Mechanism of the Fischer Tropsch Process

Abstract

Abstract When comparing the competing processes for making hydrocarbons from synthesis gas – the Fischer Tropsch CO hydrogenation and the MTG conversion -the process flow sheets show as the main difference the additional step of methanol synthesis for the MTG route. However, product selectivity is basically different for both the conversions. And from this point of view the one or the other route can be the more favourable option as fitting best the particular demand pattern. Selectivity differences fundamentally result from the different kinds of chemistry which are involved: Hydrogenation on special metal type catalysts in case of the Fischer Tropsch reaction and a conversion via car-benium ion intermediates on acidic sites, which is additionally constrained by shape selectivity in case of the MTG process. The kinetics of the mechanism of Fischer Tropsch hydrocarbon formation from CO and H 2 are modelled in this paper as a “non trivial polymerization” which occurs on the surface of the solid catalyst. The term “non trivial” relates to the fact that producing a straight chain aliphatic “polymethylene” respectively a “methylen oligomer”, from CO and H 2 involves as the repeating polymerization step of prolonging the growing chain by one CH 2 a set of reactions, of activation and transfer of hydrogen, activation of CO and splitting of the C/O bond and formation of a new C/C bond. A kinetic model of this surface polymerization is presented in the paper. It starts with the ideal system and adopts stepwise additional assumptions which regard essential features of observed types of product distributions. The ideal system is described with only one parameter, the quotient of the rate constants of chain propagation and chain termination and it is related easily to the macrokinetics of the CO consumption rate. Actual real cases of product distribution are described as extensions of the ideal model, taking into account the following functions (1) Formation of 3 kinds of primary products (olefins, paraffins and alcohols plus aldehydes) (2) Methyl branching during chain growth (3) Carbon number dependence of chain prolongation rate constants (4) Carbon number dependence of chain branching rate constants. In addition experimental product distributions can seriously be affected by instationarity of the system, errors during sampling and analysis and by-product formation or secondary reactions of the compounds which are produced through the Fischer Tropsch CO hydrogenation.