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.

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.

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.[19]

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[20][21]. 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[22][23][24][25]. 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[26]. 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[27][28]. 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.

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