3.4. Influence of baffle sizes
As mass transfer measurements are now possible during the
E.coli cultivations with a rather high precision, the question
whether or not further changes in the mixing equipment will lead to
different performance can now be answered. In Fig. 9 the influence
of the baffle size is addressed. It can be seen that different baffles
led to different kLa values at the same specific power inputs.
These results confirm the qualitative knowledge that an agitator
only works properly when the impeller and the baffle system
harmonize. Although wider baffle bars lead to higher energy consumptions
at the same stirrer speed, their energetic efficiency with
respect to the mass transfer rates, as characterized by the kLa value
at a given power input, is smaller within the relevant specific power
input interval.
Interestingly, curved baffle bars in form of half pipes,which were explored after it became evident
that curved impeller blades led to a better performance, did not
depict an improvement with respect to the oxygen mass transfer
efficiency.
This does not necessarily mean that smaller baffles are to be preferred.
In cultures suffering from excessive foam formation, lower
impeller speeds might be favorable. In the experiments reported in
this paper, where all processes were run on one and the same trajectory,
the consumption of antifoam agent was significantly lower
(−30%) in the experiments with 30 mm baffle rods as compared
to the experiments performed with 15 mm rods. Where antifoam agents lead to problems during downstream processing, it is thus
worth thinking about wider baffle rods.
3.4. Influence of baffle sizesAs mass transfer measurements are now possible during theE.coli cultivations with a rather high precision, the questionwhether or not further changes in the mixing equipment will lead todifferent performance can now be answered. In Fig. 9 the influenceof the baffle size is addressed. It can be seen that different bafflesled to different kLa values at the same specific power inputs.These results confirm the qualitative knowledge that an agitatoronly works properly when the impeller and the baffle systemharmonize. Although wider baffle bars lead to higher energy consumptionsat the same stirrer speed, their energetic efficiency withrespect to the mass transfer rates, as characterized by the kLa valueat a given power input, is smaller within the relevant specific powerinput interval.Interestingly, curved baffle bars in form of half pipes,which were explored after it became evidentthat curved impeller blades led to a better performance, did notdepict an improvement with respect to the oxygen mass transferefficiency.This does not necessarily mean that smaller baffles are to be preferred.In cultures suffering from excessive foam formation, lowerimpeller speeds might be favorable. In the experiments reported inthis paper, where all processes were run on one and the same trajectory,the consumption of antifoam agent was significantly lower(−30%) in the experiments with 30 mm baffle rods as comparedto the experiments performed with 15 mm rods. Where antifoam agents lead to problems during downstream processing, it is thus
worth thinking about wider baffle rods.
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