Modeling of CO2-Based Fischer–Tropsch Synthesis over a Cu/Zn/K-Promoted Fe Catalyst: Influence of Reaction Kinetics and Multi-Fixed-Bed Reactor Design

Abstract

CO2 hydrogenation by reverse water–gas shift (RWGS) directly combined with subsequent Fischer–Tropsch synthesis (FTS) in a single fixed-bed reactor represents a promising route for converting renewable hydrogen and captured carbon dioxide into hydrocarbons. However, the attainable CO2 conversion is limited by thermodynamic constraints of the RWGS, product inhibition of FTS, and intraparticle diffusion limitations. In the present work, intrinsic and effective reaction models were developed for a potassium-promoted FeCuZnK catalyst to investigate the interaction between intrinsic catalyst kinetics and internal diffusion phenomena. An intrinsic Langmuir–Hinshelwood–Hougen–Watson (LHHW) model was established using fine catalyst particles (dp ≤ 150 µm) and subsequently extended to coarse catalyst particles (dp ≈ 2 mm) by introducing an effectiveness factor. The intrinsic model identified water as the dominant inhibiting species, whereas the already high RWGS activity of the FeCuZnK catalyst leads to a rapid approach of the thermodynamic equilibrium, indicating that an increase in activity would only provide limited improvements. The effective model accurately reproduced the behavior of technical catalyst particles and was subsequently applied to multi-reactor concepts with intermediate water removal. A five-stage reactor cascade increased the attainable CO2 conversion from approximately 55% to 85% under otherwise identical operating conditions. The results demonstrate that reactor design and water management provide greater potential for process intensification than further increases in intrinsic catalyst activity alone.