October 1955
INDUSTRIAL AND ENGINEERING CHEMISTRY
(IO). The acetylene concentration of the constant boiling mixture a t 40’ F. is approximately lo%, the pressure being 670 pounds per square inch absolute ( 7 ) . Ethane-Acetylene. The compositions of the ethane-acetylene azeotropes at -35O and 0’ F. are definitely greater than 30% acetylene. TERNARY. Throughout the composition range studied the acetylene and ethylene concentrations in the equilibrium vapor phase are greater than in the liquid. With acetylene concentrations up t o about 10 to 15% it is the most volatile constituent. EQUILIBRIUM VOLATILITY RATIOS. The equilibrium volatility ratios ( K = y/z) for ethane are always less than 1, the lowest value being 0.7, and increase as the acetylene concentration in the liquid is varied between 0 and 20%. The over-all spread of acetylene K’s is between 2 and 1. The volatility ratios decrease with increasing acetylene concentrations. The range of ethylene K’s is between 1.5 and 1. At any given temperature and pressure the ratio remains fairly constant. Comparison of Phillips and Published Data. The isobar for 465 pounds per square inch absolute a t 40’ F. was determined in order t o establish a tie-in between these data and the results of the complete study a t 40” and 60” F. made by McCurdy and Katz ( 7 ) . The agreement between the two sets of data is limited. As shown in Figure 3, Phillips’ bubble point and dew point curves are definitely displaced from the published curves ( 7 ) . The equilibrium phases appear to contain less ethane, from 1 t o 5%, than is indicated by McCurdy and Katz. The variations of ethane and acetylene K’s a t 40” F. and 465 pounds per square inch absolute appear to be the opposite of
2215
those a t -35’ and 0” F. This reversal may result from the fact that 40’ F. is less than 10’ F. below the critical temperature of ethylene. ACKNOWLEDGMENT
The authors wish to acknowledge their indebtedness to the Phillips Petroleum Co. for permission to publish this paper, to F. P. Spessard for the use of his unpublished information, and to K. H. Hachmuth for his suggestions. LITERATURE CITED
(1) .4m. SOC.Testing Materials, Committee D-2, “ASTM Standards on Petroleum Products and Lubricants,” Method D 1020-52. (2) Ibid.. Tentative Method D 1268-531’. (3) Churchill, S. W., Collamore, W. G., and Katz, D . L., Oil dl. Gas J., 41, No. 13, 33 (1942). (4) Hanson, G. H., Hogan, R. J., Ruehlen, F. X., and Cines, M. R. “Phase Equilibria. Collected Research Papers for 1953,” p. 37, Am. Inst. Chem. Engrs., 1953. (5) Kuenen, J. P., Phil. Mag., 44, 174 (1897). (6) McCurdy, J. L., Ph.D. thesis, University of Michigan, 1943. (7) McCurdy, J. L., and Katz, D. L., IND.ENG.CHEM.,36, 675 (1944). (8) Matthews, C. S., and Hurd, C . O., Trans. Am. Inst. Chcm. Engrs., 43, 25 (1947). (9) Quinn, J. C., 8c.D. thesis, Massachusetts Institute of Technology, 1940. (10) Spessard, F. P., private communication. (11) York, R., Jr., Trans. Am, Inst. Chem. Engrs., 40, 227 (1944). RECEIVED for review February 23, 1955.
ACCEPTED J u n e 15, 1955.
Vapor-Liquid Equilibrium Data for Binary Mixtures of Paraffins and Aromatics H. S. MYERS C . F. Braun & Co., Alhambra, Culif. I X T U R E S of paraffins and aromatics show nonideal volatility characteristics. When the aromatic is benzene, the nonideality is especially pronounced. B u t equilibrium-data for mixtures of aromatics and paraffins are not very complete. This work was undertaken to fill in some of the missing paraffinaromatic data, in order t h a t the nonideal behavior could be better understood and predicted. Complete vapor-liquid equilibrium data are reported for five binary mixtures at atmospheric pressure, pentane-benzene, hexane-benzene, benzene-heptane, hexane-toluene, and heptaneethylbenzene. I n addition, hexane-toluene has been studied at 300 mm. Hg and at 150 mm. Hg absolute pressure, and heptaneethylbenzene at 300 mm. and 100 mm. Data for the system heptane-toluene have also been measured and have been reported previously (1). APPARATUS
The equilibrium apparatus has been described in detail in a n earlier article ( 1 ) . Briefly, the still is of the vapor-recirculating type. It uses a vapor jacket t o maintain adiabatic operation. All parts are made from g l a ~ except s for the Teflon sample valves. The heating elements are fused into a borosilicate glass tube and are completely enclosed with glass. Except for sampling, operation is entirely automatic.
PURIFICATION O F HYDROCARBONS
Pentane. Phillips commercial-grade pentane was the source of n-pentane. This stock vas fractionated in a 70-plate Oldershaw column a t a reflux ratio of 40 to 1. A 25 t o 83% heartcut was blended for the equilibrium studies. Hexane. Phillips supply a technical grade of n-hexane that has a purity of about 950j0. The impurity is primarily benzene. T o remove the benzene, 1250 ml. of this stock were passed through a silica gel column. The column had a 1-inch inside diameter and was 82 inches high, packed with 1025 grams of 100- t o 200mesh silica gel. Methanol was used as a desorbent. Removal of the benzene seemed t o be entirely complete after the silica gel treatment. But as a n extra precaution the extracted hexane was fractionated in a packed column that had 100 theoretical plates when tested with heptane-methylcyclohexane at total reflux. This column has a packed height of 71 inches, and the diameter of the packed section is 17 mm. Stainless-steel helices, inch in diameter and 25 to 31 gage are used as packing. The column is vacuum-jacketed and silvered. And, in addition, two heating mantles extend the entire length of the column. For the hexane purification, a reflux ratio of 40 to 1 was used. A 4 to 97y0 heartcut was blended as pure hexane. Heptane. n-Heptane sold by Phillips as reference fuel grade
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 47, No. 10
has a purity of greater than 99%. This material was refractionated in a 70-plate Oldershaw column a t a reflux ratio of 40 to 1. A 5 to 95y0 heartcut was blended from this distillation. Benzene. Baker and Adamson, reagent grade benzene wa9 refractionated in the 100-plate, helix-packed column a t 60 to 1 reflux ratio. A 28 to 89% heartcut was blended. Toluene. The toluene was Baker and Adamson reagent grade, redistilled through the 100-plate helix-packed column at 60 t o 1 reflux ratio. A 55 t o 95% heartcut was blended for the equilibrium studies. Ethylbcnzene. Phillips pure grade ethylbenzene has a specified purity of 99 mole %. This stock was refractionated in the 70-plate Oldershaw column at a reflux ratio of 40 to 1. A 15 to 80% heartcut was blended. Physical properties for all the hydrocarbons used in this study are listed in Table I.
Table I. Comparison of Physical Properties of Hydrocarbons Used in t h i s Work with NBS-API Values Refractive Index a t 20' C . Boiling Point, ' C . Hydrocarbon Pentane Hexane Heptane Benzene Toluene Ethylbenzene
ai
Experimental
XBS-API
Exuenmental
NBS-API
1.3575 1.3750 1.3877 1,5011 1.4969 1.4959
1.35748 1.37486 1.38764 1.50110 1.49693 1.49594
36.05 68.55 98.4.5 80.15 110.6 136,2
36,074 68.742 98.428 80.101 110.625 136.189
ANALYSIS AND EQUILIBRIUM MEASUREMENTS
80
For binary mixtures of aromatics and paraffins, analysis by refractive index is simple and accurate. This method was used for all the mixtures reported in this work. Tables showing the experimental calibrations and giving the experimental vaporliquid equilibrium data for the five paraffin-aromatic mixtures are available (3). For each run the relative volatilities and the activity coefficients have been computed from the experimental points. Relative volatilities are defined in the conventional manner.
79
78 77
."
i 3t
76
75 74
3 +
73 72
7, 70
69 680
10
20
30
40 50 MOLE 9 . HEXANE
60
70
80
90
I 0
Where
= mole fraction of more volatile component in vapor y~ = mole fraction of less volatile component in vapor Z A = mole fraction of more volatile component in liquid ZB = mole fraction of less volatile component in liquid YA
Activity coefficients are computed from the relation
Where
y = activity coefficient y = mole fraction in vapor z = mole fraction in liquid P = vapor pressure of pure component a t equilibrium temperature x = total pressure of system
Vapor pressures for the pure components are taken from values published by the National Bureau of Standards, Washington 25, D. C., API Research Project 44,December 1952. All the data were checked for thermodynamic consistency by the form of the Duhem equation advocated by Dodge ( 2 ) . For each binary mixture, the paraffin activity-coefficient curve was assumed t o be correct, and the consistent aromatic curve was computed by graphically integrating the relation.
Where z is the liquid mole-fraction of Component A . The choice of the paraffin curve as the basis for the comparison is purely
October 1955 112
INDUSTRIAL AND ENGINEERING CHEMISTRY
2211
FIGURE 4 TEMPERATURE-COMPOSITION DIAGRAMS FOR THE SYSTEM
M O L E % HEPTANE
MOLE X HEXANE
arbitrary. It would be equally valid to assume that the experimental aromatic curve is correct, and to compute a consistent paraffin curve. Table I1 summarizes the results of the thermodynamic check. 'Except for the pentane-benzene system, agreement is goodespecially when it is realized that this procedure throws all the errors in the aromatic curve. If the paraffin curve is shifted slightly, the deviations can be distributed between both the paraffin curve and the aromatic curve. This will reduce the magnitude of the deviations to about half the values shown in Table I1 The Dodge equation is strictly correct only for isothermal data. This may be the reason for the larger errors in the pentanebenzene system, since this system covers the largest temperature range of all the systems studied. The Van Laar equations, as modified by Gilliland, attempt to correct for temperature. And so the pentane-benzene data were also analyzed with the Van Laar equations. The procedure i s described in a n earlier article ( I ) . The results are shown as
dotted lines on Figure 6. For this system the Van Laar technique seems to fit the experimental data much better than does the Dodge procedure. At the 10% point the experimental curve for benzene deviates only about 4.9% from the Van Laar curve. The Dodge equations and the Van Laar equations are merely two of many possible integrated forms of the basic Duhem equation. The fact that experimental data do not precisely fit either
Table 11. Summary of Deviations from Thermodynamic Consistency Assuming Paraffin Curve to B e Correct and Computing Aromatic Curve by Dodge Method (2) y
System Pentane-benzene Hexane-benzene Benzene-heptane Hexane-toluene Heptane-ethylbenzene
Aromatic at 10% Aromatic % Deviation Measured Computed from Computed Value 1.70 1.48 +14.8 1.375 1.24 1 ..40
1,265
1.36 1.32 1.29 1.22
+1.1 -6.4 f8.5 +3.7
INDUSTRIAL AND ENGINEERING CHEMISTRY
2218 I
l
l
I
FIGURE 6
I
-,
ACTIVITY COEFFICIENTS FOR T H E SYSTEM PENTANE-BENZENE
Vol. 47, No. 10
I
FICYRE 7
ACTIVITY COEFFICIENTS FOR THE SYSTEM
I
HEXANE- BENZENE
24
2.2
20
h
$
1.8
4
2
20
40
SO
80
10
MOLE% P E N T I N E ! N L I Q U I D FIGURE 9
ACTIVITY COEFFICIENTS FOR THE SYSTEM HEXANE.TOLUENE
.
1.6
5
F I C U R E 10
of these forms does not necessarily prove the data are thermodynamically inconsistent, The various consistency equations are valuable tools for checking and smoothing experimental data. B u t they should not be relied upon completely. For the present study, nearly all the experimental errors that show up in the computation of activity coefficients are analytical. Temperatures are believed to be accurate to better than f0.1"C. Pressures to about k0.5 mm. Hg. Neither of these variations have much effect on the magnitude of the activity coefficients. Vapor pressures published by the Bureau of Standards for these hydrocarbons probably also have good accuracy. The analytical technique used for these mixtures is estimated to have an accuracy of about = t O . l % . At a liquid composition of 50% this could introduce a maximum uncertainty of about 0.770 in the determination of the activity coefficient. At 10% the uncertainty in gamma would increase to about 2.7% and a t 570, 5.10/,. At a liquid composition of 2%, an uncertainty of as much as %yo in the activity coefficient is possible. This shows why the experimental determination of terminal activity Ir coefficients is difficult.
ACTIVITY COEFFICIENTS FOR THE SYSTEM HEPTANCETHYLBENZENE
I
j I
October 1955
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figures 1 through 5 plot the experimental temperature-composition data for each of the systems. I n Figures 6 through 10, activity coefficients are plotted against liquid composition. CONCLUSIONS
All five paraffin-aromatic systems studied in this myork show considerable deviations from ideality. There is no evidence of azeotrope formation, although the temperature-composition diagrams for both hexane-benzene and benzene-heptane pinch together closely a t the low boiling ends of the diagrams. I REFERENCES (1)
Hipkin, IT. G., and Myers, H. S., IND.ENG.CHEM.,46, 2524 (1954).
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(2) Dodge, B. F., Chemical Engineering, Thermodynamics, 1st ed., p. 562, McGraw-Hill, New York (1944). (3) Myers, H. S., American Documentation Institute, Document No. 4595 (1955). (4) Sieg, L., &ern.-1ng.-Tech., 22, 322 (1950). ( 5 ) Tongberg, C. O., and Johnston, F., IND. ENG.CHEM..25, 733 (1933). RECEIVED for review December 13, 1954. ACCEPTED April 18, 1955. A more detailed form of this paper (or extended version, or material supplementary to this article) has been deposited as Document No. 4595 with the AD1 Auxiliary Publications Project, Photoduplication Service, Library of Congress, Washington 25, D. C. A copy may be secured by citing the document number and b y remitting $2.50 for photoprints or $1.75 for 35mm. microfilm. Advance payment is required. Make checks o r money orders payable t o Chief, Photoduplication Service, Library of Congress.
Phase Equilibria in Hydrocarbon N
Systems VOLUMETRIC BEHAVIOR OF n-HEPTANE W. B. NICHOLS, H. H. REAMER, AND B. H. SAGE California Institute of Technology, Pasadena, Calif.
I
^\FORMATION concerning the influence of pressure and temperature upon the specific volume of the lighter paraffin hydrocarbons is of interest in a number of industrial operation*. Rossini (7') summarized the physical properties of nheptane a t atmospheric pressure. Beattie ( 1 , IO) and others studied the volumetric behavior of n-heptane a t elevated pressures. Eduljee, Newitt, and Weale (4)investigated the volumetric behavior of its liquid phase at temperatures below 140" F . and a t pressures u p to approximately 75,000 pounds per square
Table I.
inch. Gilliland and Parekh ( 5 )determined the effect of pressure upon the enthalpy of this hydrocarbon for pressures below 1000 pounds per square inch. Stuart, Yu, and Coull ( I S ) calculated the thermodynamic properties from existing data for pressures up t o 300 pounds per square inch. The freezing point of nheptane was determined by Streiff ( 1 2 ) and others and the boiling point was established by Smith and Matheson (9). All these data established the volumetric behavior of this compound with fair accuracy, despite disagreement of the
Experimental Volumetric Measurements for n-Heptane (Sample weight 0.248666 lb.)
Pressure, Lb./Sq. Inch Abs.
Specific Volume, Cu. Ft./Lb.
9979.7 9536.9 8955.4 8083.0 6965.7 5927.6 4968.6 3942.3 2911.7 1911.5 913.8 36.5 25.2
40" F. 0.021575 0.021636 0.021695 0.021786 0.021915 0.022055 0.022193 0.022330 0.022480 0.022744 0.022826 0.022977 0.022981
9874.3 9485.7 9037.2 8015.0 7065.0 5915.2 5006.9 3956.2 3033.8 2014.7 947.8 34.4 24.1
160' F. 0.022861 0,022922 0.022996 0.023152 0.023308 0.023498 0.023692 0.023903 0.024137 0.024410 0,024732 0.025023 0.025031
Pressure, Lb./Sq. Inch Abs.
Specific Volume, Cu. Ft./Lb.
9494.7 9073.2 8059.2 7077.5 6081.1 5033.9 3932.6 2896.7 1964.5 868.5 103.2 36.7 21.9
100' F. 0,022280 0.022324 0 022459 0 . 022598 0 022741 0.022904 0,023093 0,023292 0.023475 0.023724 0 023911 0.023939 0,023941
10011.5 9471.2 9002.0 8006.0 7070.0 6057.7 5001.5 4007.1 3018.7 2012.3 978.9 34.1 25.9 17.9 17.6
220' F. 0.023486 0.023553 0.023663 0.023839 0.024042 0.024265 0,024518 0,024800 0.025107 0,025436 0.026852 0.026378 0.026388 0.029025 0.062026
Pressure, Lb./Sq. Inch Abs. 9604.0 9032.2 7163.8 5348.3 3081.5 931.9 122.9 61.2 39.9 39.3
9768.3 9549.2 9105.6 8079.7 7171.9 6057.0 4982.8 4006.4 2987.9 1968.1 934.9 446.2 241.7 158.3 155.3 154.7
Specific Volume Cu. F t . / i b .
280' F. 0.024162 0.024266 0,024725 0.025246 0.025932 0.027127 0.027839 0.027941 0,034996 0.059339
.
0.025501 0.025549 0.026678 0.0259~6 0.026300 0.026741 0.027229 0,027805 0,028501 0.029420 0.030856 0.031898 0.032540 0.037729 0.054006 0.075550
Pressure Lb./Sq. Idch Abs. 10028,l 9586.7 9160.2 8046,l 7072.6 6011.5 5024.3 4023.5 2998.9 1962.7 884.5 340.2 132.0 86.7 84.7 84.3
340° F. 0.024764 0.024861 0.024977 0.025253 0.025588 0.025877 0.026282 0.026690 0.027216 0.027858 0.028834 0.029494 0.029855 0.037815 0.055259 0.074289
9879.2 9527.0 9099.0 8089.8 7044.6 5935.3 4984.5 3921.4 2947.6 1919.5 1404.2 886.6 493 5 390.6 263.7 262.4 260.9 t
Specific Volume, Cu. Ft./Lb.
460° F. 0.026153 0.026296 0.026394 0,026761 0.027194 0.027783 0.028286 0.029032 0.029921 0.031284 0.033731 0.032270 0.035590 0.0363 64 0.050854 0,064656 n ,088502
