Catalytic hydrocracking of waste vegetable oil transition metal-based catalysts : selective production of jet fuel range alkanes.
The performance of transition metal-based catalysts on amorphous supports has been investigated for the high pressure (120 bar) hydrocracking of waste vegetable (cooking) oil in a fixed-bed tubular reactor between 400-450 °C. The study focused on the effect of the operating parameters (reaction temperature, type of transition metal catalyst and amorphous support, the sulphided state of the catalyst and the use of regenerated catalyst) on the yield of transportation fuel n-alkanes (C5-C18), and the primary product, kerosene (jet fuel) range n-alkanes using the One-Variable-At-A-Time approach. The objectives included characterising the feedstock and catalysts, determining the optimum catalyst and operating conditions to produce kerosene range n-alkanes and to estimate activation energies through a reaction kinetic study. Comparative studies of the results from commercially produced Ru/Al2O3 and Ni/Al2O3 catalysts and laboratory prepared Ni-Mo/SiO2 and Co-Mo/SiO2 were undertaken. All tested catalysts were effective in achieving kerosene range n-alkanes in the liquid product while achieving oil conversions > 62 wt.%. While the laboratory catalysts only had liquid and gas products, the commercial catalysts also had a waxy residue product indicating a lower hydrotreating activity on the metallic sites. Furthermore, both laboratory catalysts had higher n-alkane yields indicating a higher activity for hydrocracking reactions on the acidic sites on the SiO2 than the acidic sites on the Al2O3 support. The best yield of kerosene range n-alkanes obtained from the experimental design was 5.84 wt.% using the fresh Ni-Mo/SiO2 catalyst at 450 °C, with an oil conversion of 90.32 wt.%. An increase in oil conversion and liquid n-alkane products with an increase in reaction temperature for all tested catalysts indicated that hydroprocessing (hydrotreating and hydrocracking) reactions are favoured at higher temperatures. Furthermore, sulphiding the catalyst prior to use was found to greatly increase the catalyst activity in promoting hydroprocessing reactions. Results from the use of regenerated catalysts show a small decrease in the yield of kerosene range n-alkanes when compared to the corresponding fresh catalyst. This suggests that regeneration of the spent catalyst and subsequent re-use may be a feasible option. A simple kinetic model of the hydroprocessing reactions was developed and kinetic parameters were identified by regression of the experimental data. The regenerated Ni/Al2O3 catalyst had the lowest estimated activation energy of 14.37 kJ/mol, while the fresh Ni-Mo/SiO2 catalyst had the highest estimated activation energy of 51.75 kJ/mol from the studied catalysts.