467848 Use of Stability Diagrams to Predict Catalyst Speciation during Fischer Tropsch Reduction and Reaction

Wednesday, November 16, 2016
Grand Ballroom B (Hilton San Francisco Union Square)
Joshua Gorimbo1,2, Xiaojun Lu2, Xinying Liu2, Yali Yao2, David Glasser2 and Diane Hildebrandt2, (1)Chemical and Metallurgical Engineering, University of the Witwatersrand, Johannesburg, South Africa, (2)MaPS Engineering, University of South Africa, Johannesburg, South Africa

An understanding of the speciation of iron catalyst precursor during reduction is an important piece of knowledge in Fischer-Tropsch (FT) reaction. The thermodynamics of the precursor speciation is governed by the reducing gases used, and this tends to influence the activity, selectivity and the catalyst life span. Economic benefits can potentially be obtained if the same syngas used for the normal FT reaction can also be used to activate the catalyst at the normal FT reaction temperature.

During catalyst activation or FT synthesis, the interaction of reducing agents or syngas with Fe-based catalyst results in the formation of several gaseous components such as CO, H2, CO2, H2O, and CH4. The partial pressure of each gaseous component determines the predominant state of the catalyst. Catalyst speciation happens due to different partial pressures of the gaseous components, and as a result, the stable iron phases formed during FT synthesis are those that are in equilibrium with the gas composition. Although the ratio of PH2/PH2O and PCO/PCO2 or the partial pressure of water and carbon dioxide does not have any appreciable effect on the deactivation by oxidation on the FT reaction rate in Cobalt catalysts, in the iron based catalyst deactivation by oxidation is predominant. Based on thermodynamic calculations of the FT system, the maximum allowable oxygen partial pressure during catalyst activation is 10-45 bar. The oxygen partial pressure in therefore dependent on the PH2/PH2O and PCO2/PCO  ratios, and the equilibrium constants. Plots of O2 partial pressure against PH2/PH2O , PCO2/PCO  ratios have been used to predict the stability of the different species.

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