Phase Equilibria in Hydrocarbon Systems VI. Thermodynamic

Phase Equilibria in Hydrocarbon Systems VI. Thermodynamic Properties of Normal Pentane. B. H. Sage, W. N. Lacey, and J. G. Schaafsma. Ind. Eng. Chem...
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Phase Equilibria in Hydrocarbon Systems VI. Thermodynamic Properties of Xormal Pentane' B. H. SAGE,W. N. LACEY,AND J. G. SCHAAFSMA, California Institute of Technology, Pasadena, Calif.

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PART of the investigation being carried on by Research Project 37 of the American Petroleum Institute is concerned with the equilibrium behavior of comparatively simple hydrocarbon systems consisting of two and three constituents. *Pentane is a desirable hydrocarbon for inclusion in such studies. Before using i t in mixtures, however, the properties of the substance itself should be known over the entire pressure and temperature range used in the rest of the work. I n the above project, studies are made a t pressures up to 3000 pounds per square inch absolute and a t temperatures from 70" to 220" F. Previous investigators have studied n-pentane and have obtained portions of the information desired. Rose-Innes and Young (3) investigated the pressure-volume-temperature relations from 176" F. to above the critical temperature and to pressures of about 1000 pounds per square inch absolute. These authors have studied the superheated gas region down to a temperature of 104" F. while Young (6) has made corresponding studies for the saturated gas. The density of the s a t u r a t e d l i q u i d has been measured a t much lower temperatures by Young (6) and 0 625 by Timmermans (6). Pattee and Brown (S) report throttling data for the superheated 0 600 gas for t e m p e r a t u r e s from 200" to over 900" F. a t pressures up to 1000 pounds per y square inch. 8 0675 The n-pentane used in the present work was o b t a i n e d from the Philgas Company who 0 550 submitted the following special analysis: 99.3 per cent n-peniI tane, and 0.7 per cent isopen-

tane. This material was not further purified except that dissolved gases, mainly air, were removed by prolonged boiling under reduced pressure. The boiling point a t atmospheric pressure was in good agreement with that given by other investigators. The vapor pressure of the material was found to be independent of the amount placed in the apparatus. The methods and apparatus used in this investigation have been described by Sage, Schaafsma, and Lacey ( 4 ) .

EXPERIMEETAL RESULTS The specific gravity2 of n-pentane in the condensed liquid region was measured as a function of pressure a t a series of temperatures. The appearance or disappearance of the gas phase was determined by the sharp break in the pressurespecific gravity relation a t that point. The results are shown in Figure 1. The values shown on the saturated liquid line a t the left of the figure agree very well with those obtained by Young (6), except a t the higher temperatures where deviations up to 0.1 per cent were found. The measured vapor pressures also agreed within experimental error with those of Young. Figure 2 gives values for the specific heat a t constant pressure for liquid p e n t a n e under a total pressure of 1000 pounds per square inch absolute as a function of temperature, having been determined by the adiabatic e x p a n s i o n method (4). Each point shown was the average of seven separate m e a s u r e m e n t s . The

I I A

Parts I to V of this series appeared in January, February, June, August, and November, 1934.

500

1

FIGURE

TEMPERATURE

FIGURE

2. SPECIFIC HEATAT

I

Q525 I000 PRESSURE

IS00

2000 POUNDS PER SQUARL INCH

2500

1. SPECIFIC GRAVITYOF CONDENSED LIQUID

3 Specific gravitiea here reported are defined a8 the weight of a unit volume of the substance at the 8Decified temperature and pressure divided by the weight of the eame volume of water at maximum density for one atrnosphere preseure.

OF.

C O N S T A S T PRESSURE FOR

TO PRESSURE FIGURE3. RATIOSOF FUGACITY

CONDENSEDLIQCID

48

INDUSTRIAL AND ENGINEERING CHEMISTRY

January, 1935

TABLE 1. P" Satd. vapor Satd. liquid 10 20 30 40 50 60 70 80 90 100 200 400 600 800 1000 1250 1500 1750

P H Y S I C A L AKD

THERMAL PROPERTIES

70° F.; 8.49 LB./SQ. IN. SATK.PRESSURE 8 u f/P h ..... ..... 0.972 ,.. 0.02560 0.972 6.48 0.0132

..... .....

..... ..... ..... 0.02556 0.02551 0.02544 0.02537 0.02530 0.02523 0.02514 0.02506 0.02498 0.02491 0.02485 0.02480 0.02474 0.02470

..... ..... .....

... ...

...

.....

.....

.. .. .. .. ..

....

...

....

0.0858 0,04431 0.02364 0.01681

0.00898

6.71 6.96 7.47 7.98 8.50 9.02 9.69 10.36

0.01198 0.01 156 0.01074 0.00994 0,00915 0,00837 0.00742 0.00649

0 00789

11.72

0.00471

0.00739

i3:i3

0.00300

0.00721

14:56

0.00139

.....

....

0 01147

.....

.....

..... .....

...

...

.....

O F P E N T A X E SUPERHEATED

CONDENSED

LIQCID REGIOS

loo0 F.; 15.62 LE./% IN. SATN.PRESSURE 130' F.; 26.37 LB./SQ. Ir.SATN.PRESSURE h 8 u //P h 8 c f/P

.....

0.02632 8.070

..... ..... .....

0.9574 0.9574 0.9705

177.4 26.16

.....

..... ..... ..... ..... .....

.....

..... ..... ..... .....

..... .....

0.1536 0.07932 0.04221 0.02997

0.9362 0.9362 0.9742 0.9502

.....

.....

.....

.....

.....

...

...

... ... ... ...

..... ..... .....

.....

.....

.....

0.01586 0.01388

31.06

0.04017

0.01294

32:39

o'Oiki3

0.01255

33 j 6

:

0.03659

.....

..... .....

...

.....

..... ..... .....

.....

0.04808 0.04762 0.04673 0.04585 0.04499 0.04416 0.04312 0.04211

.....

0.02552 0.02543 0.02535 0.02628 0.02522

.....

3.098 0.02709 8.530 4.151

0.3190 0.04846

... ... ...

26.36 26.59 27.06 27.54 28.03 28.52 29.14 29.77

0.02037

.....

..... .....

GAS AND

49

..... .....

0.02i04 0.02698 0.02687 0.02677 0.02668 0.02660 0.02648 0.02637 0.02626 0.02616 0.02605 0.02596 0.02587 0.02580

.....

191.4 46.48

0.3297 0.0838

....

.....

.....

....

.....

.... ....

..... .....

0.02679

44.63 46.83 47.24 47.66 48.10 48.54 49.11 49.69

0,08333 0.08282 0.08183 0.08086 0.07991 0.07899 0.07788 0.07679

0,02247

50.90

0.07470

0.02086

52.14

0.07271

0.02016

53.42

0,07081

.....

..... .....

..... 0.2525 0.1302 0.06923 0.04915

.....

0.03325

..... ..... .....

.....

.....

....

.....

....

160' F.;42 15 LB./SQ.IN. SATS. PRESSCRE 1.90' F.; 64.13 LB./SQ. Is.S A T S . PRESSCRE 220' F.: 94.02 LB./SQ. I S . S A T N . PREBBCRE 222.4 0.3672 0.890 0.8521 Satd. vapor 1.974 0,9102 206.8 0.3429 1.309 0.8830 237.6 0.3709 0.02889 0.8830 0.03016 0.8521 90.30 0.1541 114.51 0.1895 Satd. liquid 0.02797 0.9102 67.76 0.1187 ... ..... 9.933 0.9844 8 . 997 0.9779 9.467 0.9814 10 ... .... ..... 20 4 293 0.9662 ... ..... 4.640 0.9629 4.884 0 9689 d.031 0,9446 2.860 0.9351 ... ..... 3.199 0.9531 30 2.092 0.9144 ... .... 2.224 0.9266 ,.. ..... .... 2 353 0.9373 40 50 ..... ..... ,.. ..... ... ..... 1.846 0.9216 f.739 0.9086 . . . ..... . . . . ..... ... ... ..... 1.412 0,8907 60 ..... 1,607 0,9058 .... ..... 70 .,.. ... ... 1.264 0.8901 .... ..... ... 1.082 0.8744 80 90 .... ..... ... ,.. ..... 0.939 0.8588 100 0.02792 0.3906 67.85 0.11839 0.0'2895 0 5724 90.33 0.15380 0,03015 0.8027 114.51 , , , . . 200 68.00 90.42 0,15312 0.03003 0.4134 114.51 0.18921 0.02785 0.2013 0.11781 0.02886 0.2949 400 0.02980 0.2194 114.55 0.18768 0.11668 0.02869 0.1565 0.02771 0.1067 68.32 90.63 0.15180 0.02960 0,1551 0.11558 0.01852 0.02757 O.Oi582 68.67 600 0 1107 90.87 0.15054 114.64 0,18619 0.11452 0.02744 10.02833 800 .., . 69.03 ..... 91.13 0.1493% . .,.. 0,02940 114.77 0.18478 1000 0.02733 0.05114 69.41 0.11349 0.02822 0.07472 91.43 0.14816 0 02921 0.1046 114.95 0,18344 0.02720 0.11225 0.02805 ..... z9.91 ..... 91.83 0.14677 0,02898 1250 115.23 0.18187 0.11105 0.03709 0,03953 1500 10.43 0.02789 0.05760 92.26 0,14545 0.02877 0:08048 115.55 0.18038 1750 0.02696 .., . . ... ..... IO. 02774 . ,., 0.0285-i ..... .... ..... 2000 0.02685 0.03432 71.52 0.02760 0.04988 93.21 0.14296 0,10877 . ,.. 116.32 0.17762 0.06964 0.0267% 9.02746 2250 ..... ..... , .., ..... 0,02838 0.02822 .... 2500 0.02661 0.03175 7i:66 0.10662 0.02733 0.04789 94.'26 0.14063 0.02809 0.06398 117.21 o i7i07 2750 0.02651 ..... ..... 0.02720 ., . . ... ..... ..... ..... ..... 0.02796 3000 0.02642 0.03056 73 8 5 0.02711 0.04415 95.33 0.13541 0,10457 0.02784 0.06123 118.18 0.17268 a P = pressure, Ib. per s q . in. ahs.; u = spezific volume, cu. i t per l b . ; / = fugacity, Ib. per s q . i n . ; h = hezt content, B . t. u . per lh.; s = entropy, B.t. u. per lb. per ' F. ah:.

....

.. ..

.

:

average deviation of the experimental points from those in Figure 2 was about 2 per cent. It is believed that the results given are trustworthy to about 1 per cent. The apparatus used in the present work was designed primarily for measurements a t pressures from 100 to 3000 pound. per square inch. It was therefore thought undesirable to attempt to improve upon the vapor pressures or the specific gravities of saturated and superheated pentane gas obtained by Young ( 6 ) , since the pressures involved all lie below 100 pounds per square inch for the temperature range here considered. However, as a general check, isotherms were determined at five temperatures for the wperheated gas region. The results. not reported here, were in substantial agreement with those of Young. Pressure measurements below 50 pounds per square inch absolute were made by using a mercury manometer. This work indicated that perhaps Young's values of saturated specific volume a t the lower temperatures might be 1 or 2 per cent high. Values of (6u/6T), obtained for the superheated gas were slightly larger than Young's. However, his figures have been used for this lowpressure region in the results reported in the following section. CALCCLATED RESULTS

The ratio of fugacity to pressure, as a function of pressure, has been calculated (4) for a series of temperatures, as shown in Figure 3. An assumption involved in these calculations was that ( 6 z / 6 p ) was ~ a constant below 10 pounds per square inch absolute, where z = pVV/RT. The data of Young were used in the superheated gas region u p to saturation pressure.

.....

...

. .

The values given agree very 17-ellwith those found for propane (4) when compared a t the same reduced pressures and temperatures, and with the generalized values given by Lewis and Kay (1) for the superheated gas and saturation regions. Fugacity values are also found in Tables I and 11. TABLE11.

PHYSIC.4L AND THERMAL PROPERTIES OF PENTANE SATURATED LIQUID AND SATURATED GAS ---S.ATuRATED

p

ta

10 20 30 40 50 60 70 80 90 100 a

a4

h

77.8 7.71 113.7 4.02 183:6 1 3 7 , 9 2.736 195.3 156.4 2.076 2 0 4 , 8 172.1 1,672 213.1 185.2 1.398 220.0 196.6 1,199 225.0 206.9 1.048 231.0 216.4 0.931 235.7 225.1 0.837 240.0 1 = temperature i n ' F.:

Gas--s

-SATCRATED

//P u 0.0969 0.02580 0.'3234 0.9484 0.02666 0.3328 0,9295 0.02731 0.3408 0.9133 0,02786 0.3489 0,8995 0.02836 0.3551 0,8875 0,02882 0.3603 0.8764 0.02922 0.3651 0.8657 0.02962 0.3692 0.8559 0.03001 0.3728 0 . 8467 0.03037 other units as in Table I.

LIQUID----

h 12.18 35.20 51.95 65.09 76.64 86,57 95.48 103.80 111.55 118.78

s

0,0229 0.0647 0,0929 0.1142 0.1329 0.1482 0.1618 0.1742 0.1856 0.1958

Calculation of values of specific heat content and specific entropy for condensed and saturated liquid pentane have been carried out by methods previously described (4), using the data of the authors. Corresponding calculations for saturated gas were made, combining Young's data with those of the authors. The values presented in Tables I and I1 are given to one more significant figure than would be justified by the accuracy of the specific heat data. This is necessary to show adequately isothermal changes which are not affected in accuracy by the specific heats. The values of both specific heat content and specific entropy were arbitrarily taken as zero for saturated liquid pentane at 60" F.

I N D U S T R I A L A N I3 E K G 1 N E E R I N G C H E illI S T R Y

When the methods of calculation (4) used above were a p plied to Young's data in the superheated gas region, i t was found that incompatible results were obtained for isothermal changes in heat content. This is due to the large effect of small errors in the determination of (6v/6T)=a t the pressures encountered in this region. For this region i t was thought that extrapolation of the results of the throttling experiments of Pattee and Brown (2) above 200" F. to lower temperatures would give more accurate results; it was therefore used to calculate the values of heat content and entropy in the superheated gas region. The values for entropy were thus between those obtained from the data of Young and from the data of the authors. Because of the uncertainty introduced in the extrapolation, these values for the superheated gas region have been omitted from the tabulation but are shown in Figure 4. The experimental and calculated results are summarized in the temperature entropy diagram of Figure 4. The values shown for the two-phase and superheated gas regions are

~ ' o I 2. 7 , NO. 1

characterized by more uncertainty than is the case for the condensed liquid region. It is believed that the two-phase region above 130' F. is accurate to about 1.5 percent and that the superheated gas region may be relied upon to about 5 per cent. ACKNOWLEDGMENT This investigation was made possible by financial support received from the American Petroleum Institute. LITERATURE CITED Lewis, W.K., and Kay, W. C., Oil Gas J.,32,No.45.4 (1934). Pattee, E.C.,and Brown, G. G.. IND.ENQ.CHEN.,26,511(1934). Rose-Innes. J., and Young, S., Phil. Mag., (5)47,353 (1899). . . Sage, B. H., Schaafsma, J. G., and Lacey, W. N., IND.ENQ. CHEM.,26, 1218 (1934). ( 5 ) Timmermans, J., Sci. Proc. Rou. Dublin SOC.,13, 310 (1912). (6) Young, S., Ibid., 12, 374 (1910). RECEIVED September 18,1934.

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