For biodiesel production, transesterification is applied for vegetable oils (Reaction 4.7.1) in which triglycerides, the esters of glycerin with fatty acids, and free fatty acids exist. Generally, oils are mixed with methanol using alkali catalyst such as potassium hydroxide or sodium hydroxide, and its mixture is stirred at 60-70oC for 1 h. After the reaction, the lower and upper portions are phase-separated with glycerin in the lower and estrified products in the upper which is washed to be fatty acid methyl esters as biodiesel. Since free fatty acids are contained in the waste oils, they can react with a catalyst to produce saponified products (Reaction 4.7.2), thus reducing the yield of biodiesel.

Due to such a disadvantage of the alkali catalyst method, it is reported that free fatty acids are first esterified by acid catalyst, followed by alkali catalyst method. As non-catalytic methods, ion-exchanged resin catalyst method, lipase catalyst method and supercritical
methanol method are proposed. In addition, to promote efficiency of esterification, the two-step supercritical methanol method is also proposed by hydrolyzing triglycerides with subcritical water followed by esterification of fatty acids with methanol in its supercritical state (Reactions

4.7.3

and 4.7.4, Fig. 4.7.1). This process is the only one to handle the low-grade waste oils with w