Proceedings of the 11th Australasian Heat and Mass Transfer Conference, AHMTC11 9-10th July 2018, RMIT University, Melbourne, Australia

SINGLE VS MULTI-STAGE VACUUM MEMBRANE DISTILLATION: AN ENERGETIC ANALYSIS

Amr Omar1, Amir Nashed1, Robert Taylor1 1School of Mechanical and Manufacturing Engineering, The University of New South Wales, Sydney, Australia

ABSTRACT deficit by 2030 [3]. Aside from water conservation, one of the Energy and clean water demand is on rise due to best ways to meet this demand is to further develop and population and economic growth. Since solar energy deploy desalination systems in water stressed areas. At resources are ubiquitous around the world, hybrid energy- present, more than 14,000 desalination plants have been water technologies driven by low-grade thermal energy installed around the globe, with a cumulative capacity to represent a potential way to meet this demand while produce over 60.5 billion liters of water daily [4, 5]. minimizing environmental impacts. Membrane distillation Membrane distillation (MD) is a promising technology (MD) technology is particularly well-suited to remote/off-grid that can be used to desalinate seawater or brackish applications due to its modularity and ability to treat high groundwater in remote areas where low grade heat source is salinity water with low fouling potential. Compared to other available. Vacuum membrane distillation (VMD) in particular, MD configurations, when high permeate flux and gain output is a very promising process compared to other MD ratio are required, a Vacuum Membrane Distillation (VMD) configurations due to its high flux and high Gain Output Ratio can be employed. However, unlike multi-effect distillation and (GOR), i.e. GOR is defined as how much permeate is produced multi-stage flash, VMD systems have not yet been fully per one kg of steam, with minimum heat losses [6, 7]. optimized for specific heat consumption (SHC) or water However, one of VMD biggest limitation is the extremely high recovery rate (RR). In this study, a techno-economic specific heat consumption (SHC) due to difficulties in fully assessment, using the Chemical Engineering Plant Cost Index recovering the heat from the brine or the permeate. These (CEPCI) method, of single versus multi-stage VMD deficiencies have limited the commercialization of VMD and configurations with respect to these aspects was carried out. requires some further development. One way to solve the Single stage VMD was considered as the baseline VMD’s shortcomings is – potentially – by using a multi-stage configuration. The effect of brine recirculation on the SHC is scheme. examined for the proposed multi-stage configurations. It was found that for a VMD process, brine recirculation is the most Only a limited amount of work has gone into optimizing effective method to reuse the brine heat, which resulted in VMD modules, so there is some room for technical reducing the SHC by 60%. Moreover, single stage VMD with improvement. One way to address the high SHC issue with brine recirculation was found to be superior than multi-stage VMD was by using a multi-stage scheme, such as the Memsys configurations achieving the lowest SHC and LCOW. As an module [8], to efficiently re-use the latent heat of the understanding of the VMD technology, this work represents produced vapor. The Memsys multi-effect VMD (ME-VMD) the first step in critically evaluating the performance of configuration has been reported to provide a significant alternative VMD multi-stage configurations to aid in module improvement in the GOR and the SHC over a standard VMD design, scale-up and process optimization. design. Zhao et al. confirmed this with an experimental study of a four-stage Memsys module [9]. The four-stage module 1 INTRODUCTION Both water and energy are necessary to sustain and grow achieved a GOR of 2.79 and a permeate flux of 3 �/� ℎ. In a any civilization. According to the United Nations Environment later study, Lee and Kim performed a numerical simulation on �/��� Programme, about one third of the world’s population still a 20-stage ME-VMD system. They reported 3.79 �/�ℎ suffers from shortages of energy and fresh water supply [1]. water production (permeate flux of 25.76 ) and 1.16 $/� Due to development and population growth, the global water water production cost [10]. It was also estimated that demand is projected to increase by 55% in 2050 [2], and it is the water production cost can also be slashed in half (to 0.52 $/� expected that the world will face a 40% global freshwater ) if a waste heat source is used. This value indicates that

1 AHMTC11 20 stages VMD system is cost competitive with reverse brine recirculation (Figure 1a), inter-stage heating multi-stage osmosis, which has a standard water production cost of 0.63 VMD without brine recirculation (Figure 1b), first-stage $/�. However, such configuration may be more difficult than heating multi-stage VMD with brine recirculation (Figure 1c) other MD designs such as hollow fiber or spiral wound, which and inter-stage heating multi-stage VMD with brine have superior packing densities. recirculation (Figure 1d). Researchers have also looked into different ways to 2 METHODOLOGY efficiently use the waste heat in the brine. The thermal energy 2.1 PROCESS CONFIGURATION of the brine can be recovered in two ways: concentration Figure 1 shows the process diagrams of the four cascades or batch recirculation. Concentration cascade works configurations analyzed in this study. In all the proposed when MD modules are linked in series and when the brine configurations, the latent heat of condensation is partially from one stage becomes the feed for the next. A recirculation recovered by using the feed water as the coolant. To ensure a heat recovery method loops the brine from the last stage back fair comparison between the configurations, the feed flowrate to heat the feed input at the first stage [11]. at the first stage was controlled such that the flux entering the As far as the author’s knowledge, only one study by Zhang last stage is kept constant at 200 ��/� ℎ. In the first-stage et al. [12] looked into recovering the energy of the brine using heating scheme, the feed water is heated to 85℃, then the the concentration cascade method. Their study compared a brine from one stage feeds to the adjacent module. Whereas, first-stage heating multi-stage VMD with an inter-stage in the inter-stage heating scheme, the feed temperature for heating multi-stage VMD, i.e. reheating the brine before every module is kept constant at 85℃. For brine recirculation entering the successive MD module stages. Their results show configurations, a portion of the last-stage brine is recirculated that even though inter-stage heating resulted in a relatively back and mixed with the feed water before entering the first- high RR, it had a lower GOR, which adversely impacted the stage heater. operating costs. Furthermore, their design did not fully The VMD system is designed to operate at a constant feed recover the brine’s energy and was optimized for the RR but salinity of 90 g/kg to avoid salt crystallization and the not necessarily for the SHC. associated fouling. Such salinity value can damage the internal This study extends on Zhang et al. work by further pore structure of the membrane [13]. In addition, high feed exploring the concept of brine recirculation and its effect on salinity can reduce the water recovery rate by 10% and reduce the SHC. In addition, the study will explore and compare the the permeate flux by more than 50% in the long run due to performance of a single stage module with first-stage and fouling, which reduces the hydrophobicity of the membranes inter-stage heating multi-stage VMD. Four configurations will [13]. The feed salinity was controlled by continuously be assessed and critically compared in terms of SHC, RR and discharging a portion of the brine and mixing it with the feed LCOW (Figure 1): first-stage heating multi-stage VMD without water. (a) (b)

(c) (d)

Figure 1: (a) First-stage heating multi-stage VMD (b) inter-stage heating multi-stage VMD (c) first-stage heating multi-stage VMD with brine recirculation (d) inter-stage heating multi-stage VMD with brine recirculation

2 AHMTC11 2.2 THERMODYNAMIC MODEL � = 232.9275 + 1.48892� A detailed thermodynamic model was developed to + 207.7633��� + 535.0972� analyze the multi-stage VMD. Fresh water properties were + 0.160057� ��� modelled using a Matlab function XSteam created by Magnus − 5.36505� � (5) Holmgren [14]. Salt water properties were based on − 1109.82� ��� polynomial coefficients given by El-Dessouky and Ettouney − 0.00322� − 504.672��� [15]. Table 1 shows the assumptions and design parameters − 562.0407567� − 273.15 used. Where � is the permeate flux in ��/� ℎ, � is the feed Table 1: Model inputs, assumptions and design parameters temperature in ℃, ��� is the feed velocity in �/�, � is the Parameter Symbol Value feed salinity in kg/kg and � is the brine temperature in ℃. It should be noted that these correlations are valid at a feed Initial feed water temperature � 25 ℃ temperature higher than 50 ℃. The pinch temperature Initial feed water salinity � 30 g/kg method was used to model the condenser. Area of a single module � 10 � The overall thermal performance of the VMD Feed flux at the last stage � 200 ��/� ℎ configuration is estimated by calculating the specific heat Heater exit temperature � 85 ℃ consumption and water recovery rate as follows: Vacuum Absolute Pressure � 10 kPa �̇ = �̇ � � − � Water heater’s efficiency � 0.95 Condenser’s pinch temperature Δ 10 ℃ + �̇ � � (6) Maximum feed salinity � 90 g/kg − � The mathematical model is based on mass, salinity and energy balance equations applied on each component as �̇ ��� = (7) follows: ���� ���� �̇ = �̇ (1) �� = × 100 (8) �̇ �̇ � �̇ � = �̇ � (2) where is the total heat input in kW, is the specific heat at constant pressure in kJ/kgK, ��� is the specific heat �̇ ℎ + �̇ = �̇ ℎ + �̇ (3) consumption in ��ℎ/� , ���� is the total water production rate in �/ℎ. Water recovery rate �� is computed by dividing where �̇ is the mass flowrate in kg/s, � is the salinity in g/kg, the total distillate produced over the feed water flowrate at ℎ is the specific enthalpy in kJ/kg, �̇ is the heat input in kW the first module stage. and �̇ is the useful power output in kW. 2.3 ECONOMIC MODEL VMD mathematical formulation was based on a CFD model for a hollow fiber module developed by our team with Since the most efficient desalination plant is not likely to the collaboration with the chemical engineering department be the most cost-effective, an economic analysis should be at the University of New South Wales. A similar CFD model considered. The cost model used was derived from different was developed by Lian and Nashed for a direct contact references and publication dates. For this reason, a chemical membrane distillation, which uses a surface response model engineering plant cost index (CEPCI) factor is used to update to develop a quadratic correlation for the permeate flux and the economic assessment of the plant. The CEPCI factor brine temperature [16]. Equation (4) and equation (5) accounts for the inflation rate, technology development and presents the permeate flux and brine temperature correlation other factors that affect the cost of the plant [17]. Each for VMD hollow fiber at a vacuum absolute pressure of 10 kPa. component investment cost is multiplied by a correction factor that is represented as: � = 4.291653 − 0.27917� − 9.49664��� ����� + 3.280176� � = (9) ����� + 0.553509� ��� − 0.20996� � (4) where � is the cost correction factor. The installation cost − 2.31714� ��� of the desalination plant is assumed to be 20% of the total + 0.003912� − 83.7156��� equipment cost [18]. The operating cost accounts for the cost of fuel burned, chemical treatment and brine discharge [19]. + 59.99451346� The maintenance cost was assumed to be 2% of the equipment cost [20] plus 10% of the total VMD investment cost [19]. The economic model design parameters are

3 AHMTC11 tabulated in Table 2. Note that all prices are given in US dollars. Table 2: Economic model design parameters [19, 21] Parameter Symbol Value Interest rate � 10% Inflation rate ��� 8% Operating years �� 20 Membrane cost ���� 90 $/�

Fuel cost ���� 2.53 $/�� Brine disposal cost ���� 0.0015 $/� Chemical treatment cost ���� 0.018 $/� The capital investment cost for the condenser and water heaters were based on previous works [21, 22]. After obtaining the capital investment cost for all components, the LCOW can be estimated using Koner et al. model [23]: � − ��� � = (10) 1 + ��� 1 1 ��� = 1 − (11) � (1 + ��)

� + � + ���� + � ���� = (12) ��� × ����(24 × 365) where � is the real interest rate, ��� is the present worth factor, � is the capital investment cost in USD and subscripts ���, ���, ��� and ��� presents the equipment, installation, operating and maintenance, respectively. Finally, ���� is the levelized cost of water in $/�.

2.4 MODEL ALGORITHIM Figure 2: Flowchart algorithm for the Matlab code The mathematical formulation mentioned above is an implicit problem. Thus, for a given stage, all the downstream 3 RESULTS AND DISCUSSION properties (permeate flux, brine temperature and brine 3.1 SINGLE STAGE VMD salinity) can only be calculated if all the feed properties (feed The RR, SHC and LCOW of a single stage VMD without mass flowrate, feed temperature and feed salinity) are known. brine recirculation was found to be 4.6%, 1373 ��ℎ/� and Initially, an initial guess for the feed properties is used and an 14.3 $/�, respectively. With brine recirculation, a significant iterative procedure was conducted with Matlab using the reduction of 60% and 52% in the SHC and LCOW, respectively, algorithm illustrated in Figure 2. There are two conditions that was observed, i.e. 540 ��ℎ/� and 6.8 $/�. This because should be satisfied to run the model: a minimum feed the inlet temperature at the heater is higher when brine is temperature to the modules of 50℃ and a maximum feed recirculated. Thus, less heat is required in the heater, which salinity of 90 g/kg. decreases the SHC and LCOW. However, the RR was slightly It is favorable to recirculate most of the brine and mix it lower, i.e. 4.4%, because the feed salinity increased when with the feed water as a means of recycling the energy from brine is recirculated. As a result, the permeate flux decreases the brine. However, the salinity at the first stage will increase slightly. when recirculating the brine. To keep the salinity at 90 g/kg, 3.2 MULTI-STAGE VMD an optimization method is used to estimate the ratio between Figure 3 shows how the RR, SHC and LCOW behave when the recirculated brine and compensated feed water. Initially, increasing the number of stages in the first-stage heating the recirculated brine flowrate is assumed before mixing with multi-stage VMD without brine recirculation. The maximum the cooling feed water. Then, the permeate flux, brine number of modules for this configuration are three modules, temperature and brine salinity are calculated at every stage. If since the brine temperature at the last stage is 42℃, which the feed at the last stage is lower than the 90 g/kg salinity does not satisfy the 50℃ minimum feed temperature criteria, more brine is recirculated and a new iteration starts. condition. As expected, RR increases and SHC decreases as the This loop is terminated when the feed salinity at the last stage number of modules increases, since a greater membrane reaches 90 g/kg. This will ensure that most of the brine energy length is used, resulting in more water production from the is recovered.

4 AHMTC11 same feed flowrate and heat input. The LCOW decreases recirculation, it was found that the RR increases in a when two stages are used due to the increase in water logarithmic rate when increasing the number of stages (Figure production. However, due to temperature pinching, i.e. the 5). This is due to the increase feed salinity at each stage, which loss of the driving force as the feed water temperature keeps slightly lowers the permeate flux. Furthermore, the SHC decreasing from one stage to the other, the increase in water decreases significantly before flattening at the 11th stage, after production by adding additional stages becomes less which it increases linearly. When a two-stage system is used, significant resulting in an increase in the LCOW. the water production and the total heat input increases by 100% and 37%, respectively. Whereas adding a third-stage, 14.5 7 1400 the water production and the total heat input increases by ) 1350 3 50% and 27%, respectively. This substantial increase in water )

3 14 6.5 1300 production compared to the additional heat added results in

13.5 1250 a significant drop in the SHC when few modules are used, i.e. 6 using up to 11 stages. The reason behind that is that at every 1200 13 stage, the feed salinity increases and the feed velocity 1150 5.5 decreases, which reduces the permeate flux from each 12.5 1100 module [24, 25, 26]. In addition, the brine temperature exiting

Module Recovery Rate (%) 5 1050 each stage is lower compared to the previous stage, resulting

Levelized Cost of Water ($/m 12 1000 more total heat added. Therefore, the increase in water Specific Heat Consumption (kWh/m

11.5 4.5 950 production cannot offset the increase in the total heat added 1 2 3 anymore and results in a minimum point at the 11th stage. Number of Modules in Series Furthermore, as more stages are added, i.e. more than 11 Figure 3: First-stage heating multi-stage VMD without stages, the curve shifts and increases linearly. It should be brine recirculation performance noted that since more modules are used, more initial feed Figure 4 presents the performance of the first-stage water is required to satisfy the 200 ��/�ℎ feed flux at the heating multi-stage VMD with brine recirculation. Due to the last stage, which will significantly increase the heat input rate feed water temperature and brine salinity limits discussed at every heater. Hence, when using more than 11 stages, the earlier, the maximum number of modules that can be used SHC increases. However, if the feed flowrate was the same for with this configuration is six. Similar to the previous all the different stage-systems, the curve will exponentially configuration, the RR increases when more modules are used. decrease and follow a similar trend as the one shown in Figure However, the SHC increases as more modules are used. As the 5 from one-stage to eleven-stage system. Similarly, the LCOW number of modules increases, the brine temperature from shows a same behavior as the SHC. the last stage decreases. Accordingly, an increasingly less amount of heat can be recovered from the brine as the 15 70 1400 )

number of stages increases. Therefore, more heat input is 3 14 1300

) 60 required by the heater to reach the desired feed inlet 3 13 1200 temperature, i.e. 85℃, resulting in a higher SHC. As a result of 50 the additional heat input, the LCOW also increases when the 12 1100 number of stages increases. 40 11 1000 30 12 7 720 10 900

) 20 700 3 9 800 Module Recovery Rate (%) )

3 11 6.5 680 Levelized Cost of Water ($/m 8 10 700 660 Specific Heat Consumption (kWh/m 10 6 7 0 600 640 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 Number of Modules in Series 9 5.5 620 600 Figure 5: Inter-stage heating multi-stage VMD without 8 5 580 brine recirculation performance

Module Recovery Rate (%) 560 Figure 6 presents the inter-stage heating multi-stage VMD

Levelized Cost of Water ($/m 7 4.5 with brine recirculation performance. The RR trend is similar

540 Specific Heat Consumption (kWh/m

6 4 520 to the previous configuration’s RR trend, but the SHC and 1 2 3 4 5 6 LCOW increases linearly. Since the brine is recirculated in this Number of Modules in Series configuration and the feed temperature at each stage is Figure 4: First-stage heating multi-stage VMD with brine constant at 85℃, the input temperature to the first heater is recirculation performance almost constant. The feed salinity is the only input parameter For the inter-stage heating configuration, regardless with that varies across each stage. This essentially changes the or without brine recirculation, the maximum number of brine temperature at the last stage, which affects the input stages that can be used are 39 stages. Without brine temperature at the first stage heater after brine is

5 AHMTC11 recirculated. Hence, as the heat input at the first stage heater fact that more water is produced with the same heat input. changes, it results in increasing trends for both SHC and LCOW. However, the same configuration with brine recirculation has However, it has to be noted that if the same feed flowrate is a higher SHC (623 ��ℎ/�) than using a single module with used for all the different stage-systems, an opposite trend for brine recirculation (540 ��ℎ/�). This is because the brine SHC and LCOW occurs. This is because when the same feed recirculated in the multi-stage scheme has a temperature of flow rate is used for a large number of stage-system as for a 53.7℃ when compared with the single module’s brine fewer stage-system, additional heat is added at each stage temperature of 68℃, resulting in more heat being added for without any corresponding increase in the permeate flux. As the multi-stage configuration. When comparing the free a result, for fewer stage-systems, SHC and LCOW will be waste heat LCOW for the multi-stage versus single stage significantly high, simply due to the unnecessary high feed configuration, LCOW is always higher for the multi-stage flow rate. configuration. Multi-stage schemes produce much more distillate than single-stage VMD. As a result, a larger 8.2 70 720 condenser and more cooling feed water is required to )

700 3 8 condense the additional permeate vapor, which increases the

) 60 3 680 cost significantly. 7.8 50 660 7.6 An inter-stage heating multi-stage VMD without brine 640 40 recirculation has a lower SHC (792 ��ℎ/�) than a single 7.4 620 30 stage module (1373 ��ℎ/� ). In the inter-stage heating 600 7.2 configuration, water production is multiplied by the number 20 580 7 of modules used, while the heat added to the system is linear.

Module Recovery Rate (%) 560 Levelized Cost of Water ($/m The brine that exits from each stage has a high temperature, 6.8 10 540 Specific Heat Consumption (kWh/m which allows the next stage to reach its desired module inlet 6.6 0 520 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 temperature with less heat input. For example, a single stage Number of Modules in Series module consumes 126 kW of heat, whereas two and three Figure 6: Inter-stage heating multi-stage VMD with brine stages consume 172 kW and 218 kW of heat, respectively. recirculation performance Furthermore, the same configuration with brine recirculation 3.3 COMPARATIVE ANALYSIS has a similar SHC (549 ��ℎ/�) as the single stage with brine To have a fair comparison among the different recirculation (540 ��ℎ/�). The reason behind that is due to configurations discussed in the previous section, a fixed change in the feed salinity in case of the multi-stage number of stages is used in this analysis. Figure 7 presents the configuration. The increase in the feed salinity slightly reduces comparative results of using three stages for the different the permeate flux from each module. Since the heat input is multi-stage configurations relative to a single stage VMD with a constant value multiplied by the number of heaters used, and without brine recirculation. Furthermore, to consider the inter-stage heating will have a slightly higher SHC than the impact of the input energy, the LCOW was calculated for two single stage module. Furthermore, the free waste heat LCOW different cases: fuel or free waste heat energy inputs. is similar for inter-stage heating and single stage VMD. Although the cost increases when using a multi-stage scheme, Figure 7 shows that all configurations with brine water production also increases, which compensates for the recirculation have a lower SHC than those without brine increase in cost. Nevertheless, although the LCOW is low for recirculation. For instance, brine recirculation has lowered the the inter-stage heating configuration, such system is complex SHC of a single stage module by 60%. This is due to the high due to the presence of heaters at every stage. Zhang et al. heat recovery from the brine that reduces the heat input given have mentioned that such complexity, makes such to the system. The LCOW with fuel burned is lower for brine configurations less commercially desirable even if it shows recirculation configurations because less fuel is burned in the better performance when compared with single stage heaters, e.g. brine recirculation decreased the LCOW for a configurations [12]. single stage module by 52%. Conversely, when free waste heat is used, the LCOW stays almost constant regardless of whether When comparing first-stage heating with inter-stage the brine undergoes recirculation or not. This is because the heating configuration, inter-stage heating shows a significant heaters operating cost has the highest weight among other improvement in SHC and LCOW. This outcome agrees with cost components. Hence, eliminating the heater’s operating Zhang et al. [12] study. The main reason for that is the ability cost, significantly drops the LCOW and gives a clear to operate the modules at their maximum efficiency. For representation of the system’s LCOW. Moreover, without instance, the water production across each module for first- brine recirculation, a first-stage heating multi-stage VMD has stage heating with brine recirculation are 88 �/ℎ, 35 �/ℎ and a lower SHC (961 ��ℎ/�) than the single module 16 �/ℎ, whereas the water production for inter-stage heating configuration (1373 ��ℎ/�). This can be explained by the are around 103 �/ℎ across each module.

6 AHMTC11

Figure 7: Comparative analysis among the different configurations

4 CONCLUSION � Specific heat at constant pressure (��/���) This study applies a thermo-economic model on two ���� Water Produced (�/ℎ) different VMD cascade configurations, first-stage heating � CEPCI correction factor multi-stage VMD and inter-stage heating multi-stage VMD as ℎ Specific enthalpy (��/��) compared to a single stage module. Brine recirculation was � Interest rate (%) also employed on the aforementioned configurations and ��� Inflation rate (%) compared with a single module VMD. This study has � Flux (��/�ℎ) presented the following outcomes: ���� Levelized cost of water ($/�) Ø The results show that recovering the heat from brine �̇ Mass flowrate (��/�) recirculation, reduces the SHC and LCOW. Brine � Pressure (���) recirculation method have reduced the SHC and LCOW for ��� Present worth factor ̇ a single stage module by 60% and 52%, respectively. � Heat Rate (��) Ø With brine recirculation, it was found that a multi-stage �� Recovery rate (%) cascade system provides little added value over the � Salinity (�/��) performance of single-stage VMD module. In case of no ��� Specific heat consumption (��ℎ/� ) brine recirculation, there is a significant improvement in � Temperature (℃) SHC and LCOW when using a multi-stage system than a ��� Velocity (�/�) ̇ single stage VMD. � Power (��) Ø In case of multi-stage scheme, it is suggested to use a few �� Operating years stage-system with an optimal low initial feed flowrate � Investment cost ($) bounded by the maximum recovery rate. Ø Single stage VMD with brine recirculation shows a similar Subscripts performance when compared with the best multi-stage ��� Equipment configuration (inter-stage heating) without the additional � Saturated liquid complexity of a multi-stage scheme. ��� Installation In arid developing areas, such as the rural areas of Australia, ��� Maintenance Middle East and North Africa, fresh water production requires ��� Membrane high energy consumption. At 1.9 $/�, the LCOW realized ��� Operating from a small-scale, single stage VMD module would be ���� Permeate practical use where the cost of delivered water can be ����� Chemical treatment significantly high. � Vacuum

NOMENCLATURE Abbreviations CEPCI Chemical engineering plant cost index Symbols GOR Gain output ratio � Area (� ) LCOW Levelized cost of water

7 AHMTC11 MD Membrane distillation [13] J. Bush, J. Vanneste and T. Cath, "Membrane distillation RR Recovery rate for concentration of hypersaline brines from the Great SHC Specific heat consumption Salt Lake: Effects of scaling and fouling on performance, VMD Vacuum membrane distillation efficiency and salt rejection," Separation and Purification Technology, vol. 170, pp. 78-91, 2016. Greek Symbols [14] M. Holmgren, XSteam for Matlab, www.x-eng.com, � Efficiency 2007. Δ Pinch temperature [15] H. El-Dessouky and H. Ettouney, Fundamentals of Salt

Water Desalination, Kuwait: ELSEVIER, 2002. ACKNOWLEDGEMENTS [16] B. Lian, A. Nashed, A. Sproul, V. Chen, Y. Wang and G. The authors would like to acknowledge the financial support Leslie, "Optimisation of solar powered membrane from the Australian Research Council (ARC LP160100622). distillation system using CFD and TRNSYS coupled REFERENCES model," in APC Che 2015 Congress Incorporating Chemeca, Melbourne, 2015. [1] United Nations Environmental Programme, "Water [17] D. Mignard, "Correlating the chemical engineering plant Scarcity," 2014. cost index with macro-economic indicators," Chemical Engineering Research and Design, vol. 92, pp. 285-294, [2] Organisation for Economic Co-operation and 2014. Development (OECD), "Environmental Outlook 2050," 2012. [18] M. Peters, K. Timmerhaus and R. West, Plant design and economics for chemical engineers, New York: McGraw- [3] 2030 Water Resources Group, "Charting our Water Hill, 1968. Future: Economic Frameworks to inform Decision- Making," 2009. [19] K. Park, J. Kim, H. Kim, K. Lee, D. Yang and K. Kim, "Economic analysis of geothermal energy and VMD [4] D. Kibona, G. Kidulile and F. Rwabukambara, desalination hybrid process," Clean Technology, vol. 20, "Environment, Climate Warming and Water no. 1, pp. 13-21, 2014. Management," 2009. [20] J. Spelling, Hybrid solar gas-turbine power plants: A [5] E. Curry, "Water Scarcity and the Recognition of the thermodynamic analysis, 2013. Human Right to Safe Freshwater," Journal of International Human Right, vol. 9, pp. 103-121, 2010. [21] M. Saghafifar, A. Omar, S. Erfanmoghaddam and M. Gadalla, "Thermo-economic analysis of recuperated [6] M. El-Bourawi, Z. Ding, R. Ma and M. Khayet, "A Maisotsenko bottoming cycle using air saturator: Framework for better understanding membrane Comparative analysis," Applied themral engineering, distillation separation process," Journal of membrane vol. 111, pp. 431-444, 2017. science, vol. 285, no. 1-2, pp. 4-29, 2006. [22] A. Omar, M. Saghafifar and M. Gadalla, "Thermo- [7] A. Alkhudhiri, N. Darwish and N. Hilal, "Membrane economic analysis of air saturator integration in distillation: a comprehensive review," Desalination, vol. conventional combined power cycles," Applied thermal 287, pp. 2-18, 2012. engineering, vol. 107, pp. 1107-1122, 2016. [8] W. Heinzl and J. Scharfe, "Membrane Distillation [23] P. Koner, V. Dutta and K. Chopra, "Economic viability of Device". Patent WO 2010127819, 2010. stand-alone solar photovoltaic system in comparison [9] K. Zhao, W. Heinzl, M. Wenzel, S. Buttner, F. Bollen, G. with diesel-powered system for India," Energy Lange, S. Heinzl and N. Sarda, "Experimental study of Economics, vol. 24, no. 2, pp. 155-165, 2002. the memsys vacuum-multi-effect-membrane- [24] J. Lee and W. Kim, "Numerical modeling of the vacuum distillation (V-MEMD) module," Desalination, vol. 323, membrane distillation process," Desalination, vol. 331, pp. 150-160, 2013. pp. 46-55, 2013. [10] J. Lee and W. Kim, "Numerical study on multi-stage [25] Y. Zhang, Y. Peng, S. Ji and S. Wang, "Numerical vacuum membrane distillation with economic simulation of 3D hollow-fiber vacuum membrane evaluation," Desalination, vol. 339, pp. 54-67, 2014. distillation by computational fluid dynamics," Chemical [11] F. He, J. Gilron and K. Sirkar, "High water recovery in Engineering Science, vol. 152, pp. 172-185, 2016. direct contact membrane distillation using series of [26] M. Abu-Zeid, Y. Zhang, H. Dong, L. Zhang, H. Chen and L. cascades," Desalination, vol. 323, pp. 48-54, 2013. Hou, "A comprehensive review of vacuum membrane [12] Y. Zhang, Y. Peng, S. Ji, J. Qi and S. Wang, "Numerical distillation technique," Desalination, vol. 356, pp. 1-14, modeling and economic evaluation of two multi-effect 2015. vacuum membrane distillation (ME-VMD) processes," Desalination, vol. 419, pp. 39-48, 2017.

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