Application of equilibrium analysis to a Fischer-Tropsch product

Abstract

A challenge to the simulation of the Fischer–Tropsch reaction system is the lack of reliable kinetic models which correctly account for the deviations from ideal polymerisation behaviour. The current trend in literature is to attribute these deviations to kinetic effects only. This ignores that the reactions in Fischer–Tropsch synthesis contain many aspects of an equilibrium-controlled process. This arises since the rate of chain growth is much faster than the rate of monomer formation from CO. Consequently, the production distribution and the observed trends in reaction behaviour can be described by thermodynamics. A model has been developed to describe the reaction kinetics wherein each rate expression for n-paraffin and 1-olefin formation is expressed as equilibrium-limited. This arises because of the linear variation of the ideal gas Gibbs free energy of formation with carbon number. It was therefore shown that the high methane yield, low ethene yield and the decrease in olefin-to-paraffin ratio with carbon number could effectively be captured in a light weight and tractable manner. The model preserves the polymerisation character of reaction and has been extended to include products of carbon number up to C81. The equilibrium basis of the model also successfully describes the dominant trends in the product distribution as a function of CO conversion, temperature and pressure.