The Conversion of Low Grade Tallow into Biodiesel-Grade Methyl Ester

The conversion of low grade tallow with free fatty acid contents of 11.0-26.0% into biodiesel-grade methyl ester was investigated on a laboratory scale. Methanolysis of low grade tallow could not be carried out by the standard base-catalysed process, because the high free fatty acid content of low grade tallow caused incomplete methanolysis and prevented phase separation. The problem was able to be partly overcome by adding more potassium hydroxide to neutralise the free fatty acids, 50-100% more methanol and some water to promote phase separation. However, even with the modified method only about 55% of the starting material could be recovered as methyl ester and this fell to 35 and 14% when the free fatty acid content of tallow increased to 20 and 26%, respectively. It was shown that yield losses were caused mainly by the dissolution of methyl ester in the glycerol phase, and to a lesser extent to hydrolysis of the triglycerides. In order to prevent yield losses caused by the dissolution of methyl ester in the glycerol phase the possibility of esterifying free fatty acids either before or after base catalysed methanolysis was investigated. Both methods involving methanolysis and esterification gave good conversions of low grade tallow to methyl ester, and in each case yields above 90% were obtained by relatively simple processes.