Model Feed for Hydrotreating of Fat for Biodiesel Production

Biodiesel production by the transesterification of oils and fats with an alcohol to fatty acid alkyl esters is rapidly increasing worldwide. Plant oils are usually suited for transesterification, but feedstocks from waste products like trap greases and abattoir wastes are difficult to react due to high contents free fatty acids (FFA).

These must be removed or esterified, and also salts, water and organic impurities like sterols may further complicate the process. Fatty feedstocks could also be converted to diesel via deoxygenation of the fats by hydrotreating. By reacting the feedstock over a catalyst with hydrogen at moderate pressures and temperatures over 300°C, hydrogenation or decarboxylation to alkane products results.[1-3] This strategy fits well with processes at petroleum refineries and has already been industrialised [4], and it could be applied together with hydrotreating of eg. vacuum gas oil.[1,5] The reaction was followed by using a model feed: 0.81 g tripalmitin and 0.09 g oleic acid was mixed with 8.1 g n-tetradecane as solvent and 0.03 g each of n-dodecane and n-docosane.

The feed was reacted with 15 to 40 bar H2 over 0.2 g of 5 wt% Pt/γ-Al2O3 catalyst in a closed, stirred autoclave at temperatures between 250 and 375°C, and samples were taken out for GC analysis after 1, 2, 5, and 20 h. Yields of pentadecane to octadecane were used to quantify the conversions of FFA and tripalmitin by either route as shown in Fig. 1: Fig. 1: Hydrotreating pathways from oleic acid and tripalmitin Decarboxylation gave CO2 as a byproduct, while hydrogenation of oxygen functionalities yielded H2O.

Propane from the glycerol backbone of tripalmitin resulted as by-product. This procedure allows for monitoring and distinguishing hydrogenation (yielding hexadecane and octadecane) from decarboxylation (yielding pentadecane and heptadecane) of triglyceride and FFA respectively. Even in this hydrogen-rich atmosphere, the dominant reaction above 300ºC was decarboxylation of the acid and ester functionalities, limiting H2 consumption, and complete conversion resulted within a number of hours.

At 375°C isomerisation and C-C-scission of alkanes became marked at reaction times above 5 h, representing a practical upper temperature limit for the use of this model mixture. The reaction was also tested over 5 wt % Ni and Pd on γ-Al2O3, and while Pd gave a little higher conversion than Pt, Ni resulted in lower conversions and a much higher degree of hydrogenation than with the Pt catalyst.

This protocol represents a facile method of studying hydrotreating of waste fats and oils for biodiesel production, which may be a viable alternative to current dominating transesterification technology. 1. Huber, G.W., O’Connor, P. and Corma, A., Appl. Catal. A. 329 (2007) 120 2. Kubickova, I., Snåre, M., Eränen, K., Mäki-Arvela, P. and Murzin, D.

Yu., Catal. Today, 106 (2005) 197 3. Mäki-Arvela, P., Kubickova, I., Snåre, M., Eränen, K. and Murzin, D. Yu., Energy Fuels, 21 (2007) 31 4. Mikkonen, S., Hydrocarbons Process., February (2008) 63 5. Stumborg, M., Wong, A. and Hogan, E., Bioresour. Technol. 56 (1996) 13