Kinetic modeling of the Fischer-Tropsch synthesis

The objective of this work was to develop a kinetic model to describe the conversions and selectivity typical of iron based Fischer-Tropsch catalysts. The model accounts for the formation of paraffins and olefins, and disappearance rates can be calculated from stoichiometry. There was no significant difference in model results assuming steady-state adsorption or equilibrium, but the equilibrium model has the advantage of requiring fewer parameters. The final model requires 8 constants for Fischer-Tropsch, plus constants for the water-gas shift. The model was tested in simulations of three reactor types: gas phase and slurry stirred tank and plug flow reactors. The gas phase stirred tank is a simple, well defined reactor used in the model development, while the plug flow reactor was chosen to represent the laboratory fixed bed. The slurry reactor is another laboratory reactor, but the reaction occurs on catalyst in the liquid phase formed by condensing high molecular weight products. Vapor-liquid equilibrium with simultaneous chemical reaction was used to account for the condensation.; Non-Schulz-Flory distributions were obtained in slurry reactor simulations due to olefin read-sorption followed by chain growth of high molecular weight olefins in the liquid phase. This also gives the trend of decreasing olefin content with carbon number. Both of the gas phase reactors exhibited nearly classical Schulz-Flory distributions. In the fixed bed, conversion increases along the length of the reactor, which leads to changes in selectivity with position, however, the deviations from classical distributions are significant only at low carbon numbers. Selectivity is weakly dependent on space velocity (conversion) in the fixed bed reactor, because most of the reaction takes place in the upper part of the catalyst bed, where reactant concentrations are high. In contrast, in the gas phase CSTR and slurry reactors, all the catalyst is exposed to the exit concentration of reactants, so these reactors show a strong dependence of selectivity on conversion. This does not agree with experimental studies, which show similar selectivity behavior between fixed bed and slurry reactors, as well as a weak dependence of selectivity on conversion.