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T. J. RIGAS’, D. F, MASON, and GEORGE THODOS The Technological Institute, Northwestern University, Evanston, 111.
Vapor-Liquid Equilibria Microsampling Technique Applied to a New Variable-Volume Cell Basic parts of this unit are: Equilibrium cell into which the system to be studied is charged and from which samples of coexisting phases are withdrawn Expansion system providing for expansion of the high pressure samples into sample bulbs for analysis Charging system with which components of system are introduced into equilibrium cell Pressurizing system used to adjust and measure pressure within equilibriumchamber of cell Vacuum system to allow evacuation of parts of equipment
CONSIDERABLE
experimental background on vapor-liquid equilibrium relationships is available in the literature. Additional information is ’desirable for better understanding of phase behavior, particularly for systems of more than two components a t elevated temperatures and pressures. Data can be obtained for binary systems more easily by means other than direct measurement of composition. Ternary and higher order systems require the measurement of temperature, pressure, and composition of both phases, in accordance with Gibbs’s phase rule. The study of more than two components is tedious and time-consuming. To minimize the effort involved in obtaining such measurements, it is desirable to explore a wide range of temperature and pressure with a minimum of material charging and handling. This objective has been accomplished by the design and construction of a variable-volume cell from which minute samples of the vapor and liquid phases can be withdrawn. The cell is capable of handling multicomponent systems at elevated temperatures and pressures with minimum time and effort.
The equilibrium and mercury chambers are separated by an annular movable piston (Figure 4) extending from the outside of the hollow cylinder to the inside diameter of the main body. Grooves on this piston accommodate two rubber O-rings: One effects a running seal on the outside surface of the hollow cylinder, while the other effects a similar seal on the inside cylindricaI surface of the main body. A Teflon guide attached on top of the piston permits it to move smoothly inside the main body. The stirring mechanism consists of a magnetic stirrer within the hollow cylinder and a magnetic drive outside the main body (Figure 5 ) . The magnetic stirrer is pivoted in a conical indentation of a perforated support at the bottom of the equilibrium chamber. The upper support of the stirrer is effected with a spider-web type bearing inside the upper section of the hollow cylinder. T o ensure proper mixing, four two-bladed fans are located along the stem of the stirrer. A small Alnico magnet with its axis perpendicular to the stem is press-fitted into the lower end of this unit. This stirrer is rotated by magnetically coupling it to an externally located rotating magnetic drive, which consists of a split ring made of high permeability iron and supported on ball bearing tracks outside the main body.
system to be investigated is charged and brought to thermodynamic equilibrium and from which small samples of the coexisting liquid and vapor phases are withdrawn for analysis. The cell consists of the main body, the head, and the cap, and when assembled is almost 10 inches high (Figure 3). The outside diameters of the main body and cap are approximately 4 and 5 inches, respectively. High pressure seal of the enclosure is effected with a Teflon gasket compressed by a tongue and groove arrangement between the head and the main body. The cap fits over the head, threading on the outside of the main body. Compression of the Teflon gasket is effected by eight a/8-inch Allen bolts spaced symmetrically on the head and acting through a compression ring to distribute the stresses uniformly. The central section of the main body consists of a hollow cylinder provided with radial openings, inwardly inclined, to allow for adequate communication and mixing of the material inside and outside the cylinder. A groove at the top of the cylinder accommodates a rubber O-ring which provides a low pressure differential seal between the mercury and equilibrium chambers. T o facilitate construction, the hollow cylinder was machined separately and press-fitted into the base to become an integral part of the main body. Mercury
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Experimental Equipment
The experimental unit is diagrammatically presented in Figure 1. Equilibrium CeU. The most important part of this unit is the equilibrium cell (Figure 2) into which the 1 Present address, Visking Co., Union Carbide Corp., Chicago, Ill.
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6Charqa lo Unit
Figure 1. This experimental unit has been tested and found reliable for procurement of vapor-liquid equilibrium data of multicomponent mixtures VOL. 50, NO. 9
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Diametrically opposed, two .4lnico magnets are press-fitted into this ring with unlike poles pointing toward the center. When finally assembled, the magnets of the stirrer and drive are at the same level and effect a coupling through the cell wall, permitting rotation of the stirrer whenever the drive is mechanically rotated. Four valves are incorporated into the
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equilibrium cell for withdrawal of vapor and liquid samples. Each consists of a threaded stem, to which is attached a nonturning needle-type plunger that effects a seal against a port opening in the cell body. The plunger and the stem are connected with a brass sleeve which permits translation of the rotational motion of the stem into an axial movement of the plunger. A Teflon
INDUSTRIAL AND ENGINEERING CHEMISTRY
gasket between two steel followers is compressed by a gland nut to give a tight seal against the surfaces of the plunger and cell body. This design permits lubrication of the threaded parts without danger of contaminating the contents of the equilibrium chamber and samples. Liquid and vapor samples are withdrawn and trapped between valves CL-1 and CL-2 and CV-1 and CV-2, respectively. For control and measurement of temperature, four thermocouple wells are provided, which extend within '/4 inch of the liquid sample withdrawal port. Two are located in the main body and the other two in the head. The bulk of the equilibrium cell was constructed from stainless steel, Types 304 and 416. T o minimize binding, these materials were chosen so that no rubbing contacting surfaces were of the same material. The split ring of the magnetic drive was made of highpermeability iron, the compression ring of hardened tool steel, and the plungers of the valves of hardened stainless steel, Type SAE 51440-F. Expansion System. This is used for expanding the high pressure vapor and liquid samples trapped in the equilibrium cell into sample bulbs. The liquid phase sample is expanded through valve SL-1 into a mercury-filled expansion bulb, 125 ml. in volume, connected with rubber tubing to a leveling bulb. A portion of the expanded sample is then transferred to a sample bulb through valve SL-2 and taken for analysis. The high pressure vapor phase sample is expanded through valve SV-1 into a 500-ml. expansion bulb and introduced through valve SV-2 into another sample bulb. Charging System. A known quantity of pure materials can be introduced into the equilibrium cell by means of this system. Its essential components are a mercury displacement pump and a charging cell. The charging pump is of the mercury displacement type, manufactured by the Ruska Instrument Corp., designed to withstand pressures up to 10,000 p.s.i. and calibrated to 0.01 rnl. This pump enables the transfer of mercury from the mercury reservoir into the 80-ml. charging cell. A Dewar flask surrounds this cell, permitting liquefaction of the charged material whenever necessary. A pressure gage (2000 p.s.i.) between the pump and the charging cell indicates the pressure in that part of the system. Through valves F-l and F-2, the feed components are introduced into the evacuated charging cell and then forced by mercury displacement through valve F-3 into the equilibrium cell. An electrical contact indicates when mercury has reached a certain level above the charging cell-a precaution against entry of mercury further into the system. Pressurizing System. This part of the unit provides means for indicating the operating pressure and for varying
V A R l AB L E-VO LUME C E L L
Figure 3. The main parts of the equilibrium cell consist o f t h e main body, head, and cap
the pressure within the equilibrium chamber. It includes a mercury volumetric pump calibrated within 0.01 ml. (Ruska Instrument Corp.), and rated at 25,000 p.s.i. This section of the system is filled with mercury, which can be transferred by this pump into and out of the mercury chamber of the equilibrium cell. This operation changes the equilibrium chamber volume and consequently the pressure of the system. Two Heise gages (750 and 7500 p s i . ) indicate the pressure of the system. The 750-p.s.i. gage can be disconnected by means of valve M-4 when pressure becomes excessive. These gages can be calibrated with a deadweight gage connected to the system through M-6. Vacuum System. This system provides for evacuating the necessary parts of the unit as needed. A two-stage mercury diffusion pump connected to a mechanical fore-pump is capable of producing pressures considerably less than 1 micron. The pressure in the system is determined with a mercury manometer and a thermocouple gage. A mercury trap prevents entry of mercury into this system. An air thermostat is provided for temperature control of the equilibrium cell. The thermostatic enclosure consists of an inner Transite shell completely surrounded by Fiberfrax insulation, Strip heaters, mounted inside the Transite walls, provide the necessary heat to maintain a constant temperature. Power to these heaters i s derived from a motorcontrolled Variac. Temperature control is obtained by a Minneapolis Honeywell ElectroniK proportional controller and recorder. Control can be maintained with respect to any of five thermocouples, four of which are in the cell proper and one in the air bath surrounding the cell. Temperatures can be measured within 0.2'F. and control can be effected within 0.5'F.
Figure 4. The assembly of the piston included a Teflon guide kept in place by a retaining ring with four screws and O-rings located on the inside and outside perimeters
Operation of Unit
To charge the cell, the equilibrium chamber and charging system are evacuated. The procedure for introducing these substances into the cell varies with the type of system and properties of the components. I n general, each component is liquefied in $he charging cell by means of a proper cooling bath. Then a definite volume is displaced into the equilibrium chamber. The temperature of the equilibrium cell is maintained and controHed ar a desired level and the pressure adjusted by the pressurizing mercury pump. To facilitate attainment of equilibrium, the contents of the equilibrium chamber are agitated with the magnetic stirrer. Equilibrium is assumed to exist when the pressure of the system has remained constant for 0.5 to 1 hour. T o reach this condition, 2 to 4 hours are normally required. Samples of the coexisting liquid and vapor phases are trapped in the evacuated space between valves CL-1 and CL-2, and CV-1 and CV-2. The volume of the withdrawn liquid sample is 0.20 ml. ; that of the vapor sample is 0.75 ml. Equilibrium chamber volume can vary from 18 to 120 ml. Samples can be withdrawn and isolated very rapidly (within 3 seconds). During sampling, stirring is discontinued to avoid possible splashing of the liquid on the seat of the cell vapor valve, CV-1. Although pressure decreases slightly during withdrawal, the trapped vapor and liquid are representative of the original conditions, as sample volumes are small in comparison to the equilibrium chamber volume, and time of withdrawal is small. Each trapped sample is expanded into the evacuated sample expansion system. The pressure in this system must be low enough to ensure that each sample exists only in the gaseous phase. Sufficient
time is allowed for complete mixing of each sample. This process can be somewhat expedited by manipulating the mercury leveling bulbs. Portions of the homogeneous gas samples are then transferred into the sample bulbs for analysis. The present sample expansion system should be capable of handling systems containing components as heavy as nonanes. The composition of each sample is determined by mass spectrometric analysis. Pressure of the system can be conveniently changed with the pressurizing pump. After withdrawal of the coexisting liquid and vapor samples, the pressure in the equilibrium chamber is changed and the system is allowed to reach equilibrium a t the new pressure.
Figure 5. Agitation within the cell i s made with this stirring device consisting of a mechanically operated drive that couples magnetically to a rotating stem provided with two-bladed fans VOL. 50, NO. 9
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---- r(uHIIts of Sage, Hicks and Lacey - results of this investigation
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results of Sage, Hcks and Locey results of this investigation
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ew Pressure, psi0 Methane Mole Fraction
Figure 6. The dependability of data resulting from the experimental unit has been tested on the methane-n-butane system, for which data are reported in the literature
Figure 7. Vapor liquid equilibrium constants calculated from the experimental data are in close agreement with those reported by Sage, Hicks, and Lacey
I t is possible to obtain a series of equilibrium points from a single initial charge, and to add materials a t any time, to vary the over-all composition of the system.
The equilibrium cell and charging system were evacuated to less than 1 micron, purged with methane, and reevacuated. Methane was charged as gas at 1000 p.s.i.a.; n-butane was first liquefied with an ice-water bath and charged as liquid. The over-all composition of the charge in the cell was approximately 26.5% n-butane, based on literature PVT data for methane ( I ) and a generalized liquid density correlation (3) The temperature of the cell was known and controlled to within zk0.2° F. with a maximum temperature gradient of 0.1 'F. between the liquid and vapor sampling ports. The pressure measurements were accurate to rt2 p.s.i. Analyses of liquid and vapor phases were correct to 0.005 mole fraction. Thermodynamic equilibrium was assumed to exist when the pressure of the system remained constant for 1 hour. Depending on the magnitude of pressure change imposed on the system
System Methanen-Butane at 100' F.
The binary system methane-n-butane was investigated a t 100' F. and at pressures ranging from 960 p.s.i.a. to the critical pressure of the system, to test the reliability of the equipment and the experimental technique. This particular system was selected because reliable data are available (2). The methane and n-butane were supplied by the Matheson Co., Inc. The methane was reported to be of C.P. grade, 99.3% purity, while the n-butane was instrument grade, 99.7% purity. Mass spectrometer analysis indicated actual purities to be equal to or better than those reported by the supplier.
Table 1.
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Comparison of Results of This Investigation and Those of Sage, Hicks and Lacey (2) Liouid Phase. Mole Fraction Methane Smoothed Values This Exptl. invest. (2) 0.311 0.302 0.311 0.375 0.388 0.388 0.470 0.457 0.476 0.523 0.537 0,538 0.558 0.568 0.553 0.634 0.658 0.652 0.677 0.656 0.677 0.735 0.723
Vapor Phase, Mole Fraction Methane Equilibrium Constants Smoothed T'alues MethThis Pressure, ane n-Butane Exptl. invest. P.S.I.A. (8) 2.83 0.176 0.879 0.879 0,879 960 2.26 0.199 0.878 0.880 0.878 1185 1.82 0.252 0.871 0.863 0.868 1432 1.58 0.323 0.853 0.847 0.850 1617 1.49 0.378 0.846 0.834 0.833 1692 1.23 0.566 0.800 0.811 0.806 1833 1.16 0.663 0.786 0,800 0.786 1861 0.735 1895a 0.723 1912b Critical composition and pressure Critical composition and pressure, this investigation. reported by Sage, Hicks, and Lacey (2).
and proximity to the critical pressure, 2 to 4 hours was required to assure equilibrium. During equilibration, the contents of the equilibrium chamber were agitated with the magnetic stirrer rotating intermittently a t approximately 100 r.p.m. to keep the temperature gradient between the liquid and vapor sampling ports within 0.1 O F. Samples of the coexisting phases ai seven pressures: ranging Krom 960 to 1861 p.s.i.a., were obtained and their composition was determined by mass spectrometer analysis (Table I). A pressurecomposition diagram for this system at 100' F. is shown in Figure 6. Vapor-liquid equilibrium constants resulting from the experimental data are shown in Figure 7. Critical pressure and composition were established by extrapolating the equilibrium constant curves to unity at a pressure consistent to that resulting from the extrapolation of the pressure-composition curves. This procedure produced a critical pressure of 1895 p.s.i.a. and a critical composition of 0.735 mole fraction methane. Experimental and smoothed values from this investigation are compared with smoothed data reported by Sage, Hicks, and Lacey (2) (Table I). Average deviation of experimental points from smoothed values was 0.003 mole fraction for the liquid phase; of the vapor phase, 0.002 mole fraction. The pressurecomposition curves of this investigation and those of Sage, Hicks, and Lacey (2) agree closely a t the lower pressures and gradually reach a maximum deviation a t the critical point. The critical point reported by Sage and others is 1912 p,s i.a. and 0.723 mole fraction methane, compared to 1895 p.s.i.a. and 0.735 mole fraction methane for this investigation. This difference could well result from small traces of ethane present in the methane used in these studies. This unit is reliable, and convenient for obtaining vapor-liquid equilibrium data. Although the cell is limited to a maximum operating temperature of 300' F., this limitation can be extended by use of gaskets capable of withstanding higher temperatures. The single feed charge feature permits determination of a number of experimental points with minimum effort and time. Acknowledgment
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The authors thank the National Science Foundation for financial assistance that made this work possible. literature Cited (1) Olds, R. H., Reamer, H. H., Sage, B. H., Lacey, W. N., IND. ENG.
CHEM.35, 922 (1943). Sage, B. H., Hicks, B. L., Lacey, W. N., Ibid.,32,1085 (194C). (3) Watson, K. M., Ibid., 35, 398 (1943). RECEIVED for review December 27, 1957 (2)
ACCEPTEDApril 14, 1958
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