Synthesis Of Biodiesel In Supercritical Fluids

Abstract

Alternative fuels are becoming important due to diminishing fossil fuel reserves and the environmental hazards associated with exhaust gases. Biodiesel is an attractive alternative fuel because it is environmentally friendly and can be synthesized from edible/non-edible oils. Though several methods are available for synthesis, transesterification is the preferred route for biodiesel synthesis. The current techniques for transesterification of the oils to biodiesel are based on acid/alkali catalysis. However, these methods do not work for oils with high free fatty acid content and also require an additional downstream step for separation of catalyst from the product.

In this work, we investigate the synthesis of biodiesel by two techniques: in supercritical methanol and ethanol without any catalyst; in supercritical carbon dioxide using enzyme as catalyst. In the first technique, the transesterification of refined grade edible oil like sunflower oil, palm oil and groundnut oil, and crude non-edible oils like Pongamia pinnata and Jatropha curcas was investigated in supercritical methanol and ethanol without using any catalyst from 200 oC to 400 oC at 200 bar. The variables affecting the conversion during transesterification, such as molar ratio of alcohol to oil, temperature and time were investigated in supercritical methanol and ethanol. The conversion to biodiesel increased from 30-40% at a molar ratio of 10 to 80-85% at a molar ratio of 45 to 50. Very high conversions (96-97%) were obtained in supercritical methanol and ethanol within 30 minutes at 350 oC. The kinetics of the reaction was modeled, the rate coefficients and activation energies were determined.

As an enzymatic transesterification in organic solvents is mass transfer limited, supercritical fluids are found to be a better alternative because of their gas like diffusivity. Among potentially interesting solvents for enzymatic synthesis, carbon dioxide is the most obvious choice in supercritical fluids, because it is non-toxic, nonflammable and easily available. Because the products and the enzyme do not dissolve in carbon dioxide at room conditions, separation can easily be achieved by reduction of pressure Thus, the enzymatic transesterification for production of biodiesel in supercritical fluids under mild conditions is of commercial interest. Therefore, biodiesel was synthesized using immobilized enzyme (Lipase, Novozym-435) in supercritical carbon dioxide. The effect of reaction variables like temperature, molar ratio, enzyme loading and kinetics of the reaction was investigated. The conversion to biodiesel was found to be lower in supercritical carbon dioxide than that in supercritical alcohol.