I
L. WARREN BRANDT and LOWELL STROUD Helium Activity, Bureau of Mines, U. S. Department of the Interior, Amarillo, Tex.
Phase Equilibria in Natural Gas Systems Apparatus with Windowed Cell for 800 P.S.I.G. and Temperatures to -320” F. This apparatus can be operated at any temperature from room temperature to -320” F. Equilibrium vapor and liquid samples can be obtained at conditions near the dew and bubble points of the system studied, without danger of contaminating one phase with the other T H E technical literature contains many references to methods for determining vapor-liquid equilibria in systems a t elevated temperatures ( 2 , 3 ) . The methods usually employ a vapor-recirculating equilibrium still of the Othmer type, or a modification thereof, for studies of vapor-liquid equilibria in materials that are liquids at ordinary temperatures, although some workers have studied gases and gas mixtures in the low-temperature range-i.e., down to -320’ F. Bloomer and Parent (5) have described the Institute of Gas Technology phaseequilibrium apparatus and reviewed apparatus of other workers in this field. During studies of the vapor-liquid equilibria of helium-bearing natural gases a t this laboratory, a windowed equilibrium cell was needed, especially to obtain samples of equilibrium vapor and liquid phases near the dew point and bubble point of the system under study. With a nonwindowed cell, one phase may be contaminated with the other, unless special precautions are taken. Although several “optical” cells, which permit visibility of their contents during experimentation, have been described (4, 5, 9, 7 7 , 79), none were directly applicable to this study. Cells of glass capillary tubing, containing steel balls to permit magnetic stirring, utilize the dew point-bubble point method to obtain data and therefore are limited to two-component systems. For calculations in the present work the natural-gas systems are considered as four- and sixcomponent systems; so this method was not applicable. Other cells, nonoptical in construction (7, 70, 73, 75, 77, 78), but providing devices to determine liquid level and to stir the cell contents, were considered too complex. Most of these cells could not be used in the temperature range desired, because their use of mercury limits them to temperatures above -40’ F.
A phase-equilibrium apparatus was designed and built in this laboratory. The work was undertaken to provide an apparatus incorporating a windowed cell, a stirring device, and a temperaturecontrol system to permit obtaining lowtemperature phase-equilibrium data more rapidly and conveniently than was previously possible in the range from room temperature to - 320’ F. (limited by the boiling point of the coolant used) and a t pressures to 800 p.s.i.g. The apparatus follows, with slight modification, the “once-through flow” method of Steckel and Zinn (76) for determining vapor-liquid equilibria. Constant-pressure conditions during sampling are maintained by supplying additional feed gas to the cell while samples are being removed; this gas is supplied through a pressure regulator upstream from the cell, and slow flow rates are maintained during sampling. It is unnecessary to maintain the liquid in the cell at a certain level, because with the flow rates utilized in the apparatus, the compositions of the equilibrium phases are essentially unaffected by slight changes in the liquid level in the cell. The apparatus has been utilized a t this laboratory in studies of heliumbearing natural gases over a 30-month period and has proved satisfactory in every respect. A new apparatus of similar design, but with some constructional improvements over the present cell, utilizes a windowed cell for working pressures to 4000 p.s.i. Phase-equilibrium data obtained in these investigations will be presented later.
Description of Apparatus Figure 1 is a front view of the completed appatatus. The unit is completely self-contained, requiring for operation only a 110-volt power source and a 15-liter container of liquid air or
liquid nitrogen to supply cell cooling. The apparatus has a windowed cell, pressure- and temperature-control equipment, and a Thyratron-controlled circuit to permit stirring the cell contents during experimentation. A trap containing potassium hydroxide pellets removes water vapor and carbon dioxide from the feed gas when necessary. In the interest of safety, the cell contents are observed by reflection of the illuminated cell in a mirror, with the cell facing away from the observer a t an angle of 150’. The viewing window in the front panel is Lucite l / ~inch thick. Also provided, but not visible in Figure 1, are lines for sampling the equilibrium liquid and vapor phases, and a drain line to permit rapid emptying of liquids from the cell when cell conditions are changed for a new experiment. The entire apparatus is assembled in a cabinet of the relayrack type, mounted on casters for easy movement. Over-all dimensions are 17 X 44 X 85 inches. Gas-Flow Diagram. The gas first enters trap A (Figure 2 ) , where carbon dioxide and water are removed; then the pressure of the gas is regulated by the pressure regulator, B (Grove regulator, Model 94, Grove Regulator Go., Oakland, Calif.). This regulator maintains the pressure within 0.5% of the desired value. Valve 1 is used for measuring the gas pressure by means of an auxiliary deadweight gage and also for calibrating gages C, D, E, and F. The full-scale pressure ranges of these gages vary from 1500 p.s.i.g. at C to 300 p.s.i.g. a t F, and have an accuracy 0.5% of their full-scale ranges. This arrangement, with valves 2 to 5, permits reading the cell pressure on the gage having the appropriate range for the greatest accuracy. The gas enters equilibrium cell G through valve 6. H is the clearglass Dewar flask surrounding the cell when the apparatus is in operation. VOL.
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Figure 1. The apparatus is completely selfcontained A.
Gage panel Pressure regulator (mounted behind panel) C. Switches controlling coolant-transfer circuit D. Valve admitting gas to equilibrium cell E. Temperature reiorder-controller F. Door providing access to an ice bath for thermocouple reference junction G. Switch for auxiliary measurements of cell temperature H. Viewing window 1. Temperature-control relay circuit K. Stirring circuit L. Variable voltage transformers for adjusting heating and cooling rates M. Blower control
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Figure 3. The experimental cell chamber, of rectangular cross section, is machined from brass stock
GAS INLET
Figure 2. Equilibrium liquid and vapor phases are sampled after through this gus-flow system
Valve 7 allows sampling of the vapor phase. The vapor-outlet line is copper inch in outside diameter. tubing 3//16 Valve 8 controls the liquid sampling line, which is l/le-inch stainless steel capillary tubing, 0.031 inch in inside diameter. Capillary tubing prevents fractionation of the liquid phase during sampling. Valve 9 controls the liquid drain line,
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which is copper tubing ‘/8 inch in outside diameter. Valves 7 , 8, and 9 are mounted on the right side of the cabinet, for easy sampling. Cell, The experimental cell chamber of rectangular cross section, 11/, inches deep by 3/4 inch wide by 7 ’ / 2 inches long, was machined from brass stock. The face plate and pressure glass were
INDUSTRIAL AND ENGINEERING CHEMISTRY
obtained from a Jerguson reflex gage. Figure 3 (an assembly view of the completed cell) shows some of the design details. Lead gaskets l / 3 2 inch thick are utilized as the pressure seal for the glass against the cell body and fiber covergaskets cushion the glass against the window housing. A cell housing was fabricated from No. 22 sheet copper, conforming to the shape indicated in the top view of Figure 3. A bottom plate ofsheet copper was silver-brazed to the housing, and into this plate were drilled 14 holes to accept lengths of copper tubing inch in outside diameter. These tubes were cut to the appropriate length to reach from the bottom plate to a point about ‘/z inch below the top of the housing. In the completed cell these tubes serve as “coolant return” lines and are an important part of the temperature-control system. Two lengths of 3/s-inch copper tubing to serve as coolant delivery lines were silver-brazed to the bottom plate; two other lengths serve as heater wells for two 100-watt heaters (Figure 3). The upper ends of the coolant delivery tubes were bent through 90°, so that the openings faced each other a t the top of the cell. To the lower ends of these tubes, which extend about ‘/a inch below the cell housing. are attached two “coolantpump” housings for 30-watt cartridge
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heaters (Figure 3). These housings are attached by short lengths of rubber tubing to minimize heat transfer to the cell body when the heaters are immersed in coolant. When these heaters are energized, coolant is pumped to the top of the cell to provide cooling. The gas inlet to the cell is copper tubing 3/16 inch in outside diameter, formed into a coil 1 inch in outside diameter, and about 4 inches long. The relatively large surface area of the coil is effective in bringing the inlet gas to equilibrium with the cell conditions before entry pf the gas into the cell chamber. Finally, as the last step in construction, the void space in the cell housing is filled with molten 50/50 tinlead solder to a level even with the upper ends of the coolant return tubes. The solder, when frozen, “unitizes” the entire assembly, creating metallic contact between the various parts to assist in maintaining temperature control. Three copper-constantan thermocouples are provided in the cell assembly for temperature measurement and control-one near the top of the cell, one near the bottom, and one at the center. The thermocouples were made by drilling a hole just large enough to accept the size 24, B.S. gage, thermocouple wire used (Minneapolis-Honeywell Regulator Co. No. 9BlN4) in the center of and parallel to the length of 8 / 8 2 X inch brass machine screws. The thermocouple bead was formed and silverbrazed to the end of the screw. The screws were inserted into sleeves in the cell housing (Figure 3) and screwed snugly into a threaded recess in the brass cell block. The thermocouples used were calibrated against a platinum resistance thermometer, standardized by the National Bureau of Standards. Temperature Control. Cell temperature is maintained at the desired value in the range from room temperature to the boiling point of the coolant used, by means of a relay ciicuit of conventional design. To avoid fire hazard, hydrocarbon baths are not used. The present method of temperature control, using the coolant pumps to provide cell cooling, is an improvement over an earlier method devised at this laboratory for use with a nonwindowed cell (8). When the apparatus is in operation, a Dewar flask surrounds the cell. Coolant level is maintained at a distance below the cell such that the heaters in the coolant pump assembly are completely immersed. Coolant is transferred automatically from its container into the Dewar flask surrounding the cell when the apparatus is in operation. A Floatron magnetic switch (Revere Corp. of America, Wallingford, Conn.) operates through a cold-cathode relay circuit to energize a 30-watt cartridge
heater immersed in the coolant in the 15-liter container. Except for the mag-. netic switch, the coolant transfer circuit is similar to that devised by Quinnel and Futch (72). A vent line from the container is closed while the heater is on, so the pressure increase from the vaporizing liquid transfers the coolant. When the desired level of liquid is restored in the Dewar, the switch automatically turns off the heater and energizes an electrically operated valve in the vent line to depressure the container. T o avoid subjecting the container to high pressures if the delivery line becomes plugged, the vent valve is connected by means of rubber tubing, which will blow off a t a pressure of 5 to 7 p.s.i.g. The thermocouple located near the middle of the cell is used for temperature control and is connected to the input terminal block of the recorder-controller, The temperature at the top and bottom of the cell can be measured by the other two thermocouples by means of switch G (Figure 1). The temperature recorder-controller is calibrated for use with copperconstantan thermocouples for the range $150’ to -350’ F. A MinneapolisHoneywell Electrovane on-off controller controls the cell temperature through a power relay, which switches electrical input to the cell heaters and coolant pumps. A time-delay relay (5 to 10 seconds) is used ahead of the power relay to avoid chatter of the power relay. If the cell warms above the desired temperature, the Electrovane switches the power relay to provide input to the coolant-pump heaters, which pump coolant to the top of the cell in an action similar to that of a coffee percolator. The coolant flows downward through the coolant return tubes and cools the cell until the Electrovane contacts open. When this occurs, current is suvplied through the power relay to the two 100watt heaters in the cell housing to warm the cell. Normally, temperature control is maintained by balancing rhe heat input to the coolant pumps against the heat leak into the cell from the room. Input to the cell heaters is utilized primarily to raise the cell temperature rapidly when new experimental conditions are established. Inputs to the heaters and coolant pumps are controlled by variable voltage transformers. Temperature control within approximately 10.5’ F. is maintained with this circuit. External measurements of the upper and lower thermocouple outputs indicate that no significant temperature differential exists between the top and bottom of the cell. Still better temperature control has recently been obtained in the 4000-p.s.i. apparatus by using an Electrovane unit in a recorder calibrated for a range from
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0 to 6 mv., copper-constantan thermocouples, and a reference junction a t 32’ F. The contents of the cell are stirred during a run by a magnetic stirrer, driven by a Thyratron circuit. The method and circuit have been described (7). Briefly, two coils, a “power” coil and a “trigger” coil, fit over the vapor outlet line. A soft-iron armature, to which is attached a length of drawn copper wire terminating in a suitable stirring blade, is inserted inside the vapor outlet line. The Thyratron drives the armature in an up and down motion and stirs the cell contents. To prevent accumulation of dew and frost on the cell during use, a cylindrical plastic shield encloses the portions of the cell and other apparatus projecting above the Dewar flask. The lip of the Dewar fits loosely inside this shield. During the initial cool-down of the cell, boil-off of coolant sweeps all moisture-laden air from the system. The slow boil-off of coolant that persists after the cell temperature is established at rhe desired value prevents flow of moist air from the room into the space surrounding the cell. Thus all exposed apparatus in the Dewar is kept free of water and ice. A blower prevents accumulation of dew on the outer surface of the Dewar. Operation of Apparatus
A cylinder of the gas to be studied is connected to the gas inlet of the apparatus. The cylinder valve is opened and the system is purged slowly for 10 to 15 minutes a t slightly above atmospheric pressure. During this time flow is established through both the vapor and liquid outlet lines. After the system is purged, the cell inlet and vapor and liquid outlet valves are closed, and full cylinder pressure is applied to the potassium hydroxide-filled trap ( A , Figure 2). The valves on the gage panel are then opened to permit reading the pressure on the appropriate gage, and the pressure regulator is set at the desired pressure, while the gas is slowly vented downstream from this gage. A 15-liter container of liquid air or liquid nitrogen is next placed in position in the cabinet, and the coolant transfer power switch is closed. The automatic float-level controller is bypassed temporarily to promote rapid cooling of the cell, and transfer of coolant is allowed to continue until the lower end of the cell is immersed. To hasten further the cooldown of the cell, the temperaturecontrol circuit may be turned on and the Electrovane in the recorder set for the desired cell temperature. The coolant pumps are thus energized. The cell can be brought from room temperature to -300’ F. in about 10 minutes. When the cell temperature is within a few deVOL. 50, NO. 5
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Figure 4. Phase diagram for methane-nitrogen system, a t 500 p.s.i.a., shows agreement with d a t a of Bloomer and Parent grees of the desired value, the coolant transfer circuit is placed in service for automatic control of the coolant level. After about 5 minutes, during which minor variations in the cell temperature disappear, the cell inlet valve is opened to admit gas to the cell. Conditions of temperature and pressure may be such that a liquid phase will form in the cell. The stirring-control power switch ( K , Figure 1) now is closed and the stirring rate adjusted. After 10 to 15 minutes, with a few minutes’ stirring to establish equilibrium, samples of the liquid and vapor phases are withdrawn. Liquidphase samples are taken through the capillary line at a fairly rapid flow rate to avoid fractionation of the Iiquids during sampling. Samples are withdrawn into evacuated gas-sampling bottles of about 50-cc. capacity (6). The bottles
are filled to slightly above atmospheric pressure and the samples analyzed by means of a mass spectrometer (Consolidated Electrodynamics Model 21-103). Analyses are reported to the nearest 0.170, which is well within the limit of accuracy of the spectrometer. When experimental cell conditions are changed to a new temperature and/ or pressure, the cell inlet valve is closed, and the liquids contained in the cell are discarded by venting to atmosphere through the liquid dr& line. The cell pressure is allowed to drop nearly to atmospheric pressure; then the vapor and liquid outlet lines are closed, the new cell conditions are established, and the experiment is repeated at the new conditions. By using the window, the operator can easily observe the quantities of the equilibrium phases in the cell; this permits surer and faster experimentation than with a nonwindowed cell. The apparatus also permits sampling a solid phase when such is present in the cell. Although the method is crude, it is useful when it is desired to know the composition of a solid phase in the cell. The procedure the authors have utilized successfully has been to drain the liquids from the cell at a slow rate of flow, leaving the solid phase deposited on the stirrer blade. The stirrer blade is raised, and the liquid outlet line is swept free of liquid, then the cell temperature i s raised to the melting point of the solid, and the resulting liquid is sampled through the liquid sampling line. The apparatus does not require an excessive amount of coolant; a 15-Iiter container of liquid air will bring the cell down to -2OOOto -300°F.andmaintain its temperature in that range 4 to 6
Table 1.
Phase Equilibrium Data for Methane-Nitrogen System a t 500 P.S.1.A Mole % Nitrogen Vapor Liquid Temp., F. B-Pa This work B-Pa This work O
-137.8 - 140.7 -144.9 - 145 - 147 - 149 -151.9 -155.7 - 166 - 170 - 175 -175.8 -179.6 - 181 - 189 -195.4 -200.9 -213 -213.3 -213.5 -222.2 -226.7 -228.4 a
6.11 10.02 (15.5)
...
...
...
... ...
13.2 16.4 20.1
...
... 40.7
. . e
(25.5) 28 88 I
... ...
50.88 (55.0)
... I . .
69.70 (74.5)
...
84.22 (85.0) (92.0) 95.15 (97.0)
. a .
43.0 49.0
... ...
57.8 63.6
... ... 83.9 ... ... ... 1 . .
4 . .
(2.5) (3.5) 6:11
... ... ..*
10.02 (12.0)
...
. . I
3)
(2;: 28.88
...
(443 50.88
b
(66: 69.70 84.22 (92.5) 95.15
... ... ...
5.4 6.6 8.5
... ... 18.6 20.2 24.3
... ...
30.7 36.6
... ... 70.8 ... ... ... ... ...
Experimental values obtained by Bloomer and Parent. Figures in paremhem read from
a plot of Bloomer and Parent data.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
hours. During this period, samples of the equilibrium phases can be obtained for several different pressure-temperature conditions.
Performance T o check the performance of the apparatus, nine runs were made with mixtures of methane and nitrogen at 500 p.s.i.a. (Table I and Figure 4), and the data compared with data reported by Bloomer and Parent (5). A continuous sample of the equilibrium vapor phase was passed through a thermal conductivity recorder to establish the fact that true physical equilibrium conditions were obtained before withdrawal of sample for mass spectrometer analysis.
Acknowledgment The authors thank W. M. Deaton, Chief, Division of Research, for many helpful suggestions concerning the construction of this apparatus, and the machine shop crew for their work in building the equipment. Thanks also are due Herbert E. Bruce, Charles A. Seitz, and W. J. Boone for assistance in this work.
Liferafure Cifed (1) Benedict, M., Solomon, E., Rubin, L. D., IND. ENG. CHEM.37, 55 (1945). (2) Bennett, C. O., Smith, J. M., Ibid., 47, 664 (1955). (3) Ibid., 48, 676 (1956). (4) Bloomer, 0. T., Gami, D. C., Parent, J. D., Inst. Gas Technology, Research Bull. 22 (July 1953). (5) Bloomer, 0. T., Parent, J. D., Ibid., 17, (4pril 1952). (6) Brandt, L. W., Chemist-Analyst 45, 106 (1956). (7) Brandt, L. W., Deaton, 1%’. M., Reo. Sei. Instr. 27, 714 (1956). (8) Brandt, L. W., Stroud, L., Deaton, W. M., U. S. Bur. Mines, R e p . Invest. 5121 (March 1955). (9) Gore, T. L., Davis, P. C., Kurata, F., Pet. Trans., AIME 195, 279 (1952); T.P. 3438. (10) Katz, D. L., Hachmuth, K. H., IKD. ENG.CHEM. 29, 1072 (1937). (11) Kay, W. B., Ibid.,28, 1014 (1936). (12) Quinnel, E. H., Futch, A. H., Rev. Sci.Instr. 21, 400 (1950). (13) Ruhemann, M., Proc. Roy. SOC. (London)A171, 121 (1939). (14) Sage, B. H., Lacey, W. N., Am. Inst. Mining Met. Engrs., Tech. Pub. 2269 (September 1947). (15) Sage, B. H., Lacey, W. N., IXD. ENG.CHEM. 26, 103 (1934). (16) Steckel, F., Zinn, N., J . Chem. Znd. (U.S.S.A.) 16, 24 (1939). (17) Stutzman, L. F., Brown, G. M., Chem. Eng. Progr. 45, 139 (1949). (18) Verschoyle, T. T. H., Phil. Trans. Roy. 5’06. (London) 230A, 189 (1931). (19) Wells, F. LV,, Roof, J. G., Rev. Sci. Instr. 26, 403 (1955).
RECEIVED for review May 7, 1957 ACCEPTED October 21, 1957 Reference to a specific commercial part is given for convenience only. Similar devices of other manufacture would be
