TEMPERATURE POLARIZATION IN MEMBRANE DISTILLATION OF WATER USING A POROUS HYDROPHOBIC MEMBRANE

The rate of water transport across porous membranes with a hydrophobic matrix (porosity epsilon almost-equal-to 0.5; characteristic length of voids xi almost-equal-to 150 nm; thickness delta = 30 mum) caused by a temperature difference between the bulk phases is studied. The voids of the membrane are filled with water vapour. The effective temperature difference (T' - T'') across the membrane is smaller than the temperature difference (T' - T'') between the bulk phases (''temperature polarization'') caused by heat conduction and by heat flow associated with the water transport. An experimental method is described and tested to determine the temperature polarization coefficient tau (= (T' - T'')/(T'-T'') for a given experimental situation. The phenomenological transport coefficient of the membrane for water vapour has to be known from independent experiments. This method is a simple alternative to the method developed by Schofield et al. (J. Membrane Sci. 33, 299 (1987)). The dominating factor determining the temperature polarization coefficient is the value of the thickness of the two boundary layer of heat conduction at the interfaces of the membrane.