Acidic catalyst using triglycerides with alcohol, the most commonly used is sulphuric acid and some authors prefer sulfonic acid. This type of catalyst gives very high yield in esters but the reaction is very slow, requiring almost always more than one day finishing.
Special processes are required if the oil or fat contains significant amounts of FFAs. Used cooking oils typically contain 2-7% FFAs and animal fats contain from 5-30% FFAs. Some very low quality feed stocks, such as trap grease, can approach 100% FFAs. When an alkali catalyst is added to these feed stocks, the free fatty acids react with the catalyst to form Soap and water as shown in the reaction below:
Up to about 5% FFAs, the reaction can still be catalyzed with an alkali catalyst but additional catalyst must be added to compensate for that lost to soap. The soap created during the reaction is either removed with the glycerol or is washed out during the water wash. When the FFA level is above 5%, the soap inhibits separation of the glycerol from the methyl esters and contributes to emulsion formation during the water wash. For these cases, an acid catalyst such as sulphuric acid can be used to esterifies the FFAs to methyl esters as shown in the following reaction:-
As shown in Figure 3, this process can be used as a pre-treatment to convert the FFAs to Methyl esters and thereby reduce the FFA level. Then, the low FFA pre-treated oil can be Trans-esterifies with an alkali catalyst to convert the triglycerides to methyl esters. As shown in the reaction, water is formed and, if it accumulates, it can stop the reaction well before completion. Kawahara and Ono propose allowing the alcohol to separate from the pre-treated oil or fat following the reaction. Removal
of this alcohol also removes the water formed by the esterification reaction and allows for a second step of esterification or proceeding directly to alkali catalysed trans-esterification. Note that the methanol-water mixture will also contain some dissolved oil and FFAs that should be recovered and reprocessed.
Jeromin et al. has described using acidic ion exchange resins in a packed bed for the pretreatment. Haas et al. have shown that acid-catalyzed esterification can be used to produce biodiesel from low-grade by-products of the oil refining industry such as soap stock. Soap stock is a mixture of water, soaps, and oil, is dried, sponified, and then esterifies with methanol or some other simple alcohol using an inorganic acid as a catalyst. The procedure relies on a large excess of alcohol and the cost of recovering this alcohol determines the feasibility of the process.
Lipases are enzymes used to catalyze some reaction such as hydrolysis of glycerol, alcoholysis and acidolysis, but it has been discovered that they can be used as catalyst for Transesterification and esterification reactions too. Biocompatibility, biodegradability and Environmental acceptability of the biotechnical procedure are the desired properties in Agricultural and medical applications. The extra cellular and the intracellular lipases are also able to catalyze the transesterification of triglycerides effectively.
Transesterification is the reaction of a lipid with an alcohol to form esters and a by -product glycerol. it is in principle the reaction of one alcohol displacing another from an ester, thermal coholysis (cleavage by an alcohol).The reaction is reversible and thus an excess of alcohol usually used to force the equilibrium to the product side. The stoichiometry for the reaction is 3:1 alcohol to lipids how ever in practice this is usually increased to 6:1 to increase product yield. A catalyst is usually used to speed up the reaction and may be basic, acid or enzymatic in nature. Transesterification of different types of oils, triglycerides react with an alcohol, generally methanol or ethanol, to produce esters and glycerine, catalyst is added to the reaction.