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 o f voids xi almost-equal-to 150 nm; thickness delta = 30 mum) caused by a temperature difference between the bulk phases is studied. The void s of the membrane are filled with water vapour. The effective temperat ure difference (T' - T'') across the membrane is smaller than the temp erature difference (T' - T'') between the bulk phases (''temperature p olarization'') caused by heat conduction and by heat flow associated w ith the water transport. An experimental method is described and teste d 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 (19 87)). The dominating factor determining the temperature polarization c oefficient is the value of the thickness of the two boundary layer of heat conduction at the interfaces of the membrane.