(722e) A Fugacity Based Kinetic Model for Supercritical Fischer –Tropsch Synthesis Over Cobalt-Based Catalytic Systems

Fischer-Tropsch synthesis (FTS) technology is used for the conversion of synthesis gas (a mixture of H2 and CO), produced from biomass, coal, or natural gas, into a broad spectrum of hydrocarbons, which later refined to ultra-clean fuels and value-added chemical feed stocks. Several studies reported the advantages of operating FTS reactors under sub-critical and supercritical conditions to provide certain benefits over the conventional media (i.e. gas-phase or liquid-phase).1 Nevertheless, optimizing such process requires understanding both the phase behavior of reaction mixture in the supercritical conditions as well as the kinetics of the reaction in such media. Few attempts were made to develop- kinetic models for the supercritical FTS conditions. Several of these models fail to provide satisfactory correlation with the experimental data because of the thermodynamic non idealities associated with high pressure supercritical conditions.2,3 Therefore, kinetic models in which concentration is represented in terms of partial pressures (based on ideal gas law) were not able to represent FT kinetics at near critical and supercritical conditions. In the present work, a kinetic model is derived based on experimental data obtained from a commercial cobalt catalyst where testing was conducted in near-critical and supercritical hexane media as well as in conventional gas-phase FTS in a fixed-bed reactor. Our developed kinetic model for carbon monoxide conversion and methane formation rates has been represented in terms of fugacities of the reactants (CO and H2). To have a better representation of the phase behavior of the supercritical phase FTS a modified Redlich-Kwong-Soave equation of state, which represents satisfactorily single-phase fugacity coefficients of pure fluids, has been extended to represent the reaction mixtures. Mixture parameters have derived from the corresponding pure-component ones by means of the classical mixing rules. The following experimental conditions have been considered in developing this model: syngas space velocity of 93.75 - 281.25 h-1, temperature range from 230 - 260°C, pressure range from 40-65 bar, and H2/CO feed ratios of 1-2, with supercritical solvent hexanes/syngas molar ratio of 3:1. The reactor was assumed to be an integral reactor and the 4th order Runge-Kutta model has used for reactor model. The kinetic parameters were optimized using non linear optimization least square method (global Levenberg?Marquardt algorithm in Mat lab). The residual sum of squares of the experimental and calculated CO conversion and methane formation rates are used as objective functions. The absolute average deviation (AAD) was applied to demonstrate the deviation between the calculated and experimental values. The sensitivity analysis has been done to estimate the valid parameter range of the kinetic model.

References 1. N. O. Elbashir, D. B. Bukur, E. Dourham, C. B. Roberts (2010) AIChE, 56(4), 997. 2. E.S. Lox, and G. Froment (1993) Ind. Eng. Chem. Res. 32, 71,. 3. N. O. Elbashir, C. B. Roberts (2004) Prep. Am. Chem. Soc. Div. Pet. Chem, 49,175.