THE EFFECT OF CONCENTRATION POLARIZATION ON THE SEPARATION OF VOLATILE ORGANIC-COMPOUNDS FROM WATER BY PERVAPORATION

Concentration polarization dominates the separation of dissolved volat ile organic compounds from water by pervaporation. This is particularl y true with hydrophobic organics, such as toluene and trichloroethylen e, for which concentration polarization is severe even in highly turbu lent membrane modules. With these compounds, measured separation facto rs can be 10 to 20% of the intrinsic separation factors in the absence of concentration polarization. As a result of concentration polarizat ion, unexpected permeation properties are observed. For example, the o rganic flux is independent of membrane thickness over a wide range, wh ereas the water flux decreases with membrane thickness. Consequently, thicker membranes are preferred over thinner ones. Also, the organic f lux is relatively independent of permeate pressure over a wide range, whereas the water flux decreases as the permeate pressure increases. T his means that the separation performance improves as the driving forc e across the membrane decreases, contrary to normal membrane behavior. These and other consequences of concentration polarization are descri bed in this paper. (C) 1997 Elsevier Science B.V.