Hydrocarbon Azeotropes with Acetonitrile
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RICHARD B. BISHOP AND WILLIAM I. DENTON Research and Development Department, Socony-Vacuum Laboratories, Paulsboro, N. J .
a
4
Fifteen different hydrocarbons representative of aromatics, n-olefins, paraffins, and naphthenes were distilled with acetonitrile. The compositions, boiling points, and some other properties of the azeotropes that formed were determined. A comparison was then made between these experimental data and data calculated assuming the nonaromatic hydrocarbons and acetonitrile were immiscible. I t was concluded that: (1) benzene and toluene are the only aromatic hydrocarbons that form azeotropes; (2) paraffins, n-olefins, and naphthenes containing up to nine carbon atoms form azeotropes; and (3) the boiling points and compositions of the nonaromatic hydrocarbon azeotropes can be approximated using the laws of distillation of immiscible liquids.
a few drops of condensate were withdrawn and the process J\ as repeated. The rate of boil-up was well below flooding velocities but sufficiently high to keep the helices thoroughly wet with reflux. Each hydrocarbon was mixed mith an equal volume of acetonitrile and then distilled. The percentage of the hvdrocarbon in the distillate was determined by diluting a known volume of the azeotrope with excess water and measuring the volume of the insoluble hydrocarbon. In some cases this was further checked by measuring the amount of acetonitrile required to effect distillation of a given amount of hydrocarbon as the azeotrope. In this procedure, two general precautions should be observed. The still head should be so constructed that when the azeotrope is heterogeneous total take-off with no holdup is obtained during the withdrawal period. If any holdup occurs in the take-off system the product will not represent the true composition of the azeotrope. Dilution with Mater should be continued until two successive additions of water give the same volume of insolubles.
T
HE recent commercial availability of acetonitrile has stimu-
lated interest in the possibility of using this material as a selective separating agent. Several references appear in the literature to the formation of azeotropes between acetonitrile and hydrocarbons. Pratt ( 9 ) used the fact that ternary azeotropes form with acetonitrile, water, and a hydrocarbon boiling between 60" and 145" C.-Le., benzene or toluene-to obtain anhydrous acetonitrile. Mair (7, 8) encountered azeotropes of acetonitrile and hydrocarbons. A number of petroleum companies ( I , 2, 4, 11) have also reported the azeotrope-forming tendency of acetonitrile. Lecat (6) has reported boiling points and compositions of azeotropes of acetonitrile \vith n-heuane and with benzene. The present investigation was conducted to determine the behavior with acetonitrile of hydrocarbons of different types, the boiling points of azeotropes, and the compositions of the azeotropes. In the case of liquids not completely miscible at the boiling point, the boiling point and composition of the binary azeotropes were calculated from the vapor pressure curves of the pure components, assuming complete immiscibility. The difference between these calculated figures and the observed data then represented thr deviation due to partial miscibility.
Table I is a summary of the data for the acetonitrile-hydrocarbon azeotropes. This table lists the boiling points and molecular weights of the hydrocarbons (6, I O ) as well as the boiling points and volume percentage compositions of the azeotropes. It may be noted that azeotropic distillates from the aromatic hydrocarbons condense as a single phase which remains homogeneous to room temperature. Azeotropic distillates from naphthenes, n-paraffins, isoparaffins, and n-olefins condense to two liquid phasw at the boiling point. After cooling to room temperature the acetonitrile layer from n-paraffin, naphthene, and isoparaffin azeotropes forms no insoluble layer on dilution with water; this indicates a very low content of hydrocarbon. In the case of the wolefins, however, an additional amount of hydrocarbon is recovered by diluting the acetonitrile layer and thus there is a considerable mutual solubility of the two components.
TABLEI. ACETONITRILE-HYDROCARBOX AZEOTROPES Hydrocarbon
EXPERIMENTAL
I
The hydrocarbons used in these studies were purchased chemicals and were of C.P. grade. All samples were analyzed bv spectroscopy and, if necessary, purified to 99+% purity. The acetonitrile mas made at these laboratories according to the method disclosed in t?. S.Patent 2,450,637 ( 3 ) and was distilled and dried with calcium chloride. The distillation column used in examining azeotropes was a 1-inch glass column, packed with Fenske helices. Under the operating conditions described below, this column had an efficiency of 30 theoretical plates as determined using a toluene-methplcyclohexanemixture and a McCabe-Thiele diagram. The still head was glass and was equipped with a magnetically controlled take-off which allowed the column to come to equilibrium a t total reflux, after which
Roiling point, 0
C.
Molecular weight
c.
Calcd. b
86 2 68 8 n-Hexane 98 4 100 2 n-Heptane 114 2 125.6 n-Octane 174 0 142.3 n-Decane 114.2 224-Trimethylpentane 99.2 124.1 128.3 2:2:5-Trimethylhexane 80.8 84.2 Cyclohexane 98.2 Mrthylcyclohexane 100.8 121.6 112.2 1-Ortme 125.2 112.2 2-Octene 172.0 140.3 1-Decene Benzene 80.1 78.1 Toluene 110.7 92.1 Mixed xylenesd 138-144 106.2 Ethylbeneened 186.2 106.2 Acetonitrile 81.6 41.1 a Number of layers in condensed aeeotrope. b Calculated assuming oomplete immiscibility. 0 Volume % hydrocarbon is accurate t o *2%. d No aaeotrope formed.
883
54 68
76 81 68 ,
.
60 68 77
ao
.... .,
..
Azeotrope pio.
Volume % Hydrocarbon E x p t l . layers" Ca1cd.b Exptl.c
Roiling Point,
56 8 69 4 77 2 81 6 68.9 76.1 62.2 71.1 28.0 r8.O 81.6 74.4 81.1 81.6 81.6
2 2 2 2 2 2 2 2 2 2 2 1 1 1 1
80 62 43 15 64
..
68 58
49
i6
.. .. ....
75 $56
3i 62 42
67
49 40 38 5
57 22 0 0
IN D U S T R I A L A N D E N G IN E E R IN G C H E M I S T X Y
a34
100
1
I
90
80
Yol. 42, No. 5
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0-
70 .
50 50
70
90 110 130 150 170 190 BOILING POINT OF PURE HYDROCARBON%.
210
Figure 1. Variation of Boiling Point of Acetonitrile Azeotrope with Boiling Point of Pure Hydrocarbons
100 I
I
I
I
I
I
I
I
I
I
I
80
60
.\ I
n-OLEFINS, PARAFFINS, NAPHTHENES I
40
20 0
50
90 110 130 150 170 190 BOILING P O I N T OF PURE HYDROCARBON-'C.
70
210
Figure 2. Effect of Boiling Point of Hydrocarbons on Volume Per Cent l-Iydrocarhonin Acetonitrile Azeotrope
D1SCI;SSIOS
The data in Table I are presented graphically in Figures 1: 2, and 3. Figure 1 plots the boiling point of the pure hydrocarbon against the boiling point of the azeotrope formed with acetonitrile. Paraffins, n-olefins, and naphthenes folloJ!- t'he same curve whereas the aromatic hydrocarbon-acetonitrile azeotropes boil 5 " to 10" C. higher t.han this curve. Thus, benzene and cgclohexane, which boil 0.7' C. apart, form azeotropes boiling 12.2' C. apart. I n all graphs the curve for aromatic hydrocarbons is shown as a broken line for comparative purposes only, because there are no other isomers of these hydrooarbons. Figure 2 s h o w the relationship between the boiling points of various types of hydrocarbons and the volume per cent hydrocarbon in the azeotrope. From this curve it is evident that, at a given boiling point, a given volume of acetonitrile will carry over more paraffins and naphthenes than aromatic hydrocarbons. This graph demonstrates that xylene and ethylbenzene form no azeotrope with acetonitrile. n-Decane yields an azeotrope containing but 5% of hydrocarbon, 2.:' -1 mith hydrocarbons of higher molecular weight the conbent may !-e expected to approach zero asymptotically. The more branched hydrocarbons yield azeotropes of a higher hydrocarbon content than normal paraffins of the same molecular weight. Figure 3 correlates molecular weight with the volume percentage of hydrocarbon in the azeotrope. For a given molecular weight the volume per cent hydrocarbon in the azeotrope increases in the order: aromatics, naphthenes, n-paraffins, and molefins, isoparaffins. Much of the effect for the last three groups of hydrocarbons is due t o the relationship between molecular lyeight, and boiling point for these hydrocarbons.
70
80
90
100 110 120 MOLECIJLAR WEIGHT
130
140
150
Figure 3. Effect of Molecular Weight on Volume Per Cent Hydrocarbon in Acetonitrile Azeotrope
Table I also contains some boiling points and compositions calculated assuming coniplet,e imniiscibility of the components. These boiling points are the temperatures at which the sum of the vapor pressures of the tIyo components equals the observed atmospheric pressure. The compositions correspond to the mole ratios of t,he two componerits indieaimedby the rat.ios of the vapor pressures of the t,wo components at the calculat,ed boiling points. I n general, all of 6he calculated boiling points arc below the experiment,ally determincd values by 1" to 3' C. The calculated compositions are 2 t,o 11% higher in hydrocarbon content t,han the observed values. These data s l i o ~t,hat although the azeotropes of acetonitrile with paraffins, cycloparaffins, arid n-olefins do not follow the l a w of distillation of immiscible liquids exactly, the boiling points and proport'ions of t'he components can be approximated by using these laws. The calculated resu1t.a will give a boiling poiiit about 2 C. lower and a composition about 8% higher in hydrocarbon coniponent~than the true Yalue. Thus, some mutual solubility exists a t the boiling points of these azeotropes and causes t,he differences rioted in this comparison. Because ternary azeotropes could be encountered, a threecomponent system vas investigated i o examine this possibility. From Figures 1, 2, and 3 it, can be predict,ed that if no ternary azeotrope is formed, a mixture of toluene, n-octane, and excess acetonitrile will distill as follows: First, the acetonitrile-n-octane azeotrope will come overhead until all the n-octane is gone; then the boiling point, will rise and toluene will distill over as an azeotrope with acet,onitrile. Experimentally such a mixture distilled into: (1) a fraction boiling a t 7 7 " C. coritaining all the n-octane in a ratio of 36% (volume) n-oct'ane to 64% (volume) acetonitrile and (2) a cut boiling a t 81" C. containing substantially all the toluene in a ratio of 22% (volume) toluene to 78% (volume) acetonitrile. The intermediate cut was negligible. Thus, the azeotrope of n-octane (boiling point 125.6" C.) boils below that of toluene (boiling point 110.7" (3.1. The graph indicates that n-nonane might also form an azeotrope boiling below that of toluene. COTC LU SION
It is concluded from this study that: (1) xylene and higher boiling aromatics do not form azeotropes with acetonitrile, whereas benzene and toluene do; (2) toluene can be separated from paraffins and naphthenes containing up to nine carbon atoms using acetonitrile as the azeotroping agent; (3) branched-chain hydrocarbons are carried over in greater amounts by a given quantity of acetonitrile than corresponding straight-chain hydrocarbons; and (4) the azeotropes formed with acetonitrile and different types of hydrocarbon differ-aromatics give homogeneous azeotropes whereas paraffins, isoparaffins, n-olefins, and napht,henes condense to two liquid layers. n-Olefins and acetonitrile exhibit considerable mutual solubility.
M a y 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY ACKNOWLEDGMENT
885
W. J. van (to Shell Development Co.), Ibid., 2,023,375 (Dee. 3 , 1 9 3 5 ) . * (5) Doss, M. P., ”Physical Constants of Hydrocarbons,” 4th ed., Texas Co., ,May 14, 1943. (6) Lecat, &I. M., Compt. rend., 222, 733 11946). (7) Mair, B. J., and Streiff, A . J., J . Eesearch A’atl. B w .Standards,
(4) Dijck,
The authors wish to express their appreciation for the work of Mary Humphries, who was responsible for the distillation of the azeotropes herein reported.
24, 395 (1940).
(8) Mair, B. J., and Willingham, C. D., Ibid., 21, 535 (1938). (9) Pratt. H. R. C. (to Imoerial Chemical Industries). U. S. Patent
LITERATURE CITED
Deanesly, R. ht. ($0Shell Developmellt CO.),u. S. Patent 2,290,636 (July 21, 1942). Ibid., 2,360,655 (Oct. 17, 1944). Denton, W. I., and Bishop, R.B. (to Socony-Vacuum Oil Co., Inc.), Ibid., 2,450,637 (Oct. 5, 1948).
2,305,106 (Dee. 15, i 9 4 2 ) .
cirC,
~ ~F. D ~ , , ~i ~~B i ~ standards, , ~1 . . 461 (1947). (11) Teter, J. W., and Merwin, W. J. (to Sinclair Refining Co.), U. S. Patent 2,411,346 (Nov. 19, 1946). ~
RECEIVED Xovember 21, 1949.
Viscosity of Highly
&
Compressed Fluids CORRELATION WITH CRITICAL CONSTANTS L. GRUNBERG Dr. Rosin Industrial Research Company, Ltd., Wembley, Middlesex, England
A. H. NISSAN‘ The Bowater Paper Corporation, Ltd., Northfleet, Kent, England T h e viscosity of highly compressed fluids is correlated with critical constants in the form of a nomograph on which the ratio of the viscosity of the compressed fluid, ~ p to , the viscosity of the fluid at moderate (atmospheric) pressure, T*, is given as a function of the reduced temperature, T, = TIT, and of the reduced density, pr = p , p c . The density was chosen as the second parameter in order to make interpolation in the critical region possible. The nomograph is applicable to highly compressed gases and to liquids near the critical point. The accuracy obtainable is of the order of *lo%. A summary of the theoretical
T
HE experimental determination of the viscosity of highly
compressed gases and of liquids near the critical point is very difficult, and comparatively few results have so far been published in the literature. Comings and Egly ( 3 ) correlated the available data and plotted a chart on which the ratio of the viscosity of the compressed gas to the viscosity of the gas at atmospheric pressure is shown as functions of the reduced pressure and the reduced temperature. Cyehara and Watson (26) also correlated the available data, expressing the results in the form of a generalized chart on which the redured viscosity-that is, the viscosity divided by the viscosity at the critical pointis shown as functions of the reduced temperature and the reduced pressure. Both these charts suffer from the same defect. For conditions under which experimental results are most difficult to obtain, namely in the critical region, the charts are both inaccurate and difficult t o interpolate. This disadvantage derives from the fact that the pressure was chosen as one of the parametera. There are several indications that by choosing the density instead of the pressure as the second parameter, this difficulty n-ould not arise. Foremost among these is the fact t h a t the viscositpdensity isotherms in the critical region are not as steep as the viscosity-pressure isotherms. Phillips (f2), for example, deduced from his experimental results on carbon di1 Present address, Bowaters Development and Research Ltd., Northfleet, Kent, England.
considerations leading to the nomograph are given. It is assumed that in a highly compressed fluid transfer of momentum may occur by two mechanisms, one translational, gas-type, and the other vibrational, liquid-type. The following equation is obtained:
is a function independent of the nature of the substance and dependent only on the reduced density.
f(p,)
oxide, that in the critical region the viscosity varies with the square of the density. Watson, Wien, and Murphy ( 1 7 ) found that in the critical region the kinematic viscosity of petroleum fractions vaiies little with the pressure, whereas the absolute viscosity varies considerably. Investigations on the relation between the viscosity and the density showed this latter property to be of great importance particularly for highly compressed gases and for liquids near the critical point. On the basis of these investigations, which are reported in this paper, the authors were able to obtain a general equation in which the ratio of the viscosity of the compressed gas or of the liquids near the critical point to the viscosity of the gas or vapor at moderate pressure and at the same temperature is given as a function of the reduced density and of the reduced temperature. Stated in general terms
where qp = ?A
the viscosity under pressure
= the viscosity of the vapor (not liquid)
pheric) pressure
P r = P _ = pc
density under test critical density
at moderate (atmos-
