Gas-Liquid Equilibria for the System Methane-Butane G. W. NEDERBRAGT Bataafsche Petroleum Maatschappij, Amsterdam, Holland
I
N SEVERAL processes of the natural gas and gasoline industry pressures higher than atmospheric are used. AS a consequence, information on gas-liquid equilibria of mix-
tures of lower hydrocarbons under pressure is of great interest to petroleum technologists. Such information can be best obtained by the study of binary mixtures. Sage, Lacey, and Schaafsma (4) examined the methanepropane system a t 20' C. and higher. From their measurements the compositions of the coexisting vapor and liquid are known a t different temperatures and pressures. The solubility of methane in hydrocarbons such as pentane and hexane was determined by various investigators, but they did not examine the composition of the gas phase a t the same time. It was therefore considered useful to study the composition of gas and liquid phase in the case of mixtures of methane with a hydrocarbon heavier than propane. As the second component n-butane was chosen.
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0-
Determinations The methane used contained 1.1 per cent impurities in the form of ethane, as shown by low-temperature rectification. The butane consisted of 96.6 per cent n-butane and 3.4 per cent isobutane. Into a 5-liter bomb about 1 kg. of this butane was introduced; above it methane was admitted. Equilibrium between the two phases was promoted by repeated shaking of the bomb. Small samples were then taken from the gas and liquid phases. The gas samples, which contained little butane, were analyzed
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" '20
It
30
"
2-a
.,
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M e t h a n e with b u i a n e Auihor
M e t h a n e with p e n t a n e , hexane. heptane.
Hill and Lacey
I,
I 0
I t e0
I +A40
I +60
I t 80
T e m p e r o h r e in " C . FIQURE 1. EQUILIBRIUM CONSTANT OF METHANE
by selective absorption in toluene, as described by van Dijck (1). The liquid samples, which contained little methane, were analyzed by a low-temperature rectification as described by Podbielniak (3).
TABLEI. Gas-LIQUID EQUILIBRIA DATAFOR METHANE-BUTANE Ab8. Pressure Atm. 10.3 10.0 10.3 19.2 19.9 20.1 29.2 29.0 30.0
Temp.
c.
-16.5 16 38 -21 17 38.5 -18 14.5 43
THE
Methane Gas Mole % ' 92.7 i 0.5 79.5 58.2 95.6 87.6 77.0 96.0 89.7 80.7
TABLE11. GAS-LIQUID EQUILIBRIA DATAFOR METHANE-BUTANE Abs. P;essure Atm. 10.0
20.0
Temp.
c.
-16.5 16 38 -21 17 39
30.0
-18 14.5 43
-Equilibrium Methane
12.8 16.6 20.0 6.7 8.6 10.1 4.8 5.8 6.C
SYSTEM
in: Liquid Mole % ' 7.5 0.3 4.8 3.0 13.7 10.2 7.6 19.7 15.4 12.3
THE
DeterminaJions were made at three temperatures-approximately -20 , 15", and 40' C.-and at three pressures-10, 20, and 30 atmospheres. The temperatures were kept constant within * l . O o C.; the pressures were measured within an accuracy of *0.2 atmosphere. The results of the analysis are shown in Table I.
Equilibrium Constants for Methane and n -Butane The ratio of mole fraction X in the gas to mole fraction x in the liquid was calculated for the two components, methane and butane, from the compositions of gas and liquid phase. This ratio X / x , known in literature as the equilibrium constant, is given in Table 11. In view of the low percentage of isobutane, the equilibrium constants of column 4 in Table I1 may be considered as applying to n-butane. No data have been published on the equilibrium constants of pure n-butane in the pressure and temperature range under examination. For methane, however, a comparison is possible with the data in the literature. The logarithms of the equilibrium constants of methane from Table I1 were plotted against the temperature (Figure l),so that three curves were obtained a t the three pressures
SYSTEM
ConstantButane
0.082 0.22 0.44 0,049 0 14 0.25 0.049 0.12 0.22
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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Sage, Lacey, and Schaafsma (4) examined methanepropane mixtures over an extensive temperature and pressure range. From the gas and liquid compositions given by them, X / z values for methane were calculated a t 10, 20, and 30 atmospheres. These values were also plotted in the graph; these constants seem to deviate from those calculated from butane-methane mixtures and to decrease with temperature. This abnormal behavior is due to the vicinity of the critical range of the propane-methane mixtures.. Hill and Lacey ( 2 ) determined the solubility of methane a t 30" C. and 20.4 atmospheres in n-pentane, n-hexane, and nheptane. The mole fraction of methane is recorded for the liquid phase, not for the gas, where it is approximately 1, since there is, only a little solvent in the gas phase. The mole fraction of the solvent in the gas was, however, found approximately by multiplying the mole fraction in the liquid by the equilibrium constants predicted by Souders, Selheimer, and Brown (6)for the component in question from the known vapor pressure and the compressibility as a gas. The mole fraction of methane in the gas phase was thus fairly accurately known, so that equilibrium constants for methane dissolved in n-pentane, n-hexane, and n-heptane could be determined. These were also plotted in the graph, after being corrected for the small difference between 20.4 and 20.0 atmospheres. I n the case of mixtures of methane with pentane, hexane, or heptane, the density of the liquid phase at a given temperature and pressure is greater than for mixtures of methane with butane.
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The equilibrium constants for methane, determined from the solubility in pentane, hexane, and heptane, are nevertheless in good agreement with the curves for the equilibrium constants of methane found in the above mentioned examination of methane-butane mixtures. The influence of the greater density of the liquid phase on the equilibrium constants of methane seems therefore to be small. It is concluded, then, that the heavy lines in the graph will also give a good approximation for the equilibrium constants of methane in natural gasolines, as these are chiefly mixtures of lower paraffin hydrocarbons. From measurements by Sage, Webster, and Lacey (6) concerning the solubility of methane in n-pentane, n-hexane, cyclohexane, and benzene, it may be concluded that the equilibrium constants of methane become higher when a mixture contains many naphthenes and increase considerably when many aromatics are present.
Literature Cited (1) Dijck, W. J. D. van, J . Inst. Petroleum Tech., 18, 145 (1932). ( 2 ) Hill, E. S., and Lacey, W. N., IND.ENQ.CHEM.,26, 1324 (1934). (3) Podbielniak, W.J., IND.ENQ.CHEM.,Anal. Ed., 3, 177 (1931). (4) Sage, B. H., Lacey, W. N., and Schaafsma, J. G., IND. ENG. CHEM.,26, 214 (1934). (5) Sage, B. H., Webster, D. C., and Lacey, W. N., Ibid., 28, 1045 (1936). (6) Souders, M.,Jr., SeIheimer, C. W., and Brown, C. G., Ibid., 24, 517 (1932). RECEIVED September 6, 1937.
Thermal Decomposition of Hexane at High Pressures J. N. PEARCE AND J. W. NEWSOME1 State University of Iowa, Iowa City, Iowa
The thermal decomposition of hexane has been studied at pressures of 14,000 to 15,000 pounds per square inch (984 to 1054 kg. per sq. ,cm.) at 10" intervals between 430" and 520" C. and for periods varying from a few minutes to 2 hours. Decomposition occurs in all cases to form compounds boiling both higher and lower than hexane : carbon is deposited in some cases. The gaseous products boiling below 100" C. are chiefly aliphatic in nature; in the products boiling above 100" C., the cycloparaffins predominate with appreciable amounts of aromatics and some olefins.
LTHOUGH a vast amount of work has been done on the thermal decomposition of paraffins at low pressures, relatively little attention has been given to similar studies a t high pressures. Huge1 and Artichevitch (6) found
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Present address, Aluminum Research Laboratories, New Kensington. Pa.
that, when hexadiene is heated to 470' C. under 500 atmospheres, 60 per cent is converted t o lower boiling products. Kat6 (7) decomposed petroleum under pressures of 50 to 300 atmospheres. He found that the loss as carbon deposit and noncondensable gases increases with the pressure, especially above 200 atmospheres. Leslie and Potthoff (8) found that the amount of olefins in the gaseous products is slightly diminished when the gas is cracked under a pressure of 500 pounds per square inch. However, the literature reveals no complete data showing the effect of very high pressures on the nature of the gaseous products. The present paper presents the results obtained in a preliminary study of the decomposition of n-hexane under very high pressures. Hexane appeared to be especially suited for such a study, since its decomposition a t low pressures was fully investigated rather recently (4). Because of mechanical restrictions the quantity of liquid products available was too small for accurate qualitative or quantitative analysis. Rice (10) assumed that the decomposition of a hydrocarbon involves the intermediate formation of free radicals and that the only stable radicals are the methyl and ethyl groups. The data of Frey and Hepp (4) on the decomposition of hexane a t low pressures agree fairly well with the predictions of Rice. The high-pressure products provide an interesting contrast to these data.
