3. Results and discussion
3.1. kLa measurements
During the E.coli cultivations the airflow was modulated
between 0.5 vvm and 4 vvm as shown in the upper part of Fig. 4. The
controller, keeping pO2 at 25%sat, then adjusts the stirrer speed to
the values shown in the lower part of the Figure.
The resulting kLa as a function of these airflow/stirrer speed values
are shown in Fig. 5. As can be seen, kLa does not react very
sensitively on the airflow modulations, it much more sensitively
responds to changes in the stirrer speed. This clearly shows that
in normal cultivation experiments it makes sense to first increase
the airflow to a representative value and then to adjust the pO2 by
changing the stirrer speed.
In order to fully exploitthe oxygen transfer capabilities oftypical
laboratory scale reactor such as the Biostat C used in these experiments,
the stirrer speed was increased up to values of 1400 rpm. At
aeration rates below 1 vvm, kLa values appear in the order of magnitude
of 2500 h−1. Even much higher kLa values are possible. This
can be followed from the graphs depicted in Fig. 5, namely that at
full aeration (4 vvm) the stirrer speed to achieve a kLa of 2500 h−1
was only 500 rpm.
In the lower part (b) of Fig. 5 the kLa measurement data are
displayed as a function of the superficial gas velocity wsg and the
specific power input. This representation was chosen to allow a
comparison with the standard kLa (s−1) correlations.These are of the form
3. Results and discussion3.1. kLa measurementsDuring the E.coli cultivations the airflow was modulatedbetween 0.5 vvm and 4 vvm as shown in the upper part of Fig. 4. Thecontroller, keeping pO2 at 25%sat, then adjusts the stirrer speed tothe values shown in the lower part of the Figure.The resulting kLa as a function of these airflow/stirrer speed valuesare shown in Fig. 5. As can be seen, kLa does not react verysensitively on the airflow modulations, it much more sensitivelyresponds to changes in the stirrer speed. This clearly shows thatin normal cultivation experiments it makes sense to first increasethe airflow to a representative value and then to adjust the pO2 bychanging the stirrer speed.In order to fully exploitthe oxygen transfer capabilities oftypicallaboratory scale reactor such as the Biostat C used in these experiments,the stirrer speed was increased up to values of 1400 rpm. Ataeration rates below 1 vvm, kLa values appear in the order of magnitudeof 2500 h−1. Even much higher kLa values are possible. Thiscan be followed from the graphs depicted in Fig. 5, namely that atfull aeration (4 vvm) the stirrer speed to achieve a kLa of 2500 h−1was only 500 rpm.In the lower part (b) of Fig. 5 the kLa measurement data aredisplayed as a function of the superficial gas velocity wsg and thespecific power input. This representation was chosen to allow acomparison with the standard kLa (s−1) correlations.These are of the form
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