(362c) Packing Low Tube-to-Particle Diameter Ratio Packed Beds for CFD Simulation

Li, G., Ansys Inc.
Mukhopadhyay, A., Ansys Inc.
Dai, Y., Ansys Inc.
Cheng, C., Ansys Inc.

Packed beds are widely used in chemical processes such as heat exchanger and catalytic reactors. Traditionally analysis of packed bed is usually based on the porous medium approach. In that approach, packing structure is usually described by some statistical, lumped parameters such as porosity, turtosity, and the flow characteristics including pressure, heat and mass transfer are computed by some semi-empirical correlations based on those lumped parameters. Such approach is acceptable if all dimensions of packed zone is much larger than the dimension of the packing object.  

For highly exothermic catalytic reactions, reactors with a low tube-to-particle diameter ratio is usually used. The homogenous porous medium approach will cause larger errors. For example, the high porosity near pipe walls causes flow by-passing, so the Ergun equation developed for the homogenous porous will not hold any more. In addition, the occurrence of run-away reactions and existence of hot spots are local phenomena, so homogenous porous model cannot capture them. As a result, there is a need to resolve the packing structure directly and simulate the flow within the packed bed with high fidelity.

With the development of high performance computing, it becomes possible to resolve flow/reactions in detail in a packed bed, especially for low tube-to-particle diameter ratio packed beds. In this presentation a random scheme of packing balls with different diameter in a tubular reactor is presented. With a final goal of conducting CFD calculation in mind, the paper also talks about meshing of such packed reactors. Possible point-to-point contact has brought a lot of challenges in meshing. Several meshing strategies will be discussed. In addition, numerical  results such as porosity distribution and pressure drop will be shown in the presentation as well.