(617db) 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.