Asymmetric PVDF hollow-fiber membranes for organic/water pervaporation separations

Asymmetric PVDF hollow-fiber pervaporation membranes, with an inner diameter of 0.05-0.06 cm, an outer diameter of 0.07-0.08 cm, and a dense layer (approximate to 3 mu m in thickness) on the inner fiber wall, have been fabricated and tested for the removal of ppm concentrations of organics from water. Membranes were made by air-drying the outside of the fibers for ca. 20 s and passing a fluid through the fiber bore. The set of casting conditions that produced the best hollow fiber, with a benzene separation factor of 1834 (for a 120 ppm benzene in-water feed solution at 25 degrees C and a downstream pressure of 0.025 atm) and a tensile strength 26.8 MPa, was a spinning solution of 25 wt% PVDF/30 wt% dimethylacetamide/45 wt% acetone and a bore fluid of 70 vol% water/25 vol% acetone/5 vol% dimethylacetamide. These membranes also effectively separated toluene, chloroform, and styrene from water. A small module containing 6-30 PVDF hollow fibers performed equally well for organic extraction from water with either a bore-side or shell-side feed when the feed-flow rate was sufficiently high to eliminate concentration polarization. Changes in organic flux and separation factor for variations in the organic feed concentration, downstream pressure, and temperature were qualitatively similar to those observed with asymmetric flat sheet PVDF pervaporation membranes.