Hydroprocessing of new and waste soybean oil for obtaining biodiesel: An operational conditions study

[Display omitted] •Fatty acid concentration determines the biofuel type.•Hydrocarbon composition in biofuel depends on the type of soybean oil, new or waste.•Obtained biofuel mainly contains n-C15, n-C16, n-C17 and n-C18 hydrocarbons. A study of the operational conditions on the hydroprocessing of new and waste soybean oil to obtain biofuel using a NiMo/Al2O3 catalyst was here performed. Fatty acid composition, moisture, peroxide and acidity values of waste and new soybean oil were analyzed. The main differences between both oils are the peroxide value and the fatty acid concentration values. The optimization of operational conditions was performed in a fixed-bed continuous flow micro reactor. Five operational conditions were separately analyzed using new soybean oil: temperature, pressure, soybean oil/heptane and sulfur concentrations and catalyst mass. The results show that the triglycerides conversion and waxes formation mainly depend on temperature, soybean oil/heptane concentration and mass catalyst. The optimal operational conditions were the following: T = 380 °C, P = 40 bar, soybean-oil/heptane concentration = 0.104 mol/L, Dimethyl disulfide = 0.05 v/v% and catalyst mass = 0.1 g. Reaction products were analyzed by FTIR spectroscopy and gas chromatography. The soybean oil hydroprocessing was analyzed in both reaction steps: the triglyceride hydrogenolysis and fatty acid deoxygenation. In the hydrogenolysis step, the fatty acid with the highest concentration is the oleic acid in both new and waste oil. In the deoxygenation step, the obtained biofuel contains mainly n-C18, n-C17, n-C16 and n-C15 hydrocarbons. n-C17 is the hydrocarbon with the highest concentration in both new and waste soybean oil. Results show that the NiMo/Al2O3 catalysts favors the decarbonylation and decarboxylation mechanisms over the hydrocracking and hydrodeoxygenation in both new and waste oil. Finally, our results shows that through a detail analysis of the operational conditions, it is possible to obtain biofuel avoiding the waxes formation. This study will allow to correctly evaluate new catalysts and other oils on hydroprocessing reactions.