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Ternary critical point determination of experimental demixion curve: Calculation method, relevance and limits

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Abstract

In many cases of miscibility gap in ternary systems, one critical point at least, stable or metastable, can be observed under isobaric and isothermal conditions. The experimental determination of this invariant point is difficult but its knowledge is essential. The authors propose a method for calculating the composition of the invariant solution starting from the composition of the liquid phases in equilibrium. The computing method is based on the barycentric properties of the conjugate solutions (binodal points) and an extension of the straight diameter method. A systematic study was carried out on a large number of ternary systems involving diverse constituents (230 sets ternary systems at various temperatures). Thus the results are presented and analyzed by means of consistency tests.
Ternary critical point determination of experimental demixion curve:
calculation method, relevance and limits
C. Goutaudier
1
, F. Bonnet
2
, R. Tenu
1
, O. Baudouin
2
, and J.J. Counioux
1
1
Laboratoire des Multimatériaux et Interfaces, UMR 5615 Université Lyon 1/CNRS, 69622 Villeurbanne, France
2
Prosim SA, Immeuble Stratège A, 51 rue Ampère, F-31670 Labège, France
Abstract.
In many cases of miscibility gap in ternary systems, one critical point at least, stable or metastable,
can be observed under isobaric and isothermal conditions. The experimental determination of this invariant
point is difficult but its knowledge is essential. The authors propose a method for calculating the composition of
the invariant solution starting from the composition of the liquid phases in equilibrium. The computing method
is based on the barycentric properties of the conjugate solutions (binodal points) and an extension of the straight
diameter method. A systematic study was carried out on a large number of ternary systems involving diverse
constituents (230 sets ternary systems at various temperatures). Thus the results are presented and analyzed by
means of consistency tests.
1 Introduction
According to the Gibbs’ phase rule, the knowledge of all
the invariant transformations permits to predict
qualitatively each of the equilibrium states of a
multiphase system. Hence, the importance of their
determination implies the development of many methods
and techniques applied to their characterization.
However, among invariant equilibria, the critical
phenomena, which we find particularly by studying the
miscibility gap in a multicomponent system, occupy a
special place. This problem can be illustrated by a
demixing zone in the polythermal diagram or, more
simply, in an isotherm of a ternary system (figure 1). The
binodal curves and surfaces are continuous in a
mathematical sense but, thermodynamically separated by
a critical point or line. This isobaric and isothermal
invariant point (κ
T
) or monovariant line (κ−κ
T
) is difficult
to reach by the direct measurement. So, a calculation
method was developed and tested on numerous
isothermal ternary systems with large demixing zone. The
semi-empirical method consists in using the experimental
results for the different tie-lines. A database was
constructed from experimental data taken from the
National Institute of Standards Technology NIST [1].
The determination of each critical point was
conducted in two steps: the first one is based on a
generalization of the straight diameter method and the
second, on the exploitation of the modulus of the
experimental tie-lines.
This contribution presents the original method of
calculation derived from the rectilinear diameter law and
the analysis of results shows its limits and its relevance.
Figure 1. Shell defining the domain of liq-liq equilibrium in
the 3D representation of the ternary system A + B + C under
constant pressure (κ
T
: ternary invariant critical point).
2 Methodology
2.1 Database
A database was built, containing more than 90 ternary
systems and around 230 sets of experimental data at
various temperatures.
Experimental data have been extracted from NIST
database [1]. The selected systems have a wide
miscibility gap with a sufficient number of experimental
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DOI: 10.1051/
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01046 (2013)
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points. The required data are the compositions of the two
conjugated liquid phases, the temperature and if they are
available the coordinates of the invariant point.
The database is organized in order to use easily the
data to test the method in order to assess its reliability.
2.2 Straight diameter method
Originally, the formulation of the so-called law of
rectilinear diameter for the determination of the critical
volume of substances became in a very important tool for
researchers in the field of critical phenomena [2]. This
semi-empirical method consists in plotting the midpoints
of each tie-line, defining the associated straight line with
the better regression parameters, and then extrapolating
this latter until its intersection with the experimental
binodal curve [3]. However, this graphical method needs
a fairly good symmetry between both sides of the binodal
curve, which provides a good linearity of the midpoints.
In order to apply this property to unsymmetrical curves,
we have performed a barycentric balance of the two
conjugate points. For each tie-line (i), the barycentric
point P
i
satisfies the following relation:
(
)
2i1ii
LλLλ1P +=
(1)
where (1-λ)
and λ represent respectively the mass
fractions of the liquids L
1i
and L
2i
in equilibrium at the
study temperature. In the pedagogic system A+B+C
(figure 1), the components of the column vectors
i
P
,
1i
L
and
2i
L
are their mass fractions in the solvent A,
the constituent B (x-axis) and the constituent C (y-axis).
Figure 2. Graphical representation of the straight diameter
method in the ternary system A+B+C (isotherm and isobar).
The straight line D
λ
corresponds to the better regression
parameters for λ
min
.
For a series of tie-lines of the same isotherm, an error
function E is then calculated as function of the parameter
λ, which optimal value λ
min
corresponds to the minimum
of E ie the best fitting parameters. The equation of the
corresponding straight line D
λ
may
be written (figure 2):
WC = a WB + b (2)
where W
B
and W
C
are respectively the mass fractions of
the constituents B and C. Obviously a and b are the
calculated parameters of the straight line D
λ
. According
to the rectilinear diameter law, this line will cut the
binodal curve near the critical point.
2.3 Modulus method
The second step of calculation is based on a particular
property of the critical points: at this point indeed, the
internodal length becomes zero. Since each tie-line can be
identified by its intersection with the straight line D
λ
previously obtained by the diameter method, the length of
the tie-line can be considered as a function of the abscissa
or the ordinate of this point. As the variation law is
unknown, it was assumed that the raising of the
internodal distance to the α
th
power (L
α
) linearly varies
with the mass fractions W
B
or W
C
of the constituents B
and C. At each tie-line (i), a couple of values (W
Bi
, W
Ci
)
can be calculated by using the equation (2).
For the series of experimental tie-lines, an error
function E is then calculated as function of the parameter
α, which optimal value α
min
corresponds to the common
minimum of the E function, with W
B
or W
C
ordinates. In
the figure 3, we have plotted the modulus L of this length
at the power α
min
as a function of W
B
or W
C
. On a very
short distance, we have then extrapolated the straight
lines obtained up to the modulus zero. So this
extrapolation leads to the coordinates of the critical point
κ
T
.
Figure 3. Power α of the tie-line’s internodal distance L as a
function of the mass percentage of component B. Extrapolation
to zero length corresponds to one of the coordinates of the
critical point κ.
3 Results
The proposed method for the determination of the critical
point has been tested on 230 experimental ternary
systems having a liquid-liquid equilibrium area. It gives
consistent results in most cases, provided to have a
sufficient number of tie-lines.
Two error functions have been tested in order to find
the most suitable regression parameters for each step of
the calculation (λ and α). The first one was the classical
01046-p.2
39
th
JEEP – 19
th
- 21
st
March 2013 – Nancy
correlation coefficient R. The second one was defined as
follow:
(
)
(
)
=
i
BcalcBi
α
calc
α
i
ww
LL
N
E
1
or
(
)
(
)
i
CcalcCi
α
calc
α
i
ww
LL
N
1
(3)
where N is the number of experimental tie-lines.
As an example, figure 4 compares the changes of the
error functions calculated from the system 2-
Propanol+Toluene+Water. The 13 experimental tie lines
were determined by Washburn at 298K under
atmospheric pressure using refractive indices
measurements [4]. In this figure the variations are plotted
in normalized arbitrary unit and lead to the quite similar
minimum value.
Figure 4. Comparison of the changes of the error functions
calculated from the system 2-Propanol+Toluene+Water. (R and
E values are plotted in normalized arbitrary unit. Experimental
data from [4]).
In these conditions, the optimal fitting parameters for the
studied case are λ = 0.527 and α = 1.837.
Then in order to assess the reliability of the
calculation method, the coordinates of the critical point
are compared with those available in the literature or
calculated by classical method using binary interaction
parameters. Typical values are given in table 1 for the
system 2-Propanol + Toluene + Water at 298K and are in
a very good agreement.
Table 1. Coordinates of the critical point κ
T
calculated from
several methods (in molar fraction ; T=298K).
Method
2-
Propanol
Toluene
Water
NRTL 0.29 0.06 0.65
UNIQUAC 0.30 0.075 0.625
This work 0.289 0.058 0.653
In contrast, the method does not work in a number of
cases. Often critical analysis of the experimental results
provides the solution easily, as shown in figure 5 for the
ternary system Benzene + Hexane + Perfluorohexane. It
should check the experimental tie-lines in order to
remove aberrant lines intersecting.
Figure 5. An example of ternary system for which the method
of rectilinear diameter cannot work [1].
4 Conclusion
The knowledge of the ternary critical point of demixion
areas is essential for any definition of an operation unit
based on liquid-liquid extraction. The original calculation
method was assessed on a large number of experimental
data. The method derives from the rectilinear diameter
law on which a barycentric balance is applied. The
quality of the conjugated experimental points is the
dominant parameter.
References
1. M. Frenkel, R.D. Chirico, V. Diky, C.D. Muzny, A.F.
Kazakov, J.W. Magee, I.M. Abdulagatov, K.
Kroenlein, C.A. Diaz-Tovar, J.W. Kang, R. Gani,
NIST ThermoData Engine Version 7.0, Database
#103B, NIST Thermodynamics Research Center
(2011)
2. L.P. Cailletet, E. Mathias, C. R. Séances Acad. Sci.
102 1002 (1886); J. Phys. Théor. Appl. 5 549 (1886)
3. S. Reif-Acherman, Quim. Nova. 33 2003 (2010)
4. E.R. Washburn, A.E. Beguin, J. Am. Chem. Soc. 62
579 (1940)
01046-p.3
... Nevertheless, it has to be noticed that for liquid-liquid calculation, there is an extreme sensitivity to the excess Gibbs enthalpy (or activity coefficients), which means, among other things, that the quality of the calculation of the critical point highly depends on the quality of the fitting of the experimental tie-lines data with the model. Previous studies have resulted in a semiempirical determination method for this specific point of the binodal curve (Goutaudier et al., 2013 Greek letters barycentric weighting parameteř power of the internodal distance in the modulus method ı conode slope  arctangent ı Ä critical point rectilinear diameter method, allows us to get away from these disruptions and explains its success (Reif-Acherman, 2010). The rectilinear diameter law was originally formulated at the end of the 19th century by Cailletet, who studied the behaviour of fluids under high pressures and tried to determine the critical volume of liquefied pure substances Mathias, 1886, 1887). ...
Article
Demixing phenomena in liquid ternary systems are widely used in chemical engineering processes. When a critical point is observed, the composition of this invariant point of major interest is experimentally difficult to determine. A simple calculation method for the critical point is proposed using the experimental tie-lines. The computing treatment is based on the application of the barycentric weighting of the binodal points in order to extend the rectilinear diameter method and the exploitation of the modulus of the experimental tie-lines. A systematic study was carried out on a large set of ternary systems taken from DECHEMA and NIST. The results are compared with those available in literature or calculated by standard methods using binary interaction parameters. Very good agreements are obtained when the experimental tie-lines are located close to the critical point.
Article
Full-text available
The formulation of the so-called law of rectilinear diameter for the determination of the critical volume of substances in the concluding decades of the nineteenth century became in a very useful and acceptably exact alternative tool for researchers in the field of critical phenomena. Its corresponding original expression, and even those of its early few modifications, were so mathematically simple that their use did not limit to exclusively contribute to remove the by then experimental obstacle for the estimating of this critical parameter, but also extended along several decades in the increasing applications of the principle of corresponding states.
  • S Reif-Acherman
S. Reif-Acherman, Quim. Nova. 33 2003 (2010)
  • E R Washburn
  • A E Beguin
E.R. Washburn, A.E. Beguin, J. Am. Chem. Soc. 62 579 (1940)
  • L P Cailletet
  • E Mathias
L.P. Cailletet, E. Mathias, C. R. Séances Acad. Sci. 102 1002 (1886);