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Ind. Eng. Chem. Res. 2001, 40, 990-992
RESEARCH NOTES Critical Distillation Experiments in a Region near the Homogeneous Ternary Azeotrope in the System Ethyl Acetate-Ethanol-Water Ian A. Furzer† Department of Chemical Engineering, University of Sydney, Sydney, NSW 2006, Australia
A nine-tray distillation column has been used to critically examine a constant-temperature region near the ternary homogeneous azeotrope in the system ethyl acetate-ethanol-water. Liquid samples withdrawn from the trays show an almost constant ethanol composition for a specific charge to the reboiler. An examination of the experimental discrete distillation lines on a portion of the ternary phase diagram showed that the constant-temperature region had been covered and an indication of a different location of the ternary homogeneous azeotrope. Process simulation studies could be expected to be difficult with existing parameters in thermodynamic models, under these conditions. Introduction The ternary system ethyl acetate-ethanol-water contains a minimum boiling point temperature homogeneous azeotrope at a total pressure of 101.3 kPa. The phase equilibria in this system is complex because of the presence of two binary homogeneous azeotropes, a binary heterogeneous azeotrope, and a vapor-liquidliquid region. Griswold et al.1 have completed a comprehensive study of the phase equilibria in this system and noted the difficulty in locating the composition of the ternary homogeneous azeotrope. This is due to the ternary homogeneous azeotrope being located close to the liquid-liquid region. The temperature gradient from the minimum boiling point ternary homogeneous azeotrope to the liquid-liquid region is small. The temperature contours around the ternary homogeneous azeotrope are flat and make the identification of the composition and temperature at the minimum difficult. Griswold et al.1 found a sizable region where the temperature was 70.3 °C at a total pressure of 101.3 kPa. Further phase equilibria studies2 found similar flat contours and a composition and temperature of the ternary homogeneous azeotrope as x(EtAc) ) 0.595 (mole fraction), x(EtOH) ) 0.115 (mole fraction), x(water) ) 0.290 (mole fraction), and T ) 70.20 °C. Figure 1 shows a portion of the phase diagram for the liquid compositions and is a part of the normal ternary triangular diagram at a total pressure of 101.3 kPa. The ternary homogeneous azeotropic composition is located in Figure 1 inside the constant-temperature region. One side of the constant-temperature region is the liquidliquid binodal curve at a temperature near 70 °C. The other side of this region is a curve, estimated from the collected data and lying in the vapor-liquid region. Distillation experiments in this system at total reflux could be expected to generate discrete distillation lines directed toward the azeotrope. Because the ternary †
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Figure 1. Portion of the phase diagram showing the constanttemperature region for the system ethyl acetate-ethanol-water.
azeotrope is a minimum boiling point temperature azeotrope, then distillation experiments should result in an approach to this ternary azeotrope in the top trays of the column. The limited experimental vapor-liquid equilibria (VLE) data in the region of the ternary azeotrope make for difficulties in parameter estimation of thermodynamic models to describe this ternary system. Furzer3 has provided details of the simulation of this system if the UNIFAC parameters were of sufficient quality in the region of the ternary homogeneous azeotrope. Other studies on this ternary system include discrete distillation lines4 directed toward the ternary homogeneous azeotrope, an examination5 of the pure component, binary and ternary VLE data available in the system, measurements of froth heights,6 and
10.1021/ie000477j CCC: $20.00 © 2001 American Chemical Society Published on Web 01/10/2001
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Figure 2. Composition profiles along the column for ethanol for five runs in the system ethyl acetate-ethanol-water.
Figure 3. Composition profiles along the column for ethyl acetate for five runs in the system ethyl acetate-ethanol-water.
optimal values of the UNIFAC interaction parameters to predict the ternary homogeneous azeotrope. Experimental Section The experimental glass column of 104 mm diameter contained nine dualflow trays with a 20% free area on a 320 mm tray spacing with 8 mm hole diameters. The column was operated at total reflux and at a total pressure of 101.3 kPa. Samples were withdrawn from all nine trays and analyzed by a calibrated gas chromatograph. The column was operated for 1 h to approach a steady-state condition before samples were withdrawn. Liquid samples withdrawn from the column were at a temperature near 70 °C and were homogeneous. Cooling of the liquid samples to room temperatures near 20 °C resulted in the appearance of a second liquid phase. The heterogeneous liquid samples were phase separated and the mass and composition of each phase determined. The mean or average composition of the original homogeneous liquid sample for component i is given by
xi )
nIxiI + nIIxiII n
n ) nI + nII
(1) (2)
Five runs at total reflux were completed, with different charges being admitted to the reboiler for each run. The composition of the charge was specially selected to lie in the constant-temperature region in Figure 1. Results Figure 2 shows the experimental ethanol composition profile along the column for runs 1-5. The top tray, tray 1, had an ethanol composition which was almost constant with all other trays, in run 4. This indicates the neighborhood of an azeotrope with an ethanol composition of 0.065 (mole fraction). Figure 3 shows the experimental ethyl acetate composition profile for runs 1-5. Run 4 shows a changing ethyl acetate composition
Figure 4. Portion of the phase diagram showing the discrete experimental distillation paths at total reflux for five runs in the system ethyl acetate-ethanol-water.
throughout the column, indicating that the composition of the azeotrope had not been reached. Figure 4 shows runs 1- 5 on a portion of the ternary phase diagram. The discrete distillation line for run 1 is directed downward toward the ternary homogeneous azeotrope, while the line for run 5 is directed upward toward this azeotrope. Of the three additional runs, run 4 shows a limited discrete distillation line, indicating an approach to the ternary homogeneous azeotrope. Figure 4 also shows the liquid-liquid binodal curve at a temperature of 20 °C. Homogeneous liquid samples withdrawn from a tray at approximately 70 °C become heterogeneous with a small aqueous-rich phase at a temperature of 20 °C. Figure 4 shows that most of the liquid samples required phase separation before analysis.
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Conclusions
Subscript
Five distillation runs have been made at total reflux to critically explore a constant-temperature region, identified by Griswold et al.1 near the ternary homogeneous azeotrope in the system ethyl acetate-ethanolwater. The design of the experimental program to approach the ternary azeotrope from both high and low ethanol compositions yielded almost constant ethanol compositions on nine trays in the column. This result clearly indicated the approach to the ternary homogeneous azeotrope, but the ethanol composition of 0.065 (mole fraction) differed from another reported2 result of 0.115 (mole fraction). The analysis of the liquid samples withdrawn from the column was made difficult because of the necessity to separate a second aqueous phase and to analyze the ester-rich phase at high ethyl acetate compositions. This may have contributed to the ethyl acetate composition profiles along the column, only approaching the ethyl acetate content of the ternary azeotrope. The discrete distillation lines for five runs have been shown on a portion of a phase diagram and cover the constant-temperature region. These lines are directed toward a ternary azeotrope which may have a composition that differs from the reported value. Process simulation of this system in this near constant-temperature region with the existing optimal thermodynamic model parameters could be expected to produce distillation lines directed toward the fitted and reported ternary homogeneous azeotrope. The experimental distillation lines shown in this paper are not directed toward that azeotrope, and deviations could be expected with existing simulation processes.
i ) component identification
Acknowledgment The data were collected by C. Cachat. Nomenclature n ) number of moles x ) composition of the mole fraction
Superscripts I ) phase I II ) phase II
Literature Cited (1) Griswold, J.; Chu, P. L.; Winsauer, W. O. Phase Equilibria in a Ethyl Alcohol-Ethyl Acetate-Water System. Ind. Eng. Chem. 1949, 41, 2352. (2) Van Zandijcke, F.; Verhoeye, L. The Vapour-Liquid Equilibrium of Ternary Systems with Limited Miscibility at Atmospheric Pressure. J. Appl. Chem. Biotechnol. 1974, 24, 709. (3) Furzer, I. A. Distillation for University Students. Department of Chemical Engineering, University of Sydney, Sydney, NSW 2006, Australia, 1986. (4) Furzer, I. A. Distillation in a Ternary Homogeneous Azeotropic System. CHEMECA 98, The 28th Australasian Chemical Engineering Conference, Port Douglas, North Queensland, Australia, Sept 28-30, 1998; Institution of Engineers: Barton, Australia, Vol. 26, p 192.4. (5) Furzer, I. A. Critical Process Modelling in Separation Systems. CHEMECA 98, The 28th Australasian Chemical Engineering Conference, Port Douglas, North Queensland, Australia, Sept 28-30, 1998; Institution of Engineers: Barton, Australia, Vol. 26, p 192.5. (6) Furzer, I. A. Froth Heights on Dualflow Trays with a Ternary Azeotropic System of Ethyl Acetate-Ethanol-Water. Ind. Eng. Chem. Res. 2000, 39, 1430-1436. (7) Furzer, I. A. Optimization of a Wastewater System Containing the Ternary Homogeneous Azeotropic System Ethyl AcetateEthanol-Water. Ind. Eng. Chem. Res. 2000, 39, 1539-1545.
Received for review May 9, 2000 Accepted November 17, 2000 IE000477J
