Airlift design choice: why is an internal draft tube commonly preferred over an external loop in airlift fermenters for sterile operation?

Introduction / Context:
Airlift fermenters can be configured with an internal draft tube (internal-loop) or with external piping (external-loop) to define riser and downcomer paths. The internal design is widely used in bioprocessing where cleanability and sterility are paramount.

Given Data / Assumptions:

• Biopharma processes require clean-in-place (CIP) and steam-in-place (SIP).
• Complex external loops increase welds, gaskets, and dead-legs.
• Capital and maintenance costs matter in scale-up.

Concept / Approach:
An internal draft tube is integrated inside the vessel, minimizing external surfaces and fittings. This geometry simplifies CIP/SIP validation, reduces contamination risk, and typically lowers fabrication cost versus adding external columns, bends, and nozzles. Hydrodynamics remain favorable for shear-sensitive cultures.

Step-by-Step Solution:

Verification / Alternative check:
Regulated facilities often prefer monolithic, easily accessible internals for documented CIP coverage and SIP lethality mapping.

Why Other Options Are Wrong:

• (a) alone or (b) alone are incomplete; both are true.
• “None”: contradicted by hygienic design practice.
• Eliminates gas distributor: false; a sparger/distributor is still required.

Common Pitfalls:
Overlooking validation burden of external loops; underestimating contamination risk posed by additional flanges and seals.

Final Answer:
Both (a) and (b)