Production of FAME from acid oil, a by-product of vegetable oil refining

Abstract

Simple alkyl FA esters have numerous uses, including serving as biodiesel, a fuel for compression ignition (diesel) engines. The use of acid-catalyzed esterification for the synthesis of FAME from acid oil, a by-product of edible vegetable oil refining that is produced from soapstock, was investigated. Soybean acid oil contained 59.3 wt% FFA, 28.0 wt% TAG, 4.4 wt% DAG, and less than 1% MAG. Maximum esterification occurred at 65°C and 26 h reaction at a molar ratio of total FA/methanol/sulfuric acid of 1∶15∶1.5. Residual unreacted species under these conditions, as a fraction of their content in unesterified acid oil, were FFA, 6.6%; TAG, 5.8%; and DAG, 2.6%. This corresponds to estimated concentrations of FFA, 3.2%; TAG, 1.3%; and DAG, 0.2%, on a mass basis, in the ester product. In an alternative approach, the acylglycerol species in soapstock were saponified prior to acidulation. High-acid (HA) acid oil made from this saponified soapstock had an FFA content of 96.2 wt% and no detectable TAG, DAG, or MAG. Optimal esterification conditions for HA acid oil at 65°C were a mole ratio of FFA/methanol/acid of 1∶1.8∶0.17, and 14 h incubation. FAME recovery under these conditions was 89% of theoretical, and the residual unesterified FFA content was approximately 20 mg/g. This was reduced to 3.5 mg/g, below the maximum FFA level allowed for biodiesel, by washing with NaCl, NaHCO₃, and Ca(OH)₂ solutions. Alternatively, by subjecting the unwashed ester layer to a second esterification, the FFA level was reduced to less than 2 mg/g. The acid value of this material exceeded the maximum allowed for biodiesel, but was reduced to an acceptable value by a brief wash with 0.5 N NaOH.