Wednesday, September 18, 2013

Process Wednesday: putting oxygen into reactors

I'm still reading Bert Hulshof's relentlessly fascinating Right First Time in Fine-Chemical Process Scale-up. Here he is, talking about fermentation, oxygen and agitators: 
Adequate and continuous supply of oxygen to the growing cells of the microorganism is a crucial factor. The dissolved oxygen profiles usually require special attention during process scale-up due to various reasons, such as maintaining the specific power input and a turbulent flow on both scales, keeping the air-flow proportional and the pressure identical. Maintaining a turbulent flow on both scales is more difficult to control than the other three requirements.  
If the viscosity, usually in the range of factors of 30 - 2000 more viscous than water, increases during the fermentation process, the Reynolds number may become too low resulting in a large decrease of oxygen transfer. A general solution to this problem is diluting the process, which affects the productivity. The dissolved oxygen (DO) profiles in 25,000-L fermenters show relatively high concentrations of oxygen in the well-mixed Rushton turbine regions, while low to very low values were measured in the radial and axial planes away from the stirrers. This may lead to relatively dead zones in even larger-scale fermenters where oxygen depletion may reduce the productivity of the process.  
Recently, progress has been made to improve oxygen transfer by introducing novel gas injection nozzles and using pure oxygen. Intensified oxygen mass transfer can be achieved by injecting gas into liquid via these nozzles at supersonic flow velocities, forming very tiny gas bubbles (size range of 100 um) and giving enhanced interfacial area and higher gas pressure in the bubbles on commercial scale. 
I tend to think much more about keeping oxygen out of reactors. [There are three sides to the fire triangle: heat, oxygen and fuel. The typical 2000 gallon reactor is full of flammable fuel, i.e. solvent, so keeping the oxygen level close to zero is important.]

Fermentation is about getting oxygen to the bugs so they can make product, so keeping oxygen levels up is important. It looks like fermentation processes run into similar issues as other agitated processes -- if the solution becomes too viscous, you can't get oxygen to the bugs and your process slows. Also, it appears that agitation can only do so much to make sure that oxygen is well-distributed throughout the reactor. (I don't think I'm wrong to say that a Rushton turbine has a relatively high power number, so you would it expect it to deliver the most mixing capability for gas/liquid systems*, so that tends to indicate the problems associated with oxygen distribution in fermentation...)

(I wonder if this is a problem in beermaking? I am going to guess "no", but I don't know very much about beermaking processes...)

*Note: IANAChE, i.e. I Am Not A Chemical Engineer.


  1. IANACE either, but I am assuming that if you stir too vigorously, you might damage the organisms and both lower productivity and complicate purification. I am not sure if that's relevant at stirring powers normally used for them, though.

  2. sugar to alcohol fermentation by yeast is anaerobic process and you definitely don't want oxygen there, hence the the bubbler ventil that lets the CO2 overpressure out but prevents air getting in. With air, the alcohol yield suffers and other bugs can grow in, like acetobacteria which would turn your alcohol into vinegar

    1. That's right -- I forgot about that. [headslap]

  3. Stewie Griffin:
    Mlikshake you are correct, but isn't the start of fermentation aerobic? Many homebrewers will oxygenate their wort so the yeast can multiply/grow quickly and then exclude any further O2 for the reasons you specify.

  4. We also know, "Viscometric Comparison of "Old Viscosity" Ale and SAE 30 Motor Oil" that the viscosity of beer, even a beer that is claimed to be viscous, is not very viscous at all.

  5. Ooh, finally a Process Wednesday I know something about. Usually I learn a lot, but that's because I'm starting from a floor of ignorance.

    But yeah, the others are right: fermentation for beer and winemaking is "anaerobic" (as long as you don't look to closely). In actuality, yeasts require oxygen for ergosterol biosynthesis, although at massively lower levels than is needed for respiration. If you try to culture yeasts indefinitely in a truly anoxic environment, they eventually stop growing. (But if they've grown already the will make alcohol for awhile.) So most fermentations start aerobically or at least microaerobically.

    For healthy, living bacteria you would have to stir amazingly fast to damage the organisms. They are so small that extremely high velocity gradients would be required to provide enough shear stress to lyse them. And in living cells, osmotic pressure will stabilize lipid bilayers so they are even harder deform/puncture.

    The mixing issue is really quite complicated. (And yes, IAAChE). The power number of an impeller would vary with how fast it's spun. So a Rushton impeller could have a power number of 0 if it is at rest, and on up from there when you turn it on. Mixing is important to avoid "dead zones" in the fermenter, as your excerpt mentions, but also because oxygen bubbles in the reactor have an annoying tendency to coalesce into larger bubble, reducing the surface area and thus the oxygen transfer, and also to float up to the top of the reactor, where they won't be doing any more O2 transfer either. So part of the job the impeller is doing is to keep bubbles small, and forcing them along less-vertical flow paths where they will spend more time in the reactor.

    I'm not a bioreactor guy, but I think it's becoming clear that for the most efficient gas/liquid mass transfer, especially at large scales, the agitated tank isn't the ideal solution. The problem is that the power needed to drive the impeller to maintain good aeration becomes ridiculous at very large scales. The problem doesn't come up too much for the very high value chemicals and proteins made by aerobic fermentation, where 10,000 - 30,000 L is as large as most commercial vessels get. But it does for people who want to make cheaper products using fermentation.

    One of the interesting areas where these issues are relevant is the area of syngas fermentation. H2 and CO, the components of syngas, are about as equally insoluble in water as oxygen, so mass transfer is a huge issue there too. It seems like the way things are going there is the use of bubble-column bioreactors, which rely on sparging gas through very small aperatures to create microbubbles, which then slowly rise through the reactor to carry out mass transfer. The rise of this bubble stream drags along adjacent fluid too, so eventually a flow pattern develops in the reactor to keep it mixed, even without any impeller. These systems are much lower power than impellers.