Comprehensive Kinetic Model for Fischer-Tropsch Synthesis Over a Re-Promoted Co/Al2O3 Catalyst

Abstract

In spite of considerable effort to develop comprehensive kinetic models for Fischer-Tropsch synthesis (FTS), there is still a debate about the reaction mechanism and possible reasons for deviations from Anderson-Schulz-Flory (ASF) distribution. The goal of this work is to develop a comprehensive kinetic model for FTS on promoted cobalt-alumina catalyst and provide an insight into this topic.

Experiments were conducted over 25% Co/0.48%Re/Al₂O₃ catalyst in 1L stirred tank slurry reactor over a range of operating conditions (i.e. temperature 220 °C, pressures 200 and 350 psig, H₂/CO feed ratio 1.4 and 2.1 and gas space velocity from 3.6 to 15 NL/g-cat/h). Rate equations were derived on the basis of Langmuir-Hinshelwood-Hougen-Watson approach using well known carbide mechanism for FTS ^[1]. In order to explain non-ASF behavior it was assumed that the desorption rate of an olefin precursor is a function of carbon number ^[2].

Kinetic parameters were estimated by minimizing multiple response objective function using Genetic algorithm and Levenberg-Marquardt method, as a global and local optimization tool, respectively. Model evaluation was based on physical meaningfulness of estimated parameters as well as on statistical relevance of the fit.

Model yielded physically meaningful values of parameters. Furthermore, it can be seen from Figure 1 (a-c) that it was also able to correctly predict deviations from the ASF distribution and olefin to paraffin ratio dependence on carbon number.

Figure 1 – Comparison of experimental data and model predictions for C_1-15 hydrocarbons at T=220 °C, P=203.3 psig, H₂/CO=2.1, SV=8.0 NL/g-cat/h (a) rates of formation for n-paraffin and 1-olefin; (b) O/P ratios (1-olefin/n-paraffin); (c) ASF plot (total rates vs. carbon number)

References

[1] LOX E. S., FROMENT G. F. (1993) Kinetics of the Fischer-Tropsch Reaction on a Precipitated Promoted Iron Catalyst. 2. Kinetic Modeling. Ind. Eng. Chem. Res. 32, 71-82.

[2] BOTES G. F. (2007) Proposal of a New Product Characterization Model for the Iron-Based Low-Temperature Fischer-Tropsch Synthesis. Energy & Fuels, 21, 1379-1389.

 

Acknowledgement

This paper was made possible by NPRP grant 08-173-2-050 from the Qatar National Research Fund (a member of Qatar Foundation). The statements made herein are solely the responsibility of the authors.

*Corresponding author: Dragomir Bukur, dragomir.bukur@qatar.tamu.edu