Evaluation and characterization of axial distribution in expanded bed. I. Bead size, bead density and local bed voidage

•The axial distributions of resin bead in an expanded bed were investigated.•The distributions of bead size, bead density and bed voidage were focused.•The influences of bed height, expansion factor and mobile phase were studied.•The relative bed height and relative bed voidage were introduced.•Empirical equations were proposed to correlate and predict the axial distributions.

Expanded bed adsorption (EBA) is an innovative chromatography technology that allows the adsorption of target proteins directly from unclarified feedstock, and the most important property of an expanded bed is the perfectly classified fluidization of resin beads in the column. Due to the variation of both size and density of bulk resin beads, the axial distributions of bead size, bead density and bed voidage are the inherent characteristics of an expanded bed. However, the understanding on these properties is quite limited. In this study, raw beads (3% crosslinked agarose containing tungsten carbide) and 2 cm-diameter nozzle column were used as the model system and mean bead size, bead density and local bed voidage along the bed height were measured systematically with the in-bed sampling method for two settled bed heights (11.5 and 23.1 cm) and different expansion factors (1.4–2.6). With the increase of bed height, mean bead size and wet density of the beads decreased from 140 to 90 μm and from 4 to 2 g/ml, respectively. The local bed voidage increased from 0.6 to 0.9 with the increasing bed height. The relative bed height and relative bed voidage were introduced to describe the general rule of axial distribution. Some empirical equations were used to correlate the mean bead size, bead density and local bed voidage along the bed height with the standard deviations of 10.6%, 6.1% and 5.5, respectively. In addition, a general equation was proposed to predict the axial distributions of bead size, bead density and local bed voidage in the column with standard deviations less than 10% for most of the experimental data, which would be useful for the characterization of resin beads distribution in an expanded bed under varying operation conditions.