Bubble column bioreactors are characteristically designed with which geometrical proportion to promote efficient gas–liquid contact and mixing?

Introduction:
Bubble columns are simple, sparged vessels with no mechanical agitators. Their hydrodynamics and mass transfer rely on gas flow, bubble size distribution, and vessel geometry. A defining feature is the tall, slender design that supports plug-like upward flow and sufficient gas residence time.

Given Data / Assumptions:

• No internal impeller; mixing is driven by rising bubbles.
• Design must maximize interfacial area and contact time.
• Geometry directly affects gas holdup, backmixing, and kla.

Concept / Approach:
A large height/diameter (H/D) ratio provides a longer path for bubbles to travel, improving contact with liquid and enhancing oxygen transfer. The slender profile also supports a more defined circulation pattern and can suppress large-scale backmixing at appropriate gas rates, aiding scale-up predictability.

Step-by-Step Solution:

Verification / Alternative check:
Empirical kla correlations for bubble columns include dependencies on superficial gas velocity and H/D. Pilot-to-plant comparisons consistently favor higher H/D for aerobic cultures with moderate viscosity.

Why Other Options Are Wrong:

• Small H/D or large D/H: Reduce bubble contact time and mass transfer.
• Small D/H wording duplicates the same disadvantage.

Common Pitfalls:
Assuming taller is always better; extreme H/D can complicate gas distribution and structural design. Sparger type, antifoam strategy, and broth rheology must also be optimized.

Final Answer:
large height to diameter ratio