Importance of the Usage Ratio in Iron-Based Fischer−Tropsch Synthesis with Recycle

The present study was undertaken to understand the fundamentals of recycling the tail gas during the Fischer−Tropsch Synthesis with an iron-based catalyst with particular emphasis on the importance of the usage ratio. The usage ratio was ca. 1.4 for the catalyst studied at our reaction conditions (240 °C, 20 bar, 5000 mL/(gcat·h)). The study shows the H2/CO ratio in the recycle gas fed to the reactor depends on both the H2/CO ratio in the fresh feed and the usage ratio of the catalyst. For an H2/CO in the fresh feed higher than the 1.4, the H2/CO ratio in the reactor increases with the increasing recycle ratio whereas the converse is true for H2/CO ratios lower than 1.4. For negligible change in the Fischer−Tropsch selectivity during external recycle, the H2/CO ratio in the fresh feed should match the usage ratio of the catalyst. The presence CO2 in this recycled gas did not change the overall CO2 selectivity; hence, CO2 is regarded as an inert at these conditions.