November 1949
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
(27) Mikeske, L. A,, Can. Patent 425,128 (Jan. 16, 1945). (28) Miner, C. S., Jr., Bryan, L. A,, Holysz, R. P., Jr., and Pedlow, G. W., Jr., IND.ENG.CHEM.,39, 1371 (1947). (29) Nicholson, V., Proc. Asphalt Paving Tech., 12, 9 (1940). (30) Roediger, J. C., U. S. Patent 2,332,260 (Oct. 19, 1943). (31) Saville, V. O., and Axon, E. O., Proc. Asphalt Paving Tech., 9, 86 (1937). (32) Suida, H., Jekel, O., and Haller, K., Asphalt u. Teer Strassenbautech., 39, 253 (1939). (33) Swanson, J. M., IND. ENG.CHEM., 36, 584 (1944). (34) Tremper, B., and Erwin, R.P,, Proc. Montana Natl. Bituminous Conf., 4, 102 (October 1938).
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(35) Tuaker, E. B., and Grubbs, H. M., U. S. Patent 2,276,436 (March 17,1942). (36) Ulrich, H., Plietz, E., rtndFerrares, O., Ibid., 2,351,241 (June 13, 1944). (37) Weetman, B., Ibid., 2,383,097 (Aug. 21, 1945). (38) Weetman, B., and Agnew, R. J., Ibid., 2,375,055 (May 1, 1945). (39) Whitacre, C. H., Ibid., 2,286,244 (June 16, 1942). (40) Williams, H. G., Brit. Patent 554,986 (July 28, 1943). (41) Winterkorn, H. F., Proc. Asphalt Paving Tech., 8, 79 (1936). (42) Winterkorn, H. F., Ibid., 9, 63 (1937). (43) Winterkorn, H. F., U. 8 . Patent 2,314,181 (March 16, 1943). RECEIVED January 14, 1949.
Equipment for Compressibility Measurements DATAONPROPANE B. J. CHERNEY', HENRY MARCHMAN, AND ROBERT YORK, J R . ~ Carnegie Institute of Technology, Pittsburgh 13, P a . Equipment is described for measuring pressure-volumetemperature characteristics of fluids from room temperature to 300" C. and from 10 to 220 atmospheres. The apparatus is suitable for measuring vapor pressures and critical properties as well as compressibilities. The limit of accuracy of the equipment is estimated to be 0.25%. This has been verified by measurements on propane, a material whose properties are well known. Some new data on propane are reported.
T
HIS institution started a program for determining thermodynamic properties of materials over a range of temperatures and pressures. Part of the basic data needed for this project is the complete and precise information on the volumetric behavior of the materials. The purpose of this paper is to describe the apparatus used in determining compressibilities. Measurements have been completed on several systems and are being prepared for publication. The apparatus for compressibility measurements of gases and liquids described here is a further development of the designs of Keyes ( 9 ) and Beattie (1). Essentially, the equipment consists of three main parts: A thermostated compressibility bomb whose effective volume can be varied by injecting or withdrawing mercury; a mercury compressor for the injecting or withdrawing of the mercury; and dead weight gage for determining the pressure on the sample. The equipment in its present form can be used for compressibility measurements a t temperatures from 30' to 300' C. and a t pressure from 10 to 220 atmospheres. With minor modifications the pressure range can be appreciably extended. A schematic layout of the equipment is shown in Figure 1. In operation the bomb, 4,is directly connected to the mercury compressor, 18, and to one of the dead weight gages, 26 or 29. The gas sample to be investigated is confined by mercury in the bomb, 4. Mercury fills the portion of the bomb not occupied by the sample, the lines 5, 7, 13, 14, 17, and 21, the mercury compressor, 18, and the air trap, 16. I n the mercury U-tube, 22, there is a mercury-oil interface near the center of the right-hand le From this interface t o the oil injectors, 24, 31, the system is with heavy U.S.P. mineral oil. The parts filled with oil are the line, 23, the gages, 26 and 29, and the oil injectors, 24 and 31. Valves 2, 11, 25, and 30 are normally closed while valves 12, 15, and 20 are
. died
1 Present address, E. I. du Pont de Nemours & Company, Wilmington, Del. 3 Present address, Monsanto Chemical Company, St. Louis, Mo.
normally open. In addition, either valve 27 or 28 is opened to connect the desired pressure gage into the system. In addition to the parts shown in the schematic diagram, certain other equipment is necessary for the measurement of compressibilities. This equipment is described in detail later, but is listed here for reference: The charging system for purification of the sample and introducing it into the bomb; the thermostat and temperature regulating system for the compressibility bomb; the dual contact device in the U-tube for detecting the mercury oil interface; and the temperature measuring system for the bomb thermostat. COMPONENT PARTS O F APPARATUS
The manifold used for charging the sample is a piece of brass of square cross section into which four Hoke bellows valves have been soldered. There is a hole through the entire length which terminates in a standard superpressure connection for joining to the bomb charging valve. Two of the Hoke valves are provided with connectors for the weighing bombs, while the other two are connected to a vacuum gage and vacuum pump, respectively. The weighing bomb (Figure 2) is made of brass and is fitted with a Hoke bellows valve. Into the bellows valve is soldered a standard superpressure fitting of stainless steel. The compressibility bomb with its charging union is shown in Figure 3. The bomb was machined from a solid hexagon bar of American Iron & Steel Institute 410 stainless steel. The construction and closure are of standard high pressure designs, the details of which are apparent from the figure. The charging union is essentially the same design as that used by Beattie ( I ) except that a closure disk of annealed copper (0.002 inch thick) is used instead of gold. The actual determination of temperature on the international centigrade scale was accomplished by a platinum resistance thermometer which was calibrated, using standard methods, against the ice point, the steam point, and the sulfur boiling point. The thermometer used for this work was a four lead 25-ohm Leeds & Northrup (No. 8163) platinum resistance thermometer. The resistance of this thermometer was determined by a Leeds & Northrup-type G-1 Mueller bridge. The temperature of the sample is determined by measuring the temperature of an agitated oil bath which surrounds the compressibility bomb. After time
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 41, No. 11
M e r c u r ) Reservoir Reservoir Shutoff Valve
Figure 1. Schematic Layout of Compressibility Equipment has been allowed for equilibrium t o be attained, it is assumed that the temperature of the sample is the same as the temperature of the oil bath. The bomb thermostati.e., the container for the constant temperature oil bath-is shown in Figure 4. This thermostat was constructed after the designs of Beattie (8)and Collins ( 6 ) . E x c e l l e n t agitation is obtained with this mechanism, the bath fluid rising in the annular space between the inner can and the wall of the container and overflowing into the center. T h e 12-ohm heater shown in Figure 4 serves as the control heater with a circuit described later. The main heat load for the oil bath is supplied by nine strip heaters spaced equally around the outside periphery of the container. These heaters arc connected in thice parallel banks of three heaters each. There is a switching arrangement by which the three h e a t e r b a n k s may be connected in all combinations of series and parallel. Fine adjustment of the heat input is acconiplished b y a variable WEIGHING B O M B transformer. The control of the temFigure 2
perature is accomplished with a circuit which is a modification of that proposed by Hull (8) and later used by Beattie (1). A schematic diagram of the circuit and arrangement of optical components are given in Figure 5. The essential parts of the control mechanism are: A flat-type
( 2 4 4 '
-CHARGING
C O N N ECTlON
T
Figure 3.
T compressibility Bomb with Charging Union and Valve
November 1949
Leeds & Northrup platinum resistance thermometer (No. 8162); a type G-1 Mueller bridge; a galvanometer PRIWRY UTP. TEPHIUiL having a sensitivity of 0.001 microampere per millimeter; the thyratron circuit and photocell unit; and the control heater (300 watts rating). The output of the thyratron, which is roughly proportional to the amount of illumination on the photocell, is passed through the control heater. After the temperature of the thermostat has been adjusted to the desired value, the bridge is adjusted to the point of zero deflection for the galvanometer. The image from the galvanometer, which is triangular in shape, is then illuminating the photocell. If the temperature detected by the resistance thermometer varies, the galvanometer deflects so as to change the amount of illumination on the photocell. This change of illumination causes a corresponding change in the output of the thyratron which compensates for the temperature variation. The equilibrium temperature of the thermostat with no heat load is about 100" C. For this reason the cooling coils shown in the figure are necessary. By circulating water through these coils, temperatures as low as 30" C. are easily attained. Below 100" C. the temperature fluctuated as much as 0,005" C. due to a SPUPGEAR variation in the cooling water rate, but above 100" C. the maximum IO"O~A.IUPELLER' fluctuation was 0.002 C. GEAR. The mercury compressor, which is t h e e s s e n t i a l d e v i c e f o r volume measurements, is shown in Figure 6. This eaubment follows the general design of Keyes (9) and consists essentially of a thermostated cylinder containing mercury and a piston whose position is determined by a large micrometer screw. The mercury compressor is colitained in an oil bath at 30" C. This bath is controlled to within 0.01 ' C. by a mercury-in-glass regulator with an on-off control. The piston is introduced into the cylinder through a stuffing box which is packed with ceresin-impregnated twine. The micrometer is a 1-inch diameter screw with a bronze driving nut. The twenty-pitch screw has a buttress thread because of the high axial load. The driving nut can be rotated by a motor and gear reducer at about 15 r.p.m., the amount of piston motion being determined by a revolution counter and a dial graduated in '/loo of a turn. By this method the position of the piston is determined theoretically to within 0.0005 inch, which corresponds to 0.0036 cc. for the 0.750-inch diameter piston. Since the micrometer screw was chased on an ordinary lathe, the accuracy of the pitch is determined by the accuracy of the lathe lead screw. In order to eliminate this source of error, the screw was calibrated in volume units by weighing the amount of mercury ejecte.d for every ten turns of the screw. This calibration was used t o determine a screw reading in corrected turns, the number of corrected turns being equal to the actual micrometer reading plus the calibration correction. As a safeguard against any air being drawn into the mercury compressor during the filling operation, an air trap was placed in the line leading to the compressor. Any air which enters the sys-
V4H.R. 1750 ORM. AaLTATOR MOTOR
, , , , , , -
\
..1
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-
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INDUSTRIAL AND ENGINEERING CHEMISTRY
_ -
BOMB WEEMOSTAT
20GA. SHEET METAL WELL
\
tem accidentally can easily be removed from this trap, obviating the necessity for dismantling the compressor. The volume measurement also depends on two other devices: the dual contact system which locates the mercury oil interface in the U-tube, and the charging union containing the thin copper disk. The dual contact system, Figure 7, ensures the location of the mercury-oil interface a t the same point for all volume readings. The purpose of the upper contact is to detect an upward drift of the meniscus. The total volume of the system which is occupied by the sample, the mercury, and compressor piston within the cylinder is maintained at a constant value except for small changes due to pressure and temperature effects. The pressure a t the sample is determined by measuring the pressure at the piston of the dead weight gage and correcting this pressure for the hydrostatic head of mercury and oil. These corrections are discussed in detail in a later section. Two dead weight gages are connected to the system. These gages were constructed according to the design of Keyes (9),and consist essentially of a closely machined piston and cylinder cornbination with a known weight resting on the piston. They were calibrated against the vapor pressure of carbon dioxide at the ice point as suggested by Bridgeman ( 5 ) . Equipping each gage with a differently sized piston makes immediately available two pressure ranges and sensitivities without the removal of a cylinder from its gage block. Measurement of a pressure involves only the determination of the equilibrium weight (or force) necessary to restrain the piston.
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FIGUPE 5. THERMOSTAT TEMPERATURE CQNTPOL DEVICE
THYRATRON CIRCUIT
I+
I /I I/
PLATIMUM P E S I STANCE
THERMOMETER
COLJTWL UTP.
4u
BOMB THERMOSTAT
BATTERY
I
Motion of the piston can be detected by the dual contact system since motion of the piston also involves motion of the mercury-oil interface in the U-tube. The pressure is measured, then, by determining, in a manner similar to that used in weighing on a balance, the equilibrium weight and multiplying this weight by a piston constant determined in the calibration. EXPERIMENTAL PROCEDURE
The general technique of measuring the pressure, volume, and temperature of a material is as follows: A weighed sample is confined in the bomb by mercury and the bomb temperature is controlled a t the desired value; then the mercury compressor screw is set at various positions. Each setting of the screw corresponds to a definite sample volume. At each setting of the compressor screw the pressure is measured using one of the dead weight gages. While the measurement of pressure is being carried out the mercury-oil interface in the Utube must be maintained at the zero position as indicated by the dual contact. This is necessary to determine the mercury and oil "heads" between the pressure age and the sample, and to determine accurately the amount o f mercury displaced from the mercury compressor to the bomb. The data taken in these three steps defines one isotherm. The desired number of isotherms is obtained by changing the control temperature and repeating the pressure and volume measurements. WEIGHITGAXD CHARGISGOF SAMPLE. The charging method described here is confined to those materials which have negligible vapor pressure a6 liquid nitrogen temperature. For other materials, it would be necessary to change the technique. I n this description it is assumed that the sample previously has been sufficiently purified. The weighing bomb is evacuated and a sample is distilled from the supply cylinder to the sample bomb by surrounding the sample bomb with liquid nitrogen. Any noncondensable gases which might have been trapped in the sample bomb are removed by evacuating the space above the sample while the sample is maintained a t liquid nitrogen temperature. Several times during this
process, the valve on the sample bomb is closed, the sample is allowed to warm to room temperature, and the process 1s repeated. After an excess of the sample has been charged to the weighing bomb, the weight is adjusted to the desired value by venting and the weight of the bomb and sample is determined on a large analytical balance. The sample is transferred from the weighing bomb to the compressibility bomb by first heating and evacuating the compressibility bomb and then distilling the sample by surrounding the compressibility bomb with liquid nitrogen. The sample weight is then determined by reweighing the weighing bomb. The sample is trapped in the bomb by closing the charging valve. The bonnet is replaced bv an adapter connected to a short transfer line that terminates at the thermostat cover plate, wbreupon the bomb is placed in the thermostat as shown in Figure 1. ZEROREADING. I n order to obtain the zero reading, the lines 5, 7 are evacuated with the vacuum pump, valve 11 being open and valve 12 closed. Valve 11 is then closed, valve 20 opened, and valve 12 cracked slightly. The transfer lines 7A and 7B are filled slowly with mercury by running the mercury Compressor screw down at a rate just sufficient to keep the level of mercury in the U-tube approximately at the zero position. When the transfer lines are filled up to the copper gasket in the charging union, valves 12 and 20 are opened exactly one turn from closure as indicated by dials on their stems. Having filled the transfer lines one can proceed with the zero reading. For this operation the bomb thermostat is controlled a t 30" C. The zero reading is the compressor screw setting that corresponds to a definite base pressure when the transfer lines arc filled up to the copper washer. The base pressure chosen for these investigations was that corresponding to zero load on the dead weight gage (about 9 atmospheres). Valve 27 is opened with zero load on the dead weight gage and the compressor screw is run down very slowly until the mercury meniscus in the U-tube is in the zero position. The reading of the screw at this point is the zero reading COMPRESSIBILITY DATA.I n order to determine the data neccssary for compressibilities the copper disk restraining the sample must be broken. This is done b closing valve 20 and running down the compressor screw until d e disk breaks. When the disk is broken valve 20 is opened one turn. Since the sample is confined by mercury, its volume can be varied a t will.
November 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
2657
CONNECTION FOR
Figure 7.
0
Figure 6.
I
2 . 3
Mercury Compressor (Volumometer)
4
Dual Contact Construction
for the vapor pressure of mercury a t the temperature and pressure of mercury a t the temperature and pressure of the sample (this pressure must be subtracted from the measured pressure); correction for the "heads" of oil and mercury between the sample and the pressure gage; and correction for the additional volume occupied by the sample due to the evaporation of mercury into the sample space. ESTIMATION O F ACCURACY
I n order to determine an isotherm, the bomb thermostat is controlled a t the desired temperature and various settings of the compressor screw are made. At each setting, the weight on the pressure gage required to restrain motion of the gage piston is determined. The pairs of data thus obtained constitute one isotherm. Any number of desired isotherms may be determined. BLANKRUN. I n order to calibrate the compressibility bomb and the mercury compressor system for changes of volume caused by changes in temperature and ressure, a blank run is necessary. The blank run is done in exactb the same manner as the actual run except that no sample is charged to the bomb. The data obtained in thisblank runenablecorrections to bemade forthe following variations: The expansion of the bomb with pressure and temperature ; the expansion of the mercury compressor and lines with pressure; and the decrease in volume of the mercury and the steel piston with pressure. Several calibrations have shown that, while essentially no hysteresis occurs in the bomb, the compressibility of the mercury compressor system may vary if great care is not used in adding mercury t o the system. For this reason the compressibility of the compressor system up to valve 12 was measured before each run. To avoid errors from this source, this change in volume must be the same a t the time of the actual run as a t the time of the blank run. It was found that by using care the chan e in apparent volume with a change in pressure of 3000 pounfs per square inch could be held constant to f 0.01 turn or f0.004cc. CALCULATIONS OF RESULTS. The details of the method of calculations for this type of equipment have been given thoroughly by Beattie (1) and by Sage and Lacey (I,@). In addition to the corrections previously listed in connection with the blank run, other necessary corrections are: Correction
The temperature is known to *O.0Zo K. for the worst case. The uncertainty in the pressure measurements by the dead weight gage is usually taken as *0.02%. The method of correcting for static head and the vapor pressure of mercury may in' troduce an error as high as 0.05%. This gives an uncertainty in the pressure of about 0.07%. Several calibrations of the equipment indicate that the volume is known to 0.04 CC. Since the smallest volume measured is 25 cc., the maximum per cent error is approximately 0.15%. The weight of the sample is determined to *0.001 gram. The smallest sample used is about 4 grams; therefore, the maximum per cent error is 0.03'%. Adding all the per cent errors gives an over-all error of about 0.25% when calculated on the compressibility factors, p = pv/RT. RESULTS
I n order t o establish the over-all accuracy of the equipment and method, some data were obtained on propane, whose compressibilities are well known. These data are presented in Table I and Table 11. The propane used was Phillips research grade having a purity of 99.99 +mole %propane. The results a t 100"and 125OC. were compared with those of Beattie, Kay, and Kaminsky (3) by plotting residuals from the Beattie-Bridgeman equation. The maximum per cent deviation a t 125" C. was 0.09770. At 100' C. the maximum per cent deviation was 0.33%. The accuracy of the
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Vol. 41, No. 1 1
since they used an older value for the molecular weight of propane. (Standard atmosphere) Vapor Pressures a t 30' C. 50' C. Authors' data 10.66 16.93 Beattie, Poffenberger, a n d Had16.90 lock ( 4 ) Sage, Schaafsma, and Lacey (IS) 10:+3 16.93 Deschner a n d Brown ( 7 ) , 10.78 1F.98 Stearns and George ( 1 4) 10. 63 16.98
TABLE IT. .~ Temp.,
50' C>.
P, atm.
z',
~
10 66 11.04 11 74 12 18 12.52 13.03 13.40 14.41 15.55 16.87
SPsciFIc
ACKNOW LEDGMER T
The conipressibility bomb thermostat and its control circuit were designed and built by H. W. Prengle. Financial support for this project was provided the Allied Chemical and Dye Corporaand the Standard Oil Company (Indiana). LITERATURE CITED
VOLUMESOF PROPANE VaPon
Temp., 100' CL
cc./s.
p , atm.
47.891 45.901 42.585 40.596 39.272 37,285 35.958 32.645 29.329 26,012
10.69 11.27 12.63 14.35 16.62 19.71 24.11 30.72 40.20
Temp.. 126' C .
cc./g.
p , atm.
59.166 55.816 49.122 42.431 36.746 29.054 22.366 15.676 9,000
15.64 18.19 21.73 26.91 35.10 49.13
11,
Beattie, J. A., Proc. Am. Acad. Arts Sei., 69, 389 (1934). Reattie, J. h., Rev.Sci. Instruments, 2, 458 (1931). Beattie, J. A., Kay, W. C., a n d Kaminskj, ,I., .I. Am. Chem.
zi,
cc./g.
42.358 35.642 28.920 22.203 15.487 8.788
Soc., 54, 1731 (1932).
Beattie, J. A , , Poffenberger, S . , and
H a d l o c k , C., J . Chem.
P h y s . , 3, 93 (1935). Bridgeman, 0. C . , J . A m . C'henz. Sac., 49, 1174 (1927). Collins, S.C., Rev. Sci. Instruments, 7 , 502 (1936).
Deschner, TV. W., a n d Brown, G. G., ISD.EKG.CHEW,32, 8 3 8 (1940).
Hull. A. W., Gen. EZec. Rev., 32,
390 (1929).
Keyes, F. G., Proc. Am. Acad. Arts. Sci., 6 8 , 505 (1933).
Prengle, H. W., D.Sc. thesis, Carnegie Institute of Technology, equipment a t higher pressures has been confirmed by Prenglc ( I O ) in some work on ethane t o be published later. In the calculation of the results, the values of the physical constants such as molecular weights, and the gas constant, E , were taken from the table of values recommended by the National Bureau of Standards (11). It was necessary to recalculate the data of Beattie, Kay, and Kaminsky before making a comparison
1948.
Rossini, F. D., et al., A.P.I. Research Project 34, ;\'atZ. Bur. Standards ( U . S.) , Circ. C-461 (November 1947). (12) Sage, B. H., a n d Lacey, W.N., Trans. Am. Inst. Mining Met. Engrs., 136, 136 (1940). (13) (14)
Sage, B. H., Schaafsma, J. G., a n d Lacoy, W. N . , IND.ENG. CHEM.,26, 1218 (1934). Stearns, W. V., and George, E. T., Ibid., 35, 602 (1943).
R ~ C E I V April E D 2 5 , 1949.
ressihility and Critical Constants of Propylene Vapor HEKRY _MARCH.MAN, H. FILLIABI PRENGLE, J R . ~ , AND R. L. MOTARD Curnegie Institute of Technology, Pittsburgh 13, P a . T h e compressibilities of propylene vapor are presented from 10 to 215 atmospheres of pressure and from 30" to 250" C. The critical properties, vapor pressure, and saturated vapor volumes are also presented. The constants for the Renedict-Webb-Rubin equation of state are reported and carrelate the data up to a density of 9.0 moles per liter with an over-all average deviation of 0.20%.
T
HE compressibility of propylene up to 85 atmospheres of pressure using a glass capillary tube apparatus has been determined by Vaughan and Graves (10). There was need for extension of the range of these data as well as a checking of their accuracy (estimated to be 1%). The critical properties of propylene have been determined by several investigators (7, IO),but the agreement is poor. The purpose of this work was to determine a more accurate set of data for propylene covering a greater pressure range, which could be used with assurance in calculating thermodynamic properties. APPARATUS AND EXPERIMEXTAL TECHNIQUE
The apparatus and experimental technique have been described in detail previously (6). The sample of propylene used was 1
Present address, Shell Oil Company, Inc., Houston 1, Tex.
Phillips research grade which has been certified by the National Bureau of Standards to be above 99.70% propylene. Confidence in the sample purity was gained by the observance of t>hebehavior in the two-phase region and in the vicinity of the critical point. The vapor pressure did not change within the limits of measurement, when the volume of the sample was changed from the saturated vapor to the saturated liquid. I n order to ensure maximum precision three different size samples were used. A 5-gram (approximate) sample was used for low pressures, a 10-gram (approximate) sample for pressures up to 110 atmospheres, and a 20-gram (approximate) sample for the high pressure range up to 215 atmospheres. This procedure eliminatrs the necessity for the determination of pressures when t,hc volume of the sample is small. I n all cases t,he overlap of the data for the various size sample was extremely good. In many cases the deviations were less than 0.05% and in no case did they exceed 0.20% (t,he value of 0.20y0 occurred for only a few isolated points). This agreement is considered excellent since the low pressure range data (up to 110 at,mospheres) were determined using a bomb borrowed from James A . Beattie; while t,he high pressure data (up to 215 atmospheres) were determined using the bomb previously described (6) involving a new bomb calibrat'ion. The critical constants were determined by the procedure used by Reattie and eo-workers (1, a,4 ) , and consists of measuring the pressure and volume along closely spaced isotherms in the vicinit,y of the critical. The critical temperature is taken as that temperature at which the finite flat,portion of the curve becomes a point, or the isotherm for which there is a point of inflection.
