A review of membrane selection for the dehydration of aqueous ethanol by pervaporation

Four broad types of membranes are categorised: organic polymers generally, crosslinked poly(vinyl alcohol), organic–inorganic hybrids and charged polymers. The best performers in terms of flux, which reaches a maximum of 5 kg/m2h, are anionic or cationic polymers, including polysalts. Polyanion and polysalt membranes are superior. Two examples are thin layers of the active polysalt membrane on a supporting membrane. The best combination for flux and selectivity is a polyethyleneimine/poly (acrylic acid) polysalt deposited on a reverse osmosis membrane, at 4 kg/m2h and 1075 respectively. It is noticeable that hybrid poly(vinyl alcohol)/inorganic membranes do not show enhanced fluxes. Very high separation factors were observed, covering a range of polymers, of neutral, anionic or cationic character. The top results (>10,000) were for charged membranes, either cationic or anionic, but not polysalts. The fluxes encountered here were miniscule, the best being caesium alginate at about 1 kg/m2h. The ideal structure for high fluxes would appear to be one containing discrete domains of oppositely charged species of optimal size. Fresh approaches are being actively studied, such as layer-by-layer deposition of oppositely charged polyelectrolytes, with due attention to appropriate separation of the sites of opposite character.