Influence of the Wall Structure On the Heat Transfer in Packed Beds with Small Tube to Particle Diameter Ratio

Introduction

Packed bed reactors are widely used in the chemical and process industry amongst others for catalytic surface reactions. The model based design of such reactors is often done with simplifications, e.g. plug flow and an equally distributed porosity over the whole packed bed which are not valid for reactors with a small tube to particle diameter ratio. Such reactors are dominated by wall effects caused by the radial porosity distribution. These wall effects constrain the heat transfer to or from the reaction zone which could lead to a sub-optimal performance, catalyst sintering or safety concerns. The effective thermal conductivity in radial direction can be considered as the superposition of the effective thermal conductivity of the solid matrix and the fluid phase between the particles. Therefore it is a function of the local porosity and the heat dispersion due to radial mixing of the fluid.

Aim of This Contribution

The aim of this contribution is to show, firstly, that it is possible to obtain a more homogeneous radial porosity distribution in monodisperse packed beds of spherical particles by imposing appropriate wall structures compared to a smooth-walled tube and secondly, that a more homogeneous void fraction distribution leads to enhanced radial mixing and as a result to a better heat transport.

Methods

For the numerical investigation the Discrete Element Method (DEM) is used for the generation of a randomly packed bed. For the calculation of the fluid flow, temperature and species field within the packed bed with a finite volume code, at least the fluid domain has to be meshed. To avoid well-known problems in the vicinity of contact points a new method is used which flattens the particles locally to avoid bad quality cells. The small gap between the particles can then be filled with cells of a reasonable quality. It is shown, that this flattening does not affect significantly the global porosity, the pressure drop, the radial nor the axial porosity distribution over the packing. Furthermore, the whole process can be easily automated which reduces the calculation time for the whole process significantly.

Results

The general influence of wall structures on the void fraction distribution of a monodisperse spheres packing is investigated numerically and experimentally. It is shown that with structured walls the radial porosity distribution tends to be more homogenous and the radial velocity is increased significantly up to 50% compared to its value in a smooth-walled tube. Based on these promising results the wall structure is optimized numerically. With this optimized wall structure the heat transfer can be increased by approximately up to 50 %.

To show the influence of structured walls on other reactor designs, different particle to tube diameter ratios, polydisperse packings of spheres and packings with non-spherical particles are investigated. Furthermore a model reaction, the heterogeneous partial oxidation of methane on platinum, is superposed to study the effect of different particles shapes, diameter ratios and wall structures.