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Ind. Eng. Chem. Process Des. Dev. 1984, 23, 132-137
Liquid-Liquid Mass Transfer in Three-phase Fluidired Beds. 3. Measurement of Individual Fluid-Phase Resistances P. Dakshinamurty," V. Subrahmanyam, R. V. Prasada Rao, and P. Vljayasaradhi Department of Chemical Engineering, Andhra University, Waltair, India
Individual fluid-phase resistances were measured in three-phase (liquid-liquid-solid) fluidized beds, made up of particles of three different sizes, for the systems isobutyl alcohol-water and methyl isobutyl ketone-water in a tube of 5.6 cm diameter, and for the system n-butyl acetate-water in tubes of 2.54 and 5.6 cm diameter using the ingenious technique of Colburn and Welsh. The (HTU), values are found to be independent of the variations of either continuous or dispersed phase flow rates, whereas (HTU), values are dependent on both of the flow rates. The presence and size of the particles have a definite effect on the (HTU)c values. The (HTU)c values were correlated satisfactorily as (HTU)~= 6.04
x
1 0 7 ,/L ~
c)m(~R,p)-~.2~(~Sc)-1.25
where m has a different value for each particle.
Liquid-liquid extraction, one of the important mass transfer operations, has been studied exhaustively in various types of contactors such as spray, packed, perforated plate, pulsed, and mechanically agitated columns etc; only very recently it was attempted in a co-current three-phase fluidized bed contactor by Dakshinamurty et al. (1975) and by Roszak and Gawronski (1979). Three-phase fluidization (gas-liquid-solid) is acquiring greater importance in recent times as a unit operation in chemical engineering. It is fluidization of solid particles by two fluids, one of which serves as the continuous phase and the other as the dispersed one. In three-phase fluidization both the fluids can be liquids or one of them a gas. The mass transfer rates of absorption in gas-liquid fluidization are found to be promising over the conventional type of towers as reported by Dakshinamurty et al. (1973, 1974). These results indicate that the three-phase liquid-liquid fluidized beds comprised of two immiscible or partially miscible liquids are of potential interest with respect to liquid-liquid extraction. Recently, Dakshinamurty et al. (1975) reported mass transfer rates of n-butyric and propionic acids from dispersed kerosene to continuous water phase in three-phase liquid-liquid fluidized beds, comprised of particles of four different sizes and observed that a three-phase fluidized bed contactor is superior in its performance to that of a spray extraction column, on the basis of magnitude of the empirical film HTU values obtained from the resolution of the (HTU)oc and (HTUIoDas suggested by Colburn (1939). This is followed by the work of cocurrent liquidliquid extraction in fluidized beds by Roszak and Gawrouski (1979), who determined mass transfer coefficients and drop size determination for the systems toluene-acetic acid-water and toluene-benzoic acid-water and observed that the mass transfer rates are increased due to the increase of interfacial area as a result of drop disintegration of the dispersed phase. However, in order to substantiate the above conclusions, a better method is to measure directly the resistances offered to mass transfer by the individual fluid phases when a partially miscible binary systems is contacted in it (three-phase fluidized bed). Hence the aim of the present investigation is to measure the individual fluid phase HTU values in cocurrent three-phase fluidized beds, comprised of three different sizes of particles, contained in borosilicate tubes of two
different diameters under different flow rates of the continuous and dispersed phases, using three partially miscible binary systems (1) isobutyl alcohol-water, (2) methyl isobutyl ketonewater, and (3) n-butyl acetate-water, and thereby to evaluate the performance of a three-phase fluidized bed contactor for liquid-liquid mass transfer and also to attempt a correlation in terms of the system variables for the prediction of the individual fluid phase (HTU)values.
Experimental Section Materials. Porcelain beads, glass beads, and rock wool shot were used as bed material, and their specifications are the same as those reported earlier by Dakshinamurty et al. (1973,1974,1979). Isobutyl alcohol, methyl isobutyl ketone and n-butyl acetate of laboratory reagent grade chemicals supplied by B.D.H., were used without any further treatment. Water supplied to the Chemical Engineering Department was used as it was. Equipment. A line diagram of the equipment used in the present work is given in Figure 1. Actually two columns of 5.6 and 2.54 cm diameter and 61 cm length were used. The description and mode of operation of the column with 5.6 cm was the same as reported earlier by Dakshinamurty et al. (1975). As far as the 2.54-cm column is concerned, the operation of it is the same as that of 5.6-cm column but for minor differences in the construction of the former, namely that there was no calming section and the distributor for the dispersed phase consisted of four nozzles arranged below the sieve plate, fixed in the bottom of the fluidization tube. The system n-butyl acetate-water was studied in the 5.6 and 2.54 cm diameter columns, while the other two systems were studied in the 5.6 cm diameter column. Method of Analysis. The system n-butyl acetatewater was estimated by the method of hydrolysis followed by Colburn (1942), i.e., by hydrolyzing it with a known amount of standard sodium hydroxide solution at 65 "C for 30 min and then titrating the rest of sodium hydroxide solution with standard hydrochloric acid solution. For isobutyl alcohol-water and methyl isobutyl ketone-water systems, refractive index was used as the method of analysis. Various samples of the relevant mixtures in each system were prepared and then the refractive index was measured using an immersion refractometer (Bellingham-
0 196-4305/84/1123-0132$01.50/06 1983 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 23, No. 1, 1984
133
Table I Range of Variables
L, g/(cm’ s): 4.12-11.0 LD g/(cm’ s): 0.131-0.626 N s c : 1576-1679 N R ~ :150-1900 systems studied: water-solvent-particle solvent: isobutyl alcohol, methyl isobutyl ketone, o r n-butyl acetate particles: porcelain beads, glass beads, o r rock wool shot Specifications of t h e Particles
Figure 1. Schematic diagram of experimental setup.
Stanely) with an accuracy of five digits. Then refractive index vs. composition graphs were constructed to analyze the various samples of the mixtures obtained in the work. Discussion and Correlation of Results The present investigation is directed toward the study of the effect of flow rates of both the continuous and dispersed phases, besides the effect of the size of the particles on the individual fluid-phase HTU values. The range of variables covered, systems studied, and properties of the bed materials used in the present work and the physical properties of the fluids are given in Table I. Operating behavior of the Tower. Visual observations of the operating behavior of the tower were made during the course of taking mass transfer data. When butyl acetate or isobutyl alcohol, or methyl isobutyl ketone, which is partially miscible with water, is admitted into a bed fluidized by water, it is observed that the drops of the solvent coming out of the bed move as discrete drops without any coalescence until they reach the separator. This operating behavior is in contrast to that of a cocurrent spray column wherein the jets of the solvent coming out of the distributor are found to coalesce, forming bigger drops as they move toward the separator. It is believed that the drops of the dispersed phase coming out of the distributor are disintegrated into smaller ones due to the play of the net inertia forces of the particles. Such a phenomenon is termed a drop disintegrating type of fluidization (similar to bubble disintegrating type of fluidization in gas-liquid fluidization) and appears to occur if the net inertia force of the particle is much higher in magnitude to the interfacial tension of the two liquids; it is observed in the beds made of porcelain beads and glass beads. However, in the bed comprised of rockwool shot, though the drop flow regimes (similar to the bubble flow regimes in gas-liquid contact) were maintained, unlike in other beds, there was variation in drop size and the majority of the drops were bigger than those in other beds, perhaps due to the tendency of coalescence to an extent. The coalescence here may be due to lower magnitude of the net inertia forces of these particles compared to the interfacial tension of the liquids. Similar observations have been made regarding the operating behavior of the tower in three-phase fluidized beds made up of the above particles by Dakshinamurty et al. (1975) while reporting mass transfer rates of butyric and propionic acids from dispersed kerosene to continuous water phase, by Dakshinamurty and Padmanabharaju (1977) while reporting heat transfer rates between water used as the hot and continuous phase and the kerosene as the dispersed phase, and by Dakshinamurty et al. (1979) while reporting bed porosities in three-phase (liquid-liquid) fluidized beds. Also, mention may be made of the findings of Dakshinamurty and co-
particle
size, cm
density g/cm3
porcelain beads glass beads rock wool shot
0.438 0.348 0.197
2.83 2.622 2.177
net inertia force, dynlcm’ 428 271 130
Physical Properties of t h e Solvent at 30 “ C
density, g/cm3 viscosity, CP R.I. diffusivity in water (cm2/s),
DL X
n -butyl
isobutyl alcohol
MIBK
0.802 5.834 1.38985 9.473
0.8 1.059 1.39180 7.889
acetate
water
0.88 1.00 1.047 0.8 1.3990 1.33520 7.5476
lo6
workers (1973, 1974) while reporting the rates of C 0 2 and O2absorption into water in gas-liquid fluidized beds made up of different particles that disintegration of gas bubbles into smaller sizes was observed in the case of the beds made up particles of higher size or density and coalescence of bubbles for beds of particles of lower size or density. Finally, it may be stated qualitatively that the drop disintegration or coalescence appears to occur if the ratio of the net inertia force of the particle to the interfacial tension of the liquids is much higher than unity or lower thanlmarginally higher than unity. Further, the net inertia force of the particle referred above may be defined as being equal to the sum of the forces due to gravitational, buoyancy, and liquid drag expressed per unit projected area of the particle corresponding to the average superficial liquid velocity under which different beds are operated. Values of individual fluid phase HTUs are calculated from the relation (HTU), X (NTU) = 2 and
for the continuous phase where P refers to either phase on which calculations are based, y* refers to the maximum solubility of the solvent (isobutylalcohol or methyl isobutyl ketone or n-butyl acetate) in water, y is the concentration of the solvent in the continuous phase (water) samples collected, respectively, and x, - x
(for dispersed phase)
where x, is the maximum solubility of water in the solvent and x concentration of wate r in the dispersed phase samples collected, and Z is the height of the tower, i.e., 61 cm. It may be stated that in the case of runs where the fluidized bed height is not 100% of the fluidization tube,
134
Ind. Eng. Chem. Process Des. Dev., Vol. 23, No. 1, 1984
L\
t&
/O0 oi
10
0.01
&/Le
0.
f
Figure 2. Variation of (HTU)c with LD/Lc. System: isobutyl alcohol-water-porcelain beads.
LDjLC
01
4
Figure 4. Variation of (HTU)c with LD/Lc. System: isobutyl alcohol-water-rock wool shot.
n
0
Figure 5. Variation of (HTU)c with LD/Lc. System: isobutyl alcohol-water (spray column).
/o
I 0 01
1
I
I
I
L O j L C -r
I
L
e
l
l
"1
-1
I
oi
Figure 3. Variation of (HTU)c with LD/Lc. System: isobutyl alcohol-water-glass beads.
i.e., 61 cm, composition of the continuous and dispersed streams a t the top of the fluidized bed were estimated by using the spray column data; then only the respective HTUs in the three-phase region were calculated. Further, the method of computation is based on the assumption that axial mixing can be neglected. Throughout the work, water is in continuous phase while the solvents are dispersed. The effect of dispersed and continuous phase flow rates and the particle size on the value of (HTU)c and (HTU)D are considered below. Effect of Dispersed and Continuous Phase Velocities. To study the variation of (HTU)c and (HTU)D with the variation of LD and Lc, (HTU)c vs. L D , (HTU)D vs. LD, and (HTU)D vs. Lc were plotted. In the case of (HTU)c vs. LD three different lines were obtained for each system with Lc values as parameters. However, when (HTU)c vs. LDILC were plotted for the different systems, as shown in Figures 2-13, only a single line was obtained for each system. It is obvious from Figures 2-13 that (HTU)c decreases as LD increases or the ratio of LD/Lc increases
IO 001
I
a02
I
I
I
004
LD/L
I
I I l l
e a om
0.1
,
C
Figure 6. Variation of (HTU)c with LD/Lc. System: methyl isobutyl ketone-water-porcelain beads.
linearly, indicating that (HTU)c is a strong function of LD and Lc or LDILC for all the systems as observed by Colburn and Welsh (1942) for isobutyl alcohol-water, and Laddha
Ind. Eng. Chem. Process Des. Dev., Vol. 23, No. 1, 1984
8 8 9
-1
10
column
Ln/Lc
0.1
-
135
io
Figure 10. Variation of (HTU)c with LD/Lc. System: butyl acetate-water-porcelain beads.
10
IO
001
a+
0.02
oa
0.06
0.1
0.2
LD/LC
Figure 7. Variation of (HTU)c with LD/Lc. System: methyl isobutyl ketone-water-glass beads.
b . $ad1 ro/mn 10
0 6.1 L V / L C --.L Figure 11. Variation of (HTU)c with LD/Lc. System: butyl acetate-water-glass beads.
10
I
I
0.01
I
I
Lo/fc
I
1
1
1
1
1
01
Figure 8. Variation of (HTU)c with LD/LC. System: methyl isobutyl ketone-water-rock wool shot.
-
1000:
: -
\o
o SmoM
faun,
-
T -
3 = 100
h
< 10
0.Of
0.1
L D l L C 4
I-0
Figure 9. Variation of (HTU)c with LD/Lc. System: methyl isobutyl ketone-water (spray column).
1
r
-
-
t
0
and Smith (1950) for isobutyl aldehyde-water and 3-pentanol-water. However, there is one difference between the
Figure 13. Variation of (HTU)c with tate-water (spray column).
System: butyl ace-
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Ind. Eng. Chem. Process Des. Dev., Vol. 23,No. 1, 1984
t IO Q
01
fD/kC
+
Qi
Figure 14. Comparison graph ( H T U c vs. LDILc. System: (1) isobutyl alcohol-water-porcelain beads; (2) isobutyl alcohol-waterglass beads; (3) isobutyl alcohol-water-rock wool shot; (4) isobutyl alcohol-water (bubble column); ( 5 ) Colburn line.
present data and those of Colburn and Welsh (1942) as far as isobutyl alcohol-water is concerned. In the former case, the slope of the line (HTU) vs. LDILC is -1.2 and that of the latter is -0.75, thereby indicating the rapid fall in the values of (HTU)c in a three-phase fluidized bed contactor compared to that of a packed column. Further, it is interesting to note from Figures 2,6, and 10, as the solubility of solvent in water increases there is a variation in the slope of the lines (HTU) vs. LDILC from -1.2 for isobutyl alcohol-water, -0.83 for methyl isobutyl ketone-water, and -0.68 for n-butyl acetate-water, indicating the effect of solubility on (HTU)c values; expressing it more explicitly, as the solubility of the solvent in water increases, the resistance to diffusion decreases. From Figure 17, wherein (HTU),, is plotted against LD/Lc, it is seen that (HTU)D is independent of both flow rates as reported by Colburn and Welsh (1942);however, Laddha and Smith (1950) have reported slight variation of (HTU)D values with LDILC. From Figure 16, wherein only the variation of (HTU)c with LD pertaining to the fluidization column of 2.54 cm is shown, it is evident that the lines tend to converge as the velocity of the continuous phase to reach saturation solubility of rz-butyl acetate in water. For the n-butyl acetate-water system, though the mass transfer rates were determined in two towers of different cross section, there is no effect of the tower cross section on the values of (HTU)c, as is evident from Figures 10, 11, and 12. From the above it is clear that lower continuous phase velocities and higher dispersed phase velocities are responsible, to realize lower values of (HTU)c. From Figures 14-16 wherein a comparison of the different systems is made, for the values of (HTU)c, it is observed that the presence and size of the particle have a marked effect on the value of (HTU)c. Its value is lowest for porcelain beads and highest for rock wool shots, thereby indicating that the higher the size of the particle, the lower will be the values of (HTU)c; also it is observed that for the spray column its value is higher than those of other systems for any given value of LDILC. The decrease in the values of (HTU)c in the case of porcelain beads is about
LdLc
0 Of
Ol
4
Figure 15. Comparison graph (HTU)c vs. LDfLC. System: (I) methyl isobutyl ketone-water-porcelain beads; (2) methyl isobutyl ketone-water-glass beads; (3) methyl isobutyl ketone-water-rock wool shot; (4) methyl isobutyl ketone-water (spray column).
10
0.01
LD/LC
0 1
---t
Figure 16. Variation of (HTU)c with LDILc. Systems: (1) butyl acetate-water (spray column); (2) butyl acetate-water-rock wool shot; (3) butyl acetate-water-glass beads; (4) butyl acetate-waterporcelain beads.
IO
I
0 01
I
I
I
I
L D / L C --t
I
O
I
I
Of
Figure 17. Variation of (HTU)Dwith LD,ILc. System: (1) isobutyl alcohol-water-porcelain beads; (2) isobutyl alcohol-water-glass beads; (3) isobutyl alcohol-water-rock wool shot; (4) isobutyl alcohol-water (spray column).
three times lower than those of bubble columns. Figure 17 indicates that particle size has an effect on the (HTU)D values, though i b magnitude is marginal for the systems with different particles; however, in the case of the bubble
Ind. Eng. Chem. Process Des. Dev., Vol. 23, No. 1, 1984
column its value is higher than those with particles. The reason for realizing lower fluid phase resistances in the case of porcelain beads is due to a drop-disintegrating type of fluidization, whereas the tendency of the drops to coalesce results in higher values of (HTU)c. Comparison of the Three-phase Fluidized Bed Contactor with that of a Packed Column. The present data for the system isobutyl alcohol-water are compared with those of a packed tower, as reported by Colburn and Welsh (1942) in Figure 14, though the present data were obtained at 30 f 1 OC, whereas those of Colburn and Welsh (1942) were obtained a t 25 OC, and the flow rates of the both may not agree. It is obvious from the Figure 14 that the line of Colburn and Welsh (1942) lies above those of the systems reported here and also the slope of the present data is much steeper than that of Colburn and Welsh (1942). These indicate clearly that the resistance offered by the continuous fluid phase to mass transfer in a three-phase fluidized bed is lower in magnitude than that of a packed tower, thereby indicating its better performance. To make the data obtained in the present work more useful, an attempt has been made to correlate the individual film values of ( H T U c in terms of the process variables such as LD, Lc,pc, D,, V,, pc, D, etc. It appears that (HT& can be accounted for its variation by the dimensionless groups (LDILC), (NRep)and (Ns,) satisfactorily as given below. (HTU)c = Kz (NR,,)"(Ns,)'(LD/Lc)" The constant K 2 and exponents m, n, and p are determined in the usual way. Substitution of the constant and exponents in the above equation results in the following form (HTU)c = 6.04 X lo5 (NRep)-0'20(NSc)-1'25 (LD/Lc)" where m = -1.2 for isobutyl alcohol-water systems, m = -0.83 for methyl isobutyl ketone-water systems, and m = -0.68 for butyl acetate-water systems. The above equation is found to represent the experimental data of all the systems barring the bubble column satisfactorily with an average percent deviation of 18%. The advantage of the above relation is that it has taken into consideration, besides the flow rates, the physical properties of the liquids and particles. Conclusions From the present study the following conclusions were reached. (1) The presence and size of the particle has a marked effect on the individual fluid phase HTU values. (2) It appears that as the size of the particle is increased, the individual fluid phase HTU values are decreased. (3)
137
The HTU values of the dispersed phase are found to be independent of the variations of either continuous phase or dispersed phase flow rates. (4) The HTU values of continuous phase are found to be dependent on flow rates of both the phases. (5) Drop coalescing type was responsible for realizing higher HTU values. (6) Disintegration of the dispersed phase drops is attributed to higher net inertia forces possessed by the particles compared to interfacial tension, whereas low net inertia forces of them compared to the interfacial tension are responsible for drop coalescence. (7) A three-phase fluidized bed contactor appears to give better performance than those of packed towers for the mass transfer between two partially miscible liquids.
Nomenclature (HTU)c = height of transfer unit of the continuous phase, cm
(HTU)oc = height of transfer unit based on the overall concentration ofthe solute in the continuous phase, cm (HTU)D = height of transfer unit of the dispersed phase, cm (HTU)oD = height of transfer unit based on the overall concentration ofthe solute in the dispersed phase, cm K 2 = constant Lc = continuous phase flow rate g/(cm2 s) L D = dispersed phase flow rate g/(cm2s) NRet= Reynolds number based on the particle diameter and terminal velocity Nsc = Schmidt number (NTU) = number of transfer units V , = terminal velocity of the particle, cm/s 2 = height of the fluidized column Greek Letters p =
viscosity g/(cm s)
p = density g/cm3
D = diffusivity cmz/s
Literature Cited Coiburn, A. P. Trans. A m . Int. Chem. Eng. 1030, 35, 211-237. Coiburn, A. P.; Welsh, D. G. Trans. Am. Inst. Chem. Eng. 1042, 38, 179. Dakshinamurty, P.; Chiranjivi. C.; Subrahmanyam, V.; Kameswararao, P. "Proceedings, International Symposium on Fluidization and its applications"; Touiouse, France, Oct 1-5, 1973; pp 429-433. Dakshinamurty, P.; Kameswararao, P.; Veerabhadrarao, K. Indian J . Techno/. 1074. 12, 276-280. Dakshinamurty, P.; Seshagirlrao, V. V. B.; Prasad, M. S. S. S.; Subrahmanyam, V. Paper presented at 28th Annual Session of Indian Inst. of Chemical Engineers Calcutta, Indla, Dec 27-30, 1975, and the same was published in Indian J. Techno/. 1980, 78(12), 501-505. Dakshinamurty, P.; Padmanabharaju. B. Paper presented at the 30th Annual Session of Indian Institute of Chemical Engineers, Chandigarh, Dec 27-30, 1977. Dakshinamurty, P.; Veerabhadrarao, K.; Venkatarao, A. B. Ind. Eng . Chem, Process Des. Dev. 1070, 18, 638. Laddha, G. S.; Smith, J. M. Chem. Eng. Prog. 1050, 4 6 , 195. Roszak; Gawnonski, Chem. Eng. J. 1070, 17, 101-109.
Receiued for reuiew December 11, 1981 Accepted April 13, 1983
