Multiple impeller reactors are widely used due to their advanced gas
utilization and an increased volumetric mass transfer coefficient.
However, with the application of Rushton impellers, gas dispersion
efficiency varies between the bottom and the upper impeller levels. The
present study analyzes the individual flow regime, power input and gas
hold-up in each compartment of a reactor equipped with four Rushton
impellers. The results indicate that the pre-dispersion of the air
introduced by the bottom impeller plays a key role in a better gas
retention efficiency of the upper impellers. In contrast, a flooded
bottom impeller adversely affects the gas dispersion of all impellers. A
novel analysis of the bubble flow in the dispersed state via a two-phase
CFD model reveals that a more homogenous distribution of air bubbles in
the upper compartments leads to high compartment gas hold-up values, but
fewer bubbles in the vicinity of the impellers. The measured and
simulated data of this study indicate that the upper impellers'
efficiency mostly depends on the flow regime of and the pre-dispersion
by the bottom impeller rather than on the upper impellers' flow regimes.
These results contribute to the understanding of essential mixing
processes and scaling of aerated bioreactors.