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Fig. 39. The influence of the thickness of the air gap on mass flux in AGMD.

Fig. 39. The influence of the thickness of the air gap on mass flux in AGMD.

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Fig. 40. Comparison of thermal efficiency between AGMD and DCMD.

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Fig. 40. Comparison of thermal efficiency between AGMD and DCMD.

The amount of the energy utilized for evaporation of the feed can be determined according to mass flux and latent heat of water. The total energy supplied is determined by the enthalpy change of the feed liquid in and out of the membrane module. Fig. 40 shows the comparison of thermal efficiency between AGMD and DCMD, which is obtained from the experiment. It is evident that AGMD is advantageous over DCMD in thermal efficiency. This should be attributed to the existence of air gap in the membrane module. It is the air gap that forms an additional heat transfer resistance by conduction from the feed to the permeate. As a result, the thermal efficiency of AGMD is improved. In addition, for both AGMD and DCMD, the thermal efficiency becomes higher with the increase of feed temperature. As we know, mass flux increases with feed temperature in an exponential way, whereas the heat loss through the membrane by conduction is linearly proportional to the temperature difference between the two sides of the membrane. Therefore, at a high feed temperature, the amount of heat energy utilized for evaporation is more than that transferred to the permeate side by conduction through the membrane.

Although AGMD is thermally efficient, the low mass flux makes it unlikely to be accepted in industry. On the other hand, while DCMD shows a relatively low thermal efficiency, this disadvantage can be overcome by setting an external heat exchanger to recover heat energy from the permeate.

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