(10c) A Thermodynamic Approach to the Olefin Products Distribution in Fischer-Tropsch Synthesis

The Fischer-Tropsch Synthesis (FTS) is a network of parallel and consecutive reactions that take place within catalyst pores filled with waxy liquid hydrocarbon products. The product distribution follows the so called ASF (Anderson-Schulz-Flory) model. Although product distribution models have been developed using a kinetic approach, the kinetic models can only describe the product distribution but cannot explain why they follow this particular distribution. On the other hand, researchers have also tried to explain the formation of each single group of products from a catalytic point of view, but this approach is also not fully able to describe the experimentally found product distributions. This work tries to explain a small but very important part of the FTS reaction namely the olefin distribution that is found in both a CSTR(Autoclave Engineers, 100ml in volume) and a PFR(Autoclave Engineers, 8mm in ID). Conventional FT experiments were carried out both in a CSTR and a PFR at typical low temperature FTS conditions. The olefin to paraffin ratios for short chain hydrocarbons (C2-C5) were found to change time even when reaction conditions are kept constant in the CSTR, while the ratio of adjacent olefins (for example C4H8/C3H6) remained unchanged (see Fig). The ratios for C_nH_2n/C_n-1H_2(n-1) from two different temperatures(465K and 485K) for the same carbon number n are found to be more of less the same. For the PFR, the temperature was varied from 485K to 523K and flow rate (FR) was varied from 1.8 to 5.4 NL/h/gcat and the ratio of adjacent olefins was also found to be constant. Furthermore, the ratio for the same carbon number n in the CSTR and the PFR are the same. A simple reaction network for FTS is assumed and studied based on the reaction characteristic: C_n-1H_2(n-1) C_nH_2n C_n-1H_2(n-1)+2 The thermodynamic model for olefin distribution in this work is developed based on: C_n-1H_2(n-1)+C_n+1H_2(n+1)=2C_nH_2n The thermodynamic equilibrium constant derived from thermodynamic calculations and experiments are compared. The results from experiments suggest that the olefin distribution in FTS might be described by thermodynamic equilibrium.