Ideal hydrocracking catalysts for the conversion of FT wax to diesel
Master Thesis
2014
Permanent link to this Item
Authors
Journal Title
Link to Journal
Journal ISSN
Volume Title
Publisher
Publisher
University of Cape Town
Department
License
Series
Abstract
The Fischer-Tropsch wax synthesis process and the subsequent upgrade of the wax to useful distillate fuels by mild hydrocracking is a well-known, economically viable method of producing liquid fuels, in particular diesel fuel. This project seeks to develop an ideal hydrocracking catalyst (i.e. a hydrocracking unit in which only primary cracking occurs) for the conversion of Fischer-Tropsch (FT) wax to diesel and to determine the effect of carbon monoxide on the activity and selectivity of the hydrocracking catalyst for possible integration of low temperature FT wax synthesis with wax hydrocracking into a single stage. Theoretically, a combination of the Fischer-Tropsch unit with an ideal hydrocracking unit can produce diesel yields of up to 80 wt. A non-ideal hydrocracking catalyst would lower the middle distillate yields due to the occurrence of secondary cracking. Primary cracking of the paraffins produced by the low temperature FT process occurs only when the activity of the metal is high and the rate limiting step occurs on the acid site. Integrating the wax synthesis process with the subsequent work up of the wax to produce distillate fuels is not without challenges, mainly the low reaction temperature and pressure (225°C and 20 bar), in which the hydrocracking catalyst is to operate. Noble metals, combined with zeolites are known to be active for hydrocracking at such conditions. Carbon monoxide, a feedstock of the FT process poisons noble metal catalysts; therefore knowledge of its effect on the hydrocracking catalysts performance is essential. The hydrocracking catalysts were tested when the metal and the acid sites were segregated (i.e. the metal supported on an inert carrier, physically mixed with the zeolite), and when the two sites are in close proximity (i.e. the metal impregnated into the zeolite). The tests were carried out both in the presence and absence of CO consistent with the FT feed ratio. The noble metals, Rh, Ru and Pd were used as co-catalysts to H-MFI-90. It was found that the physical distance between the metal and the acid sites has disturbs the balance of the two sites by introduction of a transport steps, this seen through both the activity and selectivity of the catalyst. Pd exhibited higher activity than Rh and Ru. Primary cracking was found to be unattainable when the metal and the acid sites are segregated. When the metal and the acid sites were in close proximity (impregnated catalyst), near primary hydrocracking performance was observed at metal loading of 0.9 wt Pd. Secondary cracking was aggravated upon the introduction of CO on both the segregated and impregnated catalyst.
Description
Includes bibliographical references.
Reference:
Ndimande, C. 2014. Ideal hydrocracking catalysts for the conversion of FT wax to diesel. University of Cape Town.