Figure 10. Isentropic horsepower reduction by back pressure.
Air sparger design of large fermenters is one of the least discussed topics of the fermentation industry. Most companies design their own air spargers. Some companies have designed and tried a wide variety of ideas. Agitator manufacturers insist that the air ring emit the air bubbles at the optimum radius of the first turbine. However, in air-agitated fermenters, the engineer must be creative to consider both the best mixing and oxygen transfer effects to be obtained from an air sparger. Not much is gained in mixing or mass transfer by having more than one orifice per 10 ft2 of cross-sectional area.
6.5 Comparison of Shear of Air Bubbles by Agitators and Jets
Visualize a filled fermenter with a variable speed agitator at rest with a Rushton turbine. When aeration is started, large bubbles will rise and impinge on the underside of the turbine disc and escape around the perimeter. When the agitator is rotated at very slow speed, the large bubbles will accumulate behind the turbine blades and small bubbles will trail off into the liquid. If aeration and the agitator speed are increased, the volume of air behind each blade becomes larger, and smaller bubbles trail off into the liquid from the mass of air behind each blade. Due to the design of a Rushton turbine, the liquid and the small air bubbles move horizontally (or radially) into the liquid. As the speed of the turbine increases, the fluid velocity caused by the pumping action of the turbine produces a profile of shear stresses. Theoretically, the turbine speed could be increased to obtain any fluid velocity up to sonic velocity, but the power cost would be high since the energy must move the mass of liquid. Similarly, a limit is reached by increasing the aeration under a rotating turbine. The limit results when the air rotating behind each turbine blade fills all the space in the arc back to the front face of the next turbine blade. All the blades then are spinning in an envelope of air, or the impeller is flooded. For details see Klaas van't Riet.[151
Now visualize the action of a submerged jet of air in liquid. At very low air flow velocities, the bubbles are large. They rise as independent bubbles at the orifice. When the velocity of air through the orifice increases, the air projects as a cone into the liquid and small bubbles shear off. The maximum velocity is reached when the ratio of absolute hydrostatic pressure outside the orifice divided by the absolute air pressure in the orifice is 0.528. This determines sonic velocity. Four regions of the air cone or jet are conceptually drawn in Fig. 11. Region I is called the potential core of air with a uniform velocity. The outermost annular cone, Region II, is an intermittency zone in which flow is both turbulent and non-turbulent. Region III lies between the potential core and the liquid and is characterized by a high velocity (shear) gradient and high intensity of turbulence. Region IV is the mixing zone where the fluid and air merge beyond the potential core. It is a totally turbulent pattern (see Brodkey). A cloud of very fine bubbles is produced. Oxygen mass transfer is enhanced because of the increased surface area of the very small bubbles. In low viscosity fluids, much more coalescence of bubbles will occur than in high viscosity fluids.
Figure 11. Region of mixing and shear for a submerged jet.
Comparing now the bubble dispersion produced by agitators and jets, it should be clear that agitators shear large air bubbles by moving fluid, and jets produce fine bubbles by forcing high velocity air past relatively stationary fluid. This explains the high energy cost and low efficiency of mechanical agitation. Another benefit of air agitation is that it is a variable horsepower device depending upon the quantity of air and the velocity of the jet.
Figure 11. Region of mixing and shear for a submerged jet.
The effect of shear on microorganisms must be determined experimentally. Most bacteria and yeast can withstand very high shear rates. On the other hand, filamentous organisms are less predictable; some are stable and others are ruptured very easily. In the latter case, the air velocity must be reduced below the "critical" shear rate the organism can tolerate. The shear rate of Rushton turbines can be reduced by lowering the speed and/or moving the vertical blades nearer to the shaft. The average bubble diameter will become larger and more air (SCFM) may have to be added for adequate dissolved oxygen.
Unfortunately, laboratory and/or pilot plant experiments cannot easily test air agitation because it requires long bubble residence time (tall vessels) with minimal wall drag. The short vessels of the laboratory and pilot plant are precisely where mechanical agitation is absolutely required to achieve good fermentation performance compared to tall air-agitated fermenters. If anyone with large (tall) mechanically agitated fermenters wants to experiment with air agitation, it will be necessary to remove the turbines from the agitator shaft. If the vessel has cooling coils, the mixing section is somewhat like a "draft tube" giving good top to bottom mixing. The air sparger will have to be replaced. Normally, air velocities of 0.75 Mach provide sufficient levels of increased agitation, mass transfer and shear. There is a reasonable body of literature, although scattered, about air agitation, frequently under gasliquid reactor design and occasionally in bioengineering journals. D. G. Mercer read a paper at the Second World Congress of Chemical Engineering. Dr. M. Charles (Lehigh University)1171 will be publishing his experimental work soon. Also, see Shapiro, A. H.;[18l Townsend, A. A.; Forstall and Shapiro.t20]
6.7 Other Examples of Jet Air/Liquid Mixing
The Buss loop reactor is a system to increase the dissolving of gas into a liquid which contains a dissolved chemical and a catalyst. Normally the reaction is first order and the reaction rate is dependent upon gas diffusion rate. An example is the hydrogenation of glucose to sorbitol. The rate of reaction and yield is increased as follows. The liquid is pumped from the bottom of the reactor, externally up to and through an eductor and discharged subsurface into the agitated vessel contents. The reason for its success is that a high velocity eductor mixes and shears a gas into very fine bubbles of very large surface area better than sparging beneath an agitator. The Buss company, of Bern, Switzerland, designs these systems.
A variation of this design is a production fermenter which has several eductors welded to discharge horizontally and tangentially just above the bottom dish. Broth is continuously pumped from the bottom of the fermenter to all eductors simultaneously. Sterile air is provided to the suction side of the eductor (exterior to the tank). The fermenter has an agitator of very minimal horsepower to keep the solids suspended during sterilization. The pressure of the air supply is low (about 8 psig). A low horsepower blower is used to overcome the pressure drop over the air filter and line losses. Not too surprising, however, the total horsepower of the blower, the circulation pump, and tank agitator have the same power consumption per unit liquid volume as fermenters with conventional agitators, turbines, i.e., about 15 hp/ 1000 gallons of liquid is minimal for the average commercial aerobic fermentation. Therefore, pumping non-compressible water through eductors at high velocity to shear air into small bubbles is no more efficient than agitator mixing. The reverse case of compressing a gas to expand isentropi-cally at high velocities into a liquid does have significant advantages for large volume fermenters. (See Bailey and 011is.)
In summary, the need for mechanical agitation beyond oxygen mass transfer is not clearly understood in fermentation broths which have a viscosity before inoculation of aNewtonian fluid, but change to pseudoplastic (non-Newtonian) after growth starts. Air-agitated fermenters exist in industry today for a wide range of products. It is a viable alternative to mechanically agitated systems. The advantages are the following:
1. Improved sterility because of no top- or bottom-entering agitator shaft.
2. Construction of very large fermenters is possible because the design is not limited by motor size, shaft length and its weight.
3. Refrigeration requirements are reduced 20 to 35% because of no mechanical agitation—see Table 2.
4. Since no agitator, gear box or crane rail is needed, less structural steel is used and cheaper fermenter design results.
5. No maintenance of motors, gear boxes, bearings or seals.
6. The air-agitated fermenter is a variable mixing power unit, like a variable speed drive with no motor and drive noise.
7. Air compressors can be steam driven to reduce power cost and continue to operate during power outages in large plants that have minimal power generation for controls.
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