I
FUMITAKE YOSHIDA, AKlO IKEDA,' SHUHEI IMAKAWAt2 and YOSHIHARU MlURA Chemical Engineering Department, Kyoto University, Kyoto, Japan
The Sulfite Oxidation Method
Oxygen Absorption Rates in Stirred Gas-liquid Contactors A new correlation is available which is useful for scaling up either fermentors or vessels for chemical reactions between gases and liquids f
e
THE
SULFITE OXIDATION method is regarded as the standard procedure for testing the rate of oxygen absorption in stirred gas-liquid contactors which are used for both submerged aerobic fermentation and chemical reactions between gases and liquids. I n this method, oxidation rate is measured when air is bubbled through a n aqueous sodium sulfite solution catalyzed by cupric ions. I n interpreting the results, however, opinion differs (7, 3, 5-70, 72). This investigation has three objectives: To test the merits of the sulfite oxidation method; to study the mechanism of the sulfite oxidation reaction; and to establish the relationship between agitation and the rate of gas absorption in stirred gas liquid contactors. For aqueous sodium sulfite solutions catalyzed by cupric ions, the oxygen absorption rate is controlled by mass transfer in the liquid phase. This is true because, when interfacial areas are equal, absorption rates in pure water and both sodium sulfate and sulfite solutions are identical. I n bubbling gas-liquid contactors with mechanical agitation operating under the same conditions, oxygen absorption rate in sodium sulfite solution per unit volume of liquid is faster than in pure water, especially under high agitation. This difference results from smaller bubbles and greater interfacial area in the sodium sulfite solution rather than from chemical reaction. For oxygen absorption from air by sodium sulfite solution, the gas film offers no appreciable resistance.
Experimental Bubbling agitated gas-liquid contactors were constructed of stainless steel, geometrically similar in design to those of Cooper and others (2). Three contactors were used-15, 25, and 37.5 cm. in 1 Present address, Mitsui Petrochemical Industry Co., Iwakuni, Japan. Present address, Kyowa Fermentation Industry Co., Ube, Japan.
diameter-and, unless otherwise stated, the liquid depth in each was kept equal to the tank diameter. Inside each tank, four equally spaced vertical baffles, one
tenth of the tank diameter in width, were attached to the shell. Each agitator was mounted at the axis of the tank with a clearance from the bottom equal to three
disks and one turbine
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VANED Tank Agitator
15 6
Diam., Cm. 25 10
I
T U R 5 INE
DISK
37.5 15
Vaned Disk, Type
Vanes Width, No.
W 0.1D
B
16 12
C
6
A
0.1D
0.1D 12 0.2D Turbine 12 0.2D I = 0.35 D for vaned disk and turbine
D
Four Types of Contactors Were Used Type Bubbling with mechanical agitation Bubbling without agitation Bead column, continuous trickling flow of liquids Horizontal surface with agitation
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System OZ-HZO; OrNazSOa ; air-NazSOs ; 02-Na2S04 OrHz0 ; Oz-NazSOa ; 02-NazSO4 Oz-HzO ; OrNazSOa
0 : - H ~ 0; Oz-NazSOs ; air-NazSOa ; OrNazSO4
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The complete manuscript from which this article was condensed, containing all tables and figures and additional text
Clip and mail coupon on reverse side VOL. 52, NO. 5
M A Y 1960
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8 IO8
N3 L2 ( f t 2 / m m 3 )
Figure 2. Correlation of data was obtained for mass transfer coefficients in the liquid phase, using oxygen absorption rates in water at 20" C. N, agitator speed in r.p.m.; L, tank diameter, feet. left, vaned disk A; right, turbine
tenths of the tank diameter. For each size tank the diameter of the corresponding agitator was equal to four tenths of the tank diameter. Gas was fed into the liquid through a single nozzle, usually of 4-mm. diameter, at the bottom center. The oxygen content of water was analyzed by a modified Winklei- method (77). Absorption Rate in Bubbling Oxygen-Water Contactors
For the oxvgen-water system, resistance
For the turbine, c equals 1.10, and m and n each equals 2 / 3 . Values obtained for n are smaller than the 0.95 reported ( 2 ) for sulfite oxidation with the Type A vaned disks. I t is also less than the value of unity obtained for sulfite oxidation with flat paddle agitators ( 4 ) . The effect of temperature on kLa is slight. At 20' and 40' C., values ot kLa coincide over the entire experimental range of agitator speeds. This coin-
The power input a t a given gas velocity for a given design of agitated vessel varies with W L 5 ; therefore, LV3L2is directly proportional to the power expended per unit volume. Figures 2,A and 2,B correlate kLa with i\73L2. The data points for the three vessels of different sizes fall on the same line for a given gas velocity, indicating the general applicability of such correlations. This correlation is expressed by
to mass transfer resides solely in the liquid
phase. In calculating the average volumetric mass transfer coefficient, k,,a, of the liquid phase, two assumptions were made: that bulk concentration of dissolved oxygen was uniform throughout the liquid; and that effect of desorption of dissolved nitrogen originally contained in water was negligible. kLa is defined by the relation
where C = concentration of oxygen in liquid in pounds per cubic foot or grams per kilogrdm; C* = concentration of oxygen in liquid a t equilibrium with purr oxygen in gas phase; and 6 = time in hours. On integration
kLa = cV7(N3LP)n
(3)
where V, is the superficial gas velocity in feet per hour. For vaned disk A . values of constants are :
V,.Ft./Hr. 5.4 10 20 40 90
V7
n
2.1 x 10-4 7.0 x 10-4 2.6 x 10-3 1.3 X 6,2 X lo-%
0.68 0.63 0.58 0.50 0.43
C
E
Vaned Disk A
and the values of m are related to N3L2by
c 0
m
c c L
L: \
In
E 1. n W
Y
iVL2 ria
08 0.84
1107
108
0.60
G.40
0 U
Y
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Figure 3. The over-all mass transfer coefficients for oxidation of sodium sulfite depend on agitator speed and gas rate
Address . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Vaned disk A; temperature, diameter, 25 cm; (2)
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tank
O X Y G E N A B S O R P T I O N RATES 400
For the 25-cm. tank with vaned disk
A a t 15' C. and a gas velocity of 40 feet per hour, values of kLa decrease with increasing viscosity. However, the exponential effect of the Schmidt number on these values cannot be determined from the present data because liquid viscosity also might affect the interfacial area, a, as well as values of k L .
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Oxygen Absorption in Bubbling Contactors
6C
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c
4c
During oxidation of sodium sulfite by air and by pure oxygen, using the 25-cm. bubbling contactor, the sulfite concentration was not allowed to fall below 0.04N because, according to Cooper, oxidation rate is independent of sulfite concentration from 0.35 to 1 N. The over-all mass transfer coefficient based on partial pressure differences, &a, were the same for air and oxygen (Figure 3). This indicates that the gasfilm resistance is negligible, because with pure oxygen no resistance to mass transfer is imposed by gas film. Where solubility of a gas follows Henry's law, the relationship between over-all and mass transfer coefficients is
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R.P.M. Figure 4. Sulfite oxidation is controlled by mass transfer but, because of smaller bubble size, values of k ~ aare higher than in pure water. 0.25N sodium sulfite and 0.25N sodium sulfate; gas velocity, 20 feet per hour
1 &a
cidence is fortuitous, indicating that at these two temperatures, the effects of temperature on k L and on the interfacial area a offset each other. The values of kLa a t 7" C. are about 2570 lower than at 20" C. for the same agitator speeds. Effect of the ratio for liquid depth to tank diameter is negligible for ratios less than 1 .O, but it becomes appreciable for values above unity-e.g., a t 1.4, values of kLa are reduced about 3OY0 from those a t unity. Regarding nozzle size, k t a values for the 4-mm. size were slightly higher than those for the 8-mm. size. However, this difference became negligible at high agitator speeds.
1 koa
+-H KLa
(4)
where ko = mass transfer coefficient of gas film per unit area based upon partial pressure differences; kL = mass transfer coefficient of liquid phase per unit area based upon concentration differences; H = Henry'slaw constant-i.e.,p = HC; and a = interfacial area per unit volume. l/kGa is zero where the gas film resistance is negligible. Oxygen Absorption Rates in Pure Water us. Sulfite Solution, For comparing oxygen absorption rates in sodium sulfite solution with those in pure water, the liquid-phase coefficients. kLa were calculated by Equation 4 from the sulfite oxidation data on the assumption that> the solubility of oxygen in sodium sulfite, solution at the interface before chemical
reaction takes place is the same as in sodium sulfate solution of the same normality. I n a sodium sulfite solution undergoing oxidation, the combined concentration of the sulfite and sulfate ions is constant with equal values at the interface and in the bulk. When kLa for sulfite oxidation was plotted against agitator speed on log-log coordinates, the points for a gas velocity of 40 feet per hour at 20' and 40' C. fell on the same lines, but the values of kLa at 7" C. were somewhat lower. Similar temperature effect occurs for oxygen absorption in pure water, however, at a given gas velocity the slope of the line for sulfite oxidation is considerably higher than for the absorption of oxygen in pure water (Figure 4). This diference can be explained by the occurrence of smaller bubbles in the electrolytic solution than in pure water. At high agitator speeds, much smaller bubbles were formed in sulfite solutions than in pure water (Figure 5). This was true for other electrolytic solutions also, and can be explained by the electrostatic potential of the resultant ions at the liquid surface.
Oxygen Absorption in SodiumSulfate Solutions. I t was assumed that physical properties for dilute sulfate solutions are nearly the same as for sulfite solutions. I n Figure 4, kLa values for sulfate solution, calculated using Equation 4, agree with those for the sulfite solution. This gives additional evidence that sulfite oxidation catalyzed by cupric ions is controlled by the rate of physical absorption of oxygen. Data for oxygen absorption in water differ from those for sulfate and sulfite solutions because of differences in bubble size. This difference increases with higher stirring speeds. Bubbling with No Mechanical Agitation. Without mechanical stirring, oxygen was bubbled into water and solutions of both sodium sulfate and sulfite. For the three systems, bubble sizes were the same and kLa values were nearly
Ratios of kLa for Various Agitators . Relative to Vaned Disk A (Tank dia., 25 cm.; temp. 20' C.; gas veloc. 40 ft./hr.)
kLa Ratioa Agitator Type
200 r.p.m.
400 r.p.m.
C
0.70 0.83 1.0 1.18 1.34
0.70
B A
D
0.83 1.0 1.18 1.72
Turbine a Apparently unaffected by gas velocity.
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Figure 5. At high agitutor speeds, bubble size is substantially reduced by adding sodium sulfite Temperature, 25' C.; gas velocity, 16.3 feet per hour; nozzle diameter, 4 mm.; r.p.m., 41 5 From left to right: 0.2N sodium sulfite; 0.1 N sodium sulfite; pure water
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identical at the same gas rates. These results are contradictory to those of Wise (72); however, similar results were obtained when a vessel, 15 cm. in diameter and 15 cm. in liquid depth was employed, where values of kLa for sulfite oxidation and oxygen absorption in sulfate solution were about 3070 higher than those for oxygen absorption in water. I n sulfite and sulfate solutions gas bubbles lingered when they reached the free surface. This surface effect per unit volume is greater in a shallow vessel than for a deep vessel and could result in higher values of kLa. Wise did not publish the dimensions of his apparatus. Oxygen Absorption in Contactors with Known Interfacial Areas
T o avoid uncertainty in measuring interfacial area, two types of gas-liquid contactors with known interfacial areas were employed. These procedures permitted direct evaluations of kL. One contactor was a bead column (73), in which 30 porcelain spheres, 1 inch in diameter each, were superimposed vertically; liquid trickled over the surface of the spheres. The other device was an agitated vessel, 25 cm. in diameter and 15 cm. in liquid depth, with a Type A vaned disk agitator supported at an elevation of 10 cm. above the bottom of the tank. Gas was passed over the free surface and the vessel was loosely covered. When oxygen is passed without bubbling through the bead column, k~ values for both water and sodium sulfite solution do not differ appreciably, regardless of gas rate (Figure 6). For the same interfacial area, the mass transfer coefficients for oxygen absorption are iden tical. T o test absorption on horizontal sur-
faces, an agitated vessel without bubbling was used (Figure 7). Below 150 r.p.m., k L values for sulfite oxidation are somewhat higher than those for sulfate and pure water. The difference increases with decreasing agitator speeds. Thus, when agitation is poor, rate of sulfite oxidation is not totally controlled by mass transfer. A previous report ( 9 ) shows that the k L value for sulfite oxidation decreases slightly with increasing agitator speed, and another ( 8 )that the rate of oxidation is independent of agitator speed and proportional to the 1.5 power of the partial oxygen pressure. These results disagree probably because of differences in experimental equipment and methods of mechanical agitation. None of the experiments simulates commercial gasliquid contactors.
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Figure 6. When oxygen i s absorbed in the bead column, k L values for both water and sodium sulfite solution do not differ appreciably. Temperature, 25” C. ReL, liquid Reynolds number
(73) 438
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Interpretation of the Mechanism
On the basis of unit horizontal area, rate of oxygen absorption by a deep bed of water with slow mechanical agitation and without bubbling is increased by adding sodium sulfite with a cupric ion catalyst, but not affected when water is trickled over a column of beads. This can be explained by a plausible mechanism: Over a differential element of thickness of liquid in the direction of diffusion near the gas-liquid interface, diffusion and chemical reaction proceed simultaneously, with the faster rate controlling the combined rate. In trickling liquid over a column of beads, the rate of chemical reaction in this element is negligible compared to the rate of diffusion; hence over-all rate is controlled by diffusion, because the rate of diffusion is rapid because of high turbulence in the liquid layer. I n a deep horizontal bed with slow agitation and with no bubbling, the rate of chemical reaction becomes significant relative to the slow rate of diffusion and both rates may become equally important. Here the over-all rate is controlled by both rates. In either case, when the oxygen reaches the main bulk of liquid remote from the interface, the added resistance imposed by the chemical reaction is negligible. Where oxygen is bubbled through water the rate of absorption depends only on diffusion. However, when oxygen is bubbled through a sodium sulfite solution under mechanical agitation the rate of absorption per unit interfacial area still depends only on diffusion, but rate per unit volume is accelerated because of the greater interfacial area induced by the presence of ions. The mechanism in this case is similar to that in the trickling bead column. I t is interesting to note that both diffusion and the chemical reaction depend on the concentration of dissolved oxygen and the rate of chemical reaction is independent of sulfite concentration in
ogmqo
02
2 c
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 7. When agitation i s poor, rate of sulfite oxidation i s not totally controlled by mass transfer
the presence of cupric ions. The rate of chemical reaction depends on oxygen concentration times concentration of a complex sulfite-copper ion which remains nearly constant during the course of the reaction. Acknowledgment
The authors are grateful to 0. A. Hougen of the University of Wisconsin for critically reviewing the manuscript and giving many helpful comments and suggestions regarding this paper. The assistance of Shukichi Oyama, Tadayuki Imai, Koji Nishikawa, Egi Yagi, and Yoshikazu Torigoe in the experimental work is also appreciated, literature Cited (1) Bartholomew, W. H., Karo, E. O., Sfat, M. R., Wilhelm, R . H., IND. END.CHEM.42, 1801 (1950). (2) Cooper, C. M., Fernstrom, G. A., Miller, S.A., Ibzd., 36,504 ( 1944). (3) Finn, R. K., Bid. Reu. 18, 254 (1954). (4) Friedman, A. M., Lightfoot, E. N., Jr., IND.ENG.CHEM. 49,1227 (1957). (5) Hixson, A. W., Gaden, E. L., Ibid., 42, 1792 (1950). (6) Maxon, W. D., Johnson, M. J., Ibid., 45, 2554 (1953). (7) Miyamoto, S., Bull. Chem. Sod. Japan 5, 125, 229, 321 (1930); 6 , 9 (1931); 7, 8 (1932). (8) Phillips, D. H., Johnson, M. J., IND. ENG.CHEM.51, 83 (1959). (9) Schultz, J. S., Gaden, E. L., Jr., Zbid., 48, 2209 (1956). (10) Solomons, G. L., Perkin, M. P., J . Appl. Chem. 8,251 (1958). ( 1 1) Treadwell-Hall, “Analytical Chemistry,” 8th ed., p. 704, John Wiley, New York, 1940. (12) Wise, W. S., J . Sac. Chem. Ind. (London) 69, Suppl. 1, S40 ( 1950). (13) Yoshida, F., Koyanagi, T., IND. 5 0 , 365 (1958). EXG.CHEM.
RECEIVED for review April 27, 1959 ACCEPTEDSeptember 24, 1959
