Resistances Against Oxygen Transfer

A liquid or air interface allows oxygen to be transferred from the surrounding air to the bulk liquid. The sum of the individual oxygen transfer resistance is the total oxygen transfer resistance. Since the volume of a gas bubble is small and the gas phase is well mixed, gas film resistance is nearly non-existent compared to other resistance. Therefore, the main resistance against oxygen transfer is the liquid coating surrounding the gas bubble. But, when the cells accelerate in the shape of pellets, the liquid film resistance around the cell becomes very important and essential, so that we will explore more about this topic.

How does oxygen transfer in a bioreactor work?

The oxygen transfer rate (OTR) through an interface is determined as the product of the mass transfer coefficient, the specific transfer area, and the concentration driving force multiplied by the mass transfer coefficient. The oxygen transfer in a bioreactor for cell cultivation is normally supplied by a sparger blowing air into the culture media.

Due to the limited solubility of oxygen in the medium, oxygen transfer is mostly the rate-limiting step in aerobic bioprocesses. Therefore, in the design, operation and scale-up of bioreactors, the precise measurement and prediction of the volumetric mass transfer coefficient (k(L)a) are critical.

The availability of oxygen has a significant impact on the performance of aerobic bioreactors. 

These processes are typically carried out in aqueous conditions, 

• where oxygen solubility is limited 

• due to the presence of ionic salts and nutrients, 

• but oxygen use by microbes is high

• As a result, oxygen transfer is a critical and rate-limiting phase in bioprocesses

Variations in oxygen availability have a significant impact on fermentation kinetics. The increased dissolved oxygen availability in the culture medium often leads to higher secondary metabolite yields. The oxygen transfer rate can be increased to overcome oxygen limitation (OTR). 

What is the use of pure oxygen in the fermentation process?

The use of pure oxygen in the fermentation process improves microbial cell harvest, or product build-up has gotten a lot of attention.

Now, here the question arises,

What are the principal barriers to oxygen delivery to cells during the fermentation process?

The following are the principal barriers to oxygen transfer to cells:

• Resistance to gas film formation at the bulk gas and gas-liquid interface

• Gas-liquid interaction interface resistance

• Resistance to liquid film formation between the interface and the bulk liquid phase

• Transferring oxygen to the liquid film surrounding a microbial cell requires liquid phase resistance

• Liquid film resistance in the presence of cells

• Intracellular resistance

Fermentation in aerobic conditions, oxygen transfer, and their mixing:

In the presence of oxygen, aerobic fermentation occurs. It normally happens just at the start of the fermentation process. It takes less time and is more vigorous than anaerobic fermentation. Because oxygen has a low solubility in water, oxygen restriction is a major issue in these fermentations. Therefore, the oxygen transfer rate is often increased to keep the dissolved oxygen (DO) content as high as possible (OTR). A high agitation and aeration rate is the most widely utilised technique to improve OTR. 

It provides:

• A bigger driving force.

•   Gas-liquid interfacial area

•    The longer residence time for the gas bubbles in the liquid to transfer oxygen into the liquid

The particular surface area (surface to volume ratio) of large-scale bioreactors is drastically reduced. As a result, relative surface aeration for gas exchange reduces, constituting a process limiting factor. Because oxygen has a very poor solubility in liquids, an adequate oxygen supply is critical for achieving a high cell density, especially for generating secondary metabolites in large-scale fermentations. When the dissolved oxygen level falls below the critical DO concentration, the DO level influences the growth rate, making it a limiting factor in aerobic fermentations.

With increased mechanical agitation, a high gas flow rate, or the dispersion of size-reduced gas bubbles with air-sparging devices, conventional stirred-tank fermenters give high oxygen transfer rates. 

The stirrer creates enough agitation to disperse gas bubbles by: 

• breaking them down into smaller ones 

• extending their residence time. 

• The agitator ensures uniformity and mixing. 

On the other hand, low agitation speeds may not produce enough turbulent flow to shatter and disperse bubbles. The oxygen transfer capacity is improved by greater stirrer speeds and higher gas flow rates. A large-diameter impeller’s increased tip speed, on the other hand, may kill cells due to the high shear stress. Both oxygen transmission and mixing can be aided by selecting the right agitation speed and impeller design.

The steps involved in transferring oxygen from a gas bubble to a cell:

• Transport from the bubble’s interior to the gas-liquid interface

• Cross-sectional movement across the gas-liquid interface

• Diffusion via the bubble’s surrounding liquid film, which is stationary

• Transport via a liquid in bulk

• Diffusion through the comparatively immobile liquid coating that surrounds the cells

• Crossing the liquid–cell boundary

• Diffusion through the solid to the individual cell if the cells are in a floe, clump, or solid particle

• Transfer to the reaction site via the cytoplasm

Conclusion

Sparging air bubbles beneath the impeller of an agitated fermenter provides oxygen to the fermentation media in aerobic fermenters. A rising air bubble dissolved oxygen in the fermentation media, then taken up by the cells. Because the oxygen transfer rate in a continuously stirred tank reactor varies with the power supplied for agitation of the fermentation broth, estimating the power required for effective agitation and oxygen transfer is critical for designing aerobic bioreactors. In an aqueous solution, oxygen is only sparingly soluble, and its solubility diminishes as the temperature rises. This adds to other challenges, particularly those induced by the vessel’s vast volume, where mixing will be less efficient in some areas.