In order to run the cultivations along the same trajectories as
shown above, i.e., to keep them on the same predefined pO2 level,
the culture agitated with two impellers had to be run at higher stirrer
speeds than the cultures equipped with three impellers. In Fig. 7
the kLa values adjusted by the pO2-controller are plotted as a function
of the specific power input. The Figure shows that over a wide
interval of the specific power input, the two-impeller version leads
to significantly higher kLa values than the three impeller version at
the same power input.
With increasing specific power inputs, the relative difference
becomes smaller, probably because the two impeller version is then closer to its limits with respect to its gas handling capability.
Note that the specific power inputs at which the experiments
were performed are already extremely high. The higher the power
input, the higher the gas recirculation and thus the gas load of the
impellers.
The results presented here are not obvious. Itis not only the local
energy dissipation density the impeller makes within the culture
what determines kLa. Additional to the dispersing shear the flow
generated by the impellers in order to distribute the bubbles across
the entire culture is of importance. This is a possible reason for the
fact that different impellers at the same power input often lead to
the same mass transfer rate although their shear is lower but the
flow higher or vice versa.