E. Bode et C. Hoempler, TRANSPORT RESISTANCES DURING PERVAPORATION THROUGH A COMPOSITE MEMBRANE - EXPERIMENTS AND MODEL-CALCULATIONS, Journal of membrane science, 113(1), 1996, pp. 43-56
For an investigation of transport resistances of a GFT PVA-PAN Pervap
1000 type membrane the pervaporation flux of pure water was measured a
s a function of downstream pressure, with the membrane oriented in nor
mal and in reverse position (= PVA layer facing the feed or the permea
te). Assuming equilibrium across all interfaces, it was possible to br
eak 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, a
nd of vapor by Poiseuille and Knudsen flow and eventually by surface d
iffusion along pore walls is presented for describing the 3 transport
regimes observed experimentally. The support layer was found to consum
e up to 80% of the total resistance. (b) Active layer resistance part
= resistances of the PVA layer and the slightly porous PAN skin: 3 mod
els describing the influence of the PAN skin are discussed and judged
in detail. Then the skin resistance is estimated and used to derive th
e permeability of water in PVA as a function of water activity or 'equ
ivalent partial pressure' EPP in the polymer which is then compared to
vapor permeation, vapor sorption and diffusivity data published by He
intz and Lichtenthaler. Strong discrepancies are found and discussed i
n terms of possible interface resistances, of free and bound water and
of cluster formation which would mean that in cases like this diffusi
vities can be derived from stationary state pervaporation but not from
non-stationary state kinetic sorption experiments.