Crude FA obtained by acid hydrolysis of used frying oils was purified using a PDMS based composite membrane. During processing crude FA, FA preferentially permeated through the membrane increasing FA content from 86.5% to 90% while color compounds were substantially reduced (92.8%) improving the overall purity of FA. Studies with crude FA-used frying oil system showed that a good correlation existed between reciprocal of viscosity and permeate flux, both total as well as FA fluxes. As expected, FA flux showed a significant improvement in crude FA-hexane system besides improving the FA content to the extent of 97% without significantly affecting reduction of color (83-88%). Increase in pressure as well as hexane dilution improved the FA flux but marginally affected purity and color reduction. Preferential permeation was observed over a wide of hexane dilution almost throughout the entire range of X (weight ratio of FA in feed) but to a varying degree and the positive coupling was strong at very low and high concentrations of hexane and FA. A maximal J(FA) (FA flux) of 4.4 kg/(m(2) h) was obtained at X=0.3 demonstrating an improvement of 11 fold achievable with appropriate hexane dilution at 1 MPa. In both crude FA-used frying oil and crude FA-hexane systems, the mean molar total permeate flux [ln(j(m))] showed a linear dependency on the mean molar volume [V-m] of permeate, which is in agreement with solution-diffusion or regular solution model. Crown Copyright (C) 2010 Published by Elsevier B.V. All rights reserved.