Sample-Extrusion Apparatus for High-pressure Vapor-Liquid Equilibria Compositions and Densities at Pressures up to the Critical B. 1. Rogers and J. M. Prausnitz University of California, Berkeley, Calif.
A static equilibrium apparatus has been developed for measuring compositions and densities of both phases in high-pressure vapor-liquid equilibria. The apparatus operates at pressures up to 1000 atm and temperatures from -50' to +150' C. Samples for density and composition measurements are extruded from the equilibrium cell without loss of pressure. Gas chromatography is used for chemical analysis.
THIS was undertaken to produce composition and density isotherms a t pressures up to the critical for both WORK
vapor and liquid phases of selected binary systems. Toward this end an experimental apparatus has been designed, built, and operated; it features a novel sampling technique and recently developed instrumentation (Hunt, 1962; Rogers, 1969; Rolz, 1963; Shcrwood, 1964). General Description
A schematic diagram of the complete apparatus is shown in Figure 1. It was designed to measure conipo4tions and densities a t pressures up to 1000 atni and temperatures from - 50' to 150' C. Pressure arid temperature are measured directly in the equilibrium cell, while composition and density measurements are made on samples removed from the high-pressure zone to a low-pressure analysis system. The following discushn describes the apparatus and experimental procedures developed for a study of the argonneopentane system. More detailed information on the apparatus and its operation is available (Rogers, 1969).
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T o avoid the problem of pressure-sealing a rotating shaft, the contents of the cell are agitated with a magnetic stirrer. The stirrer paddle (Figure 2) makes a 120-degree sweep of the cell and is designed to stir both phases, to agitate the vapor-liquid interface without splashing liquid up into the region near the vapor-sampling pistons, and to dislodge any vapor bubbles near the liquid-sampling space. The only areas not reached by the stirring action are the short lengths of tubing leading to the pressure-measurement and cellloading systems. T o avoid withdrawal of a two-phase sample, and to aid in loading the cell, the position of the vapor-liquid interface must be known. The interface is located by a movable thermistor whose response depends on whether it is in the vapor or the liquid phase. Pressure and Temperature Measurement
Pressure in the equilibrium cell is measured with a floating piston gage (Aminco Model 47-2211) used in conjunction with a strain gage, bidirectional, differential pressure transducer (Stathani Model I'hI385TC f 5-350) (Figure 3). The cell pressure is determined by adjusting the piston gage pressure until it is equal to the cell pressure, as indicated by a strip-chart recorder. The controller-power supply for the transducer is a Leeds & Northrup strain-gage module
Equilibrium Cell and Sampling
The equilibrium cell is machined from a stainless steel block and has a high-pressure chamber with a volume of approximately 150 cma. Figure 2 shows the cell together wit.h the sampling system. The sampling system removes samples from the cell to the low-pressure analysis system via two sets of moving pistons. A hydraulic drive moves the sampling pist'ons back and forth. Each set of pistons, one for liquid sampling and t'he other for vapor sampling, contains two pistons, between which is a small, variable volume. During sampling, this volume is extruded from the cell into a cylinder and nioves down the cylinder until the sample ports are reached ; the sample then expands through capillary t,ubing into the low-pressure zone. This technique has the priniary advantage that the equilibrium cell pressure is not disturbed during withdrawal of the samples. A tight pressure seal around the moving pistons is maintained by the use of specially fabricated O-ring seals made of Rulon LD, a heavy-duty fillet1 Teflon material (Dixon Corp.). 174
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LOW PRESSURE ANALYSIS SYSTEM
EHYDRAULIC
Figure 1 .
C E L L TEMPERATURE
Schematic of equilibrium apparatus
LIOUID
S A M P L E ~~I L - V A P O RSAMPLE
44
C A P I L L A R Y LINE T O LOW P R E S S U R E A N A L Y S I S SYSTEM
CAPILLARY LINE
END C R O S S - S E C X
SIDE CROSS-SECTION
Figure 2.
Equilibrium cell and sampling system
GE UT
TRANSDUCER
VESSEL
Figure 3.
Pressure measurement and cell-loading system
CENTRIFUGAL PUMP
Figure 4.
Thermostat system for equilibrium cell
(Model 099025). The transducer and the short tube leading from the equilibrium cell to the transducer are inside the thermostated zone. Pressure in the cell is measurable to within 1-psi accuracy. The equilibrium cell temperature is determined with four copper-constantan thermocouples embedded in either end and a t the top and bottom of the cell. The thermocouple voltage is measured with a potentiometer (Leeds BE Northrup Model K-3) in conjunction with an electronic null detector.
Cell Loading
The cell is first charged with a desired quantity of liquid neopentane and then argon is added from a high-pressure generator until the desired cell pressure is reached. The cell-loading system is shown in Figure 3. Neopentane is charged by heating the neopentane bottle with an air gun and driving the liquid into the evacuated cell. High-pressure argon is produced by liquefying argon VOL. 9 NO. 1 FEBRUARY 1970
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MERCURY RESERVOIR FOR SAMPLE PRESSURE AOJUSTMEkT
M E R C l J R Y RESERVOIR
Figure I
I
I
I
I
I
5. Low-pressure analysis system I
0
40
Figure 7.
200
240
Argon-neopentane equilibria at
50' C
80 120 PRESSURE
,
160 otm
Vapor and liquid molar densities
Figure 6.
Argon-neopentane equilibria at
50" C
Vapor and liquid compositions
gas with liquid nitrogen in a pressure generation vessel which is then heated inductively to room temperature. This method of pressure generation avoids contamination which results from direct mechanical compression. Aminco fittings, valves, and tubing are used throughout the highpressure system. Thermostat Sysiem
The temperature of the equilibrium cell is controlled by placing it in a therniostated enclosure (Figure 4), through which liquid from a temperature-controlled bath is pumped. The enclosure serves both as a thermal shield and a safety shield and is removable via a chain hoist to permit access to the cell. The inside of the enclosure is constructed of copper plates to promote temperature stability. Fiberglas insulation is inserted between the steel exterior of the enclosure and the copper plates. The equilibrium cell is mounted on tubular stainless steel supports to minimize heat conduction. An 176
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additional copper plate is placed between these supports. Heat-transfer fluid lines are hard-soldered to the copper walls of the interior. The bath contains both heating and cooling units. Cooling may be achieved with either a refrigeration unit (l'/Z-hp compressor) or water-cooling coils. Heating is achieved with several resistance heaters, one of which is connected to a Hallikainen Resistotrol temperature controller. To reduce thermal cycling and the time required to reach equilibrium, the output of this heater varies with the length of time that the Resistotrol is on. A reversing motor connected to a variable transformer links the heater to the Resistotrol. A l'/rhp centrifugal pump agitates the bath fluid and pumps it through the cell enclosure. This system controls the cell temperature to ~ k 0 . 0 5C.~ Nitrogen is fed continuously into the thermostated enclosure to avoid formation of an explosive mixture, should a leak develop in the cell. low-Pressure Analysis
Figure 5 shows the system for determining the density and composition of the samples removed from the equilibrium cell.
The samples are received by two sample bulbs, one for the liquid sample and another for the vapor sample. These bulbs are held in a constant-temperature water bath and have calibrated volumes. Any two of several calibrated bulbs can be used to give a desired sample pressure. Pressure in the sample bulbs is determined with a mercury manometer. With the pistons in sampling position the total low-pressure volume consists of the sample bulb volume, the sample volume extruded from the cell, and the volume of capillary tubing between the cell and the sample bulb. All three volumes are known from calibration. With this volume information, the respective temperatures of each volume, and the sample-bulb pressure, it is possible to calculate the number of moles of sample extruded using a truncated virial equation. The density of the contents of the cell can then be calculated, since the high-pressure sample volume is known from calibration. Once the density has been determined, composition measurements are made using on-line gas chromatography. The sample pressure is adjusted to a desired value with a variablevolume sample bulb in conjunction with the manometer. The chromatograph unit is a dual-column Varian Aerograph (Model 1520) with a Varian Aerograph (Model 477) digital integrator and printer. The chromatograph response is monitored with a strip-chart recorder. Both composition and density measurements are accurate to =tl%. The vacuum system for the complete apparatus consists of a mechanical vacuum pump used with a mercury diffusion pump. Prior t o charging or transfer operations, vacua of less than 5 microns are maintained.
Illustrative Experimental Data
Figure 6 shows a composition isotherm a t 50' C for the argon-neopentane system, while Figure 7 gives the corresponding density isotherm. At 50' C the critical parameters were: Pressure. 248.0 atm Composition. 7 3 . 5 mole yo argon Density. 11. 4 g moles/liter Acknowledgment
The authors express their gratitude to the National Science Foundation and the donors of the Petroleum Research Fund for financial support, and to previous workers who have participated in the long development of the project: A. E. Sherwood, C. E. Hunt, C. Rolz, D. R. Winterhalter, J. G. Dorward, and L. N. Lucas. literature Cited
Hunt, C. E., M.S. thesis, Department of Chemical Engineering, University of California, Berkeley, 1962. Rogers, B. L., unpublished report, Department of Chemical Engineering, University of California, Berkeley, 1969. Rolz, C., M.S. thesis, Department of Chemical Engineering, University of California, Berkeley, 1963. Sherwood, A. E., Ph.D. dissertation, Department of Chemical Engineering, University of California, Berkeley, 1964. RECEIVED for review February 26, 1969 ACCEPTEDAugust 14, 1969
Determination of Specific Volume of Polymer Melts K. K. Chee and Alfred Rudin Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada
Specific volumes of polymeric liquids can be measured accurately and conveniently using a thermostated cylindrical vessel, such as the barrel of a laboratory extrusion rheometer. A machinist's dial gage is used to measure the height of a column of weighed polymer confined under a weighted piston. Errors due to entrapped air are avoided b y using compression-molded specimens and a polytetrafluoroethylene piston head. A single measurement can be completed in about 30 minutes, and the data compare well with those from standard techniques.
SPECIFIC of polymeric liquids are generally measured by dilatometry (Bekkedahl, 1945), although a number VOLUMES
of other methods have been described. The latter techniques usually yield compressibility figures along with specific volume data and require specially constructed or rather expensive equipment (Foster et al., 1966; Hellwegge et al., 1962; Heydemann and Guicking, 1963; Parks and Richards, 1949; Rehage and Breuer, 1967). The present simple procedure using apparatus generally available in plastics laboratories completes a specific volume measurement in about 30 minutes and allows rapid, convenient temperature changes. The method does not differ in principle from that of Terry and Yang (1964), who used a universal testing machine and were able to obtain compressibilities as well as ambient pressure specific volumes. Our results seem to agree rather more closely with those from dilatometry, but this may not be a fair conclusion because the data available for comparison are very limited.
The apparatus (Figure 1) employs the barrel of a compressed gas-driven melt rheometer, in which the temperature can be controlled to 0.2"C (Bagley, 1957). The rheometer is used as a thermostat, rather than for extrusion, and any other extrusion viscometer or cylindrical vessel would be equally useful, with adequate temperature control. The diameter of the melt reservoir in the present apparatus is 1.5s cm and its length is 20.4 cm, a t room temperature. The rheometer barrel is stabilized a t the required temperature and then loaded with two polytetrafluoroethylene plugs, surmounted by a weighted steel piston. The weights may he varied, but the heaviest convenient load is about 10 kg, which corresponds to a pressure of 5 atm in the present apparatus. The plugs have flat end surfaces and fit the barrel snugly a t room temperature. Their lengths, about 1.5 cm, are not critical. After a IO-minute equilibration period the position of the top of the piston is registered with a dial gage. This gage, which can be read to 0.0002 cm, must be clamped firmly to the main stand of the apparatus. The dial gage is swung out of the way, the weights and piston are removed, and the upper polytetrafluoroethylene plug is removed by pushing upward with a wooden dowel. This VOL. 9 NO. 1 FEBRUARY 1970
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