Airlift design choice: why is an internal draft tube commonly preferred over an external loop in airlift fermenters for sterile operation?
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AIt is easier to clean and sterilize the reactor
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BIt is cheaper to construct compared to an external loop
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CBoth (a) and (b)
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DNone of the above
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EIt eliminates the need for a gas distributor entirely
Answer
Correct Answer: Both (a) and (b)
Explanation
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:
1) Compare flow-path geometries: internal vs external.2) Evaluate hygienic design: fewer crevices and seals with internal tube.3) Consider cost: less piping and fewer high-hygiene fittings lowers capex/opex.4) Conclude both cleanability and cost advantages favor internal tubes.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)