OXYGEN ABSORPTION IN LOW-FREQUENCY VERTICALLY VIBRATING LIQUID COLUMNS R. H. BUCHANAN, D. R. TEPLITZKY, AND DJOERIAMAN OEDJOE L'niuersity
of
South Wales: Kensington, Australia
The promotion of difficult gas absorption b y cyclic bubble migration has been studied using the absorption of oxygen from air b y NaZS03 solution. This contacting method was found to b e considerably more efficient than a sparger-column or a stirred reactor. The absorption coefficient Koa for the vibrating column was found to b e independent of the liquid rate and to vary as the 0.33 power of the gas rate, as the 3.5 power of the absolute temperature, as the 1.1 power of the vibrational acceleration, and as the - 1 .O power of the liquid height. N o conclusion could b e drawn on the relative magnitude of the resistances caused b y diffusion and chemical reaction in the liquid phase.
CYCLICmigration offers a good method of gas-liquid BUBBLE
contacting when countercurrent flow is not required (6). Two applications suggest themselves, both involving the absorption of a gas of low solubility: first, when a slow chemical reaction is involved, as in fermentation, and second? when absorption is followed by chemical reaction a t a solid interface, as in hydrogenation. This report, which deals only with the first case, demonstrates that a liquid column vibrating vertically a t low frequency and high intensity is equivalent in gas absorption rate to stirred reactors, shakers, sparger-equipped columns, and venturi and orifice mixers. All of these provide in different ways the requirements for promoting mass transfer: a large gas-liquid interfacial area and turbulence with consequent bulk mixing and reduction in the stagnant film thickness. The most important of the commercial contactors are the stirred and the sparger-equipped columns. Since the effectiveness of a gas-liquid contactor depends so much on the technique of operation, the equipment design, and the power consumption, it is difficult to make strict comparisons with the literature. Therefore comparative runs were made in vibration experiments, using the same column equipped with a 75-micron porous sparger at the bottom. A review of the literature shotvs that the efficiency of a gas-liquid contactor is often evaluated bl- the use of oxygen absorption from the air by iYa2S03solution in the presence of copper(I1) ion catalyst. This system was originally used by Cooper et al. (70). I t is also commonly employed to simulate fermentation, and for these reasons it was selected for this investigation. Theoretical
Two types of motion can be used to promote gas absorption: a steady rotary motion as in a stirred reactor and a recipro-
cating motion as in various types of rocking and shaking contactors or as in the vertically vibrating liquid columns considered here. The stirred tanks are widely used in industry, and their performance has been thoroughly investigated (2, 8, 70, 77, 72, 77, 79-27, 23, 23, 27, 30). I n bench-scale operations, shakers are frequently used? and some of These have
been studied by using the absorption of oxygen by Na2S03 solution ( 7 , 7. 8, 26). Hoffman and hiontgomery (73, 74) found that the shaker was about 3.5 times more effective than the rocker in promoting gas absorption. Another interesting horizontal shaker was proposed by Bjorkman ( d ) , who minimized the power consumption by operating at resonant frequency. The use of lolv-frequency vibration for promoting gasliquid contacting has been reported (28). This concerned the absorption of Con by a lime slurry in sugar carbonation. The apparatus consisted essentially of a largevertical cylinder in bvhich a n electrically vibrated C o s distributor \vas suspended. The rate of CO2 absorption was claimed to be markedly increased and capital and operating costs ion.. One of the authors carried out some preliminary work on the use of a low-frequency, vertically vibrating liquid column for gas absorption with the pure Cos-water system (5). In this experiment, the liquid \vas vertically vibrated by a diaphragm a t the bottom of a n unpacked column. Since the vibrational intensities \vere below the minimum values required for downward migration of bubbles, it \vas necessary to introduce the gas through a sparger into the bottom of the column. IVater was fed in a t the top continuously and removed a t the bottom. The number of the transfer units, .\70L, increased from 2.75 to 4.60 as the vibrational acceleration, w2A, was raised from 0 to 1800 cm./sec.Z,holding all other conditions constant. Oxygen Absorption. A slow chemical reaction occurs in the absorption of oxygen from air by Na2S03 solution, which limits the absorption rate. hiany investigators (9, 77, 27, 24, 27, 32) have studied oxygen absorption in NaZS03 solution, and there is some disagreement as to the relative magnitudes of the gas and liquid resistances. I t is the consensus of opinion however, that both resistances have some influence on the rate of absorption. The liquid phase resistance may be subdivided into two parts, that caused by physical diffusion and that caused by chemical reaction. The mechanism of the copper(11) ion-catalyzed reaction between 0 2 and SO3-- to SO?-- is not well understood, since it apparently involves a chain reaction of unusual length (3, 76). The reaction rate increases with temperature. I t doubles VOL. 2
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1963
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between 32" and 50" F., but increases more slowly between 65" and 104' F. (78,30). Experimental
Equipment. The equipment, shown in Figure 1, consisted essentially of a 30-inch long by 2.75-inch-diameter Perspex column mounted on a vertically vibrating platform. Details of the vibrator are described elsewhere (6). The top of the column was provided with inlets for liquid and air and an outlet for the unreacted gas. The liquid outlet was 0.5 inch from the base. Air was introduced centrally into the column 8 inches above the static liquid surface. This prevented direct
n
all connections are rubber
tublngr
t o vibrator
Figure
.
Absorption apparatus
A. Needle valves 8. Storage bottle C. Column
M. Micrometer
H.
T. V.
K.
R. S.
Heating mantle Feed sampling line
Rotameter Gas-liquid separator Thermometer . Valve
injection of air into the liquid and thereby ensured that all bubbles were produced by the vibration. The feed solution, on the other hand, was always introduced just below the static surface to prevent spray formation. A reflector was attached to the air outlet to minimize entrainment. Rotameters were used to measure the flow of the inlet air and inlet and outlet liquids. Seedle valves were used to control the flows, and thermometers were provided at various points to record the temperatures. The feed temperature could be varied bv means of a heating mantle around the storage bottle. For satisfactory operation, a gas-liquid separator in the liquid outlet was necessary to remove entrained bubbles and to smooth out surges in flow produced by the vibration. Procedure. The temperature of about 10 liters of freshly prepared nitrogen-blanketed 0 . 2 s Na2S03 solution containing 0.001M copper(I1) ion was adjusted to the desired value and the column filled to the required height. The flows of air and feed solution were begun at the predetermined rates, and then the vibration was commenced at the required frequency and amplitude. During the experiment the liquid rate was adjusted from time to time to maintain a constant average height in the column. Two samples were taken from the feed line, one at the beginning and the other a t the end of the experiment. Their close agreement showed, throughout the series, that oxidation did not occur in the storage bottle or before entering the column. Steady-state conditions were generally reached uithin 30 minutes and could be roughly ascertained by the constancy of the effluent temperature. Samples were usually taken after 30, 35,40, and 50 minutes to ensure the existence of steady state conditions. The following data were recorded at the steady state: the temperatures and flow rates of the feed, effluent and air. the vibrational frequency and amplitude. and the static height of the liquid column. The rate of the oxygen absorption was determined by pipetring 5 ml. of the solution quickly into 25 ml. of nitrogenblanketed 0.1 ,V iodine solution followed by back titration after 15 minutes with a standardized 0.1;\.' S a 2 S 2 0 3using starch indicator. The rate of oxygen absorption was calculated from the titration data as follows: .\"a
where
tion) = 0.00374 A t r A Ib. moles/(hr.)(cu. ft.) = volume of 0.1.V Na2S203 a t steady state condi-
Vo = .Y = L' = V, = 2
f m
u x
o . l o ~,
;
3
,+ . #
,
, ~ , .. , .i
0.08
2 00
Figure 2.
300
400 500 600 L ,Ib. /(hr.l(sqrf t. I
800
I 1000
Effect of liquid rate
I , 2. Vibrator, w z A = 1 1,000 cm./sec.*
3, 4. Sparger G, Lb./(Hr.) (Sq. Ft.)
0
+ m
A,
174
7.6 5.7 9.5
l&EC PROCESS DESIGN A N D DEVELOPMENT
( Vn - V0)NL' 20 VI.
.YA = oxygen absorbed. mmoles/(min.) (liter soluV,,
1
=
tions, ml. volume of 0.1S NazS203 for feed, ml. normality of Na2S203 solution liquid rate, ml./min. liquid volume in the column, liters
Method of Calculation. KOcompletely satisfactory method of expressing the coefficient of mass transfer or the height of a transfer unit has been reported for systems in which absorption is followed by a chemical reaction. In the case of oxygen absorption from the air with S a 2 S 0 3solutions, it is convenient, because of the unknown magnitude of the liquid resistances, to neglect it and consider the equilibrium partial pressure of O2 at the gas-liquid interface as zero. This simplification has been made with analogous systems such as NHa-HzS04 ( g ) , S02-causticsolutions (75),and chlorine-caustic solutions (22). The experimental results are, therefore, expressed here in terms of Kea, the over-all mass transfer coefficient based on the gas phase: Kca
=
~V~/PLM
where P,, is the log-mean of the bulk partial pressures of oxygen in the inlet and outlet gas phases. I n addition to the data on K,a, it is considered desirable to include also data on the absorption efficiency, since this cannot be obtained by back calculation from KGu. The percentage absorption efficiency E is defined as follows:
E =
mass oxygen absorbed/unit time mass oxygen supplied/unit time
x
100
Experimental Results
Effect of Liquid Rate. T h e effect of the liquid rate on KGu is shown in Figure 2 for the runs with vibration and with the sparger. Although the points are slightly scattered, KGa in both cases was essentially independent of the liquid rate within the range studied, L = 250 to 650 lb./(hr.)(sq. ft.). In the vibration experiments, variation in liquid rate had no apparent effect on the surface area of the bubbles, and consequently any change in KGa with liquid rate would indicate a significant change in the liquid film resistance. Here, the turbulence and the film thickness were dependent mainly on the vibrational intensity and were unrelated to the liquid rate? since the superficial liquid velocity was extremely low. The absorption efficiency, E, was independent of the liquid rate. This was expected, since there was no variation of K,a with the liquid rate. Effect of Gas Rate. The effect of the air rate, G, on K,a in the vibration and sparger experiments is compared with the data from the stirred reactors presented by Cooper et al. (70) in Figure 3. The sparger data give a slope of 0.67, which is the same as Cooper's, while that from the vibrator shows a slope of 0.33. These results all indicate the presence of a substantial gas-film diffusional resistance in this system. In the case of the vibration experiments, the observed variation in K,a \vith air rate is attributed to the increased turbulence in the gas-phase surrounding the splashing liquid as no effect on the bubble area or internal turbulence could be expected. No explanation is advanced for the low observed slope. Figure 3 also sho\vs that the vibrating liquid column was more effective than the sparger-column, except at the highest air rates. and i t \vas much more effective than the stirred reactor under all conditions. i\'ith the sparger-column it was observed that when the air rate was about 15 lb./(hr.)(sq. ft.), the froth height above the liquid exceeded the liquid height. This would be a prohibiting condition in commercial practice; thus, the vibrator has superior performance for all practical cases. As the air rate was increased, the absorption efficiency decreased markedly (Figure 4). In the vibration experiments, it decreased from 87.5 to 33.5% as the air rate increased from 1.9 to 11.4 lb.:'(hr.)(sq. f t . ) . This corresponds to a slope of -0.55. In the case of the sparger experiments, the efficiency decreased from 43.5 to 29.0%, corresponding to a slope of -0.30, for the same range of the air and liquid rates. Thus, the absorption efficiency for the vibration experiments was always higher than that for the sparger experiments. Effect of Liquid Height. Figure 5 shows that K,a decreased sharply with liquid height, h, but at different rates for the vibration and sparger columns. In the former, K,a was inversely proportional to hI.0 and, in the latter, to h0.7. These effects are in qualitative agreement with the data of Schulman and Slolstad (25) on the absorption-desorption of C O Z in a gas-bubble column. In the case of the vibrating liquid column. the observed de-
/ 0.02
2
3
5
4
6
7 8 9 1 0
15
G- Lh/(hr.)(sq.ft.)
Effect of air rate
Figure 3. 7.
Vibrator,
w2A
= 11,000 cm./sec.? L = 31 8 Ib./(hr.)(sq. ft.)
4
v
L = 254-637 lb./(hf.)(Sq. ft.
2.
Sparger
+
L = 31 8 lb./(hr.Nsq. ft)
3. Stirred vessel, data of Cooper ef al. (10)
90 80 70
60
3 W.
50
L
-
,
40
-
30
-
20
2
3
4
I
6
7
8 9 1 0
IS
G, Lb.flhs)(sq.ft.)
Figure 4. rate
Variation o f absorption efficiency with air Symbols defined in Figure 3
crease of K,a with the column height could be due to a number of factors : The cycle time increased with increasing column height and fewer bubbles migrated to the bottom of the column per unit liquid volume per unit time. The average bubble density decreased with column height and thus produced a variation in the first effect. I n addition to these observed effects, others would be expected because of the change of vibrational pressure with height and because of end effects. The absorption efficiency was essentially constant in these vibration experiments, independent of the liquid height. For the sparger column it was directly proportional to the -0.55 VOL. 2
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175
power of the liquid height (Figure 6). The absorption efficiency of the vibration experiments was always higher, in the range studied, than that of the sparger-column. Effect of Liquid Temperature. T h e effect of the liquid temperature on KGais shown in Figure 7 in which the logarithm of the log-mean value of the inlet and outlet absolute temperatures is plotted against log &a; K,a was directly proportional to the 3.5 power of the absolute temperature. This is attributed to the decrease in liquid viscosity, the increase in the reaction and diffusion rates, and the increase in the interfacial area with temperature. It is quite possible that the chemical reaction rate was the controlling factor as in the absorption of COz from air by caustic solution, where KGa varies as the sixth power of the absolute temperature (29).
Figure 8 shows that she effect of temperature on the absorption efficiency was high, as in the case with K,Q. The efficiency increased in one case from 62 to 67% as the liquid temperature was raised from 78.5' to 100.0" F. Thus, in the range studied, the efficiency was proportional to the 2.3 power of the absolute temperature. Effect of Vibration Intensity. The plots of Koa against vibrational acceleration, w2A (where w is frequency, radians/ sec., and A is amplitude, cm.), shown in Figure 9 are
0.2c
-E -2
0.15
L
a
-.
Y
2.
I
a
010
I
4 I
5
Y
0.08
0.30
I
Figure 7. 0.06 0.6
0.5
0.7
08
0.9
Liquid
Figure 5.
1.0
1.25
1.50
Hilght, t t .
50
-
L = 318 Ib./(hr.)(sq.
ft.)
Effect of liquid height
+
-
2.76
Effect of liquid temperature
G = 5.7 Ib./(hr.)(sq. ft.) w2A = 13,400cm./sm2
Vibrator, w z A = 11,000 cm./sec.Z 0 G = 3.8 Ib./(hr.)(sq. ft.) A G = 5.7 Ib./(hr.)(sq. ft.) Sparger G = 5.7Ib./(hr.)(sq. ft.) I = 318 Ib./(hr.)(sq. ft.)
60
1
2.75 2 .?4 109 T , * R .
1.73
2.72
m
w
.
P
66
-
E?
-
66
-
65-
Y
64
-
63
-
52
L
05
176
0.6
07 Liquid
0.8
0.9
1.0
1.2
1.4
Height, I t .
n 530
550
540
560
I I
T.'R.
Figure 6. Variation of absorption efficiency with liquid height
Figure 8. Variation of absorption efficiency with liquid temperature
Symbols defined in Figure 5
Conditions given in Figure 7
I&EC P R O C E S S DESIGN A N D DEVELOPMENT
9080
-
3
w ’O60
-
50
-
08
1.0
1.2
54
W’A, cm./rec!
1.8
1.6 I
2.0
IO‘
Figure 10. Variation of absorption with vibration acceleration Symbols defined in Figure 9 0.8
1.0 Vibratiomt
Figure
9.
Effect
1.2
1.4
1.6
1.8
Acceleration, W’A cm.lse&
2.0 I
IO‘
of vibrational acceleration
I = 3 1 8 Ib./(hr.)(sq. ft.) G = 3.8 lb./(hr.)(sq. ft). A G = 5.7 Ib./(hr.)(sq. ft.)
0
spring systems probably offer the most convenient means of obtaining the large vibrational accelerations a t low cost without the need for large bearings, eccentrics, and large frictional forces. References
straight lines with a n equal slope of 1.1. Some variation is to be expected, since u2A is responsible for the cyclic downward bubble migration and determines the amount of energy put into the system. The effect of w2A on the absorption efficiency is apparent (Figure 10). At the same gas and liquid rates. the efficiency increased from 63 to 87y0 as u2A increased from 10.000 to 19,500 cm./sec.* The efficiency was proportional to the 0.60 power of the vibrational acceleration, as shown in Figure 10. General Considerations. Although K,a was independent of the liquid rate and was directly proportional to the 0.33 power of the gas rate, the 1.1 power of the vibrational acceleration, and the 3.5 power of the absolute temperature, it is not possible to draw any conclusions regarding the magnitude of the two resistances in the liquid phase. The variation with the air rate is believed to be mainly due to the change in the gasfilm resistance. The cause of the other variations in &a found in the experiments could not be elucidated. The higher values of K,a and the absorption efficiency, as compared with those from the sparger-column and the stirred reactors for the same operating conditions, suggest that this device is one of the most efficient gas-liquid contactors known and that it should find commercial use. As a commercial technique for difficult gas absorptions, the vibrating column offers the following advantages : It is a self-contained unit which requires no sparger or gas distributor or other internal mixing devices. Thus, it is not subject to clogged orifices or leaking glands. I t offers a high absorption efficiency as a single-stage contactor. If a multistage countercurrent process is required, a battery of vibrating columns could be employed. This unit is also suitable for catalytic processes requiring intimate contact of gas-liquid-solid mixtures, since it can maintain an effective suspension of the fine solids and simultaneously undergo cyclic bubble migration. I n this investigation the effective power consumption could not be determined. It is believed that the power requirements in well-designed units will not be excessive. Resonant
(1) Arvis, M. A., Hodge, M. H., Rath, G. X., Ind. Eng. Chem. 49, 1237 (1957). (2) Bartholomew. rV. H.. Karow.’ E. 0.. Sfat. M. R.. TVilhelm. R. H., Zbid., 42; 1901 (1950). (3) Basset, H., Parker, TV. J., J . Chem. SOC.(London). 1951, pp. \
,
I
,
1540-60. (4) Bjorkman, A , Ind. Eng. Chem. 44, 2459 (1952). (5) Buchanan, R. H., Ph.D. Thesis, University of New South TYales. Australia. 1954. (6) Buchanan, R. H.. Jameson, G., Oedjoe, D., IND.EKG.CHEM. FUNDAUEXTALS 1, 82 (1962). (7) Carman, J., Tsuchiya. H. M.,Benedic. R. G., Kelly, S. E. Feger, V. H., Dworshack, R. G., Jackson, R. W., Appl. .Lfzcrobzol. 5. 5. 314 (1957). (8) ‘Chain, E . B:. Gualandi, G., Rend. Id. Super. Sanita 17, 84 (1954). (9)’ *‘Chemical Engineers Handbook,” J. H. Perry, ed., p. 706. McGraw-Hill, New York. 1950. (IO) Cooper, C. M., Fernstrom, G. A , , Miller, S. A., Ind. Ene., ‘ Chem. 36, 504 (1944). (11) Friedman, 4. M., Lightford, E. N., Znd. Eng. Chcm. 49, 1227 (19 57). (12) Hixon, TV. A,, Gaden, E. L., Jr., Zbid., 42, 1793 (1950). (13) Hoffman, A . N., Montgomery, J. B., Moore, J. K., Ibid., 40, 1708 (1948). (14) Zbid., 41, 1683 (1949). (15) Johnstone, H. F., Singh, A. D., Ibid., 29, 286 (1937). (16) Laidler, K. J., “Chemical Kinetics,” p. 339, McGraw-Hill, New York, 1950. (17) Maxon, LV. D., Johnson, M. J.. J . Bacferiol. 57, 235 (1949). (18) Miyamoto, S., Bull. Chem. SOC.J a p a n 2, 74 (1927). (19) Olson, B. H., Johnson, M. J., J . Bacferiol. 57, 235 (1949). (20) Peffy, R. B., Chem. Ind. (London) 66, 184 (1950). (21) Phillip, D. H., Johnson, M. J.: Znd. Eng. Chem. 51, 83 (1959). (22) Riggle, J. TY., Tepe, J. B., Ibid., 42, 1036 (1950). (23) Rushton, J. H., Oldshue, 3. Y.,Chem. Eng. Progr. 49, 161 (1953). (24) Schultz, J. S., Gaden, E. L., Ind. Eng. Chem. 48, 2209 (1956). (25) Shulman, H. L., Molstad, M. C., Ibid., 42, 1058 (1950). (26) Snyder, J. R.: Hagerty, P. F., Molstad, ?VI. C.. Ind. E n f . Chem. 48. 689 (1957). (27) Solomon, G. L.,’Perkins, M. P., J . .4ppl. Chem. 8, 4 (1958). (28) Sugar 45, 3, 60 (1950). (29) Tepe, J. B., Dodge, B. F., Trans. Am. Inst. Chem. Engrs. 39, 255 (1943). (30) Volfkovich, S.L., Bolopolski, A. P., J . Appl. Chem. (I..S.S.R.) 5 , 509-51 (1932). (31) LVesley, LV. E., Chern. Eng. Progr. 52, 286 (1956). (32) Yoshida. F.. Tueda., h.. , Imakaira. S.. Miura. Y.. Ind. Ene. ‘ Chem. 52, 435 (1960). RECEIVED for review October 16, 1961 ACCEPTED December 13, 1962 VOL. 2
NO. 3
JULY 1 9 6 3
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