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 pervaporation is dominated by boundary layer effects (concentration polarization). A simple analysis shows these effects to be much more severe in pervaporation than in ultrafiltration and reverse osmosis because of the high VOC enrichment that can be obtained by pervaporation. In pervaporation, the concentration of solute at the membrane surface is often one-tenth or less of the concentration in the bulk solution because of the 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 transport in the boundary layer. The resulting general expression is valid for compounds that are enriched in the permeate as well as for compounds that are depleted in the permeate. The effects of operating conditions on pervaporation performance are discussed, and compared to data obtained with spiral-wound modules. Experimental data demonstrate that increasing the permeate pressure in pervaporation does not necessarily reduce the VOC flux although it reduces the driving force for permeation.