processdesign - User contributions [en]

Nueces Desalination Center: Production of Drinking Water by Multi-Stage Flash Distillation

2016-03-07T21:20:25Z

Benjkg: /* Appendix B: Feed and Product Definitions - Salinity */

Team F: Rankine 672 Final Report

Authors: Kedric Daly, Ben Granger, Evan Rosati, Kathleen Zhou

Instructors: Fengqi You, David Wegerer

March 11, 2016

=Executive Summary=
Desalination of seawater to drinking water is a technology of ever-increasing importance given trends of increasingly widespread water shortages around the world. Even in the United States, water shortages are becoming increasingly common in dry, highly-populated such as California and certain areas of the Gulf Coast. Our design team, Rankine 672, was tasked with evaluating the need for and designing a desalination plant to provide purified water in a place of need. Investigation revealed that the city of Corpus Christi, Texas, is a strategic location for development of a desalination facility, as surrounding region is in need of new drinking water sources and the city has expressed serious interest in contracting the development of a large-scale desalination plant. Our proposed solution is a multi-stage flash distillation desalination facility capable of producing 20,000,000 gallons (75,708 m3) of purified drinking water for the city and surrounding region per operating day.

In this design, drinking water is produced from seawater feedstock through a 21-stage flash distillation process. By strategically locating the facility next to an existing natural gas power plant, the Nueces Bay Energy Center, pre-heated feed water can be taken directly from the outbound cooling systems of the power plant, allowing energy savings for our desalination plant. The process is designed for 16 hour-per-day operation, requires a seawater feed of 8250 m3/hr, and produces 4758 m3/hr of purified drinking water alongside a waste stream of 3485 m3/hr of concentrated brine. The waste stream is isolated from a recycle stream through a 9.0% purge; this results in an overall product yield of 57.7% on a volume basis. The process requires an additional pretreatment stream of 16.5 kg/hr Belgard EV 2030, which serves as an antiscalant, as well as 9.5 kg/hr chlorine, 352.1 kg/hr lime, and 418.7 kg/hr CO2 for post-treatment purposes.

The overall capital cost of the plant is expected to be $345MM, with a total operating cost of $103MM/yr, $59MM/yr of which comes from utilities. Due to the low price at which the product water can be sold, the 30-year net present value of the plant, assuming a 6% discount rate, is $-1,035MM. It is thus imperative this plant be constructed with public funds because it cannot return a profit to any private investor. The growing need for water however could justify the cost, because it is important that the residents of the greater Corpus Christi area have access to drinkable water.

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=Introduction=
==Background==
Clean drinking water is a basic human need that is becoming increasingly difficult to meet. Currently, over 1 billion people live in places where water is considered scarce<ref> Tapping the oceans. The Economist USA. http://www.economist.com/node/11484059. Accessed March 2, 2016. </ref>. Though often thought of as a problem exclusive to third-world countries, the issue is becoming more prevalent in the US as well, with water managers in 40 of 50 states anticipating water shortage conditions in some portion of their state in the next 10 years. These shortages are due to increasing demands on constant or diminishing freshwater resources, therefore, to correct the issue, a new supply must be found<ref> Water Supply in the US. EPA. http://www3.epa.gov/watersense/pubs/supply.html. Accessed March 2, 2016.</ref>. Desalination of seawater is therefore important as a new source for clean drinking water and prevent water shortages.

==Project Definition==
The overall goal of the project is to investigate the feasibility of a desalination plant in order to meet the needs of growing water demand. The design needs to take health, safety, and environmental concerns into account, while optimizing costs.

=Design Basis=
Site Location and Conditions
Corpus Christi, TX was chosen as the location for the proposed desalination plant. The city is currently experiencing “Moderate Shortage Conditions”. This issue additionally comes at a time of growing demand with population growth anticipated due to new industry and Eagle Ford Group crude oil production<ref>Stage 2 Measures in the Drought Contingency Plan. Available at http://www.cctexas.com/government/water/conservation/conservation-drought-plans/stage-2-measures. Accessed January 12, 2016. </ref>.

In June 2014, the city announced the Corpus Christi Desalination Demonstration Project<ref>Corpus Christi Desalination Demonstration Project. Available at http://www.cctexas.com/Assets/Departments/Water/Files/DesalFactSheet.pdf. Published June 2014. Accessed January 12, 2016.
</ref> which seeks to create a demonstration desalination plant capable of producing 20,000 gallons of water per day, with plans to later build a full scale facility generating 20,000,000 gallons of water per day. The desalination demonstration project indicates a clear need for implementation of a desalination plant of this size to accommodate growth in the Corpus Christi area.

Locating our plant in Corpus Christi will allow us to take advantage of the benefits of cogeneration through a partnership with the Nueces Bay Energy Center. By utilizing their outlet cooling water as a large portion of our feed for the desalination process, we can raise the temperature of our seawater feed, saving on energy costs in the multi stage flash distillation. The proposed location for the desalination plant is immediately west of the Nueces Bay Energy Center. See Appendix A for a map depicting the proposed location for the desalination plant along the coast of Nueces Bay.

==Feed and Product Definitions==
The proposed desalination facility’s feed and product streams are defined as shown in Table 1. Rationale for these definitions is also described below. The values reported in Table 1 are final derived numbers from HYSYS simulation modeling.

<center>
{| class = "wikitable"
|+Table 1: Feed and product stream definitions
!  
! Salt Concentration
! Temperature
! Amount
|- style="text-align: center;"
! style="text-align: right;" | Feed Stream
| 30,500 ppm || 45°C || 8250 m3/hr
|- style="text-align: center;"
! style="text-align: right;" | Product Stream
| ~ 0 ppm || 35°C || 4758 m3/hr
|- style="text-align: center;"
! style="text-align: right;" | Byproduct Stream
| 69,500 ppm || 35°C || 3485 m3/hr
|}
</center>

The process feed will be seawater from Nueces Bay, preheated through the cooling water system of the nearby Nueces Bay Energy Center. Based on calculations developed from NREL (National Renewable Energy Laboratory) data, it was estimated that the Nueces Bay Energy Center uses approximately 250 million gallons of seawater per day for process and cooling needs. Therefore, the 20 million gallon per day requirement for the desalination facility can easily be met through the power plant’s cooling water<ref>Torcellini P, Long N, Judkoff R. Consumptive Water Use for U.S. Power Consumption. NREL. 2003; TP-550-33905.
</ref>. Note that we are assuming the process will operate 67% of the time (16 hours per day).

Based on data acquired from two salinity-measuring stations in Nueces Bay, the average salinity of local seawater is 30.5 practical salinity units (psu)<ref>Stations Map & Real - Time Data. The Conrad Blucher Institute for Surveying and Science website. Available at http://www.cbi.tamucc.edu/cbi/data/. Updated January 2016. Accessed January 9, 2016. </ref>, thus setting the concentration of the process feed stream. This translates to a feed stream concentration of 30,500 ppm (parts per million)<ref name = "SALT">Definition and Units. CATD Salinity Expertise Center. Available at http://www.salinityremotesensing.ifremer.fr/home. Accessed January 12, 2016.</ref>. The composition of the salt attributed to this seawater salinity is shown in Table 6 in Appendix B.

No data on the composition of seawater around the Corpus Christi area was available, but the composition of seawater does not change significantly in different locations. The given composition is considered accurate for our feed stream<ref>Brown E, Colling A, Park D, Phillips J, Rothery D, Wright J. Seawater : Its Composition, Properties and Behaviour. Boston: Butterworth-Heinemann, 1995.
</ref>. Note that seawater salt is primarily sodium chloride - total dissolved solids are 85.7% sodium chloride by mass. As such, the dissolved solids in the feed stream is assumed in this project to be pure NaCl, making the feed composition 96.95% water and 3.05% NaCl by mass. A control system will likely be put in place to ensure feed concentration remains at this level, accounting for natural fluctuations in local seawater salinity.

The largest byproduct from the process will be a brine slurry with high salt concentrations. An upper corrosion limit of 700 ppm was set for recirculated and purged brine. This undesired byproduct will be sent to a nearby Class I non-hazardous injection well facility operated by GNI Group, which is approximately 12 miles away from the proposed desalination plant location as shown in the figure in Appendix A<ref>Texas Water Code. Availabe at http://www.statutes.legis.state.tx.us/Docs/WA/htm/WA.27.htm. Accessed January 14, 2016.
</ref><ref>Google Maps. Available at https://goo.gl/maps/3xcvLZwUCe42 Accessed January 15, 2016.</ref>. Through the incorporation of a Class I injection well, the brine will not have to be treated or diluted before injection. <ref>EPA Regulations on Class I Wells. Available at http://www.epa.gov/uic/class-i-industrial-and-municipal-waste-disposal-wells. Accessed January 14, 2016.
</ref><ref>List of Injection Well Facilities by Location. Available at http://www.ehso.com/cssepa/tsdfdeepwells.php. Accessed January 14, 2016.</ref>

There are no regulations regarding the temperature of city drinking water, so the product stream temperature has no requirements based on regulations. Outlet temperatures of 35oC are acceptable for input to the municipal water supply system. Since city water tends to be stored in non-temperature controlled tanks, the temperature will naturally cool to ambient temperature.<ref>Climate Corpus Christi- Texas. US Climate Data. Available at http://www.usclimatedata.com/climate/corpus-christi/texas/united-states/ustx0294. Accessed January 12, 2016. </ref>

==Pre-Treatment and Post-Treatment==
For pretreatment, Belgard EV 2030 is recommended to be added at a concentration of 1.8-2.0 mg/L for a system with top brine temperature approximately 110 °C<ref name="BWA 2030">BWA Water Additives. Belgard EV2030 Technical Data Sheet. BWA Water Additives. http://www.wateradditives.com/files/products/tech_data_sheets/Belgard%20EV%202030_Technical%20Data%20Sheet_Thermal_8.5x11.pdf. Accessed January 27, 2016.
</ref>. Since our process is designed to operate at a slightly higher top brine temperature, 120 °C, we will design for the upper bound concentration: 2.0 mg/L of antiscalant. With a feed flow rate of 8250 m3/hr, the process therefore requires 16.5 kg/hr of Belgard EV 2030.

For disinfection during post-treatment, chlorine addition typically requires a dose of roughly 2.0 mg/L as well<ref name="World Health Organization">World Health Organization. Desalination for safe water supply. http://www.who.int/water_sanitation_health/gdwqrevision/desalination.pdf. Accessed January 26, 2016.</ref>. With a pure water distillate flow rate of 4758 m3/hr, the process requires 9.5 kg/hr of chlorine as a 5%-15% sodium hypochlorite solution. Approximately 74 mg/L of lime (calcium hydroxide) and 88mg/L of CO2 should be added to the desalinated water to help prevent corrosion and increase alkalinity within acceptable ranges<ref name="Vouchkov">Vouchkov, N. Re-Mineralization of Desalinated Water. SunCam. https://s3.amazonaws.com/suncam/npdocs/118.pdf. Accessed February 18, 2016.</ref>. With a product stream flow rate of 4758 m3/hr, these concentrations correspond to 352.1 kg/hr of lime and 418.7 kg/hr of CO2. Both of these can be purchased, and it is likely that the CO2 can be procured from the neighboring natural gas power plant.

==Choice of Desalination Technology==
The two major technologies commonly implemented for desalination include reverse osmosis (RO), and multi-stage flash distillation (MSFD). RO utilizes high pressures to force water through a semipermeable membrane that allows for purified water to pass through, but stops organic and other wastes. Often, the pressures can reach 55 - 68 bar, which requires large pumps to maintain pressure, and introduces safety concerns to the process<ref>Stage 2 Measures in the Drought Contingency Plan. Available at http://www.cctexas.com/government/water/conservation/conservation-drought-plans/stage-2-measures. Accessed January 12, 2016.
</ref>. While RO can be less energy intensive than MSFD, and the membrane can also remove contaminants, it is susceptible to feedwater quality, membrane fouling, and requires more extensive pretreatment and posttreatment than MSFD. The cost of RO is directly proportional to the feed salinity, and so it is less expensive when desalinating low salinity water.

Multi-stage flash distillation is best selected for areas that have high salinity or low energy costs. The pretreatment process for MSFD is much easier when compared to RO because pretreatment often consists of simply adding anti-corrosives and antiscalants to the feed stream. The process works by heating water in vessels at different pressures to change the boiling point of the water and evaporate it as it passes through the system. After evaporation, the water condenses, leaving pure water as the effluent. MSFD costs are now roughly $1.00/m3, where this cost is mostly independent of salinity, but dependent on energy costs.<ref>Fritzman C, Lowenberg J, Wintgens T, Melin T. State-of-the-art Reverse Osmosis Desalination. Desalination. 2007; 216:1-76.
</ref> In both RO and MSFD a brine is produced which then must be appropriately treated and/or disposed of.

Multi-stage flash distillation was selected for this process, mainly due to the economic advantages offered by the selected location of Corpus Christi, TX. MSFD is also a well understood process that can be more easily modeled by chemical engineering software.

=Technical Approach=
Quantitative modeling of the process and all included optimization was conducted using the simulation software Aspen HYSYS. A variety of assumptions were developed to set up and define specific components of the process simulation itself. Within Aspen HYSYS, the NRTL Electrolyte property package was selected to model the behavior of salt water. Each stage of the MSF process is modeled as an individual flash vessel, with individual shell & tube heat exchangers used to model the distillate condensers present in each flash stage in the MSF system.

The HYSYS simulation model only considers the flash portion of the overall process, allowing establishment of mass and energy balances for the system. Pretreatment and post-treatment were excluded for the HYSYS model, as these steps are simple additions or removals of small concentrations of chemical components. Although the simulation models heat exchangers outside of each flash stage, in the physical design of the plant these heat exchangers will be contained within each flash stage.

One portion of the process is not modeled, which is the inlet seawater entering the heat rejection stages. As can be seen from the full process flow diagram (Appendix C), the inlet seawater in the heat rejection stage is in high excess to provide maximum cooling capacity; then a significant portion is purged to bring the stream flow rate down to what is needed for the feed stream. It is assumed that the flow rate of cooling water for those three stages is in such high excess that the cooling water does not heat up. Therefore, in the process simulation models, the inlet stream is the feed stream, which enters directly into the heat recovery portion of the process.

See Appendix D for a screenshot overview of the simulation environment model.

=Process Overview=
==Process Flowsheet==
The following diagram, Figure 1, shows a basic flowsheet for the process. Process alternatives considered for each stage are discussed below. Please see Appendix C for the full process flowsheet with finalized design decisions.

[[File:BlockFlowDiagram.png|center|700px|thumb|alt=Nueces Bay Desalination Facility Block Flow Diagram|Figure 1: Basic block flowsheet of the multi-stage flash desalination process.]]

==Major Features==
As seen above, there are three primary stages of the desalination process: Pre-treatment, the multi-stage flash distillation, and post-treatment. Seawater enters the pre-treatment phase first to make the feed suitable for the rest of the process by filtering out large particles that could accumulate on pipes, deaerating the water to limit corrosion, and adding an agent to combat fouling. The water then enters the distillation process, which is broken up into two sections: heat recovery and heat rejection.

In the heat recovery stage, there are 18 flash vessels, each of which separates steam from the salinated liquid water. Each flash vessel has saltwater as its inlet and two outlets: a vapor outlet (steam) that condenses into the final product of purified water and a liquid outlet composed of water with more dissolved solids than the inlet, which is the inlet stream for the next flash vessel. The pipe containing the feed is first mixed with recycled brine, then goes through the top sections of each of the flash vessels, starting with V-118 and ending up at V-101. In this way, the feed stream pipe first serves as a heat exchanger, heating the pre-treated seawater while simultaneously condensing the steam into the final product. The feed then enters a final heat exchanger where it is brought up to 103.6 °C before entering V-101 to be flashed.

After exiting V-118, the final flash vessel in the heat recovery portion, the water enters the heat rejection portion, which consists of three flash vessels (V-201, V-202, V-203). The flash vessels of this section operate in the same manner as in the heat recovery section, with the exception that condensation occurs using excess cooling water instead of pre-treated seawater. This technique allows excess heat to be rejected into an excess water stream rather than the feed.

The purified product stream finally enters post-treatment processing in which water is re-aerated and remineralized, disinfected, and pH is adjusted before being delivered to the city. The concentrated brine leftover at the end of the flash cycle is recycled into the feed, although a purge is required to continuously remove some salt from the system. This purged brine is disposed of by use of a Class I injection well, per regional regulations.

See Appendix E to find detailed stream tables of stream specifications.

==Process Alternatives==
===Pretreatment===
Multi-stage flash distillation requires significantly less pretreatment than reverse osmosis desalination, but helpful pretreatment techniques still exist. One very inexpensive method of pretreatment is screening/filtration, which is used to filter out large contaminants from intake seawater. Using more complicated membrane technology to accomplish large particle filtration is a possible alternative, but doing so would be much more expensive than using a standard screen or filter, so the latter choice has been selected for this process.<ref>El-Dessouky H., Alatiqi I, Ettouney H. Process synthesis: The multi-stage flash desalination system. Desalination. 1998;115(2):115-179. doi: 10.1016/S0011-9164(98)00035-6.</ref>

The most critical type of fouling in multi-stage flash distillation is scaling - therefore, the most critical step in the pretreatment process will be the inclusion of an anti-scalant. Alkaline scaling can be prevented by acid dosing or polyphosphate addition; however, acid dosing can result in increased corrosion and polyphosphates can lead to non-alkaline calcium phosphate fouling, so these additives will not be considered<ref name="Morris">Morris, R. The development of the multi-stage flash distillation process: A designer's viewpoint. Desalination. 1993;93(1-3):57-68. doi:10.1016/S0011-9164(93)80096-6. Availble at http://www.sciencedirect.com/science/article/pii/0011916493800966</ref><ref>Al-Gobaisi, D. A quarter-century of seawater desalination by large multistage flash plants in Abu Dhabi (Plant performance analysis, assessment, present efforts toward enhancement and future hopes). Desalination. 1994;99(2-3):509-512. doi: 10.1016/S0011-9164(94)00196-0.</ref>. As such, a polymer-based additive is recommended for the anti-scalant for this process. The Belgard EV series is designed for use in high-temperature seawater MSF; of these, Belgard EV 2030 is the most neutral in pH<ref>NSF International. NSF Product and Service Listings: Drinking Water Treatment Chemicals - Health Effects. NSF International. http://info.nsf.org/Certified/PwsChemicals/Listings.asp?Company=00300&. Accessed January 27, 2016.</ref><ref name="BWA 2030" /><ref>BWA Water Additives. Belgard EV2035 Technical Data Sheet. BWA Water Additives. http://www.wateradditives.com/files/products/tech_data_sheets/Belgard%20EV2035_Technical%20Data%20Sheet_Thermal_8.5x11.pdf. Accessed January 27, 2016.</ref><ref>BioLab Water Additives. Belgard EV Antiscalant for seawater distillation plants. BioLab Water Additives. http://www.lpq.com.mx/pdf/BELGARD%20EV.PDF. Accessed January 27, 2016.</ref>. As such, we have selected Belgard EV 2030 for use as an anti-scalant in our process.

Corrosion control is another significant method of pretreatment in multi-stage flash distillation plants. Using a piping material that has limited susceptibility to corrosion is typically not economically viable for MSF desalination plants due their large size, so using carbon steel for most of the piping is generally used for its economic benefits despite its susceptibility to corrosion<ref name = "Morris" />. Limiting oxygen concentration is very feasible through the use of deaerators, which can remove oxygen as well as other dissolved gases from the seawater stream. Often, an external deaeration system is used<ref name = "Morris" /><ref>Edwards - Hick Hargreaves. Seawater Deaerators. Edwards - Hick Hargreaves. http://psscorp.co.th/upload/news/20110830105340.pdf. Accessed January 26, 2016.</ref>. Given these considerations, using an external deaerator combined with a less-corrosive antiscalant like Belgard EV 2030 is much more practical for the process than using anti-corrosive piping.

Excessive foam can also be an issue in some MSF distillation facilities, so antifoaming agents are sometimes employed in the pretreatment process as well. However, many plants are operated without foaming issues, and therefore do not need anti-foaming pretreatment at all. Selection of an anti-foaming agent must be carefully weighed with selected of anti-scalant to avoid issues with additive interactions [26]. At this stage in the design process, an anti-foamer will not be included due to the risk of decreasing scalant efficacy, and the likelihood that foam will not be an issue in the desalination plant.

===Multistage Flash Distillation===
The main objective of the design project is desalinate water. This is done with the main process of multi stage flash distillation (MSF). MSF can be developed using a numerous number of different process configurations, such as Once-Through, Simple Mixer Brine Circulation, and Multi-Stage Heat Rejection Brine Circulation processes.

The most desired process for the project is the Three Stage Heat Rejection Brine Circulation as it builds on benefits of the other simpler technologies. By combining principles of heat rejection and heat recovery, the required heat transfer area of the process is greatly reduced as compared to once through processes or simple brine recycle processes. It has the highest performance ratio with the lowest specific feed flow rates, therefore greatly lowering the operating costs of the process. It is also known as the industry standard for MSF<ref name = "KSU Chapter 6">Kansas State University ChE 413. Available at: http://faculty.ksu.edu.sa/Almutaz/Documents/ChE-413/Multi-stage%20Flash%20Desalination.pdf. Accessed January 26, 2016.</ref>. In addition, since our brine disposal technique involves the pumping of the brine into a Class I well, the high brine outlet salinity is not a drawback for our process since post treatment or dilution of the brine is not necessary.

===Post Treatment===
After processing the water via multi-stage flash, the water needs to be post-treated to make the water taste as consumers would expect, since distilled water is “flat” and unpleasant, as well as to protect the consumers and the piping through which the water will be passing<ref name="Clayton">Clayton R. A review of current knowledge: Desalination for water supply. Foundation for Water Research. http://www.fwr.org/desal.pdf. Accessed January 26, 2016.</ref>. There are multiple alternatives to achieve these objectives of remineralization, aeration, corrosion control, and disinfection.

Aerating and introducing some dissolved solids back into the product water are effective ways to create a quality product that is not flat, and contains flavoring that is expected of the water when delivered to the consumer. Aeration will be accomplished via an inline spray aerator which will take up less space than cascade aeration.<ref>Minnesota Rural Water Association. Aeration. http://www.mrwa.com/WaterWorksMnl/Chapter%2011%20Aeration.pdf. Accessed January 26, 2016.</ref> Feed blending will be used to remineralize the water as opposed to a remineralization filter because it does not require additional equipment purchases, and can easily handle the large amounts of throughput required for the process.<ref name="Clayton" />

Corrosion and pH can be controlled by adding lime (calcium hydroxide) and carbon dioxide to the water. To do so, about 74 mg/L of lime and 88mg/L of CO2 should be added to the desalinated water to help prevent corrosion and increase alkalinity within acceptable ranges. Sodium hydroxide, soda ash, and sodium bicarbonate are alternatives to altering the alkalinity and pH of the product water, but lime and CO2 is an industry standard <ref name="Vouchkov" />.

Disinfection needs to be done to prevent microbial growth in the desalinated water and ensure the safety of the general public. Chlorine addition via chlorine gas or sodium hypochlorite is the most common and only requires a dose of 1.5 to 2.5 mg/liter. <ref name="World Health Organization" /> Other alternatives include ozonation, UV light, and chloramination. Chlorine, as a 5% - 15% sodium hypochlorite solution, is sufficient for our process and relatively inexpensive, and as such will be utilized in the process.

===Transport of Brine to Well===
Three major possibilities exist for transporting the brine from our desalination plant to the injection well for disposal: rail, trucking, and a pipeline. A pipeline was chosen as the most feasible and cost-effective approach.

A rail line is not available at this time, and with the cost to construct a rail line at approximately $43 million per mile<ref>About Money. How Much Do Rail Transit Projects Cost to Build and Operate? 2014. Available at: http://publictransport.about.com/od/Transit_Projects/a/How-Much-Do-Rail-Transit-Projects-Cost-To-Build-And-Operate.htm. Accessed January 27, 2016. </ref>, the enormous capital cost makes rail transport infeasible for this site. For large volume operations, pipelines are cheaper than trucking. In a study of wastewater sludge produced from wastewater treatment facilities, it was found that it was more economical for facilities producing over 1200 m3 per day to utilize pipelines rather than trucking.<ref>Marufuzzaman M, Eskioglu S, Hernandez R. Truck versus pipeline transportation cost analysis of wastewater sludge. Transportation Research. 2015; 74(A): 14-30. </ref> Our facility is producing nearly 3 times this volume at 3485 m3 per day of brine. Furthermore, brine is a less viscous fluid than wastewater sludge, decreasing pumping costs in the pipeline and further amplifying the benefit of choosing a pipeline over an injection well.

===Brine Storage===
Due to the choice of a pipeline, rather than trucking, as the method of transport of brine to the injection well, brine storage is unnecessary, so storage solutions such as holding tanks were not investigated. One method of storage compatible with a pipeline is to pump the brine through an evaporation pond. This solution is attractive as it would allow the brine to concentrate, saving some costs of pumping, however, the considerable land usage of evaporation ponds leads them to only be effective for smaller plants. The largest municipal plant utilizing this method produces only 5.7 million liters per day<ref>University of Santa Cruz Center for Integrated Water Research. Concentrate Management: Evaporation Ponds. 2010. Available at: https://ciwr.ucsc.edu/sites/default/files/ICM_D5.pdf. Accessed January 27, 2016. </ref>, whereas our desalination plant is proposed to produce 75.7 million liters per day. Therefore, evaporation ponds have been eliminated from consideration.

==Major Specifications==
===Material of Construction Selection===
While saltwater and desalinated water in general can be corrosive, there are best practices within the desalination industry for material selection. With this in mind, best practice shows carbon steel lifetime can be lengthened by monitoring the oxygen concentration in the water, specifying the paint used to coat any metal, and more expensive materials can be used in areas of extremely high corrosion.<ref name = "Morris" />. By following these best practices, it is possible to save significant amounts of money through the use of carbon steel in most areas of the plant, with specific high-corrosion areas being reinforced with, or replaced with, more corrosion resistant materials, such as Cu/Ni 70/30 alloy<ref name = "KSU Chapter 6" />.

===Heat Exchanger Sizing===
See Appendix F for heat exchanger sizing data. Heat exchanger sizes appear reasonable for placement in the flash vessels.

===Flash Vessel Sizing===
The dimensions for the flash drums were obtained through literature values. The flash drums were determined to be 18x4x3 m in width, height, and length.<ref name="KSU Chapter 6" />

Wall thickness was determined using the following conventions: Design pressure is 10% more than the operating pressure, design temperature is 50°F (27.8°C) higher than the operating temperature, a corrosion allowance of 3-mm and an effectiveness of 85% was derived from the assumption of a double-welded butt joint that was spot checked. See Appendix G for wall thicknesses calculated for each pressure vessel.

The sizing of the vessels to be constructed will be determined after talks with a manufacturer. Due to economies of scale, it may be more cost-effective to buy 21 pressure vessels with the dimensions mentioned above and a wall thickness of 17.1-mm, the highest wall thickness required in the system.

===Pipe Losses and Pressure Drop===
HYSYS was used to estimate the diameter of a pipe given a specified pressure drop per 100m of pipe. It was determined that for a pressure drop of 0.1 kPa/100m of pipe, a 3.05m diameter pipe is sufficient, which is more than manageable for manufacturers. This pressure drop is mostly insignificant and will be taken care of with the safety factors included in the pumps. The significant pressure drops in our system is accounted for via losses in the flash vessels and heat exchangers, which drive the flash process. Therefore overall pipe losses are insignificant and are built into intentional design losses.

=Economic Analysis=
==Economic Assumptions==
The sale of purified water yields a revenue stream of $49.5 million per year assuming sale to commercial entities within the city limits of Corpus Christi at $1.7915 per thousand liters.<ref>Water schedule. City of Corpus Christi Utility Business Office. http://www.cctexas.com/Assets/Departments/Financial-Services/Utilities-Business-Office/files/waterrates.pdf Accessed February 28, 2016.</ref> The primary variable costs are utilities and the brine disposal, with minimal raw materials costs due to the primary feedstock, seawater, being free, and minimal amounts of pre- and post-treatment chemicals being used. The cost of brine disposal used in this analysis was high at $2.097 per thousand liters<ref>McCurdy, R. Underground injection wells for produced water disposal. Chesapeake Energy Corporation. http://www.epa.gov/sites/production/files/documents/21_McCurdy_-_UIC_Disposal_508.pdf Accessed February 28, 2016.</ref>, an estimate for brine disposal utilizing trucking as the mode of transport. As has been shown through previous analyses, a lower cost can be achieved using a direct pipeline, however, no good estimates for this cost could be found.

The NPV was calculated using a 6% cost of capital, the approximate interest rate for a municipal bond.<ref>Zirbe, R.O. Jr., Han, X., Layton, D., Leshine, T. A history of discount rates and their use by government agencies. (2002). http://faculty.washington.edu/zerbe/docs/DISCOUNT%20RATE%20HISTORY-latesXIt.doc Accessed February 28, 2016.</ref> It was assumed that the plant can be built within two years, with production at half-scale in its first year. Additionally, one month of variable costs, $10.6 million, was set aside for working capital. See Appendix H for a full table of assumptions. A 30 year NPV calculation was chosen as the decision basis since 30 years is a conservative estimate for the lifetime of a desalination plant <ref>Tampa Bay Seawater Desalination Plant. American Water. http://www.amwater.com/files/tampabayseawaterdesalinationplantcasestudy6.16.09.pdf. Accessed February 25, 20216.</ref>. See below for estimates of NPV for the lifetime of the plant under various assumptions and design criteria.

==Optimization and Sensitivity Analysis==
Looking at the multi-stage flash process, there are a number of opportunities to optimize the process. The major process variables that were investigated were the inlet water temperature, the operating pressure drop per vessel, and the number of stages over which the process takes place. Considering these optimizations, our final process consists of 21 flash vessels (18 heat recovery and 3 heat rejection stages), each with a pressure drop of 5 kPa. See Appendix D for a screenshot of the HYSYS simulation used to model the optimized process.
===Operating Pressure and Pressure Drop===
Initially our process was designed to pump the inlet seawater and brine to a pressure of 2 bar and allow the pressure drop in each flash stage to end our process downstream at 1 bar to prevent complications associated with designing a system under vacuum. While this provided pumping advantages, the energy in the system was significantly higher due to vapor pressure considerations. Due to this fact, the required heat exchange area for certain exchangers proved to be infeasible (on the order of 80,000m2) due to the enormous capital and operating costs that would be associated with such a large heat exchanger.

The process was optimized to operate under vacuum, requiring additional pumping requirements but much lower required heat exchange area, and therefore lower utility costs. This optimization was done by specifying the pressure of the final flash vessel to 5.7kPa, so that the temperature of the water vapor (~30oC) is within approximately 5oC of the cooling water (35oC) so that condensation can occur within a reasonable approach. This also allows our system to optimally extract pure water from the brine running through the system and reduce energy requirements for cooling the product or the recycle. Our system redesign and optimization reduced the energy in the system and reduce the same heat exchanger area requirements by more than 3 orders of magnitude to approximately 800m2.

Once the process was optimized to operate in a vacuum, the pressure drop per flash stage regulated through values was then optimized. Three pressure drops per stages, 3kPa, 5kPa, and 7kPa, were tested and then an economic analysis was conducted to find the optimal value for the pressure drop. These changes had impacts on both operating and capital costs as they would change initial pumping requirements, the amount of water flowing through the process and recycle (and therefore the flash vessel sizes, heat exchanger sizing), and the efficiency of each stage.

The various pressure drops (3 kPa, 5 kPa, and 7 kPa) were tested on a system nominally at 24 stages under the assumption that the results would be generalizable to systems with a different number of stages. With increasing pressure drop, we found decreased capital cost but increased utility cost. As can be seen from Table 2 below, a pressure drop of 5 kPa proved to be the best option based on 30 year NPV, with a loss of $838.8 million, a savings of $65.5 million compared to the next-best option with a pressure drop of 7 kPa.

<center>
{| class = "wikitable"
|+Table 2: Comparison of profitability for our MSF desalination process with 24 stages with 3 different pressure drops per stage
! Number of Stages  
! Pressure Drop (kPa)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
! 30-Year NPV ($MM)
! NPV Ratio (over 5kPa)
|- style="text-align: center;"
| 24 || 3 || 362.3 || 39.1 || -905.8 || 1.08
|- style="text-align: center;"
| 24 || 5 || 255.1 || 53.6 || -838.8 || 1.00
|- style="text-align: center;"
| 24 || 7 || 209.5 || 72.2 || -904.3 || 1.08
|}
</center>

===Number of Flash Stages===
The number of stages is an important area for optimization. An increased number of stages yields better recovery of purified water, however, requires higher capital cost from the additional pressure vessel. Furthermore, with an increased amount of purified water comes increasingly concentrated brine. This must be taken into account since any brine concentration in excess of about 7% presents a more significant corrosion risk<ref name="KSU Chapter 6" />, so increased maintenance costs for the piping or higher capital costs for more corrosion-resistant materials could be necessary for brine concentrations exceeding 7%. Therefore, to recover the same amount of pure water without exceeding corrosion limitations, increasing the number of stages will reduce the overall size of each stage.

Optimizing based on NPV, it was found that 21 stages is optimal, balancing the increased profits from the additional purified water additional costs of higher capital cost. See Table 3 below to compare the 30-year NPV of the process across various numbers of flash vessels.

<center>
{| class = "wikitable"
|+Table 3: Comparison of profitability for our MSF desalination process with a 5 kPa pressure drop, 5000 m3/hr feed flow rate, with varying number of stages.
! Number of Stages  
! Pressure Drop (kPa)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
! 30-Year NPV ($MM)
! NPV Ratio (over 21 Stages)
|- style="text-align: center;"
| 17 || 5 || 213.9 || 58.7 || -813.8 || 1.016
|- style="text-align: center;"
| 18 || 5 || 218 || 57.6 || -809.9 || 1.011
|- style="text-align: center;"
| 19 || 5 || 224.3 || 58 || -806.1 || 1.007
|- style="text-align: center;"
| 20 || 5 || 229.8 || 57.9 || -813.6 || 1.016
|- style="text-align: center;"
| 21 || 5 || 226.6 || 55.9 || -800.7 || 1
|- style="text-align: center;"
| 22 || 5 || 254.7 || 54.6 || -849.5 || 1.061
|- style="text-align: center;"
| 23 || 5 || 249.3 || 54.1 || -833.1 || 1.04
|- style="text-align: center;"
| 24 || 5 || 255.1 || 53.6 || -838.8 || 1.048
|- style="text-align: center;"
| 25 || 5 || 254.9 || 52.7 || -837.4 || 1.046
|}
</center>

===Inlet Seawater Temperature===
By pairing our desalination plant with a natural gas power plant and utilizing its cooling water we forgo initial heating of our inlet stream. Normally, water pumped from the ocean would be approximately 25oC and by capturing waste heat from the power plant we can reduce utilities with our water inlet of 45oC. Through an economic comparison between two plants nominally at 21 flash stages, the plant with an inlet temperature at 25oC has a capital cost of $245MM and a utility cost of $77MM/yr and the same facility with an inlet water temperature at 45oC has a capital cost of only $226MM and a utility cost of $56MM/yr. As shown in Table 4 below, this $19MM savings in capital costs and $21MM/yr saving in utilities is largely due to reducing the size of the heat exchangers and lowering the steam required for heating duty on the inlet seawater respectively. Therefore our process is optimized by recovering the waste heat water from the gas fired power plant.

<center>
{| class = "wikitable"
|+Table 4: Seawater Inlet Temperature Process Cost Optimization
! Seawater Temperature (°C)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
|- style="text-align: center;"
| 25 || 245 || 77
|- style="text-align: center;"
| 45 || 226 || 56
|- style="text-align: center;"
| Savings || 56 || 19
|}
</center>

==Overall Process Economics==
After optimization at fixed flow rates, the process was scaled to meet the the 67% operating percentage and final product definitions as shown in Table 1, which resulted in finalized economics shown in Table 5 below. For a table of the installed costs of each process vessel, see Appendix I. For the finalized NPV calculations, see Appendix J.

<center>
{| class = "wikitable"
|+Table 5: Summarized Economics for 21-stage MSFD Process
! Economic Parameter
! Cost (USD)
|- style="text-align: center;"
| Capital Cost || 345 MM
|- style="text-align: center;"
| Total Operating Cost per Year || 103 MM
|- style="text-align: center;"
| Utility Cost per Year || 59 MM
|- style="text-align: center;"
| 30-Year NPV (6% Discount) || -1035 MM
|}
</center>

=Conclusions and Recommendations=
==Project Viability==
Table 5 above demonstrates that the process has reasonable operating and capital costs, but the 30-year NPV is negative by -1035 $MM due to the extremely low sell price capability of desalinated water as a product. Therefore, this plant would have to heavily be supported by the City of Corpus Christi to run as the product revenue will not make it self-sufficient. The acquisition of government subsidies are key operating requirements for all desalination facilities in the United States.

==Additional Concerns==
Large volumes of water, steam, and energy are needed to ensure the project runs as specified. While most of the water can easily be extracted from the ocean with little to consequence, the fuel used to generate the steam and electricity can have a significant environmental impact. As the plant is located next to a natural gas power plant, this fuel is better than coal with respect to environmental concerns, and utilizing energy from the power plant to preheat the feed stream for the desalination plant helps lower environmental impacts.

The large amounts of high pressure steam being used represents one of the the largest safety concern in the process, because a high pressure steam release could easily cause severe burns or even death. Proper precautions and control measures must be in place to prevent accidental release. Otherwise, the process does not utilize any known toxic chemicals and is relatively safe from a chemical standpoint. The flash vessels mostly operate below atmospheric pressure, so they must be constructed as vessels that can withstand vacuum without failing.

=Appendix A: Proposed Location of Desalination Plant=
Pictured below is the proposed location for the desalination plant, located on Nueces Bay in Corpus Christi, adjacent to the Nueces Bay Energy Center. The plant’s proposed location is shown in the area outlined by the small circle in the upper right of the figure.

[[File:NuecesBayLocation.png|center|600px|thumb|alt=Nueces Bay Desalination Facility Location|Figure A.1: Proposed mapped location of Nueces Bay Desalination Facility.]]
[[File:ClassIWell.png|center|700px|thumb|alt=Nueces Bay Desalination Facility Distance to Injection Well|Figure A.2: Mapped distance from proposed location to Class I Injection Well]]

=Appendix B: Feed and Product Definitions - Salinity=

<center>
{| class = "wikitable"
|+Table 6: Average Composition of Total Dissolved Solids in Seawater<ref> Physical and Thermodynamic Data. In: Dickson AG, Goyet C. Handbook of Methods for the Analysis of the Various Parameters of the Carbon Dioxide System in Sea Water.</ref><ref> Composition of Seawater. Water Treatment of Solutions. Available at http://www.lenntech.com/composition-seawater.htm. Accessed January 12, 2016.</ref>
! Compound  
! Mass Composition (kPa)
|- style="text-align: center;"
| Chlorine || 55.03%
|- style="text-align: center;"
| Sodium || 30.64%
|- style="text-align: center;"
| Sulfate || 7.70%
|- style="text-align: center;"
| Magnesium || 3.66%
|- style="text-align: center;"
| Calcium || 1.17%
|- style="text-align: center;"
| Potassium || 1.12%
|- style="text-align: center;"
| Bicarbonate || 0.36%
|- style="text-align: center;"
| Bromine || 0.19%
|- style="text-align: center;"
| Other || 0.15%
|}
</center>

[[File:Nueces Bay Salinity.JPG|center|700px|thumb|alt=Nueces Bay Salinity Tracking|Figure C.1: Salinity in Nueces Bay over the past 10 years<ref name = "SALT" />.]]

=Appendix C: Process Flow Diagram=
[[File:TeamF_FinalPFD.JPG|center|1200px|thumb|alt=Process Flow Diagram|Figure C.1: Process Flow Diagram]]

=Appendix D: HYSYS Simulation Model=
[[File:TeamF_HYSYS_Screenshot.PNG|center|1200px|thumb|alt=HYSYS Simulation Model|Figure D.1: Screenshot of HYSYS Model Simulation.]]

=Appendix E: Stream Tables=
A summary of some values is shown below:

<center>
{| class = "wikitable"
|+Table 7: Key process material streams
!  
! Stream  
! Molar Flow (105 kgmole/h)
! Mass Flow (105 kg/h)
|- style="text-align: center;"
! style="text-align: right;" | Inlet
| Feed Water || 454 || 83.6
|- style="text-align: center;"
! style="text-align: right;" | Outlet
| Brine Reject || 1.90 || 36.0
|- style="text-align: center;"
! style="text-align: right;" | Product
| Pure Water || 2.64 || 47.5
|}
</center>

<center>
{| class = "wikitable"
|+Table 8: Key process energy streams
! Stream  
! Heat Flow (kJ/h)
|- style="text-align: center;"
| Feed Water || 1.30E+11
|- style="text-align: center;"
| Q-301 || 5.73E+06
|- style="text-align: center;"
| Q-302 || 3.09E+09
|- style="text-align: center;"
| Q-304 || 4.68E+06
|- style="text-align: center;"
| Q-303 || -1.26E+09
|- style="text-align: center;"
| Brine Reject || -5.47E+10
|- style="text-align: center;"
| Pure Water || -7.46E+10
|}
(Negative value denotes energy leaving process)
</center>

=Appendix F: Heat Exchanger Sizing=

<center>
{| class = "wikitable"
|+Table 9: Utility Temperatures and Heat Transfer Coefficient<ref name="Towler">G.P. Towler, R. Sinnott. Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design. Elsevier, 2012.</ref>
!  
! HP Steam
! Cooling Water
|- style="text-align: center;"
! Temperature (°C)
| 250 || 35
|- style="text-align: center;"
! U (W/m2K)
| 4000 || 1500
|}
</center>

<center>
{| class = "wikitable"
|+Table 10: Internal Flash Heat Exchanger Sizing, condensing steam.
! Exchanger
! UA (kJ/°C-h)
! U (kJ/hr/m2°C)
! A (m2)
|- style="text-align: center;"
| E-101 || 1.47E+07 || 14400 || 1019
|- style="text-align: center;"
| E-102 || 1.53E+07 || 14400 || 1059
|- style="text-align: center;"
| E-103 || 1.59E+07 || 14400 || 1103
|- style="text-align: center;"
| E-104 || 1.66E+07 || 14400 || 1153
|- style="text-align: center;"
| E-105 || 1.74E+07 || 14400 || 1209
|- style="text-align: center;"
| E-106 || 1.83E+07 || 14400 || 1272
|- style="text-align: center;"
| E-107 || 1.93E+07 || 14400 || 1342
|- style="text-align: center;"
| E-108 || 2.05E+07 || 14400 || 1424
|- style="text-align: center;"
| E-109 || 2.19E+07 || 14400 || 1518
|- style="text-align: center;"
| E-110 || 2.35E+07 || 14400 || 1629
|- style="text-align: center;"
| E-111 || 2.54E+07 || 14400 || 1761
|- style="text-align: center;"
| E-112 || 2.77E+07 || 14400 || 1921
|- style="text-align: center;"
| E-113 || 3.05E+07 || 14400 || 2119
|- style="text-align: center;"
| E-114 || 3.42E+07 || 14400 || 2372
|- style="text-align: center;"
| E-115 || 3.90E+07 || 14400 || 2706
|- style="text-align: center;"
| E-116 || 4.56E+07 || 14400 || 3167
|- style="text-align: center;"
| E-117 || 5.55E+07 || 14400 || 3856
|- style="text-align: center;"
| E-118 || 7.20E+07 || 14400 || 5000
|}
</center>

<center>
{| class = "wikitable"
|+Table 11: Heat Exchanger Sizing for Process Heaters and Coolers
! Exchanger
! Q (kJ/h)
! U (J/s/m2K)
! U (kJ/hr/m2K)
! LMTD  
! A (m2)
|- style="text-align: center;"
| E-201 || 9.67E+08 || 1500 || 5400 || 16.03 || 11165
|- style="text-align: center;"
| E-202 || 1.26E+09 || 1500 || 5400 || 7.57 || 30921
|- style="text-align: center;"
| E-203 || 1.88E+09 || 1500 || 5400 || 0.45 || 77366
|- style="text-align: center;"
| E-302 || 3.09E+09 || 4000 || 14400 || 155 || 1385
|- style="text-align: center;"
| E-303 || 1.26E+09 || 4000 || 14400 || 209.5 || 418
|}
The high heat transfer area in the final cooler of the heat rejection stage is due to the close approach temperature.
 Other cooling / pressurization techniques will be used to reduce the area.
</center>

=Appendix G: Pressure Vessel Sizing=

=Appendix H: Expanded Economic Assumptions=

=Appendix I: Equipment Cost Summary=

=Appendix J: Economic NPV Analysis=
[[File:Team F Economics.JPG|center|1200px|thumb|alt=Economic Analysis|Figure J.1: Screenshot of process economic analysis.]]

=References=
<references/>

Nueces Desalination Center: Production of Drinking Water by Multi-Stage Flash Distillation

2016-03-07T21:19:08Z

Benjkg: /* Feed and Product Definitions */

Team F: Rankine 672 Final Report

Authors: Kedric Daly, Ben Granger, Evan Rosati, Kathleen Zhou

Instructors: Fengqi You, David Wegerer

March 11, 2016

=Executive Summary=
Desalination of seawater to drinking water is a technology of ever-increasing importance given trends of increasingly widespread water shortages around the world. Even in the United States, water shortages are becoming increasingly common in dry, highly-populated such as California and certain areas of the Gulf Coast. Our design team, Rankine 672, was tasked with evaluating the need for and designing a desalination plant to provide purified water in a place of need. Investigation revealed that the city of Corpus Christi, Texas, is a strategic location for development of a desalination facility, as surrounding region is in need of new drinking water sources and the city has expressed serious interest in contracting the development of a large-scale desalination plant. Our proposed solution is a multi-stage flash distillation desalination facility capable of producing 20,000,000 gallons (75,708 m3) of purified drinking water for the city and surrounding region per operating day.

In this design, drinking water is produced from seawater feedstock through a 21-stage flash distillation process. By strategically locating the facility next to an existing natural gas power plant, the Nueces Bay Energy Center, pre-heated feed water can be taken directly from the outbound cooling systems of the power plant, allowing energy savings for our desalination plant. The process is designed for 16 hour-per-day operation, requires a seawater feed of 8250 m3/hr, and produces 4758 m3/hr of purified drinking water alongside a waste stream of 3485 m3/hr of concentrated brine. The waste stream is isolated from a recycle stream through a 9.0% purge; this results in an overall product yield of 57.7% on a volume basis. The process requires an additional pretreatment stream of 16.5 kg/hr Belgard EV 2030, which serves as an antiscalant, as well as 9.5 kg/hr chlorine, 352.1 kg/hr lime, and 418.7 kg/hr CO2 for post-treatment purposes.

The overall capital cost of the plant is expected to be $345MM, with a total operating cost of $103MM/yr, $59MM/yr of which comes from utilities. Due to the low price at which the product water can be sold, the 30-year net present value of the plant, assuming a 6% discount rate, is $-1,035MM. It is thus imperative this plant be constructed with public funds because it cannot return a profit to any private investor. The growing need for water however could justify the cost, because it is important that the residents of the greater Corpus Christi area have access to drinkable water.

__TOC__

=Introduction=
==Background==
Clean drinking water is a basic human need that is becoming increasingly difficult to meet. Currently, over 1 billion people live in places where water is considered scarce<ref> Tapping the oceans. The Economist USA. http://www.economist.com/node/11484059. Accessed March 2, 2016. </ref>. Though often thought of as a problem exclusive to third-world countries, the issue is becoming more prevalent in the US as well, with water managers in 40 of 50 states anticipating water shortage conditions in some portion of their state in the next 10 years. These shortages are due to increasing demands on constant or diminishing freshwater resources, therefore, to correct the issue, a new supply must be found<ref> Water Supply in the US. EPA. http://www3.epa.gov/watersense/pubs/supply.html. Accessed March 2, 2016.</ref>. Desalination of seawater is therefore important as a new source for clean drinking water and prevent water shortages.

==Project Definition==
The overall goal of the project is to investigate the feasibility of a desalination plant in order to meet the needs of growing water demand. The design needs to take health, safety, and environmental concerns into account, while optimizing costs.

=Design Basis=
Site Location and Conditions
Corpus Christi, TX was chosen as the location for the proposed desalination plant. The city is currently experiencing “Moderate Shortage Conditions”. This issue additionally comes at a time of growing demand with population growth anticipated due to new industry and Eagle Ford Group crude oil production<ref>Stage 2 Measures in the Drought Contingency Plan. Available at http://www.cctexas.com/government/water/conservation/conservation-drought-plans/stage-2-measures. Accessed January 12, 2016. </ref>.

In June 2014, the city announced the Corpus Christi Desalination Demonstration Project<ref>Corpus Christi Desalination Demonstration Project. Available at http://www.cctexas.com/Assets/Departments/Water/Files/DesalFactSheet.pdf. Published June 2014. Accessed January 12, 2016.
</ref> which seeks to create a demonstration desalination plant capable of producing 20,000 gallons of water per day, with plans to later build a full scale facility generating 20,000,000 gallons of water per day. The desalination demonstration project indicates a clear need for implementation of a desalination plant of this size to accommodate growth in the Corpus Christi area.

Locating our plant in Corpus Christi will allow us to take advantage of the benefits of cogeneration through a partnership with the Nueces Bay Energy Center. By utilizing their outlet cooling water as a large portion of our feed for the desalination process, we can raise the temperature of our seawater feed, saving on energy costs in the multi stage flash distillation. The proposed location for the desalination plant is immediately west of the Nueces Bay Energy Center. See Appendix A for a map depicting the proposed location for the desalination plant along the coast of Nueces Bay.

==Feed and Product Definitions==
The proposed desalination facility’s feed and product streams are defined as shown in Table 1. Rationale for these definitions is also described below. The values reported in Table 1 are final derived numbers from HYSYS simulation modeling.

<center>
{| class = "wikitable"
|+Table 1: Feed and product stream definitions
!  
! Salt Concentration
! Temperature
! Amount
|- style="text-align: center;"
! style="text-align: right;" | Feed Stream
| 30,500 ppm || 45°C || 8250 m3/hr
|- style="text-align: center;"
! style="text-align: right;" | Product Stream
| ~ 0 ppm || 35°C || 4758 m3/hr
|- style="text-align: center;"
! style="text-align: right;" | Byproduct Stream
| 69,500 ppm || 35°C || 3485 m3/hr
|}
</center>

The process feed will be seawater from Nueces Bay, preheated through the cooling water system of the nearby Nueces Bay Energy Center. Based on calculations developed from NREL (National Renewable Energy Laboratory) data, it was estimated that the Nueces Bay Energy Center uses approximately 250 million gallons of seawater per day for process and cooling needs. Therefore, the 20 million gallon per day requirement for the desalination facility can easily be met through the power plant’s cooling water<ref>Torcellini P, Long N, Judkoff R. Consumptive Water Use for U.S. Power Consumption. NREL. 2003; TP-550-33905.
</ref>. Note that we are assuming the process will operate 67% of the time (16 hours per day).

Based on data acquired from two salinity-measuring stations in Nueces Bay, the average salinity of local seawater is 30.5 practical salinity units (psu)<ref>Stations Map & Real - Time Data. The Conrad Blucher Institute for Surveying and Science website. Available at http://www.cbi.tamucc.edu/cbi/data/. Updated January 2016. Accessed January 9, 2016. </ref>, thus setting the concentration of the process feed stream. This translates to a feed stream concentration of 30,500 ppm (parts per million)<ref name = "SALT">Definition and Units. CATD Salinity Expertise Center. Available at http://www.salinityremotesensing.ifremer.fr/home. Accessed January 12, 2016.</ref>. The composition of the salt attributed to this seawater salinity is shown in Table 6 in Appendix B.

No data on the composition of seawater around the Corpus Christi area was available, but the composition of seawater does not change significantly in different locations. The given composition is considered accurate for our feed stream<ref>Brown E, Colling A, Park D, Phillips J, Rothery D, Wright J. Seawater : Its Composition, Properties and Behaviour. Boston: Butterworth-Heinemann, 1995.
</ref>. Note that seawater salt is primarily sodium chloride - total dissolved solids are 85.7% sodium chloride by mass. As such, the dissolved solids in the feed stream is assumed in this project to be pure NaCl, making the feed composition 96.95% water and 3.05% NaCl by mass. A control system will likely be put in place to ensure feed concentration remains at this level, accounting for natural fluctuations in local seawater salinity.

The largest byproduct from the process will be a brine slurry with high salt concentrations. An upper corrosion limit of 700 ppm was set for recirculated and purged brine. This undesired byproduct will be sent to a nearby Class I non-hazardous injection well facility operated by GNI Group, which is approximately 12 miles away from the proposed desalination plant location as shown in the figure in Appendix A<ref>Texas Water Code. Availabe at http://www.statutes.legis.state.tx.us/Docs/WA/htm/WA.27.htm. Accessed January 14, 2016.
</ref><ref>Google Maps. Available at https://goo.gl/maps/3xcvLZwUCe42 Accessed January 15, 2016.</ref>. Through the incorporation of a Class I injection well, the brine will not have to be treated or diluted before injection. <ref>EPA Regulations on Class I Wells. Available at http://www.epa.gov/uic/class-i-industrial-and-municipal-waste-disposal-wells. Accessed January 14, 2016.
</ref><ref>List of Injection Well Facilities by Location. Available at http://www.ehso.com/cssepa/tsdfdeepwells.php. Accessed January 14, 2016.</ref>

There are no regulations regarding the temperature of city drinking water, so the product stream temperature has no requirements based on regulations. Outlet temperatures of 35oC are acceptable for input to the municipal water supply system. Since city water tends to be stored in non-temperature controlled tanks, the temperature will naturally cool to ambient temperature.<ref>Climate Corpus Christi- Texas. US Climate Data. Available at http://www.usclimatedata.com/climate/corpus-christi/texas/united-states/ustx0294. Accessed January 12, 2016. </ref>

==Pre-Treatment and Post-Treatment==
For pretreatment, Belgard EV 2030 is recommended to be added at a concentration of 1.8-2.0 mg/L for a system with top brine temperature approximately 110 °C<ref name="BWA 2030">BWA Water Additives. Belgard EV2030 Technical Data Sheet. BWA Water Additives. http://www.wateradditives.com/files/products/tech_data_sheets/Belgard%20EV%202030_Technical%20Data%20Sheet_Thermal_8.5x11.pdf. Accessed January 27, 2016.
</ref>. Since our process is designed to operate at a slightly higher top brine temperature, 120 °C, we will design for the upper bound concentration: 2.0 mg/L of antiscalant. With a feed flow rate of 8250 m3/hr, the process therefore requires 16.5 kg/hr of Belgard EV 2030.

For disinfection during post-treatment, chlorine addition typically requires a dose of roughly 2.0 mg/L as well<ref name="World Health Organization">World Health Organization. Desalination for safe water supply. http://www.who.int/water_sanitation_health/gdwqrevision/desalination.pdf. Accessed January 26, 2016.</ref>. With a pure water distillate flow rate of 4758 m3/hr, the process requires 9.5 kg/hr of chlorine as a 5%-15% sodium hypochlorite solution. Approximately 74 mg/L of lime (calcium hydroxide) and 88mg/L of CO2 should be added to the desalinated water to help prevent corrosion and increase alkalinity within acceptable ranges<ref name="Vouchkov">Vouchkov, N. Re-Mineralization of Desalinated Water. SunCam. https://s3.amazonaws.com/suncam/npdocs/118.pdf. Accessed February 18, 2016.</ref>. With a product stream flow rate of 4758 m3/hr, these concentrations correspond to 352.1 kg/hr of lime and 418.7 kg/hr of CO2. Both of these can be purchased, and it is likely that the CO2 can be procured from the neighboring natural gas power plant.

==Choice of Desalination Technology==
The two major technologies commonly implemented for desalination include reverse osmosis (RO), and multi-stage flash distillation (MSFD). RO utilizes high pressures to force water through a semipermeable membrane that allows for purified water to pass through, but stops organic and other wastes. Often, the pressures can reach 55 - 68 bar, which requires large pumps to maintain pressure, and introduces safety concerns to the process<ref>Stage 2 Measures in the Drought Contingency Plan. Available at http://www.cctexas.com/government/water/conservation/conservation-drought-plans/stage-2-measures. Accessed January 12, 2016.
</ref>. While RO can be less energy intensive than MSFD, and the membrane can also remove contaminants, it is susceptible to feedwater quality, membrane fouling, and requires more extensive pretreatment and posttreatment than MSFD. The cost of RO is directly proportional to the feed salinity, and so it is less expensive when desalinating low salinity water.

Multi-stage flash distillation is best selected for areas that have high salinity or low energy costs. The pretreatment process for MSFD is much easier when compared to RO because pretreatment often consists of simply adding anti-corrosives and antiscalants to the feed stream. The process works by heating water in vessels at different pressures to change the boiling point of the water and evaporate it as it passes through the system. After evaporation, the water condenses, leaving pure water as the effluent. MSFD costs are now roughly $1.00/m3, where this cost is mostly independent of salinity, but dependent on energy costs.<ref>Fritzman C, Lowenberg J, Wintgens T, Melin T. State-of-the-art Reverse Osmosis Desalination. Desalination. 2007; 216:1-76.
</ref> In both RO and MSFD a brine is produced which then must be appropriately treated and/or disposed of.

Multi-stage flash distillation was selected for this process, mainly due to the economic advantages offered by the selected location of Corpus Christi, TX. MSFD is also a well understood process that can be more easily modeled by chemical engineering software.

=Technical Approach=
Quantitative modeling of the process and all included optimization was conducted using the simulation software Aspen HYSYS. A variety of assumptions were developed to set up and define specific components of the process simulation itself. Within Aspen HYSYS, the NRTL Electrolyte property package was selected to model the behavior of salt water. Each stage of the MSF process is modeled as an individual flash vessel, with individual shell & tube heat exchangers used to model the distillate condensers present in each flash stage in the MSF system.

The HYSYS simulation model only considers the flash portion of the overall process, allowing establishment of mass and energy balances for the system. Pretreatment and post-treatment were excluded for the HYSYS model, as these steps are simple additions or removals of small concentrations of chemical components. Although the simulation models heat exchangers outside of each flash stage, in the physical design of the plant these heat exchangers will be contained within each flash stage.

One portion of the process is not modeled, which is the inlet seawater entering the heat rejection stages. As can be seen from the full process flow diagram (Appendix C), the inlet seawater in the heat rejection stage is in high excess to provide maximum cooling capacity; then a significant portion is purged to bring the stream flow rate down to what is needed for the feed stream. It is assumed that the flow rate of cooling water for those three stages is in such high excess that the cooling water does not heat up. Therefore, in the process simulation models, the inlet stream is the feed stream, which enters directly into the heat recovery portion of the process.

See Appendix D for a screenshot overview of the simulation environment model.

=Process Overview=
==Process Flowsheet==
The following diagram, Figure 1, shows a basic flowsheet for the process. Process alternatives considered for each stage are discussed below. Please see Appendix C for the full process flowsheet with finalized design decisions.

[[File:BlockFlowDiagram.png|center|700px|thumb|alt=Nueces Bay Desalination Facility Block Flow Diagram|Figure 1: Basic block flowsheet of the multi-stage flash desalination process.]]

==Major Features==
As seen above, there are three primary stages of the desalination process: Pre-treatment, the multi-stage flash distillation, and post-treatment. Seawater enters the pre-treatment phase first to make the feed suitable for the rest of the process by filtering out large particles that could accumulate on pipes, deaerating the water to limit corrosion, and adding an agent to combat fouling. The water then enters the distillation process, which is broken up into two sections: heat recovery and heat rejection.

In the heat recovery stage, there are 18 flash vessels, each of which separates steam from the salinated liquid water. Each flash vessel has saltwater as its inlet and two outlets: a vapor outlet (steam) that condenses into the final product of purified water and a liquid outlet composed of water with more dissolved solids than the inlet, which is the inlet stream for the next flash vessel. The pipe containing the feed is first mixed with recycled brine, then goes through the top sections of each of the flash vessels, starting with V-118 and ending up at V-101. In this way, the feed stream pipe first serves as a heat exchanger, heating the pre-treated seawater while simultaneously condensing the steam into the final product. The feed then enters a final heat exchanger where it is brought up to 103.6 °C before entering V-101 to be flashed.

After exiting V-118, the final flash vessel in the heat recovery portion, the water enters the heat rejection portion, which consists of three flash vessels (V-201, V-202, V-203). The flash vessels of this section operate in the same manner as in the heat recovery section, with the exception that condensation occurs using excess cooling water instead of pre-treated seawater. This technique allows excess heat to be rejected into an excess water stream rather than the feed.

The purified product stream finally enters post-treatment processing in which water is re-aerated and remineralized, disinfected, and pH is adjusted before being delivered to the city. The concentrated brine leftover at the end of the flash cycle is recycled into the feed, although a purge is required to continuously remove some salt from the system. This purged brine is disposed of by use of a Class I injection well, per regional regulations.

See Appendix E to find detailed stream tables of stream specifications.

==Process Alternatives==
===Pretreatment===
Multi-stage flash distillation requires significantly less pretreatment than reverse osmosis desalination, but helpful pretreatment techniques still exist. One very inexpensive method of pretreatment is screening/filtration, which is used to filter out large contaminants from intake seawater. Using more complicated membrane technology to accomplish large particle filtration is a possible alternative, but doing so would be much more expensive than using a standard screen or filter, so the latter choice has been selected for this process.<ref>El-Dessouky H., Alatiqi I, Ettouney H. Process synthesis: The multi-stage flash desalination system. Desalination. 1998;115(2):115-179. doi: 10.1016/S0011-9164(98)00035-6.</ref>

The most critical type of fouling in multi-stage flash distillation is scaling - therefore, the most critical step in the pretreatment process will be the inclusion of an anti-scalant. Alkaline scaling can be prevented by acid dosing or polyphosphate addition; however, acid dosing can result in increased corrosion and polyphosphates can lead to non-alkaline calcium phosphate fouling, so these additives will not be considered<ref name="Morris">Morris, R. The development of the multi-stage flash distillation process: A designer's viewpoint. Desalination. 1993;93(1-3):57-68. doi:10.1016/S0011-9164(93)80096-6. Availble at http://www.sciencedirect.com/science/article/pii/0011916493800966</ref><ref>Al-Gobaisi, D. A quarter-century of seawater desalination by large multistage flash plants in Abu Dhabi (Plant performance analysis, assessment, present efforts toward enhancement and future hopes). Desalination. 1994;99(2-3):509-512. doi: 10.1016/S0011-9164(94)00196-0.</ref>. As such, a polymer-based additive is recommended for the anti-scalant for this process. The Belgard EV series is designed for use in high-temperature seawater MSF; of these, Belgard EV 2030 is the most neutral in pH<ref>NSF International. NSF Product and Service Listings: Drinking Water Treatment Chemicals - Health Effects. NSF International. http://info.nsf.org/Certified/PwsChemicals/Listings.asp?Company=00300&. Accessed January 27, 2016.</ref><ref name="BWA 2030" /><ref>BWA Water Additives. Belgard EV2035 Technical Data Sheet. BWA Water Additives. http://www.wateradditives.com/files/products/tech_data_sheets/Belgard%20EV2035_Technical%20Data%20Sheet_Thermal_8.5x11.pdf. Accessed January 27, 2016.</ref><ref>BioLab Water Additives. Belgard EV Antiscalant for seawater distillation plants. BioLab Water Additives. http://www.lpq.com.mx/pdf/BELGARD%20EV.PDF. Accessed January 27, 2016.</ref>. As such, we have selected Belgard EV 2030 for use as an anti-scalant in our process.

Corrosion control is another significant method of pretreatment in multi-stage flash distillation plants. Using a piping material that has limited susceptibility to corrosion is typically not economically viable for MSF desalination plants due their large size, so using carbon steel for most of the piping is generally used for its economic benefits despite its susceptibility to corrosion<ref name = "Morris" />. Limiting oxygen concentration is very feasible through the use of deaerators, which can remove oxygen as well as other dissolved gases from the seawater stream. Often, an external deaeration system is used<ref name = "Morris" /><ref>Edwards - Hick Hargreaves. Seawater Deaerators. Edwards - Hick Hargreaves. http://psscorp.co.th/upload/news/20110830105340.pdf. Accessed January 26, 2016.</ref>. Given these considerations, using an external deaerator combined with a less-corrosive antiscalant like Belgard EV 2030 is much more practical for the process than using anti-corrosive piping.

Excessive foam can also be an issue in some MSF distillation facilities, so antifoaming agents are sometimes employed in the pretreatment process as well. However, many plants are operated without foaming issues, and therefore do not need anti-foaming pretreatment at all. Selection of an anti-foaming agent must be carefully weighed with selected of anti-scalant to avoid issues with additive interactions [26]. At this stage in the design process, an anti-foamer will not be included due to the risk of decreasing scalant efficacy, and the likelihood that foam will not be an issue in the desalination plant.

===Multistage Flash Distillation===
The main objective of the design project is desalinate water. This is done with the main process of multi stage flash distillation (MSF). MSF can be developed using a numerous number of different process configurations, such as Once-Through, Simple Mixer Brine Circulation, and Multi-Stage Heat Rejection Brine Circulation processes.

The most desired process for the project is the Three Stage Heat Rejection Brine Circulation as it builds on benefits of the other simpler technologies. By combining principles of heat rejection and heat recovery, the required heat transfer area of the process is greatly reduced as compared to once through processes or simple brine recycle processes. It has the highest performance ratio with the lowest specific feed flow rates, therefore greatly lowering the operating costs of the process. It is also known as the industry standard for MSF<ref name = "KSU Chapter 6">Kansas State University ChE 413. Available at: http://faculty.ksu.edu.sa/Almutaz/Documents/ChE-413/Multi-stage%20Flash%20Desalination.pdf. Accessed January 26, 2016.</ref>. In addition, since our brine disposal technique involves the pumping of the brine into a Class I well, the high brine outlet salinity is not a drawback for our process since post treatment or dilution of the brine is not necessary.

===Post Treatment===
After processing the water via multi-stage flash, the water needs to be post-treated to make the water taste as consumers would expect, since distilled water is “flat” and unpleasant, as well as to protect the consumers and the piping through which the water will be passing<ref name="Clayton">Clayton R. A review of current knowledge: Desalination for water supply. Foundation for Water Research. http://www.fwr.org/desal.pdf. Accessed January 26, 2016.</ref>. There are multiple alternatives to achieve these objectives of remineralization, aeration, corrosion control, and disinfection.

Aerating and introducing some dissolved solids back into the product water are effective ways to create a quality product that is not flat, and contains flavoring that is expected of the water when delivered to the consumer. Aeration will be accomplished via an inline spray aerator which will take up less space than cascade aeration.<ref>Minnesota Rural Water Association. Aeration. http://www.mrwa.com/WaterWorksMnl/Chapter%2011%20Aeration.pdf. Accessed January 26, 2016.</ref> Feed blending will be used to remineralize the water as opposed to a remineralization filter because it does not require additional equipment purchases, and can easily handle the large amounts of throughput required for the process.<ref name="Clayton" />

Corrosion and pH can be controlled by adding lime (calcium hydroxide) and carbon dioxide to the water. To do so, about 74 mg/L of lime and 88mg/L of CO2 should be added to the desalinated water to help prevent corrosion and increase alkalinity within acceptable ranges. Sodium hydroxide, soda ash, and sodium bicarbonate are alternatives to altering the alkalinity and pH of the product water, but lime and CO2 is an industry standard <ref name="Vouchkov" />.

Disinfection needs to be done to prevent microbial growth in the desalinated water and ensure the safety of the general public. Chlorine addition via chlorine gas or sodium hypochlorite is the most common and only requires a dose of 1.5 to 2.5 mg/liter. <ref name="World Health Organization" /> Other alternatives include ozonation, UV light, and chloramination. Chlorine, as a 5% - 15% sodium hypochlorite solution, is sufficient for our process and relatively inexpensive, and as such will be utilized in the process.

===Transport of Brine to Well===
Three major possibilities exist for transporting the brine from our desalination plant to the injection well for disposal: rail, trucking, and a pipeline. A pipeline was chosen as the most feasible and cost-effective approach.

A rail line is not available at this time, and with the cost to construct a rail line at approximately $43 million per mile<ref>About Money. How Much Do Rail Transit Projects Cost to Build and Operate? 2014. Available at: http://publictransport.about.com/od/Transit_Projects/a/How-Much-Do-Rail-Transit-Projects-Cost-To-Build-And-Operate.htm. Accessed January 27, 2016. </ref>, the enormous capital cost makes rail transport infeasible for this site. For large volume operations, pipelines are cheaper than trucking. In a study of wastewater sludge produced from wastewater treatment facilities, it was found that it was more economical for facilities producing over 1200 m3 per day to utilize pipelines rather than trucking.<ref>Marufuzzaman M, Eskioglu S, Hernandez R. Truck versus pipeline transportation cost analysis of wastewater sludge. Transportation Research. 2015; 74(A): 14-30. </ref> Our facility is producing nearly 3 times this volume at 3485 m3 per day of brine. Furthermore, brine is a less viscous fluid than wastewater sludge, decreasing pumping costs in the pipeline and further amplifying the benefit of choosing a pipeline over an injection well.

===Brine Storage===
Due to the choice of a pipeline, rather than trucking, as the method of transport of brine to the injection well, brine storage is unnecessary, so storage solutions such as holding tanks were not investigated. One method of storage compatible with a pipeline is to pump the brine through an evaporation pond. This solution is attractive as it would allow the brine to concentrate, saving some costs of pumping, however, the considerable land usage of evaporation ponds leads them to only be effective for smaller plants. The largest municipal plant utilizing this method produces only 5.7 million liters per day<ref>University of Santa Cruz Center for Integrated Water Research. Concentrate Management: Evaporation Ponds. 2010. Available at: https://ciwr.ucsc.edu/sites/default/files/ICM_D5.pdf. Accessed January 27, 2016. </ref>, whereas our desalination plant is proposed to produce 75.7 million liters per day. Therefore, evaporation ponds have been eliminated from consideration.

==Major Specifications==
===Material of Construction Selection===
While saltwater and desalinated water in general can be corrosive, there are best practices within the desalination industry for material selection. With this in mind, best practice shows carbon steel lifetime can be lengthened by monitoring the oxygen concentration in the water, specifying the paint used to coat any metal, and more expensive materials can be used in areas of extremely high corrosion.<ref name = "Morris" />. By following these best practices, it is possible to save significant amounts of money through the use of carbon steel in most areas of the plant, with specific high-corrosion areas being reinforced with, or replaced with, more corrosion resistant materials, such as Cu/Ni 70/30 alloy<ref name = "KSU Chapter 6" />.

===Heat Exchanger Sizing===
See Appendix F for heat exchanger sizing data. Heat exchanger sizes appear reasonable for placement in the flash vessels.

===Flash Vessel Sizing===
The dimensions for the flash drums were obtained through literature values. The flash drums were determined to be 18x4x3 m in width, height, and length.<ref name="KSU Chapter 6" />

Wall thickness was determined using the following conventions: Design pressure is 10% more than the operating pressure, design temperature is 50°F (27.8°C) higher than the operating temperature, a corrosion allowance of 3-mm and an effectiveness of 85% was derived from the assumption of a double-welded butt joint that was spot checked. See Appendix G for wall thicknesses calculated for each pressure vessel.

The sizing of the vessels to be constructed will be determined after talks with a manufacturer. Due to economies of scale, it may be more cost-effective to buy 21 pressure vessels with the dimensions mentioned above and a wall thickness of 17.1-mm, the highest wall thickness required in the system.

===Pipe Losses and Pressure Drop===
HYSYS was used to estimate the diameter of a pipe given a specified pressure drop per 100m of pipe. It was determined that for a pressure drop of 0.1 kPa/100m of pipe, a 3.05m diameter pipe is sufficient, which is more than manageable for manufacturers. This pressure drop is mostly insignificant and will be taken care of with the safety factors included in the pumps. The significant pressure drops in our system is accounted for via losses in the flash vessels and heat exchangers, which drive the flash process. Therefore overall pipe losses are insignificant and are built into intentional design losses.

=Economic Analysis=
==Economic Assumptions==
The sale of purified water yields a revenue stream of $49.5 million per year assuming sale to commercial entities within the city limits of Corpus Christi at $1.7915 per thousand liters.<ref>Water schedule. City of Corpus Christi Utility Business Office. http://www.cctexas.com/Assets/Departments/Financial-Services/Utilities-Business-Office/files/waterrates.pdf Accessed February 28, 2016.</ref> The primary variable costs are utilities and the brine disposal, with minimal raw materials costs due to the primary feedstock, seawater, being free, and minimal amounts of pre- and post-treatment chemicals being used. The cost of brine disposal used in this analysis was high at $2.097 per thousand liters<ref>McCurdy, R. Underground injection wells for produced water disposal. Chesapeake Energy Corporation. http://www.epa.gov/sites/production/files/documents/21_McCurdy_-_UIC_Disposal_508.pdf Accessed February 28, 2016.</ref>, an estimate for brine disposal utilizing trucking as the mode of transport. As has been shown through previous analyses, a lower cost can be achieved using a direct pipeline, however, no good estimates for this cost could be found.

The NPV was calculated using a 6% cost of capital, the approximate interest rate for a municipal bond.<ref>Zirbe, R.O. Jr., Han, X., Layton, D., Leshine, T. A history of discount rates and their use by government agencies. (2002). http://faculty.washington.edu/zerbe/docs/DISCOUNT%20RATE%20HISTORY-latesXIt.doc Accessed February 28, 2016.</ref> It was assumed that the plant can be built within two years, with production at half-scale in its first year. Additionally, one month of variable costs, $10.6 million, was set aside for working capital. See Appendix H for a full table of assumptions. A 30 year NPV calculation was chosen as the decision basis since 30 years is a conservative estimate for the lifetime of a desalination plant <ref>Tampa Bay Seawater Desalination Plant. American Water. http://www.amwater.com/files/tampabayseawaterdesalinationplantcasestudy6.16.09.pdf. Accessed February 25, 20216.</ref>. See below for estimates of NPV for the lifetime of the plant under various assumptions and design criteria.

==Optimization and Sensitivity Analysis==
Looking at the multi-stage flash process, there are a number of opportunities to optimize the process. The major process variables that were investigated were the inlet water temperature, the operating pressure drop per vessel, and the number of stages over which the process takes place. Considering these optimizations, our final process consists of 21 flash vessels (18 heat recovery and 3 heat rejection stages), each with a pressure drop of 5 kPa. See Appendix D for a screenshot of the HYSYS simulation used to model the optimized process.
===Operating Pressure and Pressure Drop===
Initially our process was designed to pump the inlet seawater and brine to a pressure of 2 bar and allow the pressure drop in each flash stage to end our process downstream at 1 bar to prevent complications associated with designing a system under vacuum. While this provided pumping advantages, the energy in the system was significantly higher due to vapor pressure considerations. Due to this fact, the required heat exchange area for certain exchangers proved to be infeasible (on the order of 80,000m2) due to the enormous capital and operating costs that would be associated with such a large heat exchanger.

The process was optimized to operate under vacuum, requiring additional pumping requirements but much lower required heat exchange area, and therefore lower utility costs. This optimization was done by specifying the pressure of the final flash vessel to 5.7kPa, so that the temperature of the water vapor (~30oC) is within approximately 5oC of the cooling water (35oC) so that condensation can occur within a reasonable approach. This also allows our system to optimally extract pure water from the brine running through the system and reduce energy requirements for cooling the product or the recycle. Our system redesign and optimization reduced the energy in the system and reduce the same heat exchanger area requirements by more than 3 orders of magnitude to approximately 800m2.

Once the process was optimized to operate in a vacuum, the pressure drop per flash stage regulated through values was then optimized. Three pressure drops per stages, 3kPa, 5kPa, and 7kPa, were tested and then an economic analysis was conducted to find the optimal value for the pressure drop. These changes had impacts on both operating and capital costs as they would change initial pumping requirements, the amount of water flowing through the process and recycle (and therefore the flash vessel sizes, heat exchanger sizing), and the efficiency of each stage.

The various pressure drops (3 kPa, 5 kPa, and 7 kPa) were tested on a system nominally at 24 stages under the assumption that the results would be generalizable to systems with a different number of stages. With increasing pressure drop, we found decreased capital cost but increased utility cost. As can be seen from Table 2 below, a pressure drop of 5 kPa proved to be the best option based on 30 year NPV, with a loss of $838.8 million, a savings of $65.5 million compared to the next-best option with a pressure drop of 7 kPa.

<center>
{| class = "wikitable"
|+Table 2: Comparison of profitability for our MSF desalination process with 24 stages with 3 different pressure drops per stage
! Number of Stages  
! Pressure Drop (kPa)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
! 30-Year NPV ($MM)
! NPV Ratio (over 5kPa)
|- style="text-align: center;"
| 24 || 3 || 362.3 || 39.1 || -905.8 || 1.08
|- style="text-align: center;"
| 24 || 5 || 255.1 || 53.6 || -838.8 || 1.00
|- style="text-align: center;"
| 24 || 7 || 209.5 || 72.2 || -904.3 || 1.08
|}
</center>

===Number of Flash Stages===
The number of stages is an important area for optimization. An increased number of stages yields better recovery of purified water, however, requires higher capital cost from the additional pressure vessel. Furthermore, with an increased amount of purified water comes increasingly concentrated brine. This must be taken into account since any brine concentration in excess of about 7% presents a more significant corrosion risk<ref name="KSU Chapter 6" />, so increased maintenance costs for the piping or higher capital costs for more corrosion-resistant materials could be necessary for brine concentrations exceeding 7%. Therefore, to recover the same amount of pure water without exceeding corrosion limitations, increasing the number of stages will reduce the overall size of each stage.

Optimizing based on NPV, it was found that 21 stages is optimal, balancing the increased profits from the additional purified water additional costs of higher capital cost. See Table 3 below to compare the 30-year NPV of the process across various numbers of flash vessels.

<center>
{| class = "wikitable"
|+Table 3: Comparison of profitability for our MSF desalination process with a 5 kPa pressure drop, 5000 m3/hr feed flow rate, with varying number of stages.
! Number of Stages  
! Pressure Drop (kPa)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
! 30-Year NPV ($MM)
! NPV Ratio (over 21 Stages)
|- style="text-align: center;"
| 17 || 5 || 213.9 || 58.7 || -813.8 || 1.016
|- style="text-align: center;"
| 18 || 5 || 218 || 57.6 || -809.9 || 1.011
|- style="text-align: center;"
| 19 || 5 || 224.3 || 58 || -806.1 || 1.007
|- style="text-align: center;"
| 20 || 5 || 229.8 || 57.9 || -813.6 || 1.016
|- style="text-align: center;"
| 21 || 5 || 226.6 || 55.9 || -800.7 || 1
|- style="text-align: center;"
| 22 || 5 || 254.7 || 54.6 || -849.5 || 1.061
|- style="text-align: center;"
| 23 || 5 || 249.3 || 54.1 || -833.1 || 1.04
|- style="text-align: center;"
| 24 || 5 || 255.1 || 53.6 || -838.8 || 1.048
|- style="text-align: center;"
| 25 || 5 || 254.9 || 52.7 || -837.4 || 1.046
|}
</center>

===Inlet Seawater Temperature===
By pairing our desalination plant with a natural gas power plant and utilizing its cooling water we forgo initial heating of our inlet stream. Normally, water pumped from the ocean would be approximately 25oC and by capturing waste heat from the power plant we can reduce utilities with our water inlet of 45oC. Through an economic comparison between two plants nominally at 21 flash stages, the plant with an inlet temperature at 25oC has a capital cost of $245MM and a utility cost of $77MM/yr and the same facility with an inlet water temperature at 45oC has a capital cost of only $226MM and a utility cost of $56MM/yr. As shown in Table 4 below, this $19MM savings in capital costs and $21MM/yr saving in utilities is largely due to reducing the size of the heat exchangers and lowering the steam required for heating duty on the inlet seawater respectively. Therefore our process is optimized by recovering the waste heat water from the gas fired power plant.

<center>
{| class = "wikitable"
|+Table 4: Seawater Inlet Temperature Process Cost Optimization
! Seawater Temperature (°C)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
|- style="text-align: center;"
| 25 || 245 || 77
|- style="text-align: center;"
| 45 || 226 || 56
|- style="text-align: center;"
| Savings || 56 || 19
|}
</center>

==Overall Process Economics==
After optimization at fixed flow rates, the process was scaled to meet the the 67% operating percentage and final product definitions as shown in Table 1, which resulted in finalized economics shown in Table 5 below. For a table of the installed costs of each process vessel, see Appendix I. For the finalized NPV calculations, see Appendix J.

<center>
{| class = "wikitable"
|+Table 5: Summarized Economics for 21-stage MSFD Process
! Economic Parameter
! Cost (USD)
|- style="text-align: center;"
| Capital Cost || 345 MM
|- style="text-align: center;"
| Total Operating Cost per Year || 103 MM
|- style="text-align: center;"
| Utility Cost per Year || 59 MM
|- style="text-align: center;"
| 30-Year NPV (6% Discount) || -1035 MM
|}
</center>

=Conclusions and Recommendations=
==Project Viability==
Table 5 above demonstrates that the process has reasonable operating and capital costs, but the 30-year NPV is negative by -1035 $MM due to the extremely low sell price capability of desalinated water as a product. Therefore, this plant would have to heavily be supported by the City of Corpus Christi to run as the product revenue will not make it self-sufficient. The acquisition of government subsidies are key operating requirements for all desalination facilities in the United States.

==Additional Concerns==
Large volumes of water, steam, and energy are needed to ensure the project runs as specified. While most of the water can easily be extracted from the ocean with little to consequence, the fuel used to generate the steam and electricity can have a significant environmental impact. As the plant is located next to a natural gas power plant, this fuel is better than coal with respect to environmental concerns, and utilizing energy from the power plant to preheat the feed stream for the desalination plant helps lower environmental impacts.

The large amounts of high pressure steam being used represents one of the the largest safety concern in the process, because a high pressure steam release could easily cause severe burns or even death. Proper precautions and control measures must be in place to prevent accidental release. Otherwise, the process does not utilize any known toxic chemicals and is relatively safe from a chemical standpoint. The flash vessels mostly operate below atmospheric pressure, so they must be constructed as vessels that can withstand vacuum without failing.

=Appendix A: Proposed Location of Desalination Plant=
Pictured below is the proposed location for the desalination plant, located on Nueces Bay in Corpus Christi, adjacent to the Nueces Bay Energy Center. The plant’s proposed location is shown in the area outlined by the small circle in the upper right of the figure.

[[File:NuecesBayLocation.png|center|600px|thumb|alt=Nueces Bay Desalination Facility Location|Figure A.1: Proposed mapped location of Nueces Bay Desalination Facility.]]
[[File:ClassIWell.png|center|700px|thumb|alt=Nueces Bay Desalination Facility Distance to Injection Well|Figure A.2: Mapped distance from proposed location to Class I Injection Well]]

=Appendix B: Feed and Product Definitions - Salinity=

<center>
{| class = "wikitable"
|+Table 6: Average Composition of Total Dissolved Solids in Seawater<ref> Physical and Thermodynamic Data. In: Dickson AG, Goyet C. Handbook of Methods for the Analysis of the Various Parameters of the Carbon Dioxide System in Sea Water.</ref><ref> Composition of Seawater. Water Treatment of Solutions. Available at http://www.lenntech.com/composition-seawater.htm. Accessed January 12, 2016.</ref>
! Compound  
! Mass Composition (kPa)
|- style="text-align: center;"
| Chlorine || 55.03%
|- style="text-align: center;"
| Sodium || 30.64%
|- style="text-align: center;"
| Sulfate || 7.70%
|- style="text-align: center;"
| Magnesium || 3.66%
|- style="text-align: center;"
| Calcium || 1.17%
|- style="text-align: center;"
| Potassium || 1.12%
|- style="text-align: center;"
| Bicarbonate || 0.36%
|- style="text-align: center;"
| Bromine || 0.19%
|- style="text-align: center;"
| Other || 0.15%
|}
</center>

[[File:Nueces Bay Salinity.JPG|center|700px|thumb|alt=Nueces Bay Salinity Tracking|Figure C.1: Salinity in Nueces Bay over the past 10 years<ref>Stations Map & Real - Time Data. The Conrad Blucher Institute for Surveying and Science website. Available at http://www.cbi.tamucc.edu/cbi/data/. Updated January 2016. Accessed January 9, 2016. </ref>.]]

=Appendix C: Process Flow Diagram=
[[File:TeamF_FinalPFD.JPG|center|1200px|thumb|alt=Process Flow Diagram|Figure C.1: Process Flow Diagram]]

=Appendix D: HYSYS Simulation Model=
[[File:TeamF_HYSYS_Screenshot.PNG|center|1200px|thumb|alt=HYSYS Simulation Model|Figure D.1: Screenshot of HYSYS Model Simulation.]]

=Appendix E: Stream Tables=
A summary of some values is shown below:

<center>
{| class = "wikitable"
|+Table 7: Key process material streams
!  
! Stream  
! Molar Flow (105 kgmole/h)
! Mass Flow (105 kg/h)
|- style="text-align: center;"
! style="text-align: right;" | Inlet
| Feed Water || 454 || 83.6
|- style="text-align: center;"
! style="text-align: right;" | Outlet
| Brine Reject || 1.90 || 36.0
|- style="text-align: center;"
! style="text-align: right;" | Product
| Pure Water || 2.64 || 47.5
|}
</center>

<center>
{| class = "wikitable"
|+Table 8: Key process energy streams
! Stream  
! Heat Flow (kJ/h)
|- style="text-align: center;"
| Feed Water || 1.30E+11
|- style="text-align: center;"
| Q-301 || 5.73E+06
|- style="text-align: center;"
| Q-302 || 3.09E+09
|- style="text-align: center;"
| Q-304 || 4.68E+06
|- style="text-align: center;"
| Q-303 || -1.26E+09
|- style="text-align: center;"
| Brine Reject || -5.47E+10
|- style="text-align: center;"
| Pure Water || -7.46E+10
|}
(Negative value denotes energy leaving process)
</center>

=Appendix F: Heat Exchanger Sizing=

<center>
{| class = "wikitable"
|+Table 9: Utility Temperatures and Heat Transfer Coefficient<ref name="Towler">G.P. Towler, R. Sinnott. Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design. Elsevier, 2012.</ref>
!  
! HP Steam
! Cooling Water
|- style="text-align: center;"
! Temperature (°C)
| 250 || 35
|- style="text-align: center;"
! U (W/m2K)
| 4000 || 1500
|}
</center>

<center>
{| class = "wikitable"
|+Table 10: Internal Flash Heat Exchanger Sizing, condensing steam.
! Exchanger
! UA (kJ/°C-h)
! U (kJ/hr/m2°C)
! A (m2)
|- style="text-align: center;"
| E-101 || 1.47E+07 || 14400 || 1019
|- style="text-align: center;"
| E-102 || 1.53E+07 || 14400 || 1059
|- style="text-align: center;"
| E-103 || 1.59E+07 || 14400 || 1103
|- style="text-align: center;"
| E-104 || 1.66E+07 || 14400 || 1153
|- style="text-align: center;"
| E-105 || 1.74E+07 || 14400 || 1209
|- style="text-align: center;"
| E-106 || 1.83E+07 || 14400 || 1272
|- style="text-align: center;"
| E-107 || 1.93E+07 || 14400 || 1342
|- style="text-align: center;"
| E-108 || 2.05E+07 || 14400 || 1424
|- style="text-align: center;"
| E-109 || 2.19E+07 || 14400 || 1518
|- style="text-align: center;"
| E-110 || 2.35E+07 || 14400 || 1629
|- style="text-align: center;"
| E-111 || 2.54E+07 || 14400 || 1761
|- style="text-align: center;"
| E-112 || 2.77E+07 || 14400 || 1921
|- style="text-align: center;"
| E-113 || 3.05E+07 || 14400 || 2119
|- style="text-align: center;"
| E-114 || 3.42E+07 || 14400 || 2372
|- style="text-align: center;"
| E-115 || 3.90E+07 || 14400 || 2706
|- style="text-align: center;"
| E-116 || 4.56E+07 || 14400 || 3167
|- style="text-align: center;"
| E-117 || 5.55E+07 || 14400 || 3856
|- style="text-align: center;"
| E-118 || 7.20E+07 || 14400 || 5000
|}
</center>

<center>
{| class = "wikitable"
|+Table 11: Heat Exchanger Sizing for Process Heaters and Coolers
! Exchanger
! Q (kJ/h)
! U (J/s/m2K)
! U (kJ/hr/m2K)
! LMTD  
! A (m2)
|- style="text-align: center;"
| E-201 || 9.67E+08 || 1500 || 5400 || 16.03 || 11165
|- style="text-align: center;"
| E-202 || 1.26E+09 || 1500 || 5400 || 7.57 || 30921
|- style="text-align: center;"
| E-203 || 1.88E+09 || 1500 || 5400 || 0.45 || 77366
|- style="text-align: center;"
| E-302 || 3.09E+09 || 4000 || 14400 || 155 || 1385
|- style="text-align: center;"
| E-303 || 1.26E+09 || 4000 || 14400 || 209.5 || 418
|}
The high heat transfer area in the final cooler of the heat rejection stage is due to the close approach temperature.
 Other cooling / pressurization techniques will be used to reduce the area.
</center>

=Appendix G: Pressure Vessel Sizing=

=Appendix H: Expanded Economic Assumptions=

=Appendix I: Equipment Cost Summary=

=Appendix J: Economic NPV Analysis=
[[File:Team F Economics.JPG|center|1200px|thumb|alt=Economic Analysis|Figure J.1: Screenshot of process economic analysis.]]

=References=
<references/>

Nueces Desalination Center: Production of Drinking Water by Multi-Stage Flash Distillation

2016-03-07T21:16:48Z

Benjkg: /* Appendix B: Feed and Product Definitions - Salinity */

Team F: Rankine 672 Final Report

Authors: Kedric Daly, Ben Granger, Evan Rosati, Kathleen Zhou

Instructors: Fengqi You, David Wegerer

March 11, 2016

=Executive Summary=
Desalination of seawater to drinking water is a technology of ever-increasing importance given trends of increasingly widespread water shortages around the world. Even in the United States, water shortages are becoming increasingly common in dry, highly-populated such as California and certain areas of the Gulf Coast. Our design team, Rankine 672, was tasked with evaluating the need for and designing a desalination plant to provide purified water in a place of need. Investigation revealed that the city of Corpus Christi, Texas, is a strategic location for development of a desalination facility, as surrounding region is in need of new drinking water sources and the city has expressed serious interest in contracting the development of a large-scale desalination plant. Our proposed solution is a multi-stage flash distillation desalination facility capable of producing 20,000,000 gallons (75,708 m3) of purified drinking water for the city and surrounding region per operating day.

In this design, drinking water is produced from seawater feedstock through a 21-stage flash distillation process. By strategically locating the facility next to an existing natural gas power plant, the Nueces Bay Energy Center, pre-heated feed water can be taken directly from the outbound cooling systems of the power plant, allowing energy savings for our desalination plant. The process is designed for 16 hour-per-day operation, requires a seawater feed of 8250 m3/hr, and produces 4758 m3/hr of purified drinking water alongside a waste stream of 3485 m3/hr of concentrated brine. The waste stream is isolated from a recycle stream through a 9.0% purge; this results in an overall product yield of 57.7% on a volume basis. The process requires an additional pretreatment stream of 16.5 kg/hr Belgard EV 2030, which serves as an antiscalant, as well as 9.5 kg/hr chlorine, 352.1 kg/hr lime, and 418.7 kg/hr CO2 for post-treatment purposes.

The overall capital cost of the plant is expected to be $345MM, with a total operating cost of $103MM/yr, $59MM/yr of which comes from utilities. Due to the low price at which the product water can be sold, the 30-year net present value of the plant, assuming a 6% discount rate, is $-1,035MM. It is thus imperative this plant be constructed with public funds because it cannot return a profit to any private investor. The growing need for water however could justify the cost, because it is important that the residents of the greater Corpus Christi area have access to drinkable water.

__TOC__

=Introduction=
==Background==
Clean drinking water is a basic human need that is becoming increasingly difficult to meet. Currently, over 1 billion people live in places where water is considered scarce<ref> Tapping the oceans. The Economist USA. http://www.economist.com/node/11484059. Accessed March 2, 2016. </ref>. Though often thought of as a problem exclusive to third-world countries, the issue is becoming more prevalent in the US as well, with water managers in 40 of 50 states anticipating water shortage conditions in some portion of their state in the next 10 years. These shortages are due to increasing demands on constant or diminishing freshwater resources, therefore, to correct the issue, a new supply must be found<ref> Water Supply in the US. EPA. http://www3.epa.gov/watersense/pubs/supply.html. Accessed March 2, 2016.</ref>. Desalination of seawater is therefore important as a new source for clean drinking water and prevent water shortages.

==Project Definition==
The overall goal of the project is to investigate the feasibility of a desalination plant in order to meet the needs of growing water demand. The design needs to take health, safety, and environmental concerns into account, while optimizing costs.

=Design Basis=
Site Location and Conditions
Corpus Christi, TX was chosen as the location for the proposed desalination plant. The city is currently experiencing “Moderate Shortage Conditions”. This issue additionally comes at a time of growing demand with population growth anticipated due to new industry and Eagle Ford Group crude oil production<ref>Stage 2 Measures in the Drought Contingency Plan. Available at http://www.cctexas.com/government/water/conservation/conservation-drought-plans/stage-2-measures. Accessed January 12, 2016. </ref>.

In June 2014, the city announced the Corpus Christi Desalination Demonstration Project<ref>Corpus Christi Desalination Demonstration Project. Available at http://www.cctexas.com/Assets/Departments/Water/Files/DesalFactSheet.pdf. Published June 2014. Accessed January 12, 2016.
</ref> which seeks to create a demonstration desalination plant capable of producing 20,000 gallons of water per day, with plans to later build a full scale facility generating 20,000,000 gallons of water per day. The desalination demonstration project indicates a clear need for implementation of a desalination plant of this size to accommodate growth in the Corpus Christi area.

Locating our plant in Corpus Christi will allow us to take advantage of the benefits of cogeneration through a partnership with the Nueces Bay Energy Center. By utilizing their outlet cooling water as a large portion of our feed for the desalination process, we can raise the temperature of our seawater feed, saving on energy costs in the multi stage flash distillation. The proposed location for the desalination plant is immediately west of the Nueces Bay Energy Center. See Appendix A for a map depicting the proposed location for the desalination plant along the coast of Nueces Bay.

==Feed and Product Definitions==
The proposed desalination facility’s feed and product streams are defined as shown in Table 1. Rationale for these definitions is also described below. The values reported in Table 1 are final derived numbers from HYSYS simulation modeling.

<center>
{| class = "wikitable"
|+Table 1: Feed and product stream definitions
!  
! Salt Concentration
! Temperature
! Amount
|- style="text-align: center;"
! style="text-align: right;" | Feed Stream
| 30,500 ppm || 45°C || 8250 m3/hr
|- style="text-align: center;"
! style="text-align: right;" | Product Stream
| ~ 0 ppm || 35°C || 4758 m3/hr
|- style="text-align: center;"
! style="text-align: right;" | Byproduct Stream
| 69,500 ppm || 35°C || 3485 m3/hr
|}
</center>

The process feed will be seawater from Nueces Bay, preheated through the cooling water system of the nearby Nueces Bay Energy Center. Based on calculations developed from NREL (National Renewable Energy Laboratory) data, it was estimated that the Nueces Bay Energy Center uses approximately 250 million gallons of seawater per day for process and cooling needs. Therefore, the 20 million gallon per day requirement for the desalination facility can easily be met through the power plant’s cooling water<ref>Torcellini P, Long N, Judkoff R. Consumptive Water Use for U.S. Power Consumption. NREL. 2003; TP-550-33905.
</ref>. Note that we are assuming the process will operate 67% of the time (16 hours per day).

Based on data acquired from two salinity-measuring stations in Nueces Bay, the average salinity of local seawater is 30.5 practical salinity units (psu)<ref>Stations Map & Real - Time Data. The Conrad Blucher Institute for Surveying and Science website. Available at http://www.cbi.tamucc.edu/cbi/data/. Updated January 2016. Accessed January 9, 2016. </ref>, thus setting the concentration of the process feed stream. This translates to a feed stream concentration of 30,500 ppm (parts per million)<ref>Definition and Units. CATD Salinity Expertise Center. Available at http://www.salinityremotesensing.ifremer.fr/home. Accessed January 12, 2016.</ref>. The composition of the salt attributed to this seawater salinity is shown in Table 6 in Appendix B.

No data on the composition of seawater around the Corpus Christi area was available, but the composition of seawater does not change significantly in different locations. The given composition is considered accurate for our feed stream<ref>Brown E, Colling A, Park D, Phillips J, Rothery D, Wright J. Seawater : Its Composition, Properties and Behaviour. Boston: Butterworth-Heinemann, 1995.
</ref>. Note that seawater salt is primarily sodium chloride - total dissolved solids are 85.7% sodium chloride by mass. As such, the dissolved solids in the feed stream is assumed in this project to be pure NaCl, making the feed composition 96.95% water and 3.05% NaCl by mass. A control system will likely be put in place to ensure feed concentration remains at this level, accounting for natural fluctuations in local seawater salinity.

The largest byproduct from the process will be a brine slurry with high salt concentrations. An upper corrosion limit of 700 ppm was set for recirculated and purged brine. This undesired byproduct will be sent to a nearby Class I non-hazardous injection well facility operated by GNI Group, which is approximately 12 miles away from the proposed desalination plant location as shown in the figure in Appendix A<ref>Texas Water Code. Availabe at http://www.statutes.legis.state.tx.us/Docs/WA/htm/WA.27.htm. Accessed January 14, 2016.
</ref><ref>Google Maps. Available at https://goo.gl/maps/3xcvLZwUCe42 Accessed January 15, 2016.</ref>. Through the incorporation of a Class I injection well, the brine will not have to be treated or diluted before injection. <ref>EPA Regulations on Class I Wells. Available at http://www.epa.gov/uic/class-i-industrial-and-municipal-waste-disposal-wells. Accessed January 14, 2016.
</ref><ref>List of Injection Well Facilities by Location. Available at http://www.ehso.com/cssepa/tsdfdeepwells.php. Accessed January 14, 2016.</ref>

There are no regulations regarding the temperature of city drinking water, so the product stream temperature has no requirements based on regulations. Outlet temperatures of 35oC are acceptable for input to the municipal water supply system. Since city water tends to be stored in non-temperature controlled tanks, the temperature will naturally cool to ambient temperature.<ref>Climate Corpus Christi- Texas. US Climate Data. Available at http://www.usclimatedata.com/climate/corpus-christi/texas/united-states/ustx0294. Accessed January 12, 2016. </ref>

==Pre-Treatment and Post-Treatment==
For pretreatment, Belgard EV 2030 is recommended to be added at a concentration of 1.8-2.0 mg/L for a system with top brine temperature approximately 110 °C<ref name="BWA 2030">BWA Water Additives. Belgard EV2030 Technical Data Sheet. BWA Water Additives. http://www.wateradditives.com/files/products/tech_data_sheets/Belgard%20EV%202030_Technical%20Data%20Sheet_Thermal_8.5x11.pdf. Accessed January 27, 2016.
</ref>. Since our process is designed to operate at a slightly higher top brine temperature, 120 °C, we will design for the upper bound concentration: 2.0 mg/L of antiscalant. With a feed flow rate of 8250 m3/hr, the process therefore requires 16.5 kg/hr of Belgard EV 2030.

For disinfection during post-treatment, chlorine addition typically requires a dose of roughly 2.0 mg/L as well<ref name="World Health Organization">World Health Organization. Desalination for safe water supply. http://www.who.int/water_sanitation_health/gdwqrevision/desalination.pdf. Accessed January 26, 2016.</ref>. With a pure water distillate flow rate of 4758 m3/hr, the process requires 9.5 kg/hr of chlorine as a 5%-15% sodium hypochlorite solution. Approximately 74 mg/L of lime (calcium hydroxide) and 88mg/L of CO2 should be added to the desalinated water to help prevent corrosion and increase alkalinity within acceptable ranges<ref name="Vouchkov">Vouchkov, N. Re-Mineralization of Desalinated Water. SunCam. https://s3.amazonaws.com/suncam/npdocs/118.pdf. Accessed February 18, 2016.</ref>. With a product stream flow rate of 4758 m3/hr, these concentrations correspond to 352.1 kg/hr of lime and 418.7 kg/hr of CO2. Both of these can be purchased, and it is likely that the CO2 can be procured from the neighboring natural gas power plant.

==Choice of Desalination Technology==
The two major technologies commonly implemented for desalination include reverse osmosis (RO), and multi-stage flash distillation (MSFD). RO utilizes high pressures to force water through a semipermeable membrane that allows for purified water to pass through, but stops organic and other wastes. Often, the pressures can reach 55 - 68 bar, which requires large pumps to maintain pressure, and introduces safety concerns to the process<ref>Stage 2 Measures in the Drought Contingency Plan. Available at http://www.cctexas.com/government/water/conservation/conservation-drought-plans/stage-2-measures. Accessed January 12, 2016.
</ref>. While RO can be less energy intensive than MSFD, and the membrane can also remove contaminants, it is susceptible to feedwater quality, membrane fouling, and requires more extensive pretreatment and posttreatment than MSFD. The cost of RO is directly proportional to the feed salinity, and so it is less expensive when desalinating low salinity water.

Multi-stage flash distillation is best selected for areas that have high salinity or low energy costs. The pretreatment process for MSFD is much easier when compared to RO because pretreatment often consists of simply adding anti-corrosives and antiscalants to the feed stream. The process works by heating water in vessels at different pressures to change the boiling point of the water and evaporate it as it passes through the system. After evaporation, the water condenses, leaving pure water as the effluent. MSFD costs are now roughly $1.00/m3, where this cost is mostly independent of salinity, but dependent on energy costs.<ref>Fritzman C, Lowenberg J, Wintgens T, Melin T. State-of-the-art Reverse Osmosis Desalination. Desalination. 2007; 216:1-76.
</ref> In both RO and MSFD a brine is produced which then must be appropriately treated and/or disposed of.

Multi-stage flash distillation was selected for this process, mainly due to the economic advantages offered by the selected location of Corpus Christi, TX. MSFD is also a well understood process that can be more easily modeled by chemical engineering software.

=Technical Approach=
Quantitative modeling of the process and all included optimization was conducted using the simulation software Aspen HYSYS. A variety of assumptions were developed to set up and define specific components of the process simulation itself. Within Aspen HYSYS, the NRTL Electrolyte property package was selected to model the behavior of salt water. Each stage of the MSF process is modeled as an individual flash vessel, with individual shell & tube heat exchangers used to model the distillate condensers present in each flash stage in the MSF system.

The HYSYS simulation model only considers the flash portion of the overall process, allowing establishment of mass and energy balances for the system. Pretreatment and post-treatment were excluded for the HYSYS model, as these steps are simple additions or removals of small concentrations of chemical components. Although the simulation models heat exchangers outside of each flash stage, in the physical design of the plant these heat exchangers will be contained within each flash stage.

One portion of the process is not modeled, which is the inlet seawater entering the heat rejection stages. As can be seen from the full process flow diagram (Appendix C), the inlet seawater in the heat rejection stage is in high excess to provide maximum cooling capacity; then a significant portion is purged to bring the stream flow rate down to what is needed for the feed stream. It is assumed that the flow rate of cooling water for those three stages is in such high excess that the cooling water does not heat up. Therefore, in the process simulation models, the inlet stream is the feed stream, which enters directly into the heat recovery portion of the process.

See Appendix D for a screenshot overview of the simulation environment model.

=Process Overview=
==Process Flowsheet==
The following diagram, Figure 1, shows a basic flowsheet for the process. Process alternatives considered for each stage are discussed below. Please see Appendix C for the full process flowsheet with finalized design decisions.

[[File:BlockFlowDiagram.png|center|700px|thumb|alt=Nueces Bay Desalination Facility Block Flow Diagram|Figure 1: Basic block flowsheet of the multi-stage flash desalination process.]]

==Major Features==
As seen above, there are three primary stages of the desalination process: Pre-treatment, the multi-stage flash distillation, and post-treatment. Seawater enters the pre-treatment phase first to make the feed suitable for the rest of the process by filtering out large particles that could accumulate on pipes, deaerating the water to limit corrosion, and adding an agent to combat fouling. The water then enters the distillation process, which is broken up into two sections: heat recovery and heat rejection.

In the heat recovery stage, there are 18 flash vessels, each of which separates steam from the salinated liquid water. Each flash vessel has saltwater as its inlet and two outlets: a vapor outlet (steam) that condenses into the final product of purified water and a liquid outlet composed of water with more dissolved solids than the inlet, which is the inlet stream for the next flash vessel. The pipe containing the feed is first mixed with recycled brine, then goes through the top sections of each of the flash vessels, starting with V-118 and ending up at V-101. In this way, the feed stream pipe first serves as a heat exchanger, heating the pre-treated seawater while simultaneously condensing the steam into the final product. The feed then enters a final heat exchanger where it is brought up to 103.6 °C before entering V-101 to be flashed.

After exiting V-118, the final flash vessel in the heat recovery portion, the water enters the heat rejection portion, which consists of three flash vessels (V-201, V-202, V-203). The flash vessels of this section operate in the same manner as in the heat recovery section, with the exception that condensation occurs using excess cooling water instead of pre-treated seawater. This technique allows excess heat to be rejected into an excess water stream rather than the feed.

The purified product stream finally enters post-treatment processing in which water is re-aerated and remineralized, disinfected, and pH is adjusted before being delivered to the city. The concentrated brine leftover at the end of the flash cycle is recycled into the feed, although a purge is required to continuously remove some salt from the system. This purged brine is disposed of by use of a Class I injection well, per regional regulations.

See Appendix E to find detailed stream tables of stream specifications.

==Process Alternatives==
===Pretreatment===
Multi-stage flash distillation requires significantly less pretreatment than reverse osmosis desalination, but helpful pretreatment techniques still exist. One very inexpensive method of pretreatment is screening/filtration, which is used to filter out large contaminants from intake seawater. Using more complicated membrane technology to accomplish large particle filtration is a possible alternative, but doing so would be much more expensive than using a standard screen or filter, so the latter choice has been selected for this process.<ref>El-Dessouky H., Alatiqi I, Ettouney H. Process synthesis: The multi-stage flash desalination system. Desalination. 1998;115(2):115-179. doi: 10.1016/S0011-9164(98)00035-6.</ref>

The most critical type of fouling in multi-stage flash distillation is scaling - therefore, the most critical step in the pretreatment process will be the inclusion of an anti-scalant. Alkaline scaling can be prevented by acid dosing or polyphosphate addition; however, acid dosing can result in increased corrosion and polyphosphates can lead to non-alkaline calcium phosphate fouling, so these additives will not be considered<ref name="Morris">Morris, R. The development of the multi-stage flash distillation process: A designer's viewpoint. Desalination. 1993;93(1-3):57-68. doi:10.1016/S0011-9164(93)80096-6. Availble at http://www.sciencedirect.com/science/article/pii/0011916493800966</ref><ref>Al-Gobaisi, D. A quarter-century of seawater desalination by large multistage flash plants in Abu Dhabi (Plant performance analysis, assessment, present efforts toward enhancement and future hopes). Desalination. 1994;99(2-3):509-512. doi: 10.1016/S0011-9164(94)00196-0.</ref>. As such, a polymer-based additive is recommended for the anti-scalant for this process. The Belgard EV series is designed for use in high-temperature seawater MSF; of these, Belgard EV 2030 is the most neutral in pH<ref>NSF International. NSF Product and Service Listings: Drinking Water Treatment Chemicals - Health Effects. NSF International. http://info.nsf.org/Certified/PwsChemicals/Listings.asp?Company=00300&. Accessed January 27, 2016.</ref><ref name="BWA 2030" /><ref>BWA Water Additives. Belgard EV2035 Technical Data Sheet. BWA Water Additives. http://www.wateradditives.com/files/products/tech_data_sheets/Belgard%20EV2035_Technical%20Data%20Sheet_Thermal_8.5x11.pdf. Accessed January 27, 2016.</ref><ref>BioLab Water Additives. Belgard EV Antiscalant for seawater distillation plants. BioLab Water Additives. http://www.lpq.com.mx/pdf/BELGARD%20EV.PDF. Accessed January 27, 2016.</ref>. As such, we have selected Belgard EV 2030 for use as an anti-scalant in our process.

Corrosion control is another significant method of pretreatment in multi-stage flash distillation plants. Using a piping material that has limited susceptibility to corrosion is typically not economically viable for MSF desalination plants due their large size, so using carbon steel for most of the piping is generally used for its economic benefits despite its susceptibility to corrosion<ref name = "Morris" />. Limiting oxygen concentration is very feasible through the use of deaerators, which can remove oxygen as well as other dissolved gases from the seawater stream. Often, an external deaeration system is used<ref name = "Morris" /><ref>Edwards - Hick Hargreaves. Seawater Deaerators. Edwards - Hick Hargreaves. http://psscorp.co.th/upload/news/20110830105340.pdf. Accessed January 26, 2016.</ref>. Given these considerations, using an external deaerator combined with a less-corrosive antiscalant like Belgard EV 2030 is much more practical for the process than using anti-corrosive piping.

Excessive foam can also be an issue in some MSF distillation facilities, so antifoaming agents are sometimes employed in the pretreatment process as well. However, many plants are operated without foaming issues, and therefore do not need anti-foaming pretreatment at all. Selection of an anti-foaming agent must be carefully weighed with selected of anti-scalant to avoid issues with additive interactions [26]. At this stage in the design process, an anti-foamer will not be included due to the risk of decreasing scalant efficacy, and the likelihood that foam will not be an issue in the desalination plant.

===Multistage Flash Distillation===
The main objective of the design project is desalinate water. This is done with the main process of multi stage flash distillation (MSF). MSF can be developed using a numerous number of different process configurations, such as Once-Through, Simple Mixer Brine Circulation, and Multi-Stage Heat Rejection Brine Circulation processes.

The most desired process for the project is the Three Stage Heat Rejection Brine Circulation as it builds on benefits of the other simpler technologies. By combining principles of heat rejection and heat recovery, the required heat transfer area of the process is greatly reduced as compared to once through processes or simple brine recycle processes. It has the highest performance ratio with the lowest specific feed flow rates, therefore greatly lowering the operating costs of the process. It is also known as the industry standard for MSF<ref name = "KSU Chapter 6">Kansas State University ChE 413. Available at: http://faculty.ksu.edu.sa/Almutaz/Documents/ChE-413/Multi-stage%20Flash%20Desalination.pdf. Accessed January 26, 2016.</ref>. In addition, since our brine disposal technique involves the pumping of the brine into a Class I well, the high brine outlet salinity is not a drawback for our process since post treatment or dilution of the brine is not necessary.

===Post Treatment===
After processing the water via multi-stage flash, the water needs to be post-treated to make the water taste as consumers would expect, since distilled water is “flat” and unpleasant, as well as to protect the consumers and the piping through which the water will be passing<ref name="Clayton">Clayton R. A review of current knowledge: Desalination for water supply. Foundation for Water Research. http://www.fwr.org/desal.pdf. Accessed January 26, 2016.</ref>. There are multiple alternatives to achieve these objectives of remineralization, aeration, corrosion control, and disinfection.

Aerating and introducing some dissolved solids back into the product water are effective ways to create a quality product that is not flat, and contains flavoring that is expected of the water when delivered to the consumer. Aeration will be accomplished via an inline spray aerator which will take up less space than cascade aeration.<ref>Minnesota Rural Water Association. Aeration. http://www.mrwa.com/WaterWorksMnl/Chapter%2011%20Aeration.pdf. Accessed January 26, 2016.</ref> Feed blending will be used to remineralize the water as opposed to a remineralization filter because it does not require additional equipment purchases, and can easily handle the large amounts of throughput required for the process.<ref name="Clayton" />

Corrosion and pH can be controlled by adding lime (calcium hydroxide) and carbon dioxide to the water. To do so, about 74 mg/L of lime and 88mg/L of CO2 should be added to the desalinated water to help prevent corrosion and increase alkalinity within acceptable ranges. Sodium hydroxide, soda ash, and sodium bicarbonate are alternatives to altering the alkalinity and pH of the product water, but lime and CO2 is an industry standard <ref name="Vouchkov" />.

Disinfection needs to be done to prevent microbial growth in the desalinated water and ensure the safety of the general public. Chlorine addition via chlorine gas or sodium hypochlorite is the most common and only requires a dose of 1.5 to 2.5 mg/liter. <ref name="World Health Organization" /> Other alternatives include ozonation, UV light, and chloramination. Chlorine, as a 5% - 15% sodium hypochlorite solution, is sufficient for our process and relatively inexpensive, and as such will be utilized in the process.

===Transport of Brine to Well===
Three major possibilities exist for transporting the brine from our desalination plant to the injection well for disposal: rail, trucking, and a pipeline. A pipeline was chosen as the most feasible and cost-effective approach.

A rail line is not available at this time, and with the cost to construct a rail line at approximately $43 million per mile<ref>About Money. How Much Do Rail Transit Projects Cost to Build and Operate? 2014. Available at: http://publictransport.about.com/od/Transit_Projects/a/How-Much-Do-Rail-Transit-Projects-Cost-To-Build-And-Operate.htm. Accessed January 27, 2016. </ref>, the enormous capital cost makes rail transport infeasible for this site. For large volume operations, pipelines are cheaper than trucking. In a study of wastewater sludge produced from wastewater treatment facilities, it was found that it was more economical for facilities producing over 1200 m3 per day to utilize pipelines rather than trucking.<ref>Marufuzzaman M, Eskioglu S, Hernandez R. Truck versus pipeline transportation cost analysis of wastewater sludge. Transportation Research. 2015; 74(A): 14-30. </ref> Our facility is producing nearly 3 times this volume at 3485 m3 per day of brine. Furthermore, brine is a less viscous fluid than wastewater sludge, decreasing pumping costs in the pipeline and further amplifying the benefit of choosing a pipeline over an injection well.

===Brine Storage===
Due to the choice of a pipeline, rather than trucking, as the method of transport of brine to the injection well, brine storage is unnecessary, so storage solutions such as holding tanks were not investigated. One method of storage compatible with a pipeline is to pump the brine through an evaporation pond. This solution is attractive as it would allow the brine to concentrate, saving some costs of pumping, however, the considerable land usage of evaporation ponds leads them to only be effective for smaller plants. The largest municipal plant utilizing this method produces only 5.7 million liters per day<ref>University of Santa Cruz Center for Integrated Water Research. Concentrate Management: Evaporation Ponds. 2010. Available at: https://ciwr.ucsc.edu/sites/default/files/ICM_D5.pdf. Accessed January 27, 2016. </ref>, whereas our desalination plant is proposed to produce 75.7 million liters per day. Therefore, evaporation ponds have been eliminated from consideration.

==Major Specifications==
===Material of Construction Selection===
While saltwater and desalinated water in general can be corrosive, there are best practices within the desalination industry for material selection. With this in mind, best practice shows carbon steel lifetime can be lengthened by monitoring the oxygen concentration in the water, specifying the paint used to coat any metal, and more expensive materials can be used in areas of extremely high corrosion.<ref name = "Morris" />. By following these best practices, it is possible to save significant amounts of money through the use of carbon steel in most areas of the plant, with specific high-corrosion areas being reinforced with, or replaced with, more corrosion resistant materials, such as Cu/Ni 70/30 alloy<ref name = "KSU Chapter 6" />.

===Heat Exchanger Sizing===
See Appendix F for heat exchanger sizing data. Heat exchanger sizes appear reasonable for placement in the flash vessels.

===Flash Vessel Sizing===
The dimensions for the flash drums were obtained through literature values. The flash drums were determined to be 18x4x3 m in width, height, and length.<ref name="KSU Chapter 6" />

Wall thickness was determined using the following conventions: Design pressure is 10% more than the operating pressure, design temperature is 50°F (27.8°C) higher than the operating temperature, a corrosion allowance of 3-mm and an effectiveness of 85% was derived from the assumption of a double-welded butt joint that was spot checked. See Appendix G for wall thicknesses calculated for each pressure vessel.

The sizing of the vessels to be constructed will be determined after talks with a manufacturer. Due to economies of scale, it may be more cost-effective to buy 21 pressure vessels with the dimensions mentioned above and a wall thickness of 17.1-mm, the highest wall thickness required in the system.

===Pipe Losses and Pressure Drop===
HYSYS was used to estimate the diameter of a pipe given a specified pressure drop per 100m of pipe. It was determined that for a pressure drop of 0.1 kPa/100m of pipe, a 3.05m diameter pipe is sufficient, which is more than manageable for manufacturers. This pressure drop is mostly insignificant and will be taken care of with the safety factors included in the pumps. The significant pressure drops in our system is accounted for via losses in the flash vessels and heat exchangers, which drive the flash process. Therefore overall pipe losses are insignificant and are built into intentional design losses.

=Economic Analysis=
==Economic Assumptions==
The sale of purified water yields a revenue stream of $49.5 million per year assuming sale to commercial entities within the city limits of Corpus Christi at $1.7915 per thousand liters.<ref>Water schedule. City of Corpus Christi Utility Business Office. http://www.cctexas.com/Assets/Departments/Financial-Services/Utilities-Business-Office/files/waterrates.pdf Accessed February 28, 2016.</ref> The primary variable costs are utilities and the brine disposal, with minimal raw materials costs due to the primary feedstock, seawater, being free, and minimal amounts of pre- and post-treatment chemicals being used. The cost of brine disposal used in this analysis was high at $2.097 per thousand liters<ref>McCurdy, R. Underground injection wells for produced water disposal. Chesapeake Energy Corporation. http://www.epa.gov/sites/production/files/documents/21_McCurdy_-_UIC_Disposal_508.pdf Accessed February 28, 2016.</ref>, an estimate for brine disposal utilizing trucking as the mode of transport. As has been shown through previous analyses, a lower cost can be achieved using a direct pipeline, however, no good estimates for this cost could be found.

The NPV was calculated using a 6% cost of capital, the approximate interest rate for a municipal bond.<ref>Zirbe, R.O. Jr., Han, X., Layton, D., Leshine, T. A history of discount rates and their use by government agencies. (2002). http://faculty.washington.edu/zerbe/docs/DISCOUNT%20RATE%20HISTORY-latesXIt.doc Accessed February 28, 2016.</ref> It was assumed that the plant can be built within two years, with production at half-scale in its first year. Additionally, one month of variable costs, $10.6 million, was set aside for working capital. See Appendix H for a full table of assumptions. A 30 year NPV calculation was chosen as the decision basis since 30 years is a conservative estimate for the lifetime of a desalination plant <ref>Tampa Bay Seawater Desalination Plant. American Water. http://www.amwater.com/files/tampabayseawaterdesalinationplantcasestudy6.16.09.pdf. Accessed February 25, 20216.</ref>. See below for estimates of NPV for the lifetime of the plant under various assumptions and design criteria.

==Optimization and Sensitivity Analysis==
Looking at the multi-stage flash process, there are a number of opportunities to optimize the process. The major process variables that were investigated were the inlet water temperature, the operating pressure drop per vessel, and the number of stages over which the process takes place. Considering these optimizations, our final process consists of 21 flash vessels (18 heat recovery and 3 heat rejection stages), each with a pressure drop of 5 kPa. See Appendix D for a screenshot of the HYSYS simulation used to model the optimized process.
===Operating Pressure and Pressure Drop===
Initially our process was designed to pump the inlet seawater and brine to a pressure of 2 bar and allow the pressure drop in each flash stage to end our process downstream at 1 bar to prevent complications associated with designing a system under vacuum. While this provided pumping advantages, the energy in the system was significantly higher due to vapor pressure considerations. Due to this fact, the required heat exchange area for certain exchangers proved to be infeasible (on the order of 80,000m2) due to the enormous capital and operating costs that would be associated with such a large heat exchanger.

The process was optimized to operate under vacuum, requiring additional pumping requirements but much lower required heat exchange area, and therefore lower utility costs. This optimization was done by specifying the pressure of the final flash vessel to 5.7kPa, so that the temperature of the water vapor (~30oC) is within approximately 5oC of the cooling water (35oC) so that condensation can occur within a reasonable approach. This also allows our system to optimally extract pure water from the brine running through the system and reduce energy requirements for cooling the product or the recycle. Our system redesign and optimization reduced the energy in the system and reduce the same heat exchanger area requirements by more than 3 orders of magnitude to approximately 800m2.

Once the process was optimized to operate in a vacuum, the pressure drop per flash stage regulated through values was then optimized. Three pressure drops per stages, 3kPa, 5kPa, and 7kPa, were tested and then an economic analysis was conducted to find the optimal value for the pressure drop. These changes had impacts on both operating and capital costs as they would change initial pumping requirements, the amount of water flowing through the process and recycle (and therefore the flash vessel sizes, heat exchanger sizing), and the efficiency of each stage.

The various pressure drops (3 kPa, 5 kPa, and 7 kPa) were tested on a system nominally at 24 stages under the assumption that the results would be generalizable to systems with a different number of stages. With increasing pressure drop, we found decreased capital cost but increased utility cost. As can be seen from Table 2 below, a pressure drop of 5 kPa proved to be the best option based on 30 year NPV, with a loss of $838.8 million, a savings of $65.5 million compared to the next-best option with a pressure drop of 7 kPa.

<center>
{| class = "wikitable"
|+Table 2: Comparison of profitability for our MSF desalination process with 24 stages with 3 different pressure drops per stage
! Number of Stages  
! Pressure Drop (kPa)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
! 30-Year NPV ($MM)
! NPV Ratio (over 5kPa)
|- style="text-align: center;"
| 24 || 3 || 362.3 || 39.1 || -905.8 || 1.08
|- style="text-align: center;"
| 24 || 5 || 255.1 || 53.6 || -838.8 || 1.00
|- style="text-align: center;"
| 24 || 7 || 209.5 || 72.2 || -904.3 || 1.08
|}
</center>

===Number of Flash Stages===
The number of stages is an important area for optimization. An increased number of stages yields better recovery of purified water, however, requires higher capital cost from the additional pressure vessel. Furthermore, with an increased amount of purified water comes increasingly concentrated brine. This must be taken into account since any brine concentration in excess of about 7% presents a more significant corrosion risk<ref name="KSU Chapter 6" />, so increased maintenance costs for the piping or higher capital costs for more corrosion-resistant materials could be necessary for brine concentrations exceeding 7%. Therefore, to recover the same amount of pure water without exceeding corrosion limitations, increasing the number of stages will reduce the overall size of each stage.

Optimizing based on NPV, it was found that 21 stages is optimal, balancing the increased profits from the additional purified water additional costs of higher capital cost. See Table 3 below to compare the 30-year NPV of the process across various numbers of flash vessels.

<center>
{| class = "wikitable"
|+Table 3: Comparison of profitability for our MSF desalination process with a 5 kPa pressure drop, 5000 m3/hr feed flow rate, with varying number of stages.
! Number of Stages  
! Pressure Drop (kPa)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
! 30-Year NPV ($MM)
! NPV Ratio (over 21 Stages)
|- style="text-align: center;"
| 17 || 5 || 213.9 || 58.7 || -813.8 || 1.016
|- style="text-align: center;"
| 18 || 5 || 218 || 57.6 || -809.9 || 1.011
|- style="text-align: center;"
| 19 || 5 || 224.3 || 58 || -806.1 || 1.007
|- style="text-align: center;"
| 20 || 5 || 229.8 || 57.9 || -813.6 || 1.016
|- style="text-align: center;"
| 21 || 5 || 226.6 || 55.9 || -800.7 || 1
|- style="text-align: center;"
| 22 || 5 || 254.7 || 54.6 || -849.5 || 1.061
|- style="text-align: center;"
| 23 || 5 || 249.3 || 54.1 || -833.1 || 1.04
|- style="text-align: center;"
| 24 || 5 || 255.1 || 53.6 || -838.8 || 1.048
|- style="text-align: center;"
| 25 || 5 || 254.9 || 52.7 || -837.4 || 1.046
|}
</center>

===Inlet Seawater Temperature===
By pairing our desalination plant with a natural gas power plant and utilizing its cooling water we forgo initial heating of our inlet stream. Normally, water pumped from the ocean would be approximately 25oC and by capturing waste heat from the power plant we can reduce utilities with our water inlet of 45oC. Through an economic comparison between two plants nominally at 21 flash stages, the plant with an inlet temperature at 25oC has a capital cost of $245MM and a utility cost of $77MM/yr and the same facility with an inlet water temperature at 45oC has a capital cost of only $226MM and a utility cost of $56MM/yr. As shown in Table 4 below, this $19MM savings in capital costs and $21MM/yr saving in utilities is largely due to reducing the size of the heat exchangers and lowering the steam required for heating duty on the inlet seawater respectively. Therefore our process is optimized by recovering the waste heat water from the gas fired power plant.

<center>
{| class = "wikitable"
|+Table 4: Seawater Inlet Temperature Process Cost Optimization
! Seawater Temperature (°C)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
|- style="text-align: center;"
| 25 || 245 || 77
|- style="text-align: center;"
| 45 || 226 || 56
|- style="text-align: center;"
| Savings || 56 || 19
|}
</center>

==Overall Process Economics==
After optimization at fixed flow rates, the process was scaled to meet the the 67% operating percentage and final product definitions as shown in Table 1, which resulted in finalized economics shown in Table 5 below. For a table of the installed costs of each process vessel, see Appendix I. For the finalized NPV calculations, see Appendix J.

<center>
{| class = "wikitable"
|+Table 5: Summarized Economics for 21-stage MSFD Process
! Economic Parameter
! Cost (USD)
|- style="text-align: center;"
| Capital Cost || 345 MM
|- style="text-align: center;"
| Total Operating Cost per Year || 103 MM
|- style="text-align: center;"
| Utility Cost per Year || 59 MM
|- style="text-align: center;"
| 30-Year NPV (6% Discount) || -1035 MM
|}
</center>

=Conclusions and Recommendations=
==Project Viability==
Table 5 above demonstrates that the process has reasonable operating and capital costs, but the 30-year NPV is negative by -1035 $MM due to the extremely low sell price capability of desalinated water as a product. Therefore, this plant would have to heavily be supported by the City of Corpus Christi to run as the product revenue will not make it self-sufficient. The acquisition of government subsidies are key operating requirements for all desalination facilities in the United States.

==Additional Concerns==
Large volumes of water, steam, and energy are needed to ensure the project runs as specified. While most of the water can easily be extracted from the ocean with little to consequence, the fuel used to generate the steam and electricity can have a significant environmental impact. As the plant is located next to a natural gas power plant, this fuel is better than coal with respect to environmental concerns, and utilizing energy from the power plant to preheat the feed stream for the desalination plant helps lower environmental impacts.

The large amounts of high pressure steam being used represents one of the the largest safety concern in the process, because a high pressure steam release could easily cause severe burns or even death. Proper precautions and control measures must be in place to prevent accidental release. Otherwise, the process does not utilize any known toxic chemicals and is relatively safe from a chemical standpoint. The flash vessels mostly operate below atmospheric pressure, so they must be constructed as vessels that can withstand vacuum without failing.

=Appendix A: Proposed Location of Desalination Plant=
Pictured below is the proposed location for the desalination plant, located on Nueces Bay in Corpus Christi, adjacent to the Nueces Bay Energy Center. The plant’s proposed location is shown in the area outlined by the small circle in the upper right of the figure.

[[File:NuecesBayLocation.png|center|600px|thumb|alt=Nueces Bay Desalination Facility Location|Figure A.1: Proposed mapped location of Nueces Bay Desalination Facility.]]
[[File:ClassIWell.png|center|700px|thumb|alt=Nueces Bay Desalination Facility Distance to Injection Well|Figure A.2: Mapped distance from proposed location to Class I Injection Well]]

=Appendix B: Feed and Product Definitions - Salinity=

<center>
{| class = "wikitable"
|+Table 6: Average Composition of Total Dissolved Solids in Seawater<ref> Physical and Thermodynamic Data. In: Dickson AG, Goyet C. Handbook of Methods for the Analysis of the Various Parameters of the Carbon Dioxide System in Sea Water.</ref><ref> Composition of Seawater. Water Treatment of Solutions. Available at http://www.lenntech.com/composition-seawater.htm. Accessed January 12, 2016.</ref>
! Compound  
! Mass Composition (kPa)
|- style="text-align: center;"
| Chlorine || 55.03%
|- style="text-align: center;"
| Sodium || 30.64%
|- style="text-align: center;"
| Sulfate || 7.70%
|- style="text-align: center;"
| Magnesium || 3.66%
|- style="text-align: center;"
| Calcium || 1.17%
|- style="text-align: center;"
| Potassium || 1.12%
|- style="text-align: center;"
| Bicarbonate || 0.36%
|- style="text-align: center;"
| Bromine || 0.19%
|- style="text-align: center;"
| Other || 0.15%
|}
</center>

[[File:Nueces Bay Salinity.JPG|center|700px|thumb|alt=Nueces Bay Salinity Tracking|Figure C.1: Salinity in Nueces Bay over the past 10 years<ref>Stations Map & Real - Time Data. The Conrad Blucher Institute for Surveying and Science website. Available at http://www.cbi.tamucc.edu/cbi/data/. Updated January 2016. Accessed January 9, 2016. </ref>.]]

=Appendix C: Process Flow Diagram=
[[File:TeamF_FinalPFD.JPG|center|1200px|thumb|alt=Process Flow Diagram|Figure C.1: Process Flow Diagram]]

=Appendix D: HYSYS Simulation Model=
[[File:TeamF_HYSYS_Screenshot.PNG|center|1200px|thumb|alt=HYSYS Simulation Model|Figure D.1: Screenshot of HYSYS Model Simulation.]]

=Appendix E: Stream Tables=
A summary of some values is shown below:

<center>
{| class = "wikitable"
|+Table 7: Key process material streams
!  
! Stream  
! Molar Flow (105 kgmole/h)
! Mass Flow (105 kg/h)
|- style="text-align: center;"
! style="text-align: right;" | Inlet
| Feed Water || 454 || 83.6
|- style="text-align: center;"
! style="text-align: right;" | Outlet
| Brine Reject || 1.90 || 36.0
|- style="text-align: center;"
! style="text-align: right;" | Product
| Pure Water || 2.64 || 47.5
|}
</center>

<center>
{| class = "wikitable"
|+Table 8: Key process energy streams
! Stream  
! Heat Flow (kJ/h)
|- style="text-align: center;"
| Feed Water || 1.30E+11
|- style="text-align: center;"
| Q-301 || 5.73E+06
|- style="text-align: center;"
| Q-302 || 3.09E+09
|- style="text-align: center;"
| Q-304 || 4.68E+06
|- style="text-align: center;"
| Q-303 || -1.26E+09
|- style="text-align: center;"
| Brine Reject || -5.47E+10
|- style="text-align: center;"
| Pure Water || -7.46E+10
|}
(Negative value denotes energy leaving process)
</center>

=Appendix F: Heat Exchanger Sizing=

<center>
{| class = "wikitable"
|+Table 9: Utility Temperatures and Heat Transfer Coefficient<ref name="Towler">G.P. Towler, R. Sinnott. Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design. Elsevier, 2012.</ref>
!  
! HP Steam
! Cooling Water
|- style="text-align: center;"
! Temperature (°C)
| 250 || 35
|- style="text-align: center;"
! U (W/m2K)
| 4000 || 1500
|}
</center>

<center>
{| class = "wikitable"
|+Table 10: Internal Flash Heat Exchanger Sizing, condensing steam.
! Exchanger
! UA (kJ/°C-h)
! U (kJ/hr/m2°C)
! A (m2)
|- style="text-align: center;"
| E-101 || 1.47E+07 || 14400 || 1019
|- style="text-align: center;"
| E-102 || 1.53E+07 || 14400 || 1059
|- style="text-align: center;"
| E-103 || 1.59E+07 || 14400 || 1103
|- style="text-align: center;"
| E-104 || 1.66E+07 || 14400 || 1153
|- style="text-align: center;"
| E-105 || 1.74E+07 || 14400 || 1209
|- style="text-align: center;"
| E-106 || 1.83E+07 || 14400 || 1272
|- style="text-align: center;"
| E-107 || 1.93E+07 || 14400 || 1342
|- style="text-align: center;"
| E-108 || 2.05E+07 || 14400 || 1424
|- style="text-align: center;"
| E-109 || 2.19E+07 || 14400 || 1518
|- style="text-align: center;"
| E-110 || 2.35E+07 || 14400 || 1629
|- style="text-align: center;"
| E-111 || 2.54E+07 || 14400 || 1761
|- style="text-align: center;"
| E-112 || 2.77E+07 || 14400 || 1921
|- style="text-align: center;"
| E-113 || 3.05E+07 || 14400 || 2119
|- style="text-align: center;"
| E-114 || 3.42E+07 || 14400 || 2372
|- style="text-align: center;"
| E-115 || 3.90E+07 || 14400 || 2706
|- style="text-align: center;"
| E-116 || 4.56E+07 || 14400 || 3167
|- style="text-align: center;"
| E-117 || 5.55E+07 || 14400 || 3856
|- style="text-align: center;"
| E-118 || 7.20E+07 || 14400 || 5000
|}
</center>

<center>
{| class = "wikitable"
|+Table 11: Heat Exchanger Sizing for Process Heaters and Coolers
! Exchanger
! Q (kJ/h)
! U (J/s/m2K)
! U (kJ/hr/m2K)
! LMTD  
! A (m2)
|- style="text-align: center;"
| E-201 || 9.67E+08 || 1500 || 5400 || 16.03 || 11165
|- style="text-align: center;"
| E-202 || 1.26E+09 || 1500 || 5400 || 7.57 || 30921
|- style="text-align: center;"
| E-203 || 1.88E+09 || 1500 || 5400 || 0.45 || 77366
|- style="text-align: center;"
| E-302 || 3.09E+09 || 4000 || 14400 || 155 || 1385
|- style="text-align: center;"
| E-303 || 1.26E+09 || 4000 || 14400 || 209.5 || 418
|}
The high heat transfer area in the final cooler of the heat rejection stage is due to the close approach temperature.
 Other cooling / pressurization techniques will be used to reduce the area.
</center>

=Appendix G: Pressure Vessel Sizing=

=Appendix H: Expanded Economic Assumptions=

=Appendix I: Equipment Cost Summary=

=Appendix J: Economic NPV Analysis=
[[File:Team F Economics.JPG|center|1200px|thumb|alt=Economic Analysis|Figure J.1: Screenshot of process economic analysis.]]

=References=
<references/>

Nueces Desalination Center: Production of Drinking Water by Multi-Stage Flash Distillation

2016-03-07T21:15:58Z

Benjkg: /* Economic Assumptions */

Team F: Rankine 672 Final Report

Authors: Kedric Daly, Ben Granger, Evan Rosati, Kathleen Zhou

Instructors: Fengqi You, David Wegerer

March 11, 2016

=Executive Summary=
Desalination of seawater to drinking water is a technology of ever-increasing importance given trends of increasingly widespread water shortages around the world. Even in the United States, water shortages are becoming increasingly common in dry, highly-populated such as California and certain areas of the Gulf Coast. Our design team, Rankine 672, was tasked with evaluating the need for and designing a desalination plant to provide purified water in a place of need. Investigation revealed that the city of Corpus Christi, Texas, is a strategic location for development of a desalination facility, as surrounding region is in need of new drinking water sources and the city has expressed serious interest in contracting the development of a large-scale desalination plant. Our proposed solution is a multi-stage flash distillation desalination facility capable of producing 20,000,000 gallons (75,708 m3) of purified drinking water for the city and surrounding region per operating day.

In this design, drinking water is produced from seawater feedstock through a 21-stage flash distillation process. By strategically locating the facility next to an existing natural gas power plant, the Nueces Bay Energy Center, pre-heated feed water can be taken directly from the outbound cooling systems of the power plant, allowing energy savings for our desalination plant. The process is designed for 16 hour-per-day operation, requires a seawater feed of 8250 m3/hr, and produces 4758 m3/hr of purified drinking water alongside a waste stream of 3485 m3/hr of concentrated brine. The waste stream is isolated from a recycle stream through a 9.0% purge; this results in an overall product yield of 57.7% on a volume basis. The process requires an additional pretreatment stream of 16.5 kg/hr Belgard EV 2030, which serves as an antiscalant, as well as 9.5 kg/hr chlorine, 352.1 kg/hr lime, and 418.7 kg/hr CO2 for post-treatment purposes.

The overall capital cost of the plant is expected to be $345MM, with a total operating cost of $103MM/yr, $59MM/yr of which comes from utilities. Due to the low price at which the product water can be sold, the 30-year net present value of the plant, assuming a 6% discount rate, is $-1,035MM. It is thus imperative this plant be constructed with public funds because it cannot return a profit to any private investor. The growing need for water however could justify the cost, because it is important that the residents of the greater Corpus Christi area have access to drinkable water.

__TOC__

=Introduction=
==Background==
Clean drinking water is a basic human need that is becoming increasingly difficult to meet. Currently, over 1 billion people live in places where water is considered scarce<ref> Tapping the oceans. The Economist USA. http://www.economist.com/node/11484059. Accessed March 2, 2016. </ref>. Though often thought of as a problem exclusive to third-world countries, the issue is becoming more prevalent in the US as well, with water managers in 40 of 50 states anticipating water shortage conditions in some portion of their state in the next 10 years. These shortages are due to increasing demands on constant or diminishing freshwater resources, therefore, to correct the issue, a new supply must be found<ref> Water Supply in the US. EPA. http://www3.epa.gov/watersense/pubs/supply.html. Accessed March 2, 2016.</ref>. Desalination of seawater is therefore important as a new source for clean drinking water and prevent water shortages.

==Project Definition==
The overall goal of the project is to investigate the feasibility of a desalination plant in order to meet the needs of growing water demand. The design needs to take health, safety, and environmental concerns into account, while optimizing costs.

=Design Basis=
Site Location and Conditions
Corpus Christi, TX was chosen as the location for the proposed desalination plant. The city is currently experiencing “Moderate Shortage Conditions”. This issue additionally comes at a time of growing demand with population growth anticipated due to new industry and Eagle Ford Group crude oil production<ref>Stage 2 Measures in the Drought Contingency Plan. Available at http://www.cctexas.com/government/water/conservation/conservation-drought-plans/stage-2-measures. Accessed January 12, 2016. </ref>.

In June 2014, the city announced the Corpus Christi Desalination Demonstration Project<ref>Corpus Christi Desalination Demonstration Project. Available at http://www.cctexas.com/Assets/Departments/Water/Files/DesalFactSheet.pdf. Published June 2014. Accessed January 12, 2016.
</ref> which seeks to create a demonstration desalination plant capable of producing 20,000 gallons of water per day, with plans to later build a full scale facility generating 20,000,000 gallons of water per day. The desalination demonstration project indicates a clear need for implementation of a desalination plant of this size to accommodate growth in the Corpus Christi area.

Locating our plant in Corpus Christi will allow us to take advantage of the benefits of cogeneration through a partnership with the Nueces Bay Energy Center. By utilizing their outlet cooling water as a large portion of our feed for the desalination process, we can raise the temperature of our seawater feed, saving on energy costs in the multi stage flash distillation. The proposed location for the desalination plant is immediately west of the Nueces Bay Energy Center. See Appendix A for a map depicting the proposed location for the desalination plant along the coast of Nueces Bay.

==Feed and Product Definitions==
The proposed desalination facility’s feed and product streams are defined as shown in Table 1. Rationale for these definitions is also described below. The values reported in Table 1 are final derived numbers from HYSYS simulation modeling.

<center>
{| class = "wikitable"
|+Table 1: Feed and product stream definitions
!  
! Salt Concentration
! Temperature
! Amount
|- style="text-align: center;"
! style="text-align: right;" | Feed Stream
| 30,500 ppm || 45°C || 8250 m3/hr
|- style="text-align: center;"
! style="text-align: right;" | Product Stream
| ~ 0 ppm || 35°C || 4758 m3/hr
|- style="text-align: center;"
! style="text-align: right;" | Byproduct Stream
| 69,500 ppm || 35°C || 3485 m3/hr
|}
</center>

The process feed will be seawater from Nueces Bay, preheated through the cooling water system of the nearby Nueces Bay Energy Center. Based on calculations developed from NREL (National Renewable Energy Laboratory) data, it was estimated that the Nueces Bay Energy Center uses approximately 250 million gallons of seawater per day for process and cooling needs. Therefore, the 20 million gallon per day requirement for the desalination facility can easily be met through the power plant’s cooling water<ref>Torcellini P, Long N, Judkoff R. Consumptive Water Use for U.S. Power Consumption. NREL. 2003; TP-550-33905.
</ref>. Note that we are assuming the process will operate 67% of the time (16 hours per day).

Based on data acquired from two salinity-measuring stations in Nueces Bay, the average salinity of local seawater is 30.5 practical salinity units (psu)<ref>Stations Map & Real - Time Data. The Conrad Blucher Institute for Surveying and Science website. Available at http://www.cbi.tamucc.edu/cbi/data/. Updated January 2016. Accessed January 9, 2016. </ref>, thus setting the concentration of the process feed stream. This translates to a feed stream concentration of 30,500 ppm (parts per million)<ref>Definition and Units. CATD Salinity Expertise Center. Available at http://www.salinityremotesensing.ifremer.fr/home. Accessed January 12, 2016.</ref>. The composition of the salt attributed to this seawater salinity is shown in Table 6 in Appendix B.

No data on the composition of seawater around the Corpus Christi area was available, but the composition of seawater does not change significantly in different locations. The given composition is considered accurate for our feed stream<ref>Brown E, Colling A, Park D, Phillips J, Rothery D, Wright J. Seawater : Its Composition, Properties and Behaviour. Boston: Butterworth-Heinemann, 1995.
</ref>. Note that seawater salt is primarily sodium chloride - total dissolved solids are 85.7% sodium chloride by mass. As such, the dissolved solids in the feed stream is assumed in this project to be pure NaCl, making the feed composition 96.95% water and 3.05% NaCl by mass. A control system will likely be put in place to ensure feed concentration remains at this level, accounting for natural fluctuations in local seawater salinity.

The largest byproduct from the process will be a brine slurry with high salt concentrations. An upper corrosion limit of 700 ppm was set for recirculated and purged brine. This undesired byproduct will be sent to a nearby Class I non-hazardous injection well facility operated by GNI Group, which is approximately 12 miles away from the proposed desalination plant location as shown in the figure in Appendix A<ref>Texas Water Code. Availabe at http://www.statutes.legis.state.tx.us/Docs/WA/htm/WA.27.htm. Accessed January 14, 2016.
</ref><ref>Google Maps. Available at https://goo.gl/maps/3xcvLZwUCe42 Accessed January 15, 2016.</ref>. Through the incorporation of a Class I injection well, the brine will not have to be treated or diluted before injection. <ref>EPA Regulations on Class I Wells. Available at http://www.epa.gov/uic/class-i-industrial-and-municipal-waste-disposal-wells. Accessed January 14, 2016.
</ref><ref>List of Injection Well Facilities by Location. Available at http://www.ehso.com/cssepa/tsdfdeepwells.php. Accessed January 14, 2016.</ref>

There are no regulations regarding the temperature of city drinking water, so the product stream temperature has no requirements based on regulations. Outlet temperatures of 35oC are acceptable for input to the municipal water supply system. Since city water tends to be stored in non-temperature controlled tanks, the temperature will naturally cool to ambient temperature.<ref>Climate Corpus Christi- Texas. US Climate Data. Available at http://www.usclimatedata.com/climate/corpus-christi/texas/united-states/ustx0294. Accessed January 12, 2016. </ref>

==Pre-Treatment and Post-Treatment==
For pretreatment, Belgard EV 2030 is recommended to be added at a concentration of 1.8-2.0 mg/L for a system with top brine temperature approximately 110 °C<ref name="BWA 2030">BWA Water Additives. Belgard EV2030 Technical Data Sheet. BWA Water Additives. http://www.wateradditives.com/files/products/tech_data_sheets/Belgard%20EV%202030_Technical%20Data%20Sheet_Thermal_8.5x11.pdf. Accessed January 27, 2016.
</ref>. Since our process is designed to operate at a slightly higher top brine temperature, 120 °C, we will design for the upper bound concentration: 2.0 mg/L of antiscalant. With a feed flow rate of 8250 m3/hr, the process therefore requires 16.5 kg/hr of Belgard EV 2030.

For disinfection during post-treatment, chlorine addition typically requires a dose of roughly 2.0 mg/L as well<ref name="World Health Organization">World Health Organization. Desalination for safe water supply. http://www.who.int/water_sanitation_health/gdwqrevision/desalination.pdf. Accessed January 26, 2016.</ref>. With a pure water distillate flow rate of 4758 m3/hr, the process requires 9.5 kg/hr of chlorine as a 5%-15% sodium hypochlorite solution. Approximately 74 mg/L of lime (calcium hydroxide) and 88mg/L of CO2 should be added to the desalinated water to help prevent corrosion and increase alkalinity within acceptable ranges<ref name="Vouchkov">Vouchkov, N. Re-Mineralization of Desalinated Water. SunCam. https://s3.amazonaws.com/suncam/npdocs/118.pdf. Accessed February 18, 2016.</ref>. With a product stream flow rate of 4758 m3/hr, these concentrations correspond to 352.1 kg/hr of lime and 418.7 kg/hr of CO2. Both of these can be purchased, and it is likely that the CO2 can be procured from the neighboring natural gas power plant.

==Choice of Desalination Technology==
The two major technologies commonly implemented for desalination include reverse osmosis (RO), and multi-stage flash distillation (MSFD). RO utilizes high pressures to force water through a semipermeable membrane that allows for purified water to pass through, but stops organic and other wastes. Often, the pressures can reach 55 - 68 bar, which requires large pumps to maintain pressure, and introduces safety concerns to the process<ref>Stage 2 Measures in the Drought Contingency Plan. Available at http://www.cctexas.com/government/water/conservation/conservation-drought-plans/stage-2-measures. Accessed January 12, 2016.
</ref>. While RO can be less energy intensive than MSFD, and the membrane can also remove contaminants, it is susceptible to feedwater quality, membrane fouling, and requires more extensive pretreatment and posttreatment than MSFD. The cost of RO is directly proportional to the feed salinity, and so it is less expensive when desalinating low salinity water.

Multi-stage flash distillation is best selected for areas that have high salinity or low energy costs. The pretreatment process for MSFD is much easier when compared to RO because pretreatment often consists of simply adding anti-corrosives and antiscalants to the feed stream. The process works by heating water in vessels at different pressures to change the boiling point of the water and evaporate it as it passes through the system. After evaporation, the water condenses, leaving pure water as the effluent. MSFD costs are now roughly $1.00/m3, where this cost is mostly independent of salinity, but dependent on energy costs.<ref>Fritzman C, Lowenberg J, Wintgens T, Melin T. State-of-the-art Reverse Osmosis Desalination. Desalination. 2007; 216:1-76.
</ref> In both RO and MSFD a brine is produced which then must be appropriately treated and/or disposed of.

Multi-stage flash distillation was selected for this process, mainly due to the economic advantages offered by the selected location of Corpus Christi, TX. MSFD is also a well understood process that can be more easily modeled by chemical engineering software.

=Technical Approach=
Quantitative modeling of the process and all included optimization was conducted using the simulation software Aspen HYSYS. A variety of assumptions were developed to set up and define specific components of the process simulation itself. Within Aspen HYSYS, the NRTL Electrolyte property package was selected to model the behavior of salt water. Each stage of the MSF process is modeled as an individual flash vessel, with individual shell & tube heat exchangers used to model the distillate condensers present in each flash stage in the MSF system.

The HYSYS simulation model only considers the flash portion of the overall process, allowing establishment of mass and energy balances for the system. Pretreatment and post-treatment were excluded for the HYSYS model, as these steps are simple additions or removals of small concentrations of chemical components. Although the simulation models heat exchangers outside of each flash stage, in the physical design of the plant these heat exchangers will be contained within each flash stage.

One portion of the process is not modeled, which is the inlet seawater entering the heat rejection stages. As can be seen from the full process flow diagram (Appendix C), the inlet seawater in the heat rejection stage is in high excess to provide maximum cooling capacity; then a significant portion is purged to bring the stream flow rate down to what is needed for the feed stream. It is assumed that the flow rate of cooling water for those three stages is in such high excess that the cooling water does not heat up. Therefore, in the process simulation models, the inlet stream is the feed stream, which enters directly into the heat recovery portion of the process.

See Appendix D for a screenshot overview of the simulation environment model.

=Process Overview=
==Process Flowsheet==
The following diagram, Figure 1, shows a basic flowsheet for the process. Process alternatives considered for each stage are discussed below. Please see Appendix C for the full process flowsheet with finalized design decisions.

[[File:BlockFlowDiagram.png|center|700px|thumb|alt=Nueces Bay Desalination Facility Block Flow Diagram|Figure 1: Basic block flowsheet of the multi-stage flash desalination process.]]

==Major Features==
As seen above, there are three primary stages of the desalination process: Pre-treatment, the multi-stage flash distillation, and post-treatment. Seawater enters the pre-treatment phase first to make the feed suitable for the rest of the process by filtering out large particles that could accumulate on pipes, deaerating the water to limit corrosion, and adding an agent to combat fouling. The water then enters the distillation process, which is broken up into two sections: heat recovery and heat rejection.

In the heat recovery stage, there are 18 flash vessels, each of which separates steam from the salinated liquid water. Each flash vessel has saltwater as its inlet and two outlets: a vapor outlet (steam) that condenses into the final product of purified water and a liquid outlet composed of water with more dissolved solids than the inlet, which is the inlet stream for the next flash vessel. The pipe containing the feed is first mixed with recycled brine, then goes through the top sections of each of the flash vessels, starting with V-118 and ending up at V-101. In this way, the feed stream pipe first serves as a heat exchanger, heating the pre-treated seawater while simultaneously condensing the steam into the final product. The feed then enters a final heat exchanger where it is brought up to 103.6 °C before entering V-101 to be flashed.

After exiting V-118, the final flash vessel in the heat recovery portion, the water enters the heat rejection portion, which consists of three flash vessels (V-201, V-202, V-203). The flash vessels of this section operate in the same manner as in the heat recovery section, with the exception that condensation occurs using excess cooling water instead of pre-treated seawater. This technique allows excess heat to be rejected into an excess water stream rather than the feed.

The purified product stream finally enters post-treatment processing in which water is re-aerated and remineralized, disinfected, and pH is adjusted before being delivered to the city. The concentrated brine leftover at the end of the flash cycle is recycled into the feed, although a purge is required to continuously remove some salt from the system. This purged brine is disposed of by use of a Class I injection well, per regional regulations.

See Appendix E to find detailed stream tables of stream specifications.

==Process Alternatives==
===Pretreatment===
Multi-stage flash distillation requires significantly less pretreatment than reverse osmosis desalination, but helpful pretreatment techniques still exist. One very inexpensive method of pretreatment is screening/filtration, which is used to filter out large contaminants from intake seawater. Using more complicated membrane technology to accomplish large particle filtration is a possible alternative, but doing so would be much more expensive than using a standard screen or filter, so the latter choice has been selected for this process.<ref>El-Dessouky H., Alatiqi I, Ettouney H. Process synthesis: The multi-stage flash desalination system. Desalination. 1998;115(2):115-179. doi: 10.1016/S0011-9164(98)00035-6.</ref>

The most critical type of fouling in multi-stage flash distillation is scaling - therefore, the most critical step in the pretreatment process will be the inclusion of an anti-scalant. Alkaline scaling can be prevented by acid dosing or polyphosphate addition; however, acid dosing can result in increased corrosion and polyphosphates can lead to non-alkaline calcium phosphate fouling, so these additives will not be considered<ref name="Morris">Morris, R. The development of the multi-stage flash distillation process: A designer's viewpoint. Desalination. 1993;93(1-3):57-68. doi:10.1016/S0011-9164(93)80096-6. Availble at http://www.sciencedirect.com/science/article/pii/0011916493800966</ref><ref>Al-Gobaisi, D. A quarter-century of seawater desalination by large multistage flash plants in Abu Dhabi (Plant performance analysis, assessment, present efforts toward enhancement and future hopes). Desalination. 1994;99(2-3):509-512. doi: 10.1016/S0011-9164(94)00196-0.</ref>. As such, a polymer-based additive is recommended for the anti-scalant for this process. The Belgard EV series is designed for use in high-temperature seawater MSF; of these, Belgard EV 2030 is the most neutral in pH<ref>NSF International. NSF Product and Service Listings: Drinking Water Treatment Chemicals - Health Effects. NSF International. http://info.nsf.org/Certified/PwsChemicals/Listings.asp?Company=00300&. Accessed January 27, 2016.</ref><ref name="BWA 2030" /><ref>BWA Water Additives. Belgard EV2035 Technical Data Sheet. BWA Water Additives. http://www.wateradditives.com/files/products/tech_data_sheets/Belgard%20EV2035_Technical%20Data%20Sheet_Thermal_8.5x11.pdf. Accessed January 27, 2016.</ref><ref>BioLab Water Additives. Belgard EV Antiscalant for seawater distillation plants. BioLab Water Additives. http://www.lpq.com.mx/pdf/BELGARD%20EV.PDF. Accessed January 27, 2016.</ref>. As such, we have selected Belgard EV 2030 for use as an anti-scalant in our process.

Corrosion control is another significant method of pretreatment in multi-stage flash distillation plants. Using a piping material that has limited susceptibility to corrosion is typically not economically viable for MSF desalination plants due their large size, so using carbon steel for most of the piping is generally used for its economic benefits despite its susceptibility to corrosion<ref name = "Morris" />. Limiting oxygen concentration is very feasible through the use of deaerators, which can remove oxygen as well as other dissolved gases from the seawater stream. Often, an external deaeration system is used<ref name = "Morris" /><ref>Edwards - Hick Hargreaves. Seawater Deaerators. Edwards - Hick Hargreaves. http://psscorp.co.th/upload/news/20110830105340.pdf. Accessed January 26, 2016.</ref>. Given these considerations, using an external deaerator combined with a less-corrosive antiscalant like Belgard EV 2030 is much more practical for the process than using anti-corrosive piping.

Excessive foam can also be an issue in some MSF distillation facilities, so antifoaming agents are sometimes employed in the pretreatment process as well. However, many plants are operated without foaming issues, and therefore do not need anti-foaming pretreatment at all. Selection of an anti-foaming agent must be carefully weighed with selected of anti-scalant to avoid issues with additive interactions [26]. At this stage in the design process, an anti-foamer will not be included due to the risk of decreasing scalant efficacy, and the likelihood that foam will not be an issue in the desalination plant.

===Multistage Flash Distillation===
The main objective of the design project is desalinate water. This is done with the main process of multi stage flash distillation (MSF). MSF can be developed using a numerous number of different process configurations, such as Once-Through, Simple Mixer Brine Circulation, and Multi-Stage Heat Rejection Brine Circulation processes.

The most desired process for the project is the Three Stage Heat Rejection Brine Circulation as it builds on benefits of the other simpler technologies. By combining principles of heat rejection and heat recovery, the required heat transfer area of the process is greatly reduced as compared to once through processes or simple brine recycle processes. It has the highest performance ratio with the lowest specific feed flow rates, therefore greatly lowering the operating costs of the process. It is also known as the industry standard for MSF<ref name = "KSU Chapter 6">Kansas State University ChE 413. Available at: http://faculty.ksu.edu.sa/Almutaz/Documents/ChE-413/Multi-stage%20Flash%20Desalination.pdf. Accessed January 26, 2016.</ref>. In addition, since our brine disposal technique involves the pumping of the brine into a Class I well, the high brine outlet salinity is not a drawback for our process since post treatment or dilution of the brine is not necessary.

===Post Treatment===
After processing the water via multi-stage flash, the water needs to be post-treated to make the water taste as consumers would expect, since distilled water is “flat” and unpleasant, as well as to protect the consumers and the piping through which the water will be passing<ref name="Clayton">Clayton R. A review of current knowledge: Desalination for water supply. Foundation for Water Research. http://www.fwr.org/desal.pdf. Accessed January 26, 2016.</ref>. There are multiple alternatives to achieve these objectives of remineralization, aeration, corrosion control, and disinfection.

Aerating and introducing some dissolved solids back into the product water are effective ways to create a quality product that is not flat, and contains flavoring that is expected of the water when delivered to the consumer. Aeration will be accomplished via an inline spray aerator which will take up less space than cascade aeration.<ref>Minnesota Rural Water Association. Aeration. http://www.mrwa.com/WaterWorksMnl/Chapter%2011%20Aeration.pdf. Accessed January 26, 2016.</ref> Feed blending will be used to remineralize the water as opposed to a remineralization filter because it does not require additional equipment purchases, and can easily handle the large amounts of throughput required for the process.<ref name="Clayton" />

Corrosion and pH can be controlled by adding lime (calcium hydroxide) and carbon dioxide to the water. To do so, about 74 mg/L of lime and 88mg/L of CO2 should be added to the desalinated water to help prevent corrosion and increase alkalinity within acceptable ranges. Sodium hydroxide, soda ash, and sodium bicarbonate are alternatives to altering the alkalinity and pH of the product water, but lime and CO2 is an industry standard <ref name="Vouchkov" />.

Disinfection needs to be done to prevent microbial growth in the desalinated water and ensure the safety of the general public. Chlorine addition via chlorine gas or sodium hypochlorite is the most common and only requires a dose of 1.5 to 2.5 mg/liter. <ref name="World Health Organization" /> Other alternatives include ozonation, UV light, and chloramination. Chlorine, as a 5% - 15% sodium hypochlorite solution, is sufficient for our process and relatively inexpensive, and as such will be utilized in the process.

===Transport of Brine to Well===
Three major possibilities exist for transporting the brine from our desalination plant to the injection well for disposal: rail, trucking, and a pipeline. A pipeline was chosen as the most feasible and cost-effective approach.

A rail line is not available at this time, and with the cost to construct a rail line at approximately $43 million per mile<ref>About Money. How Much Do Rail Transit Projects Cost to Build and Operate? 2014. Available at: http://publictransport.about.com/od/Transit_Projects/a/How-Much-Do-Rail-Transit-Projects-Cost-To-Build-And-Operate.htm. Accessed January 27, 2016. </ref>, the enormous capital cost makes rail transport infeasible for this site. For large volume operations, pipelines are cheaper than trucking. In a study of wastewater sludge produced from wastewater treatment facilities, it was found that it was more economical for facilities producing over 1200 m3 per day to utilize pipelines rather than trucking.<ref>Marufuzzaman M, Eskioglu S, Hernandez R. Truck versus pipeline transportation cost analysis of wastewater sludge. Transportation Research. 2015; 74(A): 14-30. </ref> Our facility is producing nearly 3 times this volume at 3485 m3 per day of brine. Furthermore, brine is a less viscous fluid than wastewater sludge, decreasing pumping costs in the pipeline and further amplifying the benefit of choosing a pipeline over an injection well.

===Brine Storage===
Due to the choice of a pipeline, rather than trucking, as the method of transport of brine to the injection well, brine storage is unnecessary, so storage solutions such as holding tanks were not investigated. One method of storage compatible with a pipeline is to pump the brine through an evaporation pond. This solution is attractive as it would allow the brine to concentrate, saving some costs of pumping, however, the considerable land usage of evaporation ponds leads them to only be effective for smaller plants. The largest municipal plant utilizing this method produces only 5.7 million liters per day<ref>University of Santa Cruz Center for Integrated Water Research. Concentrate Management: Evaporation Ponds. 2010. Available at: https://ciwr.ucsc.edu/sites/default/files/ICM_D5.pdf. Accessed January 27, 2016. </ref>, whereas our desalination plant is proposed to produce 75.7 million liters per day. Therefore, evaporation ponds have been eliminated from consideration.

==Major Specifications==
===Material of Construction Selection===
While saltwater and desalinated water in general can be corrosive, there are best practices within the desalination industry for material selection. With this in mind, best practice shows carbon steel lifetime can be lengthened by monitoring the oxygen concentration in the water, specifying the paint used to coat any metal, and more expensive materials can be used in areas of extremely high corrosion.<ref name = "Morris" />. By following these best practices, it is possible to save significant amounts of money through the use of carbon steel in most areas of the plant, with specific high-corrosion areas being reinforced with, or replaced with, more corrosion resistant materials, such as Cu/Ni 70/30 alloy<ref name = "KSU Chapter 6" />.

===Heat Exchanger Sizing===
See Appendix F for heat exchanger sizing data. Heat exchanger sizes appear reasonable for placement in the flash vessels.

===Flash Vessel Sizing===
The dimensions for the flash drums were obtained through literature values. The flash drums were determined to be 18x4x3 m in width, height, and length.<ref name="KSU Chapter 6" />

Wall thickness was determined using the following conventions: Design pressure is 10% more than the operating pressure, design temperature is 50°F (27.8°C) higher than the operating temperature, a corrosion allowance of 3-mm and an effectiveness of 85% was derived from the assumption of a double-welded butt joint that was spot checked. See Appendix G for wall thicknesses calculated for each pressure vessel.

The sizing of the vessels to be constructed will be determined after talks with a manufacturer. Due to economies of scale, it may be more cost-effective to buy 21 pressure vessels with the dimensions mentioned above and a wall thickness of 17.1-mm, the highest wall thickness required in the system.

===Pipe Losses and Pressure Drop===
HYSYS was used to estimate the diameter of a pipe given a specified pressure drop per 100m of pipe. It was determined that for a pressure drop of 0.1 kPa/100m of pipe, a 3.05m diameter pipe is sufficient, which is more than manageable for manufacturers. This pressure drop is mostly insignificant and will be taken care of with the safety factors included in the pumps. The significant pressure drops in our system is accounted for via losses in the flash vessels and heat exchangers, which drive the flash process. Therefore overall pipe losses are insignificant and are built into intentional design losses.

=Economic Analysis=
==Economic Assumptions==
The sale of purified water yields a revenue stream of $49.5 million per year assuming sale to commercial entities within the city limits of Corpus Christi at $1.7915 per thousand liters.<ref>Water schedule. City of Corpus Christi Utility Business Office. http://www.cctexas.com/Assets/Departments/Financial-Services/Utilities-Business-Office/files/waterrates.pdf Accessed February 28, 2016.</ref> The primary variable costs are utilities and the brine disposal, with minimal raw materials costs due to the primary feedstock, seawater, being free, and minimal amounts of pre- and post-treatment chemicals being used. The cost of brine disposal used in this analysis was high at $2.097 per thousand liters<ref>McCurdy, R. Underground injection wells for produced water disposal. Chesapeake Energy Corporation. http://www.epa.gov/sites/production/files/documents/21_McCurdy_-_UIC_Disposal_508.pdf Accessed February 28, 2016.</ref>, an estimate for brine disposal utilizing trucking as the mode of transport. As has been shown through previous analyses, a lower cost can be achieved using a direct pipeline, however, no good estimates for this cost could be found.

The NPV was calculated using a 6% cost of capital, the approximate interest rate for a municipal bond.<ref>Zirbe, R.O. Jr., Han, X., Layton, D., Leshine, T. A history of discount rates and their use by government agencies. (2002). http://faculty.washington.edu/zerbe/docs/DISCOUNT%20RATE%20HISTORY-latesXIt.doc Accessed February 28, 2016.</ref> It was assumed that the plant can be built within two years, with production at half-scale in its first year. Additionally, one month of variable costs, $10.6 million, was set aside for working capital. See Appendix H for a full table of assumptions. A 30 year NPV calculation was chosen as the decision basis since 30 years is a conservative estimate for the lifetime of a desalination plant <ref>Tampa Bay Seawater Desalination Plant. American Water. http://www.amwater.com/files/tampabayseawaterdesalinationplantcasestudy6.16.09.pdf. Accessed February 25, 20216.</ref>. See below for estimates of NPV for the lifetime of the plant under various assumptions and design criteria.

==Optimization and Sensitivity Analysis==
Looking at the multi-stage flash process, there are a number of opportunities to optimize the process. The major process variables that were investigated were the inlet water temperature, the operating pressure drop per vessel, and the number of stages over which the process takes place. Considering these optimizations, our final process consists of 21 flash vessels (18 heat recovery and 3 heat rejection stages), each with a pressure drop of 5 kPa. See Appendix D for a screenshot of the HYSYS simulation used to model the optimized process.
===Operating Pressure and Pressure Drop===
Initially our process was designed to pump the inlet seawater and brine to a pressure of 2 bar and allow the pressure drop in each flash stage to end our process downstream at 1 bar to prevent complications associated with designing a system under vacuum. While this provided pumping advantages, the energy in the system was significantly higher due to vapor pressure considerations. Due to this fact, the required heat exchange area for certain exchangers proved to be infeasible (on the order of 80,000m2) due to the enormous capital and operating costs that would be associated with such a large heat exchanger.

The process was optimized to operate under vacuum, requiring additional pumping requirements but much lower required heat exchange area, and therefore lower utility costs. This optimization was done by specifying the pressure of the final flash vessel to 5.7kPa, so that the temperature of the water vapor (~30oC) is within approximately 5oC of the cooling water (35oC) so that condensation can occur within a reasonable approach. This also allows our system to optimally extract pure water from the brine running through the system and reduce energy requirements for cooling the product or the recycle. Our system redesign and optimization reduced the energy in the system and reduce the same heat exchanger area requirements by more than 3 orders of magnitude to approximately 800m2.

Once the process was optimized to operate in a vacuum, the pressure drop per flash stage regulated through values was then optimized. Three pressure drops per stages, 3kPa, 5kPa, and 7kPa, were tested and then an economic analysis was conducted to find the optimal value for the pressure drop. These changes had impacts on both operating and capital costs as they would change initial pumping requirements, the amount of water flowing through the process and recycle (and therefore the flash vessel sizes, heat exchanger sizing), and the efficiency of each stage.

The various pressure drops (3 kPa, 5 kPa, and 7 kPa) were tested on a system nominally at 24 stages under the assumption that the results would be generalizable to systems with a different number of stages. With increasing pressure drop, we found decreased capital cost but increased utility cost. As can be seen from Table 2 below, a pressure drop of 5 kPa proved to be the best option based on 30 year NPV, with a loss of $838.8 million, a savings of $65.5 million compared to the next-best option with a pressure drop of 7 kPa.

<center>
{| class = "wikitable"
|+Table 2: Comparison of profitability for our MSF desalination process with 24 stages with 3 different pressure drops per stage
! Number of Stages  
! Pressure Drop (kPa)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
! 30-Year NPV ($MM)
! NPV Ratio (over 5kPa)
|- style="text-align: center;"
| 24 || 3 || 362.3 || 39.1 || -905.8 || 1.08
|- style="text-align: center;"
| 24 || 5 || 255.1 || 53.6 || -838.8 || 1.00
|- style="text-align: center;"
| 24 || 7 || 209.5 || 72.2 || -904.3 || 1.08
|}
</center>

===Number of Flash Stages===
The number of stages is an important area for optimization. An increased number of stages yields better recovery of purified water, however, requires higher capital cost from the additional pressure vessel. Furthermore, with an increased amount of purified water comes increasingly concentrated brine. This must be taken into account since any brine concentration in excess of about 7% presents a more significant corrosion risk<ref name="KSU Chapter 6" />, so increased maintenance costs for the piping or higher capital costs for more corrosion-resistant materials could be necessary for brine concentrations exceeding 7%. Therefore, to recover the same amount of pure water without exceeding corrosion limitations, increasing the number of stages will reduce the overall size of each stage.

Optimizing based on NPV, it was found that 21 stages is optimal, balancing the increased profits from the additional purified water additional costs of higher capital cost. See Table 3 below to compare the 30-year NPV of the process across various numbers of flash vessels.

<center>
{| class = "wikitable"
|+Table 3: Comparison of profitability for our MSF desalination process with a 5 kPa pressure drop, 5000 m3/hr feed flow rate, with varying number of stages.
! Number of Stages  
! Pressure Drop (kPa)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
! 30-Year NPV ($MM)
! NPV Ratio (over 21 Stages)
|- style="text-align: center;"
| 17 || 5 || 213.9 || 58.7 || -813.8 || 1.016
|- style="text-align: center;"
| 18 || 5 || 218 || 57.6 || -809.9 || 1.011
|- style="text-align: center;"
| 19 || 5 || 224.3 || 58 || -806.1 || 1.007
|- style="text-align: center;"
| 20 || 5 || 229.8 || 57.9 || -813.6 || 1.016
|- style="text-align: center;"
| 21 || 5 || 226.6 || 55.9 || -800.7 || 1
|- style="text-align: center;"
| 22 || 5 || 254.7 || 54.6 || -849.5 || 1.061
|- style="text-align: center;"
| 23 || 5 || 249.3 || 54.1 || -833.1 || 1.04
|- style="text-align: center;"
| 24 || 5 || 255.1 || 53.6 || -838.8 || 1.048
|- style="text-align: center;"
| 25 || 5 || 254.9 || 52.7 || -837.4 || 1.046
|}
</center>

===Inlet Seawater Temperature===
By pairing our desalination plant with a natural gas power plant and utilizing its cooling water we forgo initial heating of our inlet stream. Normally, water pumped from the ocean would be approximately 25oC and by capturing waste heat from the power plant we can reduce utilities with our water inlet of 45oC. Through an economic comparison between two plants nominally at 21 flash stages, the plant with an inlet temperature at 25oC has a capital cost of $245MM and a utility cost of $77MM/yr and the same facility with an inlet water temperature at 45oC has a capital cost of only $226MM and a utility cost of $56MM/yr. As shown in Table 4 below, this $19MM savings in capital costs and $21MM/yr saving in utilities is largely due to reducing the size of the heat exchangers and lowering the steam required for heating duty on the inlet seawater respectively. Therefore our process is optimized by recovering the waste heat water from the gas fired power plant.

<center>
{| class = "wikitable"
|+Table 4: Seawater Inlet Temperature Process Cost Optimization
! Seawater Temperature (°C)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
|- style="text-align: center;"
| 25 || 245 || 77
|- style="text-align: center;"
| 45 || 226 || 56
|- style="text-align: center;"
| Savings || 56 || 19
|}
</center>

==Overall Process Economics==
After optimization at fixed flow rates, the process was scaled to meet the the 67% operating percentage and final product definitions as shown in Table 1, which resulted in finalized economics shown in Table 5 below. For a table of the installed costs of each process vessel, see Appendix I. For the finalized NPV calculations, see Appendix J.

<center>
{| class = "wikitable"
|+Table 5: Summarized Economics for 21-stage MSFD Process
! Economic Parameter
! Cost (USD)
|- style="text-align: center;"
| Capital Cost || 345 MM
|- style="text-align: center;"
| Total Operating Cost per Year || 103 MM
|- style="text-align: center;"
| Utility Cost per Year || 59 MM
|- style="text-align: center;"
| 30-Year NPV (6% Discount) || -1035 MM
|}
</center>

=Conclusions and Recommendations=
==Project Viability==
Table 5 above demonstrates that the process has reasonable operating and capital costs, but the 30-year NPV is negative by -1035 $MM due to the extremely low sell price capability of desalinated water as a product. Therefore, this plant would have to heavily be supported by the City of Corpus Christi to run as the product revenue will not make it self-sufficient. The acquisition of government subsidies are key operating requirements for all desalination facilities in the United States.

==Additional Concerns==
Large volumes of water, steam, and energy are needed to ensure the project runs as specified. While most of the water can easily be extracted from the ocean with little to consequence, the fuel used to generate the steam and electricity can have a significant environmental impact. As the plant is located next to a natural gas power plant, this fuel is better than coal with respect to environmental concerns, and utilizing energy from the power plant to preheat the feed stream for the desalination plant helps lower environmental impacts.

The large amounts of high pressure steam being used represents one of the the largest safety concern in the process, because a high pressure steam release could easily cause severe burns or even death. Proper precautions and control measures must be in place to prevent accidental release. Otherwise, the process does not utilize any known toxic chemicals and is relatively safe from a chemical standpoint. The flash vessels mostly operate below atmospheric pressure, so they must be constructed as vessels that can withstand vacuum without failing.

=Appendix A: Proposed Location of Desalination Plant=
Pictured below is the proposed location for the desalination plant, located on Nueces Bay in Corpus Christi, adjacent to the Nueces Bay Energy Center. The plant’s proposed location is shown in the area outlined by the small circle in the upper right of the figure.

[[File:NuecesBayLocation.png|center|600px|thumb|alt=Nueces Bay Desalination Facility Location|Figure A.1: Proposed mapped location of Nueces Bay Desalination Facility.]]
[[File:ClassIWell.png|center|700px|thumb|alt=Nueces Bay Desalination Facility Distance to Injection Well|Figure A.2: Mapped distance from proposed location to Class I Injection Well]]

=Appendix B: Feed and Product Definitions - Salinity=

<center>
{| class = "wikitable"
|+Table 6: Average Composition of Total Dissolved Solids in Seawater<ref> Physical and Thermodynamic Data. In: Dickson AG, Goyet C. Handbook of Methods for the Analysis of the Various Parameters of the Carbon Dioxide System in Sea Water.</ref><ref> Composition of Seawater. Water Treatment of Solutions. Available at http://www.lenntech.com/composition-seawater.htm. Accessed January 12, 2016.</ref>
! Compound  
! Mass Composition (kPa)
|- style="text-align: center;"
| Chlorine || 55.03%
|- style="text-align: center;"
| Sodium || 30.64%
|- style="text-align: center;"
| Sulfate || 7.70%
|- style="text-align: center;"
| Magnesium || 3.66%
|- style="text-align: center;"
| Calcium || 1.17%
|- style="text-align: center;"
| Potassium || 1.12%
|- style="text-align: center;"
| Bicarbonate || 0.36%
|- style="text-align: center;"
| Bromine || 0.19%
|- style="text-align: center;"
| Other || 0.15%
|}
</center>

[[File:Nueces Bay Salinity.JPG|center|700px|thumb|alt=Nueces Bay Salinity Tracking|Figure C.1: Salinity in Nueces Bay over the past 10 years6.]]

=Appendix C: Process Flow Diagram=
[[File:TeamF_FinalPFD.JPG|center|1200px|thumb|alt=Process Flow Diagram|Figure C.1: Process Flow Diagram]]

=Appendix D: HYSYS Simulation Model=
[[File:TeamF_HYSYS_Screenshot.PNG|center|1200px|thumb|alt=HYSYS Simulation Model|Figure D.1: Screenshot of HYSYS Model Simulation.]]

=Appendix E: Stream Tables=
A summary of some values is shown below:

<center>
{| class = "wikitable"
|+Table 7: Key process material streams
!  
! Stream  
! Molar Flow (105 kgmole/h)
! Mass Flow (105 kg/h)
|- style="text-align: center;"
! style="text-align: right;" | Inlet
| Feed Water || 454 || 83.6
|- style="text-align: center;"
! style="text-align: right;" | Outlet
| Brine Reject || 1.90 || 36.0
|- style="text-align: center;"
! style="text-align: right;" | Product
| Pure Water || 2.64 || 47.5
|}
</center>

<center>
{| class = "wikitable"
|+Table 8: Key process energy streams
! Stream  
! Heat Flow (kJ/h)
|- style="text-align: center;"
| Feed Water || 1.30E+11
|- style="text-align: center;"
| Q-301 || 5.73E+06
|- style="text-align: center;"
| Q-302 || 3.09E+09
|- style="text-align: center;"
| Q-304 || 4.68E+06
|- style="text-align: center;"
| Q-303 || -1.26E+09
|- style="text-align: center;"
| Brine Reject || -5.47E+10
|- style="text-align: center;"
| Pure Water || -7.46E+10
|}
(Negative value denotes energy leaving process)
</center>

=Appendix F: Heat Exchanger Sizing=

<center>
{| class = "wikitable"
|+Table 9: Utility Temperatures and Heat Transfer Coefficient<ref name="Towler">G.P. Towler, R. Sinnott. Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design. Elsevier, 2012.</ref>
!  
! HP Steam
! Cooling Water
|- style="text-align: center;"
! Temperature (°C)
| 250 || 35
|- style="text-align: center;"
! U (W/m2K)
| 4000 || 1500
|}
</center>

<center>
{| class = "wikitable"
|+Table 10: Internal Flash Heat Exchanger Sizing, condensing steam.
! Exchanger
! UA (kJ/°C-h)
! U (kJ/hr/m2°C)
! A (m2)
|- style="text-align: center;"
| E-101 || 1.47E+07 || 14400 || 1019
|- style="text-align: center;"
| E-102 || 1.53E+07 || 14400 || 1059
|- style="text-align: center;"
| E-103 || 1.59E+07 || 14400 || 1103
|- style="text-align: center;"
| E-104 || 1.66E+07 || 14400 || 1153
|- style="text-align: center;"
| E-105 || 1.74E+07 || 14400 || 1209
|- style="text-align: center;"
| E-106 || 1.83E+07 || 14400 || 1272
|- style="text-align: center;"
| E-107 || 1.93E+07 || 14400 || 1342
|- style="text-align: center;"
| E-108 || 2.05E+07 || 14400 || 1424
|- style="text-align: center;"
| E-109 || 2.19E+07 || 14400 || 1518
|- style="text-align: center;"
| E-110 || 2.35E+07 || 14400 || 1629
|- style="text-align: center;"
| E-111 || 2.54E+07 || 14400 || 1761
|- style="text-align: center;"
| E-112 || 2.77E+07 || 14400 || 1921
|- style="text-align: center;"
| E-113 || 3.05E+07 || 14400 || 2119
|- style="text-align: center;"
| E-114 || 3.42E+07 || 14400 || 2372
|- style="text-align: center;"
| E-115 || 3.90E+07 || 14400 || 2706
|- style="text-align: center;"
| E-116 || 4.56E+07 || 14400 || 3167
|- style="text-align: center;"
| E-117 || 5.55E+07 || 14400 || 3856
|- style="text-align: center;"
| E-118 || 7.20E+07 || 14400 || 5000
|}
</center>

=Appendix G: Pressure Vessel Sizing=

=Appendix H: Expanded Economic Assumptions=

=Appendix I: Equipment Cost Summary=

=Appendix J: Economic NPV Analysis=
[[File:Team F Economics.JPG|center|1200px|thumb|alt=Economic Analysis|Figure J.1: Screenshot of process economic analysis.]]

=References=
<references/>

Nueces Desalination Center: Production of Drinking Water by Multi-Stage Flash Distillation

2016-03-07T21:14:49Z

Benjkg: /* Appendix B: Feed and Product Definitions - Salinity */

Team F: Rankine 672 Final Report

Authors: Kedric Daly, Ben Granger, Evan Rosati, Kathleen Zhou

Instructors: Fengqi You, David Wegerer

March 11, 2016

=Executive Summary=
Desalination of seawater to drinking water is a technology of ever-increasing importance given trends of increasingly widespread water shortages around the world. Even in the United States, water shortages are becoming increasingly common in dry, highly-populated such as California and certain areas of the Gulf Coast. Our design team, Rankine 672, was tasked with evaluating the need for and designing a desalination plant to provide purified water in a place of need. Investigation revealed that the city of Corpus Christi, Texas, is a strategic location for development of a desalination facility, as surrounding region is in need of new drinking water sources and the city has expressed serious interest in contracting the development of a large-scale desalination plant. Our proposed solution is a multi-stage flash distillation desalination facility capable of producing 20,000,000 gallons (75,708 m3) of purified drinking water for the city and surrounding region per operating day.

In this design, drinking water is produced from seawater feedstock through a 21-stage flash distillation process. By strategically locating the facility next to an existing natural gas power plant, the Nueces Bay Energy Center, pre-heated feed water can be taken directly from the outbound cooling systems of the power plant, allowing energy savings for our desalination plant. The process is designed for 16 hour-per-day operation, requires a seawater feed of 8250 m3/hr, and produces 4758 m3/hr of purified drinking water alongside a waste stream of 3485 m3/hr of concentrated brine. The waste stream is isolated from a recycle stream through a 9.0% purge; this results in an overall product yield of 57.7% on a volume basis. The process requires an additional pretreatment stream of 16.5 kg/hr Belgard EV 2030, which serves as an antiscalant, as well as 9.5 kg/hr chlorine, 352.1 kg/hr lime, and 418.7 kg/hr CO2 for post-treatment purposes.

The overall capital cost of the plant is expected to be $345MM, with a total operating cost of $103MM/yr, $59MM/yr of which comes from utilities. Due to the low price at which the product water can be sold, the 30-year net present value of the plant, assuming a 6% discount rate, is $-1,035MM. It is thus imperative this plant be constructed with public funds because it cannot return a profit to any private investor. The growing need for water however could justify the cost, because it is important that the residents of the greater Corpus Christi area have access to drinkable water.

__TOC__

=Introduction=
==Background==
Clean drinking water is a basic human need that is becoming increasingly difficult to meet. Currently, over 1 billion people live in places where water is considered scarce<ref> Tapping the oceans. The Economist USA. http://www.economist.com/node/11484059. Accessed March 2, 2016. </ref>. Though often thought of as a problem exclusive to third-world countries, the issue is becoming more prevalent in the US as well, with water managers in 40 of 50 states anticipating water shortage conditions in some portion of their state in the next 10 years. These shortages are due to increasing demands on constant or diminishing freshwater resources, therefore, to correct the issue, a new supply must be found<ref> Water Supply in the US. EPA. http://www3.epa.gov/watersense/pubs/supply.html. Accessed March 2, 2016.</ref>. Desalination of seawater is therefore important as a new source for clean drinking water and prevent water shortages.

==Project Definition==
The overall goal of the project is to investigate the feasibility of a desalination plant in order to meet the needs of growing water demand. The design needs to take health, safety, and environmental concerns into account, while optimizing costs.

=Design Basis=
Site Location and Conditions
Corpus Christi, TX was chosen as the location for the proposed desalination plant. The city is currently experiencing “Moderate Shortage Conditions”. This issue additionally comes at a time of growing demand with population growth anticipated due to new industry and Eagle Ford Group crude oil production<ref>Stage 2 Measures in the Drought Contingency Plan. Available at http://www.cctexas.com/government/water/conservation/conservation-drought-plans/stage-2-measures. Accessed January 12, 2016. </ref>.

In June 2014, the city announced the Corpus Christi Desalination Demonstration Project<ref>Corpus Christi Desalination Demonstration Project. Available at http://www.cctexas.com/Assets/Departments/Water/Files/DesalFactSheet.pdf. Published June 2014. Accessed January 12, 2016.
</ref> which seeks to create a demonstration desalination plant capable of producing 20,000 gallons of water per day, with plans to later build a full scale facility generating 20,000,000 gallons of water per day. The desalination demonstration project indicates a clear need for implementation of a desalination plant of this size to accommodate growth in the Corpus Christi area.

Locating our plant in Corpus Christi will allow us to take advantage of the benefits of cogeneration through a partnership with the Nueces Bay Energy Center. By utilizing their outlet cooling water as a large portion of our feed for the desalination process, we can raise the temperature of our seawater feed, saving on energy costs in the multi stage flash distillation. The proposed location for the desalination plant is immediately west of the Nueces Bay Energy Center. See Appendix A for a map depicting the proposed location for the desalination plant along the coast of Nueces Bay.

==Feed and Product Definitions==
The proposed desalination facility’s feed and product streams are defined as shown in Table 1. Rationale for these definitions is also described below. The values reported in Table 1 are final derived numbers from HYSYS simulation modeling.

<center>
{| class = "wikitable"
|+Table 1: Feed and product stream definitions
!  
! Salt Concentration
! Temperature
! Amount
|- style="text-align: center;"
! style="text-align: right;" | Feed Stream
| 30,500 ppm || 45°C || 8250 m3/hr
|- style="text-align: center;"
! style="text-align: right;" | Product Stream
| ~ 0 ppm || 35°C || 4758 m3/hr
|- style="text-align: center;"
! style="text-align: right;" | Byproduct Stream
| 69,500 ppm || 35°C || 3485 m3/hr
|}
</center>

The process feed will be seawater from Nueces Bay, preheated through the cooling water system of the nearby Nueces Bay Energy Center. Based on calculations developed from NREL (National Renewable Energy Laboratory) data, it was estimated that the Nueces Bay Energy Center uses approximately 250 million gallons of seawater per day for process and cooling needs. Therefore, the 20 million gallon per day requirement for the desalination facility can easily be met through the power plant’s cooling water<ref>Torcellini P, Long N, Judkoff R. Consumptive Water Use for U.S. Power Consumption. NREL. 2003; TP-550-33905.
</ref>. Note that we are assuming the process will operate 67% of the time (16 hours per day).

Based on data acquired from two salinity-measuring stations in Nueces Bay, the average salinity of local seawater is 30.5 practical salinity units (psu)<ref>Stations Map & Real - Time Data. The Conrad Blucher Institute for Surveying and Science website. Available at http://www.cbi.tamucc.edu/cbi/data/. Updated January 2016. Accessed January 9, 2016. </ref>, thus setting the concentration of the process feed stream. This translates to a feed stream concentration of 30,500 ppm (parts per million)<ref>Definition and Units. CATD Salinity Expertise Center. Available at http://www.salinityremotesensing.ifremer.fr/home. Accessed January 12, 2016.</ref>. The composition of the salt attributed to this seawater salinity is shown in Table 6 in Appendix B.

No data on the composition of seawater around the Corpus Christi area was available, but the composition of seawater does not change significantly in different locations. The given composition is considered accurate for our feed stream<ref>Brown E, Colling A, Park D, Phillips J, Rothery D, Wright J. Seawater : Its Composition, Properties and Behaviour. Boston: Butterworth-Heinemann, 1995.
</ref>. Note that seawater salt is primarily sodium chloride - total dissolved solids are 85.7% sodium chloride by mass. As such, the dissolved solids in the feed stream is assumed in this project to be pure NaCl, making the feed composition 96.95% water and 3.05% NaCl by mass. A control system will likely be put in place to ensure feed concentration remains at this level, accounting for natural fluctuations in local seawater salinity.

The largest byproduct from the process will be a brine slurry with high salt concentrations. An upper corrosion limit of 700 ppm was set for recirculated and purged brine. This undesired byproduct will be sent to a nearby Class I non-hazardous injection well facility operated by GNI Group, which is approximately 12 miles away from the proposed desalination plant location as shown in the figure in Appendix A<ref>Texas Water Code. Availabe at http://www.statutes.legis.state.tx.us/Docs/WA/htm/WA.27.htm. Accessed January 14, 2016.
</ref><ref>Google Maps. Available at https://goo.gl/maps/3xcvLZwUCe42 Accessed January 15, 2016.</ref>. Through the incorporation of a Class I injection well, the brine will not have to be treated or diluted before injection. <ref>EPA Regulations on Class I Wells. Available at http://www.epa.gov/uic/class-i-industrial-and-municipal-waste-disposal-wells. Accessed January 14, 2016.
</ref><ref>List of Injection Well Facilities by Location. Available at http://www.ehso.com/cssepa/tsdfdeepwells.php. Accessed January 14, 2016.</ref>

There are no regulations regarding the temperature of city drinking water, so the product stream temperature has no requirements based on regulations. Outlet temperatures of 35oC are acceptable for input to the municipal water supply system. Since city water tends to be stored in non-temperature controlled tanks, the temperature will naturally cool to ambient temperature.<ref>Climate Corpus Christi- Texas. US Climate Data. Available at http://www.usclimatedata.com/climate/corpus-christi/texas/united-states/ustx0294. Accessed January 12, 2016. </ref>

==Pre-Treatment and Post-Treatment==
For pretreatment, Belgard EV 2030 is recommended to be added at a concentration of 1.8-2.0 mg/L for a system with top brine temperature approximately 110 °C<ref name="BWA 2030">BWA Water Additives. Belgard EV2030 Technical Data Sheet. BWA Water Additives. http://www.wateradditives.com/files/products/tech_data_sheets/Belgard%20EV%202030_Technical%20Data%20Sheet_Thermal_8.5x11.pdf. Accessed January 27, 2016.
</ref>. Since our process is designed to operate at a slightly higher top brine temperature, 120 °C, we will design for the upper bound concentration: 2.0 mg/L of antiscalant. With a feed flow rate of 8250 m3/hr, the process therefore requires 16.5 kg/hr of Belgard EV 2030.

For disinfection during post-treatment, chlorine addition typically requires a dose of roughly 2.0 mg/L as well<ref name="World Health Organization">World Health Organization. Desalination for safe water supply. http://www.who.int/water_sanitation_health/gdwqrevision/desalination.pdf. Accessed January 26, 2016.</ref>. With a pure water distillate flow rate of 4758 m3/hr, the process requires 9.5 kg/hr of chlorine as a 5%-15% sodium hypochlorite solution. Approximately 74 mg/L of lime (calcium hydroxide) and 88mg/L of CO2 should be added to the desalinated water to help prevent corrosion and increase alkalinity within acceptable ranges<ref name="Vouchkov">Vouchkov, N. Re-Mineralization of Desalinated Water. SunCam. https://s3.amazonaws.com/suncam/npdocs/118.pdf. Accessed February 18, 2016.</ref>. With a product stream flow rate of 4758 m3/hr, these concentrations correspond to 352.1 kg/hr of lime and 418.7 kg/hr of CO2. Both of these can be purchased, and it is likely that the CO2 can be procured from the neighboring natural gas power plant.

==Choice of Desalination Technology==
The two major technologies commonly implemented for desalination include reverse osmosis (RO), and multi-stage flash distillation (MSFD). RO utilizes high pressures to force water through a semipermeable membrane that allows for purified water to pass through, but stops organic and other wastes. Often, the pressures can reach 55 - 68 bar, which requires large pumps to maintain pressure, and introduces safety concerns to the process<ref>Stage 2 Measures in the Drought Contingency Plan. Available at http://www.cctexas.com/government/water/conservation/conservation-drought-plans/stage-2-measures. Accessed January 12, 2016.
</ref>. While RO can be less energy intensive than MSFD, and the membrane can also remove contaminants, it is susceptible to feedwater quality, membrane fouling, and requires more extensive pretreatment and posttreatment than MSFD. The cost of RO is directly proportional to the feed salinity, and so it is less expensive when desalinating low salinity water.

Multi-stage flash distillation is best selected for areas that have high salinity or low energy costs. The pretreatment process for MSFD is much easier when compared to RO because pretreatment often consists of simply adding anti-corrosives and antiscalants to the feed stream. The process works by heating water in vessels at different pressures to change the boiling point of the water and evaporate it as it passes through the system. After evaporation, the water condenses, leaving pure water as the effluent. MSFD costs are now roughly $1.00/m3, where this cost is mostly independent of salinity, but dependent on energy costs.<ref>Fritzman C, Lowenberg J, Wintgens T, Melin T. State-of-the-art Reverse Osmosis Desalination. Desalination. 2007; 216:1-76.
</ref> In both RO and MSFD a brine is produced which then must be appropriately treated and/or disposed of.

Multi-stage flash distillation was selected for this process, mainly due to the economic advantages offered by the selected location of Corpus Christi, TX. MSFD is also a well understood process that can be more easily modeled by chemical engineering software.

=Technical Approach=
Quantitative modeling of the process and all included optimization was conducted using the simulation software Aspen HYSYS. A variety of assumptions were developed to set up and define specific components of the process simulation itself. Within Aspen HYSYS, the NRTL Electrolyte property package was selected to model the behavior of salt water. Each stage of the MSF process is modeled as an individual flash vessel, with individual shell & tube heat exchangers used to model the distillate condensers present in each flash stage in the MSF system.

The HYSYS simulation model only considers the flash portion of the overall process, allowing establishment of mass and energy balances for the system. Pretreatment and post-treatment were excluded for the HYSYS model, as these steps are simple additions or removals of small concentrations of chemical components. Although the simulation models heat exchangers outside of each flash stage, in the physical design of the plant these heat exchangers will be contained within each flash stage.

One portion of the process is not modeled, which is the inlet seawater entering the heat rejection stages. As can be seen from the full process flow diagram (Appendix C), the inlet seawater in the heat rejection stage is in high excess to provide maximum cooling capacity; then a significant portion is purged to bring the stream flow rate down to what is needed for the feed stream. It is assumed that the flow rate of cooling water for those three stages is in such high excess that the cooling water does not heat up. Therefore, in the process simulation models, the inlet stream is the feed stream, which enters directly into the heat recovery portion of the process.

See Appendix D for a screenshot overview of the simulation environment model.

=Process Overview=
==Process Flowsheet==
The following diagram, Figure 1, shows a basic flowsheet for the process. Process alternatives considered for each stage are discussed below. Please see Appendix C for the full process flowsheet with finalized design decisions.

[[File:BlockFlowDiagram.png|center|700px|thumb|alt=Nueces Bay Desalination Facility Block Flow Diagram|Figure 1: Basic block flowsheet of the multi-stage flash desalination process.]]

==Major Features==
As seen above, there are three primary stages of the desalination process: Pre-treatment, the multi-stage flash distillation, and post-treatment. Seawater enters the pre-treatment phase first to make the feed suitable for the rest of the process by filtering out large particles that could accumulate on pipes, deaerating the water to limit corrosion, and adding an agent to combat fouling. The water then enters the distillation process, which is broken up into two sections: heat recovery and heat rejection.

In the heat recovery stage, there are 18 flash vessels, each of which separates steam from the salinated liquid water. Each flash vessel has saltwater as its inlet and two outlets: a vapor outlet (steam) that condenses into the final product of purified water and a liquid outlet composed of water with more dissolved solids than the inlet, which is the inlet stream for the next flash vessel. The pipe containing the feed is first mixed with recycled brine, then goes through the top sections of each of the flash vessels, starting with V-118 and ending up at V-101. In this way, the feed stream pipe first serves as a heat exchanger, heating the pre-treated seawater while simultaneously condensing the steam into the final product. The feed then enters a final heat exchanger where it is brought up to 103.6 °C before entering V-101 to be flashed.

After exiting V-118, the final flash vessel in the heat recovery portion, the water enters the heat rejection portion, which consists of three flash vessels (V-201, V-202, V-203). The flash vessels of this section operate in the same manner as in the heat recovery section, with the exception that condensation occurs using excess cooling water instead of pre-treated seawater. This technique allows excess heat to be rejected into an excess water stream rather than the feed.

The purified product stream finally enters post-treatment processing in which water is re-aerated and remineralized, disinfected, and pH is adjusted before being delivered to the city. The concentrated brine leftover at the end of the flash cycle is recycled into the feed, although a purge is required to continuously remove some salt from the system. This purged brine is disposed of by use of a Class I injection well, per regional regulations.

See Appendix E to find detailed stream tables of stream specifications.

==Process Alternatives==
===Pretreatment===
Multi-stage flash distillation requires significantly less pretreatment than reverse osmosis desalination, but helpful pretreatment techniques still exist. One very inexpensive method of pretreatment is screening/filtration, which is used to filter out large contaminants from intake seawater. Using more complicated membrane technology to accomplish large particle filtration is a possible alternative, but doing so would be much more expensive than using a standard screen or filter, so the latter choice has been selected for this process.<ref>El-Dessouky H., Alatiqi I, Ettouney H. Process synthesis: The multi-stage flash desalination system. Desalination. 1998;115(2):115-179. doi: 10.1016/S0011-9164(98)00035-6.</ref>

The most critical type of fouling in multi-stage flash distillation is scaling - therefore, the most critical step in the pretreatment process will be the inclusion of an anti-scalant. Alkaline scaling can be prevented by acid dosing or polyphosphate addition; however, acid dosing can result in increased corrosion and polyphosphates can lead to non-alkaline calcium phosphate fouling, so these additives will not be considered<ref name="Morris">Morris, R. The development of the multi-stage flash distillation process: A designer's viewpoint. Desalination. 1993;93(1-3):57-68. doi:10.1016/S0011-9164(93)80096-6. Availble at http://www.sciencedirect.com/science/article/pii/0011916493800966</ref><ref>Al-Gobaisi, D. A quarter-century of seawater desalination by large multistage flash plants in Abu Dhabi (Plant performance analysis, assessment, present efforts toward enhancement and future hopes). Desalination. 1994;99(2-3):509-512. doi: 10.1016/S0011-9164(94)00196-0.</ref>. As such, a polymer-based additive is recommended for the anti-scalant for this process. The Belgard EV series is designed for use in high-temperature seawater MSF; of these, Belgard EV 2030 is the most neutral in pH<ref>NSF International. NSF Product and Service Listings: Drinking Water Treatment Chemicals - Health Effects. NSF International. http://info.nsf.org/Certified/PwsChemicals/Listings.asp?Company=00300&. Accessed January 27, 2016.</ref><ref name="BWA 2030" /><ref>BWA Water Additives. Belgard EV2035 Technical Data Sheet. BWA Water Additives. http://www.wateradditives.com/files/products/tech_data_sheets/Belgard%20EV2035_Technical%20Data%20Sheet_Thermal_8.5x11.pdf. Accessed January 27, 2016.</ref><ref>BioLab Water Additives. Belgard EV Antiscalant for seawater distillation plants. BioLab Water Additives. http://www.lpq.com.mx/pdf/BELGARD%20EV.PDF. Accessed January 27, 2016.</ref>. As such, we have selected Belgard EV 2030 for use as an anti-scalant in our process.

Corrosion control is another significant method of pretreatment in multi-stage flash distillation plants. Using a piping material that has limited susceptibility to corrosion is typically not economically viable for MSF desalination plants due their large size, so using carbon steel for most of the piping is generally used for its economic benefits despite its susceptibility to corrosion<ref name = "Morris" />. Limiting oxygen concentration is very feasible through the use of deaerators, which can remove oxygen as well as other dissolved gases from the seawater stream. Often, an external deaeration system is used<ref name = "Morris" /><ref>Edwards - Hick Hargreaves. Seawater Deaerators. Edwards - Hick Hargreaves. http://psscorp.co.th/upload/news/20110830105340.pdf. Accessed January 26, 2016.</ref>. Given these considerations, using an external deaerator combined with a less-corrosive antiscalant like Belgard EV 2030 is much more practical for the process than using anti-corrosive piping.

Excessive foam can also be an issue in some MSF distillation facilities, so antifoaming agents are sometimes employed in the pretreatment process as well. However, many plants are operated without foaming issues, and therefore do not need anti-foaming pretreatment at all. Selection of an anti-foaming agent must be carefully weighed with selected of anti-scalant to avoid issues with additive interactions [26]. At this stage in the design process, an anti-foamer will not be included due to the risk of decreasing scalant efficacy, and the likelihood that foam will not be an issue in the desalination plant.

===Multistage Flash Distillation===
The main objective of the design project is desalinate water. This is done with the main process of multi stage flash distillation (MSF). MSF can be developed using a numerous number of different process configurations, such as Once-Through, Simple Mixer Brine Circulation, and Multi-Stage Heat Rejection Brine Circulation processes.

The most desired process for the project is the Three Stage Heat Rejection Brine Circulation as it builds on benefits of the other simpler technologies. By combining principles of heat rejection and heat recovery, the required heat transfer area of the process is greatly reduced as compared to once through processes or simple brine recycle processes. It has the highest performance ratio with the lowest specific feed flow rates, therefore greatly lowering the operating costs of the process. It is also known as the industry standard for MSF<ref name = "KSU Chapter 6">Kansas State University ChE 413. Available at: http://faculty.ksu.edu.sa/Almutaz/Documents/ChE-413/Multi-stage%20Flash%20Desalination.pdf. Accessed January 26, 2016.</ref>. In addition, since our brine disposal technique involves the pumping of the brine into a Class I well, the high brine outlet salinity is not a drawback for our process since post treatment or dilution of the brine is not necessary.

===Post Treatment===
After processing the water via multi-stage flash, the water needs to be post-treated to make the water taste as consumers would expect, since distilled water is “flat” and unpleasant, as well as to protect the consumers and the piping through which the water will be passing<ref name="Clayton">Clayton R. A review of current knowledge: Desalination for water supply. Foundation for Water Research. http://www.fwr.org/desal.pdf. Accessed January 26, 2016.</ref>. There are multiple alternatives to achieve these objectives of remineralization, aeration, corrosion control, and disinfection.

Aerating and introducing some dissolved solids back into the product water are effective ways to create a quality product that is not flat, and contains flavoring that is expected of the water when delivered to the consumer. Aeration will be accomplished via an inline spray aerator which will take up less space than cascade aeration.<ref>Minnesota Rural Water Association. Aeration. http://www.mrwa.com/WaterWorksMnl/Chapter%2011%20Aeration.pdf. Accessed January 26, 2016.</ref> Feed blending will be used to remineralize the water as opposed to a remineralization filter because it does not require additional equipment purchases, and can easily handle the large amounts of throughput required for the process.<ref name="Clayton" />

Corrosion and pH can be controlled by adding lime (calcium hydroxide) and carbon dioxide to the water. To do so, about 74 mg/L of lime and 88mg/L of CO2 should be added to the desalinated water to help prevent corrosion and increase alkalinity within acceptable ranges. Sodium hydroxide, soda ash, and sodium bicarbonate are alternatives to altering the alkalinity and pH of the product water, but lime and CO2 is an industry standard <ref name="Vouchkov" />.

Disinfection needs to be done to prevent microbial growth in the desalinated water and ensure the safety of the general public. Chlorine addition via chlorine gas or sodium hypochlorite is the most common and only requires a dose of 1.5 to 2.5 mg/liter. <ref name="World Health Organization" /> Other alternatives include ozonation, UV light, and chloramination. Chlorine, as a 5% - 15% sodium hypochlorite solution, is sufficient for our process and relatively inexpensive, and as such will be utilized in the process.

===Transport of Brine to Well===
Three major possibilities exist for transporting the brine from our desalination plant to the injection well for disposal: rail, trucking, and a pipeline. A pipeline was chosen as the most feasible and cost-effective approach.

A rail line is not available at this time, and with the cost to construct a rail line at approximately $43 million per mile<ref>About Money. How Much Do Rail Transit Projects Cost to Build and Operate? 2014. Available at: http://publictransport.about.com/od/Transit_Projects/a/How-Much-Do-Rail-Transit-Projects-Cost-To-Build-And-Operate.htm. Accessed January 27, 2016. </ref>, the enormous capital cost makes rail transport infeasible for this site. For large volume operations, pipelines are cheaper than trucking. In a study of wastewater sludge produced from wastewater treatment facilities, it was found that it was more economical for facilities producing over 1200 m3 per day to utilize pipelines rather than trucking.<ref>Marufuzzaman M, Eskioglu S, Hernandez R. Truck versus pipeline transportation cost analysis of wastewater sludge. Transportation Research. 2015; 74(A): 14-30. </ref> Our facility is producing nearly 3 times this volume at 3485 m3 per day of brine. Furthermore, brine is a less viscous fluid than wastewater sludge, decreasing pumping costs in the pipeline and further amplifying the benefit of choosing a pipeline over an injection well.

===Brine Storage===
Due to the choice of a pipeline, rather than trucking, as the method of transport of brine to the injection well, brine storage is unnecessary, so storage solutions such as holding tanks were not investigated. One method of storage compatible with a pipeline is to pump the brine through an evaporation pond. This solution is attractive as it would allow the brine to concentrate, saving some costs of pumping, however, the considerable land usage of evaporation ponds leads them to only be effective for smaller plants. The largest municipal plant utilizing this method produces only 5.7 million liters per day<ref>University of Santa Cruz Center for Integrated Water Research. Concentrate Management: Evaporation Ponds. 2010. Available at: https://ciwr.ucsc.edu/sites/default/files/ICM_D5.pdf. Accessed January 27, 2016. </ref>, whereas our desalination plant is proposed to produce 75.7 million liters per day. Therefore, evaporation ponds have been eliminated from consideration.

==Major Specifications==
===Material of Construction Selection===
While saltwater and desalinated water in general can be corrosive, there are best practices within the desalination industry for material selection. With this in mind, best practice shows carbon steel lifetime can be lengthened by monitoring the oxygen concentration in the water, specifying the paint used to coat any metal, and more expensive materials can be used in areas of extremely high corrosion.<ref name = "Morris" />. By following these best practices, it is possible to save significant amounts of money through the use of carbon steel in most areas of the plant, with specific high-corrosion areas being reinforced with, or replaced with, more corrosion resistant materials, such as Cu/Ni 70/30 alloy<ref name = "KSU Chapter 6" />.

===Heat Exchanger Sizing===
See Appendix F for heat exchanger sizing data. Heat exchanger sizes appear reasonable for placement in the flash vessels.

===Flash Vessel Sizing===
The dimensions for the flash drums were obtained through literature values. The flash drums were determined to be 18x4x3 m in width, height, and length.<ref name="KSU Chapter 6" />

Wall thickness was determined using the following conventions: Design pressure is 10% more than the operating pressure, design temperature is 50°F (27.8°C) higher than the operating temperature, a corrosion allowance of 3-mm and an effectiveness of 85% was derived from the assumption of a double-welded butt joint that was spot checked. See Appendix G for wall thicknesses calculated for each pressure vessel.

The sizing of the vessels to be constructed will be determined after talks with a manufacturer. Due to economies of scale, it may be more cost-effective to buy 21 pressure vessels with the dimensions mentioned above and a wall thickness of 17.1-mm, the highest wall thickness required in the system.

===Pipe Losses and Pressure Drop===
HYSYS was used to estimate the diameter of a pipe given a specified pressure drop per 100m of pipe. It was determined that for a pressure drop of 0.1 kPa/100m of pipe, a 3.05m diameter pipe is sufficient, which is more than manageable for manufacturers. This pressure drop is mostly insignificant and will be taken care of with the safety factors included in the pumps. The significant pressure drops in our system is accounted for via losses in the flash vessels and heat exchangers, which drive the flash process. Therefore overall pipe losses are insignificant and are built into intentional design losses.

=Economic Analysis=
==Economic Assumptions==
The sale of purified water yields a revenue stream of $49.5 million per year assuming sale to commercial entities within the city limits of Corpus Christi at $1.7915 per thousand liters.<ref>Water schedule. City of Corpus Christi Utility Business Office. http://www.cctexas.com/Assets/Departments/Financial-Services/Utilities-Business-Office/files/waterrates.pdf Accessed February 28, 2016.</ref> The primary variable costs are utilities and the brine disposal, with minimal raw materials costs due to the primary feedstock, seawater, being free, and minimal amounts of pre- and post-treatment chemicals being used. The cost of brine disposal used in this analysis was high at $2.097 per thousand liters<ref>McCurdy, R. Underground injection wells for produced water disposal. Chesapeake Energy Corporation. http://www.epa.gov/sites/production/files/documents/21_McCurdy_-_UIC_Disposal_508.pdf Accessed February 28, 2016.</ref>, an estimate for brine disposal utilizing trucking as the mode of transport. As has been shown through previous analyses, a lower cost can be achieved using a direct pipeline, however, no good estimates for this cost could be found.

The NPV was calculated using a 6% cost of capital, the approximate interest rate for a municipal bond.<ref>Zirbe, R.O. Jr., Han, X., Layton, D., Leshine, T. A history of discount rates and their use by government agencies. (2002). http://faculty.washington.edu/zerbe/docs/DISCOUNT%20RATE%20HISTORY-latesXIt.doc Accessed February 28, 2016.</ref> It was assumed that the plant can be built within two years, with production at half-scale in its first year. Additionally, one month of variable costs, $10.6 million, was set aside for working capital. See Appendix H for a full table of assumptions. A 30 year NPV calculation was chosen as the decision basis since 30 years is a conservative estimate for the lifetime of a desalination plant [38]. See below for estimates of NPV for the lifetime of the plant under various assumptions and design criteria.

==Optimization and Sensitivity Analysis==
Looking at the multi-stage flash process, there are a number of opportunities to optimize the process. The major process variables that were investigated were the inlet water temperature, the operating pressure drop per vessel, and the number of stages over which the process takes place. Considering these optimizations, our final process consists of 21 flash vessels (18 heat recovery and 3 heat rejection stages), each with a pressure drop of 5 kPa. See Appendix D for a screenshot of the HYSYS simulation used to model the optimized process.
===Operating Pressure and Pressure Drop===
Initially our process was designed to pump the inlet seawater and brine to a pressure of 2 bar and allow the pressure drop in each flash stage to end our process downstream at 1 bar to prevent complications associated with designing a system under vacuum. While this provided pumping advantages, the energy in the system was significantly higher due to vapor pressure considerations. Due to this fact, the required heat exchange area for certain exchangers proved to be infeasible (on the order of 80,000m2) due to the enormous capital and operating costs that would be associated with such a large heat exchanger.

The process was optimized to operate under vacuum, requiring additional pumping requirements but much lower required heat exchange area, and therefore lower utility costs. This optimization was done by specifying the pressure of the final flash vessel to 5.7kPa, so that the temperature of the water vapor (~30oC) is within approximately 5oC of the cooling water (35oC) so that condensation can occur within a reasonable approach. This also allows our system to optimally extract pure water from the brine running through the system and reduce energy requirements for cooling the product or the recycle. Our system redesign and optimization reduced the energy in the system and reduce the same heat exchanger area requirements by more than 3 orders of magnitude to approximately 800m2.

Once the process was optimized to operate in a vacuum, the pressure drop per flash stage regulated through values was then optimized. Three pressure drops per stages, 3kPa, 5kPa, and 7kPa, were tested and then an economic analysis was conducted to find the optimal value for the pressure drop. These changes had impacts on both operating and capital costs as they would change initial pumping requirements, the amount of water flowing through the process and recycle (and therefore the flash vessel sizes, heat exchanger sizing), and the efficiency of each stage.

The various pressure drops (3 kPa, 5 kPa, and 7 kPa) were tested on a system nominally at 24 stages under the assumption that the results would be generalizable to systems with a different number of stages. With increasing pressure drop, we found decreased capital cost but increased utility cost. As can be seen from Table 2 below, a pressure drop of 5 kPa proved to be the best option based on 30 year NPV, with a loss of $838.8 million, a savings of $65.5 million compared to the next-best option with a pressure drop of 7 kPa.

<center>
{| class = "wikitable"
|+Table 2: Comparison of profitability for our MSF desalination process with 24 stages with 3 different pressure drops per stage
! Number of Stages  
! Pressure Drop (kPa)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
! 30-Year NPV ($MM)
! NPV Ratio (over 5kPa)
|- style="text-align: center;"
| 24 || 3 || 362.3 || 39.1 || -905.8 || 1.08
|- style="text-align: center;"
| 24 || 5 || 255.1 || 53.6 || -838.8 || 1.00
|- style="text-align: center;"
| 24 || 7 || 209.5 || 72.2 || -904.3 || 1.08
|}
</center>

===Number of Flash Stages===
The number of stages is an important area for optimization. An increased number of stages yields better recovery of purified water, however, requires higher capital cost from the additional pressure vessel. Furthermore, with an increased amount of purified water comes increasingly concentrated brine. This must be taken into account since any brine concentration in excess of about 7% presents a more significant corrosion risk<ref name="KSU Chapter 6" />, so increased maintenance costs for the piping or higher capital costs for more corrosion-resistant materials could be necessary for brine concentrations exceeding 7%. Therefore, to recover the same amount of pure water without exceeding corrosion limitations, increasing the number of stages will reduce the overall size of each stage.

Optimizing based on NPV, it was found that 21 stages is optimal, balancing the increased profits from the additional purified water additional costs of higher capital cost. See Table 3 below to compare the 30-year NPV of the process across various numbers of flash vessels.

<center>
{| class = "wikitable"
|+Table 3: Comparison of profitability for our MSF desalination process with a 5 kPa pressure drop, 5000 m3/hr feed flow rate, with varying number of stages.
! Number of Stages  
! Pressure Drop (kPa)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
! 30-Year NPV ($MM)
! NPV Ratio (over 21 Stages)
|- style="text-align: center;"
| 17 || 5 || 213.9 || 58.7 || -813.8 || 1.016
|- style="text-align: center;"
| 18 || 5 || 218 || 57.6 || -809.9 || 1.011
|- style="text-align: center;"
| 19 || 5 || 224.3 || 58 || -806.1 || 1.007
|- style="text-align: center;"
| 20 || 5 || 229.8 || 57.9 || -813.6 || 1.016
|- style="text-align: center;"
| 21 || 5 || 226.6 || 55.9 || -800.7 || 1
|- style="text-align: center;"
| 22 || 5 || 254.7 || 54.6 || -849.5 || 1.061
|- style="text-align: center;"
| 23 || 5 || 249.3 || 54.1 || -833.1 || 1.04
|- style="text-align: center;"
| 24 || 5 || 255.1 || 53.6 || -838.8 || 1.048
|- style="text-align: center;"
| 25 || 5 || 254.9 || 52.7 || -837.4 || 1.046
|}
</center>

===Inlet Seawater Temperature===
By pairing our desalination plant with a natural gas power plant and utilizing its cooling water we forgo initial heating of our inlet stream. Normally, water pumped from the ocean would be approximately 25oC and by capturing waste heat from the power plant we can reduce utilities with our water inlet of 45oC. Through an economic comparison between two plants nominally at 21 flash stages, the plant with an inlet temperature at 25oC has a capital cost of $245MM and a utility cost of $77MM/yr and the same facility with an inlet water temperature at 45oC has a capital cost of only $226MM and a utility cost of $56MM/yr. As shown in Table 4 below, this $19MM savings in capital costs and $21MM/yr saving in utilities is largely due to reducing the size of the heat exchangers and lowering the steam required for heating duty on the inlet seawater respectively. Therefore our process is optimized by recovering the waste heat water from the gas fired power plant.

<center>
{| class = "wikitable"
|+Table 4: Seawater Inlet Temperature Process Cost Optimization
! Seawater Temperature (°C)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
|- style="text-align: center;"
| 25 || 245 || 77
|- style="text-align: center;"
| 45 || 226 || 56
|- style="text-align: center;"
| Savings || 56 || 19
|}
</center>

==Overall Process Economics==
After optimization at fixed flow rates, the process was scaled to meet the the 67% operating percentage and final product definitions as shown in Table 1, which resulted in finalized economics shown in Table 5 below. For a table of the installed costs of each process vessel, see Appendix I. For the finalized NPV calculations, see Appendix J.

<center>
{| class = "wikitable"
|+Table 5: Summarized Economics for 21-stage MSFD Process
! Economic Parameter
! Cost (USD)
|- style="text-align: center;"
| Capital Cost || 345 MM
|- style="text-align: center;"
| Total Operating Cost per Year || 103 MM
|- style="text-align: center;"
| Utility Cost per Year || 59 MM
|- style="text-align: center;"
| 30-Year NPV (6% Discount) || -1035 MM
|}
</center>

=Conclusions and Recommendations=
==Project Viability==
Table 5 above demonstrates that the process has reasonable operating and capital costs, but the 30-year NPV is negative by -1035 $MM due to the extremely low sell price capability of desalinated water as a product. Therefore, this plant would have to heavily be supported by the City of Corpus Christi to run as the product revenue will not make it self-sufficient. The acquisition of government subsidies are key operating requirements for all desalination facilities in the United States.

==Additional Concerns==
Large volumes of water, steam, and energy are needed to ensure the project runs as specified. While most of the water can easily be extracted from the ocean with little to consequence, the fuel used to generate the steam and electricity can have a significant environmental impact. As the plant is located next to a natural gas power plant, this fuel is better than coal with respect to environmental concerns, and utilizing energy from the power plant to preheat the feed stream for the desalination plant helps lower environmental impacts.

The large amounts of high pressure steam being used represents one of the the largest safety concern in the process, because a high pressure steam release could easily cause severe burns or even death. Proper precautions and control measures must be in place to prevent accidental release. Otherwise, the process does not utilize any known toxic chemicals and is relatively safe from a chemical standpoint. The flash vessels mostly operate below atmospheric pressure, so they must be constructed as vessels that can withstand vacuum without failing.

=Appendix A: Proposed Location of Desalination Plant=
Pictured below is the proposed location for the desalination plant, located on Nueces Bay in Corpus Christi, adjacent to the Nueces Bay Energy Center. The plant’s proposed location is shown in the area outlined by the small circle in the upper right of the figure.

[[File:NuecesBayLocation.png|center|600px|thumb|alt=Nueces Bay Desalination Facility Location|Figure A.1: Proposed mapped location of Nueces Bay Desalination Facility.]]
[[File:ClassIWell.png|center|700px|thumb|alt=Nueces Bay Desalination Facility Distance to Injection Well|Figure A.2: Mapped distance from proposed location to Class I Injection Well]]

=Appendix B: Feed and Product Definitions - Salinity=

<center>
{| class = "wikitable"
|+Table 6: Average Composition of Total Dissolved Solids in Seawater<ref> Physical and Thermodynamic Data. In: Dickson AG, Goyet C. Handbook of Methods for the Analysis of the Various Parameters of the Carbon Dioxide System in Sea Water.</ref><ref> Composition of Seawater. Water Treatment of Solutions. Available at http://www.lenntech.com/composition-seawater.htm. Accessed January 12, 2016.</ref>
! Compound  
! Mass Composition (kPa)
|- style="text-align: center;"
| Chlorine || 55.03%
|- style="text-align: center;"
| Sodium || 30.64%
|- style="text-align: center;"
| Sulfate || 7.70%
|- style="text-align: center;"
| Magnesium || 3.66%
|- style="text-align: center;"
| Calcium || 1.17%
|- style="text-align: center;"
| Potassium || 1.12%
|- style="text-align: center;"
| Bicarbonate || 0.36%
|- style="text-align: center;"
| Bromine || 0.19%
|- style="text-align: center;"
| Other || 0.15%
|}
</center>

[[File:Nueces Bay Salinity.JPG|center|700px|thumb|alt=Nueces Bay Salinity Tracking|Figure C.1: Salinity in Nueces Bay over the past 10 years6.]]

=Appendix C: Process Flow Diagram=
[[File:TeamF_FinalPFD.JPG|center|1200px|thumb|alt=Process Flow Diagram|Figure C.1: Process Flow Diagram]]

=Appendix D: HYSYS Simulation Model=
[[File:TeamF_HYSYS_Screenshot.PNG|center|1200px|thumb|alt=HYSYS Simulation Model|Figure D.1: Screenshot of HYSYS Model Simulation.]]

=Appendix E: Stream Tables=
A summary of some values is shown below:

<center>
{| class = "wikitable"
|+Table 7: Key process material streams
!  
! Stream  
! Molar Flow (105 kgmole/h)
! Mass Flow (105 kg/h)
|- style="text-align: center;"
! style="text-align: right;" | Inlet
| Feed Water || 454 || 83.6
|- style="text-align: center;"
! style="text-align: right;" | Outlet
| Brine Reject || 1.90 || 36.0
|- style="text-align: center;"
! style="text-align: right;" | Product
| Pure Water || 2.64 || 47.5
|}
</center>

<center>
{| class = "wikitable"
|+Table 8: Key process energy streams
! Stream  
! Heat Flow (kJ/h)
|- style="text-align: center;"
| Feed Water || 1.30E+11
|- style="text-align: center;"
| Q-301 || 5.73E+06
|- style="text-align: center;"
| Q-302 || 3.09E+09
|- style="text-align: center;"
| Q-304 || 4.68E+06
|- style="text-align: center;"
| Q-303 || -1.26E+09
|- style="text-align: center;"
| Brine Reject || -5.47E+10
|- style="text-align: center;"
| Pure Water || -7.46E+10
|}
(Negative value denotes energy leaving process)
</center>

=Appendix F: Heat Exchanger Sizing=

<center>
{| class = "wikitable"
|+Table 9: Utility Temperatures and Heat Transfer Coefficient<ref name="Towler">G.P. Towler, R. Sinnott. Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design. Elsevier, 2012.</ref>
!  
! HP Steam
! Cooling Water
|- style="text-align: center;"
! Temperature (°C)
| 250 || 35
|- style="text-align: center;"
! U (W/m2K)
| 4000 || 1500
|}
</center>

<center>
{| class = "wikitable"
|+Table 10: Internal Flash Heat Exchanger Sizing, condensing steam.
! Exchanger
! UA (kJ/°C-h)
! U (kJ/hr/m2°C)
! A (m2)
|- style="text-align: center;"
| E-101 || 1.47E+07 || 14400 || 1019
|- style="text-align: center;"
| E-102 || 1.53E+07 || 14400 || 1059
|- style="text-align: center;"
| E-103 || 1.59E+07 || 14400 || 1103
|- style="text-align: center;"
| E-104 || 1.66E+07 || 14400 || 1153
|- style="text-align: center;"
| E-105 || 1.74E+07 || 14400 || 1209
|- style="text-align: center;"
| E-106 || 1.83E+07 || 14400 || 1272
|- style="text-align: center;"
| E-107 || 1.93E+07 || 14400 || 1342
|- style="text-align: center;"
| E-108 || 2.05E+07 || 14400 || 1424
|- style="text-align: center;"
| E-109 || 2.19E+07 || 14400 || 1518
|- style="text-align: center;"
| E-110 || 2.35E+07 || 14400 || 1629
|- style="text-align: center;"
| E-111 || 2.54E+07 || 14400 || 1761
|- style="text-align: center;"
| E-112 || 2.77E+07 || 14400 || 1921
|- style="text-align: center;"
| E-113 || 3.05E+07 || 14400 || 2119
|- style="text-align: center;"
| E-114 || 3.42E+07 || 14400 || 2372
|- style="text-align: center;"
| E-115 || 3.90E+07 || 14400 || 2706
|- style="text-align: center;"
| E-116 || 4.56E+07 || 14400 || 3167
|- style="text-align: center;"
| E-117 || 5.55E+07 || 14400 || 3856
|- style="text-align: center;"
| E-118 || 7.20E+07 || 14400 || 5000
|}
</center>

=Appendix G: Pressure Vessel Sizing=

=Appendix H: Expanded Economic Assumptions=

=Appendix I: Equipment Cost Summary=

=Appendix J: Economic NPV Analysis=
[[File:Team F Economics.JPG|center|1200px|thumb|alt=Economic Analysis|Figure J.1: Screenshot of process economic analysis.]]

=References=
<references/>

Nueces Desalination Center: Production of Drinking Water by Multi-Stage Flash Distillation

2016-03-07T21:11:54Z

Benjkg: /* Appendix J: Economic NPV Analysis */

Team F: Rankine 672 Final Report

Authors: Kedric Daly, Ben Granger, Evan Rosati, Kathleen Zhou

Instructors: Fengqi You, David Wegerer

March 11, 2016

=Executive Summary=
Desalination of seawater to drinking water is a technology of ever-increasing importance given trends of increasingly widespread water shortages around the world. Even in the United States, water shortages are becoming increasingly common in dry, highly-populated such as California and certain areas of the Gulf Coast. Our design team, Rankine 672, was tasked with evaluating the need for and designing a desalination plant to provide purified water in a place of need. Investigation revealed that the city of Corpus Christi, Texas, is a strategic location for development of a desalination facility, as surrounding region is in need of new drinking water sources and the city has expressed serious interest in contracting the development of a large-scale desalination plant. Our proposed solution is a multi-stage flash distillation desalination facility capable of producing 20,000,000 gallons (75,708 m3) of purified drinking water for the city and surrounding region per operating day.

In this design, drinking water is produced from seawater feedstock through a 21-stage flash distillation process. By strategically locating the facility next to an existing natural gas power plant, the Nueces Bay Energy Center, pre-heated feed water can be taken directly from the outbound cooling systems of the power plant, allowing energy savings for our desalination plant. The process is designed for 16 hour-per-day operation, requires a seawater feed of 8250 m3/hr, and produces 4758 m3/hr of purified drinking water alongside a waste stream of 3485 m3/hr of concentrated brine. The waste stream is isolated from a recycle stream through a 9.0% purge; this results in an overall product yield of 57.7% on a volume basis. The process requires an additional pretreatment stream of 16.5 kg/hr Belgard EV 2030, which serves as an antiscalant, as well as 9.5 kg/hr chlorine, 352.1 kg/hr lime, and 418.7 kg/hr CO2 for post-treatment purposes.

The overall capital cost of the plant is expected to be $345MM, with a total operating cost of $103MM/yr, $59MM/yr of which comes from utilities. Due to the low price at which the product water can be sold, the 30-year net present value of the plant, assuming a 6% discount rate, is $-1,035MM. It is thus imperative this plant be constructed with public funds because it cannot return a profit to any private investor. The growing need for water however could justify the cost, because it is important that the residents of the greater Corpus Christi area have access to drinkable water.

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=Introduction=
==Background==
Clean drinking water is a basic human need that is becoming increasingly difficult to meet. Currently, over 1 billion people live in places where water is considered scarce<ref> Tapping the oceans. The Economist USA. http://www.economist.com/node/11484059. Accessed March 2, 2016. </ref>. Though often thought of as a problem exclusive to third-world countries, the issue is becoming more prevalent in the US as well, with water managers in 40 of 50 states anticipating water shortage conditions in some portion of their state in the next 10 years. These shortages are due to increasing demands on constant or diminishing freshwater resources, therefore, to correct the issue, a new supply must be found<ref> Water Supply in the US. EPA. http://www3.epa.gov/watersense/pubs/supply.html. Accessed March 2, 2016.</ref>. Desalination of seawater is therefore important as a new source for clean drinking water and prevent water shortages.

==Project Definition==
The overall goal of the project is to investigate the feasibility of a desalination plant in order to meet the needs of growing water demand. The design needs to take health, safety, and environmental concerns into account, while optimizing costs.

=Design Basis=
Site Location and Conditions
Corpus Christi, TX was chosen as the location for the proposed desalination plant. The city is currently experiencing “Moderate Shortage Conditions”. This issue additionally comes at a time of growing demand with population growth anticipated due to new industry and Eagle Ford Group crude oil production<ref>Stage 2 Measures in the Drought Contingency Plan. Available at http://www.cctexas.com/government/water/conservation/conservation-drought-plans/stage-2-measures. Accessed January 12, 2016. </ref>.

In June 2014, the city announced the Corpus Christi Desalination Demonstration Project<ref>Corpus Christi Desalination Demonstration Project. Available at http://www.cctexas.com/Assets/Departments/Water/Files/DesalFactSheet.pdf. Published June 2014. Accessed January 12, 2016.
</ref> which seeks to create a demonstration desalination plant capable of producing 20,000 gallons of water per day, with plans to later build a full scale facility generating 20,000,000 gallons of water per day. The desalination demonstration project indicates a clear need for implementation of a desalination plant of this size to accommodate growth in the Corpus Christi area.

Locating our plant in Corpus Christi will allow us to take advantage of the benefits of cogeneration through a partnership with the Nueces Bay Energy Center. By utilizing their outlet cooling water as a large portion of our feed for the desalination process, we can raise the temperature of our seawater feed, saving on energy costs in the multi stage flash distillation. The proposed location for the desalination plant is immediately west of the Nueces Bay Energy Center. See Appendix A for a map depicting the proposed location for the desalination plant along the coast of Nueces Bay.

==Feed and Product Definitions==
The proposed desalination facility’s feed and product streams are defined as shown in Table 1. Rationale for these definitions is also described below. The values reported in Table 1 are final derived numbers from HYSYS simulation modeling.

<center>
{| class = "wikitable"
|+Table 1: Feed and product stream definitions
!  
! Salt Concentration
! Temperature
! Amount
|- style="text-align: center;"
! style="text-align: right;" | Feed Stream
| 30,500 ppm || 45°C || 8250 m3/hr
|- style="text-align: center;"
! style="text-align: right;" | Product Stream
| ~ 0 ppm || 35°C || 4758 m3/hr
|- style="text-align: center;"
! style="text-align: right;" | Byproduct Stream
| 69,500 ppm || 35°C || 3485 m3/hr
|}
</center>

The process feed will be seawater from Nueces Bay, preheated through the cooling water system of the nearby Nueces Bay Energy Center. Based on calculations developed from NREL (National Renewable Energy Laboratory) data, it was estimated that the Nueces Bay Energy Center uses approximately 250 million gallons of seawater per day for process and cooling needs. Therefore, the 20 million gallon per day requirement for the desalination facility can easily be met through the power plant’s cooling water<ref>Torcellini P, Long N, Judkoff R. Consumptive Water Use for U.S. Power Consumption. NREL. 2003; TP-550-33905.
</ref>. Note that we are assuming the process will operate 67% of the time (16 hours per day).

Based on data acquired from two salinity-measuring stations in Nueces Bay, the average salinity of local seawater is 30.5 practical salinity units (psu)<ref>Stations Map & Real - Time Data. The Conrad Blucher Institute for Surveying and Science website. Available at http://www.cbi.tamucc.edu/cbi/data/. Updated January 2016. Accessed January 9, 2016. </ref>, thus setting the concentration of the process feed stream. This translates to a feed stream concentration of 30,500 ppm (parts per million)<ref>Definition and Units. CATD Salinity Expertise Center. Available at http://www.salinityremotesensing.ifremer.fr/home. Accessed January 12, 2016.</ref>. The composition of the salt attributed to this seawater salinity is shown in Table 6 in Appendix B.

No data on the composition of seawater around the Corpus Christi area was available, but the composition of seawater does not change significantly in different locations. The given composition is considered accurate for our feed stream<ref>Brown E, Colling A, Park D, Phillips J, Rothery D, Wright J. Seawater : Its Composition, Properties and Behaviour. Boston: Butterworth-Heinemann, 1995.
</ref>. Note that seawater salt is primarily sodium chloride - total dissolved solids are 85.7% sodium chloride by mass. As such, the dissolved solids in the feed stream is assumed in this project to be pure NaCl, making the feed composition 96.95% water and 3.05% NaCl by mass. A control system will likely be put in place to ensure feed concentration remains at this level, accounting for natural fluctuations in local seawater salinity.

The largest byproduct from the process will be a brine slurry with high salt concentrations. An upper corrosion limit of 700 ppm was set for recirculated and purged brine. This undesired byproduct will be sent to a nearby Class I non-hazardous injection well facility operated by GNI Group, which is approximately 12 miles away from the proposed desalination plant location as shown in the figure in Appendix A<ref>Texas Water Code. Availabe at http://www.statutes.legis.state.tx.us/Docs/WA/htm/WA.27.htm. Accessed January 14, 2016.
</ref><ref>Google Maps. Available at https://goo.gl/maps/3xcvLZwUCe42 Accessed January 15, 2016.</ref>. Through the incorporation of a Class I injection well, the brine will not have to be treated or diluted before injection. <ref>EPA Regulations on Class I Wells. Available at http://www.epa.gov/uic/class-i-industrial-and-municipal-waste-disposal-wells. Accessed January 14, 2016.
</ref><ref>List of Injection Well Facilities by Location. Available at http://www.ehso.com/cssepa/tsdfdeepwells.php. Accessed January 14, 2016.</ref>

There are no regulations regarding the temperature of city drinking water, so the product stream temperature has no requirements based on regulations. Outlet temperatures of 35oC are acceptable for input to the municipal water supply system. Since city water tends to be stored in non-temperature controlled tanks, the temperature will naturally cool to ambient temperature.<ref>Climate Corpus Christi- Texas. US Climate Data. Available at http://www.usclimatedata.com/climate/corpus-christi/texas/united-states/ustx0294. Accessed January 12, 2016. </ref>

==Pre-Treatment and Post-Treatment==
For pretreatment, Belgard EV 2030 is recommended to be added at a concentration of 1.8-2.0 mg/L for a system with top brine temperature approximately 110 °C<ref name="BWA 2030">BWA Water Additives. Belgard EV2030 Technical Data Sheet. BWA Water Additives. http://www.wateradditives.com/files/products/tech_data_sheets/Belgard%20EV%202030_Technical%20Data%20Sheet_Thermal_8.5x11.pdf. Accessed January 27, 2016.
</ref>. Since our process is designed to operate at a slightly higher top brine temperature, 120 °C, we will design for the upper bound concentration: 2.0 mg/L of antiscalant. With a feed flow rate of 8250 m3/hr, the process therefore requires 16.5 kg/hr of Belgard EV 2030.

For disinfection during post-treatment, chlorine addition typically requires a dose of roughly 2.0 mg/L as well<ref name="World Health Organization">World Health Organization. Desalination for safe water supply. http://www.who.int/water_sanitation_health/gdwqrevision/desalination.pdf. Accessed January 26, 2016.</ref>. With a pure water distillate flow rate of 4758 m3/hr, the process requires 9.5 kg/hr of chlorine as a 5%-15% sodium hypochlorite solution. Approximately 74 mg/L of lime (calcium hydroxide) and 88mg/L of CO2 should be added to the desalinated water to help prevent corrosion and increase alkalinity within acceptable ranges<ref name="Vouchkov">Vouchkov, N. Re-Mineralization of Desalinated Water. SunCam. https://s3.amazonaws.com/suncam/npdocs/118.pdf. Accessed February 18, 2016.</ref>. With a product stream flow rate of 4758 m3/hr, these concentrations correspond to 352.1 kg/hr of lime and 418.7 kg/hr of CO2. Both of these can be purchased, and it is likely that the CO2 can be procured from the neighboring natural gas power plant.

==Choice of Desalination Technology==
The two major technologies commonly implemented for desalination include reverse osmosis (RO), and multi-stage flash distillation (MSFD). RO utilizes high pressures to force water through a semipermeable membrane that allows for purified water to pass through, but stops organic and other wastes. Often, the pressures can reach 55 - 68 bar, which requires large pumps to maintain pressure, and introduces safety concerns to the process<ref>Stage 2 Measures in the Drought Contingency Plan. Available at http://www.cctexas.com/government/water/conservation/conservation-drought-plans/stage-2-measures. Accessed January 12, 2016.
</ref>. While RO can be less energy intensive than MSFD, and the membrane can also remove contaminants, it is susceptible to feedwater quality, membrane fouling, and requires more extensive pretreatment and posttreatment than MSFD. The cost of RO is directly proportional to the feed salinity, and so it is less expensive when desalinating low salinity water.

Multi-stage flash distillation is best selected for areas that have high salinity or low energy costs. The pretreatment process for MSFD is much easier when compared to RO because pretreatment often consists of simply adding anti-corrosives and antiscalants to the feed stream. The process works by heating water in vessels at different pressures to change the boiling point of the water and evaporate it as it passes through the system. After evaporation, the water condenses, leaving pure water as the effluent. MSFD costs are now roughly $1.00/m3, where this cost is mostly independent of salinity, but dependent on energy costs.<ref>Fritzman C, Lowenberg J, Wintgens T, Melin T. State-of-the-art Reverse Osmosis Desalination. Desalination. 2007; 216:1-76.
</ref> In both RO and MSFD a brine is produced which then must be appropriately treated and/or disposed of.

Multi-stage flash distillation was selected for this process, mainly due to the economic advantages offered by the selected location of Corpus Christi, TX. MSFD is also a well understood process that can be more easily modeled by chemical engineering software.

=Technical Approach=
Quantitative modeling of the process and all included optimization was conducted using the simulation software Aspen HYSYS. A variety of assumptions were developed to set up and define specific components of the process simulation itself. Within Aspen HYSYS, the NRTL Electrolyte property package was selected to model the behavior of salt water. Each stage of the MSF process is modeled as an individual flash vessel, with individual shell & tube heat exchangers used to model the distillate condensers present in each flash stage in the MSF system.

The HYSYS simulation model only considers the flash portion of the overall process, allowing establishment of mass and energy balances for the system. Pretreatment and post-treatment were excluded for the HYSYS model, as these steps are simple additions or removals of small concentrations of chemical components. Although the simulation models heat exchangers outside of each flash stage, in the physical design of the plant these heat exchangers will be contained within each flash stage.

One portion of the process is not modeled, which is the inlet seawater entering the heat rejection stages. As can be seen from the full process flow diagram (Appendix C), the inlet seawater in the heat rejection stage is in high excess to provide maximum cooling capacity; then a significant portion is purged to bring the stream flow rate down to what is needed for the feed stream. It is assumed that the flow rate of cooling water for those three stages is in such high excess that the cooling water does not heat up. Therefore, in the process simulation models, the inlet stream is the feed stream, which enters directly into the heat recovery portion of the process.

See Appendix D for a screenshot overview of the simulation environment model.

=Process Overview=
==Process Flowsheet==
The following diagram, Figure 1, shows a basic flowsheet for the process. Process alternatives considered for each stage are discussed below. Please see Appendix C for the full process flowsheet with finalized design decisions.

[[File:BlockFlowDiagram.png|center|700px|thumb|alt=Nueces Bay Desalination Facility Block Flow Diagram|Figure 1: Basic block flowsheet of the multi-stage flash desalination process.]]

==Major Features==
As seen above, there are three primary stages of the desalination process: Pre-treatment, the multi-stage flash distillation, and post-treatment. Seawater enters the pre-treatment phase first to make the feed suitable for the rest of the process by filtering out large particles that could accumulate on pipes, deaerating the water to limit corrosion, and adding an agent to combat fouling. The water then enters the distillation process, which is broken up into two sections: heat recovery and heat rejection.

In the heat recovery stage, there are 18 flash vessels, each of which separates steam from the salinated liquid water. Each flash vessel has saltwater as its inlet and two outlets: a vapor outlet (steam) that condenses into the final product of purified water and a liquid outlet composed of water with more dissolved solids than the inlet, which is the inlet stream for the next flash vessel. The pipe containing the feed is first mixed with recycled brine, then goes through the top sections of each of the flash vessels, starting with V-118 and ending up at V-101. In this way, the feed stream pipe first serves as a heat exchanger, heating the pre-treated seawater while simultaneously condensing the steam into the final product. The feed then enters a final heat exchanger where it is brought up to 103.6 °C before entering V-101 to be flashed.

After exiting V-118, the final flash vessel in the heat recovery portion, the water enters the heat rejection portion, which consists of three flash vessels (V-201, V-202, V-203). The flash vessels of this section operate in the same manner as in the heat recovery section, with the exception that condensation occurs using excess cooling water instead of pre-treated seawater. This technique allows excess heat to be rejected into an excess water stream rather than the feed.

The purified product stream finally enters post-treatment processing in which water is re-aerated and remineralized, disinfected, and pH is adjusted before being delivered to the city. The concentrated brine leftover at the end of the flash cycle is recycled into the feed, although a purge is required to continuously remove some salt from the system. This purged brine is disposed of by use of a Class I injection well, per regional regulations.

See Appendix E to find detailed stream tables of stream specifications.

==Process Alternatives==
===Pretreatment===
Multi-stage flash distillation requires significantly less pretreatment than reverse osmosis desalination, but helpful pretreatment techniques still exist. One very inexpensive method of pretreatment is screening/filtration, which is used to filter out large contaminants from intake seawater. Using more complicated membrane technology to accomplish large particle filtration is a possible alternative, but doing so would be much more expensive than using a standard screen or filter, so the latter choice has been selected for this process.<ref>El-Dessouky H., Alatiqi I, Ettouney H. Process synthesis: The multi-stage flash desalination system. Desalination. 1998;115(2):115-179. doi: 10.1016/S0011-9164(98)00035-6.</ref>

The most critical type of fouling in multi-stage flash distillation is scaling - therefore, the most critical step in the pretreatment process will be the inclusion of an anti-scalant. Alkaline scaling can be prevented by acid dosing or polyphosphate addition; however, acid dosing can result in increased corrosion and polyphosphates can lead to non-alkaline calcium phosphate fouling, so these additives will not be considered<ref name="Morris">Morris, R. The development of the multi-stage flash distillation process: A designer's viewpoint. Desalination. 1993;93(1-3):57-68. doi:10.1016/S0011-9164(93)80096-6. Availble at http://www.sciencedirect.com/science/article/pii/0011916493800966</ref><ref>Al-Gobaisi, D. A quarter-century of seawater desalination by large multistage flash plants in Abu Dhabi (Plant performance analysis, assessment, present efforts toward enhancement and future hopes). Desalination. 1994;99(2-3):509-512. doi: 10.1016/S0011-9164(94)00196-0.</ref>. As such, a polymer-based additive is recommended for the anti-scalant for this process. The Belgard EV series is designed for use in high-temperature seawater MSF; of these, Belgard EV 2030 is the most neutral in pH<ref>NSF International. NSF Product and Service Listings: Drinking Water Treatment Chemicals - Health Effects. NSF International. http://info.nsf.org/Certified/PwsChemicals/Listings.asp?Company=00300&. Accessed January 27, 2016.</ref><ref name="BWA 2030" /><ref>BWA Water Additives. Belgard EV2035 Technical Data Sheet. BWA Water Additives. http://www.wateradditives.com/files/products/tech_data_sheets/Belgard%20EV2035_Technical%20Data%20Sheet_Thermal_8.5x11.pdf. Accessed January 27, 2016.</ref><ref>BioLab Water Additives. Belgard EV Antiscalant for seawater distillation plants. BioLab Water Additives. http://www.lpq.com.mx/pdf/BELGARD%20EV.PDF. Accessed January 27, 2016.</ref>. As such, we have selected Belgard EV 2030 for use as an anti-scalant in our process.

Corrosion control is another significant method of pretreatment in multi-stage flash distillation plants. Using a piping material that has limited susceptibility to corrosion is typically not economically viable for MSF desalination plants due their large size, so using carbon steel for most of the piping is generally used for its economic benefits despite its susceptibility to corrosion<ref name = "Morris" />. Limiting oxygen concentration is very feasible through the use of deaerators, which can remove oxygen as well as other dissolved gases from the seawater stream. Often, an external deaeration system is used<ref name = "Morris" /><ref>Edwards - Hick Hargreaves. Seawater Deaerators. Edwards - Hick Hargreaves. http://psscorp.co.th/upload/news/20110830105340.pdf. Accessed January 26, 2016.</ref>. Given these considerations, using an external deaerator combined with a less-corrosive antiscalant like Belgard EV 2030 is much more practical for the process than using anti-corrosive piping.

Excessive foam can also be an issue in some MSF distillation facilities, so antifoaming agents are sometimes employed in the pretreatment process as well. However, many plants are operated without foaming issues, and therefore do not need anti-foaming pretreatment at all. Selection of an anti-foaming agent must be carefully weighed with selected of anti-scalant to avoid issues with additive interactions [26]. At this stage in the design process, an anti-foamer will not be included due to the risk of decreasing scalant efficacy, and the likelihood that foam will not be an issue in the desalination plant.

===Multistage Flash Distillation===
The main objective of the design project is desalinate water. This is done with the main process of multi stage flash distillation (MSF). MSF can be developed using a numerous number of different process configurations, such as Once-Through, Simple Mixer Brine Circulation, and Multi-Stage Heat Rejection Brine Circulation processes.

The most desired process for the project is the Three Stage Heat Rejection Brine Circulation as it builds on benefits of the other simpler technologies. By combining principles of heat rejection and heat recovery, the required heat transfer area of the process is greatly reduced as compared to once through processes or simple brine recycle processes. It has the highest performance ratio with the lowest specific feed flow rates, therefore greatly lowering the operating costs of the process. It is also known as the industry standard for MSF<ref name = "KSU Chapter 6">Kansas State University ChE 413. Available at: http://faculty.ksu.edu.sa/Almutaz/Documents/ChE-413/Multi-stage%20Flash%20Desalination.pdf. Accessed January 26, 2016.</ref>. In addition, since our brine disposal technique involves the pumping of the brine into a Class I well, the high brine outlet salinity is not a drawback for our process since post treatment or dilution of the brine is not necessary.

===Post Treatment===
After processing the water via multi-stage flash, the water needs to be post-treated to make the water taste as consumers would expect, since distilled water is “flat” and unpleasant, as well as to protect the consumers and the piping through which the water will be passing<ref name="Clayton">Clayton R. A review of current knowledge: Desalination for water supply. Foundation for Water Research. http://www.fwr.org/desal.pdf. Accessed January 26, 2016.</ref>. There are multiple alternatives to achieve these objectives of remineralization, aeration, corrosion control, and disinfection.

Aerating and introducing some dissolved solids back into the product water are effective ways to create a quality product that is not flat, and contains flavoring that is expected of the water when delivered to the consumer. Aeration will be accomplished via an inline spray aerator which will take up less space than cascade aeration.<ref>Minnesota Rural Water Association. Aeration. http://www.mrwa.com/WaterWorksMnl/Chapter%2011%20Aeration.pdf. Accessed January 26, 2016.</ref> Feed blending will be used to remineralize the water as opposed to a remineralization filter because it does not require additional equipment purchases, and can easily handle the large amounts of throughput required for the process.<ref name="Clayton" />

Corrosion and pH can be controlled by adding lime (calcium hydroxide) and carbon dioxide to the water. To do so, about 74 mg/L of lime and 88mg/L of CO2 should be added to the desalinated water to help prevent corrosion and increase alkalinity within acceptable ranges. Sodium hydroxide, soda ash, and sodium bicarbonate are alternatives to altering the alkalinity and pH of the product water, but lime and CO2 is an industry standard <ref name="Vouchkov" />.

Disinfection needs to be done to prevent microbial growth in the desalinated water and ensure the safety of the general public. Chlorine addition via chlorine gas or sodium hypochlorite is the most common and only requires a dose of 1.5 to 2.5 mg/liter. <ref name="World Health Organization" /> Other alternatives include ozonation, UV light, and chloramination. Chlorine, as a 5% - 15% sodium hypochlorite solution, is sufficient for our process and relatively inexpensive, and as such will be utilized in the process.

===Transport of Brine to Well===
Three major possibilities exist for transporting the brine from our desalination plant to the injection well for disposal: rail, trucking, and a pipeline. A pipeline was chosen as the most feasible and cost-effective approach.

A rail line is not available at this time, and with the cost to construct a rail line at approximately $43 million per mile<ref>About Money. How Much Do Rail Transit Projects Cost to Build and Operate? 2014. Available at: http://publictransport.about.com/od/Transit_Projects/a/How-Much-Do-Rail-Transit-Projects-Cost-To-Build-And-Operate.htm. Accessed January 27, 2016. </ref>, the enormous capital cost makes rail transport infeasible for this site. For large volume operations, pipelines are cheaper than trucking. In a study of wastewater sludge produced from wastewater treatment facilities, it was found that it was more economical for facilities producing over 1200 m3 per day to utilize pipelines rather than trucking.<ref>Marufuzzaman M, Eskioglu S, Hernandez R. Truck versus pipeline transportation cost analysis of wastewater sludge. Transportation Research. 2015; 74(A): 14-30. </ref> Our facility is producing nearly 3 times this volume at 3485 m3 per day of brine. Furthermore, brine is a less viscous fluid than wastewater sludge, decreasing pumping costs in the pipeline and further amplifying the benefit of choosing a pipeline over an injection well.

===Brine Storage===
Due to the choice of a pipeline, rather than trucking, as the method of transport of brine to the injection well, brine storage is unnecessary, so storage solutions such as holding tanks were not investigated. One method of storage compatible with a pipeline is to pump the brine through an evaporation pond. This solution is attractive as it would allow the brine to concentrate, saving some costs of pumping, however, the considerable land usage of evaporation ponds leads them to only be effective for smaller plants. The largest municipal plant utilizing this method produces only 5.7 million liters per day<ref>University of Santa Cruz Center for Integrated Water Research. Concentrate Management: Evaporation Ponds. 2010. Available at: https://ciwr.ucsc.edu/sites/default/files/ICM_D5.pdf. Accessed January 27, 2016. </ref>, whereas our desalination plant is proposed to produce 75.7 million liters per day. Therefore, evaporation ponds have been eliminated from consideration.

==Major Specifications==
===Material of Construction Selection===
While saltwater and desalinated water in general can be corrosive, there are best practices within the desalination industry for material selection. With this in mind, best practice shows carbon steel lifetime can be lengthened by monitoring the oxygen concentration in the water, specifying the paint used to coat any metal, and more expensive materials can be used in areas of extremely high corrosion.<ref name = "Morris" />. By following these best practices, it is possible to save significant amounts of money through the use of carbon steel in most areas of the plant, with specific high-corrosion areas being reinforced with, or replaced with, more corrosion resistant materials, such as Cu/Ni 70/30 alloy<ref name = "KSU Chapter 6" />.

===Heat Exchanger Sizing===
See Appendix F for heat exchanger sizing data. Heat exchanger sizes appear reasonable for placement in the flash vessels.

===Flash Vessel Sizing===
The dimensions for the flash drums were obtained through literature values. The flash drums were determined to be 18x4x3 m in width, height, and length.<ref name="KSU Chapter 6" />

Wall thickness was determined using the following conventions: Design pressure is 10% more than the operating pressure, design temperature is 50°F (27.8°C) higher than the operating temperature, a corrosion allowance of 3-mm and an effectiveness of 85% was derived from the assumption of a double-welded butt joint that was spot checked. See Appendix G for wall thicknesses calculated for each pressure vessel.

The sizing of the vessels to be constructed will be determined after talks with a manufacturer. Due to economies of scale, it may be more cost-effective to buy 21 pressure vessels with the dimensions mentioned above and a wall thickness of 17.1-mm, the highest wall thickness required in the system.

===Pipe Losses and Pressure Drop===
HYSYS was used to estimate the diameter of a pipe given a specified pressure drop per 100m of pipe. It was determined that for a pressure drop of 0.1 kPa/100m of pipe, a 3.05m diameter pipe is sufficient, which is more than manageable for manufacturers. This pressure drop is mostly insignificant and will be taken care of with the safety factors included in the pumps. The significant pressure drops in our system is accounted for via losses in the flash vessels and heat exchangers, which drive the flash process. Therefore overall pipe losses are insignificant and are built into intentional design losses.

=Economic Analysis=
==Economic Assumptions==
The sale of purified water yields a revenue stream of $49.5 million per year assuming sale to commercial entities within the city limits of Corpus Christi at $1.7915 per thousand liters.<ref>Water schedule. City of Corpus Christi Utility Business Office. http://www.cctexas.com/Assets/Departments/Financial-Services/Utilities-Business-Office/files/waterrates.pdf Accessed February 28, 2016.</ref> The primary variable costs are utilities and the brine disposal, with minimal raw materials costs due to the primary feedstock, seawater, being free, and minimal amounts of pre- and post-treatment chemicals being used. The cost of brine disposal used in this analysis was high at $2.097 per thousand liters<ref>McCurdy, R. Underground injection wells for produced water disposal. Chesapeake Energy Corporation. http://www.epa.gov/sites/production/files/documents/21_McCurdy_-_UIC_Disposal_508.pdf Accessed February 28, 2016.</ref>, an estimate for brine disposal utilizing trucking as the mode of transport. As has been shown through previous analyses, a lower cost can be achieved using a direct pipeline, however, no good estimates for this cost could be found.

The NPV was calculated using a 6% cost of capital, the approximate interest rate for a municipal bond.<ref>Zirbe, R.O. Jr., Han, X., Layton, D., Leshine, T. A history of discount rates and their use by government agencies. (2002). http://faculty.washington.edu/zerbe/docs/DISCOUNT%20RATE%20HISTORY-latesXIt.doc Accessed February 28, 2016.</ref> It was assumed that the plant can be built within two years, with production at half-scale in its first year. Additionally, one month of variable costs, $10.6 million, was set aside for working capital. See Appendix H for a full table of assumptions. A 30 year NPV calculation was chosen as the decision basis since 30 years is a conservative estimate for the lifetime of a desalination plant [38]. See below for estimates of NPV for the lifetime of the plant under various assumptions and design criteria.

==Optimization and Sensitivity Analysis==
Looking at the multi-stage flash process, there are a number of opportunities to optimize the process. The major process variables that were investigated were the inlet water temperature, the operating pressure drop per vessel, and the number of stages over which the process takes place. Considering these optimizations, our final process consists of 21 flash vessels (18 heat recovery and 3 heat rejection stages), each with a pressure drop of 5 kPa. See Appendix D for a screenshot of the HYSYS simulation used to model the optimized process.
===Operating Pressure and Pressure Drop===
Initially our process was designed to pump the inlet seawater and brine to a pressure of 2 bar and allow the pressure drop in each flash stage to end our process downstream at 1 bar to prevent complications associated with designing a system under vacuum. While this provided pumping advantages, the energy in the system was significantly higher due to vapor pressure considerations. Due to this fact, the required heat exchange area for certain exchangers proved to be infeasible (on the order of 80,000m2) due to the enormous capital and operating costs that would be associated with such a large heat exchanger.

The process was optimized to operate under vacuum, requiring additional pumping requirements but much lower required heat exchange area, and therefore lower utility costs. This optimization was done by specifying the pressure of the final flash vessel to 5.7kPa, so that the temperature of the water vapor (~30oC) is within approximately 5oC of the cooling water (35oC) so that condensation can occur within a reasonable approach. This also allows our system to optimally extract pure water from the brine running through the system and reduce energy requirements for cooling the product or the recycle. Our system redesign and optimization reduced the energy in the system and reduce the same heat exchanger area requirements by more than 3 orders of magnitude to approximately 800m2.

Once the process was optimized to operate in a vacuum, the pressure drop per flash stage regulated through values was then optimized. Three pressure drops per stages, 3kPa, 5kPa, and 7kPa, were tested and then an economic analysis was conducted to find the optimal value for the pressure drop. These changes had impacts on both operating and capital costs as they would change initial pumping requirements, the amount of water flowing through the process and recycle (and therefore the flash vessel sizes, heat exchanger sizing), and the efficiency of each stage.

The various pressure drops (3 kPa, 5 kPa, and 7 kPa) were tested on a system nominally at 24 stages under the assumption that the results would be generalizable to systems with a different number of stages. With increasing pressure drop, we found decreased capital cost but increased utility cost. As can be seen from Table 2 below, a pressure drop of 5 kPa proved to be the best option based on 30 year NPV, with a loss of $838.8 million, a savings of $65.5 million compared to the next-best option with a pressure drop of 7 kPa.

<center>
{| class = "wikitable"
|+Table 2: Comparison of profitability for our MSF desalination process with 24 stages with 3 different pressure drops per stage
! Number of Stages  
! Pressure Drop (kPa)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
! 30-Year NPV ($MM)
! NPV Ratio (over 5kPa)
|- style="text-align: center;"
| 24 || 3 || 362.3 || 39.1 || -905.8 || 1.08
|- style="text-align: center;"
| 24 || 5 || 255.1 || 53.6 || -838.8 || 1.00
|- style="text-align: center;"
| 24 || 7 || 209.5 || 72.2 || -904.3 || 1.08
|}
</center>

===Number of Flash Stages===
The number of stages is an important area for optimization. An increased number of stages yields better recovery of purified water, however, requires higher capital cost from the additional pressure vessel. Furthermore, with an increased amount of purified water comes increasingly concentrated brine. This must be taken into account since any brine concentration in excess of about 7% presents a more significant corrosion risk<ref name="KSU Chapter 6" />, so increased maintenance costs for the piping or higher capital costs for more corrosion-resistant materials could be necessary for brine concentrations exceeding 7%. Therefore, to recover the same amount of pure water without exceeding corrosion limitations, increasing the number of stages will reduce the overall size of each stage.

Optimizing based on NPV, it was found that 21 stages is optimal, balancing the increased profits from the additional purified water additional costs of higher capital cost. See Table 3 below to compare the 30-year NPV of the process across various numbers of flash vessels.

<center>
{| class = "wikitable"
|+Table 3: Comparison of profitability for our MSF desalination process with a 5 kPa pressure drop, 5000 m3/hr feed flow rate, with varying number of stages.
! Number of Stages  
! Pressure Drop (kPa)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
! 30-Year NPV ($MM)
! NPV Ratio (over 21 Stages)
|- style="text-align: center;"
| 17 || 5 || 213.9 || 58.7 || -813.8 || 1.016
|- style="text-align: center;"
| 18 || 5 || 218 || 57.6 || -809.9 || 1.011
|- style="text-align: center;"
| 19 || 5 || 224.3 || 58 || -806.1 || 1.007
|- style="text-align: center;"
| 20 || 5 || 229.8 || 57.9 || -813.6 || 1.016
|- style="text-align: center;"
| 21 || 5 || 226.6 || 55.9 || -800.7 || 1
|- style="text-align: center;"
| 22 || 5 || 254.7 || 54.6 || -849.5 || 1.061
|- style="text-align: center;"
| 23 || 5 || 249.3 || 54.1 || -833.1 || 1.04
|- style="text-align: center;"
| 24 || 5 || 255.1 || 53.6 || -838.8 || 1.048
|- style="text-align: center;"
| 25 || 5 || 254.9 || 52.7 || -837.4 || 1.046
|}
</center>

===Inlet Seawater Temperature===
By pairing our desalination plant with a natural gas power plant and utilizing its cooling water we forgo initial heating of our inlet stream. Normally, water pumped from the ocean would be approximately 25oC and by capturing waste heat from the power plant we can reduce utilities with our water inlet of 45oC. Through an economic comparison between two plants nominally at 21 flash stages, the plant with an inlet temperature at 25oC has a capital cost of $245MM and a utility cost of $77MM/yr and the same facility with an inlet water temperature at 45oC has a capital cost of only $226MM and a utility cost of $56MM/yr. As shown in Table 4 below, this $19MM savings in capital costs and $21MM/yr saving in utilities is largely due to reducing the size of the heat exchangers and lowering the steam required for heating duty on the inlet seawater respectively. Therefore our process is optimized by recovering the waste heat water from the gas fired power plant.

<center>
{| class = "wikitable"
|+Table 4: Seawater Inlet Temperature Process Cost Optimization
! Seawater Temperature (°C)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
|- style="text-align: center;"
| 25 || 245 || 77
|- style="text-align: center;"
| 45 || 226 || 56
|- style="text-align: center;"
| Savings || 56 || 19
|}
</center>

==Overall Process Economics==
After optimization at fixed flow rates, the process was scaled to meet the the 67% operating percentage and final product definitions as shown in Table 1, which resulted in finalized economics shown in Table 5 below. For a table of the installed costs of each process vessel, see Appendix I. For the finalized NPV calculations, see Appendix J.

<center>
{| class = "wikitable"
|+Table 5: Summarized Economics for 21-stage MSFD Process
! Economic Parameter
! Cost (USD)
|- style="text-align: center;"
| Capital Cost || 345 MM
|- style="text-align: center;"
| Total Operating Cost per Year || 103 MM
|- style="text-align: center;"
| Utility Cost per Year || 59 MM
|- style="text-align: center;"
| 30-Year NPV (6% Discount) || -1035 MM
|}
</center>

=Conclusions and Recommendations=
==Project Viability==
Table 5 above demonstrates that the process has reasonable operating and capital costs, but the 30-year NPV is negative by -1035 $MM due to the extremely low sell price capability of desalinated water as a product. Therefore, this plant would have to heavily be supported by the City of Corpus Christi to run as the product revenue will not make it self-sufficient. The acquisition of government subsidies are key operating requirements for all desalination facilities in the United States.

==Additional Concerns==
Large volumes of water, steam, and energy are needed to ensure the project runs as specified. While most of the water can easily be extracted from the ocean with little to consequence, the fuel used to generate the steam and electricity can have a significant environmental impact. As the plant is located next to a natural gas power plant, this fuel is better than coal with respect to environmental concerns, and utilizing energy from the power plant to preheat the feed stream for the desalination plant helps lower environmental impacts.

The large amounts of high pressure steam being used represents one of the the largest safety concern in the process, because a high pressure steam release could easily cause severe burns or even death. Proper precautions and control measures must be in place to prevent accidental release. Otherwise, the process does not utilize any known toxic chemicals and is relatively safe from a chemical standpoint. The flash vessels mostly operate below atmospheric pressure, so they must be constructed as vessels that can withstand vacuum without failing.

=Appendix A: Proposed Location of Desalination Plant=
Pictured below is the proposed location for the desalination plant, located on Nueces Bay in Corpus Christi, adjacent to the Nueces Bay Energy Center. The plant’s proposed location is shown in the area outlined by the small circle in the upper right of the figure.

[[File:NuecesBayLocation.png|center|600px|thumb|alt=Nueces Bay Desalination Facility Location|Figure A.1: Proposed mapped location of Nueces Bay Desalination Facility.]]
[[File:ClassIWell.png|center|700px|thumb|alt=Nueces Bay Desalination Facility Distance to Injection Well|Figure A.2: Mapped distance from proposed location to Class I Injection Well]]

=Appendix B: Feed and Product Definitions - Salinity=

<center>
{| class = "wikitable"
|+Table 6: Average Composition of Total Dissolved Solids in Seawater39,40
! Compound  
! Mass Composition (kPa)
|- style="text-align: center;"
| Chlorine || 55.03%
|- style="text-align: center;"
| Sodium || 30.64%
|- style="text-align: center;"
| Sulfate || 7.70%
|- style="text-align: center;"
| Magnesium || 3.66%
|- style="text-align: center;"
| Calcium || 1.17%
|- style="text-align: center;"
| Potassium || 1.12%
|- style="text-align: center;"
| Bicarbonate || 0.36%
|- style="text-align: center;"
| Bromine || 0.19%
|- style="text-align: center;"
| Other || 0.15%
|}
</center>

[[File:Nueces Bay Salinity.JPG|center|700px|thumb|alt=Nueces Bay Salinity Tracking|Figure C.1: Salinity in Nueces Bay over the past 10 years6.]]

=Appendix C: Process Flow Diagram=
[[File:TeamF_FinalPFD.JPG|center|1200px|thumb|alt=Process Flow Diagram|Figure C.1: Process Flow Diagram]]

=Appendix D: HYSYS Simulation Model=
[[File:TeamF_HYSYS_Screenshot.PNG|center|1200px|thumb|alt=HYSYS Simulation Model|Figure D.1: Screenshot of HYSYS Model Simulation.]]

=Appendix E: Stream Tables=
A summary of some values is shown below:

<center>
{| class = "wikitable"
|+Table 7: Key process material streams
!  
! Stream  
! Molar Flow (105 kgmole/h)
! Mass Flow (105 kg/h)
|- style="text-align: center;"
! style="text-align: right;" | Inlet
| Feed Water || 454 || 83.6
|- style="text-align: center;"
! style="text-align: right;" | Outlet
| Brine Reject || 1.90 || 36.0
|- style="text-align: center;"
! style="text-align: right;" | Product
| Pure Water || 2.64 || 47.5
|}
</center>

<center>
{| class = "wikitable"
|+Table 8: Key process energy streams
! Stream  
! Heat Flow (kJ/h)
|- style="text-align: center;"
| Feed Water || 1.30E+11
|- style="text-align: center;"
| Q-301 || 5.73E+06
|- style="text-align: center;"
| Q-302 || 3.09E+09
|- style="text-align: center;"
| Q-304 || 4.68E+06
|- style="text-align: center;"
| Q-303 || -1.26E+09
|- style="text-align: center;"
| Brine Reject || -5.47E+10
|- style="text-align: center;"
| Pure Water || -7.46E+10
|}
(Negative value denotes energy leaving process)
</center>

=Appendix F: Heat Exchanger Sizing=

<center>
{| class = "wikitable"
|+Table 9: Utility Temperatures and Heat Transfer Coefficient<ref name="Towler">G.P. Towler, R. Sinnott. Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design. Elsevier, 2012.</ref>
!  
! HP Steam
! Cooling Water
|- style="text-align: center;"
! Temperature (°C)
| 250 || 35
|- style="text-align: center;"
! U (W/m2K)
| 4000 || 1500
|}
</center>

<center>
{| class = "wikitable"
|+Table 10: Internal Flash Heat Exchanger Sizing, condensing steam.
! Exchanger
! UA (kJ/°C-h)
! U (kJ/hr/m2°C)
! A (m2)
|- style="text-align: center;"
| E-101 || 1.47E+07 || 14400 || 1019
|- style="text-align: center;"
| E-102 || 1.53E+07 || 14400 || 1059
|- style="text-align: center;"
| E-103 || 1.59E+07 || 14400 || 1103
|- style="text-align: center;"
| E-104 || 1.66E+07 || 14400 || 1153
|- style="text-align: center;"
| E-105 || 1.74E+07 || 14400 || 1209
|- style="text-align: center;"
| E-106 || 1.83E+07 || 14400 || 1272
|- style="text-align: center;"
| E-107 || 1.93E+07 || 14400 || 1342
|- style="text-align: center;"
| E-108 || 2.05E+07 || 14400 || 1424
|- style="text-align: center;"
| E-109 || 2.19E+07 || 14400 || 1518
|- style="text-align: center;"
| E-110 || 2.35E+07 || 14400 || 1629
|- style="text-align: center;"
| E-111 || 2.54E+07 || 14400 || 1761
|- style="text-align: center;"
| E-112 || 2.77E+07 || 14400 || 1921
|- style="text-align: center;"
| E-113 || 3.05E+07 || 14400 || 2119
|- style="text-align: center;"
| E-114 || 3.42E+07 || 14400 || 2372
|- style="text-align: center;"
| E-115 || 3.90E+07 || 14400 || 2706
|- style="text-align: center;"
| E-116 || 4.56E+07 || 14400 || 3167
|- style="text-align: center;"
| E-117 || 5.55E+07 || 14400 || 3856
|- style="text-align: center;"
| E-118 || 7.20E+07 || 14400 || 5000
|}
</center>

=Appendix G: Pressure Vessel Sizing=

=Appendix H: Expanded Economic Assumptions=

=Appendix I: Equipment Cost Summary=

=Appendix J: Economic NPV Analysis=
[[File:Team F Economics.JPG|center|1200px|thumb|alt=Economic Analysis|Figure J.1: Screenshot of process economic analysis.]]

=References=
<references/>

Nueces Desalination Center: Production of Drinking Water by Multi-Stage Flash Distillation

2016-03-07T21:10:44Z

Benjkg: /* Appendix B: Feed and Product Definitions - Salinity */

Team F: Rankine 672 Final Report

Authors: Kedric Daly, Ben Granger, Evan Rosati, Kathleen Zhou

Instructors: Fengqi You, David Wegerer

March 11, 2016

=Executive Summary=
Desalination of seawater to drinking water is a technology of ever-increasing importance given trends of increasingly widespread water shortages around the world. Even in the United States, water shortages are becoming increasingly common in dry, highly-populated such as California and certain areas of the Gulf Coast. Our design team, Rankine 672, was tasked with evaluating the need for and designing a desalination plant to provide purified water in a place of need. Investigation revealed that the city of Corpus Christi, Texas, is a strategic location for development of a desalination facility, as surrounding region is in need of new drinking water sources and the city has expressed serious interest in contracting the development of a large-scale desalination plant. Our proposed solution is a multi-stage flash distillation desalination facility capable of producing 20,000,000 gallons (75,708 m3) of purified drinking water for the city and surrounding region per operating day.

In this design, drinking water is produced from seawater feedstock through a 21-stage flash distillation process. By strategically locating the facility next to an existing natural gas power plant, the Nueces Bay Energy Center, pre-heated feed water can be taken directly from the outbound cooling systems of the power plant, allowing energy savings for our desalination plant. The process is designed for 16 hour-per-day operation, requires a seawater feed of 8250 m3/hr, and produces 4758 m3/hr of purified drinking water alongside a waste stream of 3485 m3/hr of concentrated brine. The waste stream is isolated from a recycle stream through a 9.0% purge; this results in an overall product yield of 57.7% on a volume basis. The process requires an additional pretreatment stream of 16.5 kg/hr Belgard EV 2030, which serves as an antiscalant, as well as 9.5 kg/hr chlorine, 352.1 kg/hr lime, and 418.7 kg/hr CO2 for post-treatment purposes.

The overall capital cost of the plant is expected to be $345MM, with a total operating cost of $103MM/yr, $59MM/yr of which comes from utilities. Due to the low price at which the product water can be sold, the 30-year net present value of the plant, assuming a 6% discount rate, is $-1,035MM. It is thus imperative this plant be constructed with public funds because it cannot return a profit to any private investor. The growing need for water however could justify the cost, because it is important that the residents of the greater Corpus Christi area have access to drinkable water.

__TOC__

=Introduction=
==Background==
Clean drinking water is a basic human need that is becoming increasingly difficult to meet. Currently, over 1 billion people live in places where water is considered scarce<ref> Tapping the oceans. The Economist USA. http://www.economist.com/node/11484059. Accessed March 2, 2016. </ref>. Though often thought of as a problem exclusive to third-world countries, the issue is becoming more prevalent in the US as well, with water managers in 40 of 50 states anticipating water shortage conditions in some portion of their state in the next 10 years. These shortages are due to increasing demands on constant or diminishing freshwater resources, therefore, to correct the issue, a new supply must be found<ref> Water Supply in the US. EPA. http://www3.epa.gov/watersense/pubs/supply.html. Accessed March 2, 2016.</ref>. Desalination of seawater is therefore important as a new source for clean drinking water and prevent water shortages.

==Project Definition==
The overall goal of the project is to investigate the feasibility of a desalination plant in order to meet the needs of growing water demand. The design needs to take health, safety, and environmental concerns into account, while optimizing costs.

=Design Basis=
Site Location and Conditions
Corpus Christi, TX was chosen as the location for the proposed desalination plant. The city is currently experiencing “Moderate Shortage Conditions”. This issue additionally comes at a time of growing demand with population growth anticipated due to new industry and Eagle Ford Group crude oil production<ref>Stage 2 Measures in the Drought Contingency Plan. Available at http://www.cctexas.com/government/water/conservation/conservation-drought-plans/stage-2-measures. Accessed January 12, 2016. </ref>.

In June 2014, the city announced the Corpus Christi Desalination Demonstration Project<ref>Corpus Christi Desalination Demonstration Project. Available at http://www.cctexas.com/Assets/Departments/Water/Files/DesalFactSheet.pdf. Published June 2014. Accessed January 12, 2016.
</ref> which seeks to create a demonstration desalination plant capable of producing 20,000 gallons of water per day, with plans to later build a full scale facility generating 20,000,000 gallons of water per day. The desalination demonstration project indicates a clear need for implementation of a desalination plant of this size to accommodate growth in the Corpus Christi area.

Locating our plant in Corpus Christi will allow us to take advantage of the benefits of cogeneration through a partnership with the Nueces Bay Energy Center. By utilizing their outlet cooling water as a large portion of our feed for the desalination process, we can raise the temperature of our seawater feed, saving on energy costs in the multi stage flash distillation. The proposed location for the desalination plant is immediately west of the Nueces Bay Energy Center. See Appendix A for a map depicting the proposed location for the desalination plant along the coast of Nueces Bay.

==Feed and Product Definitions==
The proposed desalination facility’s feed and product streams are defined as shown in Table 1. Rationale for these definitions is also described below. The values reported in Table 1 are final derived numbers from HYSYS simulation modeling.

<center>
{| class = "wikitable"
|+Table 1: Feed and product stream definitions
!  
! Salt Concentration
! Temperature
! Amount
|- style="text-align: center;"
! style="text-align: right;" | Feed Stream
| 30,500 ppm || 45°C || 8250 m3/hr
|- style="text-align: center;"
! style="text-align: right;" | Product Stream
| ~ 0 ppm || 35°C || 4758 m3/hr
|- style="text-align: center;"
! style="text-align: right;" | Byproduct Stream
| 69,500 ppm || 35°C || 3485 m3/hr
|}
</center>

The process feed will be seawater from Nueces Bay, preheated through the cooling water system of the nearby Nueces Bay Energy Center. Based on calculations developed from NREL (National Renewable Energy Laboratory) data, it was estimated that the Nueces Bay Energy Center uses approximately 250 million gallons of seawater per day for process and cooling needs. Therefore, the 20 million gallon per day requirement for the desalination facility can easily be met through the power plant’s cooling water<ref>Torcellini P, Long N, Judkoff R. Consumptive Water Use for U.S. Power Consumption. NREL. 2003; TP-550-33905.
</ref>. Note that we are assuming the process will operate 67% of the time (16 hours per day).

Based on data acquired from two salinity-measuring stations in Nueces Bay, the average salinity of local seawater is 30.5 practical salinity units (psu)<ref>Stations Map & Real - Time Data. The Conrad Blucher Institute for Surveying and Science website. Available at http://www.cbi.tamucc.edu/cbi/data/. Updated January 2016. Accessed January 9, 2016. </ref>, thus setting the concentration of the process feed stream. This translates to a feed stream concentration of 30,500 ppm (parts per million)<ref>Definition and Units. CATD Salinity Expertise Center. Available at http://www.salinityremotesensing.ifremer.fr/home. Accessed January 12, 2016.</ref>. The composition of the salt attributed to this seawater salinity is shown in Table 6 in Appendix B.

No data on the composition of seawater around the Corpus Christi area was available, but the composition of seawater does not change significantly in different locations. The given composition is considered accurate for our feed stream<ref>Brown E, Colling A, Park D, Phillips J, Rothery D, Wright J. Seawater : Its Composition, Properties and Behaviour. Boston: Butterworth-Heinemann, 1995.
</ref>. Note that seawater salt is primarily sodium chloride - total dissolved solids are 85.7% sodium chloride by mass. As such, the dissolved solids in the feed stream is assumed in this project to be pure NaCl, making the feed composition 96.95% water and 3.05% NaCl by mass. A control system will likely be put in place to ensure feed concentration remains at this level, accounting for natural fluctuations in local seawater salinity.

The largest byproduct from the process will be a brine slurry with high salt concentrations. An upper corrosion limit of 700 ppm was set for recirculated and purged brine. This undesired byproduct will be sent to a nearby Class I non-hazardous injection well facility operated by GNI Group, which is approximately 12 miles away from the proposed desalination plant location as shown in the figure in Appendix A<ref>Texas Water Code. Availabe at http://www.statutes.legis.state.tx.us/Docs/WA/htm/WA.27.htm. Accessed January 14, 2016.
</ref><ref>Google Maps. Available at https://goo.gl/maps/3xcvLZwUCe42 Accessed January 15, 2016.</ref>. Through the incorporation of a Class I injection well, the brine will not have to be treated or diluted before injection. <ref>EPA Regulations on Class I Wells. Available at http://www.epa.gov/uic/class-i-industrial-and-municipal-waste-disposal-wells. Accessed January 14, 2016.
</ref><ref>List of Injection Well Facilities by Location. Available at http://www.ehso.com/cssepa/tsdfdeepwells.php. Accessed January 14, 2016.</ref>

There are no regulations regarding the temperature of city drinking water, so the product stream temperature has no requirements based on regulations. Outlet temperatures of 35oC are acceptable for input to the municipal water supply system. Since city water tends to be stored in non-temperature controlled tanks, the temperature will naturally cool to ambient temperature.<ref>Climate Corpus Christi- Texas. US Climate Data. Available at http://www.usclimatedata.com/climate/corpus-christi/texas/united-states/ustx0294. Accessed January 12, 2016. </ref>

==Pre-Treatment and Post-Treatment==
For pretreatment, Belgard EV 2030 is recommended to be added at a concentration of 1.8-2.0 mg/L for a system with top brine temperature approximately 110 °C<ref name="BWA 2030">BWA Water Additives. Belgard EV2030 Technical Data Sheet. BWA Water Additives. http://www.wateradditives.com/files/products/tech_data_sheets/Belgard%20EV%202030_Technical%20Data%20Sheet_Thermal_8.5x11.pdf. Accessed January 27, 2016.
</ref>. Since our process is designed to operate at a slightly higher top brine temperature, 120 °C, we will design for the upper bound concentration: 2.0 mg/L of antiscalant. With a feed flow rate of 8250 m3/hr, the process therefore requires 16.5 kg/hr of Belgard EV 2030.

For disinfection during post-treatment, chlorine addition typically requires a dose of roughly 2.0 mg/L as well<ref name="World Health Organization">World Health Organization. Desalination for safe water supply. http://www.who.int/water_sanitation_health/gdwqrevision/desalination.pdf. Accessed January 26, 2016.</ref>. With a pure water distillate flow rate of 4758 m3/hr, the process requires 9.5 kg/hr of chlorine as a 5%-15% sodium hypochlorite solution. Approximately 74 mg/L of lime (calcium hydroxide) and 88mg/L of CO2 should be added to the desalinated water to help prevent corrosion and increase alkalinity within acceptable ranges<ref name="Vouchkov">Vouchkov, N. Re-Mineralization of Desalinated Water. SunCam. https://s3.amazonaws.com/suncam/npdocs/118.pdf. Accessed February 18, 2016.</ref>. With a product stream flow rate of 4758 m3/hr, these concentrations correspond to 352.1 kg/hr of lime and 418.7 kg/hr of CO2. Both of these can be purchased, and it is likely that the CO2 can be procured from the neighboring natural gas power plant.

==Choice of Desalination Technology==
The two major technologies commonly implemented for desalination include reverse osmosis (RO), and multi-stage flash distillation (MSFD). RO utilizes high pressures to force water through a semipermeable membrane that allows for purified water to pass through, but stops organic and other wastes. Often, the pressures can reach 55 - 68 bar, which requires large pumps to maintain pressure, and introduces safety concerns to the process<ref>Stage 2 Measures in the Drought Contingency Plan. Available at http://www.cctexas.com/government/water/conservation/conservation-drought-plans/stage-2-measures. Accessed January 12, 2016.
</ref>. While RO can be less energy intensive than MSFD, and the membrane can also remove contaminants, it is susceptible to feedwater quality, membrane fouling, and requires more extensive pretreatment and posttreatment than MSFD. The cost of RO is directly proportional to the feed salinity, and so it is less expensive when desalinating low salinity water.

Multi-stage flash distillation is best selected for areas that have high salinity or low energy costs. The pretreatment process for MSFD is much easier when compared to RO because pretreatment often consists of simply adding anti-corrosives and antiscalants to the feed stream. The process works by heating water in vessels at different pressures to change the boiling point of the water and evaporate it as it passes through the system. After evaporation, the water condenses, leaving pure water as the effluent. MSFD costs are now roughly $1.00/m3, where this cost is mostly independent of salinity, but dependent on energy costs.<ref>Fritzman C, Lowenberg J, Wintgens T, Melin T. State-of-the-art Reverse Osmosis Desalination. Desalination. 2007; 216:1-76.
</ref> In both RO and MSFD a brine is produced which then must be appropriately treated and/or disposed of.

Multi-stage flash distillation was selected for this process, mainly due to the economic advantages offered by the selected location of Corpus Christi, TX. MSFD is also a well understood process that can be more easily modeled by chemical engineering software.

=Technical Approach=
Quantitative modeling of the process and all included optimization was conducted using the simulation software Aspen HYSYS. A variety of assumptions were developed to set up and define specific components of the process simulation itself. Within Aspen HYSYS, the NRTL Electrolyte property package was selected to model the behavior of salt water. Each stage of the MSF process is modeled as an individual flash vessel, with individual shell & tube heat exchangers used to model the distillate condensers present in each flash stage in the MSF system.

The HYSYS simulation model only considers the flash portion of the overall process, allowing establishment of mass and energy balances for the system. Pretreatment and post-treatment were excluded for the HYSYS model, as these steps are simple additions or removals of small concentrations of chemical components. Although the simulation models heat exchangers outside of each flash stage, in the physical design of the plant these heat exchangers will be contained within each flash stage.

One portion of the process is not modeled, which is the inlet seawater entering the heat rejection stages. As can be seen from the full process flow diagram (Appendix C), the inlet seawater in the heat rejection stage is in high excess to provide maximum cooling capacity; then a significant portion is purged to bring the stream flow rate down to what is needed for the feed stream. It is assumed that the flow rate of cooling water for those three stages is in such high excess that the cooling water does not heat up. Therefore, in the process simulation models, the inlet stream is the feed stream, which enters directly into the heat recovery portion of the process.

See Appendix D for a screenshot overview of the simulation environment model.

=Process Overview=
==Process Flowsheet==
The following diagram, Figure 1, shows a basic flowsheet for the process. Process alternatives considered for each stage are discussed below. Please see Appendix C for the full process flowsheet with finalized design decisions.

[[File:BlockFlowDiagram.png|center|700px|thumb|alt=Nueces Bay Desalination Facility Block Flow Diagram|Figure 1: Basic block flowsheet of the multi-stage flash desalination process.]]

==Major Features==
As seen above, there are three primary stages of the desalination process: Pre-treatment, the multi-stage flash distillation, and post-treatment. Seawater enters the pre-treatment phase first to make the feed suitable for the rest of the process by filtering out large particles that could accumulate on pipes, deaerating the water to limit corrosion, and adding an agent to combat fouling. The water then enters the distillation process, which is broken up into two sections: heat recovery and heat rejection.

In the heat recovery stage, there are 18 flash vessels, each of which separates steam from the salinated liquid water. Each flash vessel has saltwater as its inlet and two outlets: a vapor outlet (steam) that condenses into the final product of purified water and a liquid outlet composed of water with more dissolved solids than the inlet, which is the inlet stream for the next flash vessel. The pipe containing the feed is first mixed with recycled brine, then goes through the top sections of each of the flash vessels, starting with V-118 and ending up at V-101. In this way, the feed stream pipe first serves as a heat exchanger, heating the pre-treated seawater while simultaneously condensing the steam into the final product. The feed then enters a final heat exchanger where it is brought up to 103.6 °C before entering V-101 to be flashed.

After exiting V-118, the final flash vessel in the heat recovery portion, the water enters the heat rejection portion, which consists of three flash vessels (V-201, V-202, V-203). The flash vessels of this section operate in the same manner as in the heat recovery section, with the exception that condensation occurs using excess cooling water instead of pre-treated seawater. This technique allows excess heat to be rejected into an excess water stream rather than the feed.

The purified product stream finally enters post-treatment processing in which water is re-aerated and remineralized, disinfected, and pH is adjusted before being delivered to the city. The concentrated brine leftover at the end of the flash cycle is recycled into the feed, although a purge is required to continuously remove some salt from the system. This purged brine is disposed of by use of a Class I injection well, per regional regulations.

See Appendix E to find detailed stream tables of stream specifications.

==Process Alternatives==
===Pretreatment===
Multi-stage flash distillation requires significantly less pretreatment than reverse osmosis desalination, but helpful pretreatment techniques still exist. One very inexpensive method of pretreatment is screening/filtration, which is used to filter out large contaminants from intake seawater. Using more complicated membrane technology to accomplish large particle filtration is a possible alternative, but doing so would be much more expensive than using a standard screen or filter, so the latter choice has been selected for this process.<ref>El-Dessouky H., Alatiqi I, Ettouney H. Process synthesis: The multi-stage flash desalination system. Desalination. 1998;115(2):115-179. doi: 10.1016/S0011-9164(98)00035-6.</ref>

The most critical type of fouling in multi-stage flash distillation is scaling - therefore, the most critical step in the pretreatment process will be the inclusion of an anti-scalant. Alkaline scaling can be prevented by acid dosing or polyphosphate addition; however, acid dosing can result in increased corrosion and polyphosphates can lead to non-alkaline calcium phosphate fouling, so these additives will not be considered<ref name="Morris">Morris, R. The development of the multi-stage flash distillation process: A designer's viewpoint. Desalination. 1993;93(1-3):57-68. doi:10.1016/S0011-9164(93)80096-6. Availble at http://www.sciencedirect.com/science/article/pii/0011916493800966</ref><ref>Al-Gobaisi, D. A quarter-century of seawater desalination by large multistage flash plants in Abu Dhabi (Plant performance analysis, assessment, present efforts toward enhancement and future hopes). Desalination. 1994;99(2-3):509-512. doi: 10.1016/S0011-9164(94)00196-0.</ref>. As such, a polymer-based additive is recommended for the anti-scalant for this process. The Belgard EV series is designed for use in high-temperature seawater MSF; of these, Belgard EV 2030 is the most neutral in pH<ref>NSF International. NSF Product and Service Listings: Drinking Water Treatment Chemicals - Health Effects. NSF International. http://info.nsf.org/Certified/PwsChemicals/Listings.asp?Company=00300&. Accessed January 27, 2016.</ref><ref name="BWA 2030" /><ref>BWA Water Additives. Belgard EV2035 Technical Data Sheet. BWA Water Additives. http://www.wateradditives.com/files/products/tech_data_sheets/Belgard%20EV2035_Technical%20Data%20Sheet_Thermal_8.5x11.pdf. Accessed January 27, 2016.</ref><ref>BioLab Water Additives. Belgard EV Antiscalant for seawater distillation plants. BioLab Water Additives. http://www.lpq.com.mx/pdf/BELGARD%20EV.PDF. Accessed January 27, 2016.</ref>. As such, we have selected Belgard EV 2030 for use as an anti-scalant in our process.

Corrosion control is another significant method of pretreatment in multi-stage flash distillation plants. Using a piping material that has limited susceptibility to corrosion is typically not economically viable for MSF desalination plants due their large size, so using carbon steel for most of the piping is generally used for its economic benefits despite its susceptibility to corrosion<ref name = "Morris" />. Limiting oxygen concentration is very feasible through the use of deaerators, which can remove oxygen as well as other dissolved gases from the seawater stream. Often, an external deaeration system is used<ref name = "Morris" /><ref>Edwards - Hick Hargreaves. Seawater Deaerators. Edwards - Hick Hargreaves. http://psscorp.co.th/upload/news/20110830105340.pdf. Accessed January 26, 2016.</ref>. Given these considerations, using an external deaerator combined with a less-corrosive antiscalant like Belgard EV 2030 is much more practical for the process than using anti-corrosive piping.

Excessive foam can also be an issue in some MSF distillation facilities, so antifoaming agents are sometimes employed in the pretreatment process as well. However, many plants are operated without foaming issues, and therefore do not need anti-foaming pretreatment at all. Selection of an anti-foaming agent must be carefully weighed with selected of anti-scalant to avoid issues with additive interactions [26]. At this stage in the design process, an anti-foamer will not be included due to the risk of decreasing scalant efficacy, and the likelihood that foam will not be an issue in the desalination plant.

===Multistage Flash Distillation===
The main objective of the design project is desalinate water. This is done with the main process of multi stage flash distillation (MSF). MSF can be developed using a numerous number of different process configurations, such as Once-Through, Simple Mixer Brine Circulation, and Multi-Stage Heat Rejection Brine Circulation processes.

The most desired process for the project is the Three Stage Heat Rejection Brine Circulation as it builds on benefits of the other simpler technologies. By combining principles of heat rejection and heat recovery, the required heat transfer area of the process is greatly reduced as compared to once through processes or simple brine recycle processes. It has the highest performance ratio with the lowest specific feed flow rates, therefore greatly lowering the operating costs of the process. It is also known as the industry standard for MSF<ref name = "KSU Chapter 6">Kansas State University ChE 413. Available at: http://faculty.ksu.edu.sa/Almutaz/Documents/ChE-413/Multi-stage%20Flash%20Desalination.pdf. Accessed January 26, 2016.</ref>. In addition, since our brine disposal technique involves the pumping of the brine into a Class I well, the high brine outlet salinity is not a drawback for our process since post treatment or dilution of the brine is not necessary.

===Post Treatment===
After processing the water via multi-stage flash, the water needs to be post-treated to make the water taste as consumers would expect, since distilled water is “flat” and unpleasant, as well as to protect the consumers and the piping through which the water will be passing<ref name="Clayton">Clayton R. A review of current knowledge: Desalination for water supply. Foundation for Water Research. http://www.fwr.org/desal.pdf. Accessed January 26, 2016.</ref>. There are multiple alternatives to achieve these objectives of remineralization, aeration, corrosion control, and disinfection.

Aerating and introducing some dissolved solids back into the product water are effective ways to create a quality product that is not flat, and contains flavoring that is expected of the water when delivered to the consumer. Aeration will be accomplished via an inline spray aerator which will take up less space than cascade aeration.<ref>Minnesota Rural Water Association. Aeration. http://www.mrwa.com/WaterWorksMnl/Chapter%2011%20Aeration.pdf. Accessed January 26, 2016.</ref> Feed blending will be used to remineralize the water as opposed to a remineralization filter because it does not require additional equipment purchases, and can easily handle the large amounts of throughput required for the process.<ref name="Clayton" />

Corrosion and pH can be controlled by adding lime (calcium hydroxide) and carbon dioxide to the water. To do so, about 74 mg/L of lime and 88mg/L of CO2 should be added to the desalinated water to help prevent corrosion and increase alkalinity within acceptable ranges. Sodium hydroxide, soda ash, and sodium bicarbonate are alternatives to altering the alkalinity and pH of the product water, but lime and CO2 is an industry standard <ref name="Vouchkov" />.

Disinfection needs to be done to prevent microbial growth in the desalinated water and ensure the safety of the general public. Chlorine addition via chlorine gas or sodium hypochlorite is the most common and only requires a dose of 1.5 to 2.5 mg/liter. <ref name="World Health Organization" /> Other alternatives include ozonation, UV light, and chloramination. Chlorine, as a 5% - 15% sodium hypochlorite solution, is sufficient for our process and relatively inexpensive, and as such will be utilized in the process.

===Transport of Brine to Well===
Three major possibilities exist for transporting the brine from our desalination plant to the injection well for disposal: rail, trucking, and a pipeline. A pipeline was chosen as the most feasible and cost-effective approach.

A rail line is not available at this time, and with the cost to construct a rail line at approximately $43 million per mile<ref>About Money. How Much Do Rail Transit Projects Cost to Build and Operate? 2014. Available at: http://publictransport.about.com/od/Transit_Projects/a/How-Much-Do-Rail-Transit-Projects-Cost-To-Build-And-Operate.htm. Accessed January 27, 2016. </ref>, the enormous capital cost makes rail transport infeasible for this site. For large volume operations, pipelines are cheaper than trucking. In a study of wastewater sludge produced from wastewater treatment facilities, it was found that it was more economical for facilities producing over 1200 m3 per day to utilize pipelines rather than trucking.<ref>Marufuzzaman M, Eskioglu S, Hernandez R. Truck versus pipeline transportation cost analysis of wastewater sludge. Transportation Research. 2015; 74(A): 14-30. </ref> Our facility is producing nearly 3 times this volume at 3485 m3 per day of brine. Furthermore, brine is a less viscous fluid than wastewater sludge, decreasing pumping costs in the pipeline and further amplifying the benefit of choosing a pipeline over an injection well.

===Brine Storage===
Due to the choice of a pipeline, rather than trucking, as the method of transport of brine to the injection well, brine storage is unnecessary, so storage solutions such as holding tanks were not investigated. One method of storage compatible with a pipeline is to pump the brine through an evaporation pond. This solution is attractive as it would allow the brine to concentrate, saving some costs of pumping, however, the considerable land usage of evaporation ponds leads them to only be effective for smaller plants. The largest municipal plant utilizing this method produces only 5.7 million liters per day<ref>University of Santa Cruz Center for Integrated Water Research. Concentrate Management: Evaporation Ponds. 2010. Available at: https://ciwr.ucsc.edu/sites/default/files/ICM_D5.pdf. Accessed January 27, 2016. </ref>, whereas our desalination plant is proposed to produce 75.7 million liters per day. Therefore, evaporation ponds have been eliminated from consideration.

==Major Specifications==
===Material of Construction Selection===
While saltwater and desalinated water in general can be corrosive, there are best practices within the desalination industry for material selection. With this in mind, best practice shows carbon steel lifetime can be lengthened by monitoring the oxygen concentration in the water, specifying the paint used to coat any metal, and more expensive materials can be used in areas of extremely high corrosion.<ref name = "Morris" />. By following these best practices, it is possible to save significant amounts of money through the use of carbon steel in most areas of the plant, with specific high-corrosion areas being reinforced with, or replaced with, more corrosion resistant materials, such as Cu/Ni 70/30 alloy<ref name = "KSU Chapter 6" />.

===Heat Exchanger Sizing===
See Appendix F for heat exchanger sizing data. Heat exchanger sizes appear reasonable for placement in the flash vessels.

===Flash Vessel Sizing===
The dimensions for the flash drums were obtained through literature values. The flash drums were determined to be 18x4x3 m in width, height, and length.<ref name="KSU Chapter 6" />

Wall thickness was determined using the following conventions: Design pressure is 10% more than the operating pressure, design temperature is 50°F (27.8°C) higher than the operating temperature, a corrosion allowance of 3-mm and an effectiveness of 85% was derived from the assumption of a double-welded butt joint that was spot checked. See Appendix G for wall thicknesses calculated for each pressure vessel.

The sizing of the vessels to be constructed will be determined after talks with a manufacturer. Due to economies of scale, it may be more cost-effective to buy 21 pressure vessels with the dimensions mentioned above and a wall thickness of 17.1-mm, the highest wall thickness required in the system.

===Pipe Losses and Pressure Drop===
HYSYS was used to estimate the diameter of a pipe given a specified pressure drop per 100m of pipe. It was determined that for a pressure drop of 0.1 kPa/100m of pipe, a 3.05m diameter pipe is sufficient, which is more than manageable for manufacturers. This pressure drop is mostly insignificant and will be taken care of with the safety factors included in the pumps. The significant pressure drops in our system is accounted for via losses in the flash vessels and heat exchangers, which drive the flash process. Therefore overall pipe losses are insignificant and are built into intentional design losses.

=Economic Analysis=
==Economic Assumptions==
The sale of purified water yields a revenue stream of $49.5 million per year assuming sale to commercial entities within the city limits of Corpus Christi at $1.7915 per thousand liters.<ref>Water schedule. City of Corpus Christi Utility Business Office. http://www.cctexas.com/Assets/Departments/Financial-Services/Utilities-Business-Office/files/waterrates.pdf Accessed February 28, 2016.</ref> The primary variable costs are utilities and the brine disposal, with minimal raw materials costs due to the primary feedstock, seawater, being free, and minimal amounts of pre- and post-treatment chemicals being used. The cost of brine disposal used in this analysis was high at $2.097 per thousand liters<ref>McCurdy, R. Underground injection wells for produced water disposal. Chesapeake Energy Corporation. http://www.epa.gov/sites/production/files/documents/21_McCurdy_-_UIC_Disposal_508.pdf Accessed February 28, 2016.</ref>, an estimate for brine disposal utilizing trucking as the mode of transport. As has been shown through previous analyses, a lower cost can be achieved using a direct pipeline, however, no good estimates for this cost could be found.

The NPV was calculated using a 6% cost of capital, the approximate interest rate for a municipal bond.<ref>Zirbe, R.O. Jr., Han, X., Layton, D., Leshine, T. A history of discount rates and their use by government agencies. (2002). http://faculty.washington.edu/zerbe/docs/DISCOUNT%20RATE%20HISTORY-latesXIt.doc Accessed February 28, 2016.</ref> It was assumed that the plant can be built within two years, with production at half-scale in its first year. Additionally, one month of variable costs, $10.6 million, was set aside for working capital. See Appendix H for a full table of assumptions. A 30 year NPV calculation was chosen as the decision basis since 30 years is a conservative estimate for the lifetime of a desalination plant [38]. See below for estimates of NPV for the lifetime of the plant under various assumptions and design criteria.

==Optimization and Sensitivity Analysis==
Looking at the multi-stage flash process, there are a number of opportunities to optimize the process. The major process variables that were investigated were the inlet water temperature, the operating pressure drop per vessel, and the number of stages over which the process takes place. Considering these optimizations, our final process consists of 21 flash vessels (18 heat recovery and 3 heat rejection stages), each with a pressure drop of 5 kPa. See Appendix D for a screenshot of the HYSYS simulation used to model the optimized process.
===Operating Pressure and Pressure Drop===
Initially our process was designed to pump the inlet seawater and brine to a pressure of 2 bar and allow the pressure drop in each flash stage to end our process downstream at 1 bar to prevent complications associated with designing a system under vacuum. While this provided pumping advantages, the energy in the system was significantly higher due to vapor pressure considerations. Due to this fact, the required heat exchange area for certain exchangers proved to be infeasible (on the order of 80,000m2) due to the enormous capital and operating costs that would be associated with such a large heat exchanger.

The process was optimized to operate under vacuum, requiring additional pumping requirements but much lower required heat exchange area, and therefore lower utility costs. This optimization was done by specifying the pressure of the final flash vessel to 5.7kPa, so that the temperature of the water vapor (~30oC) is within approximately 5oC of the cooling water (35oC) so that condensation can occur within a reasonable approach. This also allows our system to optimally extract pure water from the brine running through the system and reduce energy requirements for cooling the product or the recycle. Our system redesign and optimization reduced the energy in the system and reduce the same heat exchanger area requirements by more than 3 orders of magnitude to approximately 800m2.

Once the process was optimized to operate in a vacuum, the pressure drop per flash stage regulated through values was then optimized. Three pressure drops per stages, 3kPa, 5kPa, and 7kPa, were tested and then an economic analysis was conducted to find the optimal value for the pressure drop. These changes had impacts on both operating and capital costs as they would change initial pumping requirements, the amount of water flowing through the process and recycle (and therefore the flash vessel sizes, heat exchanger sizing), and the efficiency of each stage.

The various pressure drops (3 kPa, 5 kPa, and 7 kPa) were tested on a system nominally at 24 stages under the assumption that the results would be generalizable to systems with a different number of stages. With increasing pressure drop, we found decreased capital cost but increased utility cost. As can be seen from Table 2 below, a pressure drop of 5 kPa proved to be the best option based on 30 year NPV, with a loss of $838.8 million, a savings of $65.5 million compared to the next-best option with a pressure drop of 7 kPa.

<center>
{| class = "wikitable"
|+Table 2: Comparison of profitability for our MSF desalination process with 24 stages with 3 different pressure drops per stage
! Number of Stages  
! Pressure Drop (kPa)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
! 30-Year NPV ($MM)
! NPV Ratio (over 5kPa)
|- style="text-align: center;"
| 24 || 3 || 362.3 || 39.1 || -905.8 || 1.08
|- style="text-align: center;"
| 24 || 5 || 255.1 || 53.6 || -838.8 || 1.00
|- style="text-align: center;"
| 24 || 7 || 209.5 || 72.2 || -904.3 || 1.08
|}
</center>

===Number of Flash Stages===
The number of stages is an important area for optimization. An increased number of stages yields better recovery of purified water, however, requires higher capital cost from the additional pressure vessel. Furthermore, with an increased amount of purified water comes increasingly concentrated brine. This must be taken into account since any brine concentration in excess of about 7% presents a more significant corrosion risk<ref name="KSU Chapter 6" />, so increased maintenance costs for the piping or higher capital costs for more corrosion-resistant materials could be necessary for brine concentrations exceeding 7%. Therefore, to recover the same amount of pure water without exceeding corrosion limitations, increasing the number of stages will reduce the overall size of each stage.

Optimizing based on NPV, it was found that 21 stages is optimal, balancing the increased profits from the additional purified water additional costs of higher capital cost. See Table 3 below to compare the 30-year NPV of the process across various numbers of flash vessels.

<center>
{| class = "wikitable"
|+Table 3: Comparison of profitability for our MSF desalination process with a 5 kPa pressure drop, 5000 m3/hr feed flow rate, with varying number of stages.
! Number of Stages  
! Pressure Drop (kPa)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
! 30-Year NPV ($MM)
! NPV Ratio (over 21 Stages)
|- style="text-align: center;"
| 17 || 5 || 213.9 || 58.7 || -813.8 || 1.016
|- style="text-align: center;"
| 18 || 5 || 218 || 57.6 || -809.9 || 1.011
|- style="text-align: center;"
| 19 || 5 || 224.3 || 58 || -806.1 || 1.007
|- style="text-align: center;"
| 20 || 5 || 229.8 || 57.9 || -813.6 || 1.016
|- style="text-align: center;"
| 21 || 5 || 226.6 || 55.9 || -800.7 || 1
|- style="text-align: center;"
| 22 || 5 || 254.7 || 54.6 || -849.5 || 1.061
|- style="text-align: center;"
| 23 || 5 || 249.3 || 54.1 || -833.1 || 1.04
|- style="text-align: center;"
| 24 || 5 || 255.1 || 53.6 || -838.8 || 1.048
|- style="text-align: center;"
| 25 || 5 || 254.9 || 52.7 || -837.4 || 1.046
|}
</center>

===Inlet Seawater Temperature===
By pairing our desalination plant with a natural gas power plant and utilizing its cooling water we forgo initial heating of our inlet stream. Normally, water pumped from the ocean would be approximately 25oC and by capturing waste heat from the power plant we can reduce utilities with our water inlet of 45oC. Through an economic comparison between two plants nominally at 21 flash stages, the plant with an inlet temperature at 25oC has a capital cost of $245MM and a utility cost of $77MM/yr and the same facility with an inlet water temperature at 45oC has a capital cost of only $226MM and a utility cost of $56MM/yr. As shown in Table 4 below, this $19MM savings in capital costs and $21MM/yr saving in utilities is largely due to reducing the size of the heat exchangers and lowering the steam required for heating duty on the inlet seawater respectively. Therefore our process is optimized by recovering the waste heat water from the gas fired power plant.

<center>
{| class = "wikitable"
|+Table 4: Seawater Inlet Temperature Process Cost Optimization
! Seawater Temperature (°C)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
|- style="text-align: center;"
| 25 || 245 || 77
|- style="text-align: center;"
| 45 || 226 || 56
|- style="text-align: center;"
| Savings || 56 || 19
|}
</center>

==Overall Process Economics==
After optimization at fixed flow rates, the process was scaled to meet the the 67% operating percentage and final product definitions as shown in Table 1, which resulted in finalized economics shown in Table 5 below. For a table of the installed costs of each process vessel, see Appendix I. For the finalized NPV calculations, see Appendix J.

<center>
{| class = "wikitable"
|+Table 5: Summarized Economics for 21-stage MSFD Process
! Economic Parameter
! Cost (USD)
|- style="text-align: center;"
| Capital Cost || 345 MM
|- style="text-align: center;"
| Total Operating Cost per Year || 103 MM
|- style="text-align: center;"
| Utility Cost per Year || 59 MM
|- style="text-align: center;"
| 30-Year NPV (6% Discount) || -1035 MM
|}
</center>

=Conclusions and Recommendations=
==Project Viability==
Table 5 above demonstrates that the process has reasonable operating and capital costs, but the 30-year NPV is negative by -1035 $MM due to the extremely low sell price capability of desalinated water as a product. Therefore, this plant would have to heavily be supported by the City of Corpus Christi to run as the product revenue will not make it self-sufficient. The acquisition of government subsidies are key operating requirements for all desalination facilities in the United States.

==Additional Concerns==
Large volumes of water, steam, and energy are needed to ensure the project runs as specified. While most of the water can easily be extracted from the ocean with little to consequence, the fuel used to generate the steam and electricity can have a significant environmental impact. As the plant is located next to a natural gas power plant, this fuel is better than coal with respect to environmental concerns, and utilizing energy from the power plant to preheat the feed stream for the desalination plant helps lower environmental impacts.

The large amounts of high pressure steam being used represents one of the the largest safety concern in the process, because a high pressure steam release could easily cause severe burns or even death. Proper precautions and control measures must be in place to prevent accidental release. Otherwise, the process does not utilize any known toxic chemicals and is relatively safe from a chemical standpoint. The flash vessels mostly operate below atmospheric pressure, so they must be constructed as vessels that can withstand vacuum without failing.

=Appendix A: Proposed Location of Desalination Plant=
Pictured below is the proposed location for the desalination plant, located on Nueces Bay in Corpus Christi, adjacent to the Nueces Bay Energy Center. The plant’s proposed location is shown in the area outlined by the small circle in the upper right of the figure.

[[File:NuecesBayLocation.png|center|600px|thumb|alt=Nueces Bay Desalination Facility Location|Figure A.1: Proposed mapped location of Nueces Bay Desalination Facility.]]
[[File:ClassIWell.png|center|700px|thumb|alt=Nueces Bay Desalination Facility Distance to Injection Well|Figure A.2: Mapped distance from proposed location to Class I Injection Well]]

=Appendix B: Feed and Product Definitions - Salinity=

<center>
{| class = "wikitable"
|+Table 6: Average Composition of Total Dissolved Solids in Seawater39,40
! Compound  
! Mass Composition (kPa)
|- style="text-align: center;"
| Chlorine || 55.03%
|- style="text-align: center;"
| Sodium || 30.64%
|- style="text-align: center;"
| Sulfate || 7.70%
|- style="text-align: center;"
| Magnesium || 3.66%
|- style="text-align: center;"
| Calcium || 1.17%
|- style="text-align: center;"
| Potassium || 1.12%
|- style="text-align: center;"
| Bicarbonate || 0.36%
|- style="text-align: center;"
| Bromine || 0.19%
|- style="text-align: center;"
| Other || 0.15%
|}
</center>

[[File:Nueces Bay Salinity.JPG|center|700px|thumb|alt=Nueces Bay Salinity Tracking|Figure C.1: Salinity in Nueces Bay over the past 10 years6.]]

=Appendix C: Process Flow Diagram=
[[File:TeamF_FinalPFD.JPG|center|1200px|thumb|alt=Process Flow Diagram|Figure C.1: Process Flow Diagram]]

=Appendix D: HYSYS Simulation Model=
[[File:TeamF_HYSYS_Screenshot.PNG|center|1200px|thumb|alt=HYSYS Simulation Model|Figure D.1: Screenshot of HYSYS Model Simulation.]]

=Appendix E: Stream Tables=
A summary of some values is shown below:

<center>
{| class = "wikitable"
|+Table 7: Key process material streams
!  
! Stream  
! Molar Flow (105 kgmole/h)
! Mass Flow (105 kg/h)
|- style="text-align: center;"
! style="text-align: right;" | Inlet
| Feed Water || 454 || 83.6
|- style="text-align: center;"
! style="text-align: right;" | Outlet
| Brine Reject || 1.90 || 36.0
|- style="text-align: center;"
! style="text-align: right;" | Product
| Pure Water || 2.64 || 47.5
|}
</center>

<center>
{| class = "wikitable"
|+Table 8: Key process energy streams
! Stream  
! Heat Flow (kJ/h)
|- style="text-align: center;"
| Feed Water || 1.30E+11
|- style="text-align: center;"
| Q-301 || 5.73E+06
|- style="text-align: center;"
| Q-302 || 3.09E+09
|- style="text-align: center;"
| Q-304 || 4.68E+06
|- style="text-align: center;"
| Q-303 || -1.26E+09
|- style="text-align: center;"
| Brine Reject || -5.47E+10
|- style="text-align: center;"
| Pure Water || -7.46E+10
|}
(Negative value denotes energy leaving process)
</center>

=Appendix F: Heat Exchanger Sizing=

<center>
{| class = "wikitable"
|+Table 9: Utility Temperatures and Heat Transfer Coefficient<ref name="Towler">G.P. Towler, R. Sinnott. Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design. Elsevier, 2012.</ref>
!  
! HP Steam
! Cooling Water
|- style="text-align: center;"
! Temperature (°C)
| 250 || 35
|- style="text-align: center;"
! U (W/m2K)
| 4000 || 1500
|}
</center>

<center>
{| class = "wikitable"
|+Table 10: Internal Flash Heat Exchanger Sizing, condensing steam.
! Exchanger
! UA (kJ/°C-h)
! U (kJ/hr/m2°C)
! A (m2)
|- style="text-align: center;"
| E-101 || 1.47E+07 || 14400 || 1019
|- style="text-align: center;"
| E-102 || 1.53E+07 || 14400 || 1059
|- style="text-align: center;"
| E-103 || 1.59E+07 || 14400 || 1103
|- style="text-align: center;"
| E-104 || 1.66E+07 || 14400 || 1153
|- style="text-align: center;"
| E-105 || 1.74E+07 || 14400 || 1209
|- style="text-align: center;"
| E-106 || 1.83E+07 || 14400 || 1272
|- style="text-align: center;"
| E-107 || 1.93E+07 || 14400 || 1342
|- style="text-align: center;"
| E-108 || 2.05E+07 || 14400 || 1424
|- style="text-align: center;"
| E-109 || 2.19E+07 || 14400 || 1518
|- style="text-align: center;"
| E-110 || 2.35E+07 || 14400 || 1629
|- style="text-align: center;"
| E-111 || 2.54E+07 || 14400 || 1761
|- style="text-align: center;"
| E-112 || 2.77E+07 || 14400 || 1921
|- style="text-align: center;"
| E-113 || 3.05E+07 || 14400 || 2119
|- style="text-align: center;"
| E-114 || 3.42E+07 || 14400 || 2372
|- style="text-align: center;"
| E-115 || 3.90E+07 || 14400 || 2706
|- style="text-align: center;"
| E-116 || 4.56E+07 || 14400 || 3167
|- style="text-align: center;"
| E-117 || 5.55E+07 || 14400 || 3856
|- style="text-align: center;"
| E-118 || 7.20E+07 || 14400 || 5000
|}
</center>

=Appendix G: Pressure Vessel Sizing=

=Appendix H: Expanded Economic Assumptions=

=Appendix I: Equipment Cost Summary=

=Appendix J: Economic NPV Analysis=

=References=
<references/>

File:Team F Economics.JPG

2016-03-07T21:09:56Z

Benjkg: Economic analysis of Team F's proposed MSF desalination process

Economic analysis of Team F's proposed MSF desalination process

Nueces Desalination Center: Production of Drinking Water by Multi-Stage Flash Distillation

2016-03-07T21:05:36Z

Benjkg: /* Appendix B: Feed and Product Definitions - Salinity */

Team F: Rankine 672 Final Report

Authors: Kedric Daly, Ben Granger, Evan Rosati, Kathleen Zhou

Instructors: Fengqi You, David Wegerer

March 11, 2016

=Executive Summary=
Desalination of seawater to drinking water is a technology of ever-increasing importance given trends of increasingly widespread water shortages around the world. Even in the United States, water shortages are becoming increasingly common in dry, highly-populated such as California and certain areas of the Gulf Coast. Our design team, Rankine 672, was tasked with evaluating the need for and designing a desalination plant to provide purified water in a place of need. Investigation revealed that the city of Corpus Christi, Texas, is a strategic location for development of a desalination facility, as surrounding region is in need of new drinking water sources and the city has expressed serious interest in contracting the development of a large-scale desalination plant. Our proposed solution is a multi-stage flash distillation desalination facility capable of producing 20,000,000 gallons (75,708 m3) of purified drinking water for the city and surrounding region per operating day.

In this design, drinking water is produced from seawater feedstock through a 21-stage flash distillation process. By strategically locating the facility next to an existing natural gas power plant, the Nueces Bay Energy Center, pre-heated feed water can be taken directly from the outbound cooling systems of the power plant, allowing energy savings for our desalination plant. The process is designed for 16 hour-per-day operation, requires a seawater feed of 8250 m3/hr, and produces 4758 m3/hr of purified drinking water alongside a waste stream of 3485 m3/hr of concentrated brine. The waste stream is isolated from a recycle stream through a 9.0% purge; this results in an overall product yield of 57.7% on a volume basis. The process requires an additional pretreatment stream of 16.5 kg/hr Belgard EV 2030, which serves as an antiscalant, as well as 9.5 kg/hr chlorine, 352.1 kg/hr lime, and 418.7 kg/hr CO2 for post-treatment purposes.

The overall capital cost of the plant is expected to be $345MM, with a total operating cost of $103MM/yr, $59MM/yr of which comes from utilities. Due to the low price at which the product water can be sold, the 30-year net present value of the plant, assuming a 6% discount rate, is $-1,035MM. It is thus imperative this plant be constructed with public funds because it cannot return a profit to any private investor. The growing need for water however could justify the cost, because it is important that the residents of the greater Corpus Christi area have access to drinkable water.

__TOC__

=Introduction=
==Background==
Clean drinking water is a basic human need that is becoming increasingly difficult to meet. Currently, over 1 billion people live in places where water is considered scarce<ref> Tapping the oceans. The Economist USA. http://www.economist.com/node/11484059. Accessed March 2, 2016. </ref>. Though often thought of as a problem exclusive to third-world countries, the issue is becoming more prevalent in the US as well, with water managers in 40 of 50 states anticipating water shortage conditions in some portion of their state in the next 10 years. These shortages are due to increasing demands on constant or diminishing freshwater resources, therefore, to correct the issue, a new supply must be found<ref> Water Supply in the US. EPA. http://www3.epa.gov/watersense/pubs/supply.html. Accessed March 2, 2016.</ref>. Desalination of seawater is therefore important as a new source for clean drinking water and prevent water shortages.

==Project Definition==
The overall goal of the project is to investigate the feasibility of a desalination plant in order to meet the needs of growing water demand. The design needs to take health, safety, and environmental concerns into account, while optimizing costs.

=Design Basis=
Site Location and Conditions
Corpus Christi, TX was chosen as the location for the proposed desalination plant. The city is currently experiencing “Moderate Shortage Conditions”. This issue additionally comes at a time of growing demand with population growth anticipated due to new industry and Eagle Ford Group crude oil production<ref>Stage 2 Measures in the Drought Contingency Plan. Available at http://www.cctexas.com/government/water/conservation/conservation-drought-plans/stage-2-measures. Accessed January 12, 2016. </ref>.

In June 2014, the city announced the Corpus Christi Desalination Demonstration Project<ref>Corpus Christi Desalination Demonstration Project. Available at http://www.cctexas.com/Assets/Departments/Water/Files/DesalFactSheet.pdf. Published June 2014. Accessed January 12, 2016.
</ref> which seeks to create a demonstration desalination plant capable of producing 20,000 gallons of water per day, with plans to later build a full scale facility generating 20,000,000 gallons of water per day. The desalination demonstration project indicates a clear need for implementation of a desalination plant of this size to accommodate growth in the Corpus Christi area.

Locating our plant in Corpus Christi will allow us to take advantage of the benefits of cogeneration through a partnership with the Nueces Bay Energy Center. By utilizing their outlet cooling water as a large portion of our feed for the desalination process, we can raise the temperature of our seawater feed, saving on energy costs in the multi stage flash distillation. The proposed location for the desalination plant is immediately west of the Nueces Bay Energy Center. See Appendix A for a map depicting the proposed location for the desalination plant along the coast of Nueces Bay.

==Feed and Product Definitions==
The proposed desalination facility’s feed and product streams are defined as shown in Table 1. Rationale for these definitions is also described below. The values reported in Table 1 are final derived numbers from HYSYS simulation modeling.

<center>
{| class = "wikitable"
|+Table 1: Feed and product stream definitions
!  
! Salt Concentration
! Temperature
! Amount
|- style="text-align: center;"
! style="text-align: right;" | Feed Stream
| 30,500 ppm || 45°C || 8250 m3/hr
|- style="text-align: center;"
! style="text-align: right;" | Product Stream
| ~ 0 ppm || 35°C || 4758 m3/hr
|- style="text-align: center;"
! style="text-align: right;" | Byproduct Stream
| 69,500 ppm || 35°C || 3485 m3/hr
|}
</center>

The process feed will be seawater from Nueces Bay, preheated through the cooling water system of the nearby Nueces Bay Energy Center. Based on calculations developed from NREL (National Renewable Energy Laboratory) data, it was estimated that the Nueces Bay Energy Center uses approximately 250 million gallons of seawater per day for process and cooling needs. Therefore, the 20 million gallon per day requirement for the desalination facility can easily be met through the power plant’s cooling water<ref>Torcellini P, Long N, Judkoff R. Consumptive Water Use for U.S. Power Consumption. NREL. 2003; TP-550-33905.
</ref>. Note that we are assuming the process will operate 67% of the time (16 hours per day).

Based on data acquired from two salinity-measuring stations in Nueces Bay, the average salinity of local seawater is 30.5 practical salinity units (psu)<ref>Stations Map & Real - Time Data. The Conrad Blucher Institute for Surveying and Science website. Available at http://www.cbi.tamucc.edu/cbi/data/. Updated January 2016. Accessed January 9, 2016. </ref>, thus setting the concentration of the process feed stream. This translates to a feed stream concentration of 30,500 ppm (parts per million)<ref>Definition and Units. CATD Salinity Expertise Center. Available at http://www.salinityremotesensing.ifremer.fr/home. Accessed January 12, 2016.</ref>. The composition of the salt attributed to this seawater salinity is shown in Table 6 in Appendix B.

No data on the composition of seawater around the Corpus Christi area was available, but the composition of seawater does not change significantly in different locations. The given composition is considered accurate for our feed stream<ref>Brown E, Colling A, Park D, Phillips J, Rothery D, Wright J. Seawater : Its Composition, Properties and Behaviour. Boston: Butterworth-Heinemann, 1995.
</ref>. Note that seawater salt is primarily sodium chloride - total dissolved solids are 85.7% sodium chloride by mass. As such, the dissolved solids in the feed stream is assumed in this project to be pure NaCl, making the feed composition 96.95% water and 3.05% NaCl by mass. A control system will likely be put in place to ensure feed concentration remains at this level, accounting for natural fluctuations in local seawater salinity.

The largest byproduct from the process will be a brine slurry with high salt concentrations. An upper corrosion limit of 700 ppm was set for recirculated and purged brine. This undesired byproduct will be sent to a nearby Class I non-hazardous injection well facility operated by GNI Group, which is approximately 12 miles away from the proposed desalination plant location as shown in the figure in Appendix A<ref>Texas Water Code. Availabe at http://www.statutes.legis.state.tx.us/Docs/WA/htm/WA.27.htm. Accessed January 14, 2016.
</ref><ref>Google Maps. Available at https://goo.gl/maps/3xcvLZwUCe42 Accessed January 15, 2016.</ref>. Through the incorporation of a Class I injection well, the brine will not have to be treated or diluted before injection. <ref>EPA Regulations on Class I Wells. Available at http://www.epa.gov/uic/class-i-industrial-and-municipal-waste-disposal-wells. Accessed January 14, 2016.
</ref><ref>List of Injection Well Facilities by Location. Available at http://www.ehso.com/cssepa/tsdfdeepwells.php. Accessed January 14, 2016.</ref>

There are no regulations regarding the temperature of city drinking water, so the product stream temperature has no requirements based on regulations. Outlet temperatures of 35oC are acceptable for input to the municipal water supply system. Since city water tends to be stored in non-temperature controlled tanks, the temperature will naturally cool to ambient temperature.<ref>Climate Corpus Christi- Texas. US Climate Data. Available at http://www.usclimatedata.com/climate/corpus-christi/texas/united-states/ustx0294. Accessed January 12, 2016. </ref>

==Pre-Treatment and Post-Treatment==
For pretreatment, Belgard EV 2030 is recommended to be added at a concentration of 1.8-2.0 mg/L for a system with top brine temperature approximately 110 °C<ref name="BWA 2030">BWA Water Additives. Belgard EV2030 Technical Data Sheet. BWA Water Additives. http://www.wateradditives.com/files/products/tech_data_sheets/Belgard%20EV%202030_Technical%20Data%20Sheet_Thermal_8.5x11.pdf. Accessed January 27, 2016.
</ref>. Since our process is designed to operate at a slightly higher top brine temperature, 120 °C, we will design for the upper bound concentration: 2.0 mg/L of antiscalant. With a feed flow rate of 8250 m3/hr, the process therefore requires 16.5 kg/hr of Belgard EV 2030.

For disinfection during post-treatment, chlorine addition typically requires a dose of roughly 2.0 mg/L as well<ref name="World Health Organization">World Health Organization. Desalination for safe water supply. http://www.who.int/water_sanitation_health/gdwqrevision/desalination.pdf. Accessed January 26, 2016.</ref>. With a pure water distillate flow rate of 4758 m3/hr, the process requires 9.5 kg/hr of chlorine as a 5%-15% sodium hypochlorite solution. Approximately 74 mg/L of lime (calcium hydroxide) and 88mg/L of CO2 should be added to the desalinated water to help prevent corrosion and increase alkalinity within acceptable ranges<ref name="Vouchkov">Vouchkov, N. Re-Mineralization of Desalinated Water. SunCam. https://s3.amazonaws.com/suncam/npdocs/118.pdf. Accessed February 18, 2016.</ref>. With a product stream flow rate of 4758 m3/hr, these concentrations correspond to 352.1 kg/hr of lime and 418.7 kg/hr of CO2. Both of these can be purchased, and it is likely that the CO2 can be procured from the neighboring natural gas power plant.

==Choice of Desalination Technology==
The two major technologies commonly implemented for desalination include reverse osmosis (RO), and multi-stage flash distillation (MSFD). RO utilizes high pressures to force water through a semipermeable membrane that allows for purified water to pass through, but stops organic and other wastes. Often, the pressures can reach 55 - 68 bar, which requires large pumps to maintain pressure, and introduces safety concerns to the process<ref>Stage 2 Measures in the Drought Contingency Plan. Available at http://www.cctexas.com/government/water/conservation/conservation-drought-plans/stage-2-measures. Accessed January 12, 2016.
</ref>. While RO can be less energy intensive than MSFD, and the membrane can also remove contaminants, it is susceptible to feedwater quality, membrane fouling, and requires more extensive pretreatment and posttreatment than MSFD. The cost of RO is directly proportional to the feed salinity, and so it is less expensive when desalinating low salinity water.

Multi-stage flash distillation is best selected for areas that have high salinity or low energy costs. The pretreatment process for MSFD is much easier when compared to RO because pretreatment often consists of simply adding anti-corrosives and antiscalants to the feed stream. The process works by heating water in vessels at different pressures to change the boiling point of the water and evaporate it as it passes through the system. After evaporation, the water condenses, leaving pure water as the effluent. MSFD costs are now roughly $1.00/m3, where this cost is mostly independent of salinity, but dependent on energy costs.<ref>Fritzman C, Lowenberg J, Wintgens T, Melin T. State-of-the-art Reverse Osmosis Desalination. Desalination. 2007; 216:1-76.
</ref> In both RO and MSFD a brine is produced which then must be appropriately treated and/or disposed of.

Multi-stage flash distillation was selected for this process, mainly due to the economic advantages offered by the selected location of Corpus Christi, TX. MSFD is also a well understood process that can be more easily modeled by chemical engineering software.

=Technical Approach=
Quantitative modeling of the process and all included optimization was conducted using the simulation software Aspen HYSYS. A variety of assumptions were developed to set up and define specific components of the process simulation itself. Within Aspen HYSYS, the NRTL Electrolyte property package was selected to model the behavior of salt water. Each stage of the MSF process is modeled as an individual flash vessel, with individual shell & tube heat exchangers used to model the distillate condensers present in each flash stage in the MSF system.

The HYSYS simulation model only considers the flash portion of the overall process, allowing establishment of mass and energy balances for the system. Pretreatment and post-treatment were excluded for the HYSYS model, as these steps are simple additions or removals of small concentrations of chemical components. Although the simulation models heat exchangers outside of each flash stage, in the physical design of the plant these heat exchangers will be contained within each flash stage.

One portion of the process is not modeled, which is the inlet seawater entering the heat rejection stages. As can be seen from the full process flow diagram (Appendix C), the inlet seawater in the heat rejection stage is in high excess to provide maximum cooling capacity; then a significant portion is purged to bring the stream flow rate down to what is needed for the feed stream. It is assumed that the flow rate of cooling water for those three stages is in such high excess that the cooling water does not heat up. Therefore, in the process simulation models, the inlet stream is the feed stream, which enters directly into the heat recovery portion of the process.

See Appendix D for a screenshot overview of the simulation environment model.

=Process Overview=
==Process Flowsheet==
The following diagram, Figure 1, shows a basic flowsheet for the process. Process alternatives considered for each stage are discussed below. Please see Appendix C for the full process flowsheet with finalized design decisions.

[[File:BlockFlowDiagram.png|center|700px|thumb|alt=Nueces Bay Desalination Facility Block Flow Diagram|Figure 1: Basic block flowsheet of the multi-stage flash desalination process.]]

==Major Features==
As seen above, there are three primary stages of the desalination process: Pre-treatment, the multi-stage flash distillation, and post-treatment. Seawater enters the pre-treatment phase first to make the feed suitable for the rest of the process by filtering out large particles that could accumulate on pipes, deaerating the water to limit corrosion, and adding an agent to combat fouling. The water then enters the distillation process, which is broken up into two sections: heat recovery and heat rejection.

In the heat recovery stage, there are 18 flash vessels, each of which separates steam from the salinated liquid water. Each flash vessel has saltwater as its inlet and two outlets: a vapor outlet (steam) that condenses into the final product of purified water and a liquid outlet composed of water with more dissolved solids than the inlet, which is the inlet stream for the next flash vessel. The pipe containing the feed is first mixed with recycled brine, then goes through the top sections of each of the flash vessels, starting with V-118 and ending up at V-101. In this way, the feed stream pipe first serves as a heat exchanger, heating the pre-treated seawater while simultaneously condensing the steam into the final product. The feed then enters a final heat exchanger where it is brought up to 103.6 °C before entering V-101 to be flashed.

After exiting V-118, the final flash vessel in the heat recovery portion, the water enters the heat rejection portion, which consists of three flash vessels (V-201, V-202, V-203). The flash vessels of this section operate in the same manner as in the heat recovery section, with the exception that condensation occurs using excess cooling water instead of pre-treated seawater. This technique allows excess heat to be rejected into an excess water stream rather than the feed.

The purified product stream finally enters post-treatment processing in which water is re-aerated and remineralized, disinfected, and pH is adjusted before being delivered to the city. The concentrated brine leftover at the end of the flash cycle is recycled into the feed, although a purge is required to continuously remove some salt from the system. This purged brine is disposed of by use of a Class I injection well, per regional regulations.

See Appendix E to find detailed stream tables of stream specifications.

==Process Alternatives==
===Pretreatment===
Multi-stage flash distillation requires significantly less pretreatment than reverse osmosis desalination, but helpful pretreatment techniques still exist. One very inexpensive method of pretreatment is screening/filtration, which is used to filter out large contaminants from intake seawater. Using more complicated membrane technology to accomplish large particle filtration is a possible alternative, but doing so would be much more expensive than using a standard screen or filter, so the latter choice has been selected for this process.<ref>El-Dessouky H., Alatiqi I, Ettouney H. Process synthesis: The multi-stage flash desalination system. Desalination. 1998;115(2):115-179. doi: 10.1016/S0011-9164(98)00035-6.</ref>

The most critical type of fouling in multi-stage flash distillation is scaling - therefore, the most critical step in the pretreatment process will be the inclusion of an anti-scalant. Alkaline scaling can be prevented by acid dosing or polyphosphate addition; however, acid dosing can result in increased corrosion and polyphosphates can lead to non-alkaline calcium phosphate fouling, so these additives will not be considered<ref name="Morris">Morris, R. The development of the multi-stage flash distillation process: A designer's viewpoint. Desalination. 1993;93(1-3):57-68. doi:10.1016/S0011-9164(93)80096-6. Availble at http://www.sciencedirect.com/science/article/pii/0011916493800966</ref><ref>Al-Gobaisi, D. A quarter-century of seawater desalination by large multistage flash plants in Abu Dhabi (Plant performance analysis, assessment, present efforts toward enhancement and future hopes). Desalination. 1994;99(2-3):509-512. doi: 10.1016/S0011-9164(94)00196-0.</ref>. As such, a polymer-based additive is recommended for the anti-scalant for this process. The Belgard EV series is designed for use in high-temperature seawater MSF; of these, Belgard EV 2030 is the most neutral in pH<ref>NSF International. NSF Product and Service Listings: Drinking Water Treatment Chemicals - Health Effects. NSF International. http://info.nsf.org/Certified/PwsChemicals/Listings.asp?Company=00300&. Accessed January 27, 2016.</ref><ref name="BWA 2030" /><ref>BWA Water Additives. Belgard EV2035 Technical Data Sheet. BWA Water Additives. http://www.wateradditives.com/files/products/tech_data_sheets/Belgard%20EV2035_Technical%20Data%20Sheet_Thermal_8.5x11.pdf. Accessed January 27, 2016.</ref><ref>BioLab Water Additives. Belgard EV Antiscalant for seawater distillation plants. BioLab Water Additives. http://www.lpq.com.mx/pdf/BELGARD%20EV.PDF. Accessed January 27, 2016.</ref>. As such, we have selected Belgard EV 2030 for use as an anti-scalant in our process.

Corrosion control is another significant method of pretreatment in multi-stage flash distillation plants. Using a piping material that has limited susceptibility to corrosion is typically not economically viable for MSF desalination plants due their large size, so using carbon steel for most of the piping is generally used for its economic benefits despite its susceptibility to corrosion<ref name = "Morris" />. Limiting oxygen concentration is very feasible through the use of deaerators, which can remove oxygen as well as other dissolved gases from the seawater stream. Often, an external deaeration system is used<ref name = "Morris" /><ref>Edwards - Hick Hargreaves. Seawater Deaerators. Edwards - Hick Hargreaves. http://psscorp.co.th/upload/news/20110830105340.pdf. Accessed January 26, 2016.</ref>. Given these considerations, using an external deaerator combined with a less-corrosive antiscalant like Belgard EV 2030 is much more practical for the process than using anti-corrosive piping.

Excessive foam can also be an issue in some MSF distillation facilities, so antifoaming agents are sometimes employed in the pretreatment process as well. However, many plants are operated without foaming issues, and therefore do not need anti-foaming pretreatment at all. Selection of an anti-foaming agent must be carefully weighed with selected of anti-scalant to avoid issues with additive interactions [26]. At this stage in the design process, an anti-foamer will not be included due to the risk of decreasing scalant efficacy, and the likelihood that foam will not be an issue in the desalination plant.

===Multistage Flash Distillation===
The main objective of the design project is desalinate water. This is done with the main process of multi stage flash distillation (MSF). MSF can be developed using a numerous number of different process configurations, such as Once-Through, Simple Mixer Brine Circulation, and Multi-Stage Heat Rejection Brine Circulation processes.

The most desired process for the project is the Three Stage Heat Rejection Brine Circulation as it builds on benefits of the other simpler technologies. By combining principles of heat rejection and heat recovery, the required heat transfer area of the process is greatly reduced as compared to once through processes or simple brine recycle processes. It has the highest performance ratio with the lowest specific feed flow rates, therefore greatly lowering the operating costs of the process. It is also known as the industry standard for MSF<ref name = "KSU Chapter 6">Kansas State University ChE 413. Available at: http://faculty.ksu.edu.sa/Almutaz/Documents/ChE-413/Multi-stage%20Flash%20Desalination.pdf. Accessed January 26, 2016.</ref>. In addition, since our brine disposal technique involves the pumping of the brine into a Class I well, the high brine outlet salinity is not a drawback for our process since post treatment or dilution of the brine is not necessary.

===Post Treatment===
After processing the water via multi-stage flash, the water needs to be post-treated to make the water taste as consumers would expect, since distilled water is “flat” and unpleasant, as well as to protect the consumers and the piping through which the water will be passing<ref name="Clayton">Clayton R. A review of current knowledge: Desalination for water supply. Foundation for Water Research. http://www.fwr.org/desal.pdf. Accessed January 26, 2016.</ref>. There are multiple alternatives to achieve these objectives of remineralization, aeration, corrosion control, and disinfection.

Aerating and introducing some dissolved solids back into the product water are effective ways to create a quality product that is not flat, and contains flavoring that is expected of the water when delivered to the consumer. Aeration will be accomplished via an inline spray aerator which will take up less space than cascade aeration.<ref>Minnesota Rural Water Association. Aeration. http://www.mrwa.com/WaterWorksMnl/Chapter%2011%20Aeration.pdf. Accessed January 26, 2016.</ref> Feed blending will be used to remineralize the water as opposed to a remineralization filter because it does not require additional equipment purchases, and can easily handle the large amounts of throughput required for the process.<ref name="Clayton" />

Corrosion and pH can be controlled by adding lime (calcium hydroxide) and carbon dioxide to the water. To do so, about 74 mg/L of lime and 88mg/L of CO2 should be added to the desalinated water to help prevent corrosion and increase alkalinity within acceptable ranges. Sodium hydroxide, soda ash, and sodium bicarbonate are alternatives to altering the alkalinity and pH of the product water, but lime and CO2 is an industry standard <ref name="Vouchkov" />.

Disinfection needs to be done to prevent microbial growth in the desalinated water and ensure the safety of the general public. Chlorine addition via chlorine gas or sodium hypochlorite is the most common and only requires a dose of 1.5 to 2.5 mg/liter. <ref name="World Health Organization" /> Other alternatives include ozonation, UV light, and chloramination. Chlorine, as a 5% - 15% sodium hypochlorite solution, is sufficient for our process and relatively inexpensive, and as such will be utilized in the process.

===Transport of Brine to Well===
Three major possibilities exist for transporting the brine from our desalination plant to the injection well for disposal: rail, trucking, and a pipeline. A pipeline was chosen as the most feasible and cost-effective approach.

A rail line is not available at this time, and with the cost to construct a rail line at approximately $43 million per mile<ref>About Money. How Much Do Rail Transit Projects Cost to Build and Operate? 2014. Available at: http://publictransport.about.com/od/Transit_Projects/a/How-Much-Do-Rail-Transit-Projects-Cost-To-Build-And-Operate.htm. Accessed January 27, 2016. </ref>, the enormous capital cost makes rail transport infeasible for this site. For large volume operations, pipelines are cheaper than trucking. In a study of wastewater sludge produced from wastewater treatment facilities, it was found that it was more economical for facilities producing over 1200 m3 per day to utilize pipelines rather than trucking.<ref>Marufuzzaman M, Eskioglu S, Hernandez R. Truck versus pipeline transportation cost analysis of wastewater sludge. Transportation Research. 2015; 74(A): 14-30. </ref> Our facility is producing nearly 3 times this volume at 3485 m3 per day of brine. Furthermore, brine is a less viscous fluid than wastewater sludge, decreasing pumping costs in the pipeline and further amplifying the benefit of choosing a pipeline over an injection well.

===Brine Storage===
Due to the choice of a pipeline, rather than trucking, as the method of transport of brine to the injection well, brine storage is unnecessary, so storage solutions such as holding tanks were not investigated. One method of storage compatible with a pipeline is to pump the brine through an evaporation pond. This solution is attractive as it would allow the brine to concentrate, saving some costs of pumping, however, the considerable land usage of evaporation ponds leads them to only be effective for smaller plants. The largest municipal plant utilizing this method produces only 5.7 million liters per day<ref>University of Santa Cruz Center for Integrated Water Research. Concentrate Management: Evaporation Ponds. 2010. Available at: https://ciwr.ucsc.edu/sites/default/files/ICM_D5.pdf. Accessed January 27, 2016. </ref>, whereas our desalination plant is proposed to produce 75.7 million liters per day. Therefore, evaporation ponds have been eliminated from consideration.

==Major Specifications==
===Material of Construction Selection===
While saltwater and desalinated water in general can be corrosive, there are best practices within the desalination industry for material selection. With this in mind, best practice shows carbon steel lifetime can be lengthened by monitoring the oxygen concentration in the water, specifying the paint used to coat any metal, and more expensive materials can be used in areas of extremely high corrosion.<ref name = "Morris" />. By following these best practices, it is possible to save significant amounts of money through the use of carbon steel in most areas of the plant, with specific high-corrosion areas being reinforced with, or replaced with, more corrosion resistant materials, such as Cu/Ni 70/30 alloy<ref name = "KSU Chapter 6" />.

===Heat Exchanger Sizing===
See Appendix F for heat exchanger sizing data. Heat exchanger sizes appear reasonable for placement in the flash vessels.

===Flash Vessel Sizing===
The dimensions for the flash drums were obtained through literature values. The flash drums were determined to be 18x4x3 m in width, height, and length.<ref name="KSU Chapter 6" />

Wall thickness was determined using the following conventions: Design pressure is 10% more than the operating pressure, design temperature is 50°F (27.8°C) higher than the operating temperature, a corrosion allowance of 3-mm and an effectiveness of 85% was derived from the assumption of a double-welded butt joint that was spot checked. See Appendix G for wall thicknesses calculated for each pressure vessel.

The sizing of the vessels to be constructed will be determined after talks with a manufacturer. Due to economies of scale, it may be more cost-effective to buy 21 pressure vessels with the dimensions mentioned above and a wall thickness of 17.1-mm, the highest wall thickness required in the system.

===Pipe Losses and Pressure Drop===
HYSYS was used to estimate the diameter of a pipe given a specified pressure drop per 100m of pipe. It was determined that for a pressure drop of 0.1 kPa/100m of pipe, a 3.05m diameter pipe is sufficient, which is more than manageable for manufacturers. This pressure drop is mostly insignificant and will be taken care of with the safety factors included in the pumps. The significant pressure drops in our system is accounted for via losses in the flash vessels and heat exchangers, which drive the flash process. Therefore overall pipe losses are insignificant and are built into intentional design losses.

=Economic Analysis=
==Economic Assumptions==
The sale of purified water yields a revenue stream of $49.5 million per year assuming sale to commercial entities within the city limits of Corpus Christi at $1.7915 per thousand liters.<ref>Water schedule. City of Corpus Christi Utility Business Office. http://www.cctexas.com/Assets/Departments/Financial-Services/Utilities-Business-Office/files/waterrates.pdf Accessed February 28, 2016.</ref> The primary variable costs are utilities and the brine disposal, with minimal raw materials costs due to the primary feedstock, seawater, being free, and minimal amounts of pre- and post-treatment chemicals being used. The cost of brine disposal used in this analysis was high at $2.097 per thousand liters<ref>McCurdy, R. Underground injection wells for produced water disposal. Chesapeake Energy Corporation. http://www.epa.gov/sites/production/files/documents/21_McCurdy_-_UIC_Disposal_508.pdf Accessed February 28, 2016.</ref>, an estimate for brine disposal utilizing trucking as the mode of transport. As has been shown through previous analyses, a lower cost can be achieved using a direct pipeline, however, no good estimates for this cost could be found.

The NPV was calculated using a 6% cost of capital, the approximate interest rate for a municipal bond.<ref>Zirbe, R.O. Jr., Han, X., Layton, D., Leshine, T. A history of discount rates and their use by government agencies. (2002). http://faculty.washington.edu/zerbe/docs/DISCOUNT%20RATE%20HISTORY-latesXIt.doc Accessed February 28, 2016.</ref> It was assumed that the plant can be built within two years, with production at half-scale in its first year. Additionally, one month of variable costs, $10.6 million, was set aside for working capital. See Appendix H for a full table of assumptions. A 30 year NPV calculation was chosen as the decision basis since 30 years is a conservative estimate for the lifetime of a desalination plant [38]. See below for estimates of NPV for the lifetime of the plant under various assumptions and design criteria.

==Optimization and Sensitivity Analysis==
Looking at the multi-stage flash process, there are a number of opportunities to optimize the process. The major process variables that were investigated were the inlet water temperature, the operating pressure drop per vessel, and the number of stages over which the process takes place. Considering these optimizations, our final process consists of 21 flash vessels (18 heat recovery and 3 heat rejection stages), each with a pressure drop of 5 kPa. See Appendix D for a screenshot of the HYSYS simulation used to model the optimized process.
===Operating Pressure and Pressure Drop===
Initially our process was designed to pump the inlet seawater and brine to a pressure of 2 bar and allow the pressure drop in each flash stage to end our process downstream at 1 bar to prevent complications associated with designing a system under vacuum. While this provided pumping advantages, the energy in the system was significantly higher due to vapor pressure considerations. Due to this fact, the required heat exchange area for certain exchangers proved to be infeasible (on the order of 80,000m2) due to the enormous capital and operating costs that would be associated with such a large heat exchanger.

The process was optimized to operate under vacuum, requiring additional pumping requirements but much lower required heat exchange area, and therefore lower utility costs. This optimization was done by specifying the pressure of the final flash vessel to 5.7kPa, so that the temperature of the water vapor (~30oC) is within approximately 5oC of the cooling water (35oC) so that condensation can occur within a reasonable approach. This also allows our system to optimally extract pure water from the brine running through the system and reduce energy requirements for cooling the product or the recycle. Our system redesign and optimization reduced the energy in the system and reduce the same heat exchanger area requirements by more than 3 orders of magnitude to approximately 800m2.

Once the process was optimized to operate in a vacuum, the pressure drop per flash stage regulated through values was then optimized. Three pressure drops per stages, 3kPa, 5kPa, and 7kPa, were tested and then an economic analysis was conducted to find the optimal value for the pressure drop. These changes had impacts on both operating and capital costs as they would change initial pumping requirements, the amount of water flowing through the process and recycle (and therefore the flash vessel sizes, heat exchanger sizing), and the efficiency of each stage.

The various pressure drops (3 kPa, 5 kPa, and 7 kPa) were tested on a system nominally at 24 stages under the assumption that the results would be generalizable to systems with a different number of stages. With increasing pressure drop, we found decreased capital cost but increased utility cost. As can be seen from Table 2 below, a pressure drop of 5 kPa proved to be the best option based on 30 year NPV, with a loss of $838.8 million, a savings of $65.5 million compared to the next-best option with a pressure drop of 7 kPa.

<center>
{| class = "wikitable"
|+Table 2: Comparison of profitability for our MSF desalination process with 24 stages with 3 different pressure drops per stage
! Number of Stages  
! Pressure Drop (kPa)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
! 30-Year NPV ($MM)
! NPV Ratio (over 5kPa)
|- style="text-align: center;"
| 24 || 3 || 362.3 || 39.1 || -905.8 || 1.08
|- style="text-align: center;"
| 24 || 5 || 255.1 || 53.6 || -838.8 || 1.00
|- style="text-align: center;"
| 24 || 7 || 209.5 || 72.2 || -904.3 || 1.08
|}
</center>

===Number of Flash Stages===
The number of stages is an important area for optimization. An increased number of stages yields better recovery of purified water, however, requires higher capital cost from the additional pressure vessel. Furthermore, with an increased amount of purified water comes increasingly concentrated brine. This must be taken into account since any brine concentration in excess of about 7% presents a more significant corrosion risk<ref name="KSU Chapter 6" />, so increased maintenance costs for the piping or higher capital costs for more corrosion-resistant materials could be necessary for brine concentrations exceeding 7%. Therefore, to recover the same amount of pure water without exceeding corrosion limitations, increasing the number of stages will reduce the overall size of each stage.

Optimizing based on NPV, it was found that 21 stages is optimal, balancing the increased profits from the additional purified water additional costs of higher capital cost. See Table 3 below to compare the 30-year NPV of the process across various numbers of flash vessels.

<center>
{| class = "wikitable"
|+Table 3: Comparison of profitability for our MSF desalination process with a 5 kPa pressure drop, 5000 m3/hr feed flow rate, with varying number of stages.
! Number of Stages  
! Pressure Drop (kPa)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
! 30-Year NPV ($MM)
! NPV Ratio (over 21 Stages)
|- style="text-align: center;"
| 17 || 5 || 213.9 || 58.7 || -813.8 || 1.016
|- style="text-align: center;"
| 18 || 5 || 218 || 57.6 || -809.9 || 1.011
|- style="text-align: center;"
| 19 || 5 || 224.3 || 58 || -806.1 || 1.007
|- style="text-align: center;"
| 20 || 5 || 229.8 || 57.9 || -813.6 || 1.016
|- style="text-align: center;"
| 21 || 5 || 226.6 || 55.9 || -800.7 || 1
|- style="text-align: center;"
| 22 || 5 || 254.7 || 54.6 || -849.5 || 1.061
|- style="text-align: center;"
| 23 || 5 || 249.3 || 54.1 || -833.1 || 1.04
|- style="text-align: center;"
| 24 || 5 || 255.1 || 53.6 || -838.8 || 1.048
|- style="text-align: center;"
| 25 || 5 || 254.9 || 52.7 || -837.4 || 1.046
|}
</center>

===Inlet Seawater Temperature===
By pairing our desalination plant with a natural gas power plant and utilizing its cooling water we forgo initial heating of our inlet stream. Normally, water pumped from the ocean would be approximately 25oC and by capturing waste heat from the power plant we can reduce utilities with our water inlet of 45oC. Through an economic comparison between two plants nominally at 21 flash stages, the plant with an inlet temperature at 25oC has a capital cost of $245MM and a utility cost of $77MM/yr and the same facility with an inlet water temperature at 45oC has a capital cost of only $226MM and a utility cost of $56MM/yr. As shown in Table 4 below, this $19MM savings in capital costs and $21MM/yr saving in utilities is largely due to reducing the size of the heat exchangers and lowering the steam required for heating duty on the inlet seawater respectively. Therefore our process is optimized by recovering the waste heat water from the gas fired power plant.

<center>
{| class = "wikitable"
|+Table 4: Seawater Inlet Temperature Process Cost Optimization
! Seawater Temperature (°C)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
|- style="text-align: center;"
| 25 || 245 || 77
|- style="text-align: center;"
| 45 || 226 || 56
|- style="text-align: center;"
| Savings || 56 || 19
|}
</center>

==Overall Process Economics==
After optimization at fixed flow rates, the process was scaled to meet the the 67% operating percentage and final product definitions as shown in Table 1, which resulted in finalized economics shown in Table 5 below. For a table of the installed costs of each process vessel, see Appendix I. For the finalized NPV calculations, see Appendix J.

<center>
{| class = "wikitable"
|+Table 5: Summarized Economics for 21-stage MSFD Process
! Economic Parameter
! Cost (USD)
|- style="text-align: center;"
| Capital Cost || 345 MM
|- style="text-align: center;"
| Total Operating Cost per Year || 103 MM
|- style="text-align: center;"
| Utility Cost per Year || 59 MM
|- style="text-align: center;"
| 30-Year NPV (6% Discount) || -1035 MM
|}
</center>

=Conclusions and Recommendations=
==Project Viability==
Table 5 above demonstrates that the process has reasonable operating and capital costs, but the 30-year NPV is negative by -1035 $MM due to the extremely low sell price capability of desalinated water as a product. Therefore, this plant would have to heavily be supported by the City of Corpus Christi to run as the product revenue will not make it self-sufficient. The acquisition of government subsidies are key operating requirements for all desalination facilities in the United States.

==Additional Concerns==
Large volumes of water, steam, and energy are needed to ensure the project runs as specified. While most of the water can easily be extracted from the ocean with little to consequence, the fuel used to generate the steam and electricity can have a significant environmental impact. As the plant is located next to a natural gas power plant, this fuel is better than coal with respect to environmental concerns, and utilizing energy from the power plant to preheat the feed stream for the desalination plant helps lower environmental impacts.

The large amounts of high pressure steam being used represents one of the the largest safety concern in the process, because a high pressure steam release could easily cause severe burns or even death. Proper precautions and control measures must be in place to prevent accidental release. Otherwise, the process does not utilize any known toxic chemicals and is relatively safe from a chemical standpoint. The flash vessels mostly operate below atmospheric pressure, so they must be constructed as vessels that can withstand vacuum without failing.

=Appendix A: Proposed Location of Desalination Plant=
Pictured below is the proposed location for the desalination plant, located on Nueces Bay in Corpus Christi, adjacent to the Nueces Bay Energy Center. The plant’s proposed location is shown in the area outlined by the small circle in the upper right of the figure.

[[File:NuecesBayLocation.png|center|600px|thumb|alt=Nueces Bay Desalination Facility Location|Figure A.1: Proposed mapped location of Nueces Bay Desalination Facility.]]
[[File:ClassIWell.png|center|700px|thumb|alt=Nueces Bay Desalination Facility Distance to Injection Well|Figure A.2: Mapped distance from proposed location to Class I Injection Well]]

=Appendix B: Feed and Product Definitions - Salinity=

<center>
{| class = "wikitable"
|+Table 6: Average Composition of Total Dissolved Solids in Seawater39,40
! Compound  
! Mass Composition (kPa)
|- style="text-align: center;"
| Chlorine || 55.03%
|- style="text-align: center;"
| Sodium || 30.64%
|- style="text-align: center;"
| Sulfate || 7.70%
|- style="text-align: center;"
| Magnesium || 3.66%
|- style="text-align: center;"
| Calcium || 1.17%
|- style="text-align: center;"
| Potassium || 1.12%
|- style="text-align: center;"
| Bicarbonate || 0.36%
|- style="text-align: center;"
| Bromine || 0.19%
|- style="text-align: center;"
| Other || 0.15%
|}
</center>

[[File:Nueces Bay Salinity.JPG|center|700px|thumb|alt=Nueces Bay Salinity Tracking|Figure 2: Salinity in Nueces Bay over the past 10 years6.]]

=Appendix C: Process Flow Diagram=
[[File:TeamF_FinalPFD.JPG|center|1200px|thumb|alt=Process Flow Diagram|Figure C.1: Process Flow Diagram]]

=Appendix D: HYSYS Simulation Model=
[[File:TeamF_HYSYS_Screenshot.PNG|center|1200px|thumb|alt=HYSYS Simulation Model|Figure D.1: Screenshot of HYSYS Model Simulation.]]

=Appendix E: Stream Tables=
A summary of some values is shown below:

<center>
{| class = "wikitable"
|+Table 7: Key process material streams
!  
! Stream  
! Molar Flow (105 kgmole/h)
! Mass Flow (105 kg/h)
|- style="text-align: center;"
! style="text-align: right;" | Inlet
| Feed Water || 454 || 83.6
|- style="text-align: center;"
! style="text-align: right;" | Outlet
| Brine Reject || 1.90 || 36.0
|- style="text-align: center;"
! style="text-align: right;" | Product
| Pure Water || 2.64 || 47.5
|}
</center>

<center>
{| class = "wikitable"
|+Table 8: Key process energy streams
! Stream  
! Heat Flow (kJ/h)
|- style="text-align: center;"
| Feed Water || 1.30E+11
|- style="text-align: center;"
| Q-301 || 5.73E+06
|- style="text-align: center;"
| Q-302 || 3.09E+09
|- style="text-align: center;"
| Q-304 || 4.68E+06
|- style="text-align: center;"
| Q-303 || -1.26E+09
|- style="text-align: center;"
| Brine Reject || -5.47E+10
|- style="text-align: center;"
| Pure Water || -7.46E+10
|}
(Negative value denotes energy leaving process)
</center>

=Appendix F: Heat Exchanger Sizing=

<center>
{| class = "wikitable"
|+Table 9: Utility Temperatures and Heat Transfer Coefficient<ref name="Towler">G.P. Towler, R. Sinnott. Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design. Elsevier, 2012.</ref>
!  
! HP Steam
! Cooling Water
|- style="text-align: center;"
! Temperature (°C)
| 250 || 35
|- style="text-align: center;"
! U (W/m2K)
| 4000 || 1500
|}
</center>

<center>
{| class = "wikitable"
|+Table 10: Internal Flash Heat Exchanger Sizing, condensing steam.
! Exchanger
! UA (kJ/°C-h)
! U (kJ/hr/m2°C)
! A (m2)
|- style="text-align: center;"
| E-101 || 1.47E+07 || 14400 || 1019
|- style="text-align: center;"
| E-102 || 1.53E+07 || 14400 || 1059
|- style="text-align: center;"
| E-103 || 1.59E+07 || 14400 || 1103
|- style="text-align: center;"
| E-104 || 1.66E+07 || 14400 || 1153
|- style="text-align: center;"
| E-105 || 1.74E+07 || 14400 || 1209
|- style="text-align: center;"
| E-106 || 1.83E+07 || 14400 || 1272
|- style="text-align: center;"
| E-107 || 1.93E+07 || 14400 || 1342
|- style="text-align: center;"
| E-108 || 2.05E+07 || 14400 || 1424
|- style="text-align: center;"
| E-109 || 2.19E+07 || 14400 || 1518
|- style="text-align: center;"
| E-110 || 2.35E+07 || 14400 || 1629
|- style="text-align: center;"
| E-111 || 2.54E+07 || 14400 || 1761
|- style="text-align: center;"
| E-112 || 2.77E+07 || 14400 || 1921
|- style="text-align: center;"
| E-113 || 3.05E+07 || 14400 || 2119
|- style="text-align: center;"
| E-114 || 3.42E+07 || 14400 || 2372
|- style="text-align: center;"
| E-115 || 3.90E+07 || 14400 || 2706
|- style="text-align: center;"
| E-116 || 4.56E+07 || 14400 || 3167
|- style="text-align: center;"
| E-117 || 5.55E+07 || 14400 || 3856
|- style="text-align: center;"
| E-118 || 7.20E+07 || 14400 || 5000
|}
</center>

=Appendix G: Pressure Vessel Sizing=

=Appendix H: Expanded Economic Assumptions=

=Appendix I: Equipment Cost Summary=

=Appendix J: Economic NPV Analysis=

=References=
<references/>

File:Nueces Bay Salinity.JPG

2016-03-07T21:03:28Z

Benjkg: Salinity in Nueces Bay over the past 10 years

Salinity in Nueces Bay over the past 10 years

Nueces Desalination Center: Production of Drinking Water by Multi-Stage Flash Distillation

2016-03-07T20:59:55Z

Benjkg: /* Appendix B: Feed and Product Definitions - Salinity */

Team F: Rankine 672 Final Report

Authors: Kedric Daly, Ben Granger, Evan Rosati, Kathleen Zhou

Instructors: Fengqi You, David Wegerer

March 11, 2016

=Executive Summary=
Desalination of seawater to drinking water is a technology of ever-increasing importance given trends of increasingly widespread water shortages around the world. Even in the United States, water shortages are becoming increasingly common in dry, highly-populated such as California and certain areas of the Gulf Coast. Our design team, Rankine 672, was tasked with evaluating the need for and designing a desalination plant to provide purified water in a place of need. Investigation revealed that the city of Corpus Christi, Texas, is a strategic location for development of a desalination facility, as surrounding region is in need of new drinking water sources and the city has expressed serious interest in contracting the development of a large-scale desalination plant. Our proposed solution is a multi-stage flash distillation desalination facility capable of producing 20,000,000 gallons (75,708 m3) of purified drinking water for the city and surrounding region per operating day.

In this design, drinking water is produced from seawater feedstock through a 21-stage flash distillation process. By strategically locating the facility next to an existing natural gas power plant, the Nueces Bay Energy Center, pre-heated feed water can be taken directly from the outbound cooling systems of the power plant, allowing energy savings for our desalination plant. The process is designed for 16 hour-per-day operation, requires a seawater feed of 8250 m3/hr, and produces 4758 m3/hr of purified drinking water alongside a waste stream of 3485 m3/hr of concentrated brine. The waste stream is isolated from a recycle stream through a 9.0% purge; this results in an overall product yield of 57.7% on a volume basis. The process requires an additional pretreatment stream of 16.5 kg/hr Belgard EV 2030, which serves as an antiscalant, as well as 9.5 kg/hr chlorine, 352.1 kg/hr lime, and 418.7 kg/hr CO2 for post-treatment purposes.

The overall capital cost of the plant is expected to be $345MM, with a total operating cost of $103MM/yr, $59MM/yr of which comes from utilities. Due to the low price at which the product water can be sold, the 30-year net present value of the plant, assuming a 6% discount rate, is $-1,035MM. It is thus imperative this plant be constructed with public funds because it cannot return a profit to any private investor. The growing need for water however could justify the cost, because it is important that the residents of the greater Corpus Christi area have access to drinkable water.

__TOC__

=Introduction=
==Background==
Clean drinking water is a basic human need that is becoming increasingly difficult to meet. Currently, over 1 billion people live in places where water is considered scarce<ref> Tapping the oceans. The Economist USA. http://www.economist.com/node/11484059. Accessed March 2, 2016. </ref>. Though often thought of as a problem exclusive to third-world countries, the issue is becoming more prevalent in the US as well, with water managers in 40 of 50 states anticipating water shortage conditions in some portion of their state in the next 10 years. These shortages are due to increasing demands on constant or diminishing freshwater resources, therefore, to correct the issue, a new supply must be found<ref> Water Supply in the US. EPA. http://www3.epa.gov/watersense/pubs/supply.html. Accessed March 2, 2016.</ref>. Desalination of seawater is therefore important as a new source for clean drinking water and prevent water shortages.

==Project Definition==
The overall goal of the project is to investigate the feasibility of a desalination plant in order to meet the needs of growing water demand. The design needs to take health, safety, and environmental concerns into account, while optimizing costs.

=Design Basis=
Site Location and Conditions
Corpus Christi, TX was chosen as the location for the proposed desalination plant. The city is currently experiencing “Moderate Shortage Conditions”. This issue additionally comes at a time of growing demand with population growth anticipated due to new industry and Eagle Ford Group crude oil production<ref>Stage 2 Measures in the Drought Contingency Plan. Available at http://www.cctexas.com/government/water/conservation/conservation-drought-plans/stage-2-measures. Accessed January 12, 2016. </ref>.

In June 2014, the city announced the Corpus Christi Desalination Demonstration Project<ref>Corpus Christi Desalination Demonstration Project. Available at http://www.cctexas.com/Assets/Departments/Water/Files/DesalFactSheet.pdf. Published June 2014. Accessed January 12, 2016.
</ref> which seeks to create a demonstration desalination plant capable of producing 20,000 gallons of water per day, with plans to later build a full scale facility generating 20,000,000 gallons of water per day. The desalination demonstration project indicates a clear need for implementation of a desalination plant of this size to accommodate growth in the Corpus Christi area.

Locating our plant in Corpus Christi will allow us to take advantage of the benefits of cogeneration through a partnership with the Nueces Bay Energy Center. By utilizing their outlet cooling water as a large portion of our feed for the desalination process, we can raise the temperature of our seawater feed, saving on energy costs in the multi stage flash distillation. The proposed location for the desalination plant is immediately west of the Nueces Bay Energy Center. See Appendix A for a map depicting the proposed location for the desalination plant along the coast of Nueces Bay.

==Feed and Product Definitions==
The proposed desalination facility’s feed and product streams are defined as shown in Table 1. Rationale for these definitions is also described below. The values reported in Table 1 are final derived numbers from HYSYS simulation modeling.

<center>
{| class = "wikitable"
|+Table 1: Feed and product stream definitions
!  
! Salt Concentration
! Temperature
! Amount
|- style="text-align: center;"
! style="text-align: right;" | Feed Stream
| 30,500 ppm || 45°C || 8250 m3/hr
|- style="text-align: center;"
! style="text-align: right;" | Product Stream
| ~ 0 ppm || 35°C || 4758 m3/hr
|- style="text-align: center;"
! style="text-align: right;" | Byproduct Stream
| 69,500 ppm || 35°C || 3485 m3/hr
|}
</center>

The process feed will be seawater from Nueces Bay, preheated through the cooling water system of the nearby Nueces Bay Energy Center. Based on calculations developed from NREL (National Renewable Energy Laboratory) data, it was estimated that the Nueces Bay Energy Center uses approximately 250 million gallons of seawater per day for process and cooling needs. Therefore, the 20 million gallon per day requirement for the desalination facility can easily be met through the power plant’s cooling water<ref>Torcellini P, Long N, Judkoff R. Consumptive Water Use for U.S. Power Consumption. NREL. 2003; TP-550-33905.
</ref>. Note that we are assuming the process will operate 67% of the time (16 hours per day).

Based on data acquired from two salinity-measuring stations in Nueces Bay, the average salinity of local seawater is 30.5 practical salinity units (psu)<ref>Stations Map & Real - Time Data. The Conrad Blucher Institute for Surveying and Science website. Available at http://www.cbi.tamucc.edu/cbi/data/. Updated January 2016. Accessed January 9, 2016. </ref>, thus setting the concentration of the process feed stream. This translates to a feed stream concentration of 30,500 ppm (parts per million)<ref>Definition and Units. CATD Salinity Expertise Center. Available at http://www.salinityremotesensing.ifremer.fr/home. Accessed January 12, 2016.</ref>. The composition of the salt attributed to this seawater salinity is shown in Table 6 in Appendix B.

No data on the composition of seawater around the Corpus Christi area was available, but the composition of seawater does not change significantly in different locations. The given composition is considered accurate for our feed stream<ref>Brown E, Colling A, Park D, Phillips J, Rothery D, Wright J. Seawater : Its Composition, Properties and Behaviour. Boston: Butterworth-Heinemann, 1995.
</ref>. Note that seawater salt is primarily sodium chloride - total dissolved solids are 85.7% sodium chloride by mass. As such, the dissolved solids in the feed stream is assumed in this project to be pure NaCl, making the feed composition 96.95% water and 3.05% NaCl by mass. A control system will likely be put in place to ensure feed concentration remains at this level, accounting for natural fluctuations in local seawater salinity.

The largest byproduct from the process will be a brine slurry with high salt concentrations. An upper corrosion limit of 700 ppm was set for recirculated and purged brine. This undesired byproduct will be sent to a nearby Class I non-hazardous injection well facility operated by GNI Group, which is approximately 12 miles away from the proposed desalination plant location as shown in the figure in Appendix A<ref>Texas Water Code. Availabe at http://www.statutes.legis.state.tx.us/Docs/WA/htm/WA.27.htm. Accessed January 14, 2016.
</ref><ref>Google Maps. Available at https://goo.gl/maps/3xcvLZwUCe42 Accessed January 15, 2016.</ref>. Through the incorporation of a Class I injection well, the brine will not have to be treated or diluted before injection. <ref>EPA Regulations on Class I Wells. Available at http://www.epa.gov/uic/class-i-industrial-and-municipal-waste-disposal-wells. Accessed January 14, 2016.
</ref><ref>List of Injection Well Facilities by Location. Available at http://www.ehso.com/cssepa/tsdfdeepwells.php. Accessed January 14, 2016.</ref>

There are no regulations regarding the temperature of city drinking water, so the product stream temperature has no requirements based on regulations. Outlet temperatures of 35oC are acceptable for input to the municipal water supply system. Since city water tends to be stored in non-temperature controlled tanks, the temperature will naturally cool to ambient temperature.<ref>Climate Corpus Christi- Texas. US Climate Data. Available at http://www.usclimatedata.com/climate/corpus-christi/texas/united-states/ustx0294. Accessed January 12, 2016. </ref>

==Pre-Treatment and Post-Treatment==
For pretreatment, Belgard EV 2030 is recommended to be added at a concentration of 1.8-2.0 mg/L for a system with top brine temperature approximately 110 °C<ref name="BWA 2030">BWA Water Additives. Belgard EV2030 Technical Data Sheet. BWA Water Additives. http://www.wateradditives.com/files/products/tech_data_sheets/Belgard%20EV%202030_Technical%20Data%20Sheet_Thermal_8.5x11.pdf. Accessed January 27, 2016.
</ref>. Since our process is designed to operate at a slightly higher top brine temperature, 120 °C, we will design for the upper bound concentration: 2.0 mg/L of antiscalant. With a feed flow rate of 8250 m3/hr, the process therefore requires 16.5 kg/hr of Belgard EV 2030.

For disinfection during post-treatment, chlorine addition typically requires a dose of roughly 2.0 mg/L as well<ref name="World Health Organization">World Health Organization. Desalination for safe water supply. http://www.who.int/water_sanitation_health/gdwqrevision/desalination.pdf. Accessed January 26, 2016.</ref>. With a pure water distillate flow rate of 4758 m3/hr, the process requires 9.5 kg/hr of chlorine as a 5%-15% sodium hypochlorite solution. Approximately 74 mg/L of lime (calcium hydroxide) and 88mg/L of CO2 should be added to the desalinated water to help prevent corrosion and increase alkalinity within acceptable ranges<ref name="Vouchkov">Vouchkov, N. Re-Mineralization of Desalinated Water. SunCam. https://s3.amazonaws.com/suncam/npdocs/118.pdf. Accessed February 18, 2016.</ref>. With a product stream flow rate of 4758 m3/hr, these concentrations correspond to 352.1 kg/hr of lime and 418.7 kg/hr of CO2. Both of these can be purchased, and it is likely that the CO2 can be procured from the neighboring natural gas power plant.

==Choice of Desalination Technology==
The two major technologies commonly implemented for desalination include reverse osmosis (RO), and multi-stage flash distillation (MSFD). RO utilizes high pressures to force water through a semipermeable membrane that allows for purified water to pass through, but stops organic and other wastes. Often, the pressures can reach 55 - 68 bar, which requires large pumps to maintain pressure, and introduces safety concerns to the process<ref>Stage 2 Measures in the Drought Contingency Plan. Available at http://www.cctexas.com/government/water/conservation/conservation-drought-plans/stage-2-measures. Accessed January 12, 2016.
</ref>. While RO can be less energy intensive than MSFD, and the membrane can also remove contaminants, it is susceptible to feedwater quality, membrane fouling, and requires more extensive pretreatment and posttreatment than MSFD. The cost of RO is directly proportional to the feed salinity, and so it is less expensive when desalinating low salinity water.

Multi-stage flash distillation is best selected for areas that have high salinity or low energy costs. The pretreatment process for MSFD is much easier when compared to RO because pretreatment often consists of simply adding anti-corrosives and antiscalants to the feed stream. The process works by heating water in vessels at different pressures to change the boiling point of the water and evaporate it as it passes through the system. After evaporation, the water condenses, leaving pure water as the effluent. MSFD costs are now roughly $1.00/m3, where this cost is mostly independent of salinity, but dependent on energy costs.<ref>Fritzman C, Lowenberg J, Wintgens T, Melin T. State-of-the-art Reverse Osmosis Desalination. Desalination. 2007; 216:1-76.
</ref> In both RO and MSFD a brine is produced which then must be appropriately treated and/or disposed of.

Multi-stage flash distillation was selected for this process, mainly due to the economic advantages offered by the selected location of Corpus Christi, TX. MSFD is also a well understood process that can be more easily modeled by chemical engineering software.

=Technical Approach=
Quantitative modeling of the process and all included optimization was conducted using the simulation software Aspen HYSYS. A variety of assumptions were developed to set up and define specific components of the process simulation itself. Within Aspen HYSYS, the NRTL Electrolyte property package was selected to model the behavior of salt water. Each stage of the MSF process is modeled as an individual flash vessel, with individual shell & tube heat exchangers used to model the distillate condensers present in each flash stage in the MSF system.

The HYSYS simulation model only considers the flash portion of the overall process, allowing establishment of mass and energy balances for the system. Pretreatment and post-treatment were excluded for the HYSYS model, as these steps are simple additions or removals of small concentrations of chemical components. Although the simulation models heat exchangers outside of each flash stage, in the physical design of the plant these heat exchangers will be contained within each flash stage.

One portion of the process is not modeled, which is the inlet seawater entering the heat rejection stages. As can be seen from the full process flow diagram (Appendix C), the inlet seawater in the heat rejection stage is in high excess to provide maximum cooling capacity; then a significant portion is purged to bring the stream flow rate down to what is needed for the feed stream. It is assumed that the flow rate of cooling water for those three stages is in such high excess that the cooling water does not heat up. Therefore, in the process simulation models, the inlet stream is the feed stream, which enters directly into the heat recovery portion of the process.

See Appendix D for a screenshot overview of the simulation environment model.

=Process Overview=
==Process Flowsheet==
The following diagram, Figure 1, shows a basic flowsheet for the process. Process alternatives considered for each stage are discussed below. Please see Appendix C for the full process flowsheet with finalized design decisions.

[[File:BlockFlowDiagram.png|center|700px|thumb|alt=Nueces Bay Desalination Facility Block Flow Diagram|Figure 1: Basic block flowsheet of the multi-stage flash desalination process.]]

==Major Features==
As seen above, there are three primary stages of the desalination process: Pre-treatment, the multi-stage flash distillation, and post-treatment. Seawater enters the pre-treatment phase first to make the feed suitable for the rest of the process by filtering out large particles that could accumulate on pipes, deaerating the water to limit corrosion, and adding an agent to combat fouling. The water then enters the distillation process, which is broken up into two sections: heat recovery and heat rejection.

In the heat recovery stage, there are 18 flash vessels, each of which separates steam from the salinated liquid water. Each flash vessel has saltwater as its inlet and two outlets: a vapor outlet (steam) that condenses into the final product of purified water and a liquid outlet composed of water with more dissolved solids than the inlet, which is the inlet stream for the next flash vessel. The pipe containing the feed is first mixed with recycled brine, then goes through the top sections of each of the flash vessels, starting with V-118 and ending up at V-101. In this way, the feed stream pipe first serves as a heat exchanger, heating the pre-treated seawater while simultaneously condensing the steam into the final product. The feed then enters a final heat exchanger where it is brought up to 103.6 °C before entering V-101 to be flashed.

After exiting V-118, the final flash vessel in the heat recovery portion, the water enters the heat rejection portion, which consists of three flash vessels (V-201, V-202, V-203). The flash vessels of this section operate in the same manner as in the heat recovery section, with the exception that condensation occurs using excess cooling water instead of pre-treated seawater. This technique allows excess heat to be rejected into an excess water stream rather than the feed.

The purified product stream finally enters post-treatment processing in which water is re-aerated and remineralized, disinfected, and pH is adjusted before being delivered to the city. The concentrated brine leftover at the end of the flash cycle is recycled into the feed, although a purge is required to continuously remove some salt from the system. This purged brine is disposed of by use of a Class I injection well, per regional regulations.

See Appendix E to find detailed stream tables of stream specifications.

==Process Alternatives==
===Pretreatment===
Multi-stage flash distillation requires significantly less pretreatment than reverse osmosis desalination, but helpful pretreatment techniques still exist. One very inexpensive method of pretreatment is screening/filtration, which is used to filter out large contaminants from intake seawater. Using more complicated membrane technology to accomplish large particle filtration is a possible alternative, but doing so would be much more expensive than using a standard screen or filter, so the latter choice has been selected for this process.<ref>El-Dessouky H., Alatiqi I, Ettouney H. Process synthesis: The multi-stage flash desalination system. Desalination. 1998;115(2):115-179. doi: 10.1016/S0011-9164(98)00035-6.</ref>

The most critical type of fouling in multi-stage flash distillation is scaling - therefore, the most critical step in the pretreatment process will be the inclusion of an anti-scalant. Alkaline scaling can be prevented by acid dosing or polyphosphate addition; however, acid dosing can result in increased corrosion and polyphosphates can lead to non-alkaline calcium phosphate fouling, so these additives will not be considered<ref name="Morris">Morris, R. The development of the multi-stage flash distillation process: A designer's viewpoint. Desalination. 1993;93(1-3):57-68. doi:10.1016/S0011-9164(93)80096-6. Availble at http://www.sciencedirect.com/science/article/pii/0011916493800966</ref><ref>Al-Gobaisi, D. A quarter-century of seawater desalination by large multistage flash plants in Abu Dhabi (Plant performance analysis, assessment, present efforts toward enhancement and future hopes). Desalination. 1994;99(2-3):509-512. doi: 10.1016/S0011-9164(94)00196-0.</ref>. As such, a polymer-based additive is recommended for the anti-scalant for this process. The Belgard EV series is designed for use in high-temperature seawater MSF; of these, Belgard EV 2030 is the most neutral in pH<ref>NSF International. NSF Product and Service Listings: Drinking Water Treatment Chemicals - Health Effects. NSF International. http://info.nsf.org/Certified/PwsChemicals/Listings.asp?Company=00300&. Accessed January 27, 2016.</ref><ref name="BWA 2030" /><ref>BWA Water Additives. Belgard EV2035 Technical Data Sheet. BWA Water Additives. http://www.wateradditives.com/files/products/tech_data_sheets/Belgard%20EV2035_Technical%20Data%20Sheet_Thermal_8.5x11.pdf. Accessed January 27, 2016.</ref><ref>BioLab Water Additives. Belgard EV Antiscalant for seawater distillation plants. BioLab Water Additives. http://www.lpq.com.mx/pdf/BELGARD%20EV.PDF. Accessed January 27, 2016.</ref>. As such, we have selected Belgard EV 2030 for use as an anti-scalant in our process.

Corrosion control is another significant method of pretreatment in multi-stage flash distillation plants. Using a piping material that has limited susceptibility to corrosion is typically not economically viable for MSF desalination plants due their large size, so using carbon steel for most of the piping is generally used for its economic benefits despite its susceptibility to corrosion<ref name = "Morris" />. Limiting oxygen concentration is very feasible through the use of deaerators, which can remove oxygen as well as other dissolved gases from the seawater stream. Often, an external deaeration system is used<ref name = "Morris" /><ref>Edwards - Hick Hargreaves. Seawater Deaerators. Edwards - Hick Hargreaves. http://psscorp.co.th/upload/news/20110830105340.pdf. Accessed January 26, 2016.</ref>. Given these considerations, using an external deaerator combined with a less-corrosive antiscalant like Belgard EV 2030 is much more practical for the process than using anti-corrosive piping.

Excessive foam can also be an issue in some MSF distillation facilities, so antifoaming agents are sometimes employed in the pretreatment process as well. However, many plants are operated without foaming issues, and therefore do not need anti-foaming pretreatment at all. Selection of an anti-foaming agent must be carefully weighed with selected of anti-scalant to avoid issues with additive interactions [26]. At this stage in the design process, an anti-foamer will not be included due to the risk of decreasing scalant efficacy, and the likelihood that foam will not be an issue in the desalination plant.

===Multistage Flash Distillation===
The main objective of the design project is desalinate water. This is done with the main process of multi stage flash distillation (MSF). MSF can be developed using a numerous number of different process configurations, such as Once-Through, Simple Mixer Brine Circulation, and Multi-Stage Heat Rejection Brine Circulation processes.

The most desired process for the project is the Three Stage Heat Rejection Brine Circulation as it builds on benefits of the other simpler technologies. By combining principles of heat rejection and heat recovery, the required heat transfer area of the process is greatly reduced as compared to once through processes or simple brine recycle processes. It has the highest performance ratio with the lowest specific feed flow rates, therefore greatly lowering the operating costs of the process. It is also known as the industry standard for MSF<ref name = "KSU Chapter 6">Kansas State University ChE 413. Available at: http://faculty.ksu.edu.sa/Almutaz/Documents/ChE-413/Multi-stage%20Flash%20Desalination.pdf. Accessed January 26, 2016.</ref>. In addition, since our brine disposal technique involves the pumping of the brine into a Class I well, the high brine outlet salinity is not a drawback for our process since post treatment or dilution of the brine is not necessary.

===Post Treatment===
After processing the water via multi-stage flash, the water needs to be post-treated to make the water taste as consumers would expect, since distilled water is “flat” and unpleasant, as well as to protect the consumers and the piping through which the water will be passing<ref name="Clayton">Clayton R. A review of current knowledge: Desalination for water supply. Foundation for Water Research. http://www.fwr.org/desal.pdf. Accessed January 26, 2016.</ref>. There are multiple alternatives to achieve these objectives of remineralization, aeration, corrosion control, and disinfection.

Aerating and introducing some dissolved solids back into the product water are effective ways to create a quality product that is not flat, and contains flavoring that is expected of the water when delivered to the consumer. Aeration will be accomplished via an inline spray aerator which will take up less space than cascade aeration.<ref>Minnesota Rural Water Association. Aeration. http://www.mrwa.com/WaterWorksMnl/Chapter%2011%20Aeration.pdf. Accessed January 26, 2016.</ref> Feed blending will be used to remineralize the water as opposed to a remineralization filter because it does not require additional equipment purchases, and can easily handle the large amounts of throughput required for the process.<ref name="Clayton" />

Corrosion and pH can be controlled by adding lime (calcium hydroxide) and carbon dioxide to the water. To do so, about 74 mg/L of lime and 88mg/L of CO2 should be added to the desalinated water to help prevent corrosion and increase alkalinity within acceptable ranges. Sodium hydroxide, soda ash, and sodium bicarbonate are alternatives to altering the alkalinity and pH of the product water, but lime and CO2 is an industry standard <ref name="Vouchkov" />.

Disinfection needs to be done to prevent microbial growth in the desalinated water and ensure the safety of the general public. Chlorine addition via chlorine gas or sodium hypochlorite is the most common and only requires a dose of 1.5 to 2.5 mg/liter. <ref name="World Health Organization" /> Other alternatives include ozonation, UV light, and chloramination. Chlorine, as a 5% - 15% sodium hypochlorite solution, is sufficient for our process and relatively inexpensive, and as such will be utilized in the process.

===Transport of Brine to Well===
Three major possibilities exist for transporting the brine from our desalination plant to the injection well for disposal: rail, trucking, and a pipeline. A pipeline was chosen as the most feasible and cost-effective approach.

A rail line is not available at this time, and with the cost to construct a rail line at approximately $43 million per mile<ref>About Money. How Much Do Rail Transit Projects Cost to Build and Operate? 2014. Available at: http://publictransport.about.com/od/Transit_Projects/a/How-Much-Do-Rail-Transit-Projects-Cost-To-Build-And-Operate.htm. Accessed January 27, 2016. </ref>, the enormous capital cost makes rail transport infeasible for this site. For large volume operations, pipelines are cheaper than trucking. In a study of wastewater sludge produced from wastewater treatment facilities, it was found that it was more economical for facilities producing over 1200 m3 per day to utilize pipelines rather than trucking.<ref>Marufuzzaman M, Eskioglu S, Hernandez R. Truck versus pipeline transportation cost analysis of wastewater sludge. Transportation Research. 2015; 74(A): 14-30. </ref> Our facility is producing nearly 3 times this volume at 3485 m3 per day of brine. Furthermore, brine is a less viscous fluid than wastewater sludge, decreasing pumping costs in the pipeline and further amplifying the benefit of choosing a pipeline over an injection well.

===Brine Storage===
Due to the choice of a pipeline, rather than trucking, as the method of transport of brine to the injection well, brine storage is unnecessary, so storage solutions such as holding tanks were not investigated. One method of storage compatible with a pipeline is to pump the brine through an evaporation pond. This solution is attractive as it would allow the brine to concentrate, saving some costs of pumping, however, the considerable land usage of evaporation ponds leads them to only be effective for smaller plants. The largest municipal plant utilizing this method produces only 5.7 million liters per day<ref>University of Santa Cruz Center for Integrated Water Research. Concentrate Management: Evaporation Ponds. 2010. Available at: https://ciwr.ucsc.edu/sites/default/files/ICM_D5.pdf. Accessed January 27, 2016. </ref>, whereas our desalination plant is proposed to produce 75.7 million liters per day. Therefore, evaporation ponds have been eliminated from consideration.

==Major Specifications==
===Material of Construction Selection===
While saltwater and desalinated water in general can be corrosive, there are best practices within the desalination industry for material selection. With this in mind, best practice shows carbon steel lifetime can be lengthened by monitoring the oxygen concentration in the water, specifying the paint used to coat any metal, and more expensive materials can be used in areas of extremely high corrosion.<ref name = "Morris" />. By following these best practices, it is possible to save significant amounts of money through the use of carbon steel in most areas of the plant, with specific high-corrosion areas being reinforced with, or replaced with, more corrosion resistant materials, such as Cu/Ni 70/30 alloy<ref name = "KSU Chapter 6" />.

===Heat Exchanger Sizing===
See Appendix F for heat exchanger sizing data. Heat exchanger sizes appear reasonable for placement in the flash vessels.

===Flash Vessel Sizing===
The dimensions for the flash drums were obtained through literature values. The flash drums were determined to be 18x4x3 m in width, height, and length.<ref name="KSU Chapter 6" />

Wall thickness was determined using the following conventions: Design pressure is 10% more than the operating pressure, design temperature is 50°F (27.8°C) higher than the operating temperature, a corrosion allowance of 3-mm and an effectiveness of 85% was derived from the assumption of a double-welded butt joint that was spot checked. See Appendix G for wall thicknesses calculated for each pressure vessel.

The sizing of the vessels to be constructed will be determined after talks with a manufacturer. Due to economies of scale, it may be more cost-effective to buy 21 pressure vessels with the dimensions mentioned above and a wall thickness of 17.1-mm, the highest wall thickness required in the system.

===Pipe Losses and Pressure Drop===
HYSYS was used to estimate the diameter of a pipe given a specified pressure drop per 100m of pipe. It was determined that for a pressure drop of 0.1 kPa/100m of pipe, a 3.05m diameter pipe is sufficient, which is more than manageable for manufacturers. This pressure drop is mostly insignificant and will be taken care of with the safety factors included in the pumps. The significant pressure drops in our system is accounted for via losses in the flash vessels and heat exchangers, which drive the flash process. Therefore overall pipe losses are insignificant and are built into intentional design losses.

=Economic Analysis=
==Economic Assumptions==
The sale of purified water yields a revenue stream of $49.5 million per year assuming sale to commercial entities within the city limits of Corpus Christi at $1.7915 per thousand liters.<ref>Water schedule. City of Corpus Christi Utility Business Office. http://www.cctexas.com/Assets/Departments/Financial-Services/Utilities-Business-Office/files/waterrates.pdf Accessed February 28, 2016.</ref> The primary variable costs are utilities and the brine disposal, with minimal raw materials costs due to the primary feedstock, seawater, being free, and minimal amounts of pre- and post-treatment chemicals being used. The cost of brine disposal used in this analysis was high at $2.097 per thousand liters<ref>McCurdy, R. Underground injection wells for produced water disposal. Chesapeake Energy Corporation. http://www.epa.gov/sites/production/files/documents/21_McCurdy_-_UIC_Disposal_508.pdf Accessed February 28, 2016.</ref>, an estimate for brine disposal utilizing trucking as the mode of transport. As has been shown through previous analyses, a lower cost can be achieved using a direct pipeline, however, no good estimates for this cost could be found.

The NPV was calculated using a 6% cost of capital, the approximate interest rate for a municipal bond.<ref>Zirbe, R.O. Jr., Han, X., Layton, D., Leshine, T. A history of discount rates and their use by government agencies. (2002). http://faculty.washington.edu/zerbe/docs/DISCOUNT%20RATE%20HISTORY-latesXIt.doc Accessed February 28, 2016.</ref> It was assumed that the plant can be built within two years, with production at half-scale in its first year. Additionally, one month of variable costs, $10.6 million, was set aside for working capital. See Appendix H for a full table of assumptions. A 30 year NPV calculation was chosen as the decision basis since 30 years is a conservative estimate for the lifetime of a desalination plant [38]. See below for estimates of NPV for the lifetime of the plant under various assumptions and design criteria.

==Optimization and Sensitivity Analysis==
Looking at the multi-stage flash process, there are a number of opportunities to optimize the process. The major process variables that were investigated were the inlet water temperature, the operating pressure drop per vessel, and the number of stages over which the process takes place. Considering these optimizations, our final process consists of 21 flash vessels (18 heat recovery and 3 heat rejection stages), each with a pressure drop of 5 kPa. See Appendix D for a screenshot of the HYSYS simulation used to model the optimized process.
===Operating Pressure and Pressure Drop===
Initially our process was designed to pump the inlet seawater and brine to a pressure of 2 bar and allow the pressure drop in each flash stage to end our process downstream at 1 bar to prevent complications associated with designing a system under vacuum. While this provided pumping advantages, the energy in the system was significantly higher due to vapor pressure considerations. Due to this fact, the required heat exchange area for certain exchangers proved to be infeasible (on the order of 80,000m2) due to the enormous capital and operating costs that would be associated with such a large heat exchanger.

The process was optimized to operate under vacuum, requiring additional pumping requirements but much lower required heat exchange area, and therefore lower utility costs. This optimization was done by specifying the pressure of the final flash vessel to 5.7kPa, so that the temperature of the water vapor (~30oC) is within approximately 5oC of the cooling water (35oC) so that condensation can occur within a reasonable approach. This also allows our system to optimally extract pure water from the brine running through the system and reduce energy requirements for cooling the product or the recycle. Our system redesign and optimization reduced the energy in the system and reduce the same heat exchanger area requirements by more than 3 orders of magnitude to approximately 800m2.

Once the process was optimized to operate in a vacuum, the pressure drop per flash stage regulated through values was then optimized. Three pressure drops per stages, 3kPa, 5kPa, and 7kPa, were tested and then an economic analysis was conducted to find the optimal value for the pressure drop. These changes had impacts on both operating and capital costs as they would change initial pumping requirements, the amount of water flowing through the process and recycle (and therefore the flash vessel sizes, heat exchanger sizing), and the efficiency of each stage.

The various pressure drops (3 kPa, 5 kPa, and 7 kPa) were tested on a system nominally at 24 stages under the assumption that the results would be generalizable to systems with a different number of stages. With increasing pressure drop, we found decreased capital cost but increased utility cost. As can be seen from Table 2 below, a pressure drop of 5 kPa proved to be the best option based on 30 year NPV, with a loss of $838.8 million, a savings of $65.5 million compared to the next-best option with a pressure drop of 7 kPa.

<center>
{| class = "wikitable"
|+Table 2: Comparison of profitability for our MSF desalination process with 24 stages with 3 different pressure drops per stage
! Number of Stages  
! Pressure Drop (kPa)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
! 30-Year NPV ($MM)
! NPV Ratio (over 5kPa)
|- style="text-align: center;"
| 24 || 3 || 362.3 || 39.1 || -905.8 || 1.08
|- style="text-align: center;"
| 24 || 5 || 255.1 || 53.6 || -838.8 || 1.00
|- style="text-align: center;"
| 24 || 7 || 209.5 || 72.2 || -904.3 || 1.08
|}
</center>

===Number of Flash Stages===
The number of stages is an important area for optimization. An increased number of stages yields better recovery of purified water, however, requires higher capital cost from the additional pressure vessel. Furthermore, with an increased amount of purified water comes increasingly concentrated brine. This must be taken into account since any brine concentration in excess of about 7% presents a more significant corrosion risk<ref name="KSU Chapter 6" />, so increased maintenance costs for the piping or higher capital costs for more corrosion-resistant materials could be necessary for brine concentrations exceeding 7%. Therefore, to recover the same amount of pure water without exceeding corrosion limitations, increasing the number of stages will reduce the overall size of each stage.

Optimizing based on NPV, it was found that 21 stages is optimal, balancing the increased profits from the additional purified water additional costs of higher capital cost. See Table 3 below to compare the 30-year NPV of the process across various numbers of flash vessels.

<center>
{| class = "wikitable"
|+Table 3: Comparison of profitability for our MSF desalination process with a 5 kPa pressure drop, 5000 m3/hr feed flow rate, with varying number of stages.
! Number of Stages  
! Pressure Drop (kPa)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
! 30-Year NPV ($MM)
! NPV Ratio (over 21 Stages)
|- style="text-align: center;"
| 17 || 5 || 213.9 || 58.7 || -813.8 || 1.016
|- style="text-align: center;"
| 18 || 5 || 218 || 57.6 || -809.9 || 1.011
|- style="text-align: center;"
| 19 || 5 || 224.3 || 58 || -806.1 || 1.007
|- style="text-align: center;"
| 20 || 5 || 229.8 || 57.9 || -813.6 || 1.016
|- style="text-align: center;"
| 21 || 5 || 226.6 || 55.9 || -800.7 || 1
|- style="text-align: center;"
| 22 || 5 || 254.7 || 54.6 || -849.5 || 1.061
|- style="text-align: center;"
| 23 || 5 || 249.3 || 54.1 || -833.1 || 1.04
|- style="text-align: center;"
| 24 || 5 || 255.1 || 53.6 || -838.8 || 1.048
|- style="text-align: center;"
| 25 || 5 || 254.9 || 52.7 || -837.4 || 1.046
|}
</center>

===Inlet Seawater Temperature===
By pairing our desalination plant with a natural gas power plant and utilizing its cooling water we forgo initial heating of our inlet stream. Normally, water pumped from the ocean would be approximately 25oC and by capturing waste heat from the power plant we can reduce utilities with our water inlet of 45oC. Through an economic comparison between two plants nominally at 21 flash stages, the plant with an inlet temperature at 25oC has a capital cost of $245MM and a utility cost of $77MM/yr and the same facility with an inlet water temperature at 45oC has a capital cost of only $226MM and a utility cost of $56MM/yr. As shown in Table 4 below, this $19MM savings in capital costs and $21MM/yr saving in utilities is largely due to reducing the size of the heat exchangers and lowering the steam required for heating duty on the inlet seawater respectively. Therefore our process is optimized by recovering the waste heat water from the gas fired power plant.

<center>
{| class = "wikitable"
|+Table 4: Seawater Inlet Temperature Process Cost Optimization
! Seawater Temperature (°C)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
|- style="text-align: center;"
| 25 || 245 || 77
|- style="text-align: center;"
| 45 || 226 || 56
|- style="text-align: center;"
| Savings || 56 || 19
|}
</center>

==Overall Process Economics==
After optimization at fixed flow rates, the process was scaled to meet the the 67% operating percentage and final product definitions as shown in Table 1, which resulted in finalized economics shown in Table 5 below. For a table of the installed costs of each process vessel, see Appendix I. For the finalized NPV calculations, see Appendix J.

<center>
{| class = "wikitable"
|+Table 5: Summarized Economics for 21-stage MSFD Process
! Economic Parameter
! Cost (USD)
|- style="text-align: center;"
| Capital Cost || 345 MM
|- style="text-align: center;"
| Total Operating Cost per Year || 103 MM
|- style="text-align: center;"
| Utility Cost per Year || 59 MM
|- style="text-align: center;"
| 30-Year NPV (6% Discount) || -1035 MM
|}
</center>

=Conclusions and Recommendations=
==Project Viability==
Table 5 above demonstrates that the process has reasonable operating and capital costs, but the 30-year NPV is negative by -1035 $MM due to the extremely low sell price capability of desalinated water as a product. Therefore, this plant would have to heavily be supported by the City of Corpus Christi to run as the product revenue will not make it self-sufficient. The acquisition of government subsidies are key operating requirements for all desalination facilities in the United States.

==Additional Concerns==
Large volumes of water, steam, and energy are needed to ensure the project runs as specified. While most of the water can easily be extracted from the ocean with little to consequence, the fuel used to generate the steam and electricity can have a significant environmental impact. As the plant is located next to a natural gas power plant, this fuel is better than coal with respect to environmental concerns, and utilizing energy from the power plant to preheat the feed stream for the desalination plant helps lower environmental impacts.

The large amounts of high pressure steam being used represents one of the the largest safety concern in the process, because a high pressure steam release could easily cause severe burns or even death. Proper precautions and control measures must be in place to prevent accidental release. Otherwise, the process does not utilize any known toxic chemicals and is relatively safe from a chemical standpoint. The flash vessels mostly operate below atmospheric pressure, so they must be constructed as vessels that can withstand vacuum without failing.

=Appendix A: Proposed Location of Desalination Plant=
Pictured below is the proposed location for the desalination plant, located on Nueces Bay in Corpus Christi, adjacent to the Nueces Bay Energy Center. The plant’s proposed location is shown in the area outlined by the small circle in the upper right of the figure.

[[File:NuecesBayLocation.png|center|600px|thumb|alt=Nueces Bay Desalination Facility Location|Figure A.1: Proposed mapped location of Nueces Bay Desalination Facility.]]
[[File:ClassIWell.png|center|700px|thumb|alt=Nueces Bay Desalination Facility Distance to Injection Well|Figure A.2: Mapped distance from proposed location to Class I Injection Well]]

=Appendix B: Feed and Product Definitions - Salinity=

<center>
{| class = "wikitable"
|+Table 6: Average Composition of Total Dissolved Solids in Seawater39,40
! Compound  
! Mass Composition (kPa)
|- style="text-align: center;"
| Chlorine || 55.03%
|- style="text-align: center;"
| Sodium || 30.64%
|- style="text-align: center;"
| Sulfate || 7.70%
|- style="text-align: center;"
| Magnesium || 3.66%
|- style="text-align: center;"
| Calcium || 1.17%
|- style="text-align: center;"
| Potassium || 1.12%
|- style="text-align: center;"
| Bicarbonate || 0.36%
|- style="text-align: center;"
| Bromine || 0.19%
|- style="text-align: center;"
| Other || 0.15%
|}
</center>

=Appendix C: Process Flow Diagram=
[[File:TeamF_FinalPFD.JPG|center|1200px|thumb|alt=Process Flow Diagram|Figure C.1: Process Flow Diagram]]

=Appendix D: HYSYS Simulation Model=
[[File:TeamF_HYSYS_Screenshot.PNG|center|1200px|thumb|alt=HYSYS Simulation Model|Figure D.1: Screenshot of HYSYS Model Simulation.]]

=Appendix E: Stream Tables=
A summary of some values is shown below:

<center>
{| class = "wikitable"
|+Table 7: Key process material streams
!  
! Stream  
! Molar Flow (105 kgmole/h)
! Mass Flow (105 kg/h)
|- style="text-align: center;"
! style="text-align: right;" | Inlet
| Feed Water || 454 || 83.6
|- style="text-align: center;"
! style="text-align: right;" | Outlet
| Brine Reject || 1.90 || 36.0
|- style="text-align: center;"
! style="text-align: right;" | Product
| Pure Water || 2.64 || 47.5
|}
</center>

<center>
{| class = "wikitable"
|+Table 8: Key process energy streams
! Stream  
! Heat Flow (kJ/h)
|- style="text-align: center;"
| Feed Water || 1.30E+11
|- style="text-align: center;"
| Q-301 || 5.73E+06
|- style="text-align: center;"
| Q-302 || 3.09E+09
|- style="text-align: center;"
| Q-304 || 4.68E+06
|- style="text-align: center;"
| Q-303 || -1.26E+09
|- style="text-align: center;"
| Brine Reject || -5.47E+10
|- style="text-align: center;"
| Pure Water || -7.46E+10
|}
(Negative value denotes energy leaving process)
</center>

=Appendix F: Heat Exchanger Sizing=

<center>
{| class = "wikitable"
|+Table 9: Utility Temperatures and Heat Transfer Coefficient<ref name="Towler">G.P. Towler, R. Sinnott. Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design. Elsevier, 2012.</ref>
!  
! HP Steam
! Cooling Water
|- style="text-align: center;"
! Temperature (°C)
| 250 || 35
|- style="text-align: center;"
! U (W/m2K)
| 4000 || 1500
|}
</center>

<center>
{| class = "wikitable"
|+Table 10: Internal Flash Heat Exchanger Sizing, condensing steam.
! Exchanger
! UA (kJ/°C-h)
! U (kJ/hr/m2°C)
! A (m2)
|- style="text-align: center;"
| E-101 || 1.47E+07 || 14400 || 1019
|- style="text-align: center;"
| E-102 || 1.53E+07 || 14400 || 1059
|- style="text-align: center;"
| E-103 || 1.59E+07 || 14400 || 1103
|- style="text-align: center;"
| E-104 || 1.66E+07 || 14400 || 1153
|- style="text-align: center;"
| E-105 || 1.74E+07 || 14400 || 1209
|- style="text-align: center;"
| E-106 || 1.83E+07 || 14400 || 1272
|- style="text-align: center;"
| E-107 || 1.93E+07 || 14400 || 1342
|- style="text-align: center;"
| E-108 || 2.05E+07 || 14400 || 1424
|- style="text-align: center;"
| E-109 || 2.19E+07 || 14400 || 1518
|- style="text-align: center;"
| E-110 || 2.35E+07 || 14400 || 1629
|- style="text-align: center;"
| E-111 || 2.54E+07 || 14400 || 1761
|- style="text-align: center;"
| E-112 || 2.77E+07 || 14400 || 1921
|- style="text-align: center;"
| E-113 || 3.05E+07 || 14400 || 2119
|- style="text-align: center;"
| E-114 || 3.42E+07 || 14400 || 2372
|- style="text-align: center;"
| E-115 || 3.90E+07 || 14400 || 2706
|- style="text-align: center;"
| E-116 || 4.56E+07 || 14400 || 3167
|- style="text-align: center;"
| E-117 || 5.55E+07 || 14400 || 3856
|- style="text-align: center;"
| E-118 || 7.20E+07 || 14400 || 5000
|}
</center>

=Appendix G: Pressure Vessel Sizing=

=Appendix H: Expanded Economic Assumptions=

=Appendix I: Equipment Cost Summary=

=Appendix J: Economic NPV Analysis=

=References=
<references/>

Nueces Desalination Center: Production of Drinking Water by Multi-Stage Flash Distillation

2016-03-07T20:14:15Z

Benjkg: /* Appendix E: Stream Tables */

Team F: Rankine 672 Final Report

Authors: Kedric Daly, Ben Granger, Evan Rosati, Kathleen Zhou

Instructors: Fengqi You, David Wegerer

March 11, 2016

=Executive Summary=
Desalination of seawater to drinking water is a technology of ever-increasing importance given trends of increasingly widespread water shortages around the world. Even in the United States, water shortages are becoming increasingly common in dry, highly-populated such as California and certain areas of the Gulf Coast. Our design team, Rankine 672, was tasked with evaluating the need for and designing a desalination plant to provide purified water in a place of need. Investigation revealed that the city of Corpus Christi, Texas, is a strategic location for development of a desalination facility, as surrounding region is in need of new drinking water sources and the city has expressed serious interest in contracting the development of a large-scale desalination plant. Our proposed solution is a multi-stage flash distillation desalination facility capable of producing 20,000,000 gallons (75,708 m3) of purified drinking water for the city and surrounding region per operating day.

In this design, drinking water is produced from seawater feedstock through a 21-stage flash distillation process. By strategically locating the facility next to an existing natural gas power plant, the Nueces Bay Energy Center, pre-heated feed water can be taken directly from the outbound cooling systems of the power plant, allowing energy savings for our desalination plant. The process is designed for 16 hour-per-day operation, requires a seawater feed of 8250 m3/hr, and produces 4758 m3/hr of purified drinking water alongside a waste stream of 3485 m3/hr of concentrated brine. The waste stream is isolated from a recycle stream through a 9.0% purge; this results in an overall product yield of 57.7% on a volume basis. The process requires an additional pretreatment stream of 16.5 kg/hr Belgard EV 2030, which serves as an antiscalant, as well as 9.5 kg/hr chlorine, 352.1 kg/hr lime, and 418.7 kg/hr CO2 for post-treatment purposes.

The overall capital cost of the plant is expected to be $345MM, with a total operating cost of $103MM/yr, $59MM/yr of which comes from utilities. Due to the low price at which the product water can be sold, the 30-year net present value of the plant, assuming a 6% discount rate, is $-1,035MM. It is thus imperative this plant be constructed with public funds because it cannot return a profit to any private investor. The growing need for water however could justify the cost, because it is important that the residents of the greater Corpus Christi area have access to drinkable water.

__TOC__

=Introduction=
==Background==
Clean drinking water is a basic human need that is becoming increasingly difficult to meet. Currently, over 1 billion people live in places where water is considered scarce<ref> Tapping the oceans. The Economist USA. http://www.economist.com/node/11484059. Accessed March 2, 2016. </ref>. Though often thought of as a problem exclusive to third-world countries, the issue is becoming more prevalent in the US as well, with water managers in 40 of 50 states anticipating water shortage conditions in some portion of their state in the next 10 years. These shortages are due to increasing demands on constant or diminishing freshwater resources, therefore, to correct the issue, a new supply must be found<ref> Water Supply in the US. EPA. http://www3.epa.gov/watersense/pubs/supply.html. Accessed March 2, 2016.</ref>. Desalination of seawater is therefore important as a new source for clean drinking water and prevent water shortages.

==Project Definition==
The overall goal of the project is to investigate the feasibility of a desalination plant in order to meet the needs of growing water demand. The design needs to take health, safety, and environmental concerns into account, while optimizing costs.

=Design Basis=
Site Location and Conditions
Corpus Christi, TX was chosen as the location for the proposed desalination plant. The city is currently experiencing “Moderate Shortage Conditions”. This issue additionally comes at a time of growing demand with population growth anticipated due to new industry and Eagle Ford Group crude oil production<ref>Stage 2 Measures in the Drought Contingency Plan. Available at http://www.cctexas.com/government/water/conservation/conservation-drought-plans/stage-2-measures. Accessed January 12, 2016. </ref>.

In June 2014, the city announced the Corpus Christi Desalination Demonstration Project<ref>Corpus Christi Desalination Demonstration Project. Available at http://www.cctexas.com/Assets/Departments/Water/Files/DesalFactSheet.pdf. Published June 2014. Accessed January 12, 2016.
</ref> which seeks to create a demonstration desalination plant capable of producing 20,000 gallons of water per day, with plans to later build a full scale facility generating 20,000,000 gallons of water per day. The desalination demonstration project indicates a clear need for implementation of a desalination plant of this size to accommodate growth in the Corpus Christi area.

Locating our plant in Corpus Christi will allow us to take advantage of the benefits of cogeneration through a partnership with the Nueces Bay Energy Center. By utilizing their outlet cooling water as a large portion of our feed for the desalination process, we can raise the temperature of our seawater feed, saving on energy costs in the multi stage flash distillation. The proposed location for the desalination plant is immediately west of the Nueces Bay Energy Center. See Appendix A for a map depicting the proposed location for the desalination plant along the coast of Nueces Bay.

==Feed and Product Definitions==
The proposed desalination facility’s feed and product streams are defined as shown in Table 1. Rationale for these definitions is also described below. The values reported in Table 1 are final derived numbers from HYSYS simulation modeling.

<center>
{| class = "wikitable"
|+Table 1: Feed and product stream definitions
!  
! Salt Concentration
! Temperature
! Amount
|- style="text-align: center;"
! style="text-align: right;" | Feed Stream
| 30,500 ppm || 45°C || 8250 m3/hr
|- style="text-align: center;"
! style="text-align: right;" | Product Stream
| ~ 0 ppm || 35°C || 4758 m3/hr
|- style="text-align: center;"
! style="text-align: right;" | Byproduct Stream
| 695 ppm || 35°C || 3485 m3/hr
|}
</center>

The process feed will be seawater from Nueces Bay, preheated through the cooling water system of the nearby Nueces Bay Energy Center. Based on calculations developed from NREL (National Renewable Energy Laboratory) data, it was estimated that the Nueces Bay Energy Center uses approximately 250 million gallons of seawater per day for process and cooling needs. Therefore, the 20 million gallon per day requirement for the desalination facility can easily be met through the power plant’s cooling water<ref>Torcellini P, Long N, Judkoff R. Consumptive Water Use for U.S. Power Consumption. NREL. 2003; TP-550-33905.
</ref>. Note that we are assuming the process will operate 67% of the time (16 hours per day).

Based on data acquired from two salinity-measuring stations in Nueces Bay, the average salinity of local seawater is 30.5 practical salinity units (psu)<ref>Stations Map & Real - Time Data. The Conrad Blucher Institute for Surveying and Science website. Available at http://www.cbi.tamucc.edu/cbi/data/. Updated January 2016. Accessed January 9, 2016. </ref>, thus setting the concentration of the process feed stream. This translates to a feed stream concentration of 30,500 ppm (parts per million)<ref>Definition and Units. CATD Salinity Expertise Center. Available at http://www.salinityremotesensing.ifremer.fr/home. Accessed January 12, 2016.</ref>. The composition of the salt attributed to this seawater salinity is shown in Table 6 in Appendix B.

No data on the composition of seawater around the Corpus Christi area was available, but the composition of seawater does not change significantly in different locations. The given composition is considered accurate for our feed stream<ref>Brown E, Colling A, Park D, Phillips J, Rothery D, Wright J. Seawater : Its Composition, Properties and Behaviour. Boston: Butterworth-Heinemann, 1995.
</ref>. Note that seawater salt is primarily sodium chloride - total dissolved solids are 85.7% sodium chloride by mass. As such, the dissolved solids in the feed stream is assumed in this project to be pure NaCl, making the feed composition 96.95% water and 3.05% NaCl by mass. A control system will likely be put in place to ensure feed concentration remains at this level, accounting for natural fluctuations in local seawater salinity.

The largest byproduct from the process will be a brine slurry with high salt concentrations. An upper corrosion limit of 700 ppm was set for recirculated and purged brine. This undesired byproduct will be sent to a nearby Class I non-hazardous injection well facility operated by GNI Group, which is approximately 12 miles away from the proposed desalination plant location as shown in the figure in Appendix A<ref>Texas Water Code. Availabe at http://www.statutes.legis.state.tx.us/Docs/WA/htm/WA.27.htm. Accessed January 14, 2016.
</ref><ref>Google Maps. Available at https://goo.gl/maps/3xcvLZwUCe42 Accessed January 15, 2016.</ref>. Through the incorporation of a Class I injection well, the brine will not have to be treated or diluted before injection. <ref>EPA Regulations on Class I Wells. Available at http://www.epa.gov/uic/class-i-industrial-and-municipal-waste-disposal-wells. Accessed January 14, 2016.
</ref><ref>List of Injection Well Facilities by Location. Available at http://www.ehso.com/cssepa/tsdfdeepwells.php. Accessed January 14, 2016.</ref>

There are no regulations regarding the temperature of city drinking water, so the product stream temperature has no requirements based on regulations. Outlet temperatures of 35oC are acceptable for input to the municipal water supply system. Since city water tends to be stored in non-temperature controlled tanks, the temperature will naturally cool to ambient temperature.<ref>Climate Corpus Christi- Texas. US Climate Data. Available at http://www.usclimatedata.com/climate/corpus-christi/texas/united-states/ustx0294. Accessed January 12, 2016. </ref>

==Pre-Treatment and Post-Treatment==
For pretreatment, Belgard EV 2030 is recommended to be added at a concentration of 1.8-2.0 mg/L for a system with top brine temperature approximately 110 °C<ref name="BWA 2030">BWA Water Additives. Belgard EV2030 Technical Data Sheet. BWA Water Additives. http://www.wateradditives.com/files/products/tech_data_sheets/Belgard%20EV%202030_Technical%20Data%20Sheet_Thermal_8.5x11.pdf. Accessed January 27, 2016.
</ref>. Since our process is designed to operate at a slightly higher top brine temperature, 120 °C, we will design for the upper bound concentration: 2.0 mg/L of antiscalant. With a feed flow rate of 8250 m3/hr, the process therefore requires 16.5 kg/hr of Belgard EV 2030.

For disinfection during post-treatment, chlorine addition typically requires a dose of roughly 2.0 mg/L as well<ref name="World Health Organization">World Health Organization. Desalination for safe water supply. http://www.who.int/water_sanitation_health/gdwqrevision/desalination.pdf. Accessed January 26, 2016.</ref>. With a pure water distillate flow rate of 4758 m3/hr, the process requires 9.5 kg/hr of chlorine as a 5%-15% sodium hypochlorite solution. Approximately 74 mg/L of lime (calcium hydroxide) and 88mg/L of CO2 should be added to the desalinated water to help prevent corrosion and increase alkalinity within acceptable ranges<ref name="Vouchkov">Vouchkov, N. Re-Mineralization of Desalinated Water. SunCam. https://s3.amazonaws.com/suncam/npdocs/118.pdf. Accessed February 18, 2016.</ref>. With a product stream flow rate of 4758 m3/hr, these concentrations correspond to 352.1 kg/hr of lime and 418.7 kg/hr of CO2. Both of these can be purchased, and it is likely that the CO2 can be procured from the neighboring natural gas power plant.

==Choice of Desalination Technology==
The two major technologies commonly implemented for desalination include reverse osmosis (RO), and multi-stage flash distillation (MSFD). RO utilizes high pressures to force water through a semipermeable membrane that allows for purified water to pass through, but stops organic and other wastes. Often, the pressures can reach 55 - 68 bar, which requires large pumps to maintain pressure, and introduces safety concerns to the process<ref>Stage 2 Measures in the Drought Contingency Plan. Available at http://www.cctexas.com/government/water/conservation/conservation-drought-plans/stage-2-measures. Accessed January 12, 2016.
</ref>. While RO can be less energy intensive than MSFD, and the membrane can also remove contaminants, it is susceptible to feedwater quality, membrane fouling, and requires more extensive pretreatment and posttreatment than MSFD. The cost of RO is directly proportional to the feed salinity, and so it is less expensive when desalinating low salinity water.

Multi-stage flash distillation is best selected for areas that have high salinity or low energy costs. The pretreatment process for MSFD is much easier when compared to RO because pretreatment often consists of simply adding anti-corrosives and antiscalants to the feed stream. The process works by heating water in vessels at different pressures to change the boiling point of the water and evaporate it as it passes through the system. After evaporation, the water condenses, leaving pure water as the effluent. MSFD costs are now roughly $1.00/m3, where this cost is mostly independent of salinity, but dependent on energy costs.<ref>Fritzman C, Lowenberg J, Wintgens T, Melin T. State-of-the-art Reverse Osmosis Desalination. Desalination. 2007; 216:1-76.
</ref> In both RO and MSFD a brine is produced which then must be appropriately treated and/or disposed of.

Multi-stage flash distillation was selected for this process, mainly due to the economic advantages offered by the selected location of Corpus Christi, TX. MSFD is also a well understood process that can be more easily modeled by chemical engineering software.

=Technical Approach=
Quantitative modeling of the process and all included optimization was conducted using the simulation software Aspen HYSYS. A variety of assumptions were developed to set up and define specific components of the process simulation itself. Within Aspen HYSYS, the NRTL Electrolyte property package was selected to model the behavior of salt water. Each stage of the MSF process is modeled as an individual flash vessel, with individual shell & tube heat exchangers used to model the distillate condensers present in each flash stage in the MSF system.

The HYSYS simulation model only considers the flash portion of the overall process, allowing establishment of mass and energy balances for the system. Pretreatment and post-treatment were excluded for the HYSYS model, as these steps are simple additions or removals of small concentrations of chemical components. Although the simulation models heat exchangers outside of each flash stage, in the physical design of the plant these heat exchangers will be contained within each flash stage.

One portion of the process is not modeled, which is the inlet seawater entering the heat rejection stages. As can be seen from the full process flow diagram (Appendix C), the inlet seawater in the heat rejection stage is in high excess to provide maximum cooling capacity; then a significant portion is purged to bring the stream flow rate down to what is needed for the feed stream. It is assumed that the flow rate of cooling water for those three stages is in such high excess that the cooling water does not heat up. Therefore, in the process simulation models, the inlet stream is the feed stream, which enters directly into the heat recovery portion of the process.

See Appendix D for a screenshot overview of the simulation environment model.

=Process Overview=
==Process Flowsheet==
The following diagram, Figure 1, shows a basic flowsheet for the process. Process alternatives considered for each stage are discussed below. Please see Appendix C for the full process flowsheet with finalized design decisions.

[[File:BlockFlowDiagram.png|center|700px|thumb|alt=Nueces Bay Desalination Facility Block Flow Diagram|Figure 1: Basic block flowsheet of the multi-stage flash desalination process.]]

==Major Features==
As seen above, there are three primary stages of the desalination process: Pre-treatment, the multi-stage flash distillation, and post-treatment. Seawater enters the pre-treatment phase first to make the feed suitable for the rest of the process by filtering out large particles that could accumulate on pipes, deaerating the water to limit corrosion, and adding an agent to combat fouling. The water then enters the distillation process, which is broken up into two sections: heat recovery and heat rejection.

In the heat recovery stage, there are 18 flash vessels, each of which separates steam from the salinated liquid water. Each flash vessel has saltwater as its inlet and two outlets: a vapor outlet (steam) that condenses into the final product of purified water and a liquid outlet composed of water with more dissolved solids than the inlet, which is the inlet stream for the next flash vessel. The pipe containing the feed is first mixed with recycled brine, then goes through the top sections of each of the flash vessels, starting with V-118 and ending up at V-101. In this way, the feed stream pipe first serves as a heat exchanger, heating the pre-treated seawater while simultaneously condensing the steam into the final product. The feed then enters a final heat exchanger where it is brought up to 103.6 °C before entering V-101 to be flashed.

After exiting V-118, the final flash vessel in the heat recovery portion, the water enters the heat rejection portion, which consists of three flash vessels (V-201, V-202, V-203). The flash vessels of this section operate in the same manner as in the heat recovery section, with the exception that condensation occurs using excess cooling water instead of pre-treated seawater. This technique allows excess heat to be rejected into an excess water stream rather than the feed.

The purified product stream finally enters post-treatment processing in which water is re-aerated and remineralized, disinfected, and pH is adjusted before being delivered to the city. The concentrated brine leftover at the end of the flash cycle is recycled into the feed, although a purge is required to continuously remove some salt from the system. This purged brine is disposed of by use of a Class I injection well, per regional regulations.

See Appendix E to find detailed stream tables of stream specifications.

==Process Alternatives==
===Pretreatment===
Multi-stage flash distillation requires significantly less pretreatment than reverse osmosis desalination, but helpful pretreatment techniques still exist. One very inexpensive method of pretreatment is screening/filtration, which is used to filter out large contaminants from intake seawater. Using more complicated membrane technology to accomplish large particle filtration is a possible alternative, but doing so would be much more expensive than using a standard screen or filter, so the latter choice has been selected for this process.<ref>El-Dessouky H., Alatiqi I, Ettouney H. Process synthesis: The multi-stage flash desalination system. Desalination. 1998;115(2):115-179. doi: 10.1016/S0011-9164(98)00035-6.</ref>

The most critical type of fouling in multi-stage flash distillation is scaling - therefore, the most critical step in the pretreatment process will be the inclusion of an anti-scalant. Alkaline scaling can be prevented by acid dosing or polyphosphate addition; however, acid dosing can result in increased corrosion and polyphosphates can lead to non-alkaline calcium phosphate fouling, so these additives will not be considered<ref name="Morris">Morris, R. The development of the multi-stage flash distillation process: A designer's viewpoint. Desalination. 1993;93(1-3):57-68. doi:10.1016/S0011-9164(93)80096-6. Availble at http://www.sciencedirect.com/science/article/pii/0011916493800966</ref><ref>Al-Gobaisi, D. A quarter-century of seawater desalination by large multistage flash plants in Abu Dhabi (Plant performance analysis, assessment, present efforts toward enhancement and future hopes). Desalination. 1994;99(2-3):509-512. doi: 10.1016/S0011-9164(94)00196-0.</ref>. As such, a polymer-based additive is recommended for the anti-scalant for this process. The Belgard EV series is designed for use in high-temperature seawater MSF; of these, Belgard EV 2030 is the most neutral in pH<ref>NSF International. NSF Product and Service Listings: Drinking Water Treatment Chemicals - Health Effects. NSF International. http://info.nsf.org/Certified/PwsChemicals/Listings.asp?Company=00300&. Accessed January 27, 2016.</ref><ref name="BWA 2030" /><ref>BWA Water Additives. Belgard EV2035 Technical Data Sheet. BWA Water Additives. http://www.wateradditives.com/files/products/tech_data_sheets/Belgard%20EV2035_Technical%20Data%20Sheet_Thermal_8.5x11.pdf. Accessed January 27, 2016.</ref><ref>BioLab Water Additives. Belgard EV Antiscalant for seawater distillation plants. BioLab Water Additives. http://www.lpq.com.mx/pdf/BELGARD%20EV.PDF. Accessed January 27, 2016.</ref>. As such, we have selected Belgard EV 2030 for use as an anti-scalant in our process.

Corrosion control is another significant method of pretreatment in multi-stage flash distillation plants. Using a piping material that has limited susceptibility to corrosion is typically not economically viable for MSF desalination plants due their large size, so using carbon steel for most of the piping is generally used for its economic benefits despite its susceptibility to corrosion<ref name = "Morris" />. Limiting oxygen concentration is very feasible through the use of deaerators, which can remove oxygen as well as other dissolved gases from the seawater stream. Often, an external deaeration system is used<ref name = "Morris" /><ref>Edwards - Hick Hargreaves. Seawater Deaerators. Edwards - Hick Hargreaves. http://psscorp.co.th/upload/news/20110830105340.pdf. Accessed January 26, 2016.</ref>. Given these considerations, using an external deaerator combined with a less-corrosive antiscalant like Belgard EV 2030 is much more practical for the process than using anti-corrosive piping.

Excessive foam can also be an issue in some MSF distillation facilities, so antifoaming agents are sometimes employed in the pretreatment process as well. However, many plants are operated without foaming issues, and therefore do not need anti-foaming pretreatment at all. Selection of an anti-foaming agent must be carefully weighed with selected of anti-scalant to avoid issues with additive interactions [26]. At this stage in the design process, an anti-foamer will not be included due to the risk of decreasing scalant efficacy, and the likelihood that foam will not be an issue in the desalination plant.

===Multistage Flash Distillation===
The main objective of the design project is desalinate water. This is done with the main process of multi stage flash distillation (MSF). MSF can be developed using a numerous number of different process configurations, such as Once-Through, Simple Mixer Brine Circulation, and Multi-Stage Heat Rejection Brine Circulation processes.

The most desired process for the project is the Three Stage Heat Rejection Brine Circulation as it builds on benefits of the other simpler technologies. By combining principles of heat rejection and heat recovery, the required heat transfer area of the process is greatly reduced as compared to once through processes or simple brine recycle processes. It has the highest performance ratio with the lowest specific feed flow rates, therefore greatly lowering the operating costs of the process. It is also known as the industry standard for MSF<ref name = "KSU Chapter 6">Kansas State University ChE 413. Available at: http://faculty.ksu.edu.sa/Almutaz/Documents/ChE-413/Multi-stage%20Flash%20Desalination.pdf. Accessed January 26, 2016.</ref>. In addition, since our brine disposal technique involves the pumping of the brine into a Class I well, the high brine outlet salinity is not a drawback for our process since post treatment or dilution of the brine is not necessary.

===Post Treatment===
After processing the water via multi-stage flash, the water needs to be post-treated to make the water taste as consumers would expect, since distilled water is “flat” and unpleasant, as well as to protect the consumers and the piping through which the water will be passing<ref name="Clayton">Clayton R. A review of current knowledge: Desalination for water supply. Foundation for Water Research. http://www.fwr.org/desal.pdf. Accessed January 26, 2016.</ref>. There are multiple alternatives to achieve these objectives of remineralization, aeration, corrosion control, and disinfection.

Aerating and introducing some dissolved solids back into the product water are effective ways to create a quality product that is not flat, and contains flavoring that is expected of the water when delivered to the consumer. Aeration will be accomplished via an inline spray aerator which will take up less space than cascade aeration.<ref>Minnesota Rural Water Association. Aeration. http://www.mrwa.com/WaterWorksMnl/Chapter%2011%20Aeration.pdf. Accessed January 26, 2016.</ref> Feed blending will be used to remineralize the water as opposed to a remineralization filter because it does not require additional equipment purchases, and can easily handle the large amounts of throughput required for the process.<ref name="Clayton" />

Corrosion and pH can be controlled by adding lime (calcium hydroxide) and carbon dioxide to the water. To do so, about 74 mg/L of lime and 88mg/L of CO2 should be added to the desalinated water to help prevent corrosion and increase alkalinity within acceptable ranges. Sodium hydroxide, soda ash, and sodium bicarbonate are alternatives to altering the alkalinity and pH of the product water, but lime and CO2 is an industry standard <ref name="Vouchkov" />.

Disinfection needs to be done to prevent microbial growth in the desalinated water and ensure the safety of the general public. Chlorine addition via chlorine gas or sodium hypochlorite is the most common and only requires a dose of 1.5 to 2.5 mg/liter. <ref name="World Health Organization" /> Other alternatives include ozonation, UV light, and chloramination. Chlorine, as a 5% - 15% sodium hypochlorite solution, is sufficient for our process and relatively inexpensive, and as such will be utilized in the process.

===Transport of Brine to Well===
Three major possibilities exist for transporting the brine from our desalination plant to the injection well for disposal: rail, trucking, and a pipeline. A pipeline was chosen as the most feasible and cost-effective approach.

A rail line is not available at this time, and with the cost to construct a rail line at approximately $43 million per mile<ref>About Money. How Much Do Rail Transit Projects Cost to Build and Operate? 2014. Available at: http://publictransport.about.com/od/Transit_Projects/a/How-Much-Do-Rail-Transit-Projects-Cost-To-Build-And-Operate.htm. Accessed January 27, 2016. </ref>, the enormous capital cost makes rail transport infeasible for this site. For large volume operations, pipelines are cheaper than trucking. In a study of wastewater sludge produced from wastewater treatment facilities, it was found that it was more economical for facilities producing over 1200 m3 per day to utilize pipelines rather than trucking.<ref>Marufuzzaman M, Eskioglu S, Hernandez R. Truck versus pipeline transportation cost analysis of wastewater sludge. Transportation Research. 2015; 74(A): 14-30. </ref> Our facility is producing nearly 3 times this volume at 3485 m3 per day of brine. Furthermore, brine is a less viscous fluid than wastewater sludge, decreasing pumping costs in the pipeline and further amplifying the benefit of choosing a pipeline over an injection well.

===Brine Storage===
Due to the choice of a pipeline, rather than trucking, as the method of transport of brine to the injection well, brine storage is unnecessary, so storage solutions such as holding tanks were not investigated. One method of storage compatible with a pipeline is to pump the brine through an evaporation pond. This solution is attractive as it would allow the brine to concentrate, saving some costs of pumping, however, the considerable land usage of evaporation ponds leads them to only be effective for smaller plants. The largest municipal plant utilizing this method produces only 5.7 million liters per day<ref>University of Santa Cruz Center for Integrated Water Research. Concentrate Management: Evaporation Ponds. 2010. Available at: https://ciwr.ucsc.edu/sites/default/files/ICM_D5.pdf. Accessed January 27, 2016. </ref>, whereas our desalination plant is proposed to produce 75.7 million liters per day. Therefore, evaporation ponds have been eliminated from consideration.

==Major Specifications==
===Material of Construction Selection===
While saltwater and desalinated water in general can be corrosive, there are best practices within the desalination industry for material selection. With this in mind, best practice shows carbon steel lifetime can be lengthened by monitoring the oxygen concentration in the water, specifying the paint used to coat any metal, and more expensive materials can be used in areas of extremely high corrosion.<ref name = "Morris" />. By following these best practices, it is possible to save significant amounts of money through the use of carbon steel in most areas of the plant, with specific high-corrosion areas being reinforced with, or replaced with, more corrosion resistant materials, such as Cu/Ni 70/30 alloy<ref name = "KSU Chapter 6" />.

===Heat Exchanger Sizing===
See Appendix F for heat exchanger sizing data. Heat exchanger sizes appear reasonable for placement in the flash vessels.

===Flash Vessel Sizing===
The dimensions for the flash drums were obtained through literature values. The flash drums were determined to be 18x4x3 m in width, height, and length.<ref name="KSU Chapter 6" />

Wall thickness was determined using the following conventions: Design pressure is 10% more than the operating pressure, design temperature is 50°F (27.8°C) higher than the operating temperature, a corrosion allowance of 3-mm and an effectiveness of 85% was derived from the assumption of a double-welded butt joint that was spot checked. See Appendix G for wall thicknesses calculated for each pressure vessel.

The sizing of the vessels to be constructed will be determined after talks with a manufacturer. Due to economies of scale, it may be more cost-effective to buy 21 pressure vessels with the dimensions mentioned above and a wall thickness of 17.1-mm, the highest wall thickness required in the system.

===Pipe Losses and Pressure Drop===
HYSYS was used to estimate the diameter of a pipe given a specified pressure drop per 100m of pipe. It was determined that for a pressure drop of 0.1 kPa/100m of pipe, a 3.05m diameter pipe is sufficient, which is more than manageable for manufacturers. This pressure drop is mostly insignificant and will be taken care of with the safety factors included in the pumps. The significant pressure drops in our system is accounted for via losses in the flash vessels and heat exchangers, which drive the flash process. Therefore overall pipe losses are insignificant and are built into intentional design losses.

=Economic Analysis=
==Economic Assumptions==
The sale of purified water yields a revenue stream of $49.5 million per year assuming sale to commercial entities within the city limits of Corpus Christi at $1.7915 per thousand liters.<ref>Water schedule. City of Corpus Christi Utility Business Office. http://www.cctexas.com/Assets/Departments/Financial-Services/Utilities-Business-Office/files/waterrates.pdf Accessed February 28, 2016.</ref> The primary variable costs are utilities and the brine disposal, with minimal raw materials costs due to the primary feedstock, seawater, being free, and minimal amounts of pre- and post-treatment chemicals being used. The cost of brine disposal used in this analysis was high at $2.097 per thousand liters<ref>McCurdy, R. Underground injection wells for produced water disposal. Chesapeake Energy Corporation. http://www.epa.gov/sites/production/files/documents/21_McCurdy_-_UIC_Disposal_508.pdf Accessed February 28, 2016.</ref>, an estimate for brine disposal utilizing trucking as the mode of transport. As has been shown through previous analyses, a lower cost can be achieved using a direct pipeline, however, no good estimates for this cost could be found.

The NPV was calculated using a 6% cost of capital, the approximate interest rate for a municipal bond.<ref>Zirbe, R.O. Jr., Han, X., Layton, D., Leshine, T. A history of discount rates and their use by government agencies. (2002). http://faculty.washington.edu/zerbe/docs/DISCOUNT%20RATE%20HISTORY-latesXIt.doc Accessed February 28, 2016.</ref> It was assumed that the plant can be built within two years, with production at half-scale in its first year. Additionally, one month of variable costs, $10.6 million, was set aside for working capital. See Appendix H for a full table of assumptions. A 30 year NPV calculation was chosen as the decision basis since 30 years is a conservative estimate for the lifetime of a desalination plant [38]. See below for estimates of NPV for the lifetime of the plant under various assumptions and design criteria.

==Optimization and Sensitivity Analysis==
Looking at the multi-stage flash process, there are a number of opportunities to optimize the process. The major process variables that were investigated were the inlet water temperature, the operating pressure drop per vessel, and the number of stages over which the process takes place. Considering these optimizations, our final process consists of 21 flash vessels (18 heat recovery and 3 heat rejection stages), each with a pressure drop of 5 kPa. See Appendix D for a screenshot of the HYSYS simulation used to model the optimized process.
===Operating Pressure and Pressure Drop===
Initially our process was designed to pump the inlet seawater and brine to a pressure of 2 bar and allow the pressure drop in each flash stage to end our process downstream at 1 bar to prevent complications associated with designing a system under vacuum. While this provided pumping advantages, the energy in the system was significantly higher due to vapor pressure considerations. Due to this fact, the required heat exchange area for certain exchangers proved to be infeasible (on the order of 80,000m2) due to the enormous capital and operating costs that would be associated with such a large heat exchanger.

The process was optimized to operate under vacuum, requiring additional pumping requirements but much lower required heat exchange area, and therefore lower utility costs. This optimization was done by specifying the pressure of the final flash vessel to 5.7kPa, so that the temperature of the water vapor (~30oC) is within approximately 5oC of the cooling water (35oC) so that condensation can occur within a reasonable approach. This also allows our system to optimally extract pure water from the brine running through the system and reduce energy requirements for cooling the product or the recycle. Our system redesign and optimization reduced the energy in the system and reduce the same heat exchanger area requirements by more than 3 orders of magnitude to approximately 800m2.

Once the process was optimized to operate in a vacuum, the pressure drop per flash stage regulated through values was then optimized. Three pressure drops per stages, 3kPa, 5kPa, and 7kPa, were tested and then an economic analysis was conducted to find the optimal value for the pressure drop. These changes had impacts on both operating and capital costs as they would change initial pumping requirements, the amount of water flowing through the process and recycle (and therefore the flash vessel sizes, heat exchanger sizing), and the efficiency of each stage.

The various pressure drops (3 kPa, 5 kPa, and 7 kPa) were tested on a system nominally at 24 stages under the assumption that the results would be generalizable to systems with a different number of stages. With increasing pressure drop, we found decreased capital cost but increased utility cost. As can be seen from Table 2 below, a pressure drop of 5 kPa proved to be the best option based on 30 year NPV, with a loss of $838.8 million, a savings of $65.5 million compared to the next-best option with a pressure drop of 7 kPa.

<center>
{| class = "wikitable"
|+Table 2: Comparison of profitability for our MSF desalination process with 24 stages with 3 different pressure drops per stage
! Number of Stages  
! Pressure Drop (kPa)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
! 30-Year NPV ($MM)
! NPV Ratio (over 5kPa)
|- style="text-align: center;"
| 24 || 3 || 362.3 || 39.1 || -905.8 || 1.08
|- style="text-align: center;"
| 24 || 5 || 255.1 || 53.6 || -838.8 || 1.00
|- style="text-align: center;"
| 24 || 7 || 209.5 || 72.2 || -904.3 || 1.08
|}
</center>

===Number of Flash Stages===
The number of stages is an important area for optimization. An increased number of stages yields better recovery of purified water, however, requires higher capital cost from the additional pressure vessel. Furthermore, with an increased amount of purified water comes increasingly concentrated brine. This must be taken into account since any brine concentration in excess of about 7% presents a more significant corrosion risk<ref name="KSU Chapter 6" />, so increased maintenance costs for the piping or higher capital costs for more corrosion-resistant materials could be necessary for brine concentrations exceeding 7%. Therefore, to recover the same amount of pure water without exceeding corrosion limitations, increasing the number of stages will reduce the overall size of each stage.

Optimizing based on NPV, it was found that 21 stages is optimal, balancing the increased profits from the additional purified water additional costs of higher capital cost. See Table 3 below to compare the 30-year NPV of the process across various numbers of flash vessels.

<center>
{| class = "wikitable"
|+Table 3: Comparison of profitability for our MSF desalination process with a 5 kPa pressure drop, 5000 m3/hr feed flow rate, with varying number of stages.
! Number of Stages  
! Pressure Drop (kPa)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
! 30-Year NPV ($MM)
! NPV Ratio (over 21 Stages)
|- style="text-align: center;"
| 17 || 5 || 213.9 || 58.7 || -813.8 || 1.016
|- style="text-align: center;"
| 18 || 5 || 218 || 57.6 || -809.9 || 1.011
|- style="text-align: center;"
| 19 || 5 || 224.3 || 58 || -806.1 || 1.007
|- style="text-align: center;"
| 20 || 5 || 229.8 || 57.9 || -813.6 || 1.016
|- style="text-align: center;"
| 21 || 5 || 226.6 || 55.9 || -800.7 || 1
|- style="text-align: center;"
| 22 || 5 || 254.7 || 54.6 || -849.5 || 1.061
|- style="text-align: center;"
| 23 || 5 || 249.3 || 54.1 || -833.1 || 1.04
|- style="text-align: center;"
| 24 || 5 || 255.1 || 53.6 || -838.8 || 1.048
|- style="text-align: center;"
| 25 || 5 || 254.9 || 52.7 || -837.4 || 1.046
|}
</center>

===Inlet Seawater Temperature===
By pairing our desalination plant with a natural gas power plant and utilizing its cooling water we forgo initial heating of our inlet stream. Normally, water pumped from the ocean would be approximately 25oC and by capturing waste heat from the power plant we can reduce utilities with our water inlet of 45oC. Through an economic comparison between two plants nominally at 21 flash stages, the plant with an inlet temperature at 25oC has a capital cost of $245MM and a utility cost of $77MM/yr and the same facility with an inlet water temperature at 45oC has a capital cost of only $226MM and a utility cost of $56MM/yr. As shown in Table 4 below, this $19MM savings in capital costs and $21MM/yr saving in utilities is largely due to reducing the size of the heat exchangers and lowering the steam required for heating duty on the inlet seawater respectively. Therefore our process is optimized by recovering the waste heat water from the gas fired power plant.

<center>
{| class = "wikitable"
|+Table 4: Seawater Inlet Temperature Process Cost Optimization
! Seawater Temperature (°C)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
|- style="text-align: center;"
| 25 || 245 || 77
|- style="text-align: center;"
| 45 || 226 || 56
|- style="text-align: center;"
| Savings || 56 || 19
|}
</center>

==Overall Process Economics==
After optimization at fixed flow rates, the process was scaled to meet the the 67% operating percentage and final product definitions as shown in Table 1, which resulted in finalized economics shown in Table 5 below. For a table of the installed costs of each process vessel, see Appendix I. For the finalized NPV calculations, see Appendix J.

<center>
{| class = "wikitable"
|+Table 5: Summarized Economics for 21-stage MSFD Process
! Economic Parameter
! Cost (USD)
|- style="text-align: center;"
| Capital Cost || 345 MM
|- style="text-align: center;"
| Total Operating Cost per Year || 103 MM
|- style="text-align: center;"
| Utility Cost per Year || 59 MM
|- style="text-align: center;"
| 30-Year NPV (6% Discount) || -1035 MM
|}
</center>

=Conclusions and Recommendations=
==Project Viability==
Table 5 above demonstrates that the process has reasonable operating and capital costs, but the 30-year NPV is negative by -1035 $MM due to the extremely low sell price capability of desalinated water as a product. Therefore, this plant would have to heavily be supported by the City of Corpus Christi to run as the product revenue will not make it self-sufficient. The acquisition of government subsidies are key operating requirements for all desalination facilities in the United States.

==Additional Concerns==
Large volumes of water, steam, and energy are needed to ensure the project runs as specified. While most of the water can easily be extracted from the ocean with little to consequence, the fuel used to generate the steam and electricity can have a significant environmental impact. As the plant is located next to a natural gas power plant, this fuel is better than coal with respect to environmental concerns, and utilizing energy from the power plant to preheat the feed stream for the desalination plant helps lower environmental impacts.

The large amounts of high pressure steam being used represents one of the the largest safety concern in the process, because a high pressure steam release could easily cause severe burns or even death. Proper precautions and control measures must be in place to prevent accidental release. Otherwise, the process does not utilize any known toxic chemicals and is relatively safe from a chemical standpoint. The flash vessels mostly operate below atmospheric pressure, so they must be constructed as vessels that can withstand vacuum without failing.

=Appendix A: Proposed Location of Desalination Plant=
Pictured below is the proposed location for the desalination plant, located on Nueces Bay in Corpus Christi, adjacent to the Nueces Bay Energy Center. The plant’s proposed location is shown in the area outlined by the small circle in the upper right of the figure.

[[File:NuecesBayLocation.png|center|600px|thumb|alt=Nueces Bay Desalination Facility Location|Figure A.1: Proposed mapped location of Nueces Bay Desalination Facility.]]
[[File:ClassIWell.png|center|700px|thumb|alt=Nueces Bay Desalination Facility Distance to Injection Well|Figure A.2: Mapped distance from proposed location to Class I Injection Well]]

=Appendix B: Feed and Product Definitions - Salinity=

=Appendix C: Process Flow Diagram=
[[File:TeamF_FinalPFD.JPG|center|1200px|thumb|alt=Process Flow Diagram|Figure C.1: Process Flow Diagram]]

=Appendix D: HYSYS Simulation Model=
[[File:TeamF_HYSYS_Screenshot.PNG|center|1200px|thumb|alt=HYSYS Simulation Model|Figure D.1: Screenshot of HYSYS Model Simulation.]]

=Appendix E: Stream Tables=
A summary of some values is shown below:

<center>
{| class = "wikitable"
|+Table 7: Key process material streams
!  
! Stream  
! Molar Flow (105 kgmole/h)
! Mass Flow (105 kg/h)
|- style="text-align: center;"
! style="text-align: right;" | Inlet
| Feed Water || 454 || 83.6
|- style="text-align: center;"
! style="text-align: right;" | Outlet
| Brine Reject || 1.90 || 36.0
|- style="text-align: center;"
! style="text-align: right;" | Product
| Pure Water || 2.64 || 47.5
|}
</center>

<center>
{| class = "wikitable"
|+Table 8: Key process energy streams
! Stream  
! Heat Flow (kJ/h)
|- style="text-align: center;"
| Feed Water || 1.30E+11
|- style="text-align: center;"
| Q-301 || 5.73E+06
|- style="text-align: center;"
| Q-302 || 3.09E+09
|- style="text-align: center;"
| Q-304 || 4.68E+06
|- style="text-align: center;"
| Q-303 || -1.26E+09
|- style="text-align: center;"
| Brine Reject || -5.47E+10
|- style="text-align: center;"
| Pure Water || -7.46E+10
|}
(Negative value denotes energy leaving process)
</center>

=Appendix F: Heat Exchanger Sizing=

<center>
{| class = "wikitable"
|+Table 9: Utility Temperatures and Heat Transfer Coefficient<ref name="Towler">G.P. Towler, R. Sinnott. Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design. Elsevier, 2012.</ref>
!  
! HP Steam
! Cooling Water
|- style="text-align: center;"
! Temperature (°C)
| 250 || 35
|- style="text-align: center;"
! U (W/m2K)
| 4000 || 1500
|}
</center>

<center>
{| class = "wikitable"
|+Table 10: Internal Flash Heat Exchanger Sizing, condensing steam.
! Exchanger
! UA (kJ/°C-h)
! U (kJ/hr/m2°C)
! A (m2)
|- style="text-align: center;"
| E-101 || 1.47E+07 || 14400 || 1019
|- style="text-align: center;"
| E-102 || 1.53E+07 || 14400 || 1059
|- style="text-align: center;"
| E-103 || 1.59E+07 || 14400 || 1103
|- style="text-align: center;"
| E-104 || 1.66E+07 || 14400 || 1153
|- style="text-align: center;"
| E-105 || 1.74E+07 || 14400 || 1209
|- style="text-align: center;"
| E-106 || 1.83E+07 || 14400 || 1272
|- style="text-align: center;"
| E-107 || 1.93E+07 || 14400 || 1342
|- style="text-align: center;"
| E-108 || 2.05E+07 || 14400 || 1424
|- style="text-align: center;"
| E-109 || 2.19E+07 || 14400 || 1518
|- style="text-align: center;"
| E-110 || 2.35E+07 || 14400 || 1629
|- style="text-align: center;"
| E-111 || 2.54E+07 || 14400 || 1761
|- style="text-align: center;"
| E-112 || 2.77E+07 || 14400 || 1921
|- style="text-align: center;"
| E-113 || 3.05E+07 || 14400 || 2119
|- style="text-align: center;"
| E-114 || 3.42E+07 || 14400 || 2372
|- style="text-align: center;"
| E-115 || 3.90E+07 || 14400 || 2706
|- style="text-align: center;"
| E-116 || 4.56E+07 || 14400 || 3167
|- style="text-align: center;"
| E-117 || 5.55E+07 || 14400 || 3856
|- style="text-align: center;"
| E-118 || 7.20E+07 || 14400 || 5000
|}
</center>

=Appendix G: Pressure Vessel Sizing=

=Appendix H: Expanded Economic Assumptions=

=Appendix I: Equipment Cost Summary=

=Appendix J: Economic NPV Analysis=

=References=
<references/>

Nueces Desalination Center: Production of Drinking Water by Multi-Stage Flash Distillation

2016-03-07T20:13:55Z

Benjkg: /* Appendix C: Process Flow Diagram */

Team F: Rankine 672 Final Report

Authors: Kedric Daly, Ben Granger, Evan Rosati, Kathleen Zhou

Instructors: Fengqi You, David Wegerer

March 11, 2016

=Executive Summary=
Desalination of seawater to drinking water is a technology of ever-increasing importance given trends of increasingly widespread water shortages around the world. Even in the United States, water shortages are becoming increasingly common in dry, highly-populated such as California and certain areas of the Gulf Coast. Our design team, Rankine 672, was tasked with evaluating the need for and designing a desalination plant to provide purified water in a place of need. Investigation revealed that the city of Corpus Christi, Texas, is a strategic location for development of a desalination facility, as surrounding region is in need of new drinking water sources and the city has expressed serious interest in contracting the development of a large-scale desalination plant. Our proposed solution is a multi-stage flash distillation desalination facility capable of producing 20,000,000 gallons (75,708 m3) of purified drinking water for the city and surrounding region per operating day.

In this design, drinking water is produced from seawater feedstock through a 21-stage flash distillation process. By strategically locating the facility next to an existing natural gas power plant, the Nueces Bay Energy Center, pre-heated feed water can be taken directly from the outbound cooling systems of the power plant, allowing energy savings for our desalination plant. The process is designed for 16 hour-per-day operation, requires a seawater feed of 8250 m3/hr, and produces 4758 m3/hr of purified drinking water alongside a waste stream of 3485 m3/hr of concentrated brine. The waste stream is isolated from a recycle stream through a 9.0% purge; this results in an overall product yield of 57.7% on a volume basis. The process requires an additional pretreatment stream of 16.5 kg/hr Belgard EV 2030, which serves as an antiscalant, as well as 9.5 kg/hr chlorine, 352.1 kg/hr lime, and 418.7 kg/hr CO2 for post-treatment purposes.

The overall capital cost of the plant is expected to be $345MM, with a total operating cost of $103MM/yr, $59MM/yr of which comes from utilities. Due to the low price at which the product water can be sold, the 30-year net present value of the plant, assuming a 6% discount rate, is $-1,035MM. It is thus imperative this plant be constructed with public funds because it cannot return a profit to any private investor. The growing need for water however could justify the cost, because it is important that the residents of the greater Corpus Christi area have access to drinkable water.

__TOC__

=Introduction=
==Background==
Clean drinking water is a basic human need that is becoming increasingly difficult to meet. Currently, over 1 billion people live in places where water is considered scarce<ref> Tapping the oceans. The Economist USA. http://www.economist.com/node/11484059. Accessed March 2, 2016. </ref>. Though often thought of as a problem exclusive to third-world countries, the issue is becoming more prevalent in the US as well, with water managers in 40 of 50 states anticipating water shortage conditions in some portion of their state in the next 10 years. These shortages are due to increasing demands on constant or diminishing freshwater resources, therefore, to correct the issue, a new supply must be found<ref> Water Supply in the US. EPA. http://www3.epa.gov/watersense/pubs/supply.html. Accessed March 2, 2016.</ref>. Desalination of seawater is therefore important as a new source for clean drinking water and prevent water shortages.

==Project Definition==
The overall goal of the project is to investigate the feasibility of a desalination plant in order to meet the needs of growing water demand. The design needs to take health, safety, and environmental concerns into account, while optimizing costs.

=Design Basis=
Site Location and Conditions
Corpus Christi, TX was chosen as the location for the proposed desalination plant. The city is currently experiencing “Moderate Shortage Conditions”. This issue additionally comes at a time of growing demand with population growth anticipated due to new industry and Eagle Ford Group crude oil production<ref>Stage 2 Measures in the Drought Contingency Plan. Available at http://www.cctexas.com/government/water/conservation/conservation-drought-plans/stage-2-measures. Accessed January 12, 2016. </ref>.

In June 2014, the city announced the Corpus Christi Desalination Demonstration Project<ref>Corpus Christi Desalination Demonstration Project. Available at http://www.cctexas.com/Assets/Departments/Water/Files/DesalFactSheet.pdf. Published June 2014. Accessed January 12, 2016.
</ref> which seeks to create a demonstration desalination plant capable of producing 20,000 gallons of water per day, with plans to later build a full scale facility generating 20,000,000 gallons of water per day. The desalination demonstration project indicates a clear need for implementation of a desalination plant of this size to accommodate growth in the Corpus Christi area.

Locating our plant in Corpus Christi will allow us to take advantage of the benefits of cogeneration through a partnership with the Nueces Bay Energy Center. By utilizing their outlet cooling water as a large portion of our feed for the desalination process, we can raise the temperature of our seawater feed, saving on energy costs in the multi stage flash distillation. The proposed location for the desalination plant is immediately west of the Nueces Bay Energy Center. See Appendix A for a map depicting the proposed location for the desalination plant along the coast of Nueces Bay.

==Feed and Product Definitions==
The proposed desalination facility’s feed and product streams are defined as shown in Table 1. Rationale for these definitions is also described below. The values reported in Table 1 are final derived numbers from HYSYS simulation modeling.

<center>
{| class = "wikitable"
|+Table 1: Feed and product stream definitions
!  
! Salt Concentration
! Temperature
! Amount
|- style="text-align: center;"
! style="text-align: right;" | Feed Stream
| 30,500 ppm || 45°C || 8250 m3/hr
|- style="text-align: center;"
! style="text-align: right;" | Product Stream
| ~ 0 ppm || 35°C || 4758 m3/hr
|- style="text-align: center;"
! style="text-align: right;" | Byproduct Stream
| 695 ppm || 35°C || 3485 m3/hr
|}
</center>

The process feed will be seawater from Nueces Bay, preheated through the cooling water system of the nearby Nueces Bay Energy Center. Based on calculations developed from NREL (National Renewable Energy Laboratory) data, it was estimated that the Nueces Bay Energy Center uses approximately 250 million gallons of seawater per day for process and cooling needs. Therefore, the 20 million gallon per day requirement for the desalination facility can easily be met through the power plant’s cooling water<ref>Torcellini P, Long N, Judkoff R. Consumptive Water Use for U.S. Power Consumption. NREL. 2003; TP-550-33905.
</ref>. Note that we are assuming the process will operate 67% of the time (16 hours per day).

Based on data acquired from two salinity-measuring stations in Nueces Bay, the average salinity of local seawater is 30.5 practical salinity units (psu)<ref>Stations Map & Real - Time Data. The Conrad Blucher Institute for Surveying and Science website. Available at http://www.cbi.tamucc.edu/cbi/data/. Updated January 2016. Accessed January 9, 2016. </ref>, thus setting the concentration of the process feed stream. This translates to a feed stream concentration of 30,500 ppm (parts per million)<ref>Definition and Units. CATD Salinity Expertise Center. Available at http://www.salinityremotesensing.ifremer.fr/home. Accessed January 12, 2016.</ref>. The composition of the salt attributed to this seawater salinity is shown in Table 6 in Appendix B.

No data on the composition of seawater around the Corpus Christi area was available, but the composition of seawater does not change significantly in different locations. The given composition is considered accurate for our feed stream<ref>Brown E, Colling A, Park D, Phillips J, Rothery D, Wright J. Seawater : Its Composition, Properties and Behaviour. Boston: Butterworth-Heinemann, 1995.
</ref>. Note that seawater salt is primarily sodium chloride - total dissolved solids are 85.7% sodium chloride by mass. As such, the dissolved solids in the feed stream is assumed in this project to be pure NaCl, making the feed composition 96.95% water and 3.05% NaCl by mass. A control system will likely be put in place to ensure feed concentration remains at this level, accounting for natural fluctuations in local seawater salinity.

The largest byproduct from the process will be a brine slurry with high salt concentrations. An upper corrosion limit of 700 ppm was set for recirculated and purged brine. This undesired byproduct will be sent to a nearby Class I non-hazardous injection well facility operated by GNI Group, which is approximately 12 miles away from the proposed desalination plant location as shown in the figure in Appendix A<ref>Texas Water Code. Availabe at http://www.statutes.legis.state.tx.us/Docs/WA/htm/WA.27.htm. Accessed January 14, 2016.
</ref><ref>Google Maps. Available at https://goo.gl/maps/3xcvLZwUCe42 Accessed January 15, 2016.</ref>. Through the incorporation of a Class I injection well, the brine will not have to be treated or diluted before injection. <ref>EPA Regulations on Class I Wells. Available at http://www.epa.gov/uic/class-i-industrial-and-municipal-waste-disposal-wells. Accessed January 14, 2016.
</ref><ref>List of Injection Well Facilities by Location. Available at http://www.ehso.com/cssepa/tsdfdeepwells.php. Accessed January 14, 2016.</ref>

There are no regulations regarding the temperature of city drinking water, so the product stream temperature has no requirements based on regulations. Outlet temperatures of 35oC are acceptable for input to the municipal water supply system. Since city water tends to be stored in non-temperature controlled tanks, the temperature will naturally cool to ambient temperature.<ref>Climate Corpus Christi- Texas. US Climate Data. Available at http://www.usclimatedata.com/climate/corpus-christi/texas/united-states/ustx0294. Accessed January 12, 2016. </ref>

==Pre-Treatment and Post-Treatment==
For pretreatment, Belgard EV 2030 is recommended to be added at a concentration of 1.8-2.0 mg/L for a system with top brine temperature approximately 110 °C<ref name="BWA 2030">BWA Water Additives. Belgard EV2030 Technical Data Sheet. BWA Water Additives. http://www.wateradditives.com/files/products/tech_data_sheets/Belgard%20EV%202030_Technical%20Data%20Sheet_Thermal_8.5x11.pdf. Accessed January 27, 2016.
</ref>. Since our process is designed to operate at a slightly higher top brine temperature, 120 °C, we will design for the upper bound concentration: 2.0 mg/L of antiscalant. With a feed flow rate of 8250 m3/hr, the process therefore requires 16.5 kg/hr of Belgard EV 2030.

For disinfection during post-treatment, chlorine addition typically requires a dose of roughly 2.0 mg/L as well<ref name="World Health Organization">World Health Organization. Desalination for safe water supply. http://www.who.int/water_sanitation_health/gdwqrevision/desalination.pdf. Accessed January 26, 2016.</ref>. With a pure water distillate flow rate of 4758 m3/hr, the process requires 9.5 kg/hr of chlorine as a 5%-15% sodium hypochlorite solution. Approximately 74 mg/L of lime (calcium hydroxide) and 88mg/L of CO2 should be added to the desalinated water to help prevent corrosion and increase alkalinity within acceptable ranges<ref name="Vouchkov">Vouchkov, N. Re-Mineralization of Desalinated Water. SunCam. https://s3.amazonaws.com/suncam/npdocs/118.pdf. Accessed February 18, 2016.</ref>. With a product stream flow rate of 4758 m3/hr, these concentrations correspond to 352.1 kg/hr of lime and 418.7 kg/hr of CO2. Both of these can be purchased, and it is likely that the CO2 can be procured from the neighboring natural gas power plant.

==Choice of Desalination Technology==
The two major technologies commonly implemented for desalination include reverse osmosis (RO), and multi-stage flash distillation (MSFD). RO utilizes high pressures to force water through a semipermeable membrane that allows for purified water to pass through, but stops organic and other wastes. Often, the pressures can reach 55 - 68 bar, which requires large pumps to maintain pressure, and introduces safety concerns to the process<ref>Stage 2 Measures in the Drought Contingency Plan. Available at http://www.cctexas.com/government/water/conservation/conservation-drought-plans/stage-2-measures. Accessed January 12, 2016.
</ref>. While RO can be less energy intensive than MSFD, and the membrane can also remove contaminants, it is susceptible to feedwater quality, membrane fouling, and requires more extensive pretreatment and posttreatment than MSFD. The cost of RO is directly proportional to the feed salinity, and so it is less expensive when desalinating low salinity water.

Multi-stage flash distillation is best selected for areas that have high salinity or low energy costs. The pretreatment process for MSFD is much easier when compared to RO because pretreatment often consists of simply adding anti-corrosives and antiscalants to the feed stream. The process works by heating water in vessels at different pressures to change the boiling point of the water and evaporate it as it passes through the system. After evaporation, the water condenses, leaving pure water as the effluent. MSFD costs are now roughly $1.00/m3, where this cost is mostly independent of salinity, but dependent on energy costs.<ref>Fritzman C, Lowenberg J, Wintgens T, Melin T. State-of-the-art Reverse Osmosis Desalination. Desalination. 2007; 216:1-76.
</ref> In both RO and MSFD a brine is produced which then must be appropriately treated and/or disposed of.

Multi-stage flash distillation was selected for this process, mainly due to the economic advantages offered by the selected location of Corpus Christi, TX. MSFD is also a well understood process that can be more easily modeled by chemical engineering software.

=Technical Approach=
Quantitative modeling of the process and all included optimization was conducted using the simulation software Aspen HYSYS. A variety of assumptions were developed to set up and define specific components of the process simulation itself. Within Aspen HYSYS, the NRTL Electrolyte property package was selected to model the behavior of salt water. Each stage of the MSF process is modeled as an individual flash vessel, with individual shell & tube heat exchangers used to model the distillate condensers present in each flash stage in the MSF system.

The HYSYS simulation model only considers the flash portion of the overall process, allowing establishment of mass and energy balances for the system. Pretreatment and post-treatment were excluded for the HYSYS model, as these steps are simple additions or removals of small concentrations of chemical components. Although the simulation models heat exchangers outside of each flash stage, in the physical design of the plant these heat exchangers will be contained within each flash stage.

One portion of the process is not modeled, which is the inlet seawater entering the heat rejection stages. As can be seen from the full process flow diagram (Appendix C), the inlet seawater in the heat rejection stage is in high excess to provide maximum cooling capacity; then a significant portion is purged to bring the stream flow rate down to what is needed for the feed stream. It is assumed that the flow rate of cooling water for those three stages is in such high excess that the cooling water does not heat up. Therefore, in the process simulation models, the inlet stream is the feed stream, which enters directly into the heat recovery portion of the process.

See Appendix D for a screenshot overview of the simulation environment model.

=Process Overview=
==Process Flowsheet==
The following diagram, Figure 1, shows a basic flowsheet for the process. Process alternatives considered for each stage are discussed below. Please see Appendix C for the full process flowsheet with finalized design decisions.

[[File:BlockFlowDiagram.png|center|700px|thumb|alt=Nueces Bay Desalination Facility Block Flow Diagram|Figure 1: Basic block flowsheet of the multi-stage flash desalination process.]]

==Major Features==
As seen above, there are three primary stages of the desalination process: Pre-treatment, the multi-stage flash distillation, and post-treatment. Seawater enters the pre-treatment phase first to make the feed suitable for the rest of the process by filtering out large particles that could accumulate on pipes, deaerating the water to limit corrosion, and adding an agent to combat fouling. The water then enters the distillation process, which is broken up into two sections: heat recovery and heat rejection.

In the heat recovery stage, there are 18 flash vessels, each of which separates steam from the salinated liquid water. Each flash vessel has saltwater as its inlet and two outlets: a vapor outlet (steam) that condenses into the final product of purified water and a liquid outlet composed of water with more dissolved solids than the inlet, which is the inlet stream for the next flash vessel. The pipe containing the feed is first mixed with recycled brine, then goes through the top sections of each of the flash vessels, starting with V-118 and ending up at V-101. In this way, the feed stream pipe first serves as a heat exchanger, heating the pre-treated seawater while simultaneously condensing the steam into the final product. The feed then enters a final heat exchanger where it is brought up to 103.6 °C before entering V-101 to be flashed.

After exiting V-118, the final flash vessel in the heat recovery portion, the water enters the heat rejection portion, which consists of three flash vessels (V-201, V-202, V-203). The flash vessels of this section operate in the same manner as in the heat recovery section, with the exception that condensation occurs using excess cooling water instead of pre-treated seawater. This technique allows excess heat to be rejected into an excess water stream rather than the feed.

The purified product stream finally enters post-treatment processing in which water is re-aerated and remineralized, disinfected, and pH is adjusted before being delivered to the city. The concentrated brine leftover at the end of the flash cycle is recycled into the feed, although a purge is required to continuously remove some salt from the system. This purged brine is disposed of by use of a Class I injection well, per regional regulations.

See Appendix E to find detailed stream tables of stream specifications.

==Process Alternatives==
===Pretreatment===
Multi-stage flash distillation requires significantly less pretreatment than reverse osmosis desalination, but helpful pretreatment techniques still exist. One very inexpensive method of pretreatment is screening/filtration, which is used to filter out large contaminants from intake seawater. Using more complicated membrane technology to accomplish large particle filtration is a possible alternative, but doing so would be much more expensive than using a standard screen or filter, so the latter choice has been selected for this process.<ref>El-Dessouky H., Alatiqi I, Ettouney H. Process synthesis: The multi-stage flash desalination system. Desalination. 1998;115(2):115-179. doi: 10.1016/S0011-9164(98)00035-6.</ref>

The most critical type of fouling in multi-stage flash distillation is scaling - therefore, the most critical step in the pretreatment process will be the inclusion of an anti-scalant. Alkaline scaling can be prevented by acid dosing or polyphosphate addition; however, acid dosing can result in increased corrosion and polyphosphates can lead to non-alkaline calcium phosphate fouling, so these additives will not be considered<ref name="Morris">Morris, R. The development of the multi-stage flash distillation process: A designer's viewpoint. Desalination. 1993;93(1-3):57-68. doi:10.1016/S0011-9164(93)80096-6. Availble at http://www.sciencedirect.com/science/article/pii/0011916493800966</ref><ref>Al-Gobaisi, D. A quarter-century of seawater desalination by large multistage flash plants in Abu Dhabi (Plant performance analysis, assessment, present efforts toward enhancement and future hopes). Desalination. 1994;99(2-3):509-512. doi: 10.1016/S0011-9164(94)00196-0.</ref>. As such, a polymer-based additive is recommended for the anti-scalant for this process. The Belgard EV series is designed for use in high-temperature seawater MSF; of these, Belgard EV 2030 is the most neutral in pH<ref>NSF International. NSF Product and Service Listings: Drinking Water Treatment Chemicals - Health Effects. NSF International. http://info.nsf.org/Certified/PwsChemicals/Listings.asp?Company=00300&. Accessed January 27, 2016.</ref><ref name="BWA 2030" /><ref>BWA Water Additives. Belgard EV2035 Technical Data Sheet. BWA Water Additives. http://www.wateradditives.com/files/products/tech_data_sheets/Belgard%20EV2035_Technical%20Data%20Sheet_Thermal_8.5x11.pdf. Accessed January 27, 2016.</ref><ref>BioLab Water Additives. Belgard EV Antiscalant for seawater distillation plants. BioLab Water Additives. http://www.lpq.com.mx/pdf/BELGARD%20EV.PDF. Accessed January 27, 2016.</ref>. As such, we have selected Belgard EV 2030 for use as an anti-scalant in our process.

Corrosion control is another significant method of pretreatment in multi-stage flash distillation plants. Using a piping material that has limited susceptibility to corrosion is typically not economically viable for MSF desalination plants due their large size, so using carbon steel for most of the piping is generally used for its economic benefits despite its susceptibility to corrosion<ref name = "Morris" />. Limiting oxygen concentration is very feasible through the use of deaerators, which can remove oxygen as well as other dissolved gases from the seawater stream. Often, an external deaeration system is used<ref name = "Morris" /><ref>Edwards - Hick Hargreaves. Seawater Deaerators. Edwards - Hick Hargreaves. http://psscorp.co.th/upload/news/20110830105340.pdf. Accessed January 26, 2016.</ref>. Given these considerations, using an external deaerator combined with a less-corrosive antiscalant like Belgard EV 2030 is much more practical for the process than using anti-corrosive piping.

Excessive foam can also be an issue in some MSF distillation facilities, so antifoaming agents are sometimes employed in the pretreatment process as well. However, many plants are operated without foaming issues, and therefore do not need anti-foaming pretreatment at all. Selection of an anti-foaming agent must be carefully weighed with selected of anti-scalant to avoid issues with additive interactions [26]. At this stage in the design process, an anti-foamer will not be included due to the risk of decreasing scalant efficacy, and the likelihood that foam will not be an issue in the desalination plant.

===Multistage Flash Distillation===
The main objective of the design project is desalinate water. This is done with the main process of multi stage flash distillation (MSF). MSF can be developed using a numerous number of different process configurations, such as Once-Through, Simple Mixer Brine Circulation, and Multi-Stage Heat Rejection Brine Circulation processes.

The most desired process for the project is the Three Stage Heat Rejection Brine Circulation as it builds on benefits of the other simpler technologies. By combining principles of heat rejection and heat recovery, the required heat transfer area of the process is greatly reduced as compared to once through processes or simple brine recycle processes. It has the highest performance ratio with the lowest specific feed flow rates, therefore greatly lowering the operating costs of the process. It is also known as the industry standard for MSF<ref name = "KSU Chapter 6">Kansas State University ChE 413. Available at: http://faculty.ksu.edu.sa/Almutaz/Documents/ChE-413/Multi-stage%20Flash%20Desalination.pdf. Accessed January 26, 2016.</ref>. In addition, since our brine disposal technique involves the pumping of the brine into a Class I well, the high brine outlet salinity is not a drawback for our process since post treatment or dilution of the brine is not necessary.

===Post Treatment===
After processing the water via multi-stage flash, the water needs to be post-treated to make the water taste as consumers would expect, since distilled water is “flat” and unpleasant, as well as to protect the consumers and the piping through which the water will be passing<ref name="Clayton">Clayton R. A review of current knowledge: Desalination for water supply. Foundation for Water Research. http://www.fwr.org/desal.pdf. Accessed January 26, 2016.</ref>. There are multiple alternatives to achieve these objectives of remineralization, aeration, corrosion control, and disinfection.

Aerating and introducing some dissolved solids back into the product water are effective ways to create a quality product that is not flat, and contains flavoring that is expected of the water when delivered to the consumer. Aeration will be accomplished via an inline spray aerator which will take up less space than cascade aeration.<ref>Minnesota Rural Water Association. Aeration. http://www.mrwa.com/WaterWorksMnl/Chapter%2011%20Aeration.pdf. Accessed January 26, 2016.</ref> Feed blending will be used to remineralize the water as opposed to a remineralization filter because it does not require additional equipment purchases, and can easily handle the large amounts of throughput required for the process.<ref name="Clayton" />

Corrosion and pH can be controlled by adding lime (calcium hydroxide) and carbon dioxide to the water. To do so, about 74 mg/L of lime and 88mg/L of CO2 should be added to the desalinated water to help prevent corrosion and increase alkalinity within acceptable ranges. Sodium hydroxide, soda ash, and sodium bicarbonate are alternatives to altering the alkalinity and pH of the product water, but lime and CO2 is an industry standard <ref name="Vouchkov" />.

Disinfection needs to be done to prevent microbial growth in the desalinated water and ensure the safety of the general public. Chlorine addition via chlorine gas or sodium hypochlorite is the most common and only requires a dose of 1.5 to 2.5 mg/liter. <ref name="World Health Organization" /> Other alternatives include ozonation, UV light, and chloramination. Chlorine, as a 5% - 15% sodium hypochlorite solution, is sufficient for our process and relatively inexpensive, and as such will be utilized in the process.

===Transport of Brine to Well===
Three major possibilities exist for transporting the brine from our desalination plant to the injection well for disposal: rail, trucking, and a pipeline. A pipeline was chosen as the most feasible and cost-effective approach.

A rail line is not available at this time, and with the cost to construct a rail line at approximately $43 million per mile<ref>About Money. How Much Do Rail Transit Projects Cost to Build and Operate? 2014. Available at: http://publictransport.about.com/od/Transit_Projects/a/How-Much-Do-Rail-Transit-Projects-Cost-To-Build-And-Operate.htm. Accessed January 27, 2016. </ref>, the enormous capital cost makes rail transport infeasible for this site. For large volume operations, pipelines are cheaper than trucking. In a study of wastewater sludge produced from wastewater treatment facilities, it was found that it was more economical for facilities producing over 1200 m3 per day to utilize pipelines rather than trucking.<ref>Marufuzzaman M, Eskioglu S, Hernandez R. Truck versus pipeline transportation cost analysis of wastewater sludge. Transportation Research. 2015; 74(A): 14-30. </ref> Our facility is producing nearly 3 times this volume at 3485 m3 per day of brine. Furthermore, brine is a less viscous fluid than wastewater sludge, decreasing pumping costs in the pipeline and further amplifying the benefit of choosing a pipeline over an injection well.

===Brine Storage===
Due to the choice of a pipeline, rather than trucking, as the method of transport of brine to the injection well, brine storage is unnecessary, so storage solutions such as holding tanks were not investigated. One method of storage compatible with a pipeline is to pump the brine through an evaporation pond. This solution is attractive as it would allow the brine to concentrate, saving some costs of pumping, however, the considerable land usage of evaporation ponds leads them to only be effective for smaller plants. The largest municipal plant utilizing this method produces only 5.7 million liters per day<ref>University of Santa Cruz Center for Integrated Water Research. Concentrate Management: Evaporation Ponds. 2010. Available at: https://ciwr.ucsc.edu/sites/default/files/ICM_D5.pdf. Accessed January 27, 2016. </ref>, whereas our desalination plant is proposed to produce 75.7 million liters per day. Therefore, evaporation ponds have been eliminated from consideration.

==Major Specifications==
===Material of Construction Selection===
While saltwater and desalinated water in general can be corrosive, there are best practices within the desalination industry for material selection. With this in mind, best practice shows carbon steel lifetime can be lengthened by monitoring the oxygen concentration in the water, specifying the paint used to coat any metal, and more expensive materials can be used in areas of extremely high corrosion.<ref name = "Morris" />. By following these best practices, it is possible to save significant amounts of money through the use of carbon steel in most areas of the plant, with specific high-corrosion areas being reinforced with, or replaced with, more corrosion resistant materials, such as Cu/Ni 70/30 alloy<ref name = "KSU Chapter 6" />.

===Heat Exchanger Sizing===
See Appendix F for heat exchanger sizing data. Heat exchanger sizes appear reasonable for placement in the flash vessels.

===Flash Vessel Sizing===
The dimensions for the flash drums were obtained through literature values. The flash drums were determined to be 18x4x3 m in width, height, and length.<ref name="KSU Chapter 6" />

Wall thickness was determined using the following conventions: Design pressure is 10% more than the operating pressure, design temperature is 50°F (27.8°C) higher than the operating temperature, a corrosion allowance of 3-mm and an effectiveness of 85% was derived from the assumption of a double-welded butt joint that was spot checked. See Appendix G for wall thicknesses calculated for each pressure vessel.

The sizing of the vessels to be constructed will be determined after talks with a manufacturer. Due to economies of scale, it may be more cost-effective to buy 21 pressure vessels with the dimensions mentioned above and a wall thickness of 17.1-mm, the highest wall thickness required in the system.

===Pipe Losses and Pressure Drop===
HYSYS was used to estimate the diameter of a pipe given a specified pressure drop per 100m of pipe. It was determined that for a pressure drop of 0.1 kPa/100m of pipe, a 3.05m diameter pipe is sufficient, which is more than manageable for manufacturers. This pressure drop is mostly insignificant and will be taken care of with the safety factors included in the pumps. The significant pressure drops in our system is accounted for via losses in the flash vessels and heat exchangers, which drive the flash process. Therefore overall pipe losses are insignificant and are built into intentional design losses.

=Economic Analysis=
==Economic Assumptions==
The sale of purified water yields a revenue stream of $49.5 million per year assuming sale to commercial entities within the city limits of Corpus Christi at $1.7915 per thousand liters.<ref>Water schedule. City of Corpus Christi Utility Business Office. http://www.cctexas.com/Assets/Departments/Financial-Services/Utilities-Business-Office/files/waterrates.pdf Accessed February 28, 2016.</ref> The primary variable costs are utilities and the brine disposal, with minimal raw materials costs due to the primary feedstock, seawater, being free, and minimal amounts of pre- and post-treatment chemicals being used. The cost of brine disposal used in this analysis was high at $2.097 per thousand liters<ref>McCurdy, R. Underground injection wells for produced water disposal. Chesapeake Energy Corporation. http://www.epa.gov/sites/production/files/documents/21_McCurdy_-_UIC_Disposal_508.pdf Accessed February 28, 2016.</ref>, an estimate for brine disposal utilizing trucking as the mode of transport. As has been shown through previous analyses, a lower cost can be achieved using a direct pipeline, however, no good estimates for this cost could be found.

The NPV was calculated using a 6% cost of capital, the approximate interest rate for a municipal bond.<ref>Zirbe, R.O. Jr., Han, X., Layton, D., Leshine, T. A history of discount rates and their use by government agencies. (2002). http://faculty.washington.edu/zerbe/docs/DISCOUNT%20RATE%20HISTORY-latesXIt.doc Accessed February 28, 2016.</ref> It was assumed that the plant can be built within two years, with production at half-scale in its first year. Additionally, one month of variable costs, $10.6 million, was set aside for working capital. See Appendix H for a full table of assumptions. A 30 year NPV calculation was chosen as the decision basis since 30 years is a conservative estimate for the lifetime of a desalination plant [38]. See below for estimates of NPV for the lifetime of the plant under various assumptions and design criteria.

==Optimization and Sensitivity Analysis==
Looking at the multi-stage flash process, there are a number of opportunities to optimize the process. The major process variables that were investigated were the inlet water temperature, the operating pressure drop per vessel, and the number of stages over which the process takes place. Considering these optimizations, our final process consists of 21 flash vessels (18 heat recovery and 3 heat rejection stages), each with a pressure drop of 5 kPa. See Appendix D for a screenshot of the HYSYS simulation used to model the optimized process.
===Operating Pressure and Pressure Drop===
Initially our process was designed to pump the inlet seawater and brine to a pressure of 2 bar and allow the pressure drop in each flash stage to end our process downstream at 1 bar to prevent complications associated with designing a system under vacuum. While this provided pumping advantages, the energy in the system was significantly higher due to vapor pressure considerations. Due to this fact, the required heat exchange area for certain exchangers proved to be infeasible (on the order of 80,000m2) due to the enormous capital and operating costs that would be associated with such a large heat exchanger.

The process was optimized to operate under vacuum, requiring additional pumping requirements but much lower required heat exchange area, and therefore lower utility costs. This optimization was done by specifying the pressure of the final flash vessel to 5.7kPa, so that the temperature of the water vapor (~30oC) is within approximately 5oC of the cooling water (35oC) so that condensation can occur within a reasonable approach. This also allows our system to optimally extract pure water from the brine running through the system and reduce energy requirements for cooling the product or the recycle. Our system redesign and optimization reduced the energy in the system and reduce the same heat exchanger area requirements by more than 3 orders of magnitude to approximately 800m2.

Once the process was optimized to operate in a vacuum, the pressure drop per flash stage regulated through values was then optimized. Three pressure drops per stages, 3kPa, 5kPa, and 7kPa, were tested and then an economic analysis was conducted to find the optimal value for the pressure drop. These changes had impacts on both operating and capital costs as they would change initial pumping requirements, the amount of water flowing through the process and recycle (and therefore the flash vessel sizes, heat exchanger sizing), and the efficiency of each stage.

The various pressure drops (3 kPa, 5 kPa, and 7 kPa) were tested on a system nominally at 24 stages under the assumption that the results would be generalizable to systems with a different number of stages. With increasing pressure drop, we found decreased capital cost but increased utility cost. As can be seen from Table 2 below, a pressure drop of 5 kPa proved to be the best option based on 30 year NPV, with a loss of $838.8 million, a savings of $65.5 million compared to the next-best option with a pressure drop of 7 kPa.

<center>
{| class = "wikitable"
|+Table 2: Comparison of profitability for our MSF desalination process with 24 stages with 3 different pressure drops per stage
! Number of Stages  
! Pressure Drop (kPa)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
! 30-Year NPV ($MM)
! NPV Ratio (over 5kPa)
|- style="text-align: center;"
| 24 || 3 || 362.3 || 39.1 || -905.8 || 1.08
|- style="text-align: center;"
| 24 || 5 || 255.1 || 53.6 || -838.8 || 1.00
|- style="text-align: center;"
| 24 || 7 || 209.5 || 72.2 || -904.3 || 1.08
|}
</center>

===Number of Flash Stages===
The number of stages is an important area for optimization. An increased number of stages yields better recovery of purified water, however, requires higher capital cost from the additional pressure vessel. Furthermore, with an increased amount of purified water comes increasingly concentrated brine. This must be taken into account since any brine concentration in excess of about 7% presents a more significant corrosion risk<ref name="KSU Chapter 6" />, so increased maintenance costs for the piping or higher capital costs for more corrosion-resistant materials could be necessary for brine concentrations exceeding 7%. Therefore, to recover the same amount of pure water without exceeding corrosion limitations, increasing the number of stages will reduce the overall size of each stage.

Optimizing based on NPV, it was found that 21 stages is optimal, balancing the increased profits from the additional purified water additional costs of higher capital cost. See Table 3 below to compare the 30-year NPV of the process across various numbers of flash vessels.

<center>
{| class = "wikitable"
|+Table 3: Comparison of profitability for our MSF desalination process with a 5 kPa pressure drop, 5000 m3/hr feed flow rate, with varying number of stages.
! Number of Stages  
! Pressure Drop (kPa)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
! 30-Year NPV ($MM)
! NPV Ratio (over 21 Stages)
|- style="text-align: center;"
| 17 || 5 || 213.9 || 58.7 || -813.8 || 1.016
|- style="text-align: center;"
| 18 || 5 || 218 || 57.6 || -809.9 || 1.011
|- style="text-align: center;"
| 19 || 5 || 224.3 || 58 || -806.1 || 1.007
|- style="text-align: center;"
| 20 || 5 || 229.8 || 57.9 || -813.6 || 1.016
|- style="text-align: center;"
| 21 || 5 || 226.6 || 55.9 || -800.7 || 1
|- style="text-align: center;"
| 22 || 5 || 254.7 || 54.6 || -849.5 || 1.061
|- style="text-align: center;"
| 23 || 5 || 249.3 || 54.1 || -833.1 || 1.04
|- style="text-align: center;"
| 24 || 5 || 255.1 || 53.6 || -838.8 || 1.048
|- style="text-align: center;"
| 25 || 5 || 254.9 || 52.7 || -837.4 || 1.046
|}
</center>

===Inlet Seawater Temperature===
By pairing our desalination plant with a natural gas power plant and utilizing its cooling water we forgo initial heating of our inlet stream. Normally, water pumped from the ocean would be approximately 25oC and by capturing waste heat from the power plant we can reduce utilities with our water inlet of 45oC. Through an economic comparison between two plants nominally at 21 flash stages, the plant with an inlet temperature at 25oC has a capital cost of $245MM and a utility cost of $77MM/yr and the same facility with an inlet water temperature at 45oC has a capital cost of only $226MM and a utility cost of $56MM/yr. As shown in Table 4 below, this $19MM savings in capital costs and $21MM/yr saving in utilities is largely due to reducing the size of the heat exchangers and lowering the steam required for heating duty on the inlet seawater respectively. Therefore our process is optimized by recovering the waste heat water from the gas fired power plant.

<center>
{| class = "wikitable"
|+Table 4: Seawater Inlet Temperature Process Cost Optimization
! Seawater Temperature (°C)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
|- style="text-align: center;"
| 25 || 245 || 77
|- style="text-align: center;"
| 45 || 226 || 56
|- style="text-align: center;"
| Savings || 56 || 19
|}
</center>

==Overall Process Economics==
After optimization at fixed flow rates, the process was scaled to meet the the 67% operating percentage and final product definitions as shown in Table 1, which resulted in finalized economics shown in Table 5 below. For a table of the installed costs of each process vessel, see Appendix I. For the finalized NPV calculations, see Appendix J.

<center>
{| class = "wikitable"
|+Table 5: Summarized Economics for 21-stage MSFD Process
! Economic Parameter
! Cost (USD)
|- style="text-align: center;"
| Capital Cost || 345 MM
|- style="text-align: center;"
| Total Operating Cost per Year || 103 MM
|- style="text-align: center;"
| Utility Cost per Year || 59 MM
|- style="text-align: center;"
| 30-Year NPV (6% Discount) || -1035 MM
|}
</center>

=Conclusions and Recommendations=
==Project Viability==
Table 5 above demonstrates that the process has reasonable operating and capital costs, but the 30-year NPV is negative by -1035 $MM due to the extremely low sell price capability of desalinated water as a product. Therefore, this plant would have to heavily be supported by the City of Corpus Christi to run as the product revenue will not make it self-sufficient. The acquisition of government subsidies are key operating requirements for all desalination facilities in the United States.

==Additional Concerns==
Large volumes of water, steam, and energy are needed to ensure the project runs as specified. While most of the water can easily be extracted from the ocean with little to consequence, the fuel used to generate the steam and electricity can have a significant environmental impact. As the plant is located next to a natural gas power plant, this fuel is better than coal with respect to environmental concerns, and utilizing energy from the power plant to preheat the feed stream for the desalination plant helps lower environmental impacts.

The large amounts of high pressure steam being used represents one of the the largest safety concern in the process, because a high pressure steam release could easily cause severe burns or even death. Proper precautions and control measures must be in place to prevent accidental release. Otherwise, the process does not utilize any known toxic chemicals and is relatively safe from a chemical standpoint. The flash vessels mostly operate below atmospheric pressure, so they must be constructed as vessels that can withstand vacuum without failing.

=Appendix A: Proposed Location of Desalination Plant=
Pictured below is the proposed location for the desalination plant, located on Nueces Bay in Corpus Christi, adjacent to the Nueces Bay Energy Center. The plant’s proposed location is shown in the area outlined by the small circle in the upper right of the figure.

[[File:NuecesBayLocation.png|center|600px|thumb|alt=Nueces Bay Desalination Facility Location|Figure A.1: Proposed mapped location of Nueces Bay Desalination Facility.]]
[[File:ClassIWell.png|center|700px|thumb|alt=Nueces Bay Desalination Facility Distance to Injection Well|Figure A.2: Mapped distance from proposed location to Class I Injection Well]]

=Appendix B: Feed and Product Definitions - Salinity=

=Appendix C: Process Flow Diagram=
[[File:TeamF_FinalPFD.JPG|center|1200px|thumb|alt=Process Flow Diagram|Figure C.1: Process Flow Diagram]]

=Appendix D: HYSYS Simulation Model=
[[File:TeamF_HYSYS_Screenshot.PNG|center|1200px|thumb|alt=HYSYS Simulation Model|Figure D.1: Screenshot of HYSYS Model Simulation.]]

=Appendix E: Stream Tables=
See attached Excel File “HYSYS 6.0 Flowsheet summary.xlsx”

A summary of some values is shown below:

<center>
{| class = "wikitable"
|+Table 7: Key process material streams
!  
! Stream  
! Molar Flow (105 kgmole/h)
! Mass Flow (105 kg/h)
|- style="text-align: center;"
! style="text-align: right;" | Inlet
| Feed Water || 454 || 83.6
|- style="text-align: center;"
! style="text-align: right;" | Outlet
| Brine Reject || 1.90 || 36.0
|- style="text-align: center;"
! style="text-align: right;" | Product
| Pure Water || 2.64 || 47.5
|}
</center>

<center>
{| class = "wikitable"
|+Table 8: Key process energy streams
! Stream  
! Heat Flow (kJ/h)
|- style="text-align: center;"
| Feed Water || 1.30E+11
|- style="text-align: center;"
| Q-301 || 5.73E+06
|- style="text-align: center;"
| Q-302 || 3.09E+09
|- style="text-align: center;"
| Q-304 || 4.68E+06
|- style="text-align: center;"
| Q-303 || -1.26E+09
|- style="text-align: center;"
| Brine Reject || -5.47E+10
|- style="text-align: center;"
| Pure Water || -7.46E+10
|}
(Negative value denotes energy leaving process)
</center>

=Appendix F: Heat Exchanger Sizing=

<center>
{| class = "wikitable"
|+Table 9: Utility Temperatures and Heat Transfer Coefficient<ref name="Towler">G.P. Towler, R. Sinnott. Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design. Elsevier, 2012.</ref>
!  
! HP Steam
! Cooling Water
|- style="text-align: center;"
! Temperature (°C)
| 250 || 35
|- style="text-align: center;"
! U (W/m2K)
| 4000 || 1500
|}
</center>

<center>
{| class = "wikitable"
|+Table 10: Internal Flash Heat Exchanger Sizing, condensing steam.
! Exchanger
! UA (kJ/°C-h)
! U (kJ/hr/m2°C)
! A (m2)
|- style="text-align: center;"
| E-101 || 1.47E+07 || 14400 || 1019
|- style="text-align: center;"
| E-102 || 1.53E+07 || 14400 || 1059
|- style="text-align: center;"
| E-103 || 1.59E+07 || 14400 || 1103
|- style="text-align: center;"
| E-104 || 1.66E+07 || 14400 || 1153
|- style="text-align: center;"
| E-105 || 1.74E+07 || 14400 || 1209
|- style="text-align: center;"
| E-106 || 1.83E+07 || 14400 || 1272
|- style="text-align: center;"
| E-107 || 1.93E+07 || 14400 || 1342
|- style="text-align: center;"
| E-108 || 2.05E+07 || 14400 || 1424
|- style="text-align: center;"
| E-109 || 2.19E+07 || 14400 || 1518
|- style="text-align: center;"
| E-110 || 2.35E+07 || 14400 || 1629
|- style="text-align: center;"
| E-111 || 2.54E+07 || 14400 || 1761
|- style="text-align: center;"
| E-112 || 2.77E+07 || 14400 || 1921
|- style="text-align: center;"
| E-113 || 3.05E+07 || 14400 || 2119
|- style="text-align: center;"
| E-114 || 3.42E+07 || 14400 || 2372
|- style="text-align: center;"
| E-115 || 3.90E+07 || 14400 || 2706
|- style="text-align: center;"
| E-116 || 4.56E+07 || 14400 || 3167
|- style="text-align: center;"
| E-117 || 5.55E+07 || 14400 || 3856
|- style="text-align: center;"
| E-118 || 7.20E+07 || 14400 || 5000
|}
</center>

=Appendix G: Pressure Vessel Sizing=

=Appendix H: Expanded Economic Assumptions=

=Appendix I: Equipment Cost Summary=

=Appendix J: Economic NPV Analysis=

=References=
<references/>

Nueces Desalination Center: Production of Drinking Water by Multi-Stage Flash Distillation

2016-03-07T20:13:25Z

Benjkg: /* Appendix C: Process Flow Diagram */

Team F: Rankine 672 Final Report

Authors: Kedric Daly, Ben Granger, Evan Rosati, Kathleen Zhou

Instructors: Fengqi You, David Wegerer

March 11, 2016

=Executive Summary=
Desalination of seawater to drinking water is a technology of ever-increasing importance given trends of increasingly widespread water shortages around the world. Even in the United States, water shortages are becoming increasingly common in dry, highly-populated such as California and certain areas of the Gulf Coast. Our design team, Rankine 672, was tasked with evaluating the need for and designing a desalination plant to provide purified water in a place of need. Investigation revealed that the city of Corpus Christi, Texas, is a strategic location for development of a desalination facility, as surrounding region is in need of new drinking water sources and the city has expressed serious interest in contracting the development of a large-scale desalination plant. Our proposed solution is a multi-stage flash distillation desalination facility capable of producing 20,000,000 gallons (75,708 m3) of purified drinking water for the city and surrounding region per operating day.

In this design, drinking water is produced from seawater feedstock through a 21-stage flash distillation process. By strategically locating the facility next to an existing natural gas power plant, the Nueces Bay Energy Center, pre-heated feed water can be taken directly from the outbound cooling systems of the power plant, allowing energy savings for our desalination plant. The process is designed for 16 hour-per-day operation, requires a seawater feed of 8250 m3/hr, and produces 4758 m3/hr of purified drinking water alongside a waste stream of 3485 m3/hr of concentrated brine. The waste stream is isolated from a recycle stream through a 9.0% purge; this results in an overall product yield of 57.7% on a volume basis. The process requires an additional pretreatment stream of 16.5 kg/hr Belgard EV 2030, which serves as an antiscalant, as well as 9.5 kg/hr chlorine, 352.1 kg/hr lime, and 418.7 kg/hr CO2 for post-treatment purposes.

The overall capital cost of the plant is expected to be $345MM, with a total operating cost of $103MM/yr, $59MM/yr of which comes from utilities. Due to the low price at which the product water can be sold, the 30-year net present value of the plant, assuming a 6% discount rate, is $-1,035MM. It is thus imperative this plant be constructed with public funds because it cannot return a profit to any private investor. The growing need for water however could justify the cost, because it is important that the residents of the greater Corpus Christi area have access to drinkable water.

__TOC__

=Introduction=
==Background==
Clean drinking water is a basic human need that is becoming increasingly difficult to meet. Currently, over 1 billion people live in places where water is considered scarce<ref> Tapping the oceans. The Economist USA. http://www.economist.com/node/11484059. Accessed March 2, 2016. </ref>. Though often thought of as a problem exclusive to third-world countries, the issue is becoming more prevalent in the US as well, with water managers in 40 of 50 states anticipating water shortage conditions in some portion of their state in the next 10 years. These shortages are due to increasing demands on constant or diminishing freshwater resources, therefore, to correct the issue, a new supply must be found<ref> Water Supply in the US. EPA. http://www3.epa.gov/watersense/pubs/supply.html. Accessed March 2, 2016.</ref>. Desalination of seawater is therefore important as a new source for clean drinking water and prevent water shortages.

==Project Definition==
The overall goal of the project is to investigate the feasibility of a desalination plant in order to meet the needs of growing water demand. The design needs to take health, safety, and environmental concerns into account, while optimizing costs.

=Design Basis=
Site Location and Conditions
Corpus Christi, TX was chosen as the location for the proposed desalination plant. The city is currently experiencing “Moderate Shortage Conditions”. This issue additionally comes at a time of growing demand with population growth anticipated due to new industry and Eagle Ford Group crude oil production<ref>Stage 2 Measures in the Drought Contingency Plan. Available at http://www.cctexas.com/government/water/conservation/conservation-drought-plans/stage-2-measures. Accessed January 12, 2016. </ref>.

In June 2014, the city announced the Corpus Christi Desalination Demonstration Project<ref>Corpus Christi Desalination Demonstration Project. Available at http://www.cctexas.com/Assets/Departments/Water/Files/DesalFactSheet.pdf. Published June 2014. Accessed January 12, 2016.
</ref> which seeks to create a demonstration desalination plant capable of producing 20,000 gallons of water per day, with plans to later build a full scale facility generating 20,000,000 gallons of water per day. The desalination demonstration project indicates a clear need for implementation of a desalination plant of this size to accommodate growth in the Corpus Christi area.

Locating our plant in Corpus Christi will allow us to take advantage of the benefits of cogeneration through a partnership with the Nueces Bay Energy Center. By utilizing their outlet cooling water as a large portion of our feed for the desalination process, we can raise the temperature of our seawater feed, saving on energy costs in the multi stage flash distillation. The proposed location for the desalination plant is immediately west of the Nueces Bay Energy Center. See Appendix A for a map depicting the proposed location for the desalination plant along the coast of Nueces Bay.

==Feed and Product Definitions==
The proposed desalination facility’s feed and product streams are defined as shown in Table 1. Rationale for these definitions is also described below. The values reported in Table 1 are final derived numbers from HYSYS simulation modeling.

<center>
{| class = "wikitable"
|+Table 1: Feed and product stream definitions
!  
! Salt Concentration
! Temperature
! Amount
|- style="text-align: center;"
! style="text-align: right;" | Feed Stream
| 30,500 ppm || 45°C || 8250 m3/hr
|- style="text-align: center;"
! style="text-align: right;" | Product Stream
| ~ 0 ppm || 35°C || 4758 m3/hr
|- style="text-align: center;"
! style="text-align: right;" | Byproduct Stream
| 695 ppm || 35°C || 3485 m3/hr
|}
</center>

The process feed will be seawater from Nueces Bay, preheated through the cooling water system of the nearby Nueces Bay Energy Center. Based on calculations developed from NREL (National Renewable Energy Laboratory) data, it was estimated that the Nueces Bay Energy Center uses approximately 250 million gallons of seawater per day for process and cooling needs. Therefore, the 20 million gallon per day requirement for the desalination facility can easily be met through the power plant’s cooling water<ref>Torcellini P, Long N, Judkoff R. Consumptive Water Use for U.S. Power Consumption. NREL. 2003; TP-550-33905.
</ref>. Note that we are assuming the process will operate 67% of the time (16 hours per day).

Based on data acquired from two salinity-measuring stations in Nueces Bay, the average salinity of local seawater is 30.5 practical salinity units (psu)<ref>Stations Map & Real - Time Data. The Conrad Blucher Institute for Surveying and Science website. Available at http://www.cbi.tamucc.edu/cbi/data/. Updated January 2016. Accessed January 9, 2016. </ref>, thus setting the concentration of the process feed stream. This translates to a feed stream concentration of 30,500 ppm (parts per million)<ref>Definition and Units. CATD Salinity Expertise Center. Available at http://www.salinityremotesensing.ifremer.fr/home. Accessed January 12, 2016.</ref>. The composition of the salt attributed to this seawater salinity is shown in Table 6 in Appendix B.

No data on the composition of seawater around the Corpus Christi area was available, but the composition of seawater does not change significantly in different locations. The given composition is considered accurate for our feed stream<ref>Brown E, Colling A, Park D, Phillips J, Rothery D, Wright J. Seawater : Its Composition, Properties and Behaviour. Boston: Butterworth-Heinemann, 1995.
</ref>. Note that seawater salt is primarily sodium chloride - total dissolved solids are 85.7% sodium chloride by mass. As such, the dissolved solids in the feed stream is assumed in this project to be pure NaCl, making the feed composition 96.95% water and 3.05% NaCl by mass. A control system will likely be put in place to ensure feed concentration remains at this level, accounting for natural fluctuations in local seawater salinity.

The largest byproduct from the process will be a brine slurry with high salt concentrations. An upper corrosion limit of 700 ppm was set for recirculated and purged brine. This undesired byproduct will be sent to a nearby Class I non-hazardous injection well facility operated by GNI Group, which is approximately 12 miles away from the proposed desalination plant location as shown in the figure in Appendix A<ref>Texas Water Code. Availabe at http://www.statutes.legis.state.tx.us/Docs/WA/htm/WA.27.htm. Accessed January 14, 2016.
</ref><ref>Google Maps. Available at https://goo.gl/maps/3xcvLZwUCe42 Accessed January 15, 2016.</ref>. Through the incorporation of a Class I injection well, the brine will not have to be treated or diluted before injection. <ref>EPA Regulations on Class I Wells. Available at http://www.epa.gov/uic/class-i-industrial-and-municipal-waste-disposal-wells. Accessed January 14, 2016.
</ref><ref>List of Injection Well Facilities by Location. Available at http://www.ehso.com/cssepa/tsdfdeepwells.php. Accessed January 14, 2016.</ref>

There are no regulations regarding the temperature of city drinking water, so the product stream temperature has no requirements based on regulations. Outlet temperatures of 35oC are acceptable for input to the municipal water supply system. Since city water tends to be stored in non-temperature controlled tanks, the temperature will naturally cool to ambient temperature.<ref>Climate Corpus Christi- Texas. US Climate Data. Available at http://www.usclimatedata.com/climate/corpus-christi/texas/united-states/ustx0294. Accessed January 12, 2016. </ref>

==Pre-Treatment and Post-Treatment==
For pretreatment, Belgard EV 2030 is recommended to be added at a concentration of 1.8-2.0 mg/L for a system with top brine temperature approximately 110 °C<ref name="BWA 2030">BWA Water Additives. Belgard EV2030 Technical Data Sheet. BWA Water Additives. http://www.wateradditives.com/files/products/tech_data_sheets/Belgard%20EV%202030_Technical%20Data%20Sheet_Thermal_8.5x11.pdf. Accessed January 27, 2016.
</ref>. Since our process is designed to operate at a slightly higher top brine temperature, 120 °C, we will design for the upper bound concentration: 2.0 mg/L of antiscalant. With a feed flow rate of 8250 m3/hr, the process therefore requires 16.5 kg/hr of Belgard EV 2030.

For disinfection during post-treatment, chlorine addition typically requires a dose of roughly 2.0 mg/L as well<ref name="World Health Organization">World Health Organization. Desalination for safe water supply. http://www.who.int/water_sanitation_health/gdwqrevision/desalination.pdf. Accessed January 26, 2016.</ref>. With a pure water distillate flow rate of 4758 m3/hr, the process requires 9.5 kg/hr of chlorine as a 5%-15% sodium hypochlorite solution. Approximately 74 mg/L of lime (calcium hydroxide) and 88mg/L of CO2 should be added to the desalinated water to help prevent corrosion and increase alkalinity within acceptable ranges<ref name="Vouchkov">Vouchkov, N. Re-Mineralization of Desalinated Water. SunCam. https://s3.amazonaws.com/suncam/npdocs/118.pdf. Accessed February 18, 2016.</ref>. With a product stream flow rate of 4758 m3/hr, these concentrations correspond to 352.1 kg/hr of lime and 418.7 kg/hr of CO2. Both of these can be purchased, and it is likely that the CO2 can be procured from the neighboring natural gas power plant.

==Choice of Desalination Technology==
The two major technologies commonly implemented for desalination include reverse osmosis (RO), and multi-stage flash distillation (MSFD). RO utilizes high pressures to force water through a semipermeable membrane that allows for purified water to pass through, but stops organic and other wastes. Often, the pressures can reach 55 - 68 bar, which requires large pumps to maintain pressure, and introduces safety concerns to the process<ref>Stage 2 Measures in the Drought Contingency Plan. Available at http://www.cctexas.com/government/water/conservation/conservation-drought-plans/stage-2-measures. Accessed January 12, 2016.
</ref>. While RO can be less energy intensive than MSFD, and the membrane can also remove contaminants, it is susceptible to feedwater quality, membrane fouling, and requires more extensive pretreatment and posttreatment than MSFD. The cost of RO is directly proportional to the feed salinity, and so it is less expensive when desalinating low salinity water.

Multi-stage flash distillation is best selected for areas that have high salinity or low energy costs. The pretreatment process for MSFD is much easier when compared to RO because pretreatment often consists of simply adding anti-corrosives and antiscalants to the feed stream. The process works by heating water in vessels at different pressures to change the boiling point of the water and evaporate it as it passes through the system. After evaporation, the water condenses, leaving pure water as the effluent. MSFD costs are now roughly $1.00/m3, where this cost is mostly independent of salinity, but dependent on energy costs.<ref>Fritzman C, Lowenberg J, Wintgens T, Melin T. State-of-the-art Reverse Osmosis Desalination. Desalination. 2007; 216:1-76.
</ref> In both RO and MSFD a brine is produced which then must be appropriately treated and/or disposed of.

Multi-stage flash distillation was selected for this process, mainly due to the economic advantages offered by the selected location of Corpus Christi, TX. MSFD is also a well understood process that can be more easily modeled by chemical engineering software.

=Technical Approach=
Quantitative modeling of the process and all included optimization was conducted using the simulation software Aspen HYSYS. A variety of assumptions were developed to set up and define specific components of the process simulation itself. Within Aspen HYSYS, the NRTL Electrolyte property package was selected to model the behavior of salt water. Each stage of the MSF process is modeled as an individual flash vessel, with individual shell & tube heat exchangers used to model the distillate condensers present in each flash stage in the MSF system.

The HYSYS simulation model only considers the flash portion of the overall process, allowing establishment of mass and energy balances for the system. Pretreatment and post-treatment were excluded for the HYSYS model, as these steps are simple additions or removals of small concentrations of chemical components. Although the simulation models heat exchangers outside of each flash stage, in the physical design of the plant these heat exchangers will be contained within each flash stage.

One portion of the process is not modeled, which is the inlet seawater entering the heat rejection stages. As can be seen from the full process flow diagram (Appendix C), the inlet seawater in the heat rejection stage is in high excess to provide maximum cooling capacity; then a significant portion is purged to bring the stream flow rate down to what is needed for the feed stream. It is assumed that the flow rate of cooling water for those three stages is in such high excess that the cooling water does not heat up. Therefore, in the process simulation models, the inlet stream is the feed stream, which enters directly into the heat recovery portion of the process.

See Appendix D for a screenshot overview of the simulation environment model.

=Process Overview=
==Process Flowsheet==
The following diagram, Figure 1, shows a basic flowsheet for the process. Process alternatives considered for each stage are discussed below. Please see Appendix C for the full process flowsheet with finalized design decisions.

[[File:BlockFlowDiagram.png|center|700px|thumb|alt=Nueces Bay Desalination Facility Block Flow Diagram|Figure 1: Basic block flowsheet of the multi-stage flash desalination process.]]

==Major Features==
As seen above, there are three primary stages of the desalination process: Pre-treatment, the multi-stage flash distillation, and post-treatment. Seawater enters the pre-treatment phase first to make the feed suitable for the rest of the process by filtering out large particles that could accumulate on pipes, deaerating the water to limit corrosion, and adding an agent to combat fouling. The water then enters the distillation process, which is broken up into two sections: heat recovery and heat rejection.

In the heat recovery stage, there are 18 flash vessels, each of which separates steam from the salinated liquid water. Each flash vessel has saltwater as its inlet and two outlets: a vapor outlet (steam) that condenses into the final product of purified water and a liquid outlet composed of water with more dissolved solids than the inlet, which is the inlet stream for the next flash vessel. The pipe containing the feed is first mixed with recycled brine, then goes through the top sections of each of the flash vessels, starting with V-118 and ending up at V-101. In this way, the feed stream pipe first serves as a heat exchanger, heating the pre-treated seawater while simultaneously condensing the steam into the final product. The feed then enters a final heat exchanger where it is brought up to 103.6 °C before entering V-101 to be flashed.

After exiting V-118, the final flash vessel in the heat recovery portion, the water enters the heat rejection portion, which consists of three flash vessels (V-201, V-202, V-203). The flash vessels of this section operate in the same manner as in the heat recovery section, with the exception that condensation occurs using excess cooling water instead of pre-treated seawater. This technique allows excess heat to be rejected into an excess water stream rather than the feed.

The purified product stream finally enters post-treatment processing in which water is re-aerated and remineralized, disinfected, and pH is adjusted before being delivered to the city. The concentrated brine leftover at the end of the flash cycle is recycled into the feed, although a purge is required to continuously remove some salt from the system. This purged brine is disposed of by use of a Class I injection well, per regional regulations.

See Appendix E to find detailed stream tables of stream specifications.

==Process Alternatives==
===Pretreatment===
Multi-stage flash distillation requires significantly less pretreatment than reverse osmosis desalination, but helpful pretreatment techniques still exist. One very inexpensive method of pretreatment is screening/filtration, which is used to filter out large contaminants from intake seawater. Using more complicated membrane technology to accomplish large particle filtration is a possible alternative, but doing so would be much more expensive than using a standard screen or filter, so the latter choice has been selected for this process.<ref>El-Dessouky H., Alatiqi I, Ettouney H. Process synthesis: The multi-stage flash desalination system. Desalination. 1998;115(2):115-179. doi: 10.1016/S0011-9164(98)00035-6.</ref>

The most critical type of fouling in multi-stage flash distillation is scaling - therefore, the most critical step in the pretreatment process will be the inclusion of an anti-scalant. Alkaline scaling can be prevented by acid dosing or polyphosphate addition; however, acid dosing can result in increased corrosion and polyphosphates can lead to non-alkaline calcium phosphate fouling, so these additives will not be considered<ref name="Morris">Morris, R. The development of the multi-stage flash distillation process: A designer's viewpoint. Desalination. 1993;93(1-3):57-68. doi:10.1016/S0011-9164(93)80096-6. Availble at http://www.sciencedirect.com/science/article/pii/0011916493800966</ref><ref>Al-Gobaisi, D. A quarter-century of seawater desalination by large multistage flash plants in Abu Dhabi (Plant performance analysis, assessment, present efforts toward enhancement and future hopes). Desalination. 1994;99(2-3):509-512. doi: 10.1016/S0011-9164(94)00196-0.</ref>. As such, a polymer-based additive is recommended for the anti-scalant for this process. The Belgard EV series is designed for use in high-temperature seawater MSF; of these, Belgard EV 2030 is the most neutral in pH<ref>NSF International. NSF Product and Service Listings: Drinking Water Treatment Chemicals - Health Effects. NSF International. http://info.nsf.org/Certified/PwsChemicals/Listings.asp?Company=00300&. Accessed January 27, 2016.</ref><ref name="BWA 2030" /><ref>BWA Water Additives. Belgard EV2035 Technical Data Sheet. BWA Water Additives. http://www.wateradditives.com/files/products/tech_data_sheets/Belgard%20EV2035_Technical%20Data%20Sheet_Thermal_8.5x11.pdf. Accessed January 27, 2016.</ref><ref>BioLab Water Additives. Belgard EV Antiscalant for seawater distillation plants. BioLab Water Additives. http://www.lpq.com.mx/pdf/BELGARD%20EV.PDF. Accessed January 27, 2016.</ref>. As such, we have selected Belgard EV 2030 for use as an anti-scalant in our process.

Corrosion control is another significant method of pretreatment in multi-stage flash distillation plants. Using a piping material that has limited susceptibility to corrosion is typically not economically viable for MSF desalination plants due their large size, so using carbon steel for most of the piping is generally used for its economic benefits despite its susceptibility to corrosion<ref name = "Morris" />. Limiting oxygen concentration is very feasible through the use of deaerators, which can remove oxygen as well as other dissolved gases from the seawater stream. Often, an external deaeration system is used<ref name = "Morris" /><ref>Edwards - Hick Hargreaves. Seawater Deaerators. Edwards - Hick Hargreaves. http://psscorp.co.th/upload/news/20110830105340.pdf. Accessed January 26, 2016.</ref>. Given these considerations, using an external deaerator combined with a less-corrosive antiscalant like Belgard EV 2030 is much more practical for the process than using anti-corrosive piping.

Excessive foam can also be an issue in some MSF distillation facilities, so antifoaming agents are sometimes employed in the pretreatment process as well. However, many plants are operated without foaming issues, and therefore do not need anti-foaming pretreatment at all. Selection of an anti-foaming agent must be carefully weighed with selected of anti-scalant to avoid issues with additive interactions [26]. At this stage in the design process, an anti-foamer will not be included due to the risk of decreasing scalant efficacy, and the likelihood that foam will not be an issue in the desalination plant.

===Multistage Flash Distillation===
The main objective of the design project is desalinate water. This is done with the main process of multi stage flash distillation (MSF). MSF can be developed using a numerous number of different process configurations, such as Once-Through, Simple Mixer Brine Circulation, and Multi-Stage Heat Rejection Brine Circulation processes.

The most desired process for the project is the Three Stage Heat Rejection Brine Circulation as it builds on benefits of the other simpler technologies. By combining principles of heat rejection and heat recovery, the required heat transfer area of the process is greatly reduced as compared to once through processes or simple brine recycle processes. It has the highest performance ratio with the lowest specific feed flow rates, therefore greatly lowering the operating costs of the process. It is also known as the industry standard for MSF<ref name = "KSU Chapter 6">Kansas State University ChE 413. Available at: http://faculty.ksu.edu.sa/Almutaz/Documents/ChE-413/Multi-stage%20Flash%20Desalination.pdf. Accessed January 26, 2016.</ref>. In addition, since our brine disposal technique involves the pumping of the brine into a Class I well, the high brine outlet salinity is not a drawback for our process since post treatment or dilution of the brine is not necessary.

===Post Treatment===
After processing the water via multi-stage flash, the water needs to be post-treated to make the water taste as consumers would expect, since distilled water is “flat” and unpleasant, as well as to protect the consumers and the piping through which the water will be passing<ref name="Clayton">Clayton R. A review of current knowledge: Desalination for water supply. Foundation for Water Research. http://www.fwr.org/desal.pdf. Accessed January 26, 2016.</ref>. There are multiple alternatives to achieve these objectives of remineralization, aeration, corrosion control, and disinfection.

Aerating and introducing some dissolved solids back into the product water are effective ways to create a quality product that is not flat, and contains flavoring that is expected of the water when delivered to the consumer. Aeration will be accomplished via an inline spray aerator which will take up less space than cascade aeration.<ref>Minnesota Rural Water Association. Aeration. http://www.mrwa.com/WaterWorksMnl/Chapter%2011%20Aeration.pdf. Accessed January 26, 2016.</ref> Feed blending will be used to remineralize the water as opposed to a remineralization filter because it does not require additional equipment purchases, and can easily handle the large amounts of throughput required for the process.<ref name="Clayton" />

Corrosion and pH can be controlled by adding lime (calcium hydroxide) and carbon dioxide to the water. To do so, about 74 mg/L of lime and 88mg/L of CO2 should be added to the desalinated water to help prevent corrosion and increase alkalinity within acceptable ranges. Sodium hydroxide, soda ash, and sodium bicarbonate are alternatives to altering the alkalinity and pH of the product water, but lime and CO2 is an industry standard <ref name="Vouchkov" />.

Disinfection needs to be done to prevent microbial growth in the desalinated water and ensure the safety of the general public. Chlorine addition via chlorine gas or sodium hypochlorite is the most common and only requires a dose of 1.5 to 2.5 mg/liter. <ref name="World Health Organization" /> Other alternatives include ozonation, UV light, and chloramination. Chlorine, as a 5% - 15% sodium hypochlorite solution, is sufficient for our process and relatively inexpensive, and as such will be utilized in the process.

===Transport of Brine to Well===
Three major possibilities exist for transporting the brine from our desalination plant to the injection well for disposal: rail, trucking, and a pipeline. A pipeline was chosen as the most feasible and cost-effective approach.

A rail line is not available at this time, and with the cost to construct a rail line at approximately $43 million per mile<ref>About Money. How Much Do Rail Transit Projects Cost to Build and Operate? 2014. Available at: http://publictransport.about.com/od/Transit_Projects/a/How-Much-Do-Rail-Transit-Projects-Cost-To-Build-And-Operate.htm. Accessed January 27, 2016. </ref>, the enormous capital cost makes rail transport infeasible for this site. For large volume operations, pipelines are cheaper than trucking. In a study of wastewater sludge produced from wastewater treatment facilities, it was found that it was more economical for facilities producing over 1200 m3 per day to utilize pipelines rather than trucking.<ref>Marufuzzaman M, Eskioglu S, Hernandez R. Truck versus pipeline transportation cost analysis of wastewater sludge. Transportation Research. 2015; 74(A): 14-30. </ref> Our facility is producing nearly 3 times this volume at 3485 m3 per day of brine. Furthermore, brine is a less viscous fluid than wastewater sludge, decreasing pumping costs in the pipeline and further amplifying the benefit of choosing a pipeline over an injection well.

===Brine Storage===
Due to the choice of a pipeline, rather than trucking, as the method of transport of brine to the injection well, brine storage is unnecessary, so storage solutions such as holding tanks were not investigated. One method of storage compatible with a pipeline is to pump the brine through an evaporation pond. This solution is attractive as it would allow the brine to concentrate, saving some costs of pumping, however, the considerable land usage of evaporation ponds leads them to only be effective for smaller plants. The largest municipal plant utilizing this method produces only 5.7 million liters per day<ref>University of Santa Cruz Center for Integrated Water Research. Concentrate Management: Evaporation Ponds. 2010. Available at: https://ciwr.ucsc.edu/sites/default/files/ICM_D5.pdf. Accessed January 27, 2016. </ref>, whereas our desalination plant is proposed to produce 75.7 million liters per day. Therefore, evaporation ponds have been eliminated from consideration.

==Major Specifications==
===Material of Construction Selection===
While saltwater and desalinated water in general can be corrosive, there are best practices within the desalination industry for material selection. With this in mind, best practice shows carbon steel lifetime can be lengthened by monitoring the oxygen concentration in the water, specifying the paint used to coat any metal, and more expensive materials can be used in areas of extremely high corrosion.<ref name = "Morris" />. By following these best practices, it is possible to save significant amounts of money through the use of carbon steel in most areas of the plant, with specific high-corrosion areas being reinforced with, or replaced with, more corrosion resistant materials, such as Cu/Ni 70/30 alloy<ref name = "KSU Chapter 6" />.

===Heat Exchanger Sizing===
See Appendix F for heat exchanger sizing data. Heat exchanger sizes appear reasonable for placement in the flash vessels.

===Flash Vessel Sizing===
The dimensions for the flash drums were obtained through literature values. The flash drums were determined to be 18x4x3 m in width, height, and length.<ref name="KSU Chapter 6" />

Wall thickness was determined using the following conventions: Design pressure is 10% more than the operating pressure, design temperature is 50°F (27.8°C) higher than the operating temperature, a corrosion allowance of 3-mm and an effectiveness of 85% was derived from the assumption of a double-welded butt joint that was spot checked. See Appendix G for wall thicknesses calculated for each pressure vessel.

The sizing of the vessels to be constructed will be determined after talks with a manufacturer. Due to economies of scale, it may be more cost-effective to buy 21 pressure vessels with the dimensions mentioned above and a wall thickness of 17.1-mm, the highest wall thickness required in the system.

===Pipe Losses and Pressure Drop===
HYSYS was used to estimate the diameter of a pipe given a specified pressure drop per 100m of pipe. It was determined that for a pressure drop of 0.1 kPa/100m of pipe, a 3.05m diameter pipe is sufficient, which is more than manageable for manufacturers. This pressure drop is mostly insignificant and will be taken care of with the safety factors included in the pumps. The significant pressure drops in our system is accounted for via losses in the flash vessels and heat exchangers, which drive the flash process. Therefore overall pipe losses are insignificant and are built into intentional design losses.

=Economic Analysis=
==Economic Assumptions==
The sale of purified water yields a revenue stream of $49.5 million per year assuming sale to commercial entities within the city limits of Corpus Christi at $1.7915 per thousand liters.<ref>Water schedule. City of Corpus Christi Utility Business Office. http://www.cctexas.com/Assets/Departments/Financial-Services/Utilities-Business-Office/files/waterrates.pdf Accessed February 28, 2016.</ref> The primary variable costs are utilities and the brine disposal, with minimal raw materials costs due to the primary feedstock, seawater, being free, and minimal amounts of pre- and post-treatment chemicals being used. The cost of brine disposal used in this analysis was high at $2.097 per thousand liters<ref>McCurdy, R. Underground injection wells for produced water disposal. Chesapeake Energy Corporation. http://www.epa.gov/sites/production/files/documents/21_McCurdy_-_UIC_Disposal_508.pdf Accessed February 28, 2016.</ref>, an estimate for brine disposal utilizing trucking as the mode of transport. As has been shown through previous analyses, a lower cost can be achieved using a direct pipeline, however, no good estimates for this cost could be found.

The NPV was calculated using a 6% cost of capital, the approximate interest rate for a municipal bond.<ref>Zirbe, R.O. Jr., Han, X., Layton, D., Leshine, T. A history of discount rates and their use by government agencies. (2002). http://faculty.washington.edu/zerbe/docs/DISCOUNT%20RATE%20HISTORY-latesXIt.doc Accessed February 28, 2016.</ref> It was assumed that the plant can be built within two years, with production at half-scale in its first year. Additionally, one month of variable costs, $10.6 million, was set aside for working capital. See Appendix H for a full table of assumptions. A 30 year NPV calculation was chosen as the decision basis since 30 years is a conservative estimate for the lifetime of a desalination plant [38]. See below for estimates of NPV for the lifetime of the plant under various assumptions and design criteria.

==Optimization and Sensitivity Analysis==
Looking at the multi-stage flash process, there are a number of opportunities to optimize the process. The major process variables that were investigated were the inlet water temperature, the operating pressure drop per vessel, and the number of stages over which the process takes place. Considering these optimizations, our final process consists of 21 flash vessels (18 heat recovery and 3 heat rejection stages), each with a pressure drop of 5 kPa. See Appendix D for a screenshot of the HYSYS simulation used to model the optimized process.
===Operating Pressure and Pressure Drop===
Initially our process was designed to pump the inlet seawater and brine to a pressure of 2 bar and allow the pressure drop in each flash stage to end our process downstream at 1 bar to prevent complications associated with designing a system under vacuum. While this provided pumping advantages, the energy in the system was significantly higher due to vapor pressure considerations. Due to this fact, the required heat exchange area for certain exchangers proved to be infeasible (on the order of 80,000m2) due to the enormous capital and operating costs that would be associated with such a large heat exchanger.

The process was optimized to operate under vacuum, requiring additional pumping requirements but much lower required heat exchange area, and therefore lower utility costs. This optimization was done by specifying the pressure of the final flash vessel to 5.7kPa, so that the temperature of the water vapor (~30oC) is within approximately 5oC of the cooling water (35oC) so that condensation can occur within a reasonable approach. This also allows our system to optimally extract pure water from the brine running through the system and reduce energy requirements for cooling the product or the recycle. Our system redesign and optimization reduced the energy in the system and reduce the same heat exchanger area requirements by more than 3 orders of magnitude to approximately 800m2.

Once the process was optimized to operate in a vacuum, the pressure drop per flash stage regulated through values was then optimized. Three pressure drops per stages, 3kPa, 5kPa, and 7kPa, were tested and then an economic analysis was conducted to find the optimal value for the pressure drop. These changes had impacts on both operating and capital costs as they would change initial pumping requirements, the amount of water flowing through the process and recycle (and therefore the flash vessel sizes, heat exchanger sizing), and the efficiency of each stage.

The various pressure drops (3 kPa, 5 kPa, and 7 kPa) were tested on a system nominally at 24 stages under the assumption that the results would be generalizable to systems with a different number of stages. With increasing pressure drop, we found decreased capital cost but increased utility cost. As can be seen from Table 2 below, a pressure drop of 5 kPa proved to be the best option based on 30 year NPV, with a loss of $838.8 million, a savings of $65.5 million compared to the next-best option with a pressure drop of 7 kPa.

<center>
{| class = "wikitable"
|+Table 2: Comparison of profitability for our MSF desalination process with 24 stages with 3 different pressure drops per stage
! Number of Stages  
! Pressure Drop (kPa)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
! 30-Year NPV ($MM)
! NPV Ratio (over 5kPa)
|- style="text-align: center;"
| 24 || 3 || 362.3 || 39.1 || -905.8 || 1.08
|- style="text-align: center;"
| 24 || 5 || 255.1 || 53.6 || -838.8 || 1.00
|- style="text-align: center;"
| 24 || 7 || 209.5 || 72.2 || -904.3 || 1.08
|}
</center>

===Number of Flash Stages===
The number of stages is an important area for optimization. An increased number of stages yields better recovery of purified water, however, requires higher capital cost from the additional pressure vessel. Furthermore, with an increased amount of purified water comes increasingly concentrated brine. This must be taken into account since any brine concentration in excess of about 7% presents a more significant corrosion risk<ref name="KSU Chapter 6" />, so increased maintenance costs for the piping or higher capital costs for more corrosion-resistant materials could be necessary for brine concentrations exceeding 7%. Therefore, to recover the same amount of pure water without exceeding corrosion limitations, increasing the number of stages will reduce the overall size of each stage.

Optimizing based on NPV, it was found that 21 stages is optimal, balancing the increased profits from the additional purified water additional costs of higher capital cost. See Table 3 below to compare the 30-year NPV of the process across various numbers of flash vessels.

<center>
{| class = "wikitable"
|+Table 3: Comparison of profitability for our MSF desalination process with a 5 kPa pressure drop, 5000 m3/hr feed flow rate, with varying number of stages.
! Number of Stages  
! Pressure Drop (kPa)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
! 30-Year NPV ($MM)
! NPV Ratio (over 21 Stages)
|- style="text-align: center;"
| 17 || 5 || 213.9 || 58.7 || -813.8 || 1.016
|- style="text-align: center;"
| 18 || 5 || 218 || 57.6 || -809.9 || 1.011
|- style="text-align: center;"
| 19 || 5 || 224.3 || 58 || -806.1 || 1.007
|- style="text-align: center;"
| 20 || 5 || 229.8 || 57.9 || -813.6 || 1.016
|- style="text-align: center;"
| 21 || 5 || 226.6 || 55.9 || -800.7 || 1
|- style="text-align: center;"
| 22 || 5 || 254.7 || 54.6 || -849.5 || 1.061
|- style="text-align: center;"
| 23 || 5 || 249.3 || 54.1 || -833.1 || 1.04
|- style="text-align: center;"
| 24 || 5 || 255.1 || 53.6 || -838.8 || 1.048
|- style="text-align: center;"
| 25 || 5 || 254.9 || 52.7 || -837.4 || 1.046
|}
</center>

===Inlet Seawater Temperature===
By pairing our desalination plant with a natural gas power plant and utilizing its cooling water we forgo initial heating of our inlet stream. Normally, water pumped from the ocean would be approximately 25oC and by capturing waste heat from the power plant we can reduce utilities with our water inlet of 45oC. Through an economic comparison between two plants nominally at 21 flash stages, the plant with an inlet temperature at 25oC has a capital cost of $245MM and a utility cost of $77MM/yr and the same facility with an inlet water temperature at 45oC has a capital cost of only $226MM and a utility cost of $56MM/yr. As shown in Table 4 below, this $19MM savings in capital costs and $21MM/yr saving in utilities is largely due to reducing the size of the heat exchangers and lowering the steam required for heating duty on the inlet seawater respectively. Therefore our process is optimized by recovering the waste heat water from the gas fired power plant.

<center>
{| class = "wikitable"
|+Table 4: Seawater Inlet Temperature Process Cost Optimization
! Seawater Temperature (°C)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
|- style="text-align: center;"
| 25 || 245 || 77
|- style="text-align: center;"
| 45 || 226 || 56
|- style="text-align: center;"
| Savings || 56 || 19
|}
</center>

==Overall Process Economics==
After optimization at fixed flow rates, the process was scaled to meet the the 67% operating percentage and final product definitions as shown in Table 1, which resulted in finalized economics shown in Table 5 below. For a table of the installed costs of each process vessel, see Appendix I. For the finalized NPV calculations, see Appendix J.

<center>
{| class = "wikitable"
|+Table 5: Summarized Economics for 21-stage MSFD Process
! Economic Parameter
! Cost (USD)
|- style="text-align: center;"
| Capital Cost || 345 MM
|- style="text-align: center;"
| Total Operating Cost per Year || 103 MM
|- style="text-align: center;"
| Utility Cost per Year || 59 MM
|- style="text-align: center;"
| 30-Year NPV (6% Discount) || -1035 MM
|}
</center>

=Conclusions and Recommendations=
==Project Viability==
Table 5 above demonstrates that the process has reasonable operating and capital costs, but the 30-year NPV is negative by -1035 $MM due to the extremely low sell price capability of desalinated water as a product. Therefore, this plant would have to heavily be supported by the City of Corpus Christi to run as the product revenue will not make it self-sufficient. The acquisition of government subsidies are key operating requirements for all desalination facilities in the United States.

==Additional Concerns==
Large volumes of water, steam, and energy are needed to ensure the project runs as specified. While most of the water can easily be extracted from the ocean with little to consequence, the fuel used to generate the steam and electricity can have a significant environmental impact. As the plant is located next to a natural gas power plant, this fuel is better than coal with respect to environmental concerns, and utilizing energy from the power plant to preheat the feed stream for the desalination plant helps lower environmental impacts.

The large amounts of high pressure steam being used represents one of the the largest safety concern in the process, because a high pressure steam release could easily cause severe burns or even death. Proper precautions and control measures must be in place to prevent accidental release. Otherwise, the process does not utilize any known toxic chemicals and is relatively safe from a chemical standpoint. The flash vessels mostly operate below atmospheric pressure, so they must be constructed as vessels that can withstand vacuum without failing.

=Appendix A: Proposed Location of Desalination Plant=
Pictured below is the proposed location for the desalination plant, located on Nueces Bay in Corpus Christi, adjacent to the Nueces Bay Energy Center. The plant’s proposed location is shown in the area outlined by the small circle in the upper right of the figure.

[[File:NuecesBayLocation.png|center|600px|thumb|alt=Nueces Bay Desalination Facility Location|Figure A.1: Proposed mapped location of Nueces Bay Desalination Facility.]]
[[File:ClassIWell.png|center|700px|thumb|alt=Nueces Bay Desalination Facility Distance to Injection Well|Figure A.2: Mapped distance from proposed location to Class I Injection Well]]

=Appendix B: Feed and Product Definitions - Salinity=

=Appendix C: Process Flow Diagram=
[[File:TeamF_FinalPFD.jpg|center|1200px|thumb|alt=Process Flow Diagram|Figure C.1: Process Flow Diagram]]

=Appendix D: HYSYS Simulation Model=
[[File:TeamF_HYSYS_Screenshot.PNG|center|1200px|thumb|alt=HYSYS Simulation Model|Figure D.1: Screenshot of HYSYS Model Simulation.]]

=Appendix E: Stream Tables=
See attached Excel File “HYSYS 6.0 Flowsheet summary.xlsx”

A summary of some values is shown below:

<center>
{| class = "wikitable"
|+Table 7: Key process material streams
!  
! Stream  
! Molar Flow (105 kgmole/h)
! Mass Flow (105 kg/h)
|- style="text-align: center;"
! style="text-align: right;" | Inlet
| Feed Water || 454 || 83.6
|- style="text-align: center;"
! style="text-align: right;" | Outlet
| Brine Reject || 1.90 || 36.0
|- style="text-align: center;"
! style="text-align: right;" | Product
| Pure Water || 2.64 || 47.5
|}
</center>

<center>
{| class = "wikitable"
|+Table 8: Key process energy streams
! Stream  
! Heat Flow (kJ/h)
|- style="text-align: center;"
| Feed Water || 1.30E+11
|- style="text-align: center;"
| Q-301 || 5.73E+06
|- style="text-align: center;"
| Q-302 || 3.09E+09
|- style="text-align: center;"
| Q-304 || 4.68E+06
|- style="text-align: center;"
| Q-303 || -1.26E+09
|- style="text-align: center;"
| Brine Reject || -5.47E+10
|- style="text-align: center;"
| Pure Water || -7.46E+10
|}
(Negative value denotes energy leaving process)
</center>

=Appendix F: Heat Exchanger Sizing=

<center>
{| class = "wikitable"
|+Table 9: Utility Temperatures and Heat Transfer Coefficient<ref name="Towler">G.P. Towler, R. Sinnott. Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design. Elsevier, 2012.</ref>
!  
! HP Steam
! Cooling Water
|- style="text-align: center;"
! Temperature (°C)
| 250 || 35
|- style="text-align: center;"
! U (W/m2K)
| 4000 || 1500
|}
</center>

<center>
{| class = "wikitable"
|+Table 10: Internal Flash Heat Exchanger Sizing, condensing steam.
! Exchanger
! UA (kJ/°C-h)
! U (kJ/hr/m2°C)
! A (m2)
|- style="text-align: center;"
| E-101 || 1.47E+07 || 14400 || 1019
|- style="text-align: center;"
| E-102 || 1.53E+07 || 14400 || 1059
|- style="text-align: center;"
| E-103 || 1.59E+07 || 14400 || 1103
|- style="text-align: center;"
| E-104 || 1.66E+07 || 14400 || 1153
|- style="text-align: center;"
| E-105 || 1.74E+07 || 14400 || 1209
|- style="text-align: center;"
| E-106 || 1.83E+07 || 14400 || 1272
|- style="text-align: center;"
| E-107 || 1.93E+07 || 14400 || 1342
|- style="text-align: center;"
| E-108 || 2.05E+07 || 14400 || 1424
|- style="text-align: center;"
| E-109 || 2.19E+07 || 14400 || 1518
|- style="text-align: center;"
| E-110 || 2.35E+07 || 14400 || 1629
|- style="text-align: center;"
| E-111 || 2.54E+07 || 14400 || 1761
|- style="text-align: center;"
| E-112 || 2.77E+07 || 14400 || 1921
|- style="text-align: center;"
| E-113 || 3.05E+07 || 14400 || 2119
|- style="text-align: center;"
| E-114 || 3.42E+07 || 14400 || 2372
|- style="text-align: center;"
| E-115 || 3.90E+07 || 14400 || 2706
|- style="text-align: center;"
| E-116 || 4.56E+07 || 14400 || 3167
|- style="text-align: center;"
| E-117 || 5.55E+07 || 14400 || 3856
|- style="text-align: center;"
| E-118 || 7.20E+07 || 14400 || 5000
|}
</center>

=Appendix G: Pressure Vessel Sizing=

=Appendix H: Expanded Economic Assumptions=

=Appendix I: Equipment Cost Summary=

=Appendix J: Economic NPV Analysis=

=References=
<references/>

Nueces Desalination Center: Production of Drinking Water by Multi-Stage Flash Distillation

2016-03-07T20:12:29Z

Benjkg: /* Appendix C: Process Flow Diagram */

Team F: Rankine 672 Final Report

Authors: Kedric Daly, Ben Granger, Evan Rosati, Kathleen Zhou

Instructors: Fengqi You, David Wegerer

March 11, 2016

=Executive Summary=
Desalination of seawater to drinking water is a technology of ever-increasing importance given trends of increasingly widespread water shortages around the world. Even in the United States, water shortages are becoming increasingly common in dry, highly-populated such as California and certain areas of the Gulf Coast. Our design team, Rankine 672, was tasked with evaluating the need for and designing a desalination plant to provide purified water in a place of need. Investigation revealed that the city of Corpus Christi, Texas, is a strategic location for development of a desalination facility, as surrounding region is in need of new drinking water sources and the city has expressed serious interest in contracting the development of a large-scale desalination plant. Our proposed solution is a multi-stage flash distillation desalination facility capable of producing 20,000,000 gallons (75,708 m3) of purified drinking water for the city and surrounding region per operating day.

In this design, drinking water is produced from seawater feedstock through a 21-stage flash distillation process. By strategically locating the facility next to an existing natural gas power plant, the Nueces Bay Energy Center, pre-heated feed water can be taken directly from the outbound cooling systems of the power plant, allowing energy savings for our desalination plant. The process is designed for 16 hour-per-day operation, requires a seawater feed of 8250 m3/hr, and produces 4758 m3/hr of purified drinking water alongside a waste stream of 3485 m3/hr of concentrated brine. The waste stream is isolated from a recycle stream through a 9.0% purge; this results in an overall product yield of 57.7% on a volume basis. The process requires an additional pretreatment stream of 16.5 kg/hr Belgard EV 2030, which serves as an antiscalant, as well as 9.5 kg/hr chlorine, 352.1 kg/hr lime, and 418.7 kg/hr CO2 for post-treatment purposes.

The overall capital cost of the plant is expected to be $345MM, with a total operating cost of $103MM/yr, $59MM/yr of which comes from utilities. Due to the low price at which the product water can be sold, the 30-year net present value of the plant, assuming a 6% discount rate, is $-1,035MM. It is thus imperative this plant be constructed with public funds because it cannot return a profit to any private investor. The growing need for water however could justify the cost, because it is important that the residents of the greater Corpus Christi area have access to drinkable water.

__TOC__

=Introduction=
==Background==
Clean drinking water is a basic human need that is becoming increasingly difficult to meet. Currently, over 1 billion people live in places where water is considered scarce<ref> Tapping the oceans. The Economist USA. http://www.economist.com/node/11484059. Accessed March 2, 2016. </ref>. Though often thought of as a problem exclusive to third-world countries, the issue is becoming more prevalent in the US as well, with water managers in 40 of 50 states anticipating water shortage conditions in some portion of their state in the next 10 years. These shortages are due to increasing demands on constant or diminishing freshwater resources, therefore, to correct the issue, a new supply must be found<ref> Water Supply in the US. EPA. http://www3.epa.gov/watersense/pubs/supply.html. Accessed March 2, 2016.</ref>. Desalination of seawater is therefore important as a new source for clean drinking water and prevent water shortages.

==Project Definition==
The overall goal of the project is to investigate the feasibility of a desalination plant in order to meet the needs of growing water demand. The design needs to take health, safety, and environmental concerns into account, while optimizing costs.

=Design Basis=
Site Location and Conditions
Corpus Christi, TX was chosen as the location for the proposed desalination plant. The city is currently experiencing “Moderate Shortage Conditions”. This issue additionally comes at a time of growing demand with population growth anticipated due to new industry and Eagle Ford Group crude oil production<ref>Stage 2 Measures in the Drought Contingency Plan. Available at http://www.cctexas.com/government/water/conservation/conservation-drought-plans/stage-2-measures. Accessed January 12, 2016. </ref>.

In June 2014, the city announced the Corpus Christi Desalination Demonstration Project<ref>Corpus Christi Desalination Demonstration Project. Available at http://www.cctexas.com/Assets/Departments/Water/Files/DesalFactSheet.pdf. Published June 2014. Accessed January 12, 2016.
</ref> which seeks to create a demonstration desalination plant capable of producing 20,000 gallons of water per day, with plans to later build a full scale facility generating 20,000,000 gallons of water per day. The desalination demonstration project indicates a clear need for implementation of a desalination plant of this size to accommodate growth in the Corpus Christi area.

Locating our plant in Corpus Christi will allow us to take advantage of the benefits of cogeneration through a partnership with the Nueces Bay Energy Center. By utilizing their outlet cooling water as a large portion of our feed for the desalination process, we can raise the temperature of our seawater feed, saving on energy costs in the multi stage flash distillation. The proposed location for the desalination plant is immediately west of the Nueces Bay Energy Center. See Appendix A for a map depicting the proposed location for the desalination plant along the coast of Nueces Bay.

==Feed and Product Definitions==
The proposed desalination facility’s feed and product streams are defined as shown in Table 1. Rationale for these definitions is also described below. The values reported in Table 1 are final derived numbers from HYSYS simulation modeling.

<center>
{| class = "wikitable"
|+Table 1: Feed and product stream definitions
!  
! Salt Concentration
! Temperature
! Amount
|- style="text-align: center;"
! style="text-align: right;" | Feed Stream
| 30,500 ppm || 45°C || 8250 m3/hr
|- style="text-align: center;"
! style="text-align: right;" | Product Stream
| ~ 0 ppm || 35°C || 4758 m3/hr
|- style="text-align: center;"
! style="text-align: right;" | Byproduct Stream
| 695 ppm || 35°C || 3485 m3/hr
|}
</center>

The process feed will be seawater from Nueces Bay, preheated through the cooling water system of the nearby Nueces Bay Energy Center. Based on calculations developed from NREL (National Renewable Energy Laboratory) data, it was estimated that the Nueces Bay Energy Center uses approximately 250 million gallons of seawater per day for process and cooling needs. Therefore, the 20 million gallon per day requirement for the desalination facility can easily be met through the power plant’s cooling water<ref>Torcellini P, Long N, Judkoff R. Consumptive Water Use for U.S. Power Consumption. NREL. 2003; TP-550-33905.
</ref>. Note that we are assuming the process will operate 67% of the time (16 hours per day).

Based on data acquired from two salinity-measuring stations in Nueces Bay, the average salinity of local seawater is 30.5 practical salinity units (psu)<ref>Stations Map & Real - Time Data. The Conrad Blucher Institute for Surveying and Science website. Available at http://www.cbi.tamucc.edu/cbi/data/. Updated January 2016. Accessed January 9, 2016. </ref>, thus setting the concentration of the process feed stream. This translates to a feed stream concentration of 30,500 ppm (parts per million)<ref>Definition and Units. CATD Salinity Expertise Center. Available at http://www.salinityremotesensing.ifremer.fr/home. Accessed January 12, 2016.</ref>. The composition of the salt attributed to this seawater salinity is shown in Table 6 in Appendix B.

No data on the composition of seawater around the Corpus Christi area was available, but the composition of seawater does not change significantly in different locations. The given composition is considered accurate for our feed stream<ref>Brown E, Colling A, Park D, Phillips J, Rothery D, Wright J. Seawater : Its Composition, Properties and Behaviour. Boston: Butterworth-Heinemann, 1995.
</ref>. Note that seawater salt is primarily sodium chloride - total dissolved solids are 85.7% sodium chloride by mass. As such, the dissolved solids in the feed stream is assumed in this project to be pure NaCl, making the feed composition 96.95% water and 3.05% NaCl by mass. A control system will likely be put in place to ensure feed concentration remains at this level, accounting for natural fluctuations in local seawater salinity.

The largest byproduct from the process will be a brine slurry with high salt concentrations. An upper corrosion limit of 700 ppm was set for recirculated and purged brine. This undesired byproduct will be sent to a nearby Class I non-hazardous injection well facility operated by GNI Group, which is approximately 12 miles away from the proposed desalination plant location as shown in the figure in Appendix A<ref>Texas Water Code. Availabe at http://www.statutes.legis.state.tx.us/Docs/WA/htm/WA.27.htm. Accessed January 14, 2016.
</ref><ref>Google Maps. Available at https://goo.gl/maps/3xcvLZwUCe42 Accessed January 15, 2016.</ref>. Through the incorporation of a Class I injection well, the brine will not have to be treated or diluted before injection. <ref>EPA Regulations on Class I Wells. Available at http://www.epa.gov/uic/class-i-industrial-and-municipal-waste-disposal-wells. Accessed January 14, 2016.
</ref><ref>List of Injection Well Facilities by Location. Available at http://www.ehso.com/cssepa/tsdfdeepwells.php. Accessed January 14, 2016.</ref>

There are no regulations regarding the temperature of city drinking water, so the product stream temperature has no requirements based on regulations. Outlet temperatures of 35oC are acceptable for input to the municipal water supply system. Since city water tends to be stored in non-temperature controlled tanks, the temperature will naturally cool to ambient temperature.<ref>Climate Corpus Christi- Texas. US Climate Data. Available at http://www.usclimatedata.com/climate/corpus-christi/texas/united-states/ustx0294. Accessed January 12, 2016. </ref>

==Pre-Treatment and Post-Treatment==
For pretreatment, Belgard EV 2030 is recommended to be added at a concentration of 1.8-2.0 mg/L for a system with top brine temperature approximately 110 °C<ref name="BWA 2030">BWA Water Additives. Belgard EV2030 Technical Data Sheet. BWA Water Additives. http://www.wateradditives.com/files/products/tech_data_sheets/Belgard%20EV%202030_Technical%20Data%20Sheet_Thermal_8.5x11.pdf. Accessed January 27, 2016.
</ref>. Since our process is designed to operate at a slightly higher top brine temperature, 120 °C, we will design for the upper bound concentration: 2.0 mg/L of antiscalant. With a feed flow rate of 8250 m3/hr, the process therefore requires 16.5 kg/hr of Belgard EV 2030.

For disinfection during post-treatment, chlorine addition typically requires a dose of roughly 2.0 mg/L as well<ref name="World Health Organization">World Health Organization. Desalination for safe water supply. http://www.who.int/water_sanitation_health/gdwqrevision/desalination.pdf. Accessed January 26, 2016.</ref>. With a pure water distillate flow rate of 4758 m3/hr, the process requires 9.5 kg/hr of chlorine as a 5%-15% sodium hypochlorite solution. Approximately 74 mg/L of lime (calcium hydroxide) and 88mg/L of CO2 should be added to the desalinated water to help prevent corrosion and increase alkalinity within acceptable ranges<ref name="Vouchkov">Vouchkov, N. Re-Mineralization of Desalinated Water. SunCam. https://s3.amazonaws.com/suncam/npdocs/118.pdf. Accessed February 18, 2016.</ref>. With a product stream flow rate of 4758 m3/hr, these concentrations correspond to 352.1 kg/hr of lime and 418.7 kg/hr of CO2. Both of these can be purchased, and it is likely that the CO2 can be procured from the neighboring natural gas power plant.

==Choice of Desalination Technology==
The two major technologies commonly implemented for desalination include reverse osmosis (RO), and multi-stage flash distillation (MSFD). RO utilizes high pressures to force water through a semipermeable membrane that allows for purified water to pass through, but stops organic and other wastes. Often, the pressures can reach 55 - 68 bar, which requires large pumps to maintain pressure, and introduces safety concerns to the process<ref>Stage 2 Measures in the Drought Contingency Plan. Available at http://www.cctexas.com/government/water/conservation/conservation-drought-plans/stage-2-measures. Accessed January 12, 2016.
</ref>. While RO can be less energy intensive than MSFD, and the membrane can also remove contaminants, it is susceptible to feedwater quality, membrane fouling, and requires more extensive pretreatment and posttreatment than MSFD. The cost of RO is directly proportional to the feed salinity, and so it is less expensive when desalinating low salinity water.

Multi-stage flash distillation is best selected for areas that have high salinity or low energy costs. The pretreatment process for MSFD is much easier when compared to RO because pretreatment often consists of simply adding anti-corrosives and antiscalants to the feed stream. The process works by heating water in vessels at different pressures to change the boiling point of the water and evaporate it as it passes through the system. After evaporation, the water condenses, leaving pure water as the effluent. MSFD costs are now roughly $1.00/m3, where this cost is mostly independent of salinity, but dependent on energy costs.<ref>Fritzman C, Lowenberg J, Wintgens T, Melin T. State-of-the-art Reverse Osmosis Desalination. Desalination. 2007; 216:1-76.
</ref> In both RO and MSFD a brine is produced which then must be appropriately treated and/or disposed of.

Multi-stage flash distillation was selected for this process, mainly due to the economic advantages offered by the selected location of Corpus Christi, TX. MSFD is also a well understood process that can be more easily modeled by chemical engineering software.

=Technical Approach=
Quantitative modeling of the process and all included optimization was conducted using the simulation software Aspen HYSYS. A variety of assumptions were developed to set up and define specific components of the process simulation itself. Within Aspen HYSYS, the NRTL Electrolyte property package was selected to model the behavior of salt water. Each stage of the MSF process is modeled as an individual flash vessel, with individual shell & tube heat exchangers used to model the distillate condensers present in each flash stage in the MSF system.

The HYSYS simulation model only considers the flash portion of the overall process, allowing establishment of mass and energy balances for the system. Pretreatment and post-treatment were excluded for the HYSYS model, as these steps are simple additions or removals of small concentrations of chemical components. Although the simulation models heat exchangers outside of each flash stage, in the physical design of the plant these heat exchangers will be contained within each flash stage.

One portion of the process is not modeled, which is the inlet seawater entering the heat rejection stages. As can be seen from the full process flow diagram (Appendix C), the inlet seawater in the heat rejection stage is in high excess to provide maximum cooling capacity; then a significant portion is purged to bring the stream flow rate down to what is needed for the feed stream. It is assumed that the flow rate of cooling water for those three stages is in such high excess that the cooling water does not heat up. Therefore, in the process simulation models, the inlet stream is the feed stream, which enters directly into the heat recovery portion of the process.

See Appendix D for a screenshot overview of the simulation environment model.

=Process Overview=
==Process Flowsheet==
The following diagram, Figure 1, shows a basic flowsheet for the process. Process alternatives considered for each stage are discussed below. Please see Appendix C for the full process flowsheet with finalized design decisions.

[[File:BlockFlowDiagram.png|center|700px|thumb|alt=Nueces Bay Desalination Facility Block Flow Diagram|Figure 1: Basic block flowsheet of the multi-stage flash desalination process.]]

==Major Features==
As seen above, there are three primary stages of the desalination process: Pre-treatment, the multi-stage flash distillation, and post-treatment. Seawater enters the pre-treatment phase first to make the feed suitable for the rest of the process by filtering out large particles that could accumulate on pipes, deaerating the water to limit corrosion, and adding an agent to combat fouling. The water then enters the distillation process, which is broken up into two sections: heat recovery and heat rejection.

In the heat recovery stage, there are 18 flash vessels, each of which separates steam from the salinated liquid water. Each flash vessel has saltwater as its inlet and two outlets: a vapor outlet (steam) that condenses into the final product of purified water and a liquid outlet composed of water with more dissolved solids than the inlet, which is the inlet stream for the next flash vessel. The pipe containing the feed is first mixed with recycled brine, then goes through the top sections of each of the flash vessels, starting with V-118 and ending up at V-101. In this way, the feed stream pipe first serves as a heat exchanger, heating the pre-treated seawater while simultaneously condensing the steam into the final product. The feed then enters a final heat exchanger where it is brought up to 103.6 °C before entering V-101 to be flashed.

After exiting V-118, the final flash vessel in the heat recovery portion, the water enters the heat rejection portion, which consists of three flash vessels (V-201, V-202, V-203). The flash vessels of this section operate in the same manner as in the heat recovery section, with the exception that condensation occurs using excess cooling water instead of pre-treated seawater. This technique allows excess heat to be rejected into an excess water stream rather than the feed.

The purified product stream finally enters post-treatment processing in which water is re-aerated and remineralized, disinfected, and pH is adjusted before being delivered to the city. The concentrated brine leftover at the end of the flash cycle is recycled into the feed, although a purge is required to continuously remove some salt from the system. This purged brine is disposed of by use of a Class I injection well, per regional regulations.

See Appendix E to find detailed stream tables of stream specifications.

==Process Alternatives==
===Pretreatment===
Multi-stage flash distillation requires significantly less pretreatment than reverse osmosis desalination, but helpful pretreatment techniques still exist. One very inexpensive method of pretreatment is screening/filtration, which is used to filter out large contaminants from intake seawater. Using more complicated membrane technology to accomplish large particle filtration is a possible alternative, but doing so would be much more expensive than using a standard screen or filter, so the latter choice has been selected for this process.<ref>El-Dessouky H., Alatiqi I, Ettouney H. Process synthesis: The multi-stage flash desalination system. Desalination. 1998;115(2):115-179. doi: 10.1016/S0011-9164(98)00035-6.</ref>

The most critical type of fouling in multi-stage flash distillation is scaling - therefore, the most critical step in the pretreatment process will be the inclusion of an anti-scalant. Alkaline scaling can be prevented by acid dosing or polyphosphate addition; however, acid dosing can result in increased corrosion and polyphosphates can lead to non-alkaline calcium phosphate fouling, so these additives will not be considered<ref name="Morris">Morris, R. The development of the multi-stage flash distillation process: A designer's viewpoint. Desalination. 1993;93(1-3):57-68. doi:10.1016/S0011-9164(93)80096-6. Availble at http://www.sciencedirect.com/science/article/pii/0011916493800966</ref><ref>Al-Gobaisi, D. A quarter-century of seawater desalination by large multistage flash plants in Abu Dhabi (Plant performance analysis, assessment, present efforts toward enhancement and future hopes). Desalination. 1994;99(2-3):509-512. doi: 10.1016/S0011-9164(94)00196-0.</ref>. As such, a polymer-based additive is recommended for the anti-scalant for this process. The Belgard EV series is designed for use in high-temperature seawater MSF; of these, Belgard EV 2030 is the most neutral in pH<ref>NSF International. NSF Product and Service Listings: Drinking Water Treatment Chemicals - Health Effects. NSF International. http://info.nsf.org/Certified/PwsChemicals/Listings.asp?Company=00300&. Accessed January 27, 2016.</ref><ref name="BWA 2030" /><ref>BWA Water Additives. Belgard EV2035 Technical Data Sheet. BWA Water Additives. http://www.wateradditives.com/files/products/tech_data_sheets/Belgard%20EV2035_Technical%20Data%20Sheet_Thermal_8.5x11.pdf. Accessed January 27, 2016.</ref><ref>BioLab Water Additives. Belgard EV Antiscalant for seawater distillation plants. BioLab Water Additives. http://www.lpq.com.mx/pdf/BELGARD%20EV.PDF. Accessed January 27, 2016.</ref>. As such, we have selected Belgard EV 2030 for use as an anti-scalant in our process.

Corrosion control is another significant method of pretreatment in multi-stage flash distillation plants. Using a piping material that has limited susceptibility to corrosion is typically not economically viable for MSF desalination plants due their large size, so using carbon steel for most of the piping is generally used for its economic benefits despite its susceptibility to corrosion<ref name = "Morris" />. Limiting oxygen concentration is very feasible through the use of deaerators, which can remove oxygen as well as other dissolved gases from the seawater stream. Often, an external deaeration system is used<ref name = "Morris" /><ref>Edwards - Hick Hargreaves. Seawater Deaerators. Edwards - Hick Hargreaves. http://psscorp.co.th/upload/news/20110830105340.pdf. Accessed January 26, 2016.</ref>. Given these considerations, using an external deaerator combined with a less-corrosive antiscalant like Belgard EV 2030 is much more practical for the process than using anti-corrosive piping.

Excessive foam can also be an issue in some MSF distillation facilities, so antifoaming agents are sometimes employed in the pretreatment process as well. However, many plants are operated without foaming issues, and therefore do not need anti-foaming pretreatment at all. Selection of an anti-foaming agent must be carefully weighed with selected of anti-scalant to avoid issues with additive interactions [26]. At this stage in the design process, an anti-foamer will not be included due to the risk of decreasing scalant efficacy, and the likelihood that foam will not be an issue in the desalination plant.

===Multistage Flash Distillation===
The main objective of the design project is desalinate water. This is done with the main process of multi stage flash distillation (MSF). MSF can be developed using a numerous number of different process configurations, such as Once-Through, Simple Mixer Brine Circulation, and Multi-Stage Heat Rejection Brine Circulation processes.

The most desired process for the project is the Three Stage Heat Rejection Brine Circulation as it builds on benefits of the other simpler technologies. By combining principles of heat rejection and heat recovery, the required heat transfer area of the process is greatly reduced as compared to once through processes or simple brine recycle processes. It has the highest performance ratio with the lowest specific feed flow rates, therefore greatly lowering the operating costs of the process. It is also known as the industry standard for MSF<ref name = "KSU Chapter 6">Kansas State University ChE 413. Available at: http://faculty.ksu.edu.sa/Almutaz/Documents/ChE-413/Multi-stage%20Flash%20Desalination.pdf. Accessed January 26, 2016.</ref>. In addition, since our brine disposal technique involves the pumping of the brine into a Class I well, the high brine outlet salinity is not a drawback for our process since post treatment or dilution of the brine is not necessary.

===Post Treatment===
After processing the water via multi-stage flash, the water needs to be post-treated to make the water taste as consumers would expect, since distilled water is “flat” and unpleasant, as well as to protect the consumers and the piping through which the water will be passing<ref name="Clayton">Clayton R. A review of current knowledge: Desalination for water supply. Foundation for Water Research. http://www.fwr.org/desal.pdf. Accessed January 26, 2016.</ref>. There are multiple alternatives to achieve these objectives of remineralization, aeration, corrosion control, and disinfection.

Aerating and introducing some dissolved solids back into the product water are effective ways to create a quality product that is not flat, and contains flavoring that is expected of the water when delivered to the consumer. Aeration will be accomplished via an inline spray aerator which will take up less space than cascade aeration.<ref>Minnesota Rural Water Association. Aeration. http://www.mrwa.com/WaterWorksMnl/Chapter%2011%20Aeration.pdf. Accessed January 26, 2016.</ref> Feed blending will be used to remineralize the water as opposed to a remineralization filter because it does not require additional equipment purchases, and can easily handle the large amounts of throughput required for the process.<ref name="Clayton" />

Corrosion and pH can be controlled by adding lime (calcium hydroxide) and carbon dioxide to the water. To do so, about 74 mg/L of lime and 88mg/L of CO2 should be added to the desalinated water to help prevent corrosion and increase alkalinity within acceptable ranges. Sodium hydroxide, soda ash, and sodium bicarbonate are alternatives to altering the alkalinity and pH of the product water, but lime and CO2 is an industry standard <ref name="Vouchkov" />.

Disinfection needs to be done to prevent microbial growth in the desalinated water and ensure the safety of the general public. Chlorine addition via chlorine gas or sodium hypochlorite is the most common and only requires a dose of 1.5 to 2.5 mg/liter. <ref name="World Health Organization" /> Other alternatives include ozonation, UV light, and chloramination. Chlorine, as a 5% - 15% sodium hypochlorite solution, is sufficient for our process and relatively inexpensive, and as such will be utilized in the process.

===Transport of Brine to Well===
Three major possibilities exist for transporting the brine from our desalination plant to the injection well for disposal: rail, trucking, and a pipeline. A pipeline was chosen as the most feasible and cost-effective approach.

A rail line is not available at this time, and with the cost to construct a rail line at approximately $43 million per mile<ref>About Money. How Much Do Rail Transit Projects Cost to Build and Operate? 2014. Available at: http://publictransport.about.com/od/Transit_Projects/a/How-Much-Do-Rail-Transit-Projects-Cost-To-Build-And-Operate.htm. Accessed January 27, 2016. </ref>, the enormous capital cost makes rail transport infeasible for this site. For large volume operations, pipelines are cheaper than trucking. In a study of wastewater sludge produced from wastewater treatment facilities, it was found that it was more economical for facilities producing over 1200 m3 per day to utilize pipelines rather than trucking.<ref>Marufuzzaman M, Eskioglu S, Hernandez R. Truck versus pipeline transportation cost analysis of wastewater sludge. Transportation Research. 2015; 74(A): 14-30. </ref> Our facility is producing nearly 3 times this volume at 3485 m3 per day of brine. Furthermore, brine is a less viscous fluid than wastewater sludge, decreasing pumping costs in the pipeline and further amplifying the benefit of choosing a pipeline over an injection well.

===Brine Storage===
Due to the choice of a pipeline, rather than trucking, as the method of transport of brine to the injection well, brine storage is unnecessary, so storage solutions such as holding tanks were not investigated. One method of storage compatible with a pipeline is to pump the brine through an evaporation pond. This solution is attractive as it would allow the brine to concentrate, saving some costs of pumping, however, the considerable land usage of evaporation ponds leads them to only be effective for smaller plants. The largest municipal plant utilizing this method produces only 5.7 million liters per day<ref>University of Santa Cruz Center for Integrated Water Research. Concentrate Management: Evaporation Ponds. 2010. Available at: https://ciwr.ucsc.edu/sites/default/files/ICM_D5.pdf. Accessed January 27, 2016. </ref>, whereas our desalination plant is proposed to produce 75.7 million liters per day. Therefore, evaporation ponds have been eliminated from consideration.

==Major Specifications==
===Material of Construction Selection===
While saltwater and desalinated water in general can be corrosive, there are best practices within the desalination industry for material selection. With this in mind, best practice shows carbon steel lifetime can be lengthened by monitoring the oxygen concentration in the water, specifying the paint used to coat any metal, and more expensive materials can be used in areas of extremely high corrosion.<ref name = "Morris" />. By following these best practices, it is possible to save significant amounts of money through the use of carbon steel in most areas of the plant, with specific high-corrosion areas being reinforced with, or replaced with, more corrosion resistant materials, such as Cu/Ni 70/30 alloy<ref name = "KSU Chapter 6" />.

===Heat Exchanger Sizing===
See Appendix F for heat exchanger sizing data. Heat exchanger sizes appear reasonable for placement in the flash vessels.

===Flash Vessel Sizing===
The dimensions for the flash drums were obtained through literature values. The flash drums were determined to be 18x4x3 m in width, height, and length.<ref name="KSU Chapter 6" />

Wall thickness was determined using the following conventions: Design pressure is 10% more than the operating pressure, design temperature is 50°F (27.8°C) higher than the operating temperature, a corrosion allowance of 3-mm and an effectiveness of 85% was derived from the assumption of a double-welded butt joint that was spot checked. See Appendix G for wall thicknesses calculated for each pressure vessel.

The sizing of the vessels to be constructed will be determined after talks with a manufacturer. Due to economies of scale, it may be more cost-effective to buy 21 pressure vessels with the dimensions mentioned above and a wall thickness of 17.1-mm, the highest wall thickness required in the system.

===Pipe Losses and Pressure Drop===
HYSYS was used to estimate the diameter of a pipe given a specified pressure drop per 100m of pipe. It was determined that for a pressure drop of 0.1 kPa/100m of pipe, a 3.05m diameter pipe is sufficient, which is more than manageable for manufacturers. This pressure drop is mostly insignificant and will be taken care of with the safety factors included in the pumps. The significant pressure drops in our system is accounted for via losses in the flash vessels and heat exchangers, which drive the flash process. Therefore overall pipe losses are insignificant and are built into intentional design losses.

=Economic Analysis=
==Economic Assumptions==
The sale of purified water yields a revenue stream of $49.5 million per year assuming sale to commercial entities within the city limits of Corpus Christi at $1.7915 per thousand liters.<ref>Water schedule. City of Corpus Christi Utility Business Office. http://www.cctexas.com/Assets/Departments/Financial-Services/Utilities-Business-Office/files/waterrates.pdf Accessed February 28, 2016.</ref> The primary variable costs are utilities and the brine disposal, with minimal raw materials costs due to the primary feedstock, seawater, being free, and minimal amounts of pre- and post-treatment chemicals being used. The cost of brine disposal used in this analysis was high at $2.097 per thousand liters<ref>McCurdy, R. Underground injection wells for produced water disposal. Chesapeake Energy Corporation. http://www.epa.gov/sites/production/files/documents/21_McCurdy_-_UIC_Disposal_508.pdf Accessed February 28, 2016.</ref>, an estimate for brine disposal utilizing trucking as the mode of transport. As has been shown through previous analyses, a lower cost can be achieved using a direct pipeline, however, no good estimates for this cost could be found.

The NPV was calculated using a 6% cost of capital, the approximate interest rate for a municipal bond.<ref>Zirbe, R.O. Jr., Han, X., Layton, D., Leshine, T. A history of discount rates and their use by government agencies. (2002). http://faculty.washington.edu/zerbe/docs/DISCOUNT%20RATE%20HISTORY-latesXIt.doc Accessed February 28, 2016.</ref> It was assumed that the plant can be built within two years, with production at half-scale in its first year. Additionally, one month of variable costs, $10.6 million, was set aside for working capital. See Appendix H for a full table of assumptions. A 30 year NPV calculation was chosen as the decision basis since 30 years is a conservative estimate for the lifetime of a desalination plant [38]. See below for estimates of NPV for the lifetime of the plant under various assumptions and design criteria.

==Optimization and Sensitivity Analysis==
Looking at the multi-stage flash process, there are a number of opportunities to optimize the process. The major process variables that were investigated were the inlet water temperature, the operating pressure drop per vessel, and the number of stages over which the process takes place. Considering these optimizations, our final process consists of 21 flash vessels (18 heat recovery and 3 heat rejection stages), each with a pressure drop of 5 kPa. See Appendix D for a screenshot of the HYSYS simulation used to model the optimized process.
===Operating Pressure and Pressure Drop===
Initially our process was designed to pump the inlet seawater and brine to a pressure of 2 bar and allow the pressure drop in each flash stage to end our process downstream at 1 bar to prevent complications associated with designing a system under vacuum. While this provided pumping advantages, the energy in the system was significantly higher due to vapor pressure considerations. Due to this fact, the required heat exchange area for certain exchangers proved to be infeasible (on the order of 80,000m2) due to the enormous capital and operating costs that would be associated with such a large heat exchanger.

The process was optimized to operate under vacuum, requiring additional pumping requirements but much lower required heat exchange area, and therefore lower utility costs. This optimization was done by specifying the pressure of the final flash vessel to 5.7kPa, so that the temperature of the water vapor (~30oC) is within approximately 5oC of the cooling water (35oC) so that condensation can occur within a reasonable approach. This also allows our system to optimally extract pure water from the brine running through the system and reduce energy requirements for cooling the product or the recycle. Our system redesign and optimization reduced the energy in the system and reduce the same heat exchanger area requirements by more than 3 orders of magnitude to approximately 800m2.

Once the process was optimized to operate in a vacuum, the pressure drop per flash stage regulated through values was then optimized. Three pressure drops per stages, 3kPa, 5kPa, and 7kPa, were tested and then an economic analysis was conducted to find the optimal value for the pressure drop. These changes had impacts on both operating and capital costs as they would change initial pumping requirements, the amount of water flowing through the process and recycle (and therefore the flash vessel sizes, heat exchanger sizing), and the efficiency of each stage.

The various pressure drops (3 kPa, 5 kPa, and 7 kPa) were tested on a system nominally at 24 stages under the assumption that the results would be generalizable to systems with a different number of stages. With increasing pressure drop, we found decreased capital cost but increased utility cost. As can be seen from Table 2 below, a pressure drop of 5 kPa proved to be the best option based on 30 year NPV, with a loss of $838.8 million, a savings of $65.5 million compared to the next-best option with a pressure drop of 7 kPa.

<center>
{| class = "wikitable"
|+Table 2: Comparison of profitability for our MSF desalination process with 24 stages with 3 different pressure drops per stage
! Number of Stages  
! Pressure Drop (kPa)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
! 30-Year NPV ($MM)
! NPV Ratio (over 5kPa)
|- style="text-align: center;"
| 24 || 3 || 362.3 || 39.1 || -905.8 || 1.08
|- style="text-align: center;"
| 24 || 5 || 255.1 || 53.6 || -838.8 || 1.00
|- style="text-align: center;"
| 24 || 7 || 209.5 || 72.2 || -904.3 || 1.08
|}
</center>

===Number of Flash Stages===
The number of stages is an important area for optimization. An increased number of stages yields better recovery of purified water, however, requires higher capital cost from the additional pressure vessel. Furthermore, with an increased amount of purified water comes increasingly concentrated brine. This must be taken into account since any brine concentration in excess of about 7% presents a more significant corrosion risk<ref name="KSU Chapter 6" />, so increased maintenance costs for the piping or higher capital costs for more corrosion-resistant materials could be necessary for brine concentrations exceeding 7%. Therefore, to recover the same amount of pure water without exceeding corrosion limitations, increasing the number of stages will reduce the overall size of each stage.

Optimizing based on NPV, it was found that 21 stages is optimal, balancing the increased profits from the additional purified water additional costs of higher capital cost. See Table 3 below to compare the 30-year NPV of the process across various numbers of flash vessels.

<center>
{| class = "wikitable"
|+Table 3: Comparison of profitability for our MSF desalination process with a 5 kPa pressure drop, 5000 m3/hr feed flow rate, with varying number of stages.
! Number of Stages  
! Pressure Drop (kPa)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
! 30-Year NPV ($MM)
! NPV Ratio (over 21 Stages)
|- style="text-align: center;"
| 17 || 5 || 213.9 || 58.7 || -813.8 || 1.016
|- style="text-align: center;"
| 18 || 5 || 218 || 57.6 || -809.9 || 1.011
|- style="text-align: center;"
| 19 || 5 || 224.3 || 58 || -806.1 || 1.007
|- style="text-align: center;"
| 20 || 5 || 229.8 || 57.9 || -813.6 || 1.016
|- style="text-align: center;"
| 21 || 5 || 226.6 || 55.9 || -800.7 || 1
|- style="text-align: center;"
| 22 || 5 || 254.7 || 54.6 || -849.5 || 1.061
|- style="text-align: center;"
| 23 || 5 || 249.3 || 54.1 || -833.1 || 1.04
|- style="text-align: center;"
| 24 || 5 || 255.1 || 53.6 || -838.8 || 1.048
|- style="text-align: center;"
| 25 || 5 || 254.9 || 52.7 || -837.4 || 1.046
|}
</center>

===Inlet Seawater Temperature===
By pairing our desalination plant with a natural gas power plant and utilizing its cooling water we forgo initial heating of our inlet stream. Normally, water pumped from the ocean would be approximately 25oC and by capturing waste heat from the power plant we can reduce utilities with our water inlet of 45oC. Through an economic comparison between two plants nominally at 21 flash stages, the plant with an inlet temperature at 25oC has a capital cost of $245MM and a utility cost of $77MM/yr and the same facility with an inlet water temperature at 45oC has a capital cost of only $226MM and a utility cost of $56MM/yr. As shown in Table 4 below, this $19MM savings in capital costs and $21MM/yr saving in utilities is largely due to reducing the size of the heat exchangers and lowering the steam required for heating duty on the inlet seawater respectively. Therefore our process is optimized by recovering the waste heat water from the gas fired power plant.

<center>
{| class = "wikitable"
|+Table 4: Seawater Inlet Temperature Process Cost Optimization
! Seawater Temperature (°C)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
|- style="text-align: center;"
| 25 || 245 || 77
|- style="text-align: center;"
| 45 || 226 || 56
|- style="text-align: center;"
| Savings || 56 || 19
|}
</center>

==Overall Process Economics==
After optimization at fixed flow rates, the process was scaled to meet the the 67% operating percentage and final product definitions as shown in Table 1, which resulted in finalized economics shown in Table 5 below. For a table of the installed costs of each process vessel, see Appendix I. For the finalized NPV calculations, see Appendix J.

<center>
{| class = "wikitable"
|+Table 5: Summarized Economics for 21-stage MSFD Process
! Economic Parameter
! Cost (USD)
|- style="text-align: center;"
| Capital Cost || 345 MM
|- style="text-align: center;"
| Total Operating Cost per Year || 103 MM
|- style="text-align: center;"
| Utility Cost per Year || 59 MM
|- style="text-align: center;"
| 30-Year NPV (6% Discount) || -1035 MM
|}
</center>

=Conclusions and Recommendations=
==Project Viability==
Table 5 above demonstrates that the process has reasonable operating and capital costs, but the 30-year NPV is negative by -1035 $MM due to the extremely low sell price capability of desalinated water as a product. Therefore, this plant would have to heavily be supported by the City of Corpus Christi to run as the product revenue will not make it self-sufficient. The acquisition of government subsidies are key operating requirements for all desalination facilities in the United States.

==Additional Concerns==
Large volumes of water, steam, and energy are needed to ensure the project runs as specified. While most of the water can easily be extracted from the ocean with little to consequence, the fuel used to generate the steam and electricity can have a significant environmental impact. As the plant is located next to a natural gas power plant, this fuel is better than coal with respect to environmental concerns, and utilizing energy from the power plant to preheat the feed stream for the desalination plant helps lower environmental impacts.

The large amounts of high pressure steam being used represents one of the the largest safety concern in the process, because a high pressure steam release could easily cause severe burns or even death. Proper precautions and control measures must be in place to prevent accidental release. Otherwise, the process does not utilize any known toxic chemicals and is relatively safe from a chemical standpoint. The flash vessels mostly operate below atmospheric pressure, so they must be constructed as vessels that can withstand vacuum without failing.

=Appendix A: Proposed Location of Desalination Plant=
Pictured below is the proposed location for the desalination plant, located on Nueces Bay in Corpus Christi, adjacent to the Nueces Bay Energy Center. The plant’s proposed location is shown in the area outlined by the small circle in the upper right of the figure.

[[File:NuecesBayLocation.png|center|600px|thumb|alt=Nueces Bay Desalination Facility Location|Figure A.1: Proposed mapped location of Nueces Bay Desalination Facility.]]
[[File:ClassIWell.png|center|700px|thumb|alt=Nueces Bay Desalination Facility Distance to Injection Well|Figure A.2: Mapped distance from proposed location to Class I Injection Well]]

=Appendix B: Feed and Product Definitions - Salinity=

=Appendix C: Process Flow Diagram=
[[File:TeamF_FinalPFD.png|center|1200px|thumb|alt=Process Flow Diagram|Figure C.1: Process Flow Diagram]]

=Appendix D: HYSYS Simulation Model=
[[File:TeamF_HYSYS_Screenshot.PNG|center|1200px|thumb|alt=HYSYS Simulation Model|Figure D.1: Screenshot of HYSYS Model Simulation.]]

=Appendix E: Stream Tables=
See attached Excel File “HYSYS 6.0 Flowsheet summary.xlsx”

A summary of some values is shown below:

<center>
{| class = "wikitable"
|+Table 7: Key process material streams
!  
! Stream  
! Molar Flow (105 kgmole/h)
! Mass Flow (105 kg/h)
|- style="text-align: center;"
! style="text-align: right;" | Inlet
| Feed Water || 454 || 83.6
|- style="text-align: center;"
! style="text-align: right;" | Outlet
| Brine Reject || 1.90 || 36.0
|- style="text-align: center;"
! style="text-align: right;" | Product
| Pure Water || 2.64 || 47.5
|}
</center>

<center>
{| class = "wikitable"
|+Table 8: Key process energy streams
! Stream  
! Heat Flow (kJ/h)
|- style="text-align: center;"
| Feed Water || 1.30E+11
|- style="text-align: center;"
| Q-301 || 5.73E+06
|- style="text-align: center;"
| Q-302 || 3.09E+09
|- style="text-align: center;"
| Q-304 || 4.68E+06
|- style="text-align: center;"
| Q-303 || -1.26E+09
|- style="text-align: center;"
| Brine Reject || -5.47E+10
|- style="text-align: center;"
| Pure Water || -7.46E+10
|}
(Negative value denotes energy leaving process)
</center>

=Appendix F: Heat Exchanger Sizing=

<center>
{| class = "wikitable"
|+Table 9: Utility Temperatures and Heat Transfer Coefficient<ref name="Towler">G.P. Towler, R. Sinnott. Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design. Elsevier, 2012.</ref>
!  
! HP Steam
! Cooling Water
|- style="text-align: center;"
! Temperature (°C)
| 250 || 35
|- style="text-align: center;"
! U (W/m2K)
| 4000 || 1500
|}
</center>

<center>
{| class = "wikitable"
|+Table 10: Internal Flash Heat Exchanger Sizing, condensing steam.
! Exchanger
! UA (kJ/°C-h)
! U (kJ/hr/m2°C)
! A (m2)
|- style="text-align: center;"
| E-101 || 1.47E+07 || 14400 || 1019
|- style="text-align: center;"
| E-102 || 1.53E+07 || 14400 || 1059
|- style="text-align: center;"
| E-103 || 1.59E+07 || 14400 || 1103
|- style="text-align: center;"
| E-104 || 1.66E+07 || 14400 || 1153
|- style="text-align: center;"
| E-105 || 1.74E+07 || 14400 || 1209
|- style="text-align: center;"
| E-106 || 1.83E+07 || 14400 || 1272
|- style="text-align: center;"
| E-107 || 1.93E+07 || 14400 || 1342
|- style="text-align: center;"
| E-108 || 2.05E+07 || 14400 || 1424
|- style="text-align: center;"
| E-109 || 2.19E+07 || 14400 || 1518
|- style="text-align: center;"
| E-110 || 2.35E+07 || 14400 || 1629
|- style="text-align: center;"
| E-111 || 2.54E+07 || 14400 || 1761
|- style="text-align: center;"
| E-112 || 2.77E+07 || 14400 || 1921
|- style="text-align: center;"
| E-113 || 3.05E+07 || 14400 || 2119
|- style="text-align: center;"
| E-114 || 3.42E+07 || 14400 || 2372
|- style="text-align: center;"
| E-115 || 3.90E+07 || 14400 || 2706
|- style="text-align: center;"
| E-116 || 4.56E+07 || 14400 || 3167
|- style="text-align: center;"
| E-117 || 5.55E+07 || 14400 || 3856
|- style="text-align: center;"
| E-118 || 7.20E+07 || 14400 || 5000
|}
</center>

=Appendix G: Pressure Vessel Sizing=

=Appendix H: Expanded Economic Assumptions=

=Appendix I: Equipment Cost Summary=

=Appendix J: Economic NPV Analysis=

=References=
<references/>

Nueces Desalination Center: Production of Drinking Water by Multi-Stage Flash Distillation

2016-03-07T20:11:48Z

Benjkg: /* Appendix C: Process Flow Diagram */

Team F: Rankine 672 Final Report

Authors: Kedric Daly, Ben Granger, Evan Rosati, Kathleen Zhou

Instructors: Fengqi You, David Wegerer

March 11, 2016

=Executive Summary=
Desalination of seawater to drinking water is a technology of ever-increasing importance given trends of increasingly widespread water shortages around the world. Even in the United States, water shortages are becoming increasingly common in dry, highly-populated such as California and certain areas of the Gulf Coast. Our design team, Rankine 672, was tasked with evaluating the need for and designing a desalination plant to provide purified water in a place of need. Investigation revealed that the city of Corpus Christi, Texas, is a strategic location for development of a desalination facility, as surrounding region is in need of new drinking water sources and the city has expressed serious interest in contracting the development of a large-scale desalination plant. Our proposed solution is a multi-stage flash distillation desalination facility capable of producing 20,000,000 gallons (75,708 m3) of purified drinking water for the city and surrounding region per operating day.

In this design, drinking water is produced from seawater feedstock through a 21-stage flash distillation process. By strategically locating the facility next to an existing natural gas power plant, the Nueces Bay Energy Center, pre-heated feed water can be taken directly from the outbound cooling systems of the power plant, allowing energy savings for our desalination plant. The process is designed for 16 hour-per-day operation, requires a seawater feed of 8250 m3/hr, and produces 4758 m3/hr of purified drinking water alongside a waste stream of 3485 m3/hr of concentrated brine. The waste stream is isolated from a recycle stream through a 9.0% purge; this results in an overall product yield of 57.7% on a volume basis. The process requires an additional pretreatment stream of 16.5 kg/hr Belgard EV 2030, which serves as an antiscalant, as well as 9.5 kg/hr chlorine, 352.1 kg/hr lime, and 418.7 kg/hr CO2 for post-treatment purposes.

The overall capital cost of the plant is expected to be $345MM, with a total operating cost of $103MM/yr, $59MM/yr of which comes from utilities. Due to the low price at which the product water can be sold, the 30-year net present value of the plant, assuming a 6% discount rate, is $-1,035MM. It is thus imperative this plant be constructed with public funds because it cannot return a profit to any private investor. The growing need for water however could justify the cost, because it is important that the residents of the greater Corpus Christi area have access to drinkable water.

__TOC__

=Introduction=
==Background==
Clean drinking water is a basic human need that is becoming increasingly difficult to meet. Currently, over 1 billion people live in places where water is considered scarce<ref> Tapping the oceans. The Economist USA. http://www.economist.com/node/11484059. Accessed March 2, 2016. </ref>. Though often thought of as a problem exclusive to third-world countries, the issue is becoming more prevalent in the US as well, with water managers in 40 of 50 states anticipating water shortage conditions in some portion of their state in the next 10 years. These shortages are due to increasing demands on constant or diminishing freshwater resources, therefore, to correct the issue, a new supply must be found<ref> Water Supply in the US. EPA. http://www3.epa.gov/watersense/pubs/supply.html. Accessed March 2, 2016.</ref>. Desalination of seawater is therefore important as a new source for clean drinking water and prevent water shortages.

==Project Definition==
The overall goal of the project is to investigate the feasibility of a desalination plant in order to meet the needs of growing water demand. The design needs to take health, safety, and environmental concerns into account, while optimizing costs.

=Design Basis=
Site Location and Conditions
Corpus Christi, TX was chosen as the location for the proposed desalination plant. The city is currently experiencing “Moderate Shortage Conditions”. This issue additionally comes at a time of growing demand with population growth anticipated due to new industry and Eagle Ford Group crude oil production<ref>Stage 2 Measures in the Drought Contingency Plan. Available at http://www.cctexas.com/government/water/conservation/conservation-drought-plans/stage-2-measures. Accessed January 12, 2016. </ref>.

In June 2014, the city announced the Corpus Christi Desalination Demonstration Project<ref>Corpus Christi Desalination Demonstration Project. Available at http://www.cctexas.com/Assets/Departments/Water/Files/DesalFactSheet.pdf. Published June 2014. Accessed January 12, 2016.
</ref> which seeks to create a demonstration desalination plant capable of producing 20,000 gallons of water per day, with plans to later build a full scale facility generating 20,000,000 gallons of water per day. The desalination demonstration project indicates a clear need for implementation of a desalination plant of this size to accommodate growth in the Corpus Christi area.

Locating our plant in Corpus Christi will allow us to take advantage of the benefits of cogeneration through a partnership with the Nueces Bay Energy Center. By utilizing their outlet cooling water as a large portion of our feed for the desalination process, we can raise the temperature of our seawater feed, saving on energy costs in the multi stage flash distillation. The proposed location for the desalination plant is immediately west of the Nueces Bay Energy Center. See Appendix A for a map depicting the proposed location for the desalination plant along the coast of Nueces Bay.

==Feed and Product Definitions==
The proposed desalination facility’s feed and product streams are defined as shown in Table 1. Rationale for these definitions is also described below. The values reported in Table 1 are final derived numbers from HYSYS simulation modeling.

<center>
{| class = "wikitable"
|+Table 1: Feed and product stream definitions
!  
! Salt Concentration
! Temperature
! Amount
|- style="text-align: center;"
! style="text-align: right;" | Feed Stream
| 30,500 ppm || 45°C || 8250 m3/hr
|- style="text-align: center;"
! style="text-align: right;" | Product Stream
| ~ 0 ppm || 35°C || 4758 m3/hr
|- style="text-align: center;"
! style="text-align: right;" | Byproduct Stream
| 695 ppm || 35°C || 3485 m3/hr
|}
</center>

The process feed will be seawater from Nueces Bay, preheated through the cooling water system of the nearby Nueces Bay Energy Center. Based on calculations developed from NREL (National Renewable Energy Laboratory) data, it was estimated that the Nueces Bay Energy Center uses approximately 250 million gallons of seawater per day for process and cooling needs. Therefore, the 20 million gallon per day requirement for the desalination facility can easily be met through the power plant’s cooling water<ref>Torcellini P, Long N, Judkoff R. Consumptive Water Use for U.S. Power Consumption. NREL. 2003; TP-550-33905.
</ref>. Note that we are assuming the process will operate 67% of the time (16 hours per day).

Based on data acquired from two salinity-measuring stations in Nueces Bay, the average salinity of local seawater is 30.5 practical salinity units (psu)<ref>Stations Map & Real - Time Data. The Conrad Blucher Institute for Surveying and Science website. Available at http://www.cbi.tamucc.edu/cbi/data/. Updated January 2016. Accessed January 9, 2016. </ref>, thus setting the concentration of the process feed stream. This translates to a feed stream concentration of 30,500 ppm (parts per million)<ref>Definition and Units. CATD Salinity Expertise Center. Available at http://www.salinityremotesensing.ifremer.fr/home. Accessed January 12, 2016.</ref>. The composition of the salt attributed to this seawater salinity is shown in Table 6 in Appendix B.

No data on the composition of seawater around the Corpus Christi area was available, but the composition of seawater does not change significantly in different locations. The given composition is considered accurate for our feed stream<ref>Brown E, Colling A, Park D, Phillips J, Rothery D, Wright J. Seawater : Its Composition, Properties and Behaviour. Boston: Butterworth-Heinemann, 1995.
</ref>. Note that seawater salt is primarily sodium chloride - total dissolved solids are 85.7% sodium chloride by mass. As such, the dissolved solids in the feed stream is assumed in this project to be pure NaCl, making the feed composition 96.95% water and 3.05% NaCl by mass. A control system will likely be put in place to ensure feed concentration remains at this level, accounting for natural fluctuations in local seawater salinity.

The largest byproduct from the process will be a brine slurry with high salt concentrations. An upper corrosion limit of 700 ppm was set for recirculated and purged brine. This undesired byproduct will be sent to a nearby Class I non-hazardous injection well facility operated by GNI Group, which is approximately 12 miles away from the proposed desalination plant location as shown in the figure in Appendix A<ref>Texas Water Code. Availabe at http://www.statutes.legis.state.tx.us/Docs/WA/htm/WA.27.htm. Accessed January 14, 2016.
</ref><ref>Google Maps. Available at https://goo.gl/maps/3xcvLZwUCe42 Accessed January 15, 2016.</ref>. Through the incorporation of a Class I injection well, the brine will not have to be treated or diluted before injection. <ref>EPA Regulations on Class I Wells. Available at http://www.epa.gov/uic/class-i-industrial-and-municipal-waste-disposal-wells. Accessed January 14, 2016.
</ref><ref>List of Injection Well Facilities by Location. Available at http://www.ehso.com/cssepa/tsdfdeepwells.php. Accessed January 14, 2016.</ref>

There are no regulations regarding the temperature of city drinking water, so the product stream temperature has no requirements based on regulations. Outlet temperatures of 35oC are acceptable for input to the municipal water supply system. Since city water tends to be stored in non-temperature controlled tanks, the temperature will naturally cool to ambient temperature.<ref>Climate Corpus Christi- Texas. US Climate Data. Available at http://www.usclimatedata.com/climate/corpus-christi/texas/united-states/ustx0294. Accessed January 12, 2016. </ref>

==Pre-Treatment and Post-Treatment==
For pretreatment, Belgard EV 2030 is recommended to be added at a concentration of 1.8-2.0 mg/L for a system with top brine temperature approximately 110 °C<ref name="BWA 2030">BWA Water Additives. Belgard EV2030 Technical Data Sheet. BWA Water Additives. http://www.wateradditives.com/files/products/tech_data_sheets/Belgard%20EV%202030_Technical%20Data%20Sheet_Thermal_8.5x11.pdf. Accessed January 27, 2016.
</ref>. Since our process is designed to operate at a slightly higher top brine temperature, 120 °C, we will design for the upper bound concentration: 2.0 mg/L of antiscalant. With a feed flow rate of 8250 m3/hr, the process therefore requires 16.5 kg/hr of Belgard EV 2030.

For disinfection during post-treatment, chlorine addition typically requires a dose of roughly 2.0 mg/L as well<ref name="World Health Organization">World Health Organization. Desalination for safe water supply. http://www.who.int/water_sanitation_health/gdwqrevision/desalination.pdf. Accessed January 26, 2016.</ref>. With a pure water distillate flow rate of 4758 m3/hr, the process requires 9.5 kg/hr of chlorine as a 5%-15% sodium hypochlorite solution. Approximately 74 mg/L of lime (calcium hydroxide) and 88mg/L of CO2 should be added to the desalinated water to help prevent corrosion and increase alkalinity within acceptable ranges<ref name="Vouchkov">Vouchkov, N. Re-Mineralization of Desalinated Water. SunCam. https://s3.amazonaws.com/suncam/npdocs/118.pdf. Accessed February 18, 2016.</ref>. With a product stream flow rate of 4758 m3/hr, these concentrations correspond to 352.1 kg/hr of lime and 418.7 kg/hr of CO2. Both of these can be purchased, and it is likely that the CO2 can be procured from the neighboring natural gas power plant.

==Choice of Desalination Technology==
The two major technologies commonly implemented for desalination include reverse osmosis (RO), and multi-stage flash distillation (MSFD). RO utilizes high pressures to force water through a semipermeable membrane that allows for purified water to pass through, but stops organic and other wastes. Often, the pressures can reach 55 - 68 bar, which requires large pumps to maintain pressure, and introduces safety concerns to the process<ref>Stage 2 Measures in the Drought Contingency Plan. Available at http://www.cctexas.com/government/water/conservation/conservation-drought-plans/stage-2-measures. Accessed January 12, 2016.
</ref>. While RO can be less energy intensive than MSFD, and the membrane can also remove contaminants, it is susceptible to feedwater quality, membrane fouling, and requires more extensive pretreatment and posttreatment than MSFD. The cost of RO is directly proportional to the feed salinity, and so it is less expensive when desalinating low salinity water.

Multi-stage flash distillation is best selected for areas that have high salinity or low energy costs. The pretreatment process for MSFD is much easier when compared to RO because pretreatment often consists of simply adding anti-corrosives and antiscalants to the feed stream. The process works by heating water in vessels at different pressures to change the boiling point of the water and evaporate it as it passes through the system. After evaporation, the water condenses, leaving pure water as the effluent. MSFD costs are now roughly $1.00/m3, where this cost is mostly independent of salinity, but dependent on energy costs.<ref>Fritzman C, Lowenberg J, Wintgens T, Melin T. State-of-the-art Reverse Osmosis Desalination. Desalination. 2007; 216:1-76.
</ref> In both RO and MSFD a brine is produced which then must be appropriately treated and/or disposed of.

Multi-stage flash distillation was selected for this process, mainly due to the economic advantages offered by the selected location of Corpus Christi, TX. MSFD is also a well understood process that can be more easily modeled by chemical engineering software.

=Technical Approach=
Quantitative modeling of the process and all included optimization was conducted using the simulation software Aspen HYSYS. A variety of assumptions were developed to set up and define specific components of the process simulation itself. Within Aspen HYSYS, the NRTL Electrolyte property package was selected to model the behavior of salt water. Each stage of the MSF process is modeled as an individual flash vessel, with individual shell & tube heat exchangers used to model the distillate condensers present in each flash stage in the MSF system.

The HYSYS simulation model only considers the flash portion of the overall process, allowing establishment of mass and energy balances for the system. Pretreatment and post-treatment were excluded for the HYSYS model, as these steps are simple additions or removals of small concentrations of chemical components. Although the simulation models heat exchangers outside of each flash stage, in the physical design of the plant these heat exchangers will be contained within each flash stage.

One portion of the process is not modeled, which is the inlet seawater entering the heat rejection stages. As can be seen from the full process flow diagram (Appendix C), the inlet seawater in the heat rejection stage is in high excess to provide maximum cooling capacity; then a significant portion is purged to bring the stream flow rate down to what is needed for the feed stream. It is assumed that the flow rate of cooling water for those three stages is in such high excess that the cooling water does not heat up. Therefore, in the process simulation models, the inlet stream is the feed stream, which enters directly into the heat recovery portion of the process.

See Appendix D for a screenshot overview of the simulation environment model.

=Process Overview=
==Process Flowsheet==
The following diagram, Figure 1, shows a basic flowsheet for the process. Process alternatives considered for each stage are discussed below. Please see Appendix C for the full process flowsheet with finalized design decisions.

[[File:BlockFlowDiagram.png|center|700px|thumb|alt=Nueces Bay Desalination Facility Block Flow Diagram|Figure 1: Basic block flowsheet of the multi-stage flash desalination process.]]

==Major Features==
As seen above, there are three primary stages of the desalination process: Pre-treatment, the multi-stage flash distillation, and post-treatment. Seawater enters the pre-treatment phase first to make the feed suitable for the rest of the process by filtering out large particles that could accumulate on pipes, deaerating the water to limit corrosion, and adding an agent to combat fouling. The water then enters the distillation process, which is broken up into two sections: heat recovery and heat rejection.

In the heat recovery stage, there are 18 flash vessels, each of which separates steam from the salinated liquid water. Each flash vessel has saltwater as its inlet and two outlets: a vapor outlet (steam) that condenses into the final product of purified water and a liquid outlet composed of water with more dissolved solids than the inlet, which is the inlet stream for the next flash vessel. The pipe containing the feed is first mixed with recycled brine, then goes through the top sections of each of the flash vessels, starting with V-118 and ending up at V-101. In this way, the feed stream pipe first serves as a heat exchanger, heating the pre-treated seawater while simultaneously condensing the steam into the final product. The feed then enters a final heat exchanger where it is brought up to 103.6 °C before entering V-101 to be flashed.

After exiting V-118, the final flash vessel in the heat recovery portion, the water enters the heat rejection portion, which consists of three flash vessels (V-201, V-202, V-203). The flash vessels of this section operate in the same manner as in the heat recovery section, with the exception that condensation occurs using excess cooling water instead of pre-treated seawater. This technique allows excess heat to be rejected into an excess water stream rather than the feed.

The purified product stream finally enters post-treatment processing in which water is re-aerated and remineralized, disinfected, and pH is adjusted before being delivered to the city. The concentrated brine leftover at the end of the flash cycle is recycled into the feed, although a purge is required to continuously remove some salt from the system. This purged brine is disposed of by use of a Class I injection well, per regional regulations.

See Appendix E to find detailed stream tables of stream specifications.

==Process Alternatives==
===Pretreatment===
Multi-stage flash distillation requires significantly less pretreatment than reverse osmosis desalination, but helpful pretreatment techniques still exist. One very inexpensive method of pretreatment is screening/filtration, which is used to filter out large contaminants from intake seawater. Using more complicated membrane technology to accomplish large particle filtration is a possible alternative, but doing so would be much more expensive than using a standard screen or filter, so the latter choice has been selected for this process.<ref>El-Dessouky H., Alatiqi I, Ettouney H. Process synthesis: The multi-stage flash desalination system. Desalination. 1998;115(2):115-179. doi: 10.1016/S0011-9164(98)00035-6.</ref>

The most critical type of fouling in multi-stage flash distillation is scaling - therefore, the most critical step in the pretreatment process will be the inclusion of an anti-scalant. Alkaline scaling can be prevented by acid dosing or polyphosphate addition; however, acid dosing can result in increased corrosion and polyphosphates can lead to non-alkaline calcium phosphate fouling, so these additives will not be considered<ref name="Morris">Morris, R. The development of the multi-stage flash distillation process: A designer's viewpoint. Desalination. 1993;93(1-3):57-68. doi:10.1016/S0011-9164(93)80096-6. Availble at http://www.sciencedirect.com/science/article/pii/0011916493800966</ref><ref>Al-Gobaisi, D. A quarter-century of seawater desalination by large multistage flash plants in Abu Dhabi (Plant performance analysis, assessment, present efforts toward enhancement and future hopes). Desalination. 1994;99(2-3):509-512. doi: 10.1016/S0011-9164(94)00196-0.</ref>. As such, a polymer-based additive is recommended for the anti-scalant for this process. The Belgard EV series is designed for use in high-temperature seawater MSF; of these, Belgard EV 2030 is the most neutral in pH<ref>NSF International. NSF Product and Service Listings: Drinking Water Treatment Chemicals - Health Effects. NSF International. http://info.nsf.org/Certified/PwsChemicals/Listings.asp?Company=00300&. Accessed January 27, 2016.</ref><ref name="BWA 2030" /><ref>BWA Water Additives. Belgard EV2035 Technical Data Sheet. BWA Water Additives. http://www.wateradditives.com/files/products/tech_data_sheets/Belgard%20EV2035_Technical%20Data%20Sheet_Thermal_8.5x11.pdf. Accessed January 27, 2016.</ref><ref>BioLab Water Additives. Belgard EV Antiscalant for seawater distillation plants. BioLab Water Additives. http://www.lpq.com.mx/pdf/BELGARD%20EV.PDF. Accessed January 27, 2016.</ref>. As such, we have selected Belgard EV 2030 for use as an anti-scalant in our process.

Corrosion control is another significant method of pretreatment in multi-stage flash distillation plants. Using a piping material that has limited susceptibility to corrosion is typically not economically viable for MSF desalination plants due their large size, so using carbon steel for most of the piping is generally used for its economic benefits despite its susceptibility to corrosion<ref name = "Morris" />. Limiting oxygen concentration is very feasible through the use of deaerators, which can remove oxygen as well as other dissolved gases from the seawater stream. Often, an external deaeration system is used<ref name = "Morris" /><ref>Edwards - Hick Hargreaves. Seawater Deaerators. Edwards - Hick Hargreaves. http://psscorp.co.th/upload/news/20110830105340.pdf. Accessed January 26, 2016.</ref>. Given these considerations, using an external deaerator combined with a less-corrosive antiscalant like Belgard EV 2030 is much more practical for the process than using anti-corrosive piping.

Excessive foam can also be an issue in some MSF distillation facilities, so antifoaming agents are sometimes employed in the pretreatment process as well. However, many plants are operated without foaming issues, and therefore do not need anti-foaming pretreatment at all. Selection of an anti-foaming agent must be carefully weighed with selected of anti-scalant to avoid issues with additive interactions [26]. At this stage in the design process, an anti-foamer will not be included due to the risk of decreasing scalant efficacy, and the likelihood that foam will not be an issue in the desalination plant.

===Multistage Flash Distillation===
The main objective of the design project is desalinate water. This is done with the main process of multi stage flash distillation (MSF). MSF can be developed using a numerous number of different process configurations, such as Once-Through, Simple Mixer Brine Circulation, and Multi-Stage Heat Rejection Brine Circulation processes.

The most desired process for the project is the Three Stage Heat Rejection Brine Circulation as it builds on benefits of the other simpler technologies. By combining principles of heat rejection and heat recovery, the required heat transfer area of the process is greatly reduced as compared to once through processes or simple brine recycle processes. It has the highest performance ratio with the lowest specific feed flow rates, therefore greatly lowering the operating costs of the process. It is also known as the industry standard for MSF<ref name = "KSU Chapter 6">Kansas State University ChE 413. Available at: http://faculty.ksu.edu.sa/Almutaz/Documents/ChE-413/Multi-stage%20Flash%20Desalination.pdf. Accessed January 26, 2016.</ref>. In addition, since our brine disposal technique involves the pumping of the brine into a Class I well, the high brine outlet salinity is not a drawback for our process since post treatment or dilution of the brine is not necessary.

===Post Treatment===
After processing the water via multi-stage flash, the water needs to be post-treated to make the water taste as consumers would expect, since distilled water is “flat” and unpleasant, as well as to protect the consumers and the piping through which the water will be passing<ref name="Clayton">Clayton R. A review of current knowledge: Desalination for water supply. Foundation for Water Research. http://www.fwr.org/desal.pdf. Accessed January 26, 2016.</ref>. There are multiple alternatives to achieve these objectives of remineralization, aeration, corrosion control, and disinfection.

Aerating and introducing some dissolved solids back into the product water are effective ways to create a quality product that is not flat, and contains flavoring that is expected of the water when delivered to the consumer. Aeration will be accomplished via an inline spray aerator which will take up less space than cascade aeration.<ref>Minnesota Rural Water Association. Aeration. http://www.mrwa.com/WaterWorksMnl/Chapter%2011%20Aeration.pdf. Accessed January 26, 2016.</ref> Feed blending will be used to remineralize the water as opposed to a remineralization filter because it does not require additional equipment purchases, and can easily handle the large amounts of throughput required for the process.<ref name="Clayton" />

Corrosion and pH can be controlled by adding lime (calcium hydroxide) and carbon dioxide to the water. To do so, about 74 mg/L of lime and 88mg/L of CO2 should be added to the desalinated water to help prevent corrosion and increase alkalinity within acceptable ranges. Sodium hydroxide, soda ash, and sodium bicarbonate are alternatives to altering the alkalinity and pH of the product water, but lime and CO2 is an industry standard <ref name="Vouchkov" />.

Disinfection needs to be done to prevent microbial growth in the desalinated water and ensure the safety of the general public. Chlorine addition via chlorine gas or sodium hypochlorite is the most common and only requires a dose of 1.5 to 2.5 mg/liter. <ref name="World Health Organization" /> Other alternatives include ozonation, UV light, and chloramination. Chlorine, as a 5% - 15% sodium hypochlorite solution, is sufficient for our process and relatively inexpensive, and as such will be utilized in the process.

===Transport of Brine to Well===
Three major possibilities exist for transporting the brine from our desalination plant to the injection well for disposal: rail, trucking, and a pipeline. A pipeline was chosen as the most feasible and cost-effective approach.

A rail line is not available at this time, and with the cost to construct a rail line at approximately $43 million per mile<ref>About Money. How Much Do Rail Transit Projects Cost to Build and Operate? 2014. Available at: http://publictransport.about.com/od/Transit_Projects/a/How-Much-Do-Rail-Transit-Projects-Cost-To-Build-And-Operate.htm. Accessed January 27, 2016. </ref>, the enormous capital cost makes rail transport infeasible for this site. For large volume operations, pipelines are cheaper than trucking. In a study of wastewater sludge produced from wastewater treatment facilities, it was found that it was more economical for facilities producing over 1200 m3 per day to utilize pipelines rather than trucking.<ref>Marufuzzaman M, Eskioglu S, Hernandez R. Truck versus pipeline transportation cost analysis of wastewater sludge. Transportation Research. 2015; 74(A): 14-30. </ref> Our facility is producing nearly 3 times this volume at 3485 m3 per day of brine. Furthermore, brine is a less viscous fluid than wastewater sludge, decreasing pumping costs in the pipeline and further amplifying the benefit of choosing a pipeline over an injection well.

===Brine Storage===
Due to the choice of a pipeline, rather than trucking, as the method of transport of brine to the injection well, brine storage is unnecessary, so storage solutions such as holding tanks were not investigated. One method of storage compatible with a pipeline is to pump the brine through an evaporation pond. This solution is attractive as it would allow the brine to concentrate, saving some costs of pumping, however, the considerable land usage of evaporation ponds leads them to only be effective for smaller plants. The largest municipal plant utilizing this method produces only 5.7 million liters per day<ref>University of Santa Cruz Center for Integrated Water Research. Concentrate Management: Evaporation Ponds. 2010. Available at: https://ciwr.ucsc.edu/sites/default/files/ICM_D5.pdf. Accessed January 27, 2016. </ref>, whereas our desalination plant is proposed to produce 75.7 million liters per day. Therefore, evaporation ponds have been eliminated from consideration.

==Major Specifications==
===Material of Construction Selection===
While saltwater and desalinated water in general can be corrosive, there are best practices within the desalination industry for material selection. With this in mind, best practice shows carbon steel lifetime can be lengthened by monitoring the oxygen concentration in the water, specifying the paint used to coat any metal, and more expensive materials can be used in areas of extremely high corrosion.<ref name = "Morris" />. By following these best practices, it is possible to save significant amounts of money through the use of carbon steel in most areas of the plant, with specific high-corrosion areas being reinforced with, or replaced with, more corrosion resistant materials, such as Cu/Ni 70/30 alloy<ref name = "KSU Chapter 6" />.

===Heat Exchanger Sizing===
See Appendix F for heat exchanger sizing data. Heat exchanger sizes appear reasonable for placement in the flash vessels.

===Flash Vessel Sizing===
The dimensions for the flash drums were obtained through literature values. The flash drums were determined to be 18x4x3 m in width, height, and length.<ref name="KSU Chapter 6" />

Wall thickness was determined using the following conventions: Design pressure is 10% more than the operating pressure, design temperature is 50°F (27.8°C) higher than the operating temperature, a corrosion allowance of 3-mm and an effectiveness of 85% was derived from the assumption of a double-welded butt joint that was spot checked. See Appendix G for wall thicknesses calculated for each pressure vessel.

The sizing of the vessels to be constructed will be determined after talks with a manufacturer. Due to economies of scale, it may be more cost-effective to buy 21 pressure vessels with the dimensions mentioned above and a wall thickness of 17.1-mm, the highest wall thickness required in the system.

===Pipe Losses and Pressure Drop===
HYSYS was used to estimate the diameter of a pipe given a specified pressure drop per 100m of pipe. It was determined that for a pressure drop of 0.1 kPa/100m of pipe, a 3.05m diameter pipe is sufficient, which is more than manageable for manufacturers. This pressure drop is mostly insignificant and will be taken care of with the safety factors included in the pumps. The significant pressure drops in our system is accounted for via losses in the flash vessels and heat exchangers, which drive the flash process. Therefore overall pipe losses are insignificant and are built into intentional design losses.

=Economic Analysis=
==Economic Assumptions==
The sale of purified water yields a revenue stream of $49.5 million per year assuming sale to commercial entities within the city limits of Corpus Christi at $1.7915 per thousand liters.<ref>Water schedule. City of Corpus Christi Utility Business Office. http://www.cctexas.com/Assets/Departments/Financial-Services/Utilities-Business-Office/files/waterrates.pdf Accessed February 28, 2016.</ref> The primary variable costs are utilities and the brine disposal, with minimal raw materials costs due to the primary feedstock, seawater, being free, and minimal amounts of pre- and post-treatment chemicals being used. The cost of brine disposal used in this analysis was high at $2.097 per thousand liters<ref>McCurdy, R. Underground injection wells for produced water disposal. Chesapeake Energy Corporation. http://www.epa.gov/sites/production/files/documents/21_McCurdy_-_UIC_Disposal_508.pdf Accessed February 28, 2016.</ref>, an estimate for brine disposal utilizing trucking as the mode of transport. As has been shown through previous analyses, a lower cost can be achieved using a direct pipeline, however, no good estimates for this cost could be found.

The NPV was calculated using a 6% cost of capital, the approximate interest rate for a municipal bond.<ref>Zirbe, R.O. Jr., Han, X., Layton, D., Leshine, T. A history of discount rates and their use by government agencies. (2002). http://faculty.washington.edu/zerbe/docs/DISCOUNT%20RATE%20HISTORY-latesXIt.doc Accessed February 28, 2016.</ref> It was assumed that the plant can be built within two years, with production at half-scale in its first year. Additionally, one month of variable costs, $10.6 million, was set aside for working capital. See Appendix H for a full table of assumptions. A 30 year NPV calculation was chosen as the decision basis since 30 years is a conservative estimate for the lifetime of a desalination plant [38]. See below for estimates of NPV for the lifetime of the plant under various assumptions and design criteria.

==Optimization and Sensitivity Analysis==
Looking at the multi-stage flash process, there are a number of opportunities to optimize the process. The major process variables that were investigated were the inlet water temperature, the operating pressure drop per vessel, and the number of stages over which the process takes place. Considering these optimizations, our final process consists of 21 flash vessels (18 heat recovery and 3 heat rejection stages), each with a pressure drop of 5 kPa. See Appendix D for a screenshot of the HYSYS simulation used to model the optimized process.
===Operating Pressure and Pressure Drop===
Initially our process was designed to pump the inlet seawater and brine to a pressure of 2 bar and allow the pressure drop in each flash stage to end our process downstream at 1 bar to prevent complications associated with designing a system under vacuum. While this provided pumping advantages, the energy in the system was significantly higher due to vapor pressure considerations. Due to this fact, the required heat exchange area for certain exchangers proved to be infeasible (on the order of 80,000m2) due to the enormous capital and operating costs that would be associated with such a large heat exchanger.

The process was optimized to operate under vacuum, requiring additional pumping requirements but much lower required heat exchange area, and therefore lower utility costs. This optimization was done by specifying the pressure of the final flash vessel to 5.7kPa, so that the temperature of the water vapor (~30oC) is within approximately 5oC of the cooling water (35oC) so that condensation can occur within a reasonable approach. This also allows our system to optimally extract pure water from the brine running through the system and reduce energy requirements for cooling the product or the recycle. Our system redesign and optimization reduced the energy in the system and reduce the same heat exchanger area requirements by more than 3 orders of magnitude to approximately 800m2.

Once the process was optimized to operate in a vacuum, the pressure drop per flash stage regulated through values was then optimized. Three pressure drops per stages, 3kPa, 5kPa, and 7kPa, were tested and then an economic analysis was conducted to find the optimal value for the pressure drop. These changes had impacts on both operating and capital costs as they would change initial pumping requirements, the amount of water flowing through the process and recycle (and therefore the flash vessel sizes, heat exchanger sizing), and the efficiency of each stage.

The various pressure drops (3 kPa, 5 kPa, and 7 kPa) were tested on a system nominally at 24 stages under the assumption that the results would be generalizable to systems with a different number of stages. With increasing pressure drop, we found decreased capital cost but increased utility cost. As can be seen from Table 2 below, a pressure drop of 5 kPa proved to be the best option based on 30 year NPV, with a loss of $838.8 million, a savings of $65.5 million compared to the next-best option with a pressure drop of 7 kPa.

<center>
{| class = "wikitable"
|+Table 2: Comparison of profitability for our MSF desalination process with 24 stages with 3 different pressure drops per stage
! Number of Stages  
! Pressure Drop (kPa)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
! 30-Year NPV ($MM)
! NPV Ratio (over 5kPa)
|- style="text-align: center;"
| 24 || 3 || 362.3 || 39.1 || -905.8 || 1.08
|- style="text-align: center;"
| 24 || 5 || 255.1 || 53.6 || -838.8 || 1.00
|- style="text-align: center;"
| 24 || 7 || 209.5 || 72.2 || -904.3 || 1.08
|}
</center>

===Number of Flash Stages===
The number of stages is an important area for optimization. An increased number of stages yields better recovery of purified water, however, requires higher capital cost from the additional pressure vessel. Furthermore, with an increased amount of purified water comes increasingly concentrated brine. This must be taken into account since any brine concentration in excess of about 7% presents a more significant corrosion risk<ref name="KSU Chapter 6" />, so increased maintenance costs for the piping or higher capital costs for more corrosion-resistant materials could be necessary for brine concentrations exceeding 7%. Therefore, to recover the same amount of pure water without exceeding corrosion limitations, increasing the number of stages will reduce the overall size of each stage.

Optimizing based on NPV, it was found that 21 stages is optimal, balancing the increased profits from the additional purified water additional costs of higher capital cost. See Table 3 below to compare the 30-year NPV of the process across various numbers of flash vessels.

<center>
{| class = "wikitable"
|+Table 3: Comparison of profitability for our MSF desalination process with a 5 kPa pressure drop, 5000 m3/hr feed flow rate, with varying number of stages.
! Number of Stages  
! Pressure Drop (kPa)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
! 30-Year NPV ($MM)
! NPV Ratio (over 21 Stages)
|- style="text-align: center;"
| 17 || 5 || 213.9 || 58.7 || -813.8 || 1.016
|- style="text-align: center;"
| 18 || 5 || 218 || 57.6 || -809.9 || 1.011
|- style="text-align: center;"
| 19 || 5 || 224.3 || 58 || -806.1 || 1.007
|- style="text-align: center;"
| 20 || 5 || 229.8 || 57.9 || -813.6 || 1.016
|- style="text-align: center;"
| 21 || 5 || 226.6 || 55.9 || -800.7 || 1
|- style="text-align: center;"
| 22 || 5 || 254.7 || 54.6 || -849.5 || 1.061
|- style="text-align: center;"
| 23 || 5 || 249.3 || 54.1 || -833.1 || 1.04
|- style="text-align: center;"
| 24 || 5 || 255.1 || 53.6 || -838.8 || 1.048
|- style="text-align: center;"
| 25 || 5 || 254.9 || 52.7 || -837.4 || 1.046
|}
</center>

===Inlet Seawater Temperature===
By pairing our desalination plant with a natural gas power plant and utilizing its cooling water we forgo initial heating of our inlet stream. Normally, water pumped from the ocean would be approximately 25oC and by capturing waste heat from the power plant we can reduce utilities with our water inlet of 45oC. Through an economic comparison between two plants nominally at 21 flash stages, the plant with an inlet temperature at 25oC has a capital cost of $245MM and a utility cost of $77MM/yr and the same facility with an inlet water temperature at 45oC has a capital cost of only $226MM and a utility cost of $56MM/yr. As shown in Table 4 below, this $19MM savings in capital costs and $21MM/yr saving in utilities is largely due to reducing the size of the heat exchangers and lowering the steam required for heating duty on the inlet seawater respectively. Therefore our process is optimized by recovering the waste heat water from the gas fired power plant.

<center>
{| class = "wikitable"
|+Table 4: Seawater Inlet Temperature Process Cost Optimization
! Seawater Temperature (°C)
! Capital Cost ($MM)
! Utility Cost ($MM/yr)
|- style="text-align: center;"
| 25 || 245 || 77
|- style="text-align: center;"
| 45 || 226 || 56
|- style="text-align: center;"
| Savings || 56 || 19
|}
</center>

==Overall Process Economics==
After optimization at fixed flow rates, the process was scaled to meet the the 67% operating percentage and final product definitions as shown in Table 1, which resulted in finalized economics shown in Table 5 below. For a table of the installed costs of each process vessel, see Appendix I. For the finalized NPV calculations, see Appendix J.

<center>
{| class = "wikitable"
|+Table 5: Summarized Economics for 21-stage MSFD Process
! Economic Parameter
! Cost (USD)
|- style="text-align: center;"
| Capital Cost || 345 MM
|- style="text-align: center;"
| Total Operating Cost per Year || 103 MM
|- style="text-align: center;"
| Utility Cost per Year || 59 MM
|- style="text-align: center;"
| 30-Year NPV (6% Discount) || -1035 MM
|}
</center>

=Conclusions and Recommendations=
==Project Viability==
Table 5 above demonstrates that the process has reasonable operating and capital costs, but the 30-year NPV is negative by -1035 $MM due to the extremely low sell price capability of desalinated water as a product. Therefore, this plant would have to heavily be supported by the City of Corpus Christi to run as the product revenue will not make it self-sufficient. The acquisition of government subsidies are key operating requirements for all desalination facilities in the United States.

==Additional Concerns==
Large volumes of water, steam, and energy are needed to ensure the project runs as specified. While most of the water can easily be extracted from the ocean with little to consequence, the fuel used to generate the steam and electricity can have a significant environmental impact. As the plant is located next to a natural gas power plant, this fuel is better than coal with respect to environmental concerns, and utilizing energy from the power plant to preheat the feed stream for the desalination plant helps lower environmental impacts.

The large amounts of high pressure steam being used represents one of the the largest safety concern in the process, because a high pressure steam release could easily cause severe burns or even death. Proper precautions and control measures must be in place to prevent accidental release. Otherwise, the process does not utilize any known toxic chemicals and is relatively safe from a chemical standpoint. The flash vessels mostly operate below atmospheric pressure, so they must be constructed as vessels that can withstand vacuum without failing.

=Appendix A: Proposed Location of Desalination Plant=
Pictured below is the proposed location for the desalination plant, located on Nueces Bay in Corpus Christi, adjacent to the Nueces Bay Energy Center. The plant’s proposed location is shown in the area outlined by the small circle in the upper right of the figure.

[[File:NuecesBayLocation.png|center|600px|thumb|alt=Nueces Bay Desalination Facility Location|Figure A.1: Proposed mapped location of Nueces Bay Desalination Facility.]]
[[File:ClassIWell.png|center|700px|thumb|alt=Nueces Bay Desalination Facility Distance to Injection Well|Figure A.2: Mapped distance from proposed location to Class I Injection Well]]

=Appendix B: Feed and Product Definitions - Salinity=

=Appendix C: Process Flow Diagram=
[[File:TeamF_FinalPFD.jpg|center|1200px|thumb|alt=Process Flow Diagram|Figure C.1: Process Flow Diagram]]

=Appendix D: HYSYS Simulation Model=
[[File:TeamF_HYSYS_Screenshot.PNG|center|1200px|thumb|alt=HYSYS Simulation Model|Figure D.1: Screenshot of HYSYS Model Simulation.]]

=Appendix E: Stream Tables=
See attached Excel File “HYSYS 6.0 Flowsheet summary.xlsx”

A summary of some values is shown below:

<center>
{| class = "wikitable"
|+Table 7: Key process material streams
!  
! Stream  
! Molar Flow (105 kgmole/h)
! Mass Flow (105 kg/h)
|- style="text-align: center;"
! style="text-align: right;" | Inlet
| Feed Water || 454 || 83.6
|- style="text-align: center;"
! style="text-align: right;" | Outlet
| Brine Reject || 1.90 || 36.0
|- style="text-align: center;"
! style="text-align: right;" | Product
| Pure Water || 2.64 || 47.5
|}
</center>

<center>
{| class = "wikitable"
|+Table 8: Key process energy streams
! Stream  
! Heat Flow (kJ/h)
|- style="text-align: center;"
| Feed Water || 1.30E+11
|- style="text-align: center;"
| Q-301 || 5.73E+06
|- style="text-align: center;"
| Q-302 || 3.09E+09
|- style="text-align: center;"
| Q-304 || 4.68E+06
|- style="text-align: center;"
| Q-303 || -1.26E+09
|- style="text-align: center;"
| Brine Reject || -5.47E+10
|- style="text-align: center;"
| Pure Water || -7.46E+10
|}
(Negative value denotes energy leaving process)
</center>

=Appendix F: Heat Exchanger Sizing=

<center>
{| class = "wikitable"
|+Table 9: Utility Temperatures and Heat Transfer Coefficient<ref name="Towler">G.P. Towler, R. Sinnott. Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design. Elsevier, 2012.</ref>
!  
! HP Steam
! Cooling Water
|- style="text-align: center;"
! Temperature (°C)
| 250 || 35
|- style="text-align: center;"
! U (W/m2K)
| 4000 || 1500
|}
</center>

<center>
{| class = "wikitable"
|+Table 10: Internal Flash Heat Exchanger Sizing, condensing steam.
! Exchanger
! UA (kJ/°C-h)
! U (kJ/hr/m2°C)
! A (m2)
|- style="text-align: center;"
| E-101 || 1.47E+07 || 14400 || 1019
|- style="text-align: center;"
| E-102 || 1.53E+07 || 14400 || 1059
|- style="text-align: center;"
| E-103 || 1.59E+07 || 14400 || 1103
|- style="text-align: center;"
| E-104 || 1.66E+07 || 14400 || 1153
|- style="text-align: center;"
| E-105 || 1.74E+07 || 14400 || 1209
|- style="text-align: center;"
| E-106 || 1.83E+07 || 14400 || 1272
|- style="text-align: center;"
| E-107 || 1.93E+07 || 14400 || 1342
|- style="text-align: center;"
| E-108 || 2.05E+07 || 14400 || 1424
|- style="text-align: center;"
| E-109 || 2.19E+07 || 14400 || 1518
|- style="text-align: center;"
| E-110 || 2.35E+07 || 14400 || 1629
|- style="text-align: center;"
| E-111 || 2.54E+07 || 14400 || 1761
|- style="text-align: center;"
| E-112 || 2.77E+07 || 14400 || 1921
|- style="text-align: center;"
| E-113 || 3.05E+07 || 14400 || 2119
|- style="text-align: center;"
| E-114 || 3.42E+07 || 14400 || 2372
|- style="text-align: center;"
| E-115 || 3.90E+07 || 14400 || 2706
|- style="text-align: center;"
| E-116 || 4.56E+07 || 14400 || 3167
|- style="text-align: center;"
| E-117 || 5.55E+07 || 14400 || 3856
|- style="text-align: center;"
| E-118 || 7.20E+07 || 14400 || 5000
|}
</center>

=Appendix G: Pressure Vessel Sizing=

=Appendix H: Expanded Economic Assumptions=

=Appendix I: Equipment Cost Summary=

=Appendix J: Economic NPV Analysis=

=References=
<references/>

File:TeamF FinalPFD.JPG

2016-03-07T20:10:00Z

Benjkg: Final PFD of Team F's Multi-Stage Flash Distillation Process

Final PFD of Team F's Multi-Stage Flash Distillation Process

Team F

2016-03-07T02:37:01Z

Benjkg: Redirected page to Nueces Desalination Center: Production of Drinking Water by Multi-Stage Flash Distillation

#REDIRECT [[Nueces Desalination Center: Production of Drinking Water by Multi-Stage Flash Distillation]]

Column

2016-02-21T07:33:06Z

Benjkg: /* Distillation */

[[File:Distillationcolumn_singlefeed.png|thumb|right|300px|Figure 1. Distillation Column <ref name = "Towler" />]]
Authors: Ben Granger, [2016] Scott Smith, [2015] and Harry Poppick [2015] 

Stewards: Jian Gong and Fengqi You

The column utilities in HYSYS can be used to model a wide variety of chemical separation techniques including absorption, liquid-liquid extraction and three-phase distillation. This article highlights the use of HYSYS for simulations of standard chemical distillation. The two primary distillation simulation utilities are the shortcut column, which is useful for quick and cursory simulations, and the full column utility, for more involved simulations.

==HYSYS Column Types==
{| border="1" class="wikitable"
|+ Varieties of Separators in HYSYS
! Name
! Icon
! Description
|-
! Absorption Column
| [[File:Absorber.JPG|center|35px|]]|| Utility to model absorption and stripping (liquid/vapor) separations.
|-
! Refluxed Absorption Column
| [[File:RefluxedAbsorb.JPG|center|35px|]]|| Utility to model absorption and stripping (liquid/vapor) operations with condensed top products.
|-
! Absorption with Reboil
| [[File:ReboiledAbsorber.JPG|center|35px|]]|| Utility to model absorption and stripping (liquid/vapor) operations with reboiled bottoms.
|-
! Liquid Liquid Extractor
| [[File:LiquidLiquid.JPG|center|35px|]]|| Utility to model liquid-liquid phase separations.
|-
! Three Phase Distillation
| [[File:ThreePhase.JPG|center|35px|]]|| Utility to model distillation consisting of three phases internally.
|-
! Distillation Sub-Flowsheet
| [[File:DistillationColumn.JPG|center|35px|]]|| Overall two-phase distillation utility.
|-
! Shortcut Column
| [[File:ShortcutDistillation.JPG|center|35px|]]|| Approximate model of distillation processes.
|-
! Component Splitter
| [[File:ComponentSplitter.JPG|center|35px|]]|| Utility to force components to separate with no equilibrium considerations.
|}

==Background==
Of the many different types of separating columns available in HYSYS listed in the table above, there are three main categories: absorption, extraction, and distillation. Each general type of separation technique has its own benefits. A specific separation may be simple using one technique while difficult or impossible using another; as such, understanding the nature of these separation techniques is important in selecting the best method for a given separation.

===Absorption===
Absorption is primarily used to separate a solute from a stream in the gas state. The separation is conducted by introducing an additional liquid solvent stream to absorb the solute from the gas stream. Selection of this solvent is critical to the effectiveness of the separation. Note that to reduce costs, the liquid solvent is often desirable to be extracted after this separation, so absorption is typically accompanied by a second separation process such as stripping to recover the liquid solvent from the solute. <ref name = "Wankat"> Wankat, P.C. Separation Process Engineering. Upper Saddle River: Prentice-Hall; 2012 </ref>

===Extraction===
Liquid-liquid extraction is typically used when distillation is unfeasible, such as separating components with an azeotrope or with very similar boiling points. Like absorption, liquid-liquid extraction requires the introduction an additional liquid solvent stream to separate one of the components. Again, selection of this solvent is critical to the effectiveness of the separation. Physically, this separation occurs by difference in solubility of the components in the solvent. Also similar to absorption, the use of a solvent is often accompanied by an additional separation process like distillation to recover the solvent from the solute. <ref name = "P&T"> Peters M.S., Timmerhaus K.D. Plant Design and Economics for Chemical Engineers. 5th ed. New York: McGraw Hill; 2003. </ref>

===Distillation===
Distillation is by far the most common separation process in practice, separating based on vapor-liquid equilibrium. The process is very effective in separating liquid components, as long as they have different boiling points and do not have an azeotrope. Distillation often requires high temperatures, so operating costs can be relatively high compared with other processes. However, distillation does not require any additional separation-aiding streams such as solvent, and as such does not require an additional utility to complete the separation as absorption and extraction do.

Distillation columns are accompanied by a reboiler at the bottom and a condenser at the top (see Figure 1). The condenser can be a total condenser, resulting in one liquid distillate stream, or a partial condenser, resulting in a liquid distillate stream and a gaseous overhead stream. These components are the major energy consuming components of a distillation column. The liquid exiting the condenser is split into two streams: reflux and distillate. The proportion of liquid returning to the column relative to the liquid leaving as distillate is a key design parameter of the column, known as reflux ratio.

Each stage of a distillation column exists under its own conditions and equilibrium. The design of the tray itself can also critically impact the performance of the column. This capability exists in HYSYS, though the focus of this page's tutorials is on more general operation of the column. <ref name = "Wankat" />

==Simulating Separations in HYSYS==
===Shortcut Distillation Column<ref name = "HYSYS"> Aspentech. "Aspen HYSYS V8.0 Help Documentation". Published 2014 </ref>===

The shortcut distillation utility within HYSYS allows users to quickly model a distillation separation of a stream with relatively little known information. Once the user attaches a fully defined inlet stream to the shortcut distillation column, the essential information that must be specified are as follows:

* The vapor/liquid quality of the top product
* The light key component and composition in the bottoms product
* The heavy key component and composition in the distillate (top) product
* The condenser pressure
* The reboiler pressure
* The external reflux ratio

With this information, the utility will run and, if possible, converge on a solution that will meet the key component specifications. The critical information that the utility will calculate are as follows:

* Fully specified distillate product stream (temperature, pressure, flow rate, composition)
* Fully specified bottom product stream (temperature, pressure, flow rate, composition)
* Condenser operating temperature and heat duty
* Reboiler operating temperature and heat duty
* Minimum number of trays
* Actual number of trays
* Optimum feed tray location

The primary purpose of the shortcut distillation is to allow for users to perform a first pass estimate on the performance requirements of a given system for a specified reflux ratio. The values gathered from the shortcut distillation utility can be used to assist the user when setting up the complete distillation utility in HYSYS, or when attempting to find reasonable column properties to allow the system to converge (e.g. number of trays, feed location, etc.)

===Standard Distillation Column<ref name = "HYSYS" />===
The standard distillation column, called "Distillation Sub-Flowsheet" the "Columns" tab on the equipment palette, may be used to perform a more rigorous simulation of your process than that offered by the shortcut column. In addition, the shortcut column may be used as a "first pass" analysis and then these specification estimations can used as the starting point for the standard column.

The standard distillation column provides many options for customization, allowing a user to specify a partial or total condenser, the number of passes through the column, and the type of reboiler all within the initial setup phase of creating a column. In addition, many specifications and parameters can be customized once the column is running, such as purities, recoveries, temperature profiles, column duties, and even internal specifications such as tray type, tray diameter, and weir height. This flexibility allows for much more rigorous, accurate simulation of a distillation column than using a shortcut column.

====Manipulating Column Specifications====

Column specifications can be heavily manipulated to force a column to operate under specific conditions, to produce a stream with specific purity, and even to minimize utilities. See the tutorial section for an example of this usage.

''Information taken from Towler Chapter 17'' <ref name = "Towler"> Towler, G. Sinnott, "Chemical Engineering Design". Elsevier, 2012, Chapter 17: R. Separation Columns (Distillation, Absorption, and Extraction). </ref>

Parameters that directly impact the design and operation of the distillation column include:
*Reflux ratio
*Feed point location
*Column Pressure
*Number of equilibrium stages/real stages

Assuming that we have already specified a desired specification for either the distillate or bottoms stream, we can manipulate the above parameters when running our simulation within HYSYS.

'''Reflux Ratio'''

The number of stages required for separation is directly affected by the reflux ratio that can be defined as follows:

<math> R=\frac {Flow\ returned\ as\ reflux} {Flow\ of\ top\ product\ taken\ off} </math>

Increasing the reflux ratio will reduce the total number of stages required but will increase the utility costs for the column. Therefore, when simulating a distillation column, special care should be taken to strike a balance between capital and operating costs.

When starting a new distillation column simulation within HYSYS, it is a good idea to calculate a minimum reflux ratio to advise the specification of the reflux ratio. The minimum reflux ratio can be calculated through use of a McCabe-Thiele diagram. According to Towler, a reflux ratio of 1.15 times the minimum reflux ratio should be used as a first approximation. Once this ratio is inputted into the simulation environment, it can be further manipulated to fine-tune the optimum reflux ratio for the system. <ref name = "Towler" />

'''Feed Point Location'''

The column simulation environment within HYSYS allows for manipulation of the feed point location. The general rule for designating feed point location is to have the feed enter the column at the stage which best matches the composition of the feed. There can be some uncertainty when it comes to finding the precise feed point location. Instead, a trial-and-error method may be useful when using HYSYS to determine which feed stage results in the desired distillate and bottoms conditions.

'''Column Pressure'''

When specifying the distillation column within the HYSYS simulation environment, you must specify the operating pressure as well as any pressure drops. If you are running a HYSYS simulation to get a first-pass approximation of compositions and flow rates, it is a typical practice to assume the pressure in the column is constant. If you are running a more intensive and rigorous simulation, you can approximate the pressure drop per tray using the following equation:

<math> \Delta P_{\text{tray}} = 2 \rho_L g h_w </math>

where <math> \rho_L </math> is the liquid density, g is gravitational acceleration, and <math> h_w </math> is the weir height. <ref name = "Towler" />

==Tutorials==
===Shortcut Distillation Column<ref name = "HYSYS" />===

This example will simulate the separation of benzene, toluene and p-xylene from some upstream process.<ref name = "HYSYS" />

'''Initialize''' [[File:AddComponents.PNG|thumb|center|600px|Figure 2. Adding Components in HYSYS]]
#Start Aspen HYSYS v.8 and click “new” on the main screen to open a new simulation window.
#Once on the component list screen, select “add” near the bottom of the screen
#Search the database for the desired components using the search bar, and add them to the component list by clicking “add.” In this case, we wish to add Benzene, Toluene and p-Xylene.
#On the left hand “properties” menu, click the “fluid packages” tab and select “add” to add a new package.
#Select the desired fluid package. In this case we are using Peng-Robinson
#In the bottom, left-hand corner of the screen, click “simulation” to enter the simulation environment.
'''Setup Streams''' [[File:Streams.PNG|thumb|center|300px|Figure 3. Add Streams]]
#Start by clicking the blue arrow at the top of the “palette” window and then clicking on the main flowsheet to produce a new material stream. This will create our feed stream. It is a good idea to place your other streams as well: one for distillate product and one for bottoms product.
#Add two energy streams that will be the condenser and reboiler duties.
#Double click on the newly created feed stream to pull up the worksheet for the stream. Please note that we are specifying this feed stream to demonstrate how the column would behaved with a fully specified inlet stream. The temperature, pressure, flow rate and composition of the feed stream can be specified by the user. [[File:FeedConditions.PNG|thumb|center|300px|Figure 4. Define Feed]]
#We will make the following changes within this worksheet to specify the feed stream:
#Enter “feed” into the stream name.
#Set the temperature to 20 degrees C.
#Set the pressure to 1 atm. (Be sure to select the “atm” unit from the dropdown when you begin to enter this value)
#Set the mass flow to 500 kg/hr [[File:Compositions.PNG|thumb|center|600px|Figure 5. Define Compositions]]
#On the left hand column within the worksheet, click “composition” to enter the composition tab.
#Click “edit” on the composition tab to change the composition of the feed stream
#Enter .25 to Toluene, .35 to Benzene and .40 to p-Xylene.
The worksheet should indicate that the stream is fully specified with a green “ok” indicator at the bottom. Close the worksheet.
'''Fully Specify Shortcut Column''' [[File:ConnectStreams.PNG|thumb|center|600px|Figure 6. Connect the inlet and outlet streams]]
#Create the shortcut column by clicking on the "columns" tab within the palette and selecting "shortcut column"
#Click on the simulation environment to place the column
#The shortcut column should currently appear red to represent that it is not currently specified.
#Double click on the column to pull up the column worksheet
#Under each drop down, add the appropriate stream: E.g. Under "Distillate" add the distillate product stream that was defined earlier.
#Additionally, be sure to specify the quality of the top product stream: whether it is liquid or vapor phase. In this case, select "liquid." [[File:Set Keys.PNG|thumb|center|600px|Figure 7. Set the key specifications]]
#On the left hand side of the worksheet, select "Parameters."
#This will move you to the parameters section of the column window where column specifications can be added.
#At this point it is important to identify which components are most volatile. In this case, benzene is most volatile, xylene is the least volatile and toluene rests in between the two. One rule of thumb is that more volatile components tend to be more valuable, and so in this example we will be purifying benzene in the distillate stream. In order to do this we will specify that benzene is the light key in the bottoms and that toluene is the heavy key in the distillate. This arrangement makes sense since it will force the heavier xylene and toluene products to drop out in the bottoms.
#Set the Light Key in Bottoms to Benzene and the mole fraction to .001 and set the Heavy Key in Distillate to Toluene and the mole fraction to .001.
#Set the condenser and reboiler pressure to the approximate values that they would be. In this case, we are assuming that our column is sufficiently small that atmospheric pressure is a fair assumption for both values. Enter 1 atm for both of these values. [[File:Worksheet.PNG|thumb|center|600px|Figure 8. Viewing the Worksheet]]
#Finally set the reflux ratio to a value that is higher than the minimum reflux ratio that HYSYS calculates. In this case the minimum reflux ratio was calculated to be 2.119 and so it is best to input around 2.5. This value can be tweaked later to see how performance is changed for different reflux ratios.
#At this point it is worthwhile to see what outputs the column is producing, since this is one of the primary uses of the shortcut distillation column.
#Click on the "worksheet" tab at the top of the window. This will bring you to your stream information where you can view the temperatures and flow rates of your outgoing streams.
#Now click on the "compositions" tab on the left side to view the compositions of the outgoing stream. As expected, the benzene is leaving in the distillate stream at 99.9% purity with a .1% toluene impurity, while all of the xylene is leaving out in the bottoms. [[File:Performance.PNG|thumb|center|600px|Figure 9. View the Performance Tab]]
#At this point it is valuable to record the calculated minimum number of trays, actual number of trays, as well as the condenser/reboiler temperatures.
#To view these values click on the "performance" tab
#You can observe here that the minimum number of trays calculated was around 15.7, the actual number of trays used was 35.7 the optimal feed stage was around 20.5, the condenser temperature was found to be around 80 C, while the reboiler was calculated to be around 123 C.

===Standard Distillation Column<ref name = "HYSYS" />===
====General Setup and Use====

In this example, we will be setting up a basic distillation column in HYSYS to simulate the separation of benzene and toluene.

# Start a new case in HYSYS by clicking on the button on the top menu bar.
# On the Components screen, add Benzene and Toluene. [[File:Columncomponents.PNG|thumb|center|300px|Figure 10. Component Selection in HYSYS]]
# On the Fluid Package screen, you have a wide range of options with which to simulate vapor-liquid equilibrium. For this example, choose the Peng-Robinson fluid package and enter the simulation environment. [[File:Fluidpkg.PNG|thumb|center|400px|Figure 11. Fluid Package Selection in HYSYS]]
We are using the simple distillation column and thus we only need a single feed stream into the distillation column as well as a distillate stream and a bottoms stream. We will also need 2 energy streams for the condenser and reboiler. <ref name = "HYSYS" />
# Add the material and energy streams to the simulation environment. Naming them appropriately will make it easer to keep everything straight as we move forward with the tutorial.
# Select the distillation column from the palette and place anywhere in the simulation environment.
# Switch to “Attach Mode” and attach the mass and energy streams to their appropriate locations on the distillation column. NOTE: You could also complete this step by double-clicking on the column itself and selecting each respective stream from the drop down menus.
# Double click on the column and select the condenser type. For this example, select “total condenser” and click next
# This next page is where the pressures of the column are specified. For this example, we will assume that the column is run at 1 atm with no pressure drops within the column. Therefore, specify reboiler pressure and condenser pressure to 101.3 kPa and the condenser pressure drop to 0 kPa.
# The next page has fields where temperature estimates can be made for the condenser, top stage, and reboiler. As these are optional fields, simply click next.
# The final screen of the distillation column setup is where the reflux ratio and distillate flow rates should be specified. For this example, specify the distillate flow rate as 1000 kgmol/hr and the reflux ratio as 1.5.
# Click “Done” at the bottom of the screen and the simulation page should be back in full view. [[File:Simulationview.PNG|thumb|center|400px|Figure 12. Simulation View of Distillation Column in HYSYS]]
We are now ready to specify the flow rates and compositions of the streams coming in and out of the column.
# Double click on the feed stream coming into the column. A specifications page should come up.
# This page allows you specify a number of parameters pertaining to the feed stream, including temperature, pressure, and compositions. Specify the temperature as 37C and pressure as 101.3 kPa (1 atm). In addition, specify the molar flow as 3500 kgmole/h.
# Now click on the composition tab on the left side of the window. Here specify, the benzene as 70 mass % and the toluene as 30 mass %. The stream should be fully specified. Close the window to return to the simulation. [[File:Configurationpage.PNG|thumb|center|400px|Figure 13. Column Configuration Page]]
The column and feed stream should now be fully specified. It is now time to run the simulation.
# If the solver is not already activated, click the green traffic light at the top of the simulation window.
# Now double click on the distillation column to bring up the main column window. A feed stage and total number of stages should already be specified.
# Click the “Run” button of the window to start the simulation. The small rectangle to the right of this button should light up green and say “converged” when it finds a solution. [[File:Columncompositions.PNG|thumb|center|400px|Figure 14. Outlet compositions in HYSYS]]

====Designing to Meet Specifications====
This example demonstrates how to use a distillation column simulation in HYSYS to design a column that meets preset requirements, and how to optimize the column to minimize costs. Basic setup of the column is assumed understood as described in the above tutorial. In this tutorial, the example requirements are to design a column that separates a stream of 20 mol% methanol and 80 mol% water into a distillate stream with 97% purity methanol and high recovery. In addition, both capital and operating costs are desired to be minimized.

# Create a simulation with water and methanol as the two components, using the NRTL fluid package.
# Place a standard distillation column in the simulation environment, and define a feed stream with vapor fraction 0, temperature 25 C, pressure 1 atm, flow rate 10 kgmol/h, and a composition of 0.2 mol fraction methanol and the balance of water.
# Enter column setup. Define the column as having a total condenser, 8 stages, and the feed entering at stage 4. The column should be once-through with a regular reboiler. Condenser pressure should be 1 atm, while reboiler pressure will be set at 1.3 kPa, giving a small pressure drop through the column. Optional temperature estimates can be left empty. Reflux ratio should be specified as 1.5, and liquid rate as 8.2 kgmol/h. In a normal situation, many of these values can be initially estimated using a shortcut column and then using them here, in the first run of a standard column simulation. Finally, run the column simulation, and it should easily converge. [[File:Column_converged_initially.PNG|thumb|center|800px|Figure 15. Initial column converged]]
# Now begins the refinement and optimization portion of the tutorial. To do so, in the distillation column window, switch to the Monitor tab in the Design toolbar on the left. We will add four new specifications of interest: distillate methanol purity, distillate methanol recovery, condenser duty, and reboiler duty. [[File:Monitor_tab.PNG|thumb|center|800px|Figure 16. The monitor tab in HYSYS]]
# To add a new specification, click the button labeled "Add Spec..." at the bottom of the window. This first spec will be for distillate methanol purity. In the new popup window, choose "Column Component Fraction" from the list of specs. In the new window, the name of the spec can be changed, and the specification itself can be defined. Change "Target Type" to "Stream," change "Draw" to the distillate stream, and change "Component" to methanol. Close this window, and the new specification should now be included in the list on the main column window. [[File:Making_a_new_spec.PNG|thumb|center|300px|Figure 17. Creating a new specification for distillate methanol mole fraction]]
# To add the recovery specification, the process is similar, expect the specification type is now "Column Component Recovery." To add the condenser and reboiler duty specifications, the spec type is "Column Duty," and the "Energy Stream" field should be chosen to be the energy stream of the condenser and reboiler for their respective specifications. Add these three specifications to the list as well.
# Note that currently, the distillate methanol fraction is only 0.244. To begin manipulation column specifications to meet the requirement of 97% purity, change the currently empty specified value for this spec to the current value, 0.244. Then, deselect Distillate Rate as an active spec, and make this new purity spec active in its stead. The simulation should automatically and instantly converge - if not, hit run. After converging, all values should remain the same.
# Begin increasing the specified value for distillate methanol purity gradually, until finding the approximate largest value at which the column simulation can still converge. In this example, this specified value can be increased up to 0.925 before the simulation fails to converge.
# Once this maximum has been reached, increase the total number of column stages by 1 or 2 by returning to the Connections tab in the Design toolbar. Note that the feed stage will automatically recalculate to stay in the same relative column position as before. In this example, change the number of stages to 10, and observe the feed stage automatically recalculate to 5. Hit run, and the simulation should still easily converge.
# Increase the specified value for purity again until the column simulation again reaches a limit beyond which it cannot converge. With 10 stages in this example, the column cannot converge with a specified purity of 0.95, but it can for a purity of 0.945. So, 0.945 is selected as the highest viable purity for this number of stages.
# Repeat this process, increasing the number of stages again followed by increasing the purity, until the desired purity is met. In this example simulation, a purity specification of 0.97 can be met with 14 stages, with the feed entering at stage 7.
# Recovery can be maximized in the same way, replacing Reflux Ratio as an active spec, though in this situation recovery is already 99.9%, so there is no need to maximize further. [[File:Requirements_met.PNG|thumb|center|800px|Figure 18. All design requirements met]]
# Now that the requirements are met, we can begin optimizing the number of stages and column duty. Condenser and reboiler duties are desired to be as low as possible to keep operating costs at a minimum, though number of stages should be kept relatively low as well. Note that with the current column setup, condenser duty is -1.83e5 kJ/h, and reboiler duty is 2.42e5 kJ/h.
# To begin, set recovery as an active spec by specifying its value as the current value, in this case 0.999, and then deselecting Reflux Ratio as an active spec and selecting the custom recovery specification.
# Next, we want to find the feed stage which minimizes column duty for this number of stages. Vary the feed stage to see which stage results in the minimum column duties. In this example, the optimal feed stage is stage 8, which results in condenser duty of -1.76e5 kJ/h and reboiler duty of 2.35e5 kJ/h.
# Now, increase the number of stages gradually, and find the duty-minimizing feed stage for each number of stages. For example, for a column with 17 stages, the optimal feed stage is 10, which yields condenser duty of -1.58e5 kJ/h and reboiler duty of 2.16e5 kJ/h.
# This process can be repeated indefinitely, as more stages will consistently result in decreased column duty. However, there are diminishing returns, so careful consideration of the balance of installation costs (more stages) vs operating costs (more column duty) must be made taking into account the actual economics of the distillation column. In this example, these diminishing returns are easily visible in the values for condenser duty: increasing from 14 to 17 stages reduces condenser duty by 0.18 kJ/h, increasing to 20 stages reduces condenser duty by a further 0.08 kJ/h, and increasing to 23 stages reduces condenser duty by only 0.03 kJ/h, at a final value of -1.47e5 kJ/h. A similar trend can be seen in the reboiler duty.

==References==
<references />

Column

2016-02-21T07:32:42Z

Benjkg: /* Distillation */

[[File:Distillationcolumn_singlefeed.png|thumb|right|300px|Figure 1. Distillation Column <ref name = "Towler" />]]
Authors: Ben Granger, [2016] Scott Smith, [2015] and Harry Poppick [2015] 

Stewards: Jian Gong and Fengqi You

The column utilities in HYSYS can be used to model a wide variety of chemical separation techniques including absorption, liquid-liquid extraction and three-phase distillation. This article highlights the use of HYSYS for simulations of standard chemical distillation. The two primary distillation simulation utilities are the shortcut column, which is useful for quick and cursory simulations, and the full column utility, for more involved simulations.

==HYSYS Column Types==
{| border="1" class="wikitable"
|+ Varieties of Separators in HYSYS
! Name
! Icon
! Description
|-
! Absorption Column
| [[File:Absorber.JPG|center|35px|]]|| Utility to model absorption and stripping (liquid/vapor) separations.
|-
! Refluxed Absorption Column
| [[File:RefluxedAbsorb.JPG|center|35px|]]|| Utility to model absorption and stripping (liquid/vapor) operations with condensed top products.
|-
! Absorption with Reboil
| [[File:ReboiledAbsorber.JPG|center|35px|]]|| Utility to model absorption and stripping (liquid/vapor) operations with reboiled bottoms.
|-
! Liquid Liquid Extractor
| [[File:LiquidLiquid.JPG|center|35px|]]|| Utility to model liquid-liquid phase separations.
|-
! Three Phase Distillation
| [[File:ThreePhase.JPG|center|35px|]]|| Utility to model distillation consisting of three phases internally.
|-
! Distillation Sub-Flowsheet
| [[File:DistillationColumn.JPG|center|35px|]]|| Overall two-phase distillation utility.
|-
! Shortcut Column
| [[File:ShortcutDistillation.JPG|center|35px|]]|| Approximate model of distillation processes.
|-
! Component Splitter
| [[File:ComponentSplitter.JPG|center|35px|]]|| Utility to force components to separate with no equilibrium considerations.
|}

==Background==
Of the many different types of separating columns available in HYSYS listed in the table above, there are three main categories: absorption, extraction, and distillation. Each general type of separation technique has its own benefits. A specific separation may be simple using one technique while difficult or impossible using another; as such, understanding the nature of these separation techniques is important in selecting the best method for a given separation.

===Absorption===
Absorption is primarily used to separate a solute from a stream in the gas state. The separation is conducted by introducing an additional liquid solvent stream to absorb the solute from the gas stream. Selection of this solvent is critical to the effectiveness of the separation. Note that to reduce costs, the liquid solvent is often desirable to be extracted after this separation, so absorption is typically accompanied by a second separation process such as stripping to recover the liquid solvent from the solute. <ref name = "Wankat"> Wankat, P.C. Separation Process Engineering. Upper Saddle River: Prentice-Hall; 2012 </ref>

===Extraction===
Liquid-liquid extraction is typically used when distillation is unfeasible, such as separating components with an azeotrope or with very similar boiling points. Like absorption, liquid-liquid extraction requires the introduction an additional liquid solvent stream to separate one of the components. Again, selection of this solvent is critical to the effectiveness of the separation. Physically, this separation occurs by difference in solubility of the components in the solvent. Also similar to absorption, the use of a solvent is often accompanied by an additional separation process like distillation to recover the solvent from the solute. <ref name = "P&T"> Peters M.S., Timmerhaus K.D. Plant Design and Economics for Chemical Engineers. 5th ed. New York: McGraw Hill; 2003. </ref>

===Distillation===
Distillation is by far the most common separation process in practice, separating based on vapor-liquid equilibrium. The process is very effective in separating liquid components, as long as they have different boiling points and do not have an azeotrope. Distillation often requires high temperatures, so operating costs can be relatively high compared with other processes. However, distillation does not require any additional separation-aiding streams such as solvent, and as such does not require an additional utility to complete the separation as absorption and extraction do.

Distillation columns are accompanied by a reboiler at the bottom and a condenser at the top. The condenser can be a total condenser, resulting in one liquid distillate stream, or a partial condenser, resulting in a liquid distillate stream and a gaseous overhead stream. These components are the major energy consuming components of a distillation column. The liquid exiting the condenser is split into two streams: reflux and distillate. The proportion of liquid returning to the column relative to the liquid leaving as distillate is a key design parameter of the column, known as reflux ratio.

Each stage of a distillation column exists under its own conditions and equilibrium. The design of the tray itself can also critically impact the performance of the column. This capability exists in HYSYS, though the focus of this page's tutorials is on more general operation of the column. <ref name = "Wankat" />

==Simulating Separations in HYSYS==
===Shortcut Distillation Column<ref name = "HYSYS"> Aspentech. "Aspen HYSYS V8.0 Help Documentation". Published 2014 </ref>===

The shortcut distillation utility within HYSYS allows users to quickly model a distillation separation of a stream with relatively little known information. Once the user attaches a fully defined inlet stream to the shortcut distillation column, the essential information that must be specified are as follows:

* The vapor/liquid quality of the top product
* The light key component and composition in the bottoms product
* The heavy key component and composition in the distillate (top) product
* The condenser pressure
* The reboiler pressure
* The external reflux ratio

With this information, the utility will run and, if possible, converge on a solution that will meet the key component specifications. The critical information that the utility will calculate are as follows:

* Fully specified distillate product stream (temperature, pressure, flow rate, composition)
* Fully specified bottom product stream (temperature, pressure, flow rate, composition)
* Condenser operating temperature and heat duty
* Reboiler operating temperature and heat duty
* Minimum number of trays
* Actual number of trays
* Optimum feed tray location

The primary purpose of the shortcut distillation is to allow for users to perform a first pass estimate on the performance requirements of a given system for a specified reflux ratio. The values gathered from the shortcut distillation utility can be used to assist the user when setting up the complete distillation utility in HYSYS, or when attempting to find reasonable column properties to allow the system to converge (e.g. number of trays, feed location, etc.)

===Standard Distillation Column<ref name = "HYSYS" />===
The standard distillation column, called "Distillation Sub-Flowsheet" the "Columns" tab on the equipment palette, may be used to perform a more rigorous simulation of your process than that offered by the shortcut column. In addition, the shortcut column may be used as a "first pass" analysis and then these specification estimations can used as the starting point for the standard column.

The standard distillation column provides many options for customization, allowing a user to specify a partial or total condenser, the number of passes through the column, and the type of reboiler all within the initial setup phase of creating a column. In addition, many specifications and parameters can be customized once the column is running, such as purities, recoveries, temperature profiles, column duties, and even internal specifications such as tray type, tray diameter, and weir height. This flexibility allows for much more rigorous, accurate simulation of a distillation column than using a shortcut column.

====Manipulating Column Specifications====

Column specifications can be heavily manipulated to force a column to operate under specific conditions, to produce a stream with specific purity, and even to minimize utilities. See the tutorial section for an example of this usage.

''Information taken from Towler Chapter 17'' <ref name = "Towler"> Towler, G. Sinnott, "Chemical Engineering Design". Elsevier, 2012, Chapter 17: R. Separation Columns (Distillation, Absorption, and Extraction). </ref>

Parameters that directly impact the design and operation of the distillation column include:
*Reflux ratio
*Feed point location
*Column Pressure
*Number of equilibrium stages/real stages

Assuming that we have already specified a desired specification for either the distillate or bottoms stream, we can manipulate the above parameters when running our simulation within HYSYS.

'''Reflux Ratio'''

The number of stages required for separation is directly affected by the reflux ratio that can be defined as follows:

<math> R=\frac {Flow\ returned\ as\ reflux} {Flow\ of\ top\ product\ taken\ off} </math>

Increasing the reflux ratio will reduce the total number of stages required but will increase the utility costs for the column. Therefore, when simulating a distillation column, special care should be taken to strike a balance between capital and operating costs.

When starting a new distillation column simulation within HYSYS, it is a good idea to calculate a minimum reflux ratio to advise the specification of the reflux ratio. The minimum reflux ratio can be calculated through use of a McCabe-Thiele diagram. According to Towler, a reflux ratio of 1.15 times the minimum reflux ratio should be used as a first approximation. Once this ratio is inputted into the simulation environment, it can be further manipulated to fine-tune the optimum reflux ratio for the system. <ref name = "Towler" />

'''Feed Point Location'''

The column simulation environment within HYSYS allows for manipulation of the feed point location. The general rule for designating feed point location is to have the feed enter the column at the stage which best matches the composition of the feed. There can be some uncertainty when it comes to finding the precise feed point location. Instead, a trial-and-error method may be useful when using HYSYS to determine which feed stage results in the desired distillate and bottoms conditions.

'''Column Pressure'''

When specifying the distillation column within the HYSYS simulation environment, you must specify the operating pressure as well as any pressure drops. If you are running a HYSYS simulation to get a first-pass approximation of compositions and flow rates, it is a typical practice to assume the pressure in the column is constant. If you are running a more intensive and rigorous simulation, you can approximate the pressure drop per tray using the following equation:

<math> \Delta P_{\text{tray}} = 2 \rho_L g h_w </math>

where <math> \rho_L </math> is the liquid density, g is gravitational acceleration, and <math> h_w </math> is the weir height. <ref name = "Towler" />

==Tutorials==
===Shortcut Distillation Column<ref name = "HYSYS" />===

This example will simulate the separation of benzene, toluene and p-xylene from some upstream process.<ref name = "HYSYS" />

'''Initialize''' [[File:AddComponents.PNG|thumb|center|600px|Figure 2. Adding Components in HYSYS]]
#Start Aspen HYSYS v.8 and click “new” on the main screen to open a new simulation window.
#Once on the component list screen, select “add” near the bottom of the screen
#Search the database for the desired components using the search bar, and add them to the component list by clicking “add.” In this case, we wish to add Benzene, Toluene and p-Xylene.
#On the left hand “properties” menu, click the “fluid packages” tab and select “add” to add a new package.
#Select the desired fluid package. In this case we are using Peng-Robinson
#In the bottom, left-hand corner of the screen, click “simulation” to enter the simulation environment.
'''Setup Streams''' [[File:Streams.PNG|thumb|center|300px|Figure 3. Add Streams]]
#Start by clicking the blue arrow at the top of the “palette” window and then clicking on the main flowsheet to produce a new material stream. This will create our feed stream. It is a good idea to place your other streams as well: one for distillate product and one for bottoms product.
#Add two energy streams that will be the condenser and reboiler duties.
#Double click on the newly created feed stream to pull up the worksheet for the stream. Please note that we are specifying this feed stream to demonstrate how the column would behaved with a fully specified inlet stream. The temperature, pressure, flow rate and composition of the feed stream can be specified by the user. [[File:FeedConditions.PNG|thumb|center|300px|Figure 4. Define Feed]]
#We will make the following changes within this worksheet to specify the feed stream:
#Enter “feed” into the stream name.
#Set the temperature to 20 degrees C.
#Set the pressure to 1 atm. (Be sure to select the “atm” unit from the dropdown when you begin to enter this value)
#Set the mass flow to 500 kg/hr [[File:Compositions.PNG|thumb|center|600px|Figure 5. Define Compositions]]
#On the left hand column within the worksheet, click “composition” to enter the composition tab.
#Click “edit” on the composition tab to change the composition of the feed stream
#Enter .25 to Toluene, .35 to Benzene and .40 to p-Xylene.
The worksheet should indicate that the stream is fully specified with a green “ok” indicator at the bottom. Close the worksheet.
'''Fully Specify Shortcut Column''' [[File:ConnectStreams.PNG|thumb|center|600px|Figure 6. Connect the inlet and outlet streams]]
#Create the shortcut column by clicking on the "columns" tab within the palette and selecting "shortcut column"
#Click on the simulation environment to place the column
#The shortcut column should currently appear red to represent that it is not currently specified.
#Double click on the column to pull up the column worksheet
#Under each drop down, add the appropriate stream: E.g. Under "Distillate" add the distillate product stream that was defined earlier.
#Additionally, be sure to specify the quality of the top product stream: whether it is liquid or vapor phase. In this case, select "liquid." [[File:Set Keys.PNG|thumb|center|600px|Figure 7. Set the key specifications]]
#On the left hand side of the worksheet, select "Parameters."
#This will move you to the parameters section of the column window where column specifications can be added.
#At this point it is important to identify which components are most volatile. In this case, benzene is most volatile, xylene is the least volatile and toluene rests in between the two. One rule of thumb is that more volatile components tend to be more valuable, and so in this example we will be purifying benzene in the distillate stream. In order to do this we will specify that benzene is the light key in the bottoms and that toluene is the heavy key in the distillate. This arrangement makes sense since it will force the heavier xylene and toluene products to drop out in the bottoms.
#Set the Light Key in Bottoms to Benzene and the mole fraction to .001 and set the Heavy Key in Distillate to Toluene and the mole fraction to .001.
#Set the condenser and reboiler pressure to the approximate values that they would be. In this case, we are assuming that our column is sufficiently small that atmospheric pressure is a fair assumption for both values. Enter 1 atm for both of these values. [[File:Worksheet.PNG|thumb|center|600px|Figure 8. Viewing the Worksheet]]
#Finally set the reflux ratio to a value that is higher than the minimum reflux ratio that HYSYS calculates. In this case the minimum reflux ratio was calculated to be 2.119 and so it is best to input around 2.5. This value can be tweaked later to see how performance is changed for different reflux ratios.
#At this point it is worthwhile to see what outputs the column is producing, since this is one of the primary uses of the shortcut distillation column.
#Click on the "worksheet" tab at the top of the window. This will bring you to your stream information where you can view the temperatures and flow rates of your outgoing streams.
#Now click on the "compositions" tab on the left side to view the compositions of the outgoing stream. As expected, the benzene is leaving in the distillate stream at 99.9% purity with a .1% toluene impurity, while all of the xylene is leaving out in the bottoms. [[File:Performance.PNG|thumb|center|600px|Figure 9. View the Performance Tab]]
#At this point it is valuable to record the calculated minimum number of trays, actual number of trays, as well as the condenser/reboiler temperatures.
#To view these values click on the "performance" tab
#You can observe here that the minimum number of trays calculated was around 15.7, the actual number of trays used was 35.7 the optimal feed stage was around 20.5, the condenser temperature was found to be around 80 C, while the reboiler was calculated to be around 123 C.

===Standard Distillation Column<ref name = "HYSYS" />===
====General Setup and Use====

In this example, we will be setting up a basic distillation column in HYSYS to simulate the separation of benzene and toluene.

# Start a new case in HYSYS by clicking on the button on the top menu bar.
# On the Components screen, add Benzene and Toluene. [[File:Columncomponents.PNG|thumb|center|300px|Figure 10. Component Selection in HYSYS]]
# On the Fluid Package screen, you have a wide range of options with which to simulate vapor-liquid equilibrium. For this example, choose the Peng-Robinson fluid package and enter the simulation environment. [[File:Fluidpkg.PNG|thumb|center|400px|Figure 11. Fluid Package Selection in HYSYS]]
We are using the simple distillation column and thus we only need a single feed stream into the distillation column as well as a distillate stream and a bottoms stream. We will also need 2 energy streams for the condenser and reboiler. <ref name = "HYSYS" />
# Add the material and energy streams to the simulation environment. Naming them appropriately will make it easer to keep everything straight as we move forward with the tutorial.
# Select the distillation column from the palette and place anywhere in the simulation environment.
# Switch to “Attach Mode” and attach the mass and energy streams to their appropriate locations on the distillation column. NOTE: You could also complete this step by double-clicking on the column itself and selecting each respective stream from the drop down menus.
# Double click on the column and select the condenser type. For this example, select “total condenser” and click next
# This next page is where the pressures of the column are specified. For this example, we will assume that the column is run at 1 atm with no pressure drops within the column. Therefore, specify reboiler pressure and condenser pressure to 101.3 kPa and the condenser pressure drop to 0 kPa.
# The next page has fields where temperature estimates can be made for the condenser, top stage, and reboiler. As these are optional fields, simply click next.
# The final screen of the distillation column setup is where the reflux ratio and distillate flow rates should be specified. For this example, specify the distillate flow rate as 1000 kgmol/hr and the reflux ratio as 1.5.
# Click “Done” at the bottom of the screen and the simulation page should be back in full view. [[File:Simulationview.PNG|thumb|center|400px|Figure 12. Simulation View of Distillation Column in HYSYS]]
We are now ready to specify the flow rates and compositions of the streams coming in and out of the column.
# Double click on the feed stream coming into the column. A specifications page should come up.
# This page allows you specify a number of parameters pertaining to the feed stream, including temperature, pressure, and compositions. Specify the temperature as 37C and pressure as 101.3 kPa (1 atm). In addition, specify the molar flow as 3500 kgmole/h.
# Now click on the composition tab on the left side of the window. Here specify, the benzene as 70 mass % and the toluene as 30 mass %. The stream should be fully specified. Close the window to return to the simulation. [[File:Configurationpage.PNG|thumb|center|400px|Figure 13. Column Configuration Page]]
The column and feed stream should now be fully specified. It is now time to run the simulation.
# If the solver is not already activated, click the green traffic light at the top of the simulation window.
# Now double click on the distillation column to bring up the main column window. A feed stage and total number of stages should already be specified.
# Click the “Run” button of the window to start the simulation. The small rectangle to the right of this button should light up green and say “converged” when it finds a solution. [[File:Columncompositions.PNG|thumb|center|400px|Figure 14. Outlet compositions in HYSYS]]

====Designing to Meet Specifications====
This example demonstrates how to use a distillation column simulation in HYSYS to design a column that meets preset requirements, and how to optimize the column to minimize costs. Basic setup of the column is assumed understood as described in the above tutorial. In this tutorial, the example requirements are to design a column that separates a stream of 20 mol% methanol and 80 mol% water into a distillate stream with 97% purity methanol and high recovery. In addition, both capital and operating costs are desired to be minimized.

# Create a simulation with water and methanol as the two components, using the NRTL fluid package.
# Place a standard distillation column in the simulation environment, and define a feed stream with vapor fraction 0, temperature 25 C, pressure 1 atm, flow rate 10 kgmol/h, and a composition of 0.2 mol fraction methanol and the balance of water.
# Enter column setup. Define the column as having a total condenser, 8 stages, and the feed entering at stage 4. The column should be once-through with a regular reboiler. Condenser pressure should be 1 atm, while reboiler pressure will be set at 1.3 kPa, giving a small pressure drop through the column. Optional temperature estimates can be left empty. Reflux ratio should be specified as 1.5, and liquid rate as 8.2 kgmol/h. In a normal situation, many of these values can be initially estimated using a shortcut column and then using them here, in the first run of a standard column simulation. Finally, run the column simulation, and it should easily converge. [[File:Column_converged_initially.PNG|thumb|center|800px|Figure 15. Initial column converged]]
# Now begins the refinement and optimization portion of the tutorial. To do so, in the distillation column window, switch to the Monitor tab in the Design toolbar on the left. We will add four new specifications of interest: distillate methanol purity, distillate methanol recovery, condenser duty, and reboiler duty. [[File:Monitor_tab.PNG|thumb|center|800px|Figure 16. The monitor tab in HYSYS]]
# To add a new specification, click the button labeled "Add Spec..." at the bottom of the window. This first spec will be for distillate methanol purity. In the new popup window, choose "Column Component Fraction" from the list of specs. In the new window, the name of the spec can be changed, and the specification itself can be defined. Change "Target Type" to "Stream," change "Draw" to the distillate stream, and change "Component" to methanol. Close this window, and the new specification should now be included in the list on the main column window. [[File:Making_a_new_spec.PNG|thumb|center|300px|Figure 17. Creating a new specification for distillate methanol mole fraction]]
# To add the recovery specification, the process is similar, expect the specification type is now "Column Component Recovery." To add the condenser and reboiler duty specifications, the spec type is "Column Duty," and the "Energy Stream" field should be chosen to be the energy stream of the condenser and reboiler for their respective specifications. Add these three specifications to the list as well.
# Note that currently, the distillate methanol fraction is only 0.244. To begin manipulation column specifications to meet the requirement of 97% purity, change the currently empty specified value for this spec to the current value, 0.244. Then, deselect Distillate Rate as an active spec, and make this new purity spec active in its stead. The simulation should automatically and instantly converge - if not, hit run. After converging, all values should remain the same.
# Begin increasing the specified value for distillate methanol purity gradually, until finding the approximate largest value at which the column simulation can still converge. In this example, this specified value can be increased up to 0.925 before the simulation fails to converge.
# Once this maximum has been reached, increase the total number of column stages by 1 or 2 by returning to the Connections tab in the Design toolbar. Note that the feed stage will automatically recalculate to stay in the same relative column position as before. In this example, change the number of stages to 10, and observe the feed stage automatically recalculate to 5. Hit run, and the simulation should still easily converge.
# Increase the specified value for purity again until the column simulation again reaches a limit beyond which it cannot converge. With 10 stages in this example, the column cannot converge with a specified purity of 0.95, but it can for a purity of 0.945. So, 0.945 is selected as the highest viable purity for this number of stages.
# Repeat this process, increasing the number of stages again followed by increasing the purity, until the desired purity is met. In this example simulation, a purity specification of 0.97 can be met with 14 stages, with the feed entering at stage 7.
# Recovery can be maximized in the same way, replacing Reflux Ratio as an active spec, though in this situation recovery is already 99.9%, so there is no need to maximize further. [[File:Requirements_met.PNG|thumb|center|800px|Figure 18. All design requirements met]]
# Now that the requirements are met, we can begin optimizing the number of stages and column duty. Condenser and reboiler duties are desired to be as low as possible to keep operating costs at a minimum, though number of stages should be kept relatively low as well. Note that with the current column setup, condenser duty is -1.83e5 kJ/h, and reboiler duty is 2.42e5 kJ/h.
# To begin, set recovery as an active spec by specifying its value as the current value, in this case 0.999, and then deselecting Reflux Ratio as an active spec and selecting the custom recovery specification.
# Next, we want to find the feed stage which minimizes column duty for this number of stages. Vary the feed stage to see which stage results in the minimum column duties. In this example, the optimal feed stage is stage 8, which results in condenser duty of -1.76e5 kJ/h and reboiler duty of 2.35e5 kJ/h.
# Now, increase the number of stages gradually, and find the duty-minimizing feed stage for each number of stages. For example, for a column with 17 stages, the optimal feed stage is 10, which yields condenser duty of -1.58e5 kJ/h and reboiler duty of 2.16e5 kJ/h.
# This process can be repeated indefinitely, as more stages will consistently result in decreased column duty. However, there are diminishing returns, so careful consideration of the balance of installation costs (more stages) vs operating costs (more column duty) must be made taking into account the actual economics of the distillation column. In this example, these diminishing returns are easily visible in the values for condenser duty: increasing from 14 to 17 stages reduces condenser duty by 0.18 kJ/h, increasing to 20 stages reduces condenser duty by a further 0.08 kJ/h, and increasing to 23 stages reduces condenser duty by only 0.03 kJ/h, at a final value of -1.47e5 kJ/h. A similar trend can be seen in the reboiler duty.

==References==
<references />

Column

2016-02-21T07:24:43Z

Benjkg: /* Background */

[[File:Distillationcolumn_singlefeed.png|thumb|right|300px|Figure 1. Distillation Column <ref name = "Towler" />]]
Authors: Ben Granger, [2016] Scott Smith, [2015] and Harry Poppick [2015] 

Stewards: Jian Gong and Fengqi You

The column utilities in HYSYS can be used to model a wide variety of chemical separation techniques including absorption, liquid-liquid extraction and three-phase distillation. This article highlights the use of HYSYS for simulations of standard chemical distillation. The two primary distillation simulation utilities are the shortcut column, which is useful for quick and cursory simulations, and the full column utility, for more involved simulations.

==HYSYS Column Types==
{| border="1" class="wikitable"
|+ Varieties of Separators in HYSYS
! Name
! Icon
! Description
|-
! Absorption Column
| [[File:Absorber.JPG|center|35px|]]|| Utility to model absorption and stripping (liquid/vapor) separations.
|-
! Refluxed Absorption Column
| [[File:RefluxedAbsorb.JPG|center|35px|]]|| Utility to model absorption and stripping (liquid/vapor) operations with condensed top products.
|-
! Absorption with Reboil
| [[File:ReboiledAbsorber.JPG|center|35px|]]|| Utility to model absorption and stripping (liquid/vapor) operations with reboiled bottoms.
|-
! Liquid Liquid Extractor
| [[File:LiquidLiquid.JPG|center|35px|]]|| Utility to model liquid-liquid phase separations.
|-
! Three Phase Distillation
| [[File:ThreePhase.JPG|center|35px|]]|| Utility to model distillation consisting of three phases internally.
|-
! Distillation Sub-Flowsheet
| [[File:DistillationColumn.JPG|center|35px|]]|| Overall two-phase distillation utility.
|-
! Shortcut Column
| [[File:ShortcutDistillation.JPG|center|35px|]]|| Approximate model of distillation processes.
|-
! Component Splitter
| [[File:ComponentSplitter.JPG|center|35px|]]|| Utility to force components to separate with no equilibrium considerations.
|}

==Background==
Of the many different types of separating columns available in HYSYS listed in the table above, there are three main categories: absorption, extraction, and distillation. Each general type of separation technique has its own benefits. A specific separation may be simple using one technique while difficult or impossible using another; as such, understanding the nature of these separation techniques is important in selecting the best method for a given separation.

===Absorption===
Absorption is primarily used to separate a solute from a stream in the gas state. The separation is conducted by introducing an additional liquid solvent stream to absorb the solute from the gas stream. Selection of this solvent is critical to the effectiveness of the separation. Note that to reduce costs, the liquid solvent is often desirable to be extracted after this separation, so absorption is typically accompanied by a second separation process such as stripping to recover the liquid solvent from the solute. <ref name = "Wankat"> Wankat, P.C. Separation Process Engineering. Upper Saddle River: Prentice-Hall; 2012 </ref>

===Extraction===
Liquid-liquid extraction is typically used when distillation is unfeasible, such as separating components with an azeotrope or with very similar boiling points. Like absorption, liquid-liquid extraction requires the introduction an additional liquid solvent stream to separate one of the components. Again, selection of this solvent is critical to the effectiveness of the separation. Physically, this separation occurs by difference in solubility of the components in the solvent. Also similar to absorption, the use of a solvent is often accompanied by an additional separation process like distillation to recover the solvent from the solute. <ref name = "P&T"> Peters M.S., Timmerhaus K.D. Plant Design and Economics for Chemical Engineers. 5th ed. New York: McGraw Hill; 2003. </ref>

===Distillation===
Distillation is by far the most common separation process in practice, separating based on vapor-liquid equilibrium. The process is very effective in separating liquid components, as long as they have different boiling points and do not have an azeotrope. Distillation often requires high temperatures, so operating costs can be relatively high compared with other processes. However, distillation does not require any additional separation-aiding streams such as solvent, and as such does not require an additional utility to complete the separation as absorption and extraction do. <ref name = "Wankat" />

==Simulating Separations in HYSYS==
===Shortcut Distillation Column<ref name = "HYSYS"> Aspentech. "Aspen HYSYS V8.0 Help Documentation". Published 2014 </ref>===

The shortcut distillation utility within HYSYS allows users to quickly model a distillation separation of a stream with relatively little known information. Once the user attaches a fully defined inlet stream to the shortcut distillation column, the essential information that must be specified are as follows:

* The vapor/liquid quality of the top product
* The light key component and composition in the bottoms product
* The heavy key component and composition in the distillate (top) product
* The condenser pressure
* The reboiler pressure
* The external reflux ratio

With this information, the utility will run and, if possible, converge on a solution that will meet the key component specifications. The critical information that the utility will calculate are as follows:

* Fully specified distillate product stream (temperature, pressure, flow rate, composition)
* Fully specified bottom product stream (temperature, pressure, flow rate, composition)
* Condenser operating temperature and heat duty
* Reboiler operating temperature and heat duty
* Minimum number of trays
* Actual number of trays
* Optimum feed tray location

The primary purpose of the shortcut distillation is to allow for users to perform a first pass estimate on the performance requirements of a given system for a specified reflux ratio. The values gathered from the shortcut distillation utility can be used to assist the user when setting up the complete distillation utility in HYSYS, or when attempting to find reasonable column properties to allow the system to converge (e.g. number of trays, feed location, etc.)

===Standard Distillation Column<ref name = "HYSYS" />===
The standard distillation column, called "Distillation Sub-Flowsheet" the "Columns" tab on the equipment palette, may be used to perform a more rigorous simulation of your process than that offered by the shortcut column. In addition, the shortcut column may be used as a "first pass" analysis and then these specification estimations can used as the starting point for the standard column.

The standard distillation column provides many options for customization, allowing a user to specify a partial or total condenser, the number of passes through the column, and the type of reboiler all within the initial setup phase of creating a column. In addition, many specifications and parameters can be customized once the column is running, such as purities, recoveries, temperature profiles, column duties, and even internal specifications such as tray type, tray diameter, and weir height. This flexibility allows for much more rigorous, accurate simulation of a distillation column than using a shortcut column.

====Manipulating Column Specifications====

Column specifications can be heavily manipulated to force a column to operate under specific conditions, to produce a stream with specific purity, and even to minimize utilities. See the tutorial section for an example of this usage.

''Information taken from Towler Chapter 17'' <ref name = "Towler"> Towler, G. Sinnott, "Chemical Engineering Design". Elsevier, 2012, Chapter 17: R. Separation Columns (Distillation, Absorption, and Extraction). </ref>

Parameters that directly impact the design and operation of the distillation column include:
*Reflux ratio
*Feed point location
*Column Pressure
*Number of equilibrium stages/real stages

Assuming that we have already specified a desired specification for either the distillate or bottoms stream, we can manipulate the above parameters when running our simulation within HYSYS.

'''Reflux Ratio'''

The number of stages required for separation is directly affected by the reflux ratio that can be defined as follows:

<math> R=\frac {Flow\ returned\ as\ reflux} {Flow\ of\ top\ product\ taken\ off} </math>

Increasing the reflux ratio will reduce the total number of stages required but will increase the utility costs for the column. Therefore, when simulating a distillation column, special care should be taken to strike a balance between capital and operating costs.

When starting a new distillation column simulation within HYSYS, it is a good idea to calculate a minimum reflux ratio to advise the specification of the reflux ratio. The minimum reflux ratio can be calculated through use of a McCabe-Thiele diagram. According to Towler, a reflux ratio of 1.15 times the minimum reflux ratio should be used as a first approximation. Once this ratio is inputted into the simulation environment, it can be further manipulated to fine-tune the optimum reflux ratio for the system. <ref name = "Towler" />

'''Feed Point Location'''

The column simulation environment within HYSYS allows for manipulation of the feed point location. The general rule for designating feed point location is to have the feed enter the column at the stage which best matches the composition of the feed. There can be some uncertainty when it comes to finding the precise feed point location. Instead, a trial-and-error method may be useful when using HYSYS to determine which feed stage results in the desired distillate and bottoms conditions.

'''Column Pressure'''

When specifying the distillation column within the HYSYS simulation environment, you must specify the operating pressure as well as any pressure drops. If you are running a HYSYS simulation to get a first-pass approximation of compositions and flow rates, it is a typical practice to assume the pressure in the column is constant. If you are running a more intensive and rigorous simulation, you can approximate the pressure drop per tray using the following equation:

<math> \Delta P_{\text{tray}} = 2 \rho_L g h_w </math>

where <math> \rho_L </math> is the liquid density, g is gravitational acceleration, and <math> h_w </math> is the weir height. <ref name = "Towler" />

==Tutorials==
===Shortcut Distillation Column<ref name = "HYSYS" />===

This example will simulate the separation of benzene, toluene and p-xylene from some upstream process.<ref name = "HYSYS" />

'''Initialize''' [[File:AddComponents.PNG|thumb|center|600px|Figure 2. Adding Components in HYSYS]]
#Start Aspen HYSYS v.8 and click “new” on the main screen to open a new simulation window.
#Once on the component list screen, select “add” near the bottom of the screen
#Search the database for the desired components using the search bar, and add them to the component list by clicking “add.” In this case, we wish to add Benzene, Toluene and p-Xylene.
#On the left hand “properties” menu, click the “fluid packages” tab and select “add” to add a new package.
#Select the desired fluid package. In this case we are using Peng-Robinson
#In the bottom, left-hand corner of the screen, click “simulation” to enter the simulation environment.
'''Setup Streams''' [[File:Streams.PNG|thumb|center|300px|Figure 3. Add Streams]]
#Start by clicking the blue arrow at the top of the “palette” window and then clicking on the main flowsheet to produce a new material stream. This will create our feed stream. It is a good idea to place your other streams as well: one for distillate product and one for bottoms product.
#Add two energy streams that will be the condenser and reboiler duties.
#Double click on the newly created feed stream to pull up the worksheet for the stream. Please note that we are specifying this feed stream to demonstrate how the column would behaved with a fully specified inlet stream. The temperature, pressure, flow rate and composition of the feed stream can be specified by the user. [[File:FeedConditions.PNG|thumb|center|300px|Figure 4. Define Feed]]
#We will make the following changes within this worksheet to specify the feed stream:
#Enter “feed” into the stream name.
#Set the temperature to 20 degrees C.
#Set the pressure to 1 atm. (Be sure to select the “atm” unit from the dropdown when you begin to enter this value)
#Set the mass flow to 500 kg/hr [[File:Compositions.PNG|thumb|center|600px|Figure 5. Define Compositions]]
#On the left hand column within the worksheet, click “composition” to enter the composition tab.
#Click “edit” on the composition tab to change the composition of the feed stream
#Enter .25 to Toluene, .35 to Benzene and .40 to p-Xylene.
The worksheet should indicate that the stream is fully specified with a green “ok” indicator at the bottom. Close the worksheet.
'''Fully Specify Shortcut Column''' [[File:ConnectStreams.PNG|thumb|center|600px|Figure 6. Connect the inlet and outlet streams]]
#Create the shortcut column by clicking on the "columns" tab within the palette and selecting "shortcut column"
#Click on the simulation environment to place the column
#The shortcut column should currently appear red to represent that it is not currently specified.
#Double click on the column to pull up the column worksheet
#Under each drop down, add the appropriate stream: E.g. Under "Distillate" add the distillate product stream that was defined earlier.
#Additionally, be sure to specify the quality of the top product stream: whether it is liquid or vapor phase. In this case, select "liquid." [[File:Set Keys.PNG|thumb|center|600px|Figure 7. Set the key specifications]]
#On the left hand side of the worksheet, select "Parameters."
#This will move you to the parameters section of the column window where column specifications can be added.
#At this point it is important to identify which components are most volatile. In this case, benzene is most volatile, xylene is the least volatile and toluene rests in between the two. One rule of thumb is that more volatile components tend to be more valuable, and so in this example we will be purifying benzene in the distillate stream. In order to do this we will specify that benzene is the light key in the bottoms and that toluene is the heavy key in the distillate. This arrangement makes sense since it will force the heavier xylene and toluene products to drop out in the bottoms.
#Set the Light Key in Bottoms to Benzene and the mole fraction to .001 and set the Heavy Key in Distillate to Toluene and the mole fraction to .001.
#Set the condenser and reboiler pressure to the approximate values that they would be. In this case, we are assuming that our column is sufficiently small that atmospheric pressure is a fair assumption for both values. Enter 1 atm for both of these values. [[File:Worksheet.PNG|thumb|center|600px|Figure 8. Viewing the Worksheet]]
#Finally set the reflux ratio to a value that is higher than the minimum reflux ratio that HYSYS calculates. In this case the minimum reflux ratio was calculated to be 2.119 and so it is best to input around 2.5. This value can be tweaked later to see how performance is changed for different reflux ratios.
#At this point it is worthwhile to see what outputs the column is producing, since this is one of the primary uses of the shortcut distillation column.
#Click on the "worksheet" tab at the top of the window. This will bring you to your stream information where you can view the temperatures and flow rates of your outgoing streams.
#Now click on the "compositions" tab on the left side to view the compositions of the outgoing stream. As expected, the benzene is leaving in the distillate stream at 99.9% purity with a .1% toluene impurity, while all of the xylene is leaving out in the bottoms. [[File:Performance.PNG|thumb|center|600px|Figure 9. View the Performance Tab]]
#At this point it is valuable to record the calculated minimum number of trays, actual number of trays, as well as the condenser/reboiler temperatures.
#To view these values click on the "performance" tab
#You can observe here that the minimum number of trays calculated was around 15.7, the actual number of trays used was 35.7 the optimal feed stage was around 20.5, the condenser temperature was found to be around 80 C, while the reboiler was calculated to be around 123 C.

===Standard Distillation Column<ref name = "HYSYS" />===
====General Setup and Use====

In this example, we will be setting up a basic distillation column in HYSYS to simulate the separation of benzene and toluene.

# Start a new case in HYSYS by clicking on the button on the top menu bar.
# On the Components screen, add Benzene and Toluene. [[File:Columncomponents.PNG|thumb|center|300px|Figure 10. Component Selection in HYSYS]]
# On the Fluid Package screen, you have a wide range of options with which to simulate vapor-liquid equilibrium. For this example, choose the Peng-Robinson fluid package and enter the simulation environment. [[File:Fluidpkg.PNG|thumb|center|400px|Figure 11. Fluid Package Selection in HYSYS]]
We are using the simple distillation column and thus we only need a single feed stream into the distillation column as well as a distillate stream and a bottoms stream. We will also need 2 energy streams for the condenser and reboiler. <ref name = "HYSYS" />
# Add the material and energy streams to the simulation environment. Naming them appropriately will make it easer to keep everything straight as we move forward with the tutorial.
# Select the distillation column from the palette and place anywhere in the simulation environment.
# Switch to “Attach Mode” and attach the mass and energy streams to their appropriate locations on the distillation column. NOTE: You could also complete this step by double-clicking on the column itself and selecting each respective stream from the drop down menus.
# Double click on the column and select the condenser type. For this example, select “total condenser” and click next
# This next page is where the pressures of the column are specified. For this example, we will assume that the column is run at 1 atm with no pressure drops within the column. Therefore, specify reboiler pressure and condenser pressure to 101.3 kPa and the condenser pressure drop to 0 kPa.
# The next page has fields where temperature estimates can be made for the condenser, top stage, and reboiler. As these are optional fields, simply click next.
# The final screen of the distillation column setup is where the reflux ratio and distillate flow rates should be specified. For this example, specify the distillate flow rate as 1000 kgmol/hr and the reflux ratio as 1.5.
# Click “Done” at the bottom of the screen and the simulation page should be back in full view. [[File:Simulationview.PNG|thumb|center|400px|Figure 12. Simulation View of Distillation Column in HYSYS]]
We are now ready to specify the flow rates and compositions of the streams coming in and out of the column.
# Double click on the feed stream coming into the column. A specifications page should come up.
# This page allows you specify a number of parameters pertaining to the feed stream, including temperature, pressure, and compositions. Specify the temperature as 37C and pressure as 101.3 kPa (1 atm). In addition, specify the molar flow as 3500 kgmole/h.
# Now click on the composition tab on the left side of the window. Here specify, the benzene as 70 mass % and the toluene as 30 mass %. The stream should be fully specified. Close the window to return to the simulation. [[File:Configurationpage.PNG|thumb|center|400px|Figure 13. Column Configuration Page]]
The column and feed stream should now be fully specified. It is now time to run the simulation.
# If the solver is not already activated, click the green traffic light at the top of the simulation window.
# Now double click on the distillation column to bring up the main column window. A feed stage and total number of stages should already be specified.
# Click the “Run” button of the window to start the simulation. The small rectangle to the right of this button should light up green and say “converged” when it finds a solution. [[File:Columncompositions.PNG|thumb|center|400px|Figure 14. Outlet compositions in HYSYS]]

====Designing to Meet Specifications====
This example demonstrates how to use a distillation column simulation in HYSYS to design a column that meets preset requirements, and how to optimize the column to minimize costs. Basic setup of the column is assumed understood as described in the above tutorial. In this tutorial, the example requirements are to design a column that separates a stream of 20 mol% methanol and 80 mol% water into a distillate stream with 97% purity methanol and high recovery. In addition, both capital and operating costs are desired to be minimized.

# Create a simulation with water and methanol as the two components, using the NRTL fluid package.
# Place a standard distillation column in the simulation environment, and define a feed stream with vapor fraction 0, temperature 25 C, pressure 1 atm, flow rate 10 kgmol/h, and a composition of 0.2 mol fraction methanol and the balance of water.
# Enter column setup. Define the column as having a total condenser, 8 stages, and the feed entering at stage 4. The column should be once-through with a regular reboiler. Condenser pressure should be 1 atm, while reboiler pressure will be set at 1.3 kPa, giving a small pressure drop through the column. Optional temperature estimates can be left empty. Reflux ratio should be specified as 1.5, and liquid rate as 8.2 kgmol/h. In a normal situation, many of these values can be initially estimated using a shortcut column and then using them here, in the first run of a standard column simulation. Finally, run the column simulation, and it should easily converge. [[File:Column_converged_initially.PNG|thumb|center|800px|Figure 15. Initial column converged]]
# Now begins the refinement and optimization portion of the tutorial. To do so, in the distillation column window, switch to the Monitor tab in the Design toolbar on the left. We will add four new specifications of interest: distillate methanol purity, distillate methanol recovery, condenser duty, and reboiler duty. [[File:Monitor_tab.PNG|thumb|center|800px|Figure 16. The monitor tab in HYSYS]]
# To add a new specification, click the button labeled "Add Spec..." at the bottom of the window. This first spec will be for distillate methanol purity. In the new popup window, choose "Column Component Fraction" from the list of specs. In the new window, the name of the spec can be changed, and the specification itself can be defined. Change "Target Type" to "Stream," change "Draw" to the distillate stream, and change "Component" to methanol. Close this window, and the new specification should now be included in the list on the main column window. [[File:Making_a_new_spec.PNG|thumb|center|300px|Figure 17. Creating a new specification for distillate methanol mole fraction]]
# To add the recovery specification, the process is similar, expect the specification type is now "Column Component Recovery." To add the condenser and reboiler duty specifications, the spec type is "Column Duty," and the "Energy Stream" field should be chosen to be the energy stream of the condenser and reboiler for their respective specifications. Add these three specifications to the list as well.
# Note that currently, the distillate methanol fraction is only 0.244. To begin manipulation column specifications to meet the requirement of 97% purity, change the currently empty specified value for this spec to the current value, 0.244. Then, deselect Distillate Rate as an active spec, and make this new purity spec active in its stead. The simulation should automatically and instantly converge - if not, hit run. After converging, all values should remain the same.
# Begin increasing the specified value for distillate methanol purity gradually, until finding the approximate largest value at which the column simulation can still converge. In this example, this specified value can be increased up to 0.925 before the simulation fails to converge.
# Once this maximum has been reached, increase the total number of column stages by 1 or 2 by returning to the Connections tab in the Design toolbar. Note that the feed stage will automatically recalculate to stay in the same relative column position as before. In this example, change the number of stages to 10, and observe the feed stage automatically recalculate to 5. Hit run, and the simulation should still easily converge.
# Increase the specified value for purity again until the column simulation again reaches a limit beyond which it cannot converge. With 10 stages in this example, the column cannot converge with a specified purity of 0.95, but it can for a purity of 0.945. So, 0.945 is selected as the highest viable purity for this number of stages.
# Repeat this process, increasing the number of stages again followed by increasing the purity, until the desired purity is met. In this example simulation, a purity specification of 0.97 can be met with 14 stages, with the feed entering at stage 7.
# Recovery can be maximized in the same way, replacing Reflux Ratio as an active spec, though in this situation recovery is already 99.9%, so there is no need to maximize further. [[File:Requirements_met.PNG|thumb|center|800px|Figure 18. All design requirements met]]
# Now that the requirements are met, we can begin optimizing the number of stages and column duty. Condenser and reboiler duties are desired to be as low as possible to keep operating costs at a minimum, though number of stages should be kept relatively low as well. Note that with the current column setup, condenser duty is -1.83e5 kJ/h, and reboiler duty is 2.42e5 kJ/h.
# To begin, set recovery as an active spec by specifying its value as the current value, in this case 0.999, and then deselecting Reflux Ratio as an active spec and selecting the custom recovery specification.
# Next, we want to find the feed stage which minimizes column duty for this number of stages. Vary the feed stage to see which stage results in the minimum column duties. In this example, the optimal feed stage is stage 8, which results in condenser duty of -1.76e5 kJ/h and reboiler duty of 2.35e5 kJ/h.
# Now, increase the number of stages gradually, and find the duty-minimizing feed stage for each number of stages. For example, for a column with 17 stages, the optimal feed stage is 10, which yields condenser duty of -1.58e5 kJ/h and reboiler duty of 2.16e5 kJ/h.
# This process can be repeated indefinitely, as more stages will consistently result in decreased column duty. However, there are diminishing returns, so careful consideration of the balance of installation costs (more stages) vs operating costs (more column duty) must be made taking into account the actual economics of the distillation column. In this example, these diminishing returns are easily visible in the values for condenser duty: increasing from 14 to 17 stages reduces condenser duty by 0.18 kJ/h, increasing to 20 stages reduces condenser duty by a further 0.08 kJ/h, and increasing to 23 stages reduces condenser duty by only 0.03 kJ/h, at a final value of -1.47e5 kJ/h. A similar trend can be seen in the reboiler duty.

==References==
<references />

Column

2016-02-21T07:22:37Z

Benjkg: Added background into distillation

[[File:Distillationcolumn_singlefeed.png|thumb|right|300px|Figure 1. Distillation Column <ref name = "Towler" />]]
Authors: Ben Granger, [2016] Scott Smith, [2015] and Harry Poppick [2015] 

Stewards: Jian Gong and Fengqi You

The column utilities in HYSYS can be used to model a wide variety of chemical separation techniques including absorption, liquid-liquid extraction and three-phase distillation. This article highlights the use of HYSYS for simulations of standard chemical distillation. The two primary distillation simulation utilities are the shortcut column, which is useful for quick and cursory simulations, and the full column utility, for more involved simulations.

==HYSYS Column Types==
{| border="1" class="wikitable"
|+ Varieties of Separators in HYSYS
! Name
! Icon
! Description
|-
! Absorption Column
| [[File:Absorber.JPG|center|35px|]]|| Utility to model absorption and stripping (liquid/vapor) separations.
|-
! Refluxed Absorption Column
| [[File:RefluxedAbsorb.JPG|center|35px|]]|| Utility to model absorption and stripping (liquid/vapor) operations with condensed top products.
|-
! Absorption with Reboil
| [[File:ReboiledAbsorber.JPG|center|35px|]]|| Utility to model absorption and stripping (liquid/vapor) operations with reboiled bottoms.
|-
! Liquid Liquid Extractor
| [[File:LiquidLiquid.JPG|center|35px|]]|| Utility to model liquid-liquid phase separations.
|-
! Three Phase Distillation
| [[File:ThreePhase.JPG|center|35px|]]|| Utility to model distillation consisting of three phases internally.
|-
! Distillation Sub-Flowsheet
| [[File:DistillationColumn.JPG|center|35px|]]|| Overall two-phase distillation utility.
|-
! Shortcut Column
| [[File:ShortcutDistillation.JPG|center|35px|]]|| Approximate model of distillation processes.
|-
! Component Splitter
| [[File:ComponentSplitter.JPG|center|35px|]]|| Utility to force components to separate with no equilibrium considerations.
|}

==Background==
Of the many different types of separating columns available in HYSYS listed in the table above, there are three main categories: absorption, extraction, and distillation. Each general type of separation technique has its own benefits. A specific separation may be simple using one technique while difficult or impossible using another; as such, understanding the nature of these separation techniques is important in selecting the best method for a given separation.

===Absorption===
Absorption is primarily used to separate a solute from a stream in the gas state. The separation is conducted by introducing an additional liquid solvent stream to absorb the solute from the gas stream. Note that to reduce costs, the liquid solvent is often desirable to be extracted after this separation, so absorption is typically accompanied by a second separation process such as stripping to recover the liquid solvent from the solute. <ref name = "Wankat"> Wankat, P.C. Separation Process Engineering. Upper Saddle River: Prentice-Hall; 2012 </ref>

===Extraction===
Liquid-liquid extraction is typically used when distillation is unfeasible, such as separating components with an azeotrope or with very similar boiling points. Like absorption, liquid-liquid extraction requires the introduction an additional liquid solvent stream to separate one of the components. Physically, this separation occurs by difference in solubility of the components in the solvent. Also similar to absorption, the use of a solvent is often accompanied by an additional separation process like distillation to recover the solvent from the solute. <ref name = "P&T"> Peters M.S., Timmerhaus K.D. Plant Design and Economics for Chemical Engineers. 5th ed. New York: McGraw Hill; 2003. </ref>

===Distillation===
Distillation is by far the most common separation process in practice, separating based on vapor-liquid equilibrium. The process is very effective in separating liquid components, as long as they have different boiling points and do not have an azeotrope. Distillation often requires high temperatures, so operating costs can be relatively high compared with other processes. However, distillation does not require any additional separation-aiding streams such as solvent, and as such does not require an additional utility to complete the separation as absorption and extraction do. <ref name = "Wankat" />

==Simulating Separations in HYSYS==
===Shortcut Distillation Column<ref name = "HYSYS"> Aspentech. "Aspen HYSYS V8.0 Help Documentation". Published 2014 </ref>===

The shortcut distillation utility within HYSYS allows users to quickly model a distillation separation of a stream with relatively little known information. Once the user attaches a fully defined inlet stream to the shortcut distillation column, the essential information that must be specified are as follows:

* The vapor/liquid quality of the top product
* The light key component and composition in the bottoms product
* The heavy key component and composition in the distillate (top) product
* The condenser pressure
* The reboiler pressure
* The external reflux ratio

With this information, the utility will run and, if possible, converge on a solution that will meet the key component specifications. The critical information that the utility will calculate are as follows:

* Fully specified distillate product stream (temperature, pressure, flow rate, composition)
* Fully specified bottom product stream (temperature, pressure, flow rate, composition)
* Condenser operating temperature and heat duty
* Reboiler operating temperature and heat duty
* Minimum number of trays
* Actual number of trays
* Optimum feed tray location

The primary purpose of the shortcut distillation is to allow for users to perform a first pass estimate on the performance requirements of a given system for a specified reflux ratio. The values gathered from the shortcut distillation utility can be used to assist the user when setting up the complete distillation utility in HYSYS, or when attempting to find reasonable column properties to allow the system to converge (e.g. number of trays, feed location, etc.)

===Standard Distillation Column<ref name = "HYSYS" />===
The standard distillation column, called "Distillation Sub-Flowsheet" the "Columns" tab on the equipment palette, may be used to perform a more rigorous simulation of your process than that offered by the shortcut column. In addition, the shortcut column may be used as a "first pass" analysis and then these specification estimations can used as the starting point for the standard column.

The standard distillation column provides many options for customization, allowing a user to specify a partial or total condenser, the number of passes through the column, and the type of reboiler all within the initial setup phase of creating a column. In addition, many specifications and parameters can be customized once the column is running, such as purities, recoveries, temperature profiles, column duties, and even internal specifications such as tray type, tray diameter, and weir height. This flexibility allows for much more rigorous, accurate simulation of a distillation column than using a shortcut column.

====Manipulating Column Specifications====

Column specifications can be heavily manipulated to force a column to operate under specific conditions, to produce a stream with specific purity, and even to minimize utilities. See the tutorial section for an example of this usage.

''Information taken from Towler Chapter 17'' <ref name = "Towler"> Towler, G. Sinnott, "Chemical Engineering Design". Elsevier, 2012, Chapter 17: R. Separation Columns (Distillation, Absorption, and Extraction). </ref>

Parameters that directly impact the design and operation of the distillation column include:
*Reflux ratio
*Feed point location
*Column Pressure
*Number of equilibrium stages/real stages

Assuming that we have already specified a desired specification for either the distillate or bottoms stream, we can manipulate the above parameters when running our simulation within HYSYS.

'''Reflux Ratio'''

The number of stages required for separation is directly affected by the reflux ratio that can be defined as follows:

<math> R=\frac {Flow\ returned\ as\ reflux} {Flow\ of\ top\ product\ taken\ off} </math>

Increasing the reflux ratio will reduce the total number of stages required but will increase the utility costs for the column. Therefore, when simulating a distillation column, special care should be taken to strike a balance between capital and operating costs.

When starting a new distillation column simulation within HYSYS, it is a good idea to calculate a minimum reflux ratio to advise the specification of the reflux ratio. The minimum reflux ratio can be calculated through use of a McCabe-Thiele diagram. According to Towler, a reflux ratio of 1.15 times the minimum reflux ratio should be used as a first approximation. Once this ratio is inputted into the simulation environment, it can be further manipulated to fine-tune the optimum reflux ratio for the system. <ref name = "Towler" />

'''Feed Point Location'''

The column simulation environment within HYSYS allows for manipulation of the feed point location. The general rule for designating feed point location is to have the feed enter the column at the stage which best matches the composition of the feed. There can be some uncertainty when it comes to finding the precise feed point location. Instead, a trial-and-error method may be useful when using HYSYS to determine which feed stage results in the desired distillate and bottoms conditions.

'''Column Pressure'''

When specifying the distillation column within the HYSYS simulation environment, you must specify the operating pressure as well as any pressure drops. If you are running a HYSYS simulation to get a first-pass approximation of compositions and flow rates, it is a typical practice to assume the pressure in the column is constant. If you are running a more intensive and rigorous simulation, you can approximate the pressure drop per tray using the following equation:

<math> \Delta P_{\text{tray}} = 2 \rho_L g h_w </math>

where <math> \rho_L </math> is the liquid density, g is gravitational acceleration, and <math> h_w </math> is the weir height. <ref name = "Towler" />

==Tutorials==
===Shortcut Distillation Column<ref name = "HYSYS" />===

This example will simulate the separation of benzene, toluene and p-xylene from some upstream process.<ref name = "HYSYS" />

'''Initialize''' [[File:AddComponents.PNG|thumb|center|600px|Figure 2. Adding Components in HYSYS]]
#Start Aspen HYSYS v.8 and click “new” on the main screen to open a new simulation window.
#Once on the component list screen, select “add” near the bottom of the screen
#Search the database for the desired components using the search bar, and add them to the component list by clicking “add.” In this case, we wish to add Benzene, Toluene and p-Xylene.
#On the left hand “properties” menu, click the “fluid packages” tab and select “add” to add a new package.
#Select the desired fluid package. In this case we are using Peng-Robinson
#In the bottom, left-hand corner of the screen, click “simulation” to enter the simulation environment.
'''Setup Streams''' [[File:Streams.PNG|thumb|center|300px|Figure 3. Add Streams]]
#Start by clicking the blue arrow at the top of the “palette” window and then clicking on the main flowsheet to produce a new material stream. This will create our feed stream. It is a good idea to place your other streams as well: one for distillate product and one for bottoms product.
#Add two energy streams that will be the condenser and reboiler duties.
#Double click on the newly created feed stream to pull up the worksheet for the stream. Please note that we are specifying this feed stream to demonstrate how the column would behaved with a fully specified inlet stream. The temperature, pressure, flow rate and composition of the feed stream can be specified by the user. [[File:FeedConditions.PNG|thumb|center|300px|Figure 4. Define Feed]]
#We will make the following changes within this worksheet to specify the feed stream:
#Enter “feed” into the stream name.
#Set the temperature to 20 degrees C.
#Set the pressure to 1 atm. (Be sure to select the “atm” unit from the dropdown when you begin to enter this value)
#Set the mass flow to 500 kg/hr [[File:Compositions.PNG|thumb|center|600px|Figure 5. Define Compositions]]
#On the left hand column within the worksheet, click “composition” to enter the composition tab.
#Click “edit” on the composition tab to change the composition of the feed stream
#Enter .25 to Toluene, .35 to Benzene and .40 to p-Xylene.
The worksheet should indicate that the stream is fully specified with a green “ok” indicator at the bottom. Close the worksheet.
'''Fully Specify Shortcut Column''' [[File:ConnectStreams.PNG|thumb|center|600px|Figure 6. Connect the inlet and outlet streams]]
#Create the shortcut column by clicking on the "columns" tab within the palette and selecting "shortcut column"
#Click on the simulation environment to place the column
#The shortcut column should currently appear red to represent that it is not currently specified.
#Double click on the column to pull up the column worksheet
#Under each drop down, add the appropriate stream: E.g. Under "Distillate" add the distillate product stream that was defined earlier.
#Additionally, be sure to specify the quality of the top product stream: whether it is liquid or vapor phase. In this case, select "liquid." [[File:Set Keys.PNG|thumb|center|600px|Figure 7. Set the key specifications]]
#On the left hand side of the worksheet, select "Parameters."
#This will move you to the parameters section of the column window where column specifications can be added.
#At this point it is important to identify which components are most volatile. In this case, benzene is most volatile, xylene is the least volatile and toluene rests in between the two. One rule of thumb is that more volatile components tend to be more valuable, and so in this example we will be purifying benzene in the distillate stream. In order to do this we will specify that benzene is the light key in the bottoms and that toluene is the heavy key in the distillate. This arrangement makes sense since it will force the heavier xylene and toluene products to drop out in the bottoms.
#Set the Light Key in Bottoms to Benzene and the mole fraction to .001 and set the Heavy Key in Distillate to Toluene and the mole fraction to .001.
#Set the condenser and reboiler pressure to the approximate values that they would be. In this case, we are assuming that our column is sufficiently small that atmospheric pressure is a fair assumption for both values. Enter 1 atm for both of these values. [[File:Worksheet.PNG|thumb|center|600px|Figure 8. Viewing the Worksheet]]
#Finally set the reflux ratio to a value that is higher than the minimum reflux ratio that HYSYS calculates. In this case the minimum reflux ratio was calculated to be 2.119 and so it is best to input around 2.5. This value can be tweaked later to see how performance is changed for different reflux ratios.
#At this point it is worthwhile to see what outputs the column is producing, since this is one of the primary uses of the shortcut distillation column.
#Click on the "worksheet" tab at the top of the window. This will bring you to your stream information where you can view the temperatures and flow rates of your outgoing streams.
#Now click on the "compositions" tab on the left side to view the compositions of the outgoing stream. As expected, the benzene is leaving in the distillate stream at 99.9% purity with a .1% toluene impurity, while all of the xylene is leaving out in the bottoms. [[File:Performance.PNG|thumb|center|600px|Figure 9. View the Performance Tab]]
#At this point it is valuable to record the calculated minimum number of trays, actual number of trays, as well as the condenser/reboiler temperatures.
#To view these values click on the "performance" tab
#You can observe here that the minimum number of trays calculated was around 15.7, the actual number of trays used was 35.7 the optimal feed stage was around 20.5, the condenser temperature was found to be around 80 C, while the reboiler was calculated to be around 123 C.

===Standard Distillation Column<ref name = "HYSYS" />===
====General Setup and Use====

In this example, we will be setting up a basic distillation column in HYSYS to simulate the separation of benzene and toluene.

# Start a new case in HYSYS by clicking on the button on the top menu bar.
# On the Components screen, add Benzene and Toluene. [[File:Columncomponents.PNG|thumb|center|300px|Figure 10. Component Selection in HYSYS]]
# On the Fluid Package screen, you have a wide range of options with which to simulate vapor-liquid equilibrium. For this example, choose the Peng-Robinson fluid package and enter the simulation environment. [[File:Fluidpkg.PNG|thumb|center|400px|Figure 11. Fluid Package Selection in HYSYS]]
We are using the simple distillation column and thus we only need a single feed stream into the distillation column as well as a distillate stream and a bottoms stream. We will also need 2 energy streams for the condenser and reboiler. <ref name = "HYSYS" />
# Add the material and energy streams to the simulation environment. Naming them appropriately will make it easer to keep everything straight as we move forward with the tutorial.
# Select the distillation column from the palette and place anywhere in the simulation environment.
# Switch to “Attach Mode” and attach the mass and energy streams to their appropriate locations on the distillation column. NOTE: You could also complete this step by double-clicking on the column itself and selecting each respective stream from the drop down menus.
# Double click on the column and select the condenser type. For this example, select “total condenser” and click next
# This next page is where the pressures of the column are specified. For this example, we will assume that the column is run at 1 atm with no pressure drops within the column. Therefore, specify reboiler pressure and condenser pressure to 101.3 kPa and the condenser pressure drop to 0 kPa.
# The next page has fields where temperature estimates can be made for the condenser, top stage, and reboiler. As these are optional fields, simply click next.
# The final screen of the distillation column setup is where the reflux ratio and distillate flow rates should be specified. For this example, specify the distillate flow rate as 1000 kgmol/hr and the reflux ratio as 1.5.
# Click “Done” at the bottom of the screen and the simulation page should be back in full view. [[File:Simulationview.PNG|thumb|center|400px|Figure 12. Simulation View of Distillation Column in HYSYS]]
We are now ready to specify the flow rates and compositions of the streams coming in and out of the column.
# Double click on the feed stream coming into the column. A specifications page should come up.
# This page allows you specify a number of parameters pertaining to the feed stream, including temperature, pressure, and compositions. Specify the temperature as 37C and pressure as 101.3 kPa (1 atm). In addition, specify the molar flow as 3500 kgmole/h.
# Now click on the composition tab on the left side of the window. Here specify, the benzene as 70 mass % and the toluene as 30 mass %. The stream should be fully specified. Close the window to return to the simulation. [[File:Configurationpage.PNG|thumb|center|400px|Figure 13. Column Configuration Page]]
The column and feed stream should now be fully specified. It is now time to run the simulation.
# If the solver is not already activated, click the green traffic light at the top of the simulation window.
# Now double click on the distillation column to bring up the main column window. A feed stage and total number of stages should already be specified.
# Click the “Run” button of the window to start the simulation. The small rectangle to the right of this button should light up green and say “converged” when it finds a solution. [[File:Columncompositions.PNG|thumb|center|400px|Figure 14. Outlet compositions in HYSYS]]

====Designing to Meet Specifications====
This example demonstrates how to use a distillation column simulation in HYSYS to design a column that meets preset requirements, and how to optimize the column to minimize costs. Basic setup of the column is assumed understood as described in the above tutorial. In this tutorial, the example requirements are to design a column that separates a stream of 20 mol% methanol and 80 mol% water into a distillate stream with 97% purity methanol and high recovery. In addition, both capital and operating costs are desired to be minimized.

# Create a simulation with water and methanol as the two components, using the NRTL fluid package.
# Place a standard distillation column in the simulation environment, and define a feed stream with vapor fraction 0, temperature 25 C, pressure 1 atm, flow rate 10 kgmol/h, and a composition of 0.2 mol fraction methanol and the balance of water.
# Enter column setup. Define the column as having a total condenser, 8 stages, and the feed entering at stage 4. The column should be once-through with a regular reboiler. Condenser pressure should be 1 atm, while reboiler pressure will be set at 1.3 kPa, giving a small pressure drop through the column. Optional temperature estimates can be left empty. Reflux ratio should be specified as 1.5, and liquid rate as 8.2 kgmol/h. In a normal situation, many of these values can be initially estimated using a shortcut column and then using them here, in the first run of a standard column simulation. Finally, run the column simulation, and it should easily converge. [[File:Column_converged_initially.PNG|thumb|center|800px|Figure 15. Initial column converged]]
# Now begins the refinement and optimization portion of the tutorial. To do so, in the distillation column window, switch to the Monitor tab in the Design toolbar on the left. We will add four new specifications of interest: distillate methanol purity, distillate methanol recovery, condenser duty, and reboiler duty. [[File:Monitor_tab.PNG|thumb|center|800px|Figure 16. The monitor tab in HYSYS]]
# To add a new specification, click the button labeled "Add Spec..." at the bottom of the window. This first spec will be for distillate methanol purity. In the new popup window, choose "Column Component Fraction" from the list of specs. In the new window, the name of the spec can be changed, and the specification itself can be defined. Change "Target Type" to "Stream," change "Draw" to the distillate stream, and change "Component" to methanol. Close this window, and the new specification should now be included in the list on the main column window. [[File:Making_a_new_spec.PNG|thumb|center|300px|Figure 17. Creating a new specification for distillate methanol mole fraction]]
# To add the recovery specification, the process is similar, expect the specification type is now "Column Component Recovery." To add the condenser and reboiler duty specifications, the spec type is "Column Duty," and the "Energy Stream" field should be chosen to be the energy stream of the condenser and reboiler for their respective specifications. Add these three specifications to the list as well.
# Note that currently, the distillate methanol fraction is only 0.244. To begin manipulation column specifications to meet the requirement of 97% purity, change the currently empty specified value for this spec to the current value, 0.244. Then, deselect Distillate Rate as an active spec, and make this new purity spec active in its stead. The simulation should automatically and instantly converge - if not, hit run. After converging, all values should remain the same.
# Begin increasing the specified value for distillate methanol purity gradually, until finding the approximate largest value at which the column simulation can still converge. In this example, this specified value can be increased up to 0.925 before the simulation fails to converge.
# Once this maximum has been reached, increase the total number of column stages by 1 or 2 by returning to the Connections tab in the Design toolbar. Note that the feed stage will automatically recalculate to stay in the same relative column position as before. In this example, change the number of stages to 10, and observe the feed stage automatically recalculate to 5. Hit run, and the simulation should still easily converge.
# Increase the specified value for purity again until the column simulation again reaches a limit beyond which it cannot converge. With 10 stages in this example, the column cannot converge with a specified purity of 0.95, but it can for a purity of 0.945. So, 0.945 is selected as the highest viable purity for this number of stages.
# Repeat this process, increasing the number of stages again followed by increasing the purity, until the desired purity is met. In this example simulation, a purity specification of 0.97 can be met with 14 stages, with the feed entering at stage 7.
# Recovery can be maximized in the same way, replacing Reflux Ratio as an active spec, though in this situation recovery is already 99.9%, so there is no need to maximize further. [[File:Requirements_met.PNG|thumb|center|800px|Figure 18. All design requirements met]]
# Now that the requirements are met, we can begin optimizing the number of stages and column duty. Condenser and reboiler duties are desired to be as low as possible to keep operating costs at a minimum, though number of stages should be kept relatively low as well. Note that with the current column setup, condenser duty is -1.83e5 kJ/h, and reboiler duty is 2.42e5 kJ/h.
# To begin, set recovery as an active spec by specifying its value as the current value, in this case 0.999, and then deselecting Reflux Ratio as an active spec and selecting the custom recovery specification.
# Next, we want to find the feed stage which minimizes column duty for this number of stages. Vary the feed stage to see which stage results in the minimum column duties. In this example, the optimal feed stage is stage 8, which results in condenser duty of -1.76e5 kJ/h and reboiler duty of 2.35e5 kJ/h.
# Now, increase the number of stages gradually, and find the duty-minimizing feed stage for each number of stages. For example, for a column with 17 stages, the optimal feed stage is 10, which yields condenser duty of -1.58e5 kJ/h and reboiler duty of 2.16e5 kJ/h.
# This process can be repeated indefinitely, as more stages will consistently result in decreased column duty. However, there are diminishing returns, so careful consideration of the balance of installation costs (more stages) vs operating costs (more column duty) must be made taking into account the actual economics of the distillation column. In this example, these diminishing returns are easily visible in the values for condenser duty: increasing from 14 to 17 stages reduces condenser duty by 0.18 kJ/h, increasing to 20 stages reduces condenser duty by a further 0.08 kJ/h, and increasing to 23 stages reduces condenser duty by only 0.03 kJ/h, at a final value of -1.47e5 kJ/h. A similar trend can be seen in the reboiler duty.

==References==
<references />

Column

2016-02-21T07:15:08Z

Benjkg: Added background into extraction

[[File:Distillationcolumn_singlefeed.png|thumb|right|300px|Figure 1. Distillation Column <ref name = "Towler" />]]
Authors: Ben Granger, [2016] Scott Smith, [2015] and Harry Poppick [2015] 

Stewards: Jian Gong and Fengqi You

The column utilities in HYSYS can be used to model a wide variety of chemical separation techniques including absorption, liquid-liquid extraction and three-phase distillation. This article highlights the use of HYSYS for simulations of standard chemical distillation. The two primary distillation simulation utilities are the shortcut column, which is useful for quick and cursory simulations, and the full column utility, for more involved simulations.

==HYSYS Column Types==
{| border="1" class="wikitable"
|+ Varieties of Separators in HYSYS
! Name
! Icon
! Description
|-
! Absorption Column
| [[File:Absorber.JPG|center|35px|]]|| Utility to model absorption and stripping (liquid/vapor) separations.
|-
! Refluxed Absorption Column
| [[File:RefluxedAbsorb.JPG|center|35px|]]|| Utility to model absorption and stripping (liquid/vapor) operations with condensed top products.
|-
! Absorption with Reboil
| [[File:ReboiledAbsorber.JPG|center|35px|]]|| Utility to model absorption and stripping (liquid/vapor) operations with reboiled bottoms.
|-
! Liquid Liquid Extractor
| [[File:LiquidLiquid.JPG|center|35px|]]|| Utility to model liquid-liquid phase separations.
|-
! Three Phase Distillation
| [[File:ThreePhase.JPG|center|35px|]]|| Utility to model distillation consisting of three phases internally.
|-
! Distillation Sub-Flowsheet
| [[File:DistillationColumn.JPG|center|35px|]]|| Overall two-phase distillation utility.
|-
! Shortcut Column
| [[File:ShortcutDistillation.JPG|center|35px|]]|| Approximate model of distillation processes.
|-
! Component Splitter
| [[File:ComponentSplitter.JPG|center|35px|]]|| Utility to force components to separate with no equilibrium considerations.
|}

==Background==
Of the many different types of separating columns available in HYSYS listed in the table above, there are three main categories: absorption, extraction, and distillation. Each general type of separation technique has its own benefits. A specific separation may be simple using one technique while difficult or impossible using another; as such, understanding the nature of these separation techniques is important in selecting the best method for a given separation.

===Absorption===
Absorption is primarily used to separate a solute from a stream in the gas state. The separation is conducted by introducing an additional liquid solvent stream to absorb the solute from the gas stream. Note that to reduce costs, the liquid solvent is often desirable to be extracted after this separation, so absorption is typically accompanied by a second separation process such as stripping to recover the liquid solvent from the solute. <ref name = "Wankat"> Wankat, P.C. Separation Process Engineering. Upper Saddle River: Prentice-Hall; 2012 </ref>

===Extraction===
Liquid-liquid extraction is typically used when distillation is unfeasible, such as separating components with an azeotrope or with very similar boiling points. Like absorption, liquid-liquid extraction requires the introduction an additional liquid solvent stream to separate one of the components. Physically, this separation occurs by difference in solubility of the components in the solvent. Also similar to absorption, the use of a solvent is often accompanied by an additional separation process like distillation to recover the solvent from the solute. <ref name = "P&T"> Peters M.S., Timmerhaus K.D. Plant Design and Economics for Chemical Engineers. 5th ed. New York: McGraw Hill; 2003. </ref>

===Distillation===

==Simulating Separations in HYSYS==
===Shortcut Distillation Column<ref name = "HYSYS"> Aspentech. "Aspen HYSYS V8.0 Help Documentation". Published 2014 </ref>===

The shortcut distillation utility within HYSYS allows users to quickly model a distillation separation of a stream with relatively little known information. Once the user attaches a fully defined inlet stream to the shortcut distillation column, the essential information that must be specified are as follows:

* The vapor/liquid quality of the top product
* The light key component and composition in the bottoms product
* The heavy key component and composition in the distillate (top) product
* The condenser pressure
* The reboiler pressure
* The external reflux ratio

With this information, the utility will run and, if possible, converge on a solution that will meet the key component specifications. The critical information that the utility will calculate are as follows:

* Fully specified distillate product stream (temperature, pressure, flow rate, composition)
* Fully specified bottom product stream (temperature, pressure, flow rate, composition)
* Condenser operating temperature and heat duty
* Reboiler operating temperature and heat duty
* Minimum number of trays
* Actual number of trays
* Optimum feed tray location

The primary purpose of the shortcut distillation is to allow for users to perform a first pass estimate on the performance requirements of a given system for a specified reflux ratio. The values gathered from the shortcut distillation utility can be used to assist the user when setting up the complete distillation utility in HYSYS, or when attempting to find reasonable column properties to allow the system to converge (e.g. number of trays, feed location, etc.)

===Standard Distillation Column<ref name = "HYSYS" />===
The standard distillation column, called "Distillation Sub-Flowsheet" the "Columns" tab on the equipment palette, may be used to perform a more rigorous simulation of your process than that offered by the shortcut column. In addition, the shortcut column may be used as a "first pass" analysis and then these specification estimations can used as the starting point for the standard column.

The standard distillation column provides many options for customization, allowing a user to specify a partial or total condenser, the number of passes through the column, and the type of reboiler all within the initial setup phase of creating a column. In addition, many specifications and parameters can be customized once the column is running, such as purities, recoveries, temperature profiles, column duties, and even internal specifications such as tray type, tray diameter, and weir height. This flexibility allows for much more rigorous, accurate simulation of a distillation column than using a shortcut column.

====Manipulating Column Specifications====

Column specifications can be heavily manipulated to force a column to operate under specific conditions, to produce a stream with specific purity, and even to minimize utilities. See the tutorial section for an example of this usage.

''Information taken from Towler Chapter 17'' <ref name = "Towler"> Towler, G. Sinnott, "Chemical Engineering Design". Elsevier, 2012, Chapter 17: R. Separation Columns (Distillation, Absorption, and Extraction). </ref>

Parameters that directly impact the design and operation of the distillation column include:
*Reflux ratio
*Feed point location
*Column Pressure
*Number of equilibrium stages/real stages

Assuming that we have already specified a desired specification for either the distillate or bottoms stream, we can manipulate the above parameters when running our simulation within HYSYS.

'''Reflux Ratio'''

The number of stages required for separation is directly affected by the reflux ratio that can be defined as follows:

<math> R=\frac {Flow\ returned\ as\ reflux} {Flow\ of\ top\ product\ taken\ off} </math>

Increasing the reflux ratio will reduce the total number of stages required but will increase the utility costs for the column. Therefore, when simulating a distillation column, special care should be taken to strike a balance between capital and operating costs.

When starting a new distillation column simulation within HYSYS, it is a good idea to calculate a minimum reflux ratio to advise the specification of the reflux ratio. The minimum reflux ratio can be calculated through use of a McCabe-Thiele diagram. According to Towler, a reflux ratio of 1.15 times the minimum reflux ratio should be used as a first approximation. Once this ratio is inputted into the simulation environment, it can be further manipulated to fine-tune the optimum reflux ratio for the system. <ref name = "Towler" />

'''Feed Point Location'''

The column simulation environment within HYSYS allows for manipulation of the feed point location. The general rule for designating feed point location is to have the feed enter the column at the stage which best matches the composition of the feed. There can be some uncertainty when it comes to finding the precise feed point location. Instead, a trial-and-error method may be useful when using HYSYS to determine which feed stage results in the desired distillate and bottoms conditions.

'''Column Pressure'''

When specifying the distillation column within the HYSYS simulation environment, you must specify the operating pressure as well as any pressure drops. If you are running a HYSYS simulation to get a first-pass approximation of compositions and flow rates, it is a typical practice to assume the pressure in the column is constant. If you are running a more intensive and rigorous simulation, you can approximate the pressure drop per tray using the following equation:

<math> \Delta P_{\text{tray}} = 2 \rho_L g h_w </math>

where <math> \rho_L </math> is the liquid density, g is gravitational acceleration, and <math> h_w </math> is the weir height. <ref name = "Towler" />

==Tutorials==
===Shortcut Distillation Column<ref name = "HYSYS" />===

This example will simulate the separation of benzene, toluene and p-xylene from some upstream process.<ref name = "HYSYS" />

'''Initialize''' [[File:AddComponents.PNG|thumb|center|600px|Figure 2. Adding Components in HYSYS]]
#Start Aspen HYSYS v.8 and click “new” on the main screen to open a new simulation window.
#Once on the component list screen, select “add” near the bottom of the screen
#Search the database for the desired components using the search bar, and add them to the component list by clicking “add.” In this case, we wish to add Benzene, Toluene and p-Xylene.
#On the left hand “properties” menu, click the “fluid packages” tab and select “add” to add a new package.
#Select the desired fluid package. In this case we are using Peng-Robinson
#In the bottom, left-hand corner of the screen, click “simulation” to enter the simulation environment.
'''Setup Streams''' [[File:Streams.PNG|thumb|center|300px|Figure 3. Add Streams]]
#Start by clicking the blue arrow at the top of the “palette” window and then clicking on the main flowsheet to produce a new material stream. This will create our feed stream. It is a good idea to place your other streams as well: one for distillate product and one for bottoms product.
#Add two energy streams that will be the condenser and reboiler duties.
#Double click on the newly created feed stream to pull up the worksheet for the stream. Please note that we are specifying this feed stream to demonstrate how the column would behaved with a fully specified inlet stream. The temperature, pressure, flow rate and composition of the feed stream can be specified by the user. [[File:FeedConditions.PNG|thumb|center|300px|Figure 4. Define Feed]]
#We will make the following changes within this worksheet to specify the feed stream:
#Enter “feed” into the stream name.
#Set the temperature to 20 degrees C.
#Set the pressure to 1 atm. (Be sure to select the “atm” unit from the dropdown when you begin to enter this value)
#Set the mass flow to 500 kg/hr [[File:Compositions.PNG|thumb|center|600px|Figure 5. Define Compositions]]
#On the left hand column within the worksheet, click “composition” to enter the composition tab.
#Click “edit” on the composition tab to change the composition of the feed stream
#Enter .25 to Toluene, .35 to Benzene and .40 to p-Xylene.
The worksheet should indicate that the stream is fully specified with a green “ok” indicator at the bottom. Close the worksheet.
'''Fully Specify Shortcut Column''' [[File:ConnectStreams.PNG|thumb|center|600px|Figure 6. Connect the inlet and outlet streams]]
#Create the shortcut column by clicking on the "columns" tab within the palette and selecting "shortcut column"
#Click on the simulation environment to place the column
#The shortcut column should currently appear red to represent that it is not currently specified.
#Double click on the column to pull up the column worksheet
#Under each drop down, add the appropriate stream: E.g. Under "Distillate" add the distillate product stream that was defined earlier.
#Additionally, be sure to specify the quality of the top product stream: whether it is liquid or vapor phase. In this case, select "liquid." [[File:Set Keys.PNG|thumb|center|600px|Figure 7. Set the key specifications]]
#On the left hand side of the worksheet, select "Parameters."
#This will move you to the parameters section of the column window where column specifications can be added.
#At this point it is important to identify which components are most volatile. In this case, benzene is most volatile, xylene is the least volatile and toluene rests in between the two. One rule of thumb is that more volatile components tend to be more valuable, and so in this example we will be purifying benzene in the distillate stream. In order to do this we will specify that benzene is the light key in the bottoms and that toluene is the heavy key in the distillate. This arrangement makes sense since it will force the heavier xylene and toluene products to drop out in the bottoms.
#Set the Light Key in Bottoms to Benzene and the mole fraction to .001 and set the Heavy Key in Distillate to Toluene and the mole fraction to .001.
#Set the condenser and reboiler pressure to the approximate values that they would be. In this case, we are assuming that our column is sufficiently small that atmospheric pressure is a fair assumption for both values. Enter 1 atm for both of these values. [[File:Worksheet.PNG|thumb|center|600px|Figure 8. Viewing the Worksheet]]
#Finally set the reflux ratio to a value that is higher than the minimum reflux ratio that HYSYS calculates. In this case the minimum reflux ratio was calculated to be 2.119 and so it is best to input around 2.5. This value can be tweaked later to see how performance is changed for different reflux ratios.
#At this point it is worthwhile to see what outputs the column is producing, since this is one of the primary uses of the shortcut distillation column.
#Click on the "worksheet" tab at the top of the window. This will bring you to your stream information where you can view the temperatures and flow rates of your outgoing streams.
#Now click on the "compositions" tab on the left side to view the compositions of the outgoing stream. As expected, the benzene is leaving in the distillate stream at 99.9% purity with a .1% toluene impurity, while all of the xylene is leaving out in the bottoms. [[File:Performance.PNG|thumb|center|600px|Figure 9. View the Performance Tab]]
#At this point it is valuable to record the calculated minimum number of trays, actual number of trays, as well as the condenser/reboiler temperatures.
#To view these values click on the "performance" tab
#You can observe here that the minimum number of trays calculated was around 15.7, the actual number of trays used was 35.7 the optimal feed stage was around 20.5, the condenser temperature was found to be around 80 C, while the reboiler was calculated to be around 123 C.

===Standard Distillation Column<ref name = "HYSYS" />===
====General Setup and Use====

In this example, we will be setting up a basic distillation column in HYSYS to simulate the separation of benzene and toluene.

# Start a new case in HYSYS by clicking on the button on the top menu bar.
# On the Components screen, add Benzene and Toluene. [[File:Columncomponents.PNG|thumb|center|300px|Figure 10. Component Selection in HYSYS]]
# On the Fluid Package screen, you have a wide range of options with which to simulate vapor-liquid equilibrium. For this example, choose the Peng-Robinson fluid package and enter the simulation environment. [[File:Fluidpkg.PNG|thumb|center|400px|Figure 11. Fluid Package Selection in HYSYS]]
We are using the simple distillation column and thus we only need a single feed stream into the distillation column as well as a distillate stream and a bottoms stream. We will also need 2 energy streams for the condenser and reboiler. <ref name = "HYSYS" />
# Add the material and energy streams to the simulation environment. Naming them appropriately will make it easer to keep everything straight as we move forward with the tutorial.
# Select the distillation column from the palette and place anywhere in the simulation environment.
# Switch to “Attach Mode” and attach the mass and energy streams to their appropriate locations on the distillation column. NOTE: You could also complete this step by double-clicking on the column itself and selecting each respective stream from the drop down menus.
# Double click on the column and select the condenser type. For this example, select “total condenser” and click next
# This next page is where the pressures of the column are specified. For this example, we will assume that the column is run at 1 atm with no pressure drops within the column. Therefore, specify reboiler pressure and condenser pressure to 101.3 kPa and the condenser pressure drop to 0 kPa.
# The next page has fields where temperature estimates can be made for the condenser, top stage, and reboiler. As these are optional fields, simply click next.
# The final screen of the distillation column setup is where the reflux ratio and distillate flow rates should be specified. For this example, specify the distillate flow rate as 1000 kgmol/hr and the reflux ratio as 1.5.
# Click “Done” at the bottom of the screen and the simulation page should be back in full view. [[File:Simulationview.PNG|thumb|center|400px|Figure 12. Simulation View of Distillation Column in HYSYS]]
We are now ready to specify the flow rates and compositions of the streams coming in and out of the column.
# Double click on the feed stream coming into the column. A specifications page should come up.
# This page allows you specify a number of parameters pertaining to the feed stream, including temperature, pressure, and compositions. Specify the temperature as 37C and pressure as 101.3 kPa (1 atm). In addition, specify the molar flow as 3500 kgmole/h.
# Now click on the composition tab on the left side of the window. Here specify, the benzene as 70 mass % and the toluene as 30 mass %. The stream should be fully specified. Close the window to return to the simulation. [[File:Configurationpage.PNG|thumb|center|400px|Figure 13. Column Configuration Page]]
The column and feed stream should now be fully specified. It is now time to run the simulation.
# If the solver is not already activated, click the green traffic light at the top of the simulation window.
# Now double click on the distillation column to bring up the main column window. A feed stage and total number of stages should already be specified.
# Click the “Run” button of the window to start the simulation. The small rectangle to the right of this button should light up green and say “converged” when it finds a solution. [[File:Columncompositions.PNG|thumb|center|400px|Figure 14. Outlet compositions in HYSYS]]

====Designing to Meet Specifications====
This example demonstrates how to use a distillation column simulation in HYSYS to design a column that meets preset requirements, and how to optimize the column to minimize costs. Basic setup of the column is assumed understood as described in the above tutorial. In this tutorial, the example requirements are to design a column that separates a stream of 20 mol% methanol and 80 mol% water into a distillate stream with 97% purity methanol and high recovery. In addition, both capital and operating costs are desired to be minimized.

# Create a simulation with water and methanol as the two components, using the NRTL fluid package.
# Place a standard distillation column in the simulation environment, and define a feed stream with vapor fraction 0, temperature 25 C, pressure 1 atm, flow rate 10 kgmol/h, and a composition of 0.2 mol fraction methanol and the balance of water.
# Enter column setup. Define the column as having a total condenser, 8 stages, and the feed entering at stage 4. The column should be once-through with a regular reboiler. Condenser pressure should be 1 atm, while reboiler pressure will be set at 1.3 kPa, giving a small pressure drop through the column. Optional temperature estimates can be left empty. Reflux ratio should be specified as 1.5, and liquid rate as 8.2 kgmol/h. In a normal situation, many of these values can be initially estimated using a shortcut column and then using them here, in the first run of a standard column simulation. Finally, run the column simulation, and it should easily converge. [[File:Column_converged_initially.PNG|thumb|center|800px|Figure 15. Initial column converged]]
# Now begins the refinement and optimization portion of the tutorial. To do so, in the distillation column window, switch to the Monitor tab in the Design toolbar on the left. We will add four new specifications of interest: distillate methanol purity, distillate methanol recovery, condenser duty, and reboiler duty. [[File:Monitor_tab.PNG|thumb|center|800px|Figure 16. The monitor tab in HYSYS]]
# To add a new specification, click the button labeled "Add Spec..." at the bottom of the window. This first spec will be for distillate methanol purity. In the new popup window, choose "Column Component Fraction" from the list of specs. In the new window, the name of the spec can be changed, and the specification itself can be defined. Change "Target Type" to "Stream," change "Draw" to the distillate stream, and change "Component" to methanol. Close this window, and the new specification should now be included in the list on the main column window. [[File:Making_a_new_spec.PNG|thumb|center|300px|Figure 17. Creating a new specification for distillate methanol mole fraction]]
# To add the recovery specification, the process is similar, expect the specification type is now "Column Component Recovery." To add the condenser and reboiler duty specifications, the spec type is "Column Duty," and the "Energy Stream" field should be chosen to be the energy stream of the condenser and reboiler for their respective specifications. Add these three specifications to the list as well.
# Note that currently, the distillate methanol fraction is only 0.244. To begin manipulation column specifications to meet the requirement of 97% purity, change the currently empty specified value for this spec to the current value, 0.244. Then, deselect Distillate Rate as an active spec, and make this new purity spec active in its stead. The simulation should automatically and instantly converge - if not, hit run. After converging, all values should remain the same.
# Begin increasing the specified value for distillate methanol purity gradually, until finding the approximate largest value at which the column simulation can still converge. In this example, this specified value can be increased up to 0.925 before the simulation fails to converge.
# Once this maximum has been reached, increase the total number of column stages by 1 or 2 by returning to the Connections tab in the Design toolbar. Note that the feed stage will automatically recalculate to stay in the same relative column position as before. In this example, change the number of stages to 10, and observe the feed stage automatically recalculate to 5. Hit run, and the simulation should still easily converge.
# Increase the specified value for purity again until the column simulation again reaches a limit beyond which it cannot converge. With 10 stages in this example, the column cannot converge with a specified purity of 0.95, but it can for a purity of 0.945. So, 0.945 is selected as the highest viable purity for this number of stages.
# Repeat this process, increasing the number of stages again followed by increasing the purity, until the desired purity is met. In this example simulation, a purity specification of 0.97 can be met with 14 stages, with the feed entering at stage 7.
# Recovery can be maximized in the same way, replacing Reflux Ratio as an active spec, though in this situation recovery is already 99.9%, so there is no need to maximize further. [[File:Requirements_met.PNG|thumb|center|800px|Figure 18. All design requirements met]]
# Now that the requirements are met, we can begin optimizing the number of stages and column duty. Condenser and reboiler duties are desired to be as low as possible to keep operating costs at a minimum, though number of stages should be kept relatively low as well. Note that with the current column setup, condenser duty is -1.83e5 kJ/h, and reboiler duty is 2.42e5 kJ/h.
# To begin, set recovery as an active spec by specifying its value as the current value, in this case 0.999, and then deselecting Reflux Ratio as an active spec and selecting the custom recovery specification.
# Next, we want to find the feed stage which minimizes column duty for this number of stages. Vary the feed stage to see which stage results in the minimum column duties. In this example, the optimal feed stage is stage 8, which results in condenser duty of -1.76e5 kJ/h and reboiler duty of 2.35e5 kJ/h.
# Now, increase the number of stages gradually, and find the duty-minimizing feed stage for each number of stages. For example, for a column with 17 stages, the optimal feed stage is 10, which yields condenser duty of -1.58e5 kJ/h and reboiler duty of 2.16e5 kJ/h.
# This process can be repeated indefinitely, as more stages will consistently result in decreased column duty. However, there are diminishing returns, so careful consideration of the balance of installation costs (more stages) vs operating costs (more column duty) must be made taking into account the actual economics of the distillation column. In this example, these diminishing returns are easily visible in the values for condenser duty: increasing from 14 to 17 stages reduces condenser duty by 0.18 kJ/h, increasing to 20 stages reduces condenser duty by a further 0.08 kJ/h, and increasing to 23 stages reduces condenser duty by only 0.03 kJ/h, at a final value of -1.47e5 kJ/h. A similar trend can be seen in the reboiler duty.

==References==
<references />

Column

2016-02-21T07:02:04Z

Benjkg: Added background into absorption

[[File:Distillationcolumn_singlefeed.png|thumb|right|300px|Figure 1. Distillation Column <ref name = "Towler" />]]
Authors: Ben Granger, [2016] Scott Smith, [2015] and Harry Poppick [2015] 

Stewards: Jian Gong and Fengqi You

The column utilities in HYSYS can be used to model a wide variety of chemical separation techniques including absorption, liquid-liquid extraction and three-phase distillation. This article highlights the use of HYSYS for simulations of standard chemical distillation. The two primary distillation simulation utilities are the shortcut column, which is useful for quick and cursory simulations, and the full column utility, for more involved simulations.

==HYSYS Column Types==
{| border="1" class="wikitable"
|+ Varieties of Separators in HYSYS
! Name
! Icon
! Description
|-
! Absorption Column
| [[File:Absorber.JPG|center|35px|]]|| Utility to model absorption and stripping (liquid/vapor) separations.
|-
! Refluxed Absorption Column
| [[File:RefluxedAbsorb.JPG|center|35px|]]|| Utility to model absorption and stripping (liquid/vapor) operations with condensed top products.
|-
! Absorption with Reboil
| [[File:ReboiledAbsorber.JPG|center|35px|]]|| Utility to model absorption and stripping (liquid/vapor) operations with reboiled bottoms.
|-
! Liquid Liquid Extractor
| [[File:LiquidLiquid.JPG|center|35px|]]|| Utility to model liquid-liquid phase separations.
|-
! Three Phase Distillation
| [[File:ThreePhase.JPG|center|35px|]]|| Utility to model distillation consisting of three phases internally.
|-
! Distillation Sub-Flowsheet
| [[File:DistillationColumn.JPG|center|35px|]]|| Overall two-phase distillation utility.
|-
! Shortcut Column
| [[File:ShortcutDistillation.JPG|center|35px|]]|| Approximate model of distillation processes.
|-
! Component Splitter
| [[File:ComponentSplitter.JPG|center|35px|]]|| Utility to force components to separate with no equilibrium considerations.
|}

==Background==
Of the many different types of separating columns available in HYSYS listed in the table above, there are three main categories: absorption, extraction, and distillation. Each general type of separation technique has its own benefits. A specific separation may be simple using one technique while difficult or impossible using another; as such, understanding the nature of these separation techniques is important in selecting the best method for a given separation.

===Absorption===
Absorption is primarily used to separate a solute from a stream in the gas state. The separation is conducted by introducing an additional liquid solvent stream to absorb the solute from the gas stream. Note that to reduce costs, the liquid solvent is often desirable to be extracted after this separation, so absorption is typically accompanied by a second separation process such as stripping to recover the liquid solvent from the solute. <ref name = "Wankat"> Wankat, P.C. (2012). Separation Process Engineering. Upper Saddle River: Prentice-Hall. </ref>

===Extraction===

===Distillation===

==Simulating Separations in HYSYS==
===Shortcut Distillation Column<ref name = "HYSYS"> Aspentech. "Aspen HYSYS V8.0 Help Documentation". Published 2014 </ref>===

The shortcut distillation utility within HYSYS allows users to quickly model a distillation separation of a stream with relatively little known information. Once the user attaches a fully defined inlet stream to the shortcut distillation column, the essential information that must be specified are as follows:

* The vapor/liquid quality of the top product
* The light key component and composition in the bottoms product
* The heavy key component and composition in the distillate (top) product
* The condenser pressure
* The reboiler pressure
* The external reflux ratio

With this information, the utility will run and, if possible, converge on a solution that will meet the key component specifications. The critical information that the utility will calculate are as follows:

* Fully specified distillate product stream (temperature, pressure, flow rate, composition)
* Fully specified bottom product stream (temperature, pressure, flow rate, composition)
* Condenser operating temperature and heat duty
* Reboiler operating temperature and heat duty
* Minimum number of trays
* Actual number of trays
* Optimum feed tray location

The primary purpose of the shortcut distillation is to allow for users to perform a first pass estimate on the performance requirements of a given system for a specified reflux ratio. The values gathered from the shortcut distillation utility can be used to assist the user when setting up the complete distillation utility in HYSYS, or when attempting to find reasonable column properties to allow the system to converge (e.g. number of trays, feed location, etc.)

===Standard Distillation Column<ref name = "HYSYS" />===
The standard distillation column, called "Distillation Sub-Flowsheet" the "Columns" tab on the equipment palette, may be used to perform a more rigorous simulation of your process than that offered by the shortcut column. In addition, the shortcut column may be used as a "first pass" analysis and then these specification estimations can used as the starting point for the standard column.

The standard distillation column provides many options for customization, allowing a user to specify a partial or total condenser, the number of passes through the column, and the type of reboiler all within the initial setup phase of creating a column. In addition, many specifications and parameters can be customized once the column is running, such as purities, recoveries, temperature profiles, column duties, and even internal specifications such as tray type, tray diameter, and weir height. This flexibility allows for much more rigorous, accurate simulation of a distillation column than using a shortcut column.

====Manipulating Column Specifications====

Column specifications can be heavily manipulated to force a column to operate under specific conditions, to produce a stream with specific purity, and even to minimize utilities. See the tutorial section for an example of this usage.

''Information taken from Towler Chapter 17'' <ref name = "Towler"> Towler, G. Sinnott, "Chemical Engineering Design". Elsevier, 2012, Chapter 17: R. Separation Columns (Distillation, Absorption, and Extraction). </ref>

Parameters that directly impact the design and operation of the distillation column include:
*Reflux ratio
*Feed point location
*Column Pressure
*Number of equilibrium stages/real stages

Assuming that we have already specified a desired specification for either the distillate or bottoms stream, we can manipulate the above parameters when running our simulation within HYSYS.

'''Reflux Ratio'''

The number of stages required for separation is directly affected by the reflux ratio that can be defined as follows:

<math> R=\frac {Flow\ returned\ as\ reflux} {Flow\ of\ top\ product\ taken\ off} </math>

Increasing the reflux ratio will reduce the total number of stages required but will increase the utility costs for the column. Therefore, when simulating a distillation column, special care should be taken to strike a balance between capital and operating costs.

When starting a new distillation column simulation within HYSYS, it is a good idea to calculate a minimum reflux ratio to advise the specification of the reflux ratio. The minimum reflux ratio can be calculated through use of a McCabe-Thiele diagram. According to Towler, a reflux ratio of 1.15 times the minimum reflux ratio should be used as a first approximation. Once this ratio is inputted into the simulation environment, it can be further manipulated to fine-tune the optimum reflux ratio for the system. <ref name = "Towler" />

'''Feed Point Location'''

The column simulation environment within HYSYS allows for manipulation of the feed point location. The general rule for designating feed point location is to have the feed enter the column at the stage which best matches the composition of the feed. There can be some uncertainty when it comes to finding the precise feed point location. Instead, a trial-and-error method may be useful when using HYSYS to determine which feed stage results in the desired distillate and bottoms conditions.

'''Column Pressure'''

When specifying the distillation column within the HYSYS simulation environment, you must specify the operating pressure as well as any pressure drops. If you are running a HYSYS simulation to get a first-pass approximation of compositions and flow rates, it is a typical practice to assume the pressure in the column is constant. If you are running a more intensive and rigorous simulation, you can approximate the pressure drop per tray using the following equation:

<math> \Delta P_{\text{tray}} = 2 \rho_L g h_w </math>

where <math> \rho_L </math> is the liquid density, g is gravitational acceleration, and <math> h_w </math> is the weir height. <ref name = "Towler" />

==Tutorials==
===Shortcut Distillation Column<ref name = "HYSYS" />===

This example will simulate the separation of benzene, toluene and p-xylene from some upstream process.<ref name = "HYSYS" />

'''Initialize''' [[File:AddComponents.PNG|thumb|center|600px|Figure 2. Adding Components in HYSYS]]
#Start Aspen HYSYS v.8 and click “new” on the main screen to open a new simulation window.
#Once on the component list screen, select “add” near the bottom of the screen
#Search the database for the desired components using the search bar, and add them to the component list by clicking “add.” In this case, we wish to add Benzene, Toluene and p-Xylene.
#On the left hand “properties” menu, click the “fluid packages” tab and select “add” to add a new package.
#Select the desired fluid package. In this case we are using Peng-Robinson
#In the bottom, left-hand corner of the screen, click “simulation” to enter the simulation environment.
'''Setup Streams''' [[File:Streams.PNG|thumb|center|300px|Figure 3. Add Streams]]
#Start by clicking the blue arrow at the top of the “palette” window and then clicking on the main flowsheet to produce a new material stream. This will create our feed stream. It is a good idea to place your other streams as well: one for distillate product and one for bottoms product.
#Add two energy streams that will be the condenser and reboiler duties.
#Double click on the newly created feed stream to pull up the worksheet for the stream. Please note that we are specifying this feed stream to demonstrate how the column would behaved with a fully specified inlet stream. The temperature, pressure, flow rate and composition of the feed stream can be specified by the user. [[File:FeedConditions.PNG|thumb|center|300px|Figure 4. Define Feed]]
#We will make the following changes within this worksheet to specify the feed stream:
#Enter “feed” into the stream name.
#Set the temperature to 20 degrees C.
#Set the pressure to 1 atm. (Be sure to select the “atm” unit from the dropdown when you begin to enter this value)
#Set the mass flow to 500 kg/hr [[File:Compositions.PNG|thumb|center|600px|Figure 5. Define Compositions]]
#On the left hand column within the worksheet, click “composition” to enter the composition tab.
#Click “edit” on the composition tab to change the composition of the feed stream
#Enter .25 to Toluene, .35 to Benzene and .40 to p-Xylene.
The worksheet should indicate that the stream is fully specified with a green “ok” indicator at the bottom. Close the worksheet.
'''Fully Specify Shortcut Column''' [[File:ConnectStreams.PNG|thumb|center|600px|Figure 6. Connect the inlet and outlet streams]]
#Create the shortcut column by clicking on the "columns" tab within the palette and selecting "shortcut column"
#Click on the simulation environment to place the column
#The shortcut column should currently appear red to represent that it is not currently specified.
#Double click on the column to pull up the column worksheet
#Under each drop down, add the appropriate stream: E.g. Under "Distillate" add the distillate product stream that was defined earlier.
#Additionally, be sure to specify the quality of the top product stream: whether it is liquid or vapor phase. In this case, select "liquid." [[File:Set Keys.PNG|thumb|center|600px|Figure 7. Set the key specifications]]
#On the left hand side of the worksheet, select "Parameters."
#This will move you to the parameters section of the column window where column specifications can be added.
#At this point it is important to identify which components are most volatile. In this case, benzene is most volatile, xylene is the least volatile and toluene rests in between the two. One rule of thumb is that more volatile components tend to be more valuable, and so in this example we will be purifying benzene in the distillate stream. In order to do this we will specify that benzene is the light key in the bottoms and that toluene is the heavy key in the distillate. This arrangement makes sense since it will force the heavier xylene and toluene products to drop out in the bottoms.
#Set the Light Key in Bottoms to Benzene and the mole fraction to .001 and set the Heavy Key in Distillate to Toluene and the mole fraction to .001.
#Set the condenser and reboiler pressure to the approximate values that they would be. In this case, we are assuming that our column is sufficiently small that atmospheric pressure is a fair assumption for both values. Enter 1 atm for both of these values. [[File:Worksheet.PNG|thumb|center|600px|Figure 8. Viewing the Worksheet]]
#Finally set the reflux ratio to a value that is higher than the minimum reflux ratio that HYSYS calculates. In this case the minimum reflux ratio was calculated to be 2.119 and so it is best to input around 2.5. This value can be tweaked later to see how performance is changed for different reflux ratios.
#At this point it is worthwhile to see what outputs the column is producing, since this is one of the primary uses of the shortcut distillation column.
#Click on the "worksheet" tab at the top of the window. This will bring you to your stream information where you can view the temperatures and flow rates of your outgoing streams.
#Now click on the "compositions" tab on the left side to view the compositions of the outgoing stream. As expected, the benzene is leaving in the distillate stream at 99.9% purity with a .1% toluene impurity, while all of the xylene is leaving out in the bottoms. [[File:Performance.PNG|thumb|center|600px|Figure 9. View the Performance Tab]]
#At this point it is valuable to record the calculated minimum number of trays, actual number of trays, as well as the condenser/reboiler temperatures.
#To view these values click on the "performance" tab
#You can observe here that the minimum number of trays calculated was around 15.7, the actual number of trays used was 35.7 the optimal feed stage was around 20.5, the condenser temperature was found to be around 80 C, while the reboiler was calculated to be around 123 C.

===Standard Distillation Column<ref name = "HYSYS" />===
====General Setup and Use====

In this example, we will be setting up a basic distillation column in HYSYS to simulate the separation of benzene and toluene.

# Start a new case in HYSYS by clicking on the button on the top menu bar.
# On the Components screen, add Benzene and Toluene. [[File:Columncomponents.PNG|thumb|center|300px|Figure 10. Component Selection in HYSYS]]
# On the Fluid Package screen, you have a wide range of options with which to simulate vapor-liquid equilibrium. For this example, choose the Peng-Robinson fluid package and enter the simulation environment. [[File:Fluidpkg.PNG|thumb|center|400px|Figure 11. Fluid Package Selection in HYSYS]]
We are using the simple distillation column and thus we only need a single feed stream into the distillation column as well as a distillate stream and a bottoms stream. We will also need 2 energy streams for the condenser and reboiler. <ref name = "HYSYS" />
# Add the material and energy streams to the simulation environment. Naming them appropriately will make it easer to keep everything straight as we move forward with the tutorial.
# Select the distillation column from the palette and place anywhere in the simulation environment.
# Switch to “Attach Mode” and attach the mass and energy streams to their appropriate locations on the distillation column. NOTE: You could also complete this step by double-clicking on the column itself and selecting each respective stream from the drop down menus.
# Double click on the column and select the condenser type. For this example, select “total condenser” and click next
# This next page is where the pressures of the column are specified. For this example, we will assume that the column is run at 1 atm with no pressure drops within the column. Therefore, specify reboiler pressure and condenser pressure to 101.3 kPa and the condenser pressure drop to 0 kPa.
# The next page has fields where temperature estimates can be made for the condenser, top stage, and reboiler. As these are optional fields, simply click next.
# The final screen of the distillation column setup is where the reflux ratio and distillate flow rates should be specified. For this example, specify the distillate flow rate as 1000 kgmol/hr and the reflux ratio as 1.5.
# Click “Done” at the bottom of the screen and the simulation page should be back in full view. [[File:Simulationview.PNG|thumb|center|400px|Figure 12. Simulation View of Distillation Column in HYSYS]]
We are now ready to specify the flow rates and compositions of the streams coming in and out of the column.
# Double click on the feed stream coming into the column. A specifications page should come up.
# This page allows you specify a number of parameters pertaining to the feed stream, including temperature, pressure, and compositions. Specify the temperature as 37C and pressure as 101.3 kPa (1 atm). In addition, specify the molar flow as 3500 kgmole/h.
# Now click on the composition tab on the left side of the window. Here specify, the benzene as 70 mass % and the toluene as 30 mass %. The stream should be fully specified. Close the window to return to the simulation. [[File:Configurationpage.PNG|thumb|center|400px|Figure 13. Column Configuration Page]]
The column and feed stream should now be fully specified. It is now time to run the simulation.
# If the solver is not already activated, click the green traffic light at the top of the simulation window.
# Now double click on the distillation column to bring up the main column window. A feed stage and total number of stages should already be specified.
# Click the “Run” button of the window to start the simulation. The small rectangle to the right of this button should light up green and say “converged” when it finds a solution. [[File:Columncompositions.PNG|thumb|center|400px|Figure 14. Outlet compositions in HYSYS]]

====Designing to Meet Specifications====
This example demonstrates how to use a distillation column simulation in HYSYS to design a column that meets preset requirements, and how to optimize the column to minimize costs. Basic setup of the column is assumed understood as described in the above tutorial. In this tutorial, the example requirements are to design a column that separates a stream of 20 mol% methanol and 80 mol% water into a distillate stream with 97% purity methanol and high recovery. In addition, both capital and operating costs are desired to be minimized.

# Create a simulation with water and methanol as the two components, using the NRTL fluid package.
# Place a standard distillation column in the simulation environment, and define a feed stream with vapor fraction 0, temperature 25 C, pressure 1 atm, flow rate 10 kgmol/h, and a composition of 0.2 mol fraction methanol and the balance of water.
# Enter column setup. Define the column as having a total condenser, 8 stages, and the feed entering at stage 4. The column should be once-through with a regular reboiler. Condenser pressure should be 1 atm, while reboiler pressure will be set at 1.3 kPa, giving a small pressure drop through the column. Optional temperature estimates can be left empty. Reflux ratio should be specified as 1.5, and liquid rate as 8.2 kgmol/h. In a normal situation, many of these values can be initially estimated using a shortcut column and then using them here, in the first run of a standard column simulation. Finally, run the column simulation, and it should easily converge. [[File:Column_converged_initially.PNG|thumb|center|800px|Figure 15. Initial column converged]]
# Now begins the refinement and optimization portion of the tutorial. To do so, in the distillation column window, switch to the Monitor tab in the Design toolbar on the left. We will add four new specifications of interest: distillate methanol purity, distillate methanol recovery, condenser duty, and reboiler duty. [[File:Monitor_tab.PNG|thumb|center|800px|Figure 16. The monitor tab in HYSYS]]
# To add a new specification, click the button labeled "Add Spec..." at the bottom of the window. This first spec will be for distillate methanol purity. In the new popup window, choose "Column Component Fraction" from the list of specs. In the new window, the name of the spec can be changed, and the specification itself can be defined. Change "Target Type" to "Stream," change "Draw" to the distillate stream, and change "Component" to methanol. Close this window, and the new specification should now be included in the list on the main column window. [[File:Making_a_new_spec.PNG|thumb|center|300px|Figure 17. Creating a new specification for distillate methanol mole fraction]]
# To add the recovery specification, the process is similar, expect the specification type is now "Column Component Recovery." To add the condenser and reboiler duty specifications, the spec type is "Column Duty," and the "Energy Stream" field should be chosen to be the energy stream of the condenser and reboiler for their respective specifications. Add these three specifications to the list as well.
# Note that currently, the distillate methanol fraction is only 0.244. To begin manipulation column specifications to meet the requirement of 97% purity, change the currently empty specified value for this spec to the current value, 0.244. Then, deselect Distillate Rate as an active spec, and make this new purity spec active in its stead. The simulation should automatically and instantly converge - if not, hit run. After converging, all values should remain the same.
# Begin increasing the specified value for distillate methanol purity gradually, until finding the approximate largest value at which the column simulation can still converge. In this example, this specified value can be increased up to 0.925 before the simulation fails to converge.
# Once this maximum has been reached, increase the total number of column stages by 1 or 2 by returning to the Connections tab in the Design toolbar. Note that the feed stage will automatically recalculate to stay in the same relative column position as before. In this example, change the number of stages to 10, and observe the feed stage automatically recalculate to 5. Hit run, and the simulation should still easily converge.
# Increase the specified value for purity again until the column simulation again reaches a limit beyond which it cannot converge. With 10 stages in this example, the column cannot converge with a specified purity of 0.95, but it can for a purity of 0.945. So, 0.945 is selected as the highest viable purity for this number of stages.
# Repeat this process, increasing the number of stages again followed by increasing the purity, until the desired purity is met. In this example simulation, a purity specification of 0.97 can be met with 14 stages, with the feed entering at stage 7.
# Recovery can be maximized in the same way, replacing Reflux Ratio as an active spec, though in this situation recovery is already 99.9%, so there is no need to maximize further. [[File:Requirements_met.PNG|thumb|center|800px|Figure 18. All design requirements met]]
# Now that the requirements are met, we can begin optimizing the number of stages and column duty. Condenser and reboiler duties are desired to be as low as possible to keep operating costs at a minimum, though number of stages should be kept relatively low as well. Note that with the current column setup, condenser duty is -1.83e5 kJ/h, and reboiler duty is 2.42e5 kJ/h.
# To begin, set recovery as an active spec by specifying its value as the current value, in this case 0.999, and then deselecting Reflux Ratio as an active spec and selecting the custom recovery specification.
# Next, we want to find the feed stage which minimizes column duty for this number of stages. Vary the feed stage to see which stage results in the minimum column duties. In this example, the optimal feed stage is stage 8, which results in condenser duty of -1.76e5 kJ/h and reboiler duty of 2.35e5 kJ/h.
# Now, increase the number of stages gradually, and find the duty-minimizing feed stage for each number of stages. For example, for a column with 17 stages, the optimal feed stage is 10, which yields condenser duty of -1.58e5 kJ/h and reboiler duty of 2.16e5 kJ/h.
# This process can be repeated indefinitely, as more stages will consistently result in decreased column duty. However, there are diminishing returns, so careful consideration of the balance of installation costs (more stages) vs operating costs (more column duty) must be made taking into account the actual economics of the distillation column. In this example, these diminishing returns are easily visible in the values for condenser duty: increasing from 14 to 17 stages reduces condenser duty by 0.18 kJ/h, increasing to 20 stages reduces condenser duty by a further 0.08 kJ/h, and increasing to 23 stages reduces condenser duty by only 0.03 kJ/h, at a final value of -1.47e5 kJ/h. A similar trend can be seen in the reboiler duty.

==References==
<references />

Column

2016-02-21T04:17:35Z

Benjkg: Repositioning images

[[File:Distillationcolumn_singlefeed.png|thumb|right|300px|Figure 1. Distillation Column <ref name = "Towler" />]]
Authors: Ben Granger, [2016] Scott Smith, [2015] and Harry Poppick [2015] 

Stewards: Jian Gong and Fengqi You

The column utilities in HYSYS can be used to model a wide variety of chemical separation techniques including absorption, liquid-liquid extraction and three-phase distillation. This article highlights the use of HYSYS for simulations of standard chemical distillation. The two primary distillation simulation utilities are the shortcut column, which is useful for quick and cursory simulations, and the full column utility, for more involved simulations.

==HYSYS Column Types==
{| border="1" class="wikitable"
|+ Varieties of Separators in HYSYS
! Name
! Icon
! Description
|-
! Absorption Column
| [[File:Absorber.JPG|center|35px|]]|| Utility to model absorption and stripping (liquid/vapor) separations.
|-
! Refluxed Absorption Column
| [[File:RefluxedAbsorb.JPG|center|35px|]]|| Utility to model absorption and stripping (liquid/vapor) operations with condensed top products.
|-
! Absorption with Reboil
| [[File:ReboiledAbsorber.JPG|center|35px|]]|| Utility to model absorption and stripping (liquid/vapor) operations with reboiled bottoms.
|-
! Liquid Liquid Extractor
| [[File:LiquidLiquid.JPG|center|35px|]]|| Utility to model liquid-liquid phase separations.
|-
! Three Phase Distillation
| [[File:ThreePhase.JPG|center|35px|]]|| Utility to model distillation consisting of three phases internally.
|-
! Distillation Sub-Flowsheet
| [[File:DistillationColumn.JPG|center|35px|]]|| Overall two-phase distillation utility.
|-
! Shortcut Column
| [[File:ShortcutDistillation.JPG|center|35px|]]|| Approximate model of distillation processes.
|-
! Component Splitter
| [[File:ComponentSplitter.JPG|center|35px|]]|| Utility to force components to separate with no equilibrium considerations.
|}

==Simulating Separations in HYSYS==
===Shortcut Distillation Column<ref name = "HYSYS"> Aspentech. "Aspen HYSYS V8.0 Help Documentation". Published 2014 </ref>===

The shortcut distillation utility within HYSYS allows users to quickly model a distillation separation of a stream with relatively little known information. Once the user attaches a fully defined inlet stream to the shortcut distillation column, the essential information that must be specified are as follows:

* The vapor/liquid quality of the top product
* The light key component and composition in the bottoms product
* The heavy key component and composition in the distillate (top) product
* The condenser pressure
* The reboiler pressure
* The external reflux ratio

With this information, the utility will run and, if possible, converge on a solution that will meet the key component specifications. The critical information that the utility will calculate are as follows:

* Fully specified distillate product stream (temperature, pressure, flow rate, composition)
* Fully specified bottom product stream (temperature, pressure, flow rate, composition)
* Condenser operating temperature and heat duty
* Reboiler operating temperature and heat duty
* Minimum number of trays
* Actual number of trays
* Optimum feed tray location

The primary purpose of the shortcut distillation is to allow for users to perform a first pass estimate on the performance requirements of a given system for a specified reflux ratio. The values gathered from the shortcut distillation utility can be used to assist the user when setting up the complete distillation utility in HYSYS, or when attempting to find reasonable column properties to allow the system to converge (e.g. number of trays, feed location, etc.)

===Standard Distillation Column<ref name = "HYSYS" />===
The standard distillation column, called "Distillation Sub-Flowsheet" the "Columns" tab on the equipment palette, may be used to perform a more rigorous simulation of your process than that offered by the shortcut column. In addition, the shortcut column may be used as a "first pass" analysis and then these specification estimations can used as the starting point for the standard column.

The standard distillation column provides many options for customization, allowing a user to specify a partial or total condenser, the number of passes through the column, and the type of reboiler all within the initial setup phase of creating a column. In addition, many specifications and parameters can be customized once the column is running, such as purities, recoveries, temperature profiles, column duties, and even internal specifications such as tray type, tray diameter, and weir height. This flexibility allows for much more rigorous, accurate simulation of a distillation column than using a shortcut column.

====Manipulating Column Specifications====

Column specifications can be heavily manipulated to force a column to operate under specific conditions, to produce a stream with specific purity, and even to minimize utilities. See the tutorial section for an example of this usage.

''Information taken from Towler Chapter 17'' <ref name = "Towler"> Towler, G. Sinnott, "Chemical Engineering Design". Elsevier, 2012, Chapter 17: R. Separation Columns (Distillation, Absorption, and Extraction). </ref>

Parameters that directly impact the design and operation of the distillation column include:
*Reflux ratio
*Feed point location
*Column Pressure
*Number of equilibrium stages/real stages

Assuming that we have already specified a desired specification for either the distillate or bottoms stream, we can manipulate the above parameters when running our simulation within HYSYS.

'''Reflux Ratio'''

The number of stages required for separation is directly affected by the reflux ratio that can be defined as follows:

<math> R=\frac {Flow\ returned\ as\ reflux} {Flow\ of\ top\ product\ taken\ off} </math>

Increasing the reflux ratio will reduce the total number of stages required but will increase the utility costs for the column. Therefore, when simulating a distillation column, special care should be taken to strike a balance between capital and operating costs.

When starting a new distillation column simulation within HYSYS, it is a good idea to calculate a minimum reflux ratio to advise the specification of the reflux ratio. The minimum reflux ratio can be calculated through use of a McCabe-Thiele diagram. According to Towler, a reflux ratio of 1.15 times the minimum reflux ratio should be used as a first approximation. Once this ratio is inputted into the simulation environment, it can be further manipulated to fine-tune the optimum reflux ratio for the system. <ref name = "Towler" />

'''Feed Point Location'''

The column simulation environment within HYSYS allows for manipulation of the feed point location. The general rule for designating feed point location is to have the feed enter the column at the stage which best matches the composition of the feed. There can be some uncertainty when it comes to finding the precise feed point location. Instead, a trial-and-error method may be useful when using HYSYS to determine which feed stage results in the desired distillate and bottoms conditions.

'''Column Pressure'''

When specifying the distillation column within the HYSYS simulation environment, you must specify the operating pressure as well as any pressure drops. If you are running a HYSYS simulation to get a first-pass approximation of compositions and flow rates, it is a typical practice to assume the pressure in the column is constant. If you are running a more intensive and rigorous simulation, you can approximate the pressure drop per tray using the following equation:

<math> \Delta P_{\text{tray}} = 2 \rho_L g h_w </math>

where <math> \rho_L </math> is the liquid density, g is gravitational acceleration, and <math> h_w </math> is the weir height. <ref name = "Towler" />

==Tutorials==
===Shortcut Distillation Column<ref name = "HYSYS" />===

This example will simulate the separation of benzene, toluene and p-xylene from some upstream process.<ref name = "HYSYS" />

'''Initialize''' [[File:AddComponents.PNG|thumb|center|600px|Figure 2. Adding Components in HYSYS]]
#Start Aspen HYSYS v.8 and click “new” on the main screen to open a new simulation window.
#Once on the component list screen, select “add” near the bottom of the screen
#Search the database for the desired components using the search bar, and add them to the component list by clicking “add.” In this case, we wish to add Benzene, Toluene and p-Xylene.
#On the left hand “properties” menu, click the “fluid packages” tab and select “add” to add a new package.
#Select the desired fluid package. In this case we are using Peng-Robinson
#In the bottom, left-hand corner of the screen, click “simulation” to enter the simulation environment.
'''Setup Streams''' [[File:Streams.PNG|thumb|center|300px|Figure 3. Add Streams]]
#Start by clicking the blue arrow at the top of the “palette” window and then clicking on the main flowsheet to produce a new material stream. This will create our feed stream. It is a good idea to place your other streams as well: one for distillate product and one for bottoms product.
#Add two energy streams that will be the condenser and reboiler duties.
#Double click on the newly created feed stream to pull up the worksheet for the stream. Please note that we are specifying this feed stream to demonstrate how the column would behaved with a fully specified inlet stream. The temperature, pressure, flow rate and composition of the feed stream can be specified by the user. [[File:FeedConditions.PNG|thumb|center|300px|Figure 4. Define Feed]]
#We will make the following changes within this worksheet to specify the feed stream:
#Enter “feed” into the stream name.
#Set the temperature to 20 degrees C.
#Set the pressure to 1 atm. (Be sure to select the “atm” unit from the dropdown when you begin to enter this value)
#Set the mass flow to 500 kg/hr [[File:Compositions.PNG|thumb|center|600px|Figure 5. Define Compositions]]
#On the left hand column within the worksheet, click “composition” to enter the composition tab.
#Click “edit” on the composition tab to change the composition of the feed stream
#Enter .25 to Toluene, .35 to Benzene and .40 to p-Xylene.
The worksheet should indicate that the stream is fully specified with a green “ok” indicator at the bottom. Close the worksheet.
'''Fully Specify Shortcut Column''' [[File:ConnectStreams.PNG|thumb|center|600px|Figure 6. Connect the inlet and outlet streams]]
#Create the shortcut column by clicking on the "columns" tab within the palette and selecting "shortcut column"
#Click on the simulation environment to place the column
#The shortcut column should currently appear red to represent that it is not currently specified.
#Double click on the column to pull up the column worksheet
#Under each drop down, add the appropriate stream: E.g. Under "Distillate" add the distillate product stream that was defined earlier.
#Additionally, be sure to specify the quality of the top product stream: whether it is liquid or vapor phase. In this case, select "liquid." [[File:Set Keys.PNG|thumb|center|600px|Figure 7. Set the key specifications]]
#On the left hand side of the worksheet, select "Parameters."
#This will move you to the parameters section of the column window where column specifications can be added.
#At this point it is important to identify which components are most volatile. In this case, benzene is most volatile, xylene is the least volatile and toluene rests in between the two. One rule of thumb is that more volatile components tend to be more valuable, and so in this example we will be purifying benzene in the distillate stream. In order to do this we will specify that benzene is the light key in the bottoms and that toluene is the heavy key in the distillate. This arrangement makes sense since it will force the heavier xylene and toluene products to drop out in the bottoms.
#Set the Light Key in Bottoms to Benzene and the mole fraction to .001 and set the Heavy Key in Distillate to Toluene and the mole fraction to .001.
#Set the condenser and reboiler pressure to the approximate values that they would be. In this case, we are assuming that our column is sufficiently small that atmospheric pressure is a fair assumption for both values. Enter 1 atm for both of these values. [[File:Worksheet.PNG|thumb|center|600px|Figure 8. Viewing the Worksheet]]
#Finally set the reflux ratio to a value that is higher than the minimum reflux ratio that HYSYS calculates. In this case the minimum reflux ratio was calculated to be 2.119 and so it is best to input around 2.5. This value can be tweaked later to see how performance is changed for different reflux ratios.
#At this point it is worthwhile to see what outputs the column is producing, since this is one of the primary uses of the shortcut distillation column.
#Click on the "worksheet" tab at the top of the window. This will bring you to your stream information where you can view the temperatures and flow rates of your outgoing streams.
#Now click on the "compositions" tab on the left side to view the compositions of the outgoing stream. As expected, the benzene is leaving in the distillate stream at 99.9% purity with a .1% toluene impurity, while all of the xylene is leaving out in the bottoms. [[File:Performance.PNG|thumb|center|600px|Figure 9. View the Performance Tab]]
#At this point it is valuable to record the calculated minimum number of trays, actual number of trays, as well as the condenser/reboiler temperatures.
#To view these values click on the "performance" tab
#You can observe here that the minimum number of trays calculated was around 15.7, the actual number of trays used was 35.7 the optimal feed stage was around 20.5, the condenser temperature was found to be around 80 C, while the reboiler was calculated to be around 123 C.

===Standard Distillation Column<ref name = "HYSYS" />===
====General Setup and Use====

In this example, we will be setting up a basic distillation column in HYSYS to simulate the separation of benzene and toluene.

# Start a new case in HYSYS by clicking on the button on the top menu bar.
# On the Components screen, add Benzene and Toluene. [[File:Columncomponents.PNG|thumb|center|300px|Figure 10. Component Selection in HYSYS]]
# On the Fluid Package screen, you have a wide range of options with which to simulate vapor-liquid equilibrium. For this example, choose the Peng-Robinson fluid package and enter the simulation environment. [[File:Fluidpkg.PNG|thumb|center|400px|Figure 11. Fluid Package Selection in HYSYS]]
We are using the simple distillation column and thus we only need a single feed stream into the distillation column as well as a distillate stream and a bottoms stream. We will also need 2 energy streams for the condenser and reboiler. <ref name = "HYSYS" />
# Add the material and energy streams to the simulation environment. Naming them appropriately will make it easer to keep everything straight as we move forward with the tutorial.
# Select the distillation column from the palette and place anywhere in the simulation environment.
# Switch to “Attach Mode” and attach the mass and energy streams to their appropriate locations on the distillation column. NOTE: You could also complete this step by double-clicking on the column itself and selecting each respective stream from the drop down menus.
# Double click on the column and select the condenser type. For this example, select “total condenser” and click next
# This next page is where the pressures of the column are specified. For this example, we will assume that the column is run at 1 atm with no pressure drops within the column. Therefore, specify reboiler pressure and condenser pressure to 101.3 kPa and the condenser pressure drop to 0 kPa.
# The next page has fields where temperature estimates can be made for the condenser, top stage, and reboiler. As these are optional fields, simply click next.
# The final screen of the distillation column setup is where the reflux ratio and distillate flow rates should be specified. For this example, specify the distillate flow rate as 1000 kgmol/hr and the reflux ratio as 1.5.
# Click “Done” at the bottom of the screen and the simulation page should be back in full view. [[File:Simulationview.PNG|thumb|center|400px|Figure 12. Simulation View of Distillation Column in HYSYS]]
We are now ready to specify the flow rates and compositions of the streams coming in and out of the column.
# Double click on the feed stream coming into the column. A specifications page should come up.
# This page allows you specify a number of parameters pertaining to the feed stream, including temperature, pressure, and compositions. Specify the temperature as 37C and pressure as 101.3 kPa (1 atm). In addition, specify the molar flow as 3500 kgmole/h.
# Now click on the composition tab on the left side of the window. Here specify, the benzene as 70 mass % and the toluene as 30 mass %. The stream should be fully specified. Close the window to return to the simulation. [[File:Configurationpage.PNG|thumb|center|400px|Figure 13. Column Configuration Page]]
The column and feed stream should now be fully specified. It is now time to run the simulation.
# If the solver is not already activated, click the green traffic light at the top of the simulation window.
# Now double click on the distillation column to bring up the main column window. A feed stage and total number of stages should already be specified.
# Click the “Run” button of the window to start the simulation. The small rectangle to the right of this button should light up green and say “converged” when it finds a solution. [[File:Columncompositions.PNG|thumb|center|400px|Figure 14. Outlet compositions in HYSYS]]

====Designing to Meet Specifications====
This example demonstrates how to use a distillation column simulation in HYSYS to design a column that meets preset requirements, and how to optimize the column to minimize costs. Basic setup of the column is assumed understood as described in the above tutorial. In this tutorial, the example requirements are to design a column that separates a stream of 20 mol% methanol and 80 mol% water into a distillate stream with 97% purity methanol and high recovery. In addition, both capital and operating costs are desired to be minimized.

# Create a simulation with water and methanol as the two components, using the NRTL fluid package.
# Place a standard distillation column in the simulation environment, and define a feed stream with vapor fraction 0, temperature 25 C, pressure 1 atm, flow rate 10 kgmol/h, and a composition of 0.2 mol fraction methanol and the balance of water.
# Enter column setup. Define the column as having a total condenser, 8 stages, and the feed entering at stage 4. The column should be once-through with a regular reboiler. Condenser pressure should be 1 atm, while reboiler pressure will be set at 1.3 kPa, giving a small pressure drop through the column. Optional temperature estimates can be left empty. Reflux ratio should be specified as 1.5, and liquid rate as 8.2 kgmol/h. In a normal situation, many of these values can be initially estimated using a shortcut column and then using them here, in the first run of a standard column simulation. Finally, run the column simulation, and it should easily converge. [[File:Column_converged_initially.PNG|thumb|center|800px|Figure 15. Initial column converged]]
# Now begins the refinement and optimization portion of the tutorial. To do so, in the distillation column window, switch to the Monitor tab in the Design toolbar on the left. We will add four new specifications of interest: distillate methanol purity, distillate methanol recovery, condenser duty, and reboiler duty. [[File:Monitor_tab.PNG|thumb|center|800px|Figure 16. The monitor tab in HYSYS]]
# To add a new specification, click the button labeled "Add Spec..." at the bottom of the window. This first spec will be for distillate methanol purity. In the new popup window, choose "Column Component Fraction" from the list of specs. In the new window, the name of the spec can be changed, and the specification itself can be defined. Change "Target Type" to "Stream," change "Draw" to the distillate stream, and change "Component" to methanol. Close this window, and the new specification should now be included in the list on the main column window. [[File:Making_a_new_spec.PNG|thumb|center|300px|Figure 17. Creating a new specification for distillate methanol mole fraction]]
# To add the recovery specification, the process is similar, expect the specification type is now "Column Component Recovery." To add the condenser and reboiler duty specifications, the spec type is "Column Duty," and the "Energy Stream" field should be chosen to be the energy stream of the condenser and reboiler for their respective specifications. Add these three specifications to the list as well.
# Note that currently, the distillate methanol fraction is only 0.244. To begin manipulation column specifications to meet the requirement of 97% purity, change the currently empty specified value for this spec to the current value, 0.244. Then, deselect Distillate Rate as an active spec, and make this new purity spec active in its stead. The simulation should automatically and instantly converge - if not, hit run. After converging, all values should remain the same.
# Begin increasing the specified value for distillate methanol purity gradually, until finding the approximate largest value at which the column simulation can still converge. In this example, this specified value can be increased up to 0.925 before the simulation fails to converge.
# Once this maximum has been reached, increase the total number of column stages by 1 or 2 by returning to the Connections tab in the Design toolbar. Note that the feed stage will automatically recalculate to stay in the same relative column position as before. In this example, change the number of stages to 10, and observe the feed stage automatically recalculate to 5. Hit run, and the simulation should still easily converge.
# Increase the specified value for purity again until the column simulation again reaches a limit beyond which it cannot converge. With 10 stages in this example, the column cannot converge with a specified purity of 0.95, but it can for a purity of 0.945. So, 0.945 is selected as the highest viable purity for this number of stages.
# Repeat this process, increasing the number of stages again followed by increasing the purity, until the desired purity is met. In this example simulation, a purity specification of 0.97 can be met with 14 stages, with the feed entering at stage 7.
# Recovery can be maximized in the same way, replacing Reflux Ratio as an active spec, though in this situation recovery is already 99.9%, so there is no need to maximize further. [[File:Requirements_met.PNG|thumb|center|800px|Figure 18. All design requirements met]]
# Now that the requirements are met, we can begin optimizing the number of stages and column duty. Condenser and reboiler duties are desired to be as low as possible to keep operating costs at a minimum, though number of stages should be kept relatively low as well. Note that with the current column setup, condenser duty is -1.83e5 kJ/h, and reboiler duty is 2.42e5 kJ/h.
# To begin, set recovery as an active spec by specifying its value as the current value, in this case 0.999, and then deselecting Reflux Ratio as an active spec and selecting the custom recovery specification.
# Next, we want to find the feed stage which minimizes column duty for this number of stages. Vary the feed stage to see which stage results in the minimum column duties. In this example, the optimal feed stage is stage 8, which results in condenser duty of -1.76e5 kJ/h and reboiler duty of 2.35e5 kJ/h.
# Now, increase the number of stages gradually, and find the duty-minimizing feed stage for each number of stages. For example, for a column with 17 stages, the optimal feed stage is 10, which yields condenser duty of -1.58e5 kJ/h and reboiler duty of 2.16e5 kJ/h.
# This process can be repeated indefinitely, as more stages will consistently result in decreased column duty. However, there are diminishing returns, so careful consideration of the balance of installation costs (more stages) vs operating costs (more column duty) must be made taking into account the actual economics of the distillation column. In this example, these diminishing returns are easily visible in the values for condenser duty: increasing from 14 to 17 stages reduces condenser duty by 0.18 kJ/h, increasing to 20 stages reduces condenser duty by a further 0.08 kJ/h, and increasing to 23 stages reduces condenser duty by only 0.03 kJ/h, at a final value of -1.47e5 kJ/h. A similar trend can be seen in the reboiler duty.

==References==
<references />

Column

2016-02-06T06:03:01Z

Benjkg:

[[File:Distillationcolumn_singlefeed.png|thumb|right|300px|Figure 1. Distillation Column <ref name = "Towler" />]]
Authors: Ben Granger, [2016] Scott Smith, [2015] and Harry Poppick [2015] 

Stewards: Jian Gong and Fengqi You

The column utilities in HYSYS can be used to model a wide variety of chemical separation techniques including absorption, liquid-liquid extraction and three-phase distillation. This article highlights the use of HYSYS for simulations of standard chemical distillation. The two primary distillation simulation utilities are the shortcut column, which is useful for quick and cursory simulations, and the full column utility, for more involved simulations.

==HYSYS Column Types==
{| border="1" class="wikitable"
|+ Varieties of Separators in HYSYS
! Name
! Icon
! Description
|-
! Absorption Column
| [[File:Absorber.JPG|center|35px|]]|| Utility to model absorption and stripping (liquid/vapor) separations.
|-
! Refluxed Absorption Column
| [[File:RefluxedAbsorb.JPG|center|35px|]]|| Utility to model absorption and stripping (liquid/vapor) operations with condensed top products.
|-
! Absorption with Reboil
| [[File:ReboiledAbsorber.JPG|center|35px|]]|| Utility to model absorption and stripping (liquid/vapor) operations with reboiled bottoms.
|-
! Liquid Liquid Extractor
| [[File:LiquidLiquid.JPG|center|35px|]]|| Utility to model liquid-liquid phase separations.
|-
! Three Phase Distillation
| [[File:ThreePhase.JPG|center|35px|]]|| Utility to model distillation consisting of three phases internally.
|-
! Distillation Sub-Flowsheet
| [[File:DistillationColumn.JPG|center|35px|]]|| Overall two-phase distillation utility.
|-
! Shortcut Column
| [[File:ShortcutDistillation.JPG|center|35px|]]|| Approximate model of distillation processes.
|-
! Component Splitter
| [[File:ComponentSplitter.JPG|center|35px|]]|| Utility to force components to separate with no equilibrium considerations.
|}

==Simulating Separations in HYSYS==
===Shortcut Distillation Column<ref name = "HYSYS"> Aspentech. "Aspen HYSYS V8.0 Help Documentation". Published 2014 </ref>===

The shortcut distillation utility within HYSYS allows users to quickly model a distillation separation of a stream with relatively little known information. Once the user attaches a fully defined inlet stream to the shortcut distillation column, the essential information that must be specified are as follows:

* The vapor/liquid quality of the top product
* The light key component and composition in the bottoms product
* The heavy key component and composition in the distillate (top) product
* The condenser pressure
* The reboiler pressure
* The external reflux ratio

With this information, the utility will run and, if possible, converge on a solution that will meet the key component specifications. The critical information that the utility will calculate are as follows:

* Fully specified distillate product stream (temperature, pressure, flow rate, composition)
* Fully specified bottom product stream (temperature, pressure, flow rate, composition)
* Condenser operating temperature and heat duty
* Reboiler operating temperature and heat duty
* Minimum number of trays
* Actual number of trays
* Optimum feed tray location

The primary purpose of the shortcut distillation is to allow for users to perform a first pass estimate on the performance requirements of a given system for a specified reflux ratio. The values gathered from the shortcut distillation utility can be used to assist the user when setting up the complete distillation utility in HYSYS, or when attempting to find reasonable column properties to allow the system to converge (e.g. number of trays, feed location, etc.)

===Standard Distillation Column<ref name = "HYSYS" />===
The standard distillation column, called "Distillation Sub-Flowsheet" the "Columns" tab on the equipment palette, may be used to perform a more rigorous simulation of your process than that offered by the shortcut column. In addition, the shortcut column may be used as a "first pass" analysis and then these specification estimations can used as the starting point for the standard column.

The standard distillation column provides many options for customization, allowing a user to specify a partial or total condenser, the number of passes through the column, and the type of reboiler all within the initial setup phase of creating a column. In addition, many specifications and parameters can be customized once the column is running, such as purities, recoveries, temperature profiles, column duties, and even internal specifications such as tray type, tray diameter, and weir height. This flexibility allows for much more rigorous, accurate simulation of a distillation column than using a shortcut column.

====Manipulating Column Specifications====

Column specifications can be heavily manipulated to force a column to operate under specific conditions, to produce a stream with specific purity, and even to minimize utilities. See the tutorial section for an example of this usage.

''Information taken from Towler Chapter 17'' <ref name = "Towler"> Towler, G. Sinnott, "Chemical Engineering Design". Elsevier, 2012, Chapter 17: R. Separation Columns (Distillation, Absorption, and Extraction). </ref>

Parameters that directly impact the design and operation of the distillation column include:
*Reflux ratio
*Feed point location
*Column Pressure
*Number of equilibrium stages/real stages

Assuming that we have already specified a desired specification for either the distillate or bottoms stream, we can manipulate the above parameters when running our simulation within HYSYS.

'''Reflux Ratio'''

The number of stages required for separation is directly affected by the reflux ratio that can be defined as follows:

<math> R=\frac {Flow\ returned\ as\ reflux} {Flow\ of\ top\ product\ taken\ off} </math>

Increasing the reflux ratio will reduce the total number of stages required but will increase the utility costs for the column. Therefore, when simulating a distillation column, special care should be taken to strike a balance between capital and operating costs.

When starting a new distillation column simulation within HYSYS, it is a good idea to calculate a minimum reflux ratio to advise the specification of the reflux ratio. The minimum reflux ratio can be calculated through use of a McCabe-Thiele diagram. According to Towler, a reflux ratio of 1.15 times the minimum reflux ratio should be used as a first approximation. Once this ratio is inputted into the simulation environment, it can be further manipulated to fine-tune the optimum reflux ratio for the system. <ref name = "Towler" />

'''Feed Point Location'''

The column simulation environment within HYSYS allows for manipulation of the feed point location. The general rule for designating feed point location is to have the feed enter the column at the stage which best matches the composition of the feed. There can be some uncertainty when it comes to finding the precise feed point location. Instead, a trial-and-error method may be useful when using HYSYS to determine which feed stage results in the desired distillate and bottoms conditions.

'''Column Pressure'''

When specifying the distillation column within the HYSYS simulation environment, you must specify the operating pressure as well as any pressure drops. If you are running a HYSYS simulation to get a first-pass approximation of compositions and flow rates, it is a typical practice to assume the pressure in the column is constant. If you are running a more intensive and rigorous simulation, you can approximate the pressure drop per tray using the following equation:

<math> \Delta P_{\text{tray}} = 2 \rho_L g h_w </math>

where <math> \rho_L </math> is the liquid density, g is gravitational acceleration, and <math> h_w </math> is the weir height. <ref name = "Towler" />

==Tutorials==
===Shortcut Distillation Column<ref name = "HYSYS" />===

This example will simulate the separation of benzene, toluene and p-xylene from some upstream process.<ref name = "HYSYS" />

'''Initialize'''

[[File:AddComponents.PNG|thumb|center|600px|Figure 2. Adding Components in HYSYS]]

#Start Aspen HYSYS v.8 and click “new” on the main screen to open a new simulation window.
#Once on the component list screen, select “add” near the bottom of the screen
#Search the database for the desired components using the search bar, and add them to the component list by clicking “add.” In this case, we wish to add Benzene, Toluene and p-Xylene.
#On the left hand “properties” menu, click the “fluid packages” tab and select “add” to add a new package.
#Select the desired fluid package. In this case we are using Peng-Robinson
#In the bottom, left-hand corner of the screen, click “simulation” to enter the simulation environment.

'''Setup Streams'''

[[File:Streams.PNG|thumb|center|300px|Figure 3. Add Streams]]

#Start by clicking the blue arrow at the top of the “palette” window and then clicking on the main flowsheet to produce a new material stream. This will create our feed stream. It is a good idea to place your other streams as well: one for distillate product and one for bottoms product.
#Add two energy streams that will be the condenser and reboiler duties.
#Double click on the newly created feed stream to pull up the worksheet for the stream. Please note that we are specifying this feed stream to demonstrate how the column would behaved with a fully specified inlet stream. The temperature, pressure, flow rate and composition of the feed stream can be specified by the user.

[[File:FeedConditions.PNG|thumb|center|300px|Figure 4. Define Feed]]

#We will make the following changes within this worksheet to specify the feed stream:
#Enter “feed” into the stream name.
#Set the temperature to 20 degrees C.
#Set the pressure to 1 atm. (Be sure to select the “atm” unit from the dropdown when you begin to enter this value)
#Set the mass flow to 500 kg/hr

[[File:Compositions.PNG|thumb|center|600px|Figure 5. Define Compositions]]

#On the left hand column within the worksheet, click “composition” to enter the composition tab.
#Click “edit” on the composition tab to change the composition of the feed stream
#Enter .25 to Toluene, .35 to Benzene and .40 to p-Xylene.

The worksheet should indicate that the stream is fully specified with a green “ok” indicator at the bottom. Close the worksheet.

'''Fully Specify Shortcut Column'''

[[File:ConnectStreams.PNG|thumb|center|600px|Figure 6. Connect the inlet and outlet streams]]

#Create the shortcut column by clicking on the "columns" tab within the palette and selecting "shortcut column"
#Click on the simulation environment to place the column
#The shortcut column should currently appear red to represent that it is not currently specified.
#Double click on the column to pull up the column worksheet
#Under each drop down, add the appropriate stream: E.g. Under "Distillate" add the distillate product stream that was defined earlier.
#Additionally, be sure to specify the quality of the top product stream: whether it is liquid or vapor phase. In this case, select "liquid."

[[File:Set Keys.PNG|thumb|center|600px|Figure 7. Set the key specifications]]

#On the left hand side of the worksheet, select "Parameters."
#This will move you to the parameters section of the column window where column specifications can be added.
#At this point it is important to identify which components are most volatile. In this case, benzene is most volatile, xylene is the least volatile and toluene rests in between the two. One rule of thumb is that more volatile components tend to be more valuable, and so in this example we will be purifying benzene in the distillate stream. In order to do this we will specify that benzene is the light key in the bottoms and that toluene is the heavy key in the distillate. This arrangement makes sense since it will force the heavier xylene and toluene products to drop out in the bottoms.
#Set the Light Key in Bottoms to Benzene and the mole fraction to .001 and set the Heavy Key in Distillate to Toluene and the mole fraction to .001.
#Set the condenser and reboiler pressure to the approximate values that they would be. In this case, we are assuming that our column is sufficiently small that atmospheric pressure is a fair assumption for both values. Enter 1 atm for both of these values.

[[File:Worksheet.PNG|thumb|center|600px|Figure 8. Viewing the Worksheet]]

#Finally set the reflux ratio to a value that is higher than the minimum reflux ratio that HYSYS calculates. In this case the minimum reflux ratio was calculated to be 2.119 and so it is best to input around 2.5. This value can be tweaked later to see how performance is changed for different reflux ratios.
#At this point it is worthwhile to see what outputs the column is producing, since this is one of the primary uses of the shortcut distillation column.
#Click on the "worksheet" tab at the top of the window. This will bring you to your stream information where you can view the temperatures and flow rates of your outgoing streams.
#Now click on the "compositions" tab on the left side to view the compositions of the outgoing stream. As expected, the benzene is leaving in the distillate stream at 99.9% purity with a .1% toluene impurity, while all of the xylene is leaving out in the bottoms.

[[File:Performance.PNG|thumb|center|600px|Figure 9. View the Performance Tab]]

#At this point it is valuable to record the calculated minimum number of trays, actual number of trays, as well as the condenser/reboiler temperatures.
#To view these values click on the "performance" tab
#You can observe here that the minimum number of trays calculated was around 15.7, the actual number of trays used was 35.7 the optimal feed stage was around 20.5, the condenser temperature was found to be around 80 C, while the reboiler was calculated to be around 123 C.

===Standard Distillation Column<ref name = "HYSYS" />===
====General Setup and Use====

In this example, we will be setting up a basic distillation column in HYSYS to simulate the separation of benzene and toluene.

# Start a new case in HYSYS by clicking on the button on the top menu bar.
# On the Components screen, add Benzene and Toluene.
# On the Fluid Package screen, you have a wide range of options with which to simulate vapor-liquid equilibrium. For this example, choose the Peng-Robinson fluid package and enter the simulation environment.

[[File:Columncomponents.PNG|thumb|right|300px|Figure 10. Component Selection in HYSYS]] [[File:Fluidpkg.PNG|thumb|right|400px|Figure 12. Fluid Package Selection in HYSYS]]

We are using the simple distillation column and thus we only need a single feed stream into the distillation column as well as a distillate stream and a bottoms stream. We will also need 2 energy streams for the condenser and reboiler. <ref name = "HYSYS" />

# Add the material and energy streams to the simulation environment. Naming them appropriately will make it easer to keep everything straight as we move forward with the tutorial.
# Select the distillation column from the palette and place anywhere in the simulation environment.
# Switch to “Attach Mode” and attach the mass and energy streams to their appropriate locations on the distillation column. NOTE: You could also complete this step by double-clicking on the column itself and selecting each respective stream from the drop down menus.
# Double click on the column and select the condenser type. For this example, select “total condenser” and click next
# This next page is where the pressures of the column are specified. For this example, we will assume that the column is run at 1 atm with no pressure drops within the column. Therefore, specify reboiler pressure and condenser pressure to 101.3 kPa and the condenser pressure drop to 0 kPa.
# The next page has fields where temperature estimates can be made for the condenser, top stage, and reboiler. As these are optional fields, simply click next.
# The final screen of the distillation column setup is where the reflux ratio and distillate flow rates should be specified. For this example, specify the distillate flow rate as 1000 kgmol/hr and the reflux ratio as 1.5.
# Click “Done” at the bottom of the screen and the simulation page should be back in full view.

[[File:Simulationview.PNG|thumb|center|400px|Figure 11. Simulation View of Distillation Column in HYSYS]]

We are now ready to specify the flow rates and compositions of the streams coming in and out of the column.
# Double click on the feed stream coming into the column. A specifications page should come up.
# This page allows you specify a number of parameters pertaining to the feed stream, including temperature, pressure, and compositions. Specify the temperature as 37C and pressure as 101.3 kPa (1 atm). In addition, specify the molar flow as 3500 kgmole/h.
# Now click on the composition tab on the left side of the window. Here specify, the benzene as 70 mass % and the toluene as 30 mass %. The stream should be fully specified. Close the window to return to the simulation.

[[File:Configurationpage.PNG|thumb|center|400px|Figure 13. Column Configuration Page]]

The column and feed stream should now be fully specified. It is now time to run the simulation.
# If the solver is not already activated, click the green traffic light at the top of the simulation window.
# Now double click on the distillation column to bring up the main column window. A feed stage and total number of stages should already be specified.
# Click the “Run” button of the window to start the simulation. The small rectangle to the right of this button should light up green and say “converged” when it finds a solution.

[[File:Columncompositions.PNG|thumb|center|400px|Figure 14. Outlet compositions in HYSYS]]

====Designing to Meet Specifications====
This example demonstrates how to use a distillation column simulation in HYSYS to design a column that meets preset requirements, and how to optimize the column to minimize costs. Basic setup of the column is assumed understood as described in the above tutorial. In this tutorial, the example requirements are to design a column that separates a stream of 20 mol% methanol and 80 mol% water into a distillate stream with 97% purity methanol and high recovery. In addition, both capital and operating costs are desired to be minimized.

# Create a simulation with water and methanol as the two components, using the NRTL fluid package.
# Place a standard distillation column in the simulation environment, and define a feed stream with vapor fraction 0, temperature 25 C, pressure 1 atm, flow rate 10 kgmol/h, and a composition of 0.2 mol fraction methanol and the balance of water.
# Enter column setup. Define the column as having a total condenser, 8 stages, and the feed entering at stage 4. The column should be once-through with a regular reboiler. Condenser pressure should be 1 atm, while reboiler pressure will be set at 1.3 kPa, giving a small pressure drop through the column. Optional temperature estimates can be left empty. Reflux ratio should be specified as 1.5, and liquid rate as 8.2 kgmol/h. In a normal situation, many of these values can be initially estimated using a shortcut column and then using them here, in the first run of a standard column simulation. Finally, run the column simulation, and it should easily converge.
[[File:Column_converged_initially.PNG|thumb|center|800px|Figure 15. Initial column converged]]
# Now begins the refinement and optimization portion of the tutorial. To do so, in the distillation column window, switch to the Monitor tab in the Design toolbar on the left. We will add four new specifications of interest: distillate methanol purity, distillate methanol recovery, condenser duty, and reboiler duty.
[[File:Monitor_tab.PNG|thumb|center|800px|Figure 16. The monitor tab in HYSYS]]
# To add a new specification, click the button labeled "Add Spec..." at the bottom of the window. This first spec will be for distillate methanol purity. In the new popup window, choose "Column Component Fraction" from the list of specs. In the new window, the name of the spec can be changed, and the specification itself can be defined. Change "Target Type" to "Stream," change "Draw" to the distillate stream, and change "Component" to methanol. Close this window, and the new specification should now be included in the list on the main column window.
[[File:Making_a_new_spec.PNG|thumb|center|300px|Figure 17. Creating a new specification for distillate methanol mole fraction]]
# To add the recovery specification, the process is similar, expect the specification type is now "Column Component Recovery." To add the condenser and reboiler duty specifications, the spec type is "Column Duty," and the "Energy Stream" field should be chosen to be the energy stream of the condenser and reboiler for their respective specifications. Add these three specifications to the list as well.
# Note that currently, the distillate methanol fraction is only 0.244. To begin manipulation column specifications to meet the requirement of 97% purity, change the currently empty specified value for this spec to the current value, 0.244. Then, deselect Distillate Rate as an active spec, and make this new purity spec active in its stead. The simulation should automatically and instantly converge - if not, hit run. After converging, all values should remain the same.
# Begin increasing the specified value for distillate methanol purity gradually, until finding the approximate largest value at which the column simulation can still converge. In this example, this specified value can be increased up to 0.925 before the simulation fails to converge.
# Once this maximum has been reached, increase the total number of column stages by 1 or 2 by returning to the Connections tab in the Design toolbar. Note that the feed stage will automatically recalculate to stay in the same relative column position as before. In this example, change the number of stages to 10, and observe the feed stage automatically recalculate to 5. Hit run, and the simulation should still easily converge.
# Increase the specified value for purity again until the column simulation again reaches a limit beyond which it cannot converge. With 10 stages in this example, the column cannot converge with a specified purity of 0.95, but it can for a purity of 0.945. So, 0.945 is selected as the highest viable purity for this number of stages.
# Repeat this process, increasing the number of stages again followed by increasing the purity, until the desired purity is met. In this example simulation, a purity specification of 0.97 can be met with 14 stages, with the feed entering at stage 7.
# Recovery can be maximized in the same way, replacing Reflux Ratio as an active spec, though in this situation recovery is already 99.9%, so there is no need to maximize further.
[[File:Requirements_met.PNG|thumb|center|800px|Figure 18. All design requirements met]]
# Now that the requirements are met, we can begin optimizing the number of stages and column duty. Condenser and reboiler duties are desired to be as low as possible to keep operating costs at a minimum, though number of stages should be kept relatively low as well. Note that with the current column setup, condenser duty is -1.83e5 kJ/h, and reboiler duty is 2.42e5 kJ/h.
# To begin, set recovery as an active spec by specifying its value as the current value, in this case 0.999, and then deselecting Reflux Ratio as an active spec and selecting the custom recovery specification.
# Next, we want to find the feed stage which minimizes column duty for this number of stages. Vary the feed stage to see which stage results in the minimum column duties. In this example, the optimal feed stage is stage 8, which results in condenser duty of -1.76e5 kJ/h and reboiler duty of 2.35e5 kJ/h.
# Now, increase the number of stages gradually, and find the duty-minimizing feed stage for each number of stages. For example, for a column with 17 stages, the optimal feed stage is 10, which yields condenser duty of -1.58e5 kJ/h and reboiler duty of 2.16e5 kJ/h.
# This process can be repeated indefinitely, as more stages will consistently result in decreased column duty. However, there are diminishing returns, so careful consideration of the balance of installation costs (more stages) vs operating costs (more column duty) must be made taking into account the actual economics of the distillation column. In this example, these diminishing returns are easily visible in the values for condenser duty: increasing from 14 to 17 stages reduces condenser duty by 0.18 kJ/h, increasing to 20 stages reduces condenser duty by a further 0.08 kJ/h, and increasing to 23 stages reduces condenser duty by only 0.03 kJ/h, at a final value of -1.47e5 kJ/h. A similar trend can be seen in the reboiler duty.

==References==
<references />

Column

2016-02-06T06:02:24Z

Benjkg:

[[File:Distillationcolumn_singlefeed.png|thumb|right|300px|Figure 1. Distillation Column <ref name = "Towler" />]]
Authors: Ben Granger, [2016] Scott Smith, [2015] and Harry Poppick [2015] 

Stewards: Jian Gong and Fengqi You

The column utilities in HYSYS can be used to model a wide variety of chemical separation techniques including absorption, liquid-liquid extraction and three-phase distillation. This article highlights the use of HYSYS for simulations of standard chemical distillation. The two primary distillation simulation utilities are the shortcut column, which is useful for quick and cursory simulations, and the full column utility, for more involved simulations.

==HYSYS Column Types==
{| border="1" class="wikitable"
|+ Varieties of Separators in HYSYS
! Name
! Icon
! Description
|-
! Absorption Column
| [[File:Absorber.JPG|center|35px|]]|| Utility to model absorption and stripping (liquid/vapor) separations.
|-
! Refluxed Absorption Column
| [[File:RefluxedAbsorb.JPG|center|35px|]]|| Utility to model absorption and stripping (liquid/vapor) operations with condensed top products.
|-
! Absorption with Reboil
| [[File:ReboiledAbsorber.JPG|center|35px|]]|| Utility to model absorption and stripping (liquid/vapor) operations with reboiled bottoms.
|-
! Liquid Liquid Extractor
| [[File:LiquidLiquid.JPG|center|35px|]]|| Utility to model liquid-liquid phase separations.
|-
! Three Phase Distillation
| [[File:ThreePhase.JPG|center|35px|]]|| Utility to model distillation consisting of three phases internally.
|-
! Distillation Sub-Flowsheet
| [[File:DistillationColumn.JPG|center|35px|]]|| Overall two-phase distillation utility.
|-
! Shortcut Column
| [[File:ShortcutDistillation.JPG|center|35px|]]|| Approximate model of distillation processes.
|-
! Component Splitter
| [[File:ComponentSplitter.JPG|center|35px|]]|| Utility to force components to separate with no equilibrium considerations.
|}

==Simulating Separations in HYSYS==
===Shortcut Distillation Column<ref name = "HYSYS"> Aspentech. "Aspen HYSYS V8.0 Help Documentation". Published 2014 </ref>===

The shortcut distillation utility within HYSYS allows users to quickly model a distillation separation of a stream with relatively little known information. Once the user attaches a fully defined inlet stream to the shortcut distillation column, the essential information that must be specified are as follows:

* The vapor/liquid quality of the top product
* The light key component and composition in the bottoms product
* The heavy key component and composition in the distillate (top) product
* The condenser pressure
* The reboiler pressure
* The external reflux ratio

With this information, the utility will run and, if possible, converge on a solution that will meet the key component specifications. The critical information that the utility will calculate are as follows:

* Fully specified distillate product stream (temperature, pressure, flow rate, composition)
* Fully specified bottom product stream (temperature, pressure, flow rate, composition)
* Condenser operating temperature and heat duty
* Reboiler operating temperature and heat duty
* Minimum number of trays
* Actual number of trays
* Optimum feed tray location

The primary purpose of the shortcut distillation is to allow for users to perform a first pass estimate on the performance requirements of a given system for a specified reflux ratio. The values gathered from the shortcut distillation utility can be used to assist the user when setting up the complete distillation utility in HYSYS, or when attempting to find reasonable column properties to allow the system to converge (e.g. number of trays, feed location, etc.)

===Standard Distillation Column<ref name = "HYSYS" />===
The standard distillation column, called "Distillation Sub-Flowsheet" the "Columns" tab on the equipment palette, may be used to perform a more rigorous simulation of your process than that offered by the shortcut column. In addition, the shortcut column may be used as a "first pass" analysis and then these specification estimations can used as the starting point for the standard column.

The standard distillation column provides many options for customization, allowing a user to specify a partial or total condenser, the number of passes through the column, and the type of reboiler all within the initial setup phase of creating a column. In addition, many specifications and parameters can be customized once the column is running, such as purities, recoveries, temperature profiles, column duties, and even internal specifications such as tray type, tray diameter, and weir height. This flexibility allows for much more rigorous, accurate simulation of a distillation column than using a shortcut column.

====Manipulating Column Specifications====

Column specifications can be heavily manipulated to force a column to operate under specific conditions, to produce a stream with specific purity, and even to minimize utilities. See the tutorial section for an example of this usage.

''Information taken from Towler Chapter 17'' <ref name = "Towler"> Towler, G. Sinnott, "Chemical Engineering Design". Elsevier, 2012, Chapter 17: R. Separation Columns (Distillation, Absorption, and Extraction). </ref>

Parameters that directly impact the design and operation of the distillation column include:
*Reflux ratio
*Feed point location
*Column Pressure
*Number of equilibrium stages/real stages

Assuming that we have already specified a desired specification for either the distillate or bottoms stream, we can manipulate the above parameters when running our simulation within HYSYS.

'''Reflux Ratio'''

The number of stages required for separation is directly affected by the reflux ratio that can be defined as follows:

<math> R=\frac {Flow\ returned\ as\ reflux} {Flow\ of\ top\ product\ taken\ off} </math>

Increasing the reflux ratio will reduce the total number of stages required but will increase the utility costs for the column. Therefore, when simulating a distillation column, special care should be taken to strike a balance between capital and operating costs.

When starting a new distillation column simulation within HYSYS, it is a good idea to calculate a minimum reflux ratio to advise the specification of the reflux ratio. The minimum reflux ratio can be calculated through use of a McCabe-Thiele diagram. According to Towler, a reflux ratio of 1.15 times the minimum reflux ratio should be used as a first approximation. Once this ratio is inputted into the simulation environment, it can be further manipulated to fine-tune the optimum reflux ratio for the system. <ref name = "Towler" />

'''Feed Point Location'''

The column simulation environment within HYSYS allows for manipulation of the feed point location. The general rule for designating feed point location is to have the feed enter the column at the stage which best matches the composition of the feed. There can be some uncertainty when it comes to finding the precise feed point location. Instead, a trial-and-error method may be useful when using HYSYS to determine which feed stage results in the desired distillate and bottoms conditions.

'''Column Pressure'''

When specifying the distillation column within the HYSYS simulation environment, you must specify the operating pressure as well as any pressure drops. If you are running a HYSYS simulation to get a first-pass approximation of compositions and flow rates, it is a typical practice to assume the pressure in the column is constant. If you are running a more intensive and rigorous simulation, you can approximate the pressure drop per tray using the following equation:

<math> \Delta P_{\text{tray}} = 2 \rho_L g h_w </math>

where <math> \rho_L </math> is the liquid density, g is gravitational acceleration, and <math> h_w </math> is the weir height. <ref name = "Towler" />

==Tutorials==
===Shortcut Distillation Column<ref name = "HYSYS" />===

This example will simulate the separation of benzene, toluene and p-xylene from some upstream process.<ref name = "HYSYS" />

'''Initialize'''

[[File:AddComponents.PNG|thumb|center|600px|Figure 2. Adding Components in HYSYS]]

#Start Aspen HYSYS v.8 and click “new” on the main screen to open a new simulation window.
#Once on the component list screen, select “add” near the bottom of the screen
#Search the database for the desired components using the search bar, and add them to the component list by clicking “add.” In this case, we wish to add Benzene, Toluene and p-Xylene.
#On the left hand “properties” menu, click the “fluid packages” tab and select “add” to add a new package.
#Select the desired fluid package. In this case we are using Peng-Robinson
#In the bottom, left-hand corner of the screen, click “simulation” to enter the simulation environment.

'''Setup Streams'''

[[File:Streams.PNG|thumb|center|300px|Figure 3. Add Streams]]

#Start by clicking the blue arrow at the top of the “palette” window and then clicking on the main flowsheet to produce a new material stream. This will create our feed stream. It is a good idea to place your other streams as well: one for distillate product and one for bottoms product.
#Add two energy streams that will be the condenser and reboiler duties.
#Double click on the newly created feed stream to pull up the worksheet for the stream. Please note that we are specifying this feed stream to demonstrate how the column would behaved with a fully specified inlet stream. The temperature, pressure, flow rate and composition of the feed stream can be specified by the user.

[[File:FeedConditions.PNG|thumb|center|300px|Figure 4. Define Feed]]

#We will make the following changes within this worksheet to specify the feed stream:
#Enter “feed” into the stream name.
#Set the temperature to 20 degrees C.
#Set the pressure to 1 atm. (Be sure to select the “atm” unit from the dropdown when you begin to enter this value)
#Set the mass flow to 500 kg/hr

[[File:Compositions.PNG|thumb|center|600px|Figure 5. Define Compositions]]

#On the left hand column within the worksheet, click “composition” to enter the composition tab.
#Click “edit” on the composition tab to change the composition of the feed stream
#Enter .25 to Toluene, .35 to Benzene and .40 to p-Xylene.

The worksheet should indicate that the stream is fully specified with a green “ok” indicator at the bottom. Close the worksheet.

'''Fully Specify Shortcut Column'''

[[File:ConnectStreams.PNG|thumb|center|600px|Figure 6. Connect the inlet and outlet streams]]

#Create the shortcut column by clicking on the "columns" tab within the palette and selecting "shortcut column"
#Click on the simulation environment to place the column
#The shortcut column should currently appear red to represent that it is not currently specified.
#Double click on the column to pull up the column worksheet
#Under each drop down, add the appropriate stream: E.g. Under "Distillate" add the distillate product stream that was defined earlier.
#Additionally, be sure to specify the quality of the top product stream: whether it is liquid or vapor phase. In this case, select "liquid."

[[File:Set Keys.PNG|thumb|center|600px|Figure 7. Set the key specifications]]

#On the left hand side of the worksheet, select "Parameters."
#This will move you to the parameters section of the column window where column specifications can be added.
#At this point it is important to identify which components are most volatile. In this case, benzene is most volatile, xylene is the least volatile and toluene rests in between the two. One rule of thumb is that more volatile components tend to be more valuable, and so in this example we will be purifying benzene in the distillate stream. In order to do this we will specify that benzene is the light key in the bottoms and that toluene is the heavy key in the distillate. This arrangement makes sense since it will force the heavier xylene and toluene products to drop out in the bottoms.
#Set the Light Key in Bottoms to Benzene and the mole fraction to .001 and set the Heavy Key in Distillate to Toluene and the mole fraction to .001.
#Set the condenser and reboiler pressure to the approximate values that they would be. In this case, we are assuming that our column is sufficiently small that atmospheric pressure is a fair assumption for both values. Enter 1 atm for both of these values.

[[File:Worksheet.PNG|thumb|center|600px|Figure 8. Viewing the Worksheet]]

#Finally set the reflux ratio to a value that is higher than the minimum reflux ratio that HYSYS calculates. In this case the minimum reflux ratio was calculated to be 2.119 and so it is best to input around 2.5. This value can be tweaked later to see how performance is changed for different reflux ratios.
#At this point it is worthwhile to see what outputs the column is producing, since this is one of the primary uses of the shortcut distillation column.
#Click on the "worksheet" tab at the top of the window. This will bring you to your stream information where you can view the temperatures and flow rates of your outgoing streams.
#Now click on the "compositions" tab on the left side to view the compositions of the outgoing stream. As expected, the benzene is leaving in the distillate stream at 99.9% purity with a .1% toluene impurity, while all of the xylene is leaving out in the bottoms.

[[File:Performance.PNG|thumb|center|600px|Figure 9. View the Performance Tab]]

#At this point it is valuable to record the calculated minimum number of trays, actual number of trays, as well as the condenser/reboiler temperatures.
#To view these values click on the "performance" tab
#You can observe here that the minimum number of trays calculated was around 15.7, the actual number of trays used was 35.7 the optimal feed stage was around 20.5, the condenser temperature was found to be around 80 C, while the reboiler was calculated to be around 123 C.

===Standard Distillation Column<ref name = "HYSYS" />===
====General Setup and Use====

In this example, we will be setting up a basic distillation column in HYSYS to simulate the separation of benzene and toluene.

# Start a new case in HYSYS by clicking on the button on the top menu bar.
# On the Components screen, add Benzene and Toluene.
# On the Fluid Package screen, you have a wide range of options with which to simulate vapor-liquid equilibrium. For this example, choose the Peng-Robinson fluid package and enter the simulation environment.

[[File:Columncomponents.PNG|thumb|right|300px|Figure 10. Component Selection in HYSYS]] [[File:Fluidpkg.PNG|thumb|right|400px|Figure 12. Fluid Package Selection in HYSYS]]

We are using the simple distillation column and thus we only need a single feed stream into the distillation column as well as a distillate stream and a bottoms stream. We will also need 2 energy streams for the condenser and reboiler. <ref name = "HYSYS" />

# Add the material and energy streams to the simulation environment. Naming them appropriately will make it easer to keep everything straight as we move forward with the tutorial.
# Select the distillation column from the palette and place anywhere in the simulation environment.
# Switch to “Attach Mode” and attach the mass and energy streams to their appropriate locations on the distillation column. NOTE: You could also complete this step by double-clicking on the column itself and selecting each respective stream from the drop down menus.
# Double click on the column and select the condenser type. For this example, select “total condenser” and click next
# This next page is where the pressures of the column are specified. For this example, we will assume that the column is run at 1 atm with no pressure drops within the column. Therefore, specify reboiler pressure and condenser pressure to 101.3 kPa and the condenser pressure drop to 0 kPa.
# The next page has fields where temperature estimates can be made for the condenser, top stage, and reboiler. As these are optional fields, simply click next.
# The final screen of the distillation column setup is where the reflux ratio and distillate flow rates should be specified. For this example, specify the distillate flow rate as 1000 kgmol/hr and the reflux ratio as 1.5.
# Click “Done” at the bottom of the screen and the simulation page should be back in full view.

[[File:Simulationview.PNG|thumb|center|400px|Figure 11. Simulation View of Distillation Column in HYSYS]]

We are now ready to specify the flow rates and compositions of the streams coming in and out of the column.
# Double click on the feed stream coming into the column. A specifications page should come up.
# This page allows you specify a number of parameters pertaining to the feed stream, including temperature, pressure, and compositions. Specify the temperature as 37C and pressure as 101.3 kPa (1 atm). In addition, specify the molar flow as 3500 kgmole/h.
# Now click on the composition tab on the left side of the window. Here specify, the benzene as 70 mass % and the toluene as 30 mass %. The stream should be fully specified. Close the window to return to the simulation.

[[File:Configurationpage.PNG|thumb|center|400px|Figure 13. Column Configuration Page]]

The column and feed stream should now be fully specified. It is now time to run the simulation.
# If the solver is not already activated, click the green traffic light at the top of the simulation window.
# Now double click on the distillation column to bring up the main column window. A feed stage and total number of stages should already be specified.
# Click the “Run” button of the window to start the simulation. The small rectangle to the right of this button should light up green and say “converged” when it finds a solution.

[[File:Columncompositions.PNG|thumb|center|400px|Figure 14. Outlet compositions in HYSYS]]

====Designing to Meet Specifications====
This example demonstrates how to use a distillation column simulation in HYSYS to design a column that meets preset requirements, and how to optimize the column to minimize costs. Basic setup of the column is assumed understood as described in the above tutorial. In this tutorial, the example requirements are to design a column that separates a stream of 20 mol% methanol and 80 mol% water into a distillate stream with 97% purity methanol and high recovery. In addition, both capital and operating costs are desired to be minimized.

# Create a simulation with water and methanol as the two components, using the NRTL fluid package.
# Place a standard distillation column in the simulation environment, and define a feed stream with vapor fraction 0, temperature 25 C, pressure 1 atm, flow rate 10 kgmol/h, and a composition of 0.2 mol fraction methanol and the balance of water.
# Enter column setup. Define the column as having a total condenser, 8 stages, and the feed entering at stage 4. The column should be once-through with a regular reboiler. Condenser pressure should be 1 atm, while reboiler pressure will be set at 1.3 kPa, giving a small pressure drop through the column. Optional temperature estimates can be left empty. Reflux ratio should be specified as 1.5, and liquid rate as 8.2 kgmol/h. In a normal situation, many of these values can be initially estimated using a shortcut column and then using them here, in the first run of a standard column simulation. Finally, run the column simulation, and it should easily converge.
[[File:Column_converged_initially.PNG|thumb|center|400px|Figure 15. Initial column converged]]
# Now begins the refinement and optimization portion of the tutorial. To do so, in the distillation column window, switch to the Monitor tab in the Design toolbar on the left. We will add four new specifications of interest: distillate methanol purity, distillate methanol recovery, condenser duty, and reboiler duty.
[[File:Monitor_tab.PNG|thumb|center|400px|Figure 16. The monitor tab in HYSYS]]
# To add a new specification, click the button labeled "Add Spec..." at the bottom of the window. This first spec will be for distillate methanol purity. In the new popup window, choose "Column Component Fraction" from the list of specs. In the new window, the name of the spec can be changed, and the specification itself can be defined. Change "Target Type" to "Stream," change "Draw" to the distillate stream, and change "Component" to methanol. Close this window, and the new specification should now be included in the list on the main column window.
[[File:Making_a_new_spec.PNG|thumb|center|300px|Figure 17. Creating a new specification for distillate methanol mole fraction]]
# To add the recovery specification, the process is similar, expect the specification type is now "Column Component Recovery." To add the condenser and reboiler duty specifications, the spec type is "Column Duty," and the "Energy Stream" field should be chosen to be the energy stream of the condenser and reboiler for their respective specifications. Add these three specifications to the list as well.
# Note that currently, the distillate methanol fraction is only 0.244. To begin manipulation column specifications to meet the requirement of 97% purity, change the currently empty specified value for this spec to the current value, 0.244. Then, deselect Distillate Rate as an active spec, and make this new purity spec active in its stead. The simulation should automatically and instantly converge - if not, hit run. After converging, all values should remain the same.
# Begin increasing the specified value for distillate methanol purity gradually, until finding the approximate largest value at which the column simulation can still converge. In this example, this specified value can be increased up to 0.925 before the simulation fails to converge.
# Once this maximum has been reached, increase the total number of column stages by 1 or 2 by returning to the Connections tab in the Design toolbar. Note that the feed stage will automatically recalculate to stay in the same relative column position as before. In this example, change the number of stages to 10, and observe the feed stage automatically recalculate to 5. Hit run, and the simulation should still easily converge.
# Increase the specified value for purity again until the column simulation again reaches a limit beyond which it cannot converge. With 10 stages in this example, the column cannot converge with a specified purity of 0.95, but it can for a purity of 0.945. So, 0.945 is selected as the highest viable purity for this number of stages.
# Repeat this process, increasing the number of stages again followed by increasing the purity, until the desired purity is met. In this example simulation, a purity specification of 0.97 can be met with 14 stages, with the feed entering at stage 7.
# Recovery can be maximized in the same way, replacing Reflux Ratio as an active spec, though in this situation recovery is already 99.9%, so there is no need to maximize further.
[[File:Requirements_met.PNG|thumb|center|600px|Figure 18. All design requirements met]]
# Now that the requirements are met, we can begin optimizing the number of stages and column duty. Condenser and reboiler duties are desired to be as low as possible to keep operating costs at a minimum, though number of stages should be kept relatively low as well. Note that with the current column setup, condenser duty is -1.83e5 kJ/h, and reboiler duty is 2.42e5 kJ/h.
# To begin, set recovery as an active spec by specifying its value as the current value, in this case 0.999, and then deselecting Reflux Ratio as an active spec and selecting the custom recovery specification.
# Next, we want to find the feed stage which minimizes column duty for this number of stages. Vary the feed stage to see which stage results in the minimum column duties. In this example, the optimal feed stage is stage 8, which results in condenser duty of -1.76e5 kJ/h and reboiler duty of 2.35e5 kJ/h.
# Now, increase the number of stages gradually, and find the duty-minimizing feed stage for each number of stages. For example, for a column with 17 stages, the optimal feed stage is 10, which yields condenser duty of -1.58e5 kJ/h and reboiler duty of 2.16e5 kJ/h.
# This process can be repeated indefinitely, as more stages will consistently result in decreased column duty. However, there are diminishing returns, so careful consideration of the balance of installation costs (more stages) vs operating costs (more column duty) must be made taking into account the actual economics of the distillation column. In this example, these diminishing returns are easily visible in the values for condenser duty: increasing from 14 to 17 stages reduces condenser duty by 0.18 kJ/h, increasing to 20 stages reduces condenser duty by a further 0.08 kJ/h, and increasing to 23 stages reduces condenser duty by only 0.03 kJ/h, at a final value of -1.47e5 kJ/h. A similar trend can be seen in the reboiler duty.

==References==
<references />

Column

2016-02-06T06:01:24Z

Benjkg: Adding figures for tutorial

[[File:Distillationcolumn_singlefeed.png|thumb|right|300px|Figure 1. Distillation Column <ref name = "Towler" />]]
Authors: Ben Granger, [2016] Scott Smith, [2015] and Harry Poppick [2015] 

Stewards: Jian Gong and Fengqi You

The column utilities in HYSYS can be used to model a wide variety of chemical separation techniques including absorption, liquid-liquid extraction and three-phase distillation. This article highlights the use of HYSYS for simulations of standard chemical distillation. The two primary distillation simulation utilities are the shortcut column, which is useful for quick and cursory simulations, and the full column utility, for more involved simulations.

==HYSYS Column Types==
{| border="1" class="wikitable"
|+ Varieties of Separators in HYSYS
! Name
! Icon
! Description
|-
! Absorption Column
| [[File:Absorber.JPG|center|35px|]]|| Utility to model absorption and stripping (liquid/vapor) separations.
|-
! Refluxed Absorption Column
| [[File:RefluxedAbsorb.JPG|center|35px|]]|| Utility to model absorption and stripping (liquid/vapor) operations with condensed top products.
|-
! Absorption with Reboil
| [[File:ReboiledAbsorber.JPG|center|35px|]]|| Utility to model absorption and stripping (liquid/vapor) operations with reboiled bottoms.
|-
! Liquid Liquid Extractor
| [[File:LiquidLiquid.JPG|center|35px|]]|| Utility to model liquid-liquid phase separations.
|-
! Three Phase Distillation
| [[File:ThreePhase.JPG|center|35px|]]|| Utility to model distillation consisting of three phases internally.
|-
! Distillation Sub-Flowsheet
| [[File:DistillationColumn.JPG|center|35px|]]|| Overall two-phase distillation utility.
|-
! Shortcut Column
| [[File:ShortcutDistillation.JPG|center|35px|]]|| Approximate model of distillation processes.
|-
! Component Splitter
| [[File:ComponentSplitter.JPG|center|35px|]]|| Utility to force components to separate with no equilibrium considerations.
|}

==Simulating Separations in HYSYS==
===Shortcut Distillation Column<ref name = "HYSYS"> Aspentech. "Aspen HYSYS V8.0 Help Documentation". Published 2014 </ref>===

The shortcut distillation utility within HYSYS allows users to quickly model a distillation separation of a stream with relatively little known information. Once the user attaches a fully defined inlet stream to the shortcut distillation column, the essential information that must be specified are as follows:

* The vapor/liquid quality of the top product
* The light key component and composition in the bottoms product
* The heavy key component and composition in the distillate (top) product
* The condenser pressure
* The reboiler pressure
* The external reflux ratio

With this information, the utility will run and, if possible, converge on a solution that will meet the key component specifications. The critical information that the utility will calculate are as follows:

* Fully specified distillate product stream (temperature, pressure, flow rate, composition)
* Fully specified bottom product stream (temperature, pressure, flow rate, composition)
* Condenser operating temperature and heat duty
* Reboiler operating temperature and heat duty
* Minimum number of trays
* Actual number of trays
* Optimum feed tray location

The primary purpose of the shortcut distillation is to allow for users to perform a first pass estimate on the performance requirements of a given system for a specified reflux ratio. The values gathered from the shortcut distillation utility can be used to assist the user when setting up the complete distillation utility in HYSYS, or when attempting to find reasonable column properties to allow the system to converge (e.g. number of trays, feed location, etc.)

===Standard Distillation Column<ref name = "HYSYS" />===
The standard distillation column, called "Distillation Sub-Flowsheet" the "Columns" tab on the equipment palette, may be used to perform a more rigorous simulation of your process than that offered by the shortcut column. In addition, the shortcut column may be used as a "first pass" analysis and then these specification estimations can used as the starting point for the standard column.

The standard distillation column provides many options for customization, allowing a user to specify a partial or total condenser, the number of passes through the column, and the type of reboiler all within the initial setup phase of creating a column. In addition, many specifications and parameters can be customized once the column is running, such as purities, recoveries, temperature profiles, column duties, and even internal specifications such as tray type, tray diameter, and weir height. This flexibility allows for much more rigorous, accurate simulation of a distillation column than using a shortcut column.

====Manipulating Column Specifications====

Column specifications can be heavily manipulated to force a column to operate under specific conditions, to produce a stream with specific purity, and even to minimize utilities. See the tutorial section for an example of this usage.

''Information taken from Towler Chapter 17'' <ref name = "Towler"> Towler, G. Sinnott, "Chemical Engineering Design". Elsevier, 2012, Chapter 17: R. Separation Columns (Distillation, Absorption, and Extraction). </ref>

Parameters that directly impact the design and operation of the distillation column include:
*Reflux ratio
*Feed point location
*Column Pressure
*Number of equilibrium stages/real stages

Assuming that we have already specified a desired specification for either the distillate or bottoms stream, we can manipulate the above parameters when running our simulation within HYSYS.

'''Reflux Ratio'''

The number of stages required for separation is directly affected by the reflux ratio that can be defined as follows:

<math> R=\frac {Flow\ returned\ as\ reflux} {Flow\ of\ top\ product\ taken\ off} </math>

Increasing the reflux ratio will reduce the total number of stages required but will increase the utility costs for the column. Therefore, when simulating a distillation column, special care should be taken to strike a balance between capital and operating costs.

When starting a new distillation column simulation within HYSYS, it is a good idea to calculate a minimum reflux ratio to advise the specification of the reflux ratio. The minimum reflux ratio can be calculated through use of a McCabe-Thiele diagram. According to Towler, a reflux ratio of 1.15 times the minimum reflux ratio should be used as a first approximation. Once this ratio is inputted into the simulation environment, it can be further manipulated to fine-tune the optimum reflux ratio for the system. <ref name = "Towler" />

'''Feed Point Location'''

The column simulation environment within HYSYS allows for manipulation of the feed point location. The general rule for designating feed point location is to have the feed enter the column at the stage which best matches the composition of the feed. There can be some uncertainty when it comes to finding the precise feed point location. Instead, a trial-and-error method may be useful when using HYSYS to determine which feed stage results in the desired distillate and bottoms conditions.

'''Column Pressure'''

When specifying the distillation column within the HYSYS simulation environment, you must specify the operating pressure as well as any pressure drops. If you are running a HYSYS simulation to get a first-pass approximation of compositions and flow rates, it is a typical practice to assume the pressure in the column is constant. If you are running a more intensive and rigorous simulation, you can approximate the pressure drop per tray using the following equation:

<math> \Delta P_{\text{tray}} = 2 \rho_L g h_w </math>

where <math> \rho_L </math> is the liquid density, g is gravitational acceleration, and <math> h_w </math> is the weir height. <ref name = "Towler" />

==Tutorials==
===Shortcut Distillation Column<ref name = "HYSYS" />===

This example will simulate the separation of benzene, toluene and p-xylene from some upstream process.<ref name = "HYSYS" />

'''Initialize'''

[[File:AddComponents.PNG|thumb|center|600px|Figure 2. Adding Components in HYSYS]]

#Start Aspen HYSYS v.8 and click “new” on the main screen to open a new simulation window.
#Once on the component list screen, select “add” near the bottom of the screen
#Search the database for the desired components using the search bar, and add them to the component list by clicking “add.” In this case, we wish to add Benzene, Toluene and p-Xylene.
#On the left hand “properties” menu, click the “fluid packages” tab and select “add” to add a new package.
#Select the desired fluid package. In this case we are using Peng-Robinson
#In the bottom, left-hand corner of the screen, click “simulation” to enter the simulation environment.

'''Setup Streams'''

[[File:Streams.PNG|thumb|center|300px|Figure 3. Add Streams]]

#Start by clicking the blue arrow at the top of the “palette” window and then clicking on the main flowsheet to produce a new material stream. This will create our feed stream. It is a good idea to place your other streams as well: one for distillate product and one for bottoms product.
#Add two energy streams that will be the condenser and reboiler duties.
#Double click on the newly created feed stream to pull up the worksheet for the stream. Please note that we are specifying this feed stream to demonstrate how the column would behaved with a fully specified inlet stream. The temperature, pressure, flow rate and composition of the feed stream can be specified by the user.

[[File:FeedConditions.PNG|thumb|center|300px|Figure 4. Define Feed]]

#We will make the following changes within this worksheet to specify the feed stream:
#Enter “feed” into the stream name.
#Set the temperature to 20 degrees C.
#Set the pressure to 1 atm. (Be sure to select the “atm” unit from the dropdown when you begin to enter this value)
#Set the mass flow to 500 kg/hr

[[File:Compositions.PNG|thumb|center|600px|Figure 5. Define Compositions]]

#On the left hand column within the worksheet, click “composition” to enter the composition tab.
#Click “edit” on the composition tab to change the composition of the feed stream
#Enter .25 to Toluene, .35 to Benzene and .40 to p-Xylene.

The worksheet should indicate that the stream is fully specified with a green “ok” indicator at the bottom. Close the worksheet.

'''Fully Specify Shortcut Column'''

[[File:ConnectStreams.PNG|thumb|center|600px|Figure 6. Connect the inlet and outlet streams]]

#Create the shortcut column by clicking on the "columns" tab within the palette and selecting "shortcut column"
#Click on the simulation environment to place the column
#The shortcut column should currently appear red to represent that it is not currently specified.
#Double click on the column to pull up the column worksheet
#Under each drop down, add the appropriate stream: E.g. Under "Distillate" add the distillate product stream that was defined earlier.
#Additionally, be sure to specify the quality of the top product stream: whether it is liquid or vapor phase. In this case, select "liquid."

[[File:Set Keys.PNG|thumb|center|600px|Figure 7. Set the key specifications]]

#On the left hand side of the worksheet, select "Parameters."
#This will move you to the parameters section of the column window where column specifications can be added.
#At this point it is important to identify which components are most volatile. In this case, benzene is most volatile, xylene is the least volatile and toluene rests in between the two. One rule of thumb is that more volatile components tend to be more valuable, and so in this example we will be purifying benzene in the distillate stream. In order to do this we will specify that benzene is the light key in the bottoms and that toluene is the heavy key in the distillate. This arrangement makes sense since it will force the heavier xylene and toluene products to drop out in the bottoms.
#Set the Light Key in Bottoms to Benzene and the mole fraction to .001 and set the Heavy Key in Distillate to Toluene and the mole fraction to .001.
#Set the condenser and reboiler pressure to the approximate values that they would be. In this case, we are assuming that our column is sufficiently small that atmospheric pressure is a fair assumption for both values. Enter 1 atm for both of these values.

[[File:Worksheet.PNG|thumb|center|600px|Figure 8. Viewing the Worksheet]]

#Finally set the reflux ratio to a value that is higher than the minimum reflux ratio that HYSYS calculates. In this case the minimum reflux ratio was calculated to be 2.119 and so it is best to input around 2.5. This value can be tweaked later to see how performance is changed for different reflux ratios.
#At this point it is worthwhile to see what outputs the column is producing, since this is one of the primary uses of the shortcut distillation column.
#Click on the "worksheet" tab at the top of the window. This will bring you to your stream information where you can view the temperatures and flow rates of your outgoing streams.
#Now click on the "compositions" tab on the left side to view the compositions of the outgoing stream. As expected, the benzene is leaving in the distillate stream at 99.9% purity with a .1% toluene impurity, while all of the xylene is leaving out in the bottoms.

[[File:Performance.PNG|thumb|center|600px|Figure 9. View the Performance Tab]]

#At this point it is valuable to record the calculated minimum number of trays, actual number of trays, as well as the condenser/reboiler temperatures.
#To view these values click on the "performance" tab
#You can observe here that the minimum number of trays calculated was around 15.7, the actual number of trays used was 35.7 the optimal feed stage was around 20.5, the condenser temperature was found to be around 80 C, while the reboiler was calculated to be around 123 C.

===Standard Distillation Column<ref name = "HYSYS" />===
====General Setup and Use====

In this example, we will be setting up a basic distillation column in HYSYS to simulate the separation of benzene and toluene.

# Start a new case in HYSYS by clicking on the button on the top menu bar.
# On the Components screen, add Benzene and Toluene.
# On the Fluid Package screen, you have a wide range of options with which to simulate vapor-liquid equilibrium. For this example, choose the Peng-Robinson fluid package and enter the simulation environment.

[[File:Columncomponents.PNG|thumb|right|300px|Figure 10. Component Selection in HYSYS]] [[File:Fluidpkg.PNG|thumb|right|400px|Figure 12. Fluid Package Selection in HYSYS]]

We are using the simple distillation column and thus we only need a single feed stream into the distillation column as well as a distillate stream and a bottoms stream. We will also need 2 energy streams for the condenser and reboiler. <ref name = "HYSYS" />

# Add the material and energy streams to the simulation environment. Naming them appropriately will make it easer to keep everything straight as we move forward with the tutorial.
# Select the distillation column from the palette and place anywhere in the simulation environment.
# Switch to “Attach Mode” and attach the mass and energy streams to their appropriate locations on the distillation column. NOTE: You could also complete this step by double-clicking on the column itself and selecting each respective stream from the drop down menus.
# Double click on the column and select the condenser type. For this example, select “total condenser” and click next
# This next page is where the pressures of the column are specified. For this example, we will assume that the column is run at 1 atm with no pressure drops within the column. Therefore, specify reboiler pressure and condenser pressure to 101.3 kPa and the condenser pressure drop to 0 kPa.
# The next page has fields where temperature estimates can be made for the condenser, top stage, and reboiler. As these are optional fields, simply click next.
# The final screen of the distillation column setup is where the reflux ratio and distillate flow rates should be specified. For this example, specify the distillate flow rate as 1000 kgmol/hr and the reflux ratio as 1.5.
# Click “Done” at the bottom of the screen and the simulation page should be back in full view.

[[File:Simulationview.PNG|thumb|center|400px|Figure 11. Simulation View of Distillation Column in HYSYS]]

We are now ready to specify the flow rates and compositions of the streams coming in and out of the column.
# Double click on the feed stream coming into the column. A specifications page should come up.
# This page allows you specify a number of parameters pertaining to the feed stream, including temperature, pressure, and compositions. Specify the temperature as 37C and pressure as 101.3 kPa (1 atm). In addition, specify the molar flow as 3500 kgmole/h.
# Now click on the composition tab on the left side of the window. Here specify, the benzene as 70 mass % and the toluene as 30 mass %. The stream should be fully specified. Close the window to return to the simulation.

[[File:Configurationpage.PNG|thumb|center|400px|Figure 13. Column Configuration Page]]

The column and feed stream should now be fully specified. It is now time to run the simulation.
# If the solver is not already activated, click the green traffic light at the top of the simulation window.
# Now double click on the distillation column to bring up the main column window. A feed stage and total number of stages should already be specified.
# Click the “Run” button of the window to start the simulation. The small rectangle to the right of this button should light up green and say “converged” when it finds a solution.

[[File:Columncompositions.PNG|thumb|center|400px|Figure 14. Outlet compositions in HYSYS]]

====Designing to Meet Specifications====
This example demonstrates how to use a distillation column simulation in HYSYS to design a column that meets preset requirements, and how to optimize the column to minimize costs. Basic setup of the column is assumed understood as described in the above tutorial. In this tutorial, the example requirements are to design a column that separates a stream of 20 mol% methanol and 80 mol% water into a distillate stream with 97% purity methanol and high recovery. In addition, both capital and operating costs are desired to be minimized.

# Create a simulation with water and methanol as the two components, using the NRTL fluid package.
# Place a standard distillation column in the simulation environment, and define a feed stream with vapor fraction 0, temperature 25 C, pressure 1 atm, flow rate 10 kgmol/h, and a composition of 0.2 mol fraction methanol and the balance of water.
# Enter column setup. Define the column as having a total condenser, 8 stages, and the feed entering at stage 4. The column should be once-through with a regular reboiler. Condenser pressure should be 1 atm, while reboiler pressure will be set at 1.3 kPa, giving a small pressure drop through the column. Optional temperature estimates can be left empty. Reflux ratio should be specified as 1.5, and liquid rate as 8.2 kgmol/h. In a normal situation, many of these values can be initially estimated using a shortcut column and then using them here, in the first run of a standard column simulation. Finally, run the column simulation, and it should easily converge.
[[File:Column_converged_initially.PNG|thumb|center}400px|Figure 15. Initial column converged]]
# Now begins the refinement and optimization portion of the tutorial. To do so, in the distillation column window, switch to the Monitor tab in the Design toolbar on the left. We will add four new specifications of interest: distillate methanol purity, distillate methanol recovery, condenser duty, and reboiler duty.
[[File:Monitor_tab.PNG|thumb|center|400px|Figure 16. The monitor tab in HYSYS]]
# To add a new specification, click the button labeled "Add Spec..." at the bottom of the window. This first spec will be for distillate methanol purity. In the new popup window, choose "Column Component Fraction" from the list of specs. In the new window, the name of the spec can be changed, and the specification itself can be defined. Change "Target Type" to "Stream," change "Draw" to the distillate stream, and change "Component" to methanol. Close this window, and the new specification should now be included in the list on the main column window.
[[File:Making_a_new_spec.PNG|thumb|center|400px|Figure 17. Creating a new specification for distillate methanol mole fraction]]
# To add the recovery specification, the process is similar, expect the specification type is now "Column Component Recovery." To add the condenser and reboiler duty specifications, the spec type is "Column Duty," and the "Energy Stream" field should be chosen to be the energy stream of the condenser and reboiler for their respective specifications. Add these three specifications to the list as well.
# Note that currently, the distillate methanol fraction is only 0.244. To begin manipulation column specifications to meet the requirement of 97% purity, change the currently empty specified value for this spec to the current value, 0.244. Then, deselect Distillate Rate as an active spec, and make this new purity spec active in its stead. The simulation should automatically and instantly converge - if not, hit run. After converging, all values should remain the same.
# Begin increasing the specified value for distillate methanol purity gradually, until finding the approximate largest value at which the column simulation can still converge. In this example, this specified value can be increased up to 0.925 before the simulation fails to converge.
# Once this maximum has been reached, increase the total number of column stages by 1 or 2 by returning to the Connections tab in the Design toolbar. Note that the feed stage will automatically recalculate to stay in the same relative column position as before. In this example, change the number of stages to 10, and observe the feed stage automatically recalculate to 5. Hit run, and the simulation should still easily converge.
# Increase the specified value for purity again until the column simulation again reaches a limit beyond which it cannot converge. With 10 stages in this example, the column cannot converge with a specified purity of 0.95, but it can for a purity of 0.945. So, 0.945 is selected as the highest viable purity for this number of stages.
# Repeat this process, increasing the number of stages again followed by increasing the purity, until the desired purity is met. In this example simulation, a purity specification of 0.97 can be met with 14 stages, with the feed entering at stage 7.
# Recovery can be maximized in the same way, replacing Reflux Ratio as an active spec, though in this situation recovery is already 99.9%, so there is no need to maximize further.
[[File:Requirements_met.PNG|thumb|center|400px|Figure 18. All design requirements met]]
# Now that the requirements are met, we can begin optimizing the number of stages and column duty. Condenser and reboiler duties are desired to be as low as possible to keep operating costs at a minimum, though number of stages should be kept relatively low as well. Note that with the current column setup, condenser duty is -1.83e5 kJ/h, and reboiler duty is 2.42e5 kJ/h.
# To begin, set recovery as an active spec by specifying its value as the current value, in this case 0.999, and then deselecting Reflux Ratio as an active spec and selecting the custom recovery specification.
# Next, we want to find the feed stage which minimizes column duty for this number of stages. Vary the feed stage to see which stage results in the minimum column duties. In this example, the optimal feed stage is stage 8, which results in condenser duty of -1.76e5 kJ/h and reboiler duty of 2.35e5 kJ/h.
# Now, increase the number of stages gradually, and find the duty-minimizing feed stage for each number of stages. For example, for a column with 17 stages, the optimal feed stage is 10, which yields condenser duty of -1.58e5 kJ/h and reboiler duty of 2.16e5 kJ/h.
# This process can be repeated indefinitely, as more stages will consistently result in decreased column duty. However, there are diminishing returns, so careful consideration of the balance of installation costs (more stages) vs operating costs (more column duty) must be made taking into account the actual economics of the distillation column. In this example, these diminishing returns are easily visible in the values for condenser duty: increasing from 14 to 17 stages reduces condenser duty by 0.18 kJ/h, increasing to 20 stages reduces condenser duty by a further 0.08 kJ/h, and increasing to 23 stages reduces condenser duty by only 0.03 kJ/h, at a final value of -1.47e5 kJ/h. A similar trend can be seen in the reboiler duty.

==References==
<references />

File:Requirements met.PNG

2016-02-06T05:57:52Z

Benjkg: HYSYS screenshot showing a distillation column with all requirements met which were set forth in the Design to Meet Specifications tutorial in the Column simulation page.

HYSYS screenshot showing a distillation column with all requirements met which were set forth in the Design to Meet Specifications tutorial in the Column simulation page.

File:Making a new spec.PNG

2016-02-06T05:57:03Z

Benjkg: The window used to create a new specification for a distillation column in HYSYS. This specification is being used to monitor and control the methanol fraction in the distillate.

The window used to create a new specification for a distillation column in HYSYS. This specification is being used to monitor and control the methanol fraction in the distillate.

File:Monitor tab.PNG

2016-02-06T05:55:58Z

Benjkg: Benjkg uploaded a new version of "File:Monitor tab.PNG": The "Monitor" tab in the standard distillation column window. This tab displays the values and properties of all specifications being monitored for the distillation column.

The "Monitor" tab in the standard distillation column window. This tab displays the values and properties of all specifications being monitored for the distillation column.

File:Monitor tab.PNG

2016-02-06T05:55:58Z

Benjkg: The "Monitor" tab in the standard distillation column window. This tab displays the values and properties of all specifications being monitored for the distillation column.

The "Monitor" tab in the standard distillation column window. This tab displays the values and properties of all specifications being monitored for the distillation column.

File:Column converged initially.PNG

2016-02-06T05:50:38Z

Benjkg: Standard distillation column, set up as described in the Design to Meet Specifications Tutorial in the Column simulation page. The column separates methanol from water, has 8 stages, and has successfully converged.

Standard distillation column, set up as described in the Design to Meet Specifications Tutorial in the Column simulation page. The column separates methanol from water, has 8 stages, and has successfully converged.

Column

2016-02-06T05:44:38Z

Benjkg: Added some more text

[[File:Distillationcolumn_singlefeed.png|thumb|right|300px|Figure 1. Distillation Column <ref name = "Towler" />]]
Authors: Ben Granger, [2016] Scott Smith, [2015] and Harry Poppick [2015] 

Stewards: Jian Gong and Fengqi You

The column utilities in HYSYS can be used to model a wide variety of chemical separation techniques including absorption, liquid-liquid extraction and three-phase distillation. This article highlights the use of HYSYS for simulations of standard chemical distillation. The two primary distillation simulation utilities are the shortcut column, which is useful for quick and cursory simulations, and the full column utility, for more involved simulations.

==HYSYS Column Types==
{| border="1" class="wikitable"
|+ Varieties of Separators in HYSYS
! Name
! Icon
! Description
|-
! Absorption Column
| [[File:Absorber.JPG|center|35px|]]|| Utility to model absorption and stripping (liquid/vapor) separations.
|-
! Refluxed Absorption Column
| [[File:RefluxedAbsorb.JPG|center|35px|]]|| Utility to model absorption and stripping (liquid/vapor) operations with condensed top products.
|-
! Absorption with Reboil
| [[File:ReboiledAbsorber.JPG|center|35px|]]|| Utility to model absorption and stripping (liquid/vapor) operations with reboiled bottoms.
|-
! Liquid Liquid Extractor
| [[File:LiquidLiquid.JPG|center|35px|]]|| Utility to model liquid-liquid phase separations.
|-
! Three Phase Distillation
| [[File:ThreePhase.JPG|center|35px|]]|| Utility to model distillation consisting of three phases internally.
|-
! Distillation Sub-Flowsheet
| [[File:DistillationColumn.JPG|center|35px|]]|| Overall two-phase distillation utility.
|-
! Shortcut Column
| [[File:ShortcutDistillation.JPG|center|35px|]]|| Approximate model of distillation processes.
|-
! Component Splitter
| [[File:ComponentSplitter.JPG|center|35px|]]|| Utility to force components to separate with no equilibrium considerations.
|}

==Simulating Separations in HYSYS==
===Shortcut Distillation Column<ref name = "HYSYS"> Aspentech. "Aspen HYSYS V8.0 Help Documentation". Published 2014 </ref>===

The shortcut distillation utility within HYSYS allows users to quickly model a distillation separation of a stream with relatively little known information. Once the user attaches a fully defined inlet stream to the shortcut distillation column, the essential information that must be specified are as follows:

* The vapor/liquid quality of the top product
* The light key component and composition in the bottoms product
* The heavy key component and composition in the distillate (top) product
* The condenser pressure
* The reboiler pressure
* The external reflux ratio

With this information, the utility will run and, if possible, converge on a solution that will meet the key component specifications. The critical information that the utility will calculate are as follows:

* Fully specified distillate product stream (temperature, pressure, flow rate, composition)
* Fully specified bottom product stream (temperature, pressure, flow rate, composition)
* Condenser operating temperature and heat duty
* Reboiler operating temperature and heat duty
* Minimum number of trays
* Actual number of trays
* Optimum feed tray location

The primary purpose of the shortcut distillation is to allow for users to perform a first pass estimate on the performance requirements of a given system for a specified reflux ratio. The values gathered from the shortcut distillation utility can be used to assist the user when setting up the complete distillation utility in HYSYS, or when attempting to find reasonable column properties to allow the system to converge (e.g. number of trays, feed location, etc.)

===Standard Distillation Column<ref name = "HYSYS" />===
The standard distillation column, called "Distillation Sub-Flowsheet" the "Columns" tab on the equipment palette, may be used to perform a more rigorous simulation of your process than that offered by the shortcut column. In addition, the shortcut column may be used as a "first pass" analysis and then these specification estimations can used as the starting point for the standard column.

The standard distillation column provides many options for customization, allowing a user to specify a partial or total condenser, the number of passes through the column, and the type of reboiler all within the initial setup phase of creating a column. In addition, many specifications and parameters can be customized once the column is running, such as purities, recoveries, temperature profiles, column duties, and even internal specifications such as tray type, tray diameter, and weir height. This flexibility allows for much more rigorous, accurate simulation of a distillation column than using a shortcut column.

====Manipulating Column Specifications====

Column specifications can be heavily manipulated to force a column to operate under specific conditions, to produce a stream with specific purity, and even to minimize utilities. See the tutorial section for an example of this usage.

''Information taken from Towler Chapter 17'' <ref name = "Towler"> Towler, G. Sinnott, "Chemical Engineering Design". Elsevier, 2012, Chapter 17: R. Separation Columns (Distillation, Absorption, and Extraction). </ref>

Parameters that directly impact the design and operation of the distillation column include:
*Reflux ratio
*Feed point location
*Column Pressure
*Number of equilibrium stages/real stages

Assuming that we have already specified a desired specification for either the distillate or bottoms stream, we can manipulate the above parameters when running our simulation within HYSYS.

'''Reflux Ratio'''

The number of stages required for separation is directly affected by the reflux ratio that can be defined as follows:

<math> R=\frac {Flow\ returned\ as\ reflux} {Flow\ of\ top\ product\ taken\ off} </math>

Increasing the reflux ratio will reduce the total number of stages required but will increase the utility costs for the column. Therefore, when simulating a distillation column, special care should be taken to strike a balance between capital and operating costs.

When starting a new distillation column simulation within HYSYS, it is a good idea to calculate a minimum reflux ratio to advise the specification of the reflux ratio. The minimum reflux ratio can be calculated through use of a McCabe-Thiele diagram. According to Towler, a reflux ratio of 1.15 times the minimum reflux ratio should be used as a first approximation. Once this ratio is inputted into the simulation environment, it can be further manipulated to fine-tune the optimum reflux ratio for the system. <ref name = "Towler" />

'''Feed Point Location'''

The column simulation environment within HYSYS allows for manipulation of the feed point location. The general rule for designating feed point location is to have the feed enter the column at the stage which best matches the composition of the feed. There can be some uncertainty when it comes to finding the precise feed point location. Instead, a trial-and-error method may be useful when using HYSYS to determine which feed stage results in the desired distillate and bottoms conditions.

'''Column Pressure'''

When specifying the distillation column within the HYSYS simulation environment, you must specify the operating pressure as well as any pressure drops. If you are running a HYSYS simulation to get a first-pass approximation of compositions and flow rates, it is a typical practice to assume the pressure in the column is constant. If you are running a more intensive and rigorous simulation, you can approximate the pressure drop per tray using the following equation:

<math> \Delta P_{\text{tray}} = 2 \rho_L g h_w </math>

where <math> \rho_L </math> is the liquid density, g is gravitational acceleration, and <math> h_w </math> is the weir height. <ref name = "Towler" />

==Tutorials==
===Shortcut Distillation Column<ref name = "HYSYS" />===

This example will simulate the separation of benzene, toluene and p-xylene from some upstream process.<ref name = "HYSYS" />

'''Initialize'''

[[File:AddComponents.PNG|thumb|center|600px|Figure 2. Adding Components in HYSYS]]

#Start Aspen HYSYS v.8 and click “new” on the main screen to open a new simulation window.
#Once on the component list screen, select “add” near the bottom of the screen
#Search the database for the desired components using the search bar, and add them to the component list by clicking “add.” In this case, we wish to add Benzene, Toluene and p-Xylene.
#On the left hand “properties” menu, click the “fluid packages” tab and select “add” to add a new package.
#Select the desired fluid package. In this case we are using Peng-Robinson
#In the bottom, left-hand corner of the screen, click “simulation” to enter the simulation environment.

'''Setup Streams'''

[[File:Streams.PNG|thumb|center|300px|Figure 3. Add Streams]]

#Start by clicking the blue arrow at the top of the “palette” window and then clicking on the main flowsheet to produce a new material stream. This will create our feed stream. It is a good idea to place your other streams as well: one for distillate product and one for bottoms product.
#Add two energy streams that will be the condenser and reboiler duties.
#Double click on the newly created feed stream to pull up the worksheet for the stream. Please note that we are specifying this feed stream to demonstrate how the column would behaved with a fully specified inlet stream. The temperature, pressure, flow rate and composition of the feed stream can be specified by the user.

[[File:FeedConditions.PNG|thumb|center|300px|Figure 4. Define Feed]]

#We will make the following changes within this worksheet to specify the feed stream:
#Enter “feed” into the stream name.
#Set the temperature to 20 degrees C.
#Set the pressure to 1 atm. (Be sure to select the “atm” unit from the dropdown when you begin to enter this value)
#Set the mass flow to 500 kg/hr

[[File:Compositions.PNG|thumb|center|600px|Figure 5. Define Compositions]]

#On the left hand column within the worksheet, click “composition” to enter the composition tab.
#Click “edit” on the composition tab to change the composition of the feed stream
#Enter .25 to Toluene, .35 to Benzene and .40 to p-Xylene.

The worksheet should indicate that the stream is fully specified with a green “ok” indicator at the bottom. Close the worksheet.

'''Fully Specify Shortcut Column'''

[[File:ConnectStreams.PNG|thumb|center|600px|Figure 6. Connect the inlet and outlet streams]]

#Create the shortcut column by clicking on the "columns" tab within the palette and selecting "shortcut column"
#Click on the simulation environment to place the column
#The shortcut column should currently appear red to represent that it is not currently specified.
#Double click on the column to pull up the column worksheet
#Under each drop down, add the appropriate stream: E.g. Under "Distillate" add the distillate product stream that was defined earlier.
#Additionally, be sure to specify the quality of the top product stream: whether it is liquid or vapor phase. In this case, select "liquid."

[[File:Set Keys.PNG|thumb|center|600px|Figure 7. Set the key specifications]]

#On the left hand side of the worksheet, select "Parameters."
#This will move you to the parameters section of the column window where column specifications can be added.
#At this point it is important to identify which components are most volatile. In this case, benzene is most volatile, xylene is the least volatile and toluene rests in between the two. One rule of thumb is that more volatile components tend to be more valuable, and so in this example we will be purifying benzene in the distillate stream. In order to do this we will specify that benzene is the light key in the bottoms and that toluene is the heavy key in the distillate. This arrangement makes sense since it will force the heavier xylene and toluene products to drop out in the bottoms.
#Set the Light Key in Bottoms to Benzene and the mole fraction to .001 and set the Heavy Key in Distillate to Toluene and the mole fraction to .001.
#Set the condenser and reboiler pressure to the approximate values that they would be. In this case, we are assuming that our column is sufficiently small that atmospheric pressure is a fair assumption for both values. Enter 1 atm for both of these values.

[[File:Worksheet.PNG|thumb|center|600px|Figure 8. Viewing the Worksheet]]

#Finally set the reflux ratio to a value that is higher than the minimum reflux ratio that HYSYS calculates. In this case the minimum reflux ratio was calculated to be 2.119 and so it is best to input around 2.5. This value can be tweaked later to see how performance is changed for different reflux ratios.
#At this point it is worthwhile to see what outputs the column is producing, since this is one of the primary uses of the shortcut distillation column.
#Click on the "worksheet" tab at the top of the window. This will bring you to your stream information where you can view the temperatures and flow rates of your outgoing streams.
#Now click on the "compositions" tab on the left side to view the compositions of the outgoing stream. As expected, the benzene is leaving in the distillate stream at 99.9% purity with a .1% toluene impurity, while all of the xylene is leaving out in the bottoms.

[[File:Performance.PNG|thumb|center|600px|Figure 9. View the Performance Tab]]

#At this point it is valuable to record the calculated minimum number of trays, actual number of trays, as well as the condenser/reboiler temperatures.
#To view these values click on the "performance" tab
#You can observe here that the minimum number of trays calculated was around 15.7, the actual number of trays used was 35.7 the optimal feed stage was around 20.5, the condenser temperature was found to be around 80 C, while the reboiler was calculated to be around 123 C.

===Standard Distillation Column<ref name = "HYSYS" />===
====General Setup and Use====

In this example, we will be setting up a basic distillation column in HYSYS to simulate the separation of benzene and toluene.

# Start a new case in HYSYS by clicking on the button on the top menu bar.
# On the Components screen, add Benzene and Toluene.
# On the Fluid Package screen, you have a wide range of options with which to simulate vapor-liquid equilibrium. For this example, choose the Peng-Robinson fluid package and enter the simulation environment.

[[File:Columncomponents.PNG|thumb|right|300px|Figure 10. Component Selection in HYSYS]] [[File:Fluidpkg.PNG|thumb|right|400px|Figure 12. Fluid Package Selection in HYSYS]]

We are using the simple distillation column and thus we only need a single feed stream into the distillation column as well as a distillate stream and a bottoms stream. We will also need 2 energy streams for the condenser and reboiler. <ref name = "HYSYS" />

# Add the material and energy streams to the simulation environment. Naming them appropriately will make it easer to keep everything straight as we move forward with the tutorial.
# Select the distillation column from the palette and place anywhere in the simulation environment.
# Switch to “Attach Mode” and attach the mass and energy streams to their appropriate locations on the distillation column. NOTE: You could also complete this step by double-clicking on the column itself and selecting each respective stream from the drop down menus.
# Double click on the column and select the condenser type. For this example, select “total condenser” and click next
# This next page is where the pressures of the column are specified. For this example, we will assume that the column is run at 1 atm with no pressure drops within the column. Therefore, specify reboiler pressure and condenser pressure to 101.3 kPa and the condenser pressure drop to 0 kPa.
# The next page has fields where temperature estimates can be made for the condenser, top stage, and reboiler. As these are optional fields, simply click next.
# The final screen of the distillation column setup is where the reflux ratio and distillate flow rates should be specified. For this example, specify the distillate flow rate as 1000 kgmol/hr and the reflux ratio as 1.5.
# Click “Done” at the bottom of the screen and the simulation page should be back in full view.

[[File:Simulationview.PNG|thumb|center|400px|Figure 11. Simulation View of Distillation Column in HYSYS]]

We are now ready to specify the flow rates and compositions of the streams coming in and out of the column.
# Double click on the feed stream coming into the column. A specifications page should come up.
# This page allows you specify a number of parameters pertaining to the feed stream, including temperature, pressure, and compositions. Specify the temperature as 37C and pressure as 101.3 kPa (1 atm). In addition, specify the molar flow as 3500 kgmole/h.
# Now click on the composition tab on the left side of the window. Here specify, the benzene as 70 mass % and the toluene as 30 mass %. The stream should be fully specified. Close the window to return to the simulation.

[[File:Configurationpage.PNG|thumb|center|400px|Figure 13. Column Configuration Page]]

The column and feed stream should now be fully specified. It is now time to run the simulation.
# If the solver is not already activated, click the green traffic light at the top of the simulation window.
# Now double click on the distillation column to bring up the main column window. A feed stage and total number of stages should already be specified.
# Click the “Run” button of the window to start the simulation. The small rectangle to the right of this button should light up green and say “converged” when it finds a solution.

[[File:Columncompositions.PNG|thumb|center|400px|Figure 14. Outlet compositions in HYSYS]]

====Designing to Meet Specifications====
This example demonstrates how to use a distillation column simulation in HYSYS to design a column that meets preset requirements, and how to optimize the column to minimize costs. Basic setup of the column is assumed understood as described in the above tutorial. In this tutorial, the example requirements are to design a column that separates a stream of 20 mol% methanol and 80 mol% water into a distillate stream with 97% purity methanol and high recovery. In addition, both capital and operating costs are desired to be minimized.

# Create a simulation with water and methanol as the two components, using the NRTL fluid package.
# Place a standard distillation column in the simulation environment, and define a feed stream with vapor fraction 0, temperature 25 C, pressure 1 atm, flow rate 10 kgmol/h, and a composition of 0.2 mol fraction methanol and the balance of water.
# Enter column setup. Define the column as having a total condenser, 8 stages, and the feed entering at stage 4. The column should be once-through with a regular reboiler. Condenser pressure should be 1 atm, while reboiler pressure will be set at 1.3 kPa, giving a small pressure drop through the column. Optional temperature estimates can be left empty. Reflux ratio should be specified as 1.5, and liquid rate as 8.2 kgmol/h. In a normal situation, many of these values can be initially estimated using a shortcut column and then using them here, in the first run of a standard column simulation. Finally, run the column simulation, and it should easily converge.
# Now begins the refinement and optimization portion of the tutorial. To do so, in the distillation column window, switch to the Monitor tab in the Design toolbar on the left. We will add four new specifications of interest: distillate methanol purity, distillate methanol recovery, condenser duty, and reboiler duty.
# To add a new specification, click the button labeled "Add Spec..." at the bottom of the window. This first spec will be for distillate methanol purity. In the new popup window, choose "Column Component Fraction" from the list of specs. In the new window, the name of the spec can be changed, and the specification itself can be defined. Change "Target Type" to "Stream," change "Draw" to the distillate stream, and change "Component" to methanol. Close this window, and the new specification should now be included in the list on the main column window.
# To add the recovery specification, the process is similar, expect the specification type is now "Column Component Recovery." To add the condenser and reboiler duty specifications, the spec type is "Column Duty," and the "Energy Stream" field should be chosen to be the energy stream of the condenser and reboiler for their respective specifications. Add these three specifications to the list as well.
# Note that currently, the distillate methanol fraction is only 0.244. To begin manipulation column specifications to meet the requirement of 97% purity, change the currently empty specified value for this spec to the current value, 0.244. Then, deselect Distillate Rate as an active spec, and make this new purity spec active in its stead. The simulation should automatically and instantly converge - if not, hit run. After converging, all values should remain the same.
# Begin increasing the specified value for distillate methanol purity gradually, until finding the approximate largest value at which the column simulation can still converge. In this example, this specified value can be increased up to 0.925 before the simulation fails to converge.
# Once this maximum has been reached, increase the total number of column stages by 1 or 2 by returning to the Connections tab in the Design toolbar. Note that the feed stage will automatically recalculate to stay in the same relative column position as before. In this example, change the number of stages to 10, and observe the feed stage automatically recalculate to 5. Hit run, and the simulation should still easily converge.
# Increase the specified value for purity again until the column simulation again reaches a limit beyond which it cannot converge. With 10 stages in this example, the column cannot converge with a specified purity of 0.95, but it can for a purity of 0.945. So, 0.945 is selected as the highest viable purity for this number of stages.
# Repeat this process, increasing the number of stages again followed by increasing the purity, until the desired purity is met. In this example simulation, a purity specification of 0.97 can be met with 14 stages, with the feed entering at stage 7.
# Recovery can be maximized in the same way, replacing Reflux Ratio as an active spec, though in this situation recovery is already 99.9%, so there is no need to maximize further.
# Now that the requirements are met, we can begin optimizing the number of stages and column duty. Condenser and reboiler duties are desired to be as low as possible to keep operating costs at a minimum, though number of stages should be kept relatively low as well. Note that with the current column setup, condenser duty is -1.83e5 kJ/h, and reboiler duty is 2.42e5 kJ/h.
# To begin, set recovery as an active spec by specifying its value as the current value, in this case 0.999, and then deselecting Reflux Ratio as an active spec and selecting the custom recovery specification.
# Next, we want to find the feed stage which minimizes column duty for this number of stages. Vary the feed stage to see which stage results in the minimum column duties. In this example, the optimal feed stage is stage 8, which results in condenser duty of -1.76e5 kJ/h and reboiler duty of 2.35e5 kJ/h.
# Now, increase the number of stages gradually, and find the duty-minimizing feed stage for each number of stages. For example, for a column with 17 stages, the optimal feed stage is 10, which yields condenser duty of -1.58e5 kJ/h and reboiler duty of 2.16e5 kJ/h.
# This process can be repeated indefinitely, as more stages will consistently result in decreased column duty. However, there are diminishing returns, so careful consideration of the balance of installation costs (more stages) vs operating costs (more column duty) must be made taking into account the actual economics of the distillation column. In this example, these diminishing returns are easily visible in the values for condenser duty: increasing from 14 to 17 stages reduces condenser duty by 0.18 kJ/h, increasing to 20 stages reduces condenser duty by a further 0.08 kJ/h, and increasing to 23 stages reduces condenser duty by only 0.03 kJ/h, at a final value of -1.47e5 kJ/h. A similar trend can be seen in the reboiler duty.

==References==
<references />

Column

2016-02-06T05:31:33Z

Benjkg: Added text for Manipulating Column Specifications section. Figures to be added later.

[[File:Distillationcolumn_singlefeed.png|thumb|right|300px|Figure 1. Distillation Column <ref name = "Towler" />]]
Authors: Ben Granger, [2016] Scott Smith, [2015] and Harry Poppick [2015] 

Stewards: Jian Gong and Fengqi You

The column utilities in HYSYS can be used to model a wide variety of chemical separation techniques including absorption, liquid-liquid extraction and three-phase distillation. This article highlights the use of HYSYS for simulations of standard chemical distillation. The two primary distillation simulation utilities are the shortcut column, which is useful for quick and cursory simulations, and the full column utility, for more involved simulations.

==HYSYS Column Types==
{| border="1" class="wikitable"
|+ Varieties of Separators in HYSYS
! Name
! Icon
! Description
|-
! Absorption Column
| [[File:Absorber.JPG|center|35px|]]|| Utility to model absorption and stripping (liquid/vapor) separations.
|-
! Refluxed Absorption Column
| [[File:RefluxedAbsorb.JPG|center|35px|]]|| Utility to model absorption and stripping (liquid/vapor) operations with condensed top products.
|-
! Absorption with Reboil
| [[File:ReboiledAbsorber.JPG|center|35px|]]|| Utility to model absorption and stripping (liquid/vapor) operations with reboiled bottoms.
|-
! Liquid Liquid Extractor
| [[File:LiquidLiquid.JPG|center|35px|]]|| Utility to model liquid-liquid phase separations.
|-
! Three Phase Distillation
| [[File:ThreePhase.JPG|center|35px|]]|| Utility to model distillation consisting of three phases internally.
|-
! Distillation Sub-Flowsheet
| [[File:DistillationColumn.JPG|center|35px|]]|| Overall two-phase distillation utility.
|-
! Shortcut Column
| [[File:ShortcutDistillation.JPG|center|35px|]]|| Approximate model of distillation processes.
|-
! Component Splitter
| [[File:ComponentSplitter.JPG|center|35px|]]|| Utility to force components to separate with no equilibrium considerations.
|}

==Simulating Separations in HYSYS==
===Shortcut Distillation Column<ref name = "HYSYS"> Aspentech. "Aspen HYSYS V8.0 Help Documentation". Published 2014 </ref>===

The shortcut distillation utility within HYSYS allows users to quickly model a distillation separation of a stream with relatively little known information. Once the user attaches a fully defined inlet stream to the shortcut distillation column, the essential information that must be specified are as follows:

* The vapor/liquid quality of the top product
* The light key component and composition in the bottoms product
* The heavy key component and composition in the distillate (top) product
* The condenser pressure
* The reboiler pressure
* The external reflux ratio

With this information, the utility will run and, if possible, converge on a solution that will meet the key component specifications. The critical information that the utility will calculate are as follows:

* Fully specified distillate product stream (temperature, pressure, flow rate, composition)
* Fully specified bottom product stream (temperature, pressure, flow rate, composition)
* Condenser operating temperature and heat duty
* Reboiler operating temperature and heat duty
* Minimum number of trays
* Actual number of trays
* Optimum feed tray location

The primary purpose of the shortcut distillation is to allow for users to perform a first pass estimate on the performance requirements of a given system for a specified reflux ratio. The values gathered from the shortcut distillation utility can be used to assist the user when setting up the complete distillation utility in HYSYS, or when attempting to find reasonable column properties to allow the system to converge (e.g. number of trays, feed location, etc.)

===Standard Distillation Column<ref name = "HYSYS" />===
The standard distillation column, found under the "Columns" tab on the equipment palette, may be used to perform a more rigorous simulation of your process than that offered by the shortcut column. In addition, the shortcut column may be used as a "first pass" analysis and then these specification estimations can used as the starting point for the standard column.

====Manipulating Column Specifications====

''Information taken from Towler Chapter 17'' <ref name = "Towler"> Towler, G. Sinnott, "Chemical Engineering Design". Elsevier, 2012, Chapter 17: R. Separation Columns (Distillation, Absorption, and Extraction). </ref>

Parameters that directly impact the design and operation of the distillation column include:
*Reflux ratio
*Feed point location
*Column Pressure
*Number of equilibrium stages/real stages

Assuming that we have already specified a desired specification for either the distillate or bottoms stream, we can manipulate the above parameters when running our simulation within HYSYS.

'''Reflux Ratio'''

The number of stages required for separation is directly affected by the reflux ratio that can be defined as follows:

<math> R=\frac {Flow\ returned\ as\ reflux} {Flow\ of\ top\ product\ taken\ off} </math>

Increasing the reflux ratio will reduce the total number of stages required but will increase the utility costs for the column. Therefore, when simulating a distillation column, special care should be taken to strike a balance between capital and operating costs.

When starting a new distillation column simulation within HYSYS, it is a good idea to calculate a minimum reflux ratio to advise the specification of the reflux ratio. The minimum reflux ratio can be calculated through use of a McCabe-Thiele diagram. According to Towler, a reflux ratio of 1.15 times the minimum reflux ratio should be used as a first approximation. Once this ratio is inputted into the simulation environment, it can be further manipulated to fine-tune the optimum reflux ratio for the system. <ref name = "Towler" />

'''Feed Point Location'''

The column simulation environment within HYSYS allows for manipulation of the feed point location. The general rule for designating feed point location is to have the feed enter the column at the stage which best matches the composition of the feed. There can be some uncertainty when it comes to finding the precise feed point location. Instead, a trial-and-error method may be useful when using HYSYS to determine which feed stage results in the desired distillate and bottoms conditions.

'''Column Pressure'''

When specifying the distillation column within the HYSYS simulation environment, you must specify the operating pressure as well as any pressure drops. If you are running a HYSYS simulation to get a first-pass approximation of compositions and flow rates, it is a typical practice to assume the pressure in the column is constant. If you are running a more intensive and rigorous simulation, you can approximate the pressure drop per tray using the following equation:

<math> \Delta P_{\text{tray}} = 2 \rho_L g h_w </math>

where <math> \rho_L </math> is the liquid density, g is gravitational acceleration, and <math> h_w </math> is the weir height. <ref name = "Towler" />

==Tutorials==
===Shortcut Distillation Column<ref name = "HYSYS" />===

This example will simulate the separation of benzene, toluene and p-xylene from some upstream process.<ref name = "HYSYS" />

'''Initialize'''

[[File:AddComponents.PNG|thumb|center|600px|Figure 2. Adding Components in HYSYS]]

#Start Aspen HYSYS v.8 and click “new” on the main screen to open a new simulation window.
#Once on the component list screen, select “add” near the bottom of the screen
#Search the database for the desired components using the search bar, and add them to the component list by clicking “add.” In this case, we wish to add Benzene, Toluene and p-Xylene.
#On the left hand “properties” menu, click the “fluid packages” tab and select “add” to add a new package.
#Select the desired fluid package. In this case we are using Peng-Robinson
#In the bottom, left-hand corner of the screen, click “simulation” to enter the simulation environment.

'''Setup Streams'''

[[File:Streams.PNG|thumb|center|300px|Figure 3. Add Streams]]

#Start by clicking the blue arrow at the top of the “palette” window and then clicking on the main flowsheet to produce a new material stream. This will create our feed stream. It is a good idea to place your other streams as well: one for distillate product and one for bottoms product.
#Add two energy streams that will be the condenser and reboiler duties.
#Double click on the newly created feed stream to pull up the worksheet for the stream. Please note that we are specifying this feed stream to demonstrate how the column would behaved with a fully specified inlet stream. The temperature, pressure, flow rate and composition of the feed stream can be specified by the user.

[[File:FeedConditions.PNG|thumb|center|300px|Figure 4. Define Feed]]

#We will make the following changes within this worksheet to specify the feed stream:
#Enter “feed” into the stream name.
#Set the temperature to 20 degrees C.
#Set the pressure to 1 atm. (Be sure to select the “atm” unit from the dropdown when you begin to enter this value)
#Set the mass flow to 500 kg/hr

[[File:Compositions.PNG|thumb|center|600px|Figure 5. Define Compositions]]

#On the left hand column within the worksheet, click “composition” to enter the composition tab.
#Click “edit” on the composition tab to change the composition of the feed stream
#Enter .25 to Toluene, .35 to Benzene and .40 to p-Xylene.

The worksheet should indicate that the stream is fully specified with a green “ok” indicator at the bottom. Close the worksheet.

'''Fully Specify Shortcut Column'''

[[File:ConnectStreams.PNG|thumb|center|600px|Figure 6. Connect the inlet and outlet streams]]

#Create the shortcut column by clicking on the "columns" tab within the palette and selecting "shortcut column"
#Click on the simulation environment to place the column
#The shortcut column should currently appear red to represent that it is not currently specified.
#Double click on the column to pull up the column worksheet
#Under each drop down, add the appropriate stream: E.g. Under "Distillate" add the distillate product stream that was defined earlier.
#Additionally, be sure to specify the quality of the top product stream: whether it is liquid or vapor phase. In this case, select "liquid."

[[File:Set Keys.PNG|thumb|center|600px|Figure 7. Set the key specifications]]

#On the left hand side of the worksheet, select "Parameters."
#This will move you to the parameters section of the column window where column specifications can be added.
#At this point it is important to identify which components are most volatile. In this case, benzene is most volatile, xylene is the least volatile and toluene rests in between the two. One rule of thumb is that more volatile components tend to be more valuable, and so in this example we will be purifying benzene in the distillate stream. In order to do this we will specify that benzene is the light key in the bottoms and that toluene is the heavy key in the distillate. This arrangement makes sense since it will force the heavier xylene and toluene products to drop out in the bottoms.
#Set the Light Key in Bottoms to Benzene and the mole fraction to .001 and set the Heavy Key in Distillate to Toluene and the mole fraction to .001.
#Set the condenser and reboiler pressure to the approximate values that they would be. In this case, we are assuming that our column is sufficiently small that atmospheric pressure is a fair assumption for both values. Enter 1 atm for both of these values.

[[File:Worksheet.PNG|thumb|center|600px|Figure 8. Viewing the Worksheet]]

#Finally set the reflux ratio to a value that is higher than the minimum reflux ratio that HYSYS calculates. In this case the minimum reflux ratio was calculated to be 2.119 and so it is best to input around 2.5. This value can be tweaked later to see how performance is changed for different reflux ratios.
#At this point it is worthwhile to see what outputs the column is producing, since this is one of the primary uses of the shortcut distillation column.
#Click on the "worksheet" tab at the top of the window. This will bring you to your stream information where you can view the temperatures and flow rates of your outgoing streams.
#Now click on the "compositions" tab on the left side to view the compositions of the outgoing stream. As expected, the benzene is leaving in the distillate stream at 99.9% purity with a .1% toluene impurity, while all of the xylene is leaving out in the bottoms.

[[File:Performance.PNG|thumb|center|600px|Figure 9. View the Performance Tab]]

#At this point it is valuable to record the calculated minimum number of trays, actual number of trays, as well as the condenser/reboiler temperatures.
#To view these values click on the "performance" tab
#You can observe here that the minimum number of trays calculated was around 15.7, the actual number of trays used was 35.7 the optimal feed stage was around 20.5, the condenser temperature was found to be around 80 C, while the reboiler was calculated to be around 123 C.

===Standard Distillation Column<ref name = "HYSYS" />===
====General Setup and Use====

In this example, we will be setting up a basic distillation column in HYSYS to simulate the separation of benzene and toluene.

# Start a new case in HYSYS by clicking on the button on the top menu bar.
# On the Components screen, add Benzene and Toluene.
# On the Fluid Package screen, you have a wide range of options with which to simulate vapor-liquid equilibrium. For this example, choose the Peng-Robinson fluid package and enter the simulation environment.

[[File:Columncomponents.PNG|thumb|right|300px|Figure 10. Component Selection in HYSYS]] [[File:Fluidpkg.PNG|thumb|right|400px|Figure 12. Fluid Package Selection in HYSYS]]

We are using the simple distillation column and thus we only need a single feed stream into the distillation column as well as a distillate stream and a bottoms stream. We will also need 2 energy streams for the condenser and reboiler. <ref name = "HYSYS" />

# Add the material and energy streams to the simulation environment. Naming them appropriately will make it easer to keep everything straight as we move forward with the tutorial.
# Select the distillation column from the palette and place anywhere in the simulation environment.
# Switch to “Attach Mode” and attach the mass and energy streams to their appropriate locations on the distillation column. NOTE: You could also complete this step by double-clicking on the column itself and selecting each respective stream from the drop down menus.
# Double click on the column and select the condenser type. For this example, select “total condenser” and click next
# This next page is where the pressures of the column are specified. For this example, we will assume that the column is run at 1 atm with no pressure drops within the column. Therefore, specify reboiler pressure and condenser pressure to 101.3 kPa and the condenser pressure drop to 0 kPa.
# The next page has fields where temperature estimates can be made for the condenser, top stage, and reboiler. As these are optional fields, simply click next.
# The final screen of the distillation column setup is where the reflux ratio and distillate flow rates should be specified. For this example, specify the distillate flow rate as 1000 kgmol/hr and the reflux ratio as 1.5.
# Click “Done” at the bottom of the screen and the simulation page should be back in full view.

[[File:Simulationview.PNG|thumb|center|400px|Figure 11. Simulation View of Distillation Column in HYSYS]]

We are now ready to specify the flow rates and compositions of the streams coming in and out of the column.
# Double click on the feed stream coming into the column. A specifications page should come up.
# This page allows you specify a number of parameters pertaining to the feed stream, including temperature, pressure, and compositions. Specify the temperature as 37C and pressure as 101.3 kPa (1 atm). In addition, specify the molar flow as 3500 kgmole/h.
# Now click on the composition tab on the left side of the window. Here specify, the benzene as 70 mass % and the toluene as 30 mass %. The stream should be fully specified. Close the window to return to the simulation.

[[File:Configurationpage.PNG|thumb|center|400px|Figure 13. Column Configuration Page]]

The column and feed stream should now be fully specified. It is now time to run the simulation.
# If the solver is not already activated, click the green traffic light at the top of the simulation window.
# Now double click on the distillation column to bring up the main column window. A feed stage and total number of stages should already be specified.
# Click the “Run” button of the window to start the simulation. The small rectangle to the right of this button should light up green and say “converged” when it finds a solution.

[[File:Columncompositions.PNG|thumb|center|400px|Figure 14. Outlet compositions in HYSYS]]

====Manipulating Column Specifications====
This example demonstrates how to use a distillation column simulation in HYSYS to design a column that meets preset requirements, and how to optimize the column to minimize costs. Basic setup of the column is assumed understood as described in the above tutorial. In this tutorial, the example requirements are to design a column that separates a stream of 20 mol% methanol and 80 mol% water into a distillate stream with 97% purity methanol and high recovery. In addition, both capital and operating costs are desired to be minimized.

# Create a simulation with water and methanol as the two components, using the NRTL fluid package.
# Place a standard distillation column in the simulation environment, and define a feed stream with vapor fraction 0, temperature 25 C, pressure 1 atm, flow rate 10 kgmol/h, and a composition of 0.2 mol fraction methanol and the balance of water.
# Enter column setup. Define the column as having a total condenser, 8 stages, and the feed entering at stage 4. The column should be once-through with a regular reboiler. Condenser pressure should be 1 atm, while reboiler pressure will be set at 1.3 kPa, giving a small pressure drop through the column. Optional temperature estimates can be left empty. Reflux ratio should be specified as 1.5, and liquid rate as 8.2 kgmol/h. In a normal situation, many of these values can be initially estimated using a shortcut column and then using them here, in the first run of a standard column simulation. Finally, run the column simulation, and it should easily converge.
# Now begins the refinement and optimization portion of the tutorial. To do so, in the distillation column window, switch to the Monitor tab in the Design toolbar on the left. We will add four new specifications of interest: distillate methanol purity, distillate methanol recovery, condenser duty, and reboiler duty.
# To add a new specification, click the button labeled "Add Spec..." at the bottom of the window. This first spec will be for distillate methanol purity. In the new popup window, choose "Column Component Fraction" from the list of specs. In the new window, the name of the spec can be changed, and the specification itself can be defined. Change "Target Type" to "Stream," change "Draw" to the distillate stream, and change "Component" to methanol. Close this window, and the new specification should now be included in the list on the main column window.
# To add the recovery specification, the process is similar, expect the specification type is now "Column Component Recovery." To add the condenser and reboiler duty specifications, the spec type is "Column Duty," and the "Energy Stream" field should be chosen to be the energy stream of the condenser and reboiler for their respective specifications. Add these three specifications to the list as well.
# Note that currently, the distillate methanol fraction is only 0.244. To begin manipulation column specifications to meet the requirement of 97% purity, change the currently empty specified value for this spec to the current value, 0.244. Then, deselect Distillate Rate as an active spec, and make this new purity spec active in its stead. The simulation should automatically and instantly converge - if not, hit run. After converging, all values should remain the same.
# Begin increasing the specified value for distillate methanol purity gradually, until finding the approximate largest value at which the column simulation can still converge. In this example, this specified value can be increased up to 0.925 before the simulation fails to converge.
# Once this maximum has been reached, increase the total number of column stages by 1 or 2 by returning to the Connections tab in the Design toolbar. Note that the feed stage will automatically recalculate to stay in the same relative column position as before. In this example, change the number of stages to 10, and observe the feed stage automatically recalculate to 5. Hit run, and the simulation should still easily converge.
# Increase the specified value for purity again until the column simulation again reaches a limit beyond which it cannot converge. With 10 stages in this example, the column cannot converge with a specified purity of 0.95, but it can for a purity of 0.945. So, 0.945 is selected as the highest viable purity for this number of stages.
# Repeat this process, increasing the number of stages again followed by increasing the purity, until the desired purity is met. In this example simulation, a purity specification of 0.97 can be met with 14 stages, with the feed entering at stage 7.
# Recovery can be maximized in the same way, replacing Reflux Ratio as an active spec, though in this situation recovery is already 99.9%, so there is no need to maximize further.
# Now that the requirements are met, we can begin optimizing the number of stages and column duty. Condenser and reboiler duties are desired to be as low as possible to keep operating costs at a minimum, though number of stages should be kept relatively low as well. Note that with the current column setup, condenser duty is -1.83e5 kJ/h, and reboiler duty is 2.42e5 kJ/h.
# To begin, set recovery as an active spec by specifying its value as the current value, in this case 0.999, and then deselecting Reflux Ratio as an active spec and selecting the custom recovery specification.
# Next, we want to find the feed stage which minimizes column duty for this number of stages. Vary the feed stage to see which stage results in the minimum column duties. In this example, the optimal feed stage is stage 8, which results in condenser duty of -1.76e5 kJ/h and reboiler duty of 2.35e5 kJ/h.
# Now, increase the number of stages gradually, and find the duty-minimizing feed stage for each number of stages. For example, for a column with 17 stages, the optimal feed stage is 10, which yields condenser duty of -1.58e5 kJ/h and reboiler duty of 2.16e5 kJ/h.
# This process can be repeated indefinitely, as more stages will consistently result in decreased column duty. However, there are diminishing returns, so careful consideration of the balance of installation costs (more stages) vs operating costs (more column duty) must be made taking into account the actual economics of the distillation column. In this example, these diminishing returns are easily visible in the values for condenser duty: increasing from 14 to 17 stages reduces condenser duty by 0.18 kJ/h, increasing to 20 stages reduces condenser duty by a further 0.08 kJ/h, and increasing to 23 stages reduces condenser duty by only 0.03 kJ/h, at a final value of -1.47e5 kJ/h. A similar trend can be seen in the reboiler duty.

==References==
<references />

Column

2016-02-06T04:41:49Z

Benjkg: This first edit is to reorganize the page to better accommodate future edits