Separation of Organic Liquid Mixtures bv Thermal Diffusion J
A. LETCHER JONES AND ERNEST C. MILBERGER Chemical and Physical Research Division, The Standard Oil Co. (Ohio), Cleveland 6, Ohio
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RACTICALLY all experimental investigations of thermal other than those of hydrogen and their compounds extremely questionable. It is known, however, t h a t the process has been diffusion reported prior to 1939 dealt with the separation of successfully applied to the separation of light and heavy uranium either inorganic solutes from aqueous solution or gases of different hexafluorides in the liquid phase (Zd), although the difference in molecular weights from each other. The reported separations appear to follow in principle the kinetic theory explanation as volume between these isotopes is very small. proposed by Chapman (Z), which states that a mixture of two Schafer and Corte ($8) proposed t h a t the separation of orthoand para-hydrogen in the gaseous phase by thermal diffusion is a gases when subjected to a temperature gradient will show a transport effect dependent upon molecular mass and interaction result of a difference in entropy of the two forms of hydrogen. between molecules. In the case of gases, Chapman’s theory Para-hydrogen has a higher entropy than ortho-hydrogen. holds reasonably well and isotopes have been successfully Schafer and Corte suggest t h a t in a thermal diffusion column the separated by many different investigators in accordance with the molecules will concentrate in such a manner as to produce maxitheory. Investigators working with inorganic aqueous solutions mum entropy. This means t h a t molecules of higher entropy have also been able to separate isotopes such as zinc and other collect a t the cold wall. Consequently, ortho-hydrogen collects metals on a mass difference basis (16-14,18). a t the top of the Clusius and Dickel type of apparatus. I n 1940 Korsching, IVirtz, and Masch (15)reported the separaKramers and Broeder (16) have considered the phenomenon from a standpoint of cage models as applied t o hydrocarbon liquid tion of water from 95.6% ethyl alcohol and found t h a t the water accumulated at the cold wall and, hence, downward in the Clusius mixtures. They conclude t h a t in systems of this type, molecular and Dickel (3) types of apparatus. The direction of separation mass is less important in determining the separation effect than the energy required for a molecule to escape from its “hole” in the was contrary to t h a t observed with inorganic isotopes in solution and also water-deuterium oxide and CeHe-CeDs mixtures. I n the liquid. Their experiments show t h a t the sequence of separation alcohol-water case the component with the lower molecular of hydrocarbon mixtures from top to bottom in a Clusius and weight concentrates downward. They stated t h a t no satisDickel type of column will be: light normal paraffins, heavy factory explanation could be found for this anomaly, Clusius normal paraffins, branched paraffins, naphthenes and monocyclic and Dickel ( 4 ) suggest, however, t h a t the water in the liquid aromatics, and bicyclic aromatics. state may be polymerized; conTrevoy and Drickamer (65) sequently, the heavier polymeric have also interpreted studies of molecule behaves according t o the the thermal diffusion separations k i n e t i c- t h e o r y explanation of of hydrocarbons in terms of the Chapman. DeGroot (6)proposed cage model theory of liquids. The PACHING h J T a different explanation based upon experimental results they present density changes in the convection are consistent with the assumption streams. t h a t the mobility of a molecule in Niyogi (60)reported that a 1 to a given system is inversely pro1 mixture of n-heptane and n-butyl INNER TUBE portional to the product of the alrohol is separated by thermal molecular mass and molecular diffusion with the latter lowercross section in the direction of molecular-weight component conflow. centrating at the “heavy” end of No kinetic theory for liquid the column. H e shows by dielecthermal diffusion y e t proposen by OUTER TUBE,tric constant and molar polarizaany investigators applies rigortion values t h a t n-butyl alcohol is ously t o either the experimental not in a polymerized or associated results t h a t have been reported by other investigators or those state in the n-heptane mixture. His conclusion is that “it is the included in this paper. density and not the molecular Several papers and patents weight which determines the inhave ameared since 1950 which ICHROME WIRE crease in concentration of a liquid illustrate that liquid thermal difHEATING COIL a t the bottom of the column.” fusion is capable of producing INSULATION Wirtz (68) in 1943 stated t h a t hitherto unexpected separations the separation of isotopes in the of the components of organic liquid phase appears to be more liquids on a practical basis ( 7 - 1 1 ) . a function of the volume of the I n the absence of adequate theory, molecules than of their relative i t has been necessary for investimasses. T h o s e w i t h s m a l l e r gators to determine the range of volume concentrate a t the cold applicability of the phenomenon wall. This explanation would F i g u r e 1. Fractionating Thermal Diffusion by direct experiment. The magmake the separation of isotopes Column nitude of the effect in organic
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INDUSTRIAL AND ENGINEERING CHEMISTRY
liquids has been found t o be large and the apparatus required t o perform effective separations is relatively simple. APPARATUS
The thermal diffusion apparatus used in this study is illustrated diagrammatically in Figure 1. This column is constructed from concentric metal tubes and utilizes the Clusius and Dickel type of thermal circulation. The length of the annular spacing between the tubes is 5 feet. The average distance between the walls (sometimes referred to as slit width or annular spacing thickness) is 0.0115 inch. The mean slit diameter is 0.63 inch. The total volume of the annular space is 22.5 ml. The column has no reservoirs. It is equipped with withdrawal ports located at 6-inch intervals along the length of the column, in order t h a t the contents may be withdrawn in 10 separate fractions after a concentration gradient has been established between the ends of the apparatus. This is accomplished by draining the upper tenth of the apparatus first and successively removing the remaining fractions, working from top to bottom of the apparatus. The outer tube of the concentric pair is heated by electrical resistance (spirally wound Nichrome wire) and controlled by means of a variable transformer, The inner wall is cooled by circulating cold water entering the innermost tube a t the bottom of the column, Water flow is adjusted so t h a t the temperature rise through the column is less than 10' C. The cold-wall temperatures referred to represent the averages of the inlet and outlet water temperatures. The hot-wall temperatures are measured by means of thermocouples attached t o the outer wall at intervals along the length of the column. The annular spacing is maintained essentially uniform by means of small spacers attached to the innermost tube a t intervals along the length of the column. Apparatus of essentially this design is available from The M. Fink Co., 5413 hrchmere Ave., Cleveland 9, Ohio. RE AGENTS
Purity of the liquids used was checked by measurement of index of refraction. Close agreement was obtained between measured values and literature values. No attempts were made to purify them further by distillation or other methods.
Vol. 45, No. 12
The data show t h a t this mixture was easily separated by liquid thermal diffusion and t h a t the degree of purity of the material in the end fractions was greater than t h a t of the starting materials before mixing (2,4-dimethylpentane before mixing, ny = 1.3794; top fraction, n:: = 1.3792. Cyclohexane before mixing, nz8 = 1.4240; bottom fraction, ring = 1.4243). The average composition in the uppermost 30% of the contents of the columnwasequivalent in purity to the 2,4-dimethylpentane used to prepare the mixture; the same fact is observed for cyclohexane in the lowermost 30% of the apparatus. It is significant t h a t this separation by liquid thermal diffusion is independent of the relative boiling points of the components. The component with the higher molecular weight concentrates a t the top of the column instead of the bottom. ISOMERS
The separation of a pair ofliquidsof identicalmolecularweightsthe cis and trans isomers of 1,2-dimethylcyclohexane-isillustrated in Figure 3. The sample was obtained from API Hydrocarbon Research Project 15 at Ohio State University. The structurally more compact cis isomer concentrated at the bottom of the column, the trans a t the top. The measured refractive index values for fractions 1, 2, and 10 are lower and higher than those reported by API-RP45 for the trans and cis, respectively. These results indicate t h a t the sample of isomer either contained a t least two different impurities in addition to the cis- and tians1,2-dimethylcyclohexane or t h a t the end fractions produced by thermal difiusion represent higher purity cis and trans isomers than those used by API-RP45 for the refractive index values reported. I n any case, these isomers are unmistakably separated by liquid thermal diffusion. Another example of isomer separation is illustrated in Figure 4. I n this case, 50-50 volume % mixtures of the three isomeric pairs of xylene were charged to the apparatus a pair a t a time. The ortho-meta and the ortho-para pairs both result in significant separations by thermal diffusion. The meta-para mixture, however, yields essentially no separation. In the two binary mixtures involving o-xylene, the ortho isomer concentrated at
EXPERIMENTAL RESULTS
The large majority of the two-component mixtures studied represent liquid pairs which behave as very nearly ideal solutions. Most of them have little tendency for association, polymerization, hydrogen bonding, or chemical interaction. When refractive index is plotted against different volume concentrations, a n essentially straightrline relationship results. I n cases where t h e difference in index of refraction between components was small, measurements of other properties such as viscosity and infrared absorption were utilized to determine concentrations. I n the three-component systems composition of the fractions was determined by means of a Beckman IR-2 infrared spectrophotometer. BINARY SYSTEMS
A typical example of the composition pattern t h a t is obtained by subjecting a binary mixture to liquid thermal diffusion, in the apparatus described, is illustrated in Figure 2 . T h e liquid charged t o the apparatus was a 50-50 volume % mixture of 2,4dimethylpentane and cyclohexane. The refractive indices of the pure (99.9%) components, the mixture charged, and each of t h e ten fractions obtained are shown graphically. The fractions are numbered progressively from the top to the bottom and represent the entire contents of the apparatus. The normal boiling points of 2,sdimethylpentane and cyclohexane are 80.60' and 80.74' C., respectively. The separation of this pair of liquids by ordinary fractional distillation is not believed possible with present-day distillation equipment because of the small difference between their normal boiling points
1 TOP
Figure 2.
2
3
4
5
8
7
8
9
1
0 BOTTOM
FRACTIONS Fractionation of %,4-Dimethylpentane and Cyclohexane
December 1953
INDUSTRIAL AND ENGINEERING CHEMISTRY 1.493
the bottom of the column in both cases. Since both m- and p xylene diffuse in a very similar manner relative to the ortho isomer, it would be expected that the separation of meta from para might be difficult.
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1.495
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Final Composition, Vel. % % Top Bottom Sepn 95 10 75.0 5 90 58 11.4 42 55.5 42.5 13.1 44.5 57.5
Components ?& Wt. Denaity n-Heptane 50 100 0.6837 Triptane 50 100 0.6900 50 114 0.8919 Iso-octane 50 114 0.7029 n-Octane 50 142 0.9905 2-Methylnaphthalene 1-Methylnaphthalene 50 142 1.0163 trans-1,2-Dimethylcyclo40 112 0.7756 100 0 hexane cis-1,2-Dimethylcyclohex60 112 0.7968 0 100 ane p-Xylene 50 106 0.8609 92 0 50 108 0.8799 8 100 +Xylenea 50 108 0.8639 100 m-Xylene 19 0 81 +Xylene' 50 106 0.8799 50 50 50 106 0.8609 p-Xylene 50 106 0.8639 50 50 m-Xylene 5 o-Xylene contains paraffinic impurity, see Figure 12.
.
1.498
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0 1.499
1.500
Table I lists the separation data for seven different binary solutions of isomers. The components of each pair are listed with respect to the direction of separation, when a separation is observed-for example, n-heptane concentrates a t the top of the column and triptane (2,2,3-trimethylbutane) at the bottom. The top and bottom concentrations listed represent the upper and lowermost tenths, respectively, of the total volume of the apparatus. The percentage separation is a relative value obtained by dividing the index of refraction difference between the upper and lower fractions by the index of refraction difference between pure components: A ~ (between D top and bottom fractions) X 100 = A ~ (between D pure compounds)
1.430
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1,435
IL 0
Figure 3.
I
2
3 . 4 5 6 FRACTIONS
7
2
3
Figure 4.
4 5 6 FRACTl O N S
7
8
9
Binary Xylene Pairs
the same temperature conditions (50" C. hot wall, 20" C. cold wall). The separations do not represent the maximum obtainable in the apparatus because steady-state conditions were not reached in any of the experiments. A 48-hour run is not the same fraction of a steady-state run for all of the mixtures involved. Therefore the relative separations obtained are qualitative in all cases. EFFECT O F TIME
V
a
1
% separation
These mixtures were each processed for 48 hours under essentially
2
7
1.494
TABLE I. GEOMETRICAL ISOMERS Vol. Mol.
2691
8
9
c i s - tmns-1,2-Dimethylcyclohexanc
IO
Several studies were made to show how the concentration changes along the entire length of the column with time. Figure 6 shows the change in refractive index in the upper half of the column using a 50-50 volume % ' mixture of cetane (n-hexadecane) and methylnaphthalene (1,2isomer mixture) as a charge stock. Curve A represents the composition of 2 or 3 drops of liquid from the extreme end of the column. Curves 1, 2, 3, etc., represent the first, second, third, etc., tenths of the total volume of the apparatus starting from the top. To obtain this series of curves, four separate experiments were run in the column with the entire contents being withdrawn in fractions a t the end of 27, 43, 88, and 168 hours. The study shows that a steady state had not been reached at the end of 168 hours. The entire upper tenth of the apparatus (curve 1) contained very nearly pure cetane, however, at the end of this time. Other systems studied show essentially the same time-composition pattern. The slope of the curves under fixed temperature conditions is a function of the viscosity of the medium being processed. The time required to reach a steady state is longer for more viscous media. This time may be substantially shortened by operating a t higher temperature levels. EFFECT O F RELATIVE CONCENTRATIONS
Debye and Bueche (6) and DeGroot ( 6 ) have suggested that studies of the effects of relative concentrations of components might shed further light on the mechanism of liquid thermal diffusion. Figure 6 shows the results of such a study using the binary system of cetane and cumene (isopropylbenzene). Volume per cent ratios of feed concentrations of 5 to 95,25 to 75,
INDUSTRIAL AND ENGINEERING CHEMISTRY
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50 to 50, 75 to 25, and 95 to 5 were charged to the column separately and each concentration was processed for 48 hours. The figure shows the resulting concentrations in fractions 1 and 10 (upper and lowermost) at the end of this time. Several other systems have been studied in this manner and the results shown for the cetane-cumene system may be considered normal behavior. These experiments show that in binary mixtures of hydrocarbons the direction of separation, if it occuis, is not a function of the relative concentrations of the two components. The magnitude of separation is dependent upon the relative concentrations, jvith the maximum being obtained from 50-50 volume % mixtures. It appears significant that this maximum occurs vith respect to relative volumes rather than the relative number of moles of the components. This same effect has heen observed for all vstems studied.
40
80
120
Vol. 45, No. 12
was observed anytvhere along the length of the column. The same fact was observed for t,he 90% octadecane mixture. This shows that the failure to observe separat'ions a t 50 mole % and higher concentrations of octadecane cannot be at'tributed to an equimolar compound of octadecane and benzene. Measurements of density, refractive index, and molar polarization, over the entire concentration range, fail to show any abnormalities for mixtures of these two components. The reason for the failure to get, separation when there is an excess molar concentration of octadecane is not understood. Korsching, \T-irt,z, and 3Iasch (1.5) and Tan Nes ( 2 6 ) have reported failure to separate mixtures of benzene and cyclohexane. In the light of the results obtained wit,h benzene and octadecane it was believed that perhaps benzene and cyclohexane might separate a t some relative concentration of the two. Figure 9 shows the results of this study. X o separation was observed a t any relative Concentration. The reason for the failure of benzene and cyclohexane to separate may be different from t,hat for the octadecane-benzene mixture. Several investigators have reported abnormal proper'ties for mixtures of cyclohexane and benzene. A volume increase occurs upon mixing (29). The freezing point lowering of benzene by cyclohexane does riot follow ideal behavior (19). A straight-line relationship is not obtained between index of refraction and volume per cent composition ( 2 1 ) . The pair forms an azeotrope ( 1 7 ) and x-ray studies of liquid mixtures indicate highly organized groups containing both types of molecules ( 1 ).
I60
TIME, HOURS
Figure 5. A.
Effect of Time on Separation
First few drops from top of columns 1, 2, 3, etc. lst, 2nd, 3rd tenths of total volume of apparatus from top down i
Trevoy and Drickamer (26) have reported that a 50-50 mole % mixture of benzene and octadecane yields essentially no separation. This result was surprising because good separation had been obtained from a 50-50 volume % mixture of benzene and hexadecane in this laboratory. The apparatus used by Trevoy and Drickamer was different in design from that described in this paper. A run was made on a 50-50 mole % mixture of benzene and octadecane to determine if the apparatus difference was responsible for the results they obtained. The results of Trevoy and Drickamer were completely confirmed. Xhen the experiment was repeated, however, using a 50-50 volume % mixture rather than the 50-50 mole % mixture, completely different results were obtained. A large separation of benzene from octadecane was obtained with benzene concentrating a t the bottom of the column. The results of a study of the effect of relative concentrations on this system are shown in Figure 7. This shows t h a t whenever there is an excess of moles of benzene, normal separation occurs. Maximum separa' mixtures. Separation tion is obtained from 50-50 volume % drops to zero at equimolar concentrations and no separation is observed when there is an excess molar concentration of octadecane. Figure 8 shows the composition distribution of the entire contents of the column for six different concentrations of octa' octadecane decane and benzene In the 10,25,and 50 volume % mixtures, essentially pure benzene concentrated a t the bottom of the column. Concentrations approaching a 50-50 mole mixture collected a t the top of the column. The 78 volume % mixture represents a 50-50 mole mixture. No concentration difference
0.41
,/ " /
I
CHARGE COMPOSITION (VOL. FRACTION CETANE)
Figure 6. Effect of Relative Concentrations
In the light of these abnormalities for benzene and cyclohexane mixtures, i t appears reasonable that the individual molecules of each component are not free to diffuse independently in this system. Table I1 shows the relative tendencies of benzene, toluene, In-xylene, and mesitylene (1,3,5-trimethylbenzene) to separate from cetane. There is no obvious correlation between the degrees of separation and the number of methyl groups substituted in the benzene ring. Benzene and mesitylene are definitely eafiier to separate from cetane than toluene and m-xylene. The fact that both benzene and mesitylene are symmetrical molecules may in some way be responsible for their greater separation tendencies. The small molar volume of benzene might explain why it separates more easily from cetane than does mesitylene. The separation tendencies of several pairs of paraffins, cycloparaffins, and aromatics are shown in Table 111. Benzene and methylcyclohexane both separate from n-heptane with approximately equal ease, the cyclic compounds being the cold wall or bottom products in both cases. Benzene and methylcyclo-
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1953
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TABLE11. SEPARATIONOF BINARY LIQUID MIXTURESBY THERMAL DIFFUSION Vol. Components Cetane Benzene Cetane Toluene Cetane m-Xylene Cetane Mesitylene
Mol. Wt. 226 78 226 92 226 106 226 120
% 50 50 50 50 50 50 50 50
Density 0 7734 0 8789 0 7734 0 8652 0 7734 0.8639 0 7734 0 8653
Final Composition, Vol. % Top Bottom 72 4 37 5 27 6 62 5 60 0 44 5 40 0 54 5 60 0 42 0 40.0 58 0 61.0 35 8 39 0 64 2
111.
PARAFFINS,
Vol. %
Components n-Heptane Beneene n-Heptane lMethyloyclohexane Methyoyclohexane Benzene Methylcyclohexane Toluene Cyclohexane Benzene Cetane Cyclohexane iso-octane Methylcyclohexane
35 14.5 17.5 24 9
CYCLOPARAFFINS, AND AROMATICS
Mol.
Wt. 100.2 78.0 100.2 98 98 78 98 92 94 78 226 84 114 98
50 50 50
50 50 50 50 56 50 50
50 50 50 50
Density 0.683'7 0.8789 0.6837 0.7689 0.7689 0.8789 0 7689 0 8652 0 7783 0 8789 0 7734 0 7783 0 6919 0 7689
t
% ' Sepn.
hexane also separate from each other, benzene being the bottom product. Toluene, on the other hand, does not separate from methylcyclohexane. Benzene does not separate from cyclohexane, as shown earlier. Cetane and cyclohexane also fail to separate. The components of this pair have the nearest density values of any pair studied. It may also be significant that their temperature-density curves intersect a t approximately the average temperature of the column operation (32" '2,).
TABLE
OCTADECANE
Final Composition, - Vol. % % Top Bottom Sepn. 95.0 3.0 84.0 5.0 97.0 9?.? 9.0 85.4 a.a 91.0 53.2 27.5 19.3 46.8 72.5 50 50 0 50 50 50 50 0 50 50 50 50 0 50 50 52 8 37 5 14 9 47 2 62 5
0.2
BENZENE
Figure 7.
1.0
(VOL. FRACTION OCTADECANE)
Effect of Relative Concentrations of Octadecane and Benzene
symmetry, the mass effect appears to predominate. If there is no mass difference, little or no separation would be expected. Yet Table I shows that isomers separate remarkably well in many cases. This separation tendency must be attributed to the influence of configuration on the relative diffusional movement of organic molecules in liquid media. In gases a t ordinary pressures, the molecules are sufficiently far apart, so that their configuration does not greatly influence their ability to move in the medium and the mass effect predominates. Thermal diffusion of aqueous solutions of inorganic salts shows a similar effect. In this case, the ions are perhaps solvated to the extent" that the shape of particles is essentially spherical, the 1.44
90 %
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0.8
0.6
0.4
CHARGE COMPOSITION
78%
Although Table I11 shows three pairs u hich fail to show separation, such pairs are comparatively rare. In hydrocarbon systems, failure to obtain separation has been found only with binary mixtures. In mixtures containing mote than two components, separation has always been observed. Table IV contains the separation data for three pairs of liquid components which have the same or very nearly the same molecular symmetry. In all three cases the component of higher molecular weight concentrates toward the bottom of the column. This indicates that if there is no shape difference between molecules, separation behavior is much like that observed with gases and aqueous systems. ~
TABLE IV. PAIRS OF Components n-Octane n-Decane n-Heptane Cetane Toluene Chlorobenzene
THE
Vol.
Mol.
% 50
Wt. 114 142.3 100.2 226 92 112.6
50
50 50 50 50
~
~~
SAMEMOLECULAR SYMMETRY Final Composition, Vel. %
Density 0.7029 0.7299 0 6837 0.7734 0.8652 1 070
Top 60 40 74.0 26.0 65.0 35 0
Bottom 27.5 72 5 22 0 78.0 38.0 62 0
% Sepn. 30.5 52.5 23.2
This illustrates the definite effect that molecular configuration has on the ability of organic molecules of this type to migrate in organic liquid media under the influence of a temperature gradient. If there is no pronounced difference in molecular
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FRACTIONS Figure 8.
Octadecane-Benzene Mix tu res Time, 48 hours
9
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
2694 25
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1.45
Vol. 45, No. 12
subjected t o thermal diffusion. The normal paraffin is the uppermost product, the aromatic the lowermost, and the cycloparaffin intermediate. No detectable amount of normal paraffin was found in the bottom 30% of the length of the apparatus. The uppermost 10% of the length contained essentially no aromatic, about 3% of cycloparaffin, and 97% normal paraffin. The distribution shown also demonstrates that it is easier to separate the normal paraffin from either the cycloparaf3.n or the aromatic than the cycloparaffin from the aromatic. I n the presence of the normal paraffin, both cyclic components behave very much alike. In its absence, in the lower part of the column, the aromatic is the predominant cold wall or bottom product.
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principal differences b e h e e n molecular aggregates being size and weight. In nonassociating organic liquid media the molecules are sufficiently close together for differences in configuration to influence their individual movement. I n the majority of separations reported in this study, these differences are of sufficient magnitude to cause the molecules to concentrate in the direction opposite to that predicted by the theory of thermal diffusion as applied t o gases, with the higher molecular neight components being top products. The separation of a monocyclic from a bicyclic aromatic, a very compact molecule (carbon tetrachloride) from a paraffin and from an aromatic, and an azeotropic mixture of two alcohols is shown in Table T'. The azeotropic mixture was prepared by mixing the approximate composition of the azeotrope and then collecting t h a t which x a s distilled over a t a constant temperature. This constant boiling distillate was charged to the thermal diffusion column. The relatively easy separation of the components of this azeotropic mixture demonstrates that the individual molecular types are free to be concentrated by thermal diffusion.
BINARY LIQUIDMIXTURES BY TABLE V. SEPARATION-OF THERMAL DIFFUSION Components Cumene blethylnaphthalene Benzene CClr Cetane CCl, Benzyl alcohol Ethylene glycol
Vol.
%
50 50 50 50 50 50 44 56
Mol. Wt. 120
142 1;: 226
154 108 62
Density 0,8633 1.010 0.8789 1.595 0,7734 1.595 1.040 1.113
Final Composition, Val. % Top 66.0 34.0 90.0 10.0
100 0 59.0
41.0
Bottom 31.0 69.0 2.0
98.0 0 100 30.3 69.7
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2
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3
4
5
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8
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Figure 4 shows that in binary mixtures of the xylene isomers, the meta-para pair failed t o separate. When all three isomers are present together, all three separate from each other. Figure 11 shows the distribution resulting from the thermal diffusion of an equivolume mixture of the three isomers. The para isomer concentrates predominantly toward the top, the ortho toward the bottom, and the meta has its highest concentration near the middle of the apparatus. The flattening of the p-xylene concentration curve near the top of the column results from the fact that a paraffinic impurity was found to be present in the mixture, which had a greater tend-
[
9-0 80
% Sepn. 34.8 88
100 29
TERNARY MIXTURES
The relative distribution of molecular types in a few threecomponent systems has also been investigated. Figure 10 shows how composition along the length of the column is distributed when a mixture of equal volumes of a normal paraffin, a cycloparaffin, and an aromatic of comparable molecular weights is
10
FRACTl O N S n-Octane-MethylcyclohexaneCumene Mixtures
FRACT10 N S Figure 11. Ternary Xylene Mixture
December 1953
INDUSTRIAL AND ENGINEERING CHEMISTRY
ency to go to the top than any of the xylenes. The existence of the impurity was indicated by infrared analysis of the fractions obtained in this experiment. The xylene isomers used to prepare the mixture were reagent grade chemicals obtained from a reputable supplier. Comparison of the refractive indices of the separate “pure” isomers with literature values as shown in Table VI indicated that the o-xylene might be the one containing the paraffinic impurity.
TABLE VI. REFRACTIVE INDICES OF XYLENE ISOMERS AT 25” C. Literature Values (API-44),
Isomers Used,
nY
nv 0-X lene 1,5000
1.5029 1.4940 1.4933
na-i!ylene 1.4948 p-Xylene 1.4925
wall. This creates an unstable condition. The effect of temperature makes the liquid near the cold wall more dense than that near the hot wall. If thermal diffusion concentrates the less dense of two components into the cold wall region, the direction in which it may concentrate along the length of the column will depend upon the magnitude of the thermal diffusion effect and the time the system is allowed to operate. De Groot (6), Prigogine (ll),van Velden, van der Voort, and Gorter (27) have recognized and discussed thermal diffusion systems in which the direction of concentration of components produces a density gradient between walls opposed to that produced by the temperature gradient alone. This phenomenon has been given the name, “The Forgotten Effect,’’ probably because so many previous investigators forgot to take i t into consideration.
. t PURE TOLUENE
1.4583
In order to confirm this possibility, the column was iilled with o-xylene as obtained from the supplier. Figure 12 shows the refractive index values obtained for the ten individual fractions and the infrared analysis of the uppermost fraction. These data indicate that the o-xylene contained approximately 4% of a paraffinic impurity. The material contained in the lower 60% of the apparatus was o-xylene of very high purity ( n v = 1.5020). This experiment serves very nicely t o illustrate how thermal diffusion may be used both as a means of purification and as a check for purity when i t is in question. The failure t o find any difference in physical properties between individual fractions is not a proof of purity--e.g., cyclohexanebenzene. On the other hand, if differences in properties between fractions are found, i t is certain that the material in question cannot be pure. Frequently large differences between fractions may be obtained after materials have been subjected to exhaustive treatment of combinations of other separation techniques such as distillation, solvent extraction, and adsorption. In all the separations reported in this paper the components with the higher densities concentrate a t the bottom of the column. There is no correlation, however, between the degree of separation and the magnitude of density difference between components. The density effect is one of the things t h a t have made interpretation of the results from thermal diffusion columns difficult, It is not correct t o assume that those components which concentrate a t the top or the bottom of a thermal diffusion column are also more concentrated on the hot and cold walls. Unfortunately, the less dense of two components will sometimes concentrate on the cold
5 4 % 0-XYLENE 5% m 11
-
3%P-
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z2 1.47-
”
3 8 % PARAFFIN IMPURITY
d
x
w
2-
1.48-
w
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1.49
-
1
3
L
2
PURE
1.50-
1.5 I
0-XYLENE
/BOTTOM PRODUCT
1.45N
./’
41
J
’‘‘’’?:-I
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TIME IN MINUTES1.4543
1
2
3
4
5
6
7
8
FRACTIONS Figure 12. Purification of o-Xylene
9
b ,
+
PURE CYCLOHEXANE
Figure 13.
Forgotten Effect
-
50% toluene-Lio%oyclohexane n y feed 1.4563
Figure 13 illustrates the separation behavior with respect to time of an equivolume mixture of cyclohexane and toluene in the thermal diffusion column. This is a typical forgotten effect pair. The experimental points were obtained by removing small volumes of liquid from the extreme ends of the apparatus a t different time intervals and measuring their respective refractive indices. At the beginning of the operation, toluene began t o increase in concentration at the top of the column and cyclohexane at the bottom. This shows that cyclohexane diffuses away from the hot wall to a greater extent than does toluene. The toluene-enriched hot wall stream is carried by thermal convection to the top of the column. As the concentration difference between walls increases with time, the toluene-enriched layer next to the hot wall becomes more dense than that slightly farther away from the wall and thus begins to creep to the lower end of the column. On the cold wall, the cyclohexaneenriched layer begins to creep upward. After a period of time the amount of material reaching the ends of the apparatus owing to this density difference resulting from concentration may exceed that of thermal convection. This explains why the curve8 cross after about 80 minutes of operation. In the separation of isomers shown in Table I n-heptane separates easily from triptane, with the branched isomer concentrating a t the bottom of the column. In the case of n-octane and iso-octane, a much smaller separation is observed, with the branched isomer concentrating a t the top of the column. The latter case was investigated more carefully and it was found that the octane isomers represent a forgotten effect pair. In both cases, the branched isomers concentrate on the cold wall and the normal isomers on the hot wall. In the case of the octanes, the iso-octane is less dense than the normal octane and eventually ends up in higher concentration a t the top of the column, in spite of thermal convection.
z
0
2695
2696
INDUSTRIAL AND ENGINEERING CHEMISTRY
The property of specific heat per unit volume has been found useful in predicting Rhich of two components should concentrate near the hot or cold wall. In general, thp component with the higher specific heat per unit volume will be in higher concentration near the hot wall and the one a i t h the lower specific heat will be a cold wall product in binary hydrocarbon mixtures. It is not surprising that the component which requires the lesser amount of energy to raise its temperature would be the one to diffuse preferentially away from the hot wall, since temperature is a function of the degree of molecular agitation. There are some apparent exceptions to this principle among the separations reported in this paper. I n a few cases, the component with the higher specific heat has concentrated in the lover part of the apparatus. These exceptions could be a result of forgotten effect pairs of components where density has been a governing factor in determining whether a component nil1 accumulate a t the top or bottom of the apparatus. Some of the exceptions have been examined in this respect and have been found to be forgotten effects. This technique has not been applied to all, however, and it is not known whether or not the property of specific heat is an infallible criterion for predicting the direction of separation of two components between the hot and cold walls of the apparatus. SUMMARY
Small scale thermal diffusion apparatus may be used for performing laboratory fractionations of organic liquids. This apparatus is relatively simple and may be constructed from commercially available metal tubing. Binary liquid mixtures difficult or impossible to separate by conventional techniques in some cases may be separated easily by liquid thermal diffusion because the phenomenon depends upon molecular properties different from those by 1% hich other separation processes are governed. For instance, distillation azeotropes and mixtures of close boiling components have been easily separated by this technique. Mass differences between molecules are not necessary in order to obtain effective fractionation of components by liquid thermal diffusion, The successful separation of mixtures of isomers demonstrates this. Molecular configuration appears to be the most important factor involved in the separation of the components of organic liquids by thermal diffusion. The magnitude of separation between two components is dependent upon their relative concentrations. In the octadecane-benzene system no separation is obtained when octadecane is in molar excess, but large separations are obtained when benzene is in excess. Density is an important criterion in determining whether or not a component may concentrate a t the top or bottom of the apparatus. However, the existence of a density differenre between components does not necessarily guarantee a separation
Vol. 45, No. 12
and is of little value in predicting the magnitude of separation if it does occur. ACKNOWLEDGMENT
The authors wish to acknowledge the valuable advice contributed by E. C. Hughes in the conduct of this investigation. Appreciation is also expressed to the numerous members of The Standard Oil Co. (Ohio) Chemical and Physical Research Laboratories who contributed to this study, and especially to R. W.Foreman, H. A. Dinsmore, C. W,Seelbach, and R. A. Gardner. LITERATURE CITED
(1) Bell, P. H., a n d Davey, W. P., J . Chem. Phys., 9,441-50 (1941). (2) Chapman, S., Phil.M a g . , 38,182 (1919). (3) Clusius, K., a n d Dickel, G., ,~ratzLrzLis.senscke~, 26, 546 (1938). (4) Ibid., 27, 148 (1939). (5) Debye, P . , a n d Bueche, -4.M., “High Polymer Physics,” pp. 497-527, Brooklyn, N. Y., Chemical Publishing Co., 1948. (6) DeGroot, S. R., “L’Effet Soret,” Amsterdam, PI‘. V. A-oordHollandsche Uitgevers Maatschappij, 1945. (7) Jones, A. L., Petroleum Processing, 6 , 1 3 2 (1951). (8) Jones, -4.I,,,a n d Foreman, R. W., IND.Erc. CHEJI.,44, 2249 (1952). (9) Jones, A. L., and Hughes, E. C., U. S. P a t e n t 2,541,069 (Feb. ’ 13, 1951). (10) Ibid., 2,541,070 (Feb. 13, 1951). (11) I b i d . , 2,541,071 (Feb. 13, 1951). (12) Jones, R. C., a n d F u r r y , R. €I., Rev. Mad. Phys., 18, S o . 2, 151-224 (1946). (13) Korsching, H., Nuturwissenscha~te,,,31, 348-9 (1943). (14) Korsching, H., and Wirta, K., I b i d . , 27, 367 (1939). (15) Korsohing, H., Wirta, K . , and Masch, L. W., Bel.., 73B, 24969 (1940). (16) Kramers, € I . , and Broeder, d. J., A d . Chim. Acta, 2 , 687-92 (1948). (17) Alarschner, R. F., and Cropper, W.P., 1x1).ESG. CHEJI., 38, 262-8 (1946). (18) l t u r i n , 4 . N., Cspekhi Khim., 10, 671-9 (1941). (19) Piegisi, I
