Pervaporation of volatile organic compounds from water -: I -: Influence of permeate pressure on selectivity

Hydrophobic pervaporation is developing within several interesting areas of application related to the recovery of volatile organic compounds from dilute water solutions. So far this process has only been commercialised to a limited extent. For optimisation and scale-up, models are needed to predict the influence of key process parameters such as temperature, hydrodynamic conditions on the feed side of the membrane and permeate pressure on the partial fluxes and the selectivity. In this study, pervaporation experiments were performed with multicomponent mixtures of five esters, one aldehyde and four alcohols, all at ppm levels in aqueous solution. A model was developed for predictions of the influence of permeate pressure on the overall separation factor. The model includes the effect of concentration polarisation on the feed side of the membrane and its dependence on permeate pressure, the variation in separation properties of the membrane with permeate pressure and the properties of the individual permeants. The results support the theory that shrinking of modified silicone rubber membranes used for pervaporation reduces the separation ability of the membrane in terms of reduced hydrophobicity. In the case of very hydrophobic permeants, this effect causes a considerable reduction in the selectivity. (C) 2001 Elsevier Science B.V. All rights reserved.