Studying of the Effect of Gas Recycle Ratio on the Formation of C₅–C₁₈ Alkenes in the Fischer–Tropsch Synthesis

The synthesis of C₅–C₁₈ alkenes in the presence of a zeolite-containing Co–Al₂O₃/SiO₂/ZSM-5/Al₂O₃ catalyst in flow and recycle flow operation modes at a temperature of 250°C, a pressure of 2.0 MPa, a gas hourly space velocity (GHSV) of 1000 h⁻¹, an H₂/CO ratio of 1.70 in the feed gas, and recycle ratios of 4, 8, and 16 has been studied. It has been found that the process parameters (selectivity and productivity with respect to C₅₊ hydrocarbons) pass through a maximum at a recycle ratio of 8. The use of gas recycling, unlike the flow synthesis mode, makes it possible to control the product composition. An increase in the recycle ratio in a range of 4–16 leads to an increase in the content of synthesized C₅–C₂₀ alkenes from 53.9 to 65.7 wt %. The use of a zeolite-containing catalyst, compared with a Co–Al₂O₃/SiO₂ catalyst, intensifies the formation of C₈–C₁₂ alkenes by 3.3 times: their content increases from 13.5 to 44.2 wt % at identical recycle ratios, pressures, and an H₂/CO ratio of 1.70 in the feed gas. It has been found that with an increase in the recycle ratio, the deactivation rate of the zeolite-containing catalyst decreases; this fact can be attributed to a decrease in the partial pressure of water in the reaction volume.