Production of Fatty Acid Methyl Esters via the In Situ Transesterification of Soybean Oil in Carbon Dioxide-Expanded Methanol

Authors: Wyatt, Victor; Haas, Michael

Submitted to: Journal of the American Oil Chemists' Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/26/2009
Publication Date: 10/1/2009
Citation: Wyatt, V.T., Haas, M.J. 2009. Production of Fatty Acid Methyl Esters via the In Situ Transesterification of Soybean Oil in Carbon Dioxide-Expanded Methanol. Journal of the American Oil Chemists' Society. 86(10):1009-1016.

Interpretive Summary: Fatty acid methyl esters, more commonly known as biodiesel, are a renewable fuel that is becoming widely accepted as a replacement for petroleum-based diesel fuel. It is domestically-produced from triacylglycerides (fats and oils) in the presence of an alcohol and a base. When using refined fats and oils as feedstocks their high cost can constitute as much as 80 percent of the total costs of biodiesel production. Therefore, it is desirable to identify low-cost feedstocks or to find ways to lower fuel production costs when using refined feeds. In an attempt to lower production costs a method termed in situ transesterification was developed which allows biodiesel to be produced directly from lipid-bearing materials. This method eliminates the costs associated with prior isolation and purification of the oil. Although biodiesel can be successfully produced by in situ transesterification, the alcohol requirement can be substantially greater than that of the standard protocol. Here we attempted to lower the alcohol requirement of the in situ process by conducting the reaction in the presence of pressurized carbon dioxide. The fatty acid methyl esters that constitute biodiesel were successfully produced in this manner, and the alcohol requirement was reduced by 33 percent. The introduction of carbon dioxide into the system also increased the rate of reaction by as much as 2.5 fold.

Technical Abstract: The production of fatty acid methyl esters (FAME) by direct alkali- and acid-catalyzed in situ transesterification of soybean flakes in CO2-expanded methanol was examined at various temperatures and pressures. Attempts to synthesize FAME from soy flakes via alkaline catalysis, using sodium methoxide as a reactant, in gas-expanded methanol were unsuccessful. However, performing the reactions in 54 mL of a 1.2 N sulfuric acid-methanol mixture expanded to 50% mole fraction with CO2 resulted in an 88.3(+/-1.5) % conversion of the triacylglycerol (TAG) in 22.5g soy flakes to FAME within 10 hours. Decreasing the total volume of the liquid phase by one-third from 54 mL to 36 mL, while keeping the mmol of acid and all other variables constant, resulted in an 88.2(+/-5.0)% conversion of triglyceride to FAME. Our estimation by quantitative HPLC analysis showed that 99.8% and 92.3% of the total mass balance for FAME, TAG, and free fatty acid (FFA) could be accounted for in the reactions performed at the liquid-phase volumes of 54 and 36 mL, respectively. The overall results show that the introduction of CO2 into the system increases the rate of reaction by as much as 2.5 fold in comparison to control reactions without CO2. Additionally, we observed a direct correlation between FAME production yield and the concentration of acid catalyst. We also determined by nitrogen analysis that at least 82.9% of protein is retained in the lipid-free soy flakes after acid-catalyzed in situ transesterification in CO2-expanded methanol.