At the present time the so called Rushton turbomixer, rotated by a shaft centrally arranged in the fermenter, and consisting of 6 rectangular straight blades radially fixed to a circular plate is mainly used in bioreactors (fermenters). If the height of the bioreactor is multiple of the diameter, a system consisting of 2-4 turbomixers fixed to a common shaft is used.
The air to be dispersed is injected below the lower mixer through a perforated loop expansion pipe, nozzles, or a central nozzle (Fejes, G.: Industrial mixers, p 52-55).
The turbomixers usually make up 1/3 of the diameter of the fermenter and disperse the air efficiently by the intensive turbulence and shear forces generated around the row of blades. Because of the high local energy dissipation, despite the high specific power consumption of the turbomixers, the proportion of energy invented in the zones farther from the mixer is minimal, and the axial transport capacity of the mixer is low, which causes problems increasing as the volume of the bioreactors grows.
There are also known two winged or multi-winged propeller mixers with inclined blades or blades according to the geometry of a helical surface. The mixing system is built up from these mixers.
SEM type mixers utilize the flow properties of the thin propeller wings. EKATO mixers utilize the interference phenomena of parallel double wing blades arranged at an angle and at a spacing above each other (Interming and Interprop mixers, Fejes, G.: Industrial mixers, p 65).
The energy dissipation of propeller mixers with large diameter ratio compared with the diameter of the fermenter is more uniform, and the axial transport capacity is high. Therefore, with the same power consumption they can mix the liquid more efficiently and evenly in high fermenters, but their dispersion capacity is reduced. This is counterbalanced by the use of several phases.
Suction mixers, consisting of hollow mixing elements fixed to a rotating tubular shaft suitable for mixing, dispersion and partly for transport of the gas, are also known. The hollow mixing elements are mostly pipes cut at an angle of 45.degree.. At the ends of these pipes, at an adequate speed, pressure drop occurs, sucking in the gas usually through the hollow tubular shaft. The gas is atomized by the shear forces generated in the liquid by the sharp pipe-ends (Fejes, G.: Industrial mixers, p 57). These mixers are not used in the fermenting industry because of their limited suction capacity. Such suction mixers are also known, where the hollow elements are nearly semi-circular channels open on the side opposite the direction of advance, and at a diameter which is nearly the same as that of the container. Thus they are suitable for the atomization of relatively large amount of gas. However, because of their low circulation capacity, they are used only in the yeast industry and sometimes in processes not requiring intensive mixing of the liquid.
The purpose of mixing in the reactors is the homogeneous distribution of the solid, liquid and gaseous phases for intensification of the material and heat transfer processes. As a result of mixing, significant velocity, gradients and turbulence are caused in the space between the mixing elements and the reactor wall provided with baffle plates. In the case of fermentation processes, the turbulence proportional to the velocity gradient and shear forces increase the dispersiveness of the injected air bubbles, and reduce the thickness of the boundary layers between the microorganisms, culture medium and air bubbles, thereby improve and speed up the material- and heat transfer processes taking place on the boundary surfaces of the phases.
A three-phase system of the microorganisms, culture medium and injected air can be brought about in the bioreactors, where the flow space and its effect on the transfer of material are made extremely complicated by the various interactions, such as a change in the rheological properties of the fermenting liquid in consequence of the metabolism of the microorganisms. The problem is further complicated by diversity and contradictions of the requirements. E.g. in a significant part of the fermentation processes intensive turbulence and shear are required for dispersion of the air and oil droplets, microblending the culture medium and biomass and cutting up the agglomeration. At the same time, however, the intensive mixing facilitates the formation of stable foams which partly directly and partly as a result of the use of foam-inhibiting materials reduces the oxygen transfer, and aeration of the carbon dioxide, and may mechanically damage the microorganisms, or may bring about production-reducing morphological changes.
It is a characteristic of the complexity of the mixing processes taking place in the bioreactors, that each basic operation: dispersion, suspension, dissolution, homogenization, etc. has an important role in the processes, i.e. essentially each fermentation process has its associated specific requirements which can differ significantly according to the type and strain. Thus, the effects of the basic operations should remain within relatively narrow limits in order that, besides affording the required beneficial effect, the adverse effects should remain minimal. In respect of the turbomixers used in the majority of the bioreactors, it is equally unfavorable to expend the major proportion of the mixing energy for the generation of turbulence, so that dissipation about 70% of the mixing energy takes place in the immediate vicinity of the turbine blades, and these conditions can be changed only to a minor degree.
In the case of fermenting liquids forming intensively aerated viscous and stable foams of non newtonian properties, the circulation and turbulence generated by small diameter turbomixers may decrease relatively quickly. The circulation could be intensified by increasing the turbomixer's diameter, but this is limited by the disproportionate growth of the mixing power, which--according to the known relationship--increases with the 5th power of the mixer's diameter. Therefore, the diameter of the turbomixer must not exceed 40% of the apparatus even in case of small fermenter with a volume below 40 m.sup.3. Thus their characteristic feature is the small diameter ratio. On the other hand, this causes additional problems, as the reactor volume and viscosity of the fermenting liquid are increased in the wake of insufficiently mixed zones.
The diameters of propeller mixers--with regard to their much lower rate of power input--may approach the diameter of the reactor. Therefore, the use of propeller mixers of high diameter ratio making up 60-70% of the apparatus' diameter is becoming widespread in bioreactors, although the dispersion capacity is lower because they are more suitable for the efficient mixing of the viscous fermenting liquids.
To provide an efficient mixer is difficult because properties of the viscous fermenting liquids containing microorganisms and air bubbles are often extremely different from those of Newtonian liquids. Some scientists have found that the turbomixer with smaller diameter is capable of an 8-times higher rate of oxygen absorption, than the turbomixers of greater diameter with the same energy input, although such differences cannot be detected in clear water (Steel, R.-Maxon, W. D.: Biotechn. and Bioeng. 2, 231, 1962). These incompletely understood phenomena dependent on the properties of cultures and composition of the culture media also justify the construction of mixing systems, whose mixing effect can be controlled within wide limits and can be modified in respect of every mixing operation.
On the other hand, a common characteristic of the described mixers is that any of them is suitable for producing mainly a certain mixing effect which could limit optimization of the processes.
The efficiency of the mixing for the apparatus depends on the magnitude of the introduced energy and construction of the mixing system. The dissolved oxygen concentration can be improved to the required level generally with the known mixers by increasing the amount of mixing energy and the injected air. However, the disproportionately increasing demand for energy and its cost, intensification of the foam formation and impairment of the microorganisms may increasingly limit the economic factors with the increasing dimensions of the reactor.
The known multi-stage turbine consisting usually of the same elements, and other mixing systems in consequence of the mentioned capabilities and restrictions of the constructions do not provide adequate flexibility for satisfying the specific requirements of the various microorganisms.