Estimating interfacial boundary-layer concentration (C_o*): in an aerated bioreactor, how is the average C_o* around bubbles typically determined for k_La measurements?
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Aultra-small dissolved oxygen probes inserted into the bubble boundary layer
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Blaser based photographic systems measuring oxygen gradients at interfaces
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Cmeasuring the steady state concentration of oxygen in the bulk liquid prior to inoculation of the reactor
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Dmeasuring the steady state concentration of oxygen in the bulk liquid after inoculation of the reactor
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Eby assuming C_o* equals zero during vigorous aeration
Answer
Correct Answer: measuring the steady state concentration of oxygen in the bulk liquid prior to inoculation of the reactor
Explanation
Introduction:Determining k_La often uses clean (microbe-free) water or medium to obtain the saturation concentration C* under given aeration and agitation. This practical approach avoids biological oxygen uptake and provides a reproducible reference for mass-transfer studies.
Given Data / Assumptions:
- Before inoculation, biological consumption is negligible.
- Under steady aeration/mixing, the bulk DO equals the saturation concentration C*.
- C_o* used in OTR expressions is taken from these baseline measurements.
Concept / Approach:In practice, one measures the bulk dissolved oxygen at steady state in clean medium under the same gas and agitation conditions. This value approximates C* (or C_o* in certain notations), the driving concentration at the interface for mass transfer calculations. Direct micro-scale measurement at bubble interfaces is not common due to instrumentation limits and disturbance of the boundary layer.
Step-by-Step Solution:Aerate and agitate clean medium until DO stabilizes at the saturation value.Record this steady-state bulk DO as C* for subsequent k_La testing.Use dynamic or steady-state methods post-inoculation, referencing this baseline for driving-force estimates.
Verification / Alternative check:Kla determinations frequently start with water or buffer to establish C*; dynamic gassing-out methods further validate k_La using this reference.
Why Other Options Are Wrong:
- Ultra-small probes or laser imaging: Not standard or practical at the bubble boundary layer for routine k_La work.
- After inoculation: DO is depressed by uptake, not equal to C*.
- Assuming C_o* = 0: Physically incorrect; saturation concentration is nonzero.
Common Pitfalls:Confusing bulk DO under biological consumption with saturation values; always measure C* before inoculation or by re-saturating depleted broth.
Final Answer:measuring the steady state concentration of oxygen in the bulk liquid prior to inoculation of the reactor