Intrinsic kinetics of the gas–solid Fischer–Tropsch and water gas shift reactions over a precipitated iron catalyst

Abstract

The kinetics of the gas–solid Fischer–Tropsch synthesis over a commercial Fe–Cu–K–SiO 2 catalyst was studied in a continuous spinning basket reactor. Experimental conditions were varied as follows: reactor pressure of 0.8–4.0 MPa, H 2/CO feed ratio of 0.25–4.0, and space velocity of 0.5–2.0×10 −3 Nm 3 kg cat −1 s −1 at a constant temperature of 523 K. A number of Langmuir–Hinshelwood–Hougen–Watson type rate equations were derived on the basis of a detailed set of possible reaction mechanisms originating from the carbide mechanism for the hydrocarbon formation and the formate mechanism for the water gas shift reaction, respectively. 14 models for the Fischer–Tropsch reaction rate and two water gas shift reaction rate models were fitted to the experimental reaction rates. Bartlett’s test was used to reduce the set of Fischer–Tropsch rate equations to 3 models, which were statistically indistinguishable. It could be concluded that the reaction rate of the Fischer–Tropsch synthesis is controlled by the formation of the monomer species (methylene) by hydrogenation of associatively adsorbed CO, whereas the carbon dioxide formation rate (water gas shift) is determined by the formation of a formate intermediate species from adsorbed CO and dissociated hydrogen. Simulations using the optimal kinetic models derived showed good agreement both with experimental data and with some kinetic models from literature.