Ultrafiltration polymeric membranes for the purification of biodiesel from ethanol

Abstract

The emerging industry of biodiesel obtained from ethanol and vegetable oil, presents a number of difficulties in the purification step of the obtained esters. Current processes use large amounts of water to remove the by-product of the transesterification reaction. This paper presents an alternative to the washing process of biodiesel, which involves the use of ultrafiltration membranes resistant to solvents and alkalis. For that, two lab-made hydrophobic polymeric membranes were prepared from poly(vinylidene fluoride) and poly(sulfone) as main materials. The synthesized membranes were used to reduce glycerol content of biodiesel solutions obtained from semi-refined soybean oil and ethanol, catalyzed by sodium hydroxide. The poly(vinylidene fluoride) membrane reached a glycerol rejection up to 67% (at 30 °C and 5 bar) from a biodiesel sample with 0.5 wt % of water added. Under the same operation conditions, the poly(sulfone) membrane showed a lower separation performance, with glycerol rejection of 48%. Moreover, stability tests demonstrated that poly(vinylidene fluoride) membrane was more stable to solvents, alkalis and temperature, compared to the poly(sulfone) membrane. A remarkable flux recovery after more than 45 cycles of biodiesel permeation (≈105 hs) was verified in poly(vinylidene fluoride) membrane, indicative of the high stability and low fouling of the membrane.