The liquid mixture to be treated and a suitable, insoluble solvent are contacted intimately. 

The constituents of liquid mixtures are distributed between the two phases resulting in some degree of separation (which can be improved by a multistage contact) and the phases are separated from one another based on the density differences of the liquid phases.  For example, acetone may be preferentially extracted from a solution in water with the help of chloroform.  The resulting chloroform phase contains a large part of acetone, but little water.

The solution to be extracted is called the feed.  The liquid extraction liquid is called the solvent.  The residual liquid solution from which the solute is removed is called the raffinate.  The extracted solvent rich product is called the extract.  The extract phase contains the desired product in a larger proportion.

Thus, if a solution of acetic acid in water is contacted with a solvent such as ethyl acetate then two phases will results.  The extract (ester/organic layer) will contain most of acetic acid in ethyl acetate with a small amount water.  The raffinate (aqueous layer) will contain a weak acetic acid solution with a small amount of ethyl acetate.  The amount of water in the extract and ethyl acetate in the raffinate depends upon their solubilties in one another.

The Distribution Coefficient

In dilute solutions at equilibrium, the concentration of the solute in the two phases is called the distribution coefficient or distribution constant ‘K’.
K= C_E/C_R
Where the C_E  and  C_R  are the concentrations of the solute in the extract and the raffinate phases respectively.  The distribution coefficient can also be given as the weight fraction of the solute in the two phases in equilibrium contact:
K’ = y^*/x 
Where y^* is the weight fraction of the solute in the extract and x is the weight fraction of the solute in the raffinate.
In liquid-liquid extraction, the liquid-liquid equilibrium must be considered.  This is best represented by equating the chemical potential of both liquid phases:

This relationship reduces to an expression, which is dependent only on the liquid mole fractions and their respective activity coefficients:

Activity coefficient models, such as UNIFAC, UNIQUAC (universal quasichemical), and NRTL can be used to determine the mole fractions.  All three models above are applicable to liquid-liquid equilibrium, the choice of which model to use depends on which properties are available.  For a multi-component system, the UNIQUAC equation for the liquid-phase activity coefficient is:

The combinatorial and residual activities are based on the statistical mechanical theory which describes how local compositions result from the size and energy differences between the molecules in the mixture.

When to Chose Liquid-Liquid Extraction

• When large volumes of water must be removed to complete a separation.  The large latent heat of vaporization of water can make this an energy intensive process.
• When two or more liquids form a close boiling azeotrope, the desired final concentration may not be possible via distillation.
• When one or more of the components are considered thermally sensitive or unstable, distillation may not be a feasible option.

In such cases, extraction is attractive and often preferred. The separation of acetic acid from dilute solutions of water is usually more economical via extraction followed by distillation.  Distillation would be feasible for this separation, but by using extraction first, the amount of water that must be vaporized is reduced significantly. 

Rohit Ramesh Rewagad and Piyush Khatavkar
Bachelor of Technology in Chemical Eng.
Laxminarayan Institute of Technology, Nagpur India
Email: piyushkhatavkar ”at” gmail.com