LIQUID-LIQUID EXTRACTION PRINCIPLE

Separation processes - general

 Mechanical separations e.g. filtration of a solid from a
suspension in a liquid, centrifugation, screening etc
 Mass transfer operations e.g. distillation, extraction etc

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Mass transfer operations – nature of
interface between phases
 Gas-liquid contact e.g. absorption, evaporation,
distillation etc
 Liquid-liquid contact e.g. extraction
 Liquid-solid contact e.g. crystallisation, adsorption
 Gas-solid contact e.g. adsorption, drying etc

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Mass transfer operations – controlling
transport phenomenon
 Mass transfer controlling e.g.distillation, absorption,
extraction, adsorption etc
 Mass transfer and heat transfer controlling e.g. drying,
crystallisation
 Heat transfer controlling e.g. evaporation

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Methods of operation
 Non steady state – concentration changes with time
e.g. batch processes
 Steady state
 Stage
 Differential contact

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When both phases are flowing:
 Co-current contact

Stage 1 Stage 2

 Cross flow

1 2

Counter-current flow

1 2

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Choice of separation process
Factors to be considered:
 Feasibility
 Product value
 Cost
 Product quality
 selectivity

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Liquid-liquid extraction principles
Feed phase contains a component, i, which is to be
removed.
Addition of a second phase (solvent phase) which is
immiscible with feed phase but component i is soluble
in both phases.
Some of component i (solute) is transferred from the
feed phase to the solvent phase.
After extraction the feed and solvent phases are called
the raffinate (R) and extract (E) phases respectively.

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Normally one of the two phases is an organic phase while the
other is an aqueous phase.

Under equilibrium conditions the distribution of solute i over

the two phases is determined by the distribution law.
After the extraction the two phases can be separated
because of their immiscibility.
Component i is then separated from the extract phase by a
technique such as distillation and the solvent is
regenerated.
Further extractions may be carried out to remove more
component i.
Liquid liquid extraction can also be used to remove a
component from an organic phase by adding an aqueous
phase.

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Example - Penicillin G
6-aminopenicillanic acid (6-APA) is manufactured
by GSK in Irvine. It is used to manufacture
amoxicillin and ‘Augmentin’.

Fermentation products (penicillin G broth) are

filtered (microfiltration) and extracted at low pH
with amyl acetate or methyl isobutyl ketone. The
penicillin G is then extracted further at a higher
pH into an aqueous phosphate buffer.

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Extractants
The efficiency of a liquid liquid extraction can be
enhanced by adding one or more extractants to the
solvent phase.
The extractant interacts with component i increasing the
capacity of the solvent for i.
To recover the solute from the extract phase the
extractant-solute complex has to be degraded.

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Distribution coefficient
K = mass fraction solute in E phase
mass fraction solute in R phase

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Immiscible liquids
e.g. water – chloroform

Consider a feed of water/acetone(solute).

K = mass fraction acetone in chloroform phase
mass fraction acetone in water phase
K = kg acetone/kg chloroform = y/x
kg acetone/kg water
K = 1.72
i.e. acetone is preferentially soluble in the chloroform phase

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Partially miscible liquids
E.g. water – MIBK
Consider a solute acetone.
Need to use a triangular phase diagram to show
equilibrium compositions of MIBK-acetone-water
mixtures.
Characteristics are single phase and two phase regions,
tie lines connecting equilibrium phase compositions in
two phase region.

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Triangular phase diagrams
B

%S %A
P

%B
A S

Each apex of triangle represents 100% pure component

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continued B

E
M

A S

A mixture of overall composition M will split into two phases – E & R. R

phase is in equilibrium with E phase
R/E = line ME/line MR

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Choice of solvent
Factors to be considered:
 Selectivity
 Distribution coefficient
 Insolubility of solvent
 Recoverability of solute from solvent
 Density difference between liquid phases
 Interfacial tension
 Chemical reactivity
 Cost
 Viscosity, vapour pressure
 Flammability, toxicity
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Selectivity
β = (mass fraction B in E)/(mass fraction A in E)
(mass fraction B in R)/(mass fraction A in R)

β>1

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Distribution coefficient
K = y/x

Large values are desirable since less solvent is required

for a given degree of extraction

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Insolubility of solvent
B

Preferred solvent – A and S have limited solubility

A S

S very soluble in A and A very soluble in S

A S
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Recoverability of solvent and solute
 No azeotrope formed between solvent and solute
 Mixtures should have a high relative volatility
 Solvent should have a small latent heat of
vapourisation

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Density
A density difference is required between the two phases.

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Interfacial tension
The larger the interfacial tension between the two
phases, the more readily coalescence of emulsions will
occur to give two distinct liquid phases but the more
difficult will be the dispersion of one liquid in the
other to give efficient solute extraction.

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Chemical reactivity
Solvent should be stable and inert.

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Physical properties
For material handling:
 Low viscosity
 Low vapour pressure
 Non-flammable (high flash point)
 Non-toxic

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Mass balances

For counter-current contact with immiscible solvents a

simple mass balance for solute B at steady state gives the
operating line:
yn+1 = a/s(xn – xF) + y1

,where yn+1 = kgB/kgS in solvent feed

a = mass component A
s = mass solvent
xn = kgB/kgA after n stages
xF = kgB/kgA in feed
y1 = kgB/kgS in extract after first stage
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continued
A graphical procedure may be used to analyse these
systems. The number of theoretical stages (n)
required to pass from xF to xn is found by drawing
in ‘steps’ between the operating line and the
equilibrium curve (yn, xn).
In practice equilibrium conditions may not be
attained and extraction efficiency will be less than
100% thus requiring more stages in practice than
the above analysis would suggest.
Also partial miscibility of the solvents has to be
considered in the separation process.
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continued
y
y1,xF

yn+1,xn
x

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Operation
 Batch
 Continuous
 Single/multi stage contact

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Equipment
 Mixer-settler units
 Columns
 Centrifugal contactors

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