Kinetics, catalyst design, and hydrodynamic analysis in Fischer–Tropsch synthesis: Fixed Bed vs Fluidized Bed Reactors

Abstract

While fixed beds maximize contacting between the fluid and solid phase, commercial processes operate with fluidized beds for highly exothermic reactions, like Fischer–Tropsch (FT), to maximize heat transfer thereby minimizing catalyst sintering and deactivation. However, conversion is lower due to gulf-streaming and poorer gas–solids contacting. In experimental reactors ( < 10 mm) gulf-streaming is negligible and gas–solids contacting is excellent as bubbles are small. Here, we measure CO conversion over 3 FT catalysts — Fe/Ce/SiO 2 -K, Cu, Fe/Zr/SiO 2 -K, Cu, Fe/Catalox-K, Cu — across the same reactor operating in the fluidized bed and fixed bed regime. Flow rates were increased up to a maximum of 5 times the minimum fluidization velocity ( U mf ). The reactor operated at 2 MPa, with a H 2 /CO molar ratio of 2, and 275 °C ≤ T ≤ 325 °C . The average CO conversion was higher in the fixed bed but temperature control was more difficult, which could account for some of the differences. The highest selectivity (80 % to 84 %) was with the Fe/Catalox-K, Cu at 275 °C, 2 to 4 × U mf , in the fixed bed reactor. A first order reaction model characterized CO conversion well ( R 2 > 0 . 91 ) for the Fe–Catalox and Fe–Zr catalysts with activation energies > 70 kJ mol −1 . Conversion was essentially independent of temperature for the Fe–Ce composition in both the fixed bed and fluidized bed. BET surface area decreased 22 % to 52 % after the reaction. XRD analysis after the reaction revealed a 4 % to 10 % decrease in crystallinity, along with the presence of various Fe carbide compounds. Post-reaction SEM-EDS images indicated particle agglomeration and carbon deposition on the catalyst surface. Despite these morphological changes, the impact on CO conversion seemed minimal. Residence time distribution tests confirmed that gas was essentially in plug flow for both the fixed bed and fluidized bed configurations. • When a fluidized bed operates 2 X Umf, conversion is the same as a fixed bed. • A first order kinetic model characterizes CO conversion in Fischer–Tropsch, R 2 > 0 . 9 . • At 325 °C, the catalyst activity reached up to 83% conversion in the fluidized bed. • E a > 70 kJmol − 1 for most catalysts and deactivation over 24 h was negligible. • CO conversion over the Fe Ce on SiO 2 catalyst was unusual, E a < < 70 kJ mol −1 .

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