Transport resistances during pervaporation through a composite membrane: Experiments and model calculations

For an investigation of transport resistances of a GFT PVA-PAN Pervap 1000 type membrane the pervaporation flux of pure water was measured as a function of downstream pressure, with the membrane oriented in normal and in reverse position (= PVA layer facing the feed or the permeate). Assuming equilibrium across all interfaces, it was possible to break up the total resistance into (a) a support layer resistance part, and (b) an active layer resistance part. (a) Support layer resistance part = resistance of the support excluding its upstream skin: A model of liquid water transported by capillary forces through small pores, and of vapor by Poiseuille and Knudsen flow and eventually by surface diffusion along pore walls is presented for describing the 3 transport regimes observed experimentally. The support layer was found to consume up to 80% of the total resistance. (b) Active layer resistance part = resistances of the PVA layer and the slightly porous PAN skin: 3 models describing the influence of the PAN skin are discussed and judged in detail. Then the skin resistance is estimated and used to derive the permeability of water in PVA as a function of water activity or 'equivalent partial pressure' EPP in the polymer which is then compared to vapor permeation, vapor sorption and diffusivity data published by Heintz and Lichtenthaler. Strong discrepancies are found and discussed in terms of possible interface resistances, of free and bound water and of cluster formation which would mean that in cases like this diffusivities can be derived from stationary state pervaporation but not from non-stationary state kinetic sorption experiments.