Numerical study of cylindrical particles' orientation in narrow packed beds

Abstract

Narrow packed beds are usually employed when either the removal of heat from highly exothermic chemical reactions or the supply of heat to highly endothermic reactions is necessary. Both cases impose restrictive constrains for the tube diameter to be relatively small, while the need for high gas flow rates constrain the particle diameter to be fairly large to have a reasonable pressure drop. Packed bed with small tube-to-particle diameter (Dc/Dp) ratios are strongly affected by the so-called wall effect which influences both the porosity and particles' arrangement. So far research on the structure of such packed beds focus mostly on porosity distribution. It is justified when spherical particles are considered, however, for other geometries as, for example, cylindrical particles, information about orientation distribution is of high interest. The research main focus is the study of the orientation of particles dumped randomly into a cylindrical column. The packed structure is simulated with a numerical algorithm in which particles are added sequentially and move until mechanical equilibrium is reached. The present paper is aimed at examining narrow packed beds for a wide range of Dc/Dp ratio from 2 to 7 and for particle height-to-diameter ratio (Hp/Dp) ranging from 0.33 to 2. The results are supplemented with global porosity distributions and reproducibility tests of the packing structures. Selected results are also compared with those obtained for Raschig rings. Orientation distributions of Raschig rings slightly differ from the ones of full cylinders for Hp/Dp ≤ 1. The reproducibility tests indicated that during independent repackings of the container the averaged variability can be even as high as 50% for the narrowest bed investigated. The axial porosity profiles are barely sensitive to the number of particles – its effect is visible only within first ten layers of particles.