Applications of supercritical fluids for biodiesel production

Abstract

The direction to opt for renewable fuels has become essential due to the overconsumption of fossil fuels and their emissions challenging the safe and clean environment. Biodiesel, preliminarily the fatty acid alkyl esters, is resultant of transesterification of oils and fats with alcohols. Owing to the problems associated with the conventional biodiesel production in the presence of basic catalysts, non-catalytic supercritical processes eliminates mass transfer resistances, enhancing reaction rates approaching near complete conversion. Supercritical transesterification processes require temperature greater than critical temperature and pressure to attain the desirable biodiesel yields. High alcohol-to-oil ratio with temperatures higher than 300°C and residence times within minutes are used to maximize the biodiesel yields with methanol/ethanol for different oil feedstocks. Green technologies by implementing enzymatic catalysts such as lipases in supercritical carbon dioxide (SCCO2) have received attention due to enhanced interactions between the reactant molecules. The key findings of enzymatic transesterification in SCCO2 and their combination with ionic liquids are presented in this chapter along with the operating parameters at maximum biodiesel yields. To improve the biodiesel economy, products superior to glycerol can be synthesized by replacing conventional alcohols with other solvents. Solvents such as methyl acetate (MeOAc) and dimethyl carbonate (DMC) are applied to get the triacetin and glycerol carbonate, which have more economic value. The challenges involved in enzymatic and non-catalytic supercritical processes both in terms of operation and economics are discussed.