Diagnosing mass transfer limitation from agitation effects: if increasing stirring speed from 300 rpm to 500 rpm doubles the reaction rate, what is the most reasonable conclusion?

Introduction / Context:
In gas–liquid or liquid–solid bioreactions, overall rate can be limited by intrinsic enzyme/cellular kinetics or by transport of substrates/products through boundary layers. Observing how rate responds to agitation provides a practical diagnostic for transport limitation during process development.

Given Data / Assumptions:

• Stirring speed increase: 300 → 500 rpm.
• Measured overall reaction rate doubles.
• Other variables (temperature, composition) unchanged.

Concept / Approach:
If rate increases substantially with agitation, hydrodynamics—and thus mass transfer coefficients—have become more favorable. When boundary-layer resistance dominates at low rpm, increasing rpm thins boundary layers and enhances interfacial renewal, raising flux and overall rate. Once mass transfer ceases to limit, further rpm increases do little, and rate plateaus at the kinetic limit.

Step-by-Step Solution:
1) At 300 rpm, agitation is weaker; film resistance is larger; interfacial area and kL are lower.2) Raising to 500 rpm increases turbulence and circulation, reducing resistance and boosting kLa or kL.3) The observed doubling of rate indicates transport control was significant at 300 rpm.4) Therefore, the logical conclusion is mass transfer limitation at the lower speed.

Verification / Alternative check:
Plotting rate versus rpm typically shows a rising section (transport-limited) and a plateau (kinetic-limited). The data point lies on the rising section, consistent with transport limitation at 300 rpm.

Why Other Options Are Wrong:
Option B: If 500 rpm were transport-limited, increasing rpm would still increase rate; the conclusion concerns the lower rpm. Option C: Clear agitation dependence contradicts “no mass transfer.” Option D: Even at 300 rpm, bulk convection exists; diffusion-only is implausible. Option E: Intrinsic kinetics do not change with rpm absent temperature or chemistry changes.

Common Pitfalls:
Confusing enhanced kLa with altered intrinsic kinetics; agitation changes transport, not enzyme turnover numbers.

Final Answer:
The reaction was mass transfer limited at 300 rpm