FERMENTATION RESEARCH & ENGINEERING Power Requirements
. . . of
a Typical Actinomycete Fermentation
I
R. STEEL and W. D. MAXON The Upjohn Co., Kalamazoo, Mich.
Differences in power patterns of fermentations indicate factors which may be important in scale-up procedure contains very few references to studies made on the power requirements for mixing cultures of filamentous organisms. Wegrich and Shurter ( 8 ) scaled up the penicillin fermentation on the basis of power measurements and mentioned that the power varied during the fermentation; however, they did not give any idea of the magnitude of the variation. Chain and Gualandi (2) reported a 30% increase in power requirement with the addition of 2y0 dry weight of mycelium of Penicillium chrysogenum. Although studies of Rushton and Oldshue (7) have shown that power varies with equipment characteristics, such as agitator speed and impeller diameter, and with fluid properties, such as viscosity and density, little attempt has been made to demonstrate the use of these correlations in predicting power consumption of fiIamentous fermentation broths. Power correlation for a fully baffled tank stirred by turbine impellers generally takes the form shown in Figure 1. Predicted effects of impeller speed, impeller diameter, viscosity, and density differ considerably between the laminar and turbulent regions. Similar relationships were found by Metzner and Otto ( 4 )to apply for mixing of certain non-Newtonian fluids. Other work by Metzner and Taylor (5) has pointed out how the unusual properties of non-Newtonians influence the mixing of these fluids. Of particular interest was the observation that both laminar and turbulent flow conditions could exist a t the same time in the tank, thus making analysis of power input, flow, and mixing efficiency difficult and limiting the application of correlations based on average conditions such as the Reynolds number. The present work shows that power may alter significantly during the novobiocin fermentation, and these changes
T H E AVAILABLE LITERATURE
Table I.
Fermentor Pilot-Scale
Operating Volume, Gal.
No. of
No. of
Impellersa
Blades
Liquid Height, Ft.
Liquid Heieht/ Ta>k Diameter
2 1
4 4
1.0 1.8
0.72
3 3
4 5
17 25.6
1.55 1.97
5.3b 66c
Production a
12,000 24,000
Flat blade turbines.
Fermentor Dimensions
20 liters.
Experimentul Power measurements were made during the course of normal fermentations in pilot plant and production fermentors ranging from 5 to 24,000 gallons of operating volume. The fermentor di-
I1
a
IO
NR, = -
0.41
0.37
D ~ ID0 N?
Figure 1. Changes in Dower number with Reynolds number indicate mixing characteristics of turbine impellers in fully baffled cylindrical 10,000 ° tank
TURBULENT /P'KZPN3D5
2
I .o
0.40 0.40
mensions are given in Table I. These fermentors were not geometrically similar, and there were differences in some of the dimensional ratios. The novobiocin fermentation was selected for these studies since it is fairly representative of actinomycete fermentations in general, it gives good growth, and the broths exhibit nonNewtonian behavior. Power measurements on 5-gallon tanks were made with a spring-loaded torque measuring device described by Nelson, Maxon, and Elferdink (6). For the 66-gallon tanks. Dower was calculated " from voltage, amperage, and power factor, and the data were corrected for motor efficiency and no-load power. For this size of tank, the no-load power accounted for about 40y0 of the total
TRANSITION
P=KlpoN2D31
I1.o0.1
1.0
250 liters.
appear to be related to alterations in physical properties of the broth. No attempt was made to correlate the data, since this was not the object of the study; critical examination of the results indicated that the factors contributing to power requirement in this system probably consisted of a complex of interacting variables. Because of the lack of power data in the literature, it was believed that these findings might be of interest to other investigators.
4
Impeller Diameter/ Tank Diameter
1000
PO VOL. 53, NO. 9
SEPTEMBER 1961
739
v)
z
0 -I
-I
a W
0
0 \
a* I
a w
30 a a
0 k
U
t W
a
FERMENTATION T I M E ,HOURS Figure 2. Changes in fluid physical properties during novobiocin fermentations in 66-gallon fermentor may influence power requirement For high power, agitator speed = 2 6 6 r.p.m., oir rate = 0.875 cubic feet per minute; for low power, agitator speed = 190 r.p.m., air rote = 0.61 2 cubic feet per minute A. Normal fermentation. E. Two liters of antifoam added during first dov
power. The maximum deviation between replicate measurements was =k 15%. For the 12,000- and 24,000-gallon tanks, a recording wattmeter was used to obtain power data. The recorder pen fluctuated continuously over a range which accounted for about 0.05 hp. per 100 gallons with the 12,000-gallon fermentors and 0.07 hp. per 100 gallons with the 24,000-gallon tanks. These power measurements were corrected for the efficiency of the motor and for noload power losses. The no-load power measurements were 6 and 8%, respectively, of the total power drawn by the agitator. The power values presented here thus give the actual power input to the agitator. Another variable measured in this work was termed the gas-retention value. This was determined by removing the gas bubbles from a 125-ml. beer sample by stirring it and then again determining the volume; the change in volume expressed as a percentage of the initial volume gave the relative amount of space occupied by gas bubbles. The rheologic behavior of broth samples containing appreciable amounts of mycelial growth could be classified as non-Yewtonian. Measurements of apparent viscosity were made after removing the air bubbles from a sample of beer, measuring 100 ml. into a 150-ml. beaker, and then subjecting the sample to a shearing stress of 150 grams in a modified Stormer viscometer. The time necessary for the paddle fork turning in the liquid to complete 100 revolutions was measured, and the data were converted to apparent viscosity expressed in centipoises using a nomogram ( 3 ) .
740
The apparent viscosity values given in this report are only relative measures. These measurements of gas retention and apparent viscosity can be criticized on the basis that they bear an unknown relationship to actual conditions in the fermentor; in fact, both of these will vary throughout the fermentor. Results
66-Gallon Tanks. The fermentations were operated a t an agitator speed of 266 r.p.m. and air rate of 0.875 cubic foot per minute ; periodically during the fermentation the agitator speed and air rate were altered to 190 r.p.m. and 0.612 cubic foot per minute, respectively, to obtain the data for a lower power input. The reverse situation was also tested where the fermentation was operated a t lower speed, and periodically during the fermentation the speed and air rate were raised to obtain the data at high power. I n both cases, the results were essentially similar. The results (Figure 2A) show that fermentations started at high power
input (0.9 hp. per 100 gallons) showed an increase in power requirement which reached a peak (1.35 hp. per 100 gallons) a t about 40 hours, and then declined gradually until the end of the fermentation. I n contrast, there was very little change in power requirement for fermentations operated a t low power input (0.5 hp. per 100 gallons). The apparent viscosiiy increased to a maximum ar about 80 hours and then decreased. Gas retention increased gradually during the fermentation to a value of 20y0 ; in some runs it decreased slightly in the later stages of the fermentation. The initial increase in power appeared to be related, within certain limits, to the increase in apparen; viscosity; also, the decrease in power in the later stages of the fermentation may be related to the increase in gas retention. However, it is not possible from these data to determine the extent of these relationships. Although the real reasons for the difference in behavior between high and low power input are not known, hypothetical cases may be considered. For example, it may be that at high power input the entire tank is mixed and both laminar and turbulent regions are present (see diagram, below) ; hence an increase in apparent viscosity may increase the size of the laminar zone, thus increasing the power requirement. The increase in power occurred a t a time when the gas retention was low; an increase in gas retention would be expected to reduce both fluid density and apparent viscosity, thus resulting in a decrease in power regardless of whether mixing was laminar or turbulent. At low power input, mixing may be localized to a laminar region about the impeller; although apparent viscosity has an effect on power in the zone about the impeller, an increase in viscosity may reduce the size of this region, thus counteracting the effect. In another run, two liters of antifoam were added to the tank during the first day of fermentation (Figure 2B). For this reason the gas retention value remained at a low level, about 3%: for the duration of the fermentation. The
QUIESCENT T R B U L E N T P=K2pN3D5 , rL A M I N A R : P = K , p a N 2 D 3 7 ~
:
Hypothetica I models represent effect of power input on mixing in pilot fermentors
INDUSTRIAL AND ENGINEERING CHEMISTRY
HIGH POWER
LOW POWER
P=7
P O W E R REQUIREMENTS
Figure 3. Influence of changes in physical properties on power requirement of 5-galIon novobiocin fermentation was related to power input level
0
40 80 I20 FERMENTATION TIME, HOURS
power input a t the start of the fermentation was 0.9 hp. per 100 gallons; this increased to 1.42 hp. per 100 gallons at about 40 hours and remained constant for the remainder of the fermentation. Comparison of Figures 2A and 2B indicates the marked influence of gas retention on power requirement. The nature of this effect is not clear, since power is influenced by apparent viscosity if mixing is laminar and by fluid density if mixing is turbulent, and a change in gas retention would influence both of these factors. The results also indicate that wide differences in apparent viscosity, from 50 to 160 cp., were without influence on the power requirement (Figure 2B). There are at least two possible explanations for this behavior: The apparent viscosity of a de-aerated sample measured outside the fermentor may be quite unrelated to the apparent viscosity of a highly aerated material inside the fermentor. I n addition, the apparent viscosity under the shear stress chosen for the sample measurement may not be proportional in some instances to the apparent viscosity under the shear stress levels existing in the fermentor. 5-Gallon Tanks. In general, the results obtained on the 5-gallon scale (Figure 3) were somewhat similar to those obtained with the 66-gallon tanks. Fermentations operated a t low power input (0.5 hp. per 100 gallons) showed little change or, qerhaps, a slight decrease in power during the fermentation; those operated a t high power input (0.9 hp. per 100 gallons) generally showed an increase in power at 25 to 30 hours followed by a decrease to a level equal to or lower than the initial power input. The decrease in power after 30 hours occurred while the apparent viscosity was increasing, but during the same period gas retention increased from 4 to 12%. 12,000-Gallon Tanks. Results obtained with 12,000-gallon fermentors are
shown in Figure 4. These fermentations were operated a t an agitator speed of 83 r.p.m. and air rate of 600 cubic feet per minute. At the start of the fermentation, the power drawn by the agitator was about 0.55 hp. per 100 gallons; this increased to about 1.0 hp. per 100 gallons a t 30 to 35 hours and then decreased to about the original power level (Figure 4A). I n these two fermentations the maximum apparent viscosities were widely different (Figure 4B), 170 compared with 360 cp., yet the peak power consumption was about the same in both cases. This result is somewhat similar to that previously given for the 66-gallon tanks (Figure 2B). Another observation made on this equipment was that the power requirement doubled even when the power input at the start of the fermentation was low. For example, at an agitator speed of 63 r.p.m. and air rate of 400 cubic feet per minute, power input was 0.35 hp. per 100 gallons at 20 hours but altered to 0.76 hp. per 100 gallons at 46 hours. This result is in contrast to that observed in pilot tanks where power was independent of apparent viscosity
m z 0
and gas retention at this level of power input. The reason for this difference in behavior is probably that in the large tank, even at low agitator speeds, there is more generalized mixing and bulk flow than in pilot tanks. 24,000-Gallon Tanks. In the case of the 24,000-gallon fermentors, the general pattern for power consumption was about the same as that obtained for the 12,000-gallon tanks. At a given air rate, power increased about 50% and then gradually decreased to about the initial value as the fermentation proceeded (Figure 5). As might be expected, the power requirement decreased as the air flow rate was increased because of either a reduction in fluid density and an increase in local hold-up about the impeller and/or a decrease in apparent viscosity. Other fermentations in this size of equipment were run at an agitator speed of 88 r.p.m. and air rate of 900 cubic feet per minute (Figure 6). An increase in power consumption occurred up to about 30 hours and appeared to correspond with the increase in apparent viscosity. Between 30 and 80 hours there was little change in apparent viscosity (120 to 130 cp.), yet there was a reduction in power requirement from 1.0 to about 0.8 hp. per 100 gallons. Over the same time interval, the gas retention values increased to a maximum of about 30%. One other factor which influences the power requirement of fermentations is the addition of antifoam agent. Figure 7 shows the actual power record and the effect of oil addition on power. The bottom of the tracing corresponds to 0.72 hp. per 100 gallons and the top corresponds to 0.77 hp. per 100 gallons. Upon the addition of antifoam, power increased to 1.02 hp. per 100 gallons. This effect was pointed out in earlier work by Wegrich and Shurter ( 8 ) and Bungay, Simons, and Hosler ( 7 ) .
-
cl 0
2
Figure 4. Changes Q in power consumption (A) and apparent viscosity (B) & 0 6 during novobiocin -* fermentations in 3 12,000 gallon fer- rT 0 4 63 RPM mentor were re- U 2 - 400.CEM lated only within cer- L tain limits 2 02-
?!
a
u
-
5
-
-
0
Lo
160z W
rT
a U a
AGITATOR SPEED,83 RCM AIR R A T E , 600 CEM. 40
240-
0 v)
- i
0 0
320-
4 e
U
80-
80 I20 0 40 FERMENTATION T I M E ,HOURS
VOL. 53, NO. 9
80
SEPTEMBER 1961
10
741
Table I1 gives the value of the exponent in the relationship : Power
=
K (N)"
for a variety of conditions, where X is agitator speed. In most cases, its value is 3.0, but it is sometimes greater or less 1.2
i W 4
. 8
-
1.0
a: I T
(r
g
RATE CfM.
0.8 900 1200
g a
0
-
1500
0.6
4 W
AGITATOR
0.0
I
SPEED; 88 RPM
I
I
I
40
I
I
80
FERMENTATION
I20
TIME, HOURS
Figure 5. Agitator power consumption during novobiocin fermentation in 24,000-gallon fermentor was reduced by increases in air rate within range examined
than this value. Both the nature of the beer and the tank size seem to have an effect. In the case of the 66-gallon fermentor, the exponent decreased with increase in agitator speed with low viscosity beer but remained constant with high viscosity beer. Power was not always proportional to N3. This relationship is expected for that portion of the tank which is turbulent, whereas for that portion which is laminar, power varies with i V . However, the fraction of the tank that is turbulent increases as a function of agitator speed, thus effectively increasing the exponent on N . O n the other hand, density ( p ) may be a n inverse function of N , thus decreasing the exponent. T h e effect of N on p is dependent on such beer characteristics as apparent viscosity (pa) and surface tension. Also, for non-Newtonian broths, a n increase in agitator speed may influence p a , since the latter is a function of shear rate and shear rate is a function of N . Accordingly, the effect of agitator speed on power cannot always be expressed as a simple equation. Discussion
AIR RATE
900 CFM 1180
x.0 8 -
I
I
-
n
= . a w
k
n
U
120
06L
0 p:
$
0
I60 Lo
04-
5>u 0
2!! >
/ P iL /p
0
.
0
I
When pilot tanks were operated at low power input (0.5 hp. per 100 gallons), the power required for the subsequent fermentation period was independent of changes in apparent viscosity and gas retention. However, this should not be misconstrued as indicating turbulent mixing, since a t even higher agitator speeds, power was not inde-
Table II.
, GAS RETENTION
.
.
.
.
40 EO I20 FERMENTATION TIME .HOURS
Figure 6. Within certain limits, changes in apparent viscosity and gas retention during novobiocin fermentation in 24,000-gallon fermentor influenced power consumption
Effect of Agitator Speed on Power Input
Nature of the beer and tank size are factors
Operating Tank Volume, Gal. 5 66 66 12,000 12,000 24,000
Viscosity
11
Low Low
3.0 1.7-3.0
High
2.7
Low
High
2.1 3.4
Low
3.1
Figure 7. Power requirement of 12,000-gallon fermentor increased about 36% upon addition of antifoam agent
l-
pendent of apparent viscosity. For example, when fermentations were operated a t high power input (0.9 hp. per 100 gallons) then the poizer requirements changed during the fermentation with alteration in the physical properties of the culture. I n production-scale equipment, the power requirements altered during the fermenlation regardlesr of the level of power input at the start of the fermentation. This difference in equipment performance indicates differences in the type of mixing on these scales, and this may be a n important factor for consideration in scale-up work. T h e significance of these differences will remain obscure until further progress has been made toward our understanding of the factors involved in mixing aerated filamentous broths and the ultimate influence of these factors on yield. All of the factors contributing to power requirements of fermentation broths are not clear Further, the use ofgeneral correlations, obtained from model systems, for predicting power requirements does not appear feasible. Perhaps if measurements of apparent viscosity and fluid density which were actually representative of those conditions existing in the fermentor could be obtained, then prediction of power might be possible. Nevertheless, flow conditions in a fermentor alter with change in the physical properties of the culture, and this results in alteration of the power requirement at various stages of the fermentation. Also, at any one time both turbulent and laminar regions may be present in a fermentor; hence any over-all power correlation? would appear to have limited usefulness. The results of the present work are largely qualitative and descriptive, bul perhaps they will help to emphasize the complexity of the problem of mixing nonNewtonian fermentation broths. literature Cited (1) Bungay, H. R., Simons, C. F., Hosler, P., J . Biochem. Microbiol. Techno[. Eng. 2, 143 (1960). (2) Chain, E. B., Gualandi, P., Rend. i.rt. super. sanitd 17, 5 (1954). (3) Chemical Engineer's Handbook, J. H. Perry, ed., 3rd ed., p. 1201, McGrawHill, New York, 1950. (4) Metzner, A. B., Otto, R. E., A.Z.Ch.E. Journal 3, 3 (1957). (5) Metzner, A. B., Taylor, J. S., Zbid.. 6, 109 (1960). (6) Nelson, H. A, Maxon, W. D., Elferdink, T. H.. IND.ENG. CHEM.48, 2183 (1956). (7) Rushton, J. H., Oldshue, J. Y . , Chem. Eng. Progr. 49, 161 (1953). (8) Wegrich, 0. G., Shurter, R. A,, IND. ENG.CHEM.45, 1153 (1953).
RECEIVED for review December 30, 1960 ACCEPTED April 5: 1961 Division of Agricultural and Food Chemistry, 138th Meeting, ACS, New York, September 1960.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
