CFD Simulation of Reaction and Heat Transfer Near the Wall of a Fixed Bed

Abstract

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 the simulations a near-wall section of an entire bed is used to create a simulation geometry that can be handled in the available computational domain. A full bed model was also available for validation of the wall-segment model. In the wall-segment approach a section of the bed is modeled in more detail, allowing for a relatively smaller control volume size and a more detailed view of the flow and heat transfer patterns. A simple model of a steam reforming process is used in the CFD simulation to incorporate the effect of reaction rate on temperature profiles in the bed. Simulations were performed under realistic industrial conditions of high temperature, pressure and gas flow rate, with gas properties corresponding to those of steam reforming. A constant wall heat flux was imposed, and various shapes of particles studied with heat sinks on the surface to simulate the reforming endothermic reaction, which is mainly confined to the surface of the pellet. Results will be presented showing the existence and effect of temperature profiles on the catalyst particles, and the effect on the local heat transfer rates of different gas compositions, corresponding to conditions at different locations along the catalyst tube. Local deactivation of catalyst particles can also lead to wall hot spots, or 'giraffe-necking' which can be well-reproduced by the simulations.