Analysis of Particle and Reactor-Scale Transport-Kinetic Interactions of CO2 Methanation

The design of an externally cooled fixed-bed reactor for CO₂ methanation has been performed using microscopic forms of reaction engineering models that account for both particle-scale and reactor-scale transport-kinetic interactions. This is an important reaction for the clean manufacture of CH₄. The predictions of various multicomponent species flux models were compared to analyze the transport-kinetics interactions for a spherical catalyst particle shape. Flux models that included Knudsen diffusion resulted in an increase in both the CO₂ conversion and CH₄ yield as the pore diameter was reduced from 40 nm to 10 nm, which suggests the development of novel nanoporous catalytic materials. Both 1-dimensional (1-D) and 2-dimensional (2-D) models for different catalyst shapes were also simulated to quantify particle-scale performance for CO₂ methanation. A spherical catalyst shape was shown to produce a higher CH₄ average concentration, while a novel 4-hole cylindrical shape has the lowest diffusion limitation. The performance of an externally cooled 1-D heterogeneous fixed bed reactor was also simulated using spherical catalyst pellets. It is shown that the high exothermicity of the CO₂ methanation reaction creates significant hot spot formation. In order to control the axial reactor temperature within a user-prescribed bound, the optimal catalyst loading profile was determined by using an energy conservation equation-based approach. It is shown that the reactor hot spot temperature can be controlled within a reasonable range, which provides the basis for development of practical reactor designs for this important reaction.