headspace sparging OR sparging gas though sparger directly?
What about calibration of DO probe? Is there any advantage though calibration by air saturation compared to O2 saturation?
Headspace sparging is never advocated. However, using a sparger might be of some use. It is upto you to find out. I feel you should work on some other method of sparging. It is always advisable to sparge a mixture of 95% Nitrogen and 5% oxygen to prevent any oxidative damage to the metabolism mainly..
I agree with the two answers on the submerse sparging - it's advantageous because of the higher oxygen transfer rate compared with surface aeration through headspace, which is always present to a certain degree. In order to compare different spargers and their oxygen transfer characteristics, you can determine the kLa value under your operating conditions. Depending on the type of sparger you will find that any kind of driled hole sparger (common sparger) offers lower oxygen transfer rates than microspargers, because latter produce much smaller bubbles resulting in larger specific transfer surface area. Therefore, you can reduce the gassing rate using microspargers and still achieve similar kLa values. However, smaller bubbles often lead to more stable foam, which may require addition of antifoam agents.
As to the calibration: in general there is no reason to use pure O2 gassing for O2 probe calibration. During your cultivation, you want to maintain a certain level of oxygen concentration (in case of CHO cells in the order of 1.5 - 4 mgO2/L for instance), which corresponds to 30% and 60% of air (!) saturation - depending on medium composition, temperature etc. Furthermore, some DO probes and/or transmitters cannot measure oxygen saturation concentrations of pure oxygen (i.e. 5-times the value of air).
Hope this helps.
Regards,
Stephan
Dear Ahmed
I agree that sparging will result in much better transfer rates but please take in consideration that this also causes more shear stress. When comparing micro and macro spargers also take your desired cell densities and transfer rates in consideration. As mentioned before a micro sparger will prove beneficial to reduce the flow rates but can cause excessive foaming especially when increasing the flow rate when reaching higher cell counts later in the process.
Instead of using antifoam agents it has proven more beneficial switching to macrospargers and higher agitation with down-flow stirrer to increase kLa. Most CHO lines today can handle quite high shear forces.
An overlay (headspace aeration) can be useful but many industry processes today are trying to avoid it if possible. Nevertheless if your process doesn't require high transfer rates a pure overlay can prove beneficial to reduce shear stress.
Another important point is also CO2 stripping. When using CO2 for pH regulation and introducing high amounts of Nitrogen as proposed by Jai this will lead to pretty high usage of CO2.
As you can see there are many different aspects affecting your control depending on your process conditions. Perhaps you could share some more information about your process and bioreactor setup.
Dear All
Thanks for your information,
For DO calibration we have obtained a more controllable result by air saturation. While, by O2 saturation the system shows more fluctuations to control the DO; prior to calibration, the MettlerToledo probe was polarized. But still during the culture, (especially, in earlier times) the DO alters between 20-100% (for a set-point of 70% air saturation).
By applying a stirring of 100 rpm and using a microsparger, no foam is observed. Although, the variation in DO is still annoying
Practically, we have observed a faster and reliable pH control by adding CO2 through the reactor headspace than sparging by microsparger (In a 3 L bioreactor). I was wondering what if we sparge CO2 though the reactor headspace and other gases N2, Air, O2) though the sparger, simultaneously. Also, is there an increase in tension when using the gas mixer (for mixing 3 gases of compared to an N2, Air, O2) compared to single air supplying?
How important is to control the pH acidity by CO2? What if we only try the traditional technique of adding acid to the reactor?
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