Fischer-Tropsch Synthesis: Simulation of the Diffusion of H2, CO, and Hydrocarbons in Liquid Phase Water and Hydrocarbons

Fischer-Tropsch Synthesis: Simulation of the diffusion of H₂, CO, and hydrocarbons in liquid phase water and hydrocarbons

Xiaojun Lu, Diane Hildebrandt, and David Glasser

Material and Process Synthesis; College of Science, Engineering, and Technology, University of South Africa

Abstract

As the major products of the Fishcer-Tropsch synthesis, H₂O and liquid hydrocarbons have been discovered by researchers to have different effects to both the reaction rate and product selectivity.  The effects of hydrocarbons on the performance of the FT reaction are: slowing down the overall reaction rate, increasing the selectivity to CH₄ and favouring the selectivity to the alkanes; while H₂O improves the overall reaction rate and increases the selectivity to alkenes. Although the phenomena have been clearly shown, the mechanism for the different roles of hydrocarbons and H₂O on the reaction rate and product selectivity are not completely understood. Some researchers have attributed such effects to the catalyst properties or to the mass transportation. A recent publication by Hibbitts and Iglesia etc. [1] explained the effect of H₂O on the performance of Fischer-Tropsch reaction from the catalytic perspective at the reaction mechanism level.

Most of the experimental results suggest that these effects should not be attributed to the change of the catalyst properties as the changes are fast and reversible. Before the catalysts cause a change  in kinetics, the concentration of the reactants or the products close to the surface of the catalyst need to change first. Therefore the mass transfer plays a very important role in the change of the overall reaction rate and product selectivity. The experimental and simulation data for the diffusion of the reactants (H₂ and CO) in the liquid hydrocarbons cannot be used to interpret the experimental results observed for the effects from liquid hydrocarbons and H₂O. The diffusion coefficient of H₂ is always larger than that of CO, in both the liquid hydrocarbons and the H₂O. To further complicate matters the data for the diffusion of H₂ and CO, in H₂O, at high temperature and pressure is not available in the literature. Due to nonideality, water formed by reaction is not entirely in the gas phase while the proportion of water in the liquid phase depends on the operating conditions and the composition of liquid phase hydrocarbons. Our experimental work, [2] conducted in a CSTR with a supported cobalt catalyst for FT reaction, suggests that a considerable proportion of water is in the liquid phase and that temperature has a big effect in this proportion. This work will simulate the diffusion of both H₂ and CO in liquid H₂O and liquid hydrocarbons under the experimental conditions. It will also investigate the diffusion of hydrocarbons out of the liquid layer coated on the catalyst. We expect this study will give us a clear and fundamental understanding of the different effects of hydrocarbons and H₂O on both reaction rate and product selectivity in the Fischer-Tropsch reaction.

Keywords: Fischer-Tropsch Synthesis, H₂ diffusion, CO diffusion, diffusion of hydrocarbons

Reference:

1.      Hibbitts, D. D.; Loveless, B. T.; Neurock, M.; Iglesia, E. Mechanistic Role of Water on the Rate and Selectivity of Fischer¨CTropsch Synthesis on Ruthenium Catalysts. Angew. Chem. 2013, 125, 12499-12504.

2.     Lu, X.; Zhu, X.; Hildebrandt, D.; Glasser, D.; Liu, X. A New Way to Look at Fischer-Tropsch Synthesis Using Flushing Experiment. Ind. Eng. Chem. Res. 2011, 50, 4359¨C4365.