Desalination - Team G - Revision history

WFu at 18:16, 11 March 2016

2016-03-11T18:16:36Z

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WFu at 18:14, 11 March 2016

2016-03-11T18:14:19Z

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WFu at 18:04, 11 March 2016

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WFu at 17:52, 11 March 2016

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 [[File:Watson_PFD_appendixB.JPG|frame|center|border|<div align=center> <div>]]
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 =References=
 . Global Agenda Council on Water. World Economic Forum website. http://www.weforum.org/communities/global-agenda-council-on-water. Accessed January 14, 2016.

WFu at 17:45, 11 March 2016

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	==Appendix A: Block Flow Diagram==		==Appendix A: Block Flow Diagram==
	[[File:Watson appendixA.JPG\|frame\|center\|border\|<div align=center> <div>]]		[[File:Watson appendixA.JPG\|frame\|center\|border\|<div align=center> <div>]]
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			==Appendix B: Process flow diagram==
			[[File:Watson_PFD_appendixB.JPG\|frame\|center\|border\|<div align=center> <div>]]
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	=References=		=References=

WFu at 17:44, 11 March 2016

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	=Appendices=		=Appendices=
	==Appendix A==		==Appendix A: Block Flow Diagram==
			[[File:Watson appendixA.JPG\|frame\|center\|border\|<div align=center> <div>]]

			<div align=left>
	=References=		=References=
	1. Global Agenda Council on Water. World Economic Forum website. http://www.weforum.org/communities/global-agenda-council-on-water. Accessed January 14, 2016.		1. Global Agenda Council on Water. World Economic Forum website. http://www.weforum.org/communities/global-agenda-council-on-water. Accessed January 14, 2016.

WFu at 17:43, 11 March 2016

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	Once the overall design equipment and strategy was selected, mass and energy balances were calculated and optimized for yield and cost. To determine these values, temperatures, and flow rates, we made a number of assumptions and set points in our process. The feed and waste concentrations were held constants, at the composition of Oregon sea water, and the maximum allowable waste concentration. The flash inlet was held at 1 atm and 98°C, in order to maximize energy carried by the stream without premature boiling. The distillate flow rate was held constant in line with our initial problem statement and project goals. Finally, phase data was obtained from Aspen+. While true seawater will contain other contaminants, these have small effects on thermodynamic properties. Pretreatment and material selection will take additional contaminants into consideration, but they are neglected in mass and energy calculations.		Once the overall design equipment and strategy was selected, mass and energy balances were calculated and optimized for yield and cost. To determine these values, temperatures, and flow rates, we made a number of assumptions and set points in our process. The feed and waste concentrations were held constants, at the composition of Oregon sea water, and the maximum allowable waste concentration. The flash inlet was held at 1 atm and 98°C, in order to maximize energy carried by the stream without premature boiling. The distillate flow rate was held constant in line with our initial problem statement and project goals. Finally, phase data was obtained from Aspen+. While true seawater will contain other contaminants, these have small effects on thermodynamic properties. Pretreatment and material selection will take additional contaminants into consideration, but they are neglected in mass and energy calculations.

			[[File:Watson_PFD_appendixB.JPG\|frame\|center\|border\|<div align=center> ASPEN+ simulation <div>]]
			<div align=left>

	===Pressure Considerations and Yield===		===Pressure Considerations and Yield===

WFu at 17:40, 11 March 2016

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	Profitability of the plant was assessed by calculating the net present value (NPV) and internal rate of return (IRR). The price of purified water was assumed to be $3.00 per 1000 gallons from average water costs in Newport, Oregon and assuming we will have to sell our water at a lower price to treatment facilities before being sold for public use.<sup>24</sup> Assuming a 5-year MACRS depreciation schedule, this plant is not profitable by a large margin, losing up to 7 billion dollar over a 20 year window. Main sources of the imbalance are high utility costs ($150 MM for high pressure steam). If profit is the goal of this process, we would not recommend pursuing this project. However, if there a pressing need and investment for purified water, and a potential source of local energy from renewable sources, this process may be a feasible project, though expensive. The full economic analysis can be found in Appendix J. A sensitivity analysis revealed that lowering high energy requirements is imperative to drive down prohibitive costs. In addition, subsidies would be necessary to execute this process. Sensitivity to several process parameters is represented in Figure 3.		Profitability of the plant was assessed by calculating the net present value (NPV) and internal rate of return (IRR). The price of purified water was assumed to be $3.00 per 1000 gallons from average water costs in Newport, Oregon and assuming we will have to sell our water at a lower price to treatment facilities before being sold for public use.<sup>24</sup> Assuming a 5-year MACRS depreciation schedule, this plant is not profitable by a large margin, losing up to 7 billion dollar over a 20 year window. Main sources of the imbalance are high utility costs ($150 MM for high pressure steam). If profit is the goal of this process, we would not recommend pursuing this project. However, if there a pressing need and investment for purified water, and a potential source of local energy from renewable sources, this process may be a feasible project, though expensive. The full economic analysis can be found in Appendix J. A sensitivity analysis revealed that lowering high energy requirements is imperative to drive down prohibitive costs. In addition, subsidies would be necessary to execute this process. Sensitivity to several process parameters is represented in Figure 3.

			[[File:Watson f3.JPG\|frame\|center\|border\|<div align=center> Figure 3. Sensitivity analysis based on change in the 20 year projection. <div>]]
	FIGURE 3
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	=Conclusion=		=Conclusion=

WFu at 17:38, 11 March 2016

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	[[File:Watson f1.JPG\|frame\|center\|border\|<div align=center> Figure 1. a) Flash conditions with respect to pressure in the last drum. b) Process metrics with respect to pressure in the last drum. <div>]]		[[File:Watson f1.JPG\|frame\|center\|border\|<div align=center> Figure 1. a) Flash conditions with respect to pressure in the last drum. b) Process metrics with respect to pressure in the last drum. <div>]]
	<div align=left>~~FIGURE 1~~		<div align=left>

	At first inspection, it appears advantageous to operate at the lowest possible pressure to obtain the highest vaporization yield. However, dilution for waste proved to be a more significant factor in overall process yield than the yield over the flash drum unit. Operating at the lowest possible pressure maximizes vapor yield, but creates a more concentrated recycle stream. This higher concentration requires more process feed to dilute to waste conditions, lowering the overall process yield. For this reason, it is desirable to produce a recycle stream as close to waste concentration as possible, minimizing the amount of process feed needed to dilute to waste conditions. Figure 1b shows the effect of flash pressure on overall process metrics. It becomes clear that the overall yield increases with pressure as an asymptote. Above a certain pressure, the recycle stream becomes too dilute to create a waste concentration of 40 g/kg, creating a negative feed requirement for this calculation. Because we would like to release waste of 40 g/kg, we focus on the feasible solutions below 0.4 atm. Figure 1b also shows the small effect on heating requirements as the pressure is changed. Because the amount of water vaporized is held constant, this energy is largely representative of the energy needed to vaporize that amount of water. Figure 1a shows that at higher operating pressures and lower vaporization yields, the brine recycle will remain hot. Therefore, despite increased recycle rates, the higher temperature keeps the energy requirement nearly constant. With these considerations, we will operate at a flash pressure that limits the vaporization yield, keeping the recycle stream near waste concentrations. When operating at a pressure of 0.3 atm, an overall yield of approximately 62% can be achieved. This higher pressure will also provide energy savings in vacuum creation.		At first inspection, it appears advantageous to operate at the lowest possible pressure to obtain the highest vaporization yield. However, dilution for waste proved to be a more significant factor in overall process yield than the yield over the flash drum unit. Operating at the lowest possible pressure maximizes vapor yield, but creates a more concentrated recycle stream. This higher concentration requires more process feed to dilute to waste conditions, lowering the overall process yield. For this reason, it is desirable to produce a recycle stream as close to waste concentration as possible, minimizing the amount of process feed needed to dilute to waste conditions. Figure 1b shows the effect of flash pressure on overall process metrics. It becomes clear that the overall yield increases with pressure as an asymptote. Above a certain pressure, the recycle stream becomes too dilute to create a waste concentration of 40 g/kg, creating a negative feed requirement for this calculation. Because we would like to release waste of 40 g/kg, we focus on the feasible solutions below 0.4 atm. Figure 1b also shows the small effect on heating requirements as the pressure is changed. Because the amount of water vaporized is held constant, this energy is largely representative of the energy needed to vaporize that amount of water. Figure 1a shows that at higher operating pressures and lower vaporization yields, the brine recycle will remain hot. Therefore, despite increased recycle rates, the higher temperature keeps the energy requirement nearly constant. With these considerations, we will operate at a flash pressure that limits the vaporization yield, keeping the recycle stream near waste concentrations. When operating at a pressure of 0.3 atm, an overall yield of approximately 62% can be achieved. This higher pressure will also provide energy savings in vacuum creation.
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	The flash unit, where all flash stages occur, and makes up the bulk of the process. It consists of 9 vertical flash drums connected in series, each with a condenser in the upper portion. The drums are held at low pressure, allowing the volume to fill with saturated water vapor. This vapor condenses on heat exchange pipes in the top of the drum, and condenses, falling onto a collection tray. Once overall mass balances were calculated, detailed mass and energy balances on the major flash unit were analyzed. First, the equilibrium in each stage was characterized. Connected equilibrium stages exhibit linearly decreasing temperature.<sup>19</sup> Optimization of mass balances called for a pressure of 0.3 atm in the last drum to achieve the highest yield. This produces the following equilibrium conditions across all 9 drums (the number of drums eventually selected).		The flash unit, where all flash stages occur, and makes up the bulk of the process. It consists of 9 vertical flash drums connected in series, each with a condenser in the upper portion. The drums are held at low pressure, allowing the volume to fill with saturated water vapor. This vapor condenses on heat exchange pipes in the top of the drum, and condenses, falling onto a collection tray. Once overall mass balances were calculated, detailed mass and energy balances on the major flash unit were analyzed. First, the equilibrium in each stage was characterized. Connected equilibrium stages exhibit linearly decreasing temperature.<sup>19</sup> Optimization of mass balances called for a pressure of 0.3 atm in the last drum to achieve the highest yield. This produces the following equilibrium conditions across all 9 drums (the number of drums eventually selected).

			[[File:Watson f2.JPG\|frame\|center\|border\|<div align=center> Figure 2. a) Final equilibrium temperature and pressure as a function of the number of flash drums. b) Cost of increasing number of drums. <div>]]
	FIGURE 2
			<div align=left>

	Interestingly, this equilibrium profile creates nearly constant vapor flow rates in each drum. These flow rates, along with the density of saturated vapor in each drum, were used to calculate the minimum chamber diameter to allow enough vapor-liquid interface. The minimum diameter for the lowest pressure drum was approximately 6 m, and the minimum for preceding drums varied minimally. For manufacturing simplicity, we have decided to build all 9 drums using a diameter and height of 6 m. As explained previously, drums will be constructed of stainless steel to prevent corrosion. Because each drum will be operated in vacuum conditions, the pressure on drum walls will never exceed 1 atm. We therefore calculated the thickness of material based on structural feasibility, requiring a thickness of 3 cm. Pricing of the drums was approximated using the required weight of stainless steel. In addition to drum material, the cost of each condensing unit was approximated using the required heat exchange area to condense the distillate. This calculation was performed under the assumption that the surge tank outlet (See Appendix L) will be used as the cooling stream on the tube side. Approximate cost of each condensing system was derived heuristically from the heat transfer area. All of these equilibrium and costing calculations were repeated for a growing number of stages. Figure 2b indicates the total cost of the flash unit for different numbers of stages. Using this, we selected 9 tanks, within the region of diminishing returns. Once this number was selected, we began specific design of each condensing region of the drums. Notably, the last three drums require excessive heat transfer area that exceeds 5000 square meters. The last drum, which requires nearly 19000 square meters, can be remedied by using the process seawater feed to cool, which is at a significantly lower temperature than the surge tank. Unfortunately, the process feed does not have the heat capacity to cool the 7th and 8th drums. For this initial design and economic analysis, these drums retain an unrealistic heat transfer area. In implementation, additional cooling water would be required. The cost of demisters and trays were also considered. The approximate price for a 6-meter diameter demister is $500. Both the demister and condensing tray price were considered negligible compared to the overall price of the flash chambers.		Interestingly, this equilibrium profile creates nearly constant vapor flow rates in each drum. These flow rates, along with the density of saturated vapor in each drum, were used to calculate the minimum chamber diameter to allow enough vapor-liquid interface. The minimum diameter for the lowest pressure drum was approximately 6 m, and the minimum for preceding drums varied minimally. For manufacturing simplicity, we have decided to build all 9 drums using a diameter and height of 6 m. As explained previously, drums will be constructed of stainless steel to prevent corrosion. Because each drum will be operated in vacuum conditions, the pressure on drum walls will never exceed 1 atm. We therefore calculated the thickness of material based on structural feasibility, requiring a thickness of 3 cm. Pricing of the drums was approximated using the required weight of stainless steel. In addition to drum material, the cost of each condensing unit was approximated using the required heat exchange area to condense the distillate. This calculation was performed under the assumption that the surge tank outlet (See Appendix L) will be used as the cooling stream on the tube side. Approximate cost of each condensing system was derived heuristically from the heat transfer area. All of these equilibrium and costing calculations were repeated for a growing number of stages. Figure 2b indicates the total cost of the flash unit for different numbers of stages. Using this, we selected 9 tanks, within the region of diminishing returns. Once this number was selected, we began specific design of each condensing region of the drums. Notably, the last three drums require excessive heat transfer area that exceeds 5000 square meters. The last drum, which requires nearly 19000 square meters, can be remedied by using the process seawater feed to cool, which is at a significantly lower temperature than the surge tank. Unfortunately, the process feed does not have the heat capacity to cool the 7th and 8th drums. For this initial design and economic analysis, these drums retain an unrealistic heat transfer area. In implementation, additional cooling water would be required. The cost of demisters and trays were also considered. The approximate price for a 6-meter diameter demister is $500. Both the demister and condensing tray price were considered negligible compared to the overall price of the flash chambers.

WFu at 17:37, 11 March 2016

2016-03-11T17:37:29Z

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	Aspen+ phase data revealed that because the energy used to vaporize the water is carried in the inlet stream, the amount of water flashed depends almost completely on the pressure in the last flash stage, or the lowest pressure in the process. Because of this, the mass balances over the entire process are largely dependent on the equilibrium conditions in the last drum. Therefore, for overall balances, we treated the connected series of flash drums as one unit. This assumption is based on the adiabatic nature of the drums, and the assumption that the brine reaches phase equilibrium before leaving the unit. This yields a simplified block diagram for the purpose of calculating overall mass balances, as shown in Appendix A. Figure 1a shows conditions at a range of vacuum pressures. As pressure is decreased, the yield of vaporization increases, which corresponds to an increase in the outlet concentration of NaCl for recycle. Temperature decreases with pressure to maintain vapor-liquid saturation conditions. The temperature profile is critical in designing the multistage flash unit, as higher temperatures through the pressure gradient will release hot distillate that can be captured in preheating.		Aspen+ phase data revealed that because the energy used to vaporize the water is carried in the inlet stream, the amount of water flashed depends almost completely on the pressure in the last flash stage, or the lowest pressure in the process. Because of this, the mass balances over the entire process are largely dependent on the equilibrium conditions in the last drum. Therefore, for overall balances, we treated the connected series of flash drums as one unit. This assumption is based on the adiabatic nature of the drums, and the assumption that the brine reaches phase equilibrium before leaving the unit. This yields a simplified block diagram for the purpose of calculating overall mass balances, as shown in Appendix A. Figure 1a shows conditions at a range of vacuum pressures. As pressure is decreased, the yield of vaporization increases, which corresponds to an increase in the outlet concentration of NaCl for recycle. Temperature decreases with pressure to maintain vapor-liquid saturation conditions. The temperature profile is critical in designing the multistage flash unit, as higher temperatures through the pressure gradient will release hot distillate that can be captured in preheating.

	[[File:Watson f1.~~jpg~~\|frame\|center\|border\|<div align=center> Figure 1. a) Flash conditions with respect to pressure in the last drum. b) Process metrics with respect to pressure in the last drum. <div>]]		[[File:Watson f1.JPG\|frame\|center\|border\|<div align=center> Figure 1. a) Flash conditions with respect to pressure in the last drum. b) Process metrics with respect to pressure in the last drum. <div>]]
	<div align=left>FIGURE 1		<div align=left>FIGURE 1