(450e) Short Chain Hydrocarbons Growth Probability in Fischer-Tropsch Synthesis over Cobalt Catalyst

The light hydrocarbon (C₁‑C₆) product distributions from a fixed bed Fischer‑Tropsch reactor with a 10% Co/TiO₂ catalyst were analysed over an extended period of time as the activity of the catalyst changed. These short chain hydrocarbon product distributions were studied starting at 200 ^oC with syngas for 48 hours and then increasing the temperature to 220 ^oC. The feed was then switched back and forth from syngas to syngas + extra N₂ with increased total pressure in such a way as to keep the reactant partial pressures the same in both cases.

The results showed that at the higher temperature (220 ^oC), more short chain paraffins were obtained than short chain olefin products. The chain growth probability (α) values for the olefins (O₃‑O₆) were smaller than those for the paraffins (P₃‑P₆) under all the FT reaction conditions studied. The results showed that the co-feeding of N₂ to the syngas during FT runs slightly increased the α values for most of the FT runs. At low reaction conversion the olefin selectivity was higher than that of the selectivity to paraffin. The light paraffin α values were high reaching 1. The data also showed that light olefins α values increased with increasing selectivity to the olefins asymptotically reaching a value of approximately 0.79 whereas the values for the paraffins were not affected by these changes in conditions. This asymptotic behaviour might be indicative of an approach to reaction equilibrium amongst the olefin species.

Overall, these results show that the change operating conditions caused by loss of activity with time affects olefin and paraffin product distributions differently. This suggests that olefin and paraffin product distributions should be considered separately in the development of models for FT processes.