(670f) Effect of Hydrogen On Catalytic Decarboxylation of BROWN Grease to Green Diesel

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.