R. A. E
d
PURDUE UNIVERSITY, LAFAYETTE, I N D (Present Address, National Defense Research Committee, Washington, D. C.)
b-? G U L F RESEARCH & D E V E L O P M E N T C O M P A N Y , PITTSBURGH, P A .
Liquids may b e divided into five classes based on their hydrogen-bond-forming tendencies. O n the basis of this classification, a systematic summary of deviations from Raoult’s law can b e derived and the tendency for the formation of azeotropes can be predicted. This summary will minimize the experimental work required in the selection of an entrainer to effect a desired separation b y azeotropic distillation.-NGulf’s pilot plant for azeotropic distillation i s shown at the right.
or internal pressure, of which the former is the more important. The concept of hydrogen bonds (Figure 1) is that hydrogen can coordinate between two molecules of oxygen, nitrogen, or fluorine. It can also coordinate between one of these donor atoms (oxygen, nitrogen, or fluorine) and a carbon atom, provided a sufficient number of negative atoms or groups are attached to the carbon atom. Hydrogen cannot coordinate between two carbon atoms. The strength of hydrogen bonds depends upon the nature of the atoms between which the hydrogen is coordinating. They may be classified generally into strong bonds and weak bonds as follows:
EOTROPIC is the term applied to distillations or rectifications which involve the formation of constant-boiling mixtures. The word “azeotrope” is synonymous with “constant-boiling mixture”. An understanding of azeotropes is desirable for two reasons. First, they often occur in fractionation and make a given separation impossible by ordinary rectification. Secondly, they may be utilized to separate mixtures not ordinarily separable by straight rectification. The formation of constant-boiling mixtures is a function of the difference in temperature between the two components and the nonideality of the resulting solution. In an ideal solution there is no change in the average intermolecular forces of attraction on mixing; there is no change in internal energy or in volume, no heat is absorbed or evolved, and the vapor pressure of the mixture is a linear function of the molar composition (Raoult’s law). Raoult’s law is sometimes taken as the definition of an ideal solution; although it is necessary, it is not a sufficient condition. Since the formation of constant-boiling mixtures is a function of the nonideality of the mixture, a correlation of the deviations from ideality for various mixtures would greatly aid in the prediction and selection of azeotrope formers. iMany attempts have been made to predict and correlate the phenomenon of constant-boiling mixtures (1,3, 4 6 , 7 , 9, 10, 12). In the light of present knowledge on the subject, the deviation from ideality can be attributed to the effects of hydrogen bonds
STROXG O-cHO N-HO 0-HN
WEAK
N-HN
HCCli HCCl-CCI HCNOn HCCN
Based on hydrogen bonds, it is possible to arrange organic liquids into five classifications. It is then possible t o generalize regarding binary systems composed of combinations of these classes. Thus, by assigning two liquids to their proper hydrogen bond classes, the nature of the deviations from ideality and the likelihood of azeotrope formation may be predicted. 871
INDUSTRIAL AND ENGINEERING CHEMISTRY
872
CLASSIFICATION
OF LIQUIDS
Liquids may be classified into the following groups based their pot,ent)ialit>ies for forming hydrogen bonds:
011
CLASSI. Liquids capable of forming three-dimcnsional networks of strong hydrogen bonds-e.g., water, glycol, glycerol. amino alcohols, hydroxylamine, hydroxy acids, polyphenols, amides, etc. Compounds such as nitromethane and acetonitrile also form three-dimensional networks of hydrogcn bonds, but the bonds are much weaker than those involving OH an(! NH groups. Therefore, these types of compounds are placed i n class 11. CLASS11. Other liquids composed of n:oliwdes containing both active hydrogen atoms and donor atoms (oxygen, nitrogen, and fluorine)-e.g., alcohols, acids, phenols, primary and wcontiary amines, oximes, nitro compounds with whydropen atoms, nitriles with a-hydrogen atoms, ammonia, hptlrazine, hydrogen fluoride, hydrogen cyanide, etc. Cr~s1 s 11. Liquids composed of molecules containing darior atoms but no active hydrogen atoms-e.g., ethers, ketones, aldehydes, esters, tertiary amines (including pyridine type), nitro compounds and nitriles without whydroqen atoms, etc. CLASSIV. Liquids composed of molecules containin;? activt. hydrogen atoms but no donor atoms. These are moleoulrs having two or three chlorine atoms on the same carbon as 21 hydrogen atom, or one chlorine on the same carbon atom and one or more chlorine‘ -toms on adjacent carbon atoms-e.g., CHCls, CH2C12, CHsCHC12, CAzCl-C&(11, CH~Cl--CHCl--(’IT,Cl, CH2CI--CHCl2, etc. CLASSV. All other liquids-Le., liquids having no hydrogenbond-forming capabilities--e.g., hydrocarbons, carbon disulfide, sulfides, mercaptans, halohydrocarbons not in class I\-, now metallic elements such as iodine, phosphorus, sulfur, etc.
ALCOHOL
WATER
O , R
O X A L I C ACID
- H c 0R’
HC O
d
-H+,O-
H
R
AMMONIA
HYDROGEN
0- H
FLUORIDE
H-F+ H-F+H-F+H-F
‘=- c’
0 H-0
\b-cH-O
. \c-cl’/ OH
Figure 1.
Vol. 36, No. 10
0
‘0-H
Conceptaf Hydrogen Bonds
Classes I and I1 include the liquids usually called “associated” or “abnormal” liquids. The concept of associated liquids has been based on deviations from certain physicochemical relations, such as the Ramsay-Shields surface tension equation, Tiouton’s rule, etc., which are obeyed by normal liquids like hydrocarbons, carbon tetrachloride, etc. No explanation of, or mechanism for, the association was given until the advent of the hydrogen bond concept, although the general relation to the presence of OH and NH groups had been recognized for some time. The term “hydrogen-bonded liquid” should now replace “associated liquid”, although the term “association” should still prove useful to describe the general phenomenon. Class I and class I1 liquids have relatively high dielectric constants. Classes I, 11, and I11 include water-soluble substances. In general, any liquid (or solid) that can form hydrogen bonds with
Table I. Summary of Deviations from Raoult’s L a w CLASSE s DEVIATIOSS HYDROGEX BOND~SG H bonds broken only I 1’ ) Always deviations; 1 V, frequently limited 11 1- ) solubility I11 IT Aiways - deviations €I bonds formed only I IV \ Always deviations; I H bonds both broken ailid I1 I V )’ IV, frequently limited formed, but dissociation solubilit,y of ciass I or I1 liquid is
+ + + + +
+I I + 11 1
+
1
I
I 111 } I1 + I1 1
+
+
+
+
rime impoItant effwt Usually T deiiatioiis, E€ lmnds both brokcn and verycomplicatedgi~ougs, former] some - dc~iatioiisgive some max. aseotmpcs
I1 -t I11 J
+ I11 +v I + IV } IT: + V 1
111
Quasi-ideal
111
mays
TV
I.+\‘
;
systc.rris. al- No H bonds involved deviations 0 1 ’ ideal; azeotropes. if :in?, will be minima
+
water will dissolve in water (01. tend t o dissolve in water). ( I1 liquicl-e.g., alcohols-can form hydrogen honds with watri, in two ways: from oxygen of water to hydrogen of alcohol, and from oxygen of alcohol to hydrogen of water. Class TI1 liquids-e.g., ketones-can form hydrogen bonds with water in only one way: from oxygen of ket>one t,o hydrogen of water. While all class I1 and class 111 liquids tend t o dissolve in \vatel’, the extent to which they actually do dissolve depends on hon. many CH2 groups have to be pulled into so1ut)ion by the hydrogen-bond-forming groups. For instance, the lower alcohols arc completely soluble in water- -n-butyl about lo%, n-amyl about 3%, etc. When the hydrocarbon part of the molecule is so large that the solubility is very small-e.g., decyl alcohol-the tendency of the hydrogen-hond-forming group to dissolve in watei. oauses the liquid to form a monomolecular oriented layer on thc water surface. Ethyl ether is soluble in water only about 7qlO2 but it has four CH2 groups to pull into solution just as n-butyl alcohol does. However, methyl et,her with 6nly two CH2 groups and dioxane with four CH2 groups, but two ether oxygens, are rompletely soluble in water. Even high-molecular-weight suhstances, such as carbohydrates, proteins: gums, and other hydrophilic materials, are soluble in water since they can form many hydrogen bonds per molecule. The old generalization that polar substances dissolve polar substances and nonpolar dissolve nonpolar is obviously incorrect. This generalization is based on a few fragmentary observations such as the following: Water (polar) dissolves glycerol (polar); benzene (nonpolar) dissolves naphthalene (nonpolar) ; water (polar) does not dissolve benzene (nonpolar). The following, however, are contrary: Water (polar, p = 1.8) does not dissolve ethyl iodide (polar, p = 1.8). Ethyl iodide (polar, p = 1.8) does dissolve naphthalene (nonpolar, p = 0 ) . Water (polar, p = 1.8) does dissolve pyrazine (nonpolar, p = 0). Thew examples further demonstrate the idea developed above t,liat solubility is largely a hydrogen bond phenomenon and has little, if any, relation t80polarity (dipole moment) of substances. Where hydrogen bonds are not involved, solubility is determined by internal pressures, and the internal pressure also has a minor effect on solubility when hydrogen bonding is the dominating factor. Any correlation of properties such as solubility, vapor pressure, etc., based on internal pressures of the components of the binary mixtures would hold only when no hydrogen bonds are either broken or formed on mixing. Internal pressure data measured from the properties of two pure compounds cannot be expected t,o apply to the binary mixture when additional and probahly dominant, forces of hydrogen bonds are rreat,ed upon mixing.
INDUSTRIAL AND ENGINEERING CHEMISTRY
October, 1944
873
water, boiling a t 87.8" C. and having a composition of 47 mole % dioxane (IS), is an example of I 111. Water forms a few maximum azeotropes with certain compounds-for instance, with formic acid, ethylenediamine, and hydrazine. Water also forms maximum azeotropes with the strong acids-hydrochloric, hydrobromic, hydriodic, and nitric. Because of the insolubility of class IV and class V liquids in water, heterogeneous or two-phase minimum azeotropes are invariably formed by these classes of compounds with water. An example of IT $. 111 is methanol (class 11) and acetone (class 111) which gives a minimum azeotrope boiling a t 54.6' C. and containing 14 mole % methanol. The dissociation of the methanol is the principal effect. An example of binary mixtures consisting of class I1 and class I V liquids is methanol (class 11) and chloroform (class 1V) which yields a minimum azeotrope boiling a t 53.5" C. and containing 35 mole % methanol. All combinations of class 111, IV, and V liquids, except I11 IV, are quasi-ideal solutions and only occadionally show a minimum azeotrope. They occur when the boiling points of the two constituents are very close. An example of 111 V which gives a minimum azeotrope is acetone (class 111) and n-hexane (class V), boiling a t 49.8" C. and containing 68 mole % acetone. An example of two class V liquids forming a minimum azeotrope is benzene-methylcyclopentane boiling a t 71.4"C. and containing 90 mole % methylcyclopentane ( 2 ) . These azeotropes are caused by internal pressure differences (Table 11). Minimum azeot,ropesare much more numerous than maximum azeotropes. About three thousand minimum azeotropes have been recorded in the literature but only about two hundred fifty maximum azeotropes ( 6 ) . All known maximum azeotropes may be classified as follows; as far a s present knowledge goes, all other combinations of liquids yield minimum azeotropes, if any:
+
COMPOS11 ION
Figure 2.
Vapor Pressure Curves
When deviations from ideal are insufficient to give an azeotrope. pure component ' 6 has the hishest vapor pressure, D . When deviations are sufficient to give an azeotrope, some mixture of components E and F such as K,will have the highest vapor pressure,
id.
The solubility of ionic substances, such as salts, is determined by the dielectric constant of the solvent; and since hydrogen-bonded liquids have high dielectric constants, they nre more or less solvents for salts. Class I V liquids might be expected to be soluble in water, but the hydrogen bonds formed between water and these liquids (from oxygen of water to active hydrogen of class IV liquid) ~ m u l dbe quite weak and do not compensate for its water hydrogen bonds which are broken. Class V liquids are all very insoluble in water, since many of the water hydrogen bonds would have t,o be broken to make room for the class V molecules and no compensat,inghydrogen bonds are formed. By t,his system of classification it is possible to predict t'he nature of the deviations from Raoult's law, and t h e hydrogen honding in a mixture makes it possible to judge approximately the extent of these deviations, The actual formation of an azeotrope or constant-boiling mixture depends upon (a) the magnitude of the deviations from Itaoult's law and (b) the difference in boiling point between the twd pure components. The smaller the deviations from Raoult's law for a pair of substances, the smaller the difference in boiling point must be before :in azeotrope will form (Figure 2). When a system which shows posit,ive (+) deviations fro:n Ilaoult's law forms an azeot.rope, it will be a minimum-boiling :izeotrope. When the deviations are negative (-), the azeotrope, if formed, will be maximum-boiling. On the basis of t>he liquid classification presented above, a systematic summary of tleviations from Raoult's law is derived in Table I. These rules may be exemplified by the following actual cases. An example of I1 V is ethanol (class 11) and benzene (class V) which gives a minimum azeot,rope boiling a t 68.2' C. and containing 44.8 mole % et>hanol. .In example of I11 I V is acetone (class 111) and chloroform (class IV) which gives a maximuni azeot,rope boiling a t 64.5" ('. arid containing 34.5 mole % acetone. Roiling point relations for water solutions are well known. The minimum constant-boiling mixture ethanol-water, boiling nt 78.1" C. and yielding 89.2 mole % ethanol, is an example of I 11. The minimurn constant-boiling mixture 1,4-dioxane-
+
+
+
Table I I . Compound Neopentane hopentane 1 3 -Pent an e ri-Cktane C'yclohexane C'rubon tetrachloride 1:thyl bromide Iknzene I odohenzene .icetone
Internal Pressures
Atmospheres 1,535 1,842 2,020 2,420 2,780 3,070 3,150 3,450 3,640 3,040
Compound Atmospheres Carbon disulfide 4,140 Bromoform 4,220 Methylene iodide 4,950 Bromine 5,700 Phosphorus 7,000 Sulfur 7,420 Iodine 7,600 Mercury 38,750 Sodium chloride 72,900
+
+
+
-
1. W a t e r strong acids-e.g., water HCl, HBr,
N BUT A N 0 L
WATER
+
HI, "03.
+
2. Water certain associated liquids-e.g., water formic acid, hydrazine, ethylenediamine. 3. Donor liquids (class 111) nonassociated liquids c o n t a i n i n g active h y d r o g e n s (class IV)--e.g., acetone chloroform, cyclohexanone bromoform, butyl acetate 1,2,2trichloropropane. 4. Organic acids amines-e.g,, 0 20 40 60 80 100 triacetic acid MOLE PERCENT e t h y l a m i n e , proFigure 3. Diagram of a Heterogznzous pionic acid pyriAzeotrope, Water-+Butanol dine.~. 5.. P h e n o l s amines-e. g . , phenol aniline, o-cresol dimethylaniline. donor liquids containing oxygen-e.g., 6. Organic acids diethyl ketone, butyric acid cyclohexanone. formic acid 7. Phenols donor liquids containing oxygen-e.g., phenol methylhexylketone, o-cresol ethyloxalate. 8. Phenols alcohols-e.g., phenol n-octanol, o-cresol glyrol.
+
+
+
+
+
+ ~
+
+
++ + +
+ +
+
+
+
+
+
+
The above discussion applies only to homogeneous azeotropes. Limited solubility always gives heterogeneous minimum azeotropes. An example of a heterogeneous azeotrope is shown in Figure 3 for water (class I) and n-butanol (class 111) (14). All maximum azeotropes are homogeneous.
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
814
ENTRAINERS
which will form a minimum azeotrope with water but PIO dzeotrope with acetic acid. An example 1s butyl acetate (1 i), shown in Figure 6 (rzght). To an acetic acid-water mixture Iiavirig B composition represented oy yoint B enough outyl acetate i s added lantil the composition becomes thai indicated by point .P Rectification of this mixture yields the 'sutyi acetatc-.vtLirr azeotrope boiling a t 90.6" C. and leaves pure aretic acid.
1. TQseparate a closely boiling pair or a maximum azeotrope:
The entrainer forms a binary minimum azeotrope with only one component. b. The entrainer forms binary minimum azeotropes with each component, but one minimum is sufficiently lower than the other. c. The entrainer forms a ternary minimum azeotrope which is sufficiently lower than any binary azeotropes. The ratio of the original Components in the ternary must be different from their ratio before the entrainer was added. 2. To separate a minimum azeotrope: a. The entrainer forms a binary minimum azeotrope with one component which is sufficiently lower than the original minimum azeotrope. b. The entrainer forms a ternary minimum azeotrope which is sufficiently lower than any binary minimum azeotrope and in which the ratio of the original components is different from their ratio in the binary minimum azeotrope. a.
SEPARATION OF AZEOTROPES
Figure 4 (Eejt) indicates graphically how the minimum azeotrope acetone-methanol can be separated by azeotropic distillation. Methanol and acetone form a minimum azeotrope boiling at 54.6" C. and containing 86 mole % acetone. Methylene chloride appears to be the only compound that will form a minimum azeotrope with methanol, boiling lower than the methanolacetone azeotrope, and yet form no azeotrope with acetone. Indeed, methylene chloride tends toward a maximum azeotrope with acetone (although it does not form one), as would be expected of a mixture of a class I11 and class IV liquid. If rnethylene chloride is added to the acetone-methanol azeotrope until a composition indicated by point A is reached, rectification of this mixture yields the minimum azeotrope methanol-methylene chloride boiling a t 39.2" C. The composition of the remaining liquid moves rectilinearly away from the minimum azeotrope composition and ultimately leaves pure acetone in the still pot. SEPARATION OF CLOSE-BOILING MIXTURES
Azeotropic distillation can often be used advantageously to separate mixtures of close-boiling liquids. Although no azeotrope forms in the binary system acetic acid-water, it is difficult
BUTYL ACETATE 125 OS.
1 \\\\;A & f , , ~
,
METHANOL 64.70C
l
,
MIN. ACETONE 54.6OC 5 6 . 4 O C
Figure 4.
WATER l0O'C.
B
i/'
Separation of Azeotropes
,
,.
No. 10
to separate these two liquids completely by ordinary xctification To effect this separation, it is necessary oniy to find an entrainer
The purpose of an entrainer in azeotropic distillation is either to separate one component of a closely boiling pair or to separate one component of an azeotrope. I n ordinary batch rectification it is necessary to produce minimum-boiling azeotropes to sffect the distillation favorably. Since the lowest boiling component or the minimum azeotrope boils first, the formation of maximum boiling azeotropes will have no effect on the course of distillation. The general methods of using entrainers to effect a separation may be summarized as follows:
METHYLENE CHLORIDE 41.5 OC.
'[d.36,
Hydrocarbons usually occur as mixtures, and the isolation oi x relatively pure compound from other similar-boiling hydrocarbons is a problem of considerable importance. The manufacture of butadiene, benzene, toluene, and xylene from petroleum fractions, to mention just a few, necessitates separations from simiiar-boiling hydrocarbons. A given entrainer may form azeotropes with each type of hydrocarbon in a specific boiling range; however, the azeotropes will not all have the same boiling point. The amount of lowering between the boiling points of the hydrocarbons and their azeotropes increases in the order aromatic, olefin, naphthene, paraffin (8). Such an entrainer, which forms azeotropes with ail the hydrocarbon types, is referred to as it nonselective entrainer. Some liquids, however, form azeotropes with some types alia fail to form azeotropes with other types of hydrocarbons 111 t h e same boiling range, Such a liquid is known as a selective entrainer. For example, all the types of hydrocarbons (paraffin. naphthene, olefin, aromatic) boiling in the range 100-110" C. form minimum azeotropes with methanol, a. nonselective enkrainer. Methyl ethyl ketone, on the other h m d , i s a selective entrainer for this range of hydrocarbons; it forms minimum azeotropes with the paraffin, naphthene, and olefin types but noi with the toluene. The proximity of the boiling points of the entrainer and the hydrocarbons is important in selecting an entrainer. When the boiling point of an entrainer is considerably below that of a closeboiling hydrocarbon mixture, first, the azeotrope boils a few degrees below the pure entrainer to give a large spread between the undesired hydrocarbon azeotrope (nonaromatic) and the desired hydrocarbon (aromatic) ; secondly, the composition of the azeotrope is low in removed hydrocarbon. On the other hand, when the boiling point of an entrainer is considerably above that of a close-boiling hydrocarbon mixture, first, the azeotrope boils but a few degrees below the hydrocarbon mixture to give a small spread between the nonaromatic azeotrope and the aromatic compound; secondly, the composition of the azeotrope is high in removed hydrocarbon. Thus both lower- and higher-boiling entrainers have advantages. Since a successful azeotropic distillation must effect a greater ease of separation than is possible by ordinary, fractional distillation, temperature spread must be obtained even a t some sacrifice of hydrocarbon recovery efficiency. A commercially suitable entrainer, therefore, invariably has a boiling point below that of the hvdrocarbon mixture lo be se.oarated. Entrainers suitable for the separation of closeboiling hydrocarbon mixtures show the following desirable properties (8):
..-. . ACETIC ACID 118 OC.
1. Boil within a limited range (0-30' C.) of the hydrocarbon which it is desired to separate. 2. Form, on mixing with the hydrocarbon a large positive deviation from Raoult's law, to give a minimum azeotrope with one or more of the hydrocarbon types in the mixture. 3. Be soluble in the hydrocarbon a t the diatillation temperature and for some degreeb below it.
October, 1944
I N D U S T R I A L AND ENGINEERING CHEMISTRY
875
1. Be easily separated trom the hydrocarbons with whish it forma an azeotrope. 5 . Be inexpensive and readily obtainable. 6. Be stable at the distillation temperature. 7. Be nonreactive with the hydrocwhoris or the rolumn rnaterial
A hydrocarbon mixture to be separated by azeotropic distillation should be a close-boiling fraction. This can be accomplished by fractional distillation into concentrates prior to azeotropic distillation. Contamination with material of a dzerent boiling range tends to nullify the effects obtained by azeotroping.
The ideal entrainer will usually boil 10’ to 30” below the hydrocarbon mixture. This will give a large temperature difference between the hydrocarbon-entrainer azeotrope and the unaffected hydrocarbon, and thus make the separation easy. Complete solubility of entrainer in the hydrocarbon is to be desired, not only a t the distillation temperature, but for some degrees below it, so that two phases will not exist a t reflux in the overhead assembly. In a commercial process it is necessary to recover the entrainer from the azeotrope in order to recycle it to the azeotroping column. This is an economic aspect and demands an entrainer which can be separated easily and cheaply. The easiest separation occurs when the entrainer and hydrocarbon are insoluble at room temperature. When they are miscible, it is desirable that the entrainer be water soluble so that water washing can be accomplished. The water-entrainer mixture should be easily separated. In the case where the entrainer is water insoluble, a different washing component must be used. I t should be inexpensive and easily removed from the entrainer.
LITERATURE CITED (1) Ewell, R. H., and Welch, L. M., J. Am. Chem. SOC.,63, 2475-8 (1941). (2) Griswold, J., and Ludwig, E. E., IND. ENG.CHEM.,35, 117-19 (1943). (3) Hildebrand, J. H., ”Solubility”, A.C.S. Monograph Series, 2nd ed., Reinhold Pub. Corp., 1936. (4) Kireev, V. A., Acta Physdcochim. U.S.S.R., 14, 371-86 (1941). ( 5 ) Lecat, M., Compt. rend., 189,990-2 (1929). (6) Lecat, M., “L’azeotropisme”,Brussels, 1918. (7) Lemond, H., Compt. rend., 202,1069-71 (1936). (8) Mair, B. J., Glasgow, A. R., Jr., and Rossini, F. D., J . Rasearch Nat2. Bur. Standards, 27, 3 9 4 3 (1941). (9) Mariller, C., Chimie & Industvie, 44, 186-90 (1940). (IO) Mund, W., Bull. BOC. chim. Belg., 38, 322-8 (1923). ENG.CHEM.,27,250-5 (1935). (11) Othmer, D. F., IND. (12) Rabcewicz-Zubkowski, I., Rocrnibi Chew., 13, 16-19 (1933). (13) Smith, E. R., and Wozciechowski, M., J . Research Natl. Bur. Standards, 18, 461-5 (1937). (14) Stockhard. J. S., arid Hull. C. M., IND. ENQ.CHEM.,23, 1438-40 (1931). PREBENTED before the Division of Industrial and Engineering Chemistry a t the 107th Meeting of the AHERICAN CHEMICAL SOCIBTY, Cleveland. Ohio.
TERNARY LIQUID EQUILIBRIA Predicted from Binary Vapor-Liquid Data ROBERT E. TREYBAL New York University, University Heights, New York, N. Y. Present address, M. W. Kellogg Company, New York, N. Y.
HE increasing use of the unit operlttion liquid-liquid extraction as a means of separating the components of a binary solution has made it necessary to have at hand for design purposes a large amount of solubility data for ternary liquid systems. The lack of such data will prevent the application of the various design methods,which have been described in detail. (18); for the proper evaluation of a proposed solvent extraction process, the complete ternary liquid phase diagram must be known. In the absence of such data, it will frequently be desirable to predict from the properties of the components of a sys-
T
tem whether a certain solvent will be useful in such processes, in order to eliminate unnecessary and tedious laboratory work in determining the complete phrase diagram. Any method of prediction of this sort must be based on properties which are readily and abundantly available if it is to be of general utility. Furthermore, unless it is to yield very precise results, it should not be too laborious. Othmer and Tobias (16)described a method of predicting the distribution of a solute between two immiscible solvents based 011 partial pressure data for the two binary solutions formed with
The graphical method proposed by Hildebrand (10) for predicting ternary liquid equilibria from activity coefficients of binary solutions has been applied to a number of systems, using constant pressure and constant temperature vapor-liquid equilibrium data, and azeotropic measurements. No data on the ternary system are necessary for its application. The results a t low concentration compare favorably with the observed data, and prove the method to be reliable in predicting which solvent will be useful in solvent extraction processes. The use of the van Laar equations for extending the applicability of the method is discussed. Two modifications to allow for mu-
tual solubility of the contacted solvents are tested; these frequently reproduce the distribution curve a t high concentrations more accurately and thus make it possible to estimate roughly the position of the plait point. A simple analytical method of calculating an index of the usefulness of a proposed solvent in extraction processes for which no ternary data are available is described. The distribution coefficient of Hand (9) is shown to be predictable in certain cases. A modification of his empirical method of plotting distribution data is presented which makes it possible, in the case of some systems, to determine accurately the distribution curve w i t h one measured tie line.
