Graphical Representation of Liquid-Liquid Phase Equilibria

GBH Enterprises, Ltd.

Process Engineering Guide:
GBHE-PEG-MAS-614

Graphical Representation of
Liquid-Liquid Phase Equilibria

Information contained in this publication or as otherwise supplied to Users is
believed to be accurate and correct at time of going to press, and is given in
good faith, but it is for the User to satisfy itself of the suitability of the information
for its own particular purpose. GBHE gives no warranty as to the fitness of this
information for any particular purpose and any implied warranty or condition
(statutory or otherwise) is excluded except to the extent that exclusion is
prevented by law. GBHE will accept no liability resulting from reliance on this
information. Freedom under Patent, Copyright and Designs cannot be assumed.

Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown
Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass
Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance
Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals
Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com

Process Engineering Guide:

Graphical Representation of
Liquid-Liquid Phase Equilibria

CONTENTS
0

INTRODUCTION/PURPOSE

2

1

SCOPE

2

2

FIELD OF APPLICATION

2

3

DEFINITIONS

2

4

GRAPHICAL REPRESENTATIONS OF PHYSICAL
PROPERTIES

3

4.1 Use of Composition Diagrams
3
4.2 Ternary Systems with Immiscible Liquids
4.3 Graphical Design Using Ternary Diagrams

4
5

APPENDICES
A

INTERPOLATION AND CORRELATION OF THE LINES

10


FIGURES
1

TRIANGULAR CO-ORDINATES

3

2

TYPE 1 SYSTEM: ONE PAIR OF PARTIALLYMISCIBLE LIQUIDS 4

3

TYPE 2 SYSTEM: TWO PAIR OF PARTIALLYMISCIBLE LIQUIDS 5

4

DESIGN OF COUNTERCURRENT EXTRACTION SYSTEM
WITHOUT REFLUX – TYPE 1 SYSTEM

6

5

BLOCK DIAGRAM OF REFLUXED LIQUID-LIQUID EXTRACTION 7

6

DESIGN OF COUNTERCURRENT SYSTEM WITH REFLUX

8

7

CONSTRUCTION OF THE CONJUGATE LINE

11

DOCUMENTS REFERRED TO IN THIS PROCESS
ENGINEERING GUIDE

12


0 INTRODUCTIONS / PURPOSE
Current trends indicate that the process industry will need to meet tighter
standards on the use of energy and control of effluents in order to remain viable.
Liquid-Liquid Extraction may have an increasingly important role to play in
providing an economically acceptable solution to these demands. Liquid-Liquid
Extraction unlike distillation does not subject process material to high
temperatures and is therefore sometimes a more appropriate means of
separation when complex molecules are involved.
This Guide is one in a series of Process Engineering Guides concerning LiquidLiquid Extraction and has been produced under the auspices of GBH
Enterprises.
1 SCOPE
This Process Engineering Guide (PEG) describes the phase equilibria and mass
balance effects that make Liquid-Liquid Extraction possible and how these can
be represented in a graphical form.
2 FIELD OF APPLICATION
This Guide applies to the Process Engineering community in GBH Enterprises
worldwide.
3 DEFINITIONS
For the purposes of this Guide the following definitions apply:
Extract

This is the exit stream from the process being substantially Solvent
material into which the Solute has transferred.

Feed

This is the inlet stream to the unit in which the substance to be
extracted is originally dissolved.

Liquid-Liquid
Extraction
This is the unit operation by which a substance or substances may
be substantially passed from solution in one liquid to solution in
another by the contacting of the liquids. This process is also known
as Solvent Extraction.


Operating
Point
A point found on a ternary diagram from which countercurrent
Extract or Raffinate streams can be calculated by mass balance
considerations.
Raffinate

This is the exit stream from the process being substantially Feed
material from which the Solute has been transferred.

Solute

This is the substance or substances which are to be transferred
from the Feed.

Solvent

This is the second liquid phase fed to the process into which the
Solute is transferred. The Solvent must be substantially immiscible
with the Feed.

Tie Line

The line joining compositions represented on a ternary diagram that
can coexist in equilibrium with each other. A method of producing
this is given in Appendix A.

With the exception of terms used as proper nouns or titles, those terms
with initial capital letters which appear in this document and are not
defined above are defined in the Glossary of Engineering Terms.
4 GRAPHICAL REPRESENTATIONS OF PHYSICAL PROPERTIES
The simplest representation used for physical properties associated with
distillation is that of the binary diagram. This is either plotted as an X-Y
diagram with vapor compositions on the vertical axis and liquid
compositions on the horizontal axis or as an H-X diagram with
compositions shown horizontally and enthalpies shown vertically. For
Liquid-Liquid Extraction at least three components are present for the
system and these may be represented by a triangular or ternary diagram.
This PEG is concerned with explaining the use of such a diagram to
represent the physical system and outlining methods of calculation based
on the diagram.


4.1 Use of Composition Diagrams
Mixtures of organic chemicals are frequently presented in the form of an
equilateral triangle or ternary diagram. The length of the altitude of the
triangle is allowed to represent 100% composition and the perpendicular
distance of a point from a side represents component composition.
Composition can be measured in any convenient units, usually mass or
molar. Constant temperature and pressure are usually implied by these
diagrams. The effect of pressure on the system will be negligible unless it
is decreased to a level at which vaporization occurs. Increased
temperature, however, can have a marked effect, usually to decrease the
range of compositions for which two liquid phases form.
In Figure 1 the apexes of the triangle represent the pure components A, B,
and C respectively. Any point on the side of the triangle represents a
binary mixture so point D for example represents a mixture containing
60% B and 40% A. Points inside the triangle represent mixtures of all
three components such as point E which contains 40% A, 40% B and 20%
C.
FIGURE 1 TRIANGULAR CO-ORDINATES


There are other characteristics that should be noted. If F weight units of
the mixture represented by point F are added to G weight units of the
mixture at G, then the resulting mixture, H, will lie on the straight line FG
such that:

where HF and HG are the line lengths.

Thus all points on the line CD represent mixtures with constant ratios of A
to B with varying amounts of C. The nearer the point is to point C the
richer the mixture is in component C. Any mixture on the line CD from
which component C is subsequently removed will result in a mixture
whose composition is represented by point D.
4.2 Ternary Systems with Immiscible Liquids
The types of system of most interest in Liquid-Liquid Extraction are given
in Figures 2 and 3.
More complex diagrams do exist but are of limited importance.
4.2.1 Type 1 - One Pair of Partially Miscible Liquids
This common system is exemplified by the diagram shown in Figure 2.
Two of the components B and C are only partially miscible whilst
component A is completely miscible with the other two components.


FIGURE 2 TYPE 1 SYSTEM: ONE PAIR OF PARTIALLY MISCIBLE LIQUIDS

All mixtures of the components outside and on the curve DFPGE are
single liquid phase whereas mixtures falling within the curve will form two
liquid phases. The curve DFPGE represents the locus of saturated
solutions and is called the solubility or binodal curve.
A mixture of overall composition H will form two immiscible liquid solutions
of composition F and G. From the above, the ratio of F formed to G
formed will be in the same as the ratio of HG to HF. The straight line FHG
forms a Tie Line along which all mixtures will split to give two liquid phases
of composition F and G. Construction of Tie Lines is considered in
Appendix A. The area within the curve contains an infinite number of Tie
Lines of which only a few are shown. The Tie Lines are not normally
parallel and generally change their slope progressively in one direction
although systems where a reversal of slope occurs are known.
In Figure 2 Tie Lines shrink in length until they disappear at point P. This
is known as the plait point.


For a feed that is a mixture of A and C, a complete separation of A from C
is not possible to achieve by use of Solvent B. The best that can be
achieved is indicated by point G which on a Solvent-free basis gives point
X.
4.2.2 Type 2 - Two Pairs of Partially Miscible Liquids
This system is typified by the diagram shown as Figure 3. The two liquid
pairs A with B and B with C are partially miscible whereas C will dissolve
in any proportion with A.
FIGURE 3 TYPE 2 SYSTEM: TWO PAIRS OF PARTIALLY MISCIBLE LIQUIDS

The area within DEFG represents mixtures that form two liquid layers. The
composition of the layers formed are indicated by Tie Lines with the
proportion of each by the relative lengths as before. There is no plait point
in this type of system.
Unlike Type 1 systems it is possible to achieve a near complete
separation of component A from component C but refluxed operation as
indicated in GBHE-PEG-MAS-613 may be necessary.


4.3 Graphical Design Using Ternary Diagrams
Ternary diagrams can be used to design a Liquid-Liquid Extraction system
by graphical construction in a manner analogous to the Ponchon Savarit
or McCabe Thiele constructions used for distillation. These constructions
are presented below by means of two examples.
4.3.1 Example 1 - Simple Countercurrent Extraction
It is required to separate a Feed F containing 60% A and 40% B into a
Raffinate containing 10% A and an Extract containing 85% A on a
Solvent-free basis. Solvent S is to be used in a countercurrent extraction
system without reflux. Figure 4 shows the ternary diagram for A, B and S
at the required operating temperature, this is a Type 1 system. RN and E1
represent the final Raffinate and Extract products required from the
extraction.
FIGURE 4 DESIGN OF COUNTERCURRENT EXTRACTION SYSTEM
WITHOUT REFLUX - TYPE 1 SYSTEM


The method of calculation is as indicated below:
(a) RN and E1 are the products out of the extractor and F and S are the
Feeds to the extractor. From a mass balance the exit pairs must equal
the inlet pairs. From previous arguments this can only occur if the total
material flows involved in the extraction are represented by point X, the
intersection of lines FS and RNE1. Therefore, from a known Feed the
Solvent rate and Raffinate and Extract rate and compositions are
calculable. In distillation terms we have fixed product compositions,
rates and vapor traffic, all that remains is to see if the separation is
possible and in how many stages.
(b) In the distillation constructions an operating line for an X-Y diagram or
an Operating Point for an H-X diagram may be constructed by
consideration of stage wise mass balance. A similar construction is
possible for Liquid-Liquid Extraction. From the Feed and Extract end of
the extractor the Operating Point must lie on the line FE1 and from the
Raffinate and Solvent end of the extractor the Operating Point must lie
on the line RNS. These conditions can only be satisfied where the two
lines intersect, at point O.
(c) To check whether the separation is possible and in how many stages,
carry out the following:

(1) Start from E1 and find R1 by constructing the Tie Line R1E1.
(2) Join R1 to 0 to find E2 by mass balance.
(3) Find R2 by constructing the Tie Line R2E2.
(7) Find RN by constructing the Tie Line RNE4.
The construction has fortuitously found RN in exactly four stages. This would
not normally occur and a fractional number of stages would be necessary.
This is of no consequence though, since as in distillation, it is not easy to
construct equipment that gives an exact number of theoretical stages.
This type of extractor has been used by GBHE’s client, and is typified by the
installations on Higher Amines plant and on the Diphenyl Oxide plant.


4.3.2 Example 2 - Countercurrent Extraction with Reflux
Even with an infinite number of stages it is not possible to do better than
achieve an Extract that is in equilibrium with the incoming Feed. This is
particularly troublesome when the Feed is lean in Solute.
FIGURE 5 BLOCK DIAGRAM OF REFLUXED LIQUID-LIQUID EXTRACTION

These shortcomings can be overcome by use of reflux. As explained in
GBHE-PEG-MAS-613 Solute and Solvent may be separated from the
Extract and a proportion of the Solute returned to contact the Extract
phase in stages prior to introduction of the Feed. In order for this to be
possible a second liquid phase must be generated when the Solvent is
removed and the product contacted with Extract. For a Type 2 system this
will always be the case and from Figure 3 it would be possible to produce
almost pure A from a mixture of A and C by extraction with B. In Figure 4,
were the Feed more dilute in A.

It would be possible to obtain an extract phase richer in A by using reflux
than by use of simple countercurrent Liquid-Liquid Extraction but the
Extract must always contain sufficient B to cause phase separation to
occur and therefore production of nearly pure A is impossible.
For this example, a Feed of 40% A and 60% B is to be extracted using a
Solvent S. Reflux is to be supplied to the column as indicated in Figure 5.
There is an additional separation device assumed to be capable of
separating pure Solvent from a mixture of A and B leaving them
Solvent-free. The Raffinate is required to have a composition of 5% A on a
Solvent-free basis and the Extract a composition of 95% A on a Solventfree basis.
FIGUR E 6 DESIGN OF COUNTERCURRENT SYSTEM WITH REFLUX

The graphical calculation is presented in Figure 6 and takes the following route:
(a) RN, E1, F and S are located as indicated previously. The product, P
from the separation unit is found by extending line SE1 to meet the
side of the triangle.

(b)

The system is not uniquely defined as in the previous example
because there is a choice of how much reflux and hence Solvent is
to be used. The Operating Point X, for the portion of the Extractor
between the Feed and the reflux must lie on the line PS and must
be between S and E1. It is chosen such that a pinch will not occur
prior to the Feed being introduced. However the closer it is to the
point S the higher the Solvent usage and reflux rate.

(c)

Having chosen X the construction is as in the previous example:
(1) Start from E1 and find R1 by constructing the Tie Line R1E1.
(2) Join R1 to X to find E2 by mass balance.

(d)

The operating line for the section of the Extractor beyond the Feed
lies at the intersection of the lines RNS and FX at Y. Having
crossed the line FX with the Tie Line R4E4, the

Operating Point Y should be used. Continuing with X will make progress
towards the solution but will be inefficient. The construction now continues
as below:
(1) Join R4 to Y to find E5 by mass balance.
(6) Find RN by constructing the Tie Line RNE7.
The construction has fortuitously found RN in exactly seven stages. This
would not normally occur and a fractional number of stages would be
necessary. As in the previous example this is of no consequence. As in
distillation there will be a trade off between the number of stages and the
materials usage.


APPENDIX A INTERPOLATION AND CORRELATION OF TIE LINES
Knowing the shape of the binodal curve and the position of some Tie Lines, the
simplest method for the interpolation and correlation of other Tie Lines is that
based on the construction of a conjugate line. This method is outlined in Figure 7.
Through points C, D, E, F, G and C', D', E', F', G' lines parallel to the sides of the
triangle are drawn. The lines starting from the co-existing phases G and G'
intersect in T and T' respectively; those starts from F and F' in U and U', and so
on. An auxiliary line, the conjugate line, XWV. T'U' is thus obtained.
The point of intersection of the conjugate line with the binodal curve indicates the
position of the plait point.
The conjugate line being known, any Tie Line can easily be constructed. The
composition of the phase which coexists with an arbitrary phase N is found by
drawing consecutively NO parallel to AS and N'O parallel to AB. Thus N', the
phase in equilibrium with phase N, is found. The composition of N' could
equally be found with the aid of the upper part of the conjugate line.


FIGURE 7 CONSTRUCTION OF THE CONJUGATE LINE


Graphical Representation of Liquid-Liquid Phase Equilibria

Empfohlen

Empfohlen

Weitere ähnliche Inhalte

Was ist angesagt?

Was ist angesagt? (20)

Andere mochten auch

Andere mochten auch (7)

Ähnlich wie Graphical Representation of Liquid-Liquid Phase Equilibria

Ähnlich wie Graphical Representation of Liquid-Liquid Phase Equilibria (20)

Mehr von Gerard B. Hawkins

Mehr von Gerard B. Hawkins (20)

Kürzlich hochgeladen

Kürzlich hochgeladen (20)

Graphical Representation of Liquid-Liquid Phase Equilibria