Bioreactor design insight:\nIf the liquid height (working height) of an aerated bioreactor is increased while other operating conditions remain comparable, which mechanism most directly explains a potential increase in the oxygen transfer rate (OTR)?

Introduction / Context:
Oxygen transfer rate (OTR) in aerated bioreactors depends on the overall mass transfer coefficient (KLa) and the driving force (C* - C), where C* is the saturation concentration of oxygen and C is the bulk dissolved oxygen. Changing the liquid height alters hydrostatic pressure and bubble residence time, which can influence these terms. This item asks which mechanism most directly supports an OTR increase when height is increased.

Given Data / Assumptions:

• Liquid height is increased without changing gas type, sparger, or impeller type.
• Aeration is maintained; temperature is held similar.
• We compare mechanisms qualitatively rather than calculating exact OTR.

Concept / Approach:
Henry’s law implies that the saturation concentration C* of a gas in liquid increases with the partial pressure of that gas at the gas–liquid interface. In a taller column, hydrostatic pressure at the bottom is higher, increasing the local oxygen partial pressure around newly formed bubbles and thus increasing local C*. Longer bubble path length can also increase interfacial contact and gas holdup, but the most direct and universal explanation is the increased partial pressure at depth raising C* and hence the driving force (C* - C).

Step-by-Step Solution:

Verification / Alternative check:
Engineering correlations for bubble columns report positive effects of static head on C*; experimental profiles often show higher dissolved oxygen near the base at larger heights due to increased C* and contact time.

Why Other Options Are Wrong:

• Decreasing saturation concentration: height does not lower C*; it tends to increase local C* at depth.
• Decreasing gas holdup: would usually reduce interfacial area and lower KLa.
• All of the above: includes incorrect statements.
• No effect: contradicts pressure-based increases in C* and residence time.

Common Pitfalls:
Assuming that only KLa changes; in reality, both KLa and C* can vary with geometry. Also, ignoring the role of temperature (higher temperature lowers C* even if height rises).

Final Answer:
Increasing the partial pressure of oxygen at the base of the reactor