processdesign - User contributions [en]

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

2016-03-07T20:52:24Z

Evanrosati: /* 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.

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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=

=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

2016-03-07T01:46:15Z

Evanrosati: /* Post Treatment */

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.

!!!!INSERT TABLE 1!!!!

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>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>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 gas. 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>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>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.
</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>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><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>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.</ref>. 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>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.</ref><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>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>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>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>

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> Vouchkov, N. Re-Mineralization of Desalinated Water. SunCam. https://s3.amazonaws.com/suncam/npdocs/118.pdf. Accessed February 18, 2016.</ref>

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>World Health Organization. Desalination for safe water supply. http://www.who.int/water_sanitation_health/gdwqrevision/desalination.pdf. Accessed January 26, 2016.</ref>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>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. http://www.sciencedirect.com/science/article/pii/0011916493800966
</ref> 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>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>.

===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>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>

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.

!!!!INSERT TABLE 2!!!!

===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>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>, 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.

!!!! INSERT TABLE 3!!!! 

===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.

!!!! INSERT TABLE 4!!!! 

==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.

!!!! INSERT TABLE 5!!!! 

=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_PFD.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=

=Appendix F: Heat Exchanger Sizing=

=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

2016-03-07T01:45:54Z

Evanrosati: /* Post Treatment */

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.

!!!!INSERT TABLE 1!!!!

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>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>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 gas. 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>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>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.
</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>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><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>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.</ref>. 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>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.</ref><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>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>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>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>

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> Vouchkov, N. Re-Mineralization of Desalinated Water. SunCam. https://s3.amazonaws.com/suncam/npdocs/118.pdf. Accessed February 18, 2016.</ref>

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>World Health Organization. Desalination for safe water supply. http://www.who.int/water_sanitation_health/gdwqrevision/desalination.pdf. Accessed January 26, 2016.</ref>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>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. http://www.sciencedirect.com/science/article/pii/0011916493800966
</ref> 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>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>.

===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>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>

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.

!!!!INSERT TABLE 2!!!!

===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>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>, 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.

!!!! INSERT TABLE 3!!!! 

===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.

!!!! INSERT TABLE 4!!!! 

==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.

!!!! INSERT TABLE 5!!!! 

=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_PFD.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=

=Appendix F: Heat Exchanger Sizing=

=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

2016-03-06T22:10:07Z

Evanrosati: /* 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.

!!!!INSERT TABLE 1!!!!

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>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>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 gas. 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>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>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.
</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>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><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>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.</ref>. 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>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.</ref><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>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>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>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>

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> Vouchkov, N. Re-Mineralization of Desalinated Water. SunCam. https://s3.amazonaws.com/suncam/npdocs/118.pdf. Accessed February 18, 2016.</ref>

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>World Health Organization. Desalination for safe water supply. http://www.who.int/water_sanitation_health/gdwqrevision/desalination.pdf. Accessed January 26, 2016.</ref>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>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. http://www.sciencedirect.com/science/article/pii/0011916493800966
</ref> 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>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>.

===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>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>

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.

!!!!INSERT TABLE 2!!!!

===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>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>, 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.

!!!! INSERT TABLE 3!!!! 

===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.

!!!! INSERT TABLE 4!!!! 

==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.

!!!! INSERT TABLE 5!!!! 

=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_PFD.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=

=Appendix F: Heat Exchanger Sizing=

=Appendix G: Pressure Vessel Sizing=

=Appendix H: Expanded Economic Assumptions=

=Appendix I: Equipment Cost Summary=

=Appendix J: Economic NPV Analysis=

=References=
<references/>

File:TeamF PFD.png

2016-03-06T22:08:30Z

Evanrosati: Process Flow Diagram of Nueces Bay Facility

Process Flow Diagram of Nueces Bay Facility

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

2016-03-06T22:06:45Z

Evanrosati: /* Appendix D: HYSYS Simulation Model */

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.

!!!!INSERT TABLE 1!!!!

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>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>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 gas. 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>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>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.
</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>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><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>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.</ref>. 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>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.</ref><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>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>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>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>

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> Vouchkov, N. Re-Mineralization of Desalinated Water. SunCam. https://s3.amazonaws.com/suncam/npdocs/118.pdf. Accessed February 18, 2016.</ref>

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>World Health Organization. Desalination for safe water supply. http://www.who.int/water_sanitation_health/gdwqrevision/desalination.pdf. Accessed January 26, 2016.</ref>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>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. http://www.sciencedirect.com/science/article/pii/0011916493800966
</ref> 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>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>.

===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>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>

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.

!!!!INSERT TABLE 2!!!!

===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>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>, 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.

!!!! INSERT TABLE 3!!!! 

===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.

!!!! INSERT TABLE 4!!!! 

==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.

!!!! INSERT TABLE 5!!!! 

=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=

=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=

=Appendix F: Heat Exchanger Sizing=

=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

2016-03-06T22:06:19Z

Evanrosati: /* Appendix D: HYSYS Simulation Model */

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.

!!!!INSERT TABLE 1!!!!

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>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>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 gas. 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>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>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.
</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>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><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>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.</ref>. 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>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.</ref><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>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>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>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>

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> Vouchkov, N. Re-Mineralization of Desalinated Water. SunCam. https://s3.amazonaws.com/suncam/npdocs/118.pdf. Accessed February 18, 2016.</ref>

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>World Health Organization. Desalination for safe water supply. http://www.who.int/water_sanitation_health/gdwqrevision/desalination.pdf. Accessed January 26, 2016.</ref>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>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. http://www.sciencedirect.com/science/article/pii/0011916493800966
</ref> 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>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>.

===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>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>

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.

!!!!INSERT TABLE 2!!!!

===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>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>, 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.

!!!! INSERT TABLE 3!!!! 

===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.

!!!! INSERT TABLE 4!!!! 

==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.

!!!! INSERT TABLE 5!!!! 

=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=

=Appendix D: HYSYS Simulation Model=
[[File:TeamF_HYSYS_Screenshot.PNG|center|700px|thumb|alt=HYSYS Simulation Model|Figure D.1: Screenshot of HYSYS Model Simulation.]]

=Appendix E: Stream Tables=

=Appendix F: Heat Exchanger Sizing=

=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

2016-03-06T22:05:06Z

Evanrosati: /* Appendix D: HYSYS Simulation Model */

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.

!!!!INSERT TABLE 1!!!!

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>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>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 gas. 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>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>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.
</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>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><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>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.</ref>. 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>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.</ref><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>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>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>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>

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> Vouchkov, N. Re-Mineralization of Desalinated Water. SunCam. https://s3.amazonaws.com/suncam/npdocs/118.pdf. Accessed February 18, 2016.</ref>

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>World Health Organization. Desalination for safe water supply. http://www.who.int/water_sanitation_health/gdwqrevision/desalination.pdf. Accessed January 26, 2016.</ref>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>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. http://www.sciencedirect.com/science/article/pii/0011916493800966
</ref> 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>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>.

===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>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>

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.

!!!!INSERT TABLE 2!!!!

===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>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>, 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.

!!!! INSERT TABLE 3!!!! 

===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.

!!!! INSERT TABLE 4!!!! 

==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.

!!!! INSERT TABLE 5!!!! 

=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=

=Appendix D: HYSYS Simulation Model=
[[File:TeamF_HYSYS_Screenshot.png|center|700px|thumb|alt=HYSYS Simulation Model|Figure D.1: Screenshot of HYSYS Model Simulation.]]

=Appendix E: Stream Tables=

=Appendix F: Heat Exchanger Sizing=

=Appendix G: Pressure Vessel Sizing=

=Appendix H: Expanded Economic Assumptions=

=Appendix I: Equipment Cost Summary=

=Appendix J: Economic NPV Analysis=

=References=
<references/>

File:TeamF HYSYS Screenshot.PNG

2016-03-06T22:02:54Z

Evanrosati: Screenshot overview of Team F HYSYS File

Screenshot overview of Team F HYSYS File

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

2016-03-06T22:01:35Z

Evanrosati: /* Appendix A: Proposed Location of Desalination Plant */

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.

!!!!INSERT TABLE 1!!!!

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>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>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 gas. 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>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>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.
</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>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><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>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.</ref>. 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>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.</ref><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>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>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>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>

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> Vouchkov, N. Re-Mineralization of Desalinated Water. SunCam. https://s3.amazonaws.com/suncam/npdocs/118.pdf. Accessed February 18, 2016.</ref>

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>World Health Organization. Desalination for safe water supply. http://www.who.int/water_sanitation_health/gdwqrevision/desalination.pdf. Accessed January 26, 2016.</ref>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>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. http://www.sciencedirect.com/science/article/pii/0011916493800966
</ref> 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>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>.

===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>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>

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.

!!!!INSERT TABLE 2!!!!

===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>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>, 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.

!!!! INSERT TABLE 3!!!! 

===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.

!!!! INSERT TABLE 4!!!! 

==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.

!!!! INSERT TABLE 5!!!! 

=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=

=Appendix D: HYSYS Simulation Model=

=Appendix E: Stream Tables=

=Appendix F: Heat Exchanger Sizing=

=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

2016-03-06T21:59:05Z

Evanrosati: /* Appendix A: Proposed Location of Desalination Plant */

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.

!!!!INSERT TABLE 1!!!!

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>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>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 gas. 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>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>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.
</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>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><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>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.</ref>. 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>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.</ref><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>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>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>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>

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> Vouchkov, N. Re-Mineralization of Desalinated Water. SunCam. https://s3.amazonaws.com/suncam/npdocs/118.pdf. Accessed February 18, 2016.</ref>

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>World Health Organization. Desalination for safe water supply. http://www.who.int/water_sanitation_health/gdwqrevision/desalination.pdf. Accessed January 26, 2016.</ref>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>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. http://www.sciencedirect.com/science/article/pii/0011916493800966
</ref> 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>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>.

===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>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>

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.

!!!!INSERT TABLE 2!!!!

===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>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>, 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.

!!!! INSERT TABLE 3!!!! 

===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.

!!!! INSERT TABLE 4!!!! 

==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.

!!!! INSERT TABLE 5!!!! 

=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]]

=References=
<references/>

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

2016-03-06T21:58:41Z

Evanrosati: /* Appendix A: Proposed Location of Desalination Plant */

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.

!!!!INSERT TABLE 1!!!!

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>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>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 gas. 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>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>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.
</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>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><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>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.</ref>. 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>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.</ref><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>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>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>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>

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> Vouchkov, N. Re-Mineralization of Desalinated Water. SunCam. https://s3.amazonaws.com/suncam/npdocs/118.pdf. Accessed February 18, 2016.</ref>

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>World Health Organization. Desalination for safe water supply. http://www.who.int/water_sanitation_health/gdwqrevision/desalination.pdf. Accessed January 26, 2016.</ref>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>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. http://www.sciencedirect.com/science/article/pii/0011916493800966
</ref> 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>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>.

===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>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>

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.

!!!!INSERT TABLE 2!!!!

===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>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>, 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.

!!!! INSERT TABLE 3!!!! 

===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.

!!!! INSERT TABLE 4!!!! 

==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.

!!!! INSERT TABLE 5!!!! 

=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|700px|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]]

=References=
<references/>

File:ClassIWell.png

2016-03-06T21:56:13Z

Evanrosati: Distance to Class I Injection Well

Distance to Class I Injection Well

File:NuecesBayLocation.png

2016-03-06T21:55:46Z

Evanrosati: Plant Location for Nueces Bay Desalination Facility

Plant Location for Nueces Bay Desalination Facility

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

2016-03-06T21:52:18Z

Evanrosati: /* Conclusions and Recommendations */

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.

!!!!INSERT TABLE 1!!!!

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>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>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 gas. 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>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>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.
</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>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><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>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.</ref>. 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>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.</ref><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>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>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>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>

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> Vouchkov, N. Re-Mineralization of Desalinated Water. SunCam. https://s3.amazonaws.com/suncam/npdocs/118.pdf. Accessed February 18, 2016.</ref>

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>World Health Organization. Desalination for safe water supply. http://www.who.int/water_sanitation_health/gdwqrevision/desalination.pdf. Accessed January 26, 2016.</ref>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>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. http://www.sciencedirect.com/science/article/pii/0011916493800966
</ref> 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>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>.

===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>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>

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.

!!!!INSERT TABLE 2!!!!

===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>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>, 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.

!!!! INSERT TABLE 3!!!! 

===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.

!!!! INSERT TABLE 4!!!! 

==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.

!!!! INSERT TABLE 5!!!! 

=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.

=References=
<references/>

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

2016-03-06T21:51:01Z

Evanrosati: /* Optimization and Sensitivity 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.

__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.

!!!!INSERT TABLE 1!!!!

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>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>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 gas. 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>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>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.
</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>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><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>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.</ref>. 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>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.</ref><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>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>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>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>

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> Vouchkov, N. Re-Mineralization of Desalinated Water. SunCam. https://s3.amazonaws.com/suncam/npdocs/118.pdf. Accessed February 18, 2016.</ref>

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>World Health Organization. Desalination for safe water supply. http://www.who.int/water_sanitation_health/gdwqrevision/desalination.pdf. Accessed January 26, 2016.</ref>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>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. http://www.sciencedirect.com/science/article/pii/0011916493800966
</ref> 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>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>.

===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>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>

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.

!!!!INSERT TABLE 2!!!!

===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>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>, 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.

!!!! INSERT TABLE 3!!!! 

===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.

!!!! INSERT TABLE 4!!!! 

==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.

!!!! INSERT TABLE 5!!!! 

=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.

=References=
<references/>

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

2016-03-06T21:49:10Z

Evanrosati: /* Operating Pressure and Pressure Drop */

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.

!!!!INSERT TABLE 1!!!!

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>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>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 gas. 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>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>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.
</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>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><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>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.</ref>. 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>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.</ref><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>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>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>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>

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> Vouchkov, N. Re-Mineralization of Desalinated Water. SunCam. https://s3.amazonaws.com/suncam/npdocs/118.pdf. Accessed February 18, 2016.</ref>

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>World Health Organization. Desalination for safe water supply. http://www.who.int/water_sanitation_health/gdwqrevision/desalination.pdf. Accessed January 26, 2016.</ref>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>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. http://www.sciencedirect.com/science/article/pii/0011916493800966
</ref> 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>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>.

===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>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>

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.

!!!!INSERT TABLE 2!!!!

===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>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>, 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.

!!!! INSERT TABLE 3!!!! 

===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.

!!!! INSERT TABLE 4!!!! 

=References=
<references/>

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

2016-03-06T21:46:05Z

Evanrosati: /* Major Specifications */

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.

!!!!INSERT TABLE 1!!!!

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>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>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 gas. 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>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>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.
</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>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><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>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.</ref>. 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>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.</ref><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>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>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>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>

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> Vouchkov, N. Re-Mineralization of Desalinated Water. SunCam. https://s3.amazonaws.com/suncam/npdocs/118.pdf. Accessed February 18, 2016.</ref>

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>World Health Organization. Desalination for safe water supply. http://www.who.int/water_sanitation_health/gdwqrevision/desalination.pdf. Accessed January 26, 2016.</ref>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>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. http://www.sciencedirect.com/science/article/pii/0011916493800966
</ref> 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>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>.

===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>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>

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===
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.

!!!!INSERT TABLE 2!!!!

=References=
<references/>

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

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Evanrosati: /* Pre-Treatment and Post-Treatment */

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Evanrosati: /* Transport of Brine to Well */

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Evanrosati: /* Process Alternatives */

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File:BlockFlowDiagram.png

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Evanrosati: Nueces Bay Block Flow Diagram: Team F

Nueces Bay Block Flow Diagram: Team F

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

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Evanrosati:

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

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Evanrosati: /* Feed and Product Definitions */

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Evanrosati: /* Feed and Product Definitions */

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Evanrosati: /* Feed and Product Definitions */

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Evanrosati: /* Design Basis */

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Evanrosati: /* Introduction */

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

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

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Evanrosati: Created page with "Team F: Rankine 672 Final Report Authors: Kedric Daly, Ben Granger, Evan Rosati, Kathleen Zhou Instructors: Fengqi You, David Wegerer March 11, 2016"

Team F: Rankine 672 Final Report

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

Instructors: Fengqi You, David Wegerer

March 11, 2016

Main Page

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Evanrosati:

'''Welcome to the Northwestern University Chemical Process Design Open Textbook.''' 
This electronic textbook is a student-contributed open-source text covering the materials used in our chemical engineering capstone design courses at Northwestern.
 
If you have any comments or suggestions on this open textbook, please contact [//www.mccormick.northwestern.edu/directory/profiles/Fengqi-You.html Professor Fengqi You].
 
----
 
Northwestern University Chemical Process Design Open Textbook

{| class="wikitable" style="padding: 1em; text-align:left"
|- valign="top"
|width = "550pt"| '''  Chemical Process Design Theory and Method'''
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  '''Design Basis'''
# [[Define product and feed]]
# [[Preliminary market analysis and plant capacity]]
# [[Site condition and design]]
# [[Block Flow Diagram| Block flow diagram]]
 
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  '''Process Flowsheet'''
# [[Process flow diagram]]
# [[Process alternatives and flowsheeting]]
# [[Reactors]]
# [[Fluidized Bed Reactor]]
# [[Separation processes]]
# [[Process hydraulics]]
# [[Heat Transfer Equipment| Heat transfer equipment: Heat exchangers, boilers, condensers, heaters and coolers]]
# [[Pinch analysis]]
# [[Utility systems]]
# [[Pressure Vessels| Pressure vessels]]
 
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  '''Process Simulation'''
# [[Property package]]
# [[Mixer and Splitter]]
# [[Separator]]
# [[Heat exchanger]]
# [[Column]]
# [[Reactor]]
# [[Pressure changer]]
# [[Solids-involved equipment]]
 
----
  '''Process Economics'''
# [[Equipment sizing]]
# [[Estimation of capital]]
# [[Estimation of production cost and revenue]]
# [[Engineering economic analysis]]
# [[Sensitivity analysis and design optimization]]
 
----
  '''Other Process Design Considerations'''
# [[Materials of construction]]
# [[Process safety]]
# [[Process hazards]]
# [[Environmental concerns]]
# [[Process controls]]
# [[Process location and layout decisions]]
 

|width = "400pt"| '''  Chemical Process Design Projects'''
----
  '''Design projects 2014'''
* [[Design G1 | Glycerol to Propylene Glycol (G1)]]
* [[Design G2 | Glycerol to Propylene Glycol (G2)]]
* [[Design S1 | Succinic Acid to 1,4-Butanediol (S1)]]
* [[Design S2 | Succinic Acid to 1,4-Butanediol (S2)]]
* [[Drop-in Hydrogen Fueling (2014)]] for (Hydrogen Design Contest)
 
----
  '''Design projects 2015'''
* [[Ethanol to Ethylene (B1)]]
* [[Biomass to Ethylene (B2)]]
* [[Shale Gas to Ethylene (G1)]]
* [[Shale Gas to Ethylene (G2)]]
* [[Natural Gas to Hydrogen (H)]]
 
----
  '''Design projects 2016'''
* [[Team A]]
* [[Team B]]
* [[Team D]]
* [[Team E]]
* [[Nueces Desalination Center: Production of Drinking Water by Multi-Stage Flash Distillation]]
* [[Team G]]
 
----
|}

 

'''Acknowledgement'''

[[File:Centennial_Logo.jpg|left|150px]]

Process safety

2016-02-13T22:55:50Z

Evanrosati: /* Benefits of Inherent Safety Over Conventional Safety */

Authors: Anne Disabato, [2014] Tim Hanrahan, [2014] Brian Merkle, [2014] , Spencer Saldana [2015] and Evan Rosati [2016] 

Stewards: David Chen, Jian Gong, and Fengqi You

Date Presented: January 19, 2014

 


__TOC__

=Introduction=
The safe design and operation of facilities is of paramount importance to every company that is involved in the manufacture of fuels, chemicals, and pharmaceuticals. Process safety focuses on the prevention of dangerous situations, such as fires, explosions, and the release of chemicals.

The American Institute of Chemical Engineers emphasizes a culture of process safety through four pillars (American Institute of Chemical Engineers, 2015):

1. Commitment to Process Safety: a workforce that is actively involved and an organization that fully supports process safety as a core value will tend to do the right things in the right way at the right time – even when no one else is looking

2. Understanding Hazard and Risk: the foundation of a risk-based approach which will allow an organization to use this information to allocate limited resources in the most effective manner

3. Manage Risk: the ongoing execution of risk based process safety tasks. Risk management can help a company to better deal with the resultant risks and sustain long-term accident free and profitable operations

4. Learn from Experience: Metrics provide direct feedback on the workings of RBPS systems, and leading indicators provide early warning signals of ineffective process safety results. Organizations must use their mistakes and those of others as motivation for action and view as opportunities for improvement.

For the prevention and management of specific safety hazards, such as fires, explosions, or the release of toxic chemicals, please see [[Process Hazards]].

==Layers of Plant Safety==
Safety and loss prevention can be expressed in "layers" of plant safety in terms of design and implementation.<ref name="Towler">G.P. Towler, R. Sinnott. ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design''. Elsevier, 2012.</ref> Each higher layer can be activated if a lower level fails. This creates a system with subsequent levels of safety to help prevent catastrophe from occurring. This diagram shows the important of safety in process design. If a process is designed to be inherently safe, additional safety "controls" will be less important and a chemical plant will be overall safer. The goal of safety is not to reach the top of the triangle, but to stay as close to the bottom as possible. This shows the importance of inherently safe design and safety legislation and regulations to provide guidelines for safety and health concerns. Regulations provide a baseline for engineers to operate when designing a chemical plant. They have brought safety to the forefront of design, when engineers have the maximum degree of freedom for implementation, and are no longer considered to be an afterthought or strictly a controls issue. Plants can be designed to be safe without the extensive use of future adaptation, safety controls, or emergency response. Although you can never eliminate these upper layers of process safety, by designing a process smartly and safely, engineers can reduce the consequences of "walking up" the process safety ladder or triangle.
[[File:Layers_of_Plant_Safety.png|center|700px|thumb|alt=Triangle Diagram on Increasing Plant Safety Mechanisms|Layers of Plant Safety Triangle Diagram]]

=Safety Legislation=

==International Regulations==
International regulations can be considered as best practice standards that are adopted by governments through treaties and establishment through United Nation resolutions. Most international regulation agencies can only register complaints for violations, but can not implement sanctions or fines to infracting parties.

===International Labour Organization (ILO)===
After the dissolution of the League of Nations in 1946, the ILO became the first specialized agency of the United Nations upon its founding in December of that same year. <ref> ISO Photo Gallery. http://www.ilo.org/dyn/media/mediasearch.fiche?p_id=16023&p_lang=en. Accessed February 13, 2016.</ref> The ILO attempts to develop labor standards for workers in all industries. The unique tripartite structure of the ILO gives an equal voice to workers, employers and governments to ensure that the views of the social partners are closely reflected in labour standards and in shaping policies and programs. <ref> ISO "How the ISO Works" http://www.ilo.org/global/about-the-ilo/how-the-ilo-works/lang--en/index.htm. Accessed February 13, 2016.</ref> The ISO has established International Labour Standards on Occupational Safety and Health, which has developed more than 40 standards specifically dealing with occupational heath and safety.<ref> ILO. "International Labour Standards on Occupational Safety and Health". http://www.ilo.org/global/standards/subjects-covered-by-international-labour-standards/occupational-safety-and-health/lang--en/index.htm. Accessed February 13, 2016.</ref>. The important conventions are listed below:

====Occupational Safety and Health Convention, 1981 and Protocol of 2002====
#The convention provides for the adoption of national occupational safety and health policy by each participating nation-state. It includes actions taken by governments and within enterprise that operate within the governmental spaces to promote the improvement of occupational safety and health to therefore improve working conditions.
#The additional Protocol in 2002 calls for the establishment and period review of procedures for recording and notification of occupational accidents along with the publication of related statistics associated with the accidents. This Protocol is similar to the Emergency Planning and CommunityRight-To-Know Act instituted in the United States (see below).

====Occupational Heath Services Convention, 1985====
#Establishment of enterprise-level occupational health services which are entrusted with preventative functions and which are responsible for advising the employer, the workers, and their representatives in the organization on maintaining a safe and healthy work environment.

====Promotional Framework for Occupational Safety and Heath Convention, 2006====
#Aims to promote a preventative safety and health culture and to progressively achieve a safe and healthy working environment for all.
#Requires ratifying nation-states to develop, in consultation with the most representative organizations of employees and workers, including workers unions, a national policy, a national system, and a national program on occupational health and safety.
#This should be developed in accordance to the Occupational Heath Services Convention, 1985 and take into account other ILO standards.
#National systems shall provide the infrastructure for implementing national policy and programs, such as domestic laws, regulations, authority bodies, and compliance mechanisms.

====Chemicals Convention, 1990====
#Provides for the adoption and implementation of a coherent policy on the safety in the use of chemicals at work, including production, handling, storage, and transport.
#Also includes best practices for the disposal and treatment of waste chemicals and the release of chemicals resulting from work activities, maintenance, and cleaning of equipment.
#In addition, it allocates specific responsibilities to suppliers and exporting states.
#Chemicals shall be evaluated to determine their level of hazards and employers shall make these hazards known to their employees.

===International Organization for Standardization (ISO)===
The ISO is an international organization for standardization on topics ranging from quality management, environmental management, social responsibility, risk management, etc. ISO standards are adopted world-wide by organizations and governments as accepted and known standards for production. Many of the practices are self-regulating as costumers often demand certain standards from companies they purchase from.<ref>ISO. Home Page. http://www.iso.org/iso/home.html. Accessed February 13, 2016.</ref> The ISO is developing its own occupational health and safety framework shown below.

====ISO 45001 Occupational Health and Safety Management Systems<ref>ISO 45001. "Occupational heath and safety". http://www.iso.org/iso/home/standards/management-standards/iso45001.htm. Accessed February 13, 2016.</ref>====
#Designed to help organizations reduce the burden of occupational injuries and diseases by providing a framework to improve employee safety, reduce workplace risks, and create better, safer working conditions.
#Currently under development by a committee of occupational health and safety experts who will follow management systems approaches of ISO 4001 and ISO 9001.
#Will embody other International Standards such as the ILO's Occupational Health and Safety Guidelines.
#The expected publication will be released in October, 2016.

==US Safety Regulations==
Over time chemical industry regulations have been developed to ensure that the best safety practices are followed to maintain the health of both people and the environment. The development of most regulations is based around the idea that organizations have both a legal and moral obligation to safeguard the health and welfare of its employees and the public.<ref name="Towler" /> The extent of legislation varies across regions around the globe. In the United States, chemical accidents have led to the creation of regulation boards and safety oriented societies such as the Center for Chemical Plant Safety of the American Institute of Chemical Engineers to aid in the development and implementation of pant safety. <ref>Seider WD, Seader JD, Lewin DR, Seider WD. Product And Process Design Principles: Synthesis, Analysis, and Evaluation. New York: Wiley; 2004.</ref> In the US, the major federal laws relating to chemical plant safety and their regulations are as follows:

===The Occupational Safety and Health Act, 1970===
The OSH Act is administer by the Occupational Safety and Health Administration (OSHA) (see below). The Act covers all employers and their employees in the United States with coverage provided either directly by the Federal Occupational Safety and Heath Administration or by an OSHA-approved state job safety and health plan.<ref> United States Department of Labor. "Safety and Health Standards: Occupational Safety and Health". http://webapps.dol.gov/elaws/elg/osha.htm </ref> The main provisions of the act are listed below.
# Employers must supply place of employment free from toxic chemicals, excessive noise mechanical dangers, or unsanitary conditions.
#This involves the implementation of engineering controls to limit exposure to hazards and toxic substances and implementing administrative controls.
# For employees, employers must provide personal protective equipment (PPE) and training, including communications of known hazards.
# The Occupational Safety and Health Administration (OSHA) was established to promote best practices, inspect facilities for hazard analysis, set standards, and enforce the law.
# The National Institute of Occupational Safety and Health (NIOSH) was established to be an independent research institute (now under the Centers for Disease Control).
#The Act also encourages states to develop and operate their own job safety and health programs with OSHA as a monitoring agency for these "state plans," which operate under the authority of state law. The standards developed by state plans need to be at least as effective as the federal regulations.<ref> United States Department of Labor. "Safety and Health Standards: Occupational Safety and Health". http://webapps.dol.gov/elaws/elg/osha.htm </ref>
#Federal OSHA standards are categorized into four categories: General Industry, Construction, Maritime Terminals, Long-shoring, and Agriculture.

===The Toxic Substances Control Act (TSCA), 1976===
The main qualifications of the TSCA were to provide the EPA with regulating power of chemicals, specifically pertaining to the chemical industry and not including foods, drugs, cosmetics, or pesticides. <ref> Environmental Protection Agency. "Summary of the Toxic Substances Control Act." http://www.epa.gov/laws-regulations/summary-toxic-substances-control-act. Accessed February 13, 2016</ref>
# The Environmental Protection Agency (EPA) is required to regulate 75,000 chemical substances used in industry.
# The EPA has jurisdiction over the safety of the sale or development of new chemicals in the United States, including the requirement for pre-manufacture notification for new chemical substances before manufacture.
#The TSCA also addresses the production, importation, use, and disposal of specific chemicals including polychlorinated biphenyls (PCBs), asbestos, radon, and lead-based paint.<ref> Environmental Protection Agency. "Summary of the Toxic Substances Control Act." http://www.epa.gov/laws-regulations/summary-toxic-substances-control-act. Accessed February 13, 2016</ref>
# Everyone has the right and obligation to report information about any health or environmental effects caused by a chemical. This is especially important for organizations as they are required to report information to the EPA if a chemical substance is found to have substantial risk of injury to health or the environment.
====Frank R. Lautenberg Chemical Safety for the 21st Century Act, 2015====
The Toxic Substances Control Act will be revised in 2015 under this new act proposed to Congress. This Act is an attempt to eliminate some of the flaws associated with the TSCA.
#One of the biggest flaws in the TSCA is the fact that a new chemical can be used without first demonstrating safety, the idea that chemicals are safe until proven unsafe.<ref>Sheppard, Kate. "Senators Introduce Bill to Overhaul U.S. Chemical Industry." http://www.huffingtonpost.com/2015/03/10/toxic-chemicals-senate-bill_n_6842524.html. Accessed February 13, 2016</ref>
#The new bill would require safety testing before chemical implementation.
#It gives the EPA more defined power on the regulation of chemical substances and the Sustainable Chemical Program will be established. This brings the TSCA into the modern era of the chemical industry.<ref>S.697 - 114th Congress (2015-2016): Frank R. Lautenberg Chemical Safety for the 21st Century Act. Congress.gov. https://www.congress.gov/bill/114th-congress/senate-bill/697/all-info. Accessed January 27, 2016.</ref>

===The Emergency Planning and Community Right-to-Know Act (EPCRA), 1986<ref> US Government Printing Office. Emergency Panning and Community Right to Know. https://www.gpo.gov/fdsys/pkg/USCODE-2010-title42/html/USCODE-2010-title42-chap116.htm. Accessed February 13, 2016</ref>===
The Act was passed by Congress in response to concerns regarding the environmental and safety hazards posed by the storage and handling of toxic chemicals. This legislation was developed as a result of the 1984 chemical disaster in Bhopal, India.<ref>What is EPCRA? EPA. http://www.epa.gov/epcra/what-epcra. Published November 10, 2015. Accessed January 27, 2016.</ref>
# All facilities manufacturing, processing, or storing hazardous chemicals must make plans for major incidents if they were to occur and the plans must be made public so that local communities can be properly informed.
# All facilities must also produce Material Safety Data Sheets (MSDSs) to state and local officials as well as local fire departments.
# Local governments should help prepare emergency plans and review the plans annually.
# State governments are required to oversee and coordinate local planning efforts.
# Emergency Notification: Facilities must immediately report accidental releases of chemicals and "hazardous substances" in quantities greater than Reportable Quantities (RQs) defined under the Comprehensive Environmental Response, Compensation, and Liability Act to state and local officials with this information then being available to the public.
# Annually, facilities must complete and submit a toxic chemical release inventory form.

===Clean Air Act Amendments of 1990===
This Amendment to the original Clean Air Act of 1970 was designed to curb three major threats to the nation's environment and health of citizens: acid rain, urban air pollution, and toxic air emissions.<ref>Environmental Protection Agency. "1990 Clean Air Act Amendment Summary" http://www.epa.gov/clean-air-act-overview/1990-clean-air-act-amendment-summary. Accessed February 13, 2016.</ref>
# Established the U.S. Chemical Safety and Hazard Investigation Board, and independent federal agency with the goal of ensuring worker and public safety through the prevention or minimization of the effects of chemical accidents. <ref>Seider WD, Seader JD, Lewin DR, Seider WD. Product And Process Design Principles: Synthesis, Analysis, and Evaluation. New York: Wiley; 2004.</ref>
# Attempt to determine the roots and contributing causes of accidents and then provide briefs on the accidents.
# Led to the development of cap and trade systems for air pollutants.
# Gave the government significantly more power to control air emissions and administer admissions permits.

==Additional Legislative Information==
For more details on environmental legislation pertaining to release of materials to the environment or the regulations of the loss of containment, see [[Environmental concerns]].

In addition to the above federal regulations, various states and other municipalities also have enacted legislation for the regulation of chemical plants. These include more specific safety items, such as local fire codes or even put into place stricter aspects of the federal regulations. <ref name="Towler" />

Any process design or plant design must always meet the requirements of local and federal mandates and regulations. Without doing so, the wellbeing of plant employees and even the public can be placed into serious jeopardy.

=Safety Organizations and Terminology=
==Organizations==
===OSHA===
The Occupational Safety and Health Administration (OSHA) is a federal agency that focuses on the enforcement of safety and health legislation.

===EPA===
The Environmental Protection Agency (EPA) is a U.S. agency whose purpose is the protection of the health of both humans and the environment through the writing and enforcement of regulatory laws.

===DOT===
The Department of Transportation (DOT) oversees federal highway, air, and maritime transportation, and can be involved in the safe transport of chemicals.

===DOE===
The Department of Energy (DOE) is a governmental department tasked with the advancement of energy technology in the United States.

==Terminology==
===HS&E===
Health, Safety, and Environmental - This term refers to all health, safety, and environmental concerns that arise at each stage of the design process. Companies are required to analyze each part of the process from an HS&E perspective to create a safe and healthy work environment.

===MSDS===
Material Safety and Data Sheet - Every chemical has an MSDS which contains all the information regarding safe handling and how to deal with spills or other accidents involving the substance. Relevant information includes how to identify the substance, hazard information, and how to handle spills, fires, and exposure, among other things.

===FMEA===
Failure Mode and Effects Analysis - FMEA is an early stage approach to identifying critical technical risks using a semi-quantitative procedure. The analysis encompasses safety, environmental, and operational feasibility. When performing FMEA, engineers look to see places in a potential design that could fail, and then quantify how likely that failure is, how severe the results would be, and then offer potential solutions to minimize the risk. A step-by-step guide to performing FMEA is shown below (Northwestern University, 2014):

# Brainstorm for failure modes
# For each FM, rate severity of impact (SEV, 1 - 10).
# For each FM, brainstorm for possible causes (there may be multiple).
# For each cause, rate likelihood of occurring (OCC, 1 - 10).
# Rate the probability that the systems currently in place will detect and prevent the problem before it has an impact (DET, 10 - 1). Do not assume that something that will be added to the design later will take care of the problem.
# Overall Risk Probability Number RPN = SEV x OCC x DET. Most practitioners use the 1,4,7,10 scale below to increase granularity. Note that the DET scale is inverse to SEV and OCC.

[[File:Safety3.JPG|center|700px]]

Figure 1: Suggested scale to be used for quantifying risk and detection of failures in FMEA. Taken from ChE 351 Slides.

The sum of all information collected is implemented into a spreadsheet like the one shown below:

[[File:Safety4.JPG|center|700px]]

Figure 2: Example spreadsheet used to organize FMEA data.

===HAZOP===
Hazard and Operability Study - For more information regarding HAZOP, please refer to [[Process Hazards]].

===SIL===
Safety Integrity Levels - The SIL is the relative level of risk-reduction provided by a safety function, or to specify a target level of risk reduction. A SIL is determined based on a number of qualitative factors such as development process and safety life cycle management. Several methods are used such as risk matrices, risk graphs, layers of protective analysis (LOPA). Three levels of safety integrity are assigned depending on the “availability” of the safety instrumented system (SIS), as shown below (Towler et al., 2012):

[[File:Safety5.JPG|center|500px]]

Figure 3: Table of safety integrity levels based on availability of system. Taken from (Towler et al., 2012).

Redundant system means instrumentation is duplicated; higher level of redundancy of trip systems give higher SIL. The required SIL should be determined during a process hazard analysis and depends on risk of operator exposure and injury.

1 Instrument
* if it signals, plant goes down
* Probability of incident = probability of instrument failure
* Probability of spurious trip = false positive rate of one instrument

2 Instruments
* 1 out of 2 voting (1oo2): one instrument signals, plant goes down
** Probability of incident reduced by duplication
** Probability of spurious trip doubled
* 2oo2 voting
** Probability of incident worse than single instrument (twice likelihood that system is down)
** Probability of spurious trip reduced

3 Instruments
* 2oo3 voting
** Best overall trade-off between reducing incident rate and spurious trip rate
** One malfunctioning instrument does not cause trip or prevent detection of real incident

=Safe Design=
==Inherently Safe Design==
Inherently safe design of a particular process can be achieved by adhering to the following six strategies (Turton et al., 2003):

1. Substitution: Avoid using or producing hazardous materials on the plant site. If the hazardous material is an intermediate product, for example, alternate chemical reaction pathways might be used. In other words, the most inherently safe strategy is to avoid the use of hazardous materials.

2. Intensification: Attempt to use less of the hazardous materials. In terms of a hazardous intermediate, the two processes could be more closely coupled, reducing or eliminating the amount of intermediate produced. The inventories of hazardous feeds or products can be reduced by enhanced scheduling techniques such as just-in-time (JIT) manufacturing.

3. Attenuation: Reducing, or attenuating, the hazards of materials can often be affected by lowering the temperature or adding stabilizing additives. By using materials under less hazardous conditions, the potential consequences of a leak can be reduced.

4. Containment: If the hazardous materials cannot be eliminated, they at least should be stored in vessels with mechanical integrity beyond any reasonably expected temperature or pressure excursion. This is an old but effective strategy to avoid leaks. However, it is not as inherently safe as substitution, intensification, or attenuation.

5. Control: If a leak of hazardous material does occur, there should be safety systems that reduce the effects. For example, chemical facilities often have emergency isolation of the site from the normal storm sewers, and large tanks for flammable liquids are surrounded by dikes that prevent any leaks from spreading to to other areas of the plant. Scrubbing systems and relief systems in general are in this category. They are essential, because they allow controlled, safe release of hazardous materials, rather than an uncontrolled release from a vessel rupture.

6. Survival: If leaks of hazardous materials do occur and they are not contained or controlled, the personnel (and the equipment) must be protected. This lowest level of the hierarchy includes fire fighting, gas masks, and so on. Although essential to the total safety of the plant, the greater the reliance on survival of leaks rather than elimination of leaks, the less inherently safe the facility.

===Safety Legislation and Process Design===
====Inherent Process Plant Safety====
Through the enactment of safety regulation shown above, process design has inherently become safer. These regulations have made safety the first and foremost important concern when designing a new chemical plant. In general, the safety of a process relies on multiple layers of protection, but the first and most important layer of protection has become the process design feature. Greater tolerances are built into the designing of processes to more effectively prevent catastrophic failures or chemical leakage. The best approach to prevent accidents is to add process design features, involving chemistry and physics, to prevent hazardous situations.
=====Examples of Inherently Safe Process Design<ref>Crowl, Daniel. Louver, Joseph. Chemical Process Safety: Fundamentals with Applications. IBN: 9780132440554</ref>=====
#Vapor Release: Vapor released from spills can be minimized by designing dikes so that flammable and toxic materials will not accumulate around leaking tanks. This also prevents potential flammable materials from building up and causing an explosion.
#Containment Building: Design can be important for the containment of toxic spills. With the addition of automatic or remote controls, personnel can leave the area if a spill or breach occurs, while the area can be continuously monitored.
#Solvent Substitution: Safety through Substitution - Substituting design with safer, less hazardous materials. Designing process for use with less toxic or flammable solvents. For example, water-based paints and adhesives as well as aqueous or dry flowable formulations for agricultural chemicals opposed to more volatile solvents that release VOCs.
#Design for Lower Temperature and Pressure Safety through Moderation - use a hazardous material under less hazardous conditions such as lower pressure and temperature conditions. This can lower the level of catastrophe if downstream safety processes do fail.

====Control Systems Safety====
Legislation has also added to subsequent layers of controls such as environmental process controls have been added to prevent release to nearby air and water systems, which would endanger surrounding ecosystems and human populations.

==Assessing Preliminary Design==
While pilot plants are necessary to design effective plant equipment, there are some dangers associated with the industrial scale of chemical plants. Scaling up without accurate literature and experimental data can be very dangerous.

In every step of process design, the Health, Safety, and Environmental (HS&E) analysis must be carried out with the available technical information (Biegler et al., 1997).

[[File:Safety1.JPG|center|800px]]

[[File:Safety2.JPG|center|700px]]

Figure 4: General steps in the design process and analysis to be carried out at each stage. Taken from (Biegler et al., 1997).

=Economic Cost of Safety=
Price of Safety installations and fire protection systems range from .5 to 1.6 % of fixed-capital investment of a plant; but expenditures are often much higher than this and it is difficult to estimate these expenditures for a given plant

In addition, designers also examine the economic impact of safety and maintenance issues. For instance, they may determine that the plant reactor configuration can be improved, and with improved operator training facilities, it can run with improved safety. These hidden costs must be considered when determining the economic feasibility of operation (Biegler et al., 1997).

==Benefits of Inherent Safety Over Conventional Safety==
The outdated method of implementing safety into process design came very late in the design process. The physical entities in the process or the process itself was changed very little and the conventional safety methods included implementing controls. Inherent process safety is instead developed very early in the design process and can lead to very significant cost savings overall. The comparison comes when looking at two ratios for safety costs, the Conventional Safety Cost Index (CSCI) <math>CSCI=\frac{C_{conventional safety}}{C_{loss}}</math> and the Inherent Safety Cost Index (ISCI) <math>ISCI=\frac{C_{inherent safety}}{C_{loss}}</math>. The values of safety cost are easy to determine as they are any additive change to a standard design. The difficulty comes with calculating the cost of losses as this must be an additive loss associated of assets, production, environmental cleanup, and potential human health losses. A diagram of the methods to calculate cost due to losses is shown below:<ref>Kahn, F. Amyotte, Paul. "I2SI: A comprehensive quantitative tool for inherent safety and cost evaluation"
. Journal of Loss Prevention. Elsevier. </ref>

[[File:FailureCost.jpg|center|700px|thumb|alt=Cost in Process Failures|Methods for Calculating Process Losses Costs]]

=Other Process Safety Considerations=
The safety and well-being of the consumers using the eventual product should be considered in process safety. The risks involved in using a product should be clearly communicated to the consumer by industrial leaders.

Human Error is another safety risk that is difficult to quantify. The intervention of well-trained operators is a vital layer in process safety (Peters et al., 2003).

=Case Study: Production of Ultrapure Hydrogen=
[[File:steam reforming.jpg|thumb|alt=A hydrogen production plant with a box steam reformer|A hydrogen production plant with a box steam reformer|300px|right|A hydrogen production plant with a box steam reformer]]
To walk through the process safety considerations when designing a chemical plant, the relatively simple example of a high purity hydrogen generation will be examined. First, various technologies are researched to determine the options that best meet the economic, physical, environmental, and safety constraints of the project. For this project, steam reforming, autothermal reforming, and partial oxidation were investigated. Technologies that lack widespread implementation have inherent safety risks as they have higher uncertainties associated with reliability, feasibility, and cost. Autothermal reforming requires oxygen and not a commercially popular method. Partial oxidation requires no catalyst, but requires high process temperatures and is a complex process to implement. After each process was analyzed and scored in a decision matrix, steam reforming was chosen as the base process technology. It was selected because it is a safe, well-known, low-emission, traditional process in use all over the world for the production of hydrogen. Steam reforming produces minimal waste compared to the alternative processes, and is capable of producing the necessary 100 MMscfd of 99.999% hydrogen gas. It is important to remember that both local and global environmental emissions are capable of harming the general public, so they should be considered safety concerns in the same way worker hazards are. Next, various reactor types, catalysts, and separation methods were evaluated with the base process chosen. In all, seven different processing stages were assessed including the initial heating of feed, the steam reformation reactor, the high and low temperature water shift reactors, the amine plant, the methanation reactor, the gas compression and cooling train, and the pressure swing adsorption unit.

Although worker safety is always the first priority in a plant, there are inherent risk associated with high temperature and pressure processes. Close attention was paid to make the preliminary plant design as inherently safe as possible. Preliminary FMEA and HAZOP analyses were conducted to identify the highest priority risks. Hazards were mitigated by substituting less hazardous materials when possible, opting to store only the necessary hazardous materials on site, lowering temperatures when possible, adding catastrophic failure controls, maximizing plant control automation when economically feasible, and necessitating worker personal protective equipment. Finally, FMEA and HAZAP analyses were repeated. These steps were repeated multiple times until a sufficient reduction in risk had been achieved.

=Case Study: Bhopal Disaster=
[[File:Bhopal.JPG|thumb|alt=The dispersal of the deadly cloud of gas from the Union Carbide plant in the Indian city of Bhopal|The dispersal of the deadly cloud of gas from the Union Carbide plant in the Indian city of Bhopal|400px|right|The dispersal of the deadly cloud of gas from the Union Carbide plant in the Indian city of Bhopal]]
On December 3, 1984 at the India Limited Pesticide Plant owned by Union Carbide in Bhopal, India, water entered a storage vessel containing over 80,000 lbs. of methyl isocyanate (MIC), a chemical intermediate in the pesticide synthesis process. This reaction caused a rapid increase in temperature accompanied by boiling, which caused toxic MIC vapors to escape from the tank. In addition, the MIC-water reaction produced methylamine and carbon dioxide gases among other toxic products which also contributed to the pressure increase (Union Carbide Corporation, 1967). These vapors passed into a scrubber and flare system that were not working at the time due to inadequate maintenance and safety practices. As a result of this accident, approximately 25 tons of MIC vapor were released, killing over 3,800 immediately and injuring roughly 20,000 in the surrounding area.

As a result of the incident, Union Carbide was forced to pay $470 million, as well as fund a hospital in Bhopal that was used specifically to treat victims of the disaster. Cleanup of the plant site and other legal action are still being determined to this day. Bhopal sparked a worldwide discussion on chemical process safety, and caused Congress to create the U.S. Chemical Safety Board (CSB). The CSB has since cited the following reasons as causes for the disaster:

* No process hazard analysis
* Poorly maintained equipment and safety system
* Lack of emergency response planning
* Inadequate training for operators

The CSB has pushed chemical safety reform since its conception, urging the chemical industry to produce inherently safer designs, use better quality equipment, and develop more thorough risk management plans. A major criticism of the process was its lack of inherent safe design. Because MIC was an intermediate, there was no reason to keep large quantities in storage. A modern design would use the intermediate as it is made (Eckerman et al., 2005).

Although chemical process safety has come a long way since 1984, industrial chemical giants still battle problems similar to Bhopal until this day. In 2008, a disaster similar to the one in Bhopal could have occurred in a plant originally designed by Union Carbide located in Institute, West Virginia after a runaway reaction caused a pressure buildup in a waste treatment vessel. The vessel exploded, killing two plant workers. Fortunately, the explosion missed a large MIC storage vessel which could have been hit by shrapnel and released tons of MIC (Blanc et al., 2009) In 2013, an ammonium nitrate explosion killed 15 and seriously injured 200 in West Texas in a blast radius similar to the one experienced in Bhopal (U.S.Chemical Safety Board, 2014).

=Case Study: Deepwater Horizon Explosion and Oil Spill=
[[File:Deepwater Horizon offshore drilling unit on fire 2010.jpg|thumb|alt=The burning Deepwater Horizon oilrig after an explosion caused by a design failure in the blowout preventer|The burning Deepwater Horizon oilrig after an explosion caused by a design failure in the blowout preventer|400px|right|The burning Deepwater Horizon oilrig after an explosion caused by a design failure in the blowout preventer]]
On April 20, 2010 on the Deepwater Horizon offshore drilling rig located in the Macondo Prospect, multiple explosions killed 11 workers and seriously injured 17. The rig burned for two days before sinking into the Gulf of Mexico. Key safety failures caused the well to spew 5 million barrels of oil into the Gulf of Mexico over the next 87 days making the incident the largest offshore oil spill in U.S. history. Finally, the well was sealed by a “static kill,” the injection of heavy fluids and cement, at the leak point 5,000 feet below the surface.

The key safety failure identified by the CSB was the blowout preventer (BOP) failure. This device that is meant to prevent the filling of annular space between the borehole and the well casing is both electrically and hydraulically powered. It is connected to a rig by a large diameter pipe called a riser. The system contains multiple pipe rams and annular preventers designed to prevent annular space buildup.

On the first night of the incident, a “kick” occurred and a mixture of oil, water, and gases began to build up in the wall and climb up the shaft. Drilling mud was injected to prevent kicks by creating a barrier. An upper annular preventer was also engaged when the buildup was discovered, but it failed. A pipe ram was activated and succeeded. However, an immense pressure buildup caused the drill pipe to buckle so it was forced off center. This buckling was later explained as a result of effective compression. This phenomenon is caused by microscopic irregularities and bends in the pipe material resulting in a higher surface area on one side of the pipe. Because the pipe was off-center, the final failsafe, the Automatic Mode Function (AMF) or deadman could not effectively shear the pipe and seal the well. This redundant control system comprised of a yellow pod and blue pod work independently to seal the well in the event of catastrophic failure when communications, electric power, and hydraulic pressure connections are cut. Both the yellow and blue pods contained 9 volt and 27 volt batteries which power solenoid valves. Unfortunately, the blue pod was miswired, so its 27 volt power supply was drained when it was to cause the blind shear blades to cut the pipe. Fortunately, a 9 volt battery in the yellow pod was also miswired which caused the blind shear ram to be engaged. However, this only partially sealed the well because of the pipe buckling. The flammable mixture erupted onto the surface of the platform and found an ignition source triggering a massive explosion. The spill was temporarily contained by a cap, and relief wells were eventually used to seal the well months later (U.S.Chemical Safety Board, 2014).

The White House Office of Energy and Climate Change Policy called the Deepwater Horizon oil spill the “worst environmental disaster the US has faced (BBC News, 2010). Over 8,000 species were estimated to be affected by the spill due to the toxicity of petroleum released, oxygen depletion, and the large quantities of Corexit, an oil dispersant used in an untested manner that is toxic to marine life (Biello et al., 2010; Butler, 2011; Froomkin, 2010).

BP, Transocean, and Halliburton were the major entities implicated in this tragedy. Investigations after the incident show that essential safety documentation including risk management and emergency procedure information were missing. Accusations were mainly aimed at BP with charges of recklessness and gross negligence (CNN Money, 2012). In January 2013, Transocean was ordered to pay $1.4 billion for US Clean Water Act violations. BP was ordered to pay $2.4 billion, but additional penalties could reach $20 billion (Department of Justice Office of Public Affairs, 2013).

=Case Study: Texas City Refinery Explosion=
[[File:BP_PLANT_EXPLOSION-1_lowres2.jpg|thumb|alt=Emergency response workers fight secondary fires caused by the isomerization unit explosion at the Texas City refinery|Emergency response workers fight secondary fires caused by the isomerization unit explosion at the Texas City refinery|228px|right|Emergency response workers fight secondary fires caused by the isomerization unit explosion at the Texas City refinery]]
On March 23, 2005 at a BP refinery in Texas City, Texas a hydrocarbon vapor cloud ignited, killing 15 workers and seriously injuring 170 others. Over the course an 11 hour period, a combination of control failures, mismanagement, and worker fatigue resulted in the buildup and release of extremely hot, combustible vapor. The key process unit in this disaster was an isomerization unit, located next to wooden trailers for workers servicing an ultracracker unit.

In the early morning on March 23rd, operators initiated startup and pumped raffinate (liquid hydrocarbons) into a raffinate splitter tower used to separate gasoline components. A liquid level indicator and multiple high level alarms monitored the tower liquid level. The level indicator could only measure up to 9 feet of liquid, and the written process called for a liquid level of about 6.5 feet. However, operators routinely filled the tower over 9 feet to minimize fluctuations and to prevent damage to a furnace. Hours later, the first high level alarm was activated and the liquid level rose, but a second alarm higher up the tower failed to trigger. The feed was halted when the liquid had risen to a level of about 13 feet, operators had no way of knowing the exact height. The lead operator relayed the startup activities to another operator and left the facility an hour before his shift ended. The morning operator arrived at 6 am to start his thirtieth consecutive day working a 12-hour shift and read a logbook that read, “Isom* Brought in some raff to unit, to pack raff with.” The day shift operator arrived an hour late, so he could not be briefed by the night shift supervisor. Recirculation then commenced in the tower, and more liquid was added to the tower. Additionally, conflicting instructions caused a liquid level regulating valve to remain closed for several hours, so liquid could not leave the tower. The furnace was then lit, and the supervisor left to attend a family medical emergency.

At noon, the liquid level had risen to 98 feet, but the improperly calibrated liquid level indicator read 8.4 feet. At 12:41 pm, a high pressure alarm caused workers to manually open a chain valve to relieve pressure by using the units pressure relief system to vent vapor into the atmosphere using an obsolete blowdown drum. Heat was also reduced in the furnace to reduce pressure. When operators became concerned about outflow rate, the liquid level regulating valve was opened to release liquid from the tower to storage. This caused the liquid in the tower to begin to boil and spill into the overhead vapor line exerting extreme pressures on the pressure relief system. At 1:14 pm, the three relief valves opened sending the liquid to the blowdown drum which overflowed into a municipal sewer setting off alarms, but a key level indicator in the blowdown drum failed. Flammable liquid erupted from the blowdown drum, formed a massive vapor cloud, and found an ignition source from a nearby idling pickup truck. The colossal blast ignited fires throughout the refinery and over half the workers in the wooden trailers adjacent to the unit were killed immediately (U.S.Chemical Safety Board, 2008).

Investigations after the incident cited multiple failures to implement safety recommendations at the Texas City Refinery. Among these, the blowdown drum was to be replaced by a modern flare to burn off hydrocarbons. However, BP’s budget cuts prevented its replacement. The training and treatment of workers was also called into question, as fatigue, poor communication, and inadequate documentation likely contributed to the disaster. Decisions like the one to operate an unsafe liquid level in order to prevent furnace damage also demonstrate the company’s fixation on the bottom line. BP was eventually fined $21 million by OSHA (New York Times, 2010; U.S.Chemical Safety Board, 2008).

=References=
==Direct Citation References==
<references/>

==Additional References==
# American Institute of Chemical Engineers. http://www.aiche.org/ccps
# Biello, David (9 June 2010). "The BP Spill's Growing Toll On the Sea Life of the Gulf". Yale Environment 360. Yale School of Forestry & Environmental Studies. Retrieved 2010-06-14.
# Blanc, P. Bhopal, 1984 – West Virginia near-miss, 2008. Psychology Today, December 2009. https://www.psychologytoday.com/blog/household-hazards/200912/bhopal-1984-west-virginia-near-miss-2008.
# Butler, J. Steven (3 March 2011). "BP Macondo Well Incident. U.S. Gulf of Mexico. Pollution Containment and Remediation Efforts" (PDF). Lillehammer Energy Claims Conference. BDO Consulting. Retrieved 17 February 2013.
# “DOJ accuses BP of ‘gross negligence’ in Gulf oil spill” CNN Money, September 2012.
# Eckerman, I. The Bhopal Saga: Causes and Consequences of the World’s Largest Industrial Disaster. Universities Press Private Limited, 2005.
# Froomkin, Dan (29 July 2010). "Scientists Find Evidence That Oil And Dispersant Mix Is Making Its Way Into The Foodchain". Huffington Post.
# "Gulf of Mexico oil leak 'worst US environment disaster'". BBC News. 30 May 2010.
# Investigation Report: BP Refinery Explosion and Fire, U.S. Chemical Safety Board, 2008.
# Lyall, Sarah. "In BP’s Record, a History of Boldness and Costly Blunders." New York Times, July 13, 2010.
# L.T. Biegler, I.E. Grossmann, A.W. Westerberg. ''Systematic Methods of Chemical Process Design''. Prentice-Hall: Upper Saddle River, 1997.
# M.S. Peters, K.D. Timmerhaus. ''Plant Design and Economics for Chemical Engineers, 5th Ed.'' McGraw-Hill: New York, 2003.
# Northwestern University. Chemical Engineering 351 Lecture Slides.
# Reflections on Bhopal After Thirty Years Video, U.S.Chemical Safety Board, December 2014.
# R.T. Turton, R.C. Bailie, W.B. Whiting, J.A. Shaeiwitz. ''Analysis, Synthesis, and Design of Chemical Processes''. Prentice Hall: Upper Saddle River, 2003.
# Union Carbide Corporation "Methyl Isocyanate" Product Information Publication, F-41443, November 1967.
# USCSB Deepwater Horizon Video, U.S.Chemical Safety Board, June 2014.
# "Transocean Agrees to Plead Guilty to Environmental Crime and Enter Civil Settlement to Resolve U.S. Clean Water Act Penalty Claims from Deepwater Horizon Incident". Department of Justice Office of Public Affairs. January 3, 2013.

Process safety

2016-02-13T22:55:28Z

Evanrosati: /* Benefits of Inherent Safety Over Conventional Safety */

Authors: Anne Disabato, [2014] Tim Hanrahan, [2014] Brian Merkle, [2014] , Spencer Saldana [2015] and Evan Rosati [2016] 

Stewards: David Chen, Jian Gong, and Fengqi You

Date Presented: January 19, 2014

 


__TOC__

=Introduction=
The safe design and operation of facilities is of paramount importance to every company that is involved in the manufacture of fuels, chemicals, and pharmaceuticals. Process safety focuses on the prevention of dangerous situations, such as fires, explosions, and the release of chemicals.

The American Institute of Chemical Engineers emphasizes a culture of process safety through four pillars (American Institute of Chemical Engineers, 2015):

1. Commitment to Process Safety: a workforce that is actively involved and an organization that fully supports process safety as a core value will tend to do the right things in the right way at the right time – even when no one else is looking

2. Understanding Hazard and Risk: the foundation of a risk-based approach which will allow an organization to use this information to allocate limited resources in the most effective manner

3. Manage Risk: the ongoing execution of risk based process safety tasks. Risk management can help a company to better deal with the resultant risks and sustain long-term accident free and profitable operations

4. Learn from Experience: Metrics provide direct feedback on the workings of RBPS systems, and leading indicators provide early warning signals of ineffective process safety results. Organizations must use their mistakes and those of others as motivation for action and view as opportunities for improvement.

For the prevention and management of specific safety hazards, such as fires, explosions, or the release of toxic chemicals, please see [[Process Hazards]].

==Layers of Plant Safety==
Safety and loss prevention can be expressed in "layers" of plant safety in terms of design and implementation.<ref name="Towler">G.P. Towler, R. Sinnott. ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design''. Elsevier, 2012.</ref> Each higher layer can be activated if a lower level fails. This creates a system with subsequent levels of safety to help prevent catastrophe from occurring. This diagram shows the important of safety in process design. If a process is designed to be inherently safe, additional safety "controls" will be less important and a chemical plant will be overall safer. The goal of safety is not to reach the top of the triangle, but to stay as close to the bottom as possible. This shows the importance of inherently safe design and safety legislation and regulations to provide guidelines for safety and health concerns. Regulations provide a baseline for engineers to operate when designing a chemical plant. They have brought safety to the forefront of design, when engineers have the maximum degree of freedom for implementation, and are no longer considered to be an afterthought or strictly a controls issue. Plants can be designed to be safe without the extensive use of future adaptation, safety controls, or emergency response. Although you can never eliminate these upper layers of process safety, by designing a process smartly and safely, engineers can reduce the consequences of "walking up" the process safety ladder or triangle.
[[File:Layers_of_Plant_Safety.png|center|700px|thumb|alt=Triangle Diagram on Increasing Plant Safety Mechanisms|Layers of Plant Safety Triangle Diagram]]

=Safety Legislation=

==International Regulations==
International regulations can be considered as best practice standards that are adopted by governments through treaties and establishment through United Nation resolutions. Most international regulation agencies can only register complaints for violations, but can not implement sanctions or fines to infracting parties.

===International Labour Organization (ILO)===
After the dissolution of the League of Nations in 1946, the ILO became the first specialized agency of the United Nations upon its founding in December of that same year. <ref> ISO Photo Gallery. http://www.ilo.org/dyn/media/mediasearch.fiche?p_id=16023&p_lang=en. Accessed February 13, 2016.</ref> The ILO attempts to develop labor standards for workers in all industries. The unique tripartite structure of the ILO gives an equal voice to workers, employers and governments to ensure that the views of the social partners are closely reflected in labour standards and in shaping policies and programs. <ref> ISO "How the ISO Works" http://www.ilo.org/global/about-the-ilo/how-the-ilo-works/lang--en/index.htm. Accessed February 13, 2016.</ref> The ISO has established International Labour Standards on Occupational Safety and Health, which has developed more than 40 standards specifically dealing with occupational heath and safety.<ref> ILO. "International Labour Standards on Occupational Safety and Health". http://www.ilo.org/global/standards/subjects-covered-by-international-labour-standards/occupational-safety-and-health/lang--en/index.htm. Accessed February 13, 2016.</ref>. The important conventions are listed below:

====Occupational Safety and Health Convention, 1981 and Protocol of 2002====
#The convention provides for the adoption of national occupational safety and health policy by each participating nation-state. It includes actions taken by governments and within enterprise that operate within the governmental spaces to promote the improvement of occupational safety and health to therefore improve working conditions.
#The additional Protocol in 2002 calls for the establishment and period review of procedures for recording and notification of occupational accidents along with the publication of related statistics associated with the accidents. This Protocol is similar to the Emergency Planning and CommunityRight-To-Know Act instituted in the United States (see below).

====Occupational Heath Services Convention, 1985====
#Establishment of enterprise-level occupational health services which are entrusted with preventative functions and which are responsible for advising the employer, the workers, and their representatives in the organization on maintaining a safe and healthy work environment.

====Promotional Framework for Occupational Safety and Heath Convention, 2006====
#Aims to promote a preventative safety and health culture and to progressively achieve a safe and healthy working environment for all.
#Requires ratifying nation-states to develop, in consultation with the most representative organizations of employees and workers, including workers unions, a national policy, a national system, and a national program on occupational health and safety.
#This should be developed in accordance to the Occupational Heath Services Convention, 1985 and take into account other ILO standards.
#National systems shall provide the infrastructure for implementing national policy and programs, such as domestic laws, regulations, authority bodies, and compliance mechanisms.

====Chemicals Convention, 1990====
#Provides for the adoption and implementation of a coherent policy on the safety in the use of chemicals at work, including production, handling, storage, and transport.
#Also includes best practices for the disposal and treatment of waste chemicals and the release of chemicals resulting from work activities, maintenance, and cleaning of equipment.
#In addition, it allocates specific responsibilities to suppliers and exporting states.
#Chemicals shall be evaluated to determine their level of hazards and employers shall make these hazards known to their employees.

===International Organization for Standardization (ISO)===
The ISO is an international organization for standardization on topics ranging from quality management, environmental management, social responsibility, risk management, etc. ISO standards are adopted world-wide by organizations and governments as accepted and known standards for production. Many of the practices are self-regulating as costumers often demand certain standards from companies they purchase from.<ref>ISO. Home Page. http://www.iso.org/iso/home.html. Accessed February 13, 2016.</ref> The ISO is developing its own occupational health and safety framework shown below.

====ISO 45001 Occupational Health and Safety Management Systems<ref>ISO 45001. "Occupational heath and safety". http://www.iso.org/iso/home/standards/management-standards/iso45001.htm. Accessed February 13, 2016.</ref>====
#Designed to help organizations reduce the burden of occupational injuries and diseases by providing a framework to improve employee safety, reduce workplace risks, and create better, safer working conditions.
#Currently under development by a committee of occupational health and safety experts who will follow management systems approaches of ISO 4001 and ISO 9001.
#Will embody other International Standards such as the ILO's Occupational Health and Safety Guidelines.
#The expected publication will be released in October, 2016.

==US Safety Regulations==
Over time chemical industry regulations have been developed to ensure that the best safety practices are followed to maintain the health of both people and the environment. The development of most regulations is based around the idea that organizations have both a legal and moral obligation to safeguard the health and welfare of its employees and the public.<ref name="Towler" /> The extent of legislation varies across regions around the globe. In the United States, chemical accidents have led to the creation of regulation boards and safety oriented societies such as the Center for Chemical Plant Safety of the American Institute of Chemical Engineers to aid in the development and implementation of pant safety. <ref>Seider WD, Seader JD, Lewin DR, Seider WD. Product And Process Design Principles: Synthesis, Analysis, and Evaluation. New York: Wiley; 2004.</ref> In the US, the major federal laws relating to chemical plant safety and their regulations are as follows:

===The Occupational Safety and Health Act, 1970===
The OSH Act is administer by the Occupational Safety and Health Administration (OSHA) (see below). The Act covers all employers and their employees in the United States with coverage provided either directly by the Federal Occupational Safety and Heath Administration or by an OSHA-approved state job safety and health plan.<ref> United States Department of Labor. "Safety and Health Standards: Occupational Safety and Health". http://webapps.dol.gov/elaws/elg/osha.htm </ref> The main provisions of the act are listed below.
# Employers must supply place of employment free from toxic chemicals, excessive noise mechanical dangers, or unsanitary conditions.
#This involves the implementation of engineering controls to limit exposure to hazards and toxic substances and implementing administrative controls.
# For employees, employers must provide personal protective equipment (PPE) and training, including communications of known hazards.
# The Occupational Safety and Health Administration (OSHA) was established to promote best practices, inspect facilities for hazard analysis, set standards, and enforce the law.
# The National Institute of Occupational Safety and Health (NIOSH) was established to be an independent research institute (now under the Centers for Disease Control).
#The Act also encourages states to develop and operate their own job safety and health programs with OSHA as a monitoring agency for these "state plans," which operate under the authority of state law. The standards developed by state plans need to be at least as effective as the federal regulations.<ref> United States Department of Labor. "Safety and Health Standards: Occupational Safety and Health". http://webapps.dol.gov/elaws/elg/osha.htm </ref>
#Federal OSHA standards are categorized into four categories: General Industry, Construction, Maritime Terminals, Long-shoring, and Agriculture.

===The Toxic Substances Control Act (TSCA), 1976===
The main qualifications of the TSCA were to provide the EPA with regulating power of chemicals, specifically pertaining to the chemical industry and not including foods, drugs, cosmetics, or pesticides. <ref> Environmental Protection Agency. "Summary of the Toxic Substances Control Act." http://www.epa.gov/laws-regulations/summary-toxic-substances-control-act. Accessed February 13, 2016</ref>
# The Environmental Protection Agency (EPA) is required to regulate 75,000 chemical substances used in industry.
# The EPA has jurisdiction over the safety of the sale or development of new chemicals in the United States, including the requirement for pre-manufacture notification for new chemical substances before manufacture.
#The TSCA also addresses the production, importation, use, and disposal of specific chemicals including polychlorinated biphenyls (PCBs), asbestos, radon, and lead-based paint.<ref> Environmental Protection Agency. "Summary of the Toxic Substances Control Act." http://www.epa.gov/laws-regulations/summary-toxic-substances-control-act. Accessed February 13, 2016</ref>
# Everyone has the right and obligation to report information about any health or environmental effects caused by a chemical. This is especially important for organizations as they are required to report information to the EPA if a chemical substance is found to have substantial risk of injury to health or the environment.
====Frank R. Lautenberg Chemical Safety for the 21st Century Act, 2015====
The Toxic Substances Control Act will be revised in 2015 under this new act proposed to Congress. This Act is an attempt to eliminate some of the flaws associated with the TSCA.
#One of the biggest flaws in the TSCA is the fact that a new chemical can be used without first demonstrating safety, the idea that chemicals are safe until proven unsafe.<ref>Sheppard, Kate. "Senators Introduce Bill to Overhaul U.S. Chemical Industry." http://www.huffingtonpost.com/2015/03/10/toxic-chemicals-senate-bill_n_6842524.html. Accessed February 13, 2016</ref>
#The new bill would require safety testing before chemical implementation.
#It gives the EPA more defined power on the regulation of chemical substances and the Sustainable Chemical Program will be established. This brings the TSCA into the modern era of the chemical industry.<ref>S.697 - 114th Congress (2015-2016): Frank R. Lautenberg Chemical Safety for the 21st Century Act. Congress.gov. https://www.congress.gov/bill/114th-congress/senate-bill/697/all-info. Accessed January 27, 2016.</ref>

===The Emergency Planning and Community Right-to-Know Act (EPCRA), 1986<ref> US Government Printing Office. Emergency Panning and Community Right to Know. https://www.gpo.gov/fdsys/pkg/USCODE-2010-title42/html/USCODE-2010-title42-chap116.htm. Accessed February 13, 2016</ref>===
The Act was passed by Congress in response to concerns regarding the environmental and safety hazards posed by the storage and handling of toxic chemicals. This legislation was developed as a result of the 1984 chemical disaster in Bhopal, India.<ref>What is EPCRA? EPA. http://www.epa.gov/epcra/what-epcra. Published November 10, 2015. Accessed January 27, 2016.</ref>
# All facilities manufacturing, processing, or storing hazardous chemicals must make plans for major incidents if they were to occur and the plans must be made public so that local communities can be properly informed.
# All facilities must also produce Material Safety Data Sheets (MSDSs) to state and local officials as well as local fire departments.
# Local governments should help prepare emergency plans and review the plans annually.
# State governments are required to oversee and coordinate local planning efforts.
# Emergency Notification: Facilities must immediately report accidental releases of chemicals and "hazardous substances" in quantities greater than Reportable Quantities (RQs) defined under the Comprehensive Environmental Response, Compensation, and Liability Act to state and local officials with this information then being available to the public.
# Annually, facilities must complete and submit a toxic chemical release inventory form.

===Clean Air Act Amendments of 1990===
This Amendment to the original Clean Air Act of 1970 was designed to curb three major threats to the nation's environment and health of citizens: acid rain, urban air pollution, and toxic air emissions.<ref>Environmental Protection Agency. "1990 Clean Air Act Amendment Summary" http://www.epa.gov/clean-air-act-overview/1990-clean-air-act-amendment-summary. Accessed February 13, 2016.</ref>
# Established the U.S. Chemical Safety and Hazard Investigation Board, and independent federal agency with the goal of ensuring worker and public safety through the prevention or minimization of the effects of chemical accidents. <ref>Seider WD, Seader JD, Lewin DR, Seider WD. Product And Process Design Principles: Synthesis, Analysis, and Evaluation. New York: Wiley; 2004.</ref>
# Attempt to determine the roots and contributing causes of accidents and then provide briefs on the accidents.
# Led to the development of cap and trade systems for air pollutants.
# Gave the government significantly more power to control air emissions and administer admissions permits.

==Additional Legislative Information==
For more details on environmental legislation pertaining to release of materials to the environment or the regulations of the loss of containment, see [[Environmental concerns]].

In addition to the above federal regulations, various states and other municipalities also have enacted legislation for the regulation of chemical plants. These include more specific safety items, such as local fire codes or even put into place stricter aspects of the federal regulations. <ref name="Towler" />

Any process design or plant design must always meet the requirements of local and federal mandates and regulations. Without doing so, the wellbeing of plant employees and even the public can be placed into serious jeopardy.

=Safety Organizations and Terminology=
==Organizations==
===OSHA===
The Occupational Safety and Health Administration (OSHA) is a federal agency that focuses on the enforcement of safety and health legislation.

===EPA===
The Environmental Protection Agency (EPA) is a U.S. agency whose purpose is the protection of the health of both humans and the environment through the writing and enforcement of regulatory laws.

===DOT===
The Department of Transportation (DOT) oversees federal highway, air, and maritime transportation, and can be involved in the safe transport of chemicals.

===DOE===
The Department of Energy (DOE) is a governmental department tasked with the advancement of energy technology in the United States.

==Terminology==
===HS&E===
Health, Safety, and Environmental - This term refers to all health, safety, and environmental concerns that arise at each stage of the design process. Companies are required to analyze each part of the process from an HS&E perspective to create a safe and healthy work environment.

===MSDS===
Material Safety and Data Sheet - Every chemical has an MSDS which contains all the information regarding safe handling and how to deal with spills or other accidents involving the substance. Relevant information includes how to identify the substance, hazard information, and how to handle spills, fires, and exposure, among other things.

===FMEA===
Failure Mode and Effects Analysis - FMEA is an early stage approach to identifying critical technical risks using a semi-quantitative procedure. The analysis encompasses safety, environmental, and operational feasibility. When performing FMEA, engineers look to see places in a potential design that could fail, and then quantify how likely that failure is, how severe the results would be, and then offer potential solutions to minimize the risk. A step-by-step guide to performing FMEA is shown below (Northwestern University, 2014):

# Brainstorm for failure modes
# For each FM, rate severity of impact (SEV, 1 - 10).
# For each FM, brainstorm for possible causes (there may be multiple).
# For each cause, rate likelihood of occurring (OCC, 1 - 10).
# Rate the probability that the systems currently in place will detect and prevent the problem before it has an impact (DET, 10 - 1). Do not assume that something that will be added to the design later will take care of the problem.
# Overall Risk Probability Number RPN = SEV x OCC x DET. Most practitioners use the 1,4,7,10 scale below to increase granularity. Note that the DET scale is inverse to SEV and OCC.

[[File:Safety3.JPG|center|700px]]

Figure 1: Suggested scale to be used for quantifying risk and detection of failures in FMEA. Taken from ChE 351 Slides.

The sum of all information collected is implemented into a spreadsheet like the one shown below:

[[File:Safety4.JPG|center|700px]]

Figure 2: Example spreadsheet used to organize FMEA data.

===HAZOP===
Hazard and Operability Study - For more information regarding HAZOP, please refer to [[Process Hazards]].

===SIL===
Safety Integrity Levels - The SIL is the relative level of risk-reduction provided by a safety function, or to specify a target level of risk reduction. A SIL is determined based on a number of qualitative factors such as development process and safety life cycle management. Several methods are used such as risk matrices, risk graphs, layers of protective analysis (LOPA). Three levels of safety integrity are assigned depending on the “availability” of the safety instrumented system (SIS), as shown below (Towler et al., 2012):

[[File:Safety5.JPG|center|500px]]

Figure 3: Table of safety integrity levels based on availability of system. Taken from (Towler et al., 2012).

Redundant system means instrumentation is duplicated; higher level of redundancy of trip systems give higher SIL. The required SIL should be determined during a process hazard analysis and depends on risk of operator exposure and injury.

1 Instrument
* if it signals, plant goes down
* Probability of incident = probability of instrument failure
* Probability of spurious trip = false positive rate of one instrument

2 Instruments
* 1 out of 2 voting (1oo2): one instrument signals, plant goes down
** Probability of incident reduced by duplication
** Probability of spurious trip doubled
* 2oo2 voting
** Probability of incident worse than single instrument (twice likelihood that system is down)
** Probability of spurious trip reduced

3 Instruments
* 2oo3 voting
** Best overall trade-off between reducing incident rate and spurious trip rate
** One malfunctioning instrument does not cause trip or prevent detection of real incident

=Safe Design=
==Inherently Safe Design==
Inherently safe design of a particular process can be achieved by adhering to the following six strategies (Turton et al., 2003):

1. Substitution: Avoid using or producing hazardous materials on the plant site. If the hazardous material is an intermediate product, for example, alternate chemical reaction pathways might be used. In other words, the most inherently safe strategy is to avoid the use of hazardous materials.

2. Intensification: Attempt to use less of the hazardous materials. In terms of a hazardous intermediate, the two processes could be more closely coupled, reducing or eliminating the amount of intermediate produced. The inventories of hazardous feeds or products can be reduced by enhanced scheduling techniques such as just-in-time (JIT) manufacturing.

3. Attenuation: Reducing, or attenuating, the hazards of materials can often be affected by lowering the temperature or adding stabilizing additives. By using materials under less hazardous conditions, the potential consequences of a leak can be reduced.

4. Containment: If the hazardous materials cannot be eliminated, they at least should be stored in vessels with mechanical integrity beyond any reasonably expected temperature or pressure excursion. This is an old but effective strategy to avoid leaks. However, it is not as inherently safe as substitution, intensification, or attenuation.

5. Control: If a leak of hazardous material does occur, there should be safety systems that reduce the effects. For example, chemical facilities often have emergency isolation of the site from the normal storm sewers, and large tanks for flammable liquids are surrounded by dikes that prevent any leaks from spreading to to other areas of the plant. Scrubbing systems and relief systems in general are in this category. They are essential, because they allow controlled, safe release of hazardous materials, rather than an uncontrolled release from a vessel rupture.

6. Survival: If leaks of hazardous materials do occur and they are not contained or controlled, the personnel (and the equipment) must be protected. This lowest level of the hierarchy includes fire fighting, gas masks, and so on. Although essential to the total safety of the plant, the greater the reliance on survival of leaks rather than elimination of leaks, the less inherently safe the facility.

===Safety Legislation and Process Design===
====Inherent Process Plant Safety====
Through the enactment of safety regulation shown above, process design has inherently become safer. These regulations have made safety the first and foremost important concern when designing a new chemical plant. In general, the safety of a process relies on multiple layers of protection, but the first and most important layer of protection has become the process design feature. Greater tolerances are built into the designing of processes to more effectively prevent catastrophic failures or chemical leakage. The best approach to prevent accidents is to add process design features, involving chemistry and physics, to prevent hazardous situations.
=====Examples of Inherently Safe Process Design<ref>Crowl, Daniel. Louver, Joseph. Chemical Process Safety: Fundamentals with Applications. IBN: 9780132440554</ref>=====
#Vapor Release: Vapor released from spills can be minimized by designing dikes so that flammable and toxic materials will not accumulate around leaking tanks. This also prevents potential flammable materials from building up and causing an explosion.
#Containment Building: Design can be important for the containment of toxic spills. With the addition of automatic or remote controls, personnel can leave the area if a spill or breach occurs, while the area can be continuously monitored.
#Solvent Substitution: Safety through Substitution - Substituting design with safer, less hazardous materials. Designing process for use with less toxic or flammable solvents. For example, water-based paints and adhesives as well as aqueous or dry flowable formulations for agricultural chemicals opposed to more volatile solvents that release VOCs.
#Design for Lower Temperature and Pressure Safety through Moderation - use a hazardous material under less hazardous conditions such as lower pressure and temperature conditions. This can lower the level of catastrophe if downstream safety processes do fail.

====Control Systems Safety====
Legislation has also added to subsequent layers of controls such as environmental process controls have been added to prevent release to nearby air and water systems, which would endanger surrounding ecosystems and human populations.

==Assessing Preliminary Design==
While pilot plants are necessary to design effective plant equipment, there are some dangers associated with the industrial scale of chemical plants. Scaling up without accurate literature and experimental data can be very dangerous.

In every step of process design, the Health, Safety, and Environmental (HS&E) analysis must be carried out with the available technical information (Biegler et al., 1997).

[[File:Safety1.JPG|center|800px]]

[[File:Safety2.JPG|center|700px]]

Figure 4: General steps in the design process and analysis to be carried out at each stage. Taken from (Biegler et al., 1997).

=Economic Cost of Safety=
Price of Safety installations and fire protection systems range from .5 to 1.6 % of fixed-capital investment of a plant; but expenditures are often much higher than this and it is difficult to estimate these expenditures for a given plant

In addition, designers also examine the economic impact of safety and maintenance issues. For instance, they may determine that the plant reactor configuration can be improved, and with improved operator training facilities, it can run with improved safety. These hidden costs must be considered when determining the economic feasibility of operation (Biegler et al., 1997).

==Benefits of Inherent Safety Over Conventional Safety==
The outdated method of implementing safety into process design came very late in the design process. The physical entities in the process or the process itself was changed very little and the conventional safety methods included implementing controls. Inherent process safety is instead developed very early in the design process and can lead to very significant cost savings overall. The comparison comes when looking at two ratios for safety costs, the Conventional Safety Cost Index (CSCI) <math>CSCI=\frac{C_{conventional safety}}{C_{loss}}</math> and the Inherent Safety Cost Index (ISCI) <math>ISCI=\frac{C_{inherent safety}}{C_{loss}}</math>. The values of safety cost are easy to determine as they are any additive change to a standard design. The difficulty comes with calculating the cost of losses as this must be an additive loss associated of assets, production, environmental cleanup, and potential human health losses. A diagram of the methods to calculate cost due to losses is shown below:<ref>Kahn, F. Amyotte, Paul. "I2SI: A comprehensive quantitative tool for inherent safety and cost evaluation"
. Journal of Loss Prevention. Elsevier.

[[File:FailureCost.jpg|center|700px|thumb|alt=Cost in Process Failures|Methods for Calculating Process Losses Costs]]

=Other Process Safety Considerations=
The safety and well-being of the consumers using the eventual product should be considered in process safety. The risks involved in using a product should be clearly communicated to the consumer by industrial leaders.

Human Error is another safety risk that is difficult to quantify. The intervention of well-trained operators is a vital layer in process safety (Peters et al., 2003).

=Case Study: Production of Ultrapure Hydrogen=
[[File:steam reforming.jpg|thumb|alt=A hydrogen production plant with a box steam reformer|A hydrogen production plant with a box steam reformer|300px|right|A hydrogen production plant with a box steam reformer]]
To walk through the process safety considerations when designing a chemical plant, the relatively simple example of a high purity hydrogen generation will be examined. First, various technologies are researched to determine the options that best meet the economic, physical, environmental, and safety constraints of the project. For this project, steam reforming, autothermal reforming, and partial oxidation were investigated. Technologies that lack widespread implementation have inherent safety risks as they have higher uncertainties associated with reliability, feasibility, and cost. Autothermal reforming requires oxygen and not a commercially popular method. Partial oxidation requires no catalyst, but requires high process temperatures and is a complex process to implement. After each process was analyzed and scored in a decision matrix, steam reforming was chosen as the base process technology. It was selected because it is a safe, well-known, low-emission, traditional process in use all over the world for the production of hydrogen. Steam reforming produces minimal waste compared to the alternative processes, and is capable of producing the necessary 100 MMscfd of 99.999% hydrogen gas. It is important to remember that both local and global environmental emissions are capable of harming the general public, so they should be considered safety concerns in the same way worker hazards are. Next, various reactor types, catalysts, and separation methods were evaluated with the base process chosen. In all, seven different processing stages were assessed including the initial heating of feed, the steam reformation reactor, the high and low temperature water shift reactors, the amine plant, the methanation reactor, the gas compression and cooling train, and the pressure swing adsorption unit.

Although worker safety is always the first priority in a plant, there are inherent risk associated with high temperature and pressure processes. Close attention was paid to make the preliminary plant design as inherently safe as possible. Preliminary FMEA and HAZOP analyses were conducted to identify the highest priority risks. Hazards were mitigated by substituting less hazardous materials when possible, opting to store only the necessary hazardous materials on site, lowering temperatures when possible, adding catastrophic failure controls, maximizing plant control automation when economically feasible, and necessitating worker personal protective equipment. Finally, FMEA and HAZAP analyses were repeated. These steps were repeated multiple times until a sufficient reduction in risk had been achieved.

=Case Study: Bhopal Disaster=
[[File:Bhopal.JPG|thumb|alt=The dispersal of the deadly cloud of gas from the Union Carbide plant in the Indian city of Bhopal|The dispersal of the deadly cloud of gas from the Union Carbide plant in the Indian city of Bhopal|400px|right|The dispersal of the deadly cloud of gas from the Union Carbide plant in the Indian city of Bhopal]]
On December 3, 1984 at the India Limited Pesticide Plant owned by Union Carbide in Bhopal, India, water entered a storage vessel containing over 80,000 lbs. of methyl isocyanate (MIC), a chemical intermediate in the pesticide synthesis process. This reaction caused a rapid increase in temperature accompanied by boiling, which caused toxic MIC vapors to escape from the tank. In addition, the MIC-water reaction produced methylamine and carbon dioxide gases among other toxic products which also contributed to the pressure increase (Union Carbide Corporation, 1967). These vapors passed into a scrubber and flare system that were not working at the time due to inadequate maintenance and safety practices. As a result of this accident, approximately 25 tons of MIC vapor were released, killing over 3,800 immediately and injuring roughly 20,000 in the surrounding area.

As a result of the incident, Union Carbide was forced to pay $470 million, as well as fund a hospital in Bhopal that was used specifically to treat victims of the disaster. Cleanup of the plant site and other legal action are still being determined to this day. Bhopal sparked a worldwide discussion on chemical process safety, and caused Congress to create the U.S. Chemical Safety Board (CSB). The CSB has since cited the following reasons as causes for the disaster:

* No process hazard analysis
* Poorly maintained equipment and safety system
* Lack of emergency response planning
* Inadequate training for operators

The CSB has pushed chemical safety reform since its conception, urging the chemical industry to produce inherently safer designs, use better quality equipment, and develop more thorough risk management plans. A major criticism of the process was its lack of inherent safe design. Because MIC was an intermediate, there was no reason to keep large quantities in storage. A modern design would use the intermediate as it is made (Eckerman et al., 2005).

Although chemical process safety has come a long way since 1984, industrial chemical giants still battle problems similar to Bhopal until this day. In 2008, a disaster similar to the one in Bhopal could have occurred in a plant originally designed by Union Carbide located in Institute, West Virginia after a runaway reaction caused a pressure buildup in a waste treatment vessel. The vessel exploded, killing two plant workers. Fortunately, the explosion missed a large MIC storage vessel which could have been hit by shrapnel and released tons of MIC (Blanc et al., 2009) In 2013, an ammonium nitrate explosion killed 15 and seriously injured 200 in West Texas in a blast radius similar to the one experienced in Bhopal (U.S.Chemical Safety Board, 2014).

=Case Study: Deepwater Horizon Explosion and Oil Spill=
[[File:Deepwater Horizon offshore drilling unit on fire 2010.jpg|thumb|alt=The burning Deepwater Horizon oilrig after an explosion caused by a design failure in the blowout preventer|The burning Deepwater Horizon oilrig after an explosion caused by a design failure in the blowout preventer|400px|right|The burning Deepwater Horizon oilrig after an explosion caused by a design failure in the blowout preventer]]
On April 20, 2010 on the Deepwater Horizon offshore drilling rig located in the Macondo Prospect, multiple explosions killed 11 workers and seriously injured 17. The rig burned for two days before sinking into the Gulf of Mexico. Key safety failures caused the well to spew 5 million barrels of oil into the Gulf of Mexico over the next 87 days making the incident the largest offshore oil spill in U.S. history. Finally, the well was sealed by a “static kill,” the injection of heavy fluids and cement, at the leak point 5,000 feet below the surface.

The key safety failure identified by the CSB was the blowout preventer (BOP) failure. This device that is meant to prevent the filling of annular space between the borehole and the well casing is both electrically and hydraulically powered. It is connected to a rig by a large diameter pipe called a riser. The system contains multiple pipe rams and annular preventers designed to prevent annular space buildup.

On the first night of the incident, a “kick” occurred and a mixture of oil, water, and gases began to build up in the wall and climb up the shaft. Drilling mud was injected to prevent kicks by creating a barrier. An upper annular preventer was also engaged when the buildup was discovered, but it failed. A pipe ram was activated and succeeded. However, an immense pressure buildup caused the drill pipe to buckle so it was forced off center. This buckling was later explained as a result of effective compression. This phenomenon is caused by microscopic irregularities and bends in the pipe material resulting in a higher surface area on one side of the pipe. Because the pipe was off-center, the final failsafe, the Automatic Mode Function (AMF) or deadman could not effectively shear the pipe and seal the well. This redundant control system comprised of a yellow pod and blue pod work independently to seal the well in the event of catastrophic failure when communications, electric power, and hydraulic pressure connections are cut. Both the yellow and blue pods contained 9 volt and 27 volt batteries which power solenoid valves. Unfortunately, the blue pod was miswired, so its 27 volt power supply was drained when it was to cause the blind shear blades to cut the pipe. Fortunately, a 9 volt battery in the yellow pod was also miswired which caused the blind shear ram to be engaged. However, this only partially sealed the well because of the pipe buckling. The flammable mixture erupted onto the surface of the platform and found an ignition source triggering a massive explosion. The spill was temporarily contained by a cap, and relief wells were eventually used to seal the well months later (U.S.Chemical Safety Board, 2014).

The White House Office of Energy and Climate Change Policy called the Deepwater Horizon oil spill the “worst environmental disaster the US has faced (BBC News, 2010). Over 8,000 species were estimated to be affected by the spill due to the toxicity of petroleum released, oxygen depletion, and the large quantities of Corexit, an oil dispersant used in an untested manner that is toxic to marine life (Biello et al., 2010; Butler, 2011; Froomkin, 2010).

BP, Transocean, and Halliburton were the major entities implicated in this tragedy. Investigations after the incident show that essential safety documentation including risk management and emergency procedure information were missing. Accusations were mainly aimed at BP with charges of recklessness and gross negligence (CNN Money, 2012). In January 2013, Transocean was ordered to pay $1.4 billion for US Clean Water Act violations. BP was ordered to pay $2.4 billion, but additional penalties could reach $20 billion (Department of Justice Office of Public Affairs, 2013).

=Case Study: Texas City Refinery Explosion=
[[File:BP_PLANT_EXPLOSION-1_lowres2.jpg|thumb|alt=Emergency response workers fight secondary fires caused by the isomerization unit explosion at the Texas City refinery|Emergency response workers fight secondary fires caused by the isomerization unit explosion at the Texas City refinery|228px|right|Emergency response workers fight secondary fires caused by the isomerization unit explosion at the Texas City refinery]]
On March 23, 2005 at a BP refinery in Texas City, Texas a hydrocarbon vapor cloud ignited, killing 15 workers and seriously injuring 170 others. Over the course an 11 hour period, a combination of control failures, mismanagement, and worker fatigue resulted in the buildup and release of extremely hot, combustible vapor. The key process unit in this disaster was an isomerization unit, located next to wooden trailers for workers servicing an ultracracker unit.

In the early morning on March 23rd, operators initiated startup and pumped raffinate (liquid hydrocarbons) into a raffinate splitter tower used to separate gasoline components. A liquid level indicator and multiple high level alarms monitored the tower liquid level. The level indicator could only measure up to 9 feet of liquid, and the written process called for a liquid level of about 6.5 feet. However, operators routinely filled the tower over 9 feet to minimize fluctuations and to prevent damage to a furnace. Hours later, the first high level alarm was activated and the liquid level rose, but a second alarm higher up the tower failed to trigger. The feed was halted when the liquid had risen to a level of about 13 feet, operators had no way of knowing the exact height. The lead operator relayed the startup activities to another operator and left the facility an hour before his shift ended. The morning operator arrived at 6 am to start his thirtieth consecutive day working a 12-hour shift and read a logbook that read, “Isom* Brought in some raff to unit, to pack raff with.” The day shift operator arrived an hour late, so he could not be briefed by the night shift supervisor. Recirculation then commenced in the tower, and more liquid was added to the tower. Additionally, conflicting instructions caused a liquid level regulating valve to remain closed for several hours, so liquid could not leave the tower. The furnace was then lit, and the supervisor left to attend a family medical emergency.

At noon, the liquid level had risen to 98 feet, but the improperly calibrated liquid level indicator read 8.4 feet. At 12:41 pm, a high pressure alarm caused workers to manually open a chain valve to relieve pressure by using the units pressure relief system to vent vapor into the atmosphere using an obsolete blowdown drum. Heat was also reduced in the furnace to reduce pressure. When operators became concerned about outflow rate, the liquid level regulating valve was opened to release liquid from the tower to storage. This caused the liquid in the tower to begin to boil and spill into the overhead vapor line exerting extreme pressures on the pressure relief system. At 1:14 pm, the three relief valves opened sending the liquid to the blowdown drum which overflowed into a municipal sewer setting off alarms, but a key level indicator in the blowdown drum failed. Flammable liquid erupted from the blowdown drum, formed a massive vapor cloud, and found an ignition source from a nearby idling pickup truck. The colossal blast ignited fires throughout the refinery and over half the workers in the wooden trailers adjacent to the unit were killed immediately (U.S.Chemical Safety Board, 2008).

Investigations after the incident cited multiple failures to implement safety recommendations at the Texas City Refinery. Among these, the blowdown drum was to be replaced by a modern flare to burn off hydrocarbons. However, BP’s budget cuts prevented its replacement. The training and treatment of workers was also called into question, as fatigue, poor communication, and inadequate documentation likely contributed to the disaster. Decisions like the one to operate an unsafe liquid level in order to prevent furnace damage also demonstrate the company’s fixation on the bottom line. BP was eventually fined $21 million by OSHA (New York Times, 2010; U.S.Chemical Safety Board, 2008).

=References=
==Direct Citation References==
<references/>

==Additional References==
# American Institute of Chemical Engineers. http://www.aiche.org/ccps
# Biello, David (9 June 2010). "The BP Spill's Growing Toll On the Sea Life of the Gulf". Yale Environment 360. Yale School of Forestry & Environmental Studies. Retrieved 2010-06-14.
# Blanc, P. Bhopal, 1984 – West Virginia near-miss, 2008. Psychology Today, December 2009. https://www.psychologytoday.com/blog/household-hazards/200912/bhopal-1984-west-virginia-near-miss-2008.
# Butler, J. Steven (3 March 2011). "BP Macondo Well Incident. U.S. Gulf of Mexico. Pollution Containment and Remediation Efforts" (PDF). Lillehammer Energy Claims Conference. BDO Consulting. Retrieved 17 February 2013.
# “DOJ accuses BP of ‘gross negligence’ in Gulf oil spill” CNN Money, September 2012.
# Eckerman, I. The Bhopal Saga: Causes and Consequences of the World’s Largest Industrial Disaster. Universities Press Private Limited, 2005.
# Froomkin, Dan (29 July 2010). "Scientists Find Evidence That Oil And Dispersant Mix Is Making Its Way Into The Foodchain". Huffington Post.
# "Gulf of Mexico oil leak 'worst US environment disaster'". BBC News. 30 May 2010.
# Investigation Report: BP Refinery Explosion and Fire, U.S. Chemical Safety Board, 2008.
# Lyall, Sarah. "In BP’s Record, a History of Boldness and Costly Blunders." New York Times, July 13, 2010.
# L.T. Biegler, I.E. Grossmann, A.W. Westerberg. ''Systematic Methods of Chemical Process Design''. Prentice-Hall: Upper Saddle River, 1997.
# M.S. Peters, K.D. Timmerhaus. ''Plant Design and Economics for Chemical Engineers, 5th Ed.'' McGraw-Hill: New York, 2003.
# Northwestern University. Chemical Engineering 351 Lecture Slides.
# Reflections on Bhopal After Thirty Years Video, U.S.Chemical Safety Board, December 2014.
# R.T. Turton, R.C. Bailie, W.B. Whiting, J.A. Shaeiwitz. ''Analysis, Synthesis, and Design of Chemical Processes''. Prentice Hall: Upper Saddle River, 2003.
# Union Carbide Corporation "Methyl Isocyanate" Product Information Publication, F-41443, November 1967.
# USCSB Deepwater Horizon Video, U.S.Chemical Safety Board, June 2014.
# "Transocean Agrees to Plead Guilty to Environmental Crime and Enter Civil Settlement to Resolve U.S. Clean Water Act Penalty Claims from Deepwater Horizon Incident". Department of Justice Office of Public Affairs. January 3, 2013.

File:FailureCost.jpg

2016-02-13T22:45:50Z

Evanrosati: Determining cost of loss

Determining cost of loss

Process safety

2016-02-13T22:45:09Z

Evanrosati: /* Benefits of Inherent Safety Over Conventional Safety */

Authors: Anne Disabato, [2014] Tim Hanrahan, [2014] Brian Merkle, [2014] , Spencer Saldana [2015] and Evan Rosati [2016] 

Stewards: David Chen, Jian Gong, and Fengqi You

Date Presented: January 19, 2014

 


__TOC__

=Introduction=
The safe design and operation of facilities is of paramount importance to every company that is involved in the manufacture of fuels, chemicals, and pharmaceuticals. Process safety focuses on the prevention of dangerous situations, such as fires, explosions, and the release of chemicals.

The American Institute of Chemical Engineers emphasizes a culture of process safety through four pillars (American Institute of Chemical Engineers, 2015):

1. Commitment to Process Safety: a workforce that is actively involved and an organization that fully supports process safety as a core value will tend to do the right things in the right way at the right time – even when no one else is looking

2. Understanding Hazard and Risk: the foundation of a risk-based approach which will allow an organization to use this information to allocate limited resources in the most effective manner

3. Manage Risk: the ongoing execution of risk based process safety tasks. Risk management can help a company to better deal with the resultant risks and sustain long-term accident free and profitable operations

4. Learn from Experience: Metrics provide direct feedback on the workings of RBPS systems, and leading indicators provide early warning signals of ineffective process safety results. Organizations must use their mistakes and those of others as motivation for action and view as opportunities for improvement.

For the prevention and management of specific safety hazards, such as fires, explosions, or the release of toxic chemicals, please see [[Process Hazards]].

==Layers of Plant Safety==
Safety and loss prevention can be expressed in "layers" of plant safety in terms of design and implementation.<ref name="Towler">G.P. Towler, R. Sinnott. ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design''. Elsevier, 2012.</ref> Each higher layer can be activated if a lower level fails. This creates a system with subsequent levels of safety to help prevent catastrophe from occurring. This diagram shows the important of safety in process design. If a process is designed to be inherently safe, additional safety "controls" will be less important and a chemical plant will be overall safer. The goal of safety is not to reach the top of the triangle, but to stay as close to the bottom as possible. This shows the importance of inherently safe design and safety legislation and regulations to provide guidelines for safety and health concerns. Regulations provide a baseline for engineers to operate when designing a chemical plant. They have brought safety to the forefront of design, when engineers have the maximum degree of freedom for implementation, and are no longer considered to be an afterthought or strictly a controls issue. Plants can be designed to be safe without the extensive use of future adaptation, safety controls, or emergency response. Although you can never eliminate these upper layers of process safety, by designing a process smartly and safely, engineers can reduce the consequences of "walking up" the process safety ladder or triangle.
[[File:Layers_of_Plant_Safety.png|center|700px|thumb|alt=Triangle Diagram on Increasing Plant Safety Mechanisms|Layers of Plant Safety Triangle Diagram]]

=Safety Legislation=

==International Regulations==
International regulations can be considered as best practice standards that are adopted by governments through treaties and establishment through United Nation resolutions. Most international regulation agencies can only register complaints for violations, but can not implement sanctions or fines to infracting parties.

===International Labour Organization (ILO)===
After the dissolution of the League of Nations in 1946, the ILO became the first specialized agency of the United Nations upon its founding in December of that same year. <ref> ISO Photo Gallery. http://www.ilo.org/dyn/media/mediasearch.fiche?p_id=16023&p_lang=en. Accessed February 13, 2016.</ref> The ILO attempts to develop labor standards for workers in all industries. The unique tripartite structure of the ILO gives an equal voice to workers, employers and governments to ensure that the views of the social partners are closely reflected in labour standards and in shaping policies and programs. <ref> ISO "How the ISO Works" http://www.ilo.org/global/about-the-ilo/how-the-ilo-works/lang--en/index.htm. Accessed February 13, 2016.</ref> The ISO has established International Labour Standards on Occupational Safety and Health, which has developed more than 40 standards specifically dealing with occupational heath and safety.<ref> ILO. "International Labour Standards on Occupational Safety and Health". http://www.ilo.org/global/standards/subjects-covered-by-international-labour-standards/occupational-safety-and-health/lang--en/index.htm. Accessed February 13, 2016.</ref>. The important conventions are listed below:

====Occupational Safety and Health Convention, 1981 and Protocol of 2002====
#The convention provides for the adoption of national occupational safety and health policy by each participating nation-state. It includes actions taken by governments and within enterprise that operate within the governmental spaces to promote the improvement of occupational safety and health to therefore improve working conditions.
#The additional Protocol in 2002 calls for the establishment and period review of procedures for recording and notification of occupational accidents along with the publication of related statistics associated with the accidents. This Protocol is similar to the Emergency Planning and CommunityRight-To-Know Act instituted in the United States (see below).

====Occupational Heath Services Convention, 1985====
#Establishment of enterprise-level occupational health services which are entrusted with preventative functions and which are responsible for advising the employer, the workers, and their representatives in the organization on maintaining a safe and healthy work environment.

====Promotional Framework for Occupational Safety and Heath Convention, 2006====
#Aims to promote a preventative safety and health culture and to progressively achieve a safe and healthy working environment for all.
#Requires ratifying nation-states to develop, in consultation with the most representative organizations of employees and workers, including workers unions, a national policy, a national system, and a national program on occupational health and safety.
#This should be developed in accordance to the Occupational Heath Services Convention, 1985 and take into account other ILO standards.
#National systems shall provide the infrastructure for implementing national policy and programs, such as domestic laws, regulations, authority bodies, and compliance mechanisms.

====Chemicals Convention, 1990====
#Provides for the adoption and implementation of a coherent policy on the safety in the use of chemicals at work, including production, handling, storage, and transport.
#Also includes best practices for the disposal and treatment of waste chemicals and the release of chemicals resulting from work activities, maintenance, and cleaning of equipment.
#In addition, it allocates specific responsibilities to suppliers and exporting states.
#Chemicals shall be evaluated to determine their level of hazards and employers shall make these hazards known to their employees.

===International Organization for Standardization (ISO)===
The ISO is an international organization for standardization on topics ranging from quality management, environmental management, social responsibility, risk management, etc. ISO standards are adopted world-wide by organizations and governments as accepted and known standards for production. Many of the practices are self-regulating as costumers often demand certain standards from companies they purchase from.<ref>ISO. Home Page. http://www.iso.org/iso/home.html. Accessed February 13, 2016.</ref> The ISO is developing its own occupational health and safety framework shown below.

====ISO 45001 Occupational Health and Safety Management Systems<ref>ISO 45001. "Occupational heath and safety". http://www.iso.org/iso/home/standards/management-standards/iso45001.htm. Accessed February 13, 2016.</ref>====
#Designed to help organizations reduce the burden of occupational injuries and diseases by providing a framework to improve employee safety, reduce workplace risks, and create better, safer working conditions.
#Currently under development by a committee of occupational health and safety experts who will follow management systems approaches of ISO 4001 and ISO 9001.
#Will embody other International Standards such as the ILO's Occupational Health and Safety Guidelines.
#The expected publication will be released in October, 2016.

==US Safety Regulations==
Over time chemical industry regulations have been developed to ensure that the best safety practices are followed to maintain the health of both people and the environment. The development of most regulations is based around the idea that organizations have both a legal and moral obligation to safeguard the health and welfare of its employees and the public.<ref name="Towler" /> The extent of legislation varies across regions around the globe. In the United States, chemical accidents have led to the creation of regulation boards and safety oriented societies such as the Center for Chemical Plant Safety of the American Institute of Chemical Engineers to aid in the development and implementation of pant safety. <ref>Seider WD, Seader JD, Lewin DR, Seider WD. Product And Process Design Principles: Synthesis, Analysis, and Evaluation. New York: Wiley; 2004.</ref> In the US, the major federal laws relating to chemical plant safety and their regulations are as follows:

===The Occupational Safety and Health Act, 1970===
The OSH Act is administer by the Occupational Safety and Health Administration (OSHA) (see below). The Act covers all employers and their employees in the United States with coverage provided either directly by the Federal Occupational Safety and Heath Administration or by an OSHA-approved state job safety and health plan.<ref> United States Department of Labor. "Safety and Health Standards: Occupational Safety and Health". http://webapps.dol.gov/elaws/elg/osha.htm </ref> The main provisions of the act are listed below.
# Employers must supply place of employment free from toxic chemicals, excessive noise mechanical dangers, or unsanitary conditions.
#This involves the implementation of engineering controls to limit exposure to hazards and toxic substances and implementing administrative controls.
# For employees, employers must provide personal protective equipment (PPE) and training, including communications of known hazards.
# The Occupational Safety and Health Administration (OSHA) was established to promote best practices, inspect facilities for hazard analysis, set standards, and enforce the law.
# The National Institute of Occupational Safety and Health (NIOSH) was established to be an independent research institute (now under the Centers for Disease Control).
#The Act also encourages states to develop and operate their own job safety and health programs with OSHA as a monitoring agency for these "state plans," which operate under the authority of state law. The standards developed by state plans need to be at least as effective as the federal regulations.<ref> United States Department of Labor. "Safety and Health Standards: Occupational Safety and Health". http://webapps.dol.gov/elaws/elg/osha.htm </ref>
#Federal OSHA standards are categorized into four categories: General Industry, Construction, Maritime Terminals, Long-shoring, and Agriculture.

===The Toxic Substances Control Act (TSCA), 1976===
The main qualifications of the TSCA were to provide the EPA with regulating power of chemicals, specifically pertaining to the chemical industry and not including foods, drugs, cosmetics, or pesticides. <ref> Environmental Protection Agency. "Summary of the Toxic Substances Control Act." http://www.epa.gov/laws-regulations/summary-toxic-substances-control-act. Accessed February 13, 2016</ref>
# The Environmental Protection Agency (EPA) is required to regulate 75,000 chemical substances used in industry.
# The EPA has jurisdiction over the safety of the sale or development of new chemicals in the United States, including the requirement for pre-manufacture notification for new chemical substances before manufacture.
#The TSCA also addresses the production, importation, use, and disposal of specific chemicals including polychlorinated biphenyls (PCBs), asbestos, radon, and lead-based paint.<ref> Environmental Protection Agency. "Summary of the Toxic Substances Control Act." http://www.epa.gov/laws-regulations/summary-toxic-substances-control-act. Accessed February 13, 2016</ref>
# Everyone has the right and obligation to report information about any health or environmental effects caused by a chemical. This is especially important for organizations as they are required to report information to the EPA if a chemical substance is found to have substantial risk of injury to health or the environment.
====Frank R. Lautenberg Chemical Safety for the 21st Century Act, 2015====
The Toxic Substances Control Act will be revised in 2015 under this new act proposed to Congress. This Act is an attempt to eliminate some of the flaws associated with the TSCA.
#One of the biggest flaws in the TSCA is the fact that a new chemical can be used without first demonstrating safety, the idea that chemicals are safe until proven unsafe.<ref>Sheppard, Kate. "Senators Introduce Bill to Overhaul U.S. Chemical Industry." http://www.huffingtonpost.com/2015/03/10/toxic-chemicals-senate-bill_n_6842524.html. Accessed February 13, 2016</ref>
#The new bill would require safety testing before chemical implementation.
#It gives the EPA more defined power on the regulation of chemical substances and the Sustainable Chemical Program will be established. This brings the TSCA into the modern era of the chemical industry.<ref>S.697 - 114th Congress (2015-2016): Frank R. Lautenberg Chemical Safety for the 21st Century Act. Congress.gov. https://www.congress.gov/bill/114th-congress/senate-bill/697/all-info. Accessed January 27, 2016.</ref>

===The Emergency Planning and Community Right-to-Know Act (EPCRA), 1986<ref> US Government Printing Office. Emergency Panning and Community Right to Know. https://www.gpo.gov/fdsys/pkg/USCODE-2010-title42/html/USCODE-2010-title42-chap116.htm. Accessed February 13, 2016</ref>===
The Act was passed by Congress in response to concerns regarding the environmental and safety hazards posed by the storage and handling of toxic chemicals. This legislation was developed as a result of the 1984 chemical disaster in Bhopal, India.<ref>What is EPCRA? EPA. http://www.epa.gov/epcra/what-epcra. Published November 10, 2015. Accessed January 27, 2016.</ref>
# All facilities manufacturing, processing, or storing hazardous chemicals must make plans for major incidents if they were to occur and the plans must be made public so that local communities can be properly informed.
# All facilities must also produce Material Safety Data Sheets (MSDSs) to state and local officials as well as local fire departments.
# Local governments should help prepare emergency plans and review the plans annually.
# State governments are required to oversee and coordinate local planning efforts.
# Emergency Notification: Facilities must immediately report accidental releases of chemicals and "hazardous substances" in quantities greater than Reportable Quantities (RQs) defined under the Comprehensive Environmental Response, Compensation, and Liability Act to state and local officials with this information then being available to the public.
# Annually, facilities must complete and submit a toxic chemical release inventory form.

===Clean Air Act Amendments of 1990===
This Amendment to the original Clean Air Act of 1970 was designed to curb three major threats to the nation's environment and health of citizens: acid rain, urban air pollution, and toxic air emissions.<ref>Environmental Protection Agency. "1990 Clean Air Act Amendment Summary" http://www.epa.gov/clean-air-act-overview/1990-clean-air-act-amendment-summary. Accessed February 13, 2016.</ref>
# Established the U.S. Chemical Safety and Hazard Investigation Board, and independent federal agency with the goal of ensuring worker and public safety through the prevention or minimization of the effects of chemical accidents. <ref>Seider WD, Seader JD, Lewin DR, Seider WD. Product And Process Design Principles: Synthesis, Analysis, and Evaluation. New York: Wiley; 2004.</ref>
# Attempt to determine the roots and contributing causes of accidents and then provide briefs on the accidents.
# Led to the development of cap and trade systems for air pollutants.
# Gave the government significantly more power to control air emissions and administer admissions permits.

==Additional Legislative Information==
For more details on environmental legislation pertaining to release of materials to the environment or the regulations of the loss of containment, see [[Environmental concerns]].

In addition to the above federal regulations, various states and other municipalities also have enacted legislation for the regulation of chemical plants. These include more specific safety items, such as local fire codes or even put into place stricter aspects of the federal regulations. <ref name="Towler" />

Any process design or plant design must always meet the requirements of local and federal mandates and regulations. Without doing so, the wellbeing of plant employees and even the public can be placed into serious jeopardy.

=Safety Organizations and Terminology=
==Organizations==
===OSHA===
The Occupational Safety and Health Administration (OSHA) is a federal agency that focuses on the enforcement of safety and health legislation.

===EPA===
The Environmental Protection Agency (EPA) is a U.S. agency whose purpose is the protection of the health of both humans and the environment through the writing and enforcement of regulatory laws.

===DOT===
The Department of Transportation (DOT) oversees federal highway, air, and maritime transportation, and can be involved in the safe transport of chemicals.

===DOE===
The Department of Energy (DOE) is a governmental department tasked with the advancement of energy technology in the United States.

==Terminology==
===HS&E===
Health, Safety, and Environmental - This term refers to all health, safety, and environmental concerns that arise at each stage of the design process. Companies are required to analyze each part of the process from an HS&E perspective to create a safe and healthy work environment.

===MSDS===
Material Safety and Data Sheet - Every chemical has an MSDS which contains all the information regarding safe handling and how to deal with spills or other accidents involving the substance. Relevant information includes how to identify the substance, hazard information, and how to handle spills, fires, and exposure, among other things.

===FMEA===
Failure Mode and Effects Analysis - FMEA is an early stage approach to identifying critical technical risks using a semi-quantitative procedure. The analysis encompasses safety, environmental, and operational feasibility. When performing FMEA, engineers look to see places in a potential design that could fail, and then quantify how likely that failure is, how severe the results would be, and then offer potential solutions to minimize the risk. A step-by-step guide to performing FMEA is shown below (Northwestern University, 2014):

# Brainstorm for failure modes
# For each FM, rate severity of impact (SEV, 1 - 10).
# For each FM, brainstorm for possible causes (there may be multiple).
# For each cause, rate likelihood of occurring (OCC, 1 - 10).
# Rate the probability that the systems currently in place will detect and prevent the problem before it has an impact (DET, 10 - 1). Do not assume that something that will be added to the design later will take care of the problem.
# Overall Risk Probability Number RPN = SEV x OCC x DET. Most practitioners use the 1,4,7,10 scale below to increase granularity. Note that the DET scale is inverse to SEV and OCC.

[[File:Safety3.JPG|center|700px]]

Figure 1: Suggested scale to be used for quantifying risk and detection of failures in FMEA. Taken from ChE 351 Slides.

The sum of all information collected is implemented into a spreadsheet like the one shown below:

[[File:Safety4.JPG|center|700px]]

Figure 2: Example spreadsheet used to organize FMEA data.

===HAZOP===
Hazard and Operability Study - For more information regarding HAZOP, please refer to [[Process Hazards]].

===SIL===
Safety Integrity Levels - The SIL is the relative level of risk-reduction provided by a safety function, or to specify a target level of risk reduction. A SIL is determined based on a number of qualitative factors such as development process and safety life cycle management. Several methods are used such as risk matrices, risk graphs, layers of protective analysis (LOPA). Three levels of safety integrity are assigned depending on the “availability” of the safety instrumented system (SIS), as shown below (Towler et al., 2012):

[[File:Safety5.JPG|center|500px]]

Figure 3: Table of safety integrity levels based on availability of system. Taken from (Towler et al., 2012).

Redundant system means instrumentation is duplicated; higher level of redundancy of trip systems give higher SIL. The required SIL should be determined during a process hazard analysis and depends on risk of operator exposure and injury.

1 Instrument
* if it signals, plant goes down
* Probability of incident = probability of instrument failure
* Probability of spurious trip = false positive rate of one instrument

2 Instruments
* 1 out of 2 voting (1oo2): one instrument signals, plant goes down
** Probability of incident reduced by duplication
** Probability of spurious trip doubled
* 2oo2 voting
** Probability of incident worse than single instrument (twice likelihood that system is down)
** Probability of spurious trip reduced

3 Instruments
* 2oo3 voting
** Best overall trade-off between reducing incident rate and spurious trip rate
** One malfunctioning instrument does not cause trip or prevent detection of real incident

=Safe Design=
==Inherently Safe Design==
Inherently safe design of a particular process can be achieved by adhering to the following six strategies (Turton et al., 2003):

1. Substitution: Avoid using or producing hazardous materials on the plant site. If the hazardous material is an intermediate product, for example, alternate chemical reaction pathways might be used. In other words, the most inherently safe strategy is to avoid the use of hazardous materials.

2. Intensification: Attempt to use less of the hazardous materials. In terms of a hazardous intermediate, the two processes could be more closely coupled, reducing or eliminating the amount of intermediate produced. The inventories of hazardous feeds or products can be reduced by enhanced scheduling techniques such as just-in-time (JIT) manufacturing.

3. Attenuation: Reducing, or attenuating, the hazards of materials can often be affected by lowering the temperature or adding stabilizing additives. By using materials under less hazardous conditions, the potential consequences of a leak can be reduced.

4. Containment: If the hazardous materials cannot be eliminated, they at least should be stored in vessels with mechanical integrity beyond any reasonably expected temperature or pressure excursion. This is an old but effective strategy to avoid leaks. However, it is not as inherently safe as substitution, intensification, or attenuation.

5. Control: If a leak of hazardous material does occur, there should be safety systems that reduce the effects. For example, chemical facilities often have emergency isolation of the site from the normal storm sewers, and large tanks for flammable liquids are surrounded by dikes that prevent any leaks from spreading to to other areas of the plant. Scrubbing systems and relief systems in general are in this category. They are essential, because they allow controlled, safe release of hazardous materials, rather than an uncontrolled release from a vessel rupture.

6. Survival: If leaks of hazardous materials do occur and they are not contained or controlled, the personnel (and the equipment) must be protected. This lowest level of the hierarchy includes fire fighting, gas masks, and so on. Although essential to the total safety of the plant, the greater the reliance on survival of leaks rather than elimination of leaks, the less inherently safe the facility.

===Safety Legislation and Process Design===
====Inherent Process Plant Safety====
Through the enactment of safety regulation shown above, process design has inherently become safer. These regulations have made safety the first and foremost important concern when designing a new chemical plant. In general, the safety of a process relies on multiple layers of protection, but the first and most important layer of protection has become the process design feature. Greater tolerances are built into the designing of processes to more effectively prevent catastrophic failures or chemical leakage. The best approach to prevent accidents is to add process design features, involving chemistry and physics, to prevent hazardous situations.
=====Examples of Inherently Safe Process Design<ref>Crowl, Daniel. Louver, Joseph. Chemical Process Safety: Fundamentals with Applications. IBN: 9780132440554</ref>=====
#Vapor Release: Vapor released from spills can be minimized by designing dikes so that flammable and toxic materials will not accumulate around leaking tanks. This also prevents potential flammable materials from building up and causing an explosion.
#Containment Building: Design can be important for the containment of toxic spills. With the addition of automatic or remote controls, personnel can leave the area if a spill or breach occurs, while the area can be continuously monitored.
#Solvent Substitution: Safety through Substitution - Substituting design with safer, less hazardous materials. Designing process for use with less toxic or flammable solvents. For example, water-based paints and adhesives as well as aqueous or dry flowable formulations for agricultural chemicals opposed to more volatile solvents that release VOCs.
#Design for Lower Temperature and Pressure Safety through Moderation - use a hazardous material under less hazardous conditions such as lower pressure and temperature conditions. This can lower the level of catastrophe if downstream safety processes do fail.

====Control Systems Safety====
Legislation has also added to subsequent layers of controls such as environmental process controls have been added to prevent release to nearby air and water systems, which would endanger surrounding ecosystems and human populations.

==Assessing Preliminary Design==
While pilot plants are necessary to design effective plant equipment, there are some dangers associated with the industrial scale of chemical plants. Scaling up without accurate literature and experimental data can be very dangerous.

In every step of process design, the Health, Safety, and Environmental (HS&E) analysis must be carried out with the available technical information (Biegler et al., 1997).

[[File:Safety1.JPG|center|800px]]

[[File:Safety2.JPG|center|700px]]

Figure 4: General steps in the design process and analysis to be carried out at each stage. Taken from (Biegler et al., 1997).

=Economic Cost of Safety=
Price of Safety installations and fire protection systems range from .5 to 1.6 % of fixed-capital investment of a plant; but expenditures are often much higher than this and it is difficult to estimate these expenditures for a given plant

In addition, designers also examine the economic impact of safety and maintenance issues. For instance, they may determine that the plant reactor configuration can be improved, and with improved operator training facilities, it can run with improved safety. These hidden costs must be considered when determining the economic feasibility of operation (Biegler et al., 1997).

==Benefits of Inherent Safety Over Conventional Safety==
The outdated method of implementing safety into process design came very late in the design process. The physical entities in the process or the process itself was changed very little and the conventional safety methods included implementing controls. Inherent process safety is instead developed very early in the design process and can lead to very significant cost savings overall. The comparison comes when looking at two ratios for safety costs, the Conventional Safety Cost Index (CSCI) <math>CSCI=\frac{C_{conventional safety}}{C_{loss}}</math> and the Inherent Safety Cost Index (ISCI) <math>ISCI=\frac{C_{inherent safety}}{C_{loss}}</math>. The values of safety cost are easy to determine as they are any additive change to a standard design. The difficulty comes with calculating the cost of losses as this must be an additive loss associated of assets, production, environmental cleanup, and potential human health losses.

=Other Process Safety Considerations=
The safety and well-being of the consumers using the eventual product should be considered in process safety. The risks involved in using a product should be clearly communicated to the consumer by industrial leaders.

Human Error is another safety risk that is difficult to quantify. The intervention of well-trained operators is a vital layer in process safety (Peters et al., 2003).

=Case Study: Production of Ultrapure Hydrogen=
[[File:steam reforming.jpg|thumb|alt=A hydrogen production plant with a box steam reformer|A hydrogen production plant with a box steam reformer|300px|right|A hydrogen production plant with a box steam reformer]]
To walk through the process safety considerations when designing a chemical plant, the relatively simple example of a high purity hydrogen generation will be examined. First, various technologies are researched to determine the options that best meet the economic, physical, environmental, and safety constraints of the project. For this project, steam reforming, autothermal reforming, and partial oxidation were investigated. Technologies that lack widespread implementation have inherent safety risks as they have higher uncertainties associated with reliability, feasibility, and cost. Autothermal reforming requires oxygen and not a commercially popular method. Partial oxidation requires no catalyst, but requires high process temperatures and is a complex process to implement. After each process was analyzed and scored in a decision matrix, steam reforming was chosen as the base process technology. It was selected because it is a safe, well-known, low-emission, traditional process in use all over the world for the production of hydrogen. Steam reforming produces minimal waste compared to the alternative processes, and is capable of producing the necessary 100 MMscfd of 99.999% hydrogen gas. It is important to remember that both local and global environmental emissions are capable of harming the general public, so they should be considered safety concerns in the same way worker hazards are. Next, various reactor types, catalysts, and separation methods were evaluated with the base process chosen. In all, seven different processing stages were assessed including the initial heating of feed, the steam reformation reactor, the high and low temperature water shift reactors, the amine plant, the methanation reactor, the gas compression and cooling train, and the pressure swing adsorption unit.

Although worker safety is always the first priority in a plant, there are inherent risk associated with high temperature and pressure processes. Close attention was paid to make the preliminary plant design as inherently safe as possible. Preliminary FMEA and HAZOP analyses were conducted to identify the highest priority risks. Hazards were mitigated by substituting less hazardous materials when possible, opting to store only the necessary hazardous materials on site, lowering temperatures when possible, adding catastrophic failure controls, maximizing plant control automation when economically feasible, and necessitating worker personal protective equipment. Finally, FMEA and HAZAP analyses were repeated. These steps were repeated multiple times until a sufficient reduction in risk had been achieved.

=Case Study: Bhopal Disaster=
[[File:Bhopal.JPG|thumb|alt=The dispersal of the deadly cloud of gas from the Union Carbide plant in the Indian city of Bhopal|The dispersal of the deadly cloud of gas from the Union Carbide plant in the Indian city of Bhopal|400px|right|The dispersal of the deadly cloud of gas from the Union Carbide plant in the Indian city of Bhopal]]
On December 3, 1984 at the India Limited Pesticide Plant owned by Union Carbide in Bhopal, India, water entered a storage vessel containing over 80,000 lbs. of methyl isocyanate (MIC), a chemical intermediate in the pesticide synthesis process. This reaction caused a rapid increase in temperature accompanied by boiling, which caused toxic MIC vapors to escape from the tank. In addition, the MIC-water reaction produced methylamine and carbon dioxide gases among other toxic products which also contributed to the pressure increase (Union Carbide Corporation, 1967). These vapors passed into a scrubber and flare system that were not working at the time due to inadequate maintenance and safety practices. As a result of this accident, approximately 25 tons of MIC vapor were released, killing over 3,800 immediately and injuring roughly 20,000 in the surrounding area.

As a result of the incident, Union Carbide was forced to pay $470 million, as well as fund a hospital in Bhopal that was used specifically to treat victims of the disaster. Cleanup of the plant site and other legal action are still being determined to this day. Bhopal sparked a worldwide discussion on chemical process safety, and caused Congress to create the U.S. Chemical Safety Board (CSB). The CSB has since cited the following reasons as causes for the disaster:

* No process hazard analysis
* Poorly maintained equipment and safety system
* Lack of emergency response planning
* Inadequate training for operators

The CSB has pushed chemical safety reform since its conception, urging the chemical industry to produce inherently safer designs, use better quality equipment, and develop more thorough risk management plans. A major criticism of the process was its lack of inherent safe design. Because MIC was an intermediate, there was no reason to keep large quantities in storage. A modern design would use the intermediate as it is made (Eckerman et al., 2005).

Although chemical process safety has come a long way since 1984, industrial chemical giants still battle problems similar to Bhopal until this day. In 2008, a disaster similar to the one in Bhopal could have occurred in a plant originally designed by Union Carbide located in Institute, West Virginia after a runaway reaction caused a pressure buildup in a waste treatment vessel. The vessel exploded, killing two plant workers. Fortunately, the explosion missed a large MIC storage vessel which could have been hit by shrapnel and released tons of MIC (Blanc et al., 2009) In 2013, an ammonium nitrate explosion killed 15 and seriously injured 200 in West Texas in a blast radius similar to the one experienced in Bhopal (U.S.Chemical Safety Board, 2014).

=Case Study: Deepwater Horizon Explosion and Oil Spill=
[[File:Deepwater Horizon offshore drilling unit on fire 2010.jpg|thumb|alt=The burning Deepwater Horizon oilrig after an explosion caused by a design failure in the blowout preventer|The burning Deepwater Horizon oilrig after an explosion caused by a design failure in the blowout preventer|400px|right|The burning Deepwater Horizon oilrig after an explosion caused by a design failure in the blowout preventer]]
On April 20, 2010 on the Deepwater Horizon offshore drilling rig located in the Macondo Prospect, multiple explosions killed 11 workers and seriously injured 17. The rig burned for two days before sinking into the Gulf of Mexico. Key safety failures caused the well to spew 5 million barrels of oil into the Gulf of Mexico over the next 87 days making the incident the largest offshore oil spill in U.S. history. Finally, the well was sealed by a “static kill,” the injection of heavy fluids and cement, at the leak point 5,000 feet below the surface.

The key safety failure identified by the CSB was the blowout preventer (BOP) failure. This device that is meant to prevent the filling of annular space between the borehole and the well casing is both electrically and hydraulically powered. It is connected to a rig by a large diameter pipe called a riser. The system contains multiple pipe rams and annular preventers designed to prevent annular space buildup.

On the first night of the incident, a “kick” occurred and a mixture of oil, water, and gases began to build up in the wall and climb up the shaft. Drilling mud was injected to prevent kicks by creating a barrier. An upper annular preventer was also engaged when the buildup was discovered, but it failed. A pipe ram was activated and succeeded. However, an immense pressure buildup caused the drill pipe to buckle so it was forced off center. This buckling was later explained as a result of effective compression. This phenomenon is caused by microscopic irregularities and bends in the pipe material resulting in a higher surface area on one side of the pipe. Because the pipe was off-center, the final failsafe, the Automatic Mode Function (AMF) or deadman could not effectively shear the pipe and seal the well. This redundant control system comprised of a yellow pod and blue pod work independently to seal the well in the event of catastrophic failure when communications, electric power, and hydraulic pressure connections are cut. Both the yellow and blue pods contained 9 volt and 27 volt batteries which power solenoid valves. Unfortunately, the blue pod was miswired, so its 27 volt power supply was drained when it was to cause the blind shear blades to cut the pipe. Fortunately, a 9 volt battery in the yellow pod was also miswired which caused the blind shear ram to be engaged. However, this only partially sealed the well because of the pipe buckling. The flammable mixture erupted onto the surface of the platform and found an ignition source triggering a massive explosion. The spill was temporarily contained by a cap, and relief wells were eventually used to seal the well months later (U.S.Chemical Safety Board, 2014).

The White House Office of Energy and Climate Change Policy called the Deepwater Horizon oil spill the “worst environmental disaster the US has faced (BBC News, 2010). Over 8,000 species were estimated to be affected by the spill due to the toxicity of petroleum released, oxygen depletion, and the large quantities of Corexit, an oil dispersant used in an untested manner that is toxic to marine life (Biello et al., 2010; Butler, 2011; Froomkin, 2010).

BP, Transocean, and Halliburton were the major entities implicated in this tragedy. Investigations after the incident show that essential safety documentation including risk management and emergency procedure information were missing. Accusations were mainly aimed at BP with charges of recklessness and gross negligence (CNN Money, 2012). In January 2013, Transocean was ordered to pay $1.4 billion for US Clean Water Act violations. BP was ordered to pay $2.4 billion, but additional penalties could reach $20 billion (Department of Justice Office of Public Affairs, 2013).

=Case Study: Texas City Refinery Explosion=
[[File:BP_PLANT_EXPLOSION-1_lowres2.jpg|thumb|alt=Emergency response workers fight secondary fires caused by the isomerization unit explosion at the Texas City refinery|Emergency response workers fight secondary fires caused by the isomerization unit explosion at the Texas City refinery|228px|right|Emergency response workers fight secondary fires caused by the isomerization unit explosion at the Texas City refinery]]
On March 23, 2005 at a BP refinery in Texas City, Texas a hydrocarbon vapor cloud ignited, killing 15 workers and seriously injuring 170 others. Over the course an 11 hour period, a combination of control failures, mismanagement, and worker fatigue resulted in the buildup and release of extremely hot, combustible vapor. The key process unit in this disaster was an isomerization unit, located next to wooden trailers for workers servicing an ultracracker unit.

In the early morning on March 23rd, operators initiated startup and pumped raffinate (liquid hydrocarbons) into a raffinate splitter tower used to separate gasoline components. A liquid level indicator and multiple high level alarms monitored the tower liquid level. The level indicator could only measure up to 9 feet of liquid, and the written process called for a liquid level of about 6.5 feet. However, operators routinely filled the tower over 9 feet to minimize fluctuations and to prevent damage to a furnace. Hours later, the first high level alarm was activated and the liquid level rose, but a second alarm higher up the tower failed to trigger. The feed was halted when the liquid had risen to a level of about 13 feet, operators had no way of knowing the exact height. The lead operator relayed the startup activities to another operator and left the facility an hour before his shift ended. The morning operator arrived at 6 am to start his thirtieth consecutive day working a 12-hour shift and read a logbook that read, “Isom* Brought in some raff to unit, to pack raff with.” The day shift operator arrived an hour late, so he could not be briefed by the night shift supervisor. Recirculation then commenced in the tower, and more liquid was added to the tower. Additionally, conflicting instructions caused a liquid level regulating valve to remain closed for several hours, so liquid could not leave the tower. The furnace was then lit, and the supervisor left to attend a family medical emergency.

At noon, the liquid level had risen to 98 feet, but the improperly calibrated liquid level indicator read 8.4 feet. At 12:41 pm, a high pressure alarm caused workers to manually open a chain valve to relieve pressure by using the units pressure relief system to vent vapor into the atmosphere using an obsolete blowdown drum. Heat was also reduced in the furnace to reduce pressure. When operators became concerned about outflow rate, the liquid level regulating valve was opened to release liquid from the tower to storage. This caused the liquid in the tower to begin to boil and spill into the overhead vapor line exerting extreme pressures on the pressure relief system. At 1:14 pm, the three relief valves opened sending the liquid to the blowdown drum which overflowed into a municipal sewer setting off alarms, but a key level indicator in the blowdown drum failed. Flammable liquid erupted from the blowdown drum, formed a massive vapor cloud, and found an ignition source from a nearby idling pickup truck. The colossal blast ignited fires throughout the refinery and over half the workers in the wooden trailers adjacent to the unit were killed immediately (U.S.Chemical Safety Board, 2008).

Investigations after the incident cited multiple failures to implement safety recommendations at the Texas City Refinery. Among these, the blowdown drum was to be replaced by a modern flare to burn off hydrocarbons. However, BP’s budget cuts prevented its replacement. The training and treatment of workers was also called into question, as fatigue, poor communication, and inadequate documentation likely contributed to the disaster. Decisions like the one to operate an unsafe liquid level in order to prevent furnace damage also demonstrate the company’s fixation on the bottom line. BP was eventually fined $21 million by OSHA (New York Times, 2010; U.S.Chemical Safety Board, 2008).

=References=
==Direct Citation References==
<references/>

==Additional References==
# American Institute of Chemical Engineers. http://www.aiche.org/ccps
# Biello, David (9 June 2010). "The BP Spill's Growing Toll On the Sea Life of the Gulf". Yale Environment 360. Yale School of Forestry & Environmental Studies. Retrieved 2010-06-14.
# Blanc, P. Bhopal, 1984 – West Virginia near-miss, 2008. Psychology Today, December 2009. https://www.psychologytoday.com/blog/household-hazards/200912/bhopal-1984-west-virginia-near-miss-2008.
# Butler, J. Steven (3 March 2011). "BP Macondo Well Incident. U.S. Gulf of Mexico. Pollution Containment and Remediation Efforts" (PDF). Lillehammer Energy Claims Conference. BDO Consulting. Retrieved 17 February 2013.
# “DOJ accuses BP of ‘gross negligence’ in Gulf oil spill” CNN Money, September 2012.
# Eckerman, I. The Bhopal Saga: Causes and Consequences of the World’s Largest Industrial Disaster. Universities Press Private Limited, 2005.
# Froomkin, Dan (29 July 2010). "Scientists Find Evidence That Oil And Dispersant Mix Is Making Its Way Into The Foodchain". Huffington Post.
# "Gulf of Mexico oil leak 'worst US environment disaster'". BBC News. 30 May 2010.
# Investigation Report: BP Refinery Explosion and Fire, U.S. Chemical Safety Board, 2008.
# Lyall, Sarah. "In BP’s Record, a History of Boldness and Costly Blunders." New York Times, July 13, 2010.
# L.T. Biegler, I.E. Grossmann, A.W. Westerberg. ''Systematic Methods of Chemical Process Design''. Prentice-Hall: Upper Saddle River, 1997.
# M.S. Peters, K.D. Timmerhaus. ''Plant Design and Economics for Chemical Engineers, 5th Ed.'' McGraw-Hill: New York, 2003.
# Northwestern University. Chemical Engineering 351 Lecture Slides.
# Reflections on Bhopal After Thirty Years Video, U.S.Chemical Safety Board, December 2014.
# R.T. Turton, R.C. Bailie, W.B. Whiting, J.A. Shaeiwitz. ''Analysis, Synthesis, and Design of Chemical Processes''. Prentice Hall: Upper Saddle River, 2003.
# Union Carbide Corporation "Methyl Isocyanate" Product Information Publication, F-41443, November 1967.
# USCSB Deepwater Horizon Video, U.S.Chemical Safety Board, June 2014.
# "Transocean Agrees to Plead Guilty to Environmental Crime and Enter Civil Settlement to Resolve U.S. Clean Water Act Penalty Claims from Deepwater Horizon Incident". Department of Justice Office of Public Affairs. January 3, 2013.

Process safety

2016-02-13T21:51:47Z

Evanrosati: /* Benefits of Inherent Safety Over Conventional Safety */

Authors: Anne Disabato, [2014] Tim Hanrahan, [2014] Brian Merkle, [2014] , Spencer Saldana [2015] and Evan Rosati [2016] 

Stewards: David Chen, Jian Gong, and Fengqi You

Date Presented: January 19, 2014

 


__TOC__

=Introduction=
The safe design and operation of facilities is of paramount importance to every company that is involved in the manufacture of fuels, chemicals, and pharmaceuticals. Process safety focuses on the prevention of dangerous situations, such as fires, explosions, and the release of chemicals.

The American Institute of Chemical Engineers emphasizes a culture of process safety through four pillars (American Institute of Chemical Engineers, 2015):

1. Commitment to Process Safety: a workforce that is actively involved and an organization that fully supports process safety as a core value will tend to do the right things in the right way at the right time – even when no one else is looking

2. Understanding Hazard and Risk: the foundation of a risk-based approach which will allow an organization to use this information to allocate limited resources in the most effective manner

3. Manage Risk: the ongoing execution of risk based process safety tasks. Risk management can help a company to better deal with the resultant risks and sustain long-term accident free and profitable operations

4. Learn from Experience: Metrics provide direct feedback on the workings of RBPS systems, and leading indicators provide early warning signals of ineffective process safety results. Organizations must use their mistakes and those of others as motivation for action and view as opportunities for improvement.

For the prevention and management of specific safety hazards, such as fires, explosions, or the release of toxic chemicals, please see [[Process Hazards]].

==Layers of Plant Safety==
Safety and loss prevention can be expressed in "layers" of plant safety in terms of design and implementation.<ref name="Towler">G.P. Towler, R. Sinnott. ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design''. Elsevier, 2012.</ref> Each higher layer can be activated if a lower level fails. This creates a system with subsequent levels of safety to help prevent catastrophe from occurring. This diagram shows the important of safety in process design. If a process is designed to be inherently safe, additional safety "controls" will be less important and a chemical plant will be overall safer. The goal of safety is not to reach the top of the triangle, but to stay as close to the bottom as possible. This shows the importance of inherently safe design and safety legislation and regulations to provide guidelines for safety and health concerns. Regulations provide a baseline for engineers to operate when designing a chemical plant. They have brought safety to the forefront of design, when engineers have the maximum degree of freedom for implementation, and are no longer considered to be an afterthought or strictly a controls issue. Plants can be designed to be safe without the extensive use of future adaptation, safety controls, or emergency response. Although you can never eliminate these upper layers of process safety, by designing a process smartly and safely, engineers can reduce the consequences of "walking up" the process safety ladder or triangle.
[[File:Layers_of_Plant_Safety.png|center|700px|thumb|alt=Triangle Diagram on Increasing Plant Safety Mechanisms|Layers of Plant Safety Triangle Diagram]]

=Safety Legislation=

==International Regulations==
International regulations can be considered as best practice standards that are adopted by governments through treaties and establishment through United Nation resolutions. Most international regulation agencies can only register complaints for violations, but can not implement sanctions or fines to infracting parties.

===International Labour Organization (ILO)===
After the dissolution of the League of Nations in 1946, the ILO became the first specialized agency of the United Nations upon its founding in December of that same year. <ref> ISO Photo Gallery. http://www.ilo.org/dyn/media/mediasearch.fiche?p_id=16023&p_lang=en. Accessed February 13, 2016.</ref> The ILO attempts to develop labor standards for workers in all industries. The unique tripartite structure of the ILO gives an equal voice to workers, employers and governments to ensure that the views of the social partners are closely reflected in labour standards and in shaping policies and programs. <ref> ISO "How the ISO Works" http://www.ilo.org/global/about-the-ilo/how-the-ilo-works/lang--en/index.htm. Accessed February 13, 2016.</ref> The ISO has established International Labour Standards on Occupational Safety and Health, which has developed more than 40 standards specifically dealing with occupational heath and safety.<ref> ILO. "International Labour Standards on Occupational Safety and Health". http://www.ilo.org/global/standards/subjects-covered-by-international-labour-standards/occupational-safety-and-health/lang--en/index.htm. Accessed February 13, 2016.</ref>. The important conventions are listed below:

====Occupational Safety and Health Convention, 1981 and Protocol of 2002====
#The convention provides for the adoption of national occupational safety and health policy by each participating nation-state. It includes actions taken by governments and within enterprise that operate within the governmental spaces to promote the improvement of occupational safety and health to therefore improve working conditions.
#The additional Protocol in 2002 calls for the establishment and period review of procedures for recording and notification of occupational accidents along with the publication of related statistics associated with the accidents. This Protocol is similar to the Emergency Planning and CommunityRight-To-Know Act instituted in the United States (see below).

====Occupational Heath Services Convention, 1985====
#Establishment of enterprise-level occupational health services which are entrusted with preventative functions and which are responsible for advising the employer, the workers, and their representatives in the organization on maintaining a safe and healthy work environment.

====Promotional Framework for Occupational Safety and Heath Convention, 2006====
#Aims to promote a preventative safety and health culture and to progressively achieve a safe and healthy working environment for all.
#Requires ratifying nation-states to develop, in consultation with the most representative organizations of employees and workers, including workers unions, a national policy, a national system, and a national program on occupational health and safety.
#This should be developed in accordance to the Occupational Heath Services Convention, 1985 and take into account other ILO standards.
#National systems shall provide the infrastructure for implementing national policy and programs, such as domestic laws, regulations, authority bodies, and compliance mechanisms.

====Chemicals Convention, 1990====
#Provides for the adoption and implementation of a coherent policy on the safety in the use of chemicals at work, including production, handling, storage, and transport.
#Also includes best practices for the disposal and treatment of waste chemicals and the release of chemicals resulting from work activities, maintenance, and cleaning of equipment.
#In addition, it allocates specific responsibilities to suppliers and exporting states.
#Chemicals shall be evaluated to determine their level of hazards and employers shall make these hazards known to their employees.

===International Organization for Standardization (ISO)===
The ISO is an international organization for standardization on topics ranging from quality management, environmental management, social responsibility, risk management, etc. ISO standards are adopted world-wide by organizations and governments as accepted and known standards for production. Many of the practices are self-regulating as costumers often demand certain standards from companies they purchase from.<ref>ISO. Home Page. http://www.iso.org/iso/home.html. Accessed February 13, 2016.</ref> The ISO is developing its own occupational health and safety framework shown below.

====ISO 45001 Occupational Health and Safety Management Systems<ref>ISO 45001. "Occupational heath and safety". http://www.iso.org/iso/home/standards/management-standards/iso45001.htm. Accessed February 13, 2016.</ref>====
#Designed to help organizations reduce the burden of occupational injuries and diseases by providing a framework to improve employee safety, reduce workplace risks, and create better, safer working conditions.
#Currently under development by a committee of occupational health and safety experts who will follow management systems approaches of ISO 4001 and ISO 9001.
#Will embody other International Standards such as the ILO's Occupational Health and Safety Guidelines.
#The expected publication will be released in October, 2016.

==US Safety Regulations==
Over time chemical industry regulations have been developed to ensure that the best safety practices are followed to maintain the health of both people and the environment. The development of most regulations is based around the idea that organizations have both a legal and moral obligation to safeguard the health and welfare of its employees and the public.<ref name="Towler" /> The extent of legislation varies across regions around the globe. In the United States, chemical accidents have led to the creation of regulation boards and safety oriented societies such as the Center for Chemical Plant Safety of the American Institute of Chemical Engineers to aid in the development and implementation of pant safety. <ref>Seider WD, Seader JD, Lewin DR, Seider WD. Product And Process Design Principles: Synthesis, Analysis, and Evaluation. New York: Wiley; 2004.</ref> In the US, the major federal laws relating to chemical plant safety and their regulations are as follows:

===The Occupational Safety and Health Act, 1970===
The OSH Act is administer by the Occupational Safety and Health Administration (OSHA) (see below). The Act covers all employers and their employees in the United States with coverage provided either directly by the Federal Occupational Safety and Heath Administration or by an OSHA-approved state job safety and health plan.<ref> United States Department of Labor. "Safety and Health Standards: Occupational Safety and Health". http://webapps.dol.gov/elaws/elg/osha.htm </ref> The main provisions of the act are listed below.
# Employers must supply place of employment free from toxic chemicals, excessive noise mechanical dangers, or unsanitary conditions.
#This involves the implementation of engineering controls to limit exposure to hazards and toxic substances and implementing administrative controls.
# For employees, employers must provide personal protective equipment (PPE) and training, including communications of known hazards.
# The Occupational Safety and Health Administration (OSHA) was established to promote best practices, inspect facilities for hazard analysis, set standards, and enforce the law.
# The National Institute of Occupational Safety and Health (NIOSH) was established to be an independent research institute (now under the Centers for Disease Control).
#The Act also encourages states to develop and operate their own job safety and health programs with OSHA as a monitoring agency for these "state plans," which operate under the authority of state law. The standards developed by state plans need to be at least as effective as the federal regulations.<ref> United States Department of Labor. "Safety and Health Standards: Occupational Safety and Health". http://webapps.dol.gov/elaws/elg/osha.htm </ref>
#Federal OSHA standards are categorized into four categories: General Industry, Construction, Maritime Terminals, Long-shoring, and Agriculture.

===The Toxic Substances Control Act (TSCA), 1976===
The main qualifications of the TSCA were to provide the EPA with regulating power of chemicals, specifically pertaining to the chemical industry and not including foods, drugs, cosmetics, or pesticides. <ref> Environmental Protection Agency. "Summary of the Toxic Substances Control Act." http://www.epa.gov/laws-regulations/summary-toxic-substances-control-act. Accessed February 13, 2016</ref>
# The Environmental Protection Agency (EPA) is required to regulate 75,000 chemical substances used in industry.
# The EPA has jurisdiction over the safety of the sale or development of new chemicals in the United States, including the requirement for pre-manufacture notification for new chemical substances before manufacture.
#The TSCA also addresses the production, importation, use, and disposal of specific chemicals including polychlorinated biphenyls (PCBs), asbestos, radon, and lead-based paint.<ref> Environmental Protection Agency. "Summary of the Toxic Substances Control Act." http://www.epa.gov/laws-regulations/summary-toxic-substances-control-act. Accessed February 13, 2016</ref>
# Everyone has the right and obligation to report information about any health or environmental effects caused by a chemical. This is especially important for organizations as they are required to report information to the EPA if a chemical substance is found to have substantial risk of injury to health or the environment.
====Frank R. Lautenberg Chemical Safety for the 21st Century Act, 2015====
The Toxic Substances Control Act will be revised in 2015 under this new act proposed to Congress. This Act is an attempt to eliminate some of the flaws associated with the TSCA.
#One of the biggest flaws in the TSCA is the fact that a new chemical can be used without first demonstrating safety, the idea that chemicals are safe until proven unsafe.<ref>Sheppard, Kate. "Senators Introduce Bill to Overhaul U.S. Chemical Industry." http://www.huffingtonpost.com/2015/03/10/toxic-chemicals-senate-bill_n_6842524.html. Accessed February 13, 2016</ref>
#The new bill would require safety testing before chemical implementation.
#It gives the EPA more defined power on the regulation of chemical substances and the Sustainable Chemical Program will be established. This brings the TSCA into the modern era of the chemical industry.<ref>S.697 - 114th Congress (2015-2016): Frank R. Lautenberg Chemical Safety for the 21st Century Act. Congress.gov. https://www.congress.gov/bill/114th-congress/senate-bill/697/all-info. Accessed January 27, 2016.</ref>

===The Emergency Planning and Community Right-to-Know Act (EPCRA), 1986<ref> US Government Printing Office. Emergency Panning and Community Right to Know. https://www.gpo.gov/fdsys/pkg/USCODE-2010-title42/html/USCODE-2010-title42-chap116.htm. Accessed February 13, 2016</ref>===
The Act was passed by Congress in response to concerns regarding the environmental and safety hazards posed by the storage and handling of toxic chemicals. This legislation was developed as a result of the 1984 chemical disaster in Bhopal, India.<ref>What is EPCRA? EPA. http://www.epa.gov/epcra/what-epcra. Published November 10, 2015. Accessed January 27, 2016.</ref>
# All facilities manufacturing, processing, or storing hazardous chemicals must make plans for major incidents if they were to occur and the plans must be made public so that local communities can be properly informed.
# All facilities must also produce Material Safety Data Sheets (MSDSs) to state and local officials as well as local fire departments.
# Local governments should help prepare emergency plans and review the plans annually.
# State governments are required to oversee and coordinate local planning efforts.
# Emergency Notification: Facilities must immediately report accidental releases of chemicals and "hazardous substances" in quantities greater than Reportable Quantities (RQs) defined under the Comprehensive Environmental Response, Compensation, and Liability Act to state and local officials with this information then being available to the public.
# Annually, facilities must complete and submit a toxic chemical release inventory form.

===Clean Air Act Amendments of 1990===
This Amendment to the original Clean Air Act of 1970 was designed to curb three major threats to the nation's environment and health of citizens: acid rain, urban air pollution, and toxic air emissions.<ref>Environmental Protection Agency. "1990 Clean Air Act Amendment Summary" http://www.epa.gov/clean-air-act-overview/1990-clean-air-act-amendment-summary. Accessed February 13, 2016.</ref>
# Established the U.S. Chemical Safety and Hazard Investigation Board, and independent federal agency with the goal of ensuring worker and public safety through the prevention or minimization of the effects of chemical accidents. <ref>Seider WD, Seader JD, Lewin DR, Seider WD. Product And Process Design Principles: Synthesis, Analysis, and Evaluation. New York: Wiley; 2004.</ref>
# Attempt to determine the roots and contributing causes of accidents and then provide briefs on the accidents.
# Led to the development of cap and trade systems for air pollutants.
# Gave the government significantly more power to control air emissions and administer admissions permits.

==Additional Legislative Information==
For more details on environmental legislation pertaining to release of materials to the environment or the regulations of the loss of containment, see [[Environmental concerns]].

In addition to the above federal regulations, various states and other municipalities also have enacted legislation for the regulation of chemical plants. These include more specific safety items, such as local fire codes or even put into place stricter aspects of the federal regulations. <ref name="Towler" />

Any process design or plant design must always meet the requirements of local and federal mandates and regulations. Without doing so, the wellbeing of plant employees and even the public can be placed into serious jeopardy.

=Safety Organizations and Terminology=
==Organizations==
===OSHA===
The Occupational Safety and Health Administration (OSHA) is a federal agency that focuses on the enforcement of safety and health legislation.

===EPA===
The Environmental Protection Agency (EPA) is a U.S. agency whose purpose is the protection of the health of both humans and the environment through the writing and enforcement of regulatory laws.

===DOT===
The Department of Transportation (DOT) oversees federal highway, air, and maritime transportation, and can be involved in the safe transport of chemicals.

===DOE===
The Department of Energy (DOE) is a governmental department tasked with the advancement of energy technology in the United States.

==Terminology==
===HS&E===
Health, Safety, and Environmental - This term refers to all health, safety, and environmental concerns that arise at each stage of the design process. Companies are required to analyze each part of the process from an HS&E perspective to create a safe and healthy work environment.

===MSDS===
Material Safety and Data Sheet - Every chemical has an MSDS which contains all the information regarding safe handling and how to deal with spills or other accidents involving the substance. Relevant information includes how to identify the substance, hazard information, and how to handle spills, fires, and exposure, among other things.

===FMEA===
Failure Mode and Effects Analysis - FMEA is an early stage approach to identifying critical technical risks using a semi-quantitative procedure. The analysis encompasses safety, environmental, and operational feasibility. When performing FMEA, engineers look to see places in a potential design that could fail, and then quantify how likely that failure is, how severe the results would be, and then offer potential solutions to minimize the risk. A step-by-step guide to performing FMEA is shown below (Northwestern University, 2014):

# Brainstorm for failure modes
# For each FM, rate severity of impact (SEV, 1 - 10).
# For each FM, brainstorm for possible causes (there may be multiple).
# For each cause, rate likelihood of occurring (OCC, 1 - 10).
# Rate the probability that the systems currently in place will detect and prevent the problem before it has an impact (DET, 10 - 1). Do not assume that something that will be added to the design later will take care of the problem.
# Overall Risk Probability Number RPN = SEV x OCC x DET. Most practitioners use the 1,4,7,10 scale below to increase granularity. Note that the DET scale is inverse to SEV and OCC.

[[File:Safety3.JPG|center|700px]]

Figure 1: Suggested scale to be used for quantifying risk and detection of failures in FMEA. Taken from ChE 351 Slides.

The sum of all information collected is implemented into a spreadsheet like the one shown below:

[[File:Safety4.JPG|center|700px]]

Figure 2: Example spreadsheet used to organize FMEA data.

===HAZOP===
Hazard and Operability Study - For more information regarding HAZOP, please refer to [[Process Hazards]].

===SIL===
Safety Integrity Levels - The SIL is the relative level of risk-reduction provided by a safety function, or to specify a target level of risk reduction. A SIL is determined based on a number of qualitative factors such as development process and safety life cycle management. Several methods are used such as risk matrices, risk graphs, layers of protective analysis (LOPA). Three levels of safety integrity are assigned depending on the “availability” of the safety instrumented system (SIS), as shown below (Towler et al., 2012):

[[File:Safety5.JPG|center|500px]]

Figure 3: Table of safety integrity levels based on availability of system. Taken from (Towler et al., 2012).

Redundant system means instrumentation is duplicated; higher level of redundancy of trip systems give higher SIL. The required SIL should be determined during a process hazard analysis and depends on risk of operator exposure and injury.

1 Instrument
* if it signals, plant goes down
* Probability of incident = probability of instrument failure
* Probability of spurious trip = false positive rate of one instrument

2 Instruments
* 1 out of 2 voting (1oo2): one instrument signals, plant goes down
** Probability of incident reduced by duplication
** Probability of spurious trip doubled
* 2oo2 voting
** Probability of incident worse than single instrument (twice likelihood that system is down)
** Probability of spurious trip reduced

3 Instruments
* 2oo3 voting
** Best overall trade-off between reducing incident rate and spurious trip rate
** One malfunctioning instrument does not cause trip or prevent detection of real incident

=Safe Design=
==Inherently Safe Design==
Inherently safe design of a particular process can be achieved by adhering to the following six strategies (Turton et al., 2003):

1. Substitution: Avoid using or producing hazardous materials on the plant site. If the hazardous material is an intermediate product, for example, alternate chemical reaction pathways might be used. In other words, the most inherently safe strategy is to avoid the use of hazardous materials.

2. Intensification: Attempt to use less of the hazardous materials. In terms of a hazardous intermediate, the two processes could be more closely coupled, reducing or eliminating the amount of intermediate produced. The inventories of hazardous feeds or products can be reduced by enhanced scheduling techniques such as just-in-time (JIT) manufacturing.

3. Attenuation: Reducing, or attenuating, the hazards of materials can often be affected by lowering the temperature or adding stabilizing additives. By using materials under less hazardous conditions, the potential consequences of a leak can be reduced.

4. Containment: If the hazardous materials cannot be eliminated, they at least should be stored in vessels with mechanical integrity beyond any reasonably expected temperature or pressure excursion. This is an old but effective strategy to avoid leaks. However, it is not as inherently safe as substitution, intensification, or attenuation.

5. Control: If a leak of hazardous material does occur, there should be safety systems that reduce the effects. For example, chemical facilities often have emergency isolation of the site from the normal storm sewers, and large tanks for flammable liquids are surrounded by dikes that prevent any leaks from spreading to to other areas of the plant. Scrubbing systems and relief systems in general are in this category. They are essential, because they allow controlled, safe release of hazardous materials, rather than an uncontrolled release from a vessel rupture.

6. Survival: If leaks of hazardous materials do occur and they are not contained or controlled, the personnel (and the equipment) must be protected. This lowest level of the hierarchy includes fire fighting, gas masks, and so on. Although essential to the total safety of the plant, the greater the reliance on survival of leaks rather than elimination of leaks, the less inherently safe the facility.

===Safety Legislation and Process Design===
====Inherent Process Plant Safety====
Through the enactment of safety regulation shown above, process design has inherently become safer. These regulations have made safety the first and foremost important concern when designing a new chemical plant. In general, the safety of a process relies on multiple layers of protection, but the first and most important layer of protection has become the process design feature. Greater tolerances are built into the designing of processes to more effectively prevent catastrophic failures or chemical leakage. The best approach to prevent accidents is to add process design features, involving chemistry and physics, to prevent hazardous situations.
=====Examples of Inherently Safe Process Design<ref>Crowl, Daniel. Louver, Joseph. Chemical Process Safety: Fundamentals with Applications. IBN: 9780132440554</ref>=====
#Vapor Release: Vapor released from spills can be minimized by designing dikes so that flammable and toxic materials will not accumulate around leaking tanks. This also prevents potential flammable materials from building up and causing an explosion.
#Containment Building: Design can be important for the containment of toxic spills. With the addition of automatic or remote controls, personnel can leave the area if a spill or breach occurs, while the area can be continuously monitored.
#Solvent Substitution: Safety through Substitution - Substituting design with safer, less hazardous materials. Designing process for use with less toxic or flammable solvents. For example, water-based paints and adhesives as well as aqueous or dry flowable formulations for agricultural chemicals opposed to more volatile solvents that release VOCs.
#Design for Lower Temperature and Pressure Safety through Moderation - use a hazardous material under less hazardous conditions such as lower pressure and temperature conditions. This can lower the level of catastrophe if downstream safety processes do fail.

====Control Systems Safety====
Legislation has also added to subsequent layers of controls such as environmental process controls have been added to prevent release to nearby air and water systems, which would endanger surrounding ecosystems and human populations.

==Assessing Preliminary Design==
While pilot plants are necessary to design effective plant equipment, there are some dangers associated with the industrial scale of chemical plants. Scaling up without accurate literature and experimental data can be very dangerous.

In every step of process design, the Health, Safety, and Environmental (HS&E) analysis must be carried out with the available technical information (Biegler et al., 1997).

[[File:Safety1.JPG|center|800px]]

[[File:Safety2.JPG|center|700px]]

Figure 4: General steps in the design process and analysis to be carried out at each stage. Taken from (Biegler et al., 1997).

=Economic Cost of Safety=
Price of Safety installations and fire protection systems range from .5 to 1.6 % of fixed-capital investment of a plant; but expenditures are often much higher than this and it is difficult to estimate these expenditures for a given plant

In addition, designers also examine the economic impact of safety and maintenance issues. For instance, they may determine that the plant reactor configuration can be improved, and with improved operator training facilities, it can run with improved safety. These hidden costs must be considered when determining the economic feasibility of operation (Biegler et al., 1997).

==Benefits of Inherent Safety Over Conventional Safety==
The old practice method of implementing safety into process design came very late in the design process. The physical entities in the process or the process itself was changed very little and the conventional safety methods included implementing controls. Inherent process safety is instead developed very early in the design process and can lead to very significant cost savings overall. The comparison comes when looking at two ratios for safety costs, the Conventional Safety Cost Index (CSCI) <math>CSCI=\frac{C_{conventional safety}}{C_{loss}}</math> and the Inherent Safety Cost Index (ISCI) <math>ISCI=\frac{C_{inherent safety}}{C_{loss}}</math>. The values of safety cost are easy to determine as they are any additive change to a standard design. The difficulty comes with calculating the cost of losses as this must be an additive loss associated of assets, production, environmental cleanup, and potential human health losses.

=Other Process Safety Considerations=
The safety and well-being of the consumers using the eventual product should be considered in process safety. The risks involved in using a product should be clearly communicated to the consumer by industrial leaders.

Human Error is another safety risk that is difficult to quantify. The intervention of well-trained operators is a vital layer in process safety (Peters et al., 2003).

=Case Study: Production of Ultrapure Hydrogen=
[[File:steam reforming.jpg|thumb|alt=A hydrogen production plant with a box steam reformer|A hydrogen production plant with a box steam reformer|300px|right|A hydrogen production plant with a box steam reformer]]
To walk through the process safety considerations when designing a chemical plant, the relatively simple example of a high purity hydrogen generation will be examined. First, various technologies are researched to determine the options that best meet the economic, physical, environmental, and safety constraints of the project. For this project, steam reforming, autothermal reforming, and partial oxidation were investigated. Technologies that lack widespread implementation have inherent safety risks as they have higher uncertainties associated with reliability, feasibility, and cost. Autothermal reforming requires oxygen and not a commercially popular method. Partial oxidation requires no catalyst, but requires high process temperatures and is a complex process to implement. After each process was analyzed and scored in a decision matrix, steam reforming was chosen as the base process technology. It was selected because it is a safe, well-known, low-emission, traditional process in use all over the world for the production of hydrogen. Steam reforming produces minimal waste compared to the alternative processes, and is capable of producing the necessary 100 MMscfd of 99.999% hydrogen gas. It is important to remember that both local and global environmental emissions are capable of harming the general public, so they should be considered safety concerns in the same way worker hazards are. Next, various reactor types, catalysts, and separation methods were evaluated with the base process chosen. In all, seven different processing stages were assessed including the initial heating of feed, the steam reformation reactor, the high and low temperature water shift reactors, the amine plant, the methanation reactor, the gas compression and cooling train, and the pressure swing adsorption unit.

Although worker safety is always the first priority in a plant, there are inherent risk associated with high temperature and pressure processes. Close attention was paid to make the preliminary plant design as inherently safe as possible. Preliminary FMEA and HAZOP analyses were conducted to identify the highest priority risks. Hazards were mitigated by substituting less hazardous materials when possible, opting to store only the necessary hazardous materials on site, lowering temperatures when possible, adding catastrophic failure controls, maximizing plant control automation when economically feasible, and necessitating worker personal protective equipment. Finally, FMEA and HAZAP analyses were repeated. These steps were repeated multiple times until a sufficient reduction in risk had been achieved.

=Case Study: Bhopal Disaster=
[[File:Bhopal.JPG|thumb|alt=The dispersal of the deadly cloud of gas from the Union Carbide plant in the Indian city of Bhopal|The dispersal of the deadly cloud of gas from the Union Carbide plant in the Indian city of Bhopal|400px|right|The dispersal of the deadly cloud of gas from the Union Carbide plant in the Indian city of Bhopal]]
On December 3, 1984 at the India Limited Pesticide Plant owned by Union Carbide in Bhopal, India, water entered a storage vessel containing over 80,000 lbs. of methyl isocyanate (MIC), a chemical intermediate in the pesticide synthesis process. This reaction caused a rapid increase in temperature accompanied by boiling, which caused toxic MIC vapors to escape from the tank. In addition, the MIC-water reaction produced methylamine and carbon dioxide gases among other toxic products which also contributed to the pressure increase (Union Carbide Corporation, 1967). These vapors passed into a scrubber and flare system that were not working at the time due to inadequate maintenance and safety practices. As a result of this accident, approximately 25 tons of MIC vapor were released, killing over 3,800 immediately and injuring roughly 20,000 in the surrounding area.

As a result of the incident, Union Carbide was forced to pay $470 million, as well as fund a hospital in Bhopal that was used specifically to treat victims of the disaster. Cleanup of the plant site and other legal action are still being determined to this day. Bhopal sparked a worldwide discussion on chemical process safety, and caused Congress to create the U.S. Chemical Safety Board (CSB). The CSB has since cited the following reasons as causes for the disaster:

* No process hazard analysis
* Poorly maintained equipment and safety system
* Lack of emergency response planning
* Inadequate training for operators

The CSB has pushed chemical safety reform since its conception, urging the chemical industry to produce inherently safer designs, use better quality equipment, and develop more thorough risk management plans. A major criticism of the process was its lack of inherent safe design. Because MIC was an intermediate, there was no reason to keep large quantities in storage. A modern design would use the intermediate as it is made (Eckerman et al., 2005).

Although chemical process safety has come a long way since 1984, industrial chemical giants still battle problems similar to Bhopal until this day. In 2008, a disaster similar to the one in Bhopal could have occurred in a plant originally designed by Union Carbide located in Institute, West Virginia after a runaway reaction caused a pressure buildup in a waste treatment vessel. The vessel exploded, killing two plant workers. Fortunately, the explosion missed a large MIC storage vessel which could have been hit by shrapnel and released tons of MIC (Blanc et al., 2009) In 2013, an ammonium nitrate explosion killed 15 and seriously injured 200 in West Texas in a blast radius similar to the one experienced in Bhopal (U.S.Chemical Safety Board, 2014).

=Case Study: Deepwater Horizon Explosion and Oil Spill=
[[File:Deepwater Horizon offshore drilling unit on fire 2010.jpg|thumb|alt=The burning Deepwater Horizon oilrig after an explosion caused by a design failure in the blowout preventer|The burning Deepwater Horizon oilrig after an explosion caused by a design failure in the blowout preventer|400px|right|The burning Deepwater Horizon oilrig after an explosion caused by a design failure in the blowout preventer]]
On April 20, 2010 on the Deepwater Horizon offshore drilling rig located in the Macondo Prospect, multiple explosions killed 11 workers and seriously injured 17. The rig burned for two days before sinking into the Gulf of Mexico. Key safety failures caused the well to spew 5 million barrels of oil into the Gulf of Mexico over the next 87 days making the incident the largest offshore oil spill in U.S. history. Finally, the well was sealed by a “static kill,” the injection of heavy fluids and cement, at the leak point 5,000 feet below the surface.

The key safety failure identified by the CSB was the blowout preventer (BOP) failure. This device that is meant to prevent the filling of annular space between the borehole and the well casing is both electrically and hydraulically powered. It is connected to a rig by a large diameter pipe called a riser. The system contains multiple pipe rams and annular preventers designed to prevent annular space buildup.

On the first night of the incident, a “kick” occurred and a mixture of oil, water, and gases began to build up in the wall and climb up the shaft. Drilling mud was injected to prevent kicks by creating a barrier. An upper annular preventer was also engaged when the buildup was discovered, but it failed. A pipe ram was activated and succeeded. However, an immense pressure buildup caused the drill pipe to buckle so it was forced off center. This buckling was later explained as a result of effective compression. This phenomenon is caused by microscopic irregularities and bends in the pipe material resulting in a higher surface area on one side of the pipe. Because the pipe was off-center, the final failsafe, the Automatic Mode Function (AMF) or deadman could not effectively shear the pipe and seal the well. This redundant control system comprised of a yellow pod and blue pod work independently to seal the well in the event of catastrophic failure when communications, electric power, and hydraulic pressure connections are cut. Both the yellow and blue pods contained 9 volt and 27 volt batteries which power solenoid valves. Unfortunately, the blue pod was miswired, so its 27 volt power supply was drained when it was to cause the blind shear blades to cut the pipe. Fortunately, a 9 volt battery in the yellow pod was also miswired which caused the blind shear ram to be engaged. However, this only partially sealed the well because of the pipe buckling. The flammable mixture erupted onto the surface of the platform and found an ignition source triggering a massive explosion. The spill was temporarily contained by a cap, and relief wells were eventually used to seal the well months later (U.S.Chemical Safety Board, 2014).

The White House Office of Energy and Climate Change Policy called the Deepwater Horizon oil spill the “worst environmental disaster the US has faced (BBC News, 2010). Over 8,000 species were estimated to be affected by the spill due to the toxicity of petroleum released, oxygen depletion, and the large quantities of Corexit, an oil dispersant used in an untested manner that is toxic to marine life (Biello et al., 2010; Butler, 2011; Froomkin, 2010).

BP, Transocean, and Halliburton were the major entities implicated in this tragedy. Investigations after the incident show that essential safety documentation including risk management and emergency procedure information were missing. Accusations were mainly aimed at BP with charges of recklessness and gross negligence (CNN Money, 2012). In January 2013, Transocean was ordered to pay $1.4 billion for US Clean Water Act violations. BP was ordered to pay $2.4 billion, but additional penalties could reach $20 billion (Department of Justice Office of Public Affairs, 2013).

=Case Study: Texas City Refinery Explosion=
[[File:BP_PLANT_EXPLOSION-1_lowres2.jpg|thumb|alt=Emergency response workers fight secondary fires caused by the isomerization unit explosion at the Texas City refinery|Emergency response workers fight secondary fires caused by the isomerization unit explosion at the Texas City refinery|228px|right|Emergency response workers fight secondary fires caused by the isomerization unit explosion at the Texas City refinery]]
On March 23, 2005 at a BP refinery in Texas City, Texas a hydrocarbon vapor cloud ignited, killing 15 workers and seriously injuring 170 others. Over the course an 11 hour period, a combination of control failures, mismanagement, and worker fatigue resulted in the buildup and release of extremely hot, combustible vapor. The key process unit in this disaster was an isomerization unit, located next to wooden trailers for workers servicing an ultracracker unit.

In the early morning on March 23rd, operators initiated startup and pumped raffinate (liquid hydrocarbons) into a raffinate splitter tower used to separate gasoline components. A liquid level indicator and multiple high level alarms monitored the tower liquid level. The level indicator could only measure up to 9 feet of liquid, and the written process called for a liquid level of about 6.5 feet. However, operators routinely filled the tower over 9 feet to minimize fluctuations and to prevent damage to a furnace. Hours later, the first high level alarm was activated and the liquid level rose, but a second alarm higher up the tower failed to trigger. The feed was halted when the liquid had risen to a level of about 13 feet, operators had no way of knowing the exact height. The lead operator relayed the startup activities to another operator and left the facility an hour before his shift ended. The morning operator arrived at 6 am to start his thirtieth consecutive day working a 12-hour shift and read a logbook that read, “Isom* Brought in some raff to unit, to pack raff with.” The day shift operator arrived an hour late, so he could not be briefed by the night shift supervisor. Recirculation then commenced in the tower, and more liquid was added to the tower. Additionally, conflicting instructions caused a liquid level regulating valve to remain closed for several hours, so liquid could not leave the tower. The furnace was then lit, and the supervisor left to attend a family medical emergency.

At noon, the liquid level had risen to 98 feet, but the improperly calibrated liquid level indicator read 8.4 feet. At 12:41 pm, a high pressure alarm caused workers to manually open a chain valve to relieve pressure by using the units pressure relief system to vent vapor into the atmosphere using an obsolete blowdown drum. Heat was also reduced in the furnace to reduce pressure. When operators became concerned about outflow rate, the liquid level regulating valve was opened to release liquid from the tower to storage. This caused the liquid in the tower to begin to boil and spill into the overhead vapor line exerting extreme pressures on the pressure relief system. At 1:14 pm, the three relief valves opened sending the liquid to the blowdown drum which overflowed into a municipal sewer setting off alarms, but a key level indicator in the blowdown drum failed. Flammable liquid erupted from the blowdown drum, formed a massive vapor cloud, and found an ignition source from a nearby idling pickup truck. The colossal blast ignited fires throughout the refinery and over half the workers in the wooden trailers adjacent to the unit were killed immediately (U.S.Chemical Safety Board, 2008).

Investigations after the incident cited multiple failures to implement safety recommendations at the Texas City Refinery. Among these, the blowdown drum was to be replaced by a modern flare to burn off hydrocarbons. However, BP’s budget cuts prevented its replacement. The training and treatment of workers was also called into question, as fatigue, poor communication, and inadequate documentation likely contributed to the disaster. Decisions like the one to operate an unsafe liquid level in order to prevent furnace damage also demonstrate the company’s fixation on the bottom line. BP was eventually fined $21 million by OSHA (New York Times, 2010; U.S.Chemical Safety Board, 2008).

=References=
==Direct Citation References==
<references/>

==Additional References==
# American Institute of Chemical Engineers. http://www.aiche.org/ccps
# Biello, David (9 June 2010). "The BP Spill's Growing Toll On the Sea Life of the Gulf". Yale Environment 360. Yale School of Forestry & Environmental Studies. Retrieved 2010-06-14.
# Blanc, P. Bhopal, 1984 – West Virginia near-miss, 2008. Psychology Today, December 2009. https://www.psychologytoday.com/blog/household-hazards/200912/bhopal-1984-west-virginia-near-miss-2008.
# Butler, J. Steven (3 March 2011). "BP Macondo Well Incident. U.S. Gulf of Mexico. Pollution Containment and Remediation Efforts" (PDF). Lillehammer Energy Claims Conference. BDO Consulting. Retrieved 17 February 2013.
# “DOJ accuses BP of ‘gross negligence’ in Gulf oil spill” CNN Money, September 2012.
# Eckerman, I. The Bhopal Saga: Causes and Consequences of the World’s Largest Industrial Disaster. Universities Press Private Limited, 2005.
# Froomkin, Dan (29 July 2010). "Scientists Find Evidence That Oil And Dispersant Mix Is Making Its Way Into The Foodchain". Huffington Post.
# "Gulf of Mexico oil leak 'worst US environment disaster'". BBC News. 30 May 2010.
# Investigation Report: BP Refinery Explosion and Fire, U.S. Chemical Safety Board, 2008.
# Lyall, Sarah. "In BP’s Record, a History of Boldness and Costly Blunders." New York Times, July 13, 2010.
# L.T. Biegler, I.E. Grossmann, A.W. Westerberg. ''Systematic Methods of Chemical Process Design''. Prentice-Hall: Upper Saddle River, 1997.
# M.S. Peters, K.D. Timmerhaus. ''Plant Design and Economics for Chemical Engineers, 5th Ed.'' McGraw-Hill: New York, 2003.
# Northwestern University. Chemical Engineering 351 Lecture Slides.
# Reflections on Bhopal After Thirty Years Video, U.S.Chemical Safety Board, December 2014.
# R.T. Turton, R.C. Bailie, W.B. Whiting, J.A. Shaeiwitz. ''Analysis, Synthesis, and Design of Chemical Processes''. Prentice Hall: Upper Saddle River, 2003.
# Union Carbide Corporation "Methyl Isocyanate" Product Information Publication, F-41443, November 1967.
# USCSB Deepwater Horizon Video, U.S.Chemical Safety Board, June 2014.
# "Transocean Agrees to Plead Guilty to Environmental Crime and Enter Civil Settlement to Resolve U.S. Clean Water Act Penalty Claims from Deepwater Horizon Incident". Department of Justice Office of Public Affairs. January 3, 2013.

Process safety

2016-02-13T21:48:15Z

Evanrosati: /* Benefits of Inherent Safety Over Conventional Safety */

Authors: Anne Disabato, [2014] Tim Hanrahan, [2014] Brian Merkle, [2014] , Spencer Saldana [2015] and Evan Rosati [2016] 

Stewards: David Chen, Jian Gong, and Fengqi You

Date Presented: January 19, 2014

 


__TOC__

=Introduction=
The safe design and operation of facilities is of paramount importance to every company that is involved in the manufacture of fuels, chemicals, and pharmaceuticals. Process safety focuses on the prevention of dangerous situations, such as fires, explosions, and the release of chemicals.

The American Institute of Chemical Engineers emphasizes a culture of process safety through four pillars (American Institute of Chemical Engineers, 2015):

1. Commitment to Process Safety: a workforce that is actively involved and an organization that fully supports process safety as a core value will tend to do the right things in the right way at the right time – even when no one else is looking

2. Understanding Hazard and Risk: the foundation of a risk-based approach which will allow an organization to use this information to allocate limited resources in the most effective manner

3. Manage Risk: the ongoing execution of risk based process safety tasks. Risk management can help a company to better deal with the resultant risks and sustain long-term accident free and profitable operations

4. Learn from Experience: Metrics provide direct feedback on the workings of RBPS systems, and leading indicators provide early warning signals of ineffective process safety results. Organizations must use their mistakes and those of others as motivation for action and view as opportunities for improvement.

For the prevention and management of specific safety hazards, such as fires, explosions, or the release of toxic chemicals, please see [[Process Hazards]].

==Layers of Plant Safety==
Safety and loss prevention can be expressed in "layers" of plant safety in terms of design and implementation.<ref name="Towler">G.P. Towler, R. Sinnott. ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design''. Elsevier, 2012.</ref> Each higher layer can be activated if a lower level fails. This creates a system with subsequent levels of safety to help prevent catastrophe from occurring. This diagram shows the important of safety in process design. If a process is designed to be inherently safe, additional safety "controls" will be less important and a chemical plant will be overall safer. The goal of safety is not to reach the top of the triangle, but to stay as close to the bottom as possible. This shows the importance of inherently safe design and safety legislation and regulations to provide guidelines for safety and health concerns. Regulations provide a baseline for engineers to operate when designing a chemical plant. They have brought safety to the forefront of design, when engineers have the maximum degree of freedom for implementation, and are no longer considered to be an afterthought or strictly a controls issue. Plants can be designed to be safe without the extensive use of future adaptation, safety controls, or emergency response. Although you can never eliminate these upper layers of process safety, by designing a process smartly and safely, engineers can reduce the consequences of "walking up" the process safety ladder or triangle.
[[File:Layers_of_Plant_Safety.png|center|700px|thumb|alt=Triangle Diagram on Increasing Plant Safety Mechanisms|Layers of Plant Safety Triangle Diagram]]

=Safety Legislation=

==International Regulations==
International regulations can be considered as best practice standards that are adopted by governments through treaties and establishment through United Nation resolutions. Most international regulation agencies can only register complaints for violations, but can not implement sanctions or fines to infracting parties.

===International Labour Organization (ILO)===
After the dissolution of the League of Nations in 1946, the ILO became the first specialized agency of the United Nations upon its founding in December of that same year. <ref> ISO Photo Gallery. http://www.ilo.org/dyn/media/mediasearch.fiche?p_id=16023&p_lang=en. Accessed February 13, 2016.</ref> The ILO attempts to develop labor standards for workers in all industries. The unique tripartite structure of the ILO gives an equal voice to workers, employers and governments to ensure that the views of the social partners are closely reflected in labour standards and in shaping policies and programs. <ref> ISO "How the ISO Works" http://www.ilo.org/global/about-the-ilo/how-the-ilo-works/lang--en/index.htm. Accessed February 13, 2016.</ref> The ISO has established International Labour Standards on Occupational Safety and Health, which has developed more than 40 standards specifically dealing with occupational heath and safety.<ref> ILO. "International Labour Standards on Occupational Safety and Health". http://www.ilo.org/global/standards/subjects-covered-by-international-labour-standards/occupational-safety-and-health/lang--en/index.htm. Accessed February 13, 2016.</ref>. The important conventions are listed below:

====Occupational Safety and Health Convention, 1981 and Protocol of 2002====
#The convention provides for the adoption of national occupational safety and health policy by each participating nation-state. It includes actions taken by governments and within enterprise that operate within the governmental spaces to promote the improvement of occupational safety and health to therefore improve working conditions.
#The additional Protocol in 2002 calls for the establishment and period review of procedures for recording and notification of occupational accidents along with the publication of related statistics associated with the accidents. This Protocol is similar to the Emergency Planning and CommunityRight-To-Know Act instituted in the United States (see below).

====Occupational Heath Services Convention, 1985====
#Establishment of enterprise-level occupational health services which are entrusted with preventative functions and which are responsible for advising the employer, the workers, and their representatives in the organization on maintaining a safe and healthy work environment.

====Promotional Framework for Occupational Safety and Heath Convention, 2006====
#Aims to promote a preventative safety and health culture and to progressively achieve a safe and healthy working environment for all.
#Requires ratifying nation-states to develop, in consultation with the most representative organizations of employees and workers, including workers unions, a national policy, a national system, and a national program on occupational health and safety.
#This should be developed in accordance to the Occupational Heath Services Convention, 1985 and take into account other ILO standards.
#National systems shall provide the infrastructure for implementing national policy and programs, such as domestic laws, regulations, authority bodies, and compliance mechanisms.

====Chemicals Convention, 1990====
#Provides for the adoption and implementation of a coherent policy on the safety in the use of chemicals at work, including production, handling, storage, and transport.
#Also includes best practices for the disposal and treatment of waste chemicals and the release of chemicals resulting from work activities, maintenance, and cleaning of equipment.
#In addition, it allocates specific responsibilities to suppliers and exporting states.
#Chemicals shall be evaluated to determine their level of hazards and employers shall make these hazards known to their employees.

===International Organization for Standardization (ISO)===
The ISO is an international organization for standardization on topics ranging from quality management, environmental management, social responsibility, risk management, etc. ISO standards are adopted world-wide by organizations and governments as accepted and known standards for production. Many of the practices are self-regulating as costumers often demand certain standards from companies they purchase from.<ref>ISO. Home Page. http://www.iso.org/iso/home.html. Accessed February 13, 2016.</ref> The ISO is developing its own occupational health and safety framework shown below.

====ISO 45001 Occupational Health and Safety Management Systems<ref>ISO 45001. "Occupational heath and safety". http://www.iso.org/iso/home/standards/management-standards/iso45001.htm. Accessed February 13, 2016.</ref>====
#Designed to help organizations reduce the burden of occupational injuries and diseases by providing a framework to improve employee safety, reduce workplace risks, and create better, safer working conditions.
#Currently under development by a committee of occupational health and safety experts who will follow management systems approaches of ISO 4001 and ISO 9001.
#Will embody other International Standards such as the ILO's Occupational Health and Safety Guidelines.
#The expected publication will be released in October, 2016.

==US Safety Regulations==
Over time chemical industry regulations have been developed to ensure that the best safety practices are followed to maintain the health of both people and the environment. The development of most regulations is based around the idea that organizations have both a legal and moral obligation to safeguard the health and welfare of its employees and the public.<ref name="Towler" /> The extent of legislation varies across regions around the globe. In the United States, chemical accidents have led to the creation of regulation boards and safety oriented societies such as the Center for Chemical Plant Safety of the American Institute of Chemical Engineers to aid in the development and implementation of pant safety. <ref>Seider WD, Seader JD, Lewin DR, Seider WD. Product And Process Design Principles: Synthesis, Analysis, and Evaluation. New York: Wiley; 2004.</ref> In the US, the major federal laws relating to chemical plant safety and their regulations are as follows:

===The Occupational Safety and Health Act, 1970===
The OSH Act is administer by the Occupational Safety and Health Administration (OSHA) (see below). The Act covers all employers and their employees in the United States with coverage provided either directly by the Federal Occupational Safety and Heath Administration or by an OSHA-approved state job safety and health plan.<ref> United States Department of Labor. "Safety and Health Standards: Occupational Safety and Health". http://webapps.dol.gov/elaws/elg/osha.htm </ref> The main provisions of the act are listed below.
# Employers must supply place of employment free from toxic chemicals, excessive noise mechanical dangers, or unsanitary conditions.
#This involves the implementation of engineering controls to limit exposure to hazards and toxic substances and implementing administrative controls.
# For employees, employers must provide personal protective equipment (PPE) and training, including communications of known hazards.
# The Occupational Safety and Health Administration (OSHA) was established to promote best practices, inspect facilities for hazard analysis, set standards, and enforce the law.
# The National Institute of Occupational Safety and Health (NIOSH) was established to be an independent research institute (now under the Centers for Disease Control).
#The Act also encourages states to develop and operate their own job safety and health programs with OSHA as a monitoring agency for these "state plans," which operate under the authority of state law. The standards developed by state plans need to be at least as effective as the federal regulations.<ref> United States Department of Labor. "Safety and Health Standards: Occupational Safety and Health". http://webapps.dol.gov/elaws/elg/osha.htm </ref>
#Federal OSHA standards are categorized into four categories: General Industry, Construction, Maritime Terminals, Long-shoring, and Agriculture.

===The Toxic Substances Control Act (TSCA), 1976===
The main qualifications of the TSCA were to provide the EPA with regulating power of chemicals, specifically pertaining to the chemical industry and not including foods, drugs, cosmetics, or pesticides. <ref> Environmental Protection Agency. "Summary of the Toxic Substances Control Act." http://www.epa.gov/laws-regulations/summary-toxic-substances-control-act. Accessed February 13, 2016</ref>
# The Environmental Protection Agency (EPA) is required to regulate 75,000 chemical substances used in industry.
# The EPA has jurisdiction over the safety of the sale or development of new chemicals in the United States, including the requirement for pre-manufacture notification for new chemical substances before manufacture.
#The TSCA also addresses the production, importation, use, and disposal of specific chemicals including polychlorinated biphenyls (PCBs), asbestos, radon, and lead-based paint.<ref> Environmental Protection Agency. "Summary of the Toxic Substances Control Act." http://www.epa.gov/laws-regulations/summary-toxic-substances-control-act. Accessed February 13, 2016</ref>
# Everyone has the right and obligation to report information about any health or environmental effects caused by a chemical. This is especially important for organizations as they are required to report information to the EPA if a chemical substance is found to have substantial risk of injury to health or the environment.
====Frank R. Lautenberg Chemical Safety for the 21st Century Act, 2015====
The Toxic Substances Control Act will be revised in 2015 under this new act proposed to Congress. This Act is an attempt to eliminate some of the flaws associated with the TSCA.
#One of the biggest flaws in the TSCA is the fact that a new chemical can be used without first demonstrating safety, the idea that chemicals are safe until proven unsafe.<ref>Sheppard, Kate. "Senators Introduce Bill to Overhaul U.S. Chemical Industry." http://www.huffingtonpost.com/2015/03/10/toxic-chemicals-senate-bill_n_6842524.html. Accessed February 13, 2016</ref>
#The new bill would require safety testing before chemical implementation.
#It gives the EPA more defined power on the regulation of chemical substances and the Sustainable Chemical Program will be established. This brings the TSCA into the modern era of the chemical industry.<ref>S.697 - 114th Congress (2015-2016): Frank R. Lautenberg Chemical Safety for the 21st Century Act. Congress.gov. https://www.congress.gov/bill/114th-congress/senate-bill/697/all-info. Accessed January 27, 2016.</ref>

===The Emergency Planning and Community Right-to-Know Act (EPCRA), 1986<ref> US Government Printing Office. Emergency Panning and Community Right to Know. https://www.gpo.gov/fdsys/pkg/USCODE-2010-title42/html/USCODE-2010-title42-chap116.htm. Accessed February 13, 2016</ref>===
The Act was passed by Congress in response to concerns regarding the environmental and safety hazards posed by the storage and handling of toxic chemicals. This legislation was developed as a result of the 1984 chemical disaster in Bhopal, India.<ref>What is EPCRA? EPA. http://www.epa.gov/epcra/what-epcra. Published November 10, 2015. Accessed January 27, 2016.</ref>
# All facilities manufacturing, processing, or storing hazardous chemicals must make plans for major incidents if they were to occur and the plans must be made public so that local communities can be properly informed.
# All facilities must also produce Material Safety Data Sheets (MSDSs) to state and local officials as well as local fire departments.
# Local governments should help prepare emergency plans and review the plans annually.
# State governments are required to oversee and coordinate local planning efforts.
# Emergency Notification: Facilities must immediately report accidental releases of chemicals and "hazardous substances" in quantities greater than Reportable Quantities (RQs) defined under the Comprehensive Environmental Response, Compensation, and Liability Act to state and local officials with this information then being available to the public.
# Annually, facilities must complete and submit a toxic chemical release inventory form.

===Clean Air Act Amendments of 1990===
This Amendment to the original Clean Air Act of 1970 was designed to curb three major threats to the nation's environment and health of citizens: acid rain, urban air pollution, and toxic air emissions.<ref>Environmental Protection Agency. "1990 Clean Air Act Amendment Summary" http://www.epa.gov/clean-air-act-overview/1990-clean-air-act-amendment-summary. Accessed February 13, 2016.</ref>
# Established the U.S. Chemical Safety and Hazard Investigation Board, and independent federal agency with the goal of ensuring worker and public safety through the prevention or minimization of the effects of chemical accidents. <ref>Seider WD, Seader JD, Lewin DR, Seider WD. Product And Process Design Principles: Synthesis, Analysis, and Evaluation. New York: Wiley; 2004.</ref>
# Attempt to determine the roots and contributing causes of accidents and then provide briefs on the accidents.
# Led to the development of cap and trade systems for air pollutants.
# Gave the government significantly more power to control air emissions and administer admissions permits.

==Additional Legislative Information==
For more details on environmental legislation pertaining to release of materials to the environment or the regulations of the loss of containment, see [[Environmental concerns]].

In addition to the above federal regulations, various states and other municipalities also have enacted legislation for the regulation of chemical plants. These include more specific safety items, such as local fire codes or even put into place stricter aspects of the federal regulations. <ref name="Towler" />

Any process design or plant design must always meet the requirements of local and federal mandates and regulations. Without doing so, the wellbeing of plant employees and even the public can be placed into serious jeopardy.

=Safety Organizations and Terminology=
==Organizations==
===OSHA===
The Occupational Safety and Health Administration (OSHA) is a federal agency that focuses on the enforcement of safety and health legislation.

===EPA===
The Environmental Protection Agency (EPA) is a U.S. agency whose purpose is the protection of the health of both humans and the environment through the writing and enforcement of regulatory laws.

===DOT===
The Department of Transportation (DOT) oversees federal highway, air, and maritime transportation, and can be involved in the safe transport of chemicals.

===DOE===
The Department of Energy (DOE) is a governmental department tasked with the advancement of energy technology in the United States.

==Terminology==
===HS&E===
Health, Safety, and Environmental - This term refers to all health, safety, and environmental concerns that arise at each stage of the design process. Companies are required to analyze each part of the process from an HS&E perspective to create a safe and healthy work environment.

===MSDS===
Material Safety and Data Sheet - Every chemical has an MSDS which contains all the information regarding safe handling and how to deal with spills or other accidents involving the substance. Relevant information includes how to identify the substance, hazard information, and how to handle spills, fires, and exposure, among other things.

===FMEA===
Failure Mode and Effects Analysis - FMEA is an early stage approach to identifying critical technical risks using a semi-quantitative procedure. The analysis encompasses safety, environmental, and operational feasibility. When performing FMEA, engineers look to see places in a potential design that could fail, and then quantify how likely that failure is, how severe the results would be, and then offer potential solutions to minimize the risk. A step-by-step guide to performing FMEA is shown below (Northwestern University, 2014):

# Brainstorm for failure modes
# For each FM, rate severity of impact (SEV, 1 - 10).
# For each FM, brainstorm for possible causes (there may be multiple).
# For each cause, rate likelihood of occurring (OCC, 1 - 10).
# Rate the probability that the systems currently in place will detect and prevent the problem before it has an impact (DET, 10 - 1). Do not assume that something that will be added to the design later will take care of the problem.
# Overall Risk Probability Number RPN = SEV x OCC x DET. Most practitioners use the 1,4,7,10 scale below to increase granularity. Note that the DET scale is inverse to SEV and OCC.

[[File:Safety3.JPG|center|700px]]

Figure 1: Suggested scale to be used for quantifying risk and detection of failures in FMEA. Taken from ChE 351 Slides.

The sum of all information collected is implemented into a spreadsheet like the one shown below:

[[File:Safety4.JPG|center|700px]]

Figure 2: Example spreadsheet used to organize FMEA data.

===HAZOP===
Hazard and Operability Study - For more information regarding HAZOP, please refer to [[Process Hazards]].

===SIL===
Safety Integrity Levels - The SIL is the relative level of risk-reduction provided by a safety function, or to specify a target level of risk reduction. A SIL is determined based on a number of qualitative factors such as development process and safety life cycle management. Several methods are used such as risk matrices, risk graphs, layers of protective analysis (LOPA). Three levels of safety integrity are assigned depending on the “availability” of the safety instrumented system (SIS), as shown below (Towler et al., 2012):

[[File:Safety5.JPG|center|500px]]

Figure 3: Table of safety integrity levels based on availability of system. Taken from (Towler et al., 2012).

Redundant system means instrumentation is duplicated; higher level of redundancy of trip systems give higher SIL. The required SIL should be determined during a process hazard analysis and depends on risk of operator exposure and injury.

1 Instrument
* if it signals, plant goes down
* Probability of incident = probability of instrument failure
* Probability of spurious trip = false positive rate of one instrument

2 Instruments
* 1 out of 2 voting (1oo2): one instrument signals, plant goes down
** Probability of incident reduced by duplication
** Probability of spurious trip doubled
* 2oo2 voting
** Probability of incident worse than single instrument (twice likelihood that system is down)
** Probability of spurious trip reduced

3 Instruments
* 2oo3 voting
** Best overall trade-off between reducing incident rate and spurious trip rate
** One malfunctioning instrument does not cause trip or prevent detection of real incident

=Safe Design=
==Inherently Safe Design==
Inherently safe design of a particular process can be achieved by adhering to the following six strategies (Turton et al., 2003):

1. Substitution: Avoid using or producing hazardous materials on the plant site. If the hazardous material is an intermediate product, for example, alternate chemical reaction pathways might be used. In other words, the most inherently safe strategy is to avoid the use of hazardous materials.

2. Intensification: Attempt to use less of the hazardous materials. In terms of a hazardous intermediate, the two processes could be more closely coupled, reducing or eliminating the amount of intermediate produced. The inventories of hazardous feeds or products can be reduced by enhanced scheduling techniques such as just-in-time (JIT) manufacturing.

3. Attenuation: Reducing, or attenuating, the hazards of materials can often be affected by lowering the temperature or adding stabilizing additives. By using materials under less hazardous conditions, the potential consequences of a leak can be reduced.

4. Containment: If the hazardous materials cannot be eliminated, they at least should be stored in vessels with mechanical integrity beyond any reasonably expected temperature or pressure excursion. This is an old but effective strategy to avoid leaks. However, it is not as inherently safe as substitution, intensification, or attenuation.

5. Control: If a leak of hazardous material does occur, there should be safety systems that reduce the effects. For example, chemical facilities often have emergency isolation of the site from the normal storm sewers, and large tanks for flammable liquids are surrounded by dikes that prevent any leaks from spreading to to other areas of the plant. Scrubbing systems and relief systems in general are in this category. They are essential, because they allow controlled, safe release of hazardous materials, rather than an uncontrolled release from a vessel rupture.

6. Survival: If leaks of hazardous materials do occur and they are not contained or controlled, the personnel (and the equipment) must be protected. This lowest level of the hierarchy includes fire fighting, gas masks, and so on. Although essential to the total safety of the plant, the greater the reliance on survival of leaks rather than elimination of leaks, the less inherently safe the facility.

===Safety Legislation and Process Design===
====Inherent Process Plant Safety====
Through the enactment of safety regulation shown above, process design has inherently become safer. These regulations have made safety the first and foremost important concern when designing a new chemical plant. In general, the safety of a process relies on multiple layers of protection, but the first and most important layer of protection has become the process design feature. Greater tolerances are built into the designing of processes to more effectively prevent catastrophic failures or chemical leakage. The best approach to prevent accidents is to add process design features, involving chemistry and physics, to prevent hazardous situations.
=====Examples of Inherently Safe Process Design<ref>Crowl, Daniel. Louver, Joseph. Chemical Process Safety: Fundamentals with Applications. IBN: 9780132440554</ref>=====
#Vapor Release: Vapor released from spills can be minimized by designing dikes so that flammable and toxic materials will not accumulate around leaking tanks. This also prevents potential flammable materials from building up and causing an explosion.
#Containment Building: Design can be important for the containment of toxic spills. With the addition of automatic or remote controls, personnel can leave the area if a spill or breach occurs, while the area can be continuously monitored.
#Solvent Substitution: Safety through Substitution - Substituting design with safer, less hazardous materials. Designing process for use with less toxic or flammable solvents. For example, water-based paints and adhesives as well as aqueous or dry flowable formulations for agricultural chemicals opposed to more volatile solvents that release VOCs.
#Design for Lower Temperature and Pressure Safety through Moderation - use a hazardous material under less hazardous conditions such as lower pressure and temperature conditions. This can lower the level of catastrophe if downstream safety processes do fail.

====Control Systems Safety====
Legislation has also added to subsequent layers of controls such as environmental process controls have been added to prevent release to nearby air and water systems, which would endanger surrounding ecosystems and human populations.

==Assessing Preliminary Design==
While pilot plants are necessary to design effective plant equipment, there are some dangers associated with the industrial scale of chemical plants. Scaling up without accurate literature and experimental data can be very dangerous.

In every step of process design, the Health, Safety, and Environmental (HS&E) analysis must be carried out with the available technical information (Biegler et al., 1997).

[[File:Safety1.JPG|center|800px]]

[[File:Safety2.JPG|center|700px]]

Figure 4: General steps in the design process and analysis to be carried out at each stage. Taken from (Biegler et al., 1997).

=Economic Cost of Safety=
Price of Safety installations and fire protection systems range from .5 to 1.6 % of fixed-capital investment of a plant; but expenditures are often much higher than this and it is difficult to estimate these expenditures for a given plant

In addition, designers also examine the economic impact of safety and maintenance issues. For instance, they may determine that the plant reactor configuration can be improved, and with improved operator training facilities, it can run with improved safety. These hidden costs must be considered when determining the economic feasibility of operation (Biegler et al., 1997).

==Benefits of Inherent Safety Over Conventional Safety==
The old practice method of implementing safety into process design came very late in the design process. The physical entities in the process or the process itself was changed very little and the conventional safety methods included implementing controls. Inherent process safety is instead developed very early in the design process and can lead to very significant cost savings overall. The comparison comes when looking at two ratios for safety costs, the Conventional Safety Cost Index (CSCI) <math>CSCI=\frac{C_{conventional safety}}{C_{loss}}</math> and the Inherent Safety Cost Index (ISCI) <math>ISCI=\frac{C_{inherent safety}}{C_{loss}}</math>.

=Other Process Safety Considerations=
The safety and well-being of the consumers using the eventual product should be considered in process safety. The risks involved in using a product should be clearly communicated to the consumer by industrial leaders.

Human Error is another safety risk that is difficult to quantify. The intervention of well-trained operators is a vital layer in process safety (Peters et al., 2003).

=Case Study: Production of Ultrapure Hydrogen=
[[File:steam reforming.jpg|thumb|alt=A hydrogen production plant with a box steam reformer|A hydrogen production plant with a box steam reformer|300px|right|A hydrogen production plant with a box steam reformer]]
To walk through the process safety considerations when designing a chemical plant, the relatively simple example of a high purity hydrogen generation will be examined. First, various technologies are researched to determine the options that best meet the economic, physical, environmental, and safety constraints of the project. For this project, steam reforming, autothermal reforming, and partial oxidation were investigated. Technologies that lack widespread implementation have inherent safety risks as they have higher uncertainties associated with reliability, feasibility, and cost. Autothermal reforming requires oxygen and not a commercially popular method. Partial oxidation requires no catalyst, but requires high process temperatures and is a complex process to implement. After each process was analyzed and scored in a decision matrix, steam reforming was chosen as the base process technology. It was selected because it is a safe, well-known, low-emission, traditional process in use all over the world for the production of hydrogen. Steam reforming produces minimal waste compared to the alternative processes, and is capable of producing the necessary 100 MMscfd of 99.999% hydrogen gas. It is important to remember that both local and global environmental emissions are capable of harming the general public, so they should be considered safety concerns in the same way worker hazards are. Next, various reactor types, catalysts, and separation methods were evaluated with the base process chosen. In all, seven different processing stages were assessed including the initial heating of feed, the steam reformation reactor, the high and low temperature water shift reactors, the amine plant, the methanation reactor, the gas compression and cooling train, and the pressure swing adsorption unit.

Although worker safety is always the first priority in a plant, there are inherent risk associated with high temperature and pressure processes. Close attention was paid to make the preliminary plant design as inherently safe as possible. Preliminary FMEA and HAZOP analyses were conducted to identify the highest priority risks. Hazards were mitigated by substituting less hazardous materials when possible, opting to store only the necessary hazardous materials on site, lowering temperatures when possible, adding catastrophic failure controls, maximizing plant control automation when economically feasible, and necessitating worker personal protective equipment. Finally, FMEA and HAZAP analyses were repeated. These steps were repeated multiple times until a sufficient reduction in risk had been achieved.

=Case Study: Bhopal Disaster=
[[File:Bhopal.JPG|thumb|alt=The dispersal of the deadly cloud of gas from the Union Carbide plant in the Indian city of Bhopal|The dispersal of the deadly cloud of gas from the Union Carbide plant in the Indian city of Bhopal|400px|right|The dispersal of the deadly cloud of gas from the Union Carbide plant in the Indian city of Bhopal]]
On December 3, 1984 at the India Limited Pesticide Plant owned by Union Carbide in Bhopal, India, water entered a storage vessel containing over 80,000 lbs. of methyl isocyanate (MIC), a chemical intermediate in the pesticide synthesis process. This reaction caused a rapid increase in temperature accompanied by boiling, which caused toxic MIC vapors to escape from the tank. In addition, the MIC-water reaction produced methylamine and carbon dioxide gases among other toxic products which also contributed to the pressure increase (Union Carbide Corporation, 1967). These vapors passed into a scrubber and flare system that were not working at the time due to inadequate maintenance and safety practices. As a result of this accident, approximately 25 tons of MIC vapor were released, killing over 3,800 immediately and injuring roughly 20,000 in the surrounding area.

As a result of the incident, Union Carbide was forced to pay $470 million, as well as fund a hospital in Bhopal that was used specifically to treat victims of the disaster. Cleanup of the plant site and other legal action are still being determined to this day. Bhopal sparked a worldwide discussion on chemical process safety, and caused Congress to create the U.S. Chemical Safety Board (CSB). The CSB has since cited the following reasons as causes for the disaster:

* No process hazard analysis
* Poorly maintained equipment and safety system
* Lack of emergency response planning
* Inadequate training for operators

The CSB has pushed chemical safety reform since its conception, urging the chemical industry to produce inherently safer designs, use better quality equipment, and develop more thorough risk management plans. A major criticism of the process was its lack of inherent safe design. Because MIC was an intermediate, there was no reason to keep large quantities in storage. A modern design would use the intermediate as it is made (Eckerman et al., 2005).

Although chemical process safety has come a long way since 1984, industrial chemical giants still battle problems similar to Bhopal until this day. In 2008, a disaster similar to the one in Bhopal could have occurred in a plant originally designed by Union Carbide located in Institute, West Virginia after a runaway reaction caused a pressure buildup in a waste treatment vessel. The vessel exploded, killing two plant workers. Fortunately, the explosion missed a large MIC storage vessel which could have been hit by shrapnel and released tons of MIC (Blanc et al., 2009) In 2013, an ammonium nitrate explosion killed 15 and seriously injured 200 in West Texas in a blast radius similar to the one experienced in Bhopal (U.S.Chemical Safety Board, 2014).

=Case Study: Deepwater Horizon Explosion and Oil Spill=
[[File:Deepwater Horizon offshore drilling unit on fire 2010.jpg|thumb|alt=The burning Deepwater Horizon oilrig after an explosion caused by a design failure in the blowout preventer|The burning Deepwater Horizon oilrig after an explosion caused by a design failure in the blowout preventer|400px|right|The burning Deepwater Horizon oilrig after an explosion caused by a design failure in the blowout preventer]]
On April 20, 2010 on the Deepwater Horizon offshore drilling rig located in the Macondo Prospect, multiple explosions killed 11 workers and seriously injured 17. The rig burned for two days before sinking into the Gulf of Mexico. Key safety failures caused the well to spew 5 million barrels of oil into the Gulf of Mexico over the next 87 days making the incident the largest offshore oil spill in U.S. history. Finally, the well was sealed by a “static kill,” the injection of heavy fluids and cement, at the leak point 5,000 feet below the surface.

The key safety failure identified by the CSB was the blowout preventer (BOP) failure. This device that is meant to prevent the filling of annular space between the borehole and the well casing is both electrically and hydraulically powered. It is connected to a rig by a large diameter pipe called a riser. The system contains multiple pipe rams and annular preventers designed to prevent annular space buildup.

On the first night of the incident, a “kick” occurred and a mixture of oil, water, and gases began to build up in the wall and climb up the shaft. Drilling mud was injected to prevent kicks by creating a barrier. An upper annular preventer was also engaged when the buildup was discovered, but it failed. A pipe ram was activated and succeeded. However, an immense pressure buildup caused the drill pipe to buckle so it was forced off center. This buckling was later explained as a result of effective compression. This phenomenon is caused by microscopic irregularities and bends in the pipe material resulting in a higher surface area on one side of the pipe. Because the pipe was off-center, the final failsafe, the Automatic Mode Function (AMF) or deadman could not effectively shear the pipe and seal the well. This redundant control system comprised of a yellow pod and blue pod work independently to seal the well in the event of catastrophic failure when communications, electric power, and hydraulic pressure connections are cut. Both the yellow and blue pods contained 9 volt and 27 volt batteries which power solenoid valves. Unfortunately, the blue pod was miswired, so its 27 volt power supply was drained when it was to cause the blind shear blades to cut the pipe. Fortunately, a 9 volt battery in the yellow pod was also miswired which caused the blind shear ram to be engaged. However, this only partially sealed the well because of the pipe buckling. The flammable mixture erupted onto the surface of the platform and found an ignition source triggering a massive explosion. The spill was temporarily contained by a cap, and relief wells were eventually used to seal the well months later (U.S.Chemical Safety Board, 2014).

The White House Office of Energy and Climate Change Policy called the Deepwater Horizon oil spill the “worst environmental disaster the US has faced (BBC News, 2010). Over 8,000 species were estimated to be affected by the spill due to the toxicity of petroleum released, oxygen depletion, and the large quantities of Corexit, an oil dispersant used in an untested manner that is toxic to marine life (Biello et al., 2010; Butler, 2011; Froomkin, 2010).

BP, Transocean, and Halliburton were the major entities implicated in this tragedy. Investigations after the incident show that essential safety documentation including risk management and emergency procedure information were missing. Accusations were mainly aimed at BP with charges of recklessness and gross negligence (CNN Money, 2012). In January 2013, Transocean was ordered to pay $1.4 billion for US Clean Water Act violations. BP was ordered to pay $2.4 billion, but additional penalties could reach $20 billion (Department of Justice Office of Public Affairs, 2013).

=Case Study: Texas City Refinery Explosion=
[[File:BP_PLANT_EXPLOSION-1_lowres2.jpg|thumb|alt=Emergency response workers fight secondary fires caused by the isomerization unit explosion at the Texas City refinery|Emergency response workers fight secondary fires caused by the isomerization unit explosion at the Texas City refinery|228px|right|Emergency response workers fight secondary fires caused by the isomerization unit explosion at the Texas City refinery]]
On March 23, 2005 at a BP refinery in Texas City, Texas a hydrocarbon vapor cloud ignited, killing 15 workers and seriously injuring 170 others. Over the course an 11 hour period, a combination of control failures, mismanagement, and worker fatigue resulted in the buildup and release of extremely hot, combustible vapor. The key process unit in this disaster was an isomerization unit, located next to wooden trailers for workers servicing an ultracracker unit.

In the early morning on March 23rd, operators initiated startup and pumped raffinate (liquid hydrocarbons) into a raffinate splitter tower used to separate gasoline components. A liquid level indicator and multiple high level alarms monitored the tower liquid level. The level indicator could only measure up to 9 feet of liquid, and the written process called for a liquid level of about 6.5 feet. However, operators routinely filled the tower over 9 feet to minimize fluctuations and to prevent damage to a furnace. Hours later, the first high level alarm was activated and the liquid level rose, but a second alarm higher up the tower failed to trigger. The feed was halted when the liquid had risen to a level of about 13 feet, operators had no way of knowing the exact height. The lead operator relayed the startup activities to another operator and left the facility an hour before his shift ended. The morning operator arrived at 6 am to start his thirtieth consecutive day working a 12-hour shift and read a logbook that read, “Isom* Brought in some raff to unit, to pack raff with.” The day shift operator arrived an hour late, so he could not be briefed by the night shift supervisor. Recirculation then commenced in the tower, and more liquid was added to the tower. Additionally, conflicting instructions caused a liquid level regulating valve to remain closed for several hours, so liquid could not leave the tower. The furnace was then lit, and the supervisor left to attend a family medical emergency.

At noon, the liquid level had risen to 98 feet, but the improperly calibrated liquid level indicator read 8.4 feet. At 12:41 pm, a high pressure alarm caused workers to manually open a chain valve to relieve pressure by using the units pressure relief system to vent vapor into the atmosphere using an obsolete blowdown drum. Heat was also reduced in the furnace to reduce pressure. When operators became concerned about outflow rate, the liquid level regulating valve was opened to release liquid from the tower to storage. This caused the liquid in the tower to begin to boil and spill into the overhead vapor line exerting extreme pressures on the pressure relief system. At 1:14 pm, the three relief valves opened sending the liquid to the blowdown drum which overflowed into a municipal sewer setting off alarms, but a key level indicator in the blowdown drum failed. Flammable liquid erupted from the blowdown drum, formed a massive vapor cloud, and found an ignition source from a nearby idling pickup truck. The colossal blast ignited fires throughout the refinery and over half the workers in the wooden trailers adjacent to the unit were killed immediately (U.S.Chemical Safety Board, 2008).

Investigations after the incident cited multiple failures to implement safety recommendations at the Texas City Refinery. Among these, the blowdown drum was to be replaced by a modern flare to burn off hydrocarbons. However, BP’s budget cuts prevented its replacement. The training and treatment of workers was also called into question, as fatigue, poor communication, and inadequate documentation likely contributed to the disaster. Decisions like the one to operate an unsafe liquid level in order to prevent furnace damage also demonstrate the company’s fixation on the bottom line. BP was eventually fined $21 million by OSHA (New York Times, 2010; U.S.Chemical Safety Board, 2008).

=References=
==Direct Citation References==
<references/>

==Additional References==
# American Institute of Chemical Engineers. http://www.aiche.org/ccps
# Biello, David (9 June 2010). "The BP Spill's Growing Toll On the Sea Life of the Gulf". Yale Environment 360. Yale School of Forestry & Environmental Studies. Retrieved 2010-06-14.
# Blanc, P. Bhopal, 1984 – West Virginia near-miss, 2008. Psychology Today, December 2009. https://www.psychologytoday.com/blog/household-hazards/200912/bhopal-1984-west-virginia-near-miss-2008.
# Butler, J. Steven (3 March 2011). "BP Macondo Well Incident. U.S. Gulf of Mexico. Pollution Containment and Remediation Efforts" (PDF). Lillehammer Energy Claims Conference. BDO Consulting. Retrieved 17 February 2013.
# “DOJ accuses BP of ‘gross negligence’ in Gulf oil spill” CNN Money, September 2012.
# Eckerman, I. The Bhopal Saga: Causes and Consequences of the World’s Largest Industrial Disaster. Universities Press Private Limited, 2005.
# Froomkin, Dan (29 July 2010). "Scientists Find Evidence That Oil And Dispersant Mix Is Making Its Way Into The Foodchain". Huffington Post.
# "Gulf of Mexico oil leak 'worst US environment disaster'". BBC News. 30 May 2010.
# Investigation Report: BP Refinery Explosion and Fire, U.S. Chemical Safety Board, 2008.
# Lyall, Sarah. "In BP’s Record, a History of Boldness and Costly Blunders." New York Times, July 13, 2010.
# L.T. Biegler, I.E. Grossmann, A.W. Westerberg. ''Systematic Methods of Chemical Process Design''. Prentice-Hall: Upper Saddle River, 1997.
# M.S. Peters, K.D. Timmerhaus. ''Plant Design and Economics for Chemical Engineers, 5th Ed.'' McGraw-Hill: New York, 2003.
# Northwestern University. Chemical Engineering 351 Lecture Slides.
# Reflections on Bhopal After Thirty Years Video, U.S.Chemical Safety Board, December 2014.
# R.T. Turton, R.C. Bailie, W.B. Whiting, J.A. Shaeiwitz. ''Analysis, Synthesis, and Design of Chemical Processes''. Prentice Hall: Upper Saddle River, 2003.
# Union Carbide Corporation "Methyl Isocyanate" Product Information Publication, F-41443, November 1967.
# USCSB Deepwater Horizon Video, U.S.Chemical Safety Board, June 2014.
# "Transocean Agrees to Plead Guilty to Environmental Crime and Enter Civil Settlement to Resolve U.S. Clean Water Act Penalty Claims from Deepwater Horizon Incident". Department of Justice Office of Public Affairs. January 3, 2013.

Process safety

2016-02-13T21:41:22Z

Evanrosati: /* Economic Cost of Safety */

Authors: Anne Disabato, [2014] Tim Hanrahan, [2014] Brian Merkle, [2014] , Spencer Saldana [2015] and Evan Rosati [2016] 

Stewards: David Chen, Jian Gong, and Fengqi You

Date Presented: January 19, 2014

 


__TOC__

=Introduction=
The safe design and operation of facilities is of paramount importance to every company that is involved in the manufacture of fuels, chemicals, and pharmaceuticals. Process safety focuses on the prevention of dangerous situations, such as fires, explosions, and the release of chemicals.

The American Institute of Chemical Engineers emphasizes a culture of process safety through four pillars (American Institute of Chemical Engineers, 2015):

1. Commitment to Process Safety: a workforce that is actively involved and an organization that fully supports process safety as a core value will tend to do the right things in the right way at the right time – even when no one else is looking

2. Understanding Hazard and Risk: the foundation of a risk-based approach which will allow an organization to use this information to allocate limited resources in the most effective manner

3. Manage Risk: the ongoing execution of risk based process safety tasks. Risk management can help a company to better deal with the resultant risks and sustain long-term accident free and profitable operations

4. Learn from Experience: Metrics provide direct feedback on the workings of RBPS systems, and leading indicators provide early warning signals of ineffective process safety results. Organizations must use their mistakes and those of others as motivation for action and view as opportunities for improvement.

For the prevention and management of specific safety hazards, such as fires, explosions, or the release of toxic chemicals, please see [[Process Hazards]].

==Layers of Plant Safety==
Safety and loss prevention can be expressed in "layers" of plant safety in terms of design and implementation.<ref name="Towler">G.P. Towler, R. Sinnott. ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design''. Elsevier, 2012.</ref> Each higher layer can be activated if a lower level fails. This creates a system with subsequent levels of safety to help prevent catastrophe from occurring. This diagram shows the important of safety in process design. If a process is designed to be inherently safe, additional safety "controls" will be less important and a chemical plant will be overall safer. The goal of safety is not to reach the top of the triangle, but to stay as close to the bottom as possible. This shows the importance of inherently safe design and safety legislation and regulations to provide guidelines for safety and health concerns. Regulations provide a baseline for engineers to operate when designing a chemical plant. They have brought safety to the forefront of design, when engineers have the maximum degree of freedom for implementation, and are no longer considered to be an afterthought or strictly a controls issue. Plants can be designed to be safe without the extensive use of future adaptation, safety controls, or emergency response. Although you can never eliminate these upper layers of process safety, by designing a process smartly and safely, engineers can reduce the consequences of "walking up" the process safety ladder or triangle.
[[File:Layers_of_Plant_Safety.png|center|700px|thumb|alt=Triangle Diagram on Increasing Plant Safety Mechanisms|Layers of Plant Safety Triangle Diagram]]

=Safety Legislation=

==International Regulations==
International regulations can be considered as best practice standards that are adopted by governments through treaties and establishment through United Nation resolutions. Most international regulation agencies can only register complaints for violations, but can not implement sanctions or fines to infracting parties.

===International Labour Organization (ILO)===
After the dissolution of the League of Nations in 1946, the ILO became the first specialized agency of the United Nations upon its founding in December of that same year. <ref> ISO Photo Gallery. http://www.ilo.org/dyn/media/mediasearch.fiche?p_id=16023&p_lang=en. Accessed February 13, 2016.</ref> The ILO attempts to develop labor standards for workers in all industries. The unique tripartite structure of the ILO gives an equal voice to workers, employers and governments to ensure that the views of the social partners are closely reflected in labour standards and in shaping policies and programs. <ref> ISO "How the ISO Works" http://www.ilo.org/global/about-the-ilo/how-the-ilo-works/lang--en/index.htm. Accessed February 13, 2016.</ref> The ISO has established International Labour Standards on Occupational Safety and Health, which has developed more than 40 standards specifically dealing with occupational heath and safety.<ref> ILO. "International Labour Standards on Occupational Safety and Health". http://www.ilo.org/global/standards/subjects-covered-by-international-labour-standards/occupational-safety-and-health/lang--en/index.htm. Accessed February 13, 2016.</ref>. The important conventions are listed below:

====Occupational Safety and Health Convention, 1981 and Protocol of 2002====
#The convention provides for the adoption of national occupational safety and health policy by each participating nation-state. It includes actions taken by governments and within enterprise that operate within the governmental spaces to promote the improvement of occupational safety and health to therefore improve working conditions.
#The additional Protocol in 2002 calls for the establishment and period review of procedures for recording and notification of occupational accidents along with the publication of related statistics associated with the accidents. This Protocol is similar to the Emergency Planning and CommunityRight-To-Know Act instituted in the United States (see below).

====Occupational Heath Services Convention, 1985====
#Establishment of enterprise-level occupational health services which are entrusted with preventative functions and which are responsible for advising the employer, the workers, and their representatives in the organization on maintaining a safe and healthy work environment.

====Promotional Framework for Occupational Safety and Heath Convention, 2006====
#Aims to promote a preventative safety and health culture and to progressively achieve a safe and healthy working environment for all.
#Requires ratifying nation-states to develop, in consultation with the most representative organizations of employees and workers, including workers unions, a national policy, a national system, and a national program on occupational health and safety.
#This should be developed in accordance to the Occupational Heath Services Convention, 1985 and take into account other ILO standards.
#National systems shall provide the infrastructure for implementing national policy and programs, such as domestic laws, regulations, authority bodies, and compliance mechanisms.

====Chemicals Convention, 1990====
#Provides for the adoption and implementation of a coherent policy on the safety in the use of chemicals at work, including production, handling, storage, and transport.
#Also includes best practices for the disposal and treatment of waste chemicals and the release of chemicals resulting from work activities, maintenance, and cleaning of equipment.
#In addition, it allocates specific responsibilities to suppliers and exporting states.
#Chemicals shall be evaluated to determine their level of hazards and employers shall make these hazards known to their employees.

===International Organization for Standardization (ISO)===
The ISO is an international organization for standardization on topics ranging from quality management, environmental management, social responsibility, risk management, etc. ISO standards are adopted world-wide by organizations and governments as accepted and known standards for production. Many of the practices are self-regulating as costumers often demand certain standards from companies they purchase from.<ref>ISO. Home Page. http://www.iso.org/iso/home.html. Accessed February 13, 2016.</ref> The ISO is developing its own occupational health and safety framework shown below.

====ISO 45001 Occupational Health and Safety Management Systems<ref>ISO 45001. "Occupational heath and safety". http://www.iso.org/iso/home/standards/management-standards/iso45001.htm. Accessed February 13, 2016.</ref>====
#Designed to help organizations reduce the burden of occupational injuries and diseases by providing a framework to improve employee safety, reduce workplace risks, and create better, safer working conditions.
#Currently under development by a committee of occupational health and safety experts who will follow management systems approaches of ISO 4001 and ISO 9001.
#Will embody other International Standards such as the ILO's Occupational Health and Safety Guidelines.
#The expected publication will be released in October, 2016.

==US Safety Regulations==
Over time chemical industry regulations have been developed to ensure that the best safety practices are followed to maintain the health of both people and the environment. The development of most regulations is based around the idea that organizations have both a legal and moral obligation to safeguard the health and welfare of its employees and the public.<ref name="Towler" /> The extent of legislation varies across regions around the globe. In the United States, chemical accidents have led to the creation of regulation boards and safety oriented societies such as the Center for Chemical Plant Safety of the American Institute of Chemical Engineers to aid in the development and implementation of pant safety. <ref>Seider WD, Seader JD, Lewin DR, Seider WD. Product And Process Design Principles: Synthesis, Analysis, and Evaluation. New York: Wiley; 2004.</ref> In the US, the major federal laws relating to chemical plant safety and their regulations are as follows:

===The Occupational Safety and Health Act, 1970===
The OSH Act is administer by the Occupational Safety and Health Administration (OSHA) (see below). The Act covers all employers and their employees in the United States with coverage provided either directly by the Federal Occupational Safety and Heath Administration or by an OSHA-approved state job safety and health plan.<ref> United States Department of Labor. "Safety and Health Standards: Occupational Safety and Health". http://webapps.dol.gov/elaws/elg/osha.htm </ref> The main provisions of the act are listed below.
# Employers must supply place of employment free from toxic chemicals, excessive noise mechanical dangers, or unsanitary conditions.
#This involves the implementation of engineering controls to limit exposure to hazards and toxic substances and implementing administrative controls.
# For employees, employers must provide personal protective equipment (PPE) and training, including communications of known hazards.
# The Occupational Safety and Health Administration (OSHA) was established to promote best practices, inspect facilities for hazard analysis, set standards, and enforce the law.
# The National Institute of Occupational Safety and Health (NIOSH) was established to be an independent research institute (now under the Centers for Disease Control).
#The Act also encourages states to develop and operate their own job safety and health programs with OSHA as a monitoring agency for these "state plans," which operate under the authority of state law. The standards developed by state plans need to be at least as effective as the federal regulations.<ref> United States Department of Labor. "Safety and Health Standards: Occupational Safety and Health". http://webapps.dol.gov/elaws/elg/osha.htm </ref>
#Federal OSHA standards are categorized into four categories: General Industry, Construction, Maritime Terminals, Long-shoring, and Agriculture.

===The Toxic Substances Control Act (TSCA), 1976===
The main qualifications of the TSCA were to provide the EPA with regulating power of chemicals, specifically pertaining to the chemical industry and not including foods, drugs, cosmetics, or pesticides. <ref> Environmental Protection Agency. "Summary of the Toxic Substances Control Act." http://www.epa.gov/laws-regulations/summary-toxic-substances-control-act. Accessed February 13, 2016</ref>
# The Environmental Protection Agency (EPA) is required to regulate 75,000 chemical substances used in industry.
# The EPA has jurisdiction over the safety of the sale or development of new chemicals in the United States, including the requirement for pre-manufacture notification for new chemical substances before manufacture.
#The TSCA also addresses the production, importation, use, and disposal of specific chemicals including polychlorinated biphenyls (PCBs), asbestos, radon, and lead-based paint.<ref> Environmental Protection Agency. "Summary of the Toxic Substances Control Act." http://www.epa.gov/laws-regulations/summary-toxic-substances-control-act. Accessed February 13, 2016</ref>
# Everyone has the right and obligation to report information about any health or environmental effects caused by a chemical. This is especially important for organizations as they are required to report information to the EPA if a chemical substance is found to have substantial risk of injury to health or the environment.
====Frank R. Lautenberg Chemical Safety for the 21st Century Act, 2015====
The Toxic Substances Control Act will be revised in 2015 under this new act proposed to Congress. This Act is an attempt to eliminate some of the flaws associated with the TSCA.
#One of the biggest flaws in the TSCA is the fact that a new chemical can be used without first demonstrating safety, the idea that chemicals are safe until proven unsafe.<ref>Sheppard, Kate. "Senators Introduce Bill to Overhaul U.S. Chemical Industry." http://www.huffingtonpost.com/2015/03/10/toxic-chemicals-senate-bill_n_6842524.html. Accessed February 13, 2016</ref>
#The new bill would require safety testing before chemical implementation.
#It gives the EPA more defined power on the regulation of chemical substances and the Sustainable Chemical Program will be established. This brings the TSCA into the modern era of the chemical industry.<ref>S.697 - 114th Congress (2015-2016): Frank R. Lautenberg Chemical Safety for the 21st Century Act. Congress.gov. https://www.congress.gov/bill/114th-congress/senate-bill/697/all-info. Accessed January 27, 2016.</ref>

===The Emergency Planning and Community Right-to-Know Act (EPCRA), 1986<ref> US Government Printing Office. Emergency Panning and Community Right to Know. https://www.gpo.gov/fdsys/pkg/USCODE-2010-title42/html/USCODE-2010-title42-chap116.htm. Accessed February 13, 2016</ref>===
The Act was passed by Congress in response to concerns regarding the environmental and safety hazards posed by the storage and handling of toxic chemicals. This legislation was developed as a result of the 1984 chemical disaster in Bhopal, India.<ref>What is EPCRA? EPA. http://www.epa.gov/epcra/what-epcra. Published November 10, 2015. Accessed January 27, 2016.</ref>
# All facilities manufacturing, processing, or storing hazardous chemicals must make plans for major incidents if they were to occur and the plans must be made public so that local communities can be properly informed.
# All facilities must also produce Material Safety Data Sheets (MSDSs) to state and local officials as well as local fire departments.
# Local governments should help prepare emergency plans and review the plans annually.
# State governments are required to oversee and coordinate local planning efforts.
# Emergency Notification: Facilities must immediately report accidental releases of chemicals and "hazardous substances" in quantities greater than Reportable Quantities (RQs) defined under the Comprehensive Environmental Response, Compensation, and Liability Act to state and local officials with this information then being available to the public.
# Annually, facilities must complete and submit a toxic chemical release inventory form.

===Clean Air Act Amendments of 1990===
This Amendment to the original Clean Air Act of 1970 was designed to curb three major threats to the nation's environment and health of citizens: acid rain, urban air pollution, and toxic air emissions.<ref>Environmental Protection Agency. "1990 Clean Air Act Amendment Summary" http://www.epa.gov/clean-air-act-overview/1990-clean-air-act-amendment-summary. Accessed February 13, 2016.</ref>
# Established the U.S. Chemical Safety and Hazard Investigation Board, and independent federal agency with the goal of ensuring worker and public safety through the prevention or minimization of the effects of chemical accidents. <ref>Seider WD, Seader JD, Lewin DR, Seider WD. Product And Process Design Principles: Synthesis, Analysis, and Evaluation. New York: Wiley; 2004.</ref>
# Attempt to determine the roots and contributing causes of accidents and then provide briefs on the accidents.
# Led to the development of cap and trade systems for air pollutants.
# Gave the government significantly more power to control air emissions and administer admissions permits.

==Additional Legislative Information==
For more details on environmental legislation pertaining to release of materials to the environment or the regulations of the loss of containment, see [[Environmental concerns]].

In addition to the above federal regulations, various states and other municipalities also have enacted legislation for the regulation of chemical plants. These include more specific safety items, such as local fire codes or even put into place stricter aspects of the federal regulations. <ref name="Towler" />

Any process design or plant design must always meet the requirements of local and federal mandates and regulations. Without doing so, the wellbeing of plant employees and even the public can be placed into serious jeopardy.

=Safety Organizations and Terminology=
==Organizations==
===OSHA===
The Occupational Safety and Health Administration (OSHA) is a federal agency that focuses on the enforcement of safety and health legislation.

===EPA===
The Environmental Protection Agency (EPA) is a U.S. agency whose purpose is the protection of the health of both humans and the environment through the writing and enforcement of regulatory laws.

===DOT===
The Department of Transportation (DOT) oversees federal highway, air, and maritime transportation, and can be involved in the safe transport of chemicals.

===DOE===
The Department of Energy (DOE) is a governmental department tasked with the advancement of energy technology in the United States.

==Terminology==
===HS&E===
Health, Safety, and Environmental - This term refers to all health, safety, and environmental concerns that arise at each stage of the design process. Companies are required to analyze each part of the process from an HS&E perspective to create a safe and healthy work environment.

===MSDS===
Material Safety and Data Sheet - Every chemical has an MSDS which contains all the information regarding safe handling and how to deal with spills or other accidents involving the substance. Relevant information includes how to identify the substance, hazard information, and how to handle spills, fires, and exposure, among other things.

===FMEA===
Failure Mode and Effects Analysis - FMEA is an early stage approach to identifying critical technical risks using a semi-quantitative procedure. The analysis encompasses safety, environmental, and operational feasibility. When performing FMEA, engineers look to see places in a potential design that could fail, and then quantify how likely that failure is, how severe the results would be, and then offer potential solutions to minimize the risk. A step-by-step guide to performing FMEA is shown below (Northwestern University, 2014):

# Brainstorm for failure modes
# For each FM, rate severity of impact (SEV, 1 - 10).
# For each FM, brainstorm for possible causes (there may be multiple).
# For each cause, rate likelihood of occurring (OCC, 1 - 10).
# Rate the probability that the systems currently in place will detect and prevent the problem before it has an impact (DET, 10 - 1). Do not assume that something that will be added to the design later will take care of the problem.
# Overall Risk Probability Number RPN = SEV x OCC x DET. Most practitioners use the 1,4,7,10 scale below to increase granularity. Note that the DET scale is inverse to SEV and OCC.

[[File:Safety3.JPG|center|700px]]

Figure 1: Suggested scale to be used for quantifying risk and detection of failures in FMEA. Taken from ChE 351 Slides.

The sum of all information collected is implemented into a spreadsheet like the one shown below:

[[File:Safety4.JPG|center|700px]]

Figure 2: Example spreadsheet used to organize FMEA data.

===HAZOP===
Hazard and Operability Study - For more information regarding HAZOP, please refer to [[Process Hazards]].

===SIL===
Safety Integrity Levels - The SIL is the relative level of risk-reduction provided by a safety function, or to specify a target level of risk reduction. A SIL is determined based on a number of qualitative factors such as development process and safety life cycle management. Several methods are used such as risk matrices, risk graphs, layers of protective analysis (LOPA). Three levels of safety integrity are assigned depending on the “availability” of the safety instrumented system (SIS), as shown below (Towler et al., 2012):

[[File:Safety5.JPG|center|500px]]

Figure 3: Table of safety integrity levels based on availability of system. Taken from (Towler et al., 2012).

Redundant system means instrumentation is duplicated; higher level of redundancy of trip systems give higher SIL. The required SIL should be determined during a process hazard analysis and depends on risk of operator exposure and injury.

1 Instrument
* if it signals, plant goes down
* Probability of incident = probability of instrument failure
* Probability of spurious trip = false positive rate of one instrument

2 Instruments
* 1 out of 2 voting (1oo2): one instrument signals, plant goes down
** Probability of incident reduced by duplication
** Probability of spurious trip doubled
* 2oo2 voting
** Probability of incident worse than single instrument (twice likelihood that system is down)
** Probability of spurious trip reduced

3 Instruments
* 2oo3 voting
** Best overall trade-off between reducing incident rate and spurious trip rate
** One malfunctioning instrument does not cause trip or prevent detection of real incident

=Safe Design=
==Inherently Safe Design==
Inherently safe design of a particular process can be achieved by adhering to the following six strategies (Turton et al., 2003):

1. Substitution: Avoid using or producing hazardous materials on the plant site. If the hazardous material is an intermediate product, for example, alternate chemical reaction pathways might be used. In other words, the most inherently safe strategy is to avoid the use of hazardous materials.

2. Intensification: Attempt to use less of the hazardous materials. In terms of a hazardous intermediate, the two processes could be more closely coupled, reducing or eliminating the amount of intermediate produced. The inventories of hazardous feeds or products can be reduced by enhanced scheduling techniques such as just-in-time (JIT) manufacturing.

3. Attenuation: Reducing, or attenuating, the hazards of materials can often be affected by lowering the temperature or adding stabilizing additives. By using materials under less hazardous conditions, the potential consequences of a leak can be reduced.

4. Containment: If the hazardous materials cannot be eliminated, they at least should be stored in vessels with mechanical integrity beyond any reasonably expected temperature or pressure excursion. This is an old but effective strategy to avoid leaks. However, it is not as inherently safe as substitution, intensification, or attenuation.

5. Control: If a leak of hazardous material does occur, there should be safety systems that reduce the effects. For example, chemical facilities often have emergency isolation of the site from the normal storm sewers, and large tanks for flammable liquids are surrounded by dikes that prevent any leaks from spreading to to other areas of the plant. Scrubbing systems and relief systems in general are in this category. They are essential, because they allow controlled, safe release of hazardous materials, rather than an uncontrolled release from a vessel rupture.

6. Survival: If leaks of hazardous materials do occur and they are not contained or controlled, the personnel (and the equipment) must be protected. This lowest level of the hierarchy includes fire fighting, gas masks, and so on. Although essential to the total safety of the plant, the greater the reliance on survival of leaks rather than elimination of leaks, the less inherently safe the facility.

===Safety Legislation and Process Design===
====Inherent Process Plant Safety====
Through the enactment of safety regulation shown above, process design has inherently become safer. These regulations have made safety the first and foremost important concern when designing a new chemical plant. In general, the safety of a process relies on multiple layers of protection, but the first and most important layer of protection has become the process design feature. Greater tolerances are built into the designing of processes to more effectively prevent catastrophic failures or chemical leakage. The best approach to prevent accidents is to add process design features, involving chemistry and physics, to prevent hazardous situations.
=====Examples of Inherently Safe Process Design<ref>Crowl, Daniel. Louver, Joseph. Chemical Process Safety: Fundamentals with Applications. IBN: 9780132440554</ref>=====
#Vapor Release: Vapor released from spills can be minimized by designing dikes so that flammable and toxic materials will not accumulate around leaking tanks. This also prevents potential flammable materials from building up and causing an explosion.
#Containment Building: Design can be important for the containment of toxic spills. With the addition of automatic or remote controls, personnel can leave the area if a spill or breach occurs, while the area can be continuously monitored.
#Solvent Substitution: Safety through Substitution - Substituting design with safer, less hazardous materials. Designing process for use with less toxic or flammable solvents. For example, water-based paints and adhesives as well as aqueous or dry flowable formulations for agricultural chemicals opposed to more volatile solvents that release VOCs.
#Design for Lower Temperature and Pressure Safety through Moderation - use a hazardous material under less hazardous conditions such as lower pressure and temperature conditions. This can lower the level of catastrophe if downstream safety processes do fail.

====Control Systems Safety====
Legislation has also added to subsequent layers of controls such as environmental process controls have been added to prevent release to nearby air and water systems, which would endanger surrounding ecosystems and human populations.

==Assessing Preliminary Design==
While pilot plants are necessary to design effective plant equipment, there are some dangers associated with the industrial scale of chemical plants. Scaling up without accurate literature and experimental data can be very dangerous.

In every step of process design, the Health, Safety, and Environmental (HS&E) analysis must be carried out with the available technical information (Biegler et al., 1997).

[[File:Safety1.JPG|center|800px]]

[[File:Safety2.JPG|center|700px]]

Figure 4: General steps in the design process and analysis to be carried out at each stage. Taken from (Biegler et al., 1997).

=Economic Cost of Safety=
Price of Safety installations and fire protection systems range from .5 to 1.6 % of fixed-capital investment of a plant; but expenditures are often much higher than this and it is difficult to estimate these expenditures for a given plant

In addition, designers also examine the economic impact of safety and maintenance issues. For instance, they may determine that the plant reactor configuration can be improved, and with improved operator training facilities, it can run with improved safety. These hidden costs must be considered when determining the economic feasibility of operation (Biegler et al., 1997).

==Benefits of Inherent Safety Over Conventional Safety==
The old practice method of implementing safety into process design came very late in the design process. The physical entities in the process or the process itself was changed very little and the conventional safety methods included implementing controls. Inherent process safety is instead developed very early in the design process and can lead to very significant cost savings overall. The comparison comes when looking at two ratios for safety costs, the Conventional Safety Cost Index (CSCI) and the Inherent Safety Cost Index (ISCI).

=Other Process Safety Considerations=
The safety and well-being of the consumers using the eventual product should be considered in process safety. The risks involved in using a product should be clearly communicated to the consumer by industrial leaders.

Human Error is another safety risk that is difficult to quantify. The intervention of well-trained operators is a vital layer in process safety (Peters et al., 2003).

=Case Study: Production of Ultrapure Hydrogen=
[[File:steam reforming.jpg|thumb|alt=A hydrogen production plant with a box steam reformer|A hydrogen production plant with a box steam reformer|300px|right|A hydrogen production plant with a box steam reformer]]
To walk through the process safety considerations when designing a chemical plant, the relatively simple example of a high purity hydrogen generation will be examined. First, various technologies are researched to determine the options that best meet the economic, physical, environmental, and safety constraints of the project. For this project, steam reforming, autothermal reforming, and partial oxidation were investigated. Technologies that lack widespread implementation have inherent safety risks as they have higher uncertainties associated with reliability, feasibility, and cost. Autothermal reforming requires oxygen and not a commercially popular method. Partial oxidation requires no catalyst, but requires high process temperatures and is a complex process to implement. After each process was analyzed and scored in a decision matrix, steam reforming was chosen as the base process technology. It was selected because it is a safe, well-known, low-emission, traditional process in use all over the world for the production of hydrogen. Steam reforming produces minimal waste compared to the alternative processes, and is capable of producing the necessary 100 MMscfd of 99.999% hydrogen gas. It is important to remember that both local and global environmental emissions are capable of harming the general public, so they should be considered safety concerns in the same way worker hazards are. Next, various reactor types, catalysts, and separation methods were evaluated with the base process chosen. In all, seven different processing stages were assessed including the initial heating of feed, the steam reformation reactor, the high and low temperature water shift reactors, the amine plant, the methanation reactor, the gas compression and cooling train, and the pressure swing adsorption unit.

Although worker safety is always the first priority in a plant, there are inherent risk associated with high temperature and pressure processes. Close attention was paid to make the preliminary plant design as inherently safe as possible. Preliminary FMEA and HAZOP analyses were conducted to identify the highest priority risks. Hazards were mitigated by substituting less hazardous materials when possible, opting to store only the necessary hazardous materials on site, lowering temperatures when possible, adding catastrophic failure controls, maximizing plant control automation when economically feasible, and necessitating worker personal protective equipment. Finally, FMEA and HAZAP analyses were repeated. These steps were repeated multiple times until a sufficient reduction in risk had been achieved.

=Case Study: Bhopal Disaster=
[[File:Bhopal.JPG|thumb|alt=The dispersal of the deadly cloud of gas from the Union Carbide plant in the Indian city of Bhopal|The dispersal of the deadly cloud of gas from the Union Carbide plant in the Indian city of Bhopal|400px|right|The dispersal of the deadly cloud of gas from the Union Carbide plant in the Indian city of Bhopal]]
On December 3, 1984 at the India Limited Pesticide Plant owned by Union Carbide in Bhopal, India, water entered a storage vessel containing over 80,000 lbs. of methyl isocyanate (MIC), a chemical intermediate in the pesticide synthesis process. This reaction caused a rapid increase in temperature accompanied by boiling, which caused toxic MIC vapors to escape from the tank. In addition, the MIC-water reaction produced methylamine and carbon dioxide gases among other toxic products which also contributed to the pressure increase (Union Carbide Corporation, 1967). These vapors passed into a scrubber and flare system that were not working at the time due to inadequate maintenance and safety practices. As a result of this accident, approximately 25 tons of MIC vapor were released, killing over 3,800 immediately and injuring roughly 20,000 in the surrounding area.

As a result of the incident, Union Carbide was forced to pay $470 million, as well as fund a hospital in Bhopal that was used specifically to treat victims of the disaster. Cleanup of the plant site and other legal action are still being determined to this day. Bhopal sparked a worldwide discussion on chemical process safety, and caused Congress to create the U.S. Chemical Safety Board (CSB). The CSB has since cited the following reasons as causes for the disaster:

* No process hazard analysis
* Poorly maintained equipment and safety system
* Lack of emergency response planning
* Inadequate training for operators

The CSB has pushed chemical safety reform since its conception, urging the chemical industry to produce inherently safer designs, use better quality equipment, and develop more thorough risk management plans. A major criticism of the process was its lack of inherent safe design. Because MIC was an intermediate, there was no reason to keep large quantities in storage. A modern design would use the intermediate as it is made (Eckerman et al., 2005).

Although chemical process safety has come a long way since 1984, industrial chemical giants still battle problems similar to Bhopal until this day. In 2008, a disaster similar to the one in Bhopal could have occurred in a plant originally designed by Union Carbide located in Institute, West Virginia after a runaway reaction caused a pressure buildup in a waste treatment vessel. The vessel exploded, killing two plant workers. Fortunately, the explosion missed a large MIC storage vessel which could have been hit by shrapnel and released tons of MIC (Blanc et al., 2009) In 2013, an ammonium nitrate explosion killed 15 and seriously injured 200 in West Texas in a blast radius similar to the one experienced in Bhopal (U.S.Chemical Safety Board, 2014).

=Case Study: Deepwater Horizon Explosion and Oil Spill=
[[File:Deepwater Horizon offshore drilling unit on fire 2010.jpg|thumb|alt=The burning Deepwater Horizon oilrig after an explosion caused by a design failure in the blowout preventer|The burning Deepwater Horizon oilrig after an explosion caused by a design failure in the blowout preventer|400px|right|The burning Deepwater Horizon oilrig after an explosion caused by a design failure in the blowout preventer]]
On April 20, 2010 on the Deepwater Horizon offshore drilling rig located in the Macondo Prospect, multiple explosions killed 11 workers and seriously injured 17. The rig burned for two days before sinking into the Gulf of Mexico. Key safety failures caused the well to spew 5 million barrels of oil into the Gulf of Mexico over the next 87 days making the incident the largest offshore oil spill in U.S. history. Finally, the well was sealed by a “static kill,” the injection of heavy fluids and cement, at the leak point 5,000 feet below the surface.

The key safety failure identified by the CSB was the blowout preventer (BOP) failure. This device that is meant to prevent the filling of annular space between the borehole and the well casing is both electrically and hydraulically powered. It is connected to a rig by a large diameter pipe called a riser. The system contains multiple pipe rams and annular preventers designed to prevent annular space buildup.

On the first night of the incident, a “kick” occurred and a mixture of oil, water, and gases began to build up in the wall and climb up the shaft. Drilling mud was injected to prevent kicks by creating a barrier. An upper annular preventer was also engaged when the buildup was discovered, but it failed. A pipe ram was activated and succeeded. However, an immense pressure buildup caused the drill pipe to buckle so it was forced off center. This buckling was later explained as a result of effective compression. This phenomenon is caused by microscopic irregularities and bends in the pipe material resulting in a higher surface area on one side of the pipe. Because the pipe was off-center, the final failsafe, the Automatic Mode Function (AMF) or deadman could not effectively shear the pipe and seal the well. This redundant control system comprised of a yellow pod and blue pod work independently to seal the well in the event of catastrophic failure when communications, electric power, and hydraulic pressure connections are cut. Both the yellow and blue pods contained 9 volt and 27 volt batteries which power solenoid valves. Unfortunately, the blue pod was miswired, so its 27 volt power supply was drained when it was to cause the blind shear blades to cut the pipe. Fortunately, a 9 volt battery in the yellow pod was also miswired which caused the blind shear ram to be engaged. However, this only partially sealed the well because of the pipe buckling. The flammable mixture erupted onto the surface of the platform and found an ignition source triggering a massive explosion. The spill was temporarily contained by a cap, and relief wells were eventually used to seal the well months later (U.S.Chemical Safety Board, 2014).

The White House Office of Energy and Climate Change Policy called the Deepwater Horizon oil spill the “worst environmental disaster the US has faced (BBC News, 2010). Over 8,000 species were estimated to be affected by the spill due to the toxicity of petroleum released, oxygen depletion, and the large quantities of Corexit, an oil dispersant used in an untested manner that is toxic to marine life (Biello et al., 2010; Butler, 2011; Froomkin, 2010).

BP, Transocean, and Halliburton were the major entities implicated in this tragedy. Investigations after the incident show that essential safety documentation including risk management and emergency procedure information were missing. Accusations were mainly aimed at BP with charges of recklessness and gross negligence (CNN Money, 2012). In January 2013, Transocean was ordered to pay $1.4 billion for US Clean Water Act violations. BP was ordered to pay $2.4 billion, but additional penalties could reach $20 billion (Department of Justice Office of Public Affairs, 2013).

=Case Study: Texas City Refinery Explosion=
[[File:BP_PLANT_EXPLOSION-1_lowres2.jpg|thumb|alt=Emergency response workers fight secondary fires caused by the isomerization unit explosion at the Texas City refinery|Emergency response workers fight secondary fires caused by the isomerization unit explosion at the Texas City refinery|228px|right|Emergency response workers fight secondary fires caused by the isomerization unit explosion at the Texas City refinery]]
On March 23, 2005 at a BP refinery in Texas City, Texas a hydrocarbon vapor cloud ignited, killing 15 workers and seriously injuring 170 others. Over the course an 11 hour period, a combination of control failures, mismanagement, and worker fatigue resulted in the buildup and release of extremely hot, combustible vapor. The key process unit in this disaster was an isomerization unit, located next to wooden trailers for workers servicing an ultracracker unit.

In the early morning on March 23rd, operators initiated startup and pumped raffinate (liquid hydrocarbons) into a raffinate splitter tower used to separate gasoline components. A liquid level indicator and multiple high level alarms monitored the tower liquid level. The level indicator could only measure up to 9 feet of liquid, and the written process called for a liquid level of about 6.5 feet. However, operators routinely filled the tower over 9 feet to minimize fluctuations and to prevent damage to a furnace. Hours later, the first high level alarm was activated and the liquid level rose, but a second alarm higher up the tower failed to trigger. The feed was halted when the liquid had risen to a level of about 13 feet, operators had no way of knowing the exact height. The lead operator relayed the startup activities to another operator and left the facility an hour before his shift ended. The morning operator arrived at 6 am to start his thirtieth consecutive day working a 12-hour shift and read a logbook that read, “Isom* Brought in some raff to unit, to pack raff with.” The day shift operator arrived an hour late, so he could not be briefed by the night shift supervisor. Recirculation then commenced in the tower, and more liquid was added to the tower. Additionally, conflicting instructions caused a liquid level regulating valve to remain closed for several hours, so liquid could not leave the tower. The furnace was then lit, and the supervisor left to attend a family medical emergency.

At noon, the liquid level had risen to 98 feet, but the improperly calibrated liquid level indicator read 8.4 feet. At 12:41 pm, a high pressure alarm caused workers to manually open a chain valve to relieve pressure by using the units pressure relief system to vent vapor into the atmosphere using an obsolete blowdown drum. Heat was also reduced in the furnace to reduce pressure. When operators became concerned about outflow rate, the liquid level regulating valve was opened to release liquid from the tower to storage. This caused the liquid in the tower to begin to boil and spill into the overhead vapor line exerting extreme pressures on the pressure relief system. At 1:14 pm, the three relief valves opened sending the liquid to the blowdown drum which overflowed into a municipal sewer setting off alarms, but a key level indicator in the blowdown drum failed. Flammable liquid erupted from the blowdown drum, formed a massive vapor cloud, and found an ignition source from a nearby idling pickup truck. The colossal blast ignited fires throughout the refinery and over half the workers in the wooden trailers adjacent to the unit were killed immediately (U.S.Chemical Safety Board, 2008).

Investigations after the incident cited multiple failures to implement safety recommendations at the Texas City Refinery. Among these, the blowdown drum was to be replaced by a modern flare to burn off hydrocarbons. However, BP’s budget cuts prevented its replacement. The training and treatment of workers was also called into question, as fatigue, poor communication, and inadequate documentation likely contributed to the disaster. Decisions like the one to operate an unsafe liquid level in order to prevent furnace damage also demonstrate the company’s fixation on the bottom line. BP was eventually fined $21 million by OSHA (New York Times, 2010; U.S.Chemical Safety Board, 2008).

=References=
==Direct Citation References==
<references/>

==Additional References==
# American Institute of Chemical Engineers. http://www.aiche.org/ccps
# Biello, David (9 June 2010). "The BP Spill's Growing Toll On the Sea Life of the Gulf". Yale Environment 360. Yale School of Forestry & Environmental Studies. Retrieved 2010-06-14.
# Blanc, P. Bhopal, 1984 – West Virginia near-miss, 2008. Psychology Today, December 2009. https://www.psychologytoday.com/blog/household-hazards/200912/bhopal-1984-west-virginia-near-miss-2008.
# Butler, J. Steven (3 March 2011). "BP Macondo Well Incident. U.S. Gulf of Mexico. Pollution Containment and Remediation Efforts" (PDF). Lillehammer Energy Claims Conference. BDO Consulting. Retrieved 17 February 2013.
# “DOJ accuses BP of ‘gross negligence’ in Gulf oil spill” CNN Money, September 2012.
# Eckerman, I. The Bhopal Saga: Causes and Consequences of the World’s Largest Industrial Disaster. Universities Press Private Limited, 2005.
# Froomkin, Dan (29 July 2010). "Scientists Find Evidence That Oil And Dispersant Mix Is Making Its Way Into The Foodchain". Huffington Post.
# "Gulf of Mexico oil leak 'worst US environment disaster'". BBC News. 30 May 2010.
# Investigation Report: BP Refinery Explosion and Fire, U.S. Chemical Safety Board, 2008.
# Lyall, Sarah. "In BP’s Record, a History of Boldness and Costly Blunders." New York Times, July 13, 2010.
# L.T. Biegler, I.E. Grossmann, A.W. Westerberg. ''Systematic Methods of Chemical Process Design''. Prentice-Hall: Upper Saddle River, 1997.
# M.S. Peters, K.D. Timmerhaus. ''Plant Design and Economics for Chemical Engineers, 5th Ed.'' McGraw-Hill: New York, 2003.
# Northwestern University. Chemical Engineering 351 Lecture Slides.
# Reflections on Bhopal After Thirty Years Video, U.S.Chemical Safety Board, December 2014.
# R.T. Turton, R.C. Bailie, W.B. Whiting, J.A. Shaeiwitz. ''Analysis, Synthesis, and Design of Chemical Processes''. Prentice Hall: Upper Saddle River, 2003.
# Union Carbide Corporation "Methyl Isocyanate" Product Information Publication, F-41443, November 1967.
# USCSB Deepwater Horizon Video, U.S.Chemical Safety Board, June 2014.
# "Transocean Agrees to Plead Guilty to Environmental Crime and Enter Civil Settlement to Resolve U.S. Clean Water Act Penalty Claims from Deepwater Horizon Incident". Department of Justice Office of Public Affairs. January 3, 2013.

Process safety

2016-02-13T20:37:18Z

Evanrosati: /* Internation Labour Organization (ILO) */

Authors: Anne Disabato, [2014] Tim Hanrahan, [2014] Brian Merkle, [2014] , Spencer Saldana [2015] and Evan Rosati [2016] 

Stewards: David Chen, Jian Gong, and Fengqi You

Date Presented: January 19, 2014

 


__TOC__

=Introduction=
The safe design and operation of facilities is of paramount importance to every company that is involved in the manufacture of fuels, chemicals, and pharmaceuticals. Process safety focuses on the prevention of dangerous situations, such as fires, explosions, and the release of chemicals.

The American Institute of Chemical Engineers emphasizes a culture of process safety through four pillars (American Institute of Chemical Engineers, 2015):

1. Commitment to Process Safety: a workforce that is actively involved and an organization that fully supports process safety as a core value will tend to do the right things in the right way at the right time – even when no one else is looking

2. Understanding Hazard and Risk: the foundation of a risk-based approach which will allow an organization to use this information to allocate limited resources in the most effective manner

3. Manage Risk: the ongoing execution of risk based process safety tasks. Risk management can help a company to better deal with the resultant risks and sustain long-term accident free and profitable operations

4. Learn from Experience: Metrics provide direct feedback on the workings of RBPS systems, and leading indicators provide early warning signals of ineffective process safety results. Organizations must use their mistakes and those of others as motivation for action and view as opportunities for improvement.

For the prevention and management of specific safety hazards, such as fires, explosions, or the release of toxic chemicals, please see [[Process Hazards]].

==Layers of Plant Safety==
Safety and loss prevention can be expressed in "layers" of plant safety in terms of design and implementation.<ref name="Towler">G.P. Towler, R. Sinnott. ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design''. Elsevier, 2012.</ref> Each higher layer can be activated if a lower level fails. This creates a system with subsequent levels of safety to help prevent catastrophe from occurring. This diagram shows the important of safety in process design. If a process is designed to be inherently safe, additional safety "controls" will be less important and a chemical plant will be overall safer. The goal of safety is not to reach the top of the triangle, but to stay as close to the bottom as possible. This shows the importance of inherently safe design and safety legislation and regulations to provide guidelines for safety and health concerns. Regulations provide a baseline for engineers to operate when designing a chemical plant. They have brought safety to the forefront of design, when engineers have the maximum degree of freedom for implementation, and are no longer considered to be an afterthought or strictly a controls issue. Plants can be designed to be safe without the extensive use of future adaptation, safety controls, or emergency response. Although you can never eliminate these upper layers of process safety, by designing a process smartly and safely, engineers can reduce the consequences of "walking up" the process safety ladder or triangle.
[[File:Layers_of_Plant_Safety.png|center|700px|thumb|alt=Triangle Diagram on Increasing Plant Safety Mechanisms|Layers of Plant Safety Triangle Diagram]]

=Safety Legislation=

==International Regulations==
International regulations can be considered as best practice standards that are adopted by governments through treaties and establishment through United Nation resolutions. Most international regulation agencies can only register complaints for violations, but can not implement sanctions or fines to infracting parties.

===International Labour Organization (ILO)===
After the dissolution of the League of Nations in 1946, the ILO became the first specialized agency of the United Nations upon its founding in December of that same year. <ref> ISO Photo Gallery. http://www.ilo.org/dyn/media/mediasearch.fiche?p_id=16023&p_lang=en. Accessed February 13, 2016.</ref> The ILO attempts to develop labor standards for workers in all industries. The unique tripartite structure of the ILO gives an equal voice to workers, employers and governments to ensure that the views of the social partners are closely reflected in labour standards and in shaping policies and programs. <ref> ISO "How the ISO Works" http://www.ilo.org/global/about-the-ilo/how-the-ilo-works/lang--en/index.htm. Accessed February 13, 2016.</ref> The ISO has established International Labour Standards on Occupational Safety and Health, which has developed more than 40 standards specifically dealing with occupational heath and safety.<ref> ILO. "International Labour Standards on Occupational Safety and Health". http://www.ilo.org/global/standards/subjects-covered-by-international-labour-standards/occupational-safety-and-health/lang--en/index.htm. Accessed February 13, 2016.</ref>. The important conventions are listed below:

====Occupational Safety and Health Convention, 1981 and Protocol of 2002====
#The convention provides for the adoption of national occupational safety and health policy by each participating nation-state. It includes actions taken by governments and within enterprise that operate within the governmental spaces to promote the improvement of occupational safety and health to therefore improve working conditions.
#The additional Protocol in 2002 calls for the establishment and period review of procedures for recording and notification of occupational accidents along with the publication of related statistics associated with the accidents. This Protocol is similar to the Emergency Planning and CommunityRight-To-Know Act instituted in the United States (see below).

====Occupational Heath Services Convention, 1985====
#Establishment of enterprise-level occupational health services which are entrusted with preventative functions and which are responsible for advising the employer, the workers, and their representatives in the organization on maintaining a safe and healthy work environment.

====Promotional Framework for Occupational Safety and Heath Convention, 2006====
#Aims to promote a preventative safety and health culture and to progressively achieve a safe and healthy working environment for all.
#Requires ratifying nation-states to develop, in consultation with the most representative organizations of employees and workers, including workers unions, a national policy, a national system, and a national program on occupational health and safety.
#This should be developed in accordance to the Occupational Heath Services Convention, 1985 and take into account other ILO standards.
#National systems shall provide the infrastructure for implementing national policy and programs, such as domestic laws, regulations, authority bodies, and compliance mechanisms.

====Chemicals Convention, 1990====
#Provides for the adoption and implementation of a coherent policy on the safety in the use of chemicals at work, including production, handling, storage, and transport.
#Also includes best practices for the disposal and treatment of waste chemicals and the release of chemicals resulting from work activities, maintenance, and cleaning of equipment.
#In addition, it allocates specific responsibilities to suppliers and exporting states.
#Chemicals shall be evaluated to determine their level of hazards and employers shall make these hazards known to their employees.

===International Organization for Standardization (ISO)===
The ISO is an international organization for standardization on topics ranging from quality management, environmental management, social responsibility, risk management, etc. ISO standards are adopted world-wide by organizations and governments as accepted and known standards for production. Many of the practices are self-regulating as costumers often demand certain standards from companies they purchase from.<ref>ISO. Home Page. http://www.iso.org/iso/home.html. Accessed February 13, 2016.</ref> The ISO is developing its own occupational health and safety framework shown below.

====ISO 45001 Occupational Health and Safety Management Systems<ref>ISO 45001. "Occupational heath and safety". http://www.iso.org/iso/home/standards/management-standards/iso45001.htm. Accessed February 13, 2016.</ref>====
#Designed to help organizations reduce the burden of occupational injuries and diseases by providing a framework to improve employee safety, reduce workplace risks, and create better, safer working conditions.
#Currently under development by a committee of occupational health and safety experts who will follow management systems approaches of ISO 4001 and ISO 9001.
#Will embody other International Standards such as the ILO's Occupational Health and Safety Guidelines.
#The expected publication will be released in October, 2016.

==US Safety Regulations==
Over time chemical industry regulations have been developed to ensure that the best safety practices are followed to maintain the health of both people and the environment. The development of most regulations is based around the idea that organizations have both a legal and moral obligation to safeguard the health and welfare of its employees and the public.<ref name="Towler" /> The extent of legislation varies across regions around the globe. In the United States, chemical accidents have led to the creation of regulation boards and safety oriented societies such as the Center for Chemical Plant Safety of the American Institute of Chemical Engineers to aid in the development and implementation of pant safety. <ref>Seider WD, Seader JD, Lewin DR, Seider WD. Product And Process Design Principles: Synthesis, Analysis, and Evaluation. New York: Wiley; 2004.</ref> In the US, the major federal laws relating to chemical plant safety and their regulations are as follows:

===The Occupational Safety and Health Act, 1970===
The OSH Act is administer by the Occupational Safety and Health Administration (OSHA) (see below). The Act covers all employers and their employees in the United States with coverage provided either directly by the Federal Occupational Safety and Heath Administration or by an OSHA-approved state job safety and health plan.<ref> United States Department of Labor. "Safety and Health Standards: Occupational Safety and Health". http://webapps.dol.gov/elaws/elg/osha.htm </ref> The main provisions of the act are listed below.
# Employers must supply place of employment free from toxic chemicals, excessive noise mechanical dangers, or unsanitary conditions.
#This involves the implementation of engineering controls to limit exposure to hazards and toxic substances and implementing administrative controls.
# For employees, employers must provide personal protective equipment (PPE) and training, including communications of known hazards.
# The Occupational Safety and Health Administration (OSHA) was established to promote best practices, inspect facilities for hazard analysis, set standards, and enforce the law.
# The National Institute of Occupational Safety and Health (NIOSH) was established to be an independent research institute (now under the Centers for Disease Control).
#The Act also encourages states to develop and operate their own job safety and health programs with OSHA as a monitoring agency for these "state plans," which operate under the authority of state law. The standards developed by state plans need to be at least as effective as the federal regulations.<ref> United States Department of Labor. "Safety and Health Standards: Occupational Safety and Health". http://webapps.dol.gov/elaws/elg/osha.htm </ref>
#Federal OSHA standards are categorized into four categories: General Industry, Construction, Maritime Terminals, Long-shoring, and Agriculture.

===The Toxic Substances Control Act (TSCA), 1976===
The main qualifications of the TSCA were to provide the EPA with regulating power of chemicals, specifically pertaining to the chemical industry and not including foods, drugs, cosmetics, or pesticides. <ref> Environmental Protection Agency. "Summary of the Toxic Substances Control Act." http://www.epa.gov/laws-regulations/summary-toxic-substances-control-act. Accessed February 13, 2016</ref>
# The Environmental Protection Agency (EPA) is required to regulate 75,000 chemical substances used in industry.
# The EPA has jurisdiction over the safety of the sale or development of new chemicals in the United States, including the requirement for pre-manufacture notification for new chemical substances before manufacture.
#The TSCA also addresses the production, importation, use, and disposal of specific chemicals including polychlorinated biphenyls (PCBs), asbestos, radon, and lead-based paint.<ref> Environmental Protection Agency. "Summary of the Toxic Substances Control Act." http://www.epa.gov/laws-regulations/summary-toxic-substances-control-act. Accessed February 13, 2016</ref>
# Everyone has the right and obligation to report information about any health or environmental effects caused by a chemical. This is especially important for organizations as they are required to report information to the EPA if a chemical substance is found to have substantial risk of injury to health or the environment.
====Frank R. Lautenberg Chemical Safety for the 21st Century Act, 2015====
The Toxic Substances Control Act will be revised in 2015 under this new act proposed to Congress. This Act is an attempt to eliminate some of the flaws associated with the TSCA.
#One of the biggest flaws in the TSCA is the fact that a new chemical can be used without first demonstrating safety, the idea that chemicals are safe until proven unsafe.<ref>Sheppard, Kate. "Senators Introduce Bill to Overhaul U.S. Chemical Industry." http://www.huffingtonpost.com/2015/03/10/toxic-chemicals-senate-bill_n_6842524.html. Accessed February 13, 2016</ref>
#The new bill would require safety testing before chemical implementation.
#It gives the EPA more defined power on the regulation of chemical substances and the Sustainable Chemical Program will be established. This brings the TSCA into the modern era of the chemical industry.<ref>S.697 - 114th Congress (2015-2016): Frank R. Lautenberg Chemical Safety for the 21st Century Act. Congress.gov. https://www.congress.gov/bill/114th-congress/senate-bill/697/all-info. Accessed January 27, 2016.</ref>

===The Emergency Planning and Community Right-to-Know Act (EPCRA), 1986<ref> US Government Printing Office. Emergency Panning and Community Right to Know. https://www.gpo.gov/fdsys/pkg/USCODE-2010-title42/html/USCODE-2010-title42-chap116.htm. Accessed February 13, 2016</ref>===
The Act was passed by Congress in response to concerns regarding the environmental and safety hazards posed by the storage and handling of toxic chemicals. This legislation was developed as a result of the 1984 chemical disaster in Bhopal, India.<ref>What is EPCRA? EPA. http://www.epa.gov/epcra/what-epcra. Published November 10, 2015. Accessed January 27, 2016.</ref>
# All facilities manufacturing, processing, or storing hazardous chemicals must make plans for major incidents if they were to occur and the plans must be made public so that local communities can be properly informed.
# All facilities must also produce Material Safety Data Sheets (MSDSs) to state and local officials as well as local fire departments.
# Local governments should help prepare emergency plans and review the plans annually.
# State governments are required to oversee and coordinate local planning efforts.
# Emergency Notification: Facilities must immediately report accidental releases of chemicals and "hazardous substances" in quantities greater than Reportable Quantities (RQs) defined under the Comprehensive Environmental Response, Compensation, and Liability Act to state and local officials with this information then being available to the public.
# Annually, facilities must complete and submit a toxic chemical release inventory form.

===Clean Air Act Amendments of 1990===
This Amendment to the original Clean Air Act of 1970 was designed to curb three major threats to the nation's environment and health of citizens: acid rain, urban air pollution, and toxic air emissions.<ref>Environmental Protection Agency. "1990 Clean Air Act Amendment Summary" http://www.epa.gov/clean-air-act-overview/1990-clean-air-act-amendment-summary. Accessed February 13, 2016.</ref>
# Established the U.S. Chemical Safety and Hazard Investigation Board, and independent federal agency with the goal of ensuring worker and public safety through the prevention or minimization of the effects of chemical accidents. <ref>Seider WD, Seader JD, Lewin DR, Seider WD. Product And Process Design Principles: Synthesis, Analysis, and Evaluation. New York: Wiley; 2004.</ref>
# Attempt to determine the roots and contributing causes of accidents and then provide briefs on the accidents.
# Led to the development of cap and trade systems for air pollutants.
# Gave the government significantly more power to control air emissions and administer admissions permits.

==Additional Legislative Information==
For more details on environmental legislation pertaining to release of materials to the environment or the regulations of the loss of containment, see [[Environmental concerns]].

In addition to the above federal regulations, various states and other municipalities also have enacted legislation for the regulation of chemical plants. These include more specific safety items, such as local fire codes or even put into place stricter aspects of the federal regulations. <ref name="Towler" />

Any process design or plant design must always meet the requirements of local and federal mandates and regulations. Without doing so, the wellbeing of plant employees and even the public can be placed into serious jeopardy.

=Safety Organizations and Terminology=
==Organizations==
===OSHA===
The Occupational Safety and Health Administration (OSHA) is a federal agency that focuses on the enforcement of safety and health legislation.

===EPA===
The Environmental Protection Agency (EPA) is a U.S. agency whose purpose is the protection of the health of both humans and the environment through the writing and enforcement of regulatory laws.

===DOT===
The Department of Transportation (DOT) oversees federal highway, air, and maritime transportation, and can be involved in the safe transport of chemicals.

===DOE===
The Department of Energy (DOE) is a governmental department tasked with the advancement of energy technology in the United States.

==Terminology==
===HS&E===
Health, Safety, and Environmental - This term refers to all health, safety, and environmental concerns that arise at each stage of the design process. Companies are required to analyze each part of the process from an HS&E perspective to create a safe and healthy work environment.

===MSDS===
Material Safety and Data Sheet - Every chemical has an MSDS which contains all the information regarding safe handling and how to deal with spills or other accidents involving the substance. Relevant information includes how to identify the substance, hazard information, and how to handle spills, fires, and exposure, among other things.

===FMEA===
Failure Mode and Effects Analysis - FMEA is an early stage approach to identifying critical technical risks using a semi-quantitative procedure. The analysis encompasses safety, environmental, and operational feasibility. When performing FMEA, engineers look to see places in a potential design that could fail, and then quantify how likely that failure is, how severe the results would be, and then offer potential solutions to minimize the risk. A step-by-step guide to performing FMEA is shown below (Northwestern University, 2014):

# Brainstorm for failure modes
# For each FM, rate severity of impact (SEV, 1 - 10).
# For each FM, brainstorm for possible causes (there may be multiple).
# For each cause, rate likelihood of occurring (OCC, 1 - 10).
# Rate the probability that the systems currently in place will detect and prevent the problem before it has an impact (DET, 10 - 1). Do not assume that something that will be added to the design later will take care of the problem.
# Overall Risk Probability Number RPN = SEV x OCC x DET. Most practitioners use the 1,4,7,10 scale below to increase granularity. Note that the DET scale is inverse to SEV and OCC.

[[File:Safety3.JPG|center|700px]]

Figure 1: Suggested scale to be used for quantifying risk and detection of failures in FMEA. Taken from ChE 351 Slides.

The sum of all information collected is implemented into a spreadsheet like the one shown below:

[[File:Safety4.JPG|center|700px]]

Figure 2: Example spreadsheet used to organize FMEA data.

===HAZOP===
Hazard and Operability Study - For more information regarding HAZOP, please refer to [[Process Hazards]].

===SIL===
Safety Integrity Levels - The SIL is the relative level of risk-reduction provided by a safety function, or to specify a target level of risk reduction. A SIL is determined based on a number of qualitative factors such as development process and safety life cycle management. Several methods are used such as risk matrices, risk graphs, layers of protective analysis (LOPA). Three levels of safety integrity are assigned depending on the “availability” of the safety instrumented system (SIS), as shown below (Towler et al., 2012):

[[File:Safety5.JPG|center|500px]]

Figure 3: Table of safety integrity levels based on availability of system. Taken from (Towler et al., 2012).

Redundant system means instrumentation is duplicated; higher level of redundancy of trip systems give higher SIL. The required SIL should be determined during a process hazard analysis and depends on risk of operator exposure and injury.

1 Instrument
* if it signals, plant goes down
* Probability of incident = probability of instrument failure
* Probability of spurious trip = false positive rate of one instrument

2 Instruments
* 1 out of 2 voting (1oo2): one instrument signals, plant goes down
** Probability of incident reduced by duplication
** Probability of spurious trip doubled
* 2oo2 voting
** Probability of incident worse than single instrument (twice likelihood that system is down)
** Probability of spurious trip reduced

3 Instruments
* 2oo3 voting
** Best overall trade-off between reducing incident rate and spurious trip rate
** One malfunctioning instrument does not cause trip or prevent detection of real incident

=Safe Design=
==Inherently Safe Design==
Inherently safe design of a particular process can be achieved by adhering to the following six strategies (Turton et al., 2003):

1. Substitution: Avoid using or producing hazardous materials on the plant site. If the hazardous material is an intermediate product, for example, alternate chemical reaction pathways might be used. In other words, the most inherently safe strategy is to avoid the use of hazardous materials.

2. Intensification: Attempt to use less of the hazardous materials. In terms of a hazardous intermediate, the two processes could be more closely coupled, reducing or eliminating the amount of intermediate produced. The inventories of hazardous feeds or products can be reduced by enhanced scheduling techniques such as just-in-time (JIT) manufacturing.

3. Attenuation: Reducing, or attenuating, the hazards of materials can often be affected by lowering the temperature or adding stabilizing additives. By using materials under less hazardous conditions, the potential consequences of a leak can be reduced.

4. Containment: If the hazardous materials cannot be eliminated, they at least should be stored in vessels with mechanical integrity beyond any reasonably expected temperature or pressure excursion. This is an old but effective strategy to avoid leaks. However, it is not as inherently safe as substitution, intensification, or attenuation.

5. Control: If a leak of hazardous material does occur, there should be safety systems that reduce the effects. For example, chemical facilities often have emergency isolation of the site from the normal storm sewers, and large tanks for flammable liquids are surrounded by dikes that prevent any leaks from spreading to to other areas of the plant. Scrubbing systems and relief systems in general are in this category. They are essential, because they allow controlled, safe release of hazardous materials, rather than an uncontrolled release from a vessel rupture.

6. Survival: If leaks of hazardous materials do occur and they are not contained or controlled, the personnel (and the equipment) must be protected. This lowest level of the hierarchy includes fire fighting, gas masks, and so on. Although essential to the total safety of the plant, the greater the reliance on survival of leaks rather than elimination of leaks, the less inherently safe the facility.

===Safety Legislation and Process Design===
====Inherent Process Plant Safety====
Through the enactment of safety regulation shown above, process design has inherently become safer. These regulations have made safety the first and foremost important concern when designing a new chemical plant. In general, the safety of a process relies on multiple layers of protection, but the first and most important layer of protection has become the process design feature. Greater tolerances are built into the designing of processes to more effectively prevent catastrophic failures or chemical leakage. The best approach to prevent accidents is to add process design features, involving chemistry and physics, to prevent hazardous situations.
=====Examples of Inherently Safe Process Design<ref>Crowl, Daniel. Louver, Joseph. Chemical Process Safety: Fundamentals with Applications. IBN: 9780132440554</ref>=====
#Vapor Release: Vapor released from spills can be minimized by designing dikes so that flammable and toxic materials will not accumulate around leaking tanks. This also prevents potential flammable materials from building up and causing an explosion.
#Containment Building: Design can be important for the containment of toxic spills. With the addition of automatic or remote controls, personnel can leave the area if a spill or breach occurs, while the area can be continuously monitored.
#Solvent Substitution: Safety through Substitution - Substituting design with safer, less hazardous materials. Designing process for use with less toxic or flammable solvents. For example, water-based paints and adhesives as well as aqueous or dry flowable formulations for agricultural chemicals opposed to more volatile solvents that release VOCs.
#Design for Lower Temperature and Pressure Safety through Moderation - use a hazardous material under less hazardous conditions such as lower pressure and temperature conditions. This can lower the level of catastrophe if downstream safety processes do fail.

====Control Systems Safety====
Legislation has also added to subsequent layers of controls such as environmental process controls have been added to prevent release to nearby air and water systems, which would endanger surrounding ecosystems and human populations.

==Assessing Preliminary Design==
While pilot plants are necessary to design effective plant equipment, there are some dangers associated with the industrial scale of chemical plants. Scaling up without accurate literature and experimental data can be very dangerous.

In every step of process design, the Health, Safety, and Environmental (HS&E) analysis must be carried out with the available technical information (Biegler et al., 1997).

[[File:Safety1.JPG|center|800px]]

[[File:Safety2.JPG|center|700px]]

Figure 4: General steps in the design process and analysis to be carried out at each stage. Taken from (Biegler et al., 1997).

=Economic Cost of Safety=
Price of Safety installations and fire protection systems range from .5 to 1.6 % of fixed-capital investment of a plant; but expenditures are often much higher than this and it is difficult to estimate these expenditures for a given plant

In addition, designers also examine the economic impact of safety and maintenance issues. For instance, they may determine that the plant reactor configuration can be improved, and with improved operator training facilities, it can run with improved safety. These hidden costs must be considered when determining the economic feasibility of operation (Biegler et al., 1997).

=Other Process Safety Considerations=
The safety and well-being of the consumers using the eventual product should be considered in process safety. The risks involved in using a product should be clearly communicated to the consumer by industrial leaders.

Human Error is another safety risk that is difficult to quantify. The intervention of well-trained operators is a vital layer in process safety (Peters et al., 2003).

=Case Study: Production of Ultrapure Hydrogen=
[[File:steam reforming.jpg|thumb|alt=A hydrogen production plant with a box steam reformer|A hydrogen production plant with a box steam reformer|300px|right|A hydrogen production plant with a box steam reformer]]
To walk through the process safety considerations when designing a chemical plant, the relatively simple example of a high purity hydrogen generation will be examined. First, various technologies are researched to determine the options that best meet the economic, physical, environmental, and safety constraints of the project. For this project, steam reforming, autothermal reforming, and partial oxidation were investigated. Technologies that lack widespread implementation have inherent safety risks as they have higher uncertainties associated with reliability, feasibility, and cost. Autothermal reforming requires oxygen and not a commercially popular method. Partial oxidation requires no catalyst, but requires high process temperatures and is a complex process to implement. After each process was analyzed and scored in a decision matrix, steam reforming was chosen as the base process technology. It was selected because it is a safe, well-known, low-emission, traditional process in use all over the world for the production of hydrogen. Steam reforming produces minimal waste compared to the alternative processes, and is capable of producing the necessary 100 MMscfd of 99.999% hydrogen gas. It is important to remember that both local and global environmental emissions are capable of harming the general public, so they should be considered safety concerns in the same way worker hazards are. Next, various reactor types, catalysts, and separation methods were evaluated with the base process chosen. In all, seven different processing stages were assessed including the initial heating of feed, the steam reformation reactor, the high and low temperature water shift reactors, the amine plant, the methanation reactor, the gas compression and cooling train, and the pressure swing adsorption unit.

Although worker safety is always the first priority in a plant, there are inherent risk associated with high temperature and pressure processes. Close attention was paid to make the preliminary plant design as inherently safe as possible. Preliminary FMEA and HAZOP analyses were conducted to identify the highest priority risks. Hazards were mitigated by substituting less hazardous materials when possible, opting to store only the necessary hazardous materials on site, lowering temperatures when possible, adding catastrophic failure controls, maximizing plant control automation when economically feasible, and necessitating worker personal protective equipment. Finally, FMEA and HAZAP analyses were repeated. These steps were repeated multiple times until a sufficient reduction in risk had been achieved.

=Case Study: Bhopal Disaster=
[[File:Bhopal.JPG|thumb|alt=The dispersal of the deadly cloud of gas from the Union Carbide plant in the Indian city of Bhopal|The dispersal of the deadly cloud of gas from the Union Carbide plant in the Indian city of Bhopal|400px|right|The dispersal of the deadly cloud of gas from the Union Carbide plant in the Indian city of Bhopal]]
On December 3, 1984 at the India Limited Pesticide Plant owned by Union Carbide in Bhopal, India, water entered a storage vessel containing over 80,000 lbs. of methyl isocyanate (MIC), a chemical intermediate in the pesticide synthesis process. This reaction caused a rapid increase in temperature accompanied by boiling, which caused toxic MIC vapors to escape from the tank. In addition, the MIC-water reaction produced methylamine and carbon dioxide gases among other toxic products which also contributed to the pressure increase (Union Carbide Corporation, 1967). These vapors passed into a scrubber and flare system that were not working at the time due to inadequate maintenance and safety practices. As a result of this accident, approximately 25 tons of MIC vapor were released, killing over 3,800 immediately and injuring roughly 20,000 in the surrounding area.

As a result of the incident, Union Carbide was forced to pay $470 million, as well as fund a hospital in Bhopal that was used specifically to treat victims of the disaster. Cleanup of the plant site and other legal action are still being determined to this day. Bhopal sparked a worldwide discussion on chemical process safety, and caused Congress to create the U.S. Chemical Safety Board (CSB). The CSB has since cited the following reasons as causes for the disaster:

* No process hazard analysis
* Poorly maintained equipment and safety system
* Lack of emergency response planning
* Inadequate training for operators

The CSB has pushed chemical safety reform since its conception, urging the chemical industry to produce inherently safer designs, use better quality equipment, and develop more thorough risk management plans. A major criticism of the process was its lack of inherent safe design. Because MIC was an intermediate, there was no reason to keep large quantities in storage. A modern design would use the intermediate as it is made (Eckerman et al., 2005).

Although chemical process safety has come a long way since 1984, industrial chemical giants still battle problems similar to Bhopal until this day. In 2008, a disaster similar to the one in Bhopal could have occurred in a plant originally designed by Union Carbide located in Institute, West Virginia after a runaway reaction caused a pressure buildup in a waste treatment vessel. The vessel exploded, killing two plant workers. Fortunately, the explosion missed a large MIC storage vessel which could have been hit by shrapnel and released tons of MIC (Blanc et al., 2009) In 2013, an ammonium nitrate explosion killed 15 and seriously injured 200 in West Texas in a blast radius similar to the one experienced in Bhopal (U.S.Chemical Safety Board, 2014).

=Case Study: Deepwater Horizon Explosion and Oil Spill=
[[File:Deepwater Horizon offshore drilling unit on fire 2010.jpg|thumb|alt=The burning Deepwater Horizon oilrig after an explosion caused by a design failure in the blowout preventer|The burning Deepwater Horizon oilrig after an explosion caused by a design failure in the blowout preventer|400px|right|The burning Deepwater Horizon oilrig after an explosion caused by a design failure in the blowout preventer]]
On April 20, 2010 on the Deepwater Horizon offshore drilling rig located in the Macondo Prospect, multiple explosions killed 11 workers and seriously injured 17. The rig burned for two days before sinking into the Gulf of Mexico. Key safety failures caused the well to spew 5 million barrels of oil into the Gulf of Mexico over the next 87 days making the incident the largest offshore oil spill in U.S. history. Finally, the well was sealed by a “static kill,” the injection of heavy fluids and cement, at the leak point 5,000 feet below the surface.

The key safety failure identified by the CSB was the blowout preventer (BOP) failure. This device that is meant to prevent the filling of annular space between the borehole and the well casing is both electrically and hydraulically powered. It is connected to a rig by a large diameter pipe called a riser. The system contains multiple pipe rams and annular preventers designed to prevent annular space buildup.

On the first night of the incident, a “kick” occurred and a mixture of oil, water, and gases began to build up in the wall and climb up the shaft. Drilling mud was injected to prevent kicks by creating a barrier. An upper annular preventer was also engaged when the buildup was discovered, but it failed. A pipe ram was activated and succeeded. However, an immense pressure buildup caused the drill pipe to buckle so it was forced off center. This buckling was later explained as a result of effective compression. This phenomenon is caused by microscopic irregularities and bends in the pipe material resulting in a higher surface area on one side of the pipe. Because the pipe was off-center, the final failsafe, the Automatic Mode Function (AMF) or deadman could not effectively shear the pipe and seal the well. This redundant control system comprised of a yellow pod and blue pod work independently to seal the well in the event of catastrophic failure when communications, electric power, and hydraulic pressure connections are cut. Both the yellow and blue pods contained 9 volt and 27 volt batteries which power solenoid valves. Unfortunately, the blue pod was miswired, so its 27 volt power supply was drained when it was to cause the blind shear blades to cut the pipe. Fortunately, a 9 volt battery in the yellow pod was also miswired which caused the blind shear ram to be engaged. However, this only partially sealed the well because of the pipe buckling. The flammable mixture erupted onto the surface of the platform and found an ignition source triggering a massive explosion. The spill was temporarily contained by a cap, and relief wells were eventually used to seal the well months later (U.S.Chemical Safety Board, 2014).

The White House Office of Energy and Climate Change Policy called the Deepwater Horizon oil spill the “worst environmental disaster the US has faced (BBC News, 2010). Over 8,000 species were estimated to be affected by the spill due to the toxicity of petroleum released, oxygen depletion, and the large quantities of Corexit, an oil dispersant used in an untested manner that is toxic to marine life (Biello et al., 2010; Butler, 2011; Froomkin, 2010).

BP, Transocean, and Halliburton were the major entities implicated in this tragedy. Investigations after the incident show that essential safety documentation including risk management and emergency procedure information were missing. Accusations were mainly aimed at BP with charges of recklessness and gross negligence (CNN Money, 2012). In January 2013, Transocean was ordered to pay $1.4 billion for US Clean Water Act violations. BP was ordered to pay $2.4 billion, but additional penalties could reach $20 billion (Department of Justice Office of Public Affairs, 2013).

=Case Study: Texas City Refinery Explosion=
[[File:BP_PLANT_EXPLOSION-1_lowres2.jpg|thumb|alt=Emergency response workers fight secondary fires caused by the isomerization unit explosion at the Texas City refinery|Emergency response workers fight secondary fires caused by the isomerization unit explosion at the Texas City refinery|228px|right|Emergency response workers fight secondary fires caused by the isomerization unit explosion at the Texas City refinery]]
On March 23, 2005 at a BP refinery in Texas City, Texas a hydrocarbon vapor cloud ignited, killing 15 workers and seriously injuring 170 others. Over the course an 11 hour period, a combination of control failures, mismanagement, and worker fatigue resulted in the buildup and release of extremely hot, combustible vapor. The key process unit in this disaster was an isomerization unit, located next to wooden trailers for workers servicing an ultracracker unit.

In the early morning on March 23rd, operators initiated startup and pumped raffinate (liquid hydrocarbons) into a raffinate splitter tower used to separate gasoline components. A liquid level indicator and multiple high level alarms monitored the tower liquid level. The level indicator could only measure up to 9 feet of liquid, and the written process called for a liquid level of about 6.5 feet. However, operators routinely filled the tower over 9 feet to minimize fluctuations and to prevent damage to a furnace. Hours later, the first high level alarm was activated and the liquid level rose, but a second alarm higher up the tower failed to trigger. The feed was halted when the liquid had risen to a level of about 13 feet, operators had no way of knowing the exact height. The lead operator relayed the startup activities to another operator and left the facility an hour before his shift ended. The morning operator arrived at 6 am to start his thirtieth consecutive day working a 12-hour shift and read a logbook that read, “Isom* Brought in some raff to unit, to pack raff with.” The day shift operator arrived an hour late, so he could not be briefed by the night shift supervisor. Recirculation then commenced in the tower, and more liquid was added to the tower. Additionally, conflicting instructions caused a liquid level regulating valve to remain closed for several hours, so liquid could not leave the tower. The furnace was then lit, and the supervisor left to attend a family medical emergency.

At noon, the liquid level had risen to 98 feet, but the improperly calibrated liquid level indicator read 8.4 feet. At 12:41 pm, a high pressure alarm caused workers to manually open a chain valve to relieve pressure by using the units pressure relief system to vent vapor into the atmosphere using an obsolete blowdown drum. Heat was also reduced in the furnace to reduce pressure. When operators became concerned about outflow rate, the liquid level regulating valve was opened to release liquid from the tower to storage. This caused the liquid in the tower to begin to boil and spill into the overhead vapor line exerting extreme pressures on the pressure relief system. At 1:14 pm, the three relief valves opened sending the liquid to the blowdown drum which overflowed into a municipal sewer setting off alarms, but a key level indicator in the blowdown drum failed. Flammable liquid erupted from the blowdown drum, formed a massive vapor cloud, and found an ignition source from a nearby idling pickup truck. The colossal blast ignited fires throughout the refinery and over half the workers in the wooden trailers adjacent to the unit were killed immediately (U.S.Chemical Safety Board, 2008).

Investigations after the incident cited multiple failures to implement safety recommendations at the Texas City Refinery. Among these, the blowdown drum was to be replaced by a modern flare to burn off hydrocarbons. However, BP’s budget cuts prevented its replacement. The training and treatment of workers was also called into question, as fatigue, poor communication, and inadequate documentation likely contributed to the disaster. Decisions like the one to operate an unsafe liquid level in order to prevent furnace damage also demonstrate the company’s fixation on the bottom line. BP was eventually fined $21 million by OSHA (New York Times, 2010; U.S.Chemical Safety Board, 2008).

=References=
==Direct Citation References==
<references/>

==Additional References==
# American Institute of Chemical Engineers. http://www.aiche.org/ccps
# Biello, David (9 June 2010). "The BP Spill's Growing Toll On the Sea Life of the Gulf". Yale Environment 360. Yale School of Forestry & Environmental Studies. Retrieved 2010-06-14.
# Blanc, P. Bhopal, 1984 – West Virginia near-miss, 2008. Psychology Today, December 2009. https://www.psychologytoday.com/blog/household-hazards/200912/bhopal-1984-west-virginia-near-miss-2008.
# Butler, J. Steven (3 March 2011). "BP Macondo Well Incident. U.S. Gulf of Mexico. Pollution Containment and Remediation Efforts" (PDF). Lillehammer Energy Claims Conference. BDO Consulting. Retrieved 17 February 2013.
# “DOJ accuses BP of ‘gross negligence’ in Gulf oil spill” CNN Money, September 2012.
# Eckerman, I. The Bhopal Saga: Causes and Consequences of the World’s Largest Industrial Disaster. Universities Press Private Limited, 2005.
# Froomkin, Dan (29 July 2010). "Scientists Find Evidence That Oil And Dispersant Mix Is Making Its Way Into The Foodchain". Huffington Post.
# "Gulf of Mexico oil leak 'worst US environment disaster'". BBC News. 30 May 2010.
# Investigation Report: BP Refinery Explosion and Fire, U.S. Chemical Safety Board, 2008.
# Lyall, Sarah. "In BP’s Record, a History of Boldness and Costly Blunders." New York Times, July 13, 2010.
# L.T. Biegler, I.E. Grossmann, A.W. Westerberg. ''Systematic Methods of Chemical Process Design''. Prentice-Hall: Upper Saddle River, 1997.
# M.S. Peters, K.D. Timmerhaus. ''Plant Design and Economics for Chemical Engineers, 5th Ed.'' McGraw-Hill: New York, 2003.
# Northwestern University. Chemical Engineering 351 Lecture Slides.
# Reflections on Bhopal After Thirty Years Video, U.S.Chemical Safety Board, December 2014.
# R.T. Turton, R.C. Bailie, W.B. Whiting, J.A. Shaeiwitz. ''Analysis, Synthesis, and Design of Chemical Processes''. Prentice Hall: Upper Saddle River, 2003.
# Union Carbide Corporation "Methyl Isocyanate" Product Information Publication, F-41443, November 1967.
# USCSB Deepwater Horizon Video, U.S.Chemical Safety Board, June 2014.
# "Transocean Agrees to Plead Guilty to Environmental Crime and Enter Civil Settlement to Resolve U.S. Clean Water Act Penalty Claims from Deepwater Horizon Incident". Department of Justice Office of Public Affairs. January 3, 2013.

Process safety

2016-02-13T20:36:51Z

Evanrosati: /* Clean Air Act Amendments of 1990 */

Authors: Anne Disabato, [2014] Tim Hanrahan, [2014] Brian Merkle, [2014] , Spencer Saldana [2015] and Evan Rosati [2016] 

Stewards: David Chen, Jian Gong, and Fengqi You

Date Presented: January 19, 2014

 


__TOC__

=Introduction=
The safe design and operation of facilities is of paramount importance to every company that is involved in the manufacture of fuels, chemicals, and pharmaceuticals. Process safety focuses on the prevention of dangerous situations, such as fires, explosions, and the release of chemicals.

The American Institute of Chemical Engineers emphasizes a culture of process safety through four pillars (American Institute of Chemical Engineers, 2015):

1. Commitment to Process Safety: a workforce that is actively involved and an organization that fully supports process safety as a core value will tend to do the right things in the right way at the right time – even when no one else is looking

2. Understanding Hazard and Risk: the foundation of a risk-based approach which will allow an organization to use this information to allocate limited resources in the most effective manner

3. Manage Risk: the ongoing execution of risk based process safety tasks. Risk management can help a company to better deal with the resultant risks and sustain long-term accident free and profitable operations

4. Learn from Experience: Metrics provide direct feedback on the workings of RBPS systems, and leading indicators provide early warning signals of ineffective process safety results. Organizations must use their mistakes and those of others as motivation for action and view as opportunities for improvement.

For the prevention and management of specific safety hazards, such as fires, explosions, or the release of toxic chemicals, please see [[Process Hazards]].

==Layers of Plant Safety==
Safety and loss prevention can be expressed in "layers" of plant safety in terms of design and implementation.<ref name="Towler">G.P. Towler, R. Sinnott. ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design''. Elsevier, 2012.</ref> Each higher layer can be activated if a lower level fails. This creates a system with subsequent levels of safety to help prevent catastrophe from occurring. This diagram shows the important of safety in process design. If a process is designed to be inherently safe, additional safety "controls" will be less important and a chemical plant will be overall safer. The goal of safety is not to reach the top of the triangle, but to stay as close to the bottom as possible. This shows the importance of inherently safe design and safety legislation and regulations to provide guidelines for safety and health concerns. Regulations provide a baseline for engineers to operate when designing a chemical plant. They have brought safety to the forefront of design, when engineers have the maximum degree of freedom for implementation, and are no longer considered to be an afterthought or strictly a controls issue. Plants can be designed to be safe without the extensive use of future adaptation, safety controls, or emergency response. Although you can never eliminate these upper layers of process safety, by designing a process smartly and safely, engineers can reduce the consequences of "walking up" the process safety ladder or triangle.
[[File:Layers_of_Plant_Safety.png|center|700px|thumb|alt=Triangle Diagram on Increasing Plant Safety Mechanisms|Layers of Plant Safety Triangle Diagram]]

=Safety Legislation=

==International Regulations==
International regulations can be considered as best practice standards that are adopted by governments through treaties and establishment through United Nation resolutions. Most international regulation agencies can only register complaints for violations, but can not implement sanctions or fines to infracting parties.

===Internation Labour Organization (ILO)===
After the dissolution of the League of Nations in 1946, the ILO became the first specialized agency of the United Nations upon its founding in December of that same year. <ref> ISO Photo Gallery. http://www.ilo.org/dyn/media/mediasearch.fiche?p_id=16023&p_lang=en. Accessed February 13, 2016.</ref> The ILO attempts to develop labor standards for workers in all industries. The unique tripartite structure of the ILO gives an equal voice to workers, employers and governments to ensure that the views of the social partners are closely reflected in labour standards and in shaping policies and programs. <ref> ISO "How the ISO Works" http://www.ilo.org/global/about-the-ilo/how-the-ilo-works/lang--en/index.htm. Accessed February 13, 2016.</ref> The ISO has established International Labour Standards on Occupational Safety and Health, which has developed more than 40 standards specifically dealing with occupational heath and safety.<ref> ILO. "International Labour Standards on Occupational Safety and Health". http://www.ilo.org/global/standards/subjects-covered-by-international-labour-standards/occupational-safety-and-health/lang--en/index.htm. Accessed February 13, 2016.</ref>. The important conventions are listed below:

====Occupational Safety and Health Convention, 1981 and Protocol of 2002====
#The convention provides for the adoption of national occupational safety and health policy by each participating nation-state. It includes actions taken by governments and within enterprise that operate within the governmental spaces to promote the improvement of occupational safety and health to therefore improve working conditions.
#The additional Protocol in 2002 calls for the establishment and period review of procedures for recording and notification of occupational accidents along with the publication of related statistics associated with the accidents. This Protocol is similar to the Emergency Planning and CommunityRight-To-Know Act instituted in the United States (see below).

====Occupational Heath Services Convention, 1985====
#Establishment of enterprise-level occupational health services which are entrusted with preventative functions and which are responsible for advising the employer, the workers, and their representatives in the organization on maintaining a safe and healthy work environment.

====Promotional Framework for Occupational Safety and Heath Convention, 2006====
#Aims to promote a preventative safety and health culture and to progressively achieve a safe and healthy working environment for all.
#Requires ratifying nation-states to develop, in consultation with the most representative organizations of employees and workers, including workers unions, a national policy, a national system, and a national program on occupational health and safety.
#This should be developed in accordance to the Occupational Heath Services Convention, 1985 and take into account other ILO standards.
#National systems shall provide the infrastructure for implementing national policy and programs, such as domestic laws, regulations, authority bodies, and compliance mechanisms.

====Chemicals Convention, 1990====
#Provides for the adoption and implementation of a coherent policy on the safety in the use of chemicals at work, including production, handling, storage, and transport.
#Also includes best practices for the disposal and treatment of waste chemicals and the release of chemicals resulting from work activities, maintenance, and cleaning of equipment.
#In addition, it allocates specific responsibilities to suppliers and exporting states.
#Chemicals shall be evaluated to determine their level of hazards and employers shall make these hazards known to their employees.

===International Organization for Standardization (ISO)===
The ISO is an international organization for standardization on topics ranging from quality management, environmental management, social responsibility, risk management, etc. ISO standards are adopted world-wide by organizations and governments as accepted and known standards for production. Many of the practices are self-regulating as costumers often demand certain standards from companies they purchase from.<ref>ISO. Home Page. http://www.iso.org/iso/home.html. Accessed February 13, 2016.</ref> The ISO is developing its own occupational health and safety framework shown below.

====ISO 45001 Occupational Health and Safety Management Systems<ref>ISO 45001. "Occupational heath and safety". http://www.iso.org/iso/home/standards/management-standards/iso45001.htm. Accessed February 13, 2016.</ref>====
#Designed to help organizations reduce the burden of occupational injuries and diseases by providing a framework to improve employee safety, reduce workplace risks, and create better, safer working conditions.
#Currently under development by a committee of occupational health and safety experts who will follow management systems approaches of ISO 4001 and ISO 9001.
#Will embody other International Standards such as the ILO's Occupational Health and Safety Guidelines.
#The expected publication will be released in October, 2016.

==US Safety Regulations==
Over time chemical industry regulations have been developed to ensure that the best safety practices are followed to maintain the health of both people and the environment. The development of most regulations is based around the idea that organizations have both a legal and moral obligation to safeguard the health and welfare of its employees and the public.<ref name="Towler" /> The extent of legislation varies across regions around the globe. In the United States, chemical accidents have led to the creation of regulation boards and safety oriented societies such as the Center for Chemical Plant Safety of the American Institute of Chemical Engineers to aid in the development and implementation of pant safety. <ref>Seider WD, Seader JD, Lewin DR, Seider WD. Product And Process Design Principles: Synthesis, Analysis, and Evaluation. New York: Wiley; 2004.</ref> In the US, the major federal laws relating to chemical plant safety and their regulations are as follows:

===The Occupational Safety and Health Act, 1970===
The OSH Act is administer by the Occupational Safety and Health Administration (OSHA) (see below). The Act covers all employers and their employees in the United States with coverage provided either directly by the Federal Occupational Safety and Heath Administration or by an OSHA-approved state job safety and health plan.<ref> United States Department of Labor. "Safety and Health Standards: Occupational Safety and Health". http://webapps.dol.gov/elaws/elg/osha.htm </ref> The main provisions of the act are listed below.
# Employers must supply place of employment free from toxic chemicals, excessive noise mechanical dangers, or unsanitary conditions.
#This involves the implementation of engineering controls to limit exposure to hazards and toxic substances and implementing administrative controls.
# For employees, employers must provide personal protective equipment (PPE) and training, including communications of known hazards.
# The Occupational Safety and Health Administration (OSHA) was established to promote best practices, inspect facilities for hazard analysis, set standards, and enforce the law.
# The National Institute of Occupational Safety and Health (NIOSH) was established to be an independent research institute (now under the Centers for Disease Control).
#The Act also encourages states to develop and operate their own job safety and health programs with OSHA as a monitoring agency for these "state plans," which operate under the authority of state law. The standards developed by state plans need to be at least as effective as the federal regulations.<ref> United States Department of Labor. "Safety and Health Standards: Occupational Safety and Health". http://webapps.dol.gov/elaws/elg/osha.htm </ref>
#Federal OSHA standards are categorized into four categories: General Industry, Construction, Maritime Terminals, Long-shoring, and Agriculture.

===The Toxic Substances Control Act (TSCA), 1976===
The main qualifications of the TSCA were to provide the EPA with regulating power of chemicals, specifically pertaining to the chemical industry and not including foods, drugs, cosmetics, or pesticides. <ref> Environmental Protection Agency. "Summary of the Toxic Substances Control Act." http://www.epa.gov/laws-regulations/summary-toxic-substances-control-act. Accessed February 13, 2016</ref>
# The Environmental Protection Agency (EPA) is required to regulate 75,000 chemical substances used in industry.
# The EPA has jurisdiction over the safety of the sale or development of new chemicals in the United States, including the requirement for pre-manufacture notification for new chemical substances before manufacture.
#The TSCA also addresses the production, importation, use, and disposal of specific chemicals including polychlorinated biphenyls (PCBs), asbestos, radon, and lead-based paint.<ref> Environmental Protection Agency. "Summary of the Toxic Substances Control Act." http://www.epa.gov/laws-regulations/summary-toxic-substances-control-act. Accessed February 13, 2016</ref>
# Everyone has the right and obligation to report information about any health or environmental effects caused by a chemical. This is especially important for organizations as they are required to report information to the EPA if a chemical substance is found to have substantial risk of injury to health or the environment.
====Frank R. Lautenberg Chemical Safety for the 21st Century Act, 2015====
The Toxic Substances Control Act will be revised in 2015 under this new act proposed to Congress. This Act is an attempt to eliminate some of the flaws associated with the TSCA.
#One of the biggest flaws in the TSCA is the fact that a new chemical can be used without first demonstrating safety, the idea that chemicals are safe until proven unsafe.<ref>Sheppard, Kate. "Senators Introduce Bill to Overhaul U.S. Chemical Industry." http://www.huffingtonpost.com/2015/03/10/toxic-chemicals-senate-bill_n_6842524.html. Accessed February 13, 2016</ref>
#The new bill would require safety testing before chemical implementation.
#It gives the EPA more defined power on the regulation of chemical substances and the Sustainable Chemical Program will be established. This brings the TSCA into the modern era of the chemical industry.<ref>S.697 - 114th Congress (2015-2016): Frank R. Lautenberg Chemical Safety for the 21st Century Act. Congress.gov. https://www.congress.gov/bill/114th-congress/senate-bill/697/all-info. Accessed January 27, 2016.</ref>

===The Emergency Planning and Community Right-to-Know Act (EPCRA), 1986<ref> US Government Printing Office. Emergency Panning and Community Right to Know. https://www.gpo.gov/fdsys/pkg/USCODE-2010-title42/html/USCODE-2010-title42-chap116.htm. Accessed February 13, 2016</ref>===
The Act was passed by Congress in response to concerns regarding the environmental and safety hazards posed by the storage and handling of toxic chemicals. This legislation was developed as a result of the 1984 chemical disaster in Bhopal, India.<ref>What is EPCRA? EPA. http://www.epa.gov/epcra/what-epcra. Published November 10, 2015. Accessed January 27, 2016.</ref>
# All facilities manufacturing, processing, or storing hazardous chemicals must make plans for major incidents if they were to occur and the plans must be made public so that local communities can be properly informed.
# All facilities must also produce Material Safety Data Sheets (MSDSs) to state and local officials as well as local fire departments.
# Local governments should help prepare emergency plans and review the plans annually.
# State governments are required to oversee and coordinate local planning efforts.
# Emergency Notification: Facilities must immediately report accidental releases of chemicals and "hazardous substances" in quantities greater than Reportable Quantities (RQs) defined under the Comprehensive Environmental Response, Compensation, and Liability Act to state and local officials with this information then being available to the public.
# Annually, facilities must complete and submit a toxic chemical release inventory form.

===Clean Air Act Amendments of 1990===
This Amendment to the original Clean Air Act of 1970 was designed to curb three major threats to the nation's environment and health of citizens: acid rain, urban air pollution, and toxic air emissions.<ref>Environmental Protection Agency. "1990 Clean Air Act Amendment Summary" http://www.epa.gov/clean-air-act-overview/1990-clean-air-act-amendment-summary. Accessed February 13, 2016.</ref>
# Established the U.S. Chemical Safety and Hazard Investigation Board, and independent federal agency with the goal of ensuring worker and public safety through the prevention or minimization of the effects of chemical accidents. <ref>Seider WD, Seader JD, Lewin DR, Seider WD. Product And Process Design Principles: Synthesis, Analysis, and Evaluation. New York: Wiley; 2004.</ref>
# Attempt to determine the roots and contributing causes of accidents and then provide briefs on the accidents.
# Led to the development of cap and trade systems for air pollutants.
# Gave the government significantly more power to control air emissions and administer admissions permits.

==Additional Legislative Information==
For more details on environmental legislation pertaining to release of materials to the environment or the regulations of the loss of containment, see [[Environmental concerns]].

In addition to the above federal regulations, various states and other municipalities also have enacted legislation for the regulation of chemical plants. These include more specific safety items, such as local fire codes or even put into place stricter aspects of the federal regulations. <ref name="Towler" />

Any process design or plant design must always meet the requirements of local and federal mandates and regulations. Without doing so, the wellbeing of plant employees and even the public can be placed into serious jeopardy.

=Safety Organizations and Terminology=
==Organizations==
===OSHA===
The Occupational Safety and Health Administration (OSHA) is a federal agency that focuses on the enforcement of safety and health legislation.

===EPA===
The Environmental Protection Agency (EPA) is a U.S. agency whose purpose is the protection of the health of both humans and the environment through the writing and enforcement of regulatory laws.

===DOT===
The Department of Transportation (DOT) oversees federal highway, air, and maritime transportation, and can be involved in the safe transport of chemicals.

===DOE===
The Department of Energy (DOE) is a governmental department tasked with the advancement of energy technology in the United States.

==Terminology==
===HS&E===
Health, Safety, and Environmental - This term refers to all health, safety, and environmental concerns that arise at each stage of the design process. Companies are required to analyze each part of the process from an HS&E perspective to create a safe and healthy work environment.

===MSDS===
Material Safety and Data Sheet - Every chemical has an MSDS which contains all the information regarding safe handling and how to deal with spills or other accidents involving the substance. Relevant information includes how to identify the substance, hazard information, and how to handle spills, fires, and exposure, among other things.

===FMEA===
Failure Mode and Effects Analysis - FMEA is an early stage approach to identifying critical technical risks using a semi-quantitative procedure. The analysis encompasses safety, environmental, and operational feasibility. When performing FMEA, engineers look to see places in a potential design that could fail, and then quantify how likely that failure is, how severe the results would be, and then offer potential solutions to minimize the risk. A step-by-step guide to performing FMEA is shown below (Northwestern University, 2014):

# Brainstorm for failure modes
# For each FM, rate severity of impact (SEV, 1 - 10).
# For each FM, brainstorm for possible causes (there may be multiple).
# For each cause, rate likelihood of occurring (OCC, 1 - 10).
# Rate the probability that the systems currently in place will detect and prevent the problem before it has an impact (DET, 10 - 1). Do not assume that something that will be added to the design later will take care of the problem.
# Overall Risk Probability Number RPN = SEV x OCC x DET. Most practitioners use the 1,4,7,10 scale below to increase granularity. Note that the DET scale is inverse to SEV and OCC.

[[File:Safety3.JPG|center|700px]]

Figure 1: Suggested scale to be used for quantifying risk and detection of failures in FMEA. Taken from ChE 351 Slides.

The sum of all information collected is implemented into a spreadsheet like the one shown below:

[[File:Safety4.JPG|center|700px]]

Figure 2: Example spreadsheet used to organize FMEA data.

===HAZOP===
Hazard and Operability Study - For more information regarding HAZOP, please refer to [[Process Hazards]].

===SIL===
Safety Integrity Levels - The SIL is the relative level of risk-reduction provided by a safety function, or to specify a target level of risk reduction. A SIL is determined based on a number of qualitative factors such as development process and safety life cycle management. Several methods are used such as risk matrices, risk graphs, layers of protective analysis (LOPA). Three levels of safety integrity are assigned depending on the “availability” of the safety instrumented system (SIS), as shown below (Towler et al., 2012):

[[File:Safety5.JPG|center|500px]]

Figure 3: Table of safety integrity levels based on availability of system. Taken from (Towler et al., 2012).

Redundant system means instrumentation is duplicated; higher level of redundancy of trip systems give higher SIL. The required SIL should be determined during a process hazard analysis and depends on risk of operator exposure and injury.

1 Instrument
* if it signals, plant goes down
* Probability of incident = probability of instrument failure
* Probability of spurious trip = false positive rate of one instrument

2 Instruments
* 1 out of 2 voting (1oo2): one instrument signals, plant goes down
** Probability of incident reduced by duplication
** Probability of spurious trip doubled
* 2oo2 voting
** Probability of incident worse than single instrument (twice likelihood that system is down)
** Probability of spurious trip reduced

3 Instruments
* 2oo3 voting
** Best overall trade-off between reducing incident rate and spurious trip rate
** One malfunctioning instrument does not cause trip or prevent detection of real incident

=Safe Design=
==Inherently Safe Design==
Inherently safe design of a particular process can be achieved by adhering to the following six strategies (Turton et al., 2003):

1. Substitution: Avoid using or producing hazardous materials on the plant site. If the hazardous material is an intermediate product, for example, alternate chemical reaction pathways might be used. In other words, the most inherently safe strategy is to avoid the use of hazardous materials.

2. Intensification: Attempt to use less of the hazardous materials. In terms of a hazardous intermediate, the two processes could be more closely coupled, reducing or eliminating the amount of intermediate produced. The inventories of hazardous feeds or products can be reduced by enhanced scheduling techniques such as just-in-time (JIT) manufacturing.

3. Attenuation: Reducing, or attenuating, the hazards of materials can often be affected by lowering the temperature or adding stabilizing additives. By using materials under less hazardous conditions, the potential consequences of a leak can be reduced.

4. Containment: If the hazardous materials cannot be eliminated, they at least should be stored in vessels with mechanical integrity beyond any reasonably expected temperature or pressure excursion. This is an old but effective strategy to avoid leaks. However, it is not as inherently safe as substitution, intensification, or attenuation.

5. Control: If a leak of hazardous material does occur, there should be safety systems that reduce the effects. For example, chemical facilities often have emergency isolation of the site from the normal storm sewers, and large tanks for flammable liquids are surrounded by dikes that prevent any leaks from spreading to to other areas of the plant. Scrubbing systems and relief systems in general are in this category. They are essential, because they allow controlled, safe release of hazardous materials, rather than an uncontrolled release from a vessel rupture.

6. Survival: If leaks of hazardous materials do occur and they are not contained or controlled, the personnel (and the equipment) must be protected. This lowest level of the hierarchy includes fire fighting, gas masks, and so on. Although essential to the total safety of the plant, the greater the reliance on survival of leaks rather than elimination of leaks, the less inherently safe the facility.

===Safety Legislation and Process Design===
====Inherent Process Plant Safety====
Through the enactment of safety regulation shown above, process design has inherently become safer. These regulations have made safety the first and foremost important concern when designing a new chemical plant. In general, the safety of a process relies on multiple layers of protection, but the first and most important layer of protection has become the process design feature. Greater tolerances are built into the designing of processes to more effectively prevent catastrophic failures or chemical leakage. The best approach to prevent accidents is to add process design features, involving chemistry and physics, to prevent hazardous situations.
=====Examples of Inherently Safe Process Design<ref>Crowl, Daniel. Louver, Joseph. Chemical Process Safety: Fundamentals with Applications. IBN: 9780132440554</ref>=====
#Vapor Release: Vapor released from spills can be minimized by designing dikes so that flammable and toxic materials will not accumulate around leaking tanks. This also prevents potential flammable materials from building up and causing an explosion.
#Containment Building: Design can be important for the containment of toxic spills. With the addition of automatic or remote controls, personnel can leave the area if a spill or breach occurs, while the area can be continuously monitored.
#Solvent Substitution: Safety through Substitution - Substituting design with safer, less hazardous materials. Designing process for use with less toxic or flammable solvents. For example, water-based paints and adhesives as well as aqueous or dry flowable formulations for agricultural chemicals opposed to more volatile solvents that release VOCs.
#Design for Lower Temperature and Pressure Safety through Moderation - use a hazardous material under less hazardous conditions such as lower pressure and temperature conditions. This can lower the level of catastrophe if downstream safety processes do fail.

====Control Systems Safety====
Legislation has also added to subsequent layers of controls such as environmental process controls have been added to prevent release to nearby air and water systems, which would endanger surrounding ecosystems and human populations.

==Assessing Preliminary Design==
While pilot plants are necessary to design effective plant equipment, there are some dangers associated with the industrial scale of chemical plants. Scaling up without accurate literature and experimental data can be very dangerous.

In every step of process design, the Health, Safety, and Environmental (HS&E) analysis must be carried out with the available technical information (Biegler et al., 1997).

[[File:Safety1.JPG|center|800px]]

[[File:Safety2.JPG|center|700px]]

Figure 4: General steps in the design process and analysis to be carried out at each stage. Taken from (Biegler et al., 1997).

=Economic Cost of Safety=
Price of Safety installations and fire protection systems range from .5 to 1.6 % of fixed-capital investment of a plant; but expenditures are often much higher than this and it is difficult to estimate these expenditures for a given plant

In addition, designers also examine the economic impact of safety and maintenance issues. For instance, they may determine that the plant reactor configuration can be improved, and with improved operator training facilities, it can run with improved safety. These hidden costs must be considered when determining the economic feasibility of operation (Biegler et al., 1997).

=Other Process Safety Considerations=
The safety and well-being of the consumers using the eventual product should be considered in process safety. The risks involved in using a product should be clearly communicated to the consumer by industrial leaders.

Human Error is another safety risk that is difficult to quantify. The intervention of well-trained operators is a vital layer in process safety (Peters et al., 2003).

=Case Study: Production of Ultrapure Hydrogen=
[[File:steam reforming.jpg|thumb|alt=A hydrogen production plant with a box steam reformer|A hydrogen production plant with a box steam reformer|300px|right|A hydrogen production plant with a box steam reformer]]
To walk through the process safety considerations when designing a chemical plant, the relatively simple example of a high purity hydrogen generation will be examined. First, various technologies are researched to determine the options that best meet the economic, physical, environmental, and safety constraints of the project. For this project, steam reforming, autothermal reforming, and partial oxidation were investigated. Technologies that lack widespread implementation have inherent safety risks as they have higher uncertainties associated with reliability, feasibility, and cost. Autothermal reforming requires oxygen and not a commercially popular method. Partial oxidation requires no catalyst, but requires high process temperatures and is a complex process to implement. After each process was analyzed and scored in a decision matrix, steam reforming was chosen as the base process technology. It was selected because it is a safe, well-known, low-emission, traditional process in use all over the world for the production of hydrogen. Steam reforming produces minimal waste compared to the alternative processes, and is capable of producing the necessary 100 MMscfd of 99.999% hydrogen gas. It is important to remember that both local and global environmental emissions are capable of harming the general public, so they should be considered safety concerns in the same way worker hazards are. Next, various reactor types, catalysts, and separation methods were evaluated with the base process chosen. In all, seven different processing stages were assessed including the initial heating of feed, the steam reformation reactor, the high and low temperature water shift reactors, the amine plant, the methanation reactor, the gas compression and cooling train, and the pressure swing adsorption unit.

Although worker safety is always the first priority in a plant, there are inherent risk associated with high temperature and pressure processes. Close attention was paid to make the preliminary plant design as inherently safe as possible. Preliminary FMEA and HAZOP analyses were conducted to identify the highest priority risks. Hazards were mitigated by substituting less hazardous materials when possible, opting to store only the necessary hazardous materials on site, lowering temperatures when possible, adding catastrophic failure controls, maximizing plant control automation when economically feasible, and necessitating worker personal protective equipment. Finally, FMEA and HAZAP analyses were repeated. These steps were repeated multiple times until a sufficient reduction in risk had been achieved.

=Case Study: Bhopal Disaster=
[[File:Bhopal.JPG|thumb|alt=The dispersal of the deadly cloud of gas from the Union Carbide plant in the Indian city of Bhopal|The dispersal of the deadly cloud of gas from the Union Carbide plant in the Indian city of Bhopal|400px|right|The dispersal of the deadly cloud of gas from the Union Carbide plant in the Indian city of Bhopal]]
On December 3, 1984 at the India Limited Pesticide Plant owned by Union Carbide in Bhopal, India, water entered a storage vessel containing over 80,000 lbs. of methyl isocyanate (MIC), a chemical intermediate in the pesticide synthesis process. This reaction caused a rapid increase in temperature accompanied by boiling, which caused toxic MIC vapors to escape from the tank. In addition, the MIC-water reaction produced methylamine and carbon dioxide gases among other toxic products which also contributed to the pressure increase (Union Carbide Corporation, 1967). These vapors passed into a scrubber and flare system that were not working at the time due to inadequate maintenance and safety practices. As a result of this accident, approximately 25 tons of MIC vapor were released, killing over 3,800 immediately and injuring roughly 20,000 in the surrounding area.

As a result of the incident, Union Carbide was forced to pay $470 million, as well as fund a hospital in Bhopal that was used specifically to treat victims of the disaster. Cleanup of the plant site and other legal action are still being determined to this day. Bhopal sparked a worldwide discussion on chemical process safety, and caused Congress to create the U.S. Chemical Safety Board (CSB). The CSB has since cited the following reasons as causes for the disaster:

* No process hazard analysis
* Poorly maintained equipment and safety system
* Lack of emergency response planning
* Inadequate training for operators

The CSB has pushed chemical safety reform since its conception, urging the chemical industry to produce inherently safer designs, use better quality equipment, and develop more thorough risk management plans. A major criticism of the process was its lack of inherent safe design. Because MIC was an intermediate, there was no reason to keep large quantities in storage. A modern design would use the intermediate as it is made (Eckerman et al., 2005).

Although chemical process safety has come a long way since 1984, industrial chemical giants still battle problems similar to Bhopal until this day. In 2008, a disaster similar to the one in Bhopal could have occurred in a plant originally designed by Union Carbide located in Institute, West Virginia after a runaway reaction caused a pressure buildup in a waste treatment vessel. The vessel exploded, killing two plant workers. Fortunately, the explosion missed a large MIC storage vessel which could have been hit by shrapnel and released tons of MIC (Blanc et al., 2009) In 2013, an ammonium nitrate explosion killed 15 and seriously injured 200 in West Texas in a blast radius similar to the one experienced in Bhopal (U.S.Chemical Safety Board, 2014).

=Case Study: Deepwater Horizon Explosion and Oil Spill=
[[File:Deepwater Horizon offshore drilling unit on fire 2010.jpg|thumb|alt=The burning Deepwater Horizon oilrig after an explosion caused by a design failure in the blowout preventer|The burning Deepwater Horizon oilrig after an explosion caused by a design failure in the blowout preventer|400px|right|The burning Deepwater Horizon oilrig after an explosion caused by a design failure in the blowout preventer]]
On April 20, 2010 on the Deepwater Horizon offshore drilling rig located in the Macondo Prospect, multiple explosions killed 11 workers and seriously injured 17. The rig burned for two days before sinking into the Gulf of Mexico. Key safety failures caused the well to spew 5 million barrels of oil into the Gulf of Mexico over the next 87 days making the incident the largest offshore oil spill in U.S. history. Finally, the well was sealed by a “static kill,” the injection of heavy fluids and cement, at the leak point 5,000 feet below the surface.

The key safety failure identified by the CSB was the blowout preventer (BOP) failure. This device that is meant to prevent the filling of annular space between the borehole and the well casing is both electrically and hydraulically powered. It is connected to a rig by a large diameter pipe called a riser. The system contains multiple pipe rams and annular preventers designed to prevent annular space buildup.

On the first night of the incident, a “kick” occurred and a mixture of oil, water, and gases began to build up in the wall and climb up the shaft. Drilling mud was injected to prevent kicks by creating a barrier. An upper annular preventer was also engaged when the buildup was discovered, but it failed. A pipe ram was activated and succeeded. However, an immense pressure buildup caused the drill pipe to buckle so it was forced off center. This buckling was later explained as a result of effective compression. This phenomenon is caused by microscopic irregularities and bends in the pipe material resulting in a higher surface area on one side of the pipe. Because the pipe was off-center, the final failsafe, the Automatic Mode Function (AMF) or deadman could not effectively shear the pipe and seal the well. This redundant control system comprised of a yellow pod and blue pod work independently to seal the well in the event of catastrophic failure when communications, electric power, and hydraulic pressure connections are cut. Both the yellow and blue pods contained 9 volt and 27 volt batteries which power solenoid valves. Unfortunately, the blue pod was miswired, so its 27 volt power supply was drained when it was to cause the blind shear blades to cut the pipe. Fortunately, a 9 volt battery in the yellow pod was also miswired which caused the blind shear ram to be engaged. However, this only partially sealed the well because of the pipe buckling. The flammable mixture erupted onto the surface of the platform and found an ignition source triggering a massive explosion. The spill was temporarily contained by a cap, and relief wells were eventually used to seal the well months later (U.S.Chemical Safety Board, 2014).

The White House Office of Energy and Climate Change Policy called the Deepwater Horizon oil spill the “worst environmental disaster the US has faced (BBC News, 2010). Over 8,000 species were estimated to be affected by the spill due to the toxicity of petroleum released, oxygen depletion, and the large quantities of Corexit, an oil dispersant used in an untested manner that is toxic to marine life (Biello et al., 2010; Butler, 2011; Froomkin, 2010).

BP, Transocean, and Halliburton were the major entities implicated in this tragedy. Investigations after the incident show that essential safety documentation including risk management and emergency procedure information were missing. Accusations were mainly aimed at BP with charges of recklessness and gross negligence (CNN Money, 2012). In January 2013, Transocean was ordered to pay $1.4 billion for US Clean Water Act violations. BP was ordered to pay $2.4 billion, but additional penalties could reach $20 billion (Department of Justice Office of Public Affairs, 2013).

=Case Study: Texas City Refinery Explosion=
[[File:BP_PLANT_EXPLOSION-1_lowres2.jpg|thumb|alt=Emergency response workers fight secondary fires caused by the isomerization unit explosion at the Texas City refinery|Emergency response workers fight secondary fires caused by the isomerization unit explosion at the Texas City refinery|228px|right|Emergency response workers fight secondary fires caused by the isomerization unit explosion at the Texas City refinery]]
On March 23, 2005 at a BP refinery in Texas City, Texas a hydrocarbon vapor cloud ignited, killing 15 workers and seriously injuring 170 others. Over the course an 11 hour period, a combination of control failures, mismanagement, and worker fatigue resulted in the buildup and release of extremely hot, combustible vapor. The key process unit in this disaster was an isomerization unit, located next to wooden trailers for workers servicing an ultracracker unit.

In the early morning on March 23rd, operators initiated startup and pumped raffinate (liquid hydrocarbons) into a raffinate splitter tower used to separate gasoline components. A liquid level indicator and multiple high level alarms monitored the tower liquid level. The level indicator could only measure up to 9 feet of liquid, and the written process called for a liquid level of about 6.5 feet. However, operators routinely filled the tower over 9 feet to minimize fluctuations and to prevent damage to a furnace. Hours later, the first high level alarm was activated and the liquid level rose, but a second alarm higher up the tower failed to trigger. The feed was halted when the liquid had risen to a level of about 13 feet, operators had no way of knowing the exact height. The lead operator relayed the startup activities to another operator and left the facility an hour before his shift ended. The morning operator arrived at 6 am to start his thirtieth consecutive day working a 12-hour shift and read a logbook that read, “Isom* Brought in some raff to unit, to pack raff with.” The day shift operator arrived an hour late, so he could not be briefed by the night shift supervisor. Recirculation then commenced in the tower, and more liquid was added to the tower. Additionally, conflicting instructions caused a liquid level regulating valve to remain closed for several hours, so liquid could not leave the tower. The furnace was then lit, and the supervisor left to attend a family medical emergency.

At noon, the liquid level had risen to 98 feet, but the improperly calibrated liquid level indicator read 8.4 feet. At 12:41 pm, a high pressure alarm caused workers to manually open a chain valve to relieve pressure by using the units pressure relief system to vent vapor into the atmosphere using an obsolete blowdown drum. Heat was also reduced in the furnace to reduce pressure. When operators became concerned about outflow rate, the liquid level regulating valve was opened to release liquid from the tower to storage. This caused the liquid in the tower to begin to boil and spill into the overhead vapor line exerting extreme pressures on the pressure relief system. At 1:14 pm, the three relief valves opened sending the liquid to the blowdown drum which overflowed into a municipal sewer setting off alarms, but a key level indicator in the blowdown drum failed. Flammable liquid erupted from the blowdown drum, formed a massive vapor cloud, and found an ignition source from a nearby idling pickup truck. The colossal blast ignited fires throughout the refinery and over half the workers in the wooden trailers adjacent to the unit were killed immediately (U.S.Chemical Safety Board, 2008).

Investigations after the incident cited multiple failures to implement safety recommendations at the Texas City Refinery. Among these, the blowdown drum was to be replaced by a modern flare to burn off hydrocarbons. However, BP’s budget cuts prevented its replacement. The training and treatment of workers was also called into question, as fatigue, poor communication, and inadequate documentation likely contributed to the disaster. Decisions like the one to operate an unsafe liquid level in order to prevent furnace damage also demonstrate the company’s fixation on the bottom line. BP was eventually fined $21 million by OSHA (New York Times, 2010; U.S.Chemical Safety Board, 2008).

=References=
==Direct Citation References==
<references/>

==Additional References==
# American Institute of Chemical Engineers. http://www.aiche.org/ccps
# Biello, David (9 June 2010). "The BP Spill's Growing Toll On the Sea Life of the Gulf". Yale Environment 360. Yale School of Forestry & Environmental Studies. Retrieved 2010-06-14.
# Blanc, P. Bhopal, 1984 – West Virginia near-miss, 2008. Psychology Today, December 2009. https://www.psychologytoday.com/blog/household-hazards/200912/bhopal-1984-west-virginia-near-miss-2008.
# Butler, J. Steven (3 March 2011). "BP Macondo Well Incident. U.S. Gulf of Mexico. Pollution Containment and Remediation Efforts" (PDF). Lillehammer Energy Claims Conference. BDO Consulting. Retrieved 17 February 2013.
# “DOJ accuses BP of ‘gross negligence’ in Gulf oil spill” CNN Money, September 2012.
# Eckerman, I. The Bhopal Saga: Causes and Consequences of the World’s Largest Industrial Disaster. Universities Press Private Limited, 2005.
# Froomkin, Dan (29 July 2010). "Scientists Find Evidence That Oil And Dispersant Mix Is Making Its Way Into The Foodchain". Huffington Post.
# "Gulf of Mexico oil leak 'worst US environment disaster'". BBC News. 30 May 2010.
# Investigation Report: BP Refinery Explosion and Fire, U.S. Chemical Safety Board, 2008.
# Lyall, Sarah. "In BP’s Record, a History of Boldness and Costly Blunders." New York Times, July 13, 2010.
# L.T. Biegler, I.E. Grossmann, A.W. Westerberg. ''Systematic Methods of Chemical Process Design''. Prentice-Hall: Upper Saddle River, 1997.
# M.S. Peters, K.D. Timmerhaus. ''Plant Design and Economics for Chemical Engineers, 5th Ed.'' McGraw-Hill: New York, 2003.
# Northwestern University. Chemical Engineering 351 Lecture Slides.
# Reflections on Bhopal After Thirty Years Video, U.S.Chemical Safety Board, December 2014.
# R.T. Turton, R.C. Bailie, W.B. Whiting, J.A. Shaeiwitz. ''Analysis, Synthesis, and Design of Chemical Processes''. Prentice Hall: Upper Saddle River, 2003.
# Union Carbide Corporation "Methyl Isocyanate" Product Information Publication, F-41443, November 1967.
# USCSB Deepwater Horizon Video, U.S.Chemical Safety Board, June 2014.
# "Transocean Agrees to Plead Guilty to Environmental Crime and Enter Civil Settlement to Resolve U.S. Clean Water Act Penalty Claims from Deepwater Horizon Incident". Department of Justice Office of Public Affairs. January 3, 2013.

Process safety

2016-02-13T20:31:11Z

Evanrosati: /* The Emergency Planning and Community Right-to-Know Act (EPCRA), 1986 */

Authors: Anne Disabato, [2014] Tim Hanrahan, [2014] Brian Merkle, [2014] , Spencer Saldana [2015] and Evan Rosati [2016] 

Stewards: David Chen, Jian Gong, and Fengqi You

Date Presented: January 19, 2014

 


__TOC__

=Introduction=
The safe design and operation of facilities is of paramount importance to every company that is involved in the manufacture of fuels, chemicals, and pharmaceuticals. Process safety focuses on the prevention of dangerous situations, such as fires, explosions, and the release of chemicals.

The American Institute of Chemical Engineers emphasizes a culture of process safety through four pillars (American Institute of Chemical Engineers, 2015):

1. Commitment to Process Safety: a workforce that is actively involved and an organization that fully supports process safety as a core value will tend to do the right things in the right way at the right time – even when no one else is looking

2. Understanding Hazard and Risk: the foundation of a risk-based approach which will allow an organization to use this information to allocate limited resources in the most effective manner

3. Manage Risk: the ongoing execution of risk based process safety tasks. Risk management can help a company to better deal with the resultant risks and sustain long-term accident free and profitable operations

4. Learn from Experience: Metrics provide direct feedback on the workings of RBPS systems, and leading indicators provide early warning signals of ineffective process safety results. Organizations must use their mistakes and those of others as motivation for action and view as opportunities for improvement.

For the prevention and management of specific safety hazards, such as fires, explosions, or the release of toxic chemicals, please see [[Process Hazards]].

==Layers of Plant Safety==
Safety and loss prevention can be expressed in "layers" of plant safety in terms of design and implementation.<ref name="Towler">G.P. Towler, R. Sinnott. ''Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design''. Elsevier, 2012.</ref> Each higher layer can be activated if a lower level fails. This creates a system with subsequent levels of safety to help prevent catastrophe from occurring. This diagram shows the important of safety in process design. If a process is designed to be inherently safe, additional safety "controls" will be less important and a chemical plant will be overall safer. The goal of safety is not to reach the top of the triangle, but to stay as close to the bottom as possible. This shows the importance of inherently safe design and safety legislation and regulations to provide guidelines for safety and health concerns. Regulations provide a baseline for engineers to operate when designing a chemical plant. They have brought safety to the forefront of design, when engineers have the maximum degree of freedom for implementation, and are no longer considered to be an afterthought or strictly a controls issue. Plants can be designed to be safe without the extensive use of future adaptation, safety controls, or emergency response. Although you can never eliminate these upper layers of process safety, by designing a process smartly and safely, engineers can reduce the consequences of "walking up" the process safety ladder or triangle.
[[File:Layers_of_Plant_Safety.png|center|700px|thumb|alt=Triangle Diagram on Increasing Plant Safety Mechanisms|Layers of Plant Safety Triangle Diagram]]

=Safety Legislation=

==International Regulations==
International regulations can be considered as best practice standards that are adopted by governments through treaties and establishment through United Nation resolutions. Most international regulation agencies can only register complaints for violations, but can not implement sanctions or fines to infracting parties.

===Internation Labour Organization (ILO)===
After the dissolution of the League of Nations in 1946, the ILO became the first specialized agency of the United Nations upon its founding in December of that same year. <ref> ISO Photo Gallery. http://www.ilo.org/dyn/media/mediasearch.fiche?p_id=16023&p_lang=en. Accessed February 13, 2016.</ref> The ILO attempts to develop labor standards for workers in all industries. The unique tripartite structure of the ILO gives an equal voice to workers, employers and governments to ensure that the views of the social partners are closely reflected in labour standards and in shaping policies and programs. <ref> ISO "How the ISO Works" http://www.ilo.org/global/about-the-ilo/how-the-ilo-works/lang--en/index.htm. Accessed February 13, 2016.</ref> The ISO has established International Labour Standards on Occupational Safety and Health, which has developed more than 40 standards specifically dealing with occupational heath and safety.<ref> ILO. "International Labour Standards on Occupational Safety and Health". http://www.ilo.org/global/standards/subjects-covered-by-international-labour-standards/occupational-safety-and-health/lang--en/index.htm. Accessed February 13, 2016.</ref>. The important conventions are listed below:

====Occupational Safety and Health Convention, 1981 and Protocol of 2002====
#The convention provides for the adoption of national occupational safety and health policy by each participating nation-state. It includes actions taken by governments and within enterprise that operate within the governmental spaces to promote the improvement of occupational safety and health to therefore improve working conditions.
#The additional Protocol in 2002 calls for the establishment and period review of procedures for recording and notification of occupational accidents along with the publication of related statistics associated with the accidents. This Protocol is similar to the Emergency Planning and CommunityRight-To-Know Act instituted in the United States (see below).

====Occupational Heath Services Convention, 1985====
#Establishment of enterprise-level occupational health services which are entrusted with preventative functions and which are responsible for advising the employer, the workers, and their representatives in the organization on maintaining a safe and healthy work environment.

====Promotional Framework for Occupational Safety and Heath Convention, 2006====
#Aims to promote a preventative safety and health culture and to progressively achieve a safe and healthy working environment for all.
#Requires ratifying nation-states to develop, in consultation with the most representative organizations of employees and workers, including workers unions, a national policy, a national system, and a national program on occupational health and safety.
#This should be developed in accordance to the Occupational Heath Services Convention, 1985 and take into account other ILO standards.
#National systems shall provide the infrastructure for implementing national policy and programs, such as domestic laws, regulations, authority bodies, and compliance mechanisms.

====Chemicals Convention, 1990====
#Provides for the adoption and implementation of a coherent policy on the safety in the use of chemicals at work, including production, handling, storage, and transport.
#Also includes best practices for the disposal and treatment of waste chemicals and the release of chemicals resulting from work activities, maintenance, and cleaning of equipment.
#In addition, it allocates specific responsibilities to suppliers and exporting states.
#Chemicals shall be evaluated to determine their level of hazards and employers shall make these hazards known to their employees.

===International Organization for Standardization (ISO)===
The ISO is an international organization for standardization on topics ranging from quality management, environmental management, social responsibility, risk management, etc. ISO standards are adopted world-wide by organizations and governments as accepted and known standards for production. Many of the practices are self-regulating as costumers often demand certain standards from companies they purchase from.<ref>ISO. Home Page. http://www.iso.org/iso/home.html. Accessed February 13, 2016.</ref> The ISO is developing its own occupational health and safety framework shown below.

====ISO 45001 Occupational Health and Safety Management Systems<ref>ISO 45001. "Occupational heath and safety". http://www.iso.org/iso/home/standards/management-standards/iso45001.htm. Accessed February 13, 2016.</ref>====
#Designed to help organizations reduce the burden of occupational injuries and diseases by providing a framework to improve employee safety, reduce workplace risks, and create better, safer working conditions.
#Currently under development by a committee of occupational health and safety experts who will follow management systems approaches of ISO 4001 and ISO 9001.
#Will embody other International Standards such as the ILO's Occupational Health and Safety Guidelines.
#The expected publication will be released in October, 2016.

==US Safety Regulations==
Over time chemical industry regulations have been developed to ensure that the best safety practices are followed to maintain the health of both people and the environment. The development of most regulations is based around the idea that organizations have both a legal and moral obligation to safeguard the health and welfare of its employees and the public.<ref name="Towler" /> The extent of legislation varies across regions around the globe. In the United States, chemical accidents have led to the creation of regulation boards and safety oriented societies such as the Center for Chemical Plant Safety of the American Institute of Chemical Engineers to aid in the development and implementation of pant safety. <ref>Seider WD, Seader JD, Lewin DR, Seider WD. Product And Process Design Principles: Synthesis, Analysis, and Evaluation. New York: Wiley; 2004.</ref> In the US, the major federal laws relating to chemical plant safety and their regulations are as follows:

===The Occupational Safety and Health Act, 1970===
The OSH Act is administer by the Occupational Safety and Health Administration (OSHA) (see below). The Act covers all employers and their employees in the United States with coverage provided either directly by the Federal Occupational Safety and Heath Administration or by an OSHA-approved state job safety and health plan.<ref> United States Department of Labor. "Safety and Health Standards: Occupational Safety and Health". http://webapps.dol.gov/elaws/elg/osha.htm </ref> The main provisions of the act are listed below.
# Employers must supply place of employment free from toxic chemicals, excessive noise mechanical dangers, or unsanitary conditions.
#This involves the implementation of engineering controls to limit exposure to hazards and toxic substances and implementing administrative controls.
# For employees, employers must provide personal protective equipment (PPE) and training, including communications of known hazards.
# The Occupational Safety and Health Administration (OSHA) was established to promote best practices, inspect facilities for hazard analysis, set standards, and enforce the law.
# The National Institute of Occupational Safety and Health (NIOSH) was established to be an independent research institute (now under the Centers for Disease Control).
#The Act also encourages states to develop and operate their own job safety and health programs with OSHA as a monitoring agency for these "state plans," which operate under the authority of state law. The standards developed by state plans need to be at least as effective as the federal regulations.<ref> United States Department of Labor. "Safety and Health Standards: Occupational Safety and Health". http://webapps.dol.gov/elaws/elg/osha.htm </ref>
#Federal OSHA standards are categorized into four categories: General Industry, Construction, Maritime Terminals, Long-shoring, and Agriculture.

===The Toxic Substances Control Act (TSCA), 1976===
The main qualifications of the TSCA were to provide the EPA with regulating power of chemicals, specifically pertaining to the chemical industry and not including foods, drugs, cosmetics, or pesticides. <ref> Environmental Protection Agency. "Summary of the Toxic Substances Control Act." http://www.epa.gov/laws-regulations/summary-toxic-substances-control-act. Accessed February 13, 2016</ref>
# The Environmental Protection Agency (EPA) is required to regulate 75,000 chemical substances used in industry.
# The EPA has jurisdiction over the safety of the sale or development of new chemicals in the United States, including the requirement for pre-manufacture notification for new chemical substances before manufacture.
#The TSCA also addresses the production, importation, use, and disposal of specific chemicals including polychlorinated biphenyls (PCBs), asbestos, radon, and lead-based paint.<ref> Environmental Protection Agency. "Summary of the Toxic Substances Control Act." http://www.epa.gov/laws-regulations/summary-toxic-substances-control-act. Accessed February 13, 2016</ref>
# Everyone has the right and obligation to report information about any health or environmental effects caused by a chemical. This is especially important for organizations as they are required to report information to the EPA if a chemical substance is found to have substantial risk of injury to health or the environment.
====Frank R. Lautenberg Chemical Safety for the 21st Century Act, 2015====
The Toxic Substances Control Act will be revised in 2015 under this new act proposed to Congress. This Act is an attempt to eliminate some of the flaws associated with the TSCA.
#One of the biggest flaws in the TSCA is the fact that a new chemical can be used without first demonstrating safety, the idea that chemicals are safe until proven unsafe.<ref>Sheppard, Kate. "Senators Introduce Bill to Overhaul U.S. Chemical Industry." http://www.huffingtonpost.com/2015/03/10/toxic-chemicals-senate-bill_n_6842524.html. Accessed February 13, 2016</ref>
#The new bill would require safety testing before chemical implementation.
#It gives the EPA more defined power on the regulation of chemical substances and the Sustainable Chemical Program will be established. This brings the TSCA into the modern era of the chemical industry.<ref>S.697 - 114th Congress (2015-2016): Frank R. Lautenberg Chemical Safety for the 21st Century Act. Congress.gov. https://www.congress.gov/bill/114th-congress/senate-bill/697/all-info. Accessed January 27, 2016.</ref>

===The Emergency Planning and Community Right-to-Know Act (EPCRA), 1986<ref> US Government Printing Office. Emergency Panning and Community Right to Know. https://www.gpo.gov/fdsys/pkg/USCODE-2010-title42/html/USCODE-2010-title42-chap116.htm. Accessed February 13, 2016</ref>===
The Act was passed by Congress in response to concerns regarding the environmental and safety hazards posed by the storage and handling of toxic chemicals. This legislation was developed as a result of the 1984 chemical disaster in Bhopal, India.<ref>What is EPCRA? EPA. http://www.epa.gov/epcra/what-epcra. Published November 10, 2015. Accessed January 27, 2016.</ref>
# All facilities manufacturing, processing, or storing hazardous chemicals must make plans for major incidents if they were to occur and the plans must be made public so that local communities can be properly informed.
# All facilities must also produce Material Safety Data Sheets (MSDSs) to state and local officials as well as local fire departments.
# Local governments should help prepare emergency plans and review the plans annually.
# State governments are required to oversee and coordinate local planning efforts.
# Emergency Notification: Facilities must immediately report accidental releases of chemicals and "hazardous substances" in quantities greater than Reportable Quantities (RQs) defined under the Comprehensive Environmental Response, Compensation, and Liability Act to state and local officials with this information then being available to the public.
# Annually, facilities must complete and submit a toxic chemical release inventory form.

===Clean Air Act Amendments of 1990===
# Established the U.S. Chemical Safety and Hazard Investigation Board, and independent federal agency with the goal of ensuring worker and public safety through the prevention or minimization of the effects of chemical accidents. <ref>Seider WD, Seader JD, Lewin DR, Seider WD. Product And Process Design Principles: Synthesis, Analysis, and Evaluation. New York: Wiley; 2004.</ref>
#Attempt to determine the roots and contributing causes of accidents and then provide briefs on the accidents.

==Additional Legislative Information==
For more details on environmental legislation pertaining to release of materials to the environment or the regulations of the loss of containment, see [[Environmental concerns]].

In addition to the above federal regulations, various states and other municipalities also have enacted legislation for the regulation of chemical plants. These include more specific safety items, such as local fire codes or even put into place stricter aspects of the federal regulations. <ref name="Towler" />

Any process design or plant design must always meet the requirements of local and federal mandates and regulations. Without doing so, the wellbeing of plant employees and even the public can be placed into serious jeopardy.

=Safety Organizations and Terminology=
==Organizations==
===OSHA===
The Occupational Safety and Health Administration (OSHA) is a federal agency that focuses on the enforcement of safety and health legislation.

===EPA===
The Environmental Protection Agency (EPA) is a U.S. agency whose purpose is the protection of the health of both humans and the environment through the writing and enforcement of regulatory laws.

===DOT===
The Department of Transportation (DOT) oversees federal highway, air, and maritime transportation, and can be involved in the safe transport of chemicals.

===DOE===
The Department of Energy (DOE) is a governmental department tasked with the advancement of energy technology in the United States.

==Terminology==
===HS&E===
Health, Safety, and Environmental - This term refers to all health, safety, and environmental concerns that arise at each stage of the design process. Companies are required to analyze each part of the process from an HS&E perspective to create a safe and healthy work environment.

===MSDS===
Material Safety and Data Sheet - Every chemical has an MSDS which contains all the information regarding safe handling and how to deal with spills or other accidents involving the substance. Relevant information includes how to identify the substance, hazard information, and how to handle spills, fires, and exposure, among other things.

===FMEA===
Failure Mode and Effects Analysis - FMEA is an early stage approach to identifying critical technical risks using a semi-quantitative procedure. The analysis encompasses safety, environmental, and operational feasibility. When performing FMEA, engineers look to see places in a potential design that could fail, and then quantify how likely that failure is, how severe the results would be, and then offer potential solutions to minimize the risk. A step-by-step guide to performing FMEA is shown below (Northwestern University, 2014):

# Brainstorm for failure modes
# For each FM, rate severity of impact (SEV, 1 - 10).
# For each FM, brainstorm for possible causes (there may be multiple).
# For each cause, rate likelihood of occurring (OCC, 1 - 10).
# Rate the probability that the systems currently in place will detect and prevent the problem before it has an impact (DET, 10 - 1). Do not assume that something that will be added to the design later will take care of the problem.
# Overall Risk Probability Number RPN = SEV x OCC x DET. Most practitioners use the 1,4,7,10 scale below to increase granularity. Note that the DET scale is inverse to SEV and OCC.

[[File:Safety3.JPG|center|700px]]

Figure 1: Suggested scale to be used for quantifying risk and detection of failures in FMEA. Taken from ChE 351 Slides.

The sum of all information collected is implemented into a spreadsheet like the one shown below:

[[File:Safety4.JPG|center|700px]]

Figure 2: Example spreadsheet used to organize FMEA data.

===HAZOP===
Hazard and Operability Study - For more information regarding HAZOP, please refer to [[Process Hazards]].

===SIL===
Safety Integrity Levels - The SIL is the relative level of risk-reduction provided by a safety function, or to specify a target level of risk reduction. A SIL is determined based on a number of qualitative factors such as development process and safety life cycle management. Several methods are used such as risk matrices, risk graphs, layers of protective analysis (LOPA). Three levels of safety integrity are assigned depending on the “availability” of the safety instrumented system (SIS), as shown below (Towler et al., 2012):

[[File:Safety5.JPG|center|500px]]

Figure 3: Table of safety integrity levels based on availability of system. Taken from (Towler et al., 2012).

Redundant system means instrumentation is duplicated; higher level of redundancy of trip systems give higher SIL. The required SIL should be determined during a process hazard analysis and depends on risk of operator exposure and injury.

1 Instrument
* if it signals, plant goes down
* Probability of incident = probability of instrument failure
* Probability of spurious trip = false positive rate of one instrument

2 Instruments
* 1 out of 2 voting (1oo2): one instrument signals, plant goes down
** Probability of incident reduced by duplication
** Probability of spurious trip doubled
* 2oo2 voting
** Probability of incident worse than single instrument (twice likelihood that system is down)
** Probability of spurious trip reduced

3 Instruments
* 2oo3 voting
** Best overall trade-off between reducing incident rate and spurious trip rate
** One malfunctioning instrument does not cause trip or prevent detection of real incident

=Safe Design=
==Inherently Safe Design==
Inherently safe design of a particular process can be achieved by adhering to the following six strategies (Turton et al., 2003):

1. Substitution: Avoid using or producing hazardous materials on the plant site. If the hazardous material is an intermediate product, for example, alternate chemical reaction pathways might be used. In other words, the most inherently safe strategy is to avoid the use of hazardous materials.

2. Intensification: Attempt to use less of the hazardous materials. In terms of a hazardous intermediate, the two processes could be more closely coupled, reducing or eliminating the amount of intermediate produced. The inventories of hazardous feeds or products can be reduced by enhanced scheduling techniques such as just-in-time (JIT) manufacturing.

3. Attenuation: Reducing, or attenuating, the hazards of materials can often be affected by lowering the temperature or adding stabilizing additives. By using materials under less hazardous conditions, the potential consequences of a leak can be reduced.

4. Containment: If the hazardous materials cannot be eliminated, they at least should be stored in vessels with mechanical integrity beyond any reasonably expected temperature or pressure excursion. This is an old but effective strategy to avoid leaks. However, it is not as inherently safe as substitution, intensification, or attenuation.

5. Control: If a leak of hazardous material does occur, there should be safety systems that reduce the effects. For example, chemical facilities often have emergency isolation of the site from the normal storm sewers, and large tanks for flammable liquids are surrounded by dikes that prevent any leaks from spreading to to other areas of the plant. Scrubbing systems and relief systems in general are in this category. They are essential, because they allow controlled, safe release of hazardous materials, rather than an uncontrolled release from a vessel rupture.

6. Survival: If leaks of hazardous materials do occur and they are not contained or controlled, the personnel (and the equipment) must be protected. This lowest level of the hierarchy includes fire fighting, gas masks, and so on. Although essential to the total safety of the plant, the greater the reliance on survival of leaks rather than elimination of leaks, the less inherently safe the facility.

===Safety Legislation and Process Design===
====Inherent Process Plant Safety====
Through the enactment of safety regulation shown above, process design has inherently become safer. These regulations have made safety the first and foremost important concern when designing a new chemical plant. In general, the safety of a process relies on multiple layers of protection, but the first and most important layer of protection has become the process design feature. Greater tolerances are built into the designing of processes to more effectively prevent catastrophic failures or chemical leakage. The best approach to prevent accidents is to add process design features, involving chemistry and physics, to prevent hazardous situations.
=====Examples of Inherently Safe Process Design<ref>Crowl, Daniel. Louver, Joseph. Chemical Process Safety: Fundamentals with Applications. IBN: 9780132440554</ref>=====
#Vapor Release: Vapor released from spills can be minimized by designing dikes so that flammable and toxic materials will not accumulate around leaking tanks. This also prevents potential flammable materials from building up and causing an explosion.
#Containment Building: Design can be important for the containment of toxic spills. With the addition of automatic or remote controls, personnel can leave the area if a spill or breach occurs, while the area can be continuously monitored.
#Solvent Substitution: Safety through Substitution - Substituting design with safer, less hazardous materials. Designing process for use with less toxic or flammable solvents. For example, water-based paints and adhesives as well as aqueous or dry flowable formulations for agricultural chemicals opposed to more volatile solvents that release VOCs.
#Design for Lower Temperature and Pressure Safety through Moderation - use a hazardous material under less hazardous conditions such as lower pressure and temperature conditions. This can lower the level of catastrophe if downstream safety processes do fail.

====Control Systems Safety====
Legislation has also added to subsequent layers of controls such as environmental process controls have been added to prevent release to nearby air and water systems, which would endanger surrounding ecosystems and human populations.

==Assessing Preliminary Design==
While pilot plants are necessary to design effective plant equipment, there are some dangers associated with the industrial scale of chemical plants. Scaling up without accurate literature and experimental data can be very dangerous.

In every step of process design, the Health, Safety, and Environmental (HS&E) analysis must be carried out with the available technical information (Biegler et al., 1997).

[[File:Safety1.JPG|center|800px]]

[[File:Safety2.JPG|center|700px]]

Figure 4: General steps in the design process and analysis to be carried out at each stage. Taken from (Biegler et al., 1997).

=Economic Cost of Safety=
Price of Safety installations and fire protection systems range from .5 to 1.6 % of fixed-capital investment of a plant; but expenditures are often much higher than this and it is difficult to estimate these expenditures for a given plant

In addition, designers also examine the economic impact of safety and maintenance issues. For instance, they may determine that the plant reactor configuration can be improved, and with improved operator training facilities, it can run with improved safety. These hidden costs must be considered when determining the economic feasibility of operation (Biegler et al., 1997).

=Other Process Safety Considerations=
The safety and well-being of the consumers using the eventual product should be considered in process safety. The risks involved in using a product should be clearly communicated to the consumer by industrial leaders.

Human Error is another safety risk that is difficult to quantify. The intervention of well-trained operators is a vital layer in process safety (Peters et al., 2003).

=Case Study: Production of Ultrapure Hydrogen=
[[File:steam reforming.jpg|thumb|alt=A hydrogen production plant with a box steam reformer|A hydrogen production plant with a box steam reformer|300px|right|A hydrogen production plant with a box steam reformer]]
To walk through the process safety considerations when designing a chemical plant, the relatively simple example of a high purity hydrogen generation will be examined. First, various technologies are researched to determine the options that best meet the economic, physical, environmental, and safety constraints of the project. For this project, steam reforming, autothermal reforming, and partial oxidation were investigated. Technologies that lack widespread implementation have inherent safety risks as they have higher uncertainties associated with reliability, feasibility, and cost. Autothermal reforming requires oxygen and not a commercially popular method. Partial oxidation requires no catalyst, but requires high process temperatures and is a complex process to implement. After each process was analyzed and scored in a decision matrix, steam reforming was chosen as the base process technology. It was selected because it is a safe, well-known, low-emission, traditional process in use all over the world for the production of hydrogen. Steam reforming produces minimal waste compared to the alternative processes, and is capable of producing the necessary 100 MMscfd of 99.999% hydrogen gas. It is important to remember that both local and global environmental emissions are capable of harming the general public, so they should be considered safety concerns in the same way worker hazards are. Next, various reactor types, catalysts, and separation methods were evaluated with the base process chosen. In all, seven different processing stages were assessed including the initial heating of feed, the steam reformation reactor, the high and low temperature water shift reactors, the amine plant, the methanation reactor, the gas compression and cooling train, and the pressure swing adsorption unit.

Although worker safety is always the first priority in a plant, there are inherent risk associated with high temperature and pressure processes. Close attention was paid to make the preliminary plant design as inherently safe as possible. Preliminary FMEA and HAZOP analyses were conducted to identify the highest priority risks. Hazards were mitigated by substituting less hazardous materials when possible, opting to store only the necessary hazardous materials on site, lowering temperatures when possible, adding catastrophic failure controls, maximizing plant control automation when economically feasible, and necessitating worker personal protective equipment. Finally, FMEA and HAZAP analyses were repeated. These steps were repeated multiple times until a sufficient reduction in risk had been achieved.

=Case Study: Bhopal Disaster=
[[File:Bhopal.JPG|thumb|alt=The dispersal of the deadly cloud of gas from the Union Carbide plant in the Indian city of Bhopal|The dispersal of the deadly cloud of gas from the Union Carbide plant in the Indian city of Bhopal|400px|right|The dispersal of the deadly cloud of gas from the Union Carbide plant in the Indian city of Bhopal]]
On December 3, 1984 at the India Limited Pesticide Plant owned by Union Carbide in Bhopal, India, water entered a storage vessel containing over 80,000 lbs. of methyl isocyanate (MIC), a chemical intermediate in the pesticide synthesis process. This reaction caused a rapid increase in temperature accompanied by boiling, which caused toxic MIC vapors to escape from the tank. In addition, the MIC-water reaction produced methylamine and carbon dioxide gases among other toxic products which also contributed to the pressure increase (Union Carbide Corporation, 1967). These vapors passed into a scrubber and flare system that were not working at the time due to inadequate maintenance and safety practices. As a result of this accident, approximately 25 tons of MIC vapor were released, killing over 3,800 immediately and injuring roughly 20,000 in the surrounding area.

As a result of the incident, Union Carbide was forced to pay $470 million, as well as fund a hospital in Bhopal that was used specifically to treat victims of the disaster. Cleanup of the plant site and other legal action are still being determined to this day. Bhopal sparked a worldwide discussion on chemical process safety, and caused Congress to create the U.S. Chemical Safety Board (CSB). The CSB has since cited the following reasons as causes for the disaster:

* No process hazard analysis
* Poorly maintained equipment and safety system
* Lack of emergency response planning
* Inadequate training for operators

The CSB has pushed chemical safety reform since its conception, urging the chemical industry to produce inherently safer designs, use better quality equipment, and develop more thorough risk management plans. A major criticism of the process was its lack of inherent safe design. Because MIC was an intermediate, there was no reason to keep large quantities in storage. A modern design would use the intermediate as it is made (Eckerman et al., 2005).

Although chemical process safety has come a long way since 1984, industrial chemical giants still battle problems similar to Bhopal until this day. In 2008, a disaster similar to the one in Bhopal could have occurred in a plant originally designed by Union Carbide located in Institute, West Virginia after a runaway reaction caused a pressure buildup in a waste treatment vessel. The vessel exploded, killing two plant workers. Fortunately, the explosion missed a large MIC storage vessel which could have been hit by shrapnel and released tons of MIC (Blanc et al., 2009) In 2013, an ammonium nitrate explosion killed 15 and seriously injured 200 in West Texas in a blast radius similar to the one experienced in Bhopal (U.S.Chemical Safety Board, 2014).

=Case Study: Deepwater Horizon Explosion and Oil Spill=
[[File:Deepwater Horizon offshore drilling unit on fire 2010.jpg|thumb|alt=The burning Deepwater Horizon oilrig after an explosion caused by a design failure in the blowout preventer|The burning Deepwater Horizon oilrig after an explosion caused by a design failure in the blowout preventer|400px|right|The burning Deepwater Horizon oilrig after an explosion caused by a design failure in the blowout preventer]]
On April 20, 2010 on the Deepwater Horizon offshore drilling rig located in the Macondo Prospect, multiple explosions killed 11 workers and seriously injured 17. The rig burned for two days before sinking into the Gulf of Mexico. Key safety failures caused the well to spew 5 million barrels of oil into the Gulf of Mexico over the next 87 days making the incident the largest offshore oil spill in U.S. history. Finally, the well was sealed by a “static kill,” the injection of heavy fluids and cement, at the leak point 5,000 feet below the surface.

The key safety failure identified by the CSB was the blowout preventer (BOP) failure. This device that is meant to prevent the filling of annular space between the borehole and the well casing is both electrically and hydraulically powered. It is connected to a rig by a large diameter pipe called a riser. The system contains multiple pipe rams and annular preventers designed to prevent annular space buildup.

On the first night of the incident, a “kick” occurred and a mixture of oil, water, and gases began to build up in the wall and climb up the shaft. Drilling mud was injected to prevent kicks by creating a barrier. An upper annular preventer was also engaged when the buildup was discovered, but it failed. A pipe ram was activated and succeeded. However, an immense pressure buildup caused the drill pipe to buckle so it was forced off center. This buckling was later explained as a result of effective compression. This phenomenon is caused by microscopic irregularities and bends in the pipe material resulting in a higher surface area on one side of the pipe. Because the pipe was off-center, the final failsafe, the Automatic Mode Function (AMF) or deadman could not effectively shear the pipe and seal the well. This redundant control system comprised of a yellow pod and blue pod work independently to seal the well in the event of catastrophic failure when communications, electric power, and hydraulic pressure connections are cut. Both the yellow and blue pods contained 9 volt and 27 volt batteries which power solenoid valves. Unfortunately, the blue pod was miswired, so its 27 volt power supply was drained when it was to cause the blind shear blades to cut the pipe. Fortunately, a 9 volt battery in the yellow pod was also miswired which caused the blind shear ram to be engaged. However, this only partially sealed the well because of the pipe buckling. The flammable mixture erupted onto the surface of the platform and found an ignition source triggering a massive explosion. The spill was temporarily contained by a cap, and relief wells were eventually used to seal the well months later (U.S.Chemical Safety Board, 2014).

The White House Office of Energy and Climate Change Policy called the Deepwater Horizon oil spill the “worst environmental disaster the US has faced (BBC News, 2010). Over 8,000 species were estimated to be affected by the spill due to the toxicity of petroleum released, oxygen depletion, and the large quantities of Corexit, an oil dispersant used in an untested manner that is toxic to marine life (Biello et al., 2010; Butler, 2011; Froomkin, 2010).

BP, Transocean, and Halliburton were the major entities implicated in this tragedy. Investigations after the incident show that essential safety documentation including risk management and emergency procedure information were missing. Accusations were mainly aimed at BP with charges of recklessness and gross negligence (CNN Money, 2012). In January 2013, Transocean was ordered to pay $1.4 billion for US Clean Water Act violations. BP was ordered to pay $2.4 billion, but additional penalties could reach $20 billion (Department of Justice Office of Public Affairs, 2013).

=Case Study: Texas City Refinery Explosion=
[[File:BP_PLANT_EXPLOSION-1_lowres2.jpg|thumb|alt=Emergency response workers fight secondary fires caused by the isomerization unit explosion at the Texas City refinery|Emergency response workers fight secondary fires caused by the isomerization unit explosion at the Texas City refinery|228px|right|Emergency response workers fight secondary fires caused by the isomerization unit explosion at the Texas City refinery]]
On March 23, 2005 at a BP refinery in Texas City, Texas a hydrocarbon vapor cloud ignited, killing 15 workers and seriously injuring 170 others. Over the course an 11 hour period, a combination of control failures, mismanagement, and worker fatigue resulted in the buildup and release of extremely hot, combustible vapor. The key process unit in this disaster was an isomerization unit, located next to wooden trailers for workers servicing an ultracracker unit.

In the early morning on March 23rd, operators initiated startup and pumped raffinate (liquid hydrocarbons) into a raffinate splitter tower used to separate gasoline components. A liquid level indicator and multiple high level alarms monitored the tower liquid level. The level indicator could only measure up to 9 feet of liquid, and the written process called for a liquid level of about 6.5 feet. However, operators routinely filled the tower over 9 feet to minimize fluctuations and to prevent damage to a furnace. Hours later, the first high level alarm was activated and the liquid level rose, but a second alarm higher up the tower failed to trigger. The feed was halted when the liquid had risen to a level of about 13 feet, operators had no way of knowing the exact height. The lead operator relayed the startup activities to another operator and left the facility an hour before his shift ended. The morning operator arrived at 6 am to start his thirtieth consecutive day working a 12-hour shift and read a logbook that read, “Isom* Brought in some raff to unit, to pack raff with.” The day shift operator arrived an hour late, so he could not be briefed by the night shift supervisor. Recirculation then commenced in the tower, and more liquid was added to the tower. Additionally, conflicting instructions caused a liquid level regulating valve to remain closed for several hours, so liquid could not leave the tower. The furnace was then lit, and the supervisor left to attend a family medical emergency.

At noon, the liquid level had risen to 98 feet, but the improperly calibrated liquid level indicator read 8.4 feet. At 12:41 pm, a high pressure alarm caused workers to manually open a chain valve to relieve pressure by using the units pressure relief system to vent vapor into the atmosphere using an obsolete blowdown drum. Heat was also reduced in the furnace to reduce pressure. When operators became concerned about outflow rate, the liquid level regulating valve was opened to release liquid from the tower to storage. This caused the liquid in the tower to begin to boil and spill into the overhead vapor line exerting extreme pressures on the pressure relief system. At 1:14 pm, the three relief valves opened sending the liquid to the blowdown drum which overflowed into a municipal sewer setting off alarms, but a key level indicator in the blowdown drum failed. Flammable liquid erupted from the blowdown drum, formed a massive vapor cloud, and found an ignition source from a nearby idling pickup truck. The colossal blast ignited fires throughout the refinery and over half the workers in the wooden trailers adjacent to the unit were killed immediately (U.S.Chemical Safety Board, 2008).

Investigations after the incident cited multiple failures to implement safety recommendations at the Texas City Refinery. Among these, the blowdown drum was to be replaced by a modern flare to burn off hydrocarbons. However, BP’s budget cuts prevented its replacement. The training and treatment of workers was also called into question, as fatigue, poor communication, and inadequate documentation likely contributed to the disaster. Decisions like the one to operate an unsafe liquid level in order to prevent furnace damage also demonstrate the company’s fixation on the bottom line. BP was eventually fined $21 million by OSHA (New York Times, 2010; U.S.Chemical Safety Board, 2008).

=References=
==Direct Citation References==
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==Additional References==
# American Institute of Chemical Engineers. http://www.aiche.org/ccps
# Biello, David (9 June 2010). "The BP Spill's Growing Toll On the Sea Life of the Gulf". Yale Environment 360. Yale School of Forestry & Environmental Studies. Retrieved 2010-06-14.
# Blanc, P. Bhopal, 1984 – West Virginia near-miss, 2008. Psychology Today, December 2009. https://www.psychologytoday.com/blog/household-hazards/200912/bhopal-1984-west-virginia-near-miss-2008.
# Butler, J. Steven (3 March 2011). "BP Macondo Well Incident. U.S. Gulf of Mexico. Pollution Containment and Remediation Efforts" (PDF). Lillehammer Energy Claims Conference. BDO Consulting. Retrieved 17 February 2013.
# “DOJ accuses BP of ‘gross negligence’ in Gulf oil spill” CNN Money, September 2012.
# Eckerman, I. The Bhopal Saga: Causes and Consequences of the World’s Largest Industrial Disaster. Universities Press Private Limited, 2005.
# Froomkin, Dan (29 July 2010). "Scientists Find Evidence That Oil And Dispersant Mix Is Making Its Way Into The Foodchain". Huffington Post.
# "Gulf of Mexico oil leak 'worst US environment disaster'". BBC News. 30 May 2010.
# Investigation Report: BP Refinery Explosion and Fire, U.S. Chemical Safety Board, 2008.
# Lyall, Sarah. "In BP’s Record, a History of Boldness and Costly Blunders." New York Times, July 13, 2010.
# L.T. Biegler, I.E. Grossmann, A.W. Westerberg. ''Systematic Methods of Chemical Process Design''. Prentice-Hall: Upper Saddle River, 1997.
# M.S. Peters, K.D. Timmerhaus. ''Plant Design and Economics for Chemical Engineers, 5th Ed.'' McGraw-Hill: New York, 2003.
# Northwestern University. Chemical Engineering 351 Lecture Slides.
# Reflections on Bhopal After Thirty Years Video, U.S.Chemical Safety Board, December 2014.
# R.T. Turton, R.C. Bailie, W.B. Whiting, J.A. Shaeiwitz. ''Analysis, Synthesis, and Design of Chemical Processes''. Prentice Hall: Upper Saddle River, 2003.
# Union Carbide Corporation "Methyl Isocyanate" Product Information Publication, F-41443, November 1967.
# USCSB Deepwater Horizon Video, U.S.Chemical Safety Board, June 2014.
# "Transocean Agrees to Plead Guilty to Environmental Crime and Enter Civil Settlement to Resolve U.S. Clean Water Act Penalty Claims from Deepwater Horizon Incident". Department of Justice Office of Public Affairs. January 3, 2013.