Improvement of a multi-tubular Fischer-Tropsch reactor with gas recycle by appropriate combination of axial activity distribution and gas velocity

Fischer-Tropsch fixed bed synthesis is frequently conducted with a gas recycle and a CO conversion per pass of less than about 60%. In this paper, we have investigated the influence of a gas recycle on the performance of a multitubular reactor (tube length 12 m, diameter 3 cm) for different total CO conversions ( X CO,total ) and superficial gas velocities ( u s ). The impact of the axial distribution of the intrinsic activity of the Co catalyst in the tubes was also studied. X CO,total is 95% for the optimal value of u s of 1 m s −1 combined with a constant activity coefficient C a of 3 (which equals a Co content of the catalyst of 30 wt%); the conversion per pass is then 45%. Further improvement ( X CO,per pass = 47%) is possible by using tubes with two zones of equal length (6 m) and a suitable activity distribution ( C a,1 = 2.5, C a,1 = 3.5). An activity grading ( C a,mean = 3) with C a,initial = 2.2 until T max of 240 °C is reached at z = 2.2 m and then a stepwise rise to C a of 4 in the remaining 9.8 m leads to X CO,per pass of 49%, the maximum for the given conditions. In this paper, the influence of a gas recycle on the performance of a multi-tubular Fischer-Tropsch reactor for different total CO conversions and superficial gas velocities was investigated.