In an airlift bioreactor, what is the name of the vessel region where spent (off) gas disengages from the circulating liquid before leaving the system?

Introduction:
Airlift bioreactors circulate liquid by density differences created through gas injection, enabling gentle mixing with low shear. Understanding each hydraulic region is essential for oxygen transfer, gas hold-up, foam control, and cell viability. A key feature is the area where the injected gas separates from the liquid phase before exiting as off-gas.

Given Data / Assumptions:

• The vessel has two main circulation zones: an aerated riser and a non-aerated downcomer.
• Gas is injected at the base of the riser and must separate from liquid at the top.
• The question asks for the region name where gas disengages prior to venting.

Concept / Approach:
The top section of an airlift is deliberately expanded to lower superficial velocities and promote phase separation. This expanded headspace is called the disengagement zone. It reduces bubble carry-under into the downcomer, manages foam, and conditions off-gas leaving the reactor. Correct naming of regions is foundational for scale-up discussions and mass-transfer calculations (for example, kla estimation).

Step-by-Step Solution:

Verification / Alternative check:
Design drawings typically label the upper enlarged section as the disengagement zone; computational fluid dynamics and tracer studies show reduced gas hold-up in the downcomer, confirming effective separation at the top.

Why Other Options Are Wrong:

• Downcomer: The liquid flows downward here; it is not where gas is released.
• Air riser: This is the aerated upflow section, not the separation region.
• Flotsam: Refers to floating debris, not a bioreactor region.

Common Pitfalls:
Confusing the location of gas separation (top, expanded section) with regions defined by flow direction (riser vs downcomer). Another mistake is assuming foam breakers replace the need for a proper disengagement zone; geometry still matters for stable operation.

Final Answer:
disengagement zone