Liquid Liquid Extraction - Basic Principles

GBH Enterprises, Ltd.

Process Engineering Guide:
GBHE-PEG-MAS-613

Liquid-Liquid Extraction: Basic
Principles

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Process Engineering Guide:

Liquid-Liquid Extraction: Basic
Principles

CONTENTS

SECTION

0

INTRODUCTION/PURPOSE

2

1

SCOPE

2

2

FIELD OF APPLICATION

2

3

DEFINITIONS

2

4

LIQUID-LIQUID EXTRACTION PROCESS

3

4.1
4.2
4.3
4.4
4.5
4.6

General
Choice of Solvent
Principles of Extraction
Liquid-Liquid Extraction Equipment
Operating Modes
Operating And Design Pitfalls

3
3
4
5
6
6

5

BIBLIOGRAPHY

9

FIGURES
1

SCHEMATIC OUTLINE OF LIQUID-LIQUID EXTRACTION
PROCESS: COUNTERCURRENT FLOW IN ROTATING
DISC CONTACTOR
8

2

COUNTERCURRENT EXTRACTION WITH REFLUX

DOCUMENTS REFERRED TO IN THIS PROCESS
ENGINEERING GUIDE

8

10


0

INTRODUCTION/PURPOSE

Current trends indicate that the process industry will need to meet tighter
standards in the use of energy and in the 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. It is also a unit
operation which, unlike distillation, does not subject process material to high
temperature and is therefore sometimes a more appropriate method of
separation when complex molecules are involved.
This guide is one in a series of Process Engineering Guides concerning LiquidLiquid Extraction and has been prepared for GBH Enterprises.
1

SCOPE

This Guide describes the basic features and the underlying fundamental
principles of Liquid-Liquid Extraction. It does not give detailed advice on the
design and operation of Liquid-Liquid Extraction equipment.

2

FIELD OF APPLICATION

This Guide is applicable to the Process Engineering community in GBH
Enterprises worldwide.

3

DEFINITIONS

For the purposes of this Process Engineering 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.


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 should be substantially
immiscible with the Feed.

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

LIQUID-LIQUID EXTRACTION PROCESS

4.1

General

Liquid-Liquid Extraction is the process of extracting a Solute from a Feed by use
of a Solvent to produce an Extract and a Raffinate. In its simplest form, it may
take the guise of a single stage mixing and separation unit analogous to a single
stage flash in distillation.
The choice of Solvent is critical in effecting a Liquid-Liquid Extraction. Factors
affecting the choice are summarized later; it is usually necessary to compromise
in one area or another.
As in distillation it is frequently impossible to achieve the separation required by
use of a single stage unit, and a multistage unit is required. These units are
summarized later and modes of operation further considered.
A number of pitfalls in operation and design of Liquid-Liquid Extraction units are
briefly mentioned.
GBHE reports covering Liquid-Liquid Extraction include “An Illustrated Brochure
on Liquid-Liquid Extraction". Other useful reference books are listed in Clause 5,
Bibliography.


4.2

Choice of Solvent

The choice of Solvent is influenced by many factors some of which are listed
below:
(a)

High Selectivity:
The ability of a Solvent to extract a component or class of components in
preference to others. This factor will determine the number of extraction
stages required.

(b)

Distribution or Partition Coefficient:
The ratio of the solubility of the Solute in the Solvent compared to the
Feed. This factor will affect the selectivity and the amount of Solvent
phase required.

(c)

Density:
The greater the density difference between the Feed and the Solvent the
easier it will be to obtain phase separation.

(d)

Viscosity:
A high viscosity will inhibit both mass transfer and separation of the
phases. A low viscosity (say less than 10 cP) is desirable.

(e)

Interfacial Tension:
This affects the settling, coalescence and mass transfer coefficient of a
system. Coalescence and settling are generally aided by high interfacial
tension whilst mass transfer is hindered.

(f)

Volatility:
The Solvent is likely to need to be separated from the Solute and/or the
Feed. If this is to be done by distillation the volatility should, where
possible, be chosen to allow this separation to be easily effected.


(g)

Stability:
The Solvent should be stable at process conditions in order to minimize
losses by degradation and generation of further impurities.

(h)

Corrosivity:
If possible, there is a strong incentive to use a component that is already
in the process, such as a reactant feed stream, as the Solvent. This may
avoid additional materials handling, environmental and corrosion penalties
later in the process.

(j)

Toxicity:
The advantages of a non-toxic Solvent are self evident in considering
inherent process safety and capital cost. Some solvents now appear on
the "Environmental Red List" and should be avoided.

(k)

Cost:
The extraction process may only be a small part in the overall process and
solvent losses should not greatly affect process economics.

No Solvent is likely to meet all the above criteria and the list is not claimed to be
exhaustive. A compromise will be necessary based on overall process
economics.
A common approach to Solvent selection is to carry out a literature survey of
solvents used in similar processes. It may be necessary to consult with Physical
Property specialists or with a Liquid-Liquid Extraction expert.


4.3

Principles of Extraction

Solvent Extraction depends on a favorable distribution of the Solute between the
Solvent and Feed streams. The two important parameters which fix the number
of extraction stages and the Solvent usage are the distribution coefficient and the
selectivity factor.
The distribution or partition coefficient, K, is defined as:-

where C refers to the composition of a component in any consistent convenient
units and the subscripts E and R refer to the Extract and Raffinate phase
respectively.
A high value of K is required for the component to be extracted, this determines
the quantity of solvent required to affect the recovery. Ideally K is independent of
the concentration of the Solute and the ratio of Extract to Raffinate phase.
The separation or selectivity factor, S, is defined as:-

where the subscript!1 refers to the component to be preferentially extracted and
subscript 2 refers to the component to be preferentially retained.
The separability of component 1 from component 2 increases with increasing
separation factor. A high value of S indicates the potential for a high degree of
separation in a small number of extraction stages.
A paper by Souders and Mott [Ref 13] indicated the relative uses of fractional
distillation, extractive distillation and Liquid-Liquid Extraction. The design of a
separations facility to carry out a particular separation was said to be the same in
a case when fractional distillation was used with a relative volatility of 1.5,
extractive distillation with a relative volatility of 2.0 and Liquid-Liquid Extraction
with a Selectivity factor of 6.0.


It is possible sometimes to tailor a particular solvent to give the desired
combination of selectivity and partition coefficient. In a mixed hydrocarbon
solvent this might be achieved by varying the proportion of aromatic to paraffinic
material for example or in a chemical solvent pH might be varied to influence
solvent activity. A solvent may be tailored to "pick up" a given solute from a
particular Feed then "drop it" into a further solvent by addition of acid or alkali to
alter the pH or some chemical equilibrium constant.
Choice as to whether to make the raffinate or the extract phase continuous will
be based on a number of considerations. These include the stability of the
mixture interface which may improve mass transfer but inhibit settling, the
relative mass transfer coefficients which may be influenced by surface tension
and more mundane considerations such as inventory of flammable or toxic
materials.

4.4

Liquid-Liquid Extraction Equipment

It has already been indicated that it may require more than one stage of LiquidLiquid Extraction in order to achieve the degree of separation required. It is
possible to achieve this by removing the Extract and making the Raffinate the
Feed to another Liquid-Liquid Extraction unit using fresh Solvent. This requires a
considerable amount of Solvent to be used and as in distillation it is more usual
to employ equipment where a countercurrent flow of one phase against the other
occurs. Equipment to achieve this is summarized in 4.4.1 to 4.4.4 but covered
more extensively in a subsequent Process Engineering Guide.

4.4.1 Mixer Settlers
These units consist of a number of mixing and settling units connected
alternately in series. They are normally used when only a small number of stages
are required. Mechanical agitation is required in the mixing zone, flow of fluid
from one zone to another may either be effected by gravity or by use of pumps.
Traditionally, the mixing chamber will have been a container with an agitator
followed by a settling chamber all known as a "Box Mixer Settler". The mixing
function may be performed by pumping the fluids through a static mixer or by
mixing the fluids in an appropriate pump followed by a settling device. Scale up is
usually good in these units.


4.4.2 Packed and Plate Column Contactors
These devices are used widely for simple separations requiring less than 3
stages. The interface is controlled to be either above or below the internals
depending on whether the light or the heavy phase is to be continuous. They
have the advantage of there being no moving parts but are difficult to scale up
with confidence due to the possibility of back-mixing and bypassing of either
phase.

4.4.3 Mechanically Agitated Columns
As throughput and numbers of stages required increases, mechanically agitated
columns become the preferred unit. There are many types of unit offered by
manufacturers including the Rotating Disc Column and the Kuhni Column.
Agitation may either be by movement of the internals in the column, e.g. by
rotation, or by pulsing the flow of the feeds to and/or products from the column.
Mechanically agitated columns are more usually used in the

4.4.4 Sundry Devices
There are a number of other sundry devices for achieving multistage contacting
to effect Liquid-Liquid Extraction including the rotating Podbielniak Extractor and
RTZ (Graesser) falling bucket contractor.

4.5

Operating Modes

The extension of a single stage Liquid-Liquid Extraction operation to a multistaged countercurrent operation is most simply achieved by making the Feed
and the Solvent inputs to the extreme stages in the unit. This is shown in Figure
1 where a dense Feed enters at the top of the extractor and the less dense
Solvent enters near the bottom.
In this case the Feed is taken to be the continuous phase with an interface
control near the top of the unit, but this is a matter of choice.
A Liquid-Liquid Extraction unit is not usually an isolated piece of equipment in a
flowsheet but will exist in conjunction with other items such as distillation units to
further process the product streams.


At the end of the column where fresh Solvent is introduced, at the bottom in
Figure 1, there will be a loss of Solvent due to any solubility in the Raffinate
stream. This would normally be recovered by distillation where the Solvent might
be expected to behave as a light component due to a high activity coefficient in
the Raffinate stream. In the case where Aromatics are recovered from a mixed
hydrocarbon stream the Solvent is polar and this may be recovered in a separate
Liquid-Liquid Extraction unit by contacting with water.
At the top of the column in Figure 1 the best separation between the desired
product and any impurity will depend on the Selectivity factor and the
concentration in the Feed. It may be possible to introduce the Feed midway down
the column, such as in Figure 2, and return Solute until it becomes insoluble in
the Solvent acting as Reflux such as in distillation. An alternative to this may be
to recover a portion of the Raffinate which is either insoluble in the Solvent or
easily separable from both Solvent and Solute and to reflux that.

4.6

Operating And Design Pitfalls

Distillation is a unit operation than depends primarily on the bulk properties of the
liquid and vapor phases. These bulk properties are so different that surface
properties tend not to have a great affect on the design and operation of
distillation units unless the columns are known to be operating with systems that
support stable foams. Hence, distillation is a unit operation that is readily
amenable to simulation for both theoretical plate analysis and for mass transfer
analysis.
In Liquid-Liquid Extraction the phases have more similar bulk properties therefore
surface effects, that are generally less easily measured, have greater affect.
Impurities, particularly surfactants, may concentrate at boundaries modifying the
surface properties and therefore mass transfer, separation and coalescence.
Hence, few Liquid-Liquid Extraction designs can be considered complete without
a laboratory simulation as discussed in GBHE-PEG-MAS-602.

4.6.1 Emulsions
Stable Emulsions which may be formed when surface active contaminants
are present, are an extreme example of how surface properties of a
system can make a Liquid-Liquid Extraction unit inoperable. Laboratory
simulations should be done wherever possible using actual plant streams
and the style of equipment that is operating

4.6.2 Crud Layer
Crud, as related to Liquid-Liquid Extraction, is material that collects at the
interface of the two liquid phases. A crud layer will occur in most
processes due to the concentration of finely divided particulate matter, but
in the majority of cases it is not troublesome. Where biological crud is
occurring due to the occurrence of bacteria or fungi, or where difficulties
are experienced, provision should be made for periodically removing
material from the interface for elimination and treatment of the crud.

4.6.3 Phase Inversion
Two types of phase inversion are possible as indicated below:
(a)

It is possible that the dense phase at one end of the extractor, for
example the Feed, may be the less dense phase at the other end,
the Raffinate. This is only likely to occur when the Solute forms a
significant proportion of either the Feed, the Extract or both. Under
these circumstances, operation of a column contactor is impossible.
A series of mixer settlers may be possible but control will be difficult
as the location of the inversion may move down the units. It is
better to avoid having to operate this type of system if at all
possible.

(b)

Phase inversion may occur by which the phase that was intended
to be continuous becomes dispersed and vice versa. This may
have a number of consequences, mass transfer may decrease,
coalescence may take longer and in a column contactor the
implication is that control has been lost as the interface has moved
to the opposite end of the column to that intended.


FIGURE 1 SCHEMATIC OUTLINE OF LIQUID-LIQUID EXTRACTION
PROCESS: COUNTERCURRENT FLOW IN ROTATING DISC CONTACTOR


FIGURE 2 COUNTERCURRENT EXTRACTION WITH REFLUX


5

BIBLIOGRAPHY


DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE
This Process Engineering Guide makes reference to the following documents:
GBHE ENGINEERING GUIDES
GBH Enterprises

Glossary of Engineering Terms (referred to in Clause
3).

GBH Enterprises

Guide for laboratory extraction (referred to in 4.6).

GBHE REPORTS
An illustrated brochure on liquid-liquid extraction (referred to in 4.1).


Liquid Liquid Extraction - Basic Principles

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