollution Control Act (The Clean Water Act, 1972)''' - The Clean Water Act was initially passed to clean water used for swimming and boating, and to protect wildlife. Amendments were later added to address water quality and toxic compound emissions. Under the CWA, the EPA can set water quality standards and monitor the industries' discharge into waters.

*'''The Safe Drinking Water Act (SDWA, 1974)''' - This act allows the EPA to set standards to potential drinking water. Public water systems must operate according to these standards.

*'''The Resource Conservation and Recovery Act (RCRA, 1976)''' - The RCRA protects current and future groundwater facilities from contamination. Firms that produce waste must have a "cradle to grave" approach to managing waste, meaning that the waste producer is legally responsible for the waste from the moment it is produced until it is finally disposed.

*'''The Comprehensive Environmental Response, Compensation and Liability Act (CERCLA or Superfund, 1980)''' - This legislation creates a tax on chemical and petroleum industries in order to cover the remediation of hazardous waste sites that are uncontrolled or abandoned. The CERCLA empowers EPA to remediate waste sites by short-term removal of material or long-term remediation actions, depending on the risk that the waste imposes on life.

*The Superfund Amendments and Reauthorization Act (SARA, 1986)
*'''The Pollution Prevention Act (PPA, 1990)''' - This act promotes minimization of waste and pollution at their source. It encourages more efficient processing and raw materials and recycling through grants, technical assistant, and information.

*'''The Oil Pollution Act of 1990 (OPA, 1990)''' - This act was passed after the Exxon Valdez oil spill in the Prince William Sound in Alaska, creating a tax on oil to cover the cost of responding to spills when the corporation that creates the spill does not respond.

*'''The Department of the Environment Act (E-10)''' - The Department of Environment Act created the Department of Environment and charges the Minister of the Environment with creating programs that protect the environment, assessing environmental impacts of federal program, and reporting information to the public in Canada.

*'''The Canadian Environmental Protection Act (CEPA, C-15.31, 1999)''' - CEPA empowers Environment Canada to control toxic substances, reduce pollution, and reduce bioaccumulating toxic substances. Canada will use CEPA to implement regulations that helps it meet its commitments to the Kyoto Protocol.

*'''The Canada Water Act (C-11)''' - This law allows the Minister of the Environment to manage water resources in Canada. This includes research, implementing conservation programs, utilization of water resources, and setting water quality standards.

Wastes comprise mainly of by-products, unused reactants, and off-specification product produced by misoperation. Leaking seals and flanges, and spills and discharges also emit waste. Material is also discharged in emergency situations. Instead of considering how to treat or manage waste, designers should start by tackling the source and find ways to minimize the production of waste.

The process of designing waste management systems are: (1) source reduction, (2) waste stream recycle, (3) waste treatment to reduce environmental impact, and (4) disposal that is legally sound. Source reduction can be achieved by reducing the concentration of impurities in the feed, protecting catalysts and adsorbents from contaminants, eliminating the use of extraneous materials, increasing recovery from separation, and improving the quality of the fuel by switching to cleaner-burning fuel. Unused feed can be recycled, and off-specification products can be reprocessed. In integral processes, the waste of one process is used as the feed for another. By-products can also be sold to another company for use as raw material. Tighter control systems, alarm, and interlocks can reduce misoperation of process unit.1

Further information about environmental considerations can be found in the [[Environmental concerns]] page.

=Conclusion=
The location and layout of a plant can greatly impact its economic and operational success. Objectives such as cost minimization, distribution, room for expansion, and safety of the plant operators and local community play important roles in the site decision. The major factors affecting this decision are market and labor. National and regional environmental legislations impact the processes, and thus make some areas more desirable than others. Least important are land costs and financial incentives. When planning to build a new facility or expanding an existing site, companies utilize their site selection team to assemble a list of desirable locational characteristics and determine how the new facility will fit in with the overall company. Then with the help of consultants, a location is narrowed down and further analyses are done to justify the continuation of the project. The location should be obtained before design of the details of the process. Within the site itself, process units are arranged to allow for the most economical flow of people and material, optimal safety, and room for future expansion.

=References=
[1] G.P. Towler, R. Sinnott, ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design'', Elsevier, 2012.

[2] J.P. Blair, R. Premus. Major Factors in Industrial Location: A Review. Economic Development Quarterly. 1987;1(1):72-85.

[3] M.S. Peters, K.D. Timmerhaus, ''Plant Design and Economics for Chemical Engineers'', 5th Ed., McGraw-Hill: New York, 2003.

[4] O. Akgul, N. Shah, L.G. Papageorgiou. Economic optimisation of a UK advanced biofuel supply chain. Biomass and Bioenergy. 2012;41:57-72.

[5] L.T. Biegler, I.E. Grossmann, A.W. Westerberg, ''Systematic Methods of Chemical Process Design'', Prentice-Hall: Upper Saddle River, 1997.

[6] Summary of the National Environmental Policy Act website. http://www.epa.gov/laws-regulations/summary-national-environmental-policy-act. August 2015. Accessed February 2016.

[7] R.T. Turton, R.C. Bailie, W.B. Whiting, J.A. Shaeiwitz, ''Analysis, Synthesis, and Design of Chemical Processes'', Prentice Hall: Upper Saddle River, 2003.

[8] R.W. Schmenner, Look Beyond the Obvious in Plant Location website. https://hbr.org/1979/01/look-beyond-the-obvious-in-plant-location. January 1979. Accessed February 2016.

Process location and layout decisions

2016-02-22T02:07:34Z

Ezhuang: /* Conclusion */

Author: Ellen Zhuang [2016]

Stewards: Daniel Garcia, and Fengqi You

=Introduction=
Location is one of the first decisions in the design of a new chemical plant. It impacts profitability and scope for future expansion. If the project is a new facility, a suitable site must be found and an optimal layout of the site and process units must be planned. If the project adds to an existing site, the impact of the new addition on the existing plant must be considered. The plant also needs to accommodate for the nearby infrastructure, the services that it requires, and its environmental impacts.

=Location Selection=
===Location Decision Factors===
Initially, economists viewed the plant location decision as a cost-minimization problem. The optimal location was one where the transportation costs of raw material to the plant and product to the market was minimized. Between the early 1900s and post World War II period, most industries were sensitive to the cost of transportation. As transportation costs became less of an obstacle, the approach to the problem developed with considerations of trade-offs between transportation and factors such as wages, energy, local regulations, etc. If lower wages could balance the higher transportation costs of building the plant in a low-wage area, the low-wage location may be more desirable.

Although location is a long-term investment, a firm does not decide on a location with the sole objective of maximizing its profits or minimizing its costs. Managers may choose a “safer” location that is more likely to produce higher profits rather than a riskier location that yields the maximum investment return. Personal factors of the people involved are also influences. For example, new businesses tend to locate where the founder lives.2

In the current age, a plant's site is chosen based on several factors. These include:1,2,3,8

'''1. Raw material supply:''' The source and price of raw materials is one of the most important factors that determine the location of a plant. Facilities that produce chemicals in bulk are usually located close to the source of raw material if the costs of shipping the product is less than the costs of shipping the feed. For example, ethylene production is growing in the Middle East since cheap ethane from natural gas is readily available. Oil refineries tend to be located near areas with high population and crude oil supplies since it is expensive to transport the oil.

'''2. Location with respect to market:''' If the plant produces high-volume and low-cost products, such as cement and fertilizer, it may be better to situate the plant closer to the primary market since transportation cost is a large fraction of the sales price. If the product is low-volume and high-cost, like pharmaceuticals, then the benefits of being closer to the primary market may not be there.

'''3. Transport facilities:''' Facilities should be close to at least two major forms of transportation, whether that be road, rail, waterway, and/or seaport. For example, paper manufacturing plants in the US use various types of pulp that are delivered by truck or by train from various places in North and South America, so paper facilities need to be close to rail and major roads. Transportation by road is common for local distribution from central warehouses, while transportation by rail is more widespread for long distance transport of bulk chemicals. Pipeline is used to ship industrial gases and bulk fuels. Air freight can be efficient for shipment of personnel and essential units and supplies and for small volume products that have high value, such as pharmaceuticals. Of course, products that are delivered by air must meet aviation regulations.

'''4. Availability of labor:''' Skilled workers are usually brought to the plant from outside the area. There should be a local pool of unskilled labor that can be trained to operate the plant, and of skilled craft workers to maintain the process units. Local labor laws, trade union customs, restrictive practices for recruitment and training should also be taken into consideration. A 10% increase in unionization of a state's labor force is projected to reduce the number of expanding facilities by 30 to 45%.

'''5. Availability of utilities:''' Processes that require a substantial amount of cooling water is usually located near water sources, such as rivers or wells. Cooling water may be directly taken from the water source, or may be stored in cooling towers. Those that need large quantities of power, such as electrochemical ones, are typically close to cheap power sources.

'''6. Availability of suitable land:''' The ideal land is flat, well-drained, with suitable load-bearing characteristics. Further considerations have to be made if the land is reclaimed land near the ocean in earthquake zones. Property tax is also a factor when choosing a site since property taxes vary area to area. Under a third of plants that relocate move to regions with lower property taxes, which is the proportion that would be expected if companies move to a new location regardless of property tax. High property taxes is not as significant as other factors such as labor supply and land costs.

'''7. Environmental impact:''' Depending on the location, it may be more difficult and costly to dispose of wastes. During the project design phase, experts are typically consulted to learn more about an area's local regulations. More details about environmental regulations are found below.

'''8. Local community considerations:''' State and local planners are typically motivated by the desire to create jobs and improve the tax base. Introduction of facilities to an area is usually viewed as the most direct way to stimulate the area's economy. However, recent studies have found that communities with high-growth are already characterized by the fast growth of businesses that are already there. It is rare for a plant to completely close in one area and relocate to another, and if plants do relocate, the majority is over short distances and often within the same community. Therefore, local policymakers favor the expansion of existing plants. The opening of a new plant at a location should impost no additional risk to the local residents. For example, they should be downwind of the residential areas. Local communities also need to be able to accommodate the plant personnelles. For example, traffic, housing, and facilities must be able to accommodate the influx of workers. Additional factors are property taxes and water consumption.

'''9. Climate:''' The climate of the area may affect processes and costs. For example, plants in cold areas need more insulation and special heating. Facilities in earthquake areas need to be seismically sound. Plants in areas with high ambient humidity will usually use air cooling instead of water cooling.

'''10. Political and strategic considerations:''' Government sometimes gives capital grants, tax concessions, and other incentives to encourage plants to be built in specific areas. Physical assistance such as roads, water, and other public infrastructure are more popular than financial assistance. Companies can also globalize and take advantage of areas with preferential tariff agreements. The tax policy of an area is inversely related to growth. High personal income sometimes hinder employment growth. Personal income does not affect the cash flow of the company, but it reduces the after-tax income of its managers, and thus high personal income tax can be classified as a personal region. High state corporate taxes has also and detrimental effects on growth, but that is not always the case. Corporate tax is more important to firms with high capital expenditures.

===Analysis of the Importance of Factors===
In 1963, Morgan surveyed 17 companies and found that on average the most important factors on industrial location decisions are market, labor, and raw material. Taxes and financial incentives were of little significance. More recently, Fortune magazine surveyed among the 1000 largest US companies and found that the most important factors were market and labor. The survey also found that personal preferences of a firm's executive, tax, and central cities have great influence over the site of corporate headquarters. Surveys of interstate locational decisions found that the factors that played key roles for new firms were access to customers and the growing market, labor force, transportation, personal reasons of management, and availability of capital. The cost of land had the least important influence. For firms that expanded across states, the influential factors were labor costs and labor supply. Companies seeking expansion usually have made their production more routine, so quick access to suppliers is less of a concern. Cost minimization becomes more relevant to existing corporations.2

===Site Selection Process===
The decision for the location of a facility is part of a larger corporate planning process. Usually, a corporate planning office or a division of the company initiates the site selection process by forecasting future capacity requirements. If capacity shortages are in the forecasts, the managers may choose to outsource, increase price to reduce the demand, expand existing sites, or open a new facility. If the managers decide on a new facility, the site selection team enters the project.

The structure of the site selection team depends on the firm's organization. In companies with centralized staff, the site selection team generally consists of representatives from relevant areas, such as engineering, real estate, and transportation. In companies with strong divisions, the locational decisions may be done at the divisional level, with the corporate office supervising the process.

The site selection team determines what characteristics are important for the new location by considering how the new facility will fit in the company's overall strategy, if the company wants to target new markets, if the corporation wants to divide or integrate its functions, or how the company wants to be seen by the public. Next, potential locations are listed and studied against the desired characteristics.

Locational decisions are typically made sequentially. The first step is at the state or regional level. Then, the team studies specific communities and sites. Different locational factors are important to different stages. When selecting a general region, the site selection team focuses on factors with interregional variations such as labor, tax policies, climate, and market locations. At the more focused stage, details like inexpensive land, access to to major roads, and good schools are important. Consultants are often hired to do site analyses.

Once site options are narrowed down, the company discusses potential problems and incentives with local public officials. Construction costs are estimated, and a feasibility analysis is done to show that the project has a high rate of return.2

===Biofuel Supply Chain Example===
Biofuels is a popular potential alternative to fossil fuels. Various biomass resources, including food crops, non-food crops, and agricultural residues, are converted into biofuels. The two most common biofuels currently are ethanol and biodiesel. The supply chain for biofuel is a network consisting of several nodes: biomass cultivation sites, biofuel production plants, and demand centers. The locations of these facilities, and the location of the demand centers with respect to the biofuel demand impact transportation costs, production resource, demand, etc., and thus affects and sets constraints to the cost minimization of the supply chain. The optimal supply chain design minimize the total cost, which is a sum of the total investment cost, production cost, transportation cost, and outsourcing and import cost. The total transportation cost captures the transportation of biomass and biofuel between areas and the local bio mass transfer.4

[[File:biofuelsupplychain.png|400px|thumb|center|'''Figure 1.''' The biofuels supply chain consists of three notes: biomass cultivation sites, biofuel production plants, and biofuel demand centers.1]]

[[File:Biofueloptimal.png|300px|thumb|center|'''Figure 2.''' A Great Britain biofuel supply chain design that minimizes cost. The plants are near areas with high wheat cultivation and wheat straw collection rates. The location of the plants with respect to the demand centers is also optimized. The main modes of transportation are road, rail, and ship.4]]

=Site Layout=
The process units and buildings need to be arranged in such a way that allows for the most economical flow of materials and people. Furthermore, dangerous processes need to be a safe distance from other buildings, and the layout should be planned to allow for future expansion.

Process units are usually laid out first in an arrangement that allows for smooth flow of materials between the process steps. The distance between equipment is usually at least 30 m. Next, the location of the principal ancillary buildings are sited as to minimize the time that it takes the workers to travel between buildings. Administrative offices and laboratories are located away from hazardous processes. Control rooms are next to the processing equipment. Utility buildings are located as to minimize piping between the process units. Storage is placed between the loading and unloading facilities and next to the process units that they serve. Tanks containing hazardous material are placed at least 70 m from the plant. An example of a typical site plan is shown below.1

[[File:Site Location.png|400px|thumb|center|'''Figure 3.''' A typical site layout.1]]

===Site Layout Factors===
The main factors that are considered when planning the layout of the plant are listed below.1

'''1. Economic considerations (construction and operating costs):'''
Construction costs can be minimized by arranging process units and buildings that minimize pipes between equipment, the amount of structural steel work, etc. However, this layout may conflict with the layout that gives the optimal operation and maintenance.

'''2. Process requirements:''' Examples of process considerations that must be taken into account is the elevation of the base of columns to give enough net positive suction head to a pump.

'''3. Operation convenience:''' Process units that are attended to frequently should be placed with convenient access. Valves, heads, and sample heads should be placed where operators can easily access. If the plant anticipates replacement of equipment, space must be allowed for removal and installation.

'''4. Maintenance convenience:''' Equipment that requires maintenance should be in a location with easy access, and should have sufficient space for the maintenance tasks. For example, shell-and-tube exchangers need space so that tube bundles can be removed for cleaning and repair.

'''5. Future expansion:''' The layout should be planned to conveniently allow for future expansion of processes. Pipe racks should have space for future piping, and pipes should be oversized to allow for more flow in the future.

'''6. Modular Construction:''' Modular construction is where sections of the plant is constructed outside of the plant, and then transported to the site by road or sea. Advantages include improved quality control, reduced construction costs, less requirements for skilled labor on site. Tradeoffs are more flanged connections and possible problems with onsite assembly.

'''7. Safety:''' Escape routes for workers need to be in place at each level in process buildings. Blast walls must isolate equipment that pose hazards to confine potential explosions.

First, a conceptual flowsheet for the process is developed. The types of equipment and their connections with each other is described in a process flow diagram (PFD). Before the PFD is translated into detailed piping and instrumentation diagrams (P&ID) the layout of the process units must be planned.2,3 Scale drawings are made to show the relationships between storage space and process equipment based on the flow of materials and people, and on future expansion. Three-dimensional visualization are the layouts are then carried out with cardboard cutouts of the equipment outlines or rectangular and cylindrical blocks. When a layout of the major process units has been decided, drawings of the plan and elevation are made, and design of the structural steelwork and foundations are done. Computer-aided design has also become increasingly popular.1,3,7

=Environmental Considerations=
Laws that protect the environment restrict the waste that plants can emit in order to preserve air, land, and water quality. States, provinces, and municipals usually have additional laws to national ones. In the US, environmental policies are moving towards more uniform and strict regulations. Businesses that produce more pollution are more likely to move to regions where local regulations are more lax, and industries in areas with more stringent policies will decline. The tighter national regulations are implemented to prevent this competition between regions. For example, amendments to the Clean Air Act and the Environmental Protection Agencies in the 1970s and 1980s reduced differences in regions' air pollution controls. Congress voted on the PSD (prevention of significant deterioration) to "level the playing field" across regions.1

Some of the main environmental legislations in North America are:

*'''The National Environmental Policy Act of 1969 (NEPA)''' - Signed by President Nixon, NEPA was one of the first laws that provide a national framework for the protection of the environment. It requires that all executive federal agencies consider the environmental impact of proposed actions. Government agencies must prepare environmental assessments and environmental impact statements that describe potential environmental effects. NEPA is implemented when buildings, airports, military complexes, parkland purchases, and highways are proposed.6

*'''The Clean Air Act (CAA, 1970)''' - The CAA was implemented to control air quality on a national level. The law was amended in 1990 to address issues such as acid rain, ozone depletion, and toxic air pollution, and to increase enforcement authority. Under the National Ambient Air Quality Standards (NAAQS), the law sets limits to allowable ambient levels of seven contaminants: ozone, carbon monoxide, lead, nitrogen dioxide, sulfur dioxide, PM10, and PM2.5. The CAA also requires that the EPA set US National Emissions Standards for Hazardous Air Pollutants (NESHAP), which regulates the emission of 189 hazardous air pollutants. The Maximum Available Control Technology (MACT) was implemented to limit the emission of volatile organics, sulfur, and other inorganic compounds from oil refineries. The CAA has had significant impact on the level of air pollutants that it regulates. Particularly in the Midwest and the Northwest, water acidity has been reduced. On the other hand, reduction of ozone damage has not been as successful since nitrogen oxide, which is linked to ozone formation, is not as tightly regulated.

*'''The Federal Water Pollution Control Act (The Clean Water Act, 1972)''' - The Clean Water Act was initially passed to clean water used for swimming and boating, and to protect wildlife. Amendments were later added to address water quality and toxic compound emissions. Under the CWA, the EPA can set water quality standards and monitor the industries' discharge into waters.

*'''The Safe Drinking Water Act (SDWA, 1974)''' - This act allows the EPA to set standards to potential drinking water. Public water systems must operate according to these standards.

*'''The Resource Conservation and Recovery Act (RCRA, 1976)''' - The RCRA protects current and future groundwater facilities from contamination. Firms that produce waste must have a "cradle to grave" approach to managing waste, meaning that the waste producer is legally responsible for the waste from the moment it is produced until it is finally disposed.

*'''The Comprehensive Environmental Response, Compensation and Liability Act (CERCLA or Superfund, 1980)''' - This legislation creates a tax on chemical and petroleum industries in order to cover the remediation of hazardous waste sites that are uncontrolled or abandoned. The CERCLA empowers EPA to remediate waste sites by short-term removal of material or long-term remediation actions, depending on the risk that the waste imposes on life.

*The Superfund Amendments and Reauthorization Act (SARA, 1986)
*'''The Pollution Prevention Act (PPA, 1990)''' - This act promotes minimization of waste and pollution at their source. It encourages more efficient processing and raw materials and recycling through grants, technical assistant, and information.

*'''The Oil Pollution Act of 1990 (OPA, 1990)''' - This act was passed after the Exxon Valdez oil spill in the Prince William Sound in Alaska, creating a tax on oil to cover the cost of responding to spills when the corporation that creates the spill does not respond.

*'''The Department of the Environment Act (E-10)''' - The Department of Environment Act created the Department of Environment and charges the Minister of the Environment with creating programs that protect the environment, assessing environmental impacts of federal program, and reporting information to the public in Canada.

*'''The Canadian Environmental Protection Act (CEPA, C-15.31, 1999)''' - CEPA empowers Environment Canada to control toxic substances, reduce pollution, and reduce bioaccumulating toxic substances. Canada will use CEPA to implement regulations that helps it meet its commitments to the Kyoto Protocol.

*'''The Canada Water Act (C-11)''' - This law allows the Minister of the Environment to manage water resources in Canada. This includes research, implementing conservation programs, utilization of water resources, and setting water quality standards.

Wastes comprise mainly of by-products, unused reactants, and off-specification product produced by misoperation. Leaking seals and flanges, and spills and discharges also emit waste. Material is also discharged in emergency situations. Instead of considering how to treat or manage waste, designers should start by tackling the source and find ways to minimize the production of waste.

The process of designing waste management systems are: (1) source reduction, (2) waste stream recycle, (3) waste treatment to reduce environmental impact, and (4) disposal that is legally sound. Source reduction can be achieved by reducing the concentration of impurities in the feed, protecting catalysts and adsorbents from contaminants, eliminating the use of extraneous materials, increasing recovery from separation, and improving the quality of the fuel by switching to cleaner-burning fuel. Unused feed can be recycled, and off-specification products can be reprocessed. In integral processes, the waste of one process is used as the feed for another. By-products can also be sold to another company for use as raw material. Tighter control systems, alarm, and interlocks can reduce misoperation of process unit.1

Further information about environmental considerations can be found in the [[Environmental concerns]] page.

=Conclusion=
The location and layout of a plant can greatly impact its economic and operational success. Objectives such as cost minimization, distribution, room for expansion, and safety of the plant operators and local community play important roles in the site decision. The major factors affecting this decision are market and labor. National and regional environmental legislations impact the processes, and thus make some areas more desirable than others. Least important are land costs and financial incentives. When planning to build a new facility or expanding an existing site, companies utilize their site selection team to assemble a list of desirable locational characteristics and determine how the new facility will fit in with the overall company. Then with the help of consultants, a location is narrowed down and further analyses are done to justify the continuation of the project. The location should be obtained before design of the details of the process. Within the site itself, process units are arranged to allow for the most economical flow of people and material, optimal safety, and room for future expansion.

=References=
[1] G.P. Towler, R. Sinnott, ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design'', Elsevier, 2012.

[2] J.P. Blair, R. Premus. Major Factors in Industrial Location: A Review. Economic Development Quarterly. 1987;1(1):72-85.

[3] M.S. Peters, K.D. Timmerhaus, ''Plant Design and Economics for Chemical Engineers'', 5th Ed., McGraw-Hill: New York, 2003.

[4] O. Akgul, N. Shah, L.G. Papageorgiou. Economic optimisation of a UK advanced biofuel supply chain. Biomass and Bioenergy. 2012;41:57-72.

[5] L.T. Biegler, I.E. Grossmann, A.W. Westerberg, ''Systematic Methods of Chemical Process Design'', Prentice-Hall: Upper Saddle River, 1997.

[6] Summary of the National Environmental Policy Act website. http://www.epa.gov/laws-regulations/summary-national-environmental-policy-act. August 2015. Accessed February 2016.

[7] R.T. Turton, R.C. Bailie, W.B. Whiting, J.A. Shaeiwitz, ''Analysis, Synthesis, and Design of Chemical Processes'', Prentice Hall: Upper Saddle River, 2003.

[8] R.W. Schmenner, Look Beyond the Obvious in Plant Location website. https://hbr.org/1979/01/look-beyond-the-obvious-in-plant-location. January 1979. Accessed February 2016.

Process location and layout decisions

2016-02-22T01:57:32Z

Ezhuang: /* Location Decision Factors */

Author: Ellen Zhuang [2016]

Stewards: Daniel Garcia, and Fengqi You

=Introduction=
Location is one of the first decisions in the design of a new chemical plant. It impacts profitability and scope for future expansion. If the project is a new facility, a suitable site must be found and an optimal layout of the site and process units must be planned. If the project adds to an existing site, the impact of the new addition on the existing plant must be considered. The plant also needs to accommodate for the nearby infrastructure, the services that it requires, and its environmental impacts.

=Location Selection=
===Location Decision Factors===
Initially, economists viewed the plant location decision as a cost-minimization problem. The optimal location was one where the transportation costs of raw material to the plant and product to the market was minimized. Between the early 1900s and post World War II period, most industries were sensitive to the cost of transportation. As transportation costs became less of an obstacle, the approach to the problem developed with considerations of trade-offs between transportation and factors such as wages, energy, local regulations, etc. If lower wages could balance the higher transportation costs of building the plant in a low-wage area, the low-wage location may be more desirable.

Although location is a long-term investment, a firm does not decide on a location with the sole objective of maximizing its profits or minimizing its costs. Managers may choose a “safer” location that is more likely to produce higher profits rather than a riskier location that yields the maximum investment return. Personal factors of the people involved are also influences. For example, new businesses tend to locate where the founder lives.2

In the current age, a plant's site is chosen based on several factors. These include:1,2,3,8

'''1. Raw material supply:''' The source and price of raw materials is one of the most important factors that determine the location of a plant. Facilities that produce chemicals in bulk are usually located close to the source of raw material if the costs of shipping the product is less than the costs of shipping the feed. For example, ethylene production is growing in the Middle East since cheap ethane from natural gas is readily available. Oil refineries tend to be located near areas with high population and crude oil supplies since it is expensive to transport the oil.

'''2. Location with respect to market:''' If the plant produces high-volume and low-cost products, such as cement and fertilizer, it may be better to situate the plant closer to the primary market since transportation cost is a large fraction of the sales price. If the product is low-volume and high-cost, like pharmaceuticals, then the benefits of being closer to the primary market may not be there.

'''3. Transport facilities:''' Facilities should be close to at least two major forms of transportation, whether that be road, rail, waterway, and/or seaport. For example, paper manufacturing plants in the US use various types of pulp that are delivered by truck or by train from various places in North and South America, so paper facilities need to be close to rail and major roads. Transportation by road is common for local distribution from central warehouses, while transportation by rail is more widespread for long distance transport of bulk chemicals. Pipeline is used to ship industrial gases and bulk fuels. Air freight can be efficient for shipment of personnel and essential units and supplies and for small volume products that have high value, such as pharmaceuticals. Of course, products that are delivered by air must meet aviation regulations.

'''4. Availability of labor:''' Skilled workers are usually brought to the plant from outside the area. There should be a local pool of unskilled labor that can be trained to operate the plant, and of skilled craft workers to maintain the process units. Local labor laws, trade union customs, restrictive practices for recruitment and training should also be taken into consideration. A 10% increase in unionization of a state's labor force is projected to reduce the number of expanding facilities by 30 to 45%.

'''5. Availability of utilities:''' Processes that require a substantial amount of cooling water is usually located near water sources, such as rivers or wells. Cooling water may be directly taken from the water source, or may be stored in cooling towers. Those that need large quantities of power, such as electrochemical ones, are typically close to cheap power sources.

'''6. Availability of suitable land:''' The ideal land is flat, well-drained, with suitable load-bearing characteristics. Further considerations have to be made if the land is reclaimed land near the ocean in earthquake zones. Property tax is also a factor when choosing a site since property taxes vary area to area. Under a third of plants that relocate move to regions with lower property taxes, which is the proportion that would be expected if companies move to a new location regardless of property tax. High property taxes is not as significant as other factors such as labor supply and land costs.

'''7. Environmental impact:''' Depending on the location, it may be more difficult and costly to dispose of wastes. During the project design phase, experts are typically consulted to learn more about an area's local regulations. More details about environmental regulations are found below.

'''8. Local community considerations:''' State and local planners are typically motivated by the desire to create jobs and improve the tax base. Introduction of facilities to an area is usually viewed as the most direct way to stimulate the area's economy. However, recent studies have found that communities with high-growth are already characterized by the fast growth of businesses that are already there. It is rare for a plant to completely close in one area and relocate to another, and if plants do relocate, the majority is over short distances and often within the same community. Therefore, local policymakers favor the expansion of existing plants. The opening of a new plant at a location should impost no additional risk to the local residents. For example, they should be downwind of the residential areas. Local communities also need to be able to accommodate the plant personnelles. For example, traffic, housing, and facilities must be able to accommodate the influx of workers. Additional factors are property taxes and water consumption.

'''9. Climate:''' The climate of the area may affect processes and costs. For example, plants in cold areas need more insulation and special heating. Facilities in earthquake areas need to be seismically sound. Plants in areas with high ambient humidity will usually use air cooling instead of water cooling.

'''10. Political and strategic considerations:''' Government sometimes gives capital grants, tax concessions, and other incentives to encourage plants to be built in specific areas. Physical assistance such as roads, water, and other public infrastructure are more popular than financial assistance. Companies can also globalize and take advantage of areas with preferential tariff agreements. The tax policy of an area is inversely related to growth. High personal income sometimes hinder employment growth. Personal income does not affect the cash flow of the company, but it reduces the after-tax income of its managers, and thus high personal income tax can be classified as a personal region. High state corporate taxes has also and detrimental effects on growth, but that is not always the case. Corporate tax is more important to firms with high capital expenditures.

===Analysis of the Importance of Factors===
In 1963, Morgan surveyed 17 companies and found that on average the most important factors on industrial location decisions are market, labor, and raw material. Taxes and financial incentives were of little significance. More recently, Fortune magazine surveyed among the 1000 largest US companies and found that the most important factors were market and labor. The survey also found that personal preferences of a firm's executive, tax, and central cities have great influence over the site of corporate headquarters. Surveys of interstate locational decisions found that the factors that played key roles for new firms were access to customers and the growing market, labor force, transportation, personal reasons of management, and availability of capital. The cost of land had the least important influence. For firms that expanded across states, the influential factors were labor costs and labor supply. Companies seeking expansion usually have made their production more routine, so quick access to suppliers is less of a concern. Cost minimization becomes more relevant to existing corporations.2

===Site Selection Process===
The decision for the location of a facility is part of a larger corporate planning process. Usually, a corporate planning office or a division of the company initiates the site selection process by forecasting future capacity requirements. If capacity shortages are in the forecasts, the managers may choose to outsource, increase price to reduce the demand, expand existing sites, or open a new facility. If the managers decide on a new facility, the site selection team enters the project.

The structure of the site selection team depends on the firm's organization. In companies with centralized staff, the site selection team generally consists of representatives from relevant areas, such as engineering, real estate, and transportation. In companies with strong divisions, the locational decisions may be done at the divisional level, with the corporate office supervising the process.

The site selection team determines what characteristics are important for the new location by considering how the new facility will fit in the company's overall strategy, if the company wants to target new markets, if the corporation wants to divide or integrate its functions, or how the company wants to be seen by the public. Next, potential locations are listed and studied against the desired characteristics.

Locational decisions are typically made sequentially. The first step is at the state or regional level. Then, the team studies specific communities and sites. Different locational factors are important to different stages. When selecting a general region, the site selection team focuses on factors with interregional variations such as labor, tax policies, climate, and market locations. At the more focused stage, details like inexpensive land, access to to major roads, and good schools are important. Consultants are often hired to do site analyses.

Once site options are narrowed down, the company discusses potential problems and incentives with local public officials. Construction costs are estimated, and a feasibility analysis is done to show that the project has a high rate of return.2

===Biofuel Supply Chain Example===
Biofuels is a popular potential alternative to fossil fuels. Various biomass resources, including food crops, non-food crops, and agricultural residues, are converted into biofuels. The two most common biofuels currently are ethanol and biodiesel. The supply chain for biofuel is a network consisting of several nodes: biomass cultivation sites, biofuel production plants, and demand centers. The locations of these facilities, and the location of the demand centers with respect to the biofuel demand impact transportation costs, production resource, demand, etc., and thus affects and sets constraints to the cost minimization of the supply chain. The optimal supply chain design minimize the total cost, which is a sum of the total investment cost, production cost, transportation cost, and outsourcing and import cost. The total transportation cost captures the transportation of biomass and biofuel between areas and the local bio mass transfer.4

[[File:biofuelsupplychain.png|400px|thumb|center|'''Figure 1.''' The biofuels supply chain consists of three notes: biomass cultivation sites, biofuel production plants, and biofuel demand centers.1]]

[[File:Biofueloptimal.png|300px|thumb|center|'''Figure 2.''' A Great Britain biofuel supply chain design that minimizes cost. The plants are near areas with high wheat cultivation and wheat straw collection rates. The location of the plants with respect to the demand centers is also optimized. The main modes of transportation are road, rail, and ship.4]]

=Site Layout=
The process units and buildings need to be arranged in such a way that allows for the most economical flow of materials and people. Furthermore, dangerous processes need to be a safe distance from other buildings, and the layout should be planned to allow for future expansion.

Process units are usually laid out first in an arrangement that allows for smooth flow of materials between the process steps. The distance between equipment is usually at least 30 m. Next, the location of the principal ancillary buildings are sited as to minimize the time that it takes the workers to travel between buildings. Administrative offices and laboratories are located away from hazardous processes. Control rooms are next to the processing equipment. Utility buildings are located as to minimize piping between the process units. Storage is placed between the loading and unloading facilities and next to the process units that they serve. Tanks containing hazardous material are placed at least 70 m from the plant. An example of a typical site plan is shown below.1

[[File:Site Location.png|400px|thumb|center|'''Figure 3.''' A typical site layout.1]]

===Site Layout Factors===
The main factors that are considered when planning the layout of the plant are listed below.1

'''1. Economic considerations (construction and operating costs):'''
Construction costs can be minimized by arranging process units and buildings that minimize pipes between equipment, the amount of structural steel work, etc. However, this layout may conflict with the layout that gives the optimal operation and maintenance.

'''2. Process requirements:''' Examples of process considerations that must be taken into account is the elevation of the base of columns to give enough net positive suction head to a pump.

'''3. Operation convenience:''' Process units that are attended to frequently should be placed with convenient access. Valves, heads, and sample heads should be placed where operators can easily access. If the plant anticipates replacement of equipment, space must be allowed for removal and installation.

'''4. Maintenance convenience:''' Equipment that requires maintenance should be in a location with easy access, and should have sufficient space for the maintenance tasks. For example, shell-and-tube exchangers need space so that tube bundles can be removed for cleaning and repair.

'''5. Future expansion:''' The layout should be planned to conveniently allow for future expansion of processes. Pipe racks should have space for future piping, and pipes should be oversized to allow for more flow in the future.

'''6. Modular Construction:''' Modular construction is where sections of the plant is constructed outside of the plant, and then transported to the site by road or sea. Advantages include improved quality control, reduced construction costs, less requirements for skilled labor on site. Tradeoffs are more flanged connections and possible problems with onsite assembly.

'''7. Safety:''' Escape routes for workers need to be in place at each level in process buildings. Blast walls must isolate equipment that pose hazards to confine potential explosions.

First, a conceptual flowsheet for the process is developed. The types of equipment and their connections with each other is described in a process flow diagram (PFD). Before the PFD is translated into detailed piping and instrumentation diagrams (P&ID) the layout of the process units must be planned.2,3 Scale drawings are made to show the relationships between storage space and process equipment based on the flow of materials and people, and on future expansion. Three-dimensional visualization are the layouts are then carried out with cardboard cutouts of the equipment outlines or rectangular and cylindrical blocks. When a layout of the major process units has been decided, drawings of the plan and elevation are made, and design of the structural steelwork and foundations are done. Computer-aided design has also become increasingly popular.1,3,7

=Environmental Considerations=
Laws that protect the environment restrict the waste that plants can emit in order to preserve air, land, and water quality. States, provinces, and municipals usually have additional laws to national ones. In the US, environmental policies are moving towards more uniform and strict regulations. Businesses that produce more pollution are more likely to move to regions where local regulations are more lax, and industries in areas with more stringent policies will decline. The tighter national regulations are implemented to prevent this competition between regions. For example, amendments to the Clean Air Act and the Environmental Protection Agencies in the 1970s and 1980s reduced differences in regions' air pollution controls. Congress voted on the PSD (prevention of significant deterioration) to "level the playing field" across regions.1

Some of the main environmental legislations in North America are:

*'''The National Environmental Policy Act of 1969 (NEPA)''' - Signed by President Nixon, NEPA was one of the first laws that provide a national framework for the protection of the environment. It requires that all executive federal agencies consider the environmental impact of proposed actions. Government agencies must prepare environmental assessments and environmental impact statements that describe potential environmental effects. NEPA is implemented when buildings, airports, military complexes, parkland purchases, and highways are proposed.6

*'''The Clean Air Act (CAA, 1970)''' - The CAA was implemented to control air quality on a national level. The law was amended in 1990 to address issues such as acid rain, ozone depletion, and toxic air pollution, and to increase enforcement authority. Under the National Ambient Air Quality Standards (NAAQS), the law sets limits to allowable ambient levels of seven contaminants: ozone, carbon monoxide, lead, nitrogen dioxide, sulfur dioxide, PM10, and PM2.5. The CAA also requires that the EPA set US National Emissions Standards for Hazardous Air Pollutants (NESHAP), which regulates the emission of 189 hazardous air pollutants. The Maximum Available Control Technology (MACT) was implemented to limit the emission of volatile organics, sulfur, and other inorganic compounds from oil refineries. The CAA has had significant impact on the level of air pollutants that it regulates. Particularly in the Midwest and the Northwest, water acidity has been reduced. On the other hand, reduction of ozone damage has not been as successful since nitrogen oxide, which is linked to ozone formation, is not as tightly regulated.

*'''The Federal Water Pollution Control Act (The Clean Water Act, 1972)''' - The Clean Water Act was initially passed to clean water used for swimming and boating, and to protect wildlife. Amendments were later added to address water quality and toxic compound emissions. Under the CWA, the EPA can set water quality standards and monitor the industries' discharge into waters.

*'''The Safe Drinking Water Act (SDWA, 1974)''' - This act allows the EPA to set standards to potential drinking water. Public water systems must operate according to these standards.

*'''The Resource Conservation and Recovery Act (RCRA, 1976)''' - The RCRA protects current and future groundwater facilities from contamination. Firms that produce waste must have a "cradle to grave" approach to managing waste, meaning that the waste producer is legally responsible for the waste from the moment it is produced until it is finally disposed.

*'''The Comprehensive Environmental Response, Compensation and Liability Act (CERCLA or Superfund, 1980)''' - This legislation creates a tax on chemical and petroleum industries in order to cover the remediation of hazardous waste sites that are uncontrolled or abandoned. The CERCLA empowers EPA to remediate waste sites by short-term removal of material or long-term remediation actions, depending on the risk that the waste imposes on life.

*The Superfund Amendments and Reauthorization Act (SARA, 1986)
*'''The Pollution Prevention Act (PPA, 1990)''' - This act promotes minimization of waste and pollution at their source. It encourages more efficient processing and raw materials and recycling through grants, technical assistant, and information.

*'''The Oil Pollution Act of 1990 (OPA, 1990)''' - This act was passed after the Exxon Valdez oil spill in the Prince William Sound in Alaska, creating a tax on oil to cover the cost of responding to spills when the corporation that creates the spill does not respond.

*'''The Department of the Environment Act (E-10)''' - The Department of Environment Act created the Department of Environment and charges the Minister of the Environment with creating programs that protect the environment, assessing environmental impacts of federal program, and reporting information to the public in Canada.

*'''The Canadian Environmental Protection Act (CEPA, C-15.31, 1999)''' - CEPA empowers Environment Canada to control toxic substances, reduce pollution, and reduce bioaccumulating toxic substances. Canada will use CEPA to implement regulations that helps it meet its commitments to the Kyoto Protocol.

*'''The Canada Water Act (C-11)''' - This law allows the Minister of the Environment to manage water resources in Canada. This includes research, implementing conservation programs, utilization of water resources, and setting water quality standards.

Wastes comprise mainly of by-products, unused reactants, and off-specification product produced by misoperation. Leaking seals and flanges, and spills and discharges also emit waste. Material is also discharged in emergency situations. Instead of considering how to treat or manage waste, designers should start by tackling the source and find ways to minimize the production of waste.

The process of designing waste management systems are: (1) source reduction, (2) waste stream recycle, (3) waste treatment to reduce environmental impact, and (4) disposal that is legally sound. Source reduction can be achieved by reducing the concentration of impurities in the feed, protecting catalysts and adsorbents from contaminants, eliminating the use of extraneous materials, increasing recovery from separation, and improving the quality of the fuel by switching to cleaner-burning fuel. Unused feed can be recycled, and off-specification products can be reprocessed. In integral processes, the waste of one process is used as the feed for another. By-products can also be sold to another company for use as raw material. Tighter control systems, alarm, and interlocks can reduce misoperation of process unit.1

Further information about environmental considerations can be found in the [[Environmental concerns]] page.

=Conclusion=
The location and layout of a plant can greatly impact its economic and operational success. Objectives such as cost minimization, distribution, room for expansion, and safety of the plant operators and local community play important roles in the site decision. The major factors affecting this decision are market and labor. National and regional environmental legislations impact the processes, and thus make some areas more desirable than others. Least important is land cost. When planning to build a new facility or expanding an existing site, companies utilize their site selection team to assemble a list of desirable locational characteristics and determine how the new facility will fit in with the overall company. Then with the help of consultants, a location is narrowed down and further analyses are done to justify the continuation of the project. The location should be obtained before design of the details of the process. Within the site itself, process units are arranged to allow for the most economical flow of people and material, optimal safety, and room for future expansion.

=References=
[1] G.P. Towler, R. Sinnott, ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design'', Elsevier, 2012.

[2] J.P. Blair, R. Premus. Major Factors in Industrial Location: A Review. Economic Development Quarterly. 1987;1(1):72-85.

[3] M.S. Peters, K.D. Timmerhaus, ''Plant Design and Economics for Chemical Engineers'', 5th Ed., McGraw-Hill: New York, 2003.

[4] O. Akgul, N. Shah, L.G. Papageorgiou. Economic optimisation of a UK advanced biofuel supply chain. Biomass and Bioenergy. 2012;41:57-72.

[5] L.T. Biegler, I.E. Grossmann, A.W. Westerberg, ''Systematic Methods of Chemical Process Design'', Prentice-Hall: Upper Saddle River, 1997.

[6] Summary of the National Environmental Policy Act website. http://www.epa.gov/laws-regulations/summary-national-environmental-policy-act. August 2015. Accessed February 2016.

[7] R.T. Turton, R.C. Bailie, W.B. Whiting, J.A. Shaeiwitz, ''Analysis, Synthesis, and Design of Chemical Processes'', Prentice Hall: Upper Saddle River, 2003.

[8] R.W. Schmenner, Look Beyond the Obvious in Plant Location website. https://hbr.org/1979/01/look-beyond-the-obvious-in-plant-location. January 1979. Accessed February 2016.

Process location and layout decisions

2016-02-22T01:56:47Z

Ezhuang: /* Location Decision Factors */

Author: Ellen Zhuang [2016]

Stewards: Daniel Garcia, and Fengqi You

=Introduction=
Location is one of the first decisions in the design of a new chemical plant. It impacts profitability and scope for future expansion. If the project is a new facility, a suitable site must be found and an optimal layout of the site and process units must be planned. If the project adds to an existing site, the impact of the new addition on the existing plant must be considered. The plant also needs to accommodate for the nearby infrastructure, the services that it requires, and its environmental impacts.

=Location Selection=
===Location Decision Factors===
Initially, economists viewed the plant location decision as a cost-minimization problem. The optimal location was one where the transportation costs of raw material to the plant and product to the market was minimized. Between the early 1900s and post World War II period, most industries were sensitive to the cost of transportation. As transportation costs became less of an obstacle, the approach to the problem developed with considerations of trade-offs between transportation and factors such as wages, energy, local regulations, etc. If lower wages could balance the higher transportation costs of building the plant in a low-wage area, the low-wage location may be more desirable.

Although location is a long-term investment, a firm does not decide on a location with the sole objective of maximizing its profits or minimizes its costs. Managers may choose a “safer” location that is more likely to produce higher profits rather than a riskier location that yields the maximum investment return. Personal factors of the people involved are also influences. For example, new businesses tend to locate where the founder lives.2

In the current age, a plant's site is chosen based on several factors. These include:1,2,3,8

'''1. Raw material supply:''' The source and price of raw materials is one of the most important factors that determine the location of a plant. Facilities that produce chemicals in bulk are usually located close to the source of raw material if the costs of shipping the product is less than the costs of shipping the feed. For example, ethylene production is growing in the Middle East since cheap ethane from natural gas is readily available. Oil refineries tend to be located near areas with high population and crude oil supplies since it is expensive to transport the oil.

'''2. Location with respect to market:''' If the plant produces high-volume and low-cost products, such as cement and fertilizer, it may be better to situate the plant closer to the primary market since transportation cost is a large fraction of the sales price. If the product is low-volume and high-cost, like pharmaceuticals, then the benefits of being closer to the primary market may not be there.

'''3. Transport facilities:''' Facilities should be close to at least two major forms of transportation, whether that be road, rail, waterway, and/or seaport. For example, paper manufacturing plants in the US use various types of pulp that are delivered by truck or by train from various places in North and South America, so paper facilities need to be close to rail and major roads. Transportation by road is common for local distribution from central warehouses, while transportation by rail is more widespread for long distance transport of bulk chemicals. Pipeline is used to ship industrial gases and bulk fuels. Air freight can be efficient for shipment of personnel and essential units and supplies and for small volume products that have high value, such as pharmaceuticals. Of course, products that are delivered by air must meet aviation regulations.

'''4. Availability of labor:''' Skilled workers are usually brought to the plant from outside the area. There should be a local pool of unskilled labor that can be trained to operate the plant, and of skilled craft workers to maintain the process units. Local labor laws, trade union customs, restrictive practices for recruitment and training should also be taken into consideration. A 10% increase in unionization of a state's labor force is projected to reduce the number of expanding facilities by 30 to 45%.

'''5. Availability of utilities:''' Processes that require a substantial amount of cooling water is usually located near water sources, such as rivers or wells. Cooling water may be directly taken from the water source, or may be stored in cooling towers. Those that need large quantities of power, such as electrochemical ones, are typically close to cheap power sources.

'''6. Availability of suitable land:''' The ideal land is flat, well-drained, with suitable load-bearing characteristics. Further considerations have to be made if the land is reclaimed land near the ocean in earthquake zones. Property tax is also a factor when choosing a site since property taxes vary area to area. Under a third of plants that relocate move to regions with lower property taxes, which is the proportion that would be expected if companies move to a new location regardless of property tax. High property taxes is not as significant as other factors such as labor supply and land costs.

'''7. Environmental impact:''' Depending on the location, it may be more difficult and costly to dispose of wastes. During the project design phase, experts are typically consulted to learn more about an area's local regulations. More details about environmental regulations are found below.

'''8. Local community considerations:''' State and local planners are typically motivated by the desire to create jobs and improve the tax base. Introduction of facilities to an area is usually viewed as the most direct way to stimulate the area's economy. However, recent studies have found that communities with high-growth are already characterized by the fast growth of businesses that are already there. It is rare for a plant to completely close in one area and relocate to another, and if plants do relocate, the majority is over short distances and often within the same community. Therefore, local policymakers favor the expansion of existing plants. The opening of a new plant at a location should impost no additional risk to the local residents. For example, they should be downwind of the residential areas. Local communities also need to be able to accommodate the plant personnelles. For example, traffic, housing, and facilities must be able to accommodate the influx of workers. Additional factors are property taxes and water consumption.

'''9. Climate:''' The climate of the area may affect processes and costs. For example, plants in cold areas need more insulation and special heating. Facilities in earthquake areas need to be seismically sound. Plants in areas with high ambient humidity will usually use air cooling instead of water cooling.

'''10. Political and strategic considerations:''' Government sometimes gives capital grants, tax concessions, and other incentives to encourage plants to be built in specific areas. Physical assistance such as roads, water, and other public infrastructure are more popular than financial assistance. Companies can also globalize and take advantage of areas with preferential tariff agreements. The tax policy of an area is inversely related to growth. High personal income sometimes hinder employment growth. Personal income does not affect the cash flow of the company, but it reduces the after-tax income of its managers, and thus high personal income tax can be classified as a personal region. High state corporate taxes has also and detrimental effects on growth, but that is not always the case. Corporate tax is more important to firms with high capital expenditures.

===Analysis of the Importance of Factors===
In 1963, Morgan surveyed 17 companies and found that on average the most important factors on industrial location decisions are market, labor, and raw material. Taxes and financial incentives were of little significance. More recently, Fortune magazine surveyed among the 1000 largest US companies and found that the most important factors were market and labor. The survey also found that personal preferences of a firm's executive, tax, and central cities have great influence over the site of corporate headquarters. Surveys of interstate locational decisions found that the factors that played key roles for new firms were access to customers and the growing market, labor force, transportation, personal reasons of management, and availability of capital. The cost of land had the least important influence. For firms that expanded across states, the influential factors were labor costs and labor supply. Companies seeking expansion usually have made their production more routine, so quick access to suppliers is less of a concern. Cost minimization becomes more relevant to existing corporations.2

===Site Selection Process===
The decision for the location of a facility is part of a larger corporate planning process. Usually, a corporate planning office or a division of the company initiates the site selection process by forecasting future capacity requirements. If capacity shortages are in the forecasts, the managers may choose to outsource, increase price to reduce the demand, expand existing sites, or open a new facility. If the managers decide on a new facility, the site selection team enters the project.

The structure of the site selection team depends on the firm's organization. In companies with centralized staff, the site selection team generally consists of representatives from relevant areas, such as engineering, real estate, and transportation. In companies with strong divisions, the locational decisions may be done at the divisional level, with the corporate office supervising the process.

The site selection team determines what characteristics are important for the new location by considering how the new facility will fit in the company's overall strategy, if the company wants to target new markets, if the corporation wants to divide or integrate its functions, or how the company wants to be seen by the public. Next, potential locations are listed and studied against the desired characteristics.

Locational decisions are typically made sequentially. The first step is at the state or regional level. Then, the team studies specific communities and sites. Different locational factors are important to different stages. When selecting a general region, the site selection team focuses on factors with interregional variations such as labor, tax policies, climate, and market locations. At the more focused stage, details like inexpensive land, access to to major roads, and good schools are important. Consultants are often hired to do site analyses.

Once site options are narrowed down, the company discusses potential problems and incentives with local public officials. Construction costs are estimated, and a feasibility analysis is done to show that the project has a high rate of return.2

===Biofuel Supply Chain Example===
Biofuels is a popular potential alternative to fossil fuels. Various biomass resources, including food crops, non-food crops, and agricultural residues, are converted into biofuels. The two most common biofuels currently are ethanol and biodiesel. The supply chain for biofuel is a network consisting of several nodes: biomass cultivation sites, biofuel production plants, and demand centers. The locations of these facilities, and the location of the demand centers with respect to the biofuel demand impact transportation costs, production resource, demand, etc., and thus affects and sets constraints to the cost minimization of the supply chain. The optimal supply chain design minimize the total cost, which is a sum of the total investment cost, production cost, transportation cost, and outsourcing and import cost. The total transportation cost captures the transportation of biomass and biofuel between areas and the local bio mass transfer.4

[[File:biofuelsupplychain.png|400px|thumb|center|'''Figure 1.''' The biofuels supply chain consists of three notes: biomass cultivation sites, biofuel production plants, and biofuel demand centers.1]]

[[File:Biofueloptimal.png|300px|thumb|center|'''Figure 2.''' A Great Britain biofuel supply chain design that minimizes cost. The plants are near areas with high wheat cultivation and wheat straw collection rates. The location of the plants with respect to the demand centers is also optimized. The main modes of transportation are road, rail, and ship.4]]

=Site Layout=
The process units and buildings need to be arranged in such a way that allows for the most economical flow of materials and people. Furthermore, dangerous processes need to be a safe distance from other buildings, and the layout should be planned to allow for future expansion.

Process units are usually laid out first in an arrangement that allows for smooth flow of materials between the process steps. The distance between equipment is usually at least 30 m. Next, the location of the principal ancillary buildings are sited as to minimize the time that it takes the workers to travel between buildings. Administrative offices and laboratories are located away from hazardous processes. Control rooms are next to the processing equipment. Utility buildings are located as to minimize piping between the process units. Storage is placed between the loading and unloading facilities and next to the process units that they serve. Tanks containing hazardous material are placed at least 70 m from the plant. An example of a typical site plan is shown below.1

[[File:Site Location.png|400px|thumb|center|'''Figure 3.''' A typical site layout.1]]

===Site Layout Factors===
The main factors that are considered when planning the layout of the plant are listed below.1

'''1. Economic considerations (construction and operating costs):'''
Construction costs can be minimized by arranging process units and buildings that minimize pipes between equipment, the amount of structural steel work, etc. However, this layout may conflict with the layout that gives the optimal operation and maintenance.

'''2. Process requirements:''' Examples of process considerations that must be taken into account is the elevation of the base of columns to give enough net positive suction head to a pump.

'''3. Operation convenience:''' Process units that are attended to frequently should be placed with convenient access. Valves, heads, and sample heads should be placed where operators can easily access. If the plant anticipates replacement of equipment, space must be allowed for removal and installation.

'''4. Maintenance convenience:''' Equipment that requires maintenance should be in a location with easy access, and should have sufficient space for the maintenance tasks. For example, shell-and-tube exchangers need space so that tube bundles can be removed for cleaning and repair.

'''5. Future expansion:''' The layout should be planned to conveniently allow for future expansion of processes. Pipe racks should have space for future piping, and pipes should be oversized to allow for more flow in the future.

'''6. Modular Construction:''' Modular construction is where sections of the plant is constructed outside of the plant, and then transported to the site by road or sea. Advantages include improved quality control, reduced construction costs, less requirements for skilled labor on site. Tradeoffs are more flanged connections and possible problems with onsite assembly.

'''7. Safety:''' Escape routes for workers need to be in place at each level in process buildings. Blast walls must isolate equipment that pose hazards to confine potential explosions.

First, a conceptual flowsheet for the process is developed. The types of equipment and their connections with each other is described in a process flow diagram (PFD). Before the PFD is translated into detailed piping and instrumentation diagrams (P&ID) the layout of the process units must be planned.2,3 Scale drawings are made to show the relationships between storage space and process equipment based on the flow of materials and people, and on future expansion. Three-dimensional visualization are the layouts are then carried out with cardboard cutouts of the equipment outlines or rectangular and cylindrical blocks. When a layout of the major process units has been decided, drawings of the plan and elevation are made, and design of the structural steelwork and foundations are done. Computer-aided design has also become increasingly popular.1,3,7

=Environmental Considerations=
Laws that protect the environment restrict the waste that plants can emit in order to preserve air, land, and water quality. States, provinces, and municipals usually have additional laws to national ones. In the US, environmental policies are moving towards more uniform and strict regulations. Businesses that produce more pollution are more likely to move to regions where local regulations are more lax, and industries in areas with more stringent policies will decline. The tighter national regulations are implemented to prevent this competition between regions. For example, amendments to the Clean Air Act and the Environmental Protection Agencies in the 1970s and 1980s reduced differences in regions' air pollution controls. Congress voted on the PSD (prevention of significant deterioration) to "level the playing field" across regions.1

Some of the main environmental legislations in North America are:

*'''The National Environmental Policy Act of 1969 (NEPA)''' - Signed by President Nixon, NEPA was one of the first laws that provide a national framework for the protection of the environment. It requires that all executive federal agencies consider the environmental impact of proposed actions. Government agencies must prepare environmental assessments and environmental impact statements that describe potential environmental effects. NEPA is implemented when buildings, airports, military complexes, parkland purchases, and highways are proposed.6

*'''The Clean Air Act (CAA, 1970)''' - The CAA was implemented to control air quality on a national level. The law was amended in 1990 to address issues such as acid rain, ozone depletion, and toxic air pollution, and to increase enforcement authority. Under the National Ambient Air Quality Standards (NAAQS), the law sets limits to allowable ambient levels of seven contaminants: ozone, carbon monoxide, lead, nitrogen dioxide, sulfur dioxide, PM10, and PM2.5. The CAA also requires that the EPA set US National Emissions Standards for Hazardous Air Pollutants (NESHAP), which regulates the emission of 189 hazardous air pollutants. The Maximum Available Control Technology (MACT) was implemented to limit the emission of volatile organics, sulfur, and other inorganic compounds from oil refineries. The CAA has had significant impact on the level of air pollutants that it regulates. Particularly in the Midwest and the Northwest, water acidity has been reduced. On the other hand, reduction of ozone damage has not been as successful since nitrogen oxide, which is linked to ozone formation, is not as tightly regulated.

*'''The Federal Water Pollution Control Act (The Clean Water Act, 1972)''' - The Clean Water Act was initially passed to clean water used for swimming and boating, and to protect wildlife. Amendments were later added to address water quality and toxic compound emissions. Under the CWA, the EPA can set water quality standards and monitor the industries' discharge into waters.

*'''The Safe Drinking Water Act (SDWA, 1974)''' - This act allows the EPA to set standards to potential drinking water. Public water systems must operate according to these standards.

*'''The Resource Conservation and Recovery Act (RCRA, 1976)''' - The RCRA protects current and future groundwater facilities from contamination. Firms that produce waste must have a "cradle to grave" approach to managing waste, meaning that the waste producer is legally responsible for the waste from the moment it is produced until it is finally disposed.

*'''The Comprehensive Environmental Response, Compensation and Liability Act (CERCLA or Superfund, 1980)''' - This legislation creates a tax on chemical and petroleum industries in order to cover the remediation of hazardous waste sites that are uncontrolled or abandoned. The CERCLA empowers EPA to remediate waste sites by short-term removal of material or long-term remediation actions, depending on the risk that the waste imposes on life.

*The Superfund Amendments and Reauthorization Act (SARA, 1986)
*'''The Pollution Prevention Act (PPA, 1990)''' - This act promotes minimization of waste and pollution at their source. It encourages more efficient processing and raw materials and recycling through grants, technical assistant, and information.

*'''The Oil Pollution Act of 1990 (OPA, 1990)''' - This act was passed after the Exxon Valdez oil spill in the Prince William Sound in Alaska, creating a tax on oil to cover the cost of responding to spills when the corporation that creates the spill does not respond.

*'''The Department of the Environment Act (E-10)''' - The Department of Environment Act created the Department of Environment and charges the Minister of the Environment with creating programs that protect the environment, assessing environmental impacts of federal program, and reporting information to the public in Canada.

*'''The Canadian Environmental Protection Act (CEPA, C-15.31, 1999)''' - CEPA empowers Environment Canada to control toxic substances, reduce pollution, and reduce bioaccumulating toxic substances. Canada will use CEPA to implement regulations that helps it meet its commitments to the Kyoto Protocol.

*'''The Canada Water Act (C-11)''' - This law allows the Minister of the Environment to manage water resources in Canada. This includes research, implementing conservation programs, utilization of water resources, and setting water quality standards.

Wastes comprise mainly of by-products, unused reactants, and off-specification product produced by misoperation. Leaking seals and flanges, and spills and discharges also emit waste. Material is also discharged in emergency situations. Instead of considering how to treat or manage waste, designers should start by tackling the source and find ways to minimize the production of waste.

The process of designing waste management systems are: (1) source reduction, (2) waste stream recycle, (3) waste treatment to reduce environmental impact, and (4) disposal that is legally sound. Source reduction can be achieved by reducing the concentration of impurities in the feed, protecting catalysts and adsorbents from contaminants, eliminating the use of extraneous materials, increasing recovery from separation, and improving the quality of the fuel by switching to cleaner-burning fuel. Unused feed can be recycled, and off-specification products can be reprocessed. In integral processes, the waste of one process is used as the feed for another. By-products can also be sold to another company for use as raw material. Tighter control systems, alarm, and interlocks can reduce misoperation of process unit.1

Further information about environmental considerations can be found in the [[Environmental concerns]] page.

=Conclusion=
The location and layout of a plant can greatly impact its economic and operational success. Objectives such as cost minimization, distribution, room for expansion, and safety of the plant operators and local community play important roles in the site decision. The major factors affecting this decision are market and labor. National and regional environmental legislations impact the processes, and thus make some areas more desirable than others. Least important is land cost. When planning to build a new facility or expanding an existing site, companies utilize their site selection team to assemble a list of desirable locational characteristics and determine how the new facility will fit in with the overall company. Then with the help of consultants, a location is narrowed down and further analyses are done to justify the continuation of the project. The location should be obtained before design of the details of the process. Within the site itself, process units are arranged to allow for the most economical flow of people and material, optimal safety, and room for future expansion.

=References=
[1] G.P. Towler, R. Sinnott, ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design'', Elsevier, 2012.

[2] J.P. Blair, R. Premus. Major Factors in Industrial Location: A Review. Economic Development Quarterly. 1987;1(1):72-85.

[3] M.S. Peters, K.D. Timmerhaus, ''Plant Design and Economics for Chemical Engineers'', 5th Ed., McGraw-Hill: New York, 2003.

[4] O. Akgul, N. Shah, L.G. Papageorgiou. Economic optimisation of a UK advanced biofuel supply chain. Biomass and Bioenergy. 2012;41:57-72.

[5] L.T. Biegler, I.E. Grossmann, A.W. Westerberg, ''Systematic Methods of Chemical Process Design'', Prentice-Hall: Upper Saddle River, 1997.

[6] Summary of the National Environmental Policy Act website. http://www.epa.gov/laws-regulations/summary-national-environmental-policy-act. August 2015. Accessed February 2016.

[7] R.T. Turton, R.C. Bailie, W.B. Whiting, J.A. Shaeiwitz, ''Analysis, Synthesis, and Design of Chemical Processes'', Prentice Hall: Upper Saddle River, 2003.

[8] R.W. Schmenner, Look Beyond the Obvious in Plant Location website. https://hbr.org/1979/01/look-beyond-the-obvious-in-plant-location. January 1979. Accessed February 2016.

Process location and layout decisions

2016-02-22T01:56:36Z

Ezhuang: /* Location Decision Factors */

Author: Ellen Zhuang [2016]

Stewards: Daniel Garcia, and Fengqi You

=Introduction=
Location is one of the first decisions in the design of a new chemical plant. It impacts profitability and scope for future expansion. If the project is a new facility, a suitable site must be found and an optimal layout of the site and process units must be planned. If the project adds to an existing site, the impact of the new addition on the existing plant must be considered. The plant also needs to accommodate for the nearby infrastructure, the services that it requires, and its environmental impacts.

=Location Selection=
===Location Decision Factors===
Initially, economists viewed the plant location decision as a cost-minimization problem. The optimal location was one where the transportation costs of raw material to the plant and product to the market was minimized. Between the early 1900s and post World War II period, most industries were sensitive to the cost of transportation. As transportation costs became less of an obstacle, the approach to the problem developed with considerations of trade-offs between transportation and factors such as wages, energy, local regulations, etc.. If lower wages could balance the higher transportation costs of building the plant in a low-wage area, the low-wage location may be more desirable.

Although location is a long-term investment, a firm does not decide on a location with the sole objective of maximizing its profits or minimizes its costs. Managers may choose a “safer” location that is more likely to produce higher profits rather than a riskier location that yields the maximum investment return. Personal factors of the people involved are also influences. For example, new businesses tend to locate where the founder lives.2

In the current age, a plant's site is chosen based on several factors. These include:1,2,3,8

'''1. Raw material supply:''' The source and price of raw materials is one of the most important factors that determine the location of a plant. Facilities that produce chemicals in bulk are usually located close to the source of raw material if the costs of shipping the product is less than the costs of shipping the feed. For example, ethylene production is growing in the Middle East since cheap ethane from natural gas is readily available. Oil refineries tend to be located near areas with high population and crude oil supplies since it is expensive to transport the oil.

'''2. Location with respect to market:''' If the plant produces high-volume and low-cost products, such as cement and fertilizer, it may be better to situate the plant closer to the primary market since transportation cost is a large fraction of the sales price. If the product is low-volume and high-cost, like pharmaceuticals, then the benefits of being closer to the primary market may not be there.

'''3. Transport facilities:''' Facilities should be close to at least two major forms of transportation, whether that be road, rail, waterway, and/or seaport. For example, paper manufacturing plants in the US use various types of pulp that are delivered by truck or by train from various places in North and South America, so paper facilities need to be close to rail and major roads. Transportation by road is common for local distribution from central warehouses, while transportation by rail is more widespread for long distance transport of bulk chemicals. Pipeline is used to ship industrial gases and bulk fuels. Air freight can be efficient for shipment of personnel and essential units and supplies and for small volume products that have high value, such as pharmaceuticals. Of course, products that are delivered by air must meet aviation regulations.

'''4. Availability of labor:''' Skilled workers are usually brought to the plant from outside the area. There should be a local pool of unskilled labor that can be trained to operate the plant, and of skilled craft workers to maintain the process units. Local labor laws, trade union customs, restrictive practices for recruitment and training should also be taken into consideration. A 10% increase in unionization of a state's labor force is projected to reduce the number of expanding facilities by 30 to 45%.

'''5. Availability of utilities:''' Processes that require a substantial amount of cooling water is usually located near water sources, such as rivers or wells. Cooling water may be directly taken from the water source, or may be stored in cooling towers. Those that need large quantities of power, such as electrochemical ones, are typically close to cheap power sources.

'''6. Availability of suitable land:''' The ideal land is flat, well-drained, with suitable load-bearing characteristics. Further considerations have to be made if the land is reclaimed land near the ocean in earthquake zones. Property tax is also a factor when choosing a site since property taxes vary area to area. Under a third of plants that relocate move to regions with lower property taxes, which is the proportion that would be expected if companies move to a new location regardless of property tax. High property taxes is not as significant as other factors such as labor supply and land costs.

'''7. Environmental impact:''' Depending on the location, it may be more difficult and costly to dispose of wastes. During the project design phase, experts are typically consulted to learn more about an area's local regulations. More details about environmental regulations are found below.

'''8. Local community considerations:''' State and local planners are typically motivated by the desire to create jobs and improve the tax base. Introduction of facilities to an area is usually viewed as the most direct way to stimulate the area's economy. However, recent studies have found that communities with high-growth are already characterized by the fast growth of businesses that are already there. It is rare for a plant to completely close in one area and relocate to another, and if plants do relocate, the majority is over short distances and often within the same community. Therefore, local policymakers favor the expansion of existing plants. The opening of a new plant at a location should impost no additional risk to the local residents. For example, they should be downwind of the residential areas. Local communities also need to be able to accommodate the plant personnelles. For example, traffic, housing, and facilities must be able to accommodate the influx of workers. Additional factors are property taxes and water consumption.

'''9. Climate:''' The climate of the area may affect processes and costs. For example, plants in cold areas need more insulation and special heating. Facilities in earthquake areas need to be seismically sound. Plants in areas with high ambient humidity will usually use air cooling instead of water cooling.

'''10. Political and strategic considerations:''' Government sometimes gives capital grants, tax concessions, and other incentives to encourage plants to be built in specific areas. Physical assistance such as roads, water, and other public infrastructure are more popular than financial assistance. Companies can also globalize and take advantage of areas with preferential tariff agreements. The tax policy of an area is inversely related to growth. High personal income sometimes hinder employment growth. Personal income does not affect the cash flow of the company, but it reduces the after-tax income of its managers, and thus high personal income tax can be classified as a personal region. High state corporate taxes has also and detrimental effects on growth, but that is not always the case. Corporate tax is more important to firms with high capital expenditures.

===Analysis of the Importance of Factors===
In 1963, Morgan surveyed 17 companies and found that on average the most important factors on industrial location decisions are market, labor, and raw material. Taxes and financial incentives were of little significance. More recently, Fortune magazine surveyed among the 1000 largest US companies and found that the most important factors were market and labor. The survey also found that personal preferences of a firm's executive, tax, and central cities have great influence over the site of corporate headquarters. Surveys of interstate locational decisions found that the factors that played key roles for new firms were access to customers and the growing market, labor force, transportation, personal reasons of management, and availability of capital. The cost of land had the least important influence. For firms that expanded across states, the influential factors were labor costs and labor supply. Companies seeking expansion usually have made their production more routine, so quick access to suppliers is less of a concern. Cost minimization becomes more relevant to existing corporations.2

===Site Selection Process===
The decision for the location of a facility is part of a larger corporate planning process. Usually, a corporate planning office or a division of the company initiates the site selection process by forecasting future capacity requirements. If capacity shortages are in the forecasts, the managers may choose to outsource, increase price to reduce the demand, expand existing sites, or open a new facility. If the managers decide on a new facility, the site selection team enters the project.

The structure of the site selection team depends on the firm's organization. In companies with centralized staff, the site selection team generally consists of representatives from relevant areas, such as engineering, real estate, and transportation. In companies with strong divisions, the locational decisions may be done at the divisional level, with the corporate office supervising the process.

The site selection team determines what characteristics are important for the new location by considering how the new facility will fit in the company's overall strategy, if the company wants to target new markets, if the corporation wants to divide or integrate its functions, or how the company wants to be seen by the public. Next, potential locations are listed and studied against the desired characteristics.

Locational decisions are typically made sequentially. The first step is at the state or regional level. Then, the team studies specific communities and sites. Different locational factors are important to different stages. When selecting a general region, the site selection team focuses on factors with interregional variations such as labor, tax policies, climate, and market locations. At the more focused stage, details like inexpensive land, access to to major roads, and good schools are important. Consultants are often hired to do site analyses.

Once site options are narrowed down, the company discusses potential problems and incentives with local public officials. Construction costs are estimated, and a feasibility analysis is done to show that the project has a high rate of return.2

===Biofuel Supply Chain Example===
Biofuels is a popular potential alternative to fossil fuels. Various biomass resources, including food crops, non-food crops, and agricultural residues, are converted into biofuels. The two most common biofuels currently are ethanol and biodiesel. The supply chain for biofuel is a network consisting of several nodes: biomass cultivation sites, biofuel production plants, and demand centers. The locations of these facilities, and the location of the demand centers with respect to the biofuel demand impact transportation costs, production resource, demand, etc., and thus affects and sets constraints to the cost minimization of the supply chain. The optimal supply chain design minimize the total cost, which is a sum of the total investment cost, production cost, transportation cost, and outsourcing and import cost. The total transportation cost captures the transportation of biomass and biofuel between areas and the local bio mass transfer.4

[[File:biofuelsupplychain.png|400px|thumb|center|'''Figure 1.''' The biofuels supply chain consists of three notes: biomass cultivation sites, biofuel production plants, and biofuel demand centers.1]]

[[File:Biofueloptimal.png|300px|thumb|center|'''Figure 2.''' A Great Britain biofuel supply chain design that minimizes cost. The plants are near areas with high wheat cultivation and wheat straw collection rates. The location of the plants with respect to the demand centers is also optimized. The main modes of transportation are road, rail, and ship.4]]

=Site Layout=
The process units and buildings need to be arranged in such a way that allows for the most economical flow of materials and people. Furthermore, dangerous processes need to be a safe distance from other buildings, and the layout should be planned to allow for future expansion.

Process units are usually laid out first in an arrangement that allows for smooth flow of materials between the process steps. The distance between equipment is usually at least 30 m. Next, the location of the principal ancillary buildings are sited as to minimize the time that it takes the workers to travel between buildings. Administrative offices and laboratories are located away from hazardous processes. Control rooms are next to the processing equipment. Utility buildings are located as to minimize piping between the process units. Storage is placed between the loading and unloading facilities and next to the process units that they serve. Tanks containing hazardous material are placed at least 70 m from the plant. An example of a typical site plan is shown below.1

[[File:Site Location.png|400px|thumb|center|'''Figure 3.''' A typical site layout.1]]

===Site Layout Factors===
The main factors that are considered when planning the layout of the plant are listed below.1

'''1. Economic considerations (construction and operating costs):'''
Construction costs can be minimized by arranging process units and buildings that minimize pipes between equipment, the amount of structural steel work, etc. However, this layout may conflict with the layout that gives the optimal operation and maintenance.

'''2. Process requirements:''' Examples of process considerations that must be taken into account is the elevation of the base of columns to give enough net positive suction head to a pump.

'''3. Operation convenience:''' Process units that are attended to frequently should be placed with convenient access. Valves, heads, and sample heads should be placed where operators can easily access. If the plant anticipates replacement of equipment, space must be allowed for removal and installation.

'''4. Maintenance convenience:''' Equipment that requires maintenance should be in a location with easy access, and should have sufficient space for the maintenance tasks. For example, shell-and-tube exchangers need space so that tube bundles can be removed for cleaning and repair.

'''5. Future expansion:''' The layout should be planned to conveniently allow for future expansion of processes. Pipe racks should have space for future piping, and pipes should be oversized to allow for more flow in the future.

'''6. Modular Construction:''' Modular construction is where sections of the plant is constructed outside of the plant, and then transported to the site by road or sea. Advantages include improved quality control, reduced construction costs, less requirements for skilled labor on site. Tradeoffs are more flanged connections and possible problems with onsite assembly.

'''7. Safety:''' Escape routes for workers need to be in place at each level in process buildings. Blast walls must isolate equipment that pose hazards to confine potential explosions.

First, a conceptual flowsheet for the process is developed. The types of equipment and their connections with each other is described in a process flow diagram (PFD). Before the PFD is translated into detailed piping and instrumentation diagrams (P&ID) the layout of the process units must be planned.2,3 Scale drawings are made to show the relationships between storage space and process equipment based on the flow of materials and people, and on future expansion. Three-dimensional visualization are the layouts are then carried out with cardboard cutouts of the equipment outlines or rectangular and cylindrical blocks. When a layout of the major process units has been decided, drawings of the plan and elevation are made, and design of the structural steelwork and foundations are done. Computer-aided design has also become increasingly popular.1,3,7

=Environmental Considerations=
Laws that protect the environment restrict the waste that plants can emit in order to preserve air, land, and water quality. States, provinces, and municipals usually have additional laws to national ones. In the US, environmental policies are moving towards more uniform and strict regulations. Businesses that produce more pollution are more likely to move to regions where local regulations are more lax, and industries in areas with more stringent policies will decline. The tighter national regulations are implemented to prevent this competition between regions. For example, amendments to the Clean Air Act and the Environmental Protection Agencies in the 1970s and 1980s reduced differences in regions' air pollution controls. Congress voted on the PSD (prevention of significant deterioration) to "level the playing field" across regions.1

Some of the main environmental legislations in North America are:

*'''The National Environmental Policy Act of 1969 (NEPA)''' - Signed by President Nixon, NEPA was one of the first laws that provide a national framework for the protection of the environment. It requires that all executive federal agencies consider the environmental impact of proposed actions. Government agencies must prepare environmental assessments and environmental impact statements that describe potential environmental effects. NEPA is implemented when buildings, airports, military complexes, parkland purchases, and highways are proposed.6

*'''The Clean Air Act (CAA, 1970)''' - The CAA was implemented to control air quality on a national level. The law was amended in 1990 to address issues such as acid rain, ozone depletion, and toxic air pollution, and to increase enforcement authority. Under the National Ambient Air Quality Standards (NAAQS), the law sets limits to allowable ambient levels of seven contaminants: ozone, carbon monoxide, lead, nitrogen dioxide, sulfur dioxide, PM10, and PM2.5. The CAA also requires that the EPA set US National Emissions Standards for Hazardous Air Pollutants (NESHAP), which regulates the emission of 189 hazardous air pollutants. The Maximum Available Control Technology (MACT) was implemented to limit the emission of volatile organics, sulfur, and other inorganic compounds from oil refineries. The CAA has had significant impact on the level of air pollutants that it regulates. Particularly in the Midwest and the Northwest, water acidity has been reduced. On the other hand, reduction of ozone damage has not been as successful since nitrogen oxide, which is linked to ozone formation, is not as tightly regulated.

*'''The Federal Water Pollution Control Act (The Clean Water Act, 1972)''' - The Clean Water Act was initially passed to clean water used for swimming and boating, and to protect wildlife. Amendments were later added to address water quality and toxic compound emissions. Under the CWA, the EPA can set water quality standards and monitor the industries' discharge into waters.

*'''The Safe Drinking Water Act (SDWA, 1974)''' - This act allows the EPA to set standards to potential drinking water. Public water systems must operate according to these standards.

*'''The Resource Conservation and Recovery Act (RCRA, 1976)''' - The RCRA protects current and future groundwater facilities from contamination. Firms that produce waste must have a "cradle to grave" approach to managing waste, meaning that the waste producer is legally responsible for the waste from the moment it is produced until it is finally disposed.

*'''The Comprehensive Environmental Response, Compensation and Liability Act (CERCLA or Superfund, 1980)''' - This legislation creates a tax on chemical and petroleum industries in order to cover the remediation of hazardous waste sites that are uncontrolled or abandoned. The CERCLA empowers EPA to remediate waste sites by short-term removal of material or long-term remediation actions, depending on the risk that the waste imposes on life.

*The Superfund Amendments and Reauthorization Act (SARA, 1986)
*'''The Pollution Prevention Act (PPA, 1990)''' - This act promotes minimization of waste and pollution at their source. It encourages more efficient processing and raw materials and recycling through grants, technical assistant, and information.

*'''The Oil Pollution Act of 1990 (OPA, 1990)''' - This act was passed after the Exxon Valdez oil spill in the Prince William Sound in Alaska, creating a tax on oil to cover the cost of responding to spills when the corporation that creates the spill does not respond.

*'''The Department of the Environment Act (E-10)''' - The Department of Environment Act created the Department of Environment and charges the Minister of the Environment with creating programs that protect the environment, assessing environmental impacts of federal program, and reporting information to the public in Canada.

*'''The Canadian Environmental Protection Act (CEPA, C-15.31, 1999)''' - CEPA empowers Environment Canada to control toxic substances, reduce pollution, and reduce bioaccumulating toxic substances. Canada will use CEPA to implement regulations that helps it meet its commitments to the Kyoto Protocol.

*'''The Canada Water Act (C-11)''' - This law allows the Minister of the Environment to manage water resources in Canada. This includes research, implementing conservation programs, utilization of water resources, and setting water quality standards.

Wastes comprise mainly of by-products, unused reactants, and off-specification product produced by misoperation. Leaking seals and flanges, and spills and discharges also emit waste. Material is also discharged in emergency situations. Instead of considering how to treat or manage waste, designers should start by tackling the source and find ways to minimize the production of waste.

The process of designing waste management systems are: (1) source reduction, (2) waste stream recycle, (3) waste treatment to reduce environmental impact, and (4) disposal that is legally sound. Source reduction can be achieved by reducing the concentration of impurities in the feed, protecting catalysts and adsorbents from contaminants, eliminating the use of extraneous materials, increasing recovery from separation, and improving the quality of the fuel by switching to cleaner-burning fuel. Unused feed can be recycled, and off-specification products can be reprocessed. In integral processes, the waste of one process is used as the feed for another. By-products can also be sold to another company for use as raw material. Tighter control systems, alarm, and interlocks can reduce misoperation of process unit.1

Further information about environmental considerations can be found in the [[Environmental concerns]] page.

=Conclusion=
The location and layout of a plant can greatly impact its economic and operational success. Objectives such as cost minimization, distribution, room for expansion, and safety of the plant operators and local community play important roles in the site decision. The major factors affecting this decision are market and labor. National and regional environmental legislations impact the processes, and thus make some areas more desirable than others. Least important is land cost. When planning to build a new facility or expanding an existing site, companies utilize their site selection team to assemble a list of desirable locational characteristics and determine how the new facility will fit in with the overall company. Then with the help of consultants, a location is narrowed down and further analyses are done to justify the continuation of the project. The location should be obtained before design of the details of the process. Within the site itself, process units are arranged to allow for the most economical flow of people and material, optimal safety, and room for future expansion.

=References=
[1] G.P. Towler, R. Sinnott, ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design'', Elsevier, 2012.

[2] J.P. Blair, R. Premus. Major Factors in Industrial Location: A Review. Economic Development Quarterly. 1987;1(1):72-85.

[3] M.S. Peters, K.D. Timmerhaus, ''Plant Design and Economics for Chemical Engineers'', 5th Ed., McGraw-Hill: New York, 2003.

[4] O. Akgul, N. Shah, L.G. Papageorgiou. Economic optimisation of a UK advanced biofuel supply chain. Biomass and Bioenergy. 2012;41:57-72.

[5] L.T. Biegler, I.E. Grossmann, A.W. Westerberg, ''Systematic Methods of Chemical Process Design'', Prentice-Hall: Upper Saddle River, 1997.

[6] Summary of the National Environmental Policy Act website. http://www.epa.gov/laws-regulations/summary-national-environmental-policy-act. August 2015. Accessed February 2016.

[7] R.T. Turton, R.C. Bailie, W.B. Whiting, J.A. Shaeiwitz, ''Analysis, Synthesis, and Design of Chemical Processes'', Prentice Hall: Upper Saddle River, 2003.

[8] R.W. Schmenner, Look Beyond the Obvious in Plant Location website. https://hbr.org/1979/01/look-beyond-the-obvious-in-plant-location. January 1979. Accessed February 2016.

Process location and layout decisions

2016-02-22T01:56:00Z

Ezhuang: /* Location Decision Factors */

Author: Ellen Zhuang [2016]

Stewards: Daniel Garcia, and Fengqi You

=Introduction=
Location is one of the first decisions in the design of a new chemical plant. It impacts profitability and scope for future expansion. If the project is a new facility, a suitable site must be found and an optimal layout of the site and process units must be planned. If the project adds to an existing site, the impact of the new addition on the existing plant must be considered. The plant also needs to accommodate for the nearby infrastructure, the services that it requires, and its environmental impacts.

=Location Selection=
===Location Decision Factors===
Initially, economists viewed the plant location decision as a cost-minimization problem. The optimal location was one where the transportation costs of raw material to the plant and product to the market was minimized. Between the early 1900s and post World War II period, most industries were sensitive to the cost of transportation. As transportation costs became less of an obstacle, the approach to the problem developed with considerations of trade-offs between transportation and factors such as wages, energy, local regulations, etc., and transportation. If lower wages could balance the higher transportation costs of building the plant in a low-wage area, the low-wage location may be more desirable.

Although location is a long-term investment, a firm does not decide on a location with the sole objective of maximizing its profits or minimizes its costs. Managers may choose a “safer” location that is more likely to produce higher profits rather than a riskier location that yields the maximum investment return. Personal factors of the people involved are also influences. For example, new businesses tend to locate where the founder lives.2

In the current age, a plant's site is chosen based on several factors. These include:1,2,3,8

'''1. Raw material supply:''' The source and price of raw materials is one of the most important factors that determine the location of a plant. Facilities that produce chemicals in bulk are usually located close to the source of raw material if the costs of shipping the product is less than the costs of shipping the feed. For example, ethylene production is growing in the Middle East since cheap ethane from natural gas is readily available. Oil refineries tend to be located near areas with high population and crude oil supplies since it is expensive to transport the oil.

'''2. Location with respect to market:''' If the plant produces high-volume and low-cost products, such as cement and fertilizer, it may be better to situate the plant closer to the primary market since transportation cost is a large fraction of the sales price. If the product is low-volume and high-cost, like pharmaceuticals, then the benefits of being closer to the primary market may not be there.

'''3. Transport facilities:''' Facilities should be close to at least two major forms of transportation, whether that be road, rail, waterway, and/or seaport. For example, paper manufacturing plants in the US use various types of pulp that are delivered by truck or by train from various places in North and South America, so paper facilities need to be close to rail and major roads. Transportation by road is common for local distribution from central warehouses, while transportation by rail is more widespread for long distance transport of bulk chemicals. Pipeline is used to ship industrial gases and bulk fuels. Air freight can be efficient for shipment of personnel and essential units and supplies and for small volume products that have high value, such as pharmaceuticals. Of course, products that are delivered by air must meet aviation regulations.

'''4. Availability of labor:''' Skilled workers are usually brought to the plant from outside the area. There should be a local pool of unskilled labor that can be trained to operate the plant, and of skilled craft workers to maintain the process units. Local labor laws, trade union customs, restrictive practices for recruitment and training should also be taken into consideration. A 10% increase in unionization of a state's labor force is projected to reduce the number of expanding facilities by 30 to 45%.

'''5. Availability of utilities:''' Processes that require a substantial amount of cooling water is usually located near water sources, such as rivers or wells. Cooling water may be directly taken from the water source, or may be stored in cooling towers. Those that need large quantities of power, such as electrochemical ones, are typically close to cheap power sources.

'''6. Availability of suitable land:''' The ideal land is flat, well-drained, with suitable load-bearing characteristics. Further considerations have to be made if the land is reclaimed land near the ocean in earthquake zones. Property tax is also a factor when choosing a site since property taxes vary area to area. Under a third of plants that relocate move to regions with lower property taxes, which is the proportion that would be expected if companies move to a new location regardless of property tax. High property taxes is not as significant as other factors such as labor supply and land costs.

'''7. Environmental impact:''' Depending on the location, it may be more difficult and costly to dispose of wastes. During the project design phase, experts are typically consulted to learn more about an area's local regulations. More details about environmental regulations are found below.

'''8. Local community considerations:''' State and local planners are typically motivated by the desire to create jobs and improve the tax base. Introduction of facilities to an area is usually viewed as the most direct way to stimulate the area's economy. However, recent studies have found that communities with high-growth are already characterized by the fast growth of businesses that are already there. It is rare for a plant to completely close in one area and relocate to another, and if plants do relocate, the majority is over short distances and often within the same community. Therefore, local policymakers favor the expansion of existing plants. The opening of a new plant at a location should impost no additional risk to the local residents. For example, they should be downwind of the residential areas. Local communities also need to be able to accommodate the plant personnelles. For example, traffic, housing, and facilities must be able to accommodate the influx of workers. Additional factors are property taxes and water consumption.

'''9. Climate:''' The climate of the area may affect processes and costs. For example, plants in cold areas need more insulation and special heating. Facilities in earthquake areas need to be seismically sound. Plants in areas with high ambient humidity will usually use air cooling instead of water cooling.

'''10. Political and strategic considerations:''' Government sometimes gives capital grants, tax concessions, and other incentives to encourage plants to be built in specific areas. Physical assistance such as roads, water, and other public infrastructure are more popular than financial assistance. Companies can also globalize and take advantage of areas with preferential tariff agreements. The tax policy of an area is inversely related to growth. High personal income sometimes hinder employment growth. Personal income does not affect the cash flow of the company, but it reduces the after-tax income of its managers, and thus high personal income tax can be classified as a personal region. High state corporate taxes has also and detrimental effects on growth, but that is not always the case. Corporate tax is more important to firms with high capital expenditures.

===Analysis of the Importance of Factors===
In 1963, Morgan surveyed 17 companies and found that on average the most important factors on industrial location decisions are market, labor, and raw material. Taxes and financial incentives were of little significance. More recently, Fortune magazine surveyed among the 1000 largest US companies and found that the most important factors were market and labor. The survey also found that personal preferences of a firm's executive, tax, and central cities have great influence over the site of corporate headquarters. Surveys of interstate locational decisions found that the factors that played key roles for new firms were access to customers and the growing market, labor force, transportation, personal reasons of management, and availability of capital. The cost of land had the least important influence. For firms that expanded across states, the influential factors were labor costs and labor supply. Companies seeking expansion usually have made their production more routine, so quick access to suppliers is less of a concern. Cost minimization becomes more relevant to existing corporations.2

===Site Selection Process===
The decision for the location of a facility is part of a larger corporate planning process. Usually, a corporate planning office or a division of the company initiates the site selection process by forecasting future capacity requirements. If capacity shortages are in the forecasts, the managers may choose to outsource, increase price to reduce the demand, expand existing sites, or open a new facility. If the managers decide on a new facility, the site selection team enters the project.

The structure of the site selection team depends on the firm's organization. In companies with centralized staff, the site selection team generally consists of representatives from relevant areas, such as engineering, real estate, and transportation. In companies with strong divisions, the locational decisions may be done at the divisional level, with the corporate office supervising the process.

The site selection team determines what characteristics are important for the new location by considering how the new facility will fit in the company's overall strategy, if the company wants to target new markets, if the corporation wants to divide or integrate its functions, or how the company wants to be seen by the public. Next, potential locations are listed and studied against the desired characteristics.

Locational decisions are typically made sequentially. The first step is at the state or regional level. Then, the team studies specific communities and sites. Different locational factors are important to different stages. When selecting a general region, the site selection team focuses on factors with interregional variations such as labor, tax policies, climate, and market locations. At the more focused stage, details like inexpensive land, access to to major roads, and good schools are important. Consultants are often hired to do site analyses.

Once site options are narrowed down, the company discusses potential problems and incentives with local public officials. Construction costs are estimated, and a feasibility analysis is done to show that the project has a high rate of return.2

===Biofuel Supply Chain Example===
Biofuels is a popular potential alternative to fossil fuels. Various biomass resources, including food crops, non-food crops, and agricultural residues, are converted into biofuels. The two most common biofuels currently are ethanol and biodiesel. The supply chain for biofuel is a network consisting of several nodes: biomass cultivation sites, biofuel production plants, and demand centers. The locations of these facilities, and the location of the demand centers with respect to the biofuel demand impact transportation costs, production resource, demand, etc., and thus affects and sets constraints to the cost minimization of the supply chain. The optimal supply chain design minimize the total cost, which is a sum of the total investment cost, production cost, transportation cost, and outsourcing and import cost. The total transportation cost captures the transportation of biomass and biofuel between areas and the local bio mass transfer.4

[[File:biofuelsupplychain.png|400px|thumb|center|'''Figure 1.''' The biofuels supply chain consists of three notes: biomass cultivation sites, biofuel production plants, and biofuel demand centers.1]]

[[File:Biofueloptimal.png|300px|thumb|center|'''Figure 2.''' A Great Britain biofuel supply chain design that minimizes cost. The plants are near areas with high wheat cultivation and wheat straw collection rates. The location of the plants with respect to the demand centers is also optimized. The main modes of transportation are road, rail, and ship.4]]

=Site Layout=
The process units and buildings need to be arranged in such a way that allows for the most economical flow of materials and people. Furthermore, dangerous processes need to be a safe distance from other buildings, and the layout should be planned to allow for future expansion.

Process units are usually laid out first in an arrangement that allows for smooth flow of materials between the process steps. The distance between equipment is usually at least 30 m. Next, the location of the principal ancillary buildings are sited as to minimize the time that it takes the workers to travel between buildings. Administrative offices and laboratories are located away from hazardous processes. Control rooms are next to the processing equipment. Utility buildings are located as to minimize piping between the process units. Storage is placed between the loading and unloading facilities and next to the process units that they serve. Tanks containing hazardous material are placed at least 70 m from the plant. An example of a typical site plan is shown below.1

[[File:Site Location.png|400px|thumb|center|'''Figure 3.''' A typical site layout.1]]

===Site Layout Factors===
The main factors that are considered when planning the layout of the plant are listed below.1

'''1. Economic considerations (construction and operating costs):'''
Construction costs can be minimized by arranging process units and buildings that minimize pipes between equipment, the amount of structural steel work, etc. However, this layout may conflict with the layout that gives the optimal operation and maintenance.

'''2. Process requirements:''' Examples of process considerations that must be taken into account is the elevation of the base of columns to give enough net positive suction head to a pump.

'''3. Operation convenience:''' Process units that are attended to frequently should be placed with convenient access. Valves, heads, and sample heads should be placed where operators can easily access. If the plant anticipates replacement of equipment, space must be allowed for removal and installation.

'''4. Maintenance convenience:''' Equipment that requires maintenance should be in a location with easy access, and should have sufficient space for the maintenance tasks. For example, shell-and-tube exchangers need space so that tube bundles can be removed for cleaning and repair.

'''5. Future expansion:''' The layout should be planned to conveniently allow for future expansion of processes. Pipe racks should have space for future piping, and pipes should be oversized to allow for more flow in the future.

'''6. Modular Construction:''' Modular construction is where sections of the plant is constructed outside of the plant, and then transported to the site by road or sea. Advantages include improved quality control, reduced construction costs, less requirements for skilled labor on site. Tradeoffs are more flanged connections and possible problems with onsite assembly.

'''7. Safety:''' Escape routes for workers need to be in place at each level in process buildings. Blast walls must isolate equipment that pose hazards to confine potential explosions.

First, a conceptual flowsheet for the process is developed. The types of equipment and their connections with each other is described in a process flow diagram (PFD). Before the PFD is translated into detailed piping and instrumentation diagrams (P&ID) the layout of the process units must be planned.2,3 Scale drawings are made to show the relationships between storage space and process equipment based on the flow of materials and people, and on future expansion. Three-dimensional visualization are the layouts are then carried out with cardboard cutouts of the equipment outlines or rectangular and cylindrical blocks. When a layout of the major process units has been decided, drawings of the plan and elevation are made, and design of the structural steelwork and foundations are done. Computer-aided design has also become increasingly popular.1,3,7

=Environmental Considerations=
Laws that protect the environment restrict the waste that plants can emit in order to preserve air, land, and water quality. States, provinces, and municipals usually have additional laws to national ones. In the US, environmental policies are moving towards more uniform and strict regulations. Businesses that produce more pollution are more likely to move to regions where local regulations are more lax, and industries in areas with more stringent policies will decline. The tighter national regulations are implemented to prevent this competition between regions. For example, amendments to the Clean Air Act and the Environmental Protection Agencies in the 1970s and 1980s reduced differences in regions' air pollution controls. Congress voted on the PSD (prevention of significant deterioration) to "level the playing field" across regions.1

Some of the main environmental legislations in North America are:

*'''The National Environmental Policy Act of 1969 (NEPA)''' - Signed by President Nixon, NEPA was one of the first laws that provide a national framework for the protection of the environment. It requires that all executive federal agencies consider the environmental impact of proposed actions. Government agencies must prepare environmental assessments and environmental impact statements that describe potential environmental effects. NEPA is implemented when buildings, airports, military complexes, parkland purchases, and highways are proposed.6

*'''The Clean Air Act (CAA, 1970)''' - The CAA was implemented to control air quality on a national level. The law was amended in 1990 to address issues such as acid rain, ozone depletion, and toxic air pollution, and to increase enforcement authority. Under the National Ambient Air Quality Standards (NAAQS), the law sets limits to allowable ambient levels of seven contaminants: ozone, carbon monoxide, lead, nitrogen dioxide, sulfur dioxide, PM10, and PM2.5. The CAA also requires that the EPA set US National Emissions Standards for Hazardous Air Pollutants (NESHAP), which regulates the emission of 189 hazardous air pollutants. The Maximum Available Control Technology (MACT) was implemented to limit the emission of volatile organics, sulfur, and other inorganic compounds from oil refineries. The CAA has had significant impact on the level of air pollutants that it regulates. Particularly in the Midwest and the Northwest, water acidity has been reduced. On the other hand, reduction of ozone damage has not been as successful since nitrogen oxide, which is linked to ozone formation, is not as tightly regulated.

*'''The Federal Water Pollution Control Act (The Clean Water Act, 1972)''' - The Clean Water Act was initially passed to clean water used for swimming and boating, and to protect wildlife. Amendments were later added to address water quality and toxic compound emissions. Under the CWA, the EPA can set water quality standards and monitor the industries' discharge into waters.

*'''The Safe Drinking Water Act (SDWA, 1974)''' - This act allows the EPA to set standards to potential drinking water. Public water systems must operate according to these standards.

*'''The Resource Conservation and Recovery Act (RCRA, 1976)''' - The RCRA protects current and future groundwater facilities from contamination. Firms that produce waste must have a "cradle to grave" approach to managing waste, meaning that the waste producer is legally responsible for the waste from the moment it is produced until it is finally disposed.

*'''The Comprehensive Environmental Response, Compensation and Liability Act (CERCLA or Superfund, 1980)''' - This legislation creates a tax on chemical and petroleum industries in order to cover the remediation of hazardous waste sites that are uncontrolled or abandoned. The CERCLA empowers EPA to remediate waste sites by short-term removal of material or long-term remediation actions, depending on the risk that the waste imposes on life.

*The Superfund Amendments and Reauthorization Act (SARA, 1986)
*'''The Pollution Prevention Act (PPA, 1990)''' - This act promotes minimization of waste and pollution at their source. It encourages more efficient processing and raw materials and recycling through grants, technical assistant, and information.

*'''The Oil Pollution Act of 1990 (OPA, 1990)''' - This act was passed after the Exxon Valdez oil spill in the Prince William Sound in Alaska, creating a tax on oil to cover the cost of responding to spills when the corporation that creates the spill does not respond.

*'''The Department of the Environment Act (E-10)''' - The Department of Environment Act created the Department of Environment and charges the Minister of the Environment with creating programs that protect the environment, assessing environmental impacts of federal program, and reporting information to the public in Canada.

*'''The Canadian Environmental Protection Act (CEPA, C-15.31, 1999)''' - CEPA empowers Environment Canada to control toxic substances, reduce pollution, and reduce bioaccumulating toxic substances. Canada will use CEPA to implement regulations that helps it meet its commitments to the Kyoto Protocol.

*'''The Canada Water Act (C-11)''' - This law allows the Minister of the Environment to manage water resources in Canada. This includes research, implementing conservation programs, utilization of water resources, and setting water quality standards.

Wastes comprise mainly of by-products, unused reactants, and off-specification product produced by misoperation. Leaking seals and flanges, and spills and discharges also emit waste. Material is also discharged in emergency situations. Instead of considering how to treat or manage waste, designers should start by tackling the source and find ways to minimize the production of waste.

The process of designing waste management systems are: (1) source reduction, (2) waste stream recycle, (3) waste treatment to reduce environmental impact, and (4) disposal that is legally sound. Source reduction can be achieved by reducing the concentration of impurities in the feed, protecting catalysts and adsorbents from contaminants, eliminating the use of extraneous materials, increasing recovery from separation, and improving the quality of the fuel by switching to cleaner-burning fuel. Unused feed can be recycled, and off-specification products can be reprocessed. In integral processes, the waste of one process is used as the feed for another. By-products can also be sold to another company for use as raw material. Tighter control systems, alarm, and interlocks can reduce misoperation of process unit.1

Further information about environmental considerations can be found in the [[Environmental concerns]] page.

=Conclusion=
The location and layout of a plant can greatly impact its economic and operational success. Objectives such as cost minimization, distribution, room for expansion, and safety of the plant operators and local community play important roles in the site decision. The major factors affecting this decision are market and labor. National and regional environmental legislations impact the processes, and thus make some areas more desirable than others. Least important is land cost. When planning to build a new facility or expanding an existing site, companies utilize their site selection team to assemble a list of desirable locational characteristics and determine how the new facility will fit in with the overall company. Then with the help of consultants, a location is narrowed down and further analyses are done to justify the continuation of the project. The location should be obtained before design of the details of the process. Within the site itself, process units are arranged to allow for the most economical flow of people and material, optimal safety, and room for future expansion.

=References=
[1] G.P. Towler, R. Sinnott, ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design'', Elsevier, 2012.

[2] J.P. Blair, R. Premus. Major Factors in Industrial Location: A Review. Economic Development Quarterly. 1987;1(1):72-85.

[3] M.S. Peters, K.D. Timmerhaus, ''Plant Design and Economics for Chemical Engineers'', 5th Ed., McGraw-Hill: New York, 2003.

[4] O. Akgul, N. Shah, L.G. Papageorgiou. Economic optimisation of a UK advanced biofuel supply chain. Biomass and Bioenergy. 2012;41:57-72.

[5] L.T. Biegler, I.E. Grossmann, A.W. Westerberg, ''Systematic Methods of Chemical Process Design'', Prentice-Hall: Upper Saddle River, 1997.

[6] Summary of the National Environmental Policy Act website. http://www.epa.gov/laws-regulations/summary-national-environmental-policy-act. August 2015. Accessed February 2016.

[7] R.T. Turton, R.C. Bailie, W.B. Whiting, J.A. Shaeiwitz, ''Analysis, Synthesis, and Design of Chemical Processes'', Prentice Hall: Upper Saddle River, 2003.

[8] R.W. Schmenner, Look Beyond the Obvious in Plant Location website. https://hbr.org/1979/01/look-beyond-the-obvious-in-plant-location. January 1979. Accessed February 2016.

Process location and layout decisions

2016-02-22T01:55:37Z

Ezhuang: /* Location Decision Factors */

Author: Ellen Zhuang [2016]

Stewards: Daniel Garcia, and Fengqi You

=Introduction=
Location is one of the first decisions in the design of a new chemical plant. It impacts profitability and scope for future expansion. If the project is a new facility, a suitable site must be found and an optimal layout of the site and process units must be planned. If the project adds to an existing site, the impact of the new addition on the existing plant must be considered. The plant also needs to accommodate for the nearby infrastructure, the services that it requires, and its environmental impacts.

=Location Selection=
===Location Decision Factors===
Initially, economists viewed the plant location decision as a cost-minimization problem. The optimal location was one where the transportation costs of raw material to the plant and product to the market was minimized. Between the early 1900s and post World War II period, most industries were sensitive to the cost of transportation. As transportation costs became less of an obstacle, the approach to the problem developed with considerations of trade-offs between other factors, such wages, energy, local regulations, etc., and transportation. If lower wages could balance the higher transportation costs of building the plant in a low-wage area, the low-wage location may be more desirable.

Although location is a long-term investment, a firm does not decide on a location with the sole objective of maximizing its profits or minimizes its costs. Managers may choose a “safer” location that is more likely to produce higher profits rather than a riskier location that yields the maximum investment return. Personal factors of the people involved are also influences. For example, new businesses tend to locate where the founder lives. 2

In the current age, a plant's site is chosen based on several factors. These include:1,2,3,8

'''1. Raw material supply:''' The source and price of raw materials is one of the most important factors that determine the location of a plant. Facilities that produce chemicals in bulk are usually located close to the source of raw material if the costs of shipping the product is less than the costs of shipping the feed. For example, ethylene production is growing in the Middle East since cheap ethane from natural gas is readily available. Oil refineries tend to be located near areas with high population and crude oil supplies since it is expensive to transport the oil.

'''2. Location with respect to market:''' If the plant produces high-volume and low-cost products, such as cement and fertilizer, it may be better to situate the plant closer to the primary market since transportation cost is a large fraction of the sales price. If the product is low-volume and high-cost, like pharmaceuticals, then the benefits of being closer to the primary market may not be there.

'''3. Transport facilities:''' Facilities should be close to at least two major forms of transportation, whether that be road, rail, waterway, and/or seaport. For example, paper manufacturing plants in the US use various types of pulp that are delivered by truck or by train from various places in North and South America, so paper facilities need to be close to rail and major roads. Transportation by road is common for local distribution from central warehouses, while transportation by rail is more widespread for long distance transport of bulk chemicals. Pipeline is used to ship industrial gases and bulk fuels. Air freight can be efficient for shipment of personnel and essential units and supplies and for small volume products that have high value, such as pharmaceuticals. Of course, products that are delivered by air must meet aviation regulations.

'''4. Availability of labor:''' Skilled workers are usually brought to the plant from outside the area. There should be a local pool of unskilled labor that can be trained to operate the plant, and of skilled craft workers to maintain the process units. Local labor laws, trade union customs, restrictive practices for recruitment and training should also be taken into consideration. A 10% increase in unionization of a state's labor force is projected to reduce the number of expanding facilities by 30 to 45%.

'''5. Availability of utilities:''' Processes that require a substantial amount of cooling water is usually located near water sources, such as rivers or wells. Cooling water may be directly taken from the water source, or may be stored in cooling towers. Those that need large quantities of power, such as electrochemical ones, are typically close to cheap power sources.

'''6. Availability of suitable land:''' The ideal land is flat, well-drained, with suitable load-bearing characteristics. Further considerations have to be made if the land is reclaimed land near the ocean in earthquake zones. Property tax is also a factor when choosing a site since property taxes vary area to area. Under a third of plants that relocate move to regions with lower property taxes, which is the proportion that would be expected if companies move to a new location regardless of property tax. High property taxes is not as significant as other factors such as labor supply and land costs.

'''7. Environmental impact:''' Depending on the location, it may be more difficult and costly to dispose of wastes. During the project design phase, experts are typically consulted to learn more about an area's local regulations. More details about environmental regulations are found below.

'''8. Local community considerations:''' State and local planners are typically motivated by the desire to create jobs and improve the tax base. Introduction of facilities to an area is usually viewed as the most direct way to stimulate the area's economy. However, recent studies have found that communities with high-growth are already characterized by the fast growth of businesses that are already there. It is rare for a plant to completely close in one area and relocate to another, and if plants do relocate, the majority is over short distances and often within the same community. Therefore, local policymakers favor the expansion of existing plants. The opening of a new plant at a location should impost no additional risk to the local residents. For example, they should be downwind of the residential areas. Local communities also need to be able to accommodate the plant personnelles. For example, traffic, housing, and facilities must be able to accommodate the influx of workers. Additional factors are property taxes and water consumption.

'''9. Climate:''' The climate of the area may affect processes and costs. For example, plants in cold areas need more insulation and special heating. Facilities in earthquake areas need to be seismically sound. Plants in areas with high ambient humidity will usually use air cooling instead of water cooling.

'''10. Political and strategic considerations:''' Government sometimes gives capital grants, tax concessions, and other incentives to encourage plants to be built in specific areas. Physical assistance such as roads, water, and other public infrastructure are more popular than financial assistance. Companies can also globalize and take advantage of areas with preferential tariff agreements. The tax policy of an area is inversely related to growth. High personal income sometimes hinder employment growth. Personal income does not affect the cash flow of the company, but it reduces the after-tax income of its managers, and thus high personal income tax can be classified as a personal region. High state corporate taxes has also and detrimental effects on growth, but that is not always the case. Corporate tax is more important to firms with high capital expenditures.

===Analysis of the Importance of Factors===
In 1963, Morgan surveyed 17 companies and found that on average the most important factors on industrial location decisions are market, labor, and raw material. Taxes and financial incentives were of little significance. More recently, Fortune magazine surveyed among the 1000 largest US companies and found that the most important factors were market and labor. The survey also found that personal preferences of a firm's executive, tax, and central cities have great influence over the site of corporate headquarters. Surveys of interstate locational decisions found that the factors that played key roles for new firms were access to customers and the growing market, labor force, transportation, personal reasons of management, and availability of capital. The cost of land had the least important influence. For firms that expanded across states, the influential factors were labor costs and labor supply. Companies seeking expansion usually have made their production more routine, so quick access to suppliers is less of a concern. Cost minimization becomes more relevant to existing corporations.2

===Site Selection Process===
The decision for the location of a facility is part of a larger corporate planning process. Usually, a corporate planning office or a division of the company initiates the site selection process by forecasting future capacity requirements. If capacity shortages are in the forecasts, the managers may choose to outsource, increase price to reduce the demand, expand existing sites, or open a new facility. If the managers decide on a new facility, the site selection team enters the project.

The structure of the site selection team depends on the firm's organization. In companies with centralized staff, the site selection team generally consists of representatives from relevant areas, such as engineering, real estate, and transportation. In companies with strong divisions, the locational decisions may be done at the divisional level, with the corporate office supervising the process.

The site selection team determines what characteristics are important for the new location by considering how the new facility will fit in the company's overall strategy, if the company wants to target new markets, if the corporation wants to divide or integrate its functions, or how the company wants to be seen by the public. Next, potential locations are listed and studied against the desired characteristics.

Locational decisions are typically made sequentially. The first step is at the state or regional level. Then, the team studies specific communities and sites. Different locational factors are important to different stages. When selecting a general region, the site selection team focuses on factors with interregional variations such as labor, tax policies, climate, and market locations. At the more focused stage, details like inexpensive land, access to to major roads, and good schools are important. Consultants are often hired to do site analyses.

Once site options are narrowed down, the company discusses potential problems and incentives with local public officials. Construction costs are estimated, and a feasibility analysis is done to show that the project has a high rate of return.2

===Biofuel Supply Chain Example===
Biofuels is a popular potential alternative to fossil fuels. Various biomass resources, including food crops, non-food crops, and agricultural residues, are converted into biofuels. The two most common biofuels currently are ethanol and biodiesel. The supply chain for biofuel is a network consisting of several nodes: biomass cultivation sites, biofuel production plants, and demand centers. The locations of these facilities, and the location of the demand centers with respect to the biofuel demand impact transportation costs, production resource, demand, etc., and thus affects and sets constraints to the cost minimization of the supply chain. The optimal supply chain design minimize the total cost, which is a sum of the total investment cost, production cost, transportation cost, and outsourcing and import cost. The total transportation cost captures the transportation of biomass and biofuel between areas and the local bio mass transfer.4

[[File:biofuelsupplychain.png|400px|thumb|center|'''Figure 1.''' The biofuels supply chain consists of three notes: biomass cultivation sites, biofuel production plants, and biofuel demand centers.1]]

[[File:Biofueloptimal.png|300px|thumb|center|'''Figure 2.''' A Great Britain biofuel supply chain design that minimizes cost. The plants are near areas with high wheat cultivation and wheat straw collection rates. The location of the plants with respect to the demand centers is also optimized. The main modes of transportation are road, rail, and ship.4]]

=Site Layout=
The process units and buildings need to be arranged in such a way that allows for the most economical flow of materials and people. Furthermore, dangerous processes need to be a safe distance from other buildings, and the layout should be planned to allow for future expansion.

Process units are usually laid out first in an arrangement that allows for smooth flow of materials between the process steps. The distance between equipment is usually at least 30 m. Next, the location of the principal ancillary buildings are sited as to minimize the time that it takes the workers to travel between buildings. Administrative offices and laboratories are located away from hazardous processes. Control rooms are next to the processing equipment. Utility buildings are located as to minimize piping between the process units. Storage is placed between the loading and unloading facilities and next to the process units that they serve. Tanks containing hazardous material are placed at least 70 m from the plant. An example of a typical site plan is shown below.1

[[File:Site Location.png|400px|thumb|center|'''Figure 3.''' A typical site layout.1]]

===Site Layout Factors===
The main factors that are considered when planning the layout of the plant are listed below.1

'''1. Economic considerations (construction and operating costs):'''
Construction costs can be minimized by arranging process units and buildings that minimize pipes between equipment, the amount of structural steel work, etc. However, this layout may conflict with the layout that gives the optimal operation and maintenance.

'''2. Process requirements:''' Examples of process considerations that must be taken into account is the elevation of the base of columns to give enough net positive suction head to a pump.

'''3. Operation convenience:''' Process units that are attended to frequently should be placed with convenient access. Valves, heads, and sample heads should be placed where operators can easily access. If the plant anticipates replacement of equipment, space must be allowed for removal and installation.

'''4. Maintenance convenience:''' Equipment that requires maintenance should be in a location with easy access, and should have sufficient space for the maintenance tasks. For example, shell-and-tube exchangers need space so that tube bundles can be removed for cleaning and repair.

'''5. Future expansion:''' The layout should be planned to conveniently allow for future expansion of processes. Pipe racks should have space for future piping, and pipes should be oversized to allow for more flow in the future.

'''6. Modular Construction:''' Modular construction is where sections of the plant is constructed outside of the plant, and then transported to the site by road or sea. Advantages include improved quality control, reduced construction costs, less requirements for skilled labor on site. Tradeoffs are more flanged connections and possible problems with onsite assembly.

'''7. Safety:''' Escape routes for workers need to be in place at each level in process buildings. Blast walls must isolate equipment that pose hazards to confine potential explosions.

First, a conceptual flowsheet for the process is developed. The types of equipment and their connections with each other is described in a process flow diagram (PFD). Before the PFD is translated into detailed piping and instrumentation diagrams (P&ID) the layout of the process units must be planned.2,3 Scale drawings are made to show the relationships between storage space and process equipment based on the flow of materials and people, and on future expansion. Three-dimensional visualization are the layouts are then carried out with cardboard cutouts of the equipment outlines or rectangular and cylindrical blocks. When a layout of the major process units has been decided, drawings of the plan and elevation are made, and design of the structural steelwork and foundations are done. Computer-aided design has also become increasingly popular.1,3,7

=Environmental Considerations=
Laws that protect the environment restrict the waste that plants can emit in order to preserve air, land, and water quality. States, provinces, and municipals usually have additional laws to national ones. In the US, environmental policies are moving towards more uniform and strict regulations. Businesses that produce more pollution are more likely to move to regions where local regulations are more lax, and industries in areas with more stringent policies will decline. The tighter national regulations are implemented to prevent this competition between regions. For example, amendments to the Clean Air Act and the Environmental Protection Agencies in the 1970s and 1980s reduced differences in regions' air pollution controls. Congress voted on the PSD (prevention of significant deterioration) to "level the playing field" across regions.1

Some of the main environmental legislations in North America are:

*'''The National Environmental Policy Act of 1969 (NEPA)''' - Signed by President Nixon, NEPA was one of the first laws that provide a national framework for the protection of the environment. It requires that all executive federal agencies consider the environmental impact of proposed actions. Government agencies must prepare environmental assessments and environmental impact statements that describe potential environmental effects. NEPA is implemented when buildings, airports, military complexes, parkland purchases, and highways are proposed.6

*'''The Clean Air Act (CAA, 1970)''' - The CAA was implemented to control air quality on a national level. The law was amended in 1990 to address issues such as acid rain, ozone depletion, and toxic air pollution, and to increase enforcement authority. Under the National Ambient Air Quality Standards (NAAQS), the law sets limits to allowable ambient levels of seven contaminants: ozone, carbon monoxide, lead, nitrogen dioxide, sulfur dioxide, PM10, and PM2.5. The CAA also requires that the EPA set US National Emissions Standards for Hazardous Air Pollutants (NESHAP), which regulates the emission of 189 hazardous air pollutants. The Maximum Available Control Technology (MACT) was implemented to limit the emission of volatile organics, sulfur, and other inorganic compounds from oil refineries. The CAA has had significant impact on the level of air pollutants that it regulates. Particularly in the Midwest and the Northwest, water acidity has been reduced. On the other hand, reduction of ozone damage has not been as successful since nitrogen oxide, which is linked to ozone formation, is not as tightly regulated.

*'''The Federal Water Pollution Control Act (The Clean Water Act, 1972)''' - The Clean Water Act was initially passed to clean water used for swimming and boating, and to protect wildlife. Amendments were later added to address water quality and toxic compound emissions. Under the CWA, the EPA can set water quality standards and monitor the industries' discharge into waters.

*'''The Safe Drinking Water Act (SDWA, 1974)''' - This act allows the EPA to set standards to potential drinking water. Public water systems must operate according to these standards.

*'''The Resource Conservation and Recovery Act (RCRA, 1976)''' - The RCRA protects current and future groundwater facilities from contamination. Firms that produce waste must have a "cradle to grave" approach to managing waste, meaning that the waste producer is legally responsible for the waste from the moment it is produced until it is finally disposed.

*'''The Comprehensive Environmental Response, Compensation and Liability Act (CERCLA or Superfund, 1980)''' - This legislation creates a tax on chemical and petroleum industries in order to cover the remediation of hazardous waste sites that are uncontrolled or abandoned. The CERCLA empowers EPA to remediate waste sites by short-term removal of material or long-term remediation actions, depending on the risk that the waste imposes on life.

*The Superfund Amendments and Reauthorization Act (SARA, 1986)
*'''The Pollution Prevention Act (PPA, 1990)''' - This act promotes minimization of waste and pollution at their source. It encourages more efficient processing and raw materials and recycling through grants, technical assistant, and information.

*'''The Oil Pollution Act of 1990 (OPA, 1990)''' - This act was passed after the Exxon Valdez oil spill in the Prince William Sound in Alaska, creating a tax on oil to cover the cost of responding to spills when the corporation that creates the spill does not respond.

*'''The Department of the Environment Act (E-10)''' - The Department of Environment Act created the Department of Environment and charges the Minister of the Environment with creating programs that protect the environment, assessing environmental impacts of federal program, and reporting information to the public in Canada.

*'''The Canadian Environmental Protection Act (CEPA, C-15.31, 1999)''' - CEPA empowers Environment Canada to control toxic substances, reduce pollution, and reduce bioaccumulating toxic substances. Canada will use CEPA to implement regulations that helps it meet its commitments to the Kyoto Protocol.

*'''The Canada Water Act (C-11)''' - This law allows the Minister of the Environment to manage water resources in Canada. This includes research, implementing conservation programs, utilization of water resources, and setting water quality standards.

Wastes comprise mainly of by-products, unused reactants, and off-specification product produced by misoperation. Leaking seals and flanges, and spills and discharges also emit waste. Material is also discharged in emergency situations. Instead of considering how to treat or manage waste, designers should start by tackling the source and find ways to minimize the production of waste.

The process of designing waste management systems are: (1) source reduction, (2) waste stream recycle, (3) waste treatment to reduce environmental impact, and (4) disposal that is legally sound. Source reduction can be achieved by reducing the concentration of impurities in the feed, protecting catalysts and adsorbents from contaminants, eliminating the use of extraneous materials, increasing recovery from separation, and improving the quality of the fuel by switching to cleaner-burning fuel. Unused feed can be recycled, and off-specification products can be reprocessed. In integral processes, the waste of one process is used as the feed for another. By-products can also be sold to another company for use as raw material. Tighter control systems, alarm, and interlocks can reduce misoperation of process unit.1

Further information about environmental considerations can be found in the [[Environmental concerns]] page.

=Conclusion=
The location and layout of a plant can greatly impact its economic and operational success. Objectives such as cost minimization, distribution, room for expansion, and safety of the plant operators and local community play important roles in the site decision. The major factors affecting this decision are market and labor. National and regional environmental legislations impact the processes, and thus make some areas more desirable than others. Least important is land cost. When planning to build a new facility or expanding an existing site, companies utilize their site selection team to assemble a list of desirable locational characteristics and determine how the new facility will fit in with the overall company. Then with the help of consultants, a location is narrowed down and further analyses are done to justify the continuation of the project. The location should be obtained before design of the details of the process. Within the site itself, process units are arranged to allow for the most economical flow of people and material, optimal safety, and room for future expansion.

=References=
[1] G.P. Towler, R. Sinnott, ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design'', Elsevier, 2012.

[2] J.P. Blair, R. Premus. Major Factors in Industrial Location: A Review. Economic Development Quarterly. 1987;1(1):72-85.

[3] M.S. Peters, K.D. Timmerhaus, ''Plant Design and Economics for Chemical Engineers'', 5th Ed., McGraw-Hill: New York, 2003.

[4] O. Akgul, N. Shah, L.G. Papageorgiou. Economic optimisation of a UK advanced biofuel supply chain. Biomass and Bioenergy. 2012;41:57-72.

[5] L.T. Biegler, I.E. Grossmann, A.W. Westerberg, ''Systematic Methods of Chemical Process Design'', Prentice-Hall: Upper Saddle River, 1997.

[6] Summary of the National Environmental Policy Act website. http://www.epa.gov/laws-regulations/summary-national-environmental-policy-act. August 2015. Accessed February 2016.

[7] R.T. Turton, R.C. Bailie, W.B. Whiting, J.A. Shaeiwitz, ''Analysis, Synthesis, and Design of Chemical Processes'', Prentice Hall: Upper Saddle River, 2003.

[8] R.W. Schmenner, Look Beyond the Obvious in Plant Location website. https://hbr.org/1979/01/look-beyond-the-obvious-in-plant-location. January 1979. Accessed February 2016.

Process location and layout decisions

2016-02-22T01:54:34Z

Ezhuang:

Author: Ellen Zhuang [2016]

Stewards: Daniel Garcia, and Fengqi You

=Introduction=
Location is one of the first decisions in the design of a new chemical plant. It impacts profitability and scope for future expansion. If the project is a new facility, a suitable site must be found and an optimal layout of the site and process units must be planned. If the project adds to an existing site, the impact of the new addition on the existing plant must be considered. The plant also needs to accommodate for the nearby infrastructure, the services that it requires, and its environmental impacts.

=Location Selection=
===Location Decision Factors===
Initially, economists viewed the plant location decision as a cost-minimization problem. The optimal location was one where the transportation costs of raw material to the plant and product to the market was minimized. Between the early 1900s and post World War II period, most industries were sensitive to the cost of transportation. As transportation costs became less of an obstacle, the approach to the problem developed with considerations of trade-offs between factors such as wages, energy, local regulations, etc. and transportation. If lower wages could balance the higher transportation costs of building the plant in a low-wage area, the low-wage location may be more desirable.

Although location is a long-term investment, a firm does not decide on a location with the sole objective of maximizing its profits or minimizes its costs. Managers may choose a “safer” location that is more likely to produce higher profits rather than a riskier location that yields the maximum investment return. Personal factors of the people involved are also influences. For example, new businesses tend to locate where the founder lives. 2

In the current age, a plant's site is chosen based on several factors. These include:1,2,3,8

'''1. Raw material supply:''' The source and price of raw materials is one of the most important factors that determine the location of a plant. Facilities that produce chemicals in bulk are usually located close to the source of raw material if the costs of shipping the product is less than the costs of shipping the feed. For example, ethylene production is growing in the Middle East since cheap ethane from natural gas is readily available. Oil refineries tend to be located near areas with high population and crude oil supplies since it is expensive to transport the oil.

'''2. Location with respect to market:''' If the plant produces high-volume and low-cost products, such as cement and fertilizer, it may be better to situate the plant closer to the primary market since transportation cost is a large fraction of the sales price. If the product is low-volume and high-cost, like pharmaceuticals, then the benefits of being closer to the primary market may not be there.

'''3. Transport facilities:''' Facilities should be close to at least two major forms of transportation, whether that be road, rail, waterway, and/or seaport. For example, paper manufacturing plants in the US use various types of pulp that are delivered by truck or by train from various places in North and South America, so paper facilities need to be close to rail and major roads. Transportation by road is common for local distribution from central warehouses, while transportation by rail is more widespread for long distance transport of bulk chemicals. Pipeline is used to ship industrial gases and bulk fuels. Air freight can be efficient for shipment of personnel and essential units and supplies and for small volume products that have high value, such as pharmaceuticals. Of course, products that are delivered by air must meet aviation regulations.

'''4. Availability of labor:''' Skilled workers are usually brought to the plant from outside the area. There should be a local pool of unskilled labor that can be trained to operate the plant, and of skilled craft workers to maintain the process units. Local labor laws, trade union customs, restrictive practices for recruitment and training should also be taken into consideration. A 10% increase in unionization of a state's labor force is projected to reduce the number of expanding facilities by 30 to 45%.

'''5. Availability of utilities:''' Processes that require a substantial amount of cooling water is usually located near water sources, such as rivers or wells. Cooling water may be directly taken from the water source, or may be stored in cooling towers. Those that need large quantities of power, such as electrochemical ones, are typically close to cheap power sources.

'''6. Availability of suitable land:''' The ideal land is flat, well-drained, with suitable load-bearing characteristics. Further considerations have to be made if the land is reclaimed land near the ocean in earthquake zones. Property tax is also a factor when choosing a site since property taxes vary area to area. Under a third of plants that relocate move to regions with lower property taxes, which is the proportion that would be expected if companies move to a new location regardless of property tax. High property taxes is not as significant as other factors such as labor supply and land costs.

'''7. Environmental impact:''' Depending on the location, it may be more difficult and costly to dispose of wastes. During the project design phase, experts are typically consulted to learn more about an area's local regulations. More details about environmental regulations are found below.

'''8. Local community considerations:''' State and local planners are typically motivated by the desire to create jobs and improve the tax base. Introduction of facilities to an area is usually viewed as the most direct way to stimulate the area's economy. However, recent studies have found that communities with high-growth are already characterized by the fast growth of businesses that are already there. It is rare for a plant to completely close in one area and relocate to another, and if plants do relocate, the majority is over short distances and often within the same community. Therefore, local policymakers favor the expansion of existing plants. The opening of a new plant at a location should impost no additional risk to the local residents. For example, they should be downwind of the residential areas. Local communities also need to be able to accommodate the plant personnelles. For example, traffic, housing, and facilities must be able to accommodate the influx of workers. Additional factors are property taxes and water consumption.

'''9. Climate:''' The climate of the area may affect processes and costs. For example, plants in cold areas need more insulation and special heating. Facilities in earthquake areas need to be seismically sound. Plants in areas with high ambient humidity will usually use air cooling instead of water cooling.

'''10. Political and strategic considerations:''' Government sometimes gives capital grants, tax concessions, and other incentives to encourage plants to be built in specific areas. Physical assistance such as roads, water, and other public infrastructure are more popular than financial assistance. Companies can also globalize and take advantage of areas with preferential tariff agreements. The tax policy of an area is inversely related to growth. High personal income sometimes hinder employment growth. Personal income does not affect the cash flow of the company, but it reduces the after-tax income of its managers, and thus high personal income tax can be classified as a personal region. High state corporate taxes has also and detrimental effects on growth, but that is not always the case. Corporate tax is more important to firms with high capital expenditures.

===Analysis of the Importance of Factors===
In 1963, Morgan surveyed 17 companies and found that on average the most important factors on industrial location decisions are market, labor, and raw material. Taxes and financial incentives were of little significance. More recently, Fortune magazine surveyed among the 1000 largest US companies and found that the most important factors were market and labor. The survey also found that personal preferences of a firm's executive, tax, and central cities have great influence over the site of corporate headquarters. Surveys of interstate locational decisions found that the factors that played key roles for new firms were access to customers and the growing market, labor force, transportation, personal reasons of management, and availability of capital. The cost of land had the least important influence. For firms that expanded across states, the influential factors were labor costs and labor supply. Companies seeking expansion usually have made their production more routine, so quick access to suppliers is less of a concern. Cost minimization becomes more relevant to existing corporations.2

===Site Selection Process===
The decision for the location of a facility is part of a larger corporate planning process. Usually, a corporate planning office or a division of the company initiates the site selection process by forecasting future capacity requirements. If capacity shortages are in the forecasts, the managers may choose to outsource, increase price to reduce the demand, expand existing sites, or open a new facility. If the managers decide on a new facility, the site selection team enters the project.

The structure of the site selection team depends on the firm's organization. In companies with centralized staff, the site selection team generally consists of representatives from relevant areas, such as engineering, real estate, and transportation. In companies with strong divisions, the locational decisions may be done at the divisional level, with the corporate office supervising the process.

The site selection team determines what characteristics are important for the new location by considering how the new facility will fit in the company's overall strategy, if the company wants to target new markets, if the corporation wants to divide or integrate its functions, or how the company wants to be seen by the public. Next, potential locations are listed and studied against the desired characteristics.

Locational decisions are typically made sequentially. The first step is at the state or regional level. Then, the team studies specific communities and sites. Different locational factors are important to different stages. When selecting a general region, the site selection team focuses on factors with interregional variations such as labor, tax policies, climate, and market locations. At the more focused stage, details like inexpensive land, access to to major roads, and good schools are important. Consultants are often hired to do site analyses.

Once site options are narrowed down, the company discusses potential problems and incentives with local public officials. Construction costs are estimated, and a feasibility analysis is done to show that the project has a high rate of return.2

===Biofuel Supply Chain Example===
Biofuels is a popular potential alternative to fossil fuels. Various biomass resources, including food crops, non-food crops, and agricultural residues, are converted into biofuels. The two most common biofuels currently are ethanol and biodiesel. The supply chain for biofuel is a network consisting of several nodes: biomass cultivation sites, biofuel production plants, and demand centers. The locations of these facilities, and the location of the demand centers with respect to the biofuel demand impact transportation costs, production resource, demand, etc., and thus affects and sets constraints to the cost minimization of the supply chain. The optimal supply chain design minimize the total cost, which is a sum of the total investment cost, production cost, transportation cost, and outsourcing and import cost. The total transportation cost captures the transportation of biomass and biofuel between areas and the local bio mass transfer.4

[[File:biofuelsupplychain.png|400px|thumb|center|'''Figure 1.''' The biofuels supply chain consists of three notes: biomass cultivation sites, biofuel production plants, and biofuel demand centers.1]]

[[File:Biofueloptimal.png|300px|thumb|center|'''Figure 2.''' A Great Britain biofuel supply chain design that minimizes cost. The plants are near areas with high wheat cultivation and wheat straw collection rates. The location of the plants with respect to the demand centers is also optimized. The main modes of transportation are road, rail, and ship.4]]

=Site Layout=
The process units and buildings need to be arranged in such a way that allows for the most economical flow of materials and people. Furthermore, dangerous processes need to be a safe distance from other buildings, and the layout should be planned to allow for future expansion.

Process units are usually laid out first in an arrangement that allows for smooth flow of materials between the process steps. The distance between equipment is usually at least 30 m. Next, the location of the principal ancillary buildings are sited as to minimize the time that it takes the workers to travel between buildings. Administrative offices and laboratories are located away from hazardous processes. Control rooms are next to the processing equipment. Utility buildings are located as to minimize piping between the process units. Storage is placed between the loading and unloading facilities and next to the process units that they serve. Tanks containing hazardous material are placed at least 70 m from the plant. An example of a typical site plan is shown below.1

[[File:Site Location.png|400px|thumb|center|'''Figure 3.''' A typical site layout.1]]

===Site Layout Factors===
The main factors that are considered when planning the layout of the plant are listed below.1

'''1. Economic considerations (construction and operating costs):'''
Construction costs can be minimized by arranging process units and buildings that minimize pipes between equipment, the amount of structural steel work, etc. However, this layout may conflict with the layout that gives the optimal operation and maintenance.

'''2. Process requirements:''' Examples of process considerations that must be taken into account is the elevation of the base of columns to give enough net positive suction head to a pump.

'''3. Operation convenience:''' Process units that are attended to frequently should be placed with convenient access. Valves, heads, and sample heads should be placed where operators can easily access. If the plant anticipates replacement of equipment, space must be allowed for removal and installation.

'''4. Maintenance convenience:''' Equipment that requires maintenance should be in a location with easy access, and should have sufficient space for the maintenance tasks. For example, shell-and-tube exchangers need space so that tube bundles can be removed for cleaning and repair.

'''5. Future expansion:''' The layout should be planned to conveniently allow for future expansion of processes. Pipe racks should have space for future piping, and pipes should be oversized to allow for more flow in the future.

'''6. Modular Construction:''' Modular construction is where sections of the plant is constructed outside of the plant, and then transported to the site by road or sea. Advantages include improved quality control, reduced construction costs, less requirements for skilled labor on site. Tradeoffs are more flanged connections and possible problems with onsite assembly.

'''7. Safety:''' Escape routes for workers need to be in place at each level in process buildings. Blast walls must isolate equipment that pose hazards to confine potential explosions.

First, a conceptual flowsheet for the process is developed. The types of equipment and their connections with each other is described in a process flow diagram (PFD). Before the PFD is translated into detailed piping and instrumentation diagrams (P&ID) the layout of the process units must be planned.2,3 Scale drawings are made to show the relationships between storage space and process equipment based on the flow of materials and people, and on future expansion. Three-dimensional visualization are the layouts are then carried out with cardboard cutouts of the equipment outlines or rectangular and cylindrical blocks. When a layout of the major process units has been decided, drawings of the plan and elevation are made, and design of the structural steelwork and foundations are done. Computer-aided design has also become increasingly popular.1,3,7

=Environmental Considerations=
Laws that protect the environment restrict the waste that plants can emit in order to preserve air, land, and water quality. States, provinces, and municipals usually have additional laws to national ones. In the US, environmental policies are moving towards more uniform and strict regulations. Businesses that produce more pollution are more likely to move to regions where local regulations are more lax, and industries in areas with more stringent policies will decline. The tighter national regulations are implemented to prevent this competition between regions. For example, amendments to the Clean Air Act and the Environmental Protection Agencies in the 1970s and 1980s reduced differences in regions' air pollution controls. Congress voted on the PSD (prevention of significant deterioration) to "level the playing field" across regions.1

Some of the main environmental legislations in North America are:

*'''The National Environmental Policy Act of 1969 (NEPA)''' - Signed by President Nixon, NEPA was one of the first laws that provide a national framework for the protection of the environment. It requires that all executive federal agencies consider the environmental impact of proposed actions. Government agencies must prepare environmental assessments and environmental impact statements that describe potential environmental effects. NEPA is implemented when buildings, airports, military complexes, parkland purchases, and highways are proposed.6

*'''The Clean Air Act (CAA, 1970)''' - The CAA was implemented to control air quality on a national level. The law was amended in 1990 to address issues such as acid rain, ozone depletion, and toxic air pollution, and to increase enforcement authority. Under the National Ambient Air Quality Standards (NAAQS), the law sets limits to allowable ambient levels of seven contaminants: ozone, carbon monoxide, lead, nitrogen dioxide, sulfur dioxide, PM10, and PM2.5. The CAA also requires that the EPA set US National Emissions Standards for Hazardous Air Pollutants (NESHAP), which regulates the emission of 189 hazardous air pollutants. The Maximum Available Control Technology (MACT) was implemented to limit the emission of volatile organics, sulfur, and other inorganic compounds from oil refineries. The CAA has had significant impact on the level of air pollutants that it regulates. Particularly in the Midwest and the Northwest, water acidity has been reduced. On the other hand, reduction of ozone damage has not been as successful since nitrogen oxide, which is linked to ozone formation, is not as tightly regulated.

*'''The Federal Water Pollution Control Act (The Clean Water Act, 1972)''' - The Clean Water Act was initially passed to clean water used for swimming and boating, and to protect wildlife. Amendments were later added to address water quality and toxic compound emissions. Under the CWA, the EPA can set water quality standards and monitor the industries' discharge into waters.

*'''The Safe Drinking Water Act (SDWA, 1974)''' - This act allows the EPA to set standards to potential drinking water. Public water systems must operate according to these standards.

*'''The Resource Conservation and Recovery Act (RCRA, 1976)''' - The RCRA protects current and future groundwater facilities from contamination. Firms that produce waste must have a "cradle to grave" approach to managing waste, meaning that the waste producer is legally responsible for the waste from the moment it is produced until it is finally disposed.

*'''The Comprehensive Environmental Response, Compensation and Liability Act (CERCLA or Superfund, 1980)''' - This legislation creates a tax on chemical and petroleum industries in order to cover the remediation of hazardous waste sites that are uncontrolled or abandoned. The CERCLA empowers EPA to remediate waste sites by short-term removal of material or long-term remediation actions, depending on the risk that the waste imposes on life.

*The Superfund Amendments and Reauthorization Act (SARA, 1986)
*'''The Pollution Prevention Act (PPA, 1990)''' - This act promotes minimization of waste and pollution at their source. It encourages more efficient processing and raw materials and recycling through grants, technical assistant, and information.

*'''The Oil Pollution Act of 1990 (OPA, 1990)''' - This act was passed after the Exxon Valdez oil spill in the Prince William Sound in Alaska, creating a tax on oil to cover the cost of responding to spills when the corporation that creates the spill does not respond.

*'''The Department of the Environment Act (E-10)''' - The Department of Environment Act created the Department of Environment and charges the Minister of the Environment with creating programs that protect the environment, assessing environmental impacts of federal program, and reporting information to the public in Canada.

*'''The Canadian Environmental Protection Act (CEPA, C-15.31, 1999)''' - CEPA empowers Environment Canada to control toxic substances, reduce pollution, and reduce bioaccumulating toxic substances. Canada will use CEPA to implement regulations that helps it meet its commitments to the Kyoto Protocol.

*'''The Canada Water Act (C-11)''' - This law allows the Minister of the Environment to manage water resources in Canada. This includes research, implementing conservation programs, utilization of water resources, and setting water quality standards.

Wastes comprise mainly of by-products, unused reactants, and off-specification product produced by misoperation. Leaking seals and flanges, and spills and discharges also emit waste. Material is also discharged in emergency situations. Instead of considering how to treat or manage waste, designers should start by tackling the source and find ways to minimize the production of waste.

The process of designing waste management systems are: (1) source reduction, (2) waste stream recycle, (3) waste treatment to reduce environmental impact, and (4) disposal that is legally sound. Source reduction can be achieved by reducing the concentration of impurities in the feed, protecting catalysts and adsorbents from contaminants, eliminating the use of extraneous materials, increasing recovery from separation, and improving the quality of the fuel by switching to cleaner-burning fuel. Unused feed can be recycled, and off-specification products can be reprocessed. In integral processes, the waste of one process is used as the feed for another. By-products can also be sold to another company for use as raw material. Tighter control systems, alarm, and interlocks can reduce misoperation of process unit.1

Further information about environmental considerations can be found in the [[Environmental concerns]] page.

=Conclusion=
The location and layout of a plant can greatly impact its economic and operational success. Objectives such as cost minimization, distribution, room for expansion, and safety of the plant operators and local community play important roles in the site decision. The major factors affecting this decision are market and labor. National and regional environmental legislations impact the processes, and thus make some areas more desirable than others. Least important is land cost. When planning to build a new facility or expanding an existing site, companies utilize their site selection team to assemble a list of desirable locational characteristics and determine how the new facility will fit in with the overall company. Then with the help of consultants, a location is narrowed down and further analyses are done to justify the continuation of the project. The location should be obtained before design of the details of the process. Within the site itself, process units are arranged to allow for the most economical flow of people and material, optimal safety, and room for future expansion.

=References=
[1] G.P. Towler, R. Sinnott, ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design'', Elsevier, 2012.

[2] J.P. Blair, R. Premus. Major Factors in Industrial Location: A Review. Economic Development Quarterly. 1987;1(1):72-85.

[3] M.S. Peters, K.D. Timmerhaus, ''Plant Design and Economics for Chemical Engineers'', 5th Ed., McGraw-Hill: New York, 2003.

[4] O. Akgul, N. Shah, L.G. Papageorgiou. Economic optimisation of a UK advanced biofuel supply chain. Biomass and Bioenergy. 2012;41:57-72.

[5] L.T. Biegler, I.E. Grossmann, A.W. Westerberg, ''Systematic Methods of Chemical Process Design'', Prentice-Hall: Upper Saddle River, 1997.

[6] Summary of the National Environmental Policy Act website. http://www.epa.gov/laws-regulations/summary-national-environmental-policy-act. August 2015. Accessed February 2016.

[7] R.T. Turton, R.C. Bailie, W.B. Whiting, J.A. Shaeiwitz, ''Analysis, Synthesis, and Design of Chemical Processes'', Prentice Hall: Upper Saddle River, 2003.

[8] R.W. Schmenner, Look Beyond the Obvious in Plant Location website. https://hbr.org/1979/01/look-beyond-the-obvious-in-plant-location. January 1979. Accessed February 2016.

Process location and layout decisions

2016-02-22T01:53:58Z

Ezhuang: /* Introduction */

Author: Ellen Zhuang [2016]

Stewards: Daniel Garcia, and Fengqi You

=Introduction=
Location is one of the first decisions in the design of a new chemical plant. It impacts profitability and scope for future expansion. If the project is a new facility, a suitable site must be found and an optimal layout of the site and process units must be planned. If the project adds to an existing site, the impact of the new addition on the existing plant must be considered. The plant also needs to accommodate for the nearby infrastructure, the services that it requires, and its environmental impacts.

=Location Selection=
===Location Decision Factors===
Initially, economists viewed the plant location decision as a cost-minimization problem. The optimal location was one where the transportation costs of raw material to the plant and product to the market was minimized. Between the early 1900s and post World War II period, most industries were sensitive to the cost of transportation. As transportation costs became less of an obstacle, the approach to the problem developed with considerations of trade-offs between factors such as wages, energy, local regulations, etc. and transportation. If lower wages could balance the higher transportation costs of building the plant in a low-wage area, the low-wage location may be more desirable.

Although location is a long-term investment, a firm does not decide on a location with the sole objective of maximizing its profits or minimizes its costs. Managers may choose a “safer” location that is more likely to produce higher profits rather than a riskier location that yields the maximum investment return. Personal factors of the people involved are also influences. For example, new businesses tend to locate where the founder lives. 2

In the current age, a plant's site is chosen based on several factors. These include:1,2,3,8

'''1. Raw material supply:''' The source and price of raw materials is one of the most important factors that determine the location of a plant. Facilities that produce chemicals in bulk are usually located close to the source of raw material if the costs of shipping the product is less than the costs of shipping the feed. For example, ethylene production is growing in the Middle East since cheap ethane from natural gas is readily available. Oil refineries tend to be located near areas with high population and crude oil supplies since it is expensive to transport the oil.

'''2. Location with respect to market:''' If the plant produces high-volume and low-cost products, such as cement and fertilizer, it may be better to situate the plant closer to the primary market since transportation cost is a large fraction of the sales price. If the product is low-volume and high-cost, like pharmaceuticals, then the benefits of being closer to the primary market may not be there.

'''3. Transport facilities:''' Facilities should be close to at least two major forms of transportation, whether that be road, rail, waterway, and/or seaport. For example, paper manufacturing plants in the US use various types of pulp that are delivered by truck or by train from various places in North and South America, so paper facilities need to be close to rail and major roads. Transportation by road is common for local distribution from central warehouses, while transportation by rail is more widespread for long distance transport of bulk chemicals. Pipeline is used to ship industrial gases and bulk fuels. Air freight can be efficient for shipment of personnel and essential units and supplies and for small volume products that have high value, such as pharmaceuticals. Of course, products that are delivered by air must meet aviation regulations.

'''4. Availability of labor:''' Skilled workers are usually brought to the plant from outside the area. There should be a local pool of unskilled labor that can be trained to operate the plant, and of skilled craft workers to maintain the process units. Local labor laws, trade union customs, restrictive practices for recruitment and training should also be taken into consideration. A 10% increase in unionization of a state's labor force is projected to reduce the number of expanding facilities by 30 to 45%.

'''5. Availability of utilities:''' Processes that require a substantial amount of cooling water is usually located near water sources, such as rivers or wells. Cooling water may be directly taken from the water source, or may be stored in cooling towers. Those that need large quantities of power, such as electrochemical ones, are typically close to cheap power sources.

'''6. Availability of suitable land:''' The ideal land is flat, well-drained, with suitable load-bearing characteristics. Further considerations have to be made if the land is reclaimed land near the ocean in earthquake zones. Property tax is also a factor when choosing a site since property taxes vary area to area. Under a third of plants that relocate move to regions with lower property taxes, which is the proportion that would be expected if companies move to a new location regardless of property tax. High property taxes is not as significant as other factors such as labor supply and land costs.

'''7. Environmental impact:''' Depending on the location, it may be more difficult and costly to dispose of wastes. During the project design phase, experts are typically consulted to learn more about an area's local regulations. More details about environmental regulations are found below.

'''8. Local community considerations:''' State and local planners are typically motivated by the desire to create jobs and improve the tax base. Introduction of facilities to an area is usually viewed as the most direct way to stimulate the area's economy. However, recent studies have found that communities with high-growth are already characterized by the fast growth of businesses that are already there. It is rare for a plant to completely close in one area and relocate to another, and if plants do relocate, the majority is over short distances and often within the same community. Therefore, local policymakers favor the expansion of existing plants. The opening of a new plant at a location should impost no additional risk to the local residents. For example, they should be downwind of the residential areas. Local communities also need to be able to accommodate the plant personnelles. For example, traffic, housing, and facilities must be able to accommodate the influx of workers. Additional factors are property taxes and water consumption.

'''9. Climate:''' The climate of the area may affect processes and costs. For example, plants in cold areas need more insulation and special heating. Facilities in earthquake areas need to be seismically sound. Plants in areas with high ambient humidity will usually use air cooling instead of water cooling.

'''10. Political and strategic considerations:''' Government sometimes gives capital grants, tax concessions, and other incentives to encourage plants to be built in specific areas. Physical assistance such as roads, water, and other public infrastructure are more popular than financial assistance. Companies can also globalize and take advantage of areas with preferential tariff agreements. The tax policy of an area is inversely related to growth. High personal income sometimes hinder employment growth. Personal income does not affect the cash flow of the company, but it reduces the after-tax income of its managers, and thus high personal income tax can be classified as a personal region. High state corporate taxes has also and detrimental effects on growth, but that is not always the case. Corporate tax is more important to firms with high capital expenditures.

===Analysis of the Importance of Factors===
In 1963, Morgan surveyed 17 companies and found that on average the most important factors on industrial location decisions are market, labor, and raw material. Taxes and financial incentives were of little significance. More recently, Fortune magazine surveyed among the 1000 largest US companies and found that the most important factors were market and labor. The survey also found that personal preferences of a firm's executive, tax, and central cities have great influence over the site of corporate headquarters. Surveys of interstate locational decisions found that the factors that played key roles for new firms were access to customers and the growing market, labor force, transportation, personal reasons of management, and availability of capital. The cost of land had the least important influence. For firms that expanded across states, the influential factors were labor costs and labor supply. Companies seeking expansion usually have made their production more routine, so quick access to suppliers is less of a concern. Cost minimization becomes more relevant to existing corporations.2

===Site Selection Process===
The decision for the location of a facility is part of a larger corporate planning process. Usually, a corporate planning office or a division of the company initiates the site selection process by forecasting future capacity requirements. If capacity shortages are in the forecasts, the managers may choose to outsource, increase price to reduce the demand, expand existing sites, or open a new facility. If the managers decide on a new facility, the site selection team enters the project.

The structure of the site selection team depends on the firm's organization. In companies with centralized staff, the site selection team generally consists of representatives from relevant areas, such as engineering, real estate, and transportation. In companies with strong divisions, the locational decisions may be done at the divisional level, with the corporate office supervising the process.

The site selection team determines what characteristics are important for the new location by considering how the new facility will fit in the company's overall strategy, if the company wants to target new markets, if the corporation wants to divide or integrate its functions, or how the company wants to be seen by the public. Next, potential locations are listed and studied against the desired characteristics.

Locational decisions are typically made sequentially. The first step is at the state or regional level. Then, the team studies specific communities and sites. Different locational factors are important to different stages. When selecting a general region, the site selection team focuses on factors with interregional variations such as labor, tax policies, climate, and market locations. At the more focused stage, details like inexpensive land, access to to major roads, and good schools are important. Consultants are often hired to do site analyses.

Once site options are narrowed down, the company discusses potential problems and incentives with local public officials. Construction costs are estimated, and a feasibility analysis is done to show that the project has a high rate of return.2

===Biofuel Supply Chain Example===
Biofuels is a popular potential alternative to fossil fuels. Various biomass resources, including food crops, non-food crops, and agricultural residues, are converted into biofuels. The two most common biofuels currently are ethanol and biodiesel. The supply chain for biofuel is a network consisting of several nodes: biomass cultivation sites, biofuel production plants, and demand centers. The locations of these facilities, and the location of the demand centers with respect to the biofuel demand impact transportation costs, production resource, demand, etc., and thus affects and sets constraints to the cost minimization of the supply chain. The optimal supply chain design minimize the total cost, which is a sum of the total investment cost, production cost, transportation cost, and outsourcing and import cost. The total transportation cost captures the transportation of biomass and biofuel between areas and the local bio mass transfer.4

[[File:biofuelsupplychain.png|400px|thumb|center|'''Figure 1.''' The biofuels supply chain consists of three notes: biomass cultivation sites, biofuel production plants, and biofuel demand centers.1]]

[[File:Biofueloptimal.png|300px|thumb|center|'''Figure 2.''' A Great Britain biofuel supply chain design that minimizes cost. The plants are near areas with high wheat cultivation and wheat straw collection rates. The location of the plants with respect to the demand centers is also optimized. The main modes of transportation are road, rail, and ship.4]]

=Site Layout=
The process units and buildings need to be arranged in such a way that allows for the most economical flow of materials and people. Furthermore, dangerous processes need to be a safe distance from other buildings, and the layout should be planned to allow for future expansion.

Process units are usually laid out first in an arrangement that allows for smooth flow of materials between the process steps. The distance between equipment is usually at least 30 m. Next, the location of the principal ancillary buildings are sited as to minimize the time that it takes the workers to travel between buildings. Administrative offices and laboratories are located away from hazardous processes. Control rooms are next to the processing equipment. Utility buildings are located as to minimize piping between the process units. Storage is placed between the loading and unloading facilities and next to the process units that they serve. Tanks containing hazardous material are placed at least 70 m from the plant. An example of a typical site plan is shown below.1

[[File:Site Location.png|400px|thumb|center|'''Figure 3.''' A typical site layout.1]]

=Plant Layout=
The main factors that are considered when planning the layout of the plant are listed below.1

'''1. Economic considerations (construction and operating costs):'''
Construction costs can be minimized by arranging process units and buildings that minimize pipes between equipment, the amount of structural steel work, etc. However, this layout may conflict with the layout that gives the optimal operation and maintenance.

'''2. Process requirements:''' Examples of process considerations that must be taken into account is the elevation of the base of columns to give enough net positive suction head to a pump.

'''3. Operation convenience:''' Process units that are attended to frequently should be placed with convenient access. Valves, heads, and sample heads should be placed where operators can easily access. If the plant anticipates replacement of equipment, space must be allowed for removal and installation.

'''4. Maintenance convenience:''' Equipment that requires maintenance should be in a location with easy access, and should have sufficient space for the maintenance tasks. For example, shell-and-tube exchangers need space so that tube bundles can be removed for cleaning and repair.

'''5. Future expansion:''' The layout should be planned to conveniently allow for future expansion of processes. Pipe racks should have space for future piping, and pipes should be oversized to allow for more flow in the future.

'''6. Modular Construction:''' Modular construction is where sections of the plant is constructed outside of the plant, and then transported to the site by road or sea. Advantages include improved quality control, reduced construction costs, less requirements for skilled labor on site. Tradeoffs are more flanged connections and possible problems with onsite assembly.

'''7. Safety:''' Escape routes for workers need to be in place at each level in process buildings. Blast walls must isolate equipment that pose hazards to confine potential explosions.

First, a conceptual flowsheet for the process is developed. The types of equipment and their connections with each other is described in a process flow diagram (PFD). Before the PFD is translated into detailed piping and instrumentation diagrams (P&ID) the layout of the process units must be planned.2,3 Scale drawings are made to show the relationships between storage space and process equipment based on the flow of materials and people, and on future expansion. Three-dimensional visualization are the layouts are then carried out with cardboard cutouts of the equipment outlines or rectangular and cylindrical blocks. When a layout of the major process units has been decided, drawings of the plan and elevation are made, and design of the structural steelwork and foundations are done. Computer-aided design has also become increasingly popular.1,3,7

=Environmental Considerations=
Laws that protect the environment restrict the waste that plants can emit in order to preserve air, land, and water quality. States, provinces, and municipals usually have additional laws to national ones. In the US, environmental policies are moving towards more uniform and strict regulations. Businesses that produce more pollution are more likely to move to regions where local regulations are more lax, and industries in areas with more stringent policies will decline. The tighter national regulations are implemented to prevent this competition between regions. For example, amendments to the Clean Air Act and the Environmental Protection Agencies in the 1970s and 1980s reduced differences in regions' air pollution controls. Congress voted on the PSD (prevention of significant deterioration) to "level the playing field" across regions.1

Some of the main environmental legislations in North America are:

*'''The National Environmental Policy Act of 1969 (NEPA)''' - Signed by President Nixon, NEPA was one of the first laws that provide a national framework for the protection of the environment. It requires that all executive federal agencies consider the environmental impact of proposed actions. Government agencies must prepare environmental assessments and environmental impact statements that describe potential environmental effects. NEPA is implemented when buildings, airports, military complexes, parkland purchases, and highways are proposed.6

*'''The Clean Air Act (CAA, 1970)''' - The CAA was implemented to control air quality on a national level. The law was amended in 1990 to address issues such as acid rain, ozone depletion, and toxic air pollution, and to increase enforcement authority. Under the National Ambient Air Quality Standards (NAAQS), the law sets limits to allowable ambient levels of seven contaminants: ozone, carbon monoxide, lead, nitrogen dioxide, sulfur dioxide, PM10, and PM2.5. The CAA also requires that the EPA set US National Emissions Standards for Hazardous Air Pollutants (NESHAP), which regulates the emission of 189 hazardous air pollutants. The Maximum Available Control Technology (MACT) was implemented to limit the emission of volatile organics, sulfur, and other inorganic compounds from oil refineries. The CAA has had significant impact on the level of air pollutants that it regulates. Particularly in the Midwest and the Northwest, water acidity has been reduced. On the other hand, reduction of ozone damage has not been as successful since nitrogen oxide, which is linked to ozone formation, is not as tightly regulated.

*'''The Federal Water Pollution Control Act (The Clean Water Act, 1972)''' - The Clean Water Act was initially passed to clean water used for swimming and boating, and to protect wildlife. Amendments were later added to address water quality and toxic compound emissions. Under the CWA, the EPA can set water quality standards and monitor the industries' discharge into waters.

*'''The Safe Drinking Water Act (SDWA, 1974)''' - This act allows the EPA to set standards to potential drinking water. Public water systems must operate according to these standards.

*'''The Resource Conservation and Recovery Act (RCRA, 1976)''' - The RCRA protects current and future groundwater facilities from contamination. Firms that produce waste must have a "cradle to grave" approach to managing waste, meaning that the waste producer is legally responsible for the waste from the moment it is produced until it is finally disposed.

*'''The Comprehensive Environmental Response, Compensation and Liability Act (CERCLA or Superfund, 1980)''' - This legislation creates a tax on chemical and petroleum industries in order to cover the remediation of hazardous waste sites that are uncontrolled or abandoned. The CERCLA empowers EPA to remediate waste sites by short-term removal of material or long-term remediation actions, depending on the risk that the waste imposes on life.

*The Superfund Amendments and Reauthorization Act (SARA, 1986)
*'''The Pollution Prevention Act (PPA, 1990)''' - This act promotes minimization of waste and pollution at their source. It encourages more efficient processing and raw materials and recycling through grants, technical assistant, and information.

*'''The Oil Pollution Act of 1990 (OPA, 1990)''' - This act was passed after the Exxon Valdez oil spill in the Prince William Sound in Alaska, creating a tax on oil to cover the cost of responding to spills when the corporation that creates the spill does not respond.

*'''The Department of the Environment Act (E-10)''' - The Department of Environment Act created the Department of Environment and charges the Minister of the Environment with creating programs that protect the environment, assessing environmental impacts of federal program, and reporting information to the public in Canada.

*'''The Canadian Environmental Protection Act (CEPA, C-15.31, 1999)''' - CEPA empowers Environment Canada to control toxic substances, reduce pollution, and reduce bioaccumulating toxic substances. Canada will use CEPA to implement regulations that helps it meet its commitments to the Kyoto Protocol.

*'''The Canada Water Act (C-11)''' - This law allows the Minister of the Environment to manage water resources in Canada. This includes research, implementing conservation programs, utilization of water resources, and setting water quality standards.

Wastes comprise mainly of by-products, unused reactants, and off-specification product produced by misoperation. Leaking seals and flanges, and spills and discharges also emit waste. Material is also discharged in emergency situations. Instead of considering how to treat or manage waste, designers should start by tackling the source and find ways to minimize the production of waste.

The process of designing waste management systems are: (1) source reduction, (2) waste stream recycle, (3) waste treatment to reduce environmental impact, and (4) disposal that is legally sound. Source reduction can be achieved by reducing the concentration of impurities in the feed, protecting catalysts and adsorbents from contaminants, eliminating the use of extraneous materials, increasing recovery from separation, and improving the quality of the fuel by switching to cleaner-burning fuel. Unused feed can be recycled, and off-specification products can be reprocessed. In integral processes, the waste of one process is used as the feed for another. By-products can also be sold to another company for use as raw material. Tighter control systems, alarm, and interlocks can reduce misoperation of process unit.1

Further information about environmental considerations can be found in the [[Environmental concerns]] page.

=Conclusion=
The location and layout of a plant can greatly impact its economic and operational success. Objectives such as cost minimization, distribution, room for expansion, and safety of the plant operators and local community play important roles in the site decision. The major factors affecting this decision are market and labor. National and regional environmental legislations impact the processes, and thus make some areas more desirable than others. Least important is land cost. When planning to build a new facility or expanding an existing site, companies utilize their site selection team to assemble a list of desirable locational characteristics and determine how the new facility will fit in with the overall company. Then with the help of consultants, a location is narrowed down and further analyses are done to justify the continuation of the project. The location should be obtained before design of the details of the process. Within the site itself, process units are arranged to allow for the most economical flow of people and material, optimal safety, and room for future expansion.

=References=
[1] G.P. Towler, R. Sinnott, ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design'', Elsevier, 2012.

[2] J.P. Blair, R. Premus. Major Factors in Industrial Location: A Review. Economic Development Quarterly. 1987;1(1):72-85.

[3] M.S. Peters, K.D. Timmerhaus, ''Plant Design and Economics for Chemical Engineers'', 5th Ed., McGraw-Hill: New York, 2003.

[4] O. Akgul, N. Shah, L.G. Papageorgiou. Economic optimisation of a UK advanced biofuel supply chain. Biomass and Bioenergy. 2012;41:57-72.

[5] L.T. Biegler, I.E. Grossmann, A.W. Westerberg, ''Systematic Methods of Chemical Process Design'', Prentice-Hall: Upper Saddle River, 1997.

[6] Summary of the National Environmental Policy Act website. http://www.epa.gov/laws-regulations/summary-national-environmental-policy-act. August 2015. Accessed February 2016.

[7] R.T. Turton, R.C. Bailie, W.B. Whiting, J.A. Shaeiwitz, ''Analysis, Synthesis, and Design of Chemical Processes'', Prentice Hall: Upper Saddle River, 2003.

[8] R.W. Schmenner, Look Beyond the Obvious in Plant Location website. https://hbr.org/1979/01/look-beyond-the-obvious-in-plant-location. January 1979. Accessed February 2016.

Process location and layout decisions

2016-02-22T01:24:07Z

Ezhuang: /* Site Layout */

Author: Ellen Zhuang [2016]

Stewards: Daniel Garcia, and Fengqi You

=Introduction=
Location is one of the first decisions in the design of a new chemical plant. It impacts the profitability of the project and the scope for future expansion. If the project is a new facility, a suitable site must be found and an optimal layout of the site and process units must be planned. If the project adds to an existing site, the impact of the new addition on the existing plant must be considered. The plant also needs to accommodate for the nearby infrastructure, the services that it requires, and its environmental impacts.

=Location Selection=
===Location Decision Factors===
Initially, economists viewed the plant location decision as a cost-minimization problem. The optimal location was one where the transportation costs of raw material to the plant and product to the market was minimized. Between the early 1900s and post World War II period, most industries were sensitive to the cost of transportation. As transportation costs became less of an obstacle, the approach to the problem developed with considerations of trade-offs between factors such as wages, energy, local regulations, etc. and transportation. If lower wages could balance the higher transportation costs of building the plant in a low-wage area, the low-wage location may be more desirable.

Although location is a long-term investment, a firm does not decide on a location with the sole objective of maximizing its profits or minimizes its costs. Managers may choose a “safer” location that is more likely to produce higher profits rather than a riskier location that yields the maximum investment return. Personal factors of the people involved are also influences. For example, new businesses tend to locate where the founder lives. 2

In the current age, a plant's site is chosen based on several factors. These include:1,2,3,8

'''1. Raw material supply:''' The source and price of raw materials is one of the most important factors that determine the location of a plant. Facilities that produce chemicals in bulk are usually located close to the source of raw material if the costs of shipping the product is less than the costs of shipping the feed. For example, ethylene production is growing in the Middle East since cheap ethane from natural gas is readily available. Oil refineries tend to be located near areas with high population and crude oil supplies since it is expensive to transport the oil.

'''2. Location with respect to market:''' If the plant produces high-volume and low-cost products, such as cement and fertilizer, it may be better to situate the plant closer to the primary market since transportation cost is a large fraction of the sales price. If the product is low-volume and high-cost, like pharmaceuticals, then the benefits of being closer to the primary market may not be there.

'''3. Transport facilities:''' Facilities should be close to at least two major forms of transportation, whether that be road, rail, waterway, and/or seaport. For example, paper manufacturing plants in the US use various types of pulp that are delivered by truck or by train from various places in North and South America, so paper facilities need to be close to rail and major roads. Transportation by road is common for local distribution from central warehouses, while transportation by rail is more widespread for long distance transport of bulk chemicals. Pipeline is used to ship industrial gases and bulk fuels. Air freight can be efficient for shipment of personnel and essential units and supplies and for small volume products that have high value, such as pharmaceuticals. Of course, products that are delivered by air must meet aviation regulations.

'''4. Availability of labor:''' Skilled workers are usually brought to the plant from outside the area. There should be a local pool of unskilled labor that can be trained to operate the plant, and of skilled craft workers to maintain the process units. Local labor laws, trade union customs, restrictive practices for recruitment and training should also be taken into consideration. A 10% increase in unionization of a state's labor force is projected to reduce the number of expanding facilities by 30 to 45%.

'''5. Availability of utilities:''' Processes that require a substantial amount of cooling water is usually located near water sources, such as rivers or wells. Cooling water may be directly taken from the water source, or may be stored in cooling towers. Those that need large quantities of power, such as electrochemical ones, are typically close to cheap power sources.

'''6. Availability of suitable land:''' The ideal land is flat, well-drained, with suitable load-bearing characteristics. Further considerations have to be made if the land is reclaimed land near the ocean in earthquake zones. Property tax is also a factor when choosing a site since property taxes vary area to area. Under a third of plants that relocate move to regions with lower property taxes, which is the proportion that would be expected if companies move to a new location regardless of property tax. High property taxes is not as significant as other factors such as labor supply and land costs.

'''7. Environmental impact:''' Depending on the location, it may be more difficult and costly to dispose of wastes. During the project design phase, experts are typically consulted to learn more about an area's local regulations. More details about environmental regulations are found below.

'''8. Local community considerations:''' State and local planners are typically motivated by the desire to create jobs and improve the tax base. Introduction of facilities to an area is usually viewed as the most direct way to stimulate the area's economy. However, recent studies have found that communities with high-growth are already characterized by the fast growth of businesses that are already there. It is rare for a plant to completely close in one area and relocate to another, and if plants do relocate, the majority is over short distances and often within the same community. Therefore, local policymakers favor the expansion of existing plants. The opening of a new plant at a location should impost no additional risk to the local residents. For example, they should be downwind of the residential areas. Local communities also need to be able to accommodate the plant personnelles. For example, traffic, housing, and facilities must be able to accommodate the influx of workers. Additional factors are property taxes and water consumption.

'''9. Climate:''' The climate of the area may affect processes and costs. For example, plants in cold areas need more insulation and special heating. Facilities in earthquake areas need to be seismically sound. Plants in areas with high ambient humidity will usually use air cooling instead of water cooling.

'''10. Political and strategic considerations:''' Government sometimes gives capital grants, tax concessions, and other incentives to encourage plants to be built in specific areas. Physical assistance such as roads, water, and other public infrastructure are more popular than financial assistance. Companies can also globalize and take advantage of areas with preferential tariff agreements. The tax policy of an area is inversely related to growth. High personal income sometimes hinder employment growth. Personal income does not affect the cash flow of the company, but it reduces the after-tax income of its managers, and thus high personal income tax can be classified as a personal region. High state corporate taxes has also and detrimental effects on growth, but that is not always the case. Corporate tax is more important to firms with high capital expenditures.

===Analysis of the Importance of Factors===
In 1963, Morgan surveyed 17 companies and found that on average the most important factors on industrial location decisions are market, labor, and raw material. Taxes and financial incentives were of little significance. More recently, Fortune magazine surveyed among the 1000 largest US companies and found that the most important factors were market and labor. The survey also found that personal preferences of a firm's executive, tax, and central cities have great influence over the site of corporate headquarters. Surveys of interstate locational decisions found that the factors that played key roles for new firms were access to customers and the growing market, labor force, transportation, personal reasons of management, and availability of capital. The cost of land had the least important influence. For firms that expanded across states, the influential factors were labor costs and labor supply. Companies seeking expansion usually have made their production more routine, so quick access to suppliers is less of a concern. Cost minimization becomes more relevant to existing corporations.2

===Site Selection Process===
The decision for the location of a facility is part of a larger corporate planning process. Usually, a corporate planning office or a division of the company initiates the site selection process by forecasting future capacity requirements. If capacity shortages are in the forecasts, the managers may choose to outsource, increase price to reduce the demand, expand existing sites, or open a new facility. If the managers decide on a new facility, the site selection team enters the project.

The structure of the site selection team depends on the firm's organization. In companies with centralized staff, the site selection team generally consists of representatives from relevant areas, such as engineering, real estate, and transportation. In companies with strong divisions, the locational decisions may be done at the divisional level, with the corporate office supervising the process.

The site selection team determines what characteristics are important for the new location by considering how the new facility will fit in the company's overall strategy, if the company wants to target new markets, if the corporation wants to divide or integrate its functions, or how the company wants to be seen by the public. Next, potential locations are listed and studied against the desired characteristics.

Locational decisions are typically made sequentially. The first step is at the state or regional level. Then, the team studies specific communities and sites. Different locational factors are important to different stages. When selecting a general region, the site selection team focuses on factors with interregional variations such as labor, tax policies, climate, and market locations. At the more focused stage, details like inexpensive land, access to to major roads, and good schools are important. Consultants are often hired to do site analyses.

Once site options are narrowed down, the company discusses potential problems and incentives with local public officials. Construction costs are estimated, and a feasibility analysis is done to show that the project has a high rate of return.2

===Biofuel Supply Chain Example===
Biofuels is a popular potential alternative to fossil fuels. Various biomass resources, including food crops, non-food crops, and agricultural residues, are converted into biofuels. The two most common biofuels currently are ethanol and biodiesel. The supply chain for biofuel is a network consisting of several nodes: biomass cultivation sites, biofuel production plants, and demand centers. The locations of these facilities, and the location of the demand centers with respect to the biofuel demand impact transportation costs, production resource, demand, etc., and thus affects and sets constraints to the cost minimization of the supply chain. The optimal supply chain design minimize the total cost, which is a sum of the total investment cost, production cost, transportation cost, and outsourcing and import cost. The total transportation cost captures the transportation of biomass and biofuel between areas and the local bio mass transfer.4

[[File:biofuelsupplychain.png|400px|thumb|center|'''Figure 1.''' The biofuels supply chain consists of three notes: biomass cultivation sites, biofuel production plants, and biofuel demand centers.1]]

[[File:Biofueloptimal.png|300px|thumb|center|'''Figure 2.''' A Great Britain biofuel supply chain design that minimizes cost. The plants are near areas with high wheat cultivation and wheat straw collection rates. The location of the plants with respect to the demand centers is also optimized. The main modes of transportation are road, rail, and ship.4]]

=Site Layout=
The process units and buildings need to be arranged in such a way that allows for the most economical flow of materials and people. Furthermore, dangerous processes need to be a safe distance from other buildings, and the layout should be planned to allow for future expansion.

Process units are usually laid out first in an arrangement that allows for smooth flow of materials between the process steps. The distance between equipment is usually at least 30 m. Next, the location of the principal ancillary buildings are sited as to minimize the time that it takes the workers to travel between buildings. Administrative offices and laboratories are located away from hazardous processes. Control rooms are next to the processing equipment. Utility buildings are located as to minimize piping between the process units. Storage is placed between the loading and unloading facilities and next to the process units that they serve. Tanks containing hazardous material are placed at least 70 m from the plant. An example of a typical site plan is shown below.1

[[File:Site Location.png|400px|thumb|center|'''Figure 3.''' A typical site layout.1]]

=Plant Layout=
The main factors that are considered when planning the layout of the plant are listed below.1

'''1. Economic considerations (construction and operating costs):'''
Construction costs can be minimized by arranging process units and buildings that minimize pipes between equipment, the amount of structural steel work, etc. However, this layout may conflict with the layout that gives the optimal operation and maintenance.

'''2. Process requirements:''' Examples of process considerations that must be taken into account is the elevation of the base of columns to give enough net positive suction head to a pump.

'''3. Operation convenience:''' Process units that are attended to frequently should be placed with convenient access. Valves, heads, and sample heads should be placed where operators can easily access. If the plant anticipates replacement of equipment, space must be allowed for removal and installation.

'''4. Maintenance convenience:''' Equipment that requires maintenance should be in a location with easy access, and should have sufficient space for the maintenance tasks. For example, shell-and-tube exchangers need space so that tube bundles can be removed for cleaning and repair.

'''5. Future expansion:''' The layout should be planned to conveniently allow for future expansion of processes. Pipe racks should have space for future piping, and pipes should be oversized to allow for more flow in the future.

'''6. Modular Construction:''' Modular construction is where sections of the plant is constructed outside of the plant, and then transported to the site by road or sea. Advantages include improved quality control, reduced construction costs, less requirements for skilled labor on site. Tradeoffs are more flanged connections and possible problems with onsite assembly.

'''7. Safety:''' Escape routes for workers need to be in place at each level in process buildings. Blast walls must isolate equipment that pose hazards to confine potential explosions.

First, a conceptual flowsheet for the process is developed. The types of equipment and their connections with each other is described in a process flow diagram (PFD). Before the PFD is translated into detailed piping and instrumentation diagrams (P&ID) the layout of the process units must be planned.2,3 Scale drawings are made to show the relationships between storage space and process equipment based on the flow of materials and people, and on future expansion. Three-dimensional visualization are the layouts are then carried out with cardboard cutouts of the equipment outlines or rectangular and cylindrical blocks. When a layout of the major process units has been decided, drawings of the plan and elevation are made, and design of the structural steelwork and foundations are done. Computer-aided design has also become increasingly popular.1,3,7

=Environmental Considerations=
Laws that protect the environment restrict the waste that plants can emit in order to preserve air, land, and water quality. States, provinces, and municipals usually have additional laws to national ones. In the US, environmental policies are moving towards more uniform and strict regulations. Businesses that produce more pollution are more likely to move to regions where local regulations are more lax, and industries in areas with more stringent policies will decline. The tighter national regulations are implemented to prevent this competition between regions. For example, amendments to the Clean Air Act and the Environmental Protection Agencies in the 1970s and 1980s reduced differences in regions' air pollution controls. Congress voted on the PSD (prevention of significant deterioration) to "level the playing field" across regions.1

Some of the main environmental legislations in North America are:

*'''The National Environmental Policy Act of 1969 (NEPA)''' - Signed by President Nixon, NEPA was one of the first laws that provide a national framework for the protection of the environment. It requires that all executive federal agencies consider the environmental impact of proposed actions. Government agencies must prepare environmental assessments and environmental impact statements that describe potential environmental effects. NEPA is implemented when buildings, airports, military complexes, parkland purchases, and highways are proposed.6

*'''The Clean Air Act (CAA, 1970)''' - The CAA was implemented to control air quality on a national level. The law was amended in 1990 to address issues such as acid rain, ozone depletion, and toxic air pollution, and to increase enforcement authority. Under the National Ambient Air Quality Standards (NAAQS), the law sets limits to allowable ambient levels of seven contaminants: ozone, carbon monoxide, lead, nitrogen dioxide, sulfur dioxide, PM10, and PM2.5. The CAA also requires that the EPA set US National Emissions Standards for Hazardous Air Pollutants (NESHAP), which regulates the emission of 189 hazardous air pollutants. The Maximum Available Control Technology (MACT) was implemented to limit the emission of volatile organics, sulfur, and other inorganic compounds from oil refineries. The CAA has had significant impact on the level of air pollutants that it regulates. Particularly in the Midwest and the Northwest, water acidity has been reduced. On the other hand, reduction of ozone damage has not been as successful since nitrogen oxide, which is linked to ozone formation, is not as tightly regulated.

*'''The Federal Water Pollution Control Act (The Clean Water Act, 1972)''' - The Clean Water Act was initially passed to clean water used for swimming and boating, and to protect wildlife. Amendments were later added to address water quality and toxic compound emissions. Under the CWA, the EPA can set water quality standards and monitor the industries' discharge into waters.

*'''The Safe Drinking Water Act (SDWA, 1974)''' - This act allows the EPA to set standards to potential drinking water. Public water systems must operate according to these standards.

*'''The Resource Conservation and Recovery Act (RCRA, 1976)''' - The RCRA protects current and future groundwater facilities from contamination. Firms that produce waste must have a "cradle to grave" approach to managing waste, meaning that the waste producer is legally responsible for the waste from the moment it is produced until it is finally disposed.

*'''The Comprehensive Environmental Response, Compensation and Liability Act (CERCLA or Superfund, 1980)''' - This legislation creates a tax on chemical and petroleum industries in order to cover the remediation of hazardous waste sites that are uncontrolled or abandoned. The CERCLA empowers EPA to remediate waste sites by short-term removal of material or long-term remediation actions, depending on the risk that the waste imposes on life.

*The Superfund Amendments and Reauthorization Act (SARA, 1986)
*'''The Pollution Prevention Act (PPA, 1990)''' - This act promotes minimization of waste and pollution at their source. It encourages more efficient processing and raw materials and recycling through grants, technical assistant, and information.

*'''The Oil Pollution Act of 1990 (OPA, 1990)''' - This act was passed after the Exxon Valdez oil spill in the Prince William Sound in Alaska, creating a tax on oil to cover the cost of responding to spills when the corporation that creates the spill does not respond.

*'''The Department of the Environment Act (E-10)''' - The Department of Environment Act created the Department of Environment and charges the Minister of the Environment with creating programs that protect the environment, assessing environmental impacts of federal program, and reporting information to the public in Canada.

*'''The Canadian Environmental Protection Act (CEPA, C-15.31, 1999)''' - CEPA empowers Environment Canada to control toxic substances, reduce pollution, and reduce bioaccumulating toxic substances. Canada will use CEPA to implement regulations that helps it meet its commitments to the Kyoto Protocol.

*'''The Canada Water Act (C-11)''' - This law allows the Minister of the Environment to manage water resources in Canada. This includes research, implementing conservation programs, utilization of water resources, and setting water quality standards.

Wastes comprise mainly of by-products, unused reactants, and off-specification product produced by misoperation. Leaking seals and flanges, and spills and discharges also emit waste. Material is also discharged in emergency situations. Instead of considering how to treat or manage waste, designers should start by tackling the source and find ways to minimize the production of waste.

The process of designing waste management systems are: (1) source reduction, (2) waste stream recycle, (3) waste treatment to reduce environmental impact, and (4) disposal that is legally sound. Source reduction can be achieved by reducing the concentration of impurities in the feed, protecting catalysts and adsorbents from contaminants, eliminating the use of extraneous materials, increasing recovery from separation, and improving the quality of the fuel by switching to cleaner-burning fuel. Unused feed can be recycled, and off-specification products can be reprocessed. In integral processes, the waste of one process is used as the feed for another. By-products can also be sold to another company for use as raw material. Tighter control systems, alarm, and interlocks can reduce misoperation of process unit.1

Further information about environmental considerations can be found in the [[Environmental concerns]] page.

=Conclusion=
The location and layout of a plant can greatly impact its economic and operational success. Objectives such as cost minimization, distribution, room for expansion, and safety of the plant operators and local community play important roles in the site decision. The major factors affecting this decision are market and labor. National and regional environmental legislations impact the processes, and thus make some areas more desirable than others. Least important is land cost. When planning to build a new facility or expanding an existing site, companies utilize their site selection team to assemble a list of desirable locational characteristics and determine how the new facility will fit in with the overall company. Then with the help of consultants, a location is narrowed down and further analyses are done to justify the continuation of the project. The location should be obtained before design of the details of the process. Within the site itself, process units are arranged to allow for the most economical flow of people and material, optimal safety, and room for future expansion.

=References=
[1] G.P. Towler, R. Sinnott, ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design'', Elsevier, 2012.

[2] J.P. Blair, R. Premus. Major Factors in Industrial Location: A Review. Economic Development Quarterly. 1987;1(1):72-85.

[3] M.S. Peters, K.D. Timmerhaus, ''Plant Design and Economics for Chemical Engineers'', 5th Ed., McGraw-Hill: New York, 2003.

[4] O. Akgul, N. Shah, L.G. Papageorgiou. Economic optimisation of a UK advanced biofuel supply chain. Biomass and Bioenergy. 2012;41:57-72.

[5] L.T. Biegler, I.E. Grossmann, A.W. Westerberg, ''Systematic Methods of Chemical Process Design'', Prentice-Hall: Upper Saddle River, 1997.

[6] Summary of the National Environmental Policy Act website. http://www.epa.gov/laws-regulations/summary-national-environmental-policy-act. August 2015. Accessed February 2016.

[7] R.T. Turton, R.C. Bailie, W.B. Whiting, J.A. Shaeiwitz, ''Analysis, Synthesis, and Design of Chemical Processes'', Prentice Hall: Upper Saddle River, 2003.

[8] R.W. Schmenner, Look Beyond the Obvious in Plant Location website. https://hbr.org/1979/01/look-beyond-the-obvious-in-plant-location. January 1979. Accessed February 2016.

Process location and layout decisions

2016-02-22T01:23:52Z

Ezhuang: /* Biofuel Supply Chain Example */

Author: Ellen Zhuang [2016]

Stewards: Daniel Garcia, and Fengqi You

=Introduction=
Location is one of the first decisions in the design of a new chemical plant. It impacts the profitability of the project and the scope for future expansion. If the project is a new facility, a suitable site must be found and an optimal layout of the site and process units must be planned. If the project adds to an existing site, the impact of the new addition on the existing plant must be considered. The plant also needs to accommodate for the nearby infrastructure, the services that it requires, and its environmental impacts.

=Location Selection=
===Location Decision Factors===
Initially, economists viewed the plant location decision as a cost-minimization problem. The optimal location was one where the transportation costs of raw material to the plant and product to the market was minimized. Between the early 1900s and post World War II period, most industries were sensitive to the cost of transportation. As transportation costs became less of an obstacle, the approach to the problem developed with considerations of trade-offs between factors such as wages, energy, local regulations, etc. and transportation. If lower wages could balance the higher transportation costs of building the plant in a low-wage area, the low-wage location may be more desirable.

Although location is a long-term investment, a firm does not decide on a location with the sole objective of maximizing its profits or minimizes its costs. Managers may choose a “safer” location that is more likely to produce higher profits rather than a riskier location that yields the maximum investment return. Personal factors of the people involved are also influences. For example, new businesses tend to locate where the founder lives. 2

In the current age, a plant's site is chosen based on several factors. These include:1,2,3,8

'''1. Raw material supply:''' The source and price of raw materials is one of the most important factors that determine the location of a plant. Facilities that produce chemicals in bulk are usually located close to the source of raw material if the costs of shipping the product is less than the costs of shipping the feed. For example, ethylene production is growing in the Middle East since cheap ethane from natural gas is readily available. Oil refineries tend to be located near areas with high population and crude oil supplies since it is expensive to transport the oil.

'''2. Location with respect to market:''' If the plant produces high-volume and low-cost products, such as cement and fertilizer, it may be better to situate the plant closer to the primary market since transportation cost is a large fraction of the sales price. If the product is low-volume and high-cost, like pharmaceuticals, then the benefits of being closer to the primary market may not be there.

'''3. Transport facilities:''' Facilities should be close to at least two major forms of transportation, whether that be road, rail, waterway, and/or seaport. For example, paper manufacturing plants in the US use various types of pulp that are delivered by truck or by train from various places in North and South America, so paper facilities need to be close to rail and major roads. Transportation by road is common for local distribution from central warehouses, while transportation by rail is more widespread for long distance transport of bulk chemicals. Pipeline is used to ship industrial gases and bulk fuels. Air freight can be efficient for shipment of personnel and essential units and supplies and for small volume products that have high value, such as pharmaceuticals. Of course, products that are delivered by air must meet aviation regulations.

'''4. Availability of labor:''' Skilled workers are usually brought to the plant from outside the area. There should be a local pool of unskilled labor that can be trained to operate the plant, and of skilled craft workers to maintain the process units. Local labor laws, trade union customs, restrictive practices for recruitment and training should also be taken into consideration. A 10% increase in unionization of a state's labor force is projected to reduce the number of expanding facilities by 30 to 45%.

'''5. Availability of utilities:''' Processes that require a substantial amount of cooling water is usually located near water sources, such as rivers or wells. Cooling water may be directly taken from the water source, or may be stored in cooling towers. Those that need large quantities of power, such as electrochemical ones, are typically close to cheap power sources.

'''6. Availability of suitable land:''' The ideal land is flat, well-drained, with suitable load-bearing characteristics. Further considerations have to be made if the land is reclaimed land near the ocean in earthquake zones. Property tax is also a factor when choosing a site since property taxes vary area to area. Under a third of plants that relocate move to regions with lower property taxes, which is the proportion that would be expected if companies move to a new location regardless of property tax. High property taxes is not as significant as other factors such as labor supply and land costs.

'''7. Environmental impact:''' Depending on the location, it may be more difficult and costly to dispose of wastes. During the project design phase, experts are typically consulted to learn more about an area's local regulations. More details about environmental regulations are found below.

'''8. Local community considerations:''' State and local planners are typically motivated by the desire to create jobs and improve the tax base. Introduction of facilities to an area is usually viewed as the most direct way to stimulate the area's economy. However, recent studies have found that communities with high-growth are already characterized by the fast growth of businesses that are already there. It is rare for a plant to completely close in one area and relocate to another, and if plants do relocate, the majority is over short distances and often within the same community. Therefore, local policymakers favor the expansion of existing plants. The opening of a new plant at a location should impost no additional risk to the local residents. For example, they should be downwind of the residential areas. Local communities also need to be able to accommodate the plant personnelles. For example, traffic, housing, and facilities must be able to accommodate the influx of workers. Additional factors are property taxes and water consumption.

'''9. Climate:''' The climate of the area may affect processes and costs. For example, plants in cold areas need more insulation and special heating. Facilities in earthquake areas need to be seismically sound. Plants in areas with high ambient humidity will usually use air cooling instead of water cooling.

'''10. Political and strategic considerations:''' Government sometimes gives capital grants, tax concessions, and other incentives to encourage plants to be built in specific areas. Physical assistance such as roads, water, and other public infrastructure are more popular than financial assistance. Companies can also globalize and take advantage of areas with preferential tariff agreements. The tax policy of an area is inversely related to growth. High personal income sometimes hinder employment growth. Personal income does not affect the cash flow of the company, but it reduces the after-tax income of its managers, and thus high personal income tax can be classified as a personal region. High state corporate taxes has also and detrimental effects on growth, but that is not always the case. Corporate tax is more important to firms with high capital expenditures.

===Analysis of the Importance of Factors===
In 1963, Morgan surveyed 17 companies and found that on average the most important factors on industrial location decisions are market, labor, and raw material. Taxes and financial incentives were of little significance. More recently, Fortune magazine surveyed among the 1000 largest US companies and found that the most important factors were market and labor. The survey also found that personal preferences of a firm's executive, tax, and central cities have great influence over the site of corporate headquarters. Surveys of interstate locational decisions found that the factors that played key roles for new firms were access to customers and the growing market, labor force, transportation, personal reasons of management, and availability of capital. The cost of land had the least important influence. For firms that expanded across states, the influential factors were labor costs and labor supply. Companies seeking expansion usually have made their production more routine, so quick access to suppliers is less of a concern. Cost minimization becomes more relevant to existing corporations.2

===Site Selection Process===
The decision for the location of a facility is part of a larger corporate planning process. Usually, a corporate planning office or a division of the company initiates the site selection process by forecasting future capacity requirements. If capacity shortages are in the forecasts, the managers may choose to outsource, increase price to reduce the demand, expand existing sites, or open a new facility. If the managers decide on a new facility, the site selection team enters the project.

The structure of the site selection team depends on the firm's organization. In companies with centralized staff, the site selection team generally consists of representatives from relevant areas, such as engineering, real estate, and transportation. In companies with strong divisions, the locational decisions may be done at the divisional level, with the corporate office supervising the process.

The site selection team determines what characteristics are important for the new location by considering how the new facility will fit in the company's overall strategy, if the company wants to target new markets, if the corporation wants to divide or integrate its functions, or how the company wants to be seen by the public. Next, potential locations are listed and studied against the desired characteristics.

Locational decisions are typically made sequentially. The first step is at the state or regional level. Then, the team studies specific communities and sites. Different locational factors are important to different stages. When selecting a general region, the site selection team focuses on factors with interregional variations such as labor, tax policies, climate, and market locations. At the more focused stage, details like inexpensive land, access to to major roads, and good schools are important. Consultants are often hired to do site analyses.

Once site options are narrowed down, the company discusses potential problems and incentives with local public officials. Construction costs are estimated, and a feasibility analysis is done to show that the project has a high rate of return.2

===Biofuel Supply Chain Example===
Biofuels is a popular potential alternative to fossil fuels. Various biomass resources, including food crops, non-food crops, and agricultural residues, are converted into biofuels. The two most common biofuels currently are ethanol and biodiesel. The supply chain for biofuel is a network consisting of several nodes: biomass cultivation sites, biofuel production plants, and demand centers. The locations of these facilities, and the location of the demand centers with respect to the biofuel demand impact transportation costs, production resource, demand, etc., and thus affects and sets constraints to the cost minimization of the supply chain. The optimal supply chain design minimize the total cost, which is a sum of the total investment cost, production cost, transportation cost, and outsourcing and import cost. The total transportation cost captures the transportation of biomass and biofuel between areas and the local bio mass transfer.4

[[File:biofuelsupplychain.png|400px|thumb|center|'''Figure 1.''' The biofuels supply chain consists of three notes: biomass cultivation sites, biofuel production plants, and biofuel demand centers.1]]

[[File:Biofueloptimal.png|300px|thumb|center|'''Figure 2.''' A Great Britain biofuel supply chain design that minimizes cost. The plants are near areas with high wheat cultivation and wheat straw collection rates. The location of the plants with respect to the demand centers is also optimized. The main modes of transportation are road, rail, and ship.4]]

=Site Layout=
The process units and buildings need to be arranged in such a way that allows for the most economical flow of materials and people. Furthermore, dangerous processes need to be a safe distance from other buildings, and the layout should be planned to allow for future expansion.

Process units are usually laid out first in an arrangement that allows for smooth flow of materials between the process steps. The distance between equipment is usually at least 30 m. Next, the location of the principal ancillary buildings are sited as to minimize the time that it takes the workers to travel between buildings. Administrative offices and laboratories are located away from hazardous processes. Control rooms are next to the processing equipment. Utility buildings are located as to minimize piping between the process units. Storage is placed between the loading and unloading facilities and next to the process units that they serve. Tanks containing hazardous material are placed at least 70 m from the plant. An example of a typical site plan is shown below.1

[[File:Site Location.png|400px|thumb|center|'''Figure 3.''' A typical site layout.[1]]]

=Plant Layout=
The main factors that are considered when planning the layout of the plant are listed below.1

'''1. Economic considerations (construction and operating costs):'''
Construction costs can be minimized by arranging process units and buildings that minimize pipes between equipment, the amount of structural steel work, etc. However, this layout may conflict with the layout that gives the optimal operation and maintenance.

'''2. Process requirements:''' Examples of process considerations that must be taken into account is the elevation of the base of columns to give enough net positive suction head to a pump.

'''3. Operation convenience:''' Process units that are attended to frequently should be placed with convenient access. Valves, heads, and sample heads should be placed where operators can easily access. If the plant anticipates replacement of equipment, space must be allowed for removal and installation.

'''4. Maintenance convenience:''' Equipment that requires maintenance should be in a location with easy access, and should have sufficient space for the maintenance tasks. For example, shell-and-tube exchangers need space so that tube bundles can be removed for cleaning and repair.

'''5. Future expansion:''' The layout should be planned to conveniently allow for future expansion of processes. Pipe racks should have space for future piping, and pipes should be oversized to allow for more flow in the future.

'''6. Modular Construction:''' Modular construction is where sections of the plant is constructed outside of the plant, and then transported to the site by road or sea. Advantages include improved quality control, reduced construction costs, less requirements for skilled labor on site. Tradeoffs are more flanged connections and possible problems with onsite assembly.

'''7. Safety:''' Escape routes for workers need to be in place at each level in process buildings. Blast walls must isolate equipment that pose hazards to confine potential explosions.

First, a conceptual flowsheet for the process is developed. The types of equipment and their connections with each other is described in a process flow diagram (PFD). Before the PFD is translated into detailed piping and instrumentation diagrams (P&ID) the layout of the process units must be planned.2,3 Scale drawings are made to show the relationships between storage space and process equipment based on the flow of materials and people, and on future expansion. Three-dimensional visualization are the layouts are then carried out with cardboard cutouts of the equipment outlines or rectangular and cylindrical blocks. When a layout of the major process units has been decided, drawings of the plan and elevation are made, and design of the structural steelwork and foundations are done. Computer-aided design has also become increasingly popular.1,3,7

=Environmental Considerations=
Laws that protect the environment restrict the waste that plants can emit in order to preserve air, land, and water quality. States, provinces, and municipals usually have additional laws to national ones. In the US, environmental policies are moving towards more uniform and strict regulations. Businesses that produce more pollution are more likely to move to regions where local regulations are more lax, and industries in areas with more stringent policies will decline. The tighter national regulations are implemented to prevent this competition between regions. For example, amendments to the Clean Air Act and the Environmental Protection Agencies in the 1970s and 1980s reduced differences in regions' air pollution controls. Congress voted on the PSD (prevention of significant deterioration) to "level the playing field" across regions.1

Some of the main environmental legislations in North America are:

*'''The National Environmental Policy Act of 1969 (NEPA)''' - Signed by President Nixon, NEPA was one of the first laws that provide a national framework for the protection of the environment. It requires that all executive federal agencies consider the environmental impact of proposed actions. Government agencies must prepare environmental assessments and environmental impact statements that describe potential environmental effects. NEPA is implemented when buildings, airports, military complexes, parkland purchases, and highways are proposed.6

*'''The Clean Air Act (CAA, 1970)''' - The CAA was implemented to control air quality on a national level. The law was amended in 1990 to address issues such as acid rain, ozone depletion, and toxic air pollution, and to increase enforcement authority. Under the National Ambient Air Quality Standards (NAAQS), the law sets limits to allowable ambient levels of seven contaminants: ozone, carbon monoxide, lead, nitrogen dioxide, sulfur dioxide, PM10, and PM2.5. The CAA also requires that the EPA set US National Emissions Standards for Hazardous Air Pollutants (NESHAP), which regulates the emission of 189 hazardous air pollutants. The Maximum Available Control Technology (MACT) was implemented to limit the emission of volatile organics, sulfur, and other inorganic compounds from oil refineries. The CAA has had significant impact on the level of air pollutants that it regulates. Particularly in the Midwest and the Northwest, water acidity has been reduced. On the other hand, reduction of ozone damage has not been as successful since nitrogen oxide, which is linked to ozone formation, is not as tightly regulated.

*'''The Federal Water Pollution Control Act (The Clean Water Act, 1972)''' - The Clean Water Act was initially passed to clean water used for swimming and boating, and to protect wildlife. Amendments were later added to address water quality and toxic compound emissions. Under the CWA, the EPA can set water quality standards and monitor the industries' discharge into waters.

*'''The Safe Drinking Water Act (SDWA, 1974)''' - This act allows the EPA to set standards to potential drinking water. Public water systems must operate according to these standards.

*'''The Resource Conservation and Recovery Act (RCRA, 1976)''' - The RCRA protects current and future groundwater facilities from contamination. Firms that produce waste must have a "cradle to grave" approach to managing waste, meaning that the waste producer is legally responsible for the waste from the moment it is produced until it is finally disposed.

*'''The Comprehensive Environmental Response, Compensation and Liability Act (CERCLA or Superfund, 1980)''' - This legislation creates a tax on chemical and petroleum industries in order to cover the remediation of hazardous waste sites that are uncontrolled or abandoned. The CERCLA empowers EPA to remediate waste sites by short-term removal of material or long-term remediation actions, depending on the risk that the waste imposes on life.

*The Superfund Amendments and Reauthorization Act (SARA, 1986)
*'''The Pollution Prevention Act (PPA, 1990)''' - This act promotes minimization of waste and pollution at their source. It encourages more efficient processing and raw materials and recycling through grants, technical assistant, and information.

*'''The Oil Pollution Act of 1990 (OPA, 1990)''' - This act was passed after the Exxon Valdez oil spill in the Prince William Sound in Alaska, creating a tax on oil to cover the cost of responding to spills when the corporation that creates the spill does not respond.

*'''The Department of the Environment Act (E-10)''' - The Department of Environment Act created the Department of Environment and charges the Minister of the Environment with creating programs that protect the environment, assessing environmental impacts of federal program, and reporting information to the public in Canada.

*'''The Canadian Environmental Protection Act (CEPA, C-15.31, 1999)''' - CEPA empowers Environment Canada to control toxic substances, reduce pollution, and reduce bioaccumulating toxic substances. Canada will use CEPA to implement regulations that helps it meet its commitments to the Kyoto Protocol.

*'''The Canada Water Act (C-11)''' - This law allows the Minister of the Environment to manage water resources in Canada. This includes research, implementing conservation programs, utilization of water resources, and setting water quality standards.

Wastes comprise mainly of by-products, unused reactants, and off-specification product produced by misoperation. Leaking seals and flanges, and spills and discharges also emit waste. Material is also discharged in emergency situations. Instead of considering how to treat or manage waste, designers should start by tackling the source and find ways to minimize the production of waste.

The process of designing waste management systems are: (1) source reduction, (2) waste stream recycle, (3) waste treatment to reduce environmental impact, and (4) disposal that is legally sound. Source reduction can be achieved by reducing the concentration of impurities in the feed, protecting catalysts and adsorbents from contaminants, eliminating the use of extraneous materials, increasing recovery from separation, and improving the quality of the fuel by switching to cleaner-burning fuel. Unused feed can be recycled, and off-specification products can be reprocessed. In integral processes, the waste of one process is used as the feed for another. By-products can also be sold to another company for use as raw material. Tighter control systems, alarm, and interlocks can reduce misoperation of process unit.1

Further information about environmental considerations can be found in the [[Environmental concerns]] page.

=Conclusion=
The location and layout of a plant can greatly impact its economic and operational success. Objectives such as cost minimization, distribution, room for expansion, and safety of the plant operators and local community play important roles in the site decision. The major factors affecting this decision are market and labor. National and regional environmental legislations impact the processes, and thus make some areas more desirable than others. Least important is land cost. When planning to build a new facility or expanding an existing site, companies utilize their site selection team to assemble a list of desirable locational characteristics and determine how the new facility will fit in with the overall company. Then with the help of consultants, a location is narrowed down and further analyses are done to justify the continuation of the project. The location should be obtained before design of the details of the process. Within the site itself, process units are arranged to allow for the most economical flow of people and material, optimal safety, and room for future expansion.

=References=
[1] G.P. Towler, R. Sinnott, ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design'', Elsevier, 2012.

[2] J.P. Blair, R. Premus. Major Factors in Industrial Location: A Review. Economic Development Quarterly. 1987;1(1):72-85.

[3] M.S. Peters, K.D. Timmerhaus, ''Plant Design and Economics for Chemical Engineers'', 5th Ed., McGraw-Hill: New York, 2003.

[4] O. Akgul, N. Shah, L.G. Papageorgiou. Economic optimisation of a UK advanced biofuel supply chain. Biomass and Bioenergy. 2012;41:57-72.

[5] L.T. Biegler, I.E. Grossmann, A.W. Westerberg, ''Systematic Methods of Chemical Process Design'', Prentice-Hall: Upper Saddle River, 1997.

[6] Summary of the National Environmental Policy Act website. http://www.epa.gov/laws-regulations/summary-national-environmental-policy-act. August 2015. Accessed February 2016.

[7] R.T. Turton, R.C. Bailie, W.B. Whiting, J.A. Shaeiwitz, ''Analysis, Synthesis, and Design of Chemical Processes'', Prentice Hall: Upper Saddle River, 2003.

[8] R.W. Schmenner, Look Beyond the Obvious in Plant Location website. https://hbr.org/1979/01/look-beyond-the-obvious-in-plant-location. January 1979. Accessed February 2016.