Effect of Pressure on Enthalpy of Pentane, Heptane, and Isooctane E. R. GILLILAND AND M. D. PAREKH Massachusetts Institute of Technology, Cambridge, Mass. Company and was subjected t o fractionationin the previously mentioned column. The purity of the n-pentane so obtained was substantiated by its refractive index, density, and constancy of boiling point.
Data have been obtained on the isothermal enthalpy changes with pressure for n-heptane, n-pentane, and isooctane by an experimental technique previously described (5). The pressure and temperature ranges covered are 200 to 2900 pounds per square inch and 330' to 660' K., respectively. The data are compared with those of other investigators.
Results and Discussion The experimental data and results are presented in Tables I, 11, and 111. The plots of isothermal enthalpy changes for n-heptane, n-pentane, and isooctane (Figures 1, 2, and 3) show the experimental points. H I is molal enthalpy a t one atmosphere absolute and H i s the molal enthalpy a t P atmosphere absolute. The critical constants for n-heptane and isooctane were those given by Beattie and Kay (6)and Beattie ( I ) , respectively. For n-heptane Young's critical constants (9) were used. His vapor pressure data were also chosen t o determine vapor pressures at T R = 0.7, 0.8, and 0.9 for n-pentane and n-heptane. Table IV gives the values of the physical data used. Young's heat of Vaporization data were used to determine the molal entropy of vaporization a t TR = 0.7, 0.8, and 0.9 for these materials. Table V summarizes the values. I n Figures 1 and 2 the liquid isotherms were drawn to the corresponding vapor pressures, since the ends of these isotherms would obviously lie on the liquid saturation curve. The saturation curves were then drawn through the points thus established and the critical point, and are the upper part of the envelope curves. The molal entropy of vaporization a t T R = 0.7,0.8, and 0.9 for the hydrocarbons in Table V were subtracted from values of ( H I - H ) / T read from the saturated liquid curve a t the corresponding vapor pressures t o give values a t the same pressures on the saturated vapor curve. The curves drawn through the points thus determined and the critical point are the saturated vapor curves and are the lower part of the envelope curves in Figures 1 and 2. For
XPERIMEKTAL data on the effect of pressure on the enthalpy of benzene were presented in an earlier paper (6). This article gives similar results on n-heptane, n-pentanq, and isooctane (2,2,4-trimethylpentane). The experimental apparatus was the same as that previously described (5) with the following exceptions: In the present setup the current passing through and the voltage drop across the expansion capillary were measured by a Leeds & Northrup type K potentiometer and a calibrated shunt, and by the potentiometer and a calibrated decade resistance box, respectively. I n order t o avoid the vapor locking of the high-pressure pump while pumping a lowboiling material such as n-pentane, a slow-moving, water-cooled, low-pressure liquid pump preceded the high-pressure pump. These two pumps were connected in series with a mercury manometer and a by-pass control valve in between. The n-heptane and isooctane were donated by the Standard Oil Development Company. The physical properties of the n-heptane were measured by the National Bureau of Standards, and the values indicated that the material was of high purity. The isooctane, produced by the hydrogenation of diisobutylene, was purified by repeated distillation in a 20foot glass column packed with glass beads, and the final product had a boiling point of 99.4" C. a t 760 mm. of mercury. Crude n-pentane was obtained from Viking Distribution
E
-
PR P / P,
PR- P/P,
P,-
P/P,
FIGURE1 (Left). ISOTHERMAL ENTHALPY CHANGE FOR HEPTANE. FIGURE 2 (Center). ISOTHERMAL ENTHALPY 3 (Right). ISOTHERMAL FIGURE CHANGE FOR WPENTANE. ENTHALPY CHANCE FOR ISOOCTAXE 360
INDUSTRIAL AND ENGINEERING CHEMISTRY
March, 1942
TABLE I. EXPERIMENTAL DATAAND RESULTSFOR Rate of Flow, Grams/Min. 31.9 25.5 18.0 16.3 30.0 26.3 24.2 14.6 17.5 12.4 13.3 26.9 20.9 12.3 8.47 7.08 4.85 4.14 3.59 27.2 21.9 11.4 11.1 9.18 6.07 24.3 18.7 17.2 11.5 8.07 19.8
PR 6.00 4.00 2.00 0.80 6.00 6.00 4.00 2.00 1.50 0.80 0.296 6.00 4.00 2.00 1.50 1.20 1.00 0.90
TR 0.80
0.90
1.00
0.80
0.60 6.00 3.00 3.00 2.00 1.50 0.60 6.00 3.00 2.00 1.50 0.50
1.10
1.20
Capillary Input Gram' Cal./Sec. 35.4 , 28.4 20.0 18.7 30.2 26.6 25.4 14.4 17.3 12.3 0.93 24.0 18.4 10.4 6.74 5.36 2.31 1.24 0.78 4.28 17.0 8.05 7.64 5.39 2.08 2.68 12.1 9.44 4.37 2.10 1.32
%-HEPTANE
(HI - H ) / T , (G. Gal.)/ (G. Mole) (" K.) 15.4 15.4 15.5 15.9 12.4 12.5 12.4 12.2 12.2 12.2 0.86 9.93 9.79 9.38 8.95 8.42 5.28 3.32 2.42 1.75 7.85 7.12 6.92 5.94 4.13 1.11 6.01 5.08 3.51 2.41 0.62
TABLE 11. EXPERIMENTAL DATAAND RESULTSFOR TR 0.70
0.80
0.90
1.00
1.10
1.20
isooctane it was not possible to construct the envelope curve, as the vapor pressure data for this material are not available. The over-all probable error in the experimental results was estimated to range from *2 per cent a t the higher pressures and lower temperatures to about j=4 per cent a t the lower pressures and higher temperatures.
Comparison with Other Data Figures 4 and 5 compare the results of this investigation
Weber (solid lines) for n-heptane with those of Gilliland (0, and York (8), Edmister (S), and Watson and Smith (7), calculated from P-V-T data by different techniques; Figure 6 compares the experimental values for *pentane with those calculated by the methods of Weber and York and of Edmister and with those of Pattee and Brown (8) obtained by JouleThomson expansions. I n these plots (Ho - H ) / T is plotted against PR for convenience. Ho is the molal enthalpy a t zero pressure, and H is the molal enthalpy a t P atmospheres ab-
1.30
1.40
TR 0.732 0.836 0.942
1.047
8 1.15
4
2 I
I .o 0.8 0.6 0.4
0.2 0.1
0.2
0.4
0.6 0.8 10.
2
4
PR
FIGURE 4.
COMPARISON OF n-HEPTANE
RESULTS
6
PR
5.00 4.00 2.00 0.90 6.00 4.00 4.00 4.00 2.00 2.00 0.90 6.00 4.00 2.00 0.71 6.00 3.00 1.50 1.00 0.90 0.70 6.00 6.00 4.00 4.00 3.00 2.00 1.50 1.00 6.00 3.00 1.50 1.00 6.00 3.00 1.50 1.00 6.00 3.00 1.50
Rate of Flow, Grams/Min. 21.5 27.5 19.5 16.1
27.1 14.5 14.8 54.1 29.2 17.9 8.21 8.56 26.2 27.6 24.9 24.3 16.4 17.3 12.7 8.52 6.81 4.22 22.1 11.1 6.10 3.93 20.1 10.15 5.73 3.72 17.9 11.2 5.45
Capillary Input Gram' Cal./Sec. 28.9 37.8 27.1 23.0 40.5 32.7 33.3 33.3 23.3 34.2 23.4 31.2 16.5 17.1 62.5 30.1 18.3 7.81 4.56 9.35 5.75 23.2 22.3 14.9 15.3 10.6 5.83 3.32 1.09 17.6 7.08 1.91 0.75 13.4 4.89 1.34 0.54 10 IO9 4.41 0.99
?&-PENTANE
(HI - f U / T , ( G . Gal.)/ (G. Mole) ( " K.) 17.7 18.0 18.2 18.7 14.1 14.4 14.5 14.5 14.7 14.6 14.6 11.8 11.7 11.8 11.6 9.48 9.42 8.76 4.9 3.28 1.91 7.82 7.67 7.61 7.42 6.97 5.72 4.07 2.15 6.11 4.88 2.40 1.47 4.70 3.40 1.66 1.03 3.73 2.59 1.21
TABLE 111. EXPERIMENTAL DATAAND RESULTS FOR ISOOCTANE
IO
6
361
PR 6.80 2.91 1.48 6.80 2.91 1.48 6.80 2.91 1.48 0.43 6.80 2.91 1.48 1.19 0.97 0.89 0.81 0.72 6.80 2.91 1.78 0.81
Rate of Flow, Grams/Min. 27.4 21.9 24.1 25.4 20.8 23.7 22.4 17.6 18.5 3.17 21.5 14.1 9.98 7.88 7.60 13.3 6.38 12.5 17.5 12.3 18.6 9.50
Capillary Input Gram' Cal./Seo. 27.6 22.6 26.4 24.4 20.1 23.4 19.4 15.2 16.2 0.49 17.3 11.1 7.37 5.54 4.10 3.81 1.54 2.60 12.8 8.12 9.58 1.59
(HI - H ) / T , (G. Gal.)/ (G. Mole) (" K.) 17.3 17.8 18.8 14.4 14.5 14.9 11.6 11.6 11.7 2.10
7.99 7.20 5.64 1.84
solute. I n order to obtain (Ho - H ) / T from the measured values of (HI - H ) / T , values of (Ho - H 1 ) / T were read from calculated plots at the given reduced temperatures and added to the experimentally determined values. These plots indicate that the agreement between the experimental points and those calculated from the same source of P-V-T data by Gilliland and by Weber and York is excellent at all pressure ranges except a t very low PR,where the experimental curves were extrapolated and hence might not be accurate. Points calculated by Edmister's correlation are in fairly good agreement with the experimental curves for the entire isotherms of T R = 1.0, 1.3, and 1.4 and in the low-pressure ranges for all the isotherms. For the isotherms TR = 1.1and 1.2 in the high-pressure range, values computed by Edmis-
INDUSTRIAL AND ENGINEERING CHEMISTRY
362
Vol. 34, No. 3
IO
0
6 4
2
I.o 0.8
0.6 0.4
0.2
I
0.2
0.4
0.6 0.8 10.
2
4
6
PR
FIGURE6. COMPARISOX OF TZ-PENTANE RESULTS
PR
FIGURE5. COMP.kRISON O F %-HEPTANERESULTS TABLEIV. PHYSICAL CONSTAXTS n-Heptane n-Pentane Isooctane
T e , K. 540.0 470.3 544.2
Pc
Atm. kbs. 26.8 33 .O 25.46
Reduced Vapor Pressure at: T R = 0.7 T R = 0.8 T R = 0.9 0.166 0.443 0 .Oi76 0.189 0.472
...
....
....
TABLEV. MOLALENTROPY OF VAPORIZATION n-Heptane n-Pentane
(Gram Cal.)/(Gram Mole) (" K.), at: T R = 0.7 T R = 0.8 TR = 0.9 14.52 10.10 1i:io 13.23 8.91
ter's method are about 10 per cent lower than those found experiment ally. In general, the plot of Katson and Smith and also the data obtained by Pattee and Brown give larger deviations (*15 per cent). I n the present investigation data were obtained on the isothermal enthalpy changes with pressure on n-heptane and npentane. Previously (6) such data were obtained on pure benzene, using the same apparatus and the same experimental technique. Based on the data of these three hydrocarbons, an average plot is made of (Ho - H ) / T us. P R as shown in Figure 7 . Isotherms T R = 1.0, 1.1, and 1.2 are based on the data of all three hydrocarbons, while the remaining two isotherms are based entirely on the n-pentane data. The deviation of the experimental points from the average curves in almost all cases is well within 5 per cent. Isooctane data were neglected for this purpose since they were obtained a t odd values of the reduced temperatures, and since sufficient data were not obtained for reliable extrapolation and interpolation. However, approximate comparisons indicate that the isooctane data are about 10 per cent higher than the values read from the average plot.
FIGURE7 . SUMMARIZED RESULTS
Literature Cited (1) Beattie, J. A . , private communication, 1940. (2) Beattie, J. A., and Kay, W.C., J . Am. Chem. Soc., 59, 1588 (1933) (3) Edmister, W.C..IXD. EXG.CHEDI., 30,352 (1938). (4) Gilliland, E. R., unpublished plots, April, 1936. ( 5 ) Gilliland, E. R., and Lukes, R. V., IND.ENG.CHEM.,32, 957 (1940).
(6) Pattee, E. C., and Brown, G. G., Ibid., 26,511 (1934). (7) Watson, K. M., and Smith, R. L., Nutl. Petroleum News, 28,J u l y , 1936. (8) Weber, H. C., and York, R., IND. ENG.CHEM.,32,388 (1940) (9) Young, S., Sci. Proc. Roy. Dublin SOC.,12,374 (1910).
