During gas transfer from the bulk liquid to an air bubble in an aerated bioreactor, which step generally offers the greatest resistance for carbon dioxide (CO₂) leaving the liquid phase?

Introduction:
Understanding where mass transfer resistances arise is crucial for designing aeration systems and predicting removal or absorption of gases. For CO₂ desorption from liquid to bubbles, the controlling resistance is typically on the liquid side near the bubble surface.

Given Data / Assumptions:

• Aerated, well-mixed bulk liquid.
• Small gas bubbles rising through liquid.
• CO₂ transferring from liquid to gas.

Concept / Approach:
In two-film theory, mass transfer is controlled by thin stagnant films at phase boundaries. For sparingly soluble gases and typical bioreactor conditions, the gas-side resistance is small; the main resistance is the liquid-side film (the bubble boundary layer). Turbulence reduces, but does not eliminate, this resistance. The interface itself does not impose a separate large resistance under usual conditions; bulk liquid is well mixed and contributes little resistance compared with the liquid film.

Step-by-Step Solution:

Verification / Alternative check:
Increased agitation (higher k_La) primarily improves liquid-side coefficients, confirming the dominance of liquid film control for CO₂ transfer in typical systems.

Why Other Options Are Wrong:

• Interface: Usually not a significant independent barrier for CO₂ under normal conditions.
• Cell membrane: Relevant for intracellular transfer, not for bulk-to-bubble desorption.
• Bulk liquid: Well-mixed bulk presents relatively small resistance versus films.

Common Pitfalls:
Assuming gas-side control for CO₂ in water-like systems; gas-side control is more common for highly soluble gases absorbed into liquids under specific conditions.

Final Answer:
Moving through the bubble boundary layer