Simulation of Fischer-Tropsch Fixed-Bed Reactor in Different Reaction Media

The continuous increase in the global demand for a cleaner energy source has instigated much interest in converting natural gas to ultra-clean fuels and value-added chemicals. Fischer-Tropsch synthesis (FTS) is a key technology for converting syngas, produced from coal, biomass or natural gas, into a variety of hydrocarbon products. Although this technology has been around for decades, commercial development remains relatively slow and limited to use of few reactor configurations (e.g. fixed-bed reactor and slurry-bubble column reactor). On the lab-scale, supercritical solvents were utilized in FTS as a reaction media since they have the advantages of both the gas-phase reaction (fixed-bed reactor) and the liquid-phase reaction (slurry-bubble column reactor), while simultaneously overcoming their limitations. This work focuses on modeling the behavior in the reactor bed (‘macro-scale’ assessment) and then zooming into the catalyst pellet itself (‘micro-scale’ assessment). The aim of this research is to simulate the heat and mass transfer behavior inside the reactor bed, identify typical conditions that look at the existence and absence of both mass and heat transfer limitations, and to quantify the role of the main controlling parameters on the overall behavior of the reactor bed and on the catalyst effectiveness factor. An often used mathematical model of the fixed-bed reactor was applied to simulate the concentration and temperature profile simultaneously based on the appropriate mass and heat balances at both scales. A second-order ordinary differential equation was used for a spherical pellet in the radial coordinate for both mass and heat balances, while a one-dimensional steady state pseudo heterogeneous model was used for the reactor bed modeling in the axial direction. In addition, in both models the mass balance equation was expressed in terms of fugacity to account for the non-ideal behavior of the reaction mixture in the SCF-FTS. The thermodynamic properties of the mixture were estimated using the Soave-Redlich-Kwong equation of state (SRK-EOS). The simulation results of this study showed a high temperature rise in the gas- phase FTS relative to that in the SCF-FTS under a comparable reaction conditions. Carbon monoxide conversion was considerably higher in the SCF compared to the gas- phase reaction. The effect of the particle size on the overall catalyst effectiveness factor was also investigated in both reaction media