Kinetic modeling of the Fischer-Tropsch synthesis

Abstract

The objective of this work was to develop a kinetic model to describe the conversions and selectivity typical of iron based Fischer-Tropsch catalysts, and to apply the model in simulations of typical laboratory reactors. The model accounts for the formation of paraffins and olefins up to an arbitrary carbon number, and disappearance rates can be calculated from known stoichiometry. Equilibrium adsorption was assumed for CO, CO2, H2, and H2O, and accounts for product and reactant inhibition on rates. There was no significant difference in model results using a steady-state adsorption model and an equilibrium model, but the equilibrium model has the advantage of requiring fewer kinetic parameters. The final model requires 8 constants for Fischer-Tropsch, plus additional constants for water-gas shift kinetics (one in this study). Three types of reactors were considered: gas phase stirred tank and fixed bed (plug flow) reactors, and a stirred tank slurry reactor. The gas phase stirred tank is a simple, well defined reactor used in the model development, while the fixed bed reactor was chosen to represent the common laboratory fixed bed. The slurry reactor is another common laboratory reactor, but the reaction occurs on catalyst in the liquid phase formed by condensing the high molecular weight products of the reaction. Vapor-liquid equilibrium with simultaneous chemical reaction was used to account for the condensation. Non-Schulz-Flory distributions were obtained in the slurry reactor simulations due to olefin readsorption followed by chain growth. Higher concentrations of the high molecular weight olefins in the liquid phase cause the non-Schulz-Flory behavior. The adsorption also gives the trend of decreasing olefin content with carbon number expected from experiment. Both of the gas phase reactors (fixed bed and stirred tank) exhibited nearly classical Schulz-Flory distributions under most conditions. In the fixed bed, conversion increases along the length of the reactor, which leads to changes in selectivity with position. The integral effect of the selectivity profiles causes some carbon number dependence on α and olefin content, but the deviations from classical distributions are significant only at low carbon numbers...