1.

Distillation for azeotropic mixture

Distillation is the most widely used separation technique in the chemical and petroleum
industry. However, not all liquid mixture are amenable to ordinary
fractional distillation. When the components of the system have low relative volatilities
(1.00 < < 1.05), separation becomes difficult and expansive because a large number of
trays are required and, usually, a high reflux ratio as well.
Both equipment and utilities costs increase markedly and the
operation can become uneconomical. If the system forms
azeotropes, as in a benzene and cyclohexane system, a
different problem arises, - the azeotropic composition limit
the separation, and for a better separation this azeotrope must be bypassed in some way.
At low to moderate pressure, with the assumption of ideal-gas model for the vapor phase,
the vapor-liquid phase equilibrium (VLE) of many mixture can be adequately describe by
the following Modified Raoults Law:

When i = 1, the mixture is said to be ideal Equation 1 simplifies to Raoults Law.

Nonideal mixtures (i 1) can exhibit either positive (i > 1) or negative deviations (i <
1) from Raoults Law. In many highly nonideal mixtures these deviations become so large
that the pressure-composition (P-x, y) and temperature-composition (T-x, y) phase
diagrams exhibit a minimum or maximum azeotrope point. In the context of the T-x, y
phase diagram, these points are called the minimum boiling azeotrope (where the boiling
temperature of the azeotrope is less than that of the pure component) or maximum boiling
azeotrope (the boiling temperature of the azeotrope is higher than that of the pure
components). About 90% of the known azeotropes are of the minimum variety. At these
minimum and maximum boiling azeotrope, the liquid phase and its equilibrium vapor
phase have the same composition, i.e.,
xi = yi for i = 1, , c (2)
Two main types of azeotropes exist, i.e. the homogeneous azeotrope, where a single
liquid phase is in the equilibrium with a vapor phase; and the heterogeneous azeotropes,
where the overall liquid composition which form two liquid phases, is identical to the
vapor composition.

Most methods of distilling azeotropes and low relative volatility mixtures rely on the
addition of specially chosen chemicals to facilitate the separation. The selection of the
separating agent will be discussed later.
The five methods for separating azeotropic mixtures are:
i.
ii.

iii.
iv.

v.

Extractive distillation and homogeneous azeotropic distillation where the

liquid separating agent is completely miscible.
Heterogeneous azeotropic distillation, or more commonly, azeotropic
distillation where the liquid separating agent, called the entrainer, forms
one or more azeotropes with the other components in the mixture and
causes two liquid phases to exist over a wide range of compositions. This
immiscibility is the key to making the distillation sequence work.
Distillation using ionic salts. The salts dissociates in the liquid mixture
and alters the relative volatilities sufficiently that the separation become
possible.
Pressure-swing distillation where a series of column operating at different
pressures are used to separate binary azeotropes which change appreciably
in composition over a moderate pressure range or where a separating agent
which forms a pressure-sensitive azeotrope is added to separate a pressureinsensitive azeotrope.
Reactive distillation where the separating agent reacts preferentially and
reversibly with one of the azeotropic constitutes. The reaction product is
then distilled from the nonreacting components and the reaction is
reversed to recover the initial component.

2. Residue Curve Maps

The most simple form of distillation, called simple distillation, is a process in which a
muticomponent liquid mixture is slowly boiled in an open pot and the vapors are
continuously removed as they form. At any instant in time the vapor is in equilibrium
with the liquid remaining on the still. Because the vapor is always richer in the more
volatile components than the liquid, the liquid composition changes continuously with
time, becoming more and more concentrated in the least volatile species. A simple
distillation residue curve is a graph showing how the composition of the liquid residue
curves on the pot changes over time. A residue curve map is a collection of the liquid
residue curves originating from different initial compositions. Residue curve maps
contain the same information as phase diagrams, but represent this information in a way
that is more useful for understanding how to synthesize a distillation sequence to separate
a mixture.
All of the residue curves originate at the light (lowest boiling) pure component in a
region, move towards the intermediate boiling component, and end at the heavy (highest
boiling) pure component in the same region. The lowest temperature nodes are termed as
unstable nodes (UN), as all trajectories leave from them; while the highest temperature
points in the region are termed stable nodes (SN), as all trajectories ultimately reach

them. The point that the trajectories approach from one direction and end in a different
direction (as always is the point of intermediate boiling component) are termed saddle
point (S). Residue curve that divide the composition space into different distillation
regions are called distillation boundaries. A better understanding of the residue curve map
may be view in Fig. 1. Notice that the trajectories move from the lowest temperature
component towards the highest.

Fig. 1 Residue curve map for a ternary mixture with a distillation boundary running from pure component
D to the binary azeotrope C.

Residue curve maps would be of limited usefulness if they could only be generated
experimentally. Fortunately that is not the case. Using various references, the simple
distillation process can be described by the set of equations:

Research studies have also been done to determine the relationship between the number
of nodes (stable and unstable) and saddle points one can have in a legitimately drawn
ternary residue plot. The equation is based on topological arguments. One form for this
equation is:
4(N3 - S3) + 2(N2 - S2) + (N1 - S1) = 1

Where:
Ni = number of nodes (stable and unstable) involving i species
Si = number of saddles involving i species
Many different residue curve maps are possible when azeotropes are present. Ternary
mixtures containing only one azeotrope may exhibit six possible residue curve maps that
differ by the binary pair forming the azeotrope and by whether the azeotrope is minimum
or maximum boiling.
Even though the simple distillation process has no practical use as a method for
separating mixtures, simple distillation residue curve maps have extremely useful
applications, such as:
i.
ii.
iii.

Testing of the consistency of experimental azeotropic data;

Predict the order and content of the cuts in batch distillation;
In distillation, to check whether the given mixture is separable by distillation,
identification of the entrainers / solvents, prediction of attainable product
compositions, qualitative prediction of composition profile shape, and synthesis
of the corresponding distillation column.

By identifying the limiting separation achievable by distillation, residue curve maps are
also useful in synthesizing separation sequences combining distillation with other
methods.

3. Homogeneous Azeotropic Distillation

The most general definition of homogeneous azeotropic distillation is the separation of
any single liquid-phase mixture containing one or more azeotropes into the desired pure
component or azeotropic products by continuous distillation. Thus, in addition to
azeotropic mixtures which require the addition of a miscible separating agent in order to
be separated, homogeneous azeotropic distillation also includes self-entrained mixtures
that can be separated without the addition of a separating agent.
The first step in the synthesis of a homogeneous azeotropic distillation sequence is to
determine the separation objective. Sometimes it is desirable to recover all of the
constituents in the mixture as pure components other times it is sufficient to recover only
some of the pure components as product. In our example, we would like to recover the
cyclohexane product at 90% purity and recycle the separating agent back to the initial
separating column for further use.
The second step is to sketch the residue curve map for the mixture to be separated. The
residue curve map allows one to determine whether the goal can be reached and if so how
to reach it, or the goal needs to be redefined.

Distillation boundaries for continuous distillation are approximated by simple distillation

boundaries. It is a good approximation for mixtures with nearly simple distillation
boundaries. For a separation to be feasible by distillation, the simple distillation boundary
should not be crossed, i.e. the distillate and bottom composition should lie in the same
distillation region. A more detail calculation method involving the composition will be
discuss in the later section.
In the most common situation, a separating agent is added to separate a minimum boiling
binary azeotrope into its two constituent pure components by homogeneous azeotropic
distillation. Michael F. D. and Jeffrey P. K. presented seven of the most favorable residue
curve maps for this task. Of the seven, the map representing extractive distillation is by
far the most common and the most important. Its corresponding residue curve and
column sequences are shown in Fig. 2 below.

4. Extractive distillation
Extractive distillation is defined as distillation in the present of a miscible, high boiling,
relatively nonvolatile component, the solvent, that forms no azeotropes with the other
components in the mixture. It is widely used in the chemical and petrochemical industries
for separating azeotropic, close-boiling, and others low relative volatility mixture.
Extractive distillation works because the solvent is specially chosen to interact differently
with the components of the original mixture, thereby altering their relative volatilities.
Because these interactions occur predominantly in the liquid phase , the solvent is
continuously added near the top of the extractive distillation column so that an
appreciable amount is present in the liquid phase on all of the trays below. The mixture to
be separated is added through second feed point further down the column. In the
extractive column, the component having the greater volatility, not necessarily the
component having the lowest boiling point, is taken overhead as a relatively pure
distillate. The other component leaves with the solvent via the column bottoms. The
solvent is separated from the remaining components in a second distillation column and
then recycled back to the first column.

Fig. 2 Extractive distillation with a heavy solvent

which introduce no new azeotrope for a minimum boiling
azeotrope. In some case, B can come off the top of the first column.

There are several industrial application for homogeneous azeotropic distillation listed in
the Encyclopedia of Separation Technology by Michael F. D., Jeffrey P. K.
Extractive distillations can be divided into three categories, each correspond to the
different residue curve maps, the minimum boiling azeotropes, maximum boiling
azeotropes and the nonazeotrope mixtures. Since our benzene-cyclohexane mixture to be
separated is of the second type of mixture, i.e. the minimum boiling azeotrope, we will
focus our attention on column sequencing this type of azeotropic separation method in
the following section.
As in azeotropic distillation, design of extractive distillation system will also requires
significant preliminary work including:

Choosing the solvent

Developing or finding necessary data, such as azeotropic condition or residue
curve
Preliminary screening
Computer simulation
Small scale testing

For our example, we will consider the first four steps.

5. Solvent screening and selection

5.1 Solvent criteria
One of the most important steps in developing a successful (economical) extractive
distillation sequence is selecting a good solvent. Approaches to the selection of an
extractive distillation solvent are discussed by Berg, Ewell et al. , and Tassions. In
general, selection criteria include the following :
i.
ii.
iii.
iv.
v.
vi.

Should enhance significantly the natural relative volatility of the key component.
Should not require an excessive ratio of solvent to nonsolvent (because of cost of
handling in the column and auxiliary equipment.
Should remain soluble in the feed components and should not lead to the
formation of two phase.
Should be easily separable from the bottom product.
Should be inexpensive and readily available.
Should be stable at the temperature of the distillation and solvent separation.

vii.
viii.
ix.

Should be nonreactive with the components in the feed mixture.

Should have a low latent heat.
Should be noncorrosive and nontoxic.

Naturally no single solvent or solvent mixture satisfy all the criteria, and compromises
must be reached.

5.2 Solvent screening

Perry's handbook serve as a good reference for the solvent selection procedure, which can
be thought of as a two-step process, i.e.:
5.2.1 First level: Broad screening by functional group or chemical family
i.
ii.

iii.

iv.

Homologous series: Select candidate solvent from the high boiling homologous
series of both light and heavy key components.
Robins Chart: Select candidate solvents from groups in the Robbins Chart (part of
the chart is shown in Table 3) that tend to give positive (or no) deviations from
Raoult's law for the key component desire in the distillate and negative (or no)
deviations for the other key.
Hydrogen-bonding characteristic: are likely to cause the formation of hydrogen
bonds with the key component to be removed in the bottoms, or disruption of
hydrogen bonds with the key to be removed in the distillate. Formation and
disruption of hydrogen bonds are often associated with strong negative and
positive deviations, respectively from Raoult's Law.
Polarity characteristic: Select candidate solvents from chemical groups that tend
to show higher polarity than one key component or lower polarity than the other
key.
5.2.2 Identification of individual candidate solvents

i.

ii.
iii.

Boiling point characteristic: Select only candidate solvents that boil at least 3040oC above the key components to ensure that the solvent is relatively nonvolatile
and remains largely in the liquid phase. With this boiling point difference, the
solvent should also not form azeotropes with the other components.
Selectivity at the infinite dilution: Rank the candidate solvents according to their
selectivity at infinite dilution.
Experimental measurement of relative volatility: Rank the candidate solvents by
the increase in relative volatility caused by the addition of the solvent.

Residue curve maps are of limited usefulness at the preliminary screening stage because
there is usually insufficient information available to sketch the them, but they are
valuable and should be sketched or calculated as part of the second stage of the solvent
selection.

6 The scenario of our distillation process

6.1 The azeotropic condition
For our example which deals with the azeotropic mixture formed between benzene and
cyclohexane, we have chosen extractive distillation (one of the homogeneous azeotropic
distillation methods). The reason of choosing this method is due to the availability of
information regarding this separation technique and its tendency to operate more
efficiently, i.e. in separating and recycling the separating agent. A brief discussion of the
process is given below.
After the mixture exited as the bottom product of the flash unit, it contains mostly our
desire product of cyclohexane and also a significant amount of unreacted benzene, which
is to be recycled back to the reactor for further conversion. Our main goal now is to
further separate the remaining components in the mixture. As cyclohexane and benzene
have been encounter most of the remaining composition with the mole % of 44.86 and
54.848 respectively (Table 1), we will consider this to be a binary mixture in our further
discussion.
From the process flowsheeting, we would like to operate the distillation column at the
pressure of 150 kPa. At this condition, cyclohexane and benzene will have boiling points
of 94.34oC and 93.49oC respectively (Fig. 3). This is a typical case where conventional
distillation would struggle to perform the separation of this type of close boiling mixture.
Thus, a special type of distillation technique, i.e. extractive distillation has been chosen in
order to purify the desire product, i.e. cyclohexane to our desired purity of 99.3%.
As can be shown from Fig. 3, this binary composition will form a minimum boiling,
homogeneous azeotrope at the temperature of 91oC and the corresponding composition at
this point will be 45.5 mole % for cyclohexane and 55.5 mole % for benzene (Fig. 4).

Table 1 Properties of Stream 26 (bottom product of Flash Drum (F13)

6.2 Solvent selection for benzene-cyclohexane binary mixture

In order to perform a successive extractive distillation, a solvent needs to be chosen to

"break" the azeotrope that forms at the operating pressure of the distillation column.
Recommended solvent for the benzene-cyclohaxane mixture from the literature,,, is
aniline, with a solvent to feed ratio (S/F) of 4, which will shift the azeotropic point
toward the corner of the high-boiling component cyclohexane, and the equilibrium curve
of the original components fall below the diagonal (Fig. 5).

As was stated in the above section, the primary goal of solvent selection is to identify a
group of feasible solvents to perform a good separation. The desired product, i.e.
cyclohexane should have a purity of above 99% to meet the market standard. Aniline was
the first solvent that had been put to the simulator to be tried out, as it is of the same
homologous group as benzene. As can be shown from the result in Table 2, this solvent
will produce the desire production rate of 150 with the solvent flow rate of 3500, i.e. a
S/F ratio of 9.85. However, the product purity can only reach 70.08% and this does not
meet our product specification. As a result, other solvent may have to be researched to
perform the desire separation.
We will have to perform the solvent selection criteria as stated in the preceding section.
At the column pressure of 150 kPa, cyclohexane and benzene boil at 94.34oC and 93.49oC
respectively and form a minimum-boiling azeotrope at 91oC. The natural volatility of the
system is benzene > cyclohexane, so the favored solvents most likely will be those that
cause the benzene to be recovered in the distillate. However, in order to get a better
quality of product, we would like to recover cyclohexane as the distillate rather than from
the bottom stream. Thus, solvent to be chosen should give positive deviations from
Raoult's law for cyclohexane and negative (or zero) deviation for benzene.

Table 2 Result from computer simulation using aniline as solvent.

Table 3 Part of Robbins Chart taken from Perry's Chemical Handbook.

Turning to the Robbins Chart (Table 3), we note that solvents that may cause the positive
deviation for cyclohexane (Class 12) and negative (or zero) to benzene (Class 11) came
from the groups of 4, 7, 8 and 9, which consist of polyol, amine and ether. We further
consider the solubility, the hydrogen bonding effect, and also the homologous
characteristic of the solvent with the corresponding components in the feed mixture. As
few candidate solvents that had been put to the computer simulation, included phenol
(homologous to benzene), 1,2-benzenediol (homologous to benzene, with -OH group that
will produce hydrogen bonding), 1,3-butanediol (with -OH group that will produce
hydrogen bonding), and also 1,2-propanediol (same characteristic as with 1,3-

butanediol). 1,2-propanediol (often known as propylene glycol), seem to give the most
promising results compared to the other solvents. This result may be caused from the high
solubility of benzene in this solvent and the hydrogen bonding that were formed between
the two constituents. Simulation result of this solvent can be view in Table 4.

6.3 Construction of the residue curve

Equation 3 and 4 were used to sketch the corresponding residue curve for the three
species. From the above information, we know that these species have boiling points at
94.34 (cyclohexane), 93.49 (benzene) and 200.35oC (propylene glycol) at the pressure of
150 kPa, and an azeotrope that boils at 91oC between the two more volatile species. As
were shown from Fig. 6 and Fig. 7 there were no new azeotropes formed between the
solvent 1,2-propanediol respectively with the another two component in the feed.

Table 3 Result from computer simulation using 1,2-propanediol as solvent.

We then start to sketch our residue curve map by sketching the triangular diagram in Fig.
8, and placing the arrows pointing from the lower to higher temperatures around the edge.
The corner points for benzene and cyclohexane are single species point, and both are
unstable nodes - all residue curves leave. The corner point for propylene glycol is a single
species point which is a stable node - all residue curve enter. All three are nodes; none are
saddles, thus;
N1 = 3 and S1 = 0

We then further assume that there will be no ternary azeotrope been form among the three
constituents, i.e.,
N3 = S3 = 0

The remaining steps here require the identification of the only binary azeotrope that form
between benzene and cyclohexane, to be either a node or a saddle point. From equation 4:
4(0-0) + 2(N2 - S2) + (3-0) = 1
2(N2 - S2) = -2
N2 - S2 = -1
Thus, the only way we can satisfy the above equation is letting N2 = 0 and S2 = 1, i.e. the
binary azeotrope is a saddle point, which directs the trajectories in another direction.

6.4 Column operation

The extractive distillation unit of this cyclohexane production plant consists of two
distillation columns (Fig. 10), which we can easily classify as direct sequence columns.
The first column acts as an extractive column where the solvent is introduced at the
second stage of the column, so that it will be present throughout the column and exits
with the bottoms. As were stated above, the solvent alters the natural volatility of the
binary mixture by forming hydrogen bonds with benzene and allowing it to be recovered
as the bottom product.
The bottom product of the first column will then fed to the second column, i.e. the
solvent recovery column, to undergo the normal distillation to separate both the
components for further usage, i.e. benzene being recycled to the reactor for further
conversion while solvent to the first column for reuse. The main operation parameter of
the distillation unit is shown in Table 4.

Table 4 Distillation unit summary

Fig. 10 Extractive distillation unit for cyclohexane production plant

References:
J. M. Smith, H. C. Van Ness, M. M. Abbott, 1996, Introduction to Chemical Engineering Thermodynamics,
McGraw Hill, p.449.
Douglas M. Ruthven (Ed.), 1997, Encyclopedia of Separation Technology, Vol. 1, John Wiley, [Distillation,

Azeotropic and Extractive by Michael F. D., Jeffrey P. K.]

L. T. Biegler, E. I. Grossmann, Arthur W. Westerberg, 1997, Systematic Methods of Chemical Process
Design, Prentice Hall, [Separating Azeotropic Mixture]
Abid.
Green, Perry, 1997, Perrys Chemical Engineers Handbook, 7th Edition, McGraw Hill,. [Section 13-78
Enhanced Distillation by J. D. Seader, Jeffrey J. S., Scott D. B.]
Philip A. Schweitzer, 1988, Handbook of Separation Technique for Chemical Engineers, 2nd Edition,
McGraw Hill, [Continuous Distillation: Separation of Multicomponent Mixture by Edward C. R., John E.
M.]
James R. F., Distillation.
Robert F. G., 1972, Extractive and Azeotropic Distillation, [Rapid screening of Extractive Distillation
Solvent. Predictive and Experimental Techniques by Tassios P. D.]
Green, Perry, 1997, Perrys Chemical Engineers Handbook, 7th Edition, McGraw Hill,. [Section 13-79
Enhanced Distillation by J. D. Seader, Jeffrey J. S., Scott D. B.]
James R. Fair, Distillation [Special Distillation]
Kith-Othmer, 1965, Encyclopedia of Chemical Technology, 2nd Edition, Vol. 6, John Wiley, [Cyclohexane
by James J.Kirk]
Lee L. E., 1999, Plant Design Project Cyclohexane Production, UTM, {Chapter 7 Discussion].

**This article was graciously submitted by Dominic Foo Chwan Yee for
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questions/comments at cyfoo98@pd.jaring.my