I am providing oxygen through sparging to my system (fermented broth) and it increases drastically the DO level in the broth. I am doing also agitation in the system for breaking of the large molecules of air bubbles, formed due to sparging, hence for proper mixing. But without sparging, the agitator system individually does not help in increasing the DO level in the broth.
I have checked it for lower to higher agitation speed maintaining the time of oxidation constant and no aeration,but still the DO is not increasing.
What does it mean?
Is that my limitation of agitator system?
Agitation would increase the oxygen transport in the presence of air bubbles - i.e. with sparging. Without sparging, and assuming the O2 transfer only across the medium surface - the O2 transfer rate is easily limited due small surface area for the transfer. Of course with air bubbles via sparging, the surface area increases, then the agitation would help improve the O2 transfer rate. Also, sparging itself also contribute to mixing inside the fermentation broth. In the case mentioned, the agitation simplify cannot improve O2 transport as it already limited by the surface area for the O2 transfer.
Thank you Dr.Joburn and Nehru
Can you refer some articles or books that describes about it in more simplified and detail manner
If agitation without aeration is not affecting in increase in DO , then how to know my system is mass transfer limiting or not? If agitation is not affecting so it should not be mass transfer limiting right?
Please find out the rate of oxygen transfer with varying rates of agitation and aeration. You must establish the parameters first and then go for aeration with agitation of the broth.
Dr. Mirza Arshad Ali Beg
I recommend that you follow the kla. As it was said above agitation alone will not increase DO. Take in consideration the air flow rate of your system and increase it accordingly. Also, model the course of kla by using an appropriate equation, i.e., Mitchell & Miller, Cooper, Fernstrom & Miller, Smith, Van’t, Riet & Miller, etc; you may find them in most biochemical engineering textbooks and handbooks. Good luck.
Dr. Marcel Gutiérrez-Correa
OUR=OTR. When OUR is limiting, DO will increase. When OTR is limiting DO will be close to zero and will not increase significantly. Agitator design is irrelevant.
Is this the answer to your question?
Dear Dr. Rob Van der Lans,
If OTR is limiting , means there is no significant transfer of oxygen due to agitation,its not helping in mass transfer, then what could be its role?
can we say that we can omit the agitation system and aeration system itself is enough
Dear Jai Gosh. Not entirely clear what your message is. Medium composition is very important for the OTR. Its maximum value in a given situation that is. And OTR determines DO. Is that what you mean?
Dear Lopa Pattanaik, it may indeed by wise to study a book on fermentation technology. As I stated OTR=OUR. So the transfer rate when limiting, determines fully the OUR. The OUR is limited by the OTR or effective oxygen supply to the organism. Consider yourself on a high mountain, then you are oxygen limited and cannot perform as you wish. You have to deal with the oxygen available. But surely you keep on consuming oxygen.
When you stop the agitation OTR (in general) will be lower, and so OUR and hence growth or product formation. This is not uncommon: often oxygen transfer determines the rates in fermentation.
Maintaining DO in a fermenter depends on the mass transfer and biological activity of the microbes. Mass transfer in a system can be controlled by agitation and aeration. Sole agitation will only cause surface renewal and would help in dissolving the oxygen present in the head space of the fermenter. However, this may not provide the desirable Do condition in the fermenter. Hence cumulative effect of aeration and agitation would give you better results.
Now, few of the microbes are highly aerobic and the OUR would be very high for them. To meet the OUR one has to enhance the OTR by several methods like enriching air with oxygen etc. Often modern fermenter comes along with cascading system which can be employed. OUR is guided by the type and metabolic state of the microbe where as OTR will depend on the mass transfer coefficient of the bioreactor and other above mentioned conditions.
Media composition may affect OTR. For example if the fermentation broth has viscous rheological behavior, obviously mass transfer coefficient and the OTR is going to be affected adversely. At that point of time one need to reset the agitation aeration condition to enhance the OTR. On the other hand if the major carbon source is depleted in the media, at that point the metabolic activity of the cells may reduce resulting in lowering the OUR and that would may also enhance the DO ( which may not be desirable and indicate end of the batch!).
Two important factors usually affect the DO concentration level in the broth:
The ionic strength or ions concentration which usually reduces the DO concentration as it increases
The broth viscosity: which reduces the solubility of oxygen as it increase. High viscous broth could promote segregation of the gaseous phase.
More over high mixing speed could result in vortex formation where the mass transfer rate between the phases will be minimum.
Dear Zakaria Al-Qodah,
Maybe I misunderstand what you mean to say, but I do have some deviating opinion.
Ionic strength is not likely to change much during the experiment. But it does have a lowering effect on solubility. As such it may lower DO, but ionic strength may also increase mass transfer (smaller bubbles), resulting in a higher DO.
Broth viscosity as such does not reduce solubility unless it increases the ionic strength. Of course the presence of compounds in larger quantities may influence the measured solubility of the broth. But, indeed, higher viscosity (above 20 mPas) leads to strong coalesence, reduced mass transfer and hence lower DO. But again it is not likely to occur during the experiment. However, it is often the reason for not being able to control DO any more at a certain stage of the fermentation.
Two kinds of vortex formation may result from stirring. Vortices behind the blades result in turbulence and better dispersion of the gas, leading to higher mass transfer and higher DO. Also a vortex from the top surface may arise, possibly leading to the entrainment of gas from the headspace. This may result in a higher DO, depending on the composition of the gas.
Dear Rob Van Der lans
Thank you for your comment. However, what I do mean is that the ion concentration in the prepared broth is initially high this leads to limited oxygen solubility.
Concerning the vortex I mean that mixing which induces liquid bulk movement rather than small vorticies around
That certainly holdes tot baffleless systems. Of course it is not wise to increase stirring speed well above 2 to 3 W/kg voor other reasons.
- Badges
- Science topic
- Similar topics
- Biological Science
- Microbiology