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DANIEL HYMAN and J. M. VAN DEN BOGAERDE Central Research Division, American Cyanamid Co., Stamford, Conn.
Gas-Liquid C o n t a c t i n g
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Small Bench-Scale Stirred Reactors Small reactors can give gas-liquid contacting performance similar that found in pilot plant and full-scale vessels
CONTACTING
O F A liquid and a gas in a vessel agitated by a rotating impeller is an important operation in chemical processing. There is a high degree of empiricism in the procedures used to predict or evaluate the performance of agitated vessels, especially when more than one phase is involved. As this performance is often critically influenced by system geometry, it is good practice to preserve geometric similarity between different scales of operation. Since the general nature of the anticipated large-scale installation can usually be established fairly early in a development program, the problem is often one of “scale-down.” There have been a number of studies on gas-liquid contacting in agitated vessels scaled down to pilot plant size (2, 4, 8, 7 7 ) . But there are many advantages in doing development work on an even smaller scale and increasing numbers of bench-scale studies will probably be made. Accordingly, two benchscale agitated gas-liquid reactors were designed, each with geometries similar to conventional pilot plant and full size vessels. One held about 1000 and the other about 200 cc. The smaller one was designed in order to extend the idea of a “scaled-down” reactor to a size convenGas Inlet 4mm 1 D
ient for even preliminary chemical studies. The use of such a device could introduce a degree of physical realism (with respect to the capabilities of plant equipment) into the very early stages of a project. In addition, measurements were made of gas-liquid contacting in a typical laboratory stirred flask. Each reactor is illustrated (dimensions given in inches).
I t was decided to characterize performance of these by the oxygen mass transfer coefficient obtained during the air oxidation of aqueous sodium sulfite. The limitations of this test reaction have been recognized and discussed elsewhere (7, 5, 8, IO, 12), but its use here was believed justified in view of the essentially comparative nature of this study. The
For Reactor 1, two types of stainless steel baffles were a d d e d to a standard reaction flask
j 14-
1
* impeller
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”.ATypicalBaffle The complete manuscript from which this article was condensed, containing all tables, flgures, additional text, and summary of experiments and results
Reactor
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751
Experimental A measured air flow was passed into sodium sulfite solution (about 0.7M initially) also containing 0 . 0 0 1 ~of cupric ion. Temperature was controlled
results of these measurements showed that it is indeed possible to extend the concept of maintaining geometric Similarity in agitated systems to operations on a scale as small as 170 cc.
Table I.
Ranges
of Experimental Variables
(Vertical distances from lowest point in reactor) 1A 1B 2 2 Impeller type
6-Blade Turbine
Ratio of diam. impellerjreactor
6-Blade Turbine
0.50
Vaned Disk
0.50
4-Blade Turbine
0.47
0.42
3 HollowArm
3 2-Blade Turbine
0.31
1130-2260 296-592
vAP,
645-1990 338-1040
400-1600 210-838
1200-2400 353-707
800-3200 314-1256
800-3200 314-1256
Superficial veloc., 5.8-24W ft./hr. Vol. gas/vol. liq./ 15.4-659 hr.
4.8-413=
18.8-415"
18-360a
6 . 7-25gb
7-150b
12.3-1060
94.8-2090
61.6-1870
100-3885
50-2250
Air flow rate
Liquid depth Inches Ratio, to vessel diam. Vol. of liq., cc.
3.5-6.8 0.88-1.71 640-1340
5.5 1.38
3.3 1.38
3.0-4.5 1.26-1.90
1.6-2.6 0.32-0.53
1.62.6 0.32-0.53
965
185
170-255
245-510
245-510
3.3 0.59 1.63
1.1 0.35 1.0
0.6-2.3 0.20-0.50 0.60-2.25
0.50 0.20-0.33 0.33
0.50 0.20-0.33 0.33
0.2 0.06
0.2 0.04-0.06
0.0 0.0
0.0
Impeller height Inches 1.0-3.4 Ratio, toliq. depth 0.25-0.50 Ratio, to impeller 0.50-1.71 diam. Gas inlet height Inches 0.3-1.3 Ratio, to liq. depth 0.04-0.31 a
2.3-3.1 0.42-0.56
Based on cross section of cylindrical section.
Table II.
Impeller
Range of K ,
1-A baffles 1-B baffles 2 2 3 3
Turbine Turbine Vaneddisk Turbine Hollow-arm Turbine
0.01 -0.17 0.001-0.14 0.008-0.042 0.008-0.11 0.005-0.10 0 . 0 1 -0.08
a
0.0
Based on cross section at maximum diam.
Same Form of Equation Can Be Used to Correlate Results for Each Reactor ( K , = c 7l-a Bob2")
Vessel
2 held constant a t 5.5 in.
C 2.56 6.88 2.85 5.93 3.57 2.78
X X X X X X
lo-' IO-lO
lo-* 10-6
a
b
c
2.04 2.40 2.17 2.17 1.79 1.26
0.21 0.13 0.14 0.14 0.05 0.00
-0.47
Oa -0.86 -0.86
Ob Ob
R K , = __
0.31
Impeller speed Rev./min. Peripheral veloc., f t./min.
at 27 =t0.5' C. From three to six small samples of liquid were withdrawn during the course of a run (about 40 to 100 minutes) and analyzed for sulfite content by the conventional iodonietric technique. Least squares analysis was used to compute the rate of decrease of sulfite concentration. The rate of oxygen consumption was obtained from this and an over-all average mass transfer coefficient per volume of liquid was calculated by
Standard Error, % 4-31 4-34 +28 +28 1-15 +13
-24 -25 -22 -22 -13 -11
Z held constant a t 1 . 5 in.
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The driving force, AP,, was based on zero oxygen back-pressure from the liquid. Factors considered as independent variables were impeller speed, air rate, liquid depth, impeller height, and gas inlet height. Not all of these were varied for each reactor (Table I). Conventional statistical techniques ( 3 ) were used to determine the significant variables and correlate the results. Results
Impeller speed, air rate, and liquid depth had significant effects. The results from each reactor-impeller combination, could be correlated as shown in Table 11. The highest values of K , were obtained in Reactor 1 using the fulllength A baffles but generally similar levels of performance could be obtained in each vessel. The major influence on the transfer coefficient in every case was the impeller speed. Runs at 20" and 35" C. under various operating conditions showed no significant effect of temperature on K , : indicating a diffusive-step limitation on the rate of oxygen transfer. No significant change in K , was observed when a fritted-glass air inlet was substituted for the open-tube inlet in Reactor 1. Foaming \vas observed a t very high impeller speeds (3200 r.p.m.) in Reactor 1 and these runs were not used in the correlation of results. Erratic results were obtained in Reactor 3 for air velocities above I50 feet per hour and liquid depths above 1.5 inches; these data were not included in the correlation.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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Discussion
The data in Table I1 demonstrate the influence of reactor geometry. Thus, although one general form of equation was used, the exponents which show the effects of the operating variables on K , are significantly different between the cylindrical baffled reactors on one hand and the spherical flask on the other. In fact, further statistical analysis of the results from both cylindrical reactors showed that they could be represented by one equation with no loss of significance: K, C ' (j\'~)2.18vo0.17 (Z/T)-O.SO ( 2 )
BENCH-SCALE STIRRED REACTORS
a
Section A-A
Section A-A
Gas Inlet, 6mm I.D. VANED DISK (GLASS)
4-BLADE SIMPLE TURBINE (ST. STEEL)
Impeller 4
A
Reactor 2 was all glass, specially fabricated with thin baffles creased into the wall.
b Reactor 3 was a conventional laboratory 1000-cc. creased spherical flask with a gas inlet added
v
Reactor
Reactor where C’ X 109 is 1.22 for Reactor 1 with A baffles, 0.63 for Reactor 1 with B baffles, 0.76 for Reactor 2 with vaned disk impeller, and 1.98 for Reactor 2 with turbine impeller. These variations in C‘ probably result from departures from exact similarity between the reactors. The standard error for Equation 2 was +32ojo and -24%. Published data (2, 4, 6, 9-11, 13) give values of K , in the same range as those in Table I1 and show K , increasing with N raised to a power in the range 1.6 to 3.0 for cylindrical reactors ranging in diameter from 6 inches to 20 feet. Other published data on the effect of gas velocity indicate exponents on V , in the range 0 to 0.7 (4,6, 7). The results of Cooper and others (2) show K , proportional to the -0.5 power of Z . The exponents in Equation 2 are thus in the same ranges as reported by other investigators using similar but much larger vessels. Using the relation between K , and the operating conditions to measure the character of the fluid regime inside a reactor, these results indicate that conditions in the cylindrical reactors were similar enough so that one equation could be used to show how K , varied with N , V O , and 2. Further, since K , in these small reactors was of the same size and varied in approximately the same way as has been reported for much larger vessels, this investigation confirms the desirability of maintaining similarity in agitated vessel design even in operations on a scale as small as 170 cc. Vessels like Reactors 1 and 2 should prove useful both for preliminary studies and small-scale development programs.
HOLLOW-ARM STIRRER ( GLASS)
Nomenclature
D
= impeller diameter, in.
K,
= oxygen mass transfer coefficient,
N R
= impeller speed, r.p.m.
7’
= reactor diameter, in.
u
= volume of liqiiid in reactor, cu.
V, Z
= superficial air velocity, ft./hr.
lb. moledcu. ft.-hr.-atm. = oxygen absorption rate, lb. mole/
hr. ft
.
= liquid depth, in. APm = log mean driving force, atm.
Acknowledgment
J. R. Bartlit did the early experimental work. The assistance of A. M. Schneider and A. L. Stockett in the statistical analyses was most valuable. literature Cited (1) Carpani, R. E., Roxburgh, J. M., Canadian J . Chem. Eng. 36,73 (1958). (2) Cooper, C. M., Fernstrom, G. A., Miller, S . A., IND. ENC. CHEM.36, 504
(1944).
2-BLADE SIMPLE TURBINE (ST. STEEL)
(3) Davies, 0. L. (ed.), “Design and Analysis of Industrial Experiments,” Hafner Publishing Co., New York, 1954. (4) Elsworth, R., Williams, V., HarrisSmith, R., J . Appl. Chem. (London) 7, 261 (1957). (5) Finn, R. K., Bacterzol. Rev. 18, 254 (1954). (6) Friedman, A. M., Lightfoot, E. N., Jr., IND.END.CHEM.49, 1227 (1957). (7) F x s o n , A. W., Gaden, E. L., Jr., Ibad., 42, 1792 (1950). (8) Karow, E. O., Bartholomew, W. H., Sfat, M. R., J. Agr. Food Chern. 1, 302 (1953). (9) Oldshue, J. Y . , IND.ENG. CHEM.48, 2194 (1956). (10) Phillips, D. H., Johnson, M. J., Ibid., 51, 83 (1959). (11) Rushton, J. H., Gallagher, J. B., Oldshue, J. Y . , Chem. Eng. Progr. 52, 319 (1956). (12) Schultz, J. S., Gaden, E. L., Jr., IND.ENG.CHEM.48,2209 (1956). (13) Snyder, 3. R., Hagerty, P. F., Molstad, M. C . , Zbid., 49, 689 (1957). RECEIVED for review November 16, 1959 ACCEPTED April 25, 1960 Presented at 41st National Meeting. A.I.Ch.E., St. Paul, Minn., September 1959, VOL. 52, NO. 9
SEPTEMBER 1960
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