Effect of radial velocity profiles on axial dispersion in packed beds: asymptotic behaviour

Axial dispersion of a solute in a flow through packed beds arises from molecular diffusion, velocity variations at the interstitial-pore scale and systematic radial velocity profiles. Currently, there is not a widely accepted procedure for estimating the last contribution. One possible reason is the fact that radial velocity profiles in packed beds are intimately related on the radial porosity profile, which is not a deterministic property, and velocity variations are relatively mild and confined to a region close to the vessel walls. A widespread notion is that only in the zone about a particle radius from the wall is the fluid velocity significantly larger (wall channelling) than in the remaining of the bed core. Based on literature results and effective-model evaluations, the impacts of porosity profiles and related velocity profiles are explored in this manuscript. It is found that the dispersion of a solute can be increased several times in magnitude solely due to the wall channelling when the detailed porosity profile is considered. This can be explained in terms of small velocity differences between a second region (up to around five particle diameters) and the innermost bed region. Guidelines are also discussed for predicting likely levels of dispersion. A further aspect concerns the simplification of the treatment in the wall-zone.