VAPOR PERMEATION AND PERVAPORATION SEPARATION OF WATER ETHANOL MIXTURES THROUGH POLYIMIDE MEMBRANES

Vapor permeation (VP) and pervaporation (PV) separation of water-ethanol mixtures were investigated for a series of polyimide membranes. Membrane performance for PV was described by both the permeability coefficient of water, P(W), which was calculated using the equilibrium partial pressure as the driving force, and the separation factor at the membrane separation step, alpha(mem). For PV, the dependence of P(W) and alpha(mem) on feed composition (wt.% of ethanol, x(E)) was rather small. For VP, with a decrease in x(E), P(W) increased and alpha(VP) decreased. P(W) was larger for PV than for VP, while alpha(mem) was smaller for PV. The differences between PV and VP were large for BPDA-DDBT and BPDA-ODA/DABA (4/6) polyimides having a higher total sorption in the liquid phase, but were small for Kapton-H and BPDA-ODA/DABA (8/2) polyimide having a lower sorption. In the case of the former polymers, the swelling effect was so large for PV that it spoiled membrane performance seriously, whereas it was not so large for VP.