Ind. Eng. Chem. Process Des. Dev. 1986, 25, 862-871
882
Liquid
Vapor 0 Observed value
Mole fraction of crotonaldehyde
Figure 7. Calculated concentration profiles in the column (experiment 7 ) .
duced pressure distillation, as was expected from the equilibrium ratio. From the concentration ratios of distillate to bottoms, the Murphree vapor efficiencies of crotonaldehyde were determined, the mean of which was 0.34. The relatively low tray efficiency must be recognized in designing the distillation process of removing crotonaldehyde from ethanol. Acknowledgment This research was supported by the financial assistance of the Grant-in-Aid for Scientific Research of the Ministry of Education, Japan (No. 56550675).
Nomenclature E = Murphree vapor efficiency K = equilibrium ratio N = number of tray R = reflux ratio 3c = mole fraction in liquid x i = mole fraction of ethanol in liquid on a trace component free basis, x ? / ( x ; + x 2 ) y = mole fraction in vapor y* = y in equilibrium with 3c yz/ = mole fraction of ethanol in vapor on a trace component free basis, y 2 / ( y 1 + y z ) Subscripts n = plate number 1 = water 2 = ethanol
3 = crotonaldehyde (trace component) Registry No. Ethanol, 64-17-5; crotonaldehyde,4170-30-3.
Literature Cited Ikari, A.; Hatate, Y.; Deguchi, R. J . Chem. Eng. Jpn. 1978, 1 1 , 265. Ikari, A.; Hatate, Y.; Sakaue, S.;Kubota, Y. J . Chem. Eng. Jpn. 1984. 1 7 , 486.
Koga-n, V. 0.: Kiekiheiko Data Book; Hirata, M., Transi.; Kodansha: Tokyo, 1974;p 248.
Received for review January 24, 1985 Revised manuscript received February 24, 1986 Accepted March 26, 1986
Mass-Transfer Efficiency of Sieve Tray Extractors J. Antonlo Rocha, Jimmy L. Humphrey, and James R. Falr" Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 78712
The mass-transfer efficiency of a 0.10-m (&in.) sieve tray liquid-liquid extraction column was studied at several tray spacings and two downcomer lengths. Test systems were toluene/acetone/water and methyl isobutyl ketone (MIBK)/acetlc ackl/water. Mass transfer in both directions was studied, with both light and heavy phases dispersed. Overall efficiencies were measured and compared with those obtained from the available mechanistic models of Skelland, Treybal, and Pilhofer. These models were found not to predict accurately much of the experimental data from the present work as well as from the work of previous investigators. As a consequence, an improved model was devekped for the prediction of Murphree efficiency. Thls model places more emphasis on mass transfer during drop formation and provides improved agreement with a large bank of extractor performance data.
Liquid-liquid extraction is gaining increased attention as a commercial separation method. There are several reasons for this renaissance of an old technique. It offers potential energy savings for some separations now carried out by distillation. It can alleviate dangers of thermal degradation of sensitive materials. It can provide gross separations for products from biosynthesis. It is benefited from increased attention to the mechanisms by which material is transferred between phases in liquid-liquid contacting. This paper reports on studies of the last-named point favoring extraction and is directed toward an improved understanding of the mechanisms of mass transfer in 0196-4305/86/1125-0862$01.50/0
sieve-type extraction devices. The sieve tray extractor internals resemble those of a sieve tray distillation column. The dispersed phase passes through the perforations and the continuous phase contacts the rising or falling drops in a crossflow or countercurrent fashion (depending on the diameter to tray spacing ratio), moving from one tray to another through downcomers or upcomers. It is clear that the manner by which the drops are formed and then move through the continuous phase is crucial to the effectiveness of mass transfer. The hydraulics of sieve tray extractors have been discussed by Treybal(1980) and more recently by the present authors (Fair et al., 1984). Specific descriptions of the 0 1986 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 4, 1986 863
Table I. Data Sources for Sieve Tray Liauid-Liauid Extraction Columns column no. of diam., m system tray spacing, m data pta toluene/benzoic acidlwater 0.222 0.152 12 12 0.095 0.076, 0.152 gasoline/methyl ethyl ketonelwater 0.092 0.120 toluene/benzoic acidlwater 18 0.092 0.120 kerosene/benzoic acidlwater 18 0.152 0.106 31 toluene/diethylamine/water toluene/benzoic acidlwater 0.076, 0.152, 0.228 0.090 48 0.152 0.106 44 methyl isobutyl ketoneladipic acidlwater 0.050 toluene/benzoic acidlwater 0.609 13 0.117, 0.177, 0.584 0.219 diethyl etherlacetic acidlwater 38 0.121 42 methyl isobutyl ketonelbutyric acid/water 0.152, 0.279, 0.584
phase contacting action will be given below in connection with the development of mechanistic models for the transfer processes.
Previous Work Experimental. Data sources of overall efficiency studies are given in Table I. The table also includes information on the geometries of the test columns. A summary of each source follows. The first investigation reported was by Row et al. (1941) who used several types of columns to extract benzoic acid from toluene with water. The same system was used by Treybal and Dumoulin (1942) in a study of the effect of tray spacing and by Allerton et al. (1943),who considered kerosene in addition to toluene in an effort to delineate effects of physical properties. Moulton and Walkey (1944) used gasoline/ methyl ethyl ketonefwater and worked with high concentrations to produce large volume changes of the phases. Pyle et al. (1950) worked with the ethyl ether/acetic acidfwater system to study influences of hole diameter, percent hole area, tray spacing, solventffeed ratio, and flow rate on the capacity and efficiency of a 0.219-m (8.6-in.) column. Dispersion of the aqueous phase (using upcomers) was also studied. Their reported efficiency of 30% for a tray spacing of 0.12 m (4.7 in.) was the highest reported to that date for a sieve tray extraction column. Mayfield and Church (1952) worked with the toluenefbenzoic acidfwater and ethyl acetatefacetic acid/ water systems and obtained higher efficiencies with the latter; this system as well as the ethyl ether system used by Pyle and ceworkers is characterized by a low interfacial tension (about 12 mNfm), in contrast to the often-used toluenefwater system which has an interfacial tension in the range of 30-35 mN/m. One important contribution of Mayfield and Church was the recommendation of a “punched jet type” plate that avoids excessive wetting of the tray by the dispersed phase and thus promotes jetting from the perforations. Garner et al. (1953) extracted diethylamine from toluene with water and studied the effect of a gauze screen to promote coalescence. A second study (Garner et al., 1956) dealt with the recovery of adipic acid from methyl isobutyl ketone (MIBK) with water. The extractor utilized downcomers (MIBK dispersed) as well as upcomers (water dispersed). In the latter case, Teflon and metallic plates were used. The authors compared their data with those from the literature and showed that systems with low interfacial tensions produced higher efficiencies. They suggested that this property influences both the degree of circulation within the drops and the ease with which the solute can be transferred across the interface. Other papers dealing with overall efficiency do not give complete experimental information. For example, Murty and Rao (1968) reported on the effect of tray spacing in the MIBKIbutyric acidlwater system but only gave ranges
dispersed phase toluene gasoline toluene kerosene toluene toluene both both both MIBK
transfer directions D-C D-C D-C D-C D-C D-C both both both D-C
ref Row, 1941 Moulton, 1944 Allerton, 1943 Allerton, 1943 Garner, 1953 Treybal, 1942 Garner, 1956 Mayfield, 1952 Pyle, 1950 Murty, 1968
Light Phose Product
Sieve Tray Column
!
Movable Tank
I for
Level Control
Heavy Phase Prod uct
Llqht Phase Feed
L Figure 1. Flow diagram of experimental equipment.
of the parameters used. However, they presented a comparison of predicted overall efficiency (using an empirical relationship proposed by Treybal in 1963) against their experimental results. After the 1956 publication of Garner and co-workers attention shifted to the development of methods and correlations for predicting overall column performance. Efficiency Modeling. Mass transfer in small diameter sieve tray columns has been modeled by Treybal (1963, 1980), Skelland and co-workers (1973, 1977, 1979), and Pilhofer and co-workers (Pilhofer, 1981; Schulz and Pilhofer, 1982). Descriptions of the models, together with final correlating equations, are given by Fair et al. (1984) and by Rocha-Uribe (1984). Except for the 1963 model of Treybal, all of these models are based on presumed mechanisms of drop formation, rise, and coalescence. Additional information on the mechanistic approach will be given later in this paper.
Experimental Work A simplified flow diagram of the experimental equipment is shown in Figure 1. Key dimensions of the column and its internals are given in Table 11. Light- and heavy-phase liquids are fed from 200-L tanks by rotary gear pumps having a capacity of 9.4 Lfmin each. Flow rates are regulated by T-valves which split the flow between forward feed and recirculation. Rotameters are used to measure flow rates. Pneumatically operated valves are used for quick shutoff of all flows in and out of the column. The column comprises 12 cylindrical glass segments joined by means of flanges. Sample connections are located a t each flange, giving the opportunity of sampling a t 0.15-m intervals. The plates and downcomers are interchangeable, as shown in Figure 2. Additional features of the column are as follows. An interface control tank may be moved up or down to adjust the location of the interface at the top of the column.
864
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 4, 1986
Table 11. Key Dimensions of Column and Internals dimension, m column (glass) 0.1000 inside diam. 0.0064 wall thickness 1.8000 effective extraction ht 1.2000 disengaging ht tray spacings studied column with 10 trays 0.1600 0.3300 column with 5 trays column with 3 trays 0.5000 trays tested 0.0012 thickness, perforated area 0.0032 thickness, brass support downcomers (three per tray) inside diam., m 0.0095 0.0011 external diam., m 0.026 downcomer/column area ratio 0.1000 (most runs) downcomer length, m 0.2500 (some runs) area holes/ hole area class holes diam., m pitch, m pitch/diam. column A 54 0.0032 0.0096 3.00 0.0535 0.0032 0.0128 4.00 0.0321 B 33 C
D
E F
13 123 21 33
0.0064 0.0024 0.0048 0.0048
3.00 2.66 3.33 2.66
0.0191 0.0064 0.0159 0.0127
0.0507 0.0676 0.0460 .0.0748
Table 111. Physical Properties of Systems Used toluene/aceMIBK/acetic tone/water acid/water density of org phase, kg/m3 862 813 density of aq phase, kg/m3 992 1000 viscosity of org phase, kg/(m s) 0.560 0.69 viscosity of aq phase, kg/(m s) 1.10 1.10 (10-3) diffusion coeff, solute in org 2.60 (10-9) 1.50 (10-9) phase, m2/s diffusion coeff solute in aq 1.07 (10-9) 1.10 (10-9) phase, m2/s interfacial tensiona N/m 25.0 (10-3) 8.00 (10-3) Average value for composition range studied. 0.6 Parameter Is Tray Spacing In Meters
0.4 EO
0.2
-
1.0
-
O.I6
ToI / A c / H20
Gasket Zone
0' 0
I
I
I
I
.o
I
I
I
2.0
3.0
m Fd / Fc
Figure 3. Effect of tray spacing on overall efficiency, type A trays. Solid lines = toluene/acetone/water. Dashed lines = MIBK/acetic acid/ water. 0
0
0
0
0
0
0
o
0
e
k,o.o cm 4 Interchangeable Plate p-
6.0cm-\
-
Interchangeable Downcomer
Figure 2. Tray and downcomer geometry.
Perforated sparge rings are used to distribute both feeds. Sections of Teflon mesh are used at each end of the column to prevent entrainment of one phase by the other. A special view tray near the center of the column permits improved observation of the drop formation and coalescence mechanisms. For selected runs photographs and motion pictures are used to study drop size distribution and thus enable selection of a suitable correlation for drop size prediction. Systems Studied. Because of the recognized importance of interfacial tension as a correlating variable for efficiency, systems with both high and low interfacial tensions were chosen for study. Following the recom-
mendation of the Working Party on Distillation, Absorption, and Extraction (Misek, 1978), the toluene/acetone/water (high interfacial tension) and methyl isobutyl ketone/acetic acid/ water (low interfacial tension) systems were used. Chemicals used to prepare the test mixtures were technical grade with the following weight percent purities: toluene, 96.0 % (remainder benzene); acetone, 98.0% ; MIBK, 99.0%; acetic acid, 99.5%. Physical properties for these materials are shown in Table 111. Equilibrium Data. Several spot experimental determinations of the distribution coefficients for the solutes in both systems confirmed the data recommended by Misek (1978), and thus the latter were used for this study. The equilibrium data a t 20 "C were converted from a weight basis to mole fractions and correlated by linear regression to obtain eq 1-4. toluene(d)/ acetone/ water Y* = 3.8310X1*oo20
(1)
water(d)/acetone/toluene Y* = 0.2595X0-9942
(2)
MIBK(d)/acetic acid/water Y* = 2.8910X0*9915
(3)
water(d)/acetic acid/MIBK Y* = 0.3425X1.w80
(4)
Operating Procedure. A typical run took about 2 h. Both acetic acid and acetone concentrations in samples were analyzed by titration with 0.1 N sodium hydroxide, following procedures given in Misek (1978). For acetone analysis, hydroxylamine hydroxide was first added so that the liberated HC1 could be titrated (eq 5). Further details (CH3)2CO + NH,OH*HCl = (CH3)2C*NOH+ H20 + HC1 (5) may be found in the dissertation by Rocha-Uribe (1984).
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 4, 1986 865
0.5
Troy Spocing =0.16m
0.4 .
-
MIBK/HAc/H20
,/ -
