(435i) Computational Singular Perturbation As an Analytical Tool to Study the Fischer-Tropsch Process

Debanajan Chakrabarti, Gouthami Senthamaraikkannan and Vinay Prasad

The Fischer-Tropsch synthesis¹ process generates synthetic crude oil (syncrude) from carbonaceous sources such as natural gas, coal and biomass via a synthesis gas intermediate. The process involves a series of simultaneous polymerization and hydrogenation reaction steps, leading to a hydrocarbon and oxygen rich product comprising of C₁ â?? C₁₀₀₊ hydrocarbon and oxygenate species. The product can be refined in a manner similar to crude oil to obtain naphtha, distillate and petrochemical products. However, the process is also plagued by the formation of CO₂ and CH₄ as by-products, which decrease the process efficiency significantly. While the process parameters can generally be adjusted under steady state operation to improve the system performance by increasing the desirable product yield and decreasing the CO₂ and CH₄ selectivity, there is a limit to the extent of this improvement. However, a significant improvement in the process performance can be brought about by improvement in catalyst design and by exploiting unsteady state behaviour of the system by means of periodic operations^2,3. However, both of these strategies require an intimate understanding of the reaction behaviour.

Computational Singular Perturbation study (CSP)^4,5is a numerical technique which can be used to study the dynamic properties of a complex reaction system. It involves discretizing a kinetic model into various reaction modes, and grouping them into active and inactive modes, based on the timescales and amplitudes of each mode. This allows the establishment of a time varying hierarchy of each reaction step and intermediate species, based on their contribution to the overall reaction behaviour, at each time instant. This can enable the establishment of time-varying reduced kinetic models, to capture the dynamic behaviour of the reaction system.

The validity of this technique depends on the accuracy of the reaction mechanism and kinetic model, utilized to represent the reaction system. Based on observations in literature and experiments, the Fischer-Tropsch chain growth has been determined to be the result of a CO addition based mechanism, while accompanied by CO disporoption based methanation and water gas shift side reactions¹. We have utilized the CO-insertion based kinetic model developed by Todic et al.⁶to conduct our analysis for identifying the dominant elementary reactions and species, at different time scales and operating conditions. This would enable us to determine the conditions to promote desirable reactions, as well as to design experiments for periodic operation.

References

1. Chakrabarti, D.; Prasad, V.; de Klerk, A., Mechanism of the Fischer-Tropsch process. In Fischer-Tropsch Synthesis, Catalysts, and Catalysis: Advances and Applications, Davis, B. H.; Occelli, M. L., Eds. Taylor and Francis: Boca Raton, 2016; pp 183-222.

2. Adesina, A.; Hudgins, R.; Silveston, P., Feed composition modulation of hydrocarbon synthesis over a cobalt oxide catalyst. The Canadian Journal of Chemical Engineering 1986,64, (3), 447-454.

3. Feimer, J.; Silveston, P.; Hudgins, R., Influence of forced cycling on the fischer�tropsch synthesis. Part I. Response to feed concentration step�changes. The Canadian Journal of Chemical Engineering 1984,62, (2), 241-248.

4. Lam, S.; Goussis, D. In Understanding complex chemical kinetics with computational singular perturbation, Symposium (International) on Combustion, 1989; Elsevier: 1989; pp 931-941.

5. Zagaris, A.; Kaper, H. G.; Kaper, T. J., Analysis of the computational singular perturbation reduction method for chemical kinetics. Journal of Nonlinear Science 2004,14, (1), 59-91.

6. Todic, B.; Ma, W.; Jacobs, G.; Davis, B. H.; Bukur, D. B., CO-insertion mechanism based kinetic model of the Fischerâ??Tropsch synthesis reaction over Re-promoted Co catalyst. Catalysis Today 2014, 228, 32-39.