Production of Renewable Diesel Fuel from Biologically Based Feedstocks
Van Gerpen, Jon H.
He, B. Brian
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Renewable diesel is an emerging option to achieve the goal set by the Federal Renewable Fuel Standard of displacing 20% of our nation’s petroleum consumption with renewable alternatives by 2022. It involves converting readily available vegetable oils and animal fats to alkane hydrocarbons that can be considered to be drop-in replacements for petroleum-based fuel components. The objective of the project was to develop a model process to produce renewable diesel from triglyceride feedstocks, such as vegetable oils, in a bench scale facility. Specifically, this study was to investigate the process conditions that affect the catalytic decarboxylation of fatty acids and esters without an external supply of hydrogen. Several heterogeneous catalysts were tested for their effectiveness of oxygen removal via decarboxylation. Palladium on activated carbon (Pd/C) was found to be the most reactive catalyst, and hence was used in further investigations on the effects of process conditions, including reaction time, operating temperature and pressure, solvent application, mixing intensity and catalyst application rate. Studies revealed that the reaction temperature is the most influential process parameter affecting the reactant conversion rate and the product yield. When the operating temperature was increased from 265°C to 300°C, the reactant conversion was increased from 54%mol to approximately 98%mol after one hour of reaction. The catalyst application rate also affects the decarboxylation rate. However, this effect levels off when the catalyst concentration is 8%wt, i.e., further increase in catalyst application beyond 8% by weight did not increase the process efficiency of decarboxylation significantly. The solvent to reactant mass ratio is also important because it affects the process productivity. It was found that the effects of operating pressure and mixing intensity were negligible under the conditions of investigation. Once the most influential process parameters were identified, an optimization of the process conditions for renewable diesel production from methyl stearate, the model compound for fatty acid esters, was attempted based on a 23 full factorial central composite design (CCD). Experimental results revealed that there were no true optimal points on reactant conversion or desired product yield in the range of operating temperature (300°C - 340°C). Therefore, a conditional set of optimum process parameters were obtained instead. Under the pre-set targets of 85% product yield, the process parameters are a temperature of 355°C, a solvent to reactant mass ratio of 62:38, and a reaction time of 187 minutes. Experimental verification showed that this set of operating conditions led to a targeted product yield of 82.384.62%mol, very close to the expected 85%mol level. Experimental results showed that decarboxylation of mixed fatty acid methyl esters yielded a complex product mixture due to the presence of unsaturated methyl esters. Besides decarboxylation, other reactions, such as cracking of the unsaturated feedstock, also occurred. Further systematic investigation is needed to fully understand the chemical reactions and process parameters on the composition of the final product mixture.