Can a fermentation system experience higher OUR values than OTR. How does this affect the process parameters.
hi Nave, your answer is already public,
just google using this string (and read the hits):
"oxygen uptake rate" "than oxygen transfer rate"
In steady-state situation OUR=OTR. If OTR>OUR or OUR>OTR, dissolved oxygen concentration (DO) will increase or decrease. That is unsteady-state case. Because OUR and OTR are function of the oxygen concentration (see McHardy), any discrepancy between them result in a new steady state, and it takes rather short time. If OUR or OTR changes slowly (as a result, for instance, the biomass growth), a difference between them are very small and you can assume OUR~OTR.
I totally share the opinions of Mr. Biryukov and Mr.Mc Hardy. I also advise you to read the articles of Prof. S O Enfors concerning the impact of KLa and therefore the influence of oxygenation during a fermentation
Thanks everyone for your comments. I am also interested in knowing if the viscosity of the media has any effect on the scenario that I described.
indeed, the viscosity can influence OUR and therefore the value of KLa
The value of KLa influence on OTR, and not on OUR. But the high viscosity influence on the optimal DO level, which continuously changes during fermentation and therefore OUR also changes. In more detail see article: Biryukov V.V., Arkhipov M.Yu. Consumption of oxygen by mycelial cultures of microorganisms in a fermenter. – Theoretical Foundations of Chemical Engineering, 2008, vol.42, No.5, pp. 542-549.
I agree with Mr. Biryukov that KLa influences OTR but not on OUR. However, viscosity affects the water characteristics which in turn affects the liquid film coefficient according to the two-film theory, and so changes in viscosity will affect KLa. Also, when the OUR exceeds OTR, I agree that "Because OUR and OTR are function of the oxygen concentration (see McHardy), any discrepancy between them result in a new steady state, and it takes rather short time", this short time can be like 20 min. if the aeration is stopped in an aeration tank of an activated sludge system. During this time, the DO will gradually diminish until a very low value is reached (endogenous respiration), and if aeration is not turned back on, the system will go septic. If it is turned on again, the DO may recover to its original value. The "gassing-in" due to the re-aeration is in fact a method to determine the KLa, from which the OTR can be determined. The "gassing-out" prior to the re-aeration is in fact a method to determine the respiration rate, i.e. the OUR. The question is: would the two be the same (i.e. OTR=OUR?)
The KLa mass transfer coefficient also contributes to determine the dissolved oxygen concentration (DO) of the aerobic fermenter. The air to fermenter's media mass-transfer rate of oxygen (Na) should be dependent on the local oxygen deficit: Na = KLa·(Cs-C). Here, Cs is the DO that would apply for saturation of the fermenter's broth and C is the measured DO. A mean concentration difference can be defined (ΔCm) after accepting some convenient mixing model, so that: KLa = OTR / ΔCm, where OTR stands for oxygen transfer rate. Agitation should contribute for KLa. Such a modelling-based approach implicitly takes into account other variables that may influence KLa; including temperature (T) or pH. Further details on this kind of approach, were given at the following reference (MSc Thesis):Thesis Controlo do Oxigénio Dissolvido em Fermentadores para Minimi...
I have recently published a paper on this topic. I found that the relationship between OTR and OUR is given by:
dC/dt = KLa (C* - C) - 2xOUR
and that
OTR = KLa (C*-C) -OUR
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