Introduction to Liquid-Liquid Extraction
Liquid-liquid extraction (also known as solvent extraction) involves the separation of the constituents (solutes) of a liquid solution by contact with another insoluble liquid. Solutes are separated based on their different solubilities in different liquids. Separation is achieved when the substances constituting the original solution is transferred from the original solution to the other liquid solution.
This can be presented in the Figure below:
The Figure showed a feed liquid (the "first" liquid) containing the desirable compound that is to be separated together with other compounds. Then an immiscible extraction liquid (the "second" liquid) is added and mixed with the feed liquid through agitation. The species re-distribute themselves between the 2 liquid phases. Agitation of the 2 phases is continued until equilibrium, and then agitation is stopped and the liquids are allowed to settle until both phases are clear. The 2 phases can then be separated.
There are 2 requirements for
liquid-liquid extraction to be feasible:
component(s) to be removed from the feed must preferentially distribute in the solvent
the feed and solvent phases must be substantially immiscible
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The simplest liquid-liquid extraction involves only a ternary (i.e. 3 components) system. The solution which is to be extracted is called the feed, and the liquid with which the feed is contacted is the solvent. The feed can be considered as comprising the solute A and the "carrier" liquid C. Solvent S is a pure liquid. During contact, mass transfer of A from the feed to the solvent S occurs, with little transfer of C to S.
The solvent (with the solute) is then permitted to separate from the carrier liquid. The solvent-rich product of the operation is called the extract, and the residual liquid from which solutes has been removed is the raffinate.
In some operations, the solutes are the desired product, hence the extract stream is the desirable stream. In other applications, the solutes may be the contaminants that need to be removed, and in this instance the raffinate is the desirable product stream.
Application of Liquid-Liquid Extraction
Extraction processes are well suited to the petroleum industry because of the need to separate heat-sensitive liquid feeds according to chemical type (e.g. aliphatic, aromatic, naphthenic) rather than by molecular weight or vapour pressure.
Other major applications exist in the biochemical or pharmaceutical industry, where emphasis is on the separation of antibiotics and protein recovery. In the inorganic chemical industry, they are used to recover high-boiling components such as phosphoric acid, boric acid, and sodium hydroxide from aqueous solutions.
Some examples are given below:
of nitrobenzene after reaction of HNO3 with toluene
Extraction of methylacrylate from organic solution with perchlorethylene
Extraction of benzylalcohol from a salt solution with toluene
Removing of H2S from LPG with MDEA
Extraction of caprolactam from ammonium sulfate solution with benzene
Extraction of acrylic acid from wastewater with butanol
Removing residual alkalis from dichlorohydrazobenzene with water
Extraction of methanol from LPG with water
Extraction of chloroacetic acid from methylchloroacetate with water
From: Table 3.9, p.136 "Separation Process Technology",
J.L. Humphrey and G.E. Keller II.
A more recent application of Supercritical Fluid Extraction (SFE) is briefly discussed in later Section.
In general, extraction is preferred over distillation for the following applications:
of azeotropes or low relative volatilities are involved (and distillation cannot
Dissolved or complexed inorganic substances in organic or aqueous solutions
Removal of a component present in small concentrations, e.g. hormones in animal oil
Recovery of a high-boiling component present in small quantities in waste stream, e.g. acetic acid from cellulose acetate
Recovery of heat-sensitive materials, where low to moderate processing temperatures are needed
Solvent recovery is easy and energy savings can be realized
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