(24d) Intra-Particle Diffusion/Reaction Modeling for Strongly Endothermic Reactions in Low-N Tubes with Cfd

Interactions between reaction rates, conduction and diffusion inside catalyst particles can be complex, especially when influenced by non-uniform surface conditions produced by the flow field external to the particle. In this work a three-dimensional, realistic flow field is coupled to species and energy simulations for two highly endothermic reactions, methane steam reforming and propane dehydrogenation, which have quite different activity levels. The simulation domain was a 120-degree segment of a packed tube of tube-to-particle diameter ratio (N) = 4, packed with cylinders. The simulations employed computational fluid dynamics (CFD) and user-defined-codes, to examine packings consisting of full cylinders and hollow cylinders.

The detailed pellet surface and intra-particle temperature, species and reaction rate distributions were obtained for the near-wall particle, along the particle radius and axis. Non-uniform and non-symmetric surface and intra-particle variations were observed, contrary to the conventional modeling approaches, which may lead to mis-evaluation of the intra-particle reaction rates by traditional approaches. Close examination of the flow fields and the particle surface species distributions, along with the temperature distributions in the particles, suggest that these effects are primarily due to the strong temperature gradients at the tube wall, as well as depletion of the reactants in regions of low or stagnant flow where particles approach each other closely. Differences in behavior between the two reaction systems studied here were observed and led to differences in particle effectiveness.