Biodiesel via hydrotreating of fat

Biodiesel production via transesterification to fatty acid alkyl esters is rising rapidly worldwide due to the limited availability of fossil resources and the problems of global warming. Often, however, the use of 2nd-generation feedstock like animal waste fat and trap greases etc. is made difficult especially by the high amount of free fatty acids (FFA) in these raw materials. Another way to utilise such feedstock could be through the complete deoxygenation of the fuel, i.e. by hydrogenation and decarboxylation of the fat itself. Hydrotreating of fats is a procedure very well suited for existing petroleum refineries, and could be applied in mixture with existing hydrotreating of for instance vacuum gas oil [1,2]. As is shown in literature, both hydrogenation and decarboxylation of the fat may occur under such circumstances [2-4]. 9 g of a model mixture of 3 % oleic acid (C18:1), 7 % tripalmitin (C16:0) and 90% n-tetradecane (C14) in molar ratios was hydrotreated by 0.2 g of a 5%wt Pt/Al2O3 catalyst. The mixture was treated with 10 to 30 bar H2 in an autoclave at temperatures between 250 and 375°C, and samples for GC analysis was taken out after 1, 2, 5 and 20 hours to track the extent of reaction. Thus, the yields of pentadecane (C15) to octadecane (C18) could be monitored by GC analysis, thus indicating the conversions of FFA and tripalmitin. This procedure makes it possible to monitor and distinguish hydrogenation (resulting in hexadecane, C16, and octadecane, C18) from decarboxylation (resulting in pentadecane, C15, and heptadecane, C17) of triglyceride and fatty acid. Even in this hydrogen-rich atmosphere, the dominant reaction above 300ºC is decarboxylation of the acid and ester functionalities, thus limiting the consumption of hydrogen. Temperatures below 300°C are not well suited for the treating, as primarily the saturation of oleic acid to stearic acid (C18:0) is observed, with low conversions of fatty acid and triglyceride. [1] Stumborg, M., Wong, A. and Hogan, E., Bioresour. Technol. 56 (1996) 13 [2] Huber, G.W., O’Connor, P. and Corma, A., Appl. Catal. A. 329 (2007) 120 [3] Kubickova, I., Snåre, M., Eränen, K., Mäki-Arvela, P. and Murzin, D. Yu., Catal. Today, 106 (2005) 197 [4] Mäki-Arvela, P., Kubickova, I., Snåre, M., Eränen, K. and Murzin, D. Yu., Energy Fuels, 21 (2007) 31