4. Conclusions
1. During well controlled cultivation processes, the oxygen mass
transfer rate and the corresponding mass transfer coefficient can
be measured online to quite a high precision.
2. These measurements can be performed automatically by means
of an up-to-date automation system. The measurement procedure
does not affect the cultivation process.
3. Measurements automatically performed with professional process
control systems (here Siemens PCS7) are so precise that
they can resolve existent differences in the actual kLa values
appearing in differently equipped reactors at the same total
power input.
4. Comparisons with literature data require simultaneous estimates
of the power drawn by the impeller.At high specific power
inputs estimates can be made via heat removal measurements.
5. Mass transfer rates do not always depend on the integral power
dissipation density only. Local effects may play a non-negligible
role.
6. In the reactors used in this work, two impeller agitators perform
better than the standard three-impeller versions. This does not
only hold true with respect to mass transfer but also from the
mixing point of view.
7. Possibly, the improved top-to-bottom mixing is the main reason
for the improvements in the mass transfer rate of the two impeller
arrangement.
8. The techniques proposed are independent of scale and can be
transferred to production reactors as well.