Fischer - Tropsch Synthetic Wax Mild Hydrocracking Catalyst Activity

The Fischer-Tropsch synthetic oils are basically made up of n-paraffins and smaller percentages of olefins. Because the Fischer-Tropsch synthetic oils have the characteristics of no sulfur, nitrogen, non-metals and low aromatics, they are environmentally-friendly fuels and chemicals. The LTFT waxes display high cetane numbers (>75) but very poor cold properties. Thus, the development and utilization of hydrocracking catalysts for maximizing high quality middle distillates is one of the effective ways in LTFT waxes upgrading.The activities of various catalysts were evaluated on a fixed-bed reactor with a model compound of 56# half-refined paraffin. First of all, the activity of FC-36 and FF-22 industrial hydrocracking catalysts was investigated, and then the process conditions of FC-36 catalyst were studied. Secondly, the hydrocracking activities of NiW catalysts with amorphous SiO2-Al2O3 as supports were tested and the effects of supports on the activity were discussed. In this series of catalysts, NiW/SiO2-Al2O3-0.4 exhibited high conversion of 91% and the middle distillates yield closely 50%. It is a kind of ideal hydrocracking catalyst but with high reaction temperature. In order to improve its catalytic activity, supports with different types and contents of zeolite were added. Experiment results showed that 10%-20% ofβzeolite in the supports enhanced the catalyst activity largely, but the selectivity of middle distillates decreased about 5%. On the other hand, the existence of ZSM-5 zeolite enhanced the catalyst activity dramatically, but it produced much gaseous hydrocarbons even at low reaction temperature. For MCM-22 zeolite catalyst, it enhanced the activity of catalyst, but the isomerization degrees of products were too low. Therefore, the addition of zeolite can improve the catalytic activity, but the selectivity is also needed to be considered. These catalysts were characterized by N2 Adsorption-Desorption and Temperature Programmed Desorption (TPD). The relations between the surface properties and activity were discussed.In a word, amorphous silica-alumina based catalysts are less active than zeolite based catalysts but are more suited to maximize the middle distillates production. Interestingly, the addition of very small amounts ofβzeolite to amorphous silica-alumina could strongly improve the activity of the catalyst for waxes hydrocracking without significant middle distillates selectivity loss.