Mass transfer vs kinetics diagnosis: An alkane has a maximum solubility (saturation concentration) of 5 ppm in the medium. If the measured dissolved concentration during fermentation is 1 ppm, what conclusion about growth limitation can be made from this information alone?
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AThe growth rate is limited by the mass transfer rate of the alkane into the medium
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BThe growth rate is limited by cell kinetics and not by mass transfer
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CThe growth rate is neither mass-transfer- nor kinetically limited
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DNo conclusion on growth limitation can be drawn from these data alone
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EThe culture is substrate-inhibited by the alkane
Answer
Correct Answer: No conclusion on growth limitation can be drawn from these data alone
Explanation
Introduction / Context:Diagnosing whether a bioprocess is limited by mass transfer or by intrinsic kinetics requires comparing supply and demand. Knowing only the saturation concentration (5 ppm) and the instantaneous dissolved concentration (1 ppm) is insufficient to assign a rate limitation. This question tests your ability to recognize what additional information is needed to distinguish limitations.
Given Data / Assumptions:
- Saturation concentration C* = 5 ppm at operating T and P.
- Measured dissolved concentration C = 1 ppm in the bulk.
- No values provided for KLa, uptake kinetics (e.g., Monod constants), or critical concentration.
Concept / Approach:Mass transfer limitation is assessed by comparing the maximum transfer rate OTR (or STR for substrate) = KLa * (C* - C) with the biological uptake rate OUR (or SUR). A low measured concentration might arise because cells consume substrate quickly (kinetic demand high) or because transfer is inadequate (KLa low). Without KLa and the cell uptake parameters (μ vs substrate, or specific uptake rate qS) and the critical concentration threshold, we cannot declare the dominant limitation.
Step-by-Step Solution:
Identify given data: C* and instantaneous C only.Recognize the missing pieces: KLa (or Kla equivalent) and cellular kinetics (qS, Ks, Ccrit).Conclude that multiple scenarios are consistent with C = 1 ppm (mass-transfer-limited or kinetically limited).Therefore, no definitive conclusion can be drawn from concentration values alone.Verification / Alternative check:Perform a step-change test in gas–liquid transfer (e.g., increase agitation/aeration). If growth improves, mass transfer is implicated; if not, intrinsic kinetics or other nutrients likely limit.
Why Other Options Are Wrong:
- Declaring mass transfer or kinetics a priori requires rates, not just concentrations.
- Stating neither is limiting contradicts the possibility that either could be.
- Substrate inhibition cannot be inferred from a concentration below saturation without inhibition data.
Common Pitfalls:Equating C < C* with mass transfer limitation automatically; the key is whether supply rate meets demand at the operating point.
Final Answer:No conclusion on growth limitation can be drawn from these data alone