AGGIE CHALLENGE. Nuclear Powered Water Desalination Plant

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1 AGGIE CHALLENGE Nuclear Powered Water Desalination Plant Executive Summary Theoretical calculations were performed around a turbine, multistage jet ejector and multi effect evaporator to describe the optimal nuclear powered water desalination plant. The calculations were tabulated in Microsoft Excel, with X steam version 2.6 being used for the steam tables where the jet ejector data and calculations were based on the dissertation CFD OPTIMIZATION STUDY OF HIGH-EFFICIENCY JET EJECTORS, by Somsak Watanawanavet. This system proved to be the most efficient and productive thus far, but future research and calculations must be done to optimize the dimensions and type of nuclear reactor and the number and sizes of the jet ejectors and multi effect evaporators. AggiE Challenge Team Fall 2013 gtsteiger@tamu.edu

2 Table of Contents Project Statement 2 Project Overview.3 Economics. 3 Nuclear Reactor 4 Desalination System..7 Calculations.9 Multi-Stage Jet Ejector 16 Multi-Effect Evaporator.17 Conclusions..19 References 20 Appendix A 21 1

3 Project Statement Design a water desalination plant Conditions: 1. Energy will be provided by a nuclear plant 2. Plant will utilize thermal vapor-compression desalination (jet ejector technologies) 3. Plant must be able to supply fresh water to a large city (10 to 12 million people) Questions: 1. What size, design and type of nuclear reactor will be the most effective? 2. What is the most efficient scheme of water desalination? i.e. number of jet ejectors, additional collection means, etc. 3. Is it cost effective to produce electricity in addition to water? 2

4 Project overview Fresh water is vital to life. In areas of the world where fresh water is not easily accessible or even accessible at all, it is necessary to desalinate salt water. The concept of desalination is simple; evaporate salt water to separate the water from the salt, but this can be more complicated on a scale large enough to supply a city or small country with fresh water. One of the most common large scale means of desalination is distillation, which usually involves an enormous assortment of moving parts, pumps, turbines, et cetera accompanied by vast maintenance and capital costs. In order to optimize and simplify the process, we opted to convert the system to be motivated by a jet ejector. A jet ejector is a low maintenance device requiring no moving parts and can be easily replaced. It is driven by an enthalpy difference between the inlet and outlet of the nozzle of the jet ejector and uses a fast moving motive stream of steam to evaporate fresh water out of a supply of salt water. The efficiency of the jet ejector system is magnified when multiple jet ejectors are used in stages to progressively increase the concentration of the waste brine. The exit steam can then be fed into a series of multi effect evaporators to further increase the amount of fresh water produced. The jet ejector requires high pressure and high temperature steam to comprise the motive stream. The optimal source to provide the energy to vaporize the water, creating this high enthalpy steam, is with a nuclear reactor. This system can be adapted to produce electricity, in addition to fresh water, by inserting a turbine between the steam coming off the nuclear reactor and jet ejector. This would also include adding a separator and a line returning to the nuclear reactor to superheat the steam, between the turbine and the jet ejector, to produce a sufficient enthalpy difference in the jet ejector to drive the production of fresh water. While this adds complexity to the system, it also greatly increases the markets that can be catered to, and subsequently the potential profits, of the desalination plant. Economics When choosing a location for the desalination plant, the wholesale price of water and electricity, the region s necessity for clean water, and the region s opinion towards nuclear energy were all assessed. We determined that Los Angeles matched our desires very well. Los Angeles is in desperate need for clean water since the growing population has depleted much of its water from the Colorado River. Additionally, it is close to the coast, so an ample supply of salt water is not an issue. There are no major adversities to the use of nuclear power, and California has even started looking into the utilization of desalination technology. Therefore, the implementation of a desalination plant powered by nuclear energy should not face much opposition. When doing an economic analysis of the costs and profits associated with implementing this desalination process, the wholesale prices of water and electricity in Los Angeles had to be investigated. The wholesale price of water in the region is $0.0235/ft 3 (U.S. Energy Information Administration), and the wholesale price of electricity is $39.09/MWh (Olivenhain Municipal Water District). These prices were examined graphically so that the optimum ratio of electricity and water production could be determined. Before we are able to specify a solid cut off for the discharge pressure from the turbine, we need to account for local demand for each water and electricity, full capabilities of the plant, 3

5 Sales ($/h) potential profit for each and the production costs for each. Our preliminary analysis indicates an all electric plant would be more profitable, however fresh water production is the main objective of this project Potential Profits Total Sales Water Sales Turbine Discharge Pressure (psia) Figure 1: Potential profits for data based on single jet ejector system, with 50 degrees F of superheat, not including water produced by the multi effect evaporator. Nuclear Reactor The nuclear team objectives were to recommend a reactor type, size and design the steam and/or power production side of the plant. Throughout our decision-making process, we sought to recommend this new technology with optimal feasibility. Reactor Recommendation For the Los Angeles, CA case study, the nuclear team recommended the Diablo Canyon Nuclear Power Plant. We chose this site because of the proximity to LA as well as direct contact with ocean water. The plant contains two PWRs (Pressurized Water Reactors), which are a preferred choice of reactor type because it is commonly used and therefore easier to accept by the client than other, lesser known reactors. Table 1 shows more details about Diablo Canyon s two units. Using the general rule of thumb conversion (Equation 1) between the thermal (steam) power and electrical power produced in such a plant, we determined that Unit 1 can produce 3,366 MW(th) and Unit 2 can produce 3,354 MW(th). 4

6 Table 1. Diablo Canyon Nuclear Power Unit 1 Unit 2 Plant Type Westinghouse 4-Loop PWR Westinghouse 4-Loop PWR Operation Since License Expiration/Renewal Date 2010 Power Generation Capacity 1,122 MW(e) 1,118 MW(e) Also notice in Table 1 that the Diablo Canyon units are due for license renewal in about a decade. The company that owns Diablo Canyon, Pacific Gas and Electric, has indicated that modifications will be made to the secondary cooling cycle of each unit upon license expiration. This information allows for the possibility of integrating the desalination plant to the nuclear reactor at this time. We have explored the feasibility of integration and recommend that a tertiary loop is installed (that is, with an additional heat exchanger) between that of the secondary loop and the jet ejector. Please refer to the Visio Process Flow Diagram in Appendix A for a schematic of this tertiary loop system. As a final remark on the Diablo Canyon recommendation, we note that both units are online solely to supply power to significant portion of Californian residents. Thus, co-generation of electrical power and potable water must be considered if the next step in this project includes Diablo Canyon s reactors. A side note: Co-generation can be assisted by individual utilization of the reactor s four loops towards one of the two purposes. DEEP/DE-TOP Plant Simulation The IAEA, International Atomic Energy Agency, has many resources for the conceptual design of nuclear desalination plants. From their website, we accessed the plant simulation tools DE-TOP and DEEP. DE- TOP stands for Desalination Thermodynamic Optimization Program, and is a beta tool, which means it has limited functionality for our purposes. The other program, DEEP, is a Desalination Economic Evaluation Program. While we have not yet been able to successfully modify the programs to our custom requirements, such as modeling the jet ejector in place of a low pressure turbine, we have used DEEP to do an economical comparison between Reverse Osmosis (RO), Multi-Effect Distillation (MED), and Multi-Stage Flash (MSF). Of these three technologies, the MED option with Vapor Compression cycle and an intermediate loop is most similar to our case. Figure 2 and 3 below show our results from DEEP. Notice that the cost of water and power production in Figure 2 (top left corner) have units of $/m3 and $/MWh, respectively, and cannot be applied directly to our process due to the differences in efficiency of our novel heat exchangers and jet ejectors. Finally, we determined from DEEP that the incremental operating cost for the additional steam loop (tertiary loop) is an increase of ~$0.06/m 3 = $0.22/kgal. This is a significant operating cost increase, as compared to the wholesale price of water to LA: $3.14/kgal. However, we maintain our recommendation due to the licensing issues of integration without this buffer loop 5

7 Figure 2: DEEP schematic for MED with VC cycle and intermediate loop, using one of the Diablo Canyon units thermal power (3360 MWth). Figure 3: DEEP economic comparison between the nuclear desalination technologies available. 6

8 Turbine Work (MW) Distillate Rate (Mlbm/h) Desalination System Turbine A high pressure turbine would be inserted between the steam coming from the nuclear reactor and the jet ejector. This would take off a portion of the highest quality steam to produce electricity. The electricity can be used to supplement the nuclear plant or the desalination plant and can be sold off into the grid. This would make the plant much more versatile and would offer an avenue to increase potential profits. Our initial analysis shows that a significant amount of power and distilled water can be produced using this method. The data below are based on the same set of data as Figure Turbine Work and Distillate Flow Rate as a Function of Discharge Pressure Turbine Discharge Pressure (psia) WT MW Distillate Rate (Mlbm/h) Figure 4: For data based on single jet ejector system, with 50 degrees F of superheat, not including water produced by the multi effect evaporator. Super heat Because the steam coming out of the turbine has been downgraded and the jet ejector is an enthalpy driven instrument, adding superheat to the motive steam greatly increases the mass flow ratio, ratio of motive steam to entrailed steam, and therefore the efficiency and productivity of the jet ejector. We chose an arbitrary value of 50 o Fahrenheit of super heat. With this addition the mass flow ratio increased by an average of 50%. 7

9 Mass Flow Ratio Superheat vs. No Superheat No Superheat +50 degrees F Superheat Motive Steam Pressure (psia) Figure 5: Increase in mass flow ratio based on degrees of super heat, for data based on single jet ejector system with 50 degrees F of superheat. Multi-Effect Evaporator A multi-effect evaporator can be included at the end of the process to take the motive steam from the outlet of the jet ejector and produce additional fresh water. The premise of this is to squeeze the last bit of energy out of the motive steam to produce more fresh water instead of letting it go to waste as heat pollution. The working principles of a multi-effect evaporator are drawn in Figure 6 below. 5 lbm water vapor produced per effect From product stream of jet ejector Enter: 5 lbm steam at 120 psia Effect 1: 120 psia Effect 2: 100 psia Effect n: 14.7 psia Fresh water Brine Fresh water Brine Fresh water Brine Salt water Source 5 lbm fresh water produced per effect Total fresh water produced = 5+5+(5*n) lbm Fresh water Storage Figure 6: Working principle of a multi-effect evaporator. 8

10 Calculations Calculations for flow rates, temperatures and pressures Given: T9 = 572 F H12 = btu/lbm P9 = 1246 psia T12 = F H9 = 1182 Btu/lbm P12 = 120 psia S9 = 1.36 btu/lbm*r v4 = 60 ft/s S1 = 1.36 btu/lbm*r P4 = 120 psia η = 0.8 P3 = psia Ereactor = 461 MMbtu/h Speed of sound in steam = m/s T11 = F Cp = Add 50 degrees of superheat *if an item is highlighted it must be looked up in the steam tables as indicated For Stream 1: S7 = S2 = Vapor fraction = Liquid fraction = H1,L = look up the enthalpy of liquid value in the steam tables at P1 H1,V = look up the enthalpy of vapor value in the steam tables at P1 Isentropic enthalpy = Isentropic ΔH Real ΔH where efficiency of the turbine = η = 0.8 Enthalpy of stream 1 = Real vapor fraction = Real liquid fraction = Stream 8: T1 = T2 = T7 = Temperature of saturated vapor at P1 from steam tables T13: since T2 is the stream that goes to be superheated and the heat source is the reactor, at a constant 572 F. 50 degrees of super heat is added to T2. If T2 is greater than 572 F with the 9

11 additional 50 degrees of super heat, T13 = 572 F. If T2 plus the 50 degrees of superheat is less than 572 F, then T13 = T H13 = H at T13 in steam tables for saturated steam T8 = H8 = H at T8 in steam tables for saturated steam Stream 2: T2 = T1 M2 = H2 = H at P1 in steam tables for saturated steam Stream 11: M11 = M8 M7 Work from the turbine: Wturbine = Jet Ejector and mass flow rates: To get V12: To get V12 in m/s: Speed of Sound in Steam: Mach number: Cp where density of steam at 100 psia in units of lbm/ft^3 is : Double linear interpolation using Table 30 (p. 157) of dissertation: The actual interpolation will use variables for each value listed in table Using double linear interpolation to solve for x with a given Cp 10

12 Mach # Mass flow 0.1 x Cp Mach # Mass flow D x F A G H B K C I J Mass flow ratios Volumetric flow rate: Money from water: Money from electricity: Total Money: Given: EXAMPLE CALCULATION EVALUATED AT: P1 = 650 PSIA T9 = 572 F H12 = btu/lbm P9 = 1246 psia T12 = F H9 = 1182 Btu/lbm P12 = 120 psia S9 = 1.36 btu/lbm*r v4 = 60 ft/s S1 = 1.36 btu/lbm*r P4 = 120 psia η = 0.8 P3 = psia Ereactor = 461 MMbtu/h Speed of sound in steam = m/s T11 = F Cp = Add 50 degrees of superheat 1. Find S1,L using: S7 = S1,L = ( * Find the entropy of liquid water at T = Use that value to get: 11

13 2. Next find S1,V using S2 = S2,V = ( * Find the entropy of saturated steam at Use that value to get: 3. Calculate the isentropic vapor fraction coming off the turbine using: Vapor fraction = 4. Calculate the isentropic liquid fraction coming off the turbine using: Liquid fraction = 5. H1,L => look up the enthalpy of liquid value in the steam tables at P1 6. H1,V = look up the enthalpy of vapor value in the steam tables at P1 7. Calculate the isentropic enthalpy using: 8. Find the isentropic enthalpy change using: 9. Find the non ideal enthalpy change, where efficiency of the turbine = η = 0.8, using: 10. To find the enthalpy of stream 1 use: 11. Find the vapor fraction of stream 1 with a turbine efficiency of 0.8 using: 12. Find the liquid fraction of stream 1 with a turbine efficiency of 0.8 using 12

14 13. T1 = T2 = T7 = Temperature of saturated vapor at P1 from steam tables 14. Find the temperature of stream 8 using: 15. H8 = H at T8 in steam tables for saturated steam 16. Find T13 using: 17. H13 = H at T13 in steam tables for saturated steam 18. Find the mass flow rate of stream 8 using: 19. Find the mass flow rate of stream 7 with T7 = T1 = 495 F using: 20. Find the mass flow rate of stream 2 with T2 = T1 = 495 F using: 21. H2 = H at P1 in steam tables for saturated steam 22. Find the mass flow rate of stream 11 using: 23. Calculate the work produced by the turbine that can be used for electricity: 13

15 24. Calculate V12 in ft/s using 25. Calculate V12 in m/s using 26. Calculate Mach Number using 27. Use double linear interpolation to solve for the mass flow ratio using a table and the interpolation formula. The heat capacity is a constant and the equation can be solved for the mass flow ratio, X: Mach # Mass flow D X F A G H B K C I J Mach # Mass flow 0.1 X

16 28. The mass flows can be calculated from the previously calculated mass flows and with the following equations 29. The volumetric flow rate can be calculated using 30. The revenue from water can be calculated using 31. The revenue from water can be calculated using 32. The total revenue can be calculate by summing the two previous equations, 15

17 Multi-Stage Jet Ejector Calculations Calculation for the multi-stage jet ejector is very similar to that of a single stage, except that the calculation must be repeated over several stages. In order to optimize the system, however, a sensitivity analysis over how the motive steam will be distributed over the stages must be performed. The following assumptions were made in the multi-stage jet ejector: 1. The initial and final outlet brine concentrations are defined. 2. Each stage is set to produce equal amount of distilled water. 3. Only the evaporated steam condenses in the cold side of the heat exchanger and the motive steam leaves the cold side to purge. Since the jet ejector desalination process performance is a function of the pressure of the motive steam, this pressure must be set. Also, the amount of motive steam required to produce the desired amount of distilled water from the jet ejector is not known until the mass flow rate ratio is calculated. It is important to remember that setting the pressure of the motive steam sets the motive steam flow rate that is available. Therefore, this procedure is iterative. Outlined below is the procedure we used in our Excel calculations of the multi stage jet ejectors: 1. Set the initial and final salt concentration. In addition to this, also specify the number of stages, the inlet motive steam properties, and motive steam mass flow rate. 2. The flow rate of distilled water produced will be varied. 3. In each stage, perform the jet ejector calculation stated previously in this report. For this procedure, however, the distilled water flow rate at each stage is known while motive steam flow rate is not known and can be determined using the calculated mass flow ratio. ass lo ratio 4. Sum up all the calculated motive steam flow rate from each stage for different distilled water flow rate. 5. Determine the total motive steam flow rate that is equal to the available motive steam flow rate at the specified temperature. 6. Once the correct total motive steam flow rate is determined, the distilled water flow rate with that total motive steam flow rate is the outlet stream of the process. Tabulated in Table 2 below are our results from our multi stage jet ejector calculations. We calculated these data based on a 10 stage jet ejector. The efficiency, distillate mass per amount of motive steam, increased an average 15%. 16

18 Table 2: Multi stage Jet Ejector efficiency increase Turbine discharge pressure (psia) Distillate mass/motive steam (single) Distillate mass/motive steam (multi) Distillate rate (million lbm/hour) (single) Distillate rate (million lbm/hour) (multi) Percent increase Average Multi-Effect Evaporator Calculations There are many literatures available on multi-effect evaporator. Dr. Samsok s dissertation contains a sample calculation on multi-effect evaporator. The calculation performed in this work is very similar to the readily available literature and the dissertation. The following assumptions are made in this work: 1. There is no heat loss in each heat exchanger. 2. All the steam condenses in the condensing side. 3. The brine in each cold side is at their saturation temperature. 4. Each heat exchanger operates with ΔT=0.3 F With these assumptions, the calculation is as followed: 1. Set the initial and final brine concentration and specify the number of stages. 2. Determine the flow rate of seawater. 3. Perform the heat balance using the equation below, where, = mass flow of condensing steam (lb m /hr) = vapor enthalpy of the condensing steam (Btu/lb m ) = liquid enthalpy of the condensing steam (Btu/lb m ) = mass flow of the evaporating steam (lb m /hr) = vapor enthalpy of the evaporating steam (Btu/lb m ) = liquid enthalpy of the evaporating steam (Btu/lb m ) 4. Determine M e as M c for the next stage and use P sat at the evaporating side to calculate the properties of the steam flowing to the next effect. 5. Using mass balance on the total mass and mass of the salt, determine the brine concentration leaving each stage. 17

19 6. From the literature over the novel latent heat exchanger, the area of the heat exchanger at each stage can be determined based on the operating conditions of the heat exchanger and amount of heat that is being transferred. Table3 below summarizes our results from a multi effect evaporator with 10 effects. Our initial analysis shows that a multi effect evaporator could increase distilled water produced significantly. Table 3 Effect Pressure (psia) Condensed water (lbm/hr) Water vapor produced (lbm/hr) Total fresh water produced (lbm/hr)

20 Conclusions Over the course of the project, our team encountered challenges that would be facilitated by a few recommendations. First, since everyone on the team is at a different level in their course work, a trip to a lab to observe a jet ejector, heat exchanger and pieces of equipment needed for the desalination plant or a more detailed lecture on the basic operating principles of each would be useful. Second, a different structure for the meeting times would be helpful. For example, use the meeting times to divide up work between teams and team members because busy schedules make it difficult to meet some weeks. More interaction between the different teams would have been nice so that all team members could have a better understanding of the overall project. In addition to improvements in the class, the project is far from over. Topics for future research, in no particular order, include but are not limited to: 1. Find the most efficient number of jet ejector stages Calculated by the increase in distillate mass per amount motive steam 2. Account for momentum ratio, C p, deviation 3. Find the most efficient turbine discharge pressure Based on the most efficient number of jet ejector and multi effect evaporator stages 4. Calculate the most efficient jet ejector sizes and the sizes for each stage Inlet and outlet nozzle diameter, length Will be different for each stage, as the number of stages increase, so should the size of the jet ejector 5. Calculate the most efficient number and size of effects for the multi effect evaporator Consider heat exchanger size, material, cost Each effect may be different 6. Calculate cost of the entire multi effect system Is it a cost efficient system? 7. Research different geographical locations to maximize profits 8. In depth analysis of plant costs and profits including: Cost of nuclear reactor, turbine, jet ejectors, multi effect evaporators and other associated plant costs Full analysis of profits, how long it will take to recover from startup costs etc. 9. Decide whether co-generation of water and electricity is profitable and efficient Calculate the size, type and cost of the turbine required 10. Full plant simulation Using a program such as ASPEN or DEEP Next semester s team can also research into Russia s new floating nuclear power plant to see if a floating desalination would be worth delving into. Russia s Floating Nuclear Power Plant: 19

21 References S. Watanawanavet, CFD Optomization Study of High-Efficiency Jet Ejectors, Ph.D. dissertation, Dept. of Chem. Engr., Texas A&M Univ., College Station, TX, May iablo_canyon_power_plant_safely_shut_down.shtml pbadupws.nrc.gov/docs/ml0036/ml pdf 20

22 Appendix A 9 HE2 - Superheating HE1 Turbine Steam Trap 1 2 JE1 JE2 JE Latent Heat Exchanger #1 Latent Heat Exchanger #2 Latent Heat Exchanger # Motive Steam This stream will be condensed by heating the brine through heat exchangers, not shown explicitly 8 11 Brine (8.75 wt% NaCl) 10 E-13 Distilled Water Seawater (3.5 wt% NaCl) Figure 7: Schematic of the process flow diagram used in the example calculations. 21

23 Figure 8: Schematic of entire process flow diagram. The Nuclear plant is the portion of the diagram to the LEFT of HE2, the desalination plant is the portion of the diagram to the RIGHT of HE2. 22