The gas holdup and volumetric mass transfer coefficients were determined
in a bubble column of 150 mm diameter and 3 m height as a function of the
aeration rate in tap water and a nutrient solution of Candida boidinii in the
presence and absence of an antifoam agent [Ucolub N 115, a water-insoluble
poly(oxyethylenepropylene) copolymer]. Up to a superficial gas velocity
w SG = 3 cm s –1 the gas holdup was not influenced by the antifoam agent. Above
that the gas holdup approached a constant value of 0.15. The volumetric mass
transfer coefficient was more sensitive to the AFA. Above w SG = 1cm s –1 , k L a did
not change with the gas velocity. Depending on the antifoam concentration it
had a value 0.15 s –1 (0.1% Ucolub + 1% methanol + 1% salt solution) and 0.1 s –1
(0.1% Ucolub in water and in 1% methanol solution). Without an antifoam
agent the gas holdup was a factor of three higher at w SG = 3 cm s –1 and the k L a a
factor of four higher at w SG =2 cm s –1 [44].
During the production of penicillin G by Penicillium chrysogenum, the addition of lard oil to the cultivation medium increases the dissolved oxygen
concentration below 25% of saturation and reduces it above this value [45].
After addition of an antifoam agent to the cultivation medium, the balance between oxygen uptake rate (OUR) and oxygen transfer rate (OTR) is disturbed.
The increase in the DOC is probably caused by the stronger reduction of OUR
of the fungus (due to its diminished respiration of the fungus) than OTR. The
decrease in DOC above this value is due to the stronger reduction of the OTR
than the OUR.
The respiration rate of microorganisms can be evaluated by means of the O 2
and CO 2 balances. Nyiri and Lengyel [46, 47] observed that DOC was reduced
and OUR increased after addition of an antifoam agent to the medium. CO 2
entrapped inside the bubbles is released changing the composition of the offgas, if the foam is destroyed. This can reduce the dissolved CO 2 concentration
in the medium and enhance the respiration of the microorganisms, which
causes an increase in OUR and a decrease in DOC.
2.3.3
Influence of Antifoam Agents on Fluid Dynamics, Cell Growth and Product Formation
It is well known that fluid dynamics influence the process performance.
Therefore, bubble velocity and gas/liquid interfacial area were monitored
during the cultivation of E. coli. The effect of an AFA on the bubbles was determined by monitoring the bubble velocity with an ultrasound Doppler velocimeter (UDV) in situ [48, 49]. By adding an AFA to the cultivation medium, the
mean bubble velocity instantaneously increased by a factor of about two in the
airlift tower loop reactor during the cultivation of E. coli [50] (Fig. 1a).
After about half an hour, the bubble velocity dropped to the original
value, which indicates that the antifoam had disappeared from the cultivation medium. However, after several antifoam additions, the base line and
the maxima of the bubble velocity gradually increased. The cultivation
medium became more and more coalescence promoting. Monitoring the
intensity of the reflected ultrasound allowed the specific gas/liquid interfacial area a to be measured in situ (Fig. 1b). The specific interfacial area a
Recovery of Proteins and Microorganisms from Cultivation Media by Foam Flotation
203
in a bubble column of 150 mm diameter and 3 m height as a function of the
aeration rate in tap water and a nutrient solution of Candida boidinii in the
presence and absence of an antifoam agent [Ucolub N 115, a water-insoluble
poly(oxyethylenepropylene) copolymer]. Up to a superficial gas velocity
w SG = 3 cm s –1 the gas holdup was not influenced by the antifoam agent. Above
that the gas holdup approached a constant value of 0.15. The volumetric mass
transfer coefficient was more sensitive to the AFA. Above w SG = 1cm s –1 , k L a did
not change with the gas velocity. Depending on the antifoam concentration it
had a value 0.15 s –1 (0.1% Ucolub + 1% methanol + 1% salt solution) and 0.1 s –1
(0.1% Ucolub in water and in 1% methanol solution). Without an antifoam
agent the gas holdup was a factor of three higher at w SG = 3 cm s –1 and the k L a a
factor of four higher at w SG =2 cm s –1 [44].
During the production of penicillin G by Penicillium chrysogenum, the addition of lard oil to the cultivation medium increases the dissolved oxygen
concentration below 25% of saturation and reduces it above this value [45].
After addition of an antifoam agent to the cultivation medium, the balance between oxygen uptake rate (OUR) and oxygen transfer rate (OTR) is disturbed.
The increase in the DOC is probably caused by the stronger reduction of OUR
of the fungus (due to its diminished respiration of the fungus) than OTR. The
decrease in DOC above this value is due to the stronger reduction of the OTR
than the OUR.
The respiration rate of microorganisms can be evaluated by means of the O 2
and CO 2 balances. Nyiri and Lengyel [46, 47] observed that DOC was reduced
and OUR increased after addition of an antifoam agent to the medium. CO 2
entrapped inside the bubbles is released changing the composition of the offgas, if the foam is destroyed. This can reduce the dissolved CO 2 concentration
in the medium and enhance the respiration of the microorganisms, which
causes an increase in OUR and a decrease in DOC.
2.3.3
Influence of Antifoam Agents on Fluid Dynamics, Cell Growth and Product Formation
It is well known that fluid dynamics influence the process performance.
Therefore, bubble velocity and gas/liquid interfacial area were monitored
during the cultivation of E. coli. The effect of an AFA on the bubbles was determined by monitoring the bubble velocity with an ultrasound Doppler velocimeter (UDV) in situ [48, 49]. By adding an AFA to the cultivation medium, the
mean bubble velocity instantaneously increased by a factor of about two in the
airlift tower loop reactor during the cultivation of E. coli [50] (Fig. 1a).
After about half an hour, the bubble velocity dropped to the original
value, which indicates that the antifoam had disappeared from the cultivation medium. However, after several antifoam additions, the base line and
the maxima of the bubble velocity gradually increased. The cultivation
medium became more and more coalescence promoting. Monitoring the
intensity of the reflected ultrasound allowed the specific gas/liquid interfacial area a to be measured in situ (Fig. 1b). The specific interfacial area a
Recovery of Proteins and Microorganisms from Cultivation Media by Foam Flotation
203