Azeotropism: A Useful Tool Clarified '
K . B. FLEER' Forest Products Chemical Company, Memphis, Tennessee
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constituents. Perhaps the first observation of such a phenomenon was made by Liebig in the year 1832 ( I ) when he found that a mixture of ethylene dichloride (01der hollandischen Chemiker) and water boiled lower than did either of the two pure compounds. On the basis of this observation and the law of partial pressures of mixed gases which Dalton had set forth some thirty years earlier, Gay-Lussac (2) postulated the law, later demonstrated by Henri Regnault in 1854, that at a given temperature the total vapor pressure of a n immiscible pair of liquids i s equal to the sum of the individual vapor pressures which the pure constituents would. h u e at that temperature regardless of the relative poportion of the components i n the mixture. Still in the same year (1832) Dalton observed the first example of an azeotrope with a maximum boiling point, namely, hydrochloric acid and water, and this maximum-boiling-point mixture proved to be so constant in composition a t any given pressure that it has been used to prepare standard solutions of the acid. A very early observation of a homogenous (one phase) minimum binary azeotrope was made by Berthelot (3) who discovered that of ethanol and carbon disulfide boiling a t about 42°C. Berthelot was the first to ohserve that "in a mixture the liquids do not always distill in the orderof decreasing volatility." Despite considerable investigation, the number of known constant boiling mixtures (CBM) was very limited prior to 1900, and most of the recognized examples were explained by assuming compound formation between the components in question. Ostwald, in his monumental six-volume work (a), suggested the use of the term "hylotropic" (from the Greek hylo, substance, and tropos, changing) for such mixtures, hylotropic substances being those which can change into another substance of the same composition. Even today the fuudamental reasons for azeotrope formation are still not clearly understood, but considering the modern interpretation involving association and hydrogen bonding (5), i t would appear that the guesses of the early masters may not have been too far wrong. That a constant boiling mixture is not a molecular compound in the usual sense, however, was demonstrated in 1861 by Su Henry Roscoe (6) who showed that the composition of the CBM varied with the pressure a t which the distillation was carried out. The term "azeotropic" (privative form of the Greek zein, to boil, and tropos, changing; hence, to boil unchanged) was suggested in 1911 by Wade and Merriman (7). After the turn of the century the number of known CBM's greatly enlarged by Sydney Young (8) and ' Present Address: American Chemical Society, Washington, Mauricewas Lecat (9). In 1902 Young (10) observed that D. C. 588
HE separation of complex mixtures of chemical compounds into their relatively pure constituents has long occupied a major portion of the attention of industrial chemists and chemical engineers, whether these mixtures arose from natural sources or from synthesis. The older methods of accomplishing this result involved the rather obvious use of simple chemical reactions, although the minor constituents of the mixtures were often destroyed in such cases. Thus, in the older methods, anhydrous ethanol was often prepared by distilling the 95 per cent alcohol over burnt lime, which reacted chemically with the water and rendered i t nonvolatile. Ammonia from the carbonization of coal was absorbed in sulfuric acid to give ammonium sulfate; to obtain strong ammonia, the sulfate liquor was distilled with lime. Methanol containing small amounts of ketonic bodies such as acetone could be freed of the latter by treatment with bisulfites. After filtering off the precipitated ketone-bisulfite compounds, the methanol could be obtained pure. Another example of such roundabout methods was that of recovery of acetic acid from its dilute aqueous solution. Pyroligneous acid used to be treated with lime, the neutral solution evaporated to yield the dry calcium acetate, and the latter distilled with concentrated sulfuric acid to obtain the glacial acetic acid. Esters used to be made by addition of dehydrating agents to remove the water formed in the reaction. The rapidly growing importance in late years of volatile, liquid chemicals of all kinds, particularly organic, and the cost and difficulty of sqch procedures on the present enormous scale have caused most of these older chemical methods to be replaced by phFsical ones. Such a physical method of the widest application and of increasing industrial importance is that of azeotropic distillation, and the value of azeotropic methods in the laboratory as well as on the large scale should make a knowledge of the principles involved of the greatest importance to every chemist. To the student not intimately familiar with the theory of fractional distillation, azeotropes are sometimes confusing; but, as a matter of fact, most cases of azeotropism are simple ones which can be analyzed by the usual methods of physical chemistry. Careful consideration of the few general cases will provide a practical, working knowledge of this useful and common behavior of liquid mixtures. An azeotropic mixture is one which boils or distils without change in composition, and in general it has a boiling point higher or lower than that of any of its pure
a ternary mixture of ethanol, benzene, and water boiled a t a lower temperature than did any of the individual coustitueuts or any of the three possible binary mixtures. This was the first such ternary mixture discovered, and Young used this knowledge to develop a commercial process for the production of anhydrous ethanol. He obtained a German patent covering the process which was adopted in 1908 by Kahlbaum in Berlin. A modification of the same process was patented in this country by D. B. Keyes (11) which is the process used today for absolute alcohol, although several other entrainers for the water have been suggested. In an intensive search for examples of azeotropism, Lecat investigated several thousand mixtures and discovered and established the composition of a great number of binary and ternary CBM's. An understanding of the behavior of azeotropic mixtures is best obtained by a consideration of phase equilibrium diagrams, and for the present purpose those of binary mixtures will be sufficient. In 1876 it was set forth on theoretical grounds by Willard Gibbs (12),and empirically by D. P. Konowaloff in 1881 (13), that in completely miscible binary mixtures the vapor will be relatively richer in that component the addition of which to the mixture increases the total vapor pressure. Just how rich the vapor will be was first shown by F. M. Raoult who, in 1887, demonstrated the accuracy of what has now become known as Raoult's Law. As applied to completely miscible binary systems, this states that the partial pressure of a constituent of a binary
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Composition FIGURE 2
Any composition m boils a t temperature t and a t equilibrium gives off a vapor of composition n richer in the more volatile component A . The residue is therefore richer in the less volatile component B. By repeated distillation such as is accomplished in a fractionating column any mixture can therefore be separated into pure A and pure B, provided only that the column is mixture at any given temperature is equal to the normal sufficiently effective. uapor pressure of that constituent at the sated temperature In mixtures where the relative .vapor pressures demultiplied by the mole fraction of the constituent in the viate from Raoult's Law widely enough to give a maximixture. mum or minimum total vapor pressure, liquid-vapor With the liquid-vapor relationships so fixed for any diagrams such as Figure 2 or Figure 3 are obtained known composition, a mixture behaving reasonably in where a t one point, k, the vapor and liquid compositious accordance with Raoult's Law thus gives the familiar are identical, and the curves representing these compoboiling point-composition diagranmhown in Figure 1. sitions are thus tangent. In such &es we have azeo-
1 k
Composition FIGURE 1
,it'ion FIGURE
3
tropic mixtures of minimum or maximum boiling point, respectively. In Figure 2, any composition o to the rigbt of k boils at temperature t and gives off a vapor fi richer in A, and a residue remains, richer in B. By fractionation, any mixture with a composition lying to the right of k can therefore be separated into pure B as a residue and the pure azeotropic composition k. Likewise, any composition of to the left of k can be separated by fractionation into pure A as a residue and the pure azeotropic mixture k; hence, no matter what the composition in the still pot, excepting pure A or pure B, the distillate, or "overhead product," on fractionation is always the pure azeotrope k. The residue left behind, if any, will be pure A or B depending on wbether there was in the starting mixture an excess of A or B over that required by the azeotropic composition k. Figure 2 is in effect two Figure 1diagrams. The curve or is a Figure 1 diagram for a mixture of A and the azeotrope k, while the curve B is a Figure 1diagram for a mixture of B and the azeotrope k. At any given pressure, an azeotrope on fractionation therefore behaves as a pure component having a boiling point of T. Figure 3 is a diagram for a mixture of two components which form a CBM of maximum boiling point. Any composition o to the right of k boils a t temperature t, gives a vapor of composition p richer in B, and-leaves a residue richer in the azeotrope k. Similarly, any composition o' to the left of k can be fractionated into a distillate of pure A and a residue of the pure azeotrope k. Thus, any composition whatever yields on fractionation a residue of pure azeotrope boiling a t the temperature T and an overhead product of pure A or pure B depending on whether the starting material contained excess of A or B over that required by the azeotrope k. Again, Figure 3 is actually two Figure 1diagrams. or is a diagram for mixtures of A and k, and P is a diagram for mixtures of B and k.
Figure 4 represents a binary system of partially miscible liquids, for example, n-butanol and water, which, by virtue of its conformity to the law of Gay-Lussac (Z),behaves as a sort of pseudo-azeotropic mixture but which, for purposes of fractional distillation, may be considered as a special case along with the true azeotropes. Such a system yields a two-phase overhead product, a situation which in most problems of separation is desirable. The curves a and b represent the limits of miscibility a t any given temperature. Any composition between m and n boils a t the temperature t and gives a vapor of composition x. The vapor x condenses to a two-phase liquid of which the quantities and compositions of the two phases are determined by the relative vapor pressures (according to Gay-Lussac) and the mutual solubilities, respectively. Unlike the true azeotropes, this minimum-boiling-point composition is the result of "one-stage" equilibrium behavior and is obtained without the aid of fractionation whenever a composition lying between m and n is distilled. Simple steam distillation is a case in point. Whenever a composition such as 0 , however, lying within the limits of miscibility (i. e., to the rigbt of m) is distilled, it boils a t temperature t' and gives a vapor of composition fi richer in A . There remains a residue richer in B. On fractionation of any mixture such as o, therefore, a point in the column is reached a t which the composition exceeds the limits of solubility a t m. Above this point in the column, then, the system is a simple case such as that described in the preceding paragraph, and the overhead product again turns out to be x. Any excess of B over that required by the composition x remains as a residue of pure B. Likewise, fractionation of any mixture o' to the left of n eventually yields a product such as 9' richer in B than n so that the final result of fractionating o f is an overhead product of composition x and a residue, if any, of pure A . Hence, effective fractionation of any mixture whatever of A and B in Figure 4 yields the CBM of composition x as overhead product, and there remains a residue of pure A or pure B depending on whether A or B was in excess of that required to form the composition x. For all practical purposes of separation, therefore, mixtures of this type behave like true azeotropes. Mixtures with compositions lying between m and the curve a or between n and the curve b consist of two phases. As the temperature is raised, the point defining conditions eventually crosses the curves a or b, respectively. When this occurs the two phases become miscible, and the mixture then behaves like o or of. Figure 5 is a diagram for totally or almost totally immiscible pairs of liquids and represents the limiting case of Figure 4 in which the range of m and n has been expanded to embrace all compositions between pure A and pure B. The curves c and d no longer have any significance, and i t will be seen that in Figure 5 any mixture of A and B will boil a t temperature t and yield a distillate of composition x , according to the law of Gay-Lussac.
A few special cases of partially miscible binary mixtures are known (such as isopropanol and water) in which the diagram is a variation on Figure 4 (Ik), but these need not be taken up here. The four types of systems represented in Figures 2 to 5 comprise most of the constant boiling - mixtures encountered in plant and laboratory. A typical application of azeotropism, of which numerous exam~lesare to be found in industrial distillation practice, is the addition of a third component to a mixture in order to f o m a CBM with one of two components otherwise difficult to separate by fractionation. Acetic acid, for example, is very difficultly separated from its aqueous solution by fractionation alone. Addition of ethyl acetate, however, to a mixture of acetic acid and water allows the formation of the esterwater CBM, boiling lower than any of the other constituents. Fractionation then gives the CBM as the first product, and, when this is removed, anhydrous acetic acid remains in the still. Sufficient ethyl acetate to form the binary CBM with all of the water present in the charge need not be added since the azeotrope condenses in two phases. By continually decanting off the aqueous layer and returning the ester layer to the still, the acetic acid is finally dehydrated. An application of the azeotropic method in which no third component need be added is exemplified in the recovery of f u r f m l from its dilute aqueous solution, for which a diagram similar to Figure 4 applies. Although furfural itself boils well over 100°C., fractionation of the dilute solution yields furfural as an overhead product, minus the bulk of the water, due to the existence of the furfural-water azeotrope boiling at about 98"C., and the vaporization of all the water in the original mixture is thereby avoided. Redistillation of the overhead product again gives the furfural-water binary, from which the water layer is continually decanted until only dry furfural remains in the still. Yet another use of azeotr~~ic.~henomena which is receiving wider and wider application iwboth laboratory and plant practice is that of removal of one product from the site of reaction in syntheses involving equilibrium. Thus, in the ordinary production of esters and acetals (15, 16) the continuous azeotropic removal of water as fast as it foms allows the reaction to proceed to completion. In many cases a portion of the ester itself or the alcohol or both will serve adequately for the purpose of forming the azeotrope, but in the case of some of the lower alcohols and their esters it may be desirable to add a "third component" in the f o m of benzene or other innocuous solvent. Removal of the ester or acetal as fast as it forms serves equally well in completing these reactions, and this is often accomplished conveniently by means of an azeotrope with an excess of the alcohol. In the effort to isolate one component of a binary azeotrope, it is not always possible to select a third material which will form a new and simple binary CBM with one component of the original azeotrope. I t is usually an easy matter, however, to pick an additive material
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Composition which will f o m a ternary CBM with the two components of the first azeotrope. By the proper choice of the third component it is possible, except in rare cases, to obtain a ternary CBM of composition favorable to the isolation of the single pure component desired. Occasionally, where the rate of change of composition with change in pressure is great enough, an azeotrope can be "broken" by distillation at another pressure, as shown by Roscoe (6). Many other cases of the practical use of azeotropic behavior could be cited to show the wide applicability of this interesting property to distillation procedures. Azeotropism appears to be a property of much more general occurrence than is usually recognized, and an adequate knowledge of this department of physical chemistry will be very useful in broadly diversified work with all kinds of liquid compounds. Once the systematic treatment of the general case is clearly understood, it is usually an easy matter to analyze some new azeotropic system and to put it to good use or to select and apply a system to accomplish an otherwise difficult job. LITERATURE CITED
(1)LIEBE, J.,Ann. chim. phys., 49,184 (1832). J. L., ibid., 49, 393 (1832). (2)GAY-Luss~c, (3)BERTHELOT. C.L., Compt. rend., 57,430,985 (1863);Ann. chim. phys., 1,384(1864). (4) OSTWALD. W.. "Lehrbuch". 11, 2, 298; Trans. Chem. Soc., 85, 511 (1904). AND L. BERG,Ind. Eng. (5) EWELL,R. H., J. M . HARRISON. Chem.. 36,871(1944). (6) R q s ~ ~ " H., , ; Trans. C h m . Sac., 13, 146 (1861); 15, 270 ,A-"",. (7) WADE,J., AND R. W. MERRIDIAN,Trans. Chem. Soc., 99, 1004 (1911). (8) YOUNG,S., "Distillation Principles and Processes", Macmillan Company, New York, first edition, 1903: second edition, 1922. Also numerous papers in Trans. Chem. Soc. (9) LECAT, M., "Le Tension de vapeur des mClanges de Liquides. l'Adotronisme." Henri Lamertin. Bmxelles. 1918. Also various pipers h Rec. traa. chim.(1926,1927,'1928) S.,Trans. Chem. Soc., 81,707 (1902). (10) YOUNG,
(11) U. S. Patent 1,830,469. (12) GIBES,J. W., "Saentfic Papers," Longmans, Green & Campany. New York, 1906. (13) K O N O W A L O D.~P., , Wied.Ann., 14,34 (1881). (14) GLASSTONE, S.. "Textbook of Physical Chemistry." D. Van Nostrand Co.. Inc., 1940, pp. 722-3.
(15) MORTON. A. A., "Laboratory Technique in Organic ~ h e r n - istry," McGraw-Hill Book Company, New York, 1938, p. 64 ff. (16) REID, E. E., in P. H . G n o o c r ~ s ,"Unit Processes in Organic Synthesis." McGraw-Hill Book Company, New York, 1938, p. 552 ff.
