Liquid – Liquid EXTRACTION
UNIT 3
Explanation of Concept
Extraction is a unit operation which separate a desired substance when it is mixed with others.The mixture
is brought into intimate contact with a solvent
in which the substance of interest is soluble,but the other substances present
are insoluble
Extractions use two immiscible phases
(these are
phases that do not mix, like oil and water) to
separate the substance from one phase into the
other.
The phases are extract phase and
raffinate phase.
The residual solution from which solute
is removed
is called raffinate and the solvent rich product of operation is called
extract.
The extract phase
contains desired product in large proportion.
Difference between extraction and distillation.
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Distillation
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Extraction
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1)Distillation
or fractionation is an operation in which the constituents of a liquid
mixture are separated by using thermal energy |
1)Extraction
is an operation in which the constituents of a liquid mixture are separated
by using an insoluble liquid solvent |
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2)Distillation
utilizes the difference in vapor pressure of different components at the same
temperature to effect a separation |
2)Extraction
utilizes the difference in solubility's of different components to effect a
separation. |
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3)
Relative volatility is used as a measure of the degree of separation |
3)
In extraction selectivity is used as a measure of the degree of separation |
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4)
In distillation new phase is created by addition of heat |
4)
In extraction a new insoluble liquid phase is created by addition of a
solvent to the original mixture |
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5)
In distillation mixing and separation of phases are easy and rapid |
5)
In extraction phases are hard to mix and harder to separate |
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6)
It almost gives pure product |
6)
Extraction itself does not give pure product and needs further processing |
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7)
It require thermal energy |
7)
It requires mechanical energy |
Applications of extraction.
►Liquid-Liquid
Extraction for Biotechnology Extraction
of valuable Products from fermentation broth.
►Removal
of high boiling organics from wastewater Such as phenol, aniline and nitrated
aromatics.
►Recovery
of tightly hydrogen-bonded organics from water Such as formaldehyde, formic acid and acetic
acid.
►Purification
of heat sensitive materials Such as
pharmaceuticals, flavors, fragrances and food products.
►Recovery
of products from reactions Such as
caprolactam and adiponitrile (for nylon porduction), acrylic acids and
agrigultural chemicals.
►Neutrialization/washing of acids or bases form organic stream Such as acrylates, nitrated organics and chloro-benzene compounds
►There
are several contaminations in waste water playing important role.
►Many
kinds of organic compounds such as pesticides, herbicides, phenols, PAHC,
heterocyclic and aromatic compounds are included in the waste water.
►Industrial
and agriculture production , and people living could be the source of organic
waste water.
►Classic
poisonous substances in organic waste water include the following; Water
organic matter, Formaldehyde, Phenols, Nitrobenzene, PCB’s, PAH’s etc. Common
form of chemical extraction using organic solvent as the extractant
►Extraction
is a fundamental technique used to isolate one compound from a mixture.
Becoming familiar with its theory and correct use are essential to successful
completion of many organic experiments.
►The three most common types of extractions are:
liquid/liquid,
liquid/solid,
and
acid/base
(also known as a chemically active extraction).
►In
stage type extractors, the two phases
are allowed to mix together in order to reach equilibrium. Then the phases are
separated before they are passed countercurrent to each other to the next
stages.
Advantages
of stage type extractors
2) No axial
mixing
3) High stage efficiency
►In differential extractor, the two phases are always in continuous contact. These are compact for a given capacity and require a small floor space.
1) In plate column, packed column the phase interdispersion and countercurrent flow is achieved by the force of gravity
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Mixer-Settler
Working Principle:-
Mixer-Settlers are one of the oldest types of extraction equipment.
Mixing Section: It is the part of the equipment
which has at least two inlet pipes. One pipe for the feed and the other pipe
for the solvent. This section is a chamber in which two fluids get into
contact and an agitator is installed in order to mix the liquids thoroughly.
Baffles are also installed so that mixing is efficient and the vortex is
eliminated.
Settling Section: It is the part of the section
where the mixed liquids enter in order to separate into two different phases
based on density differences and there are weirs in order to separate both
the phases. Recirculation pipelines: some part of the separated phases may be taken and fed back to the mixing section in order to maintain the ratio of the feed and the solvent. Working of the Mixer-Settler Equipment
►The feed liquid mixture is usually called the
Aqueous and the other liquid which is introduced to separate components is
called Solvent and usually it is organic in nature.
►The aqueous solution and the Organic are fed into
the mixing section where they are thoroughly mixed. The mass transfer of the
component of interest (Solute) from the aqueous phase to the organic phase
occurs in this section. The mixture of the aqueous phase and the Organic
phase is called an Emulsion.
►This Emulsion enters into the Settling section
and if the density differences between both the phases, aqueous and organic
is large enough then the separation of the phase happens just by gravity and
it takes very less time.This increases the size of the settler section and
often makes the extraction process infeasible for industrial use in
continuous process.  SPRAY
COLUMNS
►Column extractors typically have the two phases
flowing in countercurrent pattern. For the unagitated units shown above the
light phase being dispersed / distributed (hence the heavy phase continuous),
i.e. the light liquid enters at the bottom of the column and evolve as small
droplets at the nozzle distributor. The droplets of light liquid rise through
the mass of heavier liquid, which flows downward as a continuous stream. The
droplets are collected at the top and form the stream of light liquid leaving
the top of the column. The heavy liquid on the other hand leaves the bottom
of the column.
►The choice may be reversed, whereby the heavy
stream is introduced into the light phase at the top of the column and falls
as dispersed droplets through a continuous stream of light liquid.
►This set-up merely consists of an empty shell
with provisions at the end for introducing and removing the liquids. Its
construction is the simplest but suffers from low efficiency due to poor
phase contacting and excessive back-mixing in the continuous phase. Because
of their simple construction, spray columns are still used in the industry
for simple operations such as washing and neutralization.  CONTINUOUS CONTACT AGITATORS
►Column extractors typically have the two phases
flowing in countercurrent pattern. For the unagitated units shown above the
light phase being dispersed / distributed (hence the heavy phase continuous),
i.e. the light liquid enters at the bottom of the column and evolve as small
droplets at the nozzle distributor. The droplets of light liquid rise through
the mass of heavier liquid, which flows downward as a continuous stream. The
droplets are collected at the top and form the stream of light liquid leaving
the top of the column. The heavy liquid on the other hand leaves the bottom
of the column.
►The choice may be reversed, whereby the heavy
stream is introduced into the light phase at the top of the column and falls
as dispersed droplets through a continuous stream of light liquid.
►This set-up merely consists of an empty shell
with provisions at the end for introducing and removing the liquids. Its
construction is the simplest but suffers from low efficiency due to poor
phase contacting and excessive back-mixing in the continuous phase. Because
of their simple construction, spray columns are still used in the industry
for simple operations such as washing and neutralization.  Selection criterion of solvent
►Distribution coefficient In
dilute solution at equilibrium, the ratio of the concentrations of the solute
in the two phases is called the distribution coefficient or distribution
constant ‘K’. Mathematically
𝐾=𝐶𝐸/𝐶𝑅 Where
𝐶𝐸 and 𝐶𝑅 are the concentrations of the solute
in the extract and the raffinate phase, respectively The
distribution coefficient can also be given in terms of weight fraction of the
solute in the two phases in contact in equilibrium. 𝐾′=𝑋𝐸/𝑋𝑅 Where 𝑋𝐸 and 𝑋𝑅 is the weight fractions of the solute in the extract and the raffinate phase, respectively ►This is the ratio (at equilibrium) of the concentration of solute in the extract and raffinate phases. It gives a measure of the affinity of the solute for the two phases. ►A distribution coefficient other than unity implies that the solute must have different affinity in the two phases. If only one solute is involved (such as in the recovery of an impurity from an effluent stream), only the distribution coefficient need be considered, and it is desirable for this to be as large as possible. ►If there are more than one solutes (say two solutes A and B), then consideration should be given to the selectivity of the solvent for solute A as against B. The selectivity between the 2 solutes A and B is defined as the ratio of the distribution coefficient of A to the distribution coefficient of B. For all useful extraction operation the selectivity must exceed unity. If the selectivity is unity, no separation is possible. ►Recoverability It
is always necessary to recover the solvent for re-use, and this must
ordinarily be done by other means, eg. distillation. If distillation is to be
used, the solvent should form no azeotrope with the extracted solute and
mixtures should show high relative volatility. The solvent should also be
thermally-stable under the distillation temperature.
►Other
properties
►Density: The
difference in densities of the saturated liquid phases should be larger for
the ease in physical separation of the phases by gravity
►Insolubility
of solvent:
The solvent should be insoluble in the original liquid solvent (feed
solvent). It should have a high solubility for the solute to be extracted so
that small amount of the solvent are required.
►Interfacial
tension:
The solvent should have a high interfacial tension for coalescence of
emulsions to occur more readily
►The solvent should be stable chemically. It
should be inserts towards the components of the systems.
►The solvent should not be corrosive towards
common material of construction
►The solvent should be cheap
►The solvent should be non-toxic and non-flammable
►The solvent should have low viscosity, freezing
point, vapor pressure for ease in handling and storage.
►The addition of a new solvent to a binary liquid
mixture results in different types of mixtures or systems. These are
►1) The solvent may be completely immiscible with
the feed solvent. This is an ideal case.
►2) The solvent may be partially miscible with the
feed solvent, forming one pair of partially miscible liquids. If A is the
feed solvent, C is the solute, and B is the extracting solvent, then C
dissolves in A and B completely while A and B dissolve only to a limited
extent in each other. This is the most common type of system.
►3) The solvent is miscible with the feed solvent
and thus we get a homogeneous solution by contacting them. The solvent so
selected is not suitable and should be rejected. ►4) The solvent may lead to the formation of two or three partially miscible liquids. A and C are completely soluble, while A and B, and B and C dissolve only to a limited extent in each other resulting in two pairs partially miscible. It is observed occasionally and not desired. Triangular
Diagram  
►In liquid-liquid extraction when the solvent is
partially miscible with the original solvent (diluent), the solubility and
equilibrium relations are often shown on a triangular diagram. The
composition of ternary systems can be shown by a point lying inside an
equilateral triangle.
►Consider a system consisting of C, A and B at
250C wherein acetone is the solute, water is the diluent (original solvent)
and MIK is the solvent for extracting the solute. In this system, the C is
completely miscible with the two solvents A and B and the two solvents A-B
are partially miscible with each other. Apex C represent 100% acetone and
apexes A and B represents 100 water and 100% MIK respectively.
►Along a line BC, the concentration of A is zero
and the same is true for B and C along lines AC and BA. The ternary system
represented by point P consists of three components C, A and B in the ration
of perpendiculars PL, PJ and PK respectively. The distance AD and BE
represents the solubility of B in A and that of A in B respectively.
►Every mixture of MIK and water of composition
lying between D and E forms two liquid layers and for calculating mass ration
of these two layers, center of gravity principle is applicable. When acetone
is added to the two phase/layer mixture of MIK and water, acetone gets
distributed between the phases/layer, and compositions of phases/layers
►Follow the raffinate phase and extract phase
solubility curves. The curved line ERF indicates composition of saturated MIK
layer and curved line DQF indicates the composition of saturated water layer.
The area under the bimodal solubility curve represented by the curved line
DQFRE represents a two-phase region that will split up into two layers in
equilibrium with each other. The equilibrium composition of the two phases
lies on this curve. These two layers have compositions represented by points
Q and R and QR line is a tie line. The line joining the equilibrium
composition of two phases is called as tie line. The points of Tie line must
be found experimentally.
►Plait point is the point at which the composition
of the raffinate phases is equal to that of extract phase. Here the tie line
becomes a dot. And thus the separation is not possible.
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