Ind. Eng. Chem. Process Des. Dev. 1986, 25, 859-862
859
Behavior of a Minute Amount of Crotonaldehyde in Distillation of Aqueous Ethanol Solution under Reduced Pressure Atsushi Ikarl,' Yasuo Hatate, Seiichl Sakaue,+and Osamu Toklyoshi' Department of Chemical Englneerlng, Kagoshlma University. Kagoshima 890, Japan
Distillation under reduced pressures to remove trace amounts of crotonaldehyde from an aqueous ethanol solution has been studied. The vapor-liquid equilibria of the water-ethanol system containing a minute amount of crotonaldehyde were measured at 12.7 and 25.3 kPa. The equilibrium ratio cwves of crotonaldehyde have been obtained. Distillation experiments of ethanol containing a minute amount of crotonaldehyde were performed in an Oldershaw-type column. I t was found that crotonaldehyde could be easily removed from ethanol by distillation under reduced pressures in contrast to distillation at atmospheric pressure. From the ratlos of the mole fraction in the distillate to that in the bottoms, the Murphree vapor efficiencies of crotonaldehyde have been determined. They are within the range of 0.25-0.48 and not influenced by the vapor rate and the pressure.
Distillation under reduced pressure is one of the major purification methods used with organic solvents. When a distillation refining process is designed, knowing the behavior of a trace impurity in the distillation is desirable. The behavior of a trace component during distillation can be estimated by the distillation calculation in which the vapor-liquid equilibrium ratio and the tray efficiency of the trace component are required. In previous work (Ikari et al., 1984), the authors measured the vapor-liquid equilibria of aqueous ethanol solutions containing a minute amount of furfural and the Murphree vapor efficiencies of the trace component, furfural, by distillation experiments in an Oldershaw-type column. In this paper, crotonaldehyde is used as a trace component in place of furfural. Crotonaldehyde is one of the impurities in raw ethanol and is known to be sensitive to the chameleon test. The equilibrium ratio of a minute amount of crotonaldehyde in an aqueous ethanol solution has been found to decrease rapidly as the ethanol concentration increases and reach unity when the mole fraction of ethanol becomes 0.98. Furthermore, crotonaldehyde has been confirmed to be difficult to remove from ethanol by continuous distillation a t atmospheric pressure (Ikari et al., 1978). The purpose of this paper is to investigate the possibility of removing the trace amount of crotonaldehyde from ethanol by reduced pressure distillation.
Vapor-Liquid Equilibrium Materials. Reagent-grade ethanol was fractionally distilled after being left for several days with mphenylenediamine hydrochloride. When anhydrous ethanol was required, the distillate was dehydrated by distillation after being boiled under reflux with magnesium foil and a small amount of iodine for about 3 h. The guaranteed reagent of crotonaldehyde was used after vacuum distillation in a stream of nitrogen. Measurement. A schematic diagram of an Othmertype still with a vacuum system is shown in Figure 1. The pressure was reduced by an aspirator and maintained constant within 2 mmHg by use of a needle valve and a * buffer tank of approximately 20 L. A 420-mL aqueous ethanol solution containing approximately 0.05 wt % crotonaldehyde was charged in the still. Nippon Sanso Co., Ltd., Tokyo 105, Japan. Process Development Co., Ltd., Tokyo 143, Japan.
t Japan
0196-4305/86/1125-0859$01.50/0
The apparatus was operated for 7 h under a specified pressure. After the heaters and stirrers were stopped, the pressure was returned to atmospheric pressure. When the mole fraction of ethanol was close to unity, the reduction to atmospheric pressure was carried out by introducing pure nitrogen through the upper stopcock to prevent contamination by moisture in the atmosphere. Upon cooling to room temperature, samples from both phases were drawn for analysis. The density of samples was measured at 25 OC by a digital density meter (Anton Paar, DMA-40). The concentration of ethanol is obtained by the density data of the water-ethanol system. The concentration of crotonaldehyde was measured by an absorptiometric method. The sample was diluted properly with water, and the absorbance was measured a t the range of 210-230 nm. From the absorbance a t the peak, the concentration of crotonaldehyde is determined by use of a calibration curve prepared previously. A double-beam spectrophotometer (Hitachi, Model 124) was used. The errors inherent in determination of absorbance are assumed to be 1% . The estimated error in the calibration curve is 1.1%. The combined uncertainties in the concentration of crotonaldehyde reported in this paper are estimated to be about 2%. Results. The vapor-liquid equilibria were measured a t 12.7 and 25.3 kPa. The data a t 101.3 kPa had been reported in the earlier paper, in which the equilibrium ratio curve of the trace component, crotonaldehyde, exhibited an unusual shape when the mole fraction Qf ethanol exceeded 0.98. In this study, the curve in that range was reinvestigated and its unusual shape was reproduced. The results of the vapor-liquid equilibrium measurements are shown in Figures 2-4. In Figures 2 and 3, the data of a water-ethanol system, available in the literature (Kogan, 1974), are also plotted for comparison. Figure 2 shows the vapor-liquid equilibrium relationship between the major components, water and ethanol. The coordinates of this figure, x i and y2/, are, respectively, the mole fraction of ethanol in liquid and vapor phase, on a trace component free basis: x2'
= xz/(x1
+ -4
Y2' = YZ/(Yl
+ YZ)
In the binary system, x2/ equals x 2 and y i equals y2. It is shown that the xi-y2/ curve for the system containing a minute amount of crotonaldehyde is substantially coincident with the x2-y2 curve of the water-ethanol system 0 1986 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 4, 1986
860
n Valve
r
The1
b
r U
Hot stirrer
b
Aspiratorr
Manometer
g
4
5
2
0
t-
5 1 E, 0.7 .4.- 0.4 -
Figure 1. Schematic diagram of the apparatus for vapor-liquid equilibrium measurements.
Note change of scale
2
x;
Figure 4. Equilibrium ratio curves of the trace component (crotonaldehyde). 0.6 0.4 A
This work
o This w a k
(Kogan ,1974)
o
0.2
0.6
a4
0.8 1.0
o
02
0.4
x;
0.6 x;
0.8
1.0
Figure 2. Comparison of the vapor-liquid composition curves for the major component (water-ethanol) obtained in this work with those of the biaary system (water-ethanol), in which x i equals x2 and y i equals y 2 . 1 001
90px
Pressure
We imagine that the unusual lowering of the ratio may be attributed to the formation of hemiacetal and/or acetal from crotonaldehyde and ethanol during heating in the still. However, it is better to say that the reason is not clear, because the formation is still not confirmed experimentally. When the pressure was reduced to 25.3 and 12.7 kPa in this study, these unusual behaviors of the equilibrium ratio of crotonaldehyde were not observed. When x i is larger than 0.95, the equilibrium ratio becomes nearly constant. Its value was 1.51 a t 25.3 kPa and 1.78 a t 12.7 kPa.
Distillation Experiments The apparatus and procedure are the same as in the previous paper (Ikari et al., 1984). About 700 mL of ethanol containing 0.02 w t % crotonaldehyde was charged in the still. In all trials the reflux ratio was held constant at
a.
4
Binary d,ata(K?gy,ly4)
0 0.2 Water
0.4 0.6 0.8 1.0 Ethand
x; Figure 3. Comparison of the still temperature obtained in this work with the bubble point of the water-ethanol system at the same pressure. In a binary system, x i equals x2.
a t the same pressure. Also, the x-y relation of the major components is only slightly affected by the trace component. In Figure 3, the still temperature is plotted against x i together with the bubble point of the water-ethanol system. The trace component has practically no effect on the temperature. The equilibrium ratio of the trace component (=y3/3c3) is plotted against x i in Figure 4, in which the equilibrium curve a t atmospheric pressure (Ikari et al., 1978) is represented by the dotted line for a comparison. At atmospheric pressure, the equilibrium ratio reaches unity when x i equals 0.98 and becomes lower as x i goes beyond 0.98. When x i approaches unity, however, the ratio scatters widely to 0.56-1.0. I t appears to be restored to unity.
The experimental conditions and results are shown in Table I, in which the concentration of ethanol is not described as it can be obtained by substracting the concentration of crotonaldehyde from 100%. The concentration ratio in Table I is the ratio of the mole fraction of crotonaldehyde in the distillate to that in the bottoms. The mean concentration ratio of a series of experiments a t a constant pressure is 1.25 at atmospheric pressure, but it becomes 4.34 at 25.3 kPa and, further, 6.51 at 12.7 P a . These results may be attributed to the equilibrium ratio of crotonaldehyde which is close to unity a t atmospheric pressure but becomes 1.51 and 1.78 a t 25.3 kPa and 12.7 kPa, respectively. The distillation experiments in this study were carried out under the conditions that the reflux ratio was 8, the number of plates was 10, and the distillate was returned to the bottom. If the reflux ratio and the number of plates are increased and the conventional distillation is carried out, one might expect the concentration ratio to become larger. It can be stated that a trace amount of crotonaldehyde in ethanol can be removed by reduced pressure distillation, although it is difficult a t atmospheric pressure.
Evaluation of the Tray Efficiency From the concentration ratios in Table I, the Murphree vapor efficiencies of crotonaldehyde were obtained by the following method. Derivation of Equations. A schematic model of the apparatus used in the distillation experiments is shown in Figure 5. The following assumptions are made: (1)total
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 4, 1986 861 Table I. Experimental Results of Distillation for the Ethanol-Crotonaldehyde System expt no.
concn of crotonaldehyde, mol % charge distillate bottoms Pressure = Atmospheric
vapor rate, g.cm-z.h-1
1 2
21.8 24.2
0.0127 0.0121
0.0130 0.0133
3 4 5 6 7 8 9
10
18.1 24.4 31.7 52.4 58.7 65.2 74.5 89.8
0.0130 0.0132 0.0157 0.0132 0.0123 0.0140 0.0155 0.0144
0.0326 0.0291 0.0423 0.0261 0.0317 0.0379 0.0345 0.0335
11 12 13 14
18.9 28.5 43.8 63.7
0.0130 0.0130 0.0117 0.0140
0.0407 0.0401 0.0395 0.0420
Murphree vapor efficiency of crotonaldehyde
concn ratio (distillate/ bottoms) 1.14 1.36
0.011 34 0.009 81
Pressure = 25.3 kPa 0.007 17 0.006 58 0.008 52 0.007 03 0.007 14 0.007 56 0.009 18 0.008 83
4.54 4.42 5.05 3.71 4.44 5.01 3.76 3.77
0.374 0.353 0.463 0.260 0.356 0.474 0.259 0.263
6.89 6.16 7.10 5.89
0.360 0.279 0.393 0.256
Pressure = 12.7 kPa
91
0.005 0.006 50 0.005 56 0.007 12
1 .o
Pressure [kPal
-
0.8-
-0-
-
'v f "
2 L + ~L I L'
-
I
O
0
I
I
1
1
I
I
1
I
it follows that
= AY, + Bxo
A = R/[R(l -E) + KE(R + l)] B = KE/[R(l - E) + KE(R + l ) ]
Ig.~m-~.h''l
Figure 6. Murphree vapor efficiencies of the trace component (crotonaldehyde).
R+1%+ R + 1 X O
(2) (3) (4)
Assumptions (1)and (4) yield, respectively, (5) Substituting eq 2 successively from n = 1to n = N , we get
[
A
0
condenser, (2) reflux at the bubble point, (3) constant molal overflow, and (4) reboiled vapor in equilibrium with residue. From the equilibrium relation (y,* = Kx,) and the definition of the Murphree vapor efficiency, the following equation is derived. Y, = KEx, + (1- E)Y,+~ (1) From eq 1 and the equation of operating line R 1
yN+l .= AN + B-
O
Vapor rate
Figure 5. Model of distillation experiment.
where
.
-
I
N-1 _'v N L t Li - fv 9 N+1
Yn+l
A A
0.2-
I
0
-Q-0
-
I
=
0
0.4-
D
I I
Yn+l
25.3
0.6-
1-A
From eq 5 and 6, the following equation is obtained. K ( l - A) O -X(7) AN(l - A - B) + B XN+I If the value of E is given, the concentration ratio, xo/ xN+l, can be calculated from eq 3, 4, and 7. When the
calculated value does not agree with the experimental value within the tolerance, the value of E is adjusted and the calculations are iterated. Results and Discussion. The calculated values of the Murphree efficiency of crotonaldehyde are written in the last column of Table I and plotted against the vapor rate in Figure 6. In the case of atmospheric pressure, it was difficult to obtain a reliable value since the concentration ratio was close to unity. Figure 6 shows that the tray efficiency is only slightly influenced by the vapor rate and also by the pressure. The values are within the range of 0.25-0.48 and, the mean is 0.34, which is considerably lower than that of furfural, 0.58, obtained in the previous paper (Ikari et al., 1984). One must pay attention to the lower tray efficiency in designing the distillation process of removing crotonaldehyde from ethanol. A typical diagram of the calculated concentration profiles of crotonaldehyde is shown in Figure 7.
Conclusions The vapor-liquid equilibria of the water-ethanol system containing a minute amount of crotonaldehyde were measured a t 12.7 and 25.3 kPa. The equilibrium ratio of crotonaldehyde in ethanol is 1.78 a t 12.7 kPa and 1.51 a t 25.3 kPa, whereas it was found to be approximately unity a t atmospheric pressure. The distillation experiments of ethanol containing a minute amount of crotonaldehyde were carried ogt in an Oldershaw-type column. The ratio of the mole fraction of crotonaldehyde in the distillate to that in the bottoms was about 1.3 at atmospheric pressure; however, the ratio became 4.3 a t 25.3 W a and 6.5 at 12.7 kPa. It was shown that crotonaldehyde can be removed from ethanol by re-
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
