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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.

Recycled vegetable oil (RVO) is a relatively cheap raw material for biodiesel production, but biodiesel grade methyl ester yields from RVO were found to be considerably lower than those from pure plant oil. The present paper investigates sources of yield loss during methanolysis of RVOs with free fatty acids (FFA) contents of 0.4-3.3%, and makes suggestions for the improvement of methyl ester yields. Data presented here indicated that yield losses of methyl esters during methanolysis were due to triglyceride and methyl ester hydrolysis and to the dissolution of methyl esters in the glycerol phase. Hydrolysis of triglycerides and methyl esters seemed to be the only side reaction causing yield losses, and the amount of fatty acids from hydrolysis increased with concentration of the potassium hydroxide catalyst. Dissolution of methyl esters in the glycerol phase was probably caused by the detergent effect of potassium salts of fatty acids originating from FFA in the RVO and from triglyceride hydrolysis, and the amount of dissolved methyl esters increased with FFA content of the RVO. The FFA content of the RVO had no effect on hydrolysis, and the amount of triglycerides and methyl esters hydrolysed during methanolysis remained constant with increasing FFA content of the RVO.