Liquid-Liquid Extraction Principle and Applications

The liquid-liquid extraction techniques (methods) of removing one component from a solid or liquid using a liquid solvent fall into two categories, the first one is called leaching or solid extraction, and the second is liquid extraction. 

Leaching is used to dissolve soluble matter from its mixture with an insoluble solid [ Leaching is an operation in which a particular component of the solid is leached out with the help of a solvent] whereas liquid extraction is used to separate two miscible liquids by the use of a solvent which preferentially dissolves one of them. In this chapter, our discussion is restricted to liquid extraction - the extraction of the liquid phase with a solvent.

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Liquid-Liquid Extraction Principle  

When a mixture of liquids is not easily separable by distillation i.e. when separation by distillation is ineffective or very difficult, liquid extraction is employed. Close boiling mixtures or substances that cannot withstand the temperature of distillation, even under vacuum [i.e. thermally unstable substances (heat sensitive)] may often be separated by extraction. 

In this operation, a solvent is added to the liquid mixture. As a result, two Immiscible layers are formed both containing varying amounts of different components. The isolated layers are then separated using density difference as extract phase and raffinate phase using density difference. The recovery of the solvent for re-use and extraction is followed often by distillation or evaporation. 

Liquid-liquid extraction refers to an operation in which the constituents of a liquid mixture are separated by constituents. This operation is sometimes also termed solvent extraction. Extraction utilizes the difference in solubility of the constituents/components to effect a separation. In this operation, a solute in a liquid solution is removed by contacting the solutions with another liquid solvent. 

The solvent is relatively immiscible with the solution. In liquid extraction, the feed solution to be handled represents one phase and the solvent to be used to effect separation represents the second phase. In the operation, the two Immiscible phases in contact are both liquid and so is a liquid-liquid operation. The mass transfer of the solute takes place from the feed solution to the solvent phase. 

Applications

Whenever separation by both distillation and extraction is possible the choice is usually distillation irrespective of heating and cooling requirements. In extraction, the solvent should be recovered for reuse and hence extraction is usually followed by distillation for the recovery of the solvent. 

This combined operation is more complicated and more expensive than ordinary distillation. But whenever separation of the components of a liquid mixture is very difficult or ineffective by distillation, extraction can be thought of as an alternate process it.

Thus, (i) whenever a very large amount of latent heat is required (as in the case with very dilute solutions where water must be vaporized the latent heats of organic liquids are substantially lower than that of water) 

(ii) Whenever we are dealing with a liquid mixture form an azeotrope/close boiling mixture. 

(iii) And whenever we are dealing with substances that are thermally unstable/heat-sensitive, separation by distillation is expensive, ineffective, and difficult. In such cases, extraction is attractive and preferred. 

liquid-liquid mass transfer example

(a) Recovery of acetic acid from dilute aqueous solutions is done more economically by extraction followed by distillation than distillation alone because of the high latent heat of vaporization of water (distillation would be possible but extraction reduces substantially the amount of water to be vaporized. 

(b) Long-chain fatty acid can be removed from vegetable oils economically by extraction with liquid propane than by a high vacuum distillation technique. 

(c) Separation of petroleum products that have about the same boiling range. The separation of aromatics from lube oil fractions using further as a solvent. 

(d) Recovery of penicillin from the fermentation broth using butyl acetate. 

Liquid-Liquid extraction differences in the solubility of the component

A typical liquid-liquid extraction utilizing the difference in the solubilities of the components of a liquid mixture may be described as consisting of the following steps. 

1. Contacting the feed with a solvent. 

2. Separation of the resulting phase, and

3. Removal/recovery of solvent (s) from each phase. 

In Liquid-Liquid extraction operation, the liquid solution and an insoluble solvent are brought into intimate contact, the constituents of the liquid mixture are distributed between the phases resulting in some degrees of separation (which can be improved by a multistage contact) and then the phases are separated from one another based on density difference. 

For example, Acetone may be preferentially extracted from a solution in water with the help of chloroform, and the resulting chloroform phase is found to contain a large part of acetone but little water.

>Liquid-liquid extraction equipment's 

Take these Notes is, Orginal Sources: Unit Operations-II, KA Gavhane