Performances of air gap and water gap MD desalination modules

The availability of drinking water is continuously decreasing worldwide, while demand is rapidly increasing. Most of human diseases are because of polluted or unpurified water resources. Developed and developing countries face tremendous water scarcity because of high consumption rates and pollution generated by industrial activities. The shortage of drinking water is expected to be the most significant global problem in this century, due to unsustainable consumption rates and fast population growth. Pollution of freshwater resources (rivers, lakes, and underground water) by industrial wastes has exacerbated the problem. Unfortunately, water resources are not evenly globally distributed and are not available in sufficient quantities when or where they are needed. Thus, the development of new clean water sources is a pressing problem.

Membrane distillation (MD) is a thermally driven membrane separation technology that has the future potential to compete with conventional water desalination processes, Khayet & Matsuura (2011). In MD, hot saline water passes over one side of a hydrophobic membrane and water vapor permeates the membrane and condenses on the other side. In comparison to conventional desalination processes, the MD technique offers important advantages such as low operating temperatures (40 to 90 °C); which means that waste energy and solar collection systems can be used directly to produce distillate. MD process has the theoretical ability to achieve 100% salt rejection. Moreover, the hydrostatic pressure of feed encountered in MD is very low and close to atmospheric pressure. MD membrane materials are less demanding, they have large pore size, less fouling problems, and cheaper as compare to RO membranes. There are four common configurations (designs) of MD Modules, Alkhudiri et al. (2012), namely; direct contact membrane distillation (DCMD), air gap membrane distillation (AGMD), sweeping gas membrane distillation (SGMD), and vacuum membrane distillation (VMD). In all the MD configurations, the hot feed solution is in direct contact with the hydrophobic membrane surface. However, the design of the cold side of the membrane and the way the permeate is condensed and collected are different for each design, Alklaibi & Noam (2004).

In AGMD module design, the permeated water vapor through the membrane diffuses through a stagnant air gap situated between the membrane and a cold condensation surface where it condenses to produce distilled water. The existence of the air gap reduces heat loss by conduction across the membrane between the hot and cold streams on both sides of the membrane. However, the air gap adds more resistance to vapor mass transfer (Banat & Simandl 1998; Khayet 2011; Pangarker & Sane 2011). On the other hand, when water is used to fill the gap, the configuration is said to be Water Gap Membrane Distillation or WGMD. The WGMD design is also known in literature as Liquid gap MD (LGMD) and Permeate gap MD (PGMD), and in reality, only liquid water can be used in the gap of the module to avoid distillate contamination. The WGMD design contained stagnant distilled water filling the gap behind the membrane. Using the water gap enhances the permeate flux significantly as compared to the air gap as reported by Khalifa (2015). The increase is flux ranges between 90% and 140%, mainly depending on the feed temperature, when using the water gap as compared to the air gap. In case of water gap, the liquid-vapor interface on the cold side of the membrane and reduced gap temperature promote the condensation process and enhance the flux.

Ugrozov et al. (2003) modelled the LGMD and validated through experiments on a laboratory scale setup. It was observed that the vapor mass flux increases by increasing flow rates (feed or coolant), feed solution temperature, and the membrane length. Three possible schemes of the liquid gap membrane distiller are examined for minimal power inputs installation with small productivity were reported by Ugrozov & Kataeva (2004). It has been shown that the value of power input increases directly proportional to the product of volume stream rate and the temperature difference of the streams at the entry to the membrane module. Using different materials inside the gap of MD module has been studied by Francis et al. (2013). It has been observed that employing appropriate materials (like sand, water, and sponge) between the membrane and the condensation plate in an AGMD module enhances the water vapor flux significantly. An increase in the water vapor flux of about 200–800% was observed by filling the gap with sand and deionized water at various feed water temperatures. Also, an increase in the water gap width from 9 mm to 13 mm increases the water vapor flux. Another comparative study between air and LGMD designs is presented by Essalhi & Khayet (2014) as an application of a porous composite hydrophobic/hydrophilic membrane in desalination. A membrane of hydrophobic (active layer) and hydrophilic (support Layer) nature was used. It was reported that a maximum vapor flux increase of only 6.6% is achieved when using the liquid gap as compared to air gap. The LGMD also showed higher thermal efficiency and less internal heat loss as compared to AGMD.

In the present work, an experimental comparison is performed on the main operating and design variables affecting the performance of WGMD and AGMD. The ranges of tested variables are selected for comprehensive conclusions and the effects of some variables are combined for integrity when applicable.

The membrane module was designed and fabricated to work on both configurations exchangeable, as shown in Figure 1. The hot feed water directly flows in two channels over the hydrophobic flat sheet membrane which is supported by a brass perforated plate of 1.5 mm thickness. Another brass plate is used as a cooling surface after a suitable gap behind the membrane. The gap is created by a spacer gasket between the two brass plates (the membrane support and the cooling plates). The gap width can be varied by changing the thickness of the spacer gasket. The cold plate is used for condensing the vapor passing through the membrane pores in case of AGMD design and used for cooling the water inside the gap in case of the WGMD design. Cold water with constant inlet temperature flows behind the cooling/condensation plate to keep the plate at low temperature. In case of AGMD operation mode, the condensed vapor (permeate) is collected in a sealed cavity at the bottom of the condensation plate. In case of using the module with liquid gap, the cavity port is closed and another outlet port at the top of the gap is opened to collect the permeate. Figure 2 shows the assembled module. Pressure and temperature sensors are placed at module inlets and outlet for accurate measurements. Feed water is heated using a thermostat-heating and circulating bath, and the coolant water is supplied from a refrigerated circulation bath. The effective area of the used hydrophobic flat sheet membrane is 0.00724 m², based on the opening area of the perforated support plate. The actual applied water pressure for hot and cold sides ranged between 0 and 0.3 bar, depending on the flow rate. To switch from the air gap to water gap design, the permeate outlet at the bottom of the air gap MD module is closed and the system is run until the gap is filled by distilled water rather than air. Excess distillate water starts coming out of the port at the top of the gap.

In the present work, experiments are conducted to compare the performance of WGMD and AGMD designs under different operating and design conditions, including the feed and coolant temperatures, membrane material and pore size, and gap width. In addition, energy analysis is performed for both modules.

Feed and coolant temperatures are the most effective operating conditions on the performance of the MD process. They determine the trans-membrane potential of permeation. The effects of feed water temperature on the permeate flux of air gap and water gap modules are investigated at different coolant temperatures. Figure 3 shows the variation of the system permeate flux with feed temperature at different coolant temperatures, for both air gap and water gap modes of operations. In this test, the feed temperature is varied from 50 to 90 °C, with a step of 10 °C, and three coolant temperature of 5, 15, and 24 °C were selected for comparisons. The test conditions are listed below the figure for convenience. The exponential increase of flux with feed temperature is evident. The water gap module produces higher flux compared to the air gap module at similar test conditions. The water in the gap provides better condensation and less resistance to mass transfer of vapor across the gap since the condensation takes place immediately at the cold side of the membrane interface. In general, reducing the coolant temperature increases the flux due to increasing the temperature gradient across the membrane. However, the coolant temperature has higher effect on the performance of the AGMD system than its effect on the WGMD system. For example, when coolant temperature was reduced from 25 to 5 °C, the flux increased by about 33% for AGMD system and by about 18% for WGMD system at feed temperature of 70 °C. Similarly, at 90 °C feed temperature, flux increase of 14.5% for the AGMD system and 4.3% for the WGMD system were recorded.

To test the effects of membrane material and pore size, two membranes of same mean pore size of 0.45 μm but different materials were tested, namely Polytetraflouroethylene (PTFE) and polyvinyl diflouride (PVDF). The PTFE membrane is a composite of active layer and support layer. In addition, two PTFE membranes of different pore size, 0.22 and 0.45 μm are tested for comparison. The measured membranes properties are listed in Table 1. The detailed membrane characterization process can be found in Khalifa et al. (2015).

Figure 4 shows that the flux of the PTFE membrane is higher than that of the PVDF membrane, at all tested feed temperatures. However, the WGMD configuration is more sensitive to changing the membrane type as compared to the AGMD configuration. For the AGMD module, the effect of membrane material is more prominent at higher feed temperature while for WGMD the effect membrane material is more significant at low feed temperatures. The enhanced flux with the PTFE membrane compared to the PVDF membrane can be attributed to its higher hydrophobicity (higher contact angle), higher porosity, less thickness, and different thermal properties. The effects of thermal properties are clear in case of the WGMD rather than the AGMD and this may be the reason of clear effect at different feed temperatures. For WGMD configuration; PTFE membrane flux is 160% higher than the PVDF membrane at feed temperature of 50 °C, while this percentage increase is reduced to 7% only at feed temperature of 90 °C. Calculating the same percentage increase for the AGMD configuration, about 9% increase in flux for PTFE membrane at 50C and about 4% increase at 90C, compared to the PVDF membrane, were achieved.

In order to see the effect of membrane pore size, two PTFE membranes of similar properties (contact angle, porosity, and thickness) except for the mean pore diameter were tested with the AGMD and WGMD modules. Figure 5 shows the effect of mean pore diameter of flux for both WGMD and AGMD configurations, at different feed temperatures. It is obvious that membranes with the bigger pore diameter produce more flux in both WGMD and AGMD configurations, membrane with mean pore 0.45 μm yields more system flux as compared to membrane with pore of 0.22 μm. Moreover, the effect of pore size if more prominent at low feed temperature as compared to higher feed temperature in both WGMD and AGMD modules. For instant, at 60 °C the 0.45 μm membrane flux is 50% more than the 0.22 μm and the percentage reduced to less than 10% at 90 °C. The pore size affects the mode of vapor diffusion and hence the mass transfer resistance through the pore.

The distance between the membrane cold surface and the condensation plate determines the effective gap width. Gap width is an important design parameter in the evaluating the performance of AGMD and WGMD configurations. To investigate the influence of gap width on the flux of both AGMD and WGMD, a set of experiments were conducted using different gap widths of 2, 4, 6, 8, and 12 mm, and results are displayed in Figure 6. The gap width was changed by changing the thickness of the gap gasket between the membrane support plate and the condensation plate. As a general observation, reducing the gap width increases the output flux for both AGMD and WGMD modules. However, the AGMD model was found to be more sensitive to the gap width compared to the WGMD module. Small gap of 2 mm thickness is highly recommended for efficient AGMD system; particularly at high feed temperatures. In addition, the exponential increase in flux with the increase in feed temperature is shown in the figure. At 90 °C, the flux increased by 100% for the AGMD module when the gap was reduced from 12 mm to 2 mm. On the other hand, the corresponding increase in flux is only 25% for WGMD module.

Figure 7 shows the effect of gap width on permeate flux for WGMD and AGMD configurations at constant feed temperature of 90 °C. It can be seen that increasing the gap width decreases the permeate flux in both configurations. In WGMD, the flux trend seems gradually decreasing and then becomes quite constant. For AGMD module, a more steeper variation can be seen, which means the gap width is significantly important design parameter in AGMD. The vapor diffusion path and resistance before condensation in the air gap design depends on the air gap width. On the other hand, distilled water filling the gap in the water gap design guarantee immediate condensation of vapor at membrane cold surface. However, the heat loss between the hot and cold sides of the membrane are higher in the water gap module compared to the air gap design due to the thermal properties of both gap fluids.

In addition, the specific energy consumption (SEC, kWh/m³) is another performance measure of the MD system which is calculated for both MS-AGMD and MS-WGMD systems at different feed temperatures. SEC is the energy consumed to produce one cubic meter of fresh water. The distillate flux and the consumed electric power over the experiment time are used to calculate the SEC values for both AGMD and WGMD modules.

Figure 8 shows the effect of inlet feed temperature on the GOR of both AGMD and WGMD modules. The GOR value increases with increasing the inlet feed temperature for WGMD and AGMD. The variation of the GOR value of the AGMD module seems linearly increasing with feed temperature. On the other hand, for WGMD, GOR value reaches its maximum value at feed temperature of 80 °C. GOR values for WGMD is between 0.558 and 1.25 and for AGMD it is between 0.81 and 1.3, depending on the feed temperature. Figure 9 shows the influence of inlet feed temperature on the SEC for WGMD and AGMD. At low feed temperature, the SEC is high and decreases as the feed temperature increases, for both configurations. The SEC values ranges between 9 to 4 kWh/m³ for both modules depending on the feed temperature.

An experimental comparison on the performance of WGMD and AGMD modules for water desalination was presented. The effects of the main operating and design variables were studied. The fluxes obtained from the WGMD module are higher as compared to those obtained from the AGMD module, for all tested conditions. Having stagnant water in the gap enhances the heat transfer and condensation process within the MD module; as compared to the air gap. The feed temperature is the dominant factor affecting the system flux since it determines the potential of permeation through the membrane. Gap widths from 2 to 12 mm were tested and the permeate flux increases when reducing the gap width for both water and air gap modules. The WGMD module was found to be less sensitive to the change in the gap width compared to the AGMD module because the condensation in WGMD module takes place immediately at the membrane cold surface interface. The PTFE membrane produced higher permeate flux as compared to the PVDF membrane, for both configurations WGMD and AGMD, due to its high hydrophobicity, porosity, and its smaller thickness of compared to the PVDF membrane. A maximum GOR value of about 1.3 and a minimum SEC value of about 4 were achieved at 90C. These values are promising and can be reduced more by improving the design of the modules, including energy recovery, and using multistage design.

The author acknowledges the support and fund received from the Deanship of Research, King Fahd University of Petroleum & Minerals (KFUPM) under Research Grant # IN141035.