Separation of dilute organic/water mixtures with asymmetric poly(vinylidene fluoride) membranes

An in-depth investigation of integral asymmetric poly(vinylidene: fluoride) (PVDF) membranes has been carried out for the extraction of polar and non-polar organic compounds from dilute organic-in-water feed solutions. Membrane performance for low and high-boiling non-polar organic feed components was excellent, with separation factors as high as 4900 and high organic transmembrane fluxes. Polar organic feed components such as ethanol and acetone were also separated effectively from water but the separation factors were lower than non-polar organics. There was no change in membrane performance when either the dense or porous face of a PVDF membrane contacted the feed solution as long as the feed solution flow rate was sufficiently high. The effect of membrane preparation conditions, such as casting solution composition, air humidity and temperature during film drying, and the molecular weight of PVDF, on membrane performance was quantified. Although the water flux through the resulting films changed significantly, the organic (benzene) flux was essentially independent of the fabrication method. Variations in casting conditions also changed the mean diameter of a small number of pores in the dense layer of PVDF membranes. Transmembrane water fluxes during benzene/water separations correlated with increasing pore size, indicating that such pores were providing pathways for water movement across the hydrophobic PVDF dense layer. Based on benzene swelling and diffusivity measurements in homogeneous PVDF films, pores in the dense layer of an asymmetric membrane control permeate enrichment by either a membrane distillation, pore flow, or capillary condensation mechanism. The total time for asymmetric membrane casting was reduced from 69-72 min to 7 min, while maintaining a high organic (benzene) separation factor with only a small drop in transmembrane benzene flux.