KINETICS OF THE FISCHER-TROPSCH REACTION ON A PRECIPITATED PROMOTED IRON CATALYST .2. KINETIC MODELING

A detailed kinetic model is derived using Hougen-Watson rate expressions for the Fischer-Tropsch reactions and the water gas shift reaction on a precipitated promoted iron catalyst. The model discrimination and the parameter estimation are performed according to the integral method of kinetic analysis on experiments described in part 1 of this series. In the construction of the kinetic model, the information gained from in situ catalyst characterization is used. The model proposed assumes different active sites for the water gas shift reaction on one side and for the reactions leading to n-paraffins and 1-olefins on the other side. The rate expressions for the hydrocarbon-forming reactions are based on elementary reactions corresponding to the carbide mechanism, in the assumption that two kinds of elementary reactions the ones describing the adsorption of carbon monoxide and these describing the desorption of the hydrocarbon products, are not at equilibrium. The assumption that the active sites are almost completely occupied with surface hydrocarbon intermediates greatly simplified the kinetic expressions. The rate expression for the water gas shift reaction is based on elementary reactions involving a formate surface intermediate. The two-site reaction describing the formation of the formate intermediate is proposed to be rate determining. The values of the activation energies derived correspond well with data reported in the literature.