Fluid Mixing in Fermentation Processes M , x m e is an 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. The major steps in fermentation include :
-‘
1 !
;
I
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
I
;
1
-
1.
Considerable work for several years has been devoted to establishing the role of mixing in these steps. The 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. One 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
60
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 at 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. The 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. Many 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.
E N0 INEER IN0 ADVANCES IN FERMENTATION P R ACT1CE
L
a
A primary object in aerobic fermentation is to transfer difficultly soluble oxygen from air through the gas and liquid films into the liquid and then from the main body of the liquid into the region of the cell walls. Cellular respiration and product yield depend on maintenance of oxygen concentration above a critical minimum. Oxygen mass transfer rates and yield depend upon scale and intensity of turbulence, which are a function of power absorbed. Cooper, Fernstrom, and Miller (7) obtained data on oxygen transfer in agitated vessels using the sulfite oxidation technique. These data, obtained on the laboratory scale using vane disk agitators, demonstrated that the gas transfer coefficient, KLa, is a function of power per unit volume, P,, to the 0.95 power. They verified this relationship, using flat paddles at both large and small scales, and obtained a good correlation for the particular geometry of tanks used. Modern fermentor design calls for radial-type turbine impellers. Experience in geometrically similar tanks shows the following:
5 0
0,;
4
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6
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h 3
I
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d
2
Volume, Gal.
Laboratory Pilot plant Production
2 110 6000 and 12,000
It is apparent that low power inputs
b
*
can result in higher gas transfer on scaling up and the highest inputs result in lower values than one would predict by using the Cooper exponent of 0.95. Thus, power per unit volume is not truly satisfactory for scale-up in this case. Rushton and Oldshue (4, 5) have demonstrated scale-up on a unit power basis in a different way. Upon relating heat or mass transfer numbers to a modified Reynolds number, through dimensionless groups, which are functions of fluid properties, a straight-line log-log plot is obtained. They show that a scale-up of power per unit volume is valid only in the specific case when the slope of the line is 0.75. Upon increase in volume of equipment of geometrical similarity, physical facts must be balanced for a practical scale-up. The volume of a batch increases as a cubic function, whereas the average escape velocity of the air is a function of the tank diameter squared. Thus, if air flow rate is scaled up on the ratio of volume of air per volume of batch often used to express this in bench scale fermentation equipment, the superficial air velocity will be excessive, normally causing severe foaming. O n the other hand, a strict scale-up on a superficial or linear air velocity basis might lead to
VOL.OFBATCH TYPE OF GALLONS AGITATOR 0 2 WISCONSIN TYPE 0 2 MlXCO F L A T BLADES A 110 61 II ,I a 6000 I, 0 n I I, ,I 12000
N m t
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t > I
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N I ~ , . P , 2 G. MOLES 3 O ~ / M4L : H R . 5x
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7
A bacterial fermentation was scaled up for production of vitamin
812
with excel-
lent results
Operating
Scrtle
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K L Varies ~ as p,O. QK P"O.61
PvOJ approx. (by varying r.p.m.) P,,O.s* approx. (varying no. of blades at constant r.p.m.)
limiting molar air flow, which would not satisfy the oxygen requirements, particularly in some highly aerobic bacterial fermentations. Usually a compromise is made a t air flow levels somewhere between the lower flow predicted by the superficial air velocity basis and the higher flow predicted on the basis of ratio of volume of air to volume of batch. The foregoing all points to the desirability of relating production to oxygen transfer rather than to any specific unit power or air flow basis. The over-all gas transfer can be expressed in terms of the over-all liquid film coefficient, KLa, and liquid oxygen concentrations, or in terms of the gas film coefficient, KGa, and oxygen pressures in equilibrium with the corresponding liquid concentrations. Because it is not always practical or convenient to obtain Kaa values for each fermentation medium, recourse is made to the oxygen absorption coefficient, K d w , which is defined as that coefficient obtained from measurements of the copper-catalyzed sodium sulfite-water system employed by Cooper and others (7). This coefficient is written in terms of the gas phase and is related to KGa because both are affected by the same variables-air flow and agitation. This is a calibration number which can be used to relate the fermentor physical
performance to productivity and has a somewhat more fundamental basis than horsepower per gallon. I n some cases, it is convenient to include the total pressure, P, in the over-all coefficient by writing the concentrations in terms of mole fractions as shown in the equation: Rate of absorption = Kdw X P (y
- YL)
X P can be shown to vary as Because dependency of Kdw X P on pressure is small, it can be assumed constant throughout the fermentor and the average pressure in a large fermentor can then be used for calculation. The data in the table show how this can be used to advantage. Sulfite oxidation numbers are fairly costly to obtain in production-size tanks. I t is practical to do all the work a t atmospheric pressure, relating the oxygen transfer data to power per unit volume and superficial air velocity in the tank at sparger level as parameter. Kdwx P values can then be readily calculated for other gage pressures imposed upon the tank. The data in the table relating measured and calculated values a t different pressures show that the assumption of average pressures for calculation purposes is valid. The relationship of sulfite oxidation numbers to oxygen partial pressure is linear, as expected. Many observers have correlated productivity for a number of fermentations with oxygen transfer rate. Excellent correlations were obtained by Wise (7) for scale-up of streptomycin and penicillin fermentations from 5-liter to 100-gallon size. Karow, Bartholomew, and Sfat ( 2 ) related penicillin and streptomycin K,
VOL. 52, NO. 1
JANUARY 1960
61
productivity with oxygen transfer for 5-liter, 200-gallon, and 15,000-gallon fermentors. The data correlate well. For both the streptomycin and penicillin fermentations, there is a sharp decrease in productivity below a threshold level and a broad range of good productivity which permits fairly easy scale-up. Roxburgh, Spencer, and Salans ( 3 ) demonstrated a similar scale-up is possible for the ustilagic acid fermentation, and Strohm, Dale, and Peppler (6) obtained a good correlation for yield of baker’s yeast us. oxygen transfer in agitated and nonagitated vessels ranging from laboratory sizes to a 30,000-gallon yeast propagator. About seven years ago, bacterial fermentation for the production of vitamin B ~ Q was scaled up at Pabst Laboratories. This bacterial fermentation was affected by excessive agitation. T h e translation from 110-gallon operating volume to the production scale in the 6000- and 12,000-gallon tanks was excellent. The prediction of process results from the bench scale 2-gallon fermentors is clear, but the curve is displaced (see figure). This is probably due to the fact that an order of magnitude of about two to three times the power
Air Sterilization
SUBMERGED
aerobic fermentation processes require a continuous supply of large quantities of air. Sterilization of this air is mandatory in many fermentations. For pure culture operation, incomplete destruction or inadequate removal of the microorganisms carried in the air may preclude successful operation. Many ways have been suggested for sterilizing air. Only adiabatic compression and filtration through bed of fibrous and granular materials have found widespread usage on an industrial scale. Of these, filtration through beds of fibrous materials such as glass wool is by far the more common method. I n recent years sufficient research has been carried out to permit the design of these fibrous filters on a rational basis. Major requirements which every air sterilization system must satisfy are: The system should be simple in design. I t should not be inordinately costly to operate. I t should remove or destroy air-borne contamination to the extent necessary for satisfactory fermentation performance. I t should be stable to repeated steam or chemical vapor sterilization. I t should condition the air. Its ability to maintain a sterile air supply should not be jeopardized by power failure or compressor surges.
62
Oxygen Adsorption Coefficient Can Be Used to Relate Fermentor Physical Performance to Productivity
Gage T’ol.,
Gal. 110 6000
Press. on Tank, R.P.M. P.S.I.G. 140 0 5 100 0
5
Av . Press., Atm. 1.04 1.38
Air Flow, C.f.m. Ft./hr. atm. sparger
press.
level
x 104
x
1.17
6 8 190
1.51
240
61 61 135 134
2.52 3.39 3.64 4.66
4.70
per unit of volume is required in the 2gallon tanks above that required for the 110- to 12,000-gallon sizes over an equivalent gas transfer range. I t is possible that there is a sparing action of power, which can be considered a measure of liquid turbulence, on the gas transfer requirement. A satisfactory scale-up can be accomplished without getting completely involved in the complicated nature of the fermentation process itself. The most usable concept is on the basis of oxygen transfer. literature Cited
(1) Cooper, C. M., Fernstrom, G. A,,
This latter requirement is frequently overlooked. Its consideration is paramount in the design of filters compounded from fibrous materials. Typical performance data for the removal of bacteria from air streams by fibrous filters are illustrated in Figure 1. For a particular filter, there is an intermediate air velocity at which filtration efficiency is a minimum. If the filter design is based upon a performance observed a t an operating velocity other than that a t which minimum efficiency occurs, surges or brief power failures could create periods of operation a t lower than designed for efficiencies. Minimal efficiency at an intermediate air velocity occurs because different forces act to collect air-borne particles a t difference velocities. At low velocities, gravitational, diffusional, and electrostatic forces act on the particle.
I
Air Velocity
INDUSTRIAL AND ENGINEERING CHEMISTRY
Typical filter performance
104
... 3.35 ...
Miller, S. A., IND.ENG.CHEM.36, 504 (1944). (2) Karow, E. O., Bartholomew, W. H., Sfat, M. R., J. Agr. Food Chem. 1, 302 11953).
(3j-ROXburgh, J. M., Spencer, J. F. T., Salans, H. R., Zbid.,2, 1121 (1954). ( 4 ) Rushton, J . H., Chem. Eng. Progr. 47, 485 (1951). (5) Rushton, J. H., Oldshue, J. Y.,Zbid., 49. 161 (1953). (G) Strohm, J.; Dale, H. F., Peppler, H. J., Appl. Microbiology 7,235 (1959). (7) Wise, W. S., J. Gen. Microbial. 5, 167 (1951).
W. H. BARTHOLOMEW’ Pabst Laboratories, Milwaukee, Wis. 1 Present address, International Minerals and Chemical Corp., Skokie, Ill.
Their effect is inversely proportional to air velocity. At high velocities, inertial forces come into play, which are directly proportional to air velocity. The nature of inertial effects is such that below a certain air velocity, collection due to inertial forces is zero. One set of workers (3)has shown this velocity to be approximately that at which the filtration efficiency is a minimum. I t has been suggested (4) that this velocity could be estimated by the following relation Vminimurn eiiioienoy
*
Vinertial eiieota
-+
0
1.125 1.1 d / CPPdPZ
(1)
where ,u = air viscosity, d, = fiber diameter, C = Cunningham correction factor, p p = particle density, and d, = particle diameter. For the collection of unit density, 1-micron bacterial particles from air streams at room temperature and pressure, this velocity is equal to V i n e r t l a l eifeots
Figure 1.
Calcd. Kd, X P Kdw X P
-+
0 = 0.066
dr
(2)
where velocity Vis in feet per second and the fiber diameter, d,, is in microns. Regardless of air velocity, some collection always occurs, because air-borne particles possess a finite size and will be intercepted by some fiber blocking an air stream along which a particle moves. Collection must always be greater than that due to interception, as it represents the minimum collection physically possible. In the absence of experimental data, a reasonable estimate of the mini-
