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Flooded impeller
• If the agitation speed is too low or the
air flow rate is too high, then a
phenomenom known as a flooded impeller
will occur.
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Slowly turning
impeller
Bubbles under
impeller coalesce
due to bubbles not
being dispersed and
sheared
2
• When the impeller is flooded, bubbles
will accumulate underneath the
impeller and coalesce.
• This leads to the formation of large
bubbles and poor oxygen transfer
efficiencies
3
Sparger design
• sparger design plays an important role
determining bubble size
• In stirred tank reactors, the bubble
size is determined by
– the sparger design
– the shear generated by the stirrer.
4
Sparge ring holes are aligned directly below the
impeller blades to maximize the shearing of the bubbles
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Detergents and antiforms
• Detergent-like and and oil-like
substances have dramatic effects on
bubble size.
– Detergents
– 消泡劑 (Antifoams)
– Effect of antifoams on the interfacial area
– Effect of antifoams on kL
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Hydrophilic end (親水端)
Hydrophabic end (疏水端)
帶有電荷即可防止泡泡的聯合(coalescence)
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• Antifoaming agents are most often used to
control foam levels. It include
– vegetable oils : used by the cells as a substrate
– silicone oils : biologically inert and thus can cause
problems in downstream processing.
antiform
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Effect of antifoams on the interfacial
area
• The accumulation of antifoam at the bubble
surface also leads to an increased tendency
for bubbles to coalesce.
• This effect is shown in the following
photograph showing the effect of the
addition of antifoam on bubble size in a
bubble column.
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Antifoams surrounding gas liquid interface
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