MIXING/AERATION
AS AN ESSENTIAL MASS TRANSFER FACTOR

Mixing and aeration are not the only factors that determine what mass exchange there will be in the bioreactor or how the microorganisms will grow. It is determined both by the properties of the microorganism strain and the choice of the balanced nutrient, process regime, etc. Let these factors remain for technologists, we will discuss what will be profitable for the bioreactor in this respect.

As regards the evaluation of the role of mixing, the 2 extreme points of view are rather widespread:

• Microbiologists often say: "What can be the crucial role of mixing if the partial pressure of the dissolved oxygen (Oh, what a long explanation, but it is necessary to be exact!) pO₂ is too low, then more intensive mixing is required. And if the mixer's revolutions are such when we cannot cope with foam and other disorders, then we simply do not increase them and consider pO₂ as insufficient".
• In its turn, "mixing people" say: "Oh, the matter is not so simple with mixing! Firstly, it is not correct to choose the mixing and aeration regime - the mass exchange at the same input power is decreased dramatically. Secondly, the choice of the incorrect mixing system already at a minor input power ("mixing people", in contrast to microbiologists, commonly do not say "mixer revolutions" to characterise mixing, but rather "introduced power with mixer") can cause irreversible mechanical damages in the sensitive microorganisms. Thirdly, the senseless increase of air consumption causes even the worsening of mass exchange". And there are at least 6 another arguments, how important mixing is in microorganisms' cultivation.

But what is true?

To elucidate it, first of all, let us enumerate the results of mixing:

• Air bubble dispersion;
• Mass transfer from air bubbles (i.e. oxygen supply) to the liquid and then to cells;
• Supply of the nutrient components to cells (more precisely, cell agglomerates);
• Prevention of sedimentation;
• Securing of heat transfer;
• Solubility of the nutrient's components which are less soluble.

The mass transfer process during the cultivations of microorganisms explains the following picture:

As has been already mentioned in the division "Construction of the laboratory bioreactor", the most widespread are standard Ruchton turbine type mixers. At a constant rotational speed of the mixer, they secure the highest input power. This is practically from the viewpoint of the choice of the cultivation regime. Further it will be shown that there are, however, fermentations, in which the standard turbine is not the best solution any more. Certainly, there is a fermentation in which the role of mixing is relatively trivial. However, also in these cases, different mixing/aeration regularities should be taken into account:

1. Minimal and maximal limit of the mixer rotation.

   Irrespective of the pO₂ values (or other alternative growth or respiration parameters), it is not recommended to choose a mixer rotation speed lower than the empirically determined critical limit nmin. This limit, nmin, is chosen so that the following would not appear:

   • Sedimentation;
   • "Died" zones.

   In its turn, the choice of the critical limit nmax of the mixer's maximal rotational speed is determined by the following phenomena:

   • Foaming;
   • Liquid surface fluctuations, i.e. "waving", hence, also the liquid's evaporation.
2. Mixing/aeration relationships.

   When choosing the mixing and aeration intensity values and their relative mutual interactions, the following should be taken into account:

   • To increase the intensity of oxygen and other components' transfer intensity, first of all, we recommend to begin with the mixer rotational speed increase, and, only with n > n_max, to begin increasing gradually the air amount Q that is necessary for aeration. Before this, Q is chosen to secure a stable aeration. It is normally 1 vvm (vvm - amount of the introduced air versus the bioreactor's working volume). It means that, if we define air consumption in l/min, then the amount of the introduced air Q will be the same as the bioreactor's working volume.
   • At relatively low rotational speed values of the mixer, the increase in the amount of the introduced air should be avoided as far as the "flooding" effect begins. What the "flooding" effect is and how the transfer in it from the "loading" state occurs will be explained by the following illustrations:

   It can be seen that, as the "flooding" regime sets in, air bubbles are concentrated only in the middle part of the reactor, and they are poorly dispersed. Hence, such a mixing/aeration state is very undesirable for the microorganism growth.

   It should be noted that the transfer from "loading" to "flooding" has a hysteresis nature. It means that, as the amount of the introduced air Q decreases, the "flooding" effect will be lost at an air consumption, which is lower than that when it begins.

   To continue the review of these problems, it should be noted that, as the "flooding" effect begins, the standard Rushton turbine is not the most suitable mixer's variant any more. In these cases, the most appropriate mixing systems are Chemineer CD-6 or BT-6, as well as Scaba AB 6SRGT (is called also the Smith mixer). Thereby, the mixer construction ensures an intensive grabbing of air bubbles in the radial direction also at minor rotational speed values of the mixer. As a result, the air bubbles have nothing to do but to obey the dispersion. Using such a mixer, its air amount Q, at which the "flooding" effect sets in, can be essentially increased.

3. Mechanical cultivation of sensitive microorganisms (the case in point here will be the cultivation of mycelial microorganisms, as the mixing of more sensitive cultures has other aspects, i.e. the cultivation of these cultures even in the minimal turbulent regime is not permitted).

   In the case of mixing mycelial fungi microorganisms with a standard Rushton turbine, the mass exchange in the cultivation process increases only up to a definite rotational speed of the mixer, and, with the further increase in the mixer's rotational speed, the mass exchange parameters even begin to be impaired. The reason for such a phenomenon is the irreversible mechanical damage of the cells. Certainly, this critical rotation of the mixer is not strictly fixed and depends on different factors:

   • The variety of microorganisms' strains;
   • Nutrient's composition;
   • Aeration regime;
   • The amount of the grown biomass (at a greater biomass, the critical rotational speed of the mixer commonly decreases, as, in this case, it is more difficult for mycelial microorganisms to "run away" from the locally intensive mixing zones);
   • And other factors that determine the medium's rheological properties and cell condition.

   Ekato Intermig is one of the most widespread mixers for mycelial cultures. Thus, the mixer consists of two mixers, i.e, the lower and upper ones. In this combination, axial flows are generated in the mixer, and the mixer's radial end construction ensures a sufficient radial mixing. Thereby, an even distribution of the energy introduced in the mixer throughout the reactor's volume is ensured, resulting in the decrease of the maximal shear forces.

Another construction solution for mixing mycelial cultures is the so-called "counterflow mixing system". In this mixing system, the lower mixer generates an axial flow upwards, while the upper mixer downwards. The form of the mixer's blades ensures a sufficient flow eddy, i.e., as a result of tangential components, the given mixing system ensures both even distribution of the mechanically introduced energy (i.e. low shear stresses), and also a sufficient dispersion degree.