Symposium on Physical Properties of' Hydrocarbon Mixtures Papers presented before t h e Division of Petroleum Chemistry at the 85th Meeting of the American Chemical Society, Washington, D. C., March 26 t o 31. 1933.
Vapor Pressure of Low-Boiling Paraffin Hydrocarbons in -4bsorber Oil G. L. MATHESON AND L. W. T. CUMMINGS Department of Chemical Engineering. Massachusetts Institute of Technology, Cambridge, Mass.
T
HE recovery of low-boiling hydrocarbons from
A new apparatus has been developed for the static determination of vapor-liquid equilibria. ' The vapor pressures of five low-boiling para& hydrocarbons in a n absorber have been determined over a wide range of concentration. All of the hydrocarbons exhibit positive deeiation f r o m Raoult's law. The pressure deoiafions of fhe normal Paraflins, expressed as percentage of the value calculated by Raoult's law, were found to be approximately the Same at a giren molal concentration und temperature.
natural a n d r e f i n e r y gases is u s u a l l y accomplished by absorption of these materials in a n absorber oil followed by a strippingoperation. Thedesign of equipment to effect this process has been d e v e l o p e d and r e q u i r e s information on t h e equilibrium values of the vapor pressure of the low-boiling hydrocarbon in a n absorber oil through the range of concentration to be encountered. Raoult's law is a convenient means of Icalculating the vapor pressure, but, On account of the physical dissimilarity of the light hydrocarbon and the absorber oil, deviations from the law are tc)be expected* and WYlde (2) have reported that the vapor pressure of a n impure hexane gave a small negative &viation from Raoult's Ian, in a parafinbase oil and a positive deviation of about per cent in an asphalt-base Oil :tt molal concentrations of 10 Per cent. Calineaert and Hitchcock (1 find that N-butane exhibits a smlll negative deviation when dissolved in a straw oil. It was the purpose of this investigation to make a systematic study of the behavior of a number of light hydrocarbons in a particular absorber oil over a considerable range of concentration. >
,
EXPERIMEKTAL, PROCEDURE The apparatus used was a modification of that of Calingaert and Hitchcock ( I ) , the essential difference being that provision was made f o r a g i t a t i o n of b o t h phases. This modification insured uniform c o m p o s i t i o n of the phases and made it possible to obtain equil i b r i u m in a minimum length of time. The apparatus was c o n s t r u c t e d
of Pyrex glass and is shown in Figure 1. The liquid phase, light hy-
drocarbon dissolved in absorber oil,
~a g~r ho u, n"d,- go l$a s$s ,emercury-sealed i~~~~~~,"~ ; f , "q~~~n~~~~~i.' ~ ~ ~ +'~~
R. The s t i r r e r , made of glass, ~~~~~~~~~
" , " ~ ~ ~ , ' ~ f ~ d $ ~ ~
s o l e n o i d c i r c u i t was made and broken twice a second. The vapor was circulated through the system by the fan, H , driven by a threeP base m o t o r . The exit vapor tube was sealed tangentially onto the fan enclosure. The fan and rotor were e n t i r e l y w i t h i n the system, while the field coils were placed outside. The vapor reservoir and 1; uid receiver were maintained at a constant temperature of 25.0 by water baths, G, J . Water from a constant-temperature source was circulated over the solenoid, through bath J , into bath G, and back to the constanttemperature source. The vapor volume was varied by changing the amount of mercury in the vapor reservoir, E, and the change in volume measured by buret K . The surplus mercury was stored in bottle iV. The volume of the head of the apparatus was calibrated previous to use. The vapor volume of this apparatus was capable of a 2100-cc. variation. The mercury levels in a manometer, M , were adjusted to the level in the vapor reservoir by applying pressure or vacuum through the stopcock, A B . Adjustment was f a c i l i t a t e d bythe presence of the liter flask, C. The pressure exerted by the system was then measured with the manometer, D. The difference in the mercury levels was estimated to 0.01 cm. The a b s o r b e r oil s a m p l e was weighed into the liquid-phase receiver, F , and a t t a c h e d to the apparatus. The oil was then cooled with carbon dioxide-acetone slush, and the whole apparatus was e v a c u a t e d through stopcock V . With the system completely closed off f r o m the atmosphere, the oil was permitted to warm u p t o r o o m temperature, thus expelling dissolved air, removed by a second evacuation with the oil cooled. Mercury was forced into the vapor r e s e r v o i r until it was filled completely. All of the hydrocarbons e x c e p t N-butane were admitted to the apparatus at P. A tared glass tube was filled with a particular hydrocarbon and boiled until about one-third of its contents had been removed. Dissolved air was removed in the process. The tube was sealed while boiling, weighed, and inserted into the apparaFIGURE1. EQUILIBRIUM APPARATUS tus at P , where it was held in an in723
8.
tl
INDUSTRIAL AND ENGINEERING
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CHEMISTRY
Vol. 25, No. 7
68.75 cm. of mercury a t 25.0' C.; the range of the N - p e n t a n e was 35.9" to 3 6 . l o C . , and its vapor p r e s s u r e 52.27 c m . of m e r c u r y a t 25.0' C. C r u d e hexane was fractionated and a product was obtained which had a boiling point range of 68.5' to 69.0" C. under normal barometer. T h e p u r e N-heptane, obtained by fractionation of a rich crude, had a constant boiling point of 98.3' C. a t normal barometer. The absorber oil exhibited a n appreciable vapor pressure so that it was necessary to correct the total pressure observed by that amount to obtain the vapor pressure of the light h y d r o c a r b o n . This correction was made by assuming that both constituents followed Raoult's law. The ratio of the observed pressure to the v a p o r p r e s s u r e of t h e p u r e l i g h t clined position by an iron rod. The glass tube fitting over the hydrocarbon a t the temperature in question gave its aptube containing the hydrocarbon was attached t o the apparatus with a ground-glass joint. The solenoid at P raised the iron rod, proximate mole fraction in the liquid phase. The vapor and the hydrocarbon tube slid into a horizontal position beneath pressure of the absorbel: oil was then calculated with the the iron rod. The iron rod fell on the tube Then the solenoid information on its vapor pressure a t the concentration in circuit was opened, breaking it and admitting the hydrocarbon question, and, when subtracted from the total pressure, the into the system. The N-butane was admitted t o the equilib- vapor pressure of the light hydrocarbon was obtained. This rium apparatus as a gas at V from a buret. The hydrocarbon was condensed into the liquid-phase re- correction never amounted to more than 3 per cent of the ceiver by chilling the receiver with the low-temperature bath. light hydrocarbon vapor pressure. Stopcock X was closed and the constant-temperature bath, J, The moles of light hydrocarbon in the vapor phase were was then raised into its normal position to surround the liquidphase receiver. The apparatus was then ready for a series of calculated by the gas laws. The mole fraction of the light determinations, which were made with increasing vapor volume hydrocarbon in the liquid phase was calculated by a material to cover a wide range of concentration of the light hydrocarbon balance, using the data on the weight and molecular weight in the absorber oil. of the components admitted to the equilibrium apparatus. Change in weight of the absorber oil as a result of volatilization was neglected.
DISCUSSION OF RESULTS
40
' 0
005
a10 05 020 025 030 035 MOL FRKTION OF ISO-PEMAHE Ih LIQUIO
Errors from the absorption of the hydrocarbons in the stopcock grease were avoided by using a stopcock lubricant composed of water, caramelized sugar, and soap. The lubricant is hygroscopic and the stopcocks were therefore sealed from the atmosphere with mercury. The absorber oil used was a straw oil from a Midcontinent crude. The Engler distillation curve of this oil showed 225' C. initial, 50 per cent over at 248' C., and 300" C. end point. The oil was dried by passing dry air through it for several days. It had a molecular weight of 186, determined cryoscopically in thiophene-free, twice recrystallized benzene. The N-butane was purchased from the Rlatheson Company and was fractionated in a true boiling point column until the product had a constant boiling point. It was then refractionated and the ends discarded. The vapor pressure of the sample a t 25.0' C. was found to be 189.5 cm. of mercury. Isopentane and N-pentane were prepared from crude pentane by repeated fractionation. The isopentane had a boiling range of 28.0" to 28.5' C., and a vapor pressure of
The vapor pressure of N-butane, N-pentane, isopentane, and N-hexane a t various concentrations in the absorber oil a t 25' C. are shown in Figures 2 to 5 . For convenience the vapor pressure calculated by Raoult's law is also given on the figures. All of the paraffin hydrocarbons show an appreciable positive deviation. The deviations of the normal paraffins are plotted against their concentrations in the absorber oil in Figure 6, and fall on a common curve. The curve for isopentane is similar to that of the normal paraffins but lies above it. The practical effect of the positive deviation from Raoult's law of the light hydrocarbons dissolved in the absorber oil is that a t a given total pressure an a b s o r p t i o n tower will remove less light hydroc a r b o n f r o m the inert gas than would be calculated by Raoult's law.
INDUSTRIAL AND ENGINEERING
July, 1933
CHEMISTRY
ACKNOWLEDGMEKT
125
LITERATURECITED
This work was carried out in the Research Laboratory of Applied Chemistry with the cooperation of the Humble Oil and Refining Company, for which the authors express their appreciation.
(1) Calingaert and Hitchcock, J. Am. Chem. Soc., 49, 750 (1927). (2) Filson and Wylde, IXD. ENG.CHEX.,15, 801 (1923).
RECEIVED April 10, 1933. G. L. Matheson’s present address is Standard Oil Development Company, Bayway, N. J.
Vapor-Liquid Equilibria of Hydrocarbons a t High Pressures W. K. LEWISAND C. D. LUKE D e p a r t m e n t of Chemical Engineering, Massachusetts I n s t i t u t e of Technology, Cambridge, Mass.
T
HIS laboratory (3, 7) has
recommended the use of a single reduced graphical equation of state for hydroc a r b o n s having m o r e than 3 carbon a t o m s per m o l e c u l e , e x p r e s s e d as a p l o t of the c o r r e c t i o n f a c t o r to the gas laws, p , against the r e d u c e d pressure, PR,a t constant values of r e d u c e d temperature, TR. It was r e c o g n i z e d t h a t the h y d r o c a r b o n s of low molecular weight deviate f r o m t h i s c h a r t . The c h a r t was constructed a t the critical temperature, and below, from data on h i g h e r h y d r o c a r b o n s , but, a b o v e TR = 1, t h e d a t a of Amagat (1) on ethylene were used, i n a s m u c h as they were the only data over wide ranges of t e m p e r a t u r e and pressure available to the authors a t that time. Later the authors’ attention w a s called (2) to the data of Young on N-pentane . ( I O ) and particularly isopentane
originally presented to correspond to these data of Young. The chart t h u s c o r r e c t e d is s h o w n in Figure 1. There is no change on the TR = 1 isotherm except beyond PR = 1. The isotherms f o r TR = 1-05 and 1.10 have b e e n l o w e r e d somewhat, but the diagram is o t h e r w i s e unchanged. It is b e l i e v e d that this d i a g r a m represents closely the data a t present available for hydrocarbons with m o r e than 3 carbon atoms per molecule. Along with the p chart (7) was presented a fugacity chart, on which was plotted the ratio of the fugacity to the pressure, f/P, as a f u n c t i o n of the reduced pressure, PR. This chart has been corrected f o r t h e changes in the p chart just described and is given in Figure 2. Fundamentally, of course, this chart applies only to the vapors of Pure hydrocarbons.
Curaes f o r the correction factor, p, to the gas laws, applicable to hydrocarbons with more than 3 carbon atoms per molecule, are corrected in minor details. The corresponding corrections hate also been made to the generalized fugacity plot. Preliminary experimental data on the colatility of benzene in nitrogen at high pressures are presented, and the results indicated tentatively on the fugacity plot as a means of estimating the volatility of high-boiling hydrocarbons at high pressures. The correction factor f o r the internal energy qf hydrocarbon mpors at low capor volumes has been determined by graphical integration f r o m the isometrics of those hydrocarbons .for which adequate data are available. The resulling corrections appear to be relaticely independent of the temperature, at least over considerable ranges, and differences in the correction f o r hydrocarbons having more than 3 carbon atoms per molecule are probably small at corresponding reduced conditions. (11).
VAPOR-LIQUID EQUILIBRIA13 HYDROCARBON SYSTEMS REDUCED P-V-T RELATIOSSHIPS FOR VAPORS
PURE
EIYDROCARBON
A study of these data shows that they check the original curves a t TR = 1 and below, closely up to the critical pressure, but that a t higher pressures they indicate lower values of p than those obtained from Amagat’s data on ethylene. However, a t high values of TR the differences between the data on ethylene and the pentanes are apparently within the experimental error through the limited range of overlap. It has therefore been deemed desirable to correct the p chart
Despite the importance of the subject, there are almost no data available on the volatility of hydrocarbons a t temperatures below their critical in the presence of noncondensable gases a t high pressure. This laboratory is conducting an intensive investigation of this field, and, while the work has just started, the preliminary results are so interesting and of such obvious importance to the oil industry that it seems desirable to give a preliminary presentation. The work to date has been limited to the binary mixture of benzene and nitrogen. Two sets of measurements have been
TABLEI. DATAON VOLATILITYOF BENZENE 13NITROGEN AT HIGHPRESSURES (4) TEMP. C. 100 100 125 125 150 150 175 175 200 200 O
a
Atn. 75 98 75 98 75 9s 75 98 75 98
“R 1.565 2 090 1.565 2,090 1.565 2,090 1,565 2.090 1,565 2.090
Z(CeH5) 0.9550 0.9405 0.9550 0.9405 0.9550 0.9405 0,9550 0.9406 0.9550 0.9405
Y(C5H6)
P
0.0360 0.0320 0.0664 0.0588 0.1060 0.0956
1.80 1.80 3.35 3.35 5.70 5.70 9.18 9.18 14.0 14.0
0.1665
0.1470 0.26 0.224.:
/. PIP
PR
TR
0.955 0.955 0,927 0.927 0,895 0.895 0.851 0.851 0.808 0.808
0.0376 0.0376 0.0699 0.0699 0.1190 0.1190 0.1916 0.1916 0.2922 0.2922
0.665 0,665 0.709 0.709 0.754 0.754 0.798 0,798 0.843 0.843
fP
1.72 1.72 3.11 3.11 5.10 5.10 7.82 7.82 11.31 11.31
=
fPZ u
45.6 50.6 44.7 49.8 45.9 50.2 44.9 50.0 41.6 47.4
/J. 0.608 0.516 0.596 0.508 0.612 0.512 0,598 0.510 0.555 0.484
