American Society of Agricultural and Biological Engineers

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Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  Transactions of the ASAE. Vol. 44(6): 1429–1436 . (doi: 10.13031/2013.7010) @2001
Authors:   M. Canakci, J. Van Gerpen
Keywords:   Alternative fuel, Biodiesel, Diesel, High free fatty acid, Rendered animal fat, Waste restaurant oil

Biodiesel is an alternative fuel for diesel engines consisting of the alkyl monoesters of fatty acids from vegetable oils or animal fats. Most of the biodiesel that is currently made uses soybean oil, methanol, and an alkaline catalyst. The high value of soybean oil as a food product makes production of a cost–effective fuel very challenging. However, there are large amounts of low–cost oils and fats such as restaurant waste and animal fats that could be converted to biodiesel. The problem with processing these low cost oils and fats is that they often contain large amounts of free fatty acids (FFA) that cannot be converted to biodiesel using an alkaline catalyst. In this study, a technique is described to reduce the free fatty acids content of these feedstocks using an acid–catalyzed pretreatment to esterify the free fatty acids before transesterifying the triglycerides with an alkaline catalyst to complete the reaction. Initial process development was performed with synthetic mixtures containing 20% and 40% free fatty acids, prepared using palmitic acid. Process parameters such as the molar ratio of alcohol, type of alcohol, acid catalyst amount, reaction time, and free fatty acids level were investigated to determine the best strategy for converting the free fatty acids to usable esters. The work showed that the acid level of the high free fatty acids feedstocks could be reduced to less than 1% with a 2–step pretreatment reaction. The reaction mixture was allowed to settle between steps so that the water–containing alcohol phase could be removed. The 2–step pretreatment reaction was demonstrated with actual feedstocks, including yellow grease with 12% free fatty acids and brown grease with 33% free fatty acids. After reducing the acid levels of these feedstocks to less than 1%, the transesterification reaction was completed with an alkaline catalyst to produce fuel–grade biodiesel.

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