Conclusions
The current work presents new insights into stirring and aeration of a
pilot-scale multistage Rushton impeller bioreactor. The complementary
experimental and simulation analyses indicate that the pre-dispersion of
air by the bottom impeller leads to a significantly reduced local gas
load at the upper impellers, and thus, to the superior gas utilization
efficiency and high power input of the multiple impeller stages.
However, the benefits directly depend on the bottom impeller flow
regime.
This conclusion is based on several new experimental findings:
- An individual, cumulative shift in the formation of cavities and
flooding at the upper impellers towards higher Fl numbers
- A moderate decrease of the relative power input at the upper impellers
for elevated aeration rates
- A superior gas utilization efficiency of the upper impellers, i.e.
sensitivity of the gas hold-up with respect to stirring and aeration
- An enhanced power input with each impeller level, although the gas
hold-up increases
- An individual sensitivity of gas hold-up (and power input) in the
upper bioreactor compartments with respect to stirring and aeration,
dependent on the bottom stirrer flow regime, and, independent of the
local flow regime of the upper impellers
The novel two-phase CFD simulation results support the above conclusion
and are consistent with the experimental data:
- The higher gas hold-up in the upper compartments is due to the larger
number and smaller size of recirculating gas bubbles
- A large fraction of gas bubbles in the upper compartments is
recirculating without moving through the impeller disc region
- The newly introduced local aeration rate reflects this behavior and
indicates lower local Fl numbers for the upper impellers. This
corresponds to the experimentally demonstrated shift of the flow
regime transition towards higher Fl numbers in terms of the overall
aeration rate.
The work provides novel aspects towards at mechanistic understanding of
the complex gas dispersion and distribution behavior of multiple Rushton
impeller bioreactors, particularly of the individual contribution by
each impeller stage. Moreover, this knowledge will open up new
opportunities for optimizing their design and operating conditions of
bioreactors.