267694 Effect of Hydrogen On Catalytic Decarboxylation of BROWN Grease to Green Diesel

Thursday, November 1, 2012: 2:10 PM
322 (Convention Center )
Elvan Sari1, Manhoe Kim1, Steven O. Salley1 and K. Y. Simon Ng2, (1)Chemical Engineering and Materials Science, Wayne State University, Detroit, MI, (2)Department of Chemical Engineering and Materials Science, Wayne State University, Detroit, MI

Brown grease (BG) is a mixture of waste fats and oils which may contain 50-100 % free fatty acid (FFA). BG is a potential low cost and non-food competing feedstock for biofuels. Due to its high FFA content, BG is a good candidate for decarboxylation reaction where the oxygen is removed as carbon dioxide, producing “green diesel.” This process is a low cost alternative to hydrodeoxygenation process. Approximately 75 % of BG consists of unsaturated FFAs with oleic acid having the largest portion (1). Decarboxylation of oleic acid studies showed that the hydrocarbon selectivity is low because the unsaturated C=C bond in the oleic acid tends to be saturated under the reaction conditions in the presence of H2 and decarboxylation of oleic acid yields to stearic acid and n-heptadecane as products (2). In the case of absence of H2 the key challenge is fast catalyst deactivation. We explored the conversion of brown grease to green diesel via a pre-hydrogenation/decarboxylation reaction route with minimum hydrogen consumption.  Results demonstrated that C=C bonds in FFAs can be easily saturated under 15 bar and 100 oC in the presence of Pd/C under continuous flow of 10 vol% H2 - 90 vol% Ar in a semi- batch reactor. The saturated FFA content in the BG was then converted to n-paraffins via decarboxylation by increasing the temperature to 300 oC. The main n-paraffin products consist of tridecane, pentadecane and heptadecane as a result of decarboxylation of BG FFAs. Due to the high unsaturation level of BG feedstock, formation of heavier products with carbon number more than 25 was observed. Pre-hydrogenation step helps to decrease heavier compounds yield and to increase olefinic C13-C18 compounds yield. H2- pretreatment also avoids C=C bond cleavage. Increasing H2 amount from 0.4/1 ratio of H2/BG to excess amount (4/1 ratio of H2/BG) increases the FFA conversion more than 2 times. While decarboxylation and decarbonylation reactions are the major pathways for pre-hydrotreated BG conversion, decarboxylation and dimerization are the major pathways for conversion of non-pretreated BG under stoichiometric H2 amount (H2/BG ratio of 2/1 mol/mol). Increasing BG initial concentration resulted n-paraffins selectivity to decrease and heavy compounds selectivity to increase. This is because the higher BG concentration in reaction atmosphere means the higher unsaturated FFAs concentration; therefore, more heavy compounds formation via dimerization. With a dilute solution of BG, side reactions are minimized and primary n-paraffins production increases. A continuous decarboxylation of BG over 5%Pd/C catalyst conducted in a fixed bed tubular reactor showed pretty stable behavior of BG decarboxylation for 4 days. However, decarboxylation reaction sites of Pd/C catalyst was partially poisoned by impurities in BG.

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