EOgglgllB
ELSEVIER
Fluid Phase Equilibria 103 (1995) 51 76
A data bank for azeotropic data - - status and applications
Jfirgen G m e h l i n g a,,, Jochen M e n k e b, J6rg K r a f c z y k a, Kai Fischer a
a Universitiit Oldenburg, Technische Chemie, Postfach 2503, D-261 ! l OIdenburg, Germany
b D D B S T GmbH, Industriestrafle I, D-26121 Oldenburg, Germany
Received 22 March 1994; accepted in final form 2 July 1994
Abstract
A computerized bank of azeotropic data is now available. Data from the literature have been tested before storage.
Newly measured azeotropic and zeotropic data are also included, which were obtained for the purpose of verifying
questionable literature information and also for systems for which no data had been available.
This data bank now contains approximately 36 000 entries (information) on azeotropic or zeotropic behavior or
approximately 19 000 non-electrolyte systems involving approximately 1700 compounds. It can be used reliably in
process synthesis computations, e.g. for design calculations of distillation columns, selection of the best solvents for
azeotropic distillation and also for further development of group contribution methods or for fitting reliable g E-model
parameters.
This compilation will increase the capabilities of existing software tools, such as the Dortmund Data Bank and the
integrated program packages (Gmehling, J., 1991. Development of thermodynamic models with a view to the
synthesis and design of separation processes. In: J. Gmehling (Ed.), Software Development in Chemistry 5.
Springer-Verlag, Berlin, Chapter 1.) for process design, synthesis and simulation.
Keywords: Azeotropic data; Data bank; Distillation; Selection of selective solvents; Process synthesis
I. Introduction
Distillation is the most widely used separation process (Humphrey and Seibert, 1992). Thus,
information on VLE is very important for the design of distillation equipment, since it highlights
the relative volatility ~ , which indicates the ease of separation by distillation of components i
and j. The relative volatility s 0. is also referred to as the separation factor. Separation factors
close to or equal to unity indicate that separation by distillation is difficult or impossible.
* Corresponding author.
0378-3812/95/$09.50 © 1995 - Elsevier Science B.V. All rights reserved
SSDI 0 3 7 8 - 3 8 1 2 ( 9 4 ) 0 2 5 6 9 - M
J. Gmehling et al. / FluM Phase Equilibria 103 (1995) 51 76
52
Therefore the ability to predict the relative volatility and also the possible occurrence of or an
approach to azeotropic conditions is an important tool, which enables the designer to select
between alternative options for separating azeotropic mixtures. Alternative distillation processes
can employ separation at a different pressure, by extractive or azeotropic distillation, pressure
swing distillation or hetero-azeotropic distillation. Non-distillation and hybrid processes can also
be employed. The availability of data and data handling procedures for illustrating the phase
behavior and the separability of a mixture makes it possible for the process designer to select
between these various processing options.
2. Basic thermodynamic relationships
The relative volatility e~j between components i and j in a system consisting of a liquid and a
vapor at equilibrium is defined as
Ki
o¢~/ -
K~
Yi/Xi
----
(1)
Yj ix~
Assuming ideal vapor phase behavior and neglecting the Poynting correction the relative
volatility and/or the K values can be calculated from vapor pressure data and activity
coefficients. The resulting equation is
K,
:¢o -
K:
y~/xi _ TiP s
-
y:/x:
(2a)
~'
:jPj
where P~ and P~ are the vapor pressures of the pure components i and j and 7: and 7i are the
activity coefficients; the variables x: and y, are the mole fractions of component i in the liquid
phase and the vapor phase respectively.
If equations of state are used (no simplifying assumption) the expression for the relative
volatility becomes
Ki
y:/x:
qo~/~pv
- K,
yjlxj
iq, v
(2b)
Various methods (ge models of group contribution methods) are used for either calculating or
predicting the activity coefficients for the components under given conditions (temperature,
pressure, composition). Equations of state are used for calculating the fugacity coefficients in Eq.
(2b). According to Eqs. (2a) and (2b), no separation by rectification is possible when the relative
volatility is equal to unity, while separation would be very difficult if the value of ~,7.is close to
unity. The conditions of % = 1 can occur in a binary system when the ratio P{/ps2 is equal to
the ratio 72/71. That is
P~/P~ = 72/7,
(3a)
or
log(P~/P~) = log(Tz/7, )
(3b)
F r o m the above, azeotropic points will occur in a binary system when one of the following
conditions is fulfilled.
ly,
53
J. Gmehling et al. / Fluid Phase Equilibria 103 (1995) 51- 76
\
D
t
\\
/
0
i kX2./
/
ff'~
I,--
t
0
III
t
1
//
t ;,v/..//
o
Q_
v
1
f////z
IV
,/
t
\1
t \.
p~
",:j
I,--
t
V
y
0 _L
VI
1
00
Q.
I,-0..
t
t ~\\\\X
//
O--o--~
1
0
1
0
Fig. 1. y x, P - x and T - x diagrams for the different types of binary azeotropes: I, homogeneous pressure maximum
azeotrope; II, heterogeneous pressure maximum azeotrope; Ill, homogeneous pressure minimum azeotrope; IV,
homogeneous pressure maximum azeotrope in a heterogeneous system; V, double azeotrope; VI, homogeneous
pressure minimum azeotrope in a heterogeneous system.
(1) In the case o f positive deviation from Raoult's law (pressure m a x i m u m azeotropes):
7 2 > P ] / P ~ > 1/7~
(4)
J. Gmehling et al./ Huid Phase Equilibria 103 (1995) 51-76
54
Table 1
Examples of the different types of binary azeotropic points
Type of azeotrope
Systems
Homogeneous, pressure maximum
Ethanol -water
Benzene cyclohexane
Acetone cyclohexane
Homogeneous, pressure minimum
Acetone chloroform
Methyl acetate chloroform
Water formic acid
Heterogeneous, pressure maximum
l-Butanol-water
Ethyl acetate water
Butyl acetate-water
Homogeneous, pressure maximum azeotrope
in a system with miscibility gap
2-Butanone water
2-Butanol water
Methyl acetate water
Double azeotrope
Benzene-hexafluorobenzene
Methyl acetate- butylene- 1,2-oxide
Diethylamine- methanol
Homogeneous, pressure minimum azeotrope
in a system with miscibility gap
Triethylamine-acetic acid
Hydrogen chloride-water
(2) In case of negative deviation from Raoult's law (pressure minimum azeotopes):
7~ < ps/p~ < 1/7~
(5)
It is clear from Eqs. (3a) and (3b) that components with very different vapor pressures whose
mixtures exhibit strong deviation from ideal behavior, e.g. large positive or negative logarithms
of the activity coefficients, can form azeotropes.
Eqs. (4) and (5) are valid when the curves describing the activity coefficient ratios as a
function of concentration (see Eq. (3a)) do not exhibit either maxima or minima. However,
when either a maximum or a minimum occurs and Eqs. (3a) and (3b) are fulfilled, two
azeotropic points may exist 1. For example, two azeotropic points have been observed (Gmehling et al., 1977) for the system benzene-hexafluorobenzene within the temperature range
30-80°C.
Homogeneous binary azeotropic points can also occur in almost ideal systems, when the
vapor pressures of the two components are nearly equal. Thus, azeotropic behavior also occurs
in mixtures of two alkanes and two alcohols, even for isomers. For example, azeotropes have
been found in the nearly ideal system, 2,4-dimethylpentane 2,2,3-trimethylbutane, cyclohexane-2,4-dimethylpentane, cyclohexane-2,2,3-trimethylbutane, isopropanol-tert-butanol, etc.
Two azeotropic points can also occur in the case of strong real behavior in the vapor phase, as observed for the
system acetic acid-isobutyl acetate (Christensen and Olson, 1992).
55
J. Gmehling et al. / Fluid Phase Equilibria 103 (1995) 51-76
0-01,
Bancroft point
0.00.
-0.01-
-0.02.,
280
I
320
360
Fig. 2. Vapor pressure differenceas a function of temperature and Bancroft point for the system 1-propanol-water.
Large deviations from ideal mixing can also lead to partial miscibility. In most cases,
heterogeneous azeotropes are formed when miscibility gaps occur. However, in some binary
systems both homogeneous azeotropes and miscibility gaps are found at the same temperature.
In addition, in a very few interesting cases the unusual concentration dependence of the activity
coefficients can cause a homogeneous pressure minimum azeotrope and a miscibility gap in
another concentration range.
The different types of binary azeotropes are shown in Fig. 1. Examples of the different
azeotropic systems are given in Table 1. The nature of the azeotropes often depends greatly on
temperature (pressure). A reliable knowledge of the temperature (pressure) dependence is thus
required for process synthesis. The variables of the previous equations, i.e. vapor pressures and
activity coefficients, vary with temperature; in addition the activity coefficients are concentration
dependent. F r o m the Clausius-Clapeyron and Gibbs-Helmholtz equations (Gmehling and
Brehm, 1995) the variation of the azeotropic composition with temperature can be calculated
using the heat of vaporization and the partial heats of mixing of the compounds considered:
(~Y'~
=
OrJaz.T
(YlYz)az
RrZ[l -(Oyl/OXl)az]
( A h v . , - Ahv2 + ~2 - h-~)
(6)
at any composition in a binary system.
- ~
=
-- ( 8h Elax, )T
(7)
where Ahv.i is the molar heat of vaporization of component i and ~ is the partial molar excess
enthalpy of component i.
The value of (~y~/Oxl)az is smaller than unity in systems with positive deviation from ideal
mixing (7g > 1) and greater than unity in systems with negative deviations (7i < 1).
J. Gmehling et al. / Fluid Phase Equilibria 103 (1995) 51 76
56
Usually the heat of vaporization of a compound in the mixture is larger than its partial
molar excess enthalpy 2. The heat of vaporization depends mainly on the size and the polarity
of the compound. The magnitude of the heat of vaporization directly influences the slope
(dP~/dT) of the vapor pressure curve at any point. The different slope of the vapor pressure
curve can lead to a point where the vapor pressures of two components become identical. This
point is sometimes called the Bancroft point. At the Bancroft point (temperature) for totally
ideal binary systems el2 is equal to unity at any composition and separation by distillation is
impossible, even though no azeotropic point exists (pressure maximum or minimum). In
non-ideal systems at the Bancroft point (temperature) there will be at least one azeotrope in all
cases.
The binary mixture of water and propanol, compounds with large differences in heat of
vaporization, is an example of such a system and is illustrated in Fig. 2. At temperatures
below the Bancroft point the vapor pressure of water is higher than that of 1-propanol. At
temperatures above this point the opposite situation exists.
If real vapor phase behavior and Poynting corrections are neglected, the occurrence of the
azeotropes only depends on the activity coefficients and the vapor pressures. Azeotropes can
therefore only exist when the combination of these variables satisfies the conditions of Eqs.
(3)-(5). The system acetone-tetrachloromethane is an interesting example, for which the
variation of the azeotropic composition with temperature is shown in Fig. 3(a). The plots of
both the experimental and the azeotropic data predicted with modified U N I F A C (Gmehling et
al. (1993)) vary in a parabolic manner. In this system, azeotropy is observed and predicted only
100.
80.
0
tD
QQ
60.
"'17"'.
a~
40.
~9
[-~
20.
O.
0.90
~
i
0.92
I
i
~
0.94
I
0.96
i
0.98
i
1.00
Xl
Fig. 3(a)
2 At infinite dilution the values for the partial molar excess enthalpics often show values similar to Ahv.
J. Gmehling et al. / Fluid Phase Equilibria 103 (1995) 51- 76
57
0.74
0.70
0.66
0.62
0.58
,
O.
[
20.
(b)
~
i
40.
,,
Temperature
I
60.
I
80.
,
I00.
[°C]
400.
200.
o.
-200.
-400.
-600.
0.0
(c)
0.5
1.0
xI
Fig. 3. (a) Temperature dependence of the azeotropic composition for the system acetone( 1)-tetrachloromethane(2):
, modified U N I F A C ; Q, experimental data. (b) Logarithm of the vapor pressure ratio and predicted activity
coefficient ln~)'2 (modified U N I F A C ) as a function of temperature for the binary system acetone(l) tetrachloromethane(2). (c) Experimental and predicted hE data for the system acetone(l) tetrachloromethane(2): - modified U N I F A C .
58
J. Gmehling et al. / fTuid Phase Equilibria 103 (1995) 51-76
at high acetone concentrations (x~ > 0.94) within a limited temperature range (20-100°C). The
reason for this unusual behavior is the variation of the P~/P=2 and the 72/7, ratio with
temperature. In Fig. 3(b) the vapor pressure ratio calculated using the Antoine constants from
the Dortmund Data Bank is shown in logarithmic form together with the y~: values predicted
using the modified U N I F A C method. Following Eq. (4) for systems with positive deviation
from Raoult's law, azeotropic points can only occur when the 7~ value is larger than the ratio
of
This situation only occurs experimentally between about 20 and 100°C and is predicted to
occur between 25 and 80°C using the modified U N I F A C method. The observed behavior is
caused mainly by the variation of the 7j values with temperature, which using the GibbsHelmholtz equation
(8)
81/T) )p,x =-2
can be derived directly from experimental h E data. In Fig. 3(c) these data are given for the
temperature range 5-45°C. For CC14 negative (positive) partial molar excess enthalpies at
infinite dilution,/7~, are obtained for temperatures below (above) 25°C. Therefore the observed
y~: values decrease (increase) above (below) 25°C. The h E behavior as a function of temperature
is predicted at least qualitatively by the modified U N I F A C method (see Fig. 3(c)), so that the
strange composition dependence of the azeotropic points is also predicted by the modified
U N I F A C method. In ternary and higher systems the existence of azeotropic points is less
probable, since the requirements that all the K values and relative volatilities should be equal to
unity become harder to satisfy with an increasing number of components.
Table 2
Examples of the different types of ternary azeotropic points
Type of azeotrope
Systems
Homogeneous, pressure maximum
Ethanol-benzene cyclohexane
Ethanol-ethyl acetate-cyclohexane
Benzene cyclohexane 2-propanol
Homogeneous, pressure minimum
HF-H2SiF,,-water
Homogeneous, saddle point
Acetone-chloroform methanol
Acetone-chloroform-n-hexane
Water-formic acid-acetic acid
Heterogeneous, pressure maximum
Ethanol-water benzene
2-Butanone-water-n-octane
Dichloroethane water-formic acid
Homogeneous, pressure maximum azeotrope
in a system with miscibility gap
Ethanol-ethyl acetate water
Acetone-methanol-cyclohexane (T < 323 K)
2-Propanol water-nitromethane
J. Gmehling et al. / Fluid Phase Equilibria 103 (1995) 51 76
59
For homogeneous ternary systems at the azeotropic point the following objective function F
has to be fulfilled:
F = l ,2-11 + l ,3- II +
T
11- 0
(9)
With the help of Eq. (9) and non-linear regression methods the azeotropic compositions of
homogeneous ternary systems can be calculated using thermodynamic models (gE models,
equations of state, group contribution methods). A general form of Eq. (9) for predicting the
azeotropes in multicomponent systems is the following (Gmehling and Kolbe, 1992):
_
v minimum
(10)
i j> i
(At the azeotropic point, F = 0).
However, Eqs. (9) and (10) are not directly applicable for predicting heterogeneous
azeotropes. A different procedure including stability tests has therefore been developed
(Schmidtmann, 1984; Hel3berg, 1989).
As for binary systems, different types of azeotropes have also been observed for higher
systems. Examples of the different types of ternary azeotropes are given in Table 2.
3. Scope and capabilities of the azeotropic data bank
Compilations of azeotropic data have been available in book form for quite some time (Lecat,
1918; Lecat, 1949; Horsley, 1973; Ogorodnikov et al., 1971). The great disadvantage of these
compilations is that no computerized version is available. The new data bank described here is
computerized and offers evaluated up-to-date data. Moreover, reliable techniques for analysis
and predictions have been integrated.
The purpose of the development of software for evaluating and storage of azeotropic data was
the preparation of a comprehensive computerized data bank with evaluated data as part of a
software tool involving the synthesis of separation processes. Besides the above-mentioned
compilations, all the references (approximately 3800) from the VLE part of the Dortmund Data
Bank were evaluated (Gmehling et al. 1977). The list of references was supplemented by
references found from CAS on line searches. No data were taken from secondary literature. All
the references were copied and the data were taken directly from the originals. The azeotropic
data published were either measured in rectification columns with a large number of theoretical
stages and a negligible pressure drop or (in the case of binary systems) also derived from reliable
VLE data. In the case of binary systems, the accuracy of the azeotropic data obtained with the
different experimental methods is similar. For higher systems rectification is to be preferred. The
determination of azeotropic data from ternary VLE data can lead to very poor results (Tamir
and Wisniak, 1978), especially when only a limited number of data of poor quality are used,
only a part of the concentration range is covered or too many parameters are used to correlate
the experimental data.
Various problems occurred during the evaluation of the references (published in approximately 20 different languages). Often the units of the data (mol% , wt.%) were not given, the
compounds were interchanged, or only qualitative information (greater than, smaller than,
60
J. Gmehling et al. / Fluid Phase Equilibria 103 (1995) 51 - 76
approximated values) was provided. In many cases not all the desired quantities (pressure,
temperature, composition) were given.
To minimize the number of errors, all azeotropic data were checked with the help of different
programs before storing, e.g. the azeotropic composition data for the binary systems provided
by different authors were plotted as a function of temperature. In the case of ternary systems the
azeotropic compositions for each system taken from the different sources were plotted in a
triangular diagram, whereby the experimental and predicted results were connected by a straight
line. Furthermore the azeotropic data (binary systems: T - y a z data; ternary systems: Yi.az in a
triangular diagram) were compared graphically with the results predicted using the group
contribution methods U N I F A C and modified UNIFAC.
For this purpose we used a program package (Schmidtmann, 1984; HeBberg, 1989) which
allows the calculation of homogeneous and heterogeneous azeotropes using the different gE
models (Wilson, NRTL, UNIQUAC) and group contribution methods taking into account the
real vapor phase behavior (chemical theory in the case of carboxylic acids). The output of this
package provides the comparison in the form of tables or plots. At the same time the assigned
types of azeotrope were checked.
Additionally the temperatures and pressures given for the azeotropes were checked for
every system (binary and ternary) by a linear regression analysis of the log P~ versus l I T values
when at least three, T, P data were available. Finally azeotropic data (more than 300 values)
were measured using a wire band column to check the differing statements in the various
references.
As a result of the evaluation procedure, questionable data were either removed or a poor
quality code was given to the stored data point. With the help of the pure component properties
(e.g. vapor pressures) the results of the group contribution methods and the LLE data stored in
the Dortmund Data Bank, most types of azeotrope could be assigned, at least for binary
systems.
120.
100.
Q)
80.
Q)
~
60.
40.
~
0.0
1
0.2
/
I
0.4
0.6
0.8
1.0
XI
Fig. 4. Experimental and predicted azeotropic data for the system 1-butanol(l) n-butyl acetate(2) from different
authors as a function of temperature: - - , modified UNIFAC; O, experimental.
J. Gmehling et al. / Fluid Phase Equilibria 103 (1995) 51-76
61
WATER
BENZENE
2 - PROPAN OL
Fig. 5. Published and predicted azeotropic data for the system2-propanol water benzenefrom differentauthors: O,
experimental data; [], modified UNIFAC.
The results of the different evaluation procedures are given in the following figures. In Fig. 4
the azeotropic composition for the system 1-butanol-butyl acetate as a function of temperature
is shown together with the results obtained using the modified U N I F A C method. It can be seen
that most of the data provided by the different authors are in good agreement. There is a little
disagreement for the data at approximately 390 K (atmospheric pressure) and one strongly
deviating point. This experimental data point, after a new check of the original reference, was
given a poor quality code. In this way this point can be neglected during process synthesis. Fig.
4 also shows the suitability of the modified U N I F A C method for testing the data.
Fig. 5 shows the published ternary heterogeneous azeotropic points for the system 2propanol water-benzene along with the results predicted by the modified U N I F A C method. It
can be seen that most experimental data are in good agreement with the predicted values. This
type of diagram can be used to determine whether the azeotropic compositions given by the
different authors are reliable. At the same time errors in the literature (wrong compound name,
wrong composition units (wt.% or mol.%)) can be discovered.
In addition the consistency of the data was checked with the help of graphical presentations
of the logarithm of pressure versus l I T . Fig. 6 shows the results for the system 1-butanol-butyl
acetate together with the results of the modified U N I F A C method. Similar results are shown in
Fig. 7 for the ternary system 2-propanol- water- benzene. This test procedure was also applied
automatically for all systems with more than three entries by using linear regression. By
evaluation of the pressure deviation of the regression results it was even possible to find wrong
62
J. Gmehling et al. / Fluid Phase Equilibria 103 (1995) 51 76
2.2
2.0
1.8
1.6
1.4
1.0
0.8
06
2.5
~
2.6
2.7
~
2.8
2.9
1 0 0 0 / T [KI
3.0
3.1
Fig. 6. Logarithm of pressure as a function of 1/T for the azeotropic data stored for the system 1-butanol n-butyl
acetate: - - - , modified UNIFAC; 0, experimental.
3.0
2.6
~
2.2
1.4
1.0
L
2.4
t
i
I
i
J
i
r
i
2.6
2.8
3.0
3.2
3.4
1000/ T [K]
Fig. 7. Logarithm of pressure as a function of l i T for the azeotropic data stored for the system 2-propanol-waterbenzene: - - , modified UNIFAC; O, experimental.
c o m p o u n d names in the original reference. In Fig. 8 the reported experimental azeotropic data
for the system 2-butanone(1) water(2) are shown together with the experimental L L E data
stored in the D o r t m u n d D a t a Bank; the results predicted by the modified U N I F A C m e t h o d are
also shown. It can be concluded from Fig. 8 that the system 2-but anone water forms only
h o m o g e n e o u s azeotropes in the t em pe r at ure range where the miscibility gap occurs. This
diagram demonstrates again the predictive capabilities o f the modified U N I F A C method.
63
J. Gmehling et al. / Fluid Phase Equilibria 103 (1995) 51 76
200.
!
[
"l |
I
I
!
150.
o
o~
ii
100.
50.
%%%
o~
O.
!
--50.
I
I
0.0
0.5
1.0
X1
Fig. 8. Experimental and predicted (modified UNIFAC) LLE and azeotropic data for the system 2-butanone(l) water(2) as a function of temperature: O, experimental LLE data; II, experimental azeotropic data; - - - , modified
UNIFAC;
, UNIFAC.
Table 3
Present status of the Dortmund Data Bank
Pure component properties:
For approx. 4300 compounds
References for each type of data XXX:
XXX = VLE, LLE, HE, ACT, GLE, CPE, AZD, SLE, ELE
Approx. 12000 references
Data on mixtures:
Vapor liquid equilibria (VLE)
Liquid liquid equilibria (LLE)
Heats of mixing (HE)
Activity coefficients at infinite dilution (ACT)
Gas solubilities (GLE)
Excess heat capacities (CPE)
Number of Isotherms or Isobars:
15250
8500
9700
29300 values
6800
720
Integrated:
Azeotropic data (AZD)
Solid liquid equilibria (SLE) "
VLE of electrolyte systems (ELE)
36000 values
2800
1200
" Started January 1992.
4. Exploitation of the VLE part of the Dortmund Data Bank
T h e D o r t m u n d D a t a B a n k ( D D B ) is the largest c o m p u t e r i z e d c o m p i l a t i o n o f p h a s e equilibrium d a t a in the world. It was started in 1973 at the U n i v e r s i t y o f D o r t m u n d to use the vast
64
J. Gmehling et al. / Fhdd Phase Equilibria 103 (1995) 51 76
amount of available phase equilibrium data for the development of reliable predictive methods
with a large range of applicability. It has since been successfully used for fitting the required
group interaction parameters in the different group contribution methods (Hansen et al. 1991,
Tochigi et al., 1990; Holderbaum and Gmehling 1991; Fischer, 1993; Gmehling et al. 1993). This
data bank is now continuously updated by the engineering and software company DDBST
GmbH. The present status of the D o r t m u n d Data Bank is given in Table 3. In order to build
up the data bank for azeotropic data, all the references (approximately 3800) from the VLE data
bank were evaluated in addition to the references from the different compilations of azeotropic
data, CAS on-line etc. to complete the azeotropic and zeotropic information in the computerized
data bank. In all cases the information given by the authors was used. When no information
about azeotropy for the binary system was given, the VLE data given in the reference were
evaluated by us. For binary systems a program package was developed which allows the
calculation of the azeotropic points from the experimental data. In all cases the agreement
between calculated and experimental data was checked by graphical representations, whereby
the composition range of interest was enlarged. Only in the case in which enough experimental
data close to the azeotropic point were available, was the value obtained by this evaluation
procedure stored in the data bank. The information about the evaluator (original author,
evaluator) was also stored. In total, approximately 5100 additional pieces of information on
binary systems were obtained by this procedure.
5. Experimental
Experimental data were recorded for the purpose of verifying questionable literature information and also for systems for which no data had been available.
For homogeneous azeotropes with a pressure maximum (up to a pressure of 3.5 atm) a wire
band column made from glass (supplier: N O R M A G AG, Hofheim, Germany) was generally
applied. In this column which has nearly 45 theoretical stages, the binary mixture (about 30 cm 3)
of the purified chemicals with estimated azeotropic composition is distilled at constant pressure
and a small pressure drop at nearly total reflux for approximately 60 min. Then approximately
2 cm 3 of the distillate is withdrawn and analyzed with the help of gas chromatography. At the
same time the temperature is measured with a mercury thermometer. The results were only
accepted and stored in the data bank when with different feed compositions the same results
were obtained within the error limits of the analytical method. In Table 4 the results are given
for different ethers (MTBE, ETBE, TAME, TAEE, IPTBE) which are produced by a reaction
of the C3, Ca or C5 fractions with the light alcohols (methanol, ethanol, isopropanol). All these
ethers are used as gasoline additives to increase the octane number and to reduce the CO
emission. Furthermore the data are given for those ethers with the alcohols, tert-butanol and
2-methyl-2-butanol, which are produced in a side reaction of the olefin with water. In Fig. 9 the
measured azeotropic points for the ethers with the light alcohols are shown as function of
temperature. Because of the higher heats of vaporization of the alcohols compared with the
ethers the concentration of the alcohol increases in all cases with increasing temperature
(pressure) according to Eq. (6).
A knowledge of the azeotropic composition as a function of pressure is especially important
for pressure swing distillation, for a reliable column design and for checking the influence of
J. Gmehling et al. / Fluid Phase Equilibria 103 (1995) 51- 76
65
Table 4
Experimental azeotropic data for different ether alcohol systems
System"
Temperature
(°C)
Pressure
(mmHg)
x~
MTBE( 1 ) -methanol(2)
50.8
40.0
27.0
23.0
753.8
499.3
295.2
246.3
0.6944
0.7341
0.7865
0.7994
-
762.0
298.0
None
None
ETBE(l)-ethanol(2)
66.8
49.5
32.7
26.1
762.9
397.7
194.3
144.0
0.6273
0.6943
0.7602
0.7855
ETBE(1)-tert-butanol(2)
69.7
63.6
52.2
34.7
27.5
760.9
603.0
395.9
196.8
143.6
0.7488
0.7839
0.8355
0.8946
0.9151
IPTBE(1) -isopropanol(2)
76.8
76.4
59.5
52.5
42.7
27.9
26.8
770.3
755.3
396.9
298.8
195.6
98.2
91.7
0.4694
0.4678
0.5546
0.5856
0.6451
0.7170
0.7284
I PTBE( 1) - tert-butanol(2)
77.75
60.5
43.8
772.1
397.4
195.7
0.4610
0.5698
0.6768
TAME( 1) -methanol(2)
62.4
46.2
30.7
24.5
762.7
397.7
196.6
146.0
0.2265
0.2597
0.2925
0.3057
TAME( 1)- 2-methyl-2-butanol(2)
-
753.2
97.8
None
None
TAEE( 1) -ethanol(2)
76.2
53.3
29.6
761.7
300.3
96.3
0.2356
0.3048
0.3729
TAEE(I) 2-methyl-2-butanol(2)
96.7
77.0
58.8
28.7
754.0
399.8
198.1
49.9
0.6096
0.6991
0.7797
0.8830
MTBE( 1) - tert-butanol(2)
a Abbreviations: MTBE, methyl tert-butyl ether; ETBE, ethyl tert-butyl ether; IPTBE, isopropyl tert-butyl ether;
TAME, tert-amyl methyl ether; TAEE, tert-amyl ethyl ether.
66
J. Gmehling et al. / Fluid Phase Equilibria I03 (1995) 5I 76
80.
70.
0
-
[]
60.
5o.
[]
40.
[-.
[]
30,
O
•
[]
20.
,
0.0
I
,
0.2
i
;
0.4
I
0.6
,
~
L
0.8
1.0
Xl
Fig. 9. New experimental azeotropic data for the following systems: 0, MTBE(I) methanol(2); I , ETBE(1);
ethanol(2): A, TAME(l) methanol(2); O, TAEE(I) ethanol(2); [~, IPTBE(I) isopropanol(2).
Table 5
Present status of the data bank for azeotropic data
Number of components
2
3
4
Zeotropic data
Azeotropic data
18543
15441
670
1230
45
80
19258
16751
Total
33984
1900
125
36009
Total number of systems
Number of aqueous systems
17662
486
1179
433
90
30
18931
949
Number of references: 4595
Number of compounds: 1682
pressure on the azeotropic composition (e.g. water cont ent in the heterogeneous azeotrope
a l c o h o l - w a t e r - h y d r o c a r b o n ) and the distillation lines in m u l t i c o m p o n e n t systems.
Experimental azeotropic data for systems with a pressure m i n i m u m azeotrope (e.g. 2p r o p a n o l - t e r t - b u t a n o l ) and data for higher pressures were derived from very precise P , x data
measured with the help o f a static ebulliometer ( R a r e y and Gmehling, 1993).
6. Status and structure o f the data bank
T h e present status o f the data bank for azeotropic data is given in Table 5. In total 36 009
zeotropic and azeotropic data are stored f r om 4595 references. A p p r o x i m a t e l y 48% o f the stored
J. Gmehling et al. / Fluid Phase Equilibria 103 (1995) 51- 76
67
160
140
120
O)
100
80
0
e
60
~
40
Z
20
1860
1880
1900
1920
(a)
1940
1960
1980
2000
Year
20.
15.
--
10.--
.m
o.
i
1860
(b)
1880
1900
I
I
I
I
1920
1940
1960
1980
2000
Year
Fig. 10. (a) Number of references as function of the publication year. (b) Percentage of published azeotropic data as
function of the publication year.
data show azeotropic behavior. About 18 times more data are available for binary systems than
for ternary systems. The number of available azeotropic data again decreases by a similar factor
when going from ternary to quaternary systems. More than 3700 of the data stored involve
water. Nearly 35% of the data stored in the data bank were published in the different papers of
Lecat.
68
J. Gmehling et al. / Fluid Phase Equilibria 103 (1995) 51-76
More than 90"/0 of the binary azeotropic data show a pressure maximum. In most cases
( > 80%) these are homogeneous azeotropes and in approximately 7 8°/,, of the cases heterogeneous azeotropes are reported. Less than 10% of the data stored show a pressure minimum.
As already mentioned, often only partial information (Yaz, T, P) is available. A large part of
the data were measured more than 40 years ago. The number of references and the number of
data as a function of the publication year is shown in Fig. 10. It can be concluded from Fig.
10(a) that in the last 30 years, approximately 100-120 references contained azeotropic/zeotropic
information. When the number of azeotropic data is considered it can be seen that a great part
of the available azeotropic or zeotropic entries were already published between 1940 and 1950.
These "old" data were mainly published by Lecat. Approximately 13 000 data of the data stored
were published after the publication of the different data compilations for azeotropic data
(Horsley, 1973; Ogorodnikov et al., 1971 ).
7. Applications
Rectification processes are favored because of their great advantage in comparison to other
separation methods, often also for azeotropic systems. For the separation of azeotropic systems
a complete disappearance of the azeotropic point is not required. A strong temperature
dependence of the azeotropic composition, as in the case of tetrahydrofuran-water, can be used
to separate the system by pressure swing distillation. Also heterogeneous binary azeotropes can
easily be separated in two columns. A classical example for heterogeneous azeotropic distillation
is the system l-butanol-water.
However, in most cases the separation of azeotropic systems by rectification is realized in a
different way. In the case of extractive or azeotropic distillation the separation factor is
influenced by the addition of a selective solvent. The function of the solvent is the alteration of
the separation factor (activity coefficients) of the compounds to be separated. In the case of
extractive rectification a selective high boiling solvent is used to alter the activity coefficients in
such a way that the separation factor becomes very different from unity. In the case of
azeotropic rectification a solvent is selected which forms a lower boiling azeotrope which can be
easily separated. To allow an easy separation of the new azeotrope the formation of a
heterogeneous azeotrope (for example, dehydration of ethanol using a hydrocarbon such as
benzene, cyclohexane, n-pentane or toluene) as entrainer is most desirable. The formation of
homogeneous azeotropes can also be accepted in special cases, e.g. when with the help of a water
wash the azeotrope formed can be separated (for example, separation of benzene from
cyclohexane (aliphatics from aromatics) using acetone as an entrainer).
The main tasks of process synthesis are to choose the best suited separation process, the right
separation sequence and the best suited selective solvent. A knowledge of azeotropic points or
distillation lines as a function of pressure and the selection of selective solvents are of particular
importance for the synthesis of rectification processes. Thermodynamic models - - especially
group contribution methods - - and sophisticated software tools for the analysis of phase
equilibrium behavior, the selection of selective solvents, and the calculation of distillation lines
are usually applied for this purpose. A data bank for azeotropic data expands the available
software tools for the optimal synthesis and design of separation processes, since it permits a
69
J. Gmehling et al. / Fluid Phase Equilibria 103 (1995) 51- 76
160.
oo
140.
120.
o
o
100.
o
•
80.
~
60.
40.
o
20.
O.
~
L
,
0.2
0.0
L
, e\~
0.4
0.6
1.0
0.8
xl
Fig. ll. Experimental and predicted azeotropic data for the systems acetonitrile(l) water(2) (Q) and acetone( 1)methanol(2) (©) in the temperature range 0 160°C. ( - - , modified UNIFAC.)
200.
•
G 150.
o
100.
~
50.
0.
0.6
J
i
0.7
,
I
0.8
,
{
0.9
,
1.0
Xl
Fig. 12. Experimental and predicted azeotropic data for the systems ethanol(l) 1,4-dioxane(2)(©) and acetone(l)
water(2) (O). - - , modified UNIFAC.)
careful examination of the calculated results. Using the stored azeotropic data it can be decided
directly whether pressure swing distillation can be used as an alternative separation process. Fig.
11 shows the experimental data for the systems acetonitrile-water and a c e t o n e - m e t h a n o l
together with the predicted results using the modified U N I F A C m e t h o d for the temperature
70
J. Gmehling et al. / Fluid Phase Equilibria 103 (1995) 51-76
oe,~
100.
80.
60.
°°
~
40.
~
-
20.
0.
0.4
~
I
0.5
(a)
i
I
L
0.6
0.7
O
0.8
xl
4.
3,
~ 2 . ~
1.
2.8
(b)
3.0
3.2
3.4
3.6
IO00/T [X]
Fig. 13. Experimental and predicted azeotropic compositions and pressures for the systems benzene(1)-cyclohexane(2) ( O ) and acetone(1)-cyclohexane(2) (©) in the temperature range 10-90°C ( - - , modified UNIFAC).
range 0-160°C. As can be seen, a strong temperature dependence of the azeotropic point is
observed for these systems; this means that separation by pressure swing distillation can be
applied as an alternative to other separation processes. Furthermore it can be seen that reliable
azeotropic data are obtained with the help of the modified U N I F A C method.
In Fig. 12 the azeotropic data for the systems acetone-water and ethanol-l,4-dioxane are
shown as a function of pressure. For these systems no azeotropic behavior is observed at
temperatures below 75°C (about 260 kPa) (above 75°C, 170 kPa), so that separation can be
carried out by ordinary distillation without any problem at lower (higher) pressures. Again the
behavior is predicted with the required accuracy by the modified U N I F A C method. Sometimes
the occurrence of an azeotrope can also be utilized to remove undesired compounds. The
selection of selective solvents for azeotropic distillation is particularly simple with the help of a
computerized bank of azeotropic data. The selective solvent can be found by a simple search
J. Gmehling et al. / Fluid Phase Equilibria 103 (1995) 51-76
71
3.5
3.0
~
2.5
~ 2.0
1.5
1.0
2.2
2.4
2.6
2.8
3.0
3.2
3.4
1 0 0 0 / T [K]
Fig. 14. Experimental and predicted pressures of the azeotropic points for the systems ethanol-water (©) and
ethanol-water-benzene (O) in the temperature range 25-180°C ( - - , modified UNIFAC).
WATER
308
0.29
K ~ T
bar
~ P
445
K
- -
ETHANOL
308
445
19.37
K
bar
K
BENZENE
Fig. 15. Experimental and predicted azeotropic points for the system ethanol-water-benzene in the temperature
range 35-172°C: 0 , experimental data; [~, modified UNIFAC.
72
J. Gmehling et al. / Fluid Phase Equilibria 103 (1995) 51- 76
strategy. The only requirement is that in the binary case the solvent should show azeotropic
behavior with only one of the components to be separated. At the same time the boiling point
of the "new" azeotropic system should be below that of the azeotropic system to be separated.
The formation of a heterogeneous azeotrope would be advantageous, but the formation of
homogeneous azeotropes can also be utilized. Fig. 13 shows the azeotropic composition and P,
T relation for the system acetone cyclohexane and benzene-cyclohexane. From this diagram it
can be concluded that acetone can be used to separate the sytsem benzene-cyclohexane, since
acetone forms a lower boiling azeotrope with cyclohexane. The system benzene-cyclohexane can
be considered as a typical system for the separation of aromatics from aliphatics. However, there
is the disadvantage that a homogeneous azeotrope is formed. But with the help of a water wash
the recovery of acetone is no problem.
Fig. 14 shows the results for the separation of ethanol and water with the help of benzene. It
can be seen from the log P / ( 1 / T ) representation that benzene forms a ternary lower boiling
heterogeneous azeotrope with ethanol and water, which can be used as a selective solvent for
azeotropic distillation.
From Fig. 15, which shows the azeotropic composition as a function of temperature (pressure),
it can furthermore be concluded that for this separation problem the azeotropic composition
becomes even more advantageous at higher pressures, since the water content in the azeotrope
is enlarged with increasing pressure. This also allows a heat integration of the different columns,
whereby it becomes very advantageous when the azeotropic column works at higher pressure.
In the case of extractive distillation a selective high boiling solvent is utilized which alters the
activity coefficients in such a way that the separation factor becomes very different from unity.
Group contribution methods again can be applied to investigate the selectivity of the various
solvents. Activity coefficients at infinite dilution are also extremely helpful. Fig. 16 demonstrates
the selective influence of N-formylmorpholine ( N F M ) on the separation of aromatics from
aliphatics. Again the system benzene cyclohexane has been chosen for the examination of the
3.50
3.00
2.50
L 2.00
1.50
1.00
.~
•
•
_9
-v
~
I
3.00
-
•
o_
""
qb
•
0.50
0.00
2.60
i
I
2.80
i
3.20
3.40
IO00/T [K]
Fig. 16. Experimental activity coefficientsat infinite dilution for the systems benzene(i)-NFM (O) and cyclohexane(i)-NFM (©) in the temperature range 25-100"C.
J. Gmehling et al. / Fluid Phase Equilibria 103 (1995) 51 76
73
E
AB
AB ~
~B
Extractive
Distillation
Distillation
with Electrolytes
Pressure-Swing
Distillation
Separation of Azeotropic and
Close-boiling Mixtures
(A-B)
AB
r
~S
Heterogeneous
Azeotropic
Distillation
Pressure-Swing
Distillation with
an E n t r a i n e r
lzeotropic Distillation
Using an Entrainer
Fig. 17. Different possibilities for the separation of azeotropic or close-boiling mixtures.
selectivity. It can be concluded from the diagram the N F M is suitable for the separation of
aromatics from aliphatics by extractive distillation.
The application of the methods mentioned above can also be recommended for the synthesis
of rectification processes, since the separation problems that occur can be discovered. In Fig. 17
the different possibilities for separating azeotropic close-boiling mixtures are shown as simplified
process flow sheets. For completeness, distillation with the aid of salts is also mentioned.
Although these components have not been considered in the data bank up to now, it is intended
that they will be included in future.
A computerized data bank with azeotropic data also allows us to check the model parameters
used for process simulation. In Table 6 the experimental azeotropic data are given together with
the results of this procedure using modified U N I F A C for the quinary system acetone-chlorof o r m - m e t h a n o l - e t h a n o l - b e n z e n e and the binary, ternary and quaternary subsystems. It would
also be advisable to use the procedure to check the quality of the parameters derived for process
simulation.
The calculation of a non-existing azeotropic point with the gE-model parameters used for
process simulation can cause great problems and lead to erroneous results during process
synthesis. A knowledge of all the azeotropic points as a function of pressure is especially
important for the synthesis of rectification processes.
74
J. Gmehling et al. / Fluid Phase Equilibria 103 (1995) 51 76
Table 6
Experimental and predicted azeotropic data for the system acetone(l) chloroform(2)-methanol(3) ethanol(4)
benzene(5) at 760 mmHg
System
Experimental
(tool%)
Yl
1 2
1-3
1-4
1-5
2 3
2-4
2-5
3-4
3-5
4-5
1 2 3
1-2-4
1-2 5
1 34
61.2
44.7
31.7
34.5
n.a. b
1-3-5
n.a.
1-4 5
234
2-3-5
2 4 5
345
n.a.
n.a.
n.a.
n.a.
n.a.
1 2 3-4
n.a.
1-2-3 5
!-2-4-5
1-3-4-5
2-3-4-5
1-2-3-4 5
18.2
n.a.
n.a.
n.a.
n.a.
Predicted (modified UNIFAC)
(mol%)
)'2
Y3
)'4
)'1
34.8
78.6
_a
35.6
78.7
65.1
84.3
65.0
86.3
24.0
46.5
-
Y2
61.5
46.3
33.6
34.5
22.4
48.2
10.2
12.4
60.3
43.8
Y3
)'4
m
12.0
50.0
a no azeotrope.
b not available.
8. Conclusions
A c o m p u t e r i z e d d a t a b a n k for a z e o t r o p i c d a t a has been built up. This d a t a b a n k n o w
c o n t a i n s m o r e t h a n 36 009 sets o f e x p e r i m e n t a l i n f o r m a t i o n a b o u t the z e o t r o p i c a n d a z e o t r o p i c
b e h a v i o r o f b i n a r y a n d higher n o n - e l e c t r o l y t e systems. In c o m b i n a t i o n with the o t h e r p a r t s o f
the D o r t m u n d D a t a B a n k a n d the i n t e g r a t e d p r o g r a m p a c k a g e s it is the ideal s o f t w a r e tool for
the synthesis a n d design o f s e p a r a t i o n processes.
T h e s t o r e d a z e o t r o p i c i n f o r m a t i o n is available in p r i n t e d f o r m ( G m e h l i n g et al., 1994). In
f u t u r e the d a t a b a n k will be c o n t i n u o u s l y u p d a t e d b y D D B S T G m b H a n d will be used in
J. Gmehling et al. / Fluid Phase Equilibria 103 (1995) 51 76
75
connection with the phase equilibrium information stored in the Dortmund Data Bank for
fitting recommended gE-model parameters for process design and for the further development of
group contribution methods.
Acknowledgments
The authors thank the Ministry of Research and Technology (section 425) for financial
support. Furthermore the help of Dipl. Chem. L. Dallinga, Dr. S. Partzsch, Dr. M. Schiller, Dr.
H.M. Polka, Dr. I. Shulgin and other students in building up the data bank is gratefully
acknowledged. Additionally the authors are grateful to R. B61tz for the experimental work.
List o f s y m b o l s
Ahv
gE
hE
K/
P
p,~
R
T
VLE
Xi
Ys
molar heat of vaporization (J mol 1)
molar excess Gibbs energy (J mo1-1)
molar heat of mixing (J mol 1)
K-factor for component i
pressure (kPa)
vapor pressure of component i (kPa)
gas constant (J mol 1 K - l )
absolute temperature (K)
vapor-liquid equilibrium
mole fraction of component i in the liquid phase
mole fraction of component i in the vapor phase
Greek letters
(~/)"
7i
separation factor between components i and j
activity coefficient of component i
fugacity coefficient of component i
Subscripts
1, 2, 3, i, j
az
component
value at the azeotropic point
Superscripts
L
V
-
oc
liquid phase
vapor phase
partial property
value at infinite dilution
76
J. Gmehling et al. / Fluid Phase Equilibria 103 (1995) 51- 76
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