Equilibria in Hydrocarbon

pressures up to 3000 pounds per square inch absolute. The mixtures studied ranged from the pure liquid hydrocarbons to mixtures with sufficient methan...
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Phase

XVI. Solubility

of Methane in Four Light Hydrocarbons'

Equilibria in Hydrocarbon Systems

The solubility of methane in each of the hydrocarbon liquids n-pentane, n hexane, cyclohexane, and benzene, was measured at temperatures and total pressures commonly encountered in the formations from which petroleum is procured. The specific volumes of the mixtures were determined. Several diagrams illustrate the behavior of some of the mixtures studied.

B. H. SAGE, D. C. WEBSTER, AND W. N. LACEY California Institute of Technology, Pasadena, Calif.

Q

A

S PART of an extended study of the phase

equilibria in siniple and complex systems of the hydrocarbons found in naturally occurring petroleum and natural gas mixtures, behavior of mixtures of methane with each of the following hydrocarbons has been studied: n-pentane, n-hexane, cyclohexane, and benzene. The measurements consisted of determination of the total volumes of many mixtures of known compositions at temperatures from 100" to 220" F. and a t total pressures up to 3000 pounds per square inch absolute. The mixtures studied ranged from the pure liquid hydrocarbons to mixtures with sufficient methane to have bubble points in excess of 2500 pounds per square inch absolute, each mixture being studied in both condensed and two-phase regions. Solubility values were obtained from determination of bubblepoint condition as a function of composition, temperature, and total pressure. TABLE I. Abs. Pressure, Lb./Sq. In. Bubble point

___

400

500 600 800 1000 1250 1500 1750 2000 2250 2500 2750 3000

-7.15

SPECIFIC VOLUMES O F

1 Previous articles in this series appeared during 1934 and 1935, and in earlier issues of 1936.

MIXTURESO F METHAXE.4KD FOCRLIGHTHYDROCARBONS '

. --10.01

___Methane and n-Pentane

mass CHI160' F.' 220° F. (854)O (968) (1043) 0.02860b0.03095 0.03383 0.0645 0.0792 0.1037 0.05070 0.0624 0.790 0.04144 0.05115 0.06380 0.03070 0.03742 0.04557 0.02849 0.03091 0.03540 0.02832 0.03063 0.03350 0,02815 0.03036 0.03312 0.02799 0.03011 0.03274 0.02784 0,02988 0.03239 0.02768 0.02965 0.03204 0.02754 0.02944 0.03170 0.02740 0.02923 0.03137 0.02728 0.02903 0.03104

looo F.

Properties of Samples The source of n-pentane was a sample used in an earlier study (6) and was obtained from the Phillips Petroleum Company who furnished the analysis showing 99.3 mole per cent n-pentane and 0.7 per cent isopentane. However, in the present case the material was further purified by fractional distillation in a glass-ring-packed column ( 8 ) ,4 feet long and 0.5 inch in diameter, equipped with a vacuum jacket and with

mass % C H e -27.06 mass % CHI160° F. 220' F. 100' F. 160' F. 220' F. (1945) (2064) (2026) (2228) (2327) (2152) 0.03317 0.03720 0.04420 0.03682 0.04230 0.05377 -20.31

looo F.

...

.....

0.1216 0.0874 0.0684 0.05370 0.04405 0.03718 0.03305 0,03258 0.03218 0.03183 0.03151

..

.....

0.1046 0.0814 0.06390 0.05233 0.04445 0.03848 0.03655 0.03586 0.03540 0.03450

.....

0.1230 0.0964 0.07560 0.06153 0.05181 0,04483 0.04219 0.04051 0.03927 0,03827

Methane and n-Hexane -12.33 mass % CHI- 4 . 2 4 mass % CHI100' F. 160' F. 220' F. 100' F. 160' F. 220' F. (655) (739) (795) (1623) (1775) (1855) Bubble point 0.02629 0.02802 0.02994 0.02911 0.03119 0.03396 400 0.04130 0.04840 0,05720 ................. 500 0.03343 0.03920 0.04591 600 0.02836 0.03324 0.038t30 0 : O i i i o oiio 0 : ioos 800 0.02622 0.02797 0.02993 0.0559 0.0648 0.0748 1000 0.02612 0.02782 0.02971 0.04492 0.05225 0.06006 1250 0.02601 0.02764 0.02946 0.03643 0.04240 0.04830 1500 0.02590 0.02749 0.02923 0.03107 0.03605 0.04080 1750 0.02580 0.02734 0.02902 0.02900 0.03157 0.03564 2000 0.02570 0.02721 0.02884 0.02881 0.03090 0.03363 2250 0.02562 0.02708 0.02885 0.02884 0.03062 0.03317 2500 0.02554 0.02696 0.02848 0.02850 0.03036 0.03278 2750 0.02546 0.02884 0.02831 0.02836 0.03014 0.03243 3000 0.02538 0.02672 0.02815 0.02824 0.02995 0.03212 Figures in parentheses refer t o bubble-point pressures, in pounds per b Expressed in cubic feet per pound. c

:

.....

0.1157 0.0896 0.07003 0.05726 0.04818 0.04149 0.03672 0.03577 0.03516 0.03474

.. ..

.. .....

0.1071 0.08381 0.06829 0.05748 0.04982 0.04391 0.04115 0.03993 0.03898

....

.... ....

.....

0.1270 0.09848 0.08070 0.06767 0,05828 0.05230 0,04904 0.04671 0.04508

-19.20 mass % CHI100" F. 160' F. 220' F. (2412) (2528) (2508) 0.03139 0.03414 0.03810

...

.....

o.os6i 0.0687 0:0796 0:0915 0.05512 0.06379 0.07295 0.04632 0.05356 0.06103 0.04032 0.04644 0.05270 0.03609 0.04126 0,04660 0.03297 0 03741 0.04197 0.03128 0:03440 0.03817 0.03102 0.03364 0.03720 0.03077 0.03320 0.03647 square inch sbsolute.

1045

-

hlethane and Cyclohexanemass 7 CH4-13.44 masf , C H r 100' F. 160' F .' 220' F. 100' F. 160 220OF. (2045) (2196) (2240) (2568) (2698) (2734) 0.02422 0.02549 0.02726 0.02554 0.02702 0.02902

.....

0.0776 0.0652 0.04989 0.04088 0.03395 0.02954 0.02663 0.02455 0.02408 0.02396 0.02388 0.02382 -4.31

.....

0,0881 0.0738 0.05636 0.04610 0.03825 0.03312 0.02953 0.02698 0.02544 0.02528 0.02518 0.02509

....

.....

0.0841 0.06345 0.05179 0.04260 0.03668 0.03244 0.02937 0.02725 0.02694 0.02674 0.02660

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

Methane and Benzene mass ?7 CH4-7.57 mass % CHa100' F. 160' F. 220" F. (1456) (1432) (2390) (2354) (2310) 0.02081 0.02202 0.02082 0.02192 0.02322 0.05200 0.05896 ............... 0.04300 0.04813 0.03715 0.04120 O : O 5 2 0 0:057S 0:0644 0.03004 0.03266 0.04084 0.04525 0.04987 0.02573 0.02775 0.03427 0.03782 0.04130 0.02253 0.02396 0.02916 0.03189 0.03452 0.02080 0.02199 0.02594 0.02812 0.03021 0.02073 0.02187 0.02382 0.02557 0.02727 0.02068 0.02177 0.02238 0.02375 0.02514 0.02062 0.02167 0.02135 0 02240'0.02351 0.0205J 0.02158 0.02079 0:02187 0.02311 0 . 0 2 0 ~ ~ ~ 0 . 0 2 1 50.02073 0 0.02179 0.02298 0.02048 0.02143 0.02067 0.02172 0.02287

100' F. 160' F.' 220" F.

(1448) 0.01983 0,04615 0.03852 0.03362 0.02741 0.02386 0.02120 0.01982 0.01976 0.01971 0.01966 0.01962 0.01959 0.01955

.....

.....

0.0859 0.0649 0.0737 0.0831 0.05251 0.04308 0.05989 0.0671 0.04889 0.05460 0.03704 0.04179 0.04654 0.03278 0.03686 0.04090 0.02970 0.03318 0.03672 0.02750 0.03048 0.03345 0.02591 0.02839 0.03090 0.02536 0.02672 0.02695 0.02900 0.02619 0.02869

ISDUSTRIAL 4ND ENGINEERING CHEMISTRY

1016

PRESSURE

LBS. PER

L-OL. 28, NO. 9

adequate facilities to maintain very constant reflux conditions. The middle fraction of the distillate used for this work exhibited a change in vapor pressure of less than 0.03 inch of mercury between dew and bubble points at 100" F. The samples of n-hexane and of cyclohexane were of the best grade listed by the Eastman Kodak Company and were not further purified. The benzene was thiophene-free and had a freezing range of 0.22' F. with a mean temperature of 41.76". The methane was prepared from natural gas by methods previously described ( d ) , with the additional step of fractional condensation directly as a solid a t liquid air temperature and at a pressure of 2 inches of mercury, absolute, to aid in the removal of nitrogen and similar impurities. The sample used contained less than 0.01 per cent ethane and heavier hydrocarbons and probably less than 0.2 per cent nitrogen and other inert gases.

SQ. IN

Experimental Results

a

w

a

t' L

3

0 W

5

T E M P E R AT URE

IO0

125

150

T E M P E RAT URE

'F.

175

200

a F.

D A T A FOR MIXTURE CONTAININQ 19.20 FIGURE 1. EXPERIMENTAL 'MASS PER CENT METHANE AND 80.80 PER CENT n-HEXANE

FIGURE2. SPECIFIC VOLUMESOF METHANE-WHEXANE MIXTURE

FIQURE 3. RELATION BETWEEN PRESSURE AND TEMPERATURE FOR METHANE-WHEXAKE MIXTURE AT GIVENSPECIFIC VOLUMES

The apparatus and the experimental methods used in this study were described previously (3, 4, 7). Known masses of liquid and gas were added to a steel cell whose temperature was maintained constant within 0.1' F. and whose effective volume was systematically varied by the addition or withdrawal of mercury. Phase equilibrium was maintained by a mechanical agitator within the cell. The equilibrium pressures were measured at each of a series of known cell volumes at loo", 160', and 220' F. The attainment of equilibrium was verified by agreement of data obtained upon increasing volume with those following decrease of volume. The volumes of the hydrocarbon system were known within 0.2 per cent and the pressure was measured to 1 pound per square inch. The data from over twelve hundred equilibrium measurements were graphically interpolated to even values of pressure and the results are presented in Table I. In order to illustrate the behavior of the systems studied, a mixture containing 19.20 mass per cent methane and 80.80 per cent n-hexane was chosen as a typical example, for which several diagrams were prepared from the data of Table I. Figure 1 shows the relation between pressure and specific volume of the methane-n-hexane mixture in the vicinity of bubble point. Experimental points are shown in this figure to indicate the degree of uncertainty in establishment of the curves. The dashed-line curve indicates the change of bubblepoint specific volume with pressure or, by interpolation, with temperature. The maximum pressure shown by this curve is the maximum pressure a t which liquid and gas phases can coexist at any temperature for the particular mixture in question. The effect of temperature upon specific volume of this same mixture of methane and n-hexane is shown for each of a series of pressures in Figure 2. The maximum pressure for coexistence of two phases, discussed above, would appear in this figure at the point where an isobar was tangent to the bubblepoint liquid curve. Such an isobar would obviously correspond for this mixture to a pressure greater than 2500 and less than 3000 pounds per square inch absolute, It actually occurs at a pressure of approximately 2540 pounds per square inch and a temperature of about 180' F. The nearly linear change in specific volume with temperature in the

SEPTEMBER. 1936

INDUSTRIAL AND ENGISEERING CHEhIISTRY

1041

two-phase region of Figure 2 was found in each case here reported except in mixtures of methane and n - p e n t a n e . Figure 3 shows the variation of pressure with temperature corresponding to a series of constant volumes for the same mixt u r e of m e t h a n e and n-hexane. Similarly the isochores in the twophase region are straight lines within the accuracy of the experimental measurements. The point of maximum pressure for two phases is here shown directly by the maximum in the bubble-point liquid curve. I t is interesting to note, in thismethanen-hexane mixture, the much greater spread of temperature and pressure between the states corresponding to maximum pressure for two phases, critical state, and maximum temperature for two phases (cricondentherm) MASS PERCENT as c o m p a r e d to the mixtures of FIGURE 4. COMPARISW OF BVBBLE-POIST PRESSVRES FOR MJXTCRES OF METHmethane and propane p r e 1' io u s l y AXE WITH DIFFERESTHYDROCARBONS oii THE BASISOF THE M4ss PER CEXTOF studied ( 5 ) . Thiq situation is indiMETHASEIS THE MIXTTRE cated by the fact that the range of temperature and preswre here used included only the state corresponding to maximum pressure, while the I I qeme range included all thrce states for mixtures of 0.E methane and propane. > To assist in the comparison of the solubilities of methane in the four different h y d r o c a r b o n s h e r e 2 studied as well as in tn-o others for which results were g O,E reported earlier (PI 5 ) . Figure 4 was prepared. I t shon s the mass per cent of methane present in mixtures i! a t bubble point for various pressures and for each of 2 the three temperatures studied. Since, a t the bubble point, no appreciable amount of the methane in the $ 0.4 system remains undissolved in the other hydrocarbon while no appreciable amount of the latter has been permitted to transfer to the gas phase, the mass per cent of methane in the mixture under these conditions 500 1000 1500 2000 2500 gives a true measure of the solubility at the prevailing total pressure. B U B B L E POINT PRESSURE LBS. P E R Sa. IN FIGURE5. SPECIFIC GRAVITY OF BUBBLE-POINT LIQUID . 4 ~160' F. The rapid decrease in the slope of some of the curves FOR MIXTCRES OF METHANE WITH DIFFERE-UT HYDROCARBOSS of Figure 4 a t the higher pressures indicates proximity to the critical state for the corresponding mixtures and temperatures. It is of interest to note that the change in slope 160" F. is shown for each of the six systems as a function of is more rapid a t the higher temperatures for mixtures containbubble-point pressure in Figure 5 . The approach to critical ing propane, n-pentane, n-hexane, cyclohexane, and benzene, pressure for this temperature is evident in the cases of mixtures whereas the reverse is true for the methanecrystal oil mixof methane with propane and with n-pentane. tures. This indicates that all the temperatures studied are Acknowledgment below the temperature corresponding to the point of maximum The American Petroleum Institute supported this investipressure for the methanecrystal oil system and are above those gation under its Research Project 37. J. E. Sherborne made temperatures for the other systems mentioned. The mixtures the experimental measurements for the system consisting of with benzene are noteworthy in having only a small decrease methane and n-pentane. in solubility with increase in temperature and little change in slope a t the higher pressures. Literature Cited Frolich and eo-Forkers (1) reported values of the solubility (1) Frolich, P. K., Tauch, E. J., Hogan, J. J., and Peer, A. A , , IID. ENG.CHEM.,23, 548 (1931). of methane in several of the hydrocarbons considered in this (2) Sage, B. H., Backus, H. S., and Lacey, W. N.,Zbid., 27, 686 paper for a temperature of 77" F. (25' C.). The values they (193.5). reported, except in the case of propane, are somewhat higher (3) Sa&, B: H., and Lacey, W. N.,I b i d . , 26, 103 (1934). (10 per cent) than would be predicted from extrapolation of (4) Zbid., 28, 106 (1936). (5) Sage, B. H., Lacey, W. N., and Schaafsma, J. G., Ibid.,26, 214 the present results to 77" F. As Frolich's work was carried (1934). out in the two-phase region by methods widely divergent ( 6 ) Zbid., 27, 48 (1935). from those here described and was considered trustworthy (7) Sage, B. H., Schaafsma, J. G., and Lacey, W.N., Zbid., 26, 1218 to * 5 per cent, the agreement is considered satisfactory. (1934). (8) Young, W. G . ,and Jasaitis, Z., J.Am. Chem. SOC.,58, 377 (1936). The specific gravity (compared to water a t maximum density for atmospheric pressure) of the bubble-point liquid at RECEIVED May 4, 1936.