Why does increasing stirrer speed raise oxygen transfer rate (OTR) in an aerated bioreactor?

Introduction / Context:
Volumetric oxygen transfer, kLa, is central to aerobic bioprocess performance. Agitation affects several contributors to kLa and to the overall OTR, including bubble size distribution, interfacial area, and the liquid-side mass-transfer coefficient.

Given Data / Assumptions:

• Aerated, baffled stirred tank with mechanical agitation.
• Gas rate held constant while stirrer speed increases.
• Broth rheology in the low-to-moderate viscosity range.

Concept / Approach:
Agitation promotes bubble breakup and suppresses coalescence, reducing mean bubble diameter and increasing interfacial area a. Turbulence intensifies micro-mixing and thins boundary layers, raising kL. Stronger circulation renews oxygen-depleted liquid at bubble surfaces and transfers oxygenated liquid away, maintaining driving force (C* − C). These factors act together to raise OTR as speed increases—up to a point where gas flooding, shear damage, or power limits intervene.

Step-by-Step Solution:
1) Higher rpm → higher energy dissipation → smaller bubbles → larger a.2) Turbulence increases local gradients and reduces boundary-layer thickness → higher kL.3) Enhanced circulation distributes oxygen rapidly, sustaining driving force and preventing local depletion.4) Net: OTR rises with rpm until other constraints dominate.

Verification / Alternative check:
Dynamic gassing-out tests show kLa increasing with rpm at fixed aeration; off-gas analysis confirms higher oxygen uptake capacity.

Why Other Options Are Wrong:
Each individual mechanism contributes; selecting only one understates the system behavior. “None” contradicts established mixing and transfer principles.

Common Pitfalls:
Overdriving rpm can increase shear and antifoam use, which may reduce kLa—there is a practical optimum.

Final Answer:
All of the above