Vapor-Liquid Equilibria at Subatmospheric Pressures SYSTEM DODECANE-HEXADECENE JOHN R. KEISTLERI AND MATTHEW VAN WINKLE University of Texas, Austin, Tex.
D
ESIGN of vacuum distillation equipment for separation of
higher molecular weight hydrocarbon mixtures requires a knowledge of vapor-liquid equilibria for the specific systems for which the separation is required or a general knowledge of the effect of reduced pressure on the vapor-liquid relations of similar systems. Prior to the work of Rasmussen and Van Winkle ( 4 ) no vapor-liquid equilibrium data had been reported on the higher
3.5
20
The tests indicate the dodecane to be of reasonable purity. The hexadecene properties agree with those reported except for atmospheric boiling point. This is somewhat higher a t all pressures than that of Krafft reported hy Stull(5) and by Rasmussen
(4). Thermal instability of the hexadecene was encountered when samples were maintained a t the boiling temperature for any appreciable length of time. Iodine tests indicated the presence of peroxides in the sample and it was considered possible that dimerization or condensation of the hexadecene was being catnlyzed by the peroxides. This difficulty was not encountered by Rasmussen (4). However, his work 'lras conducted with freshly prepared hexadecene and the present work was conducted with a different sample of hexadecene which had been stored approximately 1 year.
TABLE I.
PROPERTIES O F
DODECANE .4ND HEXADECENE
Dodecane Exptl. Lit.
.....
Refractive index, n%O Refractive index, nS5
1,42004
Densitv. - , d?6
0.7462
4
Boiling temp. at 760 mm O F . BromiAe N o . (6) Ojo-Purity by bromine
416.0
zoo
240 TEMPERATURE
280
320
,
360
-"E
Figure 1. Vapor Pressure Chart for Dodecane and Hexadecene molecular weight hydrocarbon systems. The results presented here are those obtained from one of a systematic series of studies being conducted for the purpose of evaluating the effect of low pressures on equilibrium characteristics of the heavy hydrocarbons.
1 44111.4420 (I)
...........
NO.
.....
..... .....
Pt.
.....
.....
545.0 70.40
0.77660 . 7 8 1 8 (1) 5 2 5 . 2 (6) 71.35
98.7 99.4-99.7
..... , . , ..
DATAOF PURECOMPONEWS TABLE 11. VAPORPRESSURE nodmane . ........
10 20
Temp., F., exptl. 194.4 220.0
50
26i: 6
Pressure hIm. HB(
PURITY O F COMPOUNDS
The purity of the dodecane and hexadecene used in this investigation was checked by index of refraction, density, boiling point, and freezing point ( 8 ) . I n addition, bromine number was determined on the hexadecene to evaluate the percentage of unsaturai tion. Results of the tests and comparison with literature values are given in Table I. 1
1.4420
The peroxides were removed by refluxing and distilling the hexadecene sample in the presence of solid ferrous sulfate. While the removal of the peroxides reduced the change of refractive index when the hexadecene was heated for a period of time, it did not eliminate the change. Because the boiling points were unchanged before, during, and after the reaction, i t was assumed that the effect on volatility would be negligible and with the correct analytical procedure, reasonably correct vapor-liquid equilibria data could be obtained.
0
160
Hexadecene Exptl. Lit.
1.41951,4197 ( 1 ) 0 . 7 4 5 8 (.1.) 0.7783 4 1 7 . 6 (6)
.....
% Purity by freezing
1.5
.....
-
40
60 100 200 300 400 760
Present address, Dow Chemical Co ., Freeport, Tex.
622
294: 4 329.8 353.3 371.7 416.0
Temp., F., lit. (6) 193.6 219.6 251.2 26916 294.8 330.3 3i0:3 417.6
Hexadecene Temp., F., Temp O F., exptl. lit.'?6) 298.0 295.2 327.1 323.6 353.8 3jO: 0 3j5:4 408: 0 401.6 448.7 440.2 494: 8 545.0
:
48i 0
525.2
I N D U S T R I A. L A N D E N G I N E E R I N G C H E MI S T R Y
March 1952
0
eo
I
40
Bo
80
I
I
I
623
ANALYSIS O F VAPOR AND LIQUID SAMPLES
00
The equilibrium samples were analyzed for composition by means of a Bausch and Lomb precision refractometer using a sodium d line light source. Because the refractive index of the hexadecene changed with temperature and time, it was necessary t o establish reproducible correction factor for the refractive index for constant time of exposure of 20 minutes a t various temeratures and various concentrations of exadecene. The correction data tabulated in I , I I 0 PO 40 eo 80 Kx) Table I11 and shown graphically in Figure 2 were determined by refluxing MOLE PER ORlT WDEOANE prepared samples of known composition Figure 3. Equilibrium Boiling for 20 minutes a t pressures of 760, 400, Point Diagrams 200,100,50,20, and 10 mm. of mercury Dodecane-hexadecene s y s t e m a t and determining their refractive insubatmospheric pressures MOLE PER CENT HMADECENE dexes. The refractive indexes of the Figure 2. Refractive Index Correction mixtures of corresponding composition Chart determined on the unheated samples, Table IV, were subtracted Based o n a 20-minute boiling t i m e from the values obtained on the heated samples of the same composition. Thus the additive correction factor indicates the increase in refractive index with heating a t the indicated pressure VAPOR PRESSURE DATA for 20 minutes. Corrections were necessary only for the liquid samples since the vapor samples evidenced no change in refracVapor pressures for both the dodecane and hexadecene were tive index with time and temperature, determined by means of a Cottrell boiling point apparatus and checked in the Colburn equilibrium still (8). The vapor pressure curve for dodecane checked the literature values as reported by Stull (6) and Rasmussen (4). The vapor pressure curve for the TABLEIV. REFRACTIVE INDEX-COMPOSITION DATA hexadecene used in this investigation differed somewhat from the Unheated samples literature values, particularly at the higher pressures. Hexadecene, Index of Refraction Experimental vapor pressure data for the two compounds are Mole % a t 25' C. tabulated in Table I1 and represented graphically in Figure 1. 0.0 1.4200
K
EXPERIMENTAL METHODS
Vapor-liquid equilibria samples for this binary were obtained through the use of a Colburn-type (8) equilibrium still. The still and the operation of the still were the same as that described by Rasmussen (4). The still was o erated under steady state conditions for a period of 20 minutes for each determination.
18.8 32.0 47.3 59.7 73.1 86.3 100.0
1.4243 1,4271 1.4301 1.4324 1.4347 1.4370 1.4393
TABLE111. CORRECTION FOR REFRACTIVIC INDEX Liquid samples Time, 20 minutes Pressure
Mm. H; 760
400
200
100
50 20
10
.
Hexadeoene, Mole %
Index of Refraction Correction at 25' C.
0 28.5 55.5 77.5 88.4 100.0 0 65.5 77.5 88.4 100.0 0 77.5 88.4 100.0 0 77.5 88.4 100.0 0 88.4 100.0 0 100.0 0 100.0
0.00000 0.00005 0.00018 0.00045 0,00086 0.00162 0.00000 0.00004 0.00018 0.00043 0.00112 0.00000 0.00010 0.00027 0.00083 0.00000 0.00004 0.00014 0.00055 0.00000 0.00008 0.00044 0.00000 0.00034 0.00000 0.00025
100
-X Vapqr-Liquid Equilibrium Diagrams
MOLE PER CENT DODECANE IN LlOUlD
Figure 4.
Dodecane-hexadecene s y s t e m a t subatmospheric pressures VAPOR-LIQUID EQUILIBRIUM DATA
The vapor-liquid equilibrium data derived from this investigation are presented in Table V, as temperature composition curves
INDUSTRIAL AND ENGINEERING CHEMISTRY
624
Vol. 44, No. 3
TABLEV. EXPERIMENTAL VAPOR-LIQUID EQUILIBRIUM DATA System dodecane-hexadecene
Mole Fraction Dodeoane --__ Liquid
Vapor
Temp., F. 370.0 339.0 325.2 308.0 292.2 280.3 270.8 261.6
494.8 473 451.7 445.7 437.2 414.0 394.9 384.3 371.7 448.7 419.4 399.2 381.0 365.2 360.3 339.2 329.8 408.0 381.4 362,O 338.4 323.4 311.4 301.0 294.4
4too M M .
0 8.1 18.3 21.5 27.1 45.0 65.6 78.7 100
0 29.4 53.9 60.6 67.7 82.5 93.0 97.0 100
327.1 304.4 288.3 271.3 264.1 257.6 245.2 236.8 220.0
9.7 19.6 33.5 48.0 66.6 83.1
4i.3 62.7 77.2 87.2 94.3 98.0 100
n
100 A I M . 0 6.9 17.9 34.0 49.4 64.9 81.5
298.0 280.0 264.1 260.8 243.2 232.8 218.8 204.4 194.4
0 37.5 63.2 80.7 90.1 95.3 98.3
I.oo
100
100
_ _ Mole Fraction Dodecane Liquid
50
hIh1.
-
Vapor
0
n
IO. 8 18.2 31.8 49.4 63.9 81.2 YO0
52.8 67.6 81.9 90.8 95.3 98.8 100
20 MM.
0
8.0 16.6 30.8 38.8 47.0 64.3 76.3 100 10 MM.
0 7.6 15.2 26.3 32.0 43.5 63.2 83.2 100
0 48. B .68.1 82.4 87.5 91.0 96.1 98. (I 100
0
49.8 68.1 80.6 85.2 90.6 96.4 99.0 100
accuracy of the methods of determining and evaluating the compositions of vapor-liquid samples resulting from the determinations of equilibria. SUMMARY AND CONCLUSIONS
in Figure 3, and as vapor-liquid composition curves in Figure 4. The points on the figures are experimental points while the curves represent the best curves u-hich could be drawn through the points. ACTIVITY COEFFICIENTS
The activity coefficient criteria applied to the experimental data are inconclusive. At the higher pressure the act,ivit)coefficients calculated for the dodecane were slightly higher than 1.0, showing less than 4% average deviation. At the lower pressures, 50, 20, and 10 mm. of mercury, the activity coefficients for dodecane became less than 1.0 with average deviations approximately -2, -9, and -16%. At all pressures investigated, the activity coefficients for the hexadecene were slightly less than 1.0 (negative deviations). Since it is impossible to have both positive and negative deviations at the same condition for two components in a binary mixture, it must be assumed that some small, unaccounted-for error in determination of composition or vapor pressure resulted in the inconsistent deviations. Examination of the data and the vapor-liquid composition plots indicates the dodecane-hexadecene system to be essentially ideal under the pressures studied. Certainly the deviations from Raoult law ideality are well within the limits of experimental
Vapor-liquid equilibrium data are presented for the system dodecane-hexadecene at pressures of 760, 400, 200, 100, 50, 20, and 10 mm. of mercury absolute pressure. Activity coefficient trend vvith pressure for this system was found to be too erratic and too close to the ideal value of 1.0 to enable definite conclusions to be drawn. Within the limits of experimental accuracy the data reported here indicate the binary system dodecane-hexadecene to follow Raoult law idrality from 760 to 10 mm. of mercury absolute pressure. LITERATURE CITED
(1) Egloff, G., “Physical Constant of Hydrocarbons,” Vol. I, pp. 80,. 284, New York, Reinhold Publishing Corp., 1939. (2) Jones, C. A., Schoenborn, E. M., and Colburn, A. P., IND. EXG. CHEM.,35, 666 (1943). (3) Mair, A. R., Streiff, A. J . , and Rossini, F. D., J . Research ,\‘atZ. Bur. Standards, 35, 355 (1945). (4)Rasmussen, R. R., and Van Winkle, M., IND.ENG.CHEM.,42, 2121 (1950). ( 5 ) Stull, D. R., Ibid., 39, 517 (1947). (6) Universal Oil Products Co., “Laboratory Test, Methods f o r Petroleum and Its Produrts,” H-25, 1947. RECEIVED for review June 25, 1951. ACCEPTEDOctober 8, 1951.. From a thesis presented in partial fulfillment of the requirements for t h e A1.S.Ch.E. degree, University of Texas.
