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Experimental study of the memsys vacuum-multi-effect-membrane-distillation (V-MEMD) module
Desalination 323 (2013) 150–160
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Desalination journal homepage: www.elsevier.com/locate/desal
Experimental study of the memsys vacuum-multi-effect-membrane-distillation (V-MEMD) module K. Zhao a, 1, W. Heinzl a,⁎, M. Wenzel b, 2, S. Büttner a, 1, F. Bollen a, 1, G. Lange a, 1, S. Heinzl a, 1, N. Sarda a, 1 a b
memsys clearwater Pte. Ltd., 82 Toh Guan Road, Water Hub, Singapore Technische Universität München, Lehrstuhl für Thermodynamik, TU München, D-85747 Garching, Germany
H I G H L I G H T S ► Memsys has successfully commercialized the V-MEMD module. ► The solar driven memsys system illustrates good operating performance. ► Memsys module has great potential in increasing the Gain Output Ratio (GOR).
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Article history: Received 8 April 2012 Received in revised form 8 August 2012 Accepted 3 December 2012 Available online 11 January 2013 Keywords: Membrane distillation V-MEMD Memsys Solar driven MD Hydrophobic membrane GOR
a b s t r a c t The memsys has successfully commercialized the vacuum-multi-effect-membrane-distillation (V-MEMD) module. This compact memsys module employs hydrophobic membranes as a separating medium and makes use of vacuum to enhance membrane distillation process. This novel V-MEMD module enables highly efficient heat recovery. Compared with conventional thermal desalination processes, the memsys module shows advantages in lower investment and operational cost, and higher energy efficiency. In this paper, solar and diesel heater were used as heating sources to drive the memsys V-MEMD module. The solar driven memsys system illustrates good operating performance with a flux at approximately 7 LMH on a sunny day using seawater as feed. The diesel heater driven memsys system was used to investigate the effects of heating, cooling and feed conditions on the module performance using tap water as feed at a relatively low operating temperature (45~60 °C). The results show that heating and cooling temperature are the main factors affecting flux and energy efficiency. The optimization of number of module stage and size of each stage were also studied. The experimental results show that the memsys module has great potential in increasing the Gain Output Ratio (GOR), which is one of the most important criteria for industrialization of MD technology. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Membrane distillation (MD) is a thermally driven process which allows water vapor transport through hydrophobic porous membranes [1,2]. MD produces high quality distillate from wastewater or seawater under non-pressurized condition and at moderate temperatures (50– 80 °C) [3,4]. Vacuum membrane distillation (VMD) is an approach to reduce heat losses and achieve higher vapor flow rates as compared with direct contact MD [5–7]. In VMD, vacuum is applied at the downstream side to maintain the pressure below the equilibrium vapor pressure, enhancing mass transfer [8]. The memsys vacuum-multi-effectmembrane-distillate (V-MEMD) technology combines the advantages of multi-effects and vacuum to achieve highly efficient heat recovery [9]. Compared with the conventional MD processes, memsys has ⁎ Corresponding author. Tel.: +65 91085258, fax: +65 66864267. E-mail address: [email protected] (W. Heinzl). 1 Tel.: +65 91085258; fax: +65 66864267. 2 Tel.: +49 8928916217; fax: +49 8928916218. 0011-9164/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.desal.2012.12.003
successfully developed a commercial membrane distillation product, named as the memsys module. The memsys module shows enormous potential in increasing the Gain Output Ratio (GOR) and reducing the energy requirement in the desalination process. In this paper, the memsys systems with 4-stage module were installed, and the systems were driven by solar thermal [10] and diesel heaters as heating sources respectively. The solar driven memsys system was used in seawater desalination process in the field to evaluate the performance and compatibility of the module. The diesel heater driven memsys system was used to investigate the effects of heating, cooling and feed conditions on the module performance using tap water as feed at a relatively low operating temperature (45– 60 °C). The module design including the number and size of the membrane stage for module optimization was also discussed. 2. Theory One set of standard memsys V-MEMD module consists of a steam raiser, multiple stages, and a condenser. The membrane frames and
K. Zhao et al. / Desalination 323 (2013) 150–160 Table 1 Terminology of memsys module. Terminology
Descriptions
Membrane frame
Two pieces of membranes (one piece size: 600 × 360 mm) are welded onto two sides of one PP (polypropylene) frame in a single production step, as shown in the Fig. 1(a) PP films (~20 μm thickness, same size as membrane) are welded onto two sides of one PP frame in a single production step, as shown in the Fig. 1(a) The membrane and foil frames are alternatively friction-welded together to form stage (number of foil frames = number of membrane frames + 1). When welding the frames together, the necessary channels are formed and supported by a PP-spacer, as shown in Fig. 1(b), two PP cover plates were used to alter the direction of feed and vapor flows Stages were stacked to form multiple stages with vacuum tight silicon gaskets, as shown in the Fig. 1(c) The steam raiser is fabricated by a certain number of membrane frames and installed in front of the first stage. The heat produced by the external heat source (e.g. solar thermal) is exchanged in the steam-raiser. The water in the steam raiser was evaporated at lower pressure (e.g. 600 mbar) and the produced hot vapor flows to the first stage. The condenser is fabricated by certain number of foil frames and installed after the final stage. The vapor produced in the final stage is condensed in the condenser, using the coolant (e.g. fresh water) flow. One set of standard memsys module consists of a steam raiser, multiple stages, and a condenser
Foil frame
Single stage
Multiple stages Steam raiser
Condenser
Memsys module
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Vapor enters the stage and flows into the parallel foil frames. The vapor in the foil frames heats up the feed and is then condensed. The small channel at the bottom of frame facilities the removal of non-condensable gases and application of vacuum. The condensed vapor then flows into a distillate channel. The heat energy of condensation is transported through the foil and is immediately converted into evaporation energy, generating new vapor in the vapor channel. The vapor leaves the vapor channels and enters the next stage. This process is repeated in the remaining stages. 2.2. Basic working principle of the memsys module In a working memsys module, each stage recovers the heat from the previous stage. Distillate is produced in each stage and in the condenser. As shown in Fig. 3, heat energy produced by the external heating source (e.g. solar thermal or other waste heat) is exchanged in the steam raiser. The water in the steam-raiser is under vacuum pressure (e.g. 600 mbar). The hot vapor generated by steam raiser flows to stage 1, meanwhile feed is also introduced to stage 1 and flows serially through the remaining stages. At the end of the final stage, feed is concentrated as brine. Vacuum is always applied to the permeate side of the membranes. The vapor pressure and temperature gradually decreases from the steam raiser to the condenser. The vapor produced in the final stage is condensed in the condenser, using the coolant (e.g. fresh water) flow. 3. Experimental
foil frames are friction-welded together to form a single stage. A number of single stages are stacked up and vacuum sealed using silicon gaskets to form multiple stages. The terminology of memsys module parts is shown in Table 1, whereas Fig. 1 shows the photos of a frame and stages.
2.1. Flow diagram in the single membrane stage Fig. 2 shows the structure of a single stage, with alternating membrane frames and foil frames for evaporation and condensation. The spaces inside each frame and between neighboring frames, act as channels. Foil frames function as a condenser and provide a means as ‘distillate channels’, while membrane frames serve as ‘vapor channels’. ‘Feed channels’ are created between foil frames and membrane frames.
(a)
(b)
3.1. Preparation of memsys frames and modules As the memsys process works at modest temperatures (b90 °C) and moderate negative pressure, all module components are made of polypropylene (PP). This minimizes corrosion and scaling and allows large-scale cost efficient production. The memsys has developed a highly automated production line for the modules. The hydrophobic membrane used in the module is made of PTFE with a pore size of ~ 0.2 μm. The effective dimension of one piece of membrane is about 335 mm × 475 mm. The condensation foil is made of PP with a thickness of ~ 20 μm. The feed channel has a width of 1.0 to 1.5 mm which can be adjusted during the welding process. There is polypropylene mesh spacer in the feed channel between each membrane and foil. Upon being welded on both sides of the frame, the effective surface area of one membrane frame is
(c)
Fig. 1. Photos of memsys frame and stages. (a) Frame, 2 pieces of membranes or PP foils were weld onto both sides of the frame; (b) single stage with welded frames and two covering plates; (c) multiple stages.
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Fig. 2. Feed, brine and vapor flows in the single stage.
Fig. 3. Basic principle of memsys process.
Fig. 4. Diagram of standard memsys testing system.
K. Zhao et al. / Desalination 323 (2013) 150–160 Table 2 Equipment list of a typical memsys testing system. Symbol Name B1 B2 B3 B4 B6 P1 P1_D
P3 P4 P5 P6 MV1 MV2 SR S1–S4 CR
Function
Heating tank
Water was pre-filled in B1 to receive the energy from external heat source Feed tank Contains the feed Brine tank Contains the brine from the final stage Distillate tank Contains the distillate from stages Cooling tank Water was pre-filled in B6 as coolant in the condenser Heating pump The heated water in B1 was recirculated through the steam raiser by P1 Refilling pump Transfers the distillate back to B1, this distillate is produced by the water in B1 and not the part of product distillate Brine pump Transfers the brine from brine tank B3 Distillate pump Transfers the distillate from distillate tank B4 Vacuum pump Provides vacuum to the whole system Cooling pump The water in B6 was recirculated through the condenser by P6 Heating control Solenoid valve MV1 is used to top up the B1 when valve the water level in the B1 is low Feed control Solenoid valve MV2 is used to top up the B2 when valve the feed level in the B2 is low Steam raiser Refer to Table 1 Stage 1–stage 4 Refer to Table 1 Condenser Refer to Table 1
about 0.31 m 2, the membrane area is around 1.25 m 2 for 11-frame stage, and 2.5 m 2 for a 17-frame stage. Modules can be of varying capacity, depending on the number of stages and the number of frames per stage. All the stages, steam raiser and condenser are sealed with vacuum silicon gaskets when vacuum is applied. 3.2. Structure of memsys testing system The four-stage module were used for the testing, each stage includes 11 frames (1.25 m 2 membrane area) or 17 frames (2.5 m 2 membranes area). The schematic of the whole system is shown in Fig. 4, the descriptions of main components and testing parameters are shown in Tables 2 and 3. As shown in Fig. 4, the solar system or diesel heater was used to provide the heat source to drive the memsys testing system. Heat was transferred to the system by heat exchanger 1. The pre-filled
Table 3 Main operating parameters and descriptions. Parameters Unit P1_1, T1_1 P1_2, T1_2 P2_1, T2_1 P3_1, T3_1 P4_1, T4_1 P5_1 P6_1, T6_1 P6_2, T6_2 P7_1–P7_5 T7_1–T7_5 F1 F2 F3 F4 F6
Descriptions
mbar, °C Inlet temperature and pressure of the water flowing into the steam raiser mbar, °C Outlet temperature and pressure of the water flowing out the steam raiser mbar, °C Feed temperature and pressure before flowing into stage 1 mbar, °C Brine temperature and pressure after flowing out the final stage mbar, °C Distillate temperature and pressure mbar Pressure produced by vacuum pump P5 mbar, °C Inlet temperature and pressure of the water flowing into the condenser mbar, °C Outlet temperature and pressure of the water flowing out the condenser mbar Vapor pressure from the steam raiser and each stage °C Vapor temperature from the steam raiser and each stage l/h Water flow rate flowing into the steam raiser l/h Feed flow rate l/h Brine flow rate (by manual measurement) l/h Distillate flow rate (by manual measurement) l/h Coolant (fresh water) flow rate flowing into the condenser
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Table 4 Formulas of main performance indicators. Parameters
Calculation formulas
Distillate GOR⁎
¼ F4 S
Product ratio
¼ F4 F2
Cooling energy ratio
½ðT6 ¼ F6⋅ F1⋅½ðT1
60⋅F4⋅L ¼ C⋅F1⋅½ðT1 1Þ−ðT1
2Þ
2Þ−ðT6 1Þ 1Þ−ðT1 2Þ
F3⋅½ðT3 1Þ−ðT2 1ÞþF4⋅½ðT4 1Þ−ðT2 1Þ F1⋅½ðT1 1Þ−ðT1 2Þ
Feed energy ratio
¼
Lost energy ratio
¼ 1− F6⋅½ðT6
2Þ−ðT6 1ÞþF3⋅½ðT3 1Þ−ðT2 1ÞþF4⋅½ðT4 1Þ−ðT2 1Þ F1⋅½ðT1 1Þ−ðT1 2Þ
Note: * S=membrane area (m2), C=water specific heat capacity (4.19 kJ/kg), L=latent heat (kWh/l, depending on the boiling temperature of feed under different vacuum pressure). * All the energy discussed in the paper is thermal energy, electricity consumption is not included.
fresh water in tank B6 recirculates as coolant and transfers heat to the external cooling water by heat exchanger 2. The pre-filled fresh water in tank B1 recirculates and receives heat from heat exchanger 1. The feed was automatically sucked in B2 through control by solenoid valve MV2, the brine was collected in the tank B3 and pumped out by pump P3. Vacuum was applied to the whole module and tanks. The distillate production is transported via siphons at the bottom of each stage by pressure differences between stages. The flux and product ratio is used to evaluate the performance and efficiency of the system. GOR and energy consumption of heating, cooling and feed were used to evaluate the energy efficiency. The calculation of energy and performance were shown in Table 4. 3.3. Operation of the memsys testing system 3.3.1. Solar driven memsys field testing system Seawater is used as external coolant and feed. Brine is ejected to the sea at ~ 40 °C. The membrane module consists of 4 stages and each stage has 11 frames. The total membrane area is around 5 m 2. The system collects thermal and electrical solar energy (12 solar thermal collectors with average thermal energy production of 80 kWh/day and 1.5 kW peak electricity by 6 solar cells). Seawater is drawn by a submerged pump approximately 1 m from the shoreline and 2 m below sea-level. Raw seawater is first pre-treated by an ultra-filtration system (Hollow fiber polyacrylonitrile membrane, MWCO is about 100,000 Da). The power needed for the electrical equipment (pumps, PLC) is supplied by solar cells, with consumption varying from 0.9–1.2 kW. 3.3.2. The memsys lab testing system using diesel heater To study the effects of heating, cooling and feed conditions, a diesel heater was used to provide the stable heating temperature T1_1. The relatively low T1_1 was selected to evaluate the performance and energy efficiency of the module. Tap water (25 °C, conductivity about 200 μS/cm) is used as external coolant and feed. The membrane module consists of 4 stages and each stage has 17 frames. The total membrane area is around 10 m 2. The optimization of number and size of stage were studied using a 2-stage system with 7, 9 and 17 frames for each stage. 4. Results and discussion 4.1. Typical seawater desalination operation of solar driven memsys field testing system The solar driven memsys system was operated for 5 to 8 h per day. Fig. 5 shows the heating and vapor temperature curves in each stage. Normally, the system requires about 30 min for temperature and
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Fig. 5. Temperature distribution in the steam raiser and each stage for the solar driven memsys MD system.
pressure stabilization until the last membrane stage starts to create vapor. T1_1 is the temperature of vapor created by hot water circulating in the solar collectors, the energy of this vapor is used to heat up
the seawater in stage 1 and generates new vapor at stage 2, then the similar process will happen in stages 3 and 4, so T1_1 is the leading temperature to drive the whole membrane distillation process, any fluctuation of T1_1 will strongly affect the temperature in the each stage and condenser. The level of sun radiation is the critical factor affecting the inlet heating temperature T1_1. Based on the weather conditions, a good product output can be obtained if the vapor temperature in stage 1 (T7_1) is higher than 65 °C, which implies heating temperature T1_1 must be higher than 70 °C. During the peak hours of the day, T1_1 can reach 80–85 °C. During cloudy moment, T1_1 could go below 60 °C, which will reduce the distillate output significantly. Fig. 6 shows the vapor pressure curves in the different stages, steam raiser and condenser. The vapor pressure curves show similar trend with vapor temperature, as shown in Fig. 6(a), the vapor vacuum pressure in stage 1 (P7_1) is always the highest and decreases in the following stage. The vapor pressure fluctuates with the vapor temperature. Higher vapor temperature leads to larger pressure differences between neighboring stages, thereby producing a higher distillate output. This solar driven memsys system produces about 35 l of distillate per hour under a good weather condition (~ 75 °C T1_1). Therefore, given the 5 m 2 membrane surface area, the flux is about 7 LMH. The pre-treated seawater was used as both the external coolant through heat exchanger 2 and feed. The quality of raw seawater, pre-treated seawater and distillate were analyzed and the results are shown in Table 5. Based on the analysis results, the ultra-treated seawater shows very low turbidity of 0.15 NTU and no suspended solids are detected, but the conductivity of pre-treated seawater remained at the same value as raw seawater. The conductivity of distillate from memsys module is 8–10 μS/cm.
Table 5 Quality analysis of raw seawater, ultra-filtrated seawater and distillate.
Fig. 6. Pressure distribution in the module for the solar driven memsys MD system. (a) Pressure distribution in the steam raiser and each stage, (b) pressure distribution in the heating and cooling part.
Testing items
Units
Conductivity TSS Turbidity SDI15
μS/cm mg/l NTU –
Sample reference Raw seawater
Seawater after UF
Distillate
47,000 10.8 5.5 5.73
47,300 0 0.15 3.93
8–10 – – –
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Fig. 7. Two testing ways of heating influence.
4.2. Effects of heating flow rate and temperature on the module performance and energy efficiency using memsys lab testing system Generally, the more vapor energy produced in the steam raiser, the more energy is available for the distillation process. Heating flow rate F1 and heating temperature T1_1 are the two main heating
Fig. 9. Influence of heating flow rate F1 to product flux and energy efficiency (a) effects of F1 on flux and GOR, (b) effects of F1 on energy and product ratio.
parameters. Fluctuation in F1 and T1_1 will affect the amount of vapor energy generated by the steam raiser and therefore affect the temperature/pressure distribution in each stage and final distillate flux. Fig. 7 shows the two testing methods which involve constant F1 with decreasing T1_1, and constant T1_1 with increasing F1.
Fig. 8. Temperature and pressure of vapor in the each stage using constant inlet heating temperature T1_1=60 °C and changing heating flow rate F1=8.0, 15.2 and 27.7 l/min. (a) Vapor temperature distribution in each stage, (b) vapor pressure distribution in each stage.
4.2.1. Effect of heating flow rate F1 In this test, the heating temperature T1_1 (60 °C), cooling temperature T6_1 (25 °C), feed temperature T2_1 (25 °C), cooling flow rate F6 (7.5 l/min), feed flow rate F2 (1.5 l/min), and the vacuum P5_1 (80 mbar) are maintained constant. The heating flow rate F1 is stepwise increased, and three testing points of F1 (8.0, 15, and 27.7 l/min) are chosen to run the test. As shown in Fig. 8, the pressure and temperature of the vapor generated by the first three stages (P7_2 to P7_4, T7_2 to T7_4) increases with F1, but the pressure and temperature from the last stage (T7_5 and P7_5) are comparably stable, which means the cooling temperature T6_1 and vacuum pressure P5_1 have stronger influence to the last stage than other stages. The pressure differences between stages 1–2, and stages 2–3 are about 10 mbar, and the pressure difference between stages 3 and 4 is about 15 mbar, which are the driving force to transport the vapor and produce the distillate. When the heating flow F1 is increased to 27.7 l/min, the pressure differences between stages 1–2 and 2–3 are slightly increased by 5 mbar and the difference between last two stages rises by 10–15 mbar, these increases in pressure differences provide higher driving force and
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Fig. 10. Temperature and pressure of vapor in each stage using constant heating flow F1 = 15 l/min and changing inlet heating temperature T1_1 = 60, 50 and 45 °C. (a) Vapor temperature distribution in each stage, (b) vapor pressure distribution in each stage.
produce more distillate. As shown in Fig. 9(a), the distillate flux is increased from 1.7 to 3.5 LMH, but GOR did not change significantly following F1. If F1 is increased, both the heating energy (C· F1· [(T1_1) − (T1_2)]) and distillate flow (F4) will be increased proportionally, so based on the equation of GOR (see Table 4), GOR would not be improved. This increased heating energy is mostly transferred to the cooling flow (the cooling ratio increased from 60 to 70% as shown in Fig. 9(b)), which means the entire energy efficiency is not improved by increasing F1. 4.2.2. Effect of heating temperature T1_1 In this test, the heating flow rate F1 (15 l/min), cooling temperature T6_1 (25 °C), feed temperature (25 °C), feed flow rate F2 (1.5 l/min), cooling flow rate F6 (14 l/min), and vacuum P5_1 (60 mbar) are maintained constant, the heating temperature T1_1 decreases at three temperature points, 60 °C, 50 °C and 45 °C, as shown in Fig. 7. The oscillation of the temperature curve is due to the inefficient internal temperature controlling system, but this effect does not influence the test results because of the constant average values. The vapor pressure and temperature from all four stages (P7_2 to P7_5, T7_2 to T7_5) decreases with T1_1. As shown in Fig. 10, T1_1 shows dominant influence on the vapor pressure and temperature from each stage. The pressure differences between each stage reduce by over 20 mbar when T1_1 decreases to 45 °C, which means that the driving force is very weak. As shown in Fig. 11(a), the distillate flux is reduced to below 1 LMH when T1_1 is 45 °C, meanwhile the GOR also
Fig. 11. Influence of inlet heating temperature to product flux and energy efficiency (a) effects of T1_1 on flux and GOR, (b) effects of T1_1 on energy and product ratio.
decreases, which implies that energy efficiency is lower at low heating temperature. GOR at heating temperature T1_1 of 50 °C and 60 °C is rather stable, but the flux at 50 °C is much lower than 60 °C, which means GOR would not be improved by increasing heating temperature T1_1, that is because temperature difference ((T1_1) − (T1_2)) and distillate flow (F4) are increased proportionally based on the equation of GOR in Table 4. As shown in Fig. 11(b), the
Fig. 12. Two testing ways of cooling influence.
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Fig. 14. Influence of cooling flow rate F6 to product flux and energy efficiency (a) effects of F6 on flux and GOR, (b) effects of F6 on energy and product ratio. Fig. 13. Temperature and pressure of vapor in each stage using constant inlet cooling temperature T6_1 and changing cooling flow rate F6. (a) Vapor temperature distribution in each stage, (b) vapor pressure distribution in each stage.
cooling, feed and lost energy ratio are quite similar at T1_1 50 °C and 60 °C. The flux is higher at 60 °C because that around 50% of feed is evaporated and condensed to distillate, which is higher than 30% product ratio achieved at 50 °C T1_1. Fig. 11(b) also shows that the energy lost for the low temperature heating (45 °C) is almost 30% which is far more than 10% normal energy lost ratio at higher temperature. This is probably due to the fact that the cooling energy ratio is only 50%, the energy is lost before the vapor transfers the energy to the next stage. 4.3. Effects of cooling flow rate and temperature on the module performance and energy efficiency using memsys lab testing system Based on the discussions in Section 4.2, the external coolant will take away over 70% of the energy from the heating side, hence the required amount of external coolant is high. Generally the ambient tap water or seawater is used as a coolant to reduce the operating cost, therefore there is no special temperature control on the cooling flow. In this section, the effects of cooling flow rate F6 and cooling temperature T6_1 (naturally be warmed during the operation, not by purposely heating up) will be discussed. The testing condition is shown in Fig. 12. 4.3.1. Effects of cooling flow rate F6 In this test, the heating temperature T1_1 (60 °C), heating flow rate F1 (15 l/min), feed temperature (25 °C), cooling temperature
T6_1 (25 °C), feed flow rate F2 (1.5 l/min), and the vacuum P5_1 (60 mbar) are maintained constant, the cooling flow rate F6 is increased, five testing points of F6 are chosen to run the test. As shown in Fig. 13, the vapor pressure and temperature from all stages (P7_2 to P7_5, T7_2 to T7_5) show little fluctuation, which means the cooling flow F6 has very limited influence on the vapor pressure and temperature distributions. Compared with normal cooling flow rate 7.5 l/min, the higher F6 (around 9 l/min) results in a mild increase in flux, but when the F6 is larger than 9 l/min, there is no improvement on flux and GOR, as shown in Fig. 14(a). In addition, the high cooling flow rate will also take more energy away from heating side and results in very low energy lost at around 5% as shown in Fig. 14(b).
4.3.2. Effects of cooling temperature T6_1 In this test, the heating flow rate F1 (15 l/min), heating temperature T1_1 (60 °C), cooling flow rate F6 (9 l/min), feed temperature (25 °C), feed flow rate F2 (1.5 l/min) are maintained constant, the cooling flow is recirculated and continuously heated up by energy from the vapor in the condenser. The temperature curve is shown in Fig. 12. The vapor pressure and temperature from all four stages (P7_2 to P7_5, T7_2 to T7_5) increases with T6_1, as shown in Fig. 15. Because the heating temperature T1_1 is constant at 60 °C, the increase of vapor temperature results in lower pressure difference and weaker driving force, therefore the flux and GOR are reduced as shown in Fig. 16(a). Since the coolant cannot absorb sufficient heat from
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Fig. 15. Temperature and pressure of vapor in each stage using constant cooling flow rate F6 and changing inlet cooling temperature T6_1=25–40 °C. (a) vapor temperature distribution in each stage, (b) vapor pressure distribution in each stage.
heating side when T6_1 is increasing, more energy is wasted (~ 20%) as shown in Fig. 16(b).
4.4. Effect of feed flow rate F2 on the module performance and energy efficiency using memsys lab testing system Feed flows into the first stage. In stage 1, feed is firstly pre-heated from ambient temperature to the boiling point and then partially evaporated. The energies of both pre-heating and evaporation are from the heating source, therefore if the temperature of feed is naturally high, a higher GOR will be achieved because energy consumption during pre-heating is reduced. In this section, the effect of feed flow rate F2 at ambient temperature is discussed. The feed flow rate is a sensitive parameter which has to be adjusted more precisely. If the feed flow rate is higher than needed, more energy will be wasted on pre-heating the feed, and the product output will be also low and it will produce extra unnecessary high temperature brine. If the feed flow is too low, some feed will be over concentrated and may result in crystallization inside the module and shortening of the module life span. In this test, the heating flow rate F1 (15 l/min), heating temperature T1_1 (60 °C), cooling flow rate F6 (9 l/min), feed temperature (25 °C), cooling temperature T6_1 (25 °C), and the vacuum P5_1 (60 mbar) are maintained constant. Different feed flow rate F2 (0.8, 1.2 and 1.5 l/min) are tested and the brine temperature T3_1 is also monitored as shown in Fig. 17.
Fig. 16. Influence of inlet cooling temperature T6_1 to product flux and energy efficiency (a) effects of T6_1 on flux and GOR, (b) effects of T6_1 on energy and product ratio.
At the low feed flow rate 0.8 l/min, 64.5% of feed is evaporated and then condensed as distillate. The temperature of brine T3_1 fluctuates because the feed is insufficient to fill up the feed channel and it causes unstable brine flow (see Fig. 17). When the feed flow is 1.2 l/min, the product ratio reduces to 40%, but the flux remains unchanged. As shown in Fig. 18, when the F2 is 1.5 l/min, the flux is slightly increased, but the product ratio and other energy efficiency have no
Fig. 17. Influence of feed flow rate F2 to feed and brine temperature.
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study the performance and pressure/temperature distribution in the modules. The number of stage is a critical configuration parameter that has an impact on the energy efficiency, the size and the investment costs of the whole MD system. More stages imply higher investment costs for the module, but it is advantageous for specific energy consumption and product rate. In this test, the heating flow rate F1 (15 l/min), heating temperature T1_1 (60 °C), cooling flow rate F6 (9 l/min), feed temperature (25 °C), feed flow rate F2 (1.5 l/min), cooling temperature T6_1 (25 °C), and the vacuum P5_1 (65 mbar) are maintained constant. The temperature/pressure distributions of two stage and four stage systems are compared in Figs. 19 and 20, the pressure and temperature in the 2-stage module are more sensitive and unstable than the 4-stage module. Since the heating temperature T1_1 and cooling temperature T6_1 are the same in the 2-stage and 4-stage modules, the vapor pressure differences in the 2-stage module are higher than the 4-stage module, hence the flux of the 2-stage module is higher than the 4-stage, but the higher flux is based on the lower product ratio and lower GOR, as shown in Table 6. The size of the stage affects the built-in number of frames which is another critical parameter for the investment cost and energy efficiency. Large transfer areas minimize the required temperature difference but it results in extra cost and higher foot print. In this test, the heating flow rate F1 (15 l/min), heating temperature T1_1 (60 °C), cooling flow rate F6 (9 l/min), feed temperature (25 °C), feed flow rate F2 (1.5 l/min), cooling temperature T6_1 (25 °C), and the vacuum P5_1 (65 mbar) are maintained constant. The performance of the 2-stage modules with different stage size (7, 9 and 17 frames) respectively is shown in Table 7. The GOR for different stage sizes are similar, but the flux of the 7-frame module is higher than the 9-frame and 17-frame modules due to higher pressure difference, which means that the module design has a significant impact on pressure distribution and module performance. Fig. 18. Influence of feed flow rate F2 to product flux and energy efficiency (a) effects of F6 on flux and GOR, (b) effects of F6 on energy and product rate.
significant change, which indicates that 1.5 l/min is an optimized feed flow rate based on these testing conditions. 4.5. Optimization of number of module stage and size of each stage As described in Section 2, the memsys module can easily be altered by inserting or removing one or more stages and replacing a different size of stage. In this test, the size of steam raiser and condenser are kept unchanged, the number and size of stage were varied to
5. Conclusions The memsys vacuum-multi-effect-membrane-distillation (V-MEMD) module is a novel, compact and high energy efficiency technology to produce high quality distillate using solar and waste heat as driving forces. The conditions (temperature and flow rate) of heating, cooling and feed are the main operating parameters affecting module performance and energy efficiency. The number of stages and the size of each stage are the key parameters in optimizing module design and system scaling-up. The experimental results show that heating and cooling temperatures have a more significant impact on module flux and energy efficiency
Fig. 19. Comparison of temperature distribution in 2 stage and 4 stage systems (a) two stage system, (b) 4 stage system.
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Fig. 20. Comparison of pressure distribution in 2 stage and 4 stage systems (a) two stage system, (b) 4 stage system.
Acknowledgments
Table 6 Performance comparison of 2 stage and 4 stage systems.
GOR Flux Cooling energy ratio (%) Feed energy ratio (%) Lost energy ratio (%) Product ratio (%)
2 stage
4 stage
1.84 3.8 78.7 14.6 6.7 24
2.79 3.0 75.8 16.7 7.5 40
Table 7 Performance comparison of the 2 stage systems with different numbers of membrane frames.
2
Membrane area (m ) Flux (l/m2·h) GOR
7 frames
9 frames
17 frames
1.88 8.7 1.66
2.50 6.4 1.52
5 3.9 1.60
under maximum vacuum capacity (e.g. 50 mbar). If the heating temperature is limited, increasing heating flow rate and optimizing feed flow rate could be the alternative ways to improve the process (e.g. increase the flux). The optimization of module design shows that the memsys module has great potential in increasing the Gain Output Ratio (GOR), which is one of the most important criteria for industrialization of MD technology.
The authors of this work and memsys wish to gratefully acknowledge the support of PUB (Public Utilities Board, Singapore) and the Aquiva foundation for a grant to partially support this study. References [1] K.W. Lawson, D.R. Lloyd, Review: membrane distillation, J. Membr. Sci. 124 (1997) 1–25. [2] M.S. El-Bourawi, Z. Ding, R. Ma, M. Khayet, A framework for better understanding membrane distillation separation process, J. Membr. Sci. 285 (2006) 4–29. [3] F. Macedonio, E. Drioli, Pressure-driven membrane operations and membrane distillation technology integration for water purification, Desalination 223 (2008) 396–409. [4] M. Khayet, Membranes and theoretical modeling of membrane distillation: a review, Adv. Colloid Interface Sci. 164 (2011) 56–88. [5] S. Cerneaux, I. Struzynska, W.M. Kujawski, M. Persin, A. Larbot, Comparison of various membrane distillation methods for desalination using hydrophobic ceramic membranes, J. Membr. Sci. 337 (2009) 55–60. [6] K.W. Lawson, D.R. Lloyd, Membrane distillation II. Direct contact MD, J. Membr. Sci. 120 (1996) 123–133. [7] T. Na, H. Zhang, W. Wang, Computational fluid dynamic numerical simulation of vacuum membrane distillation for aqueous NaCl solution, Desalination 274 (2011) 120–129. [8] K.W. Lawson, D.R. Lloyd, Membrane distillation I. Module design and performance evaluation using vacuum membrane distillation, J. Membr. Sci. 120 (1996) 111–121. [9] W. Heinzl, Membrane distillation device, patent WO 2010127819, 2010. [10] J. Mericq, S. Laborie, C. Cabassud, Evaluation of systems coupling vacuum membrane distillation and solar energy for seawater desalination, Chem. Eng. J. 166 (2011) 596–606.
Contents lists available at SciVerse ScienceDirect
Desalination journal homepage: www.elsevier.com/locate/desal
Experimental study of the memsys vacuum-multi-effect-membrane-distillation (V-MEMD) module K. Zhao a, 1, W. Heinzl a,⁎, M. Wenzel b, 2, S. Büttner a, 1, F. Bollen a, 1, G. Lange a, 1, S. Heinzl a, 1, N. Sarda a, 1 a b
memsys clearwater Pte. Ltd., 82 Toh Guan Road, Water Hub, Singapore Technische Universität München, Lehrstuhl für Thermodynamik, TU München, D-85747 Garching, Germany
H I G H L I G H T S ► Memsys has successfully commercialized the V-MEMD module. ► The solar driven memsys system illustrates good operating performance. ► Memsys module has great potential in increasing the Gain Output Ratio (GOR).
a r t i c l e
i n f o
Article history: Received 8 April 2012 Received in revised form 8 August 2012 Accepted 3 December 2012 Available online 11 January 2013 Keywords: Membrane distillation V-MEMD Memsys Solar driven MD Hydrophobic membrane GOR
a b s t r a c t The memsys has successfully commercialized the vacuum-multi-effect-membrane-distillation (V-MEMD) module. This compact memsys module employs hydrophobic membranes as a separating medium and makes use of vacuum to enhance membrane distillation process. This novel V-MEMD module enables highly efficient heat recovery. Compared with conventional thermal desalination processes, the memsys module shows advantages in lower investment and operational cost, and higher energy efficiency. In this paper, solar and diesel heater were used as heating sources to drive the memsys V-MEMD module. The solar driven memsys system illustrates good operating performance with a flux at approximately 7 LMH on a sunny day using seawater as feed. The diesel heater driven memsys system was used to investigate the effects of heating, cooling and feed conditions on the module performance using tap water as feed at a relatively low operating temperature (45~60 °C). The results show that heating and cooling temperature are the main factors affecting flux and energy efficiency. The optimization of number of module stage and size of each stage were also studied. The experimental results show that the memsys module has great potential in increasing the Gain Output Ratio (GOR), which is one of the most important criteria for industrialization of MD technology. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Membrane distillation (MD) is a thermally driven process which allows water vapor transport through hydrophobic porous membranes [1,2]. MD produces high quality distillate from wastewater or seawater under non-pressurized condition and at moderate temperatures (50– 80 °C) [3,4]. Vacuum membrane distillation (VMD) is an approach to reduce heat losses and achieve higher vapor flow rates as compared with direct contact MD [5–7]. In VMD, vacuum is applied at the downstream side to maintain the pressure below the equilibrium vapor pressure, enhancing mass transfer [8]. The memsys vacuum-multi-effectmembrane-distillate (V-MEMD) technology combines the advantages of multi-effects and vacuum to achieve highly efficient heat recovery [9]. Compared with the conventional MD processes, memsys has ⁎ Corresponding author. Tel.: +65 91085258, fax: +65 66864267. E-mail address: [email protected] (W. Heinzl). 1 Tel.: +65 91085258; fax: +65 66864267. 2 Tel.: +49 8928916217; fax: +49 8928916218. 0011-9164/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.desal.2012.12.003
successfully developed a commercial membrane distillation product, named as the memsys module. The memsys module shows enormous potential in increasing the Gain Output Ratio (GOR) and reducing the energy requirement in the desalination process. In this paper, the memsys systems with 4-stage module were installed, and the systems were driven by solar thermal [10] and diesel heaters as heating sources respectively. The solar driven memsys system was used in seawater desalination process in the field to evaluate the performance and compatibility of the module. The diesel heater driven memsys system was used to investigate the effects of heating, cooling and feed conditions on the module performance using tap water as feed at a relatively low operating temperature (45– 60 °C). The module design including the number and size of the membrane stage for module optimization was also discussed. 2. Theory One set of standard memsys V-MEMD module consists of a steam raiser, multiple stages, and a condenser. The membrane frames and
K. Zhao et al. / Desalination 323 (2013) 150–160 Table 1 Terminology of memsys module. Terminology
Descriptions
Membrane frame
Two pieces of membranes (one piece size: 600 × 360 mm) are welded onto two sides of one PP (polypropylene) frame in a single production step, as shown in the Fig. 1(a) PP films (~20 μm thickness, same size as membrane) are welded onto two sides of one PP frame in a single production step, as shown in the Fig. 1(a) The membrane and foil frames are alternatively friction-welded together to form stage (number of foil frames = number of membrane frames + 1). When welding the frames together, the necessary channels are formed and supported by a PP-spacer, as shown in Fig. 1(b), two PP cover plates were used to alter the direction of feed and vapor flows Stages were stacked to form multiple stages with vacuum tight silicon gaskets, as shown in the Fig. 1(c) The steam raiser is fabricated by a certain number of membrane frames and installed in front of the first stage. The heat produced by the external heat source (e.g. solar thermal) is exchanged in the steam-raiser. The water in the steam raiser was evaporated at lower pressure (e.g. 600 mbar) and the produced hot vapor flows to the first stage. The condenser is fabricated by certain number of foil frames and installed after the final stage. The vapor produced in the final stage is condensed in the condenser, using the coolant (e.g. fresh water) flow. One set of standard memsys module consists of a steam raiser, multiple stages, and a condenser
Foil frame
Single stage
Multiple stages Steam raiser
Condenser
Memsys module
151
Vapor enters the stage and flows into the parallel foil frames. The vapor in the foil frames heats up the feed and is then condensed. The small channel at the bottom of frame facilities the removal of non-condensable gases and application of vacuum. The condensed vapor then flows into a distillate channel. The heat energy of condensation is transported through the foil and is immediately converted into evaporation energy, generating new vapor in the vapor channel. The vapor leaves the vapor channels and enters the next stage. This process is repeated in the remaining stages. 2.2. Basic working principle of the memsys module In a working memsys module, each stage recovers the heat from the previous stage. Distillate is produced in each stage and in the condenser. As shown in Fig. 3, heat energy produced by the external heating source (e.g. solar thermal or other waste heat) is exchanged in the steam raiser. The water in the steam-raiser is under vacuum pressure (e.g. 600 mbar). The hot vapor generated by steam raiser flows to stage 1, meanwhile feed is also introduced to stage 1 and flows serially through the remaining stages. At the end of the final stage, feed is concentrated as brine. Vacuum is always applied to the permeate side of the membranes. The vapor pressure and temperature gradually decreases from the steam raiser to the condenser. The vapor produced in the final stage is condensed in the condenser, using the coolant (e.g. fresh water) flow. 3. Experimental
foil frames are friction-welded together to form a single stage. A number of single stages are stacked up and vacuum sealed using silicon gaskets to form multiple stages. The terminology of memsys module parts is shown in Table 1, whereas Fig. 1 shows the photos of a frame and stages.
2.1. Flow diagram in the single membrane stage Fig. 2 shows the structure of a single stage, with alternating membrane frames and foil frames for evaporation and condensation. The spaces inside each frame and between neighboring frames, act as channels. Foil frames function as a condenser and provide a means as ‘distillate channels’, while membrane frames serve as ‘vapor channels’. ‘Feed channels’ are created between foil frames and membrane frames.
(a)
(b)
3.1. Preparation of memsys frames and modules As the memsys process works at modest temperatures (b90 °C) and moderate negative pressure, all module components are made of polypropylene (PP). This minimizes corrosion and scaling and allows large-scale cost efficient production. The memsys has developed a highly automated production line for the modules. The hydrophobic membrane used in the module is made of PTFE with a pore size of ~ 0.2 μm. The effective dimension of one piece of membrane is about 335 mm × 475 mm. The condensation foil is made of PP with a thickness of ~ 20 μm. The feed channel has a width of 1.0 to 1.5 mm which can be adjusted during the welding process. There is polypropylene mesh spacer in the feed channel between each membrane and foil. Upon being welded on both sides of the frame, the effective surface area of one membrane frame is
(c)
Fig. 1. Photos of memsys frame and stages. (a) Frame, 2 pieces of membranes or PP foils were weld onto both sides of the frame; (b) single stage with welded frames and two covering plates; (c) multiple stages.
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Fig. 2. Feed, brine and vapor flows in the single stage.
Fig. 3. Basic principle of memsys process.
Fig. 4. Diagram of standard memsys testing system.
K. Zhao et al. / Desalination 323 (2013) 150–160 Table 2 Equipment list of a typical memsys testing system. Symbol Name B1 B2 B3 B4 B6 P1 P1_D
P3 P4 P5 P6 MV1 MV2 SR S1–S4 CR
Function
Heating tank
Water was pre-filled in B1 to receive the energy from external heat source Feed tank Contains the feed Brine tank Contains the brine from the final stage Distillate tank Contains the distillate from stages Cooling tank Water was pre-filled in B6 as coolant in the condenser Heating pump The heated water in B1 was recirculated through the steam raiser by P1 Refilling pump Transfers the distillate back to B1, this distillate is produced by the water in B1 and not the part of product distillate Brine pump Transfers the brine from brine tank B3 Distillate pump Transfers the distillate from distillate tank B4 Vacuum pump Provides vacuum to the whole system Cooling pump The water in B6 was recirculated through the condenser by P6 Heating control Solenoid valve MV1 is used to top up the B1 when valve the water level in the B1 is low Feed control Solenoid valve MV2 is used to top up the B2 when valve the feed level in the B2 is low Steam raiser Refer to Table 1 Stage 1–stage 4 Refer to Table 1 Condenser Refer to Table 1
about 0.31 m 2, the membrane area is around 1.25 m 2 for 11-frame stage, and 2.5 m 2 for a 17-frame stage. Modules can be of varying capacity, depending on the number of stages and the number of frames per stage. All the stages, steam raiser and condenser are sealed with vacuum silicon gaskets when vacuum is applied. 3.2. Structure of memsys testing system The four-stage module were used for the testing, each stage includes 11 frames (1.25 m 2 membrane area) or 17 frames (2.5 m 2 membranes area). The schematic of the whole system is shown in Fig. 4, the descriptions of main components and testing parameters are shown in Tables 2 and 3. As shown in Fig. 4, the solar system or diesel heater was used to provide the heat source to drive the memsys testing system. Heat was transferred to the system by heat exchanger 1. The pre-filled
Table 3 Main operating parameters and descriptions. Parameters Unit P1_1, T1_1 P1_2, T1_2 P2_1, T2_1 P3_1, T3_1 P4_1, T4_1 P5_1 P6_1, T6_1 P6_2, T6_2 P7_1–P7_5 T7_1–T7_5 F1 F2 F3 F4 F6
Descriptions
mbar, °C Inlet temperature and pressure of the water flowing into the steam raiser mbar, °C Outlet temperature and pressure of the water flowing out the steam raiser mbar, °C Feed temperature and pressure before flowing into stage 1 mbar, °C Brine temperature and pressure after flowing out the final stage mbar, °C Distillate temperature and pressure mbar Pressure produced by vacuum pump P5 mbar, °C Inlet temperature and pressure of the water flowing into the condenser mbar, °C Outlet temperature and pressure of the water flowing out the condenser mbar Vapor pressure from the steam raiser and each stage °C Vapor temperature from the steam raiser and each stage l/h Water flow rate flowing into the steam raiser l/h Feed flow rate l/h Brine flow rate (by manual measurement) l/h Distillate flow rate (by manual measurement) l/h Coolant (fresh water) flow rate flowing into the condenser
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Table 4 Formulas of main performance indicators. Parameters
Calculation formulas
Distillate GOR⁎
¼ F4 S
Product ratio
¼ F4 F2
Cooling energy ratio
½ðT6 ¼ F6⋅ F1⋅½ðT1
60⋅F4⋅L ¼ C⋅F1⋅½ðT1 1Þ−ðT1
2Þ
2Þ−ðT6 1Þ 1Þ−ðT1 2Þ
F3⋅½ðT3 1Þ−ðT2 1ÞþF4⋅½ðT4 1Þ−ðT2 1Þ F1⋅½ðT1 1Þ−ðT1 2Þ
Feed energy ratio
¼
Lost energy ratio
¼ 1− F6⋅½ðT6
2Þ−ðT6 1ÞþF3⋅½ðT3 1Þ−ðT2 1ÞþF4⋅½ðT4 1Þ−ðT2 1Þ F1⋅½ðT1 1Þ−ðT1 2Þ
Note: * S=membrane area (m2), C=water specific heat capacity (4.19 kJ/kg), L=latent heat (kWh/l, depending on the boiling temperature of feed under different vacuum pressure). * All the energy discussed in the paper is thermal energy, electricity consumption is not included.
fresh water in tank B6 recirculates as coolant and transfers heat to the external cooling water by heat exchanger 2. The pre-filled fresh water in tank B1 recirculates and receives heat from heat exchanger 1. The feed was automatically sucked in B2 through control by solenoid valve MV2, the brine was collected in the tank B3 and pumped out by pump P3. Vacuum was applied to the whole module and tanks. The distillate production is transported via siphons at the bottom of each stage by pressure differences between stages. The flux and product ratio is used to evaluate the performance and efficiency of the system. GOR and energy consumption of heating, cooling and feed were used to evaluate the energy efficiency. The calculation of energy and performance were shown in Table 4. 3.3. Operation of the memsys testing system 3.3.1. Solar driven memsys field testing system Seawater is used as external coolant and feed. Brine is ejected to the sea at ~ 40 °C. The membrane module consists of 4 stages and each stage has 11 frames. The total membrane area is around 5 m 2. The system collects thermal and electrical solar energy (12 solar thermal collectors with average thermal energy production of 80 kWh/day and 1.5 kW peak electricity by 6 solar cells). Seawater is drawn by a submerged pump approximately 1 m from the shoreline and 2 m below sea-level. Raw seawater is first pre-treated by an ultra-filtration system (Hollow fiber polyacrylonitrile membrane, MWCO is about 100,000 Da). The power needed for the electrical equipment (pumps, PLC) is supplied by solar cells, with consumption varying from 0.9–1.2 kW. 3.3.2. The memsys lab testing system using diesel heater To study the effects of heating, cooling and feed conditions, a diesel heater was used to provide the stable heating temperature T1_1. The relatively low T1_1 was selected to evaluate the performance and energy efficiency of the module. Tap water (25 °C, conductivity about 200 μS/cm) is used as external coolant and feed. The membrane module consists of 4 stages and each stage has 17 frames. The total membrane area is around 10 m 2. The optimization of number and size of stage were studied using a 2-stage system with 7, 9 and 17 frames for each stage. 4. Results and discussion 4.1. Typical seawater desalination operation of solar driven memsys field testing system The solar driven memsys system was operated for 5 to 8 h per day. Fig. 5 shows the heating and vapor temperature curves in each stage. Normally, the system requires about 30 min for temperature and
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Fig. 5. Temperature distribution in the steam raiser and each stage for the solar driven memsys MD system.
pressure stabilization until the last membrane stage starts to create vapor. T1_1 is the temperature of vapor created by hot water circulating in the solar collectors, the energy of this vapor is used to heat up
the seawater in stage 1 and generates new vapor at stage 2, then the similar process will happen in stages 3 and 4, so T1_1 is the leading temperature to drive the whole membrane distillation process, any fluctuation of T1_1 will strongly affect the temperature in the each stage and condenser. The level of sun radiation is the critical factor affecting the inlet heating temperature T1_1. Based on the weather conditions, a good product output can be obtained if the vapor temperature in stage 1 (T7_1) is higher than 65 °C, which implies heating temperature T1_1 must be higher than 70 °C. During the peak hours of the day, T1_1 can reach 80–85 °C. During cloudy moment, T1_1 could go below 60 °C, which will reduce the distillate output significantly. Fig. 6 shows the vapor pressure curves in the different stages, steam raiser and condenser. The vapor pressure curves show similar trend with vapor temperature, as shown in Fig. 6(a), the vapor vacuum pressure in stage 1 (P7_1) is always the highest and decreases in the following stage. The vapor pressure fluctuates with the vapor temperature. Higher vapor temperature leads to larger pressure differences between neighboring stages, thereby producing a higher distillate output. This solar driven memsys system produces about 35 l of distillate per hour under a good weather condition (~ 75 °C T1_1). Therefore, given the 5 m 2 membrane surface area, the flux is about 7 LMH. The pre-treated seawater was used as both the external coolant through heat exchanger 2 and feed. The quality of raw seawater, pre-treated seawater and distillate were analyzed and the results are shown in Table 5. Based on the analysis results, the ultra-treated seawater shows very low turbidity of 0.15 NTU and no suspended solids are detected, but the conductivity of pre-treated seawater remained at the same value as raw seawater. The conductivity of distillate from memsys module is 8–10 μS/cm.
Table 5 Quality analysis of raw seawater, ultra-filtrated seawater and distillate.
Fig. 6. Pressure distribution in the module for the solar driven memsys MD system. (a) Pressure distribution in the steam raiser and each stage, (b) pressure distribution in the heating and cooling part.
Testing items
Units
Conductivity TSS Turbidity SDI15
μS/cm mg/l NTU –
Sample reference Raw seawater
Seawater after UF
Distillate
47,000 10.8 5.5 5.73
47,300 0 0.15 3.93
8–10 – – –
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Fig. 7. Two testing ways of heating influence.
4.2. Effects of heating flow rate and temperature on the module performance and energy efficiency using memsys lab testing system Generally, the more vapor energy produced in the steam raiser, the more energy is available for the distillation process. Heating flow rate F1 and heating temperature T1_1 are the two main heating
Fig. 9. Influence of heating flow rate F1 to product flux and energy efficiency (a) effects of F1 on flux and GOR, (b) effects of F1 on energy and product ratio.
parameters. Fluctuation in F1 and T1_1 will affect the amount of vapor energy generated by the steam raiser and therefore affect the temperature/pressure distribution in each stage and final distillate flux. Fig. 7 shows the two testing methods which involve constant F1 with decreasing T1_1, and constant T1_1 with increasing F1.
Fig. 8. Temperature and pressure of vapor in the each stage using constant inlet heating temperature T1_1=60 °C and changing heating flow rate F1=8.0, 15.2 and 27.7 l/min. (a) Vapor temperature distribution in each stage, (b) vapor pressure distribution in each stage.
4.2.1. Effect of heating flow rate F1 In this test, the heating temperature T1_1 (60 °C), cooling temperature T6_1 (25 °C), feed temperature T2_1 (25 °C), cooling flow rate F6 (7.5 l/min), feed flow rate F2 (1.5 l/min), and the vacuum P5_1 (80 mbar) are maintained constant. The heating flow rate F1 is stepwise increased, and three testing points of F1 (8.0, 15, and 27.7 l/min) are chosen to run the test. As shown in Fig. 8, the pressure and temperature of the vapor generated by the first three stages (P7_2 to P7_4, T7_2 to T7_4) increases with F1, but the pressure and temperature from the last stage (T7_5 and P7_5) are comparably stable, which means the cooling temperature T6_1 and vacuum pressure P5_1 have stronger influence to the last stage than other stages. The pressure differences between stages 1–2, and stages 2–3 are about 10 mbar, and the pressure difference between stages 3 and 4 is about 15 mbar, which are the driving force to transport the vapor and produce the distillate. When the heating flow F1 is increased to 27.7 l/min, the pressure differences between stages 1–2 and 2–3 are slightly increased by 5 mbar and the difference between last two stages rises by 10–15 mbar, these increases in pressure differences provide higher driving force and
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Fig. 10. Temperature and pressure of vapor in each stage using constant heating flow F1 = 15 l/min and changing inlet heating temperature T1_1 = 60, 50 and 45 °C. (a) Vapor temperature distribution in each stage, (b) vapor pressure distribution in each stage.
produce more distillate. As shown in Fig. 9(a), the distillate flux is increased from 1.7 to 3.5 LMH, but GOR did not change significantly following F1. If F1 is increased, both the heating energy (C· F1· [(T1_1) − (T1_2)]) and distillate flow (F4) will be increased proportionally, so based on the equation of GOR (see Table 4), GOR would not be improved. This increased heating energy is mostly transferred to the cooling flow (the cooling ratio increased from 60 to 70% as shown in Fig. 9(b)), which means the entire energy efficiency is not improved by increasing F1. 4.2.2. Effect of heating temperature T1_1 In this test, the heating flow rate F1 (15 l/min), cooling temperature T6_1 (25 °C), feed temperature (25 °C), feed flow rate F2 (1.5 l/min), cooling flow rate F6 (14 l/min), and vacuum P5_1 (60 mbar) are maintained constant, the heating temperature T1_1 decreases at three temperature points, 60 °C, 50 °C and 45 °C, as shown in Fig. 7. The oscillation of the temperature curve is due to the inefficient internal temperature controlling system, but this effect does not influence the test results because of the constant average values. The vapor pressure and temperature from all four stages (P7_2 to P7_5, T7_2 to T7_5) decreases with T1_1. As shown in Fig. 10, T1_1 shows dominant influence on the vapor pressure and temperature from each stage. The pressure differences between each stage reduce by over 20 mbar when T1_1 decreases to 45 °C, which means that the driving force is very weak. As shown in Fig. 11(a), the distillate flux is reduced to below 1 LMH when T1_1 is 45 °C, meanwhile the GOR also
Fig. 11. Influence of inlet heating temperature to product flux and energy efficiency (a) effects of T1_1 on flux and GOR, (b) effects of T1_1 on energy and product ratio.
decreases, which implies that energy efficiency is lower at low heating temperature. GOR at heating temperature T1_1 of 50 °C and 60 °C is rather stable, but the flux at 50 °C is much lower than 60 °C, which means GOR would not be improved by increasing heating temperature T1_1, that is because temperature difference ((T1_1) − (T1_2)) and distillate flow (F4) are increased proportionally based on the equation of GOR in Table 4. As shown in Fig. 11(b), the
Fig. 12. Two testing ways of cooling influence.
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Fig. 14. Influence of cooling flow rate F6 to product flux and energy efficiency (a) effects of F6 on flux and GOR, (b) effects of F6 on energy and product ratio. Fig. 13. Temperature and pressure of vapor in each stage using constant inlet cooling temperature T6_1 and changing cooling flow rate F6. (a) Vapor temperature distribution in each stage, (b) vapor pressure distribution in each stage.
cooling, feed and lost energy ratio are quite similar at T1_1 50 °C and 60 °C. The flux is higher at 60 °C because that around 50% of feed is evaporated and condensed to distillate, which is higher than 30% product ratio achieved at 50 °C T1_1. Fig. 11(b) also shows that the energy lost for the low temperature heating (45 °C) is almost 30% which is far more than 10% normal energy lost ratio at higher temperature. This is probably due to the fact that the cooling energy ratio is only 50%, the energy is lost before the vapor transfers the energy to the next stage. 4.3. Effects of cooling flow rate and temperature on the module performance and energy efficiency using memsys lab testing system Based on the discussions in Section 4.2, the external coolant will take away over 70% of the energy from the heating side, hence the required amount of external coolant is high. Generally the ambient tap water or seawater is used as a coolant to reduce the operating cost, therefore there is no special temperature control on the cooling flow. In this section, the effects of cooling flow rate F6 and cooling temperature T6_1 (naturally be warmed during the operation, not by purposely heating up) will be discussed. The testing condition is shown in Fig. 12. 4.3.1. Effects of cooling flow rate F6 In this test, the heating temperature T1_1 (60 °C), heating flow rate F1 (15 l/min), feed temperature (25 °C), cooling temperature
T6_1 (25 °C), feed flow rate F2 (1.5 l/min), and the vacuum P5_1 (60 mbar) are maintained constant, the cooling flow rate F6 is increased, five testing points of F6 are chosen to run the test. As shown in Fig. 13, the vapor pressure and temperature from all stages (P7_2 to P7_5, T7_2 to T7_5) show little fluctuation, which means the cooling flow F6 has very limited influence on the vapor pressure and temperature distributions. Compared with normal cooling flow rate 7.5 l/min, the higher F6 (around 9 l/min) results in a mild increase in flux, but when the F6 is larger than 9 l/min, there is no improvement on flux and GOR, as shown in Fig. 14(a). In addition, the high cooling flow rate will also take more energy away from heating side and results in very low energy lost at around 5% as shown in Fig. 14(b).
4.3.2. Effects of cooling temperature T6_1 In this test, the heating flow rate F1 (15 l/min), heating temperature T1_1 (60 °C), cooling flow rate F6 (9 l/min), feed temperature (25 °C), feed flow rate F2 (1.5 l/min) are maintained constant, the cooling flow is recirculated and continuously heated up by energy from the vapor in the condenser. The temperature curve is shown in Fig. 12. The vapor pressure and temperature from all four stages (P7_2 to P7_5, T7_2 to T7_5) increases with T6_1, as shown in Fig. 15. Because the heating temperature T1_1 is constant at 60 °C, the increase of vapor temperature results in lower pressure difference and weaker driving force, therefore the flux and GOR are reduced as shown in Fig. 16(a). Since the coolant cannot absorb sufficient heat from
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Fig. 15. Temperature and pressure of vapor in each stage using constant cooling flow rate F6 and changing inlet cooling temperature T6_1=25–40 °C. (a) vapor temperature distribution in each stage, (b) vapor pressure distribution in each stage.
heating side when T6_1 is increasing, more energy is wasted (~ 20%) as shown in Fig. 16(b).
4.4. Effect of feed flow rate F2 on the module performance and energy efficiency using memsys lab testing system Feed flows into the first stage. In stage 1, feed is firstly pre-heated from ambient temperature to the boiling point and then partially evaporated. The energies of both pre-heating and evaporation are from the heating source, therefore if the temperature of feed is naturally high, a higher GOR will be achieved because energy consumption during pre-heating is reduced. In this section, the effect of feed flow rate F2 at ambient temperature is discussed. The feed flow rate is a sensitive parameter which has to be adjusted more precisely. If the feed flow rate is higher than needed, more energy will be wasted on pre-heating the feed, and the product output will be also low and it will produce extra unnecessary high temperature brine. If the feed flow is too low, some feed will be over concentrated and may result in crystallization inside the module and shortening of the module life span. In this test, the heating flow rate F1 (15 l/min), heating temperature T1_1 (60 °C), cooling flow rate F6 (9 l/min), feed temperature (25 °C), cooling temperature T6_1 (25 °C), and the vacuum P5_1 (60 mbar) are maintained constant. Different feed flow rate F2 (0.8, 1.2 and 1.5 l/min) are tested and the brine temperature T3_1 is also monitored as shown in Fig. 17.
Fig. 16. Influence of inlet cooling temperature T6_1 to product flux and energy efficiency (a) effects of T6_1 on flux and GOR, (b) effects of T6_1 on energy and product ratio.
At the low feed flow rate 0.8 l/min, 64.5% of feed is evaporated and then condensed as distillate. The temperature of brine T3_1 fluctuates because the feed is insufficient to fill up the feed channel and it causes unstable brine flow (see Fig. 17). When the feed flow is 1.2 l/min, the product ratio reduces to 40%, but the flux remains unchanged. As shown in Fig. 18, when the F2 is 1.5 l/min, the flux is slightly increased, but the product ratio and other energy efficiency have no
Fig. 17. Influence of feed flow rate F2 to feed and brine temperature.
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study the performance and pressure/temperature distribution in the modules. The number of stage is a critical configuration parameter that has an impact on the energy efficiency, the size and the investment costs of the whole MD system. More stages imply higher investment costs for the module, but it is advantageous for specific energy consumption and product rate. In this test, the heating flow rate F1 (15 l/min), heating temperature T1_1 (60 °C), cooling flow rate F6 (9 l/min), feed temperature (25 °C), feed flow rate F2 (1.5 l/min), cooling temperature T6_1 (25 °C), and the vacuum P5_1 (65 mbar) are maintained constant. The temperature/pressure distributions of two stage and four stage systems are compared in Figs. 19 and 20, the pressure and temperature in the 2-stage module are more sensitive and unstable than the 4-stage module. Since the heating temperature T1_1 and cooling temperature T6_1 are the same in the 2-stage and 4-stage modules, the vapor pressure differences in the 2-stage module are higher than the 4-stage module, hence the flux of the 2-stage module is higher than the 4-stage, but the higher flux is based on the lower product ratio and lower GOR, as shown in Table 6. The size of the stage affects the built-in number of frames which is another critical parameter for the investment cost and energy efficiency. Large transfer areas minimize the required temperature difference but it results in extra cost and higher foot print. In this test, the heating flow rate F1 (15 l/min), heating temperature T1_1 (60 °C), cooling flow rate F6 (9 l/min), feed temperature (25 °C), feed flow rate F2 (1.5 l/min), cooling temperature T6_1 (25 °C), and the vacuum P5_1 (65 mbar) are maintained constant. The performance of the 2-stage modules with different stage size (7, 9 and 17 frames) respectively is shown in Table 7. The GOR for different stage sizes are similar, but the flux of the 7-frame module is higher than the 9-frame and 17-frame modules due to higher pressure difference, which means that the module design has a significant impact on pressure distribution and module performance. Fig. 18. Influence of feed flow rate F2 to product flux and energy efficiency (a) effects of F6 on flux and GOR, (b) effects of F6 on energy and product rate.
significant change, which indicates that 1.5 l/min is an optimized feed flow rate based on these testing conditions. 4.5. Optimization of number of module stage and size of each stage As described in Section 2, the memsys module can easily be altered by inserting or removing one or more stages and replacing a different size of stage. In this test, the size of steam raiser and condenser are kept unchanged, the number and size of stage were varied to
5. Conclusions The memsys vacuum-multi-effect-membrane-distillation (V-MEMD) module is a novel, compact and high energy efficiency technology to produce high quality distillate using solar and waste heat as driving forces. The conditions (temperature and flow rate) of heating, cooling and feed are the main operating parameters affecting module performance and energy efficiency. The number of stages and the size of each stage are the key parameters in optimizing module design and system scaling-up. The experimental results show that heating and cooling temperatures have a more significant impact on module flux and energy efficiency
Fig. 19. Comparison of temperature distribution in 2 stage and 4 stage systems (a) two stage system, (b) 4 stage system.
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Fig. 20. Comparison of pressure distribution in 2 stage and 4 stage systems (a) two stage system, (b) 4 stage system.
Acknowledgments
Table 6 Performance comparison of 2 stage and 4 stage systems.
GOR Flux Cooling energy ratio (%) Feed energy ratio (%) Lost energy ratio (%) Product ratio (%)
2 stage
4 stage
1.84 3.8 78.7 14.6 6.7 24
2.79 3.0 75.8 16.7 7.5 40
Table 7 Performance comparison of the 2 stage systems with different numbers of membrane frames.
2
Membrane area (m ) Flux (l/m2·h) GOR
7 frames
9 frames
17 frames
1.88 8.7 1.66
2.50 6.4 1.52
5 3.9 1.60
under maximum vacuum capacity (e.g. 50 mbar). If the heating temperature is limited, increasing heating flow rate and optimizing feed flow rate could be the alternative ways to improve the process (e.g. increase the flux). The optimization of module design shows that the memsys module has great potential in increasing the Gain Output Ratio (GOR), which is one of the most important criteria for industrialization of MD technology.
The authors of this work and memsys wish to gratefully acknowledge the support of PUB (Public Utilities Board, Singapore) and the Aquiva foundation for a grant to partially support this study. References [1] K.W. Lawson, D.R. Lloyd, Review: membrane distillation, J. Membr. Sci. 124 (1997) 1–25. [2] M.S. El-Bourawi, Z. Ding, R. Ma, M. Khayet, A framework for better understanding membrane distillation separation process, J. Membr. Sci. 285 (2006) 4–29. [3] F. Macedonio, E. Drioli, Pressure-driven membrane operations and membrane distillation technology integration for water purification, Desalination 223 (2008) 396–409. [4] M. Khayet, Membranes and theoretical modeling of membrane distillation: a review, Adv. Colloid Interface Sci. 164 (2011) 56–88. [5] S. Cerneaux, I. Struzynska, W.M. Kujawski, M. Persin, A. Larbot, Comparison of various membrane distillation methods for desalination using hydrophobic ceramic membranes, J. Membr. Sci. 337 (2009) 55–60. [6] K.W. Lawson, D.R. Lloyd, Membrane distillation II. Direct contact MD, J. Membr. Sci. 120 (1996) 123–133. [7] T. Na, H. Zhang, W. Wang, Computational fluid dynamic numerical simulation of vacuum membrane distillation for aqueous NaCl solution, Desalination 274 (2011) 120–129. [8] K.W. Lawson, D.R. Lloyd, Membrane distillation I. Module design and performance evaluation using vacuum membrane distillation, J. Membr. Sci. 120 (1996) 111–121. [9] W. Heinzl, Membrane distillation device, patent WO 2010127819, 2010. [10] J. Mericq, S. Laborie, C. Cabassud, Evaluation of systems coupling vacuum membrane distillation and solar energy for seawater desalination, Chem. Eng. J. 166 (2011) 596–606.