(341d) Cfd Modeling of a Fixed Bed Reactor for Strongly Endothermic Reactions

Taskin, M. E., Worcester Polytechnic Institute
Stitt, H., Johnson Matthey

In chemical engineering processes, fixed bed reactors are frequently used catalytic systems. At present, the majority of commercial gas-phase catalytic processes are carried out in these reactors, such as steam reforming and propane dehydrogenation. Multitubular reactors with low tube-to-particle diameter ratios (N) are especially used for these types of endothermic reactions. The design and optimization of the catalyst particle for activity, pressure drop and heat transfer are particularly important for these reactions and require an understanding of the flow patterns, temperature field at the near wall region, and species distribution especially inside the catalyst particles. This information can be provided by CFD simulations in fixed bed packings. We have verified by our previous uses of CFD in fixed bed simulations that, 120-degree segments of tubes packed with spheres and cylinders can represent very well the flow and temperature profiles of full tubes, especially focusing on the center particles positioned in the middle of the near-wall region. In our previous work, the temperature-dependent heat sinks inside catalyst particles with various features were used to mimic the heat effects of the endothermic methane steam reforming reaction taking place inside the solid particles. In the present work, the above reactions were simulated with conduction, species diffusion and reaction inside the catalyst particles, by defining the solid particles as porous catalysts in the commercial CFD code Fluent 6.2. Inside the particles, the species and temperature distributions were found as quite symmetric for 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.