extraction

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SEPARATION PROCESSES III
CHE 541
N.A. Amenaghawon
Department of Chemical
Engineering,
University of Benin, Benin City

Class Information


Course Instructors : Prof. A.I. Igbafe, Prof. F.A. Aisien and N.A. Amenaghawon
[email protected]



Lectures:
500L Class
First term 2012/2013: Fridays (10am-12pm)



Course outline
• Multicomponent Distillation
• Liquid-Liquid Extraction
• Multi component gas absorption
• Solid-Liquid separation



Recommended texts

• R.E, Treybal, Mass transfer operations, 3rd edition.
• W.L, McCabe, J.C, Smith and P, Harriot, Unit operations of
chemical engineering, 5th edition, McGraw Hill International
editions, NewYork.
• J.F. Richardson, J.H. Harker and F.R. Backhurst, Coulson & Richardson’s
chemical engineering, vol. 2, 5th edition., Chemical Engineering (Oxford:
Butterworth-Heinemann, 2005).
• R.K. Sinnott, J.M. Coulson, and J.F. Richardson, Coulson & Richardson’s
chemical engineering, vol. 6, 4th ed., Chemical Engineering (Oxford: ButterworthHeinemann, 2005).
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Learning Outcomes
• Awareness on the liquid-liquid separation
techniques
• Principles of extraction liquid-liquid

• Representation of equilibrium data
• Calculations on single stage
extraction processes
• Calculations on multiple stage
extraction processes
• Principles of countercurrent
multistage extraction
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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
• Gas-liquid contact e.g. absorption,
evaporation, distillation etc
• Liquid-liquid contact e.g. extraction
• Liquid-solid contact e.g.
crystallisation, leaching, adsorption
• Gas-solid contact e.g. adsorption,
drying etc

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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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Contacting patterns

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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
• Separation of two components of a
liquid ( the “feed”) by contact with a
second immiscible liquid (the “feed”)
• Primarily used when separation by
distillation is ineffective, very difficult
or not economical e.g. close-boiling
mixture, substances that cannot
withstand high temperatures
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Liquid-liquid extraction is a useful method to separate components
(compounds) of a mixture

Liquid-liquid extraction
principle

When Liquid-liquid extraction is carried out
in a test tube or flask the two immiscible
phases are shaken together to allow
molecules to partition (dissolve) into the
preferred solvent phase
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Let's see an example.
Suppose that you have a mixture of sugar in vegetable oil (it tastes
sweet!) and you want to separate the sugar from the oil. You
observe that the sugar particles are too tiny to filter and you
suspect that the sugar is partially dissolved in the vegetable oil.

What will you do?

How about shaking the mixture
with water
Will it separate the sugar from the
oil? Sugar is much more soluble in
water than in vegetable oil, and,
as you know, water is immiscible
(=not soluble) with oil.
Did you see the result?The water
phase is the bottom layer andthe oil
phase is the top layer, because
water is denser than oil.
*You have not shaken the mixture
yet, so sugar is still in the oil phase.

By shaking the layers (phases) well, you
increase the contact area between the
two phases.The sugar will move to the
phase in which it is most soluble: the
water layer
Now the water phase tastes sweet,
because the sugar is moved to the
water phase upon shaking.**You
extracted sugar from the oil with
water.**In this example,water was
the extraction solvent ;the original
oil-sugar mixture was the solution to
be extracted; and sugar was the
compound extracted from one phase
to another. Separating the two layers
accomplishes the separation of the
sugar from the vegetable oil

Did you get it? .....the concept of liquid-liquid extraction?
Liquid-liquid extraction is based on the transfer of a solute
substance from one liquid phase into another liquid phase according
to the solubility.Extraction becomes a very useful tool if you choose
a suitable extraction solvent.You can use extraction to separate a
substance selectively from a mixture, or to remove unwanted
impurities from a solution.In the practical use, usually one phase is a
water or water-based (aqueous) solution and the other an organic
solvent which is immiscible with water.
The success of this method depends upon the difference in solubility
of a compound in various solvents. For a given compound, solubility
differences between solvents is quantified as the "distribution
coefficient"

Liquid-liquid extraction
Solvent exhibits preferential affinity or
selectivity towards one or more of the
components in the feed. Two streams result
from this contact:

a) Extract is the solvent rich solution containing the
desired extracted solute. (the remains of the solvent
stream after the solute has joined it)

b) Raffinate is the residual feed solution containing
little solute (the remains of the feed stream after the
solute has been extracted from it)
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Choice of solvent

Factors to be considered:
• Selectivity- gives an indication of the affinity of
the solvent for the solute to be extracted
• Distribution coefficient-large values are
typically desired since less solvent will be
required for extraction
• Insolubility of solvent-the higher the
insolubility of the solvent in the feed, the
higher its capacity to extract. Small amounts
of insoluble solvents are usually required
• Recoverability of solute from solvent-it is
always necessary to recover the solvent for
reuse. The solute should be recoverable from
the solvent rich extract phase
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Choice of solvent

• Density difference between liquid
phases-density difference between
both phases is necessary for separation
with a higher difference desirable
• Interfacial tension-the larger the
interfacial tension, the more readily
coalescence to form emulsion occurs
• Chemical reactivity-the solvent should
be inert and non reactive to other
components of the liquid mixture and
the materials of vessel construction
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Choice of solvent

• Viscosity, vapour pressure, freezing
point- these should all be low for the
purpose of easy handling and
storage
• Flammability, toxicity, Cost-solvent
should be non flammable, non toxic
and of low cost

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Liquid-liquid extraction
examples
Extraction of penicillin
from fermentation

broth by contact with amyl or butyl acetate
• Recovery of acetic acid from dilute aqueous
solutions by contact with ethyl acetate or
ethyl ether
• Separation of aromatics from aliphatics by
contact with triethylene glycol
• Separation of high-MW fatty acids from
vegetable oils by contact with liquid
propane
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Equipment for LLE





Mixer-Settler
Spray type extraction tower
Sieve tray extraction tower
Agitated extraction tower

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• Each mixer-settler unit provides
a single stage of extraction
• The two phases enter the
mixing section where they are
mixed using an impeller
• The two-phase solution flows
into the settling section where
they are allowed to separate by
gravity due to their density
differences
• Typical mixer settlers have
mixing times on the order of a
few minutes and settling times
of several minutes
• Mixer settlers can also be
arranged as a counter current
cascade

Mixer-Settler

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Spray type extraction tower
• Uses a sparger for
light liquid and
disperser for heavy
liquid
• Overflow is
light/underflow is
heavy

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Sieve tray extraction tower
• Light liquid
inventoried under
each tray
• Bubbles of light liquid
pass through
counterflow of heavy
liquid
• Downcomers are
used to transfer
heavy liquid down the
column
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Agitated extraction tower
• Agitation to mix
material for each
stage
• Settling occurs
outside the
agitation zone

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Representation of
equilibrium data

• Rectangular cartesian coordinates
• Triangular coordinates
• Solvent-free basis

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Equilibrium data

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Equilibrium data

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Notation

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Rectangular coordinates
• Feed = A+C
– A= Carrier liquid
– C= Solute

• Solvent = (B+c)
• Equilibrium data is represented in
x,y-fb diagrams plotted on
rectangular cartesian coordinates
fbx= weight fraction of B in Raffinate
fby = weight fraction of B in Extract
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Rectangular coordinates

• fbx= weight fraction of B in
Raffinate
• fby = weight fraction of B in
Extract
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Triangular Coordinates


The extract phase is rich in solvent and preferentially soaks up
component C (the solute), which we are trying to separate from the
other component in the feed (components A and C).



The raffinate phase is the liquid phase which is rich in the component
A (carrier liquid) and from which the solute (component C) is being
removed.



The original feed is usually a mixture of solute (component C) and
carrier liquid (component A).



The solvent-rich phase contains mostly solvent (component B) and
solute (component C) and only a small amount of carrier liquid
(component A)



The carrier liquid -rich phase contains mostly solute (component C)
and carrier liquid (component A), but also possibly some small amount
of solvent.
ChE 334: Separation Processes

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Dr Saad Al-Shahrani

Triangular Coordinates
• Triangular coordinates are
extensively used to graphically
describe the concentrations of
ternary systems in equilibrium
• Utilises the property of an equilateral
triangle
• The sum of the perpendicular
distances from any point within the
triangle to the three sides equals the
altitude of the triangle
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Triangular Coordinates
• The altitude represent
100% composition
• The distances to the
three sides represents
the percentages or
the fractions of the
three components
• Each apex of the
triangle represents
one of the pure
components
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Triangular Coordinates
• The perpendicular
distance from any
point suck as K to the
base AB represents the
percentage of C in the
mixture at K
• The distance from K to
AC= percentage of B
• The distance of K to
BC= percentage of A
• Any point on the side
of the triangle
represents a binary
mixture

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Triangular Coordinates

TYPE I: System of three liquids-one pair partially
soluble
•Liquid C dissolve completely in A and B
•Liquids A and B are only partially soluble in each other
resulting in extract and raffinate layers
•Most common type of system
•Water (A)-Chloroform (B) and Acetone (C)
•Benzene (A)-Water (B) and Acetic acid (C)

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Triangular Coordinates

TYPE II: System of three liquids-two pairs partially
soluble
•Liquids A and C dissolve completely in each other
•The pair of Liquids A-B and B-C are partially soluble in
each other resulting in extract and raffinate layers
•Chlorobenzene (A)-Water (B) and Methylethylketone (C)

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Single stage ExtractionBatch
May be batch or continuous


• Feed of mass F (A+C) with composition
xF
• Solvent of mass S1 (mainly B) with
composition ys
• F and S contact at equilibrium to give
extract E1 and raffinate R1

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Inverse Lever-arm
rule

• R kg of mixture
at point R is
added to E kg of
mixture at point
E
• The new mixture
is shown on the
RE line at point
M
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Inverse Lever-arm
rule

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Inverse Lever-arm
rule

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Single stage ExtractionBatch
May be batch or continuous


• Feed of mass F (A+C) with composition
xF
• Solvent of mass S1 (mainly B) with
composition ys
• F and S contact at equilibrium to give
extract E1 and raffinate R1

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Single stage ExtractionBatch
Adding F of composition x to S of
F

composition ys (ys=0 if the solvent is
pure) produces in the extraction stage
a mixture M of composition xM
• On settling, the mixture forms the
equilibrium phases E1 and R1 joined by
a tie line through M1

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Single stage ExtractionBatch

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Single stage ExtractionBatch
All points are located by their

respective compositions.
• F is located by xF
• S is located by ys
• M1 is located by xM1
• R1 is located by x1
• E1 is located by y1

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Single stage ExtractionBatch

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Single stage Extraction-Batch

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Single stage ExtractionBatch

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Multistage crosscurrent
extraction
Extension of single
stage extraction


• Each raffinate stream is successively
extracted with fresh solvent
• Calculations are same as that for single
stage extraction

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Example 1
It is desired to reduce the pyridine
concentration of 2000kg of an aqueous
solution from 50 to 2% in a single batch
extraction with chlorobenzene. Calculate
the amount of solvent required.

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Example 2

100kg of a solution of acetic acid (C) and
water (A) containing 30% acid is to be
extracted using 50kg of isopropyl ether.
– Determine the quantities and composition of the
various streams
– Determine the minimum and maximum amount
of solvent that can be used

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Example 3

100kg of a solution of acetic acid (C) and
water (A) containing 30% acid is to be
extracted using three times with 40kg of
isopropyl ether in each stage.
– Determine the quantities and composition of the
various streams
– How much solvent would be required if the same
final raffinate concentration were to be obtained
with one stage

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Continuous countercurrent
multistage extraction

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Triangular Coordinates

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