An optimized simulation model for iron-based Fischer–Tropsch catalyst design: Transfer limitations as functions of operating and design conditions

• Heat and mass transfer resistances for iron Fischer–Tropsch catalysts were modeled.
• Optimal conditions are 30 bar, 256 °C, and 80 μm for maximum observed rate.
• Pressure drop is not limiting under lab-scale conditions.
• Contour plots map resistances at varying pellet sizes, temperatures and pressures.
• Mass/heat transfer resistances are not limiting for pellets smaller than 250 μm.

Transfer limitations were successfully modeled and optimized for iron-based catalysts for Fischer–Tropsch synthesis. The simulation model predicts the effect of changing reaction temperatures, reaction pressures, catalyst pellet size, and the feed CO composition on pore diffusion, film heat transfer, internal heat transfer, and pressure drop. The comprehensive contour maps obtained from the model quantitatively display the effects of these various design variables to both optimize catalyst design and provide guidance for kinetic experiments. The optimization results favor higher reaction temperatures and pressures, smaller pellet sizes, and lower feed CO compositions to maintain high activity of kinetically-controlled reaction rates. The optimal temperature (255.8 °C) was constrained by the rate of catalytic deactivation. The model was validated by experimental data acquired from a fixed-bed reactor and shows excellent agreement. The model predicts the observed rate to be 79% of the intrinsic rate at 250 °C, 20 bar, equimolar H2:CO, and 425 μm pellet size, while experimental results showed this percentage was 74 ± 7% for 250–600 μm pellets. The model predicts no pore-diffusion limitations at pellet sizes smaller than 250 μm, indicating that the reaction rate is kinetically-controlled. Furthermore, the resistance due to film temperature gradients is more limiting than that due to intraparticle temperature gradients. Finally, pressure drop was well below 10% of the inlet reactor pressure under laboratory-scale conditions. The model was used to predict the effect of using smaller catalyst pellets on pressure drop for a commercial-scale reactor, which showed that acceptable operation could be expected with a pressure drop of 20% of the inlet reactor pressure.