Molecular simulation on the separation of water/ethanol azeotropic mixture by poly(vinyl alcohol) membrane

Grand canonical Monte Carlo (GCMC) simulation is used to investigate the performance of poly(vinyl alcohol) (PVA) membrane in separating the azeotropic water/ethanol mixture (95.57 wt% ethanol) over a wide range of pressures (10-1000 kPa), temperatures (298-338 K) and PVA polymerization degrees (100-1000). By calculating the sorption isotherms and the ethanol-to-water separation factors, we observe that the water/ethanol adsorption amount and separation factor decline slowly with the increase of temperature: as the polymerization degree rises, both of adsorption amounts first increase and then decrease, while the separation factor changes adversely. Concepts such as fractional free volume (FFV) and hydrogen bonding interactions are analyzed to explain the observation. As the polymerization degree increases, the FFV changing trend is similar to the one mentioned in the discussion of adsorption amount, but their inflexions are different. Hydrogen bonding interaction successfully explains this variation. We further deduce that the fact that the change of adsorption amount results from a transition from cooperation to competition between FFV and hydrogen bonding interactions. The optimal operating conditions for separation are 298 K and 101.325 kPa. Under this condition, the PVA membrane (polymerization degree 1000) has a separation factor of similar to 80 for the water/ethanol azeotropic mixture, which means that the concentration of ethanol can be refined to 99.96 wt% and anhydrous ethanol is possible to be obtained by PVA membrane separation. (C) 2010 Elsevier B.V. All rights reserved.