Correlating Azeotropic Data DONALD F. OTHMER AND EDWARD H. TEN EYCK, J R . Polytechnic Institute of Brooklyn, Brooklyn, N. Y .
*
B y the expansion of correlations previously presented, it is shown that plots on logarithmic paper give straight lines for pressures, partial pressures, compositions (of either vapor or liquid), or activity coefficients of binary or ternary azeotropic mixtures when plotted against the total pressure or when plotted against a temperature scale derived from the vapor pressure of a reference substance. The thermodynamic proof is given and depends on a derivation under conditions that x = y rather than conditions of constant temperature, pressure, or volume.
T
HE effect of pressure and temperature in changing the
composition of azeotropic mixtures has been studied by several investigators (4,6,8,9, 11, 2.9, 23, 66). The correlations derived from their work are usually tedious in use or rather specific. The methods of Licht and Denzler (9), Prigogine ( e l ) , and Nutting and Horsley (13) allow more ready presentation of experimental data but are limited to the change in total azeotropic pressure with temperature. It has been found possible, by expanding a method previously proposed (16),to extend these correlations for partial pressures, vapor compositions, activity coefficients, and other relations of azeotropic mixtures with either the total pressures on the system or the boiling or azeotropic temperatures. The method (16) using logarithmic paper was previously found very useful in correlating all of these properties of volatile solutions under conditions of constant composition, either vapor or liquid. It has been followed in the present case with adaptations for the condition of azeotropism-Le., 2 = y. Previously it was shown that a plot on logarithmic paper of the various properties of solutions versus the total pressure upon the system would give straight lines, the slopes of which represented heat quantities which were functions of the individual latent heats of the components of the solution and the heat of solution.
Azeotropic or constant boiling mixtures ( c . b . m . ) are usually defined as being those which, on boiling a t a fixed pressure, give the same composition in the vapor as that of the liquid-Le., x = y. Minimum constant boiling mixtures are further subdivided into two groups: homogeneous, wherein the vapors on condensation form a single liquid phase; and heterogeneous, wherein the vapors on condensing form two liquid phases. I n defining azeotropism under different total pressures, neither the partial pressure, the temperature, nor the composition of the liquid phase is maintained constant. The constraint on an azeotropic system is that the composition of the vapor is always the same as the composition of the liquid-Le., 2 = y. (More familiar constraints, or conditions of partial differentials are constant pressure, constant temperature, or constant composition of the liquid phase.) I n the Clausius-Clapeyron equation, for example, as previously used (16) for binary solutions, the constraint is that the system is under constant composition of the liquid phase. There may be developed under the specific condition of azeotropism correlations for the various properties of solutions in the same way as tho e developed under the specific conditions that they be a t constant values of the liquid compositions. While constant pressure, volume, or temperature are usud conditions in thermodynamic developments and properties, as for example specific heats, a similar concept to the present one is the familiar use of specific heats at, a condition of the liquid such that it is always a t the boiling point, or of vapors such that they are always saturated and a t the condensation point. Here both temperature and pressure are changing. DERIVATION OF EQUATIONS OF AZEOTROPIC SOLUTIONS
PARTIAL PRESSURES. By analogy to the equation derived by Licht and Denzler (9) and Redlich and Schutz ($3) the ClausiusClapeyron equation for one component of a binary solution a t the azeotropic point may be written:
IO0 80
In this equation pf represents the partial pressure of component 1 (usually the more volatile one) at the temperature T of the azeotrope. Lf is its molal latent heat of vaporization from the solution, and R is the gas constant.
60
L
40t
TOTAL PRESSURE 10
20
Io2
I
't
I
I
I I l l
IO' TOTAL PRESSURE,
I
I
I
I
mrn.tig
I
OF AZEOTROPE, mm.Hg
100
1000
I
I I I11111
I
I
I
I I I11111
I
I I I11111
I I11111
1 I
I l l
IO4
Figure 1. Vapor Composition of More Volatile Component 'us. Total Pressure on System Mole or weight per cent as shown in Table I1
2897
P
S
I
IO0
1000
10,000
TOTAL PRESSURE OF AZEOTROPE, mm.Hg
Figure 2. Activity Coefficient of More Volatile Component us. Total Pressure on System
INDUSTRIAL AND ENGINEERING CHEMISTRY
2898
System
1 2 3 4
SOt-?i-butanc n-Propyl alcohol-water Acetonitrile-n ater Ethyl alcohol-water0 Methyl alcohol-henzene Methyl alcohol-MEKd .4cetone-water Ethyl acetat?-ethyl alcohol 8-Picoline-phenol Chloroform-ethyl alcohol 7 -Picnlinp-phenol
5 6 7
8 9
10
11
12
HCI-water
13 14 15 16
NRr-water MEK-rvaterd Ethyl acetate-\vator E t h y l ether-water
TABLE I. SYSTEMS PLOTTED
Pressure Range", Alm.
NO.
Vol. 41, No. 12
Composition Range b , Temp. Range, Afore Volatiie c. Homogeneous Minimum Boiling Binary Azeotropes 350-2,015 -35-$3 740-5,930 87-181 150-760 34-77 100-1,450 34.2-95.3 26-149 200-11,000 100-2,040 18.4-92.3 2,570-10,320 95.8-155.8 28-1,478.5 - 1,2-91,4 200-760 146.0-187.0 35-55 652-307 147.5-190.5 200-760 Homogeneous Maximum Boiling Binary Areotropes 50-1,210 19.3-23.5 wt. % 48.7-123
Investigator$
Rcfercnce
hfatuzak and Frey Licht and Denzler Otbmor and Josefoivita Perry Horsley and Rayland Britton, Nutting, and Horsley Othmer and Morley Merriman Othmer and Savitt Scatchard a n d Raymond Othmer and Savitt
Akerlnf and Teare Bonner and Wallace 47.03-49.80 wt. yo 74.12-137.34 Bnnner, Bonner. and Guerney 39.9-73.3 200--760 65.4-72.2 mole % Othmer and Benenati 25-1,441.3 -1.8-+89.1 3.6-9.94 \Vt. 7 0 Merriman 31,700-106,000 87.5-92.5 15%. 7 0 62.5-114 hloeller Heterogeneous Minimum Boiling Ternary Azeotropes 740-5,930 72.7-82.3 u-t. 70benzene 67-135 Licht a n d Denzler (9) 17 Benzene-propyl alcohol-HZ0 10.1-15.0 wt. % n-propyl alcohol 11.5-21.2 wt. Yo ethyl alcohol 25.1-131 Licht and Denelcr (9) 118-5,660 Trichloroethylene-ethyl 18 70.5-85.1 wt. Yo trichloroethylene alcohol-water 0 The pressure scale for svstems 12, 13, and 15 should be multiplied b y 10-1 for system 16 by 102. b Compositions plotted a r e of the first, more volatile component given under "System" (except for ternary systems) and are either weight % or mole % a5 indioated under composition range. T h e Composition scale for system 1 5 should be multiplied b y 10-1. C See also Griswold et al. (6). d M E K = methyl ethyl ketone.
20
ion-i,zoo
TEMPERATURE, 'C 60 80
40
Substituting Equation 5 in 4 there results: 100
120
140
160
100
e.a
Equation 6 shows the relationship of the composition of the azeotropic vapor (or liquid since z A = yA) > 30over a range of pressures and temperatures, as a function of the latent heats and the total pressure of the z system. 17 Figure 1 is a logarithmic plot of Equation 6 for the I I I Ill # I IO I I I ! ! ( I ! ! azeotropic compositions for a group of binary miutures 10 50 100 500 1000 5000 VAWR PRESSURE OF WATER, mrn Hg (and several ternaries) versus the total pressure. The straightness of the lines indicates the constancy of the Figure 3. Vapor Composition of More Volatile Component us. ratio of latent heats and makes the plot useful as a corVapor Pressure of Reference Substance a t Same Temperature relating tool where only two points (or one point and Mole or weight per cent as shown in Table 11; s y s t e m 3 4 and 15 fall under 16 heat data) might be known. The lines represent homogeneous azeotropes of maximum and minimum boiling points, as well as heterogeneous azeotropes for both binary Similarly, the equation for the total azeotropic pressure, PA, and ternary systems. For ternary azeotropes, two lines arc realways a t the same temperature, may be written: quired to represent the three components of the system. Either mole per cents or weight per cents may be used as shown in Table I to indicate compositions (for either vapor 01 liquid, since x = y); the resulting curve6 are straight lines. The term L A is the latent heat of vaporizing 1 mole of the azeoIn certain irregular systems slightly curved lines have heen tropic mixture (which includes heat of solution of the two comobtained on a plot of this type. These are the benzene-ethyk ponents in the liquid phase). alcohol (27), acetone-carbon disulfide (G'), and benzene-ethyl Equations 1and 2 equate to: alcohol-15-ater (26) azeotropes. I t would apppar that additional experimental studies of these systems ale warranted. (3) ACTIVITYCOEFFICIEST.The activity coefficient is defined for one component as Assuming that the ratio L;'/LA is constant, the integrated equation is: J 40-
0
I
10gp: = J2 -1ogP" L A
fc
(4)
which indicates that the logarithm of the partial pressure of one component of an azeotropic mixture is a straight line function of the logarithm of the total azeotropic pressure. Dalton's law may be written: VAPORCOMPOSITIOXS. P: = &PA
(5)
is the fraction of the more volatile component in the where azeotropic vapors.
This may be combined with the above equations to give the relation for the activity coefficient against pressure in Table 11. A corresponding plot has been niadc in Figure 2 for two azeotropic mixtures; these are straight, as are many others which have been made. HEATQUANTITIES. It has been assumed that the ratio of heat quantities is constant to allow integration of the differential equations. Actually, the partial heat of solution of the pure component will change very slightly as the coriceIitration of the
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
December 1949
2899
FOR VALUESOF x = y (AZEOTROPES) TARLE 11. LOGPLOTS (All of these functions give straight lines on logarithmic paper when plotted againat t h e total pressures on t h e system or the vapor pressures of a reference substance a t t h e same temperatures. T h e slopes of these lines are equal t o t h e heat ratios in t h e last column. Only equations and plots for one component are indicated; those for a, second or subsequent volatile component are similar straight line functions.)
Property
2
Plotted Against
Logarithmic Form
Differential &)quation
log PA
-2
PY
log B:
= (1 -
P:
.PA
log p:
: y
PP
d log u: d log p ?
:y
PA
d_ log y: _ d log p A
7:
P?
d log : r d log p y
r:
PA
d1 log yA = - d log P A
PA
P:
P:
d log PA LA d log Po = Ly
~.
- d log P
