Influence of the wall structure on the void fraction distribution in packed beds

The poor heat transport in packed-bed reactors is one of the major drawbacks of this reactor type. This characteristic is to some extent due to the non-homogeneous lateral void fraction distribution (VFD) in a tubular packed bed. The objective of this work was to find out, firstly, if it is possible to obtain more homogeneous VFDs in monodisperse packed beds of spherical particles by imposing appropriate wall structures and, secondly, if a more homogeneous void fraction distribution really leads to enhanced lateral mixing. VFDs for different wall structures were determined both by experiment and numerical simulation for a packing of monodisperse spherical particles. These investigations showed that the VFD strongly depends on the wall structure. Using a hexagonal arrangement of hemispheres as a wall structure only slightly reduces the height of the peaks of the VFD with respect to the case of a plain wall. A significantly more homogeneous VFD can be obtained if a wave-like orthogonal structure is applied. In a first case study it could be shown by computational fluid dynamics that the extend of lateral mixing in a flow through a monodisperse bed of spherical particles is increased if the radial void fraction distribution is more homogeneous.

► We have investigated numerically and experimentally the effect of wall structures on the void fraction distribution.
► Based on the wall structure the voidage in the near-wall region can be reduced significantly.
► It can be shown that the radial velocity and as a result the radial mixing increases.