Transesterification of Lipids with Supercritical Alcohols for Biofuels Production
Since the early studies of Saka and Kusdiana on the noncatalytic transesterification of vegetable oils with supercritical alcohols, a large collection of reaction data for different vegetable oils and lipid-containing materials, with methanol or other alcohols, has been reported in the open literature. The wealth of information gathered in the last decade will be discussed to present general guidelines to carry out these processes under optimum conditions regarding alcohol/fatty oil molar ratio, operating temperature and pressure and contact time. Very promising have been the studies that indicate that neither water nor free fatty acids interfere with the transesterification process. On the contrary, it seems that their presence in the mixture fed to the reactor is beneficial for the transesterification reaction. This property and the absence of a catalyst simplifies the biodiesel purification, produces better quality glycerin, and much less waste than the conventional catalytic processes. The experimental studies have progressed from closed batch reactors, to windowed reactors, to continuous reactors. The problems associated to reaction system density, phase conditions, thermodynamic modeling, and effect of cosolvents have all been given considerable attention. The conditions that cause degradation of the fatty esters have also been investigated. As an additional application the process has been used in the recovery and conversion to biodiesel of the lipids contained in algae cells. Preliminary economic evaluations of large scale production processes have been made. These evaluations indicate that this technology has economic potential and is environmentally benign.
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