Fischer-Tropsch synthesis in a bench-scale two-stage multitubular fixed-bed reactor: Simulation and enhancement in conversion and diesel selectivity

摘要

A process based on a two-stage multitubular fixed-bed reactor is presented to enhance in conversion and diesel selectivity for the Fischer-Tropsch synthesis (FTS) process, which is characterized by the condensation and separation of liquid products and water from the outlet of the first-stage reactor. A two-dimensional pseudohomogeneous reactor model is proposed to simulate the temperature profile and CO conversion of the two-stage fixed-bed process. Model calculations indicate that the condensation and separation of liquid products and water between the two stages of the multitubular fixed-bed reactor plays an important role by changing temperature profile and synthesis gas partial pressure of the second-stage reactor. The model validation has been verified on the basis of the bench-scale test data in a single-stage and two-stage fixed-bed reactor, respectively, which demonstrates the understandable products distribution shift toward diesel-range hydrocarbon. The combined action of higher temperature, syngas/H_2 partial pressure and readsorption of olefins at the two-stage fixed-bed reactor make CO conversion and C_(5+) selectivity increase, while CH_4 and CO_2 selectivity keep decreasing trends. Compared with the single-stage fixed-bed reactor process, the total CO conversion increases from 79% to 89% in the two-stage fixed-bed reactor process. The high valuable diesel range (C_(12)-C_(22)) increases significantly from 67% to 78%, while the gasoline range (C_5-C_(11)) decreases from 27% to 18%. The catalysts exhibit more stability and less deactivation rate over the two-stage fixed-bed process. The results are helpful to obtain more valuable diesel products and effective utilization of the syngas by the multistage fixed-bed process without tail gas recycling, which provides a practical case for multi-stage short reactor rather than one long reactor for FTS to improve products distribution.