Evaluation and characterization of axial distribution in expanded bed: II. Liquid mixing and local effective axial dispersion

• Axial distributions of liquid mixing in a nozzle expanded bed were investigated.
• Axial distributions of local effective axial dispersion were evaluated.
• Influences of expansion factor, settled bed height and mobile phase were discussed.
• Some unique tendency of local effective Dax along the bed height was found.
• Potential mechanism was discussed to describe the axial distribution behaviors.

Expanded bed adsorption (EBA) is a promising technology to capture proteins directly from unclarified feedstock. In order to better understand liquid mixing along the bed height in expanded beds, an in-bed sampling method was used to measure residence time distribution at different bed heights. A 2 cm diameter nozzle column was tested with agarose raw beads (3% crosslinked agarose containing tungsten carbide). Two settled bed heights (11.5 and 23.1 cm) with different expansion factors (1.4–2.6) were investigated and the number of theoretical plates (N), the height equivalent of theoretical plate (HETP) and the local effective axial dispersion coefficient (Dax) were calculated for each bed height-defined zone. The effects of expansion factor, settled bed height and mobile phase were evaluated. The results showed that N increased with the increase of expansion factors, but Dax was unaffected under fixed bed heights. Dax and HETP were found similar as a function of relative bed height for two settled bed heights tested. Higher mobile phase viscosity resulted in stronger axial dispersion. In addition, the local effective Dax under the expansion factor near 2.0 had a different profile which showed minimum values at 0.6–0.8 relative bed height, and the potential mechanism was discussed. These results would be useful for the characterization of axial dispersion and modeling protein adsorption in expanded beds under varying operation conditions.