Detailed Kinetics of Fischer−Tropsch Synthesis on an Industrial Fe−Mn Catalyst

The detailed kinetics of the Fischer−Tropsch synthesis over an industrial Fe−Mn catalyst was studied in a continuous integral fixed-bed reactor under the conditions relevant to industrial operations [temperature, 540−600 K; pressure, 1.0−3.0 MPa; H2/CO feed ratio, 1.0−3.0; space velocity, (1.6−4.2) × 10-3 Nm3 kg of catalyst-1 s-1]. Reaction rate equations were derived on the basis of the Langmuir−Hinshelwood−Hougen−Watson type models for the Fischer−Tropsch reactions and the water-gas-shift reaction. Kinetic model candidates were evaluated by the global optimization of kinetic parameters, which were realized by first minimization of multiresponse objective functions with a genetic algorithm approach and second optimization with the conventional Levenberg−Marquardt method. It was found that an alkylidene mechanism based model could produce a good fit of the experimental data. This model shows that the desorption of the products and the insertion of methylene into the metal−alkylidene bond are the rate-determining steps. The activation energy for olefins formation is 97.37 kJ mol-1 and smaller than that for the paraffin formation (111.48 kJ mol-1). In this model, the readsorption and secondary reactions of olefins are taken into account, and deviations of hydrocarbon distribution from the conventional ASF distribution can therefore be quantitatively described. However, the deeper information for the olefin-to-paraffin ratio has not intrinsically been described in the present stage, leaving for the further improvements in models to consider the transportation-enhanced readsorption and secondary reaction of olefins more practically in the reactor modeling stage.