(5ci) Cfd Simulation of Transport and Reaction in Catalyst Particles
AIChE Annual Meeting
Sunday, November 12, 2006 - 2:00pm to 5:00pm
Modeling of fluid flow, heat transfer and reaction in fixed beds is an essential part of their design. This is especially critical for highly endothermic or exothermic reactions in low tube-to-particle diameter ratio (N) tubes, such as are used in steam reforming and partial oxidation. In our previous work, comparative studies were performed to show the effect of catalyst design on heat transfer performance in the near-wall region, in a steam reforming packed bed reactor tube. Computational Fluid Dynamics (CFD) was used to obtain detailed flow and temperature fields in a representative wall segment of the tube. However these simulations, in which inert packing was heated up, ignored the influence of catalyst geometry on the reaction heat effects. Present state-of-the-art in CFD does not permit the representation of species diffusion, conduction and reaction inside solid regions, such as catalyst particles. For that reason, as a first approach, to approximate the heat effects in steam reforming, temperature-dependent heat sinks were introduced into the solid particles while keeping the partial pressures constant. This was complicated by the very low effectiveness factors in these catalysts, so that the particles behaved like egg-shell catalysts. This was especially challenging for particles with internal voids. Simulations were performed under realistic industrial conditions of steam reforming. A constant wall heat flux was imposed, and various shapes of particles studied with heat sinks to simulate the reforming endothermic reaction. When heat sinks were included in the particles to represent the thermal effects of chemical reaction, the heat effects of the particles were shown to change markedly, both qualitatively and quantitatively. To improve on heat sinks approach by explicit inclusion of intraparticle effects (conduction species diffusion and reaction), the particles were defined as porous catalysts. Considering realistic 3D external flow and temperature fields in a steam reformer, the effect of diffusion into spherical and cylindrical catalyst particles was investigated. Species and temperatures were found to be quite symmetric in the non-wall particles, as is the conventional assumption. At the wall particles, however, strong deviations from uniformity and symmetry have been seen due to the strong wall temperature gradients. These observations lead us to conclude that, with the conventional assumption, the tube wall temperature and reaction rates for catalyst particles at the wall can be incorrectly evaluated, so that important design considerations as tube life and catalyst deactivation due to carbon deposition may not be predicted well. All the simulations were done by the CFD code Fluent 6.2 by implemented user-defined functions.