Fluid Mixing in Fermentation Processes M , x m e is a n essential part of fermentation processes. There are two sources of energy in a fermentation process: Mixing impellers provide circulation of fluid through the tank, and the expansion and velocity of air passing through a fermentor impart fluid motion. T h e major steps in fermentation include :
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Gas-liquid steps Physical dispersion of air in tank Oxygen transfer from gas to liquid 2. Liquid-solid steps Oxygen transfer from liquid to solid Effect of fluid shear stress on growth of organism 3 . Blending steps Blending of liquid and solids throughout entire vessel Maintenance of desired dissolved oxygen level at all points in fermentor 4. Heat transfer
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Considerable work for several years has been devoted to establishing the role of mixing in these steps. T h e over-all process result is a summation of all these effects. T h e consideration of mixing requirements on bench scale, pilot scale, or full scale involves studying the effect on each step. Among pertinent mixing variables are pumping capacity, maximum intensity of fluid shear stress, average fluid shearing stress in the tank, fluid velocities, power input, and superficial gas velocity. Any step in the fermentation process may require either minimum or maximum levels of any of these quantities. When mixer speed, impeller dl; meter, or tank size is changed, the ratios between these mixing variables are changed.
Fluid Turbulence Theory and practice of turbulence are undergoing extensive development, and all the implications of this work have not yet been established. O n e consideration is that turbulent processes proceed mainly from transfer of momentum. Thus, we may think of small elements of fluid undergoing random movements. Among their characteristics are the velocity of these elements of fluid as well as their size. Scale of turbulence refers to the size of thesc elements. Large scale turbulence is determined by the size of the container and of the mixing impeller. Thus, different large scale turbulence would be expected, in different-sized systems. Relatively large scale turbulence is transferred to small scale turbulence through the process of momentum transfer, and eventually reaches a size where energy may be lost by viscous shear stresses. Therefore, in a turbulent
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INDUSTRIAL AND ENGINEERING CHEMISTRY
ElIn scale-up to a full-size unit, these superimposed conditions would be needed if completely identical and exact mixing conditions were required
process, energy is being exchanged between various elements and eventually is reduced to a relatively small scale, where energy is dissipated to heat. Several theories are current on the role of turbulent shear stress in mixing. Most mixing processes are governed by the over-all level of fluid action in the tank and the over-all level of fluid shear stress or impeller head in the system. This, undoubtedly, is the summation of many effects and is not a complete picture of the mixing system. Therefore, the role of each process step must be carefully analyzed and the effect of mixing in these steps evaluated. Then the over-all process can be evaluated and an accurate design worked out for the large equipment.
very effective in promoting the transfer of oxygen from the liquid across the fluid film to the surface of the solids. I t also increases the interfacial area of the solids and enables oxygen and fluids to permeate the organism clumps. However, the fluid shear stress in a mixing tank is constantly working on the clumps of organisms to affect the character of their growth. This effect may be desirable or undesirable, depending upon the product. At some point, however, the fluid shear stress in the system may completely rupture the cells and change the process. Each type of growth has its own resistance to fluid shear rates, so that it is necessary to examine experimental data to determine the magnitude of these effects. Two quantities are important in considering fluid shear stress: the maximum instantaneous value that exists a t the impeller and the over-all average fluid shear stress that exists throughout the tank. I n discussing fluid shear stresq, it is important to recognize the distinction between these two effects and to consider the effects separately. Dion, Carilli, Sermonti, and Chain (7) give data on thc effect of mixing conditions on the character of growth of the organism. A large tank cannot have the same relationship between maximum fluid shear stress and minimum fluid shear stress as a small pilot tank. I t is not practical to make up a large volume from a multiplicity of small volumes. Therefore, it is necessary to determine limits on these fluid effects, and make sure that the large unit meets these requirements. Principles of impeller mixing, scale-up, gas-liquid effects, gas flooding, blending, heat transfer, effect of non-Newtonian fluids, and power consumption are also of importance.
Literature Cited (1) Dion, W. M., Carilli, A., Sermonti, G., Chain, E. B., “Effect of Mechanical Agitation on Morphology of Penicillin chrysogcnurn Thom in Agitated Fermenters,’’ Instituto Superiore di SanitB, Centro internazionale di chimica microbiologica, Rome, 1957.
Liquid-Solid Effects Liquid-solid effects are of considerable importance. T h e mixing impeller is
J. Y. OLDSHUE Mixing Equipment Co., Rochester, N. Y.
Scale-Up of Submerged Fermentations
A and very practical problem in submerged aerobic fermentation is preMAJOR
diction of results in production fermentors based on data obtained in bench scale and pilot plant fermentors. M a n y data on this problem have been presented in
the past decade. Common procedures used to scale u p from laboratory through pilot plant to production operation relate productivity to power absorbed per unit volume of fermenting medium or to oxygen transfer.
