In an airlift bioreactor, what is the correct name for the region where air (or gas) is sparged to drive circulation?

Introduction / Context:
Airlift bioreactors use gas injection to create density differences between two communicating columns, driving circulation without mechanical agitation. Knowing the named regions is essential for scale-up and mass transfer discussions.

Given Data / Assumptions:

• Typical internal- or external-loop airlift configuration.
• Gas sparged into one section to induce upward flow.
• Liquid circulates through riser and downcomer via hydrostatic head difference.

Concept / Approach:
The section receiving gas forms the riser: gas–liquid dispersion reduces bulk density, causing upward flow. After disengagement at the top (gas released in the headspace within the disengagement zone), degassed liquid returns via the downcomer, completing the loop and providing shear characteristics suitable for shear-sensitive cultures.

Step-by-Step Solution:
1) Identify gas-injected zone: that is the riser.2) Recognize the degassed return path: the downcomer.3) Note the headspace separation region at the top: the disengagement zone.4) Therefore, the air sparging region is correctly termed the air riser.

Verification / Alternative check:
Flow visualization and pressure measurements show lower gas holdup and higher liquid density in the downcomer versus the aerated riser, producing the circulation driving head.

Why Other Options Are Wrong:
Downcomer: opposite section, where degassed liquid descends.Disengagement zone: where bubbles separate from liquid at the top, not where they are introduced.None of these: incorrect because riser is the standard term.

Common Pitfalls:
Confusing riser/downcomer due to schematic orientation or assuming the tallest section is always the riser regardless of sparging position.

Final Answer:
Air riser (riser section)