(33c) Olefin Pseudo-Equilibrium in the Fischer-Tropsch Reaction

It is generally accepted that the product distribution obtained in the Fischer-Tropsch (FT) synthesis over various catalysts can be described by the Anderson-Schulz-Flory (ASF) product distribution or some slight variation of it. Most attempts to describe the FT product distribution have a kinetic basis. If it did have a kinetic basis one would expect that for very low conversions one would find that the ASF distribution would not hold (fewer than predicted higher members of the homologous series) as it would take longer to make the later products in the distribution. This does not seem to be the case.

Few attempts have been made to derive the FT product distribution from thermodynamic considerations. Overall or global thermodynamic equilibrium predicts a predominately methane product for the FT synthesis reaction. However, although the FT product distribution may not be described by overall thermodynamic equilibrium, there may be aspects of it that can be described by equilibrium.

We postulate that an equilibrium is set up in the FT reaction between species, i.e. between hydrocarbon of chain length n and that of chain length n+1, and thus although we do not have global equilibrium, we have an approach to equilibrium between the product species.

We initially postulate that an equilibrium distribution is set up between the alpha olefins or a species that leads to alpha-olefins. Thermodynamic equilibrium and reaction reversibility often are different manifestations of the same phenomena. As olefins are reactive under FT reaction conditions, these are obvious candidates to postulate that equilibrium could exist between the species. We tested this hypothesis by theoretically calculating what the product distribution would be. We also checked what the theoretical predictions would give if we assumed that equilibrium was set up between the members of the paraffin series as well as the alcohol species.

The calculated results predict that equilibrium between the species of any homologous series would results in an ASF type distribution, where the ratio of the moles of species n to that of the moles of species n+1 would be constant. However when the value of this ratio α is calculated, it is found that the experimentally measured values match those calculated and thus the distribution of the olefins. As mentioned above paraffins and alcohols would also follow a single α product distribution at Fischer-Tropsch reaction conditions However, assuming that the alcohols or paraffin distribution approaches equilibrium, leads to calculated values of α that are not consistent with the experimental results. Thus we postulate that there is a pseudo-equilibrium set up among the alpha olefins.

Keywords: Fischer-Tropsch product distribution, FT thermodynamics, olefin reactivity.