In an airlift bioreactor, what is the name of the flow region where the bulk liquid returns downward after gas–liquid separation has occurred at the top?

Introduction:
Airlift reactors exploit density differences between an aerated zone and a non-aerated zone to drive circulation without mechanical agitation. Correct identification of the downcomer and riser is essential for interpreting mixing time, kla distribution, and shear exposure for sensitive cultures.

Given Data / Assumptions:

• Gas is sparged into the riser to decrease its density.
• Separated liquid must return to the bottom to complete the loop.
• The region with downward flow is sought.

Concept / Approach:
The riser contains gas–liquid mixture rising due to buoyancy, while the downcomer contains relatively gas-free liquid descending by hydrostatic head difference. Together they establish a stable circulation loop. The disengagement zone at the top allows phase separation to sustain this density gradient.

Step-by-Step Solution:

Verification / Alternative check:
Tracer experiments and flow visualization confirm downflow in the downcomer with lower gas holdup versus the riser. Velocity profiles and pressure measurements align with the density-driven loop model.

Why Other Options Are Wrong:

• Disengagement zone: Located at the top for gas separation, not downward flow.
• Air riser: The upward flowing, aerated region.
• Flotsam: Not a technical term for flow regions in bioreactors.

Common Pitfalls:
Assuming bubbles routinely enter the downcomer; excessive carry-under indicates poor disengagement or too high gas rate, which can destabilize circulation and reduce kla efficiency.

Final Answer:
downcomer