(625b) Influence of the Wall Structure On the Heat Transfer in Packed Beds with Small Tube to Particle Diameter Ratio

Authors: 
Zobel, N., Technische Universität Berlin
Behrendt, F., Technische Universität Berlin


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.