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Liquid-Liquid Extraction
Separation of components of a liquid mixture by treatment with a solvent in which one or more of the desired components is preferentially soluble
Processing of coal tar liquids
Production of fuels in the nuclear industry
Separation of hydrocarbons in the petroleum industry
Metallurgical industry
Biotechnology industry
It is essential that the liquid-mixture feed and solvent are at least partially if not completely immiscible
Liquid-Liquid Extraction
Three stages are involved
Bringing the feed mixture and the solvent into intimate
contact
Separation of the resulting two phases
Removal and recovery of the solvent from each phase
First two stage are combined in column which
operated continuously (differential contacting)
Mixing of the two phases by agitation, followed
by settling in a separate vessel by gravity (Stage
wise equipment-mixer-settler unit)
Extraction Process
Batch or continuous
Single stage batch extraction
Mixers and settlers are separate vessels
Extraction units are followed by distillation or a similar operation in order to recover the solvent and solute
Extraction Process
Transfer mechanism of solute is molecular or eddy diffusion Concepts of phase equilibrium, interfacial area, and surface renewal are all similar in principle to those met in distillation and absorption Dispersion is effected by mechanical means including pumping and agitation, except in packed columns It is necessary to take into account the equilibrium conditions for the distribution of solute between the phases Determines the maximum degree of separation possible in a single stage
Equilibrium Data
The equilibrium condition for the distribution of one solute between two liquid phases is conveniently considered in terms of the distribution law
At equilibrium CE/CR = K ’
K ’^ is distribution constant
This relation will apply accurately only if both solvents are immiscible, and if there is no association or dissociation of the solute
If the solute forms molecules of different molecular weights, then the distribution law holds for each molecular species
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:
Equilibrium Data
A homogenous solution may be formed and the selected solvent is then unsuitable
The solvent may be completely immiscible with the initial solvent
The solvent may be partially miscible with the original solvent resulting in the formation of one pair of partially miscible liquids
The new solvent may lead to the formation of two or three partially miscible liquids
For 2nd^ case, the equilibrium relation is conveniently shown by a plot of the concentration of solute in one phase against the conc. in the second phase
For 3rd^ and 4th^ case, equilibrium data is shown by triangular diagrams
Equilibrium Data
Point 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
Within the area under the curve, the temperature and composition of one phase will fix the composition of the other
For three component at constant temperature and pressure, mixture degree of freedom is: For one liquid phase, there are two degree of freedom and two compositions must be stated For two liquid phases, there is only one degree of freedom
F C P 2
Equilibrium Data
Represents system of 4th type
A and C are miscible in all proportions
B and A, and B and C are only partially miscible
The %age of solute in one phase is plotted against the %age in the second phase in equilibrium with it
Equivalent to plotting the composition at either end of a tie line
Equilibrium relationship for the aniline-water-phenol system
Equilibrium relationship for the aniline-water-phenol system at higher temperature docsity.com
Calculation of the no of Theoretical
Stages
- Co-current Contact with Partially Miscible Solvents
- Co-current Contact with Immiscible Solvents
- Counter-current Contact with Immiscible Solvents
- Counter-current Contact with Partially Miscible Solvents
- Continuous Extraction in Columns
Co-current Contact with Partially
Miscible Solvents
Co-current Contact with Immiscible
Solvents
For the second stage
For n Stages
Counter-current Contact with
Immiscible Sol.
Solvent in the raffinate streams remains as A, and the added solvent in the extract stream as S
This is the equation of straight line of slope A/S (known as operating line), passes through pts (Xf, Y 1 ), and (Xn, Yn+1)
Counter-current Contact with
Partially Miscible Solvents
Continuous Extraction in Columns
The use of spray towers, packed towers or mechanical columns enables continuous countercurrent extraction to be obtained in a similar manner to that in gas absorption or distillation The transfer through the film on the raffinate side of the interface is brought about by a concentration difference CR – CRi On the extract side by a concentration difference CEi - CE
If equilibrium curve is a straight line
