Page 64 - Refining Biomass Residues for Sustainable Energy and Bioproducts

38 Refining Biomass Residues for Sustainable Energy and Bioproducts

requires an alcohol molecule which acts as a nucleophilic reagent that leads to the
hydrolysis of ester transesterification process involves alcoholysis of an ester in
which an alcohol is displaced away by another alcohol from an ester (Ma and
Hanna, 1999)(Fig. 2.4). This is a reversible reaction, so to increase the production
of end product, excess alcohol is added in order to shift the equilibrium to the right
side as governed by Le Chatelier’s principle. In general a catalyst (alkali or acidic)
is always required to improve the rate of reaction and further the production yield.
Lipase is a biological catalyst used in transesterification reaction, which has several
advantages over acid and alkali catalyst, but at a large scale, its application is not
viable and cost effective. Selection and application of a catalyst is mostly governed
by the inherent nature and content of FFA in the raw materials. Excess amount of
FFA content in the starting material limits the application of alkali catalyst in trans-
esterification. The transesterification reaction was first time described by Rochieder
before 1846, during the preparation of glycerol from castor oil by the process of
ethanolysis (Demirbas, 2009). Meanwhile, alcoholysis had been explored in many
countries. Several investigators have also studied the significant reaction conditions
and factors involved in the process of transesterification.
Transesterification takes place with or without catalyst by utilizing 1 or 2
degrees monohydric aliphatic alcohols containing one to eight carbons. Generally,
methanol, ethanol, propanol, butanol, and amyl alcohol are used, among which
alcohol, methanol, and ethanol are extensively employed. As compared to
methanol, ethanol is preferable, because it is produced from agricultural wastes,
which is green in nature and environmental friendly (Demirbas, 2005).
Nevertheless, methanol is also used due to its cost-effectiveness and good physical
and chemical properties such as polarity and shortest chain length. Triacylglycerols
are formed when long-chain fatty acids combine with glycerol molecules. By
transesterification process the component of fatty acids of triacylglycerols are
transformed into their relative methyl esters. The variables affecting the yield of
methyl ester include reaction temperature, pressure, lipids alcohol molar ratio, water

Figure 2.4 Catalytic transesterification of triglycerides.