Mixing studies in a model aerated bioreactor equipped with an up- or a down-pumping ‘Elephant Ear’ agitator: Power, hold-up and aerated flow field measurements

Abstract

The hydrodynamics of a model bioreactor stirred by an ‘Elephant Ears’ impeller, either up- (EEU) or down-pumping (EED) have been studied under unaerated and aerated conditions. For the latter, air flow rates of 0.01–0.05 vvm (typical for animal cell culture) and 0.5 vvm (at the lower end of those used for bacterial fermentations) were used. For all these four conditions, the detailed flow of the liquid phase was studied using particle image velocimetry (PIV) with red fluorescent seeding particles, which emitted at a different frequency from the laser light. This approach enabled exclusion of the laser light scattered by the bubbles using an optical filter. From these measurements, quantitative global flow fields, axial velocity profiles, flow numbers, turbulent kinetic energy and vorticity have been determined. In addition, power, power numbers and hold-up were obtained. For the up-pumping mode, turbulent power numbers were not affected at any aeration rate, i.e., P g/ P ≈ 1 whilst for the down-pumping EE impeller, P g/ P decreased by up to 30% at the highest rate. In addition, the hold-up was greater in the up-pumping mode. The global flow field and mean liquid velocities were not significantly affected at the lower air flow rates, but for the highest aeration rate of 0.5 vvm, the liquid velocities were decreased regardless of pumping mode. Maximum values of turbulent kinetic energy (TKE) in the discharge stream were relatively unaltered, but the distribution of TKE in the bulk changed significantly with the mean values decreasing with increasing aeration. Flow numbers decrease by 20–30% from the unaerated case at the highest air flow rate, especially for the EED. The results show that, as with other axial flow impellers, the up-pumping mode offers significant advantages over the down flow.