Optimizing production of ethyl esters of grease using 95% ethanol by response surface methodology

Abstract

Previous research suggested that ethyl esters derived from recycled restaurant grease might be a potential source of biodiesel. Accordingly, response surface methodology (RSM) was employed to optimize reaction parameterstemperature, time, level of lipase, mole ratio of reactants-in the PS-30 lipase-catalyzed transesterification reaction of grease to ethyl esters using 95% ethanol. The regression equation obtained by a modified central composite design of RSM predicted optimal reaction conditions of 38.4°C, 2.47 h, 13.7 wt% lipase (PS-30), and a mole ratio of grease to ethanol of 1:6.6. Under these conditions the predicted optimal percentage ethyl ester yield was 85.4%. Subsequent experiments using the predicted parameter combinations indicated a trend where experimental percentage yields of ethyl ester were consistently lower than predicted values. In an effort to improve the experimental yield of esters, a second portion of PS-30 lipase was added without success; however, the addition of 5% SP435 one hour after the start of the initial reaction increased the yield of esters to >96%. Neither lipase PS-30 nor lipase SP435 alone, however, gave the RSM-predicted yield of ethyl esters.Paper no. J8965 in JAOCS 76, 517-521 (1999).There is increased interest in alternative fuels derived from renewable resources rather than nonrenewable petroleum. Among renewable alternative fuels, biodiesel-simple alkyl esters (primarily methyl and ethyl) of natural fats and oilshas attracted much research interest because of its additional potential to reduce pollutants emitted by heavy-duty diesel engines (1-6). Previously, we demonstrated that selected monoalkyl esters might be used as biodiesel and could be produced by lipase-catalyzed transesterification of fats and oils with anhydrous alcohols in a solvent-free environment (7). Compared to the traditional alkali-catalyzed chemical reactions, the use of lipases in production of biodiesel may have certain advantages, such as no by-products, ease of product recovery, milder reaction conditions and catalyst recyclability. In addition, lipase use can overcome difficulty in transesterifying materials such as recycled restaurant grease, which may contain high levels of free fatty acids. Our recent research on low-temperature properties and diesel engine performance of selected monoalkyl esters derived from tallow and restaurant grease strongly suggested that ethyl esters of grease (prepared with absolute ethanol) might be an excellent biodiesel candidate (8). We found that ethyl esters of grease (ethyl greasate) had low-temperature properties-such as cloud point, pour point, cold filter plugging point and low-temperature flow test-which closely resemble methyl soyate, the predominant form of biodiesel currently marketed in the United States. Moreover, short-term diesel engine performance and emission tests in a matched dual-cylinder diesel engine indicated that a 20% blend of ethyl greasate in No. 2 diesel fuel had adequate fuel properties (8). Results from this work showed that this fue...

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