THE ROLE OF BOUNDARY-LAYERS IN THE REMOVAL OF VOLATILE ORGANIC-COMPOUNDS FROM WATER BY PERVAPORATION

Removal of volatile organic compounds (VOCs) from water by pervaporati on is dominated by boundary layer effects (concentration polarization) . A simple analysis shows these effects to be much more severe in perv aporation than in ultrafiltration and reverse osmosis because of the h igh VOC enrichment that can be obtained by pervaporation. In pervapora tion, the concentration of solute at the membrane surface is often one -tenth or less of the concentration in the bulk solution because of th e huge concentration polarization effect. In this paper, we present a rigorous treatment of concentration polarization using the resistances in-series model and include the contribution of convective flow to tr ansport in the boundary layer. The resulting general expression is val id for compounds that are enriched in the permeate as well as for comp ounds that are depleted in the permeate. The effects of operating cond itions on pervaporation performance are discussed, and compared to dat a obtained with spiral-wound modules. Experimental data demonstrate th at increasing the permeate pressure in pervaporation does not necessar ily reduce the VOC flux although it reduces the driving force for perm eation.