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

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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.

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[1]. 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[2]. 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[3].

In June 2014, the city announced the Corpus Christi Desalination Demonstration Project[4] 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[5]. 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)[6], thus setting the concentration of the process feed stream. This translates to a feed stream concentration of 30,500 ppm (parts per million)[7]. 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[8]. 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[9][10]. Through the incorporation of a Class I injection well, the brine will not have to be treated or diluted before injection. [11][12]

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.[13]

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[14]. 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[15]. 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[16]. 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[17]. 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.[18] 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.

References

  1. ^ Tapping the oceans. The Economist USA. http://www.economist.com/node/11484059. Accessed March 2, 2016.
  2. ^ Water Supply in the US. EPA. http://www3.epa.gov/watersense/pubs/supply.html. Accessed March 2, 2016.
  3. ^ 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.
  4. ^ Corpus Christi Desalination Demonstration Project. Available at http://www.cctexas.com/Assets/Departments/Water/Files/DesalFactSheet.pdf. Published June 2014. Accessed January 12, 2016.
  5. ^ Torcellini P, Long N, Judkoff R. Consumptive Water Use for U.S. Power Consumption. NREL. 2003; TP-550-33905.
  6. ^ 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.
  7. ^ Definition and Units. CATD Salinity Expertise Center. Available at http://www.salinityremotesensing.ifremer.fr/home. Accessed January 12, 2016.
  8. ^ Brown E, Colling A, Park D, Phillips J, Rothery D, Wright J. Seawater : Its Composition, Properties and Behaviour. Boston: Butterworth-Heinemann, 1995.
  9. ^ Texas Water Code. Availabe at http://www.statutes.legis.state.tx.us/Docs/WA/htm/WA.27.htm. Accessed January 14, 2016.
  10. ^ Google Maps. Available at https://goo.gl/maps/3xcvLZwUCe42 Accessed January 15, 2016.
  11. ^ 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.
  12. ^ List of Injection Well Facilities by Location. Available at http://www.ehso.com/cssepa/tsdfdeepwells.php. Accessed January 14, 2016.
  13. ^ Climate Corpus Christi- Texas. US Climate Data. Available at http://www.usclimatedata.com/climate/corpus-christi/texas/united-states/ustx0294. Accessed January 12, 2016.
  14. ^ 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.
  15. ^ World Health Organization. Desalination for safe water supply. http://www.who.int/water_sanitation_health/gdwqrevision/desalination.pdf. Accessed January 26, 2016.
  16. ^ Vouchkov, N. Re-Mineralization of Desalinated Water. SunCam. https://s3.amazonaws.com/suncam/npdocs/118.pdf. Accessed February 18, 2016.
  17. ^ 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.
  18. ^ Fritzman C, Lowenberg J, Wintgens T, Melin T. State-of-the-art Reverse Osmosis Desalination. Desalination. 2007; 216:1-76.