Large-scale airlift bioreactors: why is the air riser commonly located outside the draft tube (external-loop designs) in industrial units?

Introduction / Context:
Airlift bioreactors rely on density differences between riser and downcomer to drive circulation. Industrial-scale variants often employ external-loop configurations with the aerated segment (riser) placed outside a draft tube or external to the main column. The question asks why this placement is beneficial.

Given Data / Assumptions:

• Scale-up introduces constraints on gas distribution, heat removal, and hydrodynamics.
• We are comparing internal vs external riser placement under similar production conditions.
• Target cultures can be shear-sensitive and oxygen-demanding.

Concept / Approach:
External-loop airlifts allow engineers to tailor cross-sectional areas of riser and downcomer to tune circulation velocity, bubble residence time, and gas holdup. Better loop hydraulics improve both oxygen transfer (kLa) and overall heat removal (via jackets or external exchangers). Sparger configuration can be optimized without interfering with internals.

Step-by-Step Solution:
1) Positioning the riser externally simplifies routing of cooling surfaces and improves temperature control → better heat transfer.2) Flow path control raises circulation velocity and gas residence, aiding oxygen transfer.3) External riser geometry provides space for robust, maintainable spargers (even if compact rings are used), while preserving stable hydrodynamics.

Verification / Alternative check:
Case studies show that external-loop designs can match or exceed stirred-tank OTR at lower specific energy inputs, especially valuable for mammalian or fungal cultures.

Why Other Options Are Wrong:
Each individual reason is valid; picking any single one underestimates the multi-factor benefit. “None” is incompatible with routine design practice.

Common Pitfalls:
Assuming oxygen transfer is controlled only by sparger size; loop hydraulics and heat management are equally important.

Final Answer:
All of these