Graphical Representation of Liquid-Liquid Phase Equilibria

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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.



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 4.3 Graphical Design Using Ternary Diagrams 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 Liquid-Liquid 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. (4) Join R2 to 0 to find E3 by mass balance. (5) Find R3 by constructing the Tie Line R3E3. (6) Join R3 to 0 to find E4 by mass balance. (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 Solvent-free 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. (3) Find R2 by constructing the Tie Line R2E2. (4) Join R2 to X to find E3 by mass balance. (5) Find R3 by constructing the Tie Line R3E3. (6) Join R3 to X to find E4 by mass balance. (7) Find R4 by constructing the Tie Line R4E4.

(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. (2) Find R5 by constructing the Tie Line R5E5. (3) Join R5 to Y to find E6 by mass balance. (4) Find R6 by constructing the Tie Line R6E6. (5) Join R6 to Y to find E7 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

