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Fluid phase equilibria for the methyl chloride—water system
327
Fluid Phase Equilibria, 65 (1991) 327-338 Elsevier Science Publishers B.V., Amsterdam
Fluid phase equilibria for the methyl chloride-water
system
Richard A. Coats ‘, Joseph C. Mullins and Mark C. Thies * Department
of Chemical Engineering,
Clemson University, Clemson, SC 29634 (U.S.A.)
(Received June 11, 1990; accepted in final form December 20, 1990)
ABSTRACT Coats, R.A., Mullins, J.C. and Thies, M.C., 1991. Fluid phase equilibria for the methyl chloride-water system. Fluid Phase Equilibria, 65: 327-338. Azeotropic distillation is frequently used in the dehydration of organic-water systems to produce a completely dehydrated product. A basic requirement for the process is that the added solvent entrain, or azeotrope, the water for removal as the distillate. The purpose of this study was to determine whether methyl chloride exhibits the necessary fluid phase behavior with water to be used as an entrainer solvent for the dehydration of organic-water mixtures. A flow apparatus was used to determine liquid-liquid, liquid-liquid-vapor and vapor-liquid equilibrium compositions for the methyl chloride-water system at 313.2 and 333.2 K. Pressure vs. composition diagrams have been constructed at these two temperatures, and indicate that the methyl chloride-water system does not exhibit the type of azeotropic behavior suitable for water entrainment.
INTRODUCTION
This paper is the third in a series concerning the dehydration of organic chemicals by using light, subcritical or supercritical solvents. Earlier studies by Briones et al. (1987) and by McCully et al. (1988) focused on the use of supercritical carbon dioxide and near-critical propane as extractive solvents for the dehydration of organic-water mixtures. In this study the solvent methyl chloride (T, = 416.2 K) has been investigated. Although a number of extraction and distillation schemes can be used for the dehydration of organic-water mixtures, many processes include an azeotropic distillation step in which the solvent entrains, or azeotropes, with ‘Current address: Ethyl Corporation, P.O. Box 1028, Orangeburg, SC 29116, U.S.A. 2 Author to whom correspondence should be addressed. 0378-3812/91/$03.50
0 1991 Elsevier Science Publishers B.V.
X.YSOI”,“l (b)
Fig. 1. Pressure vs. composition diagrams of the two types of heterogeneous azeotropes that can occur with partially miscible liquids. Diagrams are at constant temperature: (a) heterogeneous azeotrope of the first kind; (b) heterogeneous azeotrope of the second kind.
the water and both exit the column as distillate. The dehydrated organic product is produced as the bottoms. A basic requirement of an azeotropic distillation process is that the solvent forms an azeotrope with water of the type shown in Fig. l(a), ‘which van Ness and Abbott (1982) refer to as a heterogeneous azeotrope of the first kind. Note that the azeotropic vapor composition at the three-phase line contains more water than the solvent-rich liquid. This is the underlying phenomenon that causes the desired entrainment effect. On the other hand, for a heterogeneous azeotrope of the second kind (Fig. l(b)), the vapor contains less water than the solvent-rich liquid and entrainment does not occur. Hoffman (1977) uses ternary phase diagrams for a number of systems to illustrate the entrainment effect. Examples of solvents that exhibit the necessary phase behavior with water (Fig. l(a)) include benzene, diethyl ether and pentane; all of these have been used to dehydrate ethanol-water solutions (Black et al., 1972). More re-
329
cently, Brignole et al. (1987) have used a group contribution equation of state to show that propane can be used to produce absolute ethanol but that carbon dioxide cannot. Such results are expected from our knowledge of these systems’ phase behavior with water, since propane exhibits behavior of the first kind (Kobayashi and Katz, 1953) and carbon dioxide exhibits behavior of the second kind (Evelein et al., 1976). The purpose of this study was to determine the type of azeotrope that methyl chloride forms with water and to evaluate its potential as an entrainer for the azeotropic distillation of organic-water solutions. To accomplish this, liquid-liquid, liquid-liquid-vapor and vapor-liquid equilibrium compositions were measured at 313.2 and 333.2 K. Of the previous studies that have been made for this system, the most comprehensive is that of Holldorff and Knapp (1988), who measured liquid-liquid and vaporliquid compositions from - 266 to 323 K. However, no measurements which focus on the azeotropic region of interest are known.
EXPERIMENTAL METHOD
An equilibrium flow apparatus was used to measure the experimental data. This apparatus was convenient for our work, since the reaction between methyl chloride and water to form methanol and hydrochloric acid becomes significant at 323-333 K. The flow apparatus also is suited particularly for overcoming the difficulties associated with accurately measuring the solubility of a minor component in a liquid phase, which include the adsorption of the minor component onto the surfaces of sampling tubing (Tsonopoulos and Wilson, 1983) and the absorption of the minor component into the polymer seals of liquid sampling valves (McCully et al., 1988). Liquid-liquid and vapor-liquid equilibria
A schematic of the experimental apparatus used for measuring liquidliquid and vapor-liquid equilibria is shown in Fig. 2. Unless otherwise noted, all fluid transfer lines are 316 stainless steel with an O.D. of 1.59 and an I.D. of 0.762 mm. For an experimental run, methyl chloride and water are delivered as compressed liquids by separate high-pressure feed pumps (Milton Roy, model no. 396). The combined constant flow rate from the two pumps ranged from 50 to 200 mL h-r. The inlet to the methyl chloride pump is cooled with a water-ice mixture to ensure the delivery of a liquid. A 500 ml sample cylinder (Whitey Co.) is used as a surge tank and serves to dampen pressure fluctuations caused by the pumps. After leaving the pumps, the two-phase mixture enters the equilibrium coil, which is used for
330 Wet-Test Meter
Cold Traps
Gas Flow Meter Oil Bath
rnwomererbng
Cold
Fig. 2. Schematic of equilibrium flow apparatus.
heating the mixture to the cell temperature. This coil consists of two 12 m sections of 1.59 mm O.D./0.76 mm I.D. tubing interconnected with a 3 m section of 3.18 mm O.D./1.59 mm I.D. tubing. The diameter change is used to promote mixing of the phases. An in-line, Type J thermocouple is used to monitor the temperature of the mixture exiting the equilibrium coil. The temperature of this mixture was always within 0.1 K of the contents of the view cell. After exiting the coil, the equilibrated, two-phase mixture enters the view cell, which functions as a phase separator. The lighter, methyl chloride-rich phase exits the top and the heavier, water-rich liquid phase exits the bottom of the cell. The methyl chloride-rich phase subsequently passes through a heated section of tubing to prevent condensation and is then expanded to atmospheric pressure across a micrometering valve (Autoclave Engineers, model no. 60VRMM). The valve is heated to prevent hydrate formation and the accompanying blockage that would occur during a Joule-Thomson expansion. The water-rich phase is expanded similarly through a micrometering valve, but no heating is required. After expansion, the effluent streams enter the sampling system and samples are collected. The methyl chloride-rich phase first flows through a series of cold traps that are maintained at O-2 IS above the normal boiling point of methyl chloride, which is 249.5 IL At these conditions, most of the
331
water in the mixture precipitates in the form of a methyl chloride hydrate and is collected. A small amount of liquid methyl chloride also occasionally forms and is collected. The volume of the gas phase exiting the cold traps is measured with a wet-test meter (Precision Scientific Co., model no. 63111). At the end of an experimental run, the cold traps are allowed to warm to ambient temperatures until only liquid water remains in the cold traps, The mass of this water is then determined. The equilibrium composition of the methyl chloride-rich phase in the view cell is then calculated by assuming that the collected liquid water is essentially pure and that the composition of the collected gas phase was in equilibrium with a hydrate phase. The expanded water-rich phase from the view cell flows into a trap maintained at ambient temperature where the liquid phase is collected. The gas phase exits the trap and is collected in a gas flow meter (Brooks Instrument, model no. 1056). Equilibrium compositions of the water-rich phase are then determined by assuming that the collected gas and liquid phases are in equilibrium. Henry’s law constants are needed for this calculation and are available in the literature (Glew and Moelwyn-Hughes, 1953). The virial equation was used to correct for non-idealities in both of the collected gas phases; the necessary pure-component second virial coefficients have been measured experimentally (O’Connell and Prausnitz, 1970) and the cross-coefficient was estimated by a method of Tsonopoulos (1974). The view cell is a liquid level gauge (Jerguson Co., model 11-T-20) with a modified cell body. The body was fabricated at Clemson from a block of Carpenter 450 stainless steel. Its fluid chamber has been machined to the original height and depth but is reduced in width from 1.6 cm to 0.95 cm. The cell windows are made of borosilicate glass (Corning Glass Works) mounted on Teflon gaskets. The equilibrium coil and the view cell are immersed in a silicone oil bath. The bath temperature is controlled to within +O.Ol K with a Yellow Springs model 71A temperature controller and thermistor, which are connected to a 750-W heating element. Thermal gradients in the bath are < 0.02 K. Two safety features are included in the apparatus. A 9.5mm thick polycarbonate shield surrounds the oil bath and protects the operator against a possible rupture of the view cell. All effluent streams containing methyl chloride are vented from the building. Liquid-liquid-vapor
equilibria
Minor modifications to the apparatus described above were necessary for measuring three-phase equilibria (see Fig. 3). To collect vapor phase samples, the feed is introduced into the methyl chloride-rich liquid (i.e. middle) phase to minimize possible entrainment in the vapor phase. The middle
332
* -
Vapor Phase to Sample Collection
Feed Middle Phase
* Aqueous Phase to Sample Collection
(4
. -l-Y-Middle Phase to Sample Collection
-.
Feed
1
t
Aqueous Phase
(b)
Fig. 3. Modifications to the apparatus for measuring three-phase equilibria: (a) configuration for collection of vapor and aqueous phases; (b) configuration for collection of methyl chloride-rich liquid (middle) phase.
phase is removed through an additional section of tubing located midway between the feed port and the bottom phase exit port (Fig. 3(a)). The flow rate of the middle phase is controlled with a micrometering valve (Hoke Inc., model no. 1666G2Y), which is heated to prevent hydrate formation during flashing. The aqueous bottom phase is collected in a similar manner. When collecting middle phase samples, the feed is introduced into the bottom aqueous phase to minimize possible contamination of the middle phase with entrained water droplets (Fig. 3(b)). Experimental
measurements
Operating temperatures of each phase in the cell and of the feed were measured with Type J thermocouples referenced to an ice-water bath. Thermocouple voltages were measured with a Keithley model 191 digital multimeter. The thermocouples and multimeter were calibrated as a unit against an NBS traceable mercury-in-glass thermometer and are believed to
333
be accurate to +O.l K. The feed, top and bottom phase thermocouples always agreed within *O.l K of each other. Owing to the presence of temperature fluctuations, which are inherent in the operation of an equilibrium flow apparatus, reported temperatures in the vapor-liquid and liquid-liquid regions are believed to be accurate to +0.2 K and in the liquid-liquid-vapor region to &OS K. Operating pressures were measured with two Bourdon tube-type Heise gauges (model CM, O-5000 psig range and model CM, O-1000 psig range) that had been calibrated against a Budenberg dead weight gauge (model 380H). Our ability to control the system pressure with the micrometering valves was dependent on the type of phase behavior that existed in the cell. Reported pressures are believed to be accurate to f 5 psi in the liquid-liquid region, kO.5 psi in the vapor-liquid region and + 2 psi at the three-phase line. Before beginning an experimental run, each phase exiting the view cell was checked for the presence of reaction products and for impurities, such as nitrogen and air, with HPLC sample injectors (Valco Instruments Co., product no. CI4W). These injectors (not shown in Fig. 2) are located in the oil bath in the tubing immediately exiting the top and bottom of the view cell. Samples were injected into a Hewlett-Packard 5890A gas chromatograph equipped with a 3.18 mm X 1.8 m nickel column packed with Chromosorb 107 and were analyzed with a thermal conductivity detector. At the highest operating temperature of 333 K, methanol was detected in the view cell if the contents were held statically in the cell overnight. However, after 5 min of operating the apparatus in the flow mode no methanol could be detected. No experimental run was begun until the contents of the cell were determined to be pure.
RESULTS AND DISCUSSION
In an earlier study we measured phase equilibria for the carbon dioxidewater system at 323.2 K with an equilibrium flow apparatus (Briones et al., 1987); comparison with accepted literature values indicated that reliable results were obtained. Thus, no additional measurements of a corroborative nature were made for this work. Equilibrium compositions for the methyl chloride-water system at 313.2 and 333.2 K are presented in Table 1 and also are shown on pressure vs. composition diagrams in Figs. 4-7. As expected, the measured liquid-liquid equilibrium data were found to be nearly independent of pressure, which is a good indication of the reproducibility of the data. For each experimental run, three to five samples were collected consecutively. Reported results
334 TABLE 1 Equilibrium
compositions
Pressure (bar)
for the methyl chloride-water
Mole fraction water in methyl chloride-rich phase
Mole fraction methyl chloride in water-rich phase
Liquid
Vapor
Liquid
0.00969 0.0105
0.00961 0.00966 0.00968 0.00973 0.00792
0.0146 0.0163
0.0103 0.0101 0.00986 0.00969 0.00930
313.2 K 69.96 35.49 9.29 8.67 a 7.22
0.00944 0.00936 0.00969 0.0101
333.2 K 69.96 35.49 16.18 14.25 a 12.73
0.0156 0.0157 0.0158 0.0157
a Three-phase
system at 313.2 and 333.2 K
line.
were obtained by averaging all samples for a given run and then by averaging all runs if more than one experimental run was made. This consecutive, cumulative sampling was found to be necessary to obtain good results. The technique helps to account for any two-phase flow existing in the sample tubing by inherently averaging the contents of the flow stream over time. To illustrate this point, all data used to obtain two of the
tvtoie
Fig. 4. Pressure
Fraction
Methyl
vs. composition
Chloride
diagram
for the methyl chloride-water
system at 313.2 K.
335 12
11
Mole
Fraction
Methyl
Chloride
Fig. 5. Enlargement of the methyl chloride-rich three-phase region at 313.2 K.
compositions in this work are presented in Table 2. Although some variation between samples is present, results from different experimental runs (and thus different days) are highly reproducible. Compositions of the minor component are believed to be accurate to + 3% of the values reported in Table 1 for both the methyl chloride-rich and water-rich phases. Enlargements of the methyl chloride-rich three-phase region are shown in Figs. 5 and 7 and indicate that the vapor phase always equilibrium
1, L-L
I
0
,
/’
I
I
I’
L-V
,I
v
,1’
,I 0.01
Mole
0.02
0.98
Fraction
Methyl
1.01 Chloride
Fig. 6. Pressure vs. composition diagram for the methyl chloride-water
system at 333.2 K.
Mole Fraction Methyl Chloride
Fig. 7. Enlargement of the methyl chloride-rich three-phase region at 333.2 K.
more methyl chloride than the methyl chloride-rich liquid phase. Although the differences in composition between the vapor and liquid phases are small, the reproducibility of the experimental runs used to generate these points was better than f l/2% of the values reported in Table 1. We have concluded, therefore, that the observed differences are significant, and that the methyl chloride-water binary system forms a heterogeneous azeotrope of the second kind (Fig. l(b)). In addition, since the vapor pressure of methyl chloride occurs at essentially the same pressure as the three-phase line, the system also may exhibit a maximum pressure (or minimum-boiling) homogeneous azeotrope as depicted by the dashed lines in Figs. 5 and 7. Holldorff and Knapp (1988) measured liquid-liquid equilibria for the methyl chloride-water system at - 312 and - 315 K; we have interpolated their results to 313.2 K for comparison. Our results for the methyl chloriderich liquid at 9.29, 35.49 and 69.96 bar are in good agreement with their reported value of 0.96 mol% water. For the water-rich liquid, our results are low relative to their reported value of 1.13 mol% methyl chloride. Holdorff and Knapp (1988) also estimated the composition of this phase by extrapolating the Henry’s law gas-liquid solubility curve to the three-phase line and obtained a value of 1.07 mol%. As shown in Table 1, there is virtually no scatter in our results for this phase; we do not have an adequate explanation for the discrepancy between our and the previous authors’ work. contained
337 CONCLUSIONS
Liquid-liquid, liquid-liquid-vapor and vapor-liquid equilibrium compositions have been measured for the methyl chloride-water system at 313.2 and 333.2 K. Measurements at the three-phase line indicate that this system forms a heterogeneous azeotrope of the second kind. Therefore, methyl chloride is not a suitable entrainer for water in an azeotropic distillation process for dehydrating organic-water mixtures.
TABLE 2 Experimental runs for the methyl chloride-water
system at 313.2 K and 69.96 bar
Mole fraction water in methyl chloride-rich liquid phase
Mole fraction methyl chloride in water-rich liquid phase
Run 1
Run 4 0.00822 0.00927 0.0109 0.00960 0.00927
0.00905 0.00965 0.00989 0.00977 0.00999 0.0100 0.00981
Mean 0.00945 Run 2
Mean 0.00974 0.00977 0.00917 0.00940 0.00960 0.00931
Run 5 0.00904 0.00989 0.00972 0.00960 0.00977
Mean 0.00945
Mean 0.00960
Run 3 0.00925 0.00952 0.00950 Mean 0.00942
Run 6 0.00887 0.00937 0.00963 0.00961 0.00926 0.00984 0.00972 0.00973 Mean 0.00950
338 ACKNOWLEDGEMENTS
The partial financial support of Hoechst-Celanese is gratefully acknowledged. The authors are indebted also to Carl Howell from Hoechst-Celanese of Charlotte for his helpful discussions.
REFERENCES Black, C., Golding, R.A. and Ditsler, D.E., 1972, Azeotropic distillation results from automatic computer calculations. In: Robert F. Gould (Ed.), Extractive and Azeotropic Distillation. Adv. Chem. Ser., 115: 64-92. Brignole, E.A., Anderson, P.M. and Fredenslund, A., 1987. Supercritical fluid extraction of alcohols from water. Ind. Eng. Chem. Res., 26: 254-261. Briones, J.A., Mullins, J.C., Thies, M.C. and Kim, B.-U., 1987. Ternary phase equilibria for acetic acid-water mixtures with supercritical carbon dioxide. Fluid Phase Equilibria, 36: 235-246. Evelein, K.A., Moore, R.G. and Heidemann, R.A., 1976. Correlation of the phase behavior in the systems hydrogen sulfide-water and carbon dioxide-water. Ind. Eng. Chem. Process Des. Dev., 15: 423-428. Glew, D.N. and Moelwyn-Hughes, E.A., 1953. Chemical statics of the methyl halides in water. Discuss. Faraday Sot., 15: 150-161. Hoffman, E.J., 1977. Azeotropic and Extractive Distillation. Robert E. Krieger, New York, Chapt. 6. Holldorff, H. and Knapp, H., 1988. Binary vapor-liquid-liquid equilibrium of dimethyl ether-water and mutual solubilities of methyl chloride and water: experimental results and data reduction. Fluid Phase Equilibria, 44: 195-209. Kobayashi, R. and Katz, D.L., 1953. Vapor-liquid equilibria for binary hydrocarbon-water systems. Ind. Eng. Chem., 45: 440-451. McCully, M.A., Mullins, J.C., Thies, M.C. and Hartley, I.J., 1988. Dehydration of acetic acid-water mixtures with near critical and supercritical fluid solvents. Sep. Sci. Technol., 23: 2065-2085. O’Connell, J.P. and Prausnitz, J.M., 1970. Thermodynamic and transport properties of water vapor at low densities. Ind. Eng. Chem. Fundam., 9: 579-584. Tsonopoulos, C., 1974. An empirical correlation of second virial coefficients. AIChE J., 20: 263-272. Tsonopoulos, C. and Wilson, G.M., 1983. High temperature mutual solubilities of hydrocarbons and water. AIChE J., 29: 990-999. Van Ness, H.C. and Abbott, M.M., 1982. Classical Thermodynamics of Nonelectrolyte Solutions. McGraw-I-Ii& New York, pp. 372-400.
Fluid Phase Equilibria, 65 (1991) 327-338 Elsevier Science Publishers B.V., Amsterdam
Fluid phase equilibria for the methyl chloride-water
system
Richard A. Coats ‘, Joseph C. Mullins and Mark C. Thies * Department
of Chemical Engineering,
Clemson University, Clemson, SC 29634 (U.S.A.)
(Received June 11, 1990; accepted in final form December 20, 1990)
ABSTRACT Coats, R.A., Mullins, J.C. and Thies, M.C., 1991. Fluid phase equilibria for the methyl chloride-water system. Fluid Phase Equilibria, 65: 327-338. Azeotropic distillation is frequently used in the dehydration of organic-water systems to produce a completely dehydrated product. A basic requirement for the process is that the added solvent entrain, or azeotrope, the water for removal as the distillate. The purpose of this study was to determine whether methyl chloride exhibits the necessary fluid phase behavior with water to be used as an entrainer solvent for the dehydration of organic-water mixtures. A flow apparatus was used to determine liquid-liquid, liquid-liquid-vapor and vapor-liquid equilibrium compositions for the methyl chloride-water system at 313.2 and 333.2 K. Pressure vs. composition diagrams have been constructed at these two temperatures, and indicate that the methyl chloride-water system does not exhibit the type of azeotropic behavior suitable for water entrainment.
INTRODUCTION
This paper is the third in a series concerning the dehydration of organic chemicals by using light, subcritical or supercritical solvents. Earlier studies by Briones et al. (1987) and by McCully et al. (1988) focused on the use of supercritical carbon dioxide and near-critical propane as extractive solvents for the dehydration of organic-water mixtures. In this study the solvent methyl chloride (T, = 416.2 K) has been investigated. Although a number of extraction and distillation schemes can be used for the dehydration of organic-water mixtures, many processes include an azeotropic distillation step in which the solvent entrains, or azeotropes, with ‘Current address: Ethyl Corporation, P.O. Box 1028, Orangeburg, SC 29116, U.S.A. 2 Author to whom correspondence should be addressed. 0378-3812/91/$03.50
0 1991 Elsevier Science Publishers B.V.
X.YSOI”,“l (b)
Fig. 1. Pressure vs. composition diagrams of the two types of heterogeneous azeotropes that can occur with partially miscible liquids. Diagrams are at constant temperature: (a) heterogeneous azeotrope of the first kind; (b) heterogeneous azeotrope of the second kind.
the water and both exit the column as distillate. The dehydrated organic product is produced as the bottoms. A basic requirement of an azeotropic distillation process is that the solvent forms an azeotrope with water of the type shown in Fig. l(a), ‘which van Ness and Abbott (1982) refer to as a heterogeneous azeotrope of the first kind. Note that the azeotropic vapor composition at the three-phase line contains more water than the solvent-rich liquid. This is the underlying phenomenon that causes the desired entrainment effect. On the other hand, for a heterogeneous azeotrope of the second kind (Fig. l(b)), the vapor contains less water than the solvent-rich liquid and entrainment does not occur. Hoffman (1977) uses ternary phase diagrams for a number of systems to illustrate the entrainment effect. Examples of solvents that exhibit the necessary phase behavior with water (Fig. l(a)) include benzene, diethyl ether and pentane; all of these have been used to dehydrate ethanol-water solutions (Black et al., 1972). More re-
329
cently, Brignole et al. (1987) have used a group contribution equation of state to show that propane can be used to produce absolute ethanol but that carbon dioxide cannot. Such results are expected from our knowledge of these systems’ phase behavior with water, since propane exhibits behavior of the first kind (Kobayashi and Katz, 1953) and carbon dioxide exhibits behavior of the second kind (Evelein et al., 1976). The purpose of this study was to determine the type of azeotrope that methyl chloride forms with water and to evaluate its potential as an entrainer for the azeotropic distillation of organic-water solutions. To accomplish this, liquid-liquid, liquid-liquid-vapor and vapor-liquid equilibrium compositions were measured at 313.2 and 333.2 K. Of the previous studies that have been made for this system, the most comprehensive is that of Holldorff and Knapp (1988), who measured liquid-liquid and vaporliquid compositions from - 266 to 323 K. However, no measurements which focus on the azeotropic region of interest are known.
EXPERIMENTAL METHOD
An equilibrium flow apparatus was used to measure the experimental data. This apparatus was convenient for our work, since the reaction between methyl chloride and water to form methanol and hydrochloric acid becomes significant at 323-333 K. The flow apparatus also is suited particularly for overcoming the difficulties associated with accurately measuring the solubility of a minor component in a liquid phase, which include the adsorption of the minor component onto the surfaces of sampling tubing (Tsonopoulos and Wilson, 1983) and the absorption of the minor component into the polymer seals of liquid sampling valves (McCully et al., 1988). Liquid-liquid and vapor-liquid equilibria
A schematic of the experimental apparatus used for measuring liquidliquid and vapor-liquid equilibria is shown in Fig. 2. Unless otherwise noted, all fluid transfer lines are 316 stainless steel with an O.D. of 1.59 and an I.D. of 0.762 mm. For an experimental run, methyl chloride and water are delivered as compressed liquids by separate high-pressure feed pumps (Milton Roy, model no. 396). The combined constant flow rate from the two pumps ranged from 50 to 200 mL h-r. The inlet to the methyl chloride pump is cooled with a water-ice mixture to ensure the delivery of a liquid. A 500 ml sample cylinder (Whitey Co.) is used as a surge tank and serves to dampen pressure fluctuations caused by the pumps. After leaving the pumps, the two-phase mixture enters the equilibrium coil, which is used for
330 Wet-Test Meter
Cold Traps
Gas Flow Meter Oil Bath
rnwomererbng
Cold
Fig. 2. Schematic of equilibrium flow apparatus.
heating the mixture to the cell temperature. This coil consists of two 12 m sections of 1.59 mm O.D./0.76 mm I.D. tubing interconnected with a 3 m section of 3.18 mm O.D./1.59 mm I.D. tubing. The diameter change is used to promote mixing of the phases. An in-line, Type J thermocouple is used to monitor the temperature of the mixture exiting the equilibrium coil. The temperature of this mixture was always within 0.1 K of the contents of the view cell. After exiting the coil, the equilibrated, two-phase mixture enters the view cell, which functions as a phase separator. The lighter, methyl chloride-rich phase exits the top and the heavier, water-rich liquid phase exits the bottom of the cell. The methyl chloride-rich phase subsequently passes through a heated section of tubing to prevent condensation and is then expanded to atmospheric pressure across a micrometering valve (Autoclave Engineers, model no. 60VRMM). The valve is heated to prevent hydrate formation and the accompanying blockage that would occur during a Joule-Thomson expansion. The water-rich phase is expanded similarly through a micrometering valve, but no heating is required. After expansion, the effluent streams enter the sampling system and samples are collected. The methyl chloride-rich phase first flows through a series of cold traps that are maintained at O-2 IS above the normal boiling point of methyl chloride, which is 249.5 IL At these conditions, most of the
331
water in the mixture precipitates in the form of a methyl chloride hydrate and is collected. A small amount of liquid methyl chloride also occasionally forms and is collected. The volume of the gas phase exiting the cold traps is measured with a wet-test meter (Precision Scientific Co., model no. 63111). At the end of an experimental run, the cold traps are allowed to warm to ambient temperatures until only liquid water remains in the cold traps, The mass of this water is then determined. The equilibrium composition of the methyl chloride-rich phase in the view cell is then calculated by assuming that the collected liquid water is essentially pure and that the composition of the collected gas phase was in equilibrium with a hydrate phase. The expanded water-rich phase from the view cell flows into a trap maintained at ambient temperature where the liquid phase is collected. The gas phase exits the trap and is collected in a gas flow meter (Brooks Instrument, model no. 1056). Equilibrium compositions of the water-rich phase are then determined by assuming that the collected gas and liquid phases are in equilibrium. Henry’s law constants are needed for this calculation and are available in the literature (Glew and Moelwyn-Hughes, 1953). The virial equation was used to correct for non-idealities in both of the collected gas phases; the necessary pure-component second virial coefficients have been measured experimentally (O’Connell and Prausnitz, 1970) and the cross-coefficient was estimated by a method of Tsonopoulos (1974). The view cell is a liquid level gauge (Jerguson Co., model 11-T-20) with a modified cell body. The body was fabricated at Clemson from a block of Carpenter 450 stainless steel. Its fluid chamber has been machined to the original height and depth but is reduced in width from 1.6 cm to 0.95 cm. The cell windows are made of borosilicate glass (Corning Glass Works) mounted on Teflon gaskets. The equilibrium coil and the view cell are immersed in a silicone oil bath. The bath temperature is controlled to within +O.Ol K with a Yellow Springs model 71A temperature controller and thermistor, which are connected to a 750-W heating element. Thermal gradients in the bath are < 0.02 K. Two safety features are included in the apparatus. A 9.5mm thick polycarbonate shield surrounds the oil bath and protects the operator against a possible rupture of the view cell. All effluent streams containing methyl chloride are vented from the building. Liquid-liquid-vapor
equilibria
Minor modifications to the apparatus described above were necessary for measuring three-phase equilibria (see Fig. 3). To collect vapor phase samples, the feed is introduced into the methyl chloride-rich liquid (i.e. middle) phase to minimize possible entrainment in the vapor phase. The middle
332
* -
Vapor Phase to Sample Collection
Feed Middle Phase
* Aqueous Phase to Sample Collection
(4
. -l-Y-Middle Phase to Sample Collection
-.
Feed
1
t
Aqueous Phase
(b)
Fig. 3. Modifications to the apparatus for measuring three-phase equilibria: (a) configuration for collection of vapor and aqueous phases; (b) configuration for collection of methyl chloride-rich liquid (middle) phase.
phase is removed through an additional section of tubing located midway between the feed port and the bottom phase exit port (Fig. 3(a)). The flow rate of the middle phase is controlled with a micrometering valve (Hoke Inc., model no. 1666G2Y), which is heated to prevent hydrate formation during flashing. The aqueous bottom phase is collected in a similar manner. When collecting middle phase samples, the feed is introduced into the bottom aqueous phase to minimize possible contamination of the middle phase with entrained water droplets (Fig. 3(b)). Experimental
measurements
Operating temperatures of each phase in the cell and of the feed were measured with Type J thermocouples referenced to an ice-water bath. Thermocouple voltages were measured with a Keithley model 191 digital multimeter. The thermocouples and multimeter were calibrated as a unit against an NBS traceable mercury-in-glass thermometer and are believed to
333
be accurate to +O.l K. The feed, top and bottom phase thermocouples always agreed within *O.l K of each other. Owing to the presence of temperature fluctuations, which are inherent in the operation of an equilibrium flow apparatus, reported temperatures in the vapor-liquid and liquid-liquid regions are believed to be accurate to +0.2 K and in the liquid-liquid-vapor region to &OS K. Operating pressures were measured with two Bourdon tube-type Heise gauges (model CM, O-5000 psig range and model CM, O-1000 psig range) that had been calibrated against a Budenberg dead weight gauge (model 380H). Our ability to control the system pressure with the micrometering valves was dependent on the type of phase behavior that existed in the cell. Reported pressures are believed to be accurate to f 5 psi in the liquid-liquid region, kO.5 psi in the vapor-liquid region and + 2 psi at the three-phase line. Before beginning an experimental run, each phase exiting the view cell was checked for the presence of reaction products and for impurities, such as nitrogen and air, with HPLC sample injectors (Valco Instruments Co., product no. CI4W). These injectors (not shown in Fig. 2) are located in the oil bath in the tubing immediately exiting the top and bottom of the view cell. Samples were injected into a Hewlett-Packard 5890A gas chromatograph equipped with a 3.18 mm X 1.8 m nickel column packed with Chromosorb 107 and were analyzed with a thermal conductivity detector. At the highest operating temperature of 333 K, methanol was detected in the view cell if the contents were held statically in the cell overnight. However, after 5 min of operating the apparatus in the flow mode no methanol could be detected. No experimental run was begun until the contents of the cell were determined to be pure.
RESULTS AND DISCUSSION
In an earlier study we measured phase equilibria for the carbon dioxidewater system at 323.2 K with an equilibrium flow apparatus (Briones et al., 1987); comparison with accepted literature values indicated that reliable results were obtained. Thus, no additional measurements of a corroborative nature were made for this work. Equilibrium compositions for the methyl chloride-water system at 313.2 and 333.2 K are presented in Table 1 and also are shown on pressure vs. composition diagrams in Figs. 4-7. As expected, the measured liquid-liquid equilibrium data were found to be nearly independent of pressure, which is a good indication of the reproducibility of the data. For each experimental run, three to five samples were collected consecutively. Reported results
334 TABLE 1 Equilibrium
compositions
Pressure (bar)
for the methyl chloride-water
Mole fraction water in methyl chloride-rich phase
Mole fraction methyl chloride in water-rich phase
Liquid
Vapor
Liquid
0.00969 0.0105
0.00961 0.00966 0.00968 0.00973 0.00792
0.0146 0.0163
0.0103 0.0101 0.00986 0.00969 0.00930
313.2 K 69.96 35.49 9.29 8.67 a 7.22
0.00944 0.00936 0.00969 0.0101
333.2 K 69.96 35.49 16.18 14.25 a 12.73
0.0156 0.0157 0.0158 0.0157
a Three-phase
system at 313.2 and 333.2 K
line.
were obtained by averaging all samples for a given run and then by averaging all runs if more than one experimental run was made. This consecutive, cumulative sampling was found to be necessary to obtain good results. The technique helps to account for any two-phase flow existing in the sample tubing by inherently averaging the contents of the flow stream over time. To illustrate this point, all data used to obtain two of the
tvtoie
Fig. 4. Pressure
Fraction
Methyl
vs. composition
Chloride
diagram
for the methyl chloride-water
system at 313.2 K.
335 12
11
Mole
Fraction
Methyl
Chloride
Fig. 5. Enlargement of the methyl chloride-rich three-phase region at 313.2 K.
compositions in this work are presented in Table 2. Although some variation between samples is present, results from different experimental runs (and thus different days) are highly reproducible. Compositions of the minor component are believed to be accurate to + 3% of the values reported in Table 1 for both the methyl chloride-rich and water-rich phases. Enlargements of the methyl chloride-rich three-phase region are shown in Figs. 5 and 7 and indicate that the vapor phase always equilibrium
1, L-L
I
0
,
/’
I
I
I’
L-V
,I
v
,1’
,I 0.01
Mole
0.02
0.98
Fraction
Methyl
1.01 Chloride
Fig. 6. Pressure vs. composition diagram for the methyl chloride-water
system at 333.2 K.
Mole Fraction Methyl Chloride
Fig. 7. Enlargement of the methyl chloride-rich three-phase region at 333.2 K.
more methyl chloride than the methyl chloride-rich liquid phase. Although the differences in composition between the vapor and liquid phases are small, the reproducibility of the experimental runs used to generate these points was better than f l/2% of the values reported in Table 1. We have concluded, therefore, that the observed differences are significant, and that the methyl chloride-water binary system forms a heterogeneous azeotrope of the second kind (Fig. l(b)). In addition, since the vapor pressure of methyl chloride occurs at essentially the same pressure as the three-phase line, the system also may exhibit a maximum pressure (or minimum-boiling) homogeneous azeotrope as depicted by the dashed lines in Figs. 5 and 7. Holldorff and Knapp (1988) measured liquid-liquid equilibria for the methyl chloride-water system at - 312 and - 315 K; we have interpolated their results to 313.2 K for comparison. Our results for the methyl chloriderich liquid at 9.29, 35.49 and 69.96 bar are in good agreement with their reported value of 0.96 mol% water. For the water-rich liquid, our results are low relative to their reported value of 1.13 mol% methyl chloride. Holdorff and Knapp (1988) also estimated the composition of this phase by extrapolating the Henry’s law gas-liquid solubility curve to the three-phase line and obtained a value of 1.07 mol%. As shown in Table 1, there is virtually no scatter in our results for this phase; we do not have an adequate explanation for the discrepancy between our and the previous authors’ work. contained
337 CONCLUSIONS
Liquid-liquid, liquid-liquid-vapor and vapor-liquid equilibrium compositions have been measured for the methyl chloride-water system at 313.2 and 333.2 K. Measurements at the three-phase line indicate that this system forms a heterogeneous azeotrope of the second kind. Therefore, methyl chloride is not a suitable entrainer for water in an azeotropic distillation process for dehydrating organic-water mixtures.
TABLE 2 Experimental runs for the methyl chloride-water
system at 313.2 K and 69.96 bar
Mole fraction water in methyl chloride-rich liquid phase
Mole fraction methyl chloride in water-rich liquid phase
Run 1
Run 4 0.00822 0.00927 0.0109 0.00960 0.00927
0.00905 0.00965 0.00989 0.00977 0.00999 0.0100 0.00981
Mean 0.00945 Run 2
Mean 0.00974 0.00977 0.00917 0.00940 0.00960 0.00931
Run 5 0.00904 0.00989 0.00972 0.00960 0.00977
Mean 0.00945
Mean 0.00960
Run 3 0.00925 0.00952 0.00950 Mean 0.00942
Run 6 0.00887 0.00937 0.00963 0.00961 0.00926 0.00984 0.00972 0.00973 Mean 0.00950
338 ACKNOWLEDGEMENTS
The partial financial support of Hoechst-Celanese is gratefully acknowledged. The authors are indebted also to Carl Howell from Hoechst-Celanese of Charlotte for his helpful discussions.
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