Thermal Properties of Pentane PAUL RAYMOND KONZ' AND GEORGE GRANGER BROWN University of Michigan, Ann Arbor, Mich.
The effect of pressure and temperature on the enthalpy of pentane has been determined by means of the Joule-Thomson expansion from pressures up to 3000 pounds per square inch and temperatures up to the point of decomposition with qparticular care in the critical region. The data are presented on a pressuretemperature plot showing lines of constant enthalpy.
I
I I
I I I I I
I I
I
FIGURE 1. FLOW DIAGRAM OF APPARATUS
ITH the increasing use of high temperature and pressure in industry, accurate information on the thermal properties of hydrocarbons is becoming most important. In order to confirm experimentally the values of enthalpy of hydrocarbon vapors a t high pressure computed by pressure-temperature-volume relations, it is desirable to use a compound generally representative of those encountered in industrial practice and of such properties that it may be investigated thoroughly in the region of the critical point and a t high reduced temperatures and pressures. %-Pentane seems to meet these requirements fully; but the previous study a n pentane (7) did not include sufficient range of pressure to serve the above object. A supply of npentane of about 99.7 per cent purity was obtained through the courtesy of the Phillips Petroleum Company.
A . The pressure on the upstream side of the orifice is measured by pressure gage M , and on the downstream side by pressure gage N . The corresponding temperatures are measured by thermocouples V and W inserted in thermocouple wells which are completely surrounded by a double pass of the va ors before and after ex ansion, as previously Ascribed (7). Kressure gapes M and N were calibrated to an accuracy of 2 pounds f o i Fessures below 1000 pounds per square inch and of 5 ounJs at higher pressures. Throttle were calibrated to *0.5" F., and thermocouples V and calorimetric thermocouples S, T,and Uwere calibrated to *0.2" F. H is a four-cylinder single-acting surge pump operated by a one-horsepower motor through a wormgear speed reducer. The pistons are so set as t o be actuated at 90" intervals for every revolution of the crank arm and thereby assure steady flow. The packing around the istons is Johns-Manville No. 323 gasoline rod packing, adjustex with reat care. If the packing was too tight, friction developed sufacient heat to vaporize the residual pentane left in the cylinder after the discharge stroke. Electric radiant furnace I is shown in cross section in Figure 2. The tube bank consists of twelve len ths of seamless steel tubes, 3 feet 8 inches long, 8/8 inch i. d., anfa/( inch 0.d., welded in return headers. The tube bank is surrounded on its sides and top by an electric heating element of Chrome1 A resistance ribbon, set in special firebrick furnace forms as indicated in Figure 2 . Resistance controls enabled the furnace to be operated from 1 t o 15 kilowatts capacity.
b
Operation of Apparatus The apparatus and manner of operation are generally similar to those used in earlier work (6, 7) except for a new furnace of electric radiant type and other minor improvements. The flow diagram is shown in Figure 1: The pentane makes a complete circuit, flowing from supply tank A through pump B , decolumn C, cooler E, high-pressure electric furnace I , Joule-Thomson throttle k, control valve P, calorimetric condenser R, two-way valve L,back to supply tank
c$B
1 Present address, The M. W. Kellogg Company, 226 Broadway, New York, N. Y.
FIGURE2. TUBEBANKIN F U R N A C ~ 617
INDUSTRIdL AND ENGINEERING CHEMISTRY
618 ~
Vol. 33, No. 5
~~
AND INTERPOLaTED JOULE-THOYSON DATA TABLE I. EXTRAPOLATED Expt. NO.
TI
T2
TI - Tn
Expt. NO.
TI
TI
Ta
PI A-1 A-2 A-3 A-4 A-5 A-6 A-7 A-8 A-9 A-10 A-11 A-12 A-13 A-14 A-15 A-16 A-17
72.9 95.9 100.0 131.0 192.0 199.6 223.5 228.5 240.2 244.8 256.8 268.8 262.7 292 * 0 294.4 312.8 315.6
78.9 101.9 106.2 136.0 196.2 203.9 227.0 231 .O 243.3 247.9 260.2 262.5 266.5 294.0 296.4 313.9 317.3
G-1 G-2 B-1 B-2 B-3
68.8 73.6 97.4 98.5 88.8 99.5 100.1 135.7 135.8
70.3 76.1 99.0 100.0 100.3 100 9 101.7 136.9 136.9
-1.2 -1.1
c-5 c-2
70.3 75.1 136.9 136.9 180.3 217.0 228.8
71.9 76.7 138.1 138.2 180.9 217.7 228.9
-1.6 -1.6 -1.2 -1.3 -0.6 -0.7 -0.1
G-1 G-2 G-3 G-4 G-5 D-1 G-6 D-2
71.9 76.7 138.1 138.2 217.7 231.7 258.6 262.7
73.5 78.2 139.3 139.5 218.4 231.7 258.9 262.4
-1.6 -1.5 -1.2 -1.3 -0.7 0.0 -0.3 0.3
H-1 H-2 H-3 H-4
67.6 134.5 204.0 264.7 399.7 416.2 429.2
69.1 135.6 204.5 264.3 392.7 406.2 412.0
-1.6
H-4
B-5 G-3 G-4
G-1 G-2 G-3 G-4
c-1
-6.0 -6.0 -6.2 -6.0 -4.2 -4.3 -3.5 -2.5 -3.1 -3.1 -3.4 -3.7 -3.8 -2.0 -2.0
-1.1 -1.7
-1.5
-1.6 -1.6
-1.5
-1.5 -1.4
-1.6
A-18 A-19 A-20 A-2 1 A-22 A-23 A-24 A-25 A-26 A47 A-28 A-29 A-30 A-31 A-32 A-33
316.8 318.2 350.3 360.5 382.7 423.0 425.6 429.0 429.3 430.8 436.7 439.8 441.0 455.8 458.6 480.3
318.4 319.4 350.7 360.7 382.0 420.8 423.3 425.8 426.8 428.0 433.4 436.6 437.9 452.3 454.9 475.4
B-6 G-5 B-7 G-6 B-8 B-9 G-7 B-10
179.6 216.3 226.3 258.0 261.0 295.5 359.6 382.2
179.8 217.0 226.6 258.3 261.0 294.9 358.3 380.4
3016, -1.6 -1.2 -0.4 -0.2 0.7 2.2 2.3 3.2 2.6 2.8 3.3 3.3
3
c-5
C-6
258.6 263.7 292.6 356.9 375.5 434.6
356.9 378.8 409.3 442.5 444.3 468.8 478.2 484.7
355.6 375.9 405.3 436.1 437.2 458.4 465.7 472.0
434.0 436.0 438.0 439.2 449.0 460.0 502.0 517.2
415.0 417.5 416.9 419.3 422.0 430.2 469.1 485.4
1864, -0.2 -0.7 -0.3 -0.3 0.0 0.6 1.3
E-1 E-2
N
-0.5 0.4 7.0
10.0
17.2
D-6
D-7 D-8 D-9
0 E-3 P E-4 B E-5
E-6 F-1
PI 1-1 1-2 1-3 1-4
1-5
496.7 496.6 513.4 514.4 514.8
435.0 435.6 457.9 458.3 460.1
61.7 61.1 55.5 56.1 54.7
1566,
1-6 1-7
1-8 1-9 1-10
533.8 533.8 569.6 600.9 600.9
484.0 484.0 527.3 563.4 563.8
-
B-11 B-12 B-13 B-14 B-15 B-16 B-17 B-18
P,
Pa
626, 49.8 49.8 42.3 37.4 37.1
PI
The expanded vapor from throttle K (Figure l), after passing through control valve P, is condensed in calorimetric condenser R and may be directed to tank A as previously described or into weigh tank X by turning the two-way valve, L, which also simultaneously controls the flow of the cooling water. The water is directed to the sewer when the condensate goes to tank A , and into weigh tank Y when the condensate goes into weigh tank X . About 2-4 hours are required to bring the furnace to the chosen tem erature. During this heating period the pentane is circulatex continuously. The temperature of the vapor entering the throttle is measured every few minutes by reading thermocouple V . As the vapor approaches the desired temperature, the current supply to the furnace is gradually decreased until constanttemperature conditions are obtained. After about 10 minutes of steady operation at constant inlet temperature and pressure, and also with steady conditions of outlet temperature and pressure as indicated by thermocouple W and pressure gage N , the double two-way valve, L, IS turned t o allow the condensate and water from the calorimeter condenser to flow into their respective weigh tanks, X and Y . During a run the temperatures are read to 0.1' F., of the vapor entering throttle V , the vapor leaving throttle W , the condensate leaving calorimeter condenser U,the water entering calorimeter condenser T, and the water leaving calorimeter condenser S. The length of such a run is 10-15 minutes and enables about seven
-
TI- Tn
Expt. No.
TI
T,
491.5 613.8 524.8 530.5 531.8 557.2 557.7 568.4 570.5 571.8 572.8 594.5 618.4 619.2 643.5 645.4
485.9 606.9 517.0 521.8 522.8 547.4 549.0 568.7 560.4 661.8 662.4 582.9 606.5 606.2 629.9 631.0
5.6 6.9 7.8 8.7 8.0 9.8 8.7 9.7 10.1 10.0 10.4 11.6 11.9 13.0 13.6 14.4
A-50 A-61 A-52 A43 A-54 A-55 A-56 A-57 A-58 A-59 A- 60 A-61 A-62 A-63 A-64 A-65
664.2 655.4 666.4 668.0 688.0 720.8 721.0 738.2 739.2 758.0 760.6 762.6 767.6 782.2 793.1 795.3
639.6 641.2 652.0 663.7 673.0 705.5 705.9 723,O 724.1 742.6 745.6 747.1 751.9 766.6 778.1 781.0
14.6 14.2 14.4 14.3 15.0 15.3 15.1 15.2 15.1 15.4 15.0 15.5 15.7 15.6 15.0 14.3
435.7 450.5 475.2 483.0 489.0 525.7 543.6 553.4
433.2 447.5 471.3 479.1 484.9 520.1 636.1 546.3
2.5 3.0 3.9 3.9 4.1 5.6 7.5 7.1
B-19 B-20 B-21 B-22 B-23 B-24 B-25 B-26
613.5 651.6 680.7 690.0 710.5 711.4 745.4 771.0
606.1 643.5 673.7 681.8 702.2 704.2 737.7 764.3
7.4 8.1 8.0 8.2 7.9 7.2 7.7 6.5
446.7 469.4 482.7 514.3 571.0 601.0
4.3 5.8 8.4 9.2 10.8 11.3
C-13 C-14
C-16 C-17 c-18
647.5 647.9 675.4 678.2 729.7 794.2
636.0 636.0 664.5 667.5 720,O 786,7
11.5 11.9 10.9 10.7 9.7 8.5
473.8 476.2 489.8 505.2 510.2 662.2 595.2
13.1 13.7 15.5 17.1 18.0 17.4 16.6
D-17 D-18 D-19 D-20 D-2 1 D-22 D-23
646.2 646.3 681.4 681.5 693.4 734.3 780.8
630,Q 631.4 667.0 668.0 680.5 723.1 770,4
19.3 14.9 14.4 13.5 12.9 11.2 10.4
621.9 532.5 533.5 565.3 567.2 572.6 572.7 576.3
490.9 503.0 604.1 540.4 542.1 548.1 549.7 552.5
31.9 29.5 29.4 24.9 25.1 24.6 23.0 23.8
E-8 E-10 E-9 F-9 F-10 E-11 E-12 E-13
612.6 644.9 645.5 666.4 667.3 671.6 722.1 780.4
591.9 629.8 626.8 649.8 651.5 656.4 708.6 768.6
20.7 19.1 18.7 16.6 15.8 15.2 13.5 11.8
600.9 620.1 621.0 635.5 652.8 653.1
563.8 585.1 585.2 602.6 621.5 622.3
37.1 35.0 35.8 32.9 31.3 30.8
1-17 1-18 1-19 1-20 1-21 1-22
686.6 690.3 728.6 769.0 808.4 840.5
658.4 662.0 703,l 745,9 786,4 820.9
28.2 28.3 25.5 23.1 22.0 20.0
- Ta
TI
1265
c-15
= 962
-
962, 19.0 18.5 21.1 19.9 27.0 29.8 32.9 31.8
-
TI
PI = 1566
PI = 1265, P2 G-7 D-3 D-4 D-5
PI -1.1
A-34 A-35 A-36 A-37 A-38 A-39 A-40 A41 A-42 A43 A-44 A-45 A46 A-47 A-48 A49
1.8
-
TI
Pa = 1864
3.7 4.9
Pi 258.3 263.9 293.2 358.3 378.2 440.0
Expt. NO.
3.1 3.5
PI
C-6 c-3 c-4 c-7
- Tt
626
F-2 F-3 F-4 F-5 F-6 E-7 F-7
F-8
-
14.7
1-11
1-12 1-13 1-14 1-16 1-16
readings to be taken for each thermocouple. At the end of the run the double two-way valve L, is turned to waste the water from the condenser and t o allow the condensate to return to tank A , and the condensate and water in containers X and Y are weighed. The maximum variation of the temperature of the vapor entering the throttle during a run is +0.5" F. The temperature of the vapor leaving the throttle keeps in step with any change in the temperature of the vapor entering the throttle. Calorimetric or enthalpy measurements were not taken with every Joule-Thomson measurement but were made over a sufficient range of temperature to determine the enthalpy value of any isenthalp. Choice of the pressure drop accompanying the expansion depends on the shape of the isenthalp. If the isenthalp is fairly straight, the pressure drop may be relatively large, but if the isenthalp is curved-for example, near the vapor pressure curve (saturated conditions) or in the vicinity of the critical point-small pressure drops are required for satisfactory results.
May, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
EFFECT FOR FIGURE 3. JOULE-THOMSON
Usually after the completion of one Joule-Thomson measurement, the initial pressure, temperature, and final pressure were adjusted for another measurement. When working a t conditions close to saturated vapor, the initial pressure and temperature were kept constant for a number of runs, and the final pressure only was readjusted for each measurement. Although much more time consuming, the latter procedure is far more atisfactory for determining values in the vicinity of the critical or the vapor pressure curve.
Treatment of Data Since the enthalpy value for n-pentane is a relative value above the selected datum of liquid pentane a t 32' F. and 1 atmosphere pressure, the enthalpy of the liquid pentane a t the temperature a t which it leaves the calorimeter relative to 32" F. was calculated from the specific heat data of Clark and Hufiman (9). The differences in the enthalpy of the vapor entering the calorimeter condenser and the liquid pentane leaving the calorimeter were calculated using the enthalpy values of water as determined by the Third Inter-
?&-PENTANE IN THE
7
national Conference on Steam Tables (IO). C o r rection for heat loss from the calorimeter was not made, as it waa found to be negligible. I n order that the JouleThomson expansion data for different ranges of pressure might follow in connecting steps (the initial pressure of one expansion to be the same as the h a 1 pressure of the next higher expansion), some of the original data were plotted with pressure and temperature as ordinates. The initial and final points of a n expansion were then connected by a straight line as a means of interpolating or extrapolating to the desired pressures, The temperatures as read from this plot and their differences corresponding to the various series of runs are given in Table I. It is estimated
LOW-PRESSURE REGION
351
? 30 i
619
I
I
I
I
I
I
CURVES FOR CONSTANT INITIAL AND FINAL PRESSURE OF JOULE-THOMSON EXPANSION. PI = INITIAL PRESSURE = LBS,/SQ IN. q=FINAL PRESSURE = LBS/SQ.IN.
100
200 300 400 500 600 700 FINAL TEMPERATURE OF JOULE-THOMSON EXPANSON Tef F
FIQURE 4. JOULE-THOMSON EFFECT FOR %PENTANE IN THE
800
HIQH-PRESSURE REQION
1
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
620
FIGURE5 .
EXTHALPY
OF
12-PENTASE
AT
TEmg..
Enthal y B. t. u./%d.
J K
M N 0 P
191.8 223.8 237.2 252.8 262.3 270.6 276.8
222 237 249 258 260 263 269
S T U
354.2 390.5
8
Expt. T:mp., KO. F.
--
7 -
308 329
Tr
- 1H / -T
PI
Fig. 8
1.25 2.20 5.50 7.70 7.90 8.05 8.20 8.40 8.65 8.68
=
. ---1H/T--
1.00
7 -
York To difference and over values Weber (18) from Fig. 8
... ... ... ...
-T, 0.45
0.75 1.00 1.28
1.64 1.80 2.06 2.55 3.02 3.41 3.72 4.12
4.37 4.53'" 4.64-
1.32 2.30 5.23 7.93 8.15 8.31 8.42 8.60 8.75 8.85
... ... ... ...
+6.6 +4.5
+4.9 +3.0 +3.2 . +3.2 4-2.7 +2.4 f2.3 f2.0
... ...
...
.,. Average +3.4
= 1.400.47 0.72 0.96 1.41 1.46 1.72 1.97 1.45 2.90 3.25 3.56 3 94 4.21 4.37 4.62
-4.4 -4.0 -4.0 -1.0 -1.3 -4.4 -4.3 -2.0 -4.0 -4.7 -4 2 -4.2
-3.7 -3.2
-2.6
Average -4.1 5
w
H
Expt. T z m p . , Enthalpy, No. F. B.t.u./Lb.
1-12 1-13 1-14 1-15 1-16 1-17 1-18 1-19 1-20 1-21 1-22
585.1 585.2 602.6 621.5 622.3 658.4 662.0 703.1 745.9 786.4 820.9
Extrapolated values.
Fig. 8
0.82 1.56 2.16 2.90 3.77 4.60 5.25 6.03 6.45 6.75 6.97 7.14 7.15 7,150
-0.355 0.55a 0.75a 0.94" 1.13a
1.32a 1.500
1.84a 2.14a 2.38'3 2.61a 2.97~ 3.24a 3.400 3.50a
I'r
= 1.10
York 70 difference and over values Weber (18) from Fig. 8
0.93 1.50 2.20 3.13 4.05 5.16 5.75 6.37 6.70 6.88 7.00 7.19 7.30 7.36 Average
+13.5 3.8 1.8 1.4 7.4 +12.1 9.5 5.6 3.8 1.9
+ + + + + + + + + 0.4 + 0.4 + 2.1 + 2.1 + 4.7
Tr = 1.50-
0.41 0.62 0.83 1.07 1.10 1.29 1.44 1.72 2.0s 2.35 2.60
2.99 3.22 3.39 3.50 Average
--
. 7 -
-- 6.5 - 2.8 1.2 -- 0.4 + 0.7 - 0.6 - 0.3 0 =t
5.1
0.000287 t 2
+ 0.315 t + 168.7
'S'ALUES FOR 12-PENTANE
- A HTr/ T -= 1.20------
Fig. 8
0.73 1.13 1.56 1.98 2.47 2.97 3.43 4.17 4.80 5.09 5.37 5.71 5.88 5.95a 7 -
+17.i +12.6 +10.7 +13.S - 2.7 2.3 4.0
=
Figure 5 also includes the experimental points previously reported (7) on the enthalpy of pentane vapor a t atmospheric pressure. The current values are about 10 per cent lower than the earlier ones.
429 438 449 460 468 491 495 516 527 589 611
O F EXPERIMENTAL AND COMPUTED a H / T TABLE 111. COMPARISON
0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.50 3.00 3.50 4.00 5.00 6.00 7.00
0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.50 3.00 3.50 4.00 5.00 6.00 7.00 8.00
Enthal y
B. t. u.pL6. 1-1 435.0 352 1-2 435.0 349 1-3 457.9 354 1-4 45S,3 351 1-5 460.1 352 1-6 484.0 374 1-7 484.0 376 1-8 527.3 436 1-9 563.4 383 1-10 563.8 418 1-11 563.8 415
%:-i 8:287 " 323.4
The isenthalps were extended to atmospheric pressure as indicated in Table 11,which also includes the enthalpy for the corresponding temperatures a t atmospheric pressures. The enthalpy a t atmospheric pressure is plotted as a function of temperature in Figure 5. The curve was obtained by smoothing the data by Running's graphical method (8). The equation of the curve in Figure 5 is:
ENTHALPY DATAAT ATMOSPHERICPRESSURE
EXPERIMENTAL
Expt. No. L
i
I
ATMOSPHERIC
PRESSURE
that these extrapolated or interpolated temperatures differ from the true temperatures by an amount less than 0.2' F. These temperature differences for a constant initial and final pressure were then plotted as a function of the final temperature (2'2) as shown in Figures 3 and 4. The curves were drawn through the points by applying Running's graphical method (8) of smoothing experimental data.
TABLE 11.
Vol. 33, No. 5
O.ZQ~ 0.445 0.59a 0.74a 0.88 1.03" 1.18" 1.455 l.70a 1.90a 2.12' 2.49 2.67a 2.s2m 2.90"
York 56 difference and over values Weber ( 1 s ) from Fig. 8
0.73 1.11 1.51 1.97 2.45 2.96 3.42 4.34 4.92 6.26 5.49 5.74 5.86 5.92
0
-1.8 -3.2 -0.5 -0.8
-0.3 -0.3
4-4.1 4-2.6 +3.3 4-2.2 +0.5 -0.3 -0.5 Average il.9
Tr = 1.60-
0.30 0.46 C.60 0.74 0.87 1 .02 1.14 1.40 1.63 1.89 2.06 2.39 2.63 2.76 2.90
+3.!
+4.0 +1.7 0 -1.1 -1.0 -3.4 -3.5 -4.1 -5.1 -2.8
-2.4 -0.7 -2.1 -0
Average + 2 . 8
-TT ---1H/TFig. 8
0.55 0.78 1.13 1.45 1.77 2.10 2.41 3.03 3.57 4.00 4.30 4.72 4.95 5.07a
= 1 . 3 0 York % difference and over w l u e s Weber (fa)from Fig, 8
0.55 0.84 1.14 1.48 1.77 2.12 2.44 3.07 3.59 4.03 4.27 4.58 4.77 4.90
-1,5
f7.7 f0.9 f2.1
0 f1.0 +1.2 f1.3
+0.6 +0.7 -0.7 -3.0 -3.6 -3.4 Average 12.8
May, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY PRESSURE
POUNDS
PER
SQUARE INCH
621
622
Vol. 33, No. 5
INDUSTRIAL AND ENGINEERING CHEMISTRY
The equation for the specific heat of pentane vapor a t atmospheric pressure is obtained by differentiating the above equation : C, =
dH
= 0.000575 t
+ 0.315
This equation is plotted in Figure 6 for comparison with the following equations which have been suggested by other investigators. Cope, Lewis, and Weber (3)suggested a general specific heat equation for hydrocarbons which, for pentane, reduces to: C , = 0.000623 t
+ 0.346
Cragoe (4) suggested a general specific heat equation for hydrocarbon vapors which may be evaluated by substituting the specific gravity of the hydrocarbon in the liquid state; for n-pentane it reduces to: C, = 0.000567 t
4-0.346
Bahlke and Kay (1) derived a general equation which, applied to n-pentane, is: C, = 0.000322 t
+ 0.350
Watson and Nelson (11) reported the specific heat of hydrocarbon vapors also &s a function of liquid density, which would indicate the curve as drawn in Figure 6 for their value of n-pentane. The reported data consist of the single value of Dixon and Greenwood (6) a t 86" C., which corresponds to 0.41 B. t. u. per pound per F. a t 187" F., and several experimental values of Sage, Webster, and Lacey (9),which lie along the line terminated by the values of 0.404 and 0.486 B. t. u. per pound per " F. a t 100" and 310" F., respectively. As a result of the comparison (Figure 6) it is believed that more confidence can be placed in the present enthalpy data than those previously reported (7) because of the fairly good agreement of the present specific heat results with the experimental heat data (6, 9) and the general equations of Bahlke and Kay and of Watson and Nelson, although they are 5 per cent lower than the equation of Cragoe and 8 per cent lower than the relation proposed by Cope, Lewis, and Weber.
Pressure-Temperature Enthalpy Chart for n-Pentane
A convenient graphical representation of the thermal properties of n-pentane is shown in Figure 7, representing a family of isenthalps plotted on coordinates of temperature and pressure. This chart was prepared by plotting the leveled data of Table I. The temperature a t atmospheric pressure (14.7 pounds per square inch) for any particular value of enthalpy is determined by the data of Table I or Figure 5. This may be considered the final temperature of a Joule-Thomson expansion down to atmospheric pressure. The initial temperature for this expansion, representing another point on the isenthalp, is that corresponding to the next higher pressure such as 626 pounds per square inch, as given in Table IV, and is determined by adding the value of the temperature difference ( T I - Ts) as read from Figure 3. A similar procedure is followed in determining the next temperature point on the
same isenthalp a t 962 pounds pressure from the curve of Figure 4. I n this manner the isenthalp can be constructed up to pressure of 3000 pounds per square inch. I n the vicinity of the vapor pressure curve the isenthalps have considerable curvature and also points of inflection, and many more points are required for satisfactory results. To obtain points for these isenthalps, two intermediate charts were constructed. Detailed data obtained in the experiments were plotted directly and isenthalps drawn. For interpolation a cross plot (lines of constant pressure on ordinates of enthalpy and temperature) was also constructed. Other experimental data were used for the construction above the vapor pressure curve. The enthalpies for the isenthalps intersecting the vapor pressure curve on the saturated liquid side were determined by subtracting the latent heat of n-pentane from the enthalpy of the saturated vapors, using the latent heat data reported by Young (13). The vapor pressure curve and critical point are also those reported by Young. The experimental Joule-Thomson measurements previously reported (7) are in excellent agreement with those determined by taking the slopes of the isenthalps from Figure 7, a t the lower pressures where the isenthalps are approximately straight lines. As a Convenient means of comparing the experimental data obtained on pentane with those data reported for other compounds, and also for the purpose of estimating the effect of pressure on the enthalpy of other compounds from the data reported here, Figures 8 and 9 were constructed; 847" Rankine was used as the critical temperature and 485.4 pounds per square inch as the critical pressure for %-pentane. The reference datum wm chosen a t zero pressure.
Discussion Since this work was completed, the correlation used in Figure 8 was modified by York and Weber (19) by replacing the ordinate A H / T by (AH/T,)q5 where 4, an empirical corre-
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FIGURE 8.
EFFECTOF PRESSURE ON EXTHALPY OF ?%-PENTANE AT' CONSTANT TEMPERATURE-VAPOR REGION
,
.”
623 properties of complex mixtures from the pseudocritical t e m p e r a t u r e and pressure in connection with a reduced enthalpy plot such as Figure 8 have not been entirely s u c c e s s f u l . This may be due in large measure to the fact that the pseudocritical temperature and pressure for a m i x t u r e occurs w i t h i n t h e twophase region, i n which case it would be expected that only those isotherms on an enthalpy plot such as Figure 8 which lie wholly in t h e single-phase region could be used for complex mixtures as well as pure hydrocarbons.
Nomenclature C, = specific heat, B. t. u./(lb.) (” F.) H = enthalpy of n-pentane O F PRESSURE ON ENTHALPY O F +PENTANE AT CONSTANT TEMP~RATURE-LIQUID FIGURE 9. EFFECT above 32OF. AND -VAPORREQIONS at atm. pressure B. t. u. PI = initial pressure of Joule-Thomson expansion, Ib./sq. in. lation factor, is a function of the critical and the reduced temabs. peratures. The introduction of 4 is an attempt to refine the Pz = final pressure of Joule-Thomson expansion, Ib. /sq. in. abs. correlation by correcting for the deviations of the individual P, = reduced pressure hydrocarbons from the reference hydrocarbon. The method TI = initial temperature of Joule-Thomson F. TI = final temperature of Joule-Thomson expansion, F. of obtaining the appropriate value of 4 was fully described I = temperature, F. (12). Since 4 is a correction factor, it may be used in conT. = critical temperature, O R. junction with Figure 8 by the following procedure: A H / T TI = reduced temperature = actual absolute temperature/ absolute critical temperature values read from Figure 8 and multiplied by 4 will be the corAHJT = ratio of decrease in enthalpy per mole above zero presrected AH/T values for the hydrocarbon in question. The sure/absolute temperature application of the correlation factor would change the AH/T + = correlation factor (19) values read from Figure 8 for the reduced temperature range from 1.00 to 1.60 by the designated average percentages for Literature Cited the following hydrocarbons : n-Hexane +2 Methane -17 (1) Bahlke, W. H., and Kay, W. B., IND.ENQ. CHEM.,21, 942 ?&-Heptane +3 REDUCED PRESSURE = PR
O
-
Ethane Propane %-Butane n-Pentane
-9
- 052 (reference hydrocarbon)
n-Ootane n-Nonane n-Decane
i-4 +6
+e
The correlation of York and Weber presented as a graph is based on calculated results from P-V-T data on propane and ethane, and has been applied with good agreement in predicting the effect of pressure on enthalpy a t constant temperature for several other substances. Table I1 compares A H / T values read from Figure 8 and calculated from the correlation of York and Weber. The calculated and experimental values are in good agreement for the entire range of temperature. The density of complex hydrocarbon mixtures may be estimated with fair precision from the pseudocritical points of the mixture as equivalent to the true critical point of a pure hydrocarbon i n connection with the reduced properties of hydrocarbon vapors; but attempts to estimate the thermal
(1929). (2) Clark, G.S.,and Huffman, H. M.. J. An. Chem. Soc., 52,4381 (1930). (3) Cope, J. Q.,Lewis, W. K., and Weber, H. C., IND. ENQ.CHEM., 23, 887 (1931). (4) Cragoe, C. S., U. S. Bur. Mines, Misc. Pub. 97 (1929). (5) Dixon. H. D.. and Greenwood.. G... Proc. Rou. SOC.(London), ~ 1 0 5199 , (i924). (6) Lindsay, J. D., and Brown, G. G., IND.ENQ. CHEM.,27, 517 (1936). (7) Pattee, E.C.,and Brown, G. G., Ibid.,26, 511 (1934). (8) Running, T. R., “Graphical Calculus”, p. 19, Ann Arbor, Mich., George Wahr, 1937. (9) Sage, B. H., Webster, D. C., and Lacey, W. N., IND. ENQ. CHEM.,29, 1309 (1937). (10) Third Intern. Conf. on Steam Tables, Mech. Eng., 57, 714 (1935). (11) Watson, K. M.,and Nelson, E. F., IND.ENQ. CHIM., 25, 880 (1933). (12) York,R.,Jr., and Weber, H. C., Ibid., 32,388 (1940). (13) Y o u n g , Sidney, Sci. Proc. Roy. Dublin Soc., 12,374 (190S-10) .
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