Integral asymmetric fluoropolymer membranes for the removal of organic compounds from dilute aqueous solutions
Description
An in-depth investigation of integral asymmetric poly(vinylidene fluoride) (PVDF) membranes has been carried out for the extraction of organic compounds from dilute aqueous solutions. Flat sheet membrane performance for low and high-boiling non-polar organic components was excellent, with separation factors as high as 4,900 and high organic fluxes. Polar organic components 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 surface 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, on membrane performance was quantified. Although the water flux through the resulting membrane 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. Water fluxes during benzene/water separation correlated with increasing pore size, indicating that such pores were providing pathways for water movement across the hydrophobic 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 pore flow or membrane distillation mechanism. The total time for asymmetric PVDF flat sheet 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 Asymmetric PVDF hollow fiber membranes, with an internal diameter of 0.05-0.06 cm, an outside diameter of 0.07-0.08 cm, and a dense layer (approximately 3 $\mu$m in thickness) on the inner fiber wall have been fabricated and tested for the removal of ppm concentration of organics from water. Membranes were made by air drying the outside of the fiber for about 20 seconds and passing a fluid through the fiber bore. The set of casting conditions that produced the best hollow fiber membranes, with a benzene separation factor of 1,834 (for a 120 ppm benzene-in-water feed solution at 25$\sp\circ$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 separated effectively 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 solution flow rate was sufficiently high to eliminate concentration polarization. Changes in organic flux and separation factor for variations in the organic feed concentration, temperature and downstream pressure were qualitatively similar to those observed for asymmetric PVDF flat sheet membranes