VAPOR-LIQUID EQUlLlBplA OF
Close-Boiling Binary Hydrocarbon Mixtures A. R. Richards and E. Hargreaves TRINIDAD LEASEHOLDS LIMITED. T R I N I D A D .
Vapor-liquid equilibrium data for binary systems of hydrocarbons boiling in the neighborhood of 80' C. are presented. A m i n i m u m constant-boiling mixture is formed by t h e system benzene-2,4-dimethylpentane, giving an azeotrope boiling a t 75.2" C. and 757 mm. and containing 54.5 mole benzene, and by the system benzene-cyclohexane, giving an azeotrope boiling a t 77.4' C. and 759 mm. and containing 51.5 mole % benzene. Analytical difficulties prevent precise evaluation of data for system cyclohexane methylcyclohexane.
E.W.1
lowed in the apparatus devised by Rosanoff, Bacon, and White (94). The results were not self-consistent. Experiments were therefore undertaken in a modification of the apparatus described by Othmer (21) which ensured better circulation of the liquid in the reboiler. The apparatus in its final form approximated closely the design subsequently reported by Othmer in a more recent communication (93). Successive additions of l e 3 0 ml. of the hydrocarbon were made to an initial charge of 200 ml. of benzene in the Othmer apparatus, and samples of liquid and condensate were drawn 6 hours after each addition. A series of mixtures of the hydrocarbons was made up by weighing, and the molal compositions were calculated. After determination of the refractive indices of these samples on an Abbe refractometer, curves were prepared which enabled samples obtained from the Othmer apparatus t o be analyzed by a single refractive index determination (Table 11).
-
T
HE separation of pure hydrocarbons from petroleum fractions by distillation usually requires a high degree of fractionation and very efficient columns. However, even under the best conditions certain separations are found t o be impossible; the separation of benzene from the low-octane cut boiling in the range 65-70' C. during precise fractionation might be cited as an example. Several binary hydrocarbon systems related t o this problem have been investigated, and vapor-liquid equilibrium data have been published on binary hydrocarbon systems of the following types: paraffin-paraffin (16)' paraffin-aromatic (36)'' aromaticaromatic (15),aromatic-naphthenic ( l 7 ) ,and paraffin-naphthenir (4). I n many cases the existence of azeotropes was reported, and in view of the ever increasing importance of supplies of aromatic blending agents for high-octane and high-performance aviation fuel, more information was required. The data are extended in this communication to the behavior of petroleum hydrocarbons boiling between 80 and 81 C. Baker's Analyzed benzene was refractionated on a still employing 4 feet of 25-mm. Stedman packing (3) at B reflux ratio of 34 to 1. This column was equivalent t o about fifty theoretical plates on total reflux. Eighty-four per cent by volume of the charge distilling at constant temperature and refractive index was dried over anhydrous sodium sulfate. The 2,4-dimethylpentane was obtained during the fractionation of a cold acid alkylate, and the fraction used in this work resulted from five complete series of fractionations on stills employing 4 feet of 3/s-inch Stedman packing a t a reflux ratio of 119 to 1. These columns were equivalent to about eighty plates on total reflux. Cyclohexane was prepared by the catalytic hydrogenation of a sample of the dry benzene; methylcyclohexane was prepared in like manner from toluene. The hydrogenated produets were fractionated in stills employing 4 feet of a/s-inch Stedman packing a t a reflux ratio of 119 to 1. Comparison of the physical constants of the fractions used in this work with the literature values is given in Table I. Any impurities in the materials used are probably paraffinic, but in view of the method of preparation it is considered unlikely that the effect of these impurities on the vapor-liquid equilibrium data could be detected. The method outlined by Cornel1 and Montonna (8) was folO
2.4 A
A
BENZENE CYCLOHEXANE
2.0
16.
2o-,1.2 0
-J
08
O
0.4
0
0.2
0.4
0.6
0.8
1.0
MOL FRACTION BENZENE IN LIQUID
Figure 1.
Activity Coefficients for binary Hydrocarbon Mixtures
The temperature in the apparatus was determined during the runs by a totally immersed Anschutz thermometer previously calibrated t o *0.05" C. against the platinum resistance thermometer which had been used for the determination of temperatures recorded in Table I. Accordingly the temperatures in Tables 111,IV, and V for the boiling points of the binary mixtures are probably correct t o *0.1 O C. The data obtained by analysis of samples from the Othmer apparatus are given in Tables 111,IV, and V, together with other data necessary t o calculate the activity coefficients as suggested by Carlson and Colburn (7). Conformity of experimental data
805
Vol. 36, No. 9
INDUSTRIAL AND ENGINEERING CHEMISTRY
806
0.2 04 06 0-8 1.0 MOL FRACTION CYCLWEXANE IN LIQUID
0
Figure3. Equilibrium Data for CyclohexaneMethylcyclohexane Mixtures a t 760 Mm. T h e curve represents Raoult's law a n d was calculated from t h e following equation, assuming a = 1.796:
2 -
!
0
0.2
0.4
0.6
0.8
1 - y
=
ax -
1 - x
1.0 boiling point, a similar equation was used for calculating the partial pressures of cyclohexane and 2,4-dimethylpentane. I n effect, t8hismethod assumes that the pressure-temperature curves for the three compounds were parallel over the small temperature ranges under consideration, and the probable errors could not make any significant change in the conformity or nonconformit>g of the activity-coefficient composition relation with a smoot8h curve. The relation between these coefficients and the composition of the mixtures is shown in Figure 1; it is evident that the technique adopted gave satisfactory results. The curveR in Figure 2 n-ere drawn from the equilibrium data in Tables I11 a.nd I v.
TABLE I. PROPERTIES OF H Y D E ~ C A R B O N S Material Cyclohexane
Freezing Point, O C . 6.4 (34) 6.4 (98)
Boiling Point (760 Mm.), C . 80.8 (33) 80.5 (8)
6.5(86) 6.25" (6)
o
0.2
0.4
6.27b
0.6
0.8
1.0
MOL FRACTION BENZENE IN LIQUID
Figure 2. Equilibrium Data for Mixtures of Benzene-2,4-Dimethylpentane a t 757 M m . (above) and of Benzene-Cyclohexane (below)
5.456 -120.6 (19) -123.4 (10)
80.06b 80.70 (I#)
-119.2 (38)
...
-120.4b
with equations of van Laar (BO), Margules ( I I ) ,or Scatchard (26) is not a necessary criterion of experimental accuracy, particularly as it is necessary to choose the correct type of equation to fit the data. The data on beneene-2,4-dimethylpentane would, for instance, be best represented by a n equation of the van Laar form; the data on benzene-cyclohexane would require an equation of the more unusual Scatchard or Margules type. However, the continuity of any function such as the activity coefficient which depends on temperature and composition, is a valuable check on the self-consistency of the results. To obtain these coefficients, the formula t =
80.09
+ 0.042683 ( p - 760) - 0.00002199 ( p - 760)'
given by Smith and Matheson (50) was used for calculating the partial pressure, p , of benzene at a temperature, t. After making a suitable correction in the first term for the slight difference in
Xlethylcyclohexane
.Lo*
.... ,, , , .... 1.'42llb
80:60b
80.098 (89) 5149 ( 1 ) 80.094(57) 5.45( I f , 14) 80.122 (39) 5.51 (37) 80.12(88)
Benzene
2,4-Dimethylpentane
80.8(6)
~Refractive I_ n d ex
80.60 (96) 80.60(91)
...
.
, , ,
, . . .
1:464w ..,.
.... 1:377Ob
n'$
1:4264(1f; .
,
I
.
1 ,'42k8C
1:5012(18~
....
i:5oiic 1.3920(19)
....
1:3820< 1.4230 (11)
101.2( 9 ) ,... 100.8(34) 100.8Ob 1:4182b 1:4233C calculated from values of n'D" and d n / d t given in refer-
-126.4 (18)
- 127.O l b
* Values for n%' encee cited. a From petroleum.
b This research.
C
Calculated.
TABLE 11. REFRACTIVE INDICES OF HYDROCARBOS MIxrLxEs Benzene-2,4Dimethylpentane Mole fraction benzene n%Q 0.000 1.3770
0,095 0.210 0.295 0.418 0.528 0.627 0.715 0.796 0.870 0.938 1.000
1.3832 1.3914 1.3986 1.4101 1.4211 1.4330 1.4447 1.4570 1.4688 1.4818 1.4944
BenzeneCyclohexaneMole fraction benzene ny 0.000 1.4212 0.154 1.4290
0.310 0.513 0.711
0.808
0.904 1.000
1.4387 1.4542 1.4693 1.4780 1.4860 1.4948
Cyclohexane-
-Methylcyclohexane Mole fraction cyclohexane 0.000
0.140 0.450 0.675 0.806
1,000
nko 1.4185 1.4188 1.4195 1.4201 1.4205 1.4211
_
EQUILIBRIUM DATAFOR BENZENE-z,4-DITABLE 111. VAPOR-LIQUID METHYLPENTANE (24M5) MIXTURES AT 757 MM. Vanor
696.3 689.8 679.0 674.8 670.7 666.5 662.2 658.3 656.1 656.1 651.9 656.1 654.3 658.3 658.3 666.5 670.7 674.8 681.1 693.9
77.3 77.0 76.5 76.3 76.1 75.9 75.7 75.5 75.4 75.4 75.2 75.4 75.3 75.5 75.5 75.9 76.1 76.3 76.6 77.2
682.8 676.3 665.9 661.6 657.7 653.1 648.9 644.9 642.7 642.7 637.7 642.7 639.7 644.9 644.9 652.2 656.7 660.7 667.1 680.0
Mole Fraction in: Liquid Vapor 0.922 0.894 0.860 0.838 0.808 0.776 0.749 0.719 0.665 0.622 0.568 0.520 0.475 0.428 0.384 0.338 0.300 0.260 0.215 0.161
0.078 0.106 0.140 0.162 0.192 0.224 0.251 0.281 0.335 0.378 0.432 0.480 0.525 0.572 0.616 0.662 0.700 0.740 0.785 0.839
0.875 0.836 0.805 0.781 0.768 0.732 0.708 0.682 0.643 0.608 0.568 0.631 0.496 0.459 0.424 0.386 0.348 0.814 0.269 0.215
0.125 0.164 0.195 0.219 0.242 0.268 0.292 0.318 0.357 0.392 0.432 0.469 0.604 0.541 0.576 0.614 0.652 0.686 0.731 0.785
~~
EQUILIBRIUM DATAFOR TABLE IV. VAPOR-LIQUID MIXTURES AT 759 MM.
762.6 758.0 746.1 741.5 716.4 700.8 698.4 698.4 698.4 700.8 703.1 703.1 705.2 707.4 711.9 716.4 718.8 727.6 732.1 734.5 739.2 741.5 746.1
80.2 80.0 79.5 79.3 78.2 77.5 77.4 77.4 77.4 77.5 77.6 77.6 77.7 77.8 78.0 78.2 78.3 78.7 78.9 79.0 79.2 79.3 79.5
750.9 745.7 734.3 729.5 704.7 689.2 687.1 687.1 687.1 689.2 691.5 691.5 693.4 695.5 700.1 704.7 706.8 716.0 721.4 723.0 727.4 729.5 734.1
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
September, 1944
0.045 0.066 0.105 0.118 0.254 0.383 0.449 0.602 0.554 0.597 0.628 0.645 0.720 0.738 0.753 0.798 0.835 0.879 0.893 0.914 0.928 0.937 0.960
0.955 0.934 0.895 0.882 0.746 0.617 0.551 0.498 0.446 0.403 0.372 0.355 0.280 0.262 0.247 0.202 0.165 0.121 0.107 0.086 0.072 0,063 0.040
0.059 0.084 0.129 0.145 0.290 0,407 0.461 0.502 0.545 0.580 0.605 0.621 0.682 0.698 0.717 0.758 0.798 0.847 0.865 0.889 0.907 0.918 0.948
0.941 0.916 0.871 0.855 0.710 0.593 0 539 0.498 0.455 0.420 0.395 0.379 0.318 0.302 0.283 0.242 0.202 0.153 0.135 0.111 0.093 0.082 0.052
Activity Coeffioiex 1.032 1.026 1.044 1.046 1.059 1.071 1.080 1.091 1.116 1.129 1.162 1.178 1.210 1.236 1.268 1.298 1.310 1.356 1.388 1.458
1.776 1.732 1.683 1.547 1.451 1.387 1.357 1.328 1.227 1.222 1.188 1.151 1.138 1.113 ‘1.096 1.077 1.074 1.064 1.055 1.042
0.591 0.605 0.671 0.689 0.745 0.788 0.813 0.838 0.907 0.942 1.000 1.045 1.087 1.134 1.182 1.231 1.246 1.302 1.343 1.427
of the formation of an azeotrope, and the deviai tion from the Raoult law curve is well within experimental error. The values of cy for each system have been calculated from the equation
where x,y = moles of one component in liquid and vapor phases in equilibrium
and are represented graphically in Figure 4. The probable errors are indicated by dotted lines. The data on binary hydrocarbon mixtures consisting of an aromatic with a nonaromatic constituent were discussed in a recent paper by Griswold and Ludwig (17)where attention was drawn t o the fact that a constant-boiling mixture or some other abnormality had been encountered in all cases so far reported. The data offered BENZENE-CYCLOHEXANE in the present paper give additional evidence that the “abnormal” behavior of benzene is the rule in such systems. The data in the literature also indicate that, while not necessarily ideal, mixtures of two hydrocarbons in the same homologous series do not ex1.306 1.275 hibit constant-boiling mixtures. Examples have 1.254 been cited of binary paraffinic mixtures and binary 1.262 1.211 aromatic mixtures, and it is evident from this work 1 153 1.118 that at least one binary naphthenic mixture does 1.089 not form an azeotrope. Consequently the difficul1.070 1.053 ties encountered in achieving a satisfactory sepa1.041 1.042 ration of pure hydrocarbons from mixtures boil1.020 ing in a range near to or below that of the aro1.015 1.015 matic compounds cannot be attributed to homol1.005 1.008 ogous azeotropic mixtures but t o azeotropes 1.006 formed by aromatics with paraffins or naphthenes, 1.004 1.003 as pointed out by Bruun (6), and also by non1.004 1.002 ideality of some paraffin-naphtbenic mixtures. 1.006
TABLE V. VAPOR-LIQUID EQUILIBRIUM DATAFOR CYCLOHEXANE-METHYLCYCLOHEXANE MIXTURES AT 760 MM.
‘ Temp., C. 98.3 96.0 94 1 91.6 90 5 87 4 84.2 83.6
Vapor Pressure, Mm. MethylcycloCyclohexane hexane 710.0 1247 665.8 1172 630.0 1112 684.2 1038 565 2 1005 512.0 916 832 464.5 817 455.7
Mole Fraction in: Liquid Vapor MethylMethylCycloCyclo- cyclocyclohexane hexane hexane hexane 0.909 0.185 0.815 0.091 0.278 0 722 0.185 0.815 0.564 0.278 0.722 0.436 0.637 0.363 0.447 0.553 0.436 0.675 0.325 0.564 0.709 0.637 0.363 0.291 0.807 0.193 0.902 0.098 0.068 0.965 0.035 0.952
Activity Coeffioient y MethylCyclocyclohexane hexane 1.240 0 9609 0.9751 1.012 1.387 0.7290 1.043 0 7290 0.9505 1002 0 9234 1.190 1020 0.8301 0.9624 0.8577
The relations between the equilibrium data and temperature for the two series of mixtures are also given in Figure 2. The minimum constant-boiling mixture formed by benzene-2,4dimethylpentane mixtures contains 54.5 mole % benzene and boils a t 75.2’ C. (757 mm.). The minimum constant-boiling mixture formed by benzene-cyclohexane mixture contains 51.5 mole % benzene and boils a t 77.4’ C. (759 mm.). Scatchard, Wood, and Mochel (87) reported isothermal data for this system, showing an azeotrope containing about 50 mole % benzene a t a boiling temperature of 70” C. The refractive indices of cyclohexane and methylcyclohexane are so close together that an accurate analysis of their mixtures could not be obtained with the Abbe refractometer. The results are given in Tables I1 and V and Figure 3. There is no indication
0.2
0
06
0.6
0.8
1.0
MOL FRACTION BENZENE IN LIQUID Figure 4.
Relative Volatilities of Binary Hydrocarbon Mixtures Benzene-cyclohexane
0 Benzene-2,4-dimethylpent.n.
808
INDUSTRIAL AND ENGINEERING CHEMISTRY ACKNOWLEDGMENT
Thanks are due to Trinidad Leaseholds Limited for permission to publish the details of this work. LITERATURE C I T E D
(1) (2) (3) (4)
Bartell, F. E., and Mack, G. L., J . Phya. Chem., 36, 65 (1932). Bell, G. H., and Davey, W. P., J . Chem. Phys., 9, 441 (1941). Bragg, L. B., IND. EXG.CHEM.,ANAL.E D . , 11, 283 (1939).. Bromiley, E. C., and Quiggle, D., I N D . ENG.CHEM.,25, 1136
(1933). (5) Bruun, J. H., and Hicks-Bruun, M .% Bur. I.Standard8 , J . Research, 5, 612 (1930). (6) Ibid., 5, 933 (1930). (7) Carlson, H. C., and Colburn, A. P., IWD.ENG.CHEM.,30, 581 (1942). (8) Cornell, L. W., and LMontonna, R. E., Ibid., 25, 1331 (1933). (9) Doss, M. P., “Physical Constants of Hydrocarbons”, 2nd ed., 1939. (10) Edgar, G., and Calingaert, G., J . Am,. Chem. SOC., 51, 1540 11929). (11) EGoff, G., “Physical Constants of Hydrocarbons”, New York, Reinhold Pub. Corp., 1937. (12) Egloff, G., and Grosse, A. V., Universal Oil Products Co., Booklet 217 (1938). (13) Francis, A. W., IND.ENG.CHEM.,33, 554 (1941). (14) Garner, F. H., and Evans, E. B., Inst. PetroZeumTech., 18, 751 (1932). (16) Griswold, J., D.Sc., thesis, M.I.T., 1931. ENQ.CHEM.,35, 247 (1943). (16) Griswold, J., IND. (17) Griswold, J., and Ludwig, E. E., Zbid., 35, 117 (1943).
Vol. 36, No. 9
(18) Hicks-Bruun. &I. M.. and Bruun. J. H.. Bur. Standard.y .I. Research, 8, 525 (1932). (19) Huffman, H. M,, and Parks, G, s., IND. ENG. CHEM,, 23, 1138 (1931). (20) Laar, J.’J. van, 2.physik. Chem., 72, 723 (1910); 83, 599 (1913) (21) Margules, M., Sitzber. Akad. Wiss. W i e n , Math. naturw. Klasse, 11, 104, 1243 (1895). (22) Othmer, D. I?., I N D .ENG.CHEM.,20, 743 (1928). (23) Ibid., 35, 614 (1943). (24) Rosanoff, M. d.,Bacon, C. W., and White, R., J . Am. (’hem. Sac., 36, 1803 (1914). (25) Sabatier, P., and Senderens, J. B., Compt. rend., 132, 1255 (1901). (26) Scatchard, G., and Hamer, W. J., J . Am. C h m . Soc., 57, 1805 (1935). J . P h y ~ Chem., . (27) Scatchard, G., Kood, 5. E., and .Mochel, J. M., 43, 119 (1938). (28) Smith, A., and Menries, A., J . A m . Chem. Sac., 32, 1453 (1910) (29) Smith, E. R., J . Research Natl. Bur. Standards, 26, 129 (1941) ( 3 0 Smith, E. R., and Matheson, H., Ibid., 20, 645 (1938). (31) Smittenberg, J., Hoop, H., and Henkes, R. A., J . Am. Chem. Soc., 60, 18 (1938). (32) Timmermans, J., Bull. sac. chim. Belo., 36, 502 (1927). (33) Timmermans, J., J . chim. phys., 23, 760 (1926). (34) Timmermans, J., and Martin, F., Ibid., 23, 733 (1926). (35) Tongberg, C. O., and Johnson, F., IND. ENG.CHEX.,25. 733 (1933). (36) W-ibaut, J. P., et al., Rec. trav. chim., 58, 329 (1939). (37) Wojciechowski, M., J. Research Natl. Bur. Standards, 19. 347 (1937). (38) Zelinskii, Ber., 34, 2802 (1901). (39) Zamsczyaski, A., J . chim. phys., 27, 503 (1930).
Soap-BoilingEquilibria for Sodium Stearate THE NEW PHASE, KETTLE WAX JAMES W. McBAIN, KENNETH GARDXER’, ~ N ROBERT D D. VOLD2 Stanford University, Calif.
T
HE phase rule in its usual form applies, strictly, to equilibria in soap systems (2, 9). This has been carefully demonstrated for a number of binary systems containing water with pure and commercial soaps, and for the ternary system sodium palmitate-sodium chloride-water (3,5, 6, 11). I n general, the data for commercial soaps in the presence of electrolytes and water rather closely conform t o the behavior calculable from their constituents (4). Heretofore ternary soap systems have been interpreted as involving equilibria only between curd, neat soap, middle soap, nigre, and Iye. The only existing studies of such ternary systems of a single pure soap are for laurate and palmitate (3, 6, 11), and they are now known to be incomplete, since at that time the waxlike island phase, “kettle wax”, had not been discovered. Data are given here for sodium stearate. I n 1940 Gardiner (1) found evidence that in the ternary systems sodium stearate-sodium chloride-water and sodium palmitate-sodium chloride-water a waxlike form exists a t soap boiling temperatures. This new kettle wax phase occupies a prominent position in the middle of the phase diagram. It is so predominant that many of the equilibria commonly assumed t o be the result of “graining out” into soap curd and lye are now shown t o be equilibria between kettle wax and lye. Only a t still higher 1 2
Present address, Lever Brothers Company, Cambridge, Mass. Present address, University of Southern California, Los Angeles, Calif.
salt concentrations (not usually employed) is sodium stearate curd in equilibrium with lye. The kettle wax phase is dull white, soft, and waxy. On cooling to about 70” C., it turns to a white solid curd, which turns again t o kettle wax upon heating. The problem has been approached by a number of independent methods; a separate study of the same system was made by Lee ( 7 ) using the vapor pressure method. Results for binary systems naturally give the starting point for the boundaries iri the ternary systems. The simplest method is visual examination (usually between crossed polaroids), but the precaution must be taken of first heating the system until it is one iimpid isotropic liquid. I n the synthetic method, temperatures a t which the various phase changes occur are observed for systems of known composition, and a boundary may be delimited b e h e e n pairs on either side of it. T h e separation of phases is greatly helped by centrifugation in a n angle centrifuge kept a t the equilibrium temperature. Where possible it is highly advantageous t o analyze both layers even though only one phace may separate completely from the other; usually, however, only the latter phase affords a good sample. It is a requirement that R straight line connecting either analysis with the total composition of the kystem must pass through the actual composition of the other pha5e as well as all mixtures of the two. These methods have been supplemented by microscopic observations using a heating stage and by calorimetric determinations which show large heat effertb
