The hydrocracking of long chain n-paraffins under Fischer-Tropsch conditions

Master Thesis


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University of Cape Town

Interest in the area of hydrocracking has grown rapidly over the years. In the early 1960's companies such as Chevron and Universal Oil Products (UOP) introduced new hydrocracking processes to manufacture high octane gasoline. The demand for transportation fuels such as diesel and jet fuel has increased significantly which results in the continuous development of hydrocracking techniques and catalysts. The conversion of normal long chain paraffins from the Fischer-Tropsch synthesis to clean distillate fuels is a particular area of interest. The objective of this project is to investigate the hydrocracking of long chain paraffins under F-T conditions. The aim is to achieve in situ, the hydrocracking of low temperature Cobalt-based F-T wax by combining F-T synthesis and hydrocracking in a single reactor. For the purpose of this thesis, it involves subjecting the hydrocracking catalyst to F-T conditions. Synthesis gas (carbon monoxide and hydrogen), the paraffin n-C16 and water were co-currently fed to a fixed-bed reactor containing only the hydrocracking catalyst. Therefore care was taken to match the experimental conditions of the hydrocracking experiments to those that prevail in the Fischer-Tropsch synthesis. Practically this means the hydrocracking of n-hexadecane was studied at the space velocity, the reaction temperature and pressure and under partial pressure of H2, CO and water, at which n-hexadecane is produced in F-T process assuming that n-hexadecane is the only hydrocarbon product and that n-hexadecane is a model compound for the low temperature F-T process. The results of this investigation show that the hydrocracking reaction over a Pd catalyst supported on H-MFI Zeolite under F-T conditions is non-ideal. At low feed (n-C16) conversions, product distributions are strongly dominated by secondary reactions. The ability of the metal site is significantly inhibited by the presence of CO and water. The product distributions show exactly this due to the increase in unsaturated and more branched species. Feed conversion in the presence of water and CO increase with increasing reaction temperature. The absence of methane in the product spectrum is an indication that the hydrogenolysis reaction is an unfavorable pathway for the catalyst used. The data obtained from this investigation suggests that the combination of low temperature Fischer-Tropsch and hydrocracking into a single reaction step is feasible.

Includes bibliographical references.