“Resistance is a measure of the mean separation distance between the bubble and organism, but it is not generally assumed to be constant within the liquid in any zone”.
Three distinct resistances at the gas-liquid interface have been visualized.
The first is concerned with gas film resistance. This encompasses the resistance between the bulk of the gas and the gas-liquid interface, and it gives a measure of stagnation that may occur inside the air bubbles.
The second manifests as internal resistance in the form of an energy barrier which excludes oxygen molecules below a threshold velocity. The third includes liquid film resistance. It extends from the gas-liquid interface into the bulk of the liquid, and nature-wise it provides a measure of stagnation in the liquid around the bubbles.
This resistance is a measure of the mean separation distance between the bubble and organism, but it is not generally assumed to be constant within the liquid in any zone. However, for a fermentation broth having high rheological property, this resistance might have a significant contribution. In that case, DO concentrations may vary from region to region in the fermenter.
The intra-clump resistance has four counterparts. The first is liquid film resistance that exists around the cell or clumps of cells—again, a stagnation effect. The second manifests as clump resistance. It is a measure of the diffusion barrier between organism surface and metabolic site. Eventually, it may become significant in a large single cell or in clumped cells.
The third one indicates cell membrane resistance to transport. It extends from the exterior surface of the cell membrane to its interior surface, and it is associated perhaps with an endergonic reaction. The fourth one is reaction resistance, perhaps also related to an energy barrier for the reaction of oxygen molecules with terminal electron carriers.
Problems to Increase “a”:
The problems of increasing the gas-liquid interfacial area “a” are not so acute in Newtonian broth. However very often it becomes a serious problem in many non-Newtonian broths, where large agitation power input is needed to increase the gas-liquid interfacial area.
Although increased power input produces a large value of a marginal effect on KL, in large-scale systems it increases the overall cost, thus becoming a problem with increasing a with minimum power expenditure. This becomes especially crucial for some high oxygen-demanding microbial systems that are shear sensitive.
Problems with such systems arise because of the tendency of the cells to be ruptured by the shear force exerted in agitation. Transport of oxygen across cell membranes has been described in many reports to be an active transport. For pellet, aggregates, and floc- forming mold as well as actinomycetes and yeast fermentations, oxygen diffusion from bulk to the mycelial pellets, aggregates, or floes frequently becomes controlling.
This means that the rate of transport of oxygen across the cell membrane rather than the rate of oxygen dissolution in the bulk becomes a controlling step, as shown in Fig. 5.3. Another important problem encountered in much mycelial fermentation is that the broth viscosity in the region of the impeller may be low, while that in the distant part of the fermenter is very high.
In effect, bulk mixing becomes poor and channeling of the air bubbles is most likely to take place, the net result being that a large part of the culture in the vessel remains under oxygen-starved conditions, thereby hampering the overall process. In such cases, DO distribution at low power expenditure becomes a problem.
Beneficial Effects of Agitation:
Many of the engineering problems described above have been overcome by providing agitation arrangement during fermentation.
This greatly reduces the resistances by imparting its fourfold functions, namely:
(1) Creating a large air-liquid interfacial area by bubble disruption;
(2) Reducing the thickness of the liquid film around the air bubble, thus enhancing oxygen diffusion and increasing the KL value;
(3) Maintaining the homogeneity of DO in the liquid by reducing resistance to bulk mixing; and