Influence of the Particle Size Distribution on Hydraulic Permeability and Eddy Dispersion in Bulk Packings

The narrow particle size distribution (PSD) of certain packing materials has been linked to a reduced eddy dispersion contribution to band broadening in chromatographic columns. It is unclear if the influence of the PSD acts mostly on the stage of the packing process or if a narrow PSD provides an additional, intrinsic advantage to the column performance. To investigate the latter proposition, we created narrow-PSD and wide-PSD random packings based on the experimental PSDs of sub-3 mu m core shell and sub-2 mu m fully porous particles, respectively, as determined by scanning electron microscopy. Unconfined packings were computer-generated with a fixed packing protocol at bed porosities from random-close to random-loose packing to simulate fluid flow and advective diffusive mass transport in the packings' interparticle void space. The comparison of wide-PSD, narrow-PSD, and monodisperse packings revealed no systematic differences in hydraulic permeability and only small differences in hydrodynamic dispersion, which originate from a slightly increased short-range interchannel contribution to eddy dispersion in wide-PSD packings. The demonstrated intrinsic influence of the PSD on dispersion in bulk packings is negligible compared with the influence of the bed porosity. Thus, the reduced eddy dispersion observed for experimental core shell packings cannot be attributed to a narrow PSD per se.