Ultrafiltration of vegetable oils - Degumming by polymeric membranes

In this work, four different membranes synthesized in our laboratories have been tested for their permeation flux, phospholipid retention and stability in hexane during membrane degumming of crude soybean oil. Membranes were made of three different polymeric materials, i.e. Polyvinylidenfluroide (PVDF), Polyethersulfone (PES) and Polysulfone (PSf), and prepared by the phase inversion process. Raw membranes were characterized by the molecular weight cut-off (MWCO) and the water permeability, L-h,L-w. Ultrafiltration (UF) of an oil-hexane miscella was performed in a stirred dead-end UF cell, pressurized with N-2. All membranes were soaked in solvent of decreasing polarities to minimize the action of solvent on pore size. Each membrane was tested with pure solvent first and the membrane permeability to pure hexane, L-h,L-h, was determined. The degumming experiments were carried out with a 25% crude soybean oil-hexane mixture right after the pure solvent test. The ratio (L-h,L-h/L-h,L-w) is used to indicate the degree of change in membrane structure due to the organic solvent. Results show that PVDF is more stable with hexane than PES and PSf In addition to membrane material, pore size influences membrane stability also. Small pore sizes give more stable membranes. During degumming, a sharp decrease in the permeate flux with lime occurs at the beginning of the permeation process. This behaviour is explained in terms of concentration polarization effects and internal fouling. PES and PSF membranes have a larger initial decrease than PVDF ones. (C) 2001 Elsevier Science B.V. All rights reserved.