Abstract
Research and development of biofuels from renewable resources are now expanding from oxygenated transportation fuels into other areas such as renewable diesel. Oxygen removal, or deoxygenation, to produce renewable diesel, is a logical way to overcome the drawback of biofuel's low energy density due to the oxygen presence.
Generally, deoxygenation can be achieved by the following chemical reactions: decarboxylation, decarbonylation and hydrodeoxygenation. This study aims to investigate the effectiveness of catalytic decarboxylation without an external supply of hydrogen. Heterogeneous catalysts, including Pd/C, Pd/Al2O3, Pt/C, Ni/SiO2, Pt/Al2O3 and Raney Nickel, were tested on their efficiencies for decarboxylation under different operating conditions, including reaction time, operating temperature and pressure, solvent application, mixing intensity (or stirring rate) and catalyst application rate. Studies on the aforementioned catalysts revealed that the Pd/C was found to be the most reactive catalyst for the decarboxylation of stearic acid, the model fatty acid. Therefore, further studies on the effects of process parameters were conducted using the Pd/C catalyst.
Process parameters were investigated on decarboxylation catalyzed by the Pd/C catalyst systematically. Apparently, the reaction temperature significantly affects the reactant conversion rate and the product yield. The conversion was increased from 54%mol at 265°C to approximately 98%mol at 300°C after one hour of reaction. In general, the decarboxylation rate of stearic acid increases as the concentration of catalyst in the reactant mixture increases. However, this effect levels off when the catalyst concentration is 8%wt or higher. Additionally, as the solvent to reactant mass ratio decreases, the reaction takes longer to complete. Experiments have found that the effects of operating pressure and mixing intensity were negligible under the conditions of investigation, therefore they were kept at constant at 250 psi and 500 rpm, respectively.
In optimizing the process conditions for the renewable diesel production, methyl stearate is chosen as the model compound for fatty acid esters. Based on a 23 full factorial central composite design (CCD) for response surface methodology (RSM), sixteen set of experiments were performed by varying temperature, solvent to reactant mass ratio (sRatio) and reaction time, which are the most influential operating parameters in decarboxylation of fatty acids. The experimental results were fitted to a second-order polynomial model using multiple regression analysis and examined statistically. The optimum process conditions for maximum product yield were obtained as temperature 355°C, sRatio 62:38, and reaction time 187 min, which corresponds to an experimental heptadecane yield of 82.38 %mol, and was close to the expected yield of 85.00 %mol.