In aerated, agitated bioreactors, “flooding” describes which operating condition with respect to gas–liquid dispersion by the impeller?

Introduction / Context:
Gas–liquid mass transfer is critical in aerobic bioprocesses. A key failure mode is “flooding,” where the agitator no longer disperses gas properly. Recognizing flooding is essential for preventing oxygen limitation and for protecting equipment from unstable hydrodynamics.

Given Data / Assumptions:

• Agitated, sparged vessel with a mechanical impeller.
• Gas flow rate and impeller speed determine dispersion quality.
• Operating viscosity and surface tension are in the typical aqueous range.

Concept / Approach:
Flooding occurs when the gas rate is too high (or impeller speed too low) for the impeller to break up and disperse bubbles. Gas then bypasses the impeller zone and rises as large pockets or channels, escaping without effective dispersion, which drastically reduces kLa and oxygen transfer.

Step-by-Step Solution:

Verification / Alternative check:
Power draw curves show a drop in effective power under gas load at flooding; visual inspection reveals large gas cavities and poor bubble dispersion.

Why Other Options Are Wrong:

• Medium spills/foam volume > broth: these describe foaming, not flooding.
• Sparging stops: that is a different failure (gas supply interruption).
• None of the above: incorrect because a precise definition exists.

Common Pitfalls:
Confusing flooding with “loading” (incipient gas accumulation) or with external foaming; flooding specifically concerns failed dispersion at the impeller.

Final Answer:
The air escapes without being distributed by the agitator