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INDUSTRIAL AND ENGINEERING CHEMISTRY
The correlation method previously developed to give straightline plots (b) is shown in Figure 4, which is a plot on logarithmic paper of
Vol. 41, No. 5
in Figure 5 and Table IV. The abnormal hump of the caproir acid system is reflected in the density curve. ACKNOW LEDGMEYT
where a is the fraction of solvent in the solvent phase, and b is the fraction of water in the water phase. The plots for two of the systems are straight lines, while that for the system containing caproic acid is a curve, as might be expected from the previously mentioned abnormality. If the suggested association actually occurs, the molecular weight of the associated component will not be the same in each phase, so the Kernst equation on which the plot is ultimately based would not hold. These solublity data have been used (1) for analyses of mixtures of acetic acid and for each of the three higher boiling acids in determinations of vapor-liquid equilibria. Specific gravities a t 20 "/20 of the respective phases in equilibrium with each other were determined, and the data are shown
Appreciation is expressed to the Bank of Mexico, S A , Mexico,
D.F., which sponsored the fellowship under which this work was done, and to Edward H. Ten Eyck who helped in preparation of the data and figures. LITERATURE CITED
(1) Othmer, D. F., and Serrano, J., unpublished work. (2) Othmer, D. F., and Tobias, P e pIND.ENG.CHEix., 34, 690, 893 (1942). (3) Othmer, D. F., White, R. E., and Trueger, E., Ibid.,33, 1240 (1941).
RECEIVED December 18, 1947. Previous articles of this series have appeared in IND. EXG.CHEV.,1941 and 1942.
of Vacuum ectification Columns USE OF BINARY TEST MIXTURES JULIAN FELDRTAN, MARY MYLES, IRVIKG WEISDER, AND 3IILTON ORCHIN U . S. B u r e a u of Mines, P i t t s b u r g h , P a . 'rwo binary tcst mixtures (a) n-dodecane and cyclohexylcyclopentane and ( b ) n-tridccane and dicyclohexyl were investigated for use in determining the efficiency of fractionating columns at subatmospheric pressures. Vapor-liquid equilibria data were presented for various compositions at pressures ranging from 20 to 330 m m . of mercury. Data were obtained b y using two different types of equilibrium stills. Analyses were made rcfractometrically. Both mixtures can be used for column evaluation. As the relative volatilitj increases as the pressure decreases, columns having more than 20-plate efficiencies at 20 m m . cannot be tested with these mixtures. Mixture ( b ) suffers certain limitations; mixture (a) deviates but slightly from ideal behavior.
HE requirements for a binary teat mixture for use in evaluating efficient distillation columns operating a t pressures from 20 t o 400 mm. of mercury are the following: The mixture should approximate ideality in the working ranges; it should be easy to analyze; the relative volatility (a-value) should be betiwen the limits of 1.05 to 1.3; the components of the mixture should be readily available and easily purified; and the boiling point should be such that complete condensation occurs a t the IoTest pressures and pyrolytic decomposition is absent a t the highest pressures. Aliphatic hydrocarbons and naphthenes form mixtures which generally do not deviate appreciably from ideality. The refractive indexes, moreover, are sufficiently different t o permit analysis by simple refractometric techniques. The normal paraffins in the boiling range desired (200' to 300" C.) have indexes of 1.42 to 1.43, while the dicyclic naphthenes in this range have indexes of about 1.47 to 1.48.
Two binary test mixtures were investigated; ( a ) n-dodecanr and cyclohexylcyclopentane and ( b ) n-tridecane and dicyclohexyl The n-dodecane can be purchased in a fairly pure state. Thp dicyclohexyl is readily obtained through the catalytic hydrogenation of biphenyl. The n-tridecane can be obtained in 75 to 80% over-all yield by Grignard synthesis from n-heptaldehyde and nhexyl chloride, followed by vapor-phase dehydration of the carbinol over alumina a t 300' C. and catalytic reduction of the olefin. The cyclohexylcyclopentane can be obtained in 3601, over-ali yield by condensation of cyclohexanone M ith cyclopentadiene (4) and catalytic reduction of the fulvene, according to the follorvina modification: The neutral chloroform solution of the fulvene obtained from the condensation is hydrogenated a t 100 pounds per square inch pressure, using 0.5 gram of Adams catalyst per mole of fulvene, or the chloroform may be removed a t lov temperature before hydrogenation. The hydrocarbon product is distilled from a Claisen flask a t 10-mm. pressure and then washed with cold concentrated sulfuric acid, followed by a water wash until neutral. The components were freed from aromatic and olefinic materials by extraction with sulfuric acid. This step was found especially necessary for removing appreciable amounts of cyclohexylbenzene from the dicyclohexyl. These last two compounds boiled too close together to permit separation by distillation Extraction with cold 100% sulfuric acid was effective in removing cyclohexylbenzene and unconverted biphenyl. Final purifications were made in columns with efficiencies of about 50 theoretical plates. The properties of the purified components are listed in Table I together with the values from the literature ( 1 ) . Composition versus refractive index curves were made up for both mixtures (Figure 1). These can be used t o determine mole per cent composition through use of an ordinary Abbe refractometer t o an accuracy of 0.1%.
May 1949
1033
INDUSTRIAL AND ENGINEERING CHEMISTRY
1.480
1.470
1.460
1.450 0 N O
1.44C
0
1.43C
100
300
400
500
600
700
800
PRESSURE, mm. Hg
Figure 2. 1.42C
200
0.2 0.4 0.6 0.8 mol F R A C T I O N , A L I P H A T I C H Y D R O C A R B O N
1.0
Figure 1. Refractive Index Composition Curves for Mixtures (a)and ( b )
The vapor pressure curves shown in Figure 2 were obtained by the dynamic method ( 2 ) . For mixture ( a ) the difference between the boiling points of the components increases as the pressure is lowered; this should increase the relative volatility. The observed boiling points of the components of mixture ( b ) approach each other and then intersect a t about 200 mm., so that the less volatile dicyclohexyl at atmospheric pressure becomes the more ,volatile component at low pressures. The relative volatility should therefore go through the value 1.00, forming azeotropes a t 1.00. Vapor-liquid equilibria data were obtained by using two equilibrium stills. One is a modified Othmer still (5) of the Kjeldahl flask variety. The modifications include the use of a bare wire coil passing into the flask through side arms fitted with ball joints. The metal terminals are sealed through the glass stoppers. Connection to the vacuum system is made on the return line from receiver to still pot. A two-way stopcock per,. Tlwrmocouple
Vapor Pressure Curves for Pure Components
mits the introduction of nitrogen before withdrawal of samples. The column of the flask is heated with a coil of insulated wire to reduce refluxing at the higher temperatures. The other equilibrium still is a modification of that described by Gillespie (3). The apparatus used appears in Figure 3. The neck of the Cottrell pump is extended, so t h a t a total immersion thermometer of the Anschutz type could be used. It is necessary to heat this neck, when high boiling liquid mixtures are used, by means of an insulated Chrome1 coil wound more tightly at the bottom than a t the top. I n addition, the portion of the apparatus containing the thermometer and entrainment chamber, in the region where the vapor is separated from the liquid at equilibrium conditions, is insulated from drafts b y a rectangular box fitted with glass windows, front and rear. A Nichrome coil heater is used to make up heat loss. Heat inputs are regulated so that the thermocouple shows a box temperature which does not rise above the thermometer reading or fall more than 10" C. below. The still pot, whose volume is 180 ml., is insulated with a n asbestos covering. Heat to the pot is supplied not only by the internal Nichrome wire heater shown but also by a coil of resistance wire wrapped around the asbestos. All resistance heaters are independently controlled by variable voltage transformers. The adjustments are easily set, so that the distillation rate is uniform and no observable superheating occurs. Temperatures are read in tenths of a degree ( " C.) on the suspended thermometer. Pressures are maintained a t *O.l mm. of mercury b means of a standard Podbielniak manostat which utilizes a soyenoid valve on a vacuum line with suitable ballast t o give minimum pressure fluctuations. At the end of a half-hour run, t h e heaters are shut off and nitrogen introduced into the apparatus by means of the two-way stopcock shown in Figure 3. The receiver has a volume of 8 ml.
NZ
TABLE I. PROPERTIES OF HYDROCARBON COMPONENTS
To
O
d
Centimeter8
Figure 3. Modification of Equilibrium Still Described by Gillespie (3)
Substance n-Dodecane Observed Literature value n-Tridecane Observed Literature value Dicyclohexyl Observed Literature value Cyclohexyl, Cyclopentane Observed Literature value a At 25O C.
Formula CnHee
Molecular Weight 170
CisHa8
184
CIZHZ?
166
Boiling Point, ' C. a t 760 Mm.
Refractive Index n%
Density dzo
215.9 216.26
1.4216 1.42156
0.7487 0.74876
234.8 236.5
1.4258 1.4250"
0.7560 0.7567
1 ,4792
1.4795
0.8870 0.8848
1,47225 1.4728
0.87659 0.8758
238-9 CiiHzo
152 215.1 215.5
INDUSTRIAL AND ENGINEERING CHEMISTRY
1034
Vol. 41, No. 5
Vapor-liquid composition and temperature data were obtained for both liquid mixtures over a composition range and pressure range a t selected intervals of composition and pressure. Average relative volatility for all compositions versus pressures is shown graphically in Figure 4. These curves confirm the predictions made above from the vapor pressure curves shown in Figure 2.
1.3
8 1.2 >k =! b
5 1.1 9 BLI
s
4 w
THE n - D O D E C A N E - C Y C L O H E X Y L C Y C L O PENTANE MIXTURE
1.c
a
The equilibrium data for this mixture were studied over the composition range 3 t o 98 50 100 150 200 250 300 350 400 mole yo of cyclohexylcyclopentane a t pressures PRESSURE, m m of Hg of 20, 30, 40, 50, 60, 80, 100, 125, 150, 200, and Figure 4. Average Relative Volatility vs. Pressure 250 mm. of mercury. The temperature data are not accurate enough t o be plotted as smooth curves. As evaluation of the precision of the experimental techniques shows an accuracy of not more than =t0.3O C., there ar? no observable minima in composition boiling point diagrams. The x-y curves for the mixture in Figure 5 indicate that there are no appreciable deviations from ideal behavior and no minimum boiling mixtures a t any composition. The smoothed results of relative volatility as a function of composition a t various pressures (Figure 6 ) demonstrate that the behavior of this binary mixture is not strictly ideal. An ideal mixture would show no variation in relative volatility %-ithcomposition. This deviation from ideality does not invalidate the usefulness of this mixture, because the variation of relative volatility with composition is not great and is constant. The logarithm of the relative volatility shows a linear relationship Tith the temperature for different compositions (Figure 7). The general behavior of this group of curves may be expressed by the following equation: log cy = -0.001027t c where c is a constant depending on the composition. The average value of c is +0.2075. The value of t for any particular pressure for this mixture is given by the expression:
,9
+
CYCLOHEXYL
Figure 5.
CYCLOPENTnhi
IN
LIQUID,
r-11
' [-0.0003796
PERCENl
30
I25
_I
F I 20 4 J
a
-
y115 bW J
I10
I05
'
"mol F ROA3C T I O N ODODECANE 4 O 5I N
Figure 6.
L I Q UQ6 ID
O7
1-
+ 0.003168
273
Much attention was paid to the equilibrium values a t high concentrations of cyclohexylcyclopentane, as this is the direction of change in the composition of a mixture during distillation. Although it was not possible to get good equilibrium data a t the very high and low concentrations of a component, because of limitations in analytical accuracy and the exaggerated effect on relative volatility of small concentration changes, the results do not indicate any unexpected deviations. Because of the limitations in accuracy 111herent in the equipment and techniques, the values for the relative volatilities over most of the concentration range are no more accurate than 10.01. The relative volatility data presented are not reliable at concentrations of cyclohexylcyclopentane over 95% or under 5%. The following example illustrates the method of applying the data on relative volatility for the evaluation of a distillation column:
Vapor-Liquid Curve for Cyclohexylcyclopentane-n-Dodecane Rlixture
I35
to
1 log p
"
O9
Relative Volatility as Function of Composition
lo
A column under test a t a total pressure of 40 mm. of mercury is operated at a pressure drop of 20 mm. Analysis of the feed shows 60% dodecane, and the overhead shows 90% cyclohexylcyclopentane. The average column pressure is 50 mm., and the average column composition, 65% cyclohexylcyclopentane. For
"
May 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 7.
Log of Relative Volatility as Function of Temperature
these values the relative volatility is 1.21 (Figure 6). Application of the Fenske equation:
n
+ 1 = log --log1 1.21
90 - X 60 40 X 10
gives a value of n = 13theoretical plates. A value of a = 1.19 and 1.23 results in a value of n = 14 and 12, respectively. The determination is accurate, therefore, t o within * 1plate.
A 25-mm. inside diameter, %foot length Podbielniak column was evaluated under total reflux conditions at 150-, loo-, and 50mm. operating pressure. The values obtained are compared in Figure 8 with the behavior of the same column operated at atmospheric pressure when tested with the n-heptane-methylcyclohexane mixture. The results are in the order expected for the particular column tested.
Figure 8.
1035
Column Efficiency of %Foot Podbielnialr Column
THE n-TRIDECANE-DICYCLOHEXY L MIXTURE
The equilibrium values for this mixture were obtained in a n Othmer-type still except for compositions of 28 and 90% tridecane; these were run in the Gillespie still. This mixture was studied over a pressure range of 375 t o 15 mm. of mercury. T h e 2-21 curves shown in Figures 9 and 10 demonstrate the interesting behavior of this mixture. At 150 mm. of mercury pressure the 2-y curve is coincident with the 45' line, which means that both liquids have identical vapor pressures in the mixture over most of the composition range. This differs from the usual azeotrope which has a fixed composition. At this pressure any mixture of
100
I-
85
80
W
a
- 70 60
LT 0
n. 5 0
3 Z
B
40
*30 20
I-
10
0
IO
Figure 9.
30 40 50 60 70 80 90 100 TRIDECANE IN LIQUID, mol PERCENT
20
x - y Curve for rz-Tridecane Dicyclohexyl Mixture
Figure 10. x-y Curve for n-Tridecane-Dicyclohexyl Mixture
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
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Vol. 41, No. 5
Rherever a curve crosses the line foi LY = 1.00. This phenomenon occurs in concentrations of tridecane greater than 75%. In t h r middle range of compositions this mixture shows nearly ideal behavior n i t h little if any change of relative volatility with composition. The logarithm of the relative volatility shows a straight-line relationship for the compositions which have the least deviation from ideal beLavior (Figure 12). At 150 mm. (178”) the relative volatilities are about the same a t all compositions. From the data, it is apparent that there are Imitations to the use of this mixture for test purposes. However, certain generalisations can be made which serve to define the range of application of the n-tridecanedicyclohexyl misture for column testing. At pressures between 150 and 350 mm. of mercury where tridecane is the mole volatile component, distillation changes the compom o l F R A C T I O N O I C Y C L O H E X Y L I N LIQUID sition of the vapors in the diiection of an azeotrope. It is theiefore necessary to limit the Figure 11. Variation of Relative Volatility with Composition of Mixture of m-Tridecane utility of this mixture a t pressures over 150 mm. to columns where the efficiencies do not yield more than 75% tridecane in the distillate. n-tridecane and dicyclohexyl distills unchanged. Below 150 inin., At pressures below 150 mm., dicyclohexyl is the more volatile comdicyclohexyl is the more volatile component. Above this presponent. Distillation a t these lower pressures changes the composure n-tridecane is the more volatile component, except for comsition avr-ay from the region of excessive abnormal behavior. At positions of more than 80% tridecane, where dicyclohexyl is the these lower pressures, the still pot should have tridecane preseiit in more volatile component up t o pressures of about 350 mm. concentration less than 65%; the overhead composition is useful Above 350 mm. (approximately), tridecane is the more volatile down t o a t least 15y0 tridecane. The behavior of the misture component over the entire composition range. beyond this low concentration of tridecane needs further study. The variation of relative volatility with composition (Figure The proper relative volatility value which is no more reliable 11) emphasizes the deviations from ideality. Azeotropes exist than *0.01 is selected in the same y a y from Figure 11 as the values for the dodecane mixture were selected. T o citr t h r hame example as that given for the other mixture, suppose a column operating a t the same pressure of 40 mm. and pressure drop of 20 mm. gave 80% dicyclohexyl a t the top and 4070 in the feed. The relative volatility is 1.10 and the number of theoretical platesisn = 18. The tridecane-dicyclohexyl mixture, despite its limitations, has some advantages in its range of applicability over the dodecane-cyclohexylcyclopentane mixture. These are the negligible change of a with coinposition and the lower values for a. This permits the testing of more efficient columns. SUIWIIARY
TITObinary test mixtures n-ere studied in cquilibrium stills a t various subatmospheric pressures. Sufficient relative volatility data were procured to permit the application of these mixtures for the evaluation of distillation equipment a t pressures as low a,s 20 mm. ACKKOWLEDGMENT
130
c
--
____
.05
The authors gratefully acknowledge the assistance of the operators of the distillation equipment, Peter and Gus Pantazoplos and Elmer Steele.
~
28.1 % T r i d e c o n e
\
\L
\i
\
’
0
LITERATURE CITED
36.%
\I “O-‘
,
1
+.05
\ ‘
I
47.% 58. %
\
\\\ 0.10
LOG
Figure 12. Relationship of Log Relative Volatility a, to Temperature for Several Mixtures of n-Tridecane-Dicyclohexyl
(1) Doss, M. P., “Physical Constants of the Principal Hydrocarbons,” 4th ed., New York, Texas Co., 1943.
Fenske, M. R., “Science of Petroleum,” ed. by Dunstan, A. E., Nash, A. W., Brooks, B. T., and Tizard, Henry, Vol. 11, p. 1634, London, Oxford University Press, 1938. (3) Gillespie, D. T. C.. IND. EXG.CHEM.,ASAL ED.,18, 576-7 (1946). (4)Kohler, E. P., and Kable, John, J. Am. Chem. SOC.,57, 917 (1935). ( 5 ) Othmer, D. F., IND.ENG.CHEM., ANAL.ED.,4, 232 (1932). (2)
RECEIVED April 17, 1948. Presented before t h e DiTision of Petrolounr Chemistry a t the 113th Meeting of the AMERICASCHEMICAL SDCIFTY, Chicago, Ill.
