Investigation of a model feed for hydrotreating of oils and fats

Biodiesel production is rising worldwide due to soaring oil prices, environmental concerns and desire to increase security of fuel supply. The most common way to produce biodiesel is by transesterification of oils and fats; however it can also be produced by the hydrotreatment of oils and fats to long-chain alkanes [1,2]. In this way feedstock of very low e.g. tall oil, a waste residue from the Kraft process, may be utilised as a feedstock.[3] Such feedstock contains a high percentage of free fatty acids (FFAs) and organic impurities, which are unwanted in transesterification processes. Hydrotreating for biodiesel production has been studied using a model feed: A mixture of oleic acid and tripalmitin was dissolved in n-tetradecane and treated at 250-375°C and 0-40 bars of H2 over a Pt/γ-Al2O3 catalyst in a 50 ml stirred autoclave. Under these conditions the reactants may undergo two different reaction pathways: Full hydrogenation to alkanes, which also produces water, or decarboxylation/decarbonylation to alkanes, which produces CO2 or CO.[4,5] These two pathways are depicted in Figure 1. Figure 1. Hydrotreating pathways from oleic acid and tripalmitin Temperatures below 300°C are not suited for the process, but between 300°C and 375°C the conversion increases with temperature. Conversion did not have a distinct dependence of pressure; however small amounts of hydrogen are necessary to ensure that all products were saturated with hydrogen. When the reaction ran in N2 gas solely conversion did take place; however, hydrogen was abstracted by the noble metal catalyst from the carbon chains of the feed to introduce alkene functionalities in the products. An intermediate in the reaction from tripalmitin is detected during the course of the reaction, namely palmitic acid. It thus appears that glycerides react to alkanes via their corresponding FFAs, either via a hydrolysis or a partial hydrogenation path. Biodiesel produced in this way have superior fuel characteristics compared to the transesterification biodiesel, especially the cetane number, cold properties and storage stability.[1] [1] S. Mikkonen, Hydrocarbons Process., February 2008, 63 [2] G.W. Huber, P. O’Connor. and A. Corma, Appl. Catal. A. 2007, 329 120 [3] M. Stumborg, A. Wong and E. Hogan, Bioresour. Technol. 1996, 56, 13 [4] I. Kubickova, M. Snåre, K. Eränen, P. Mäki-Arvela and D. Yu Murzin, Catal. Today, 2005, 106, 197 [5] P. Mäki-Arvela, I. Kubickova, M. Snåre, K. Eränen and D. Yu Murzin, Energy Fuels, 2007, 21, 31