Separation science and technology vol:39 issue:3 pages:563-580
This article explores water, methanol, and ethanol transport through hydrophilic membranes, by measuring the flux and separation factor in pervaporation as a function of the feed composition for binary water/methanol and water/ethanol mixtures. Additionally, the influence of adding a third component to the feed is examined. Because the solution-diffusion model is considered to be the basic reference model for the description of transport through pervaporation membranes, it was used for understanding differences in transport characteristics. It was found that the feed composition has a large influence on flux and selectivity of all components. The results show that for a given feed concentration, the permeability is different for binary and ternary mixtures. Permeability thus depends also on the other feed components and their concentration, which contradicts the assumptions of the solution-diffusion model. The deviations from the ideal are explained qualitatively, and may be attributed to the swelling behavior of the membrane, and to differences in size and polarity of the components. Because of the low polarity (and the larger size) of ethanol, ethanol permeation is only possible in the presence of enough water. Furthermore, it was observed for both alcohols that the water-methanol or water-ethanol separation factor in the ternary mixtures was (much) higher than in the binary mixtures, indicating that the presence of a second alcohol in the feed decreases the permeation of both alcohols.