Using pure oxygen instead of air in an aerated bioreactor generally increases oxygen transfer because which factor changes most directly in the mass-transfer driving force?

Introduction / Context:
Oxygen transfer rate (OTR) in gas-sparged reactors is often expressed as OTR = kLa * (C* − C), where C* is the saturation DO concentration and C is the bulk DO. Replacing air with pure oxygen mainly affects the driving force term, not necessarily the hydrodynamic coefficient kLa.

Given Data / Assumptions:

• Same mixer, gas rate adjusted to avoid flooding.
• Temperature and salinity unchanged.
• No intentional change in bubble size distribution beyond gas composition.

Concept / Approach:
At fixed pressure and temperature, C* is proportional to the oxygen mole fraction in the gas phase (Henry’s law). Pure oxygen has a much higher oxygen partial pressure than air (≈1.0 vs ≈0.21), so C* increases several-fold, boosting OTR. Bubble size and kLa may or may not change appreciably; the primary, guaranteed effect is the larger C* − C driving force.

Step-by-Step Solution:

Verification / Alternative check:
Measured DO rise curves and sulfite tests under identical hydrodynamics show OTR scaling with C*, consistent with Henry’s law and increased pO2.

Why Other Options Are Wrong:

• Smaller bubbles are not automatic; gas composition alone does not guarantee it.
• kLa is a hydrodynamic parameter; it does not inherently increase with pure O2.
• Partial pressure is higher, not lower, with pure oxygen.
• Broth density change is negligible and irrelevant here.

Common Pitfalls:
Attributing OTR gains to kLa rather than to C*; overlooking safety, cost, and flammability considerations with pure O2.

Final Answer:
The saturation concentration of oxygen is higher.