472
Ind. Eng.
Chem. Process Des. Dev. 1986, 25, 472-476
Gas Holdup and Mass-Transfer Characteristics in a Three-phase Bubble Column Elzo Sada,' Hldehlro Kumazawa, Choulho Lee,+ and Takahlsa Iguchl Department of Chemical Engineering, Kyoto University, Kyoto, 606, Japan
Gas holdup and mass-transfer characteristics were studied in the semibatch two- and three-phase bubble column. The effect of the gas flow rate on the gas holdup (eG) was examined experimentally in terms of the concept of slip velocity. The slip velocity was found to be proportional to (1 - E$ and the value of the exponent n was determined to be -4 and -3 for two- and three-phase bubble columns, respectively. According to these experimental findings, the gas holdup in a three-phase bukble column could be correlated within a maximum deviation of f20%. The volumetric mass-trangfer coefficient (k,a ) was found to be closely related to the gas holdup, and hence, the experimental results on k,a in the two- and three-phase bubble columns were correlated with gas holdup to the power of 0.9. The influence of suspended particles on the gas holdup and the volumetric mass-transfer coefficient in electrolyte solutions was also investigated, and the effect was found to be weaker than those in nonelectrolyte solutions.
Although three-phase bubble columns or slurry bubble columns have been widely employed for such industrial practices as chemical absorption into slurry, FischerTropsch synthesis, three-phase catalytic reaction, and so on, the influences of suspended particles on the hydrodynamics and mass-transfer characteristics have not been well understood yet. Kat0 et al. (1973) qualitatively investigated the influences of suspended particles (glass bead, 74-295 pm; magnetite, 38-175 hm) upon gas holdup and the volumetric mass-transfer coefficient and concluded that the increases in solid concentration, particle size, and solid-liquid density difference might result in a decrease in gas holdup (eG), but the influence of suspended particles was reduced when the superficial gas velocity (uG)was higher than 10 cm/s. These trends toward the influence of suspended particles on gas holdup were also confirmed by Kara et al. (1982) and Ying et al. (1980). According to these investigations, CG increases with decreasing the solid concentration and particle size and eventually becomes either equal to (Kato et al., 1973; Ying et al., 1980) or larger than that in a two-phase bubble column when the particle size is smaller than about 10 pm (Kara et al., 1982; Sada et al., 1985a, 1985b). Almost all these works and others available in the literatures concerning eG in the semibatch three-phase bubble columns are presently qualitative ones. With reference to the volumetric mass-transfer coefficient (kEa) in a three-phase bubble column, contradictory conclusions have been obtained; that is, a decrease in kEa with an increase in the solid loading has been reported (Ka!a et al., 1982; Deckwer et al., 1974),while an increase in k,a (Sada et al., l983,1985a, 1985b) and a negligible influence (Zaidi et al., 1979) have been also reported. Such discrepancies may result from the differences in experimental conditions employed, such as the physicochemical properties of liquid, particle size and loading, and density and wettability of solids. In the present work, thus, the influences of superficial gas velocity and physical properties of liquid on EG and kEa in a two-phase bubble column were first compared with previous information by using aqueous sucrose so!utions, and the effects of suspended particles on eG and k L a were *Author to whom correspondence should be addressed. Present address: Korea Research Institute of Chemical Technology, Daejeon, Korea. 0196-4305/86/1125-0472$01.50/0
Table I. Experimental Conditions temp, "C pressure, MPa sucrose concn, wt % Na2S0, concn, mol/dm3 NaCl concn, mol/dm3 KC1 concn, mol/dm3 UG, cm/s solid concn, wt % CalOH), particle d,, r m g/cm3 glass bead d,, r m P9 g/cm3 glass bead d,, r m P , g/cm3 nylon 6 particle d,, r m P , g/cm3 P9
10-25 0.1 0-60 0-1 0-2.4 2.4 2-20 0-20 7 0.24
40 2.48 96 2.48
2000 1.14
studied quantitatively by using different kinds of wettable solid particles. Moreover, these effects were also investigated in electrolyte solutions. Experimental Apparatus and Procedure The experimental setup was shown in Figure 1. All the measurements were made a t 10-25 "C and atmospheric pressure. The column was made of transparant MMA, and its diameter and height were 7.8 and 150 cm, respectively. The gas distributor was a perforated plate with 37 holes of 1-mm diameter. This bubble column was operated batchwise with respect to the liquid or slurry phase and continuously with respect to the gas phase. The liquid or slurry height free of gas bubbles was 62-75 cm, and ionexchanged water was used as the liquid phase. Pure oxygen or nitrogen was used as the gas phase. More details on the experimental conditions and information on solid particles were shown in Table 1. A solution or slurry, in which the solid and/or electrolyte concentrations were adjusted to the desired values, was charged into the column, and nitrogen gas was introduced in the column at a proper gas flow rate to evolve the dissolved oxygen. The dissolved oxygen concentration was monitored with the help of a D.O. meter (Beck", Model 123301). After the dissolved oxygen concentration reached zero and the temperature reached the desired value, the inlet gas was switched to oxygen at a flow rate which was previously 0 1986 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 2, 1986 473 BUBBLE COLUMN WATER SATURATOR G A S CYLINDER FLOW M E T E R )-WAY VALVE G 4 S FLOW M E T E R 0 0 METER RECORDER THERMOMETER WATER JACKET LIQUIO U M P L I N G NOZZLE
P
5
v
t
(HT- HSL)/HT
(1)
= (1- ~ ( H s-LHL)/HsL
(2)
= 1-
tG
- cp
(4)
Results and Discussion Gas Holdup. As the hydrodynamics, mass- and heattransfer, and dispersion characteristics in the bubble column are strongly dependent on the prevailing flow regimes, so many studies have been directed toward the flow regimes (Wallis, 1962; Govier, 1965; Freedman and Davidson, 1969; Lockett and Kirkpatrick, 1975; Miller, 1980). The concept of slip velocity, which is defined as U G / ~ Gf U L / E L
’
1
006 -iog(l
I
’
I
0.09
012
I
I
015
-€GI
Figure 2. Effect of particle size on slip velocity. 2
2
’
6
1
(
1
1
004
0
1
,
1
,
008
012
- l o g u - EG)
Figure 3. Effect of solid concentration on slip velocity for nylon particle with d, = 2000 wm.
(5)
where kEu refers to the volumetric mass-transfer coefficient based on the aerated volume and C refers to the concentration of oxygen in the liquid. The effect of the time constant of the D.O. meter on the determined kLa value can be neglected within experimental error (Sada et al., 1985a). The liquid-phase physical properties were taken from the International Critical Tables, and the variation of the apparent properties with solid concentration was not considered. All the runs for the three-phase bubble column were carried out above the critical suspension velocity which was determined from the literature of Roy et al. (1964) or visual observation. Aqueous slurries of calcium hydroxide had been degassed by a vacuum pump to remove the gas which may exist around particles.
us =
1
For the purpose of empirical correlation, the slip velocity can be described as (Wallis, 1962)
and eq 4 can be rewritten to yield
kEu = -eL{d In (C* - C)/dt)
I
003
(3)
where HsLand HL refer to the slurry and liquid height free of gas bubbles, respectively. During physical absorption of the pure oxygen in a bubble column, the material balance for oxygen in the liquid medium is written as dC/dt = (k;a/tL)(C* - C)
’
1.41 0
adjusted to the desired value by a needle valve. Then, the increase in the dissolved oxygen concentration with time was monitored by the D.O. meter and recorded. When the dissolved oxygen concentration went up to the maximum value and showed no increase with time, the recording was stopped and the aerated bed height ( H T ) was read. The values of gas, solid, and liquid holdups were determined by eq 1-3, respectively,
EL
-
- 16-
Figure 1. Schematic diagram of experimental setup.
tp
(
m
CONSTANT TEMPERATURE BATH
EG
V 0
18-
(6)
where the plus sign is taken in the case of the countercurrent and the minus sign is taken in the case of the cocurrent is widely used to characterize the flow regime in the bubble column. Although the slip velocity was primarily defined for a two-phase bubble column, Darton and Harrison (1975) showed that this definition could also be applied to a three-phase bubble column.
us
=
UbO(1
- EG)n
(7)
where UbO refers to the rising velocity of a single bubble in an infinite stagnant medium. The exponent n in eq 7 is an indication of how the rising velocity of a respective bubble is affected by neighboring bubbles as well as the surrounding medium. For the semibatch operation, the slip velocity is given as us =
UG/EG
=
UbO(1
- tG)n
(8)
Figure 2 shows relationships between us and (1- t G ) for different particle sizes. The chain line represents the corresponding results for the two-phase bubble column where ion-exchange water was used as the liquid phase. The slope of the chain line was found to be 4, and the value of the exponent n is represented by -4 for the churnturbulent flow regime. The chain line is expressed by tG/(1
- 6G)4 = 0 . 0 4 6 ~ ~
(9)
where uG is in centimeters/second. A similar conclusion has also been derived by Akita and Yoshida (1973) and Mersmann (1978). Figures 3 and 4 show relationships between us and (1 - tG) for different loadings of the nylon particle and fine calcium hydroxide particle, respectively. The slip velocity is also shown to increase with increasing solid loading and is smaller than that in the two-phase bubble column only when low concentrations of fine calcium hydroxide particles are suspended. For such low concentrations of fine particles, an increase in the gas holdups which was ascribed to the hindering effect from bubble coalescence by fine particles in the liquid film layer around the bubbles was observed like in our other work (Sada et al., 1985b).
474
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 2, 1986 3;
05
2003
vB 'i
w
without solid
-
502
Figure 6. Correlation of gas holdup in the three-phase bubble column. - l o g ( 1 - a) Figure 4. Effect of solid concentration on slip velocity for calcium hydroxide particle with d, = 7 pm.
V
2000 nylon
&5.ib7
oi
02' Et
'
03
Figure 7. Dependence on kEa on gas holdup in the three-phase bubble column.
I
I
2 cs-( ut-o'6/8) 15
Figure 5. Relation of [cc/(l - f G ) 3 ] / u C vs.
25
I 3
cB4125L't4'6.
From Figures 2-4, tG in the three-phase bubble column can be correlated as
[%/(I
- tG)31/UG =
cl
(10)
except for low concentrations ( 1 5 w t YO) of calcium hydroxide. C1 for various kinds of suspended particles was plotted against C,, with an assumption that C1 is a function of C, and the terminal velocity of suspended solid base on Stokes law
ut = d p p - ~ ~ ) d p ~ / ( l & d , and the following correlation was derived. C1 = C C -(0.125UL4'8) 2
(11)
(12)
s
The experimental results of C1 for different particles were plotted on the basis of eq 12 in Figure 5. From this figure, C, could be determined as c2
= 0.O19Ut'J'6
(13)
Accordingly, from eq 10-13, the gas holdup in the three-phase bubble column was empirically correlated as tG/(l - + ) 3 = o.019ut1/16C-(O.125Ut4'')
UG
(14)
where C, is in volume fraction and both Ut and U G are centimeters/second. The present experimental results were compared with eq 14 in Figure 6. The present results are well-represented by eq 14 with a standard deviation of 12% and maximum deviation of f20%. From the ratio of eq 9 to eq 14, one can get a relation of t~ for a two-phase bubble column to that for a three-phase bubble column
bubble columns, respectively. Equation 15 suggests that when tG for a two-phase bubble column is known a t any operating conditions, t G for a three-phase bubble column at the same operating conditions can conveniently be determined with the knowledge of the solid volume fraction and the evaluated terminal velocity of the solid particle by Stokes law. Volumetric Liquid-Side Mass-Transfer Coefficient. Numerous studies on mass transfer in the bubble c$umn have revealed that the mass-transfer coefficient ( k L )depends mainly on the mean bubble size, physical properties of the liquid medium, and the diffusivity of the absorbing gas component in the liquid medium (Calderbank and Moo-Young, 1961; Hughmark, 1967; Akita and Yoshida, 1975; Schugerl et al., 1977). Concerning the Sauter mean bubble diameter encountered in bubble columns, it is rather invariable (0.3 < d,, < 0.7 cm) even when the superficial gas velocity varies by as large as 1 order of magnitude (Calderbank, 1959; Hughmark, 1967; Akita and Yoshida, 1974; Gestrich and Krauss, 1967). So some correlation of kEa with the gas holdup instead of the superficial gas velocity is expected to be worthwhile. Figure 7 shows kLa in the thzee-phase bubble column. For the sake of comparison, k,u for a two-phase bubble column was also shown as a dashed line which was obtained by the experimental result for water at 20 "C as kEa = 0.24tc0.9
(16)
Although the values of k i u at the same tc vary with particle size and solid loading, the dependence of kLu on t G is very similar to that in a two-phast bubble column. From the data depicted in Figure 7, kLa in the three-phase bubble column can also be expressed as
where C4 refers to an arbitrary constant which can be determined from the experimental data. Here, the values of C4 were determined with the aid of the least-square [ e G / ( l - tG)3]III = 0 . 4 1 ~ ~ 1 ~ 1 6 C ~ ~ [%/(I 0 ~ ' 2 5-u%)4111 ~~b'8~ method for respective particle sizes and solid loadings and (15) wFre listed in Table 11. Figure 8 shows a parity plot of kLa observed and predicted from eq 17 and Table 11. The where subscripts I1 and I11 refer to two- and three-phase
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 2, 1986
475
Wt'l.
-11
20
I
I
Figure 8. Parity plot of kEu in the three-phase bubble column. 05,
,
,
I
-
I
\
,
[
,
I
,
I
key dp,pm c s , % ----
,
A
70
6-20
A
30
11
I
I
ref Kara
etal,
0
a s - w a t e r system
.. 4
2
3
5
7
1
0
20
UG. c m i s
Figure 9. Effect of electrolyte on gas holdup.
vO
01
02
03
cc ?red
Table 11. Values of C4in Equation 17 for Different Particle Sizes and Solid Concentrations nylon 6, glass, glass, Ca(OH),, w , wt '70 2000 Mm 96 fim 40 fim 7 0 0.24 0.24 0.24 0.24 1 0.26 0.26 5 0.24 0.21 0.22 0.24 10 0.28 20 0.21 0.33
Figure 11. Parity plot of gas holdup in the three-phase bubble column when the electrolyte or nonelectrolyte solutions were used as the liquid medium.
predicted values of kea in the three-phase bubble column coincide well with the observed ones, and hence, kEa in the bubble columns is considered to be related to t G as
kia
tG0"
(18)
irrespective of the presence of :olid particles. From Figure 7 and Table 11, moreover, kLa in a three-phase bubble column at high solid loading is larger than that in a twophase bubble column at the same t G value; however, if the solid concentration is lower than 5 w t % , kEa is approximately equal to that in the two-phase bubble column at the same t G value. Effect of Suspended Solid on eG and kEa in Electrolyte Solution. In the previous sections, the effect of suspended solid particles on EG and kEa,when the bubble coalescence-promoting media of water are used as the liquid phase, was discussed. In this section, the same effect, when the bubble coalescence-hindering media of the electrolyte solution are used as the liquid phase, will be discussed. Experimental results of tG for various electrolyte solutions were shown in Figure 9. The gas holdup in electrolyte solutions is always higher than that in water and seems to be independent of the concentration and type of electrolyte, presumably resulting from the fact that the electrolyte concentrations higher than the critical concentration of bubble coalescence hindering were used in the present work. The existing literature concerning the effect of the electrolyte on tG in an electrolyte solution has
reported some increase by about 10-25% compared to those in nonelectrolyte solutions irrespective of the gas flow rate (Akita and Yoshida, 1973; Kit0 et al., 1976; Hikita et al., 1980). But from the present work depicted in Figure 9, the ratio of t G in an electrolyte solution that in water varies from 1.0 to 1.3 with the superficial gas velocity. Further, the effect of electrolyte on t G seems to be dependent on the turbulence structure of the bubble column. The experimental results of t~ in the three-phase bubble column, where a 0.8 mol/dm3 sodium sulfate solution was used as the liquid medium, were shown in Figure 10. Except for the fact that addition of 1 wt % calcium hydroxide particles gives higher t G and addition of 20 wt % nylon 6 particles gives smaller tG,being similar to the case of water without electrolyte, the effect of suspended solids is not so clear. This may be due to an increase in the rigidity of bubbles in an electrolyte solution and resultant difficulty in disintegration and coalescence of bubbles caused by solid particles. Although eq 15 was derived for bubble columns with water as the liquid phase, with an assumption that this relation can be also employed for bubble columns containing electrolyte solutions, the experimental data on tG in the present and the previous works (Kato et al., 1973; Kara et al., 1982) were compared with the predictions by eq 15 in Figure 11. The experimental results of our own as well as other workers' are found considerably well correlated by eq 15. Figure 12 shows the quantity kEu for the three-phase bubble column where a 0.8 mol/dm3 sodium sulfate solution was used as the liquid phase. For the sake of comparison, the experimental results for the two-phase bubble columns were also shown as broken and chain lines for a nonelectrolyte solution and a 0.8 mol/dm3 sodium sulfate
476
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 2. 1986
5 -Ln
-2% --& - 4 b i z -;; : :
O B m o l I d ~/
- -
+20
,~,AS~W*T€R~
8
,/$p23-*,.':$
/S&BW / 4
2 -
/;b+GAS-O8
-
,',/a /'
/
'w
P
C, = volume fraction of solid in slurry, cm3 of solid/cm3 of
slurry
d, = average diameter of solid particle, pm
/"Aa
.a .x
Na,SO'
molldm? Na,SO< SOLUTION I
,
d,, = Sauter mean bubble diameter, cm g = gravitational acceleration, cm/s2 H = dispersion height, cm kEa = volumetric liquid-side mass-transfer coefficient, t = time, s u b o = rising velocity of single bubble, cm/s u = superficial velocity, cm/s u, = slip velocity, cm/s Ut = terminal velocity of a solid particle based on the Stokes
law, cm/s concentration of solid based on slurry, wt % Greek Symbols e = phase holdup p = viscosity, Pa.s p = density, g/cm3 u = surface tension, N/m Subscripts G = gas phase L = liquid phase p = solid particle SL = slurry T = total Superscript * = in equilibrium with the gas phase Literature Cited u; =
Akita, K.; Yoshida, F. Ind. Eng. Chem. Process Des. Dev. 1973, 12, 76. Akita, K.; Yoshida, F. I n d . Eng. Chem. Process Des. Dev. 1974, 13, 84. Calderbank, P. H. Trans. Inst. Chem. Eng. 1959, 3 7 , 173. Calderbank, P. H.; Moo-Young, M. B. Chem. Eng. Sci. 1961, 16, 39. Darton, R. C.; Harrison, D. Chem. Eng. Sci. 1975, 3 0 , 581. Deckwer, W.-D.; Burckhart. R.; 2011, G. Chem. Eng. Sci. 1974, 29, 2177. Freedman, W.; Davidson, J. F. Trans. Inst. Chem. Eng. 1969. 4 7 , T251. Gestrich, W.; Krauss, W. Int. Chem. Eng. 1976, 16, 10. Govier. G. W. Can. J. Chem. Eng. 1965, Feb, 3. Hikita, H.; Asai, S . ; Tanigawa, K.; Segawa, K.; Kitao. M. Chem. Eng. J . 1960, 20, 59. Hughmark, G. A. Ind. Eng. Chem. Process Des. Dev. 1967, 6 ,219. Kara, S.: Kelkar, B. G.; Shah, Y. T.; Carr, N. L. Ind. Eng. Chem. Process Des. Dev. 1982, 21, 584. Kato, Y.; Nishiwaki, A,; Kago, T.; Fukuda, T.; Tanaka. S . I n t . Chem. Eng. 1973, 13,562. Kito, M.; Shimada, M.; Sakai, T.; Sugiyama, S.; Wen, C. Y. "Fluidization Technology"; Hemisphere Publishing Co.: Keairns, 1976; Vol. 11, p 41 1. Lockett, M. J.; Kirkpatrick, R. D. Trans. Int. Chem. Eng. 1975, 53, 267. Mersmann, A. Ger. Chem. Eng. 1978, 1 , 1. Miller, D. N. I n d . Eng. Chem. Process Des. Dev. 1980, 19, 371. Roy, N. K.; Guha. D. K.; Rao, M. N. Chem. Eng. Sci. 1964, 19,215. Sada, E.; Kumazawa, H.; Lee, C. H. Chem. Eng. Sci. 1983, 3 8 , 2047. Sada, E.; Kumazawa, H.; Lee, C.; Fujiwara, N. I n d . Eng. Chsm. Process Des. Dev. 1985a. 24, 255. Sada, E.; Kumazawa, H.; Lee, C. AIChE J., submitted. Schugerl, K.; Lucke, J.; Oels, U. "Advances in Blochemical Engineering"; Ghose, T. K., Fiechter, A., Blakebrough, N., Ed.; Springer-Verlag: Berlin, 1977; Vol. 7, p 60. Wallis, G. B. Inst. Chem. Eng. 1962, 9. Ying, D. H.; Givens, E. N.; Weiner, R. F. Ind. Eng. Chem. Process Des. Dev. 1980, 19, 635. Zaidi, A.; Louisi, Y.; Ralek, N.; Deckwer, W.-D. Ger. Chem. Eng. 1979, 2 , 94.
Received for reuiew March 1, 1985 Accepted August 26, 1985
