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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. Slowly turning impeller. Bubbles under impeller coalesce due to bubbles not being dispersed and sheared.
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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.
Slowly turning impeller Bubbles under impeller coalesce due to bubbles not being dispersed and sheared
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
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.
Sparge ring holes are aligned directly below the impeller blades to maximize the shearing of the bubbles
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
Hydrophilic end (親水端) Hydrophabic end (疏水端) 帶有電荷即可防止泡泡的聯合(coalescence)
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
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.
