**3.4 Transesterification (alcoholysis)**

Transesterification is a process that involves the reaction of triglycerides such as vegetable oil, with alcohol in the presence of a catalyst to produce 3 moles of fatty acid esters and one mole of glycerol [17]. Catalyst is used to increase the rate and yield of the reaction. The reaction is reversible. Excess alcohol is used to shift the equilibrium to the product side. Suitable alcohols such as methanol, ethanol, propanol, butanol and amyl alcohol are used for the transesterification reaction. Among these methanol and ethanol are most frequently used because of their low cost and physical and chemical advantages (polar and shortest chain alcohol). The fatty acid methyl ester (FAME) obtained by this process can be used as an alternative fuel for diesel engines [18]. The catalyst used for transesterification may be acid or base (homogeneous or heterogeneous) and lipase enzymes. Transesterification reaction depends on various factors such as catalyst concentration, nature of the feedstock, molar ratio of alcohol-oil, agitation

#### **Figure 4.**

*Transesterification reaction [20].*

#### **Figure 5.**

*Schematic representation of transesterification.*

rate, temperature, reaction time, amount of free fatty acids and moisture content [19]. Transesterification is a reversible reaction and proceeds by mixing the reactants under heat. In this process, 1 mole of triglyceride react with 3 moles of alcohol gives 3 mole of fatty acid alkyl ester and 1 mole of glycerol in a sequence of three reversible reactions where the triglyceride are converted to diglycerides and then to monoglycerides as shown in **Figure 4**. From each step, one molecule of alkyl ester is produced (**Figure 5**).
