n-Decane-trans-D ecahydronaphthalene d
BINARY MIXTURE FOR DETERMINING EFFICIENCIES OF FRACTIONATING COLUMNS OPERATING AT REDUCED PRESSURES M. R. FENSKE, H. S. MYERS, AND DOROTHY QUIGGLE
d
School of Chemistry and Physics, The Pennsylvania State College, State College, Pa.
ARIOUS m i x t u r e s h a v e b e e n investigated and proposed for determining the efficiencies of fractionating columns at reduced pressures.
V
Coulter and Lindsay ( 4 ) have found aniline-chlorobenzene mixtures satisfactory at absolute pressures in the range of 380 to 95 mm. of mercury. Bragg and Richards (3)have used
~ ~ to about 100 mm. of mercury
*
Vapor-liquid equilibrium data, obtained in a unit having the equivalent of two theoretical plates, are presented for the binary system of n-decane-trans-Decalin (decahydronaphthalene) over the range of 50 mm. of mercury absolute pressure to atmospheric pressure. The relative volatility, alpha, of this system is constant over the composition range at a given pressure. The relative volatility increases from 1.19 at 50 mm. of mercury to 1.30 at atmospheric pressure and makes possible the testing, at total reflux, of fractionating sectionshaving the equivalent of about 34 and 22 theoretical plates at the two pressures, respectively. The use of binary mixtures of trans-Decalin and cis-Decalin and of dicyclohexyl (cyclohexylcyclohexane) and cyclohexylbenzenefor determining the efficiencies of fractionating columns is discussed.
and bottom of the fraction-
::tii
~~~~~
yryzl:eZnt
number of plates calculated, depending on whether the relative volatility existing a t the top or the bottom of the section or some average value is used in making the calculations. This factor is especi: lly important when operating at reduced pressures, where the pressure drop through the fractionating sectiqn may be .appreciable in comparison with the pressure at the top of the section. For vacuum work where pressure drop is a factor, it may be better to choose a system having a relative volatility of reasonable magnitude and, if necessary, take samples a t several points t o determine the number of plates in individual sections, rather than to use a system with a very low relative volatility and test the entire fractionating section at one time,
~
~
absolute and o-dichlorobqnzene-diethylbenzene mixtures over the range of 50 to 10 mm. Feldman and co-workers ( 6 ) have investigated the systems n-dodecane-cyclohexylcyclopentane and n-tridecane-dicyclohexyl over the ressure range of about 400 to 20 mm. of mercury absolute. $he latter system shows azeotropic properties. In the pressure range around 1mm. absolute, mixtures of esters have been suggested-for example, vapor-liquid equilibrium data have been obtained by Perry and Fuguitt (18) at 0.1 mm. on binary systems of di-%ethyl hexyl phthalate n-ith hexyl sebacate, and di-n-octyl phthalate with di-2-ethyl hexyl sebacate. Another ester system consisting of di-n-butyl phthalate and di-n-butyl azelate has been studied by Williams (W4)at 1mm. of mercury. Bishop ( a ) has suggested the use of m- and p-tricresyl phosphates. Berg and Popovac ( 1 )have used mixtures of n-octane and toluene for testing columns operating at pressures over the range of 760 to 20 mm. Othmer and cOworkers (9,16, 1 7 ) have presented the vapor-liquid equilibria at subatmospheric pressures for a number of binary mixtures of oxygenated compounds.
,
$
It has been found (14) t h a t the binary mixture of n-decanetrans-Decalin (decahydronaphthalene) meets most of the above requirements' Data On this system are presented in this paper* APPARATUS AND PROCEDURE
Though a great many types of apparatus have been suggested for the determination of vapor-liquid equilibrium data, they all attempt t o obtain a vapor sample in equilibrium urith a liquid sample one theoretical removed. very often, however, systems are encountered which cannot be analyzed accurately enough t o obtain reliable values of relative volatility when the difference in the composition of the liquid and vapor samples is that accomplished by only the single plate distillation. A portion of the error in vapor-liquid equilibrium data is probably due t o such analytical inaccuracies. No one, apparently, has Such an apparatus? when the made use Of IL multiplate relative volatility, alpha, is constant or substantially constant over the range of composition covered by the vapor and liquid samples in any one determination, would appreciably minimize the error in alpha, for the error of analysis would now be spread Over more than one Plate. For all Practical PUrPOSeS, i t may be assumed that the analytical error is of substantially the same order of magnitude Over the range of composition covered by the few plates which might be incorporated in a multip]ate still. When the relative Volatility is constant, or nearly SO, its Value may be calculated using the expression:
The preferred characteristics of a binary mixture, or its oomponents, for determining the efficiencies of columns are as follows: The components should be obtainable in a high state of purity. The components &ould be relatively inexpensive if columns of large capacity are t o be twted. The components be of the same type as those regularly used in the columns. The mixture should be stable and noncorrosive in the columns. The mixture should allow condensation without resorting to refrigerative cooling. The mixture should be capable of easy and accurateanalysis. The mixture should be as nearly ideal as possible, the components being completely miscible a t all the temperatures used, no azeotropes being formed, and the relative volatility being substantially constant, for a given pressure, Over the entire concentration range. The vapor-]iquid equilibrium diagram should be available a t the pressure used in testing, if a perfect solution is not formed and if the relative volatilities existing over the concentration range t o be worked in are not known. The boiling points of the two liquids should be close enough so that, under the conditions of test, neither component is produced in a high state of purity. When this is the case, i t is usually easier to obtain a more accurate analysis of the still and condenser samples. At the same time, the boiling points of the two liquids should not be so close that differences in pressure between the top
Or
CY
649
=
ti(%)(E) d K =
(6,7)
INDUSTRIAL AND ENGINEERING CHEMISTRY
650 where
= relative volatility n = number of theoretical plates X A , X B = mole fractions of the more volatile and less volatile comDonents. resnectivelv. - , in the liquid phase Y A ,Y B = mole fractions of the more volatile and less volatile components, respectively, in the vapor phase E.F. = enrichment factor LY
I
_
I n this equation, the relative volatility is the nth root of the enrichment factor and the analytical errors are decreased proportionately.
For procuring the equilibrium data presented in this paper, a unit having the equivalent of two theoretical plates and patterned after a n Othmer still ( I o ' ) was used. However, a vapor-liquid equilibrium apparatus having the equivalent of six theoretical plates and involving an entirely different type of design-namely, five cocurrent contacting sections and a still all arranged in the form of a column-was also employed successfully ( I O ) .
Vol. 42, No. 4
if they check the first samples procured. If thev do, the composition in the system is then changed for additional equilibrium determinations. For operation at reduced pressures the apparatus is connected to the vacuum system described below. The system methylcyclohexane-toluene has been chosen to test this two-theoretical-plate unit, as vapor-liquid equilibrium data at atmospheric pressure for the mixture are available (19). Shorn on Figure 2 is the single theoretical plate equilibrium curve for this binary system, and above this is the curve drawn from the data obtained in the two-theoretical-plate unit. Proof that this latter unit has a separating ability equivalent to two theoretical plates is demonstrated by stepping off plates on the one-theoretical-plate equilibrium curve by the conventional McCabeThiele method ( I S ) . This shows that it takes the equivalent of two theoretical plates to accomplish the separation obtained in the two-still unit, Prior to determining equilibrium data on binary mixtures for testing fractionating columns a t reduced pressures, it is the usual practice in this laboratory to study the vapor pressures of the two pure compounds and then attempt the separation of a miuture of the two by efficient fractionation a t subatmospheric pressure to investigate the possibility of the existence of azeotropes. If these data indicate that the use of the mixture might be feasible, the vapor-liquid equilibrium data are then obtained. I n the determination of vapor pressure data, a modified Cottrell boiling point apparatus (bo), minus side arm for distilling off product, is used in a vacuum system with a ballast tank and a mercury-filled U-tube manostat with electrical contacts for operating a relay cirruit having a type 43 vacuum tube-described by Hersh and co-workers (Il)-which in turn operates a capillary leak. This type of controller is simple and reliable. ,4Type FA135 Wallace and Tiernan absolute manometer permits the reading of pressures with an accuracy of +0.1 mm. of mercurv. Temperatures are measured with a Bureau of Standards calibrated copper-constantan thermocouple used in conjunction with a Leeds &? Northrup portable precision potentiometer allowing readings to bemade to nithin *0.5" C.
3-WAY STOPCOCK FOR CONDENSED VAPOR SAMPLE
RESISTANCE WINDINGS ON STILLS ARE N O T SHOWN
-
0
5 IO SCALE IN CM.
Figure 1. Two-Theoretical-PlateEquilibrium Unit Made of borosilicate glass
The two-theoretical-plate apparatus is shown in Figure 1. In operation the vapor rising from the boiling liquid in the first (lower) still is condensed and flows into the second still. Vapor from this second still is condensed and returns to the boilingliquid in this same still. A constant level of liquid is maintained in this still by means of the return line to the first still. Provision is made for sampling the liquid in the first still and the condensed vapor from the second. Khen equilibrium conditions have been attained, this vapor is then two theoretical plates removed from the liquid in the first still. The apparatus is constructed entirely of heat-resistant glass, The heat required to boil the liquid is supplied by 250-watt Chromalox ring heaters mounted a t the base of each still. To prevent condensation of the vapors before they reach the condensers, the walls of each still are wound with Kichrome resistance wire. Currents t o these windings and the Chromalox heaters are controlled by individual Variac transformers.
n-DECANE-trans-DECALIN
n-Decane and trans-Decalin were studied because their solution properties were likely to be normal; they were stable and fairly easily procurable, had about the desired boiling points for vacuum work, and could be analyzed accurately by refractive index-the difference in refractive indexes of the two pure materials was 573 points, using a refractometer reading to four decimal places. The n-decane used was prepared by the vapor-liquid extraction (in a column having the equivalent of 70 theoretical vapor-liquid extraction stages, 8) and subsequent fractional distillation (in a column having the equivalent of 100 theoretical plates and a t a reflux ratio of 40 to 1) of a n-decane concentrate obtained from Michigan straight-run naphtha (12). This material was used because it had been obtained in the course of some other cxperimental work. However, a good grade of n-decane may also be procured from the Connecticut Hard Rubber Company, New Haven, Conn. The trans-De-alin was obtained by fractionating (in a column having the equivalent of 75 theoretical plates and a t a reflux ratio of 40 to 1) Decalin obtained from E. I. du Pont de iVemours & Company This material contained both the cis and trans isomers in approximately equal proportions. Heart cuts of trans-Deedin were used.
PROPERTIES OF PURE COWONEXTS n-Decane Literature Observed (23)
A total of about 125 ml. of binary mixture has been used in procuring each set of equilibrium data. Three hours are generally sufficient for equilibrium conditions to be reached. When the samples are analyzed by means of refractive index measurements, where only small amounts of material are necessary, i t is possible t o take the samples while the unit is in operation without upsetting the equilibrium of the system to any discernible degree, Then samples may be taken again in another hour or two to see
Boiling point at 760 mm. Hg ",C. Refra'cttve index at 20' C.,
ng
174.1 1.4120
174.1 1.4119
_ _ _ _trans-Dccalin _ __ Observed 187.3 1,4692
Literatiire (23)
187.3 1,4692
Both materials are believed to have had purities of a t least
99%.
INDUSTRIAL AND ENGINEERING CHEMISTRY
April 1950 100
a given pressure, i t was unnecessary to cover the composition range a t these other pressures. Struck and Kinney (.?A%', $3) have effectively used this n-decanetrans-Decalin system in testing the efficiencies of packed fract,ionating columns a t reduced pressures. Because they wished
90
E
80
> 4
z
651
70
z
3 2 60
TABLE11. EQUILIBRIUM DATA AND RELATIVEVOLATILITY
V J
5 50
VALUES
*
FOR
BINARYSYSTEM n-DECANE-trans-DECALIN
I N TWO-
THEORETICAL-PLATE EQUILIBRIUM UKIT
I-
Mole % n-Decane Liquid Vapor c 2 theoretical plates +
240
z
{ 30
Relative Volatility, Alphaa
Atmospheric Pressure (725to 740 Mm. Hg)
n w
8.0 9.5 15.5 17.2 19.7 27.2 32.6 37.7 41.6 44.8 49.7 54.7 64.8 73.7
2 20 10
10
20 30 40 50 60 70 80 MOLE PERCENT METHYLCYCLOHEXANE IN LIQUID
90
100
Figure 2. Equilibrium Data for Binary System Methylcyclohexane-Toluene with One-Theoretical; Plate and Two-Theoretical-Plate Units
13.1 15.2 24.0 26.3 28.9 38.2 44.6 50.3 54.5 57.5 62.8 66.9 76.1 82.3 Av.
The n-decane-trans-Decalin mixtures were analyzed by means of refractive index measurements. Refractive index-composition data on known mixtures of these two compounds are listed in Table I. Vapor pressure data for trans-Decalin are presented in Table IV. The Bureau of Standards (15) has obtained the vapor pressure data for n-decane. Fractionation of an approximately 45 mole % ' mixture of n-decane in trans-Decalin a t 100 mm. of mercury absolute pressure, using a column having the equivalent of 35 theoretical plates and a reflux ratio of 40 to 1, gave no evidence of the exisb ence of an azeotropic mixture over the range of 0.5 to 99.5 mole yon-decane. Because both the vapor pressure and fractionation data showed no abnormalities, the vapor-liquid equilibrium data for the 12decane-trans-Decalin system were determined. These data were procured in the two-theoretical-plate unit over almost the entire composition range a t absolute pressures of 760, 200, and 50 mm. of mercury. The equilibrium data at these three pressures are shown in Table I1 and plotted in Figure 3. These data show that within the limits of experimental error the relative volatility, alpha, for the n-decane-trans-Decalin system is independent of composition a t any given pressure. A series of runs was also made using approximately 50 mole yo mixtures in each case, a t absolute pressures of 720, 400, 300, 250, 180, 150,and 100 mm. of mercury (Table 11). Because the data a t 760, 200, and 50 mm. showed a constant relative volatility for
41.3 41.1 41.6 41.8 41.6 41.6 41.6 41.6 8.2 12.0 15.5 21.8 21.9 29.6 33.9 38.6 45.1 53.0 53.6 61.1 64.1 69.6 73.6 83.7 86.8 88.1
42.5 42.5
TABLE I. REFRACTIVE INDEX-COMPOSITION DATAFOR SYSTEM n-DECANE-trUnS-DECALIN R.I.
0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00 50.00
1.4692 1.4658 1.4624 1.4590 1.4558 1.4527 1.4496 1.4466 1.4436 1.4407 1.4379
n$'O
Mole % n-Decane
R.I.
55.00 60.00 65.00 70.00 75.00 80.00 85.00 90.00 95.00 100.00
1.4351 1.4323 1.4296 1.4269 1.4243 1.4217 1.4192 1.4167 1,4144 1.4120
42.4 43.0
%'bo
Data obtained from smoothed ourve of original data plotted on 12 X 20 inch scale.
Pressure
1.287 1.288 Pressure
1.280 1.277 Pressure
1.273 1.273 Pressure
1.268 1.268 Pressure
Av.
42.5 42.4
Mole Yo n-Decane
720 Mm. H g Absolute 53.8 53.6 400 Mm. H g Absolute 53.9 53.9 300 Mm. H g Absolute 53.6 53.6 250 Mm. H g Absolute 53.4 53.4 200 Mm. H g Absolute 12.3 17.7 22.4 30.1 30.6 39.4 44.0 49.5 56.8 63.7 64.5 71.2 73.7 77.9 81.1 88.7 91.1 91.9
.
0
34.4 36.5 42.4 52.7 62.4 71.9 79.5
-
1.253 1.255 1.253 1.244 1.257 1.244 1.238 1,248 1.264 1.247 1.253 1.254 1.253 1.241 1,241 1.238 1.248 1.238 1.248
180 Mm. H g Absolute Pressure 53.8 1.254 53.8 1.254 150 Mm. H g Absolute Pressure 53.2 1.239 53.0 1,236 100 M m . H g Absolute Pressure 52.3 1.220 52.8 1.218 50 Mm. H g Absolute Pressure 42.2 1.181 44.9 1.190 50.5 1.178 60.7 1.178 70.1 1.188 78.3 1.187 84.6 1.190 Av. E 1.185
Calculated from equation, alpha =
theoretical plates
1.316 1.306 1.310 1.310 1.288 1.286 1.289 1.292 1.296 1.292 1.307 1.293 1.314 1.290 1.299
2 (6,7).
d(E)(2)
where n, number of
INDUSTRIAL AND ENGINEERING CHEMISTRY
652
Vol. 42, No. 4
Table I11 lists the approximate number of theoretical plates that may be determined with this mixture at total reflux and a t different absolute pressures, assuming enrichment to be such that the product contains 95 mole yo n-decane and the residue 5 mole % ' n-decane. These values range from 22 theoretical plates a t atmospheric pressure to 56 theoretical plates at 10 mm. of mercury absolute pressure. If, however, it is desired to test a column having a greater number of theoretical plates, provision can be made for sampling the unit at several points along its length. Each section of the column is then tested separately in the usual manner, the compositions being so adjuqted that the concentration gradient over the section being tested is reasonable-that is, the top and bottom of the section under test do not contain either of the components in the pure form. Such a procedure is relatively simple and extends the usefulness of a given test mixture. trans-DEChLI?i-ciJ-I)E
