Transesterification of soybean oil to biodiesel using CaO as a solid base catalyst

The most common catalysts for biodiesel production are homogeneous basic catalysts. In the present paper, a comparison is made of different basic catalysts (sodium methoxide, potassium methoxide, sodium hydroxide and potassium hydroxide) for methanolysis of sunflower oil. All the reactions were carried out under the same experimental conditions in a batch stirred reactor and the subsequent separation and purification stages in a decanter. The analytical methods included gas chromatography and the determination of fat and oil conventional parameters. The biodiesel purity was near 100 wt.% for all catalysts. However, near 100 wt.% biodiesel yields were only obtained with the methoxide catalysts. According to the material balance of the process, yield losses were due to triglyceride saponification and methyl ester dissolution in glycerol. Obtained biodiesel met the measured specifications, except for the iodine value, according to the German and EU draft standards. Although all the transesterification reactions were quite rapid and the biodiesel layers achieved nearly 100% methyl ester concentrations, the reactions using sodium hydroxide turned out the fastest.

In the conventional transesterification of fats/vegetable oils for biodiesel production, free fatty acids and water always produce negative effects, since the presence of free fatty acids and water causes soap formation, consumes catalyst and reduces catalyst effectiveness, all of which result in a low conversion. The objective of this study was, therefore, to investigate the effect of water on the yield of methyl esters in transesterification of triglycerides and methyl esterification of fatty acids as treated by catalyst-free supercritical methanol. The presence of water did not have a significant effect on the yield, as complete conversions were always achieved regardless of the content of water. In fact, the present of water at a certain amount could enhance the methyl esters formation. For the vegetable oil containing water, three types of reaction took place; transesterification and hydrolysis of triglycerides and methyl esterification of fatty acids proceeded simultaneously during the treatment to produce a high yield. These results were compared with those of methyl esters prepared by acid- and alkaline-catalyzed methods. The finding demonstrated that, by a supercritical methanol approach, crude vegetable oil as well as its wastes could be readily used for biodiesel fuel production in a simple preparation.