Table V.
Isobaric Heat Capacity as Function of Temperature and Pressure IC,( TP),cal./mole-degree]
PI08-
aure. ___ Bars 100 150 50 11.82 11.32 100 15.87" 13.07" 150 21.91 15.80 200 23.02 17.72 250 22.22 18.46 300 21.17 18.57 350 20.15 18.28 400 19.60 18.07 450 19.00 17.77 500 18.67 17.61 600 18.04 17.24 700 17.57 16.95 800 17.24 16.74 900 16.97 16.56 16.78 16.40 1000 16.61 16.26 1100 16.50 16.16 1200 16.44 16.05 1300 16.28 15 97 1400 From (I). ~
200 11.31 12.46 13.56 14.64 15.52 16.02 16.24 16.23 16.17 16.04 15.82 15.68 15.57 15.48 15.38 15.28 15.20 15.13 15.07
250 11.47 12.24 12.90 13.52 14.11 14.61 14.96 15.12 15.15 15.12 15.02 14.93 14.87 14.82 14.77 14.72 14.68 14.65 14.62
300 11.65 12.22 12.74 13.21 13.66 14.05 14.38 14.67 14.88 18.00 15.10 15.07 15.03 15.01 15.01 15.02 15.02 15.01 15.01
350 11.85 12.26 12.68 13.10 13.48 13.82 14.10 14.38 14.63 14.81 15.00 15.03 15.01 15.01 15.04 15.08 15.11 15.14 15.17
Temperature, C. 450 400 12.04 12.22 12.31 12.44 12.63 12.65 12.96 12.88 13.27 13.09 13.55 13.31 13.79 13.51 14.01 13.70 14.18 13.86 14.32 13.98 14.52 14.20 14.62 14.38 ' 14.65 14.47 14.66 14.51 14.70 14.55 14.75 14.61 14.82 14.67 14.88 14.74 14.95 14.80
these values with a more accurate set when such data become available. DEFINITIONS AND CONVERSION FACTORS
V
volume (A.U.) = volume/volumeNTp = density (A.U.) = density/densityprTp =
p
p
=
_____---.-___
500 12.40 12.54 12.72 12.90 13.05 13.21 13.36 13.50 13.64 13.77 14.01 14.20 14.34 14.42 14.47 14.51 14.56 14.60 14.66
600 12.72 12.84 12.97 13.09 13.18 13.28 13.36 13.45 13.54 13.65 13.85 14.02 14.18 14.30 14.38 14.44 14.47 14.51 14.55
700 13.02 13.12 13.22 13.31 13.39 13.46 13.53 13.59 13.65 13.72 13.85 13.97 14.08 14.20 14.30 14.38 14.45 14.50 14.55
800 13.27 13.37 13.41 13.48 13.54 13.60 13.66 13.72 13.77 13.82 13.92 14.02 14.13 14.23 14.33 14.43 14.51 14.57 14.62
900
1000
13.48 13.53 13.59 13.64 13.70 13.76 13.81 13.86 13.90 13.94 14.04 14.16 14.28 14.40 14.50 14.61 '14.70 14.77 14.82
13.67 13.71 13.77 13.82 13.88 13.93 13.97 14.01 14.04 14.08 14.18 14.30 14.42 14.55 14.68 14.79 14.90 14.99 16.05
LITERATURE CITED
(1) DeGroot, S. R., and Michels. A., Physica, 14, 218-22 (1948). (2) Kennedy, G. C., Am. J . Sci., 252, 225-41 (1954). (3) MacCormack, K. E., and Schneider. W. G., J. Chem. Phys., 18, 1269-72 (1950). (4) Ibid., pp. 1273--5. ( 5 ) Masi, J. F., private
communication. Preliminary incomplete version of chapter, "Thermodynamic Properties of Carbon Dioxide," for Bureau of Standards compilation. (6) Michels, A,. and DeGroot, S. R.. A p p l . Sci. Research, lA, 94-
v-1
N T P indicates 0" C. and I atm. = 505.833 p (gram/cc.) (mole/cc.) = 4.49202 X 10-6 p (A.U.) R = 82.0567 cc.-atm./mole-degree = 83.1357 cc.-bar/mole-degree = 1.98719 cal./mole-degree P(bars) = 0.98692 P(atm.) PV(bar-A.U.) = 0.98692 PV (atin.-A.U.) PV(bar-A.U.) = 531.97 c*al./mole p p
Vol. 41, No. 8
INDUSTRIAL AND ENGINEERING CHEMISTRY
1652
(A.U.)
102 (1948). (7) Ibid., pp. 103-6.
( 8 ) Michels, A,, and Michels, C., PTOC.Roy. Soc. (London), 153A, 201-14 (1935). (9) Ibid., 160A,348-57 (1937).
(10) Michels, A., Michels, C . , and Wouters, H. H., Ibid., 153A, 214-24 (1935). (11)
ACKNOW LEDGltl ENT
The writer wishes t o thank Elise Fisher, Applied Mathematics Division, Naval Ordnance Laboratory, for assistance with this work; most of the computation was carried out on an I.B.M. card-programmed calculator.
Price, D., Naval Ordnance Laboratory, Rept. 2876 (1953).
(12) Ibid., 3876 (1954). (13) Sweigert, R. L., Weber, P., and Allen, R. L., IND. ENG.CHEM., 38, 185-200 (1946). (14) Woolley, H. E., J. Research Natl. Bur. Standards, 52, 289-92 (1954). ACCEPTEDFebruary 21, 1955. RECEIVED for review December 11, 1954. Work of G. C. Kennedy supported by Bureau of Ordnance under contract NOrd-10449, Task 5, with Harvsrd Univemity.
Measurement and Correlation of Vapor Pressure Data for High Boiling Hvdrocarbons 1
J
€I. S. MYERS' AND M. R. FENSBE College of Chemistry and Physics, The Pennsylvania State University, University Park, Pa.
F
OR some time the petroleum industry has shown an increasing interest in better utilization of high boiling residua. As a result, the vacuum unit is rapidly becoming an integral part of nearly every refinery. This introduces a need for reliable vapor pressure data for high boiling hydrocarbons. I n designing and operating a vacuum unit, boiling points must be converted from one pressure t o another by means of a vapor pressure chart. A search of the literature shows many vapor pressure charts Present address, C. F. Braun Br Co.. Alhambra. Calif.
and nomographs. But very few of these correlations agree cloaely, and some of them are widely different, particularly in the high boiling region. For example, suppose a hydrocarbon oil boils a t 575' F. a t an absolute pressure of 1 mm. of mercury. T h e vapor pressure nomograph developed b y Lippincott ( 4 ) predicts a boiling point of 1060" F. a t atmospheric pressure. T h e nomograph of Maxwell (6) predicts 1020" F., that of Kelson ( 6 ) , 980' F., and that of Brown and Badger ( 2 ) , 970" F. This particular example, then, shows a discrepancy of 90" between . correlations.
August 1955
INDUSTRIAL AND ENGINEERING CHEMISTRY
Poor vapor pressure data probably cause more of these variations. Only a few vapor pressures have been reported in the literature below about 10 mm. of mercury absolute pressure, and many of these do not appear reliable. For high boiling hydrocarbons the data are particularly meager, and consequently the high boiling region of most existing vapor pressure charts is merely an extrapolation of data on low boiling hydrocarbons. Therefore, in order to develop a better vapor pressure correlation, it was first necessary to get vapor pressure data for high boiling hydrocarbons. An apparatus has been built to do this. It measures boiling points a t absolute pressures ranging from about 0.1 mm. of mercury to atmospheric pressure. I n addition to measuring boiling points, the apparatus can be used to obtain flash-equilibrium data, by means of a variablevolume, condensate holdup cup built into the unit. Vaporliquid equilibrium data can also be measured, as the apparatus is essentially an Othmer-type equilibrium still. DESIGN CONSIDERATIONS
Several special considerations enter into the design of such an apparatus. The major problem always encountered in vacuum distillation equipment is bumping or unsteady boiling. To stop the bumping, superheating of the boiling liquid miist be eliminated. At low absolute pressures, hydrostatic head can cause localized superheating a t the bottom of the still. A good design, therefore, should incorporate a reboiler with a large cross-sectional area, thereby permitting a low liquid depth and consequentll- a low hydrostatic head. Even heating and adequate mixing of the boiIing liquid must be assured. I n the past, most designers of this type of equipment have utilized some form of mechanical mixing along with immersion heating. For this apparatus it was decided to use a cornbi-
-
/-
MINIATURE SMRK PLUGS
!,I
I I\
y us /
WINDING NOT SHOWN
\ I d " PLATE
SPRING. LOADED TEFLON VALVE
Figure 1.
/ T m m i F
4 L L PARTS STAINLESS STEEL,UNLESS OTHERWISE NOTED.
Vacuum equilibrium still
1653
Designers and Operators of Vacuum U n i t s . . . will find the vapor pressure chart described here of particular'value. Conversion of boiling points from one pressure to another is possible with less than 2% error, for most high boiIing hydro car bo ns
-
nation heater and stirrer. With this vibrating heater, good heattransfer coefficients between the heater and the liquid would be expected, and superheating would be very unlikely. Ease of cleaning is another consideration in designing this type of equipment, as some high boiling stocks might be very difficult to remove by solvent action alone. The design, therefore, should permit easy dismantling of the reboiler, so that residues can be scrubbed out-or chipped out-if necessary. Finally, materials of construction must be chosen. Xearly every apparatus for determining boiling point and every equilibrium still reported in the literature have been made of glass. For this unit, however, stainless steel was considered safer and more practical, because of the high temperatures and low prwsures involved. APPARATUS
Figure 1 shows a detailed drawing of the vacuum equilibrium apparatus. The top of the reboiler is flanged to permit cleaning of the unit. The thermocouple, the hollow, heater-stirrer shaft, and the rod that raises and lowers the plug in the condensate cup are sealed by stuffing boxes welded onto the top flange. A tempered-glass window, sealed with a Teflon gasket, permits viewing the interior of the still. The condensate cup is silver-soldered against the inner wall o€ the reboiler directly beneath the condenser. A rectangular baffle makes the cup self-purging. The aluminum plug shown in the diagram can be raised or lowered in the cup, thereby varying the volume of condensate holdup. When the plug is raised completely out of the cup, a spring-loaded Teflon valve lifts out of its hole in the bottom of the cup, permitting the condensate to drain. Boiling points are determined with the plug in this position-that is, with no holdup in the cup. A double-walled baffle, with glass wool between the walls, insulates the cup from the hot vapors in the still and prevents the liquid in the cup from boiling. As an added precaution, the adiabatic heating winding does not cover the area adjacent to the condensate cup. Actually only about two thirds of the external area of the reboiler is heated and insulated. The remaining third, which can be considered the condensing section of the unit, does not require insulation. The heater consists of a 2-foot length of a/,,-inch copper tubing rolled into a flat coil. There is about 1/16-inch annular space between each two adjacent tubes of the coil. A 4-foot length of asbestos-covered Nichrome wire, 1.68 ohms per foot, was bent double and forced into the copper tube before this tube had been wound into the coil. The leads from the heating wire extend up through the hollow shaft of the combination heater and stirrer into an enlarged chamber, where they are coiled for about three turns each, and then soldered onto the center terminals of two model-engine sparkplugs that are sealed into the top plate. A glass-insulated solenoid coil operates the stirrer. The coil can be raised or lowered with respect to the soft-iron core, thereby increasing or decreasing the amplitude of the stirrer. The thermocouple is made from No. 24, copper-Constantan, Duplex wire. Before it was installed it was calibrated against a couple that had been standardized a t the Bureau of Standards. Over the temperature range from 20' to 300' C., it agreed with the standard couple t o within 10.2' C. Pressures from 0.1 t o 30 mm. of mercury are measured with a double-range McLeod gage. One range covers pressures from
1654
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 8
about 0.5 mm. of mcrcury and then falls off slightly. This may be true relationship, but probably the fall off is again caused by pressure drop. For this reason smoothed data reported a t 0.2 m m . 2.c of mercury have been extrapolated linearly from the values obtained at pressures above 0.5 mm. Hexadecane was chosen to test the accuracy of boiling points measured in the vacuum unit. The sample used had been fractionated in a 35-plate coluynn, and had an estimated purity of greater 1.0 than 98%. The data obtained are shown in Table I, along with those published m for hexadecane by API Research Project 0.8 44 ( 1 ) and Stull ( 7 ) . As the bureau does not list vapor pressures below E 10 mm. of mercury it was necessary to w" 0.6 calculate the boiling temperature at a 2 1 mm. of mercury from the Antoine 2 0.5 equation, using values for the constants W given, log P = 7.03044 - 1831.317/(t II: a 154.528), where P is in millimeters of w 0.4 mercury and t is in degrees centigrade c 3 J The procedure adopted for obtaining 0 these vapor pressure data consists of first m m 0.3 pumping the system down to its lowest Q 0.1 mm. of attainable pressure-about mercury-and measuring the corresponding boiling temperature a t this pressure. Then the vacuum pump is isolated from 0.2 the rest of the system by the proper values, and air is admitted to the ballast tank until the desired pressure is reached for the second temperature reading, and so on. Ordinarily 4 or 5 minute? is sufficient a t each pressure to establish equilibrium. The system is tight enough so that the pressure does not increase 0. I measurably during this time. 120 I30 140 I50 160 A series of tests was made, using the TEMPERATURE,*C. variable-holdup cup, to study the appliFigure 2. Vapor pressure curves for eicosane be€ore and after cability of the unit for obtaining equilibcondenser modification rium flash vaporization data. These studies are not discussed in this article, 0.1 to 3.0 mm. of mercury, with scale graduations every 0.01 mm. but they show that the apparatus is satisfactory for making flashThe other is for pressures from 0.1 to 30 mm., with scale graduavaporization measurements in a batchwise manner. tions every 0.1 mm. Both scales had been calibrated against a second McLeod gage. Pressures above 30 mm. are measured EXPERIMENTAL WORK with a Wallace and Tiernan sensitive manometer, which has a probable accuracy within about f 0 . 5 mm. Vapor pressure-temperature measurements were made for 26 different hydrocarbons or narrow-boiling mixtures of hydrocar-
=
i
+
PRELIMINARY TESTIIYG AND PROCEDURE
The first model of the boiling point apparatus had a condenser just half the diameter of the present condenser--3/4 inch instead of 11/s inch. Figure 2 shows that at low pressures this smaller condenser was not satisfactory. The dotted vapor pressure lines of Figure 2 were obtained with the '/,-inch condenser a t three different boilup rates. Below about 1 mm. of mercury absolute pressure, the curves fall off rapidly-the greater the boilup, the greater the deviation. With the vibrating heater, superheating did not seem likely. There W&B no bumping. Furthermore, the thermocouple could be lowered into the boiling liquid or raised into the vapor space without changing its reading. The only other possibility seemed to be pressure drop. The solid curve of Figure 2 shows the results of enlarging the diameter of the condenser. Boilup rate no longer has an a p preciable effect. The curve remains essentially linear down to
Table I. Abs. Pressure. Mrn. Hg i n
10 20 30 40 50 60 80 100 150 200 300 400 300 600 700 760
Vapor Pressure Data for Hexadecane Boiling Point of Hexa
