The effect of temperature on the Fischer-Tropsch selectivity and further mechanistic insights

Master Thesis


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University of Cape Town

Concern’s that the world’s energy supply will not be able to keep pace with rising energy demands, have surfaced periodically for much of the petrochemical industry’s nearly 150 year history, but each time the industry has responded with technological advances and innovations to satisfy the global energy needs. Future advances will most likely include the enhanced recovery of conventional oil, the production of extra-heavy oil / tar sands and the utilization of alternative energy production technologies (technologies other than crude oil refining). The Fischer-Tropsch Synthesis (FTS) discovered in 1923 by Fischer and Tropsch, is one of these alternative fuel production technologies and can briefly be defined as the means used to convert synthesis gas containing hydrogen and carbon monoxide over a group VIII metal catalyst to hydrocarbon products and water. Given the vast product spectrum possible for the FTS (paraffins, olefins, alcohols, carbonyls, acids and aromatics), a great deal of controversy still exists as to the chemical identity of the monomeric building block and the propagation of the hydrocarbon chain on the catalyst surface [van Dijk., 2001]. Several mechanisms have been published with the four most popular (alkyl, alkenyl, enol and CO-insertion), recently reviewed by Claeys and van Steen (2004). It must however, be appreciated that given the complexity of the FT reaction it is generally accepted that more than one mechanism may operate on the catalyst surface at any one time. Furthermore, process parameters such as temperature, total pressure, partial pressure, hydrogen to carbon monoxide ratio, space velocity and residence time all have an influence on the FT product selectivity. Because of this it becomes exceptionally complicated to determine the effects of just one parameter while taking the effects of the additional parameters into account.

Includes bibliographical references (p. 133-145).