Liquid Liquid Extraction:

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Liquid Liquid Extraction

• The separation of the components of a liquid
  mixture by treatment with a solvent in which one
  or more of the desired components is
  preferentially soluble is known as liquidliquid
  extraction.
• Liquid-liquid extraction, also known as solvent
  extraction, is a method to separate compounds
  based on their relative solubilities in two
  different immiscible liquids, usually water and
  an organic solvent. It is an extraction of a
  substance from one liquid phase into another
  liquid phase

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Extraction processes

• liquidliquid extraction operations, may be
  carried out either as a batch or as a continuous
  process.

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Batch Extraction
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Continuous two-stage operation
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Extraction equipment

• Mixer settler
• Spray and packed extraction tower

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Mixer settler arrangement
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Spray and packed extraction tower
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The mechanism of transfer of solute from one
phase to the second

• Molecular and eddy diffusion
• The concepts of phase equilibrium
• Interfacial area
• Surface renewal are all similar in principle to
  those met in distillation and absorption

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Dispersion is effected

• in liquidliquid extraction, dispersion is
  effected by mechanical means including pumping
  and agitation, except in standard packed columns.
• Packed columns. In formulating design criteria
  for extraction equipment, it is necessary to take
  into account the equilibrium conditions for the
  distribution of solute between the phases as this
  determines the maximum degree of separation
  possible in a single stage.

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Rate of transfer

• The transfer rate is given by
• Rate per unit interfacial area k ?C
• where k is a mass transfer coefficient and C a
  concentration driving force. A high value of k
  can be obtained only if turbulent or eddy
  conditions prevail and, although these may be
  readily achieved in the continuous phase by some
  form of agitation, it is very difficult to
  generate eddies in the drops which constitute the
  dispersed phase.

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The addition of a new solvent to a binary mixture
of a solute in a solvent may lead to the
formation of several types of mixture

• (a) A homogeneous solution may be formed and the
  selected solvent is then unsuitable.
• (b) The solvent may be completely immiscible with
  the initial solvent.
• (c) The solvent may be partially miscible with
  the original solvent resulting in the
• formation of one pair of partially miscible
  liquids.
• (d) The new solvent may lead to the formation of
  two or three partially miscible liquids.

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Equilibrium Data
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• A mixture indicated by point H consists of the
  three components A, B and C in the ratio of the
  perpendiculars HL, HJ, HK.
• The distance BN represents the solubility of
  solvent C in B, and MC that of B in C.
• The area under the curved line NPFQM, the binodal
  solubility curve, represents a two-phase region
  which will split up into two layers in
  equilibrium with each other.

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• These layers have compositions represented by
  points P and Q, and PQ is known as a tie line.
  Such lines, two of which are shown in the
  diagram, connect the compositions of two phases
  in equilibrium with each other, and these
  compositions must be found by practical
  measurement.
• There is one point on the binodal curve at F
  which represents a single phase that does not
  split into two phases. F is known as a plait
  point, and this must also be found by
  experimental measurement.

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Similarly, if an extract Y is removed, from a
mixture Z the remaining liquor will
havecomposition X.

• One of the most useful features of this method of
  representation is that, if a solution of
  composition X is mixed with one of composition Y,
  then the resulting mixture will have a
  composition shown by Z on a line XY, such that

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• The important factor in assessing the value of a
  solvent is the ratio of the concentrations of the
  desired component in the two phases, rather than
  the actual concentrations. A selectivity ratio
  may be defined in terms of either mass or mole
  fractions as

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Calculation Of The Number Of Theoretical Stages
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Co-current contact with partially miscible
solvents
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Co-current contact with immiscible solvents
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Countercurrent contact with immiscible solvents
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• This is the equation of a straight line of slope
  A/S, known as the operating line,which passes
  through the points (Xf , Y1) and (Xn, Yn1). the
  equilibrium relation, Yn against Xn, and the
  operating line are drawn in, and the number of
  stages required to pass from Xf to Xn is found by
  drawing in steps between the operating line and
  the equilibrium curve. In this example, four
  stages are required, and (Xn, Yn1) to (X4, Y5).
  It may be noted that the operating line connects
  the compositions of the raffinate stream leaving
  and the fresh solvent stream entering a unit, Xn
  and Yn1, respectively.

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