Application of microwave heating to pervaporation: A case study for separation of ethanol-water mixtures

Membrane pervaporation experiments for dewatering of water-ethanol mixtures were conducted, using a polymeric hydrophilic membrane, under microwave and conventional heating in a multimode microwave oven and a convection oven, respectively. Three feed temperatures (33.5, 45.5 and 51.5 °C) and two feed compositions (5.5 wt% and 20 wt% water in the feed) were considered. At 20 wt% water content, higher water fluxes through the membrane were obtained in the convection oven. At lower water content in the feed (5.5 wt%), the opposite effect was observed; the water fluxes were higher under microwave heating over the considered temperature range. These differences may arise from the different dielectric properties and consequently thermal behaviour of the feed mixtures under microwave heating. Microwave coupling with ethanol is stronger than with water. Moreover, unlike water, the dielectric loss factor of ethanol increases with temperature, which makes microwave dissipation preponderant in hot areas. Hence, high ethanol concentrations in the feed can easily induce thermal gradients.