Aeration in stirred tanks — Why does a flooded impeller often lead to poor oxygen transfer rates (low k_La)?

Introduction:
In gas–liquid mixing, an impeller can become flooded when the incoming gas rate is so high that the impeller stalls in a gas cavity and loses effective liquid pumping. Flooding degrades gas dispersion, enlarges bubbles, and lowers gas–liquid interfacial area, thereby reducing oxygen transfer (k_La).

Given Data / Assumptions:

• Gassed stirred tank with radial-flow turbine or similar.
• High superficial gas velocity relative to impeller capacity.
• Flooding characterized by gas cavity around the impeller.

Concept / Approach:
Under flooding, bubbles coalesce under the impeller rather than being finely dispersed. Larger bubbles present lower area per volume and escape faster, diminishing residence time and interfacial area. The impeller’s circulation collapses, further lowering mass transfer. Optimal k_La requires balanced gas rate and impeller power to sustain dispersion without complete flooding.

Step-by-Step Solution:
Recognize flooding condition: gas cavity forms; liquid pumping drops.Identify consequence: increased bubble coalescence and larger average bubble size.Relate to k_La: interfacial area a decreases and liquid-side film renewal diminishes, lowering k_La.

Verification / Alternative check:
Empirical correlations show a peak in k_La versus aeration rate at fixed power; beyond a threshold, further gas increases induce flooding and reduce k_La—matching the explanation of coalescence and poor dispersion.

Why Other Options Are Wrong:

• (b) Excessive breakup would, if anything, increase interfacial area; the problem is coalescence.
• (c) Flooding does not increase contact time; large bubbles disengage rapidly.
• (d) Complete surface clogging by cells is not the general mechanism.
• (e) Turbulence does not vanish; it is misdirected and less effective at dispersion.

Common Pitfalls:
Confusing gas loading (beneficial up to a point) with flooding (detrimental); ignoring impeller type and submergence depth effects on flooding onset.

Final Answer:
Bubbles tend to coalesce beneath the impeller, reducing interfacial area.