The word extraction itself defines the principle of the operation where the component is extracted from the solution or a mixture. The technique of removing one component from a solid or liquid by using a liquid solvent is known as extraction process. The technique can be divided into two categories based on the dissolving soluble matter from its mixture with an insoluble solid with help of a solvent which is called as leaching whereas liquid extraction is used to separate two miscible liquids by the use of a solvent which preferentially dissolves one of them.
A solvent employed in a liquid-liquid extraction operation should preferably have low viscosity and high interfacial tension. Separation by solvent extraction is not possible if the selectivity is 1.Solvent extraction should preferably be carried out at temperatures below the critical solution temperature above the critical solution temperature equal to the critical solution temperature. On a distribution curve plait point is that point where y = x, at the plait point selectivity is 1.
There is a direct relationship between the distribution coefficient and the solvent requirement as the distribution coefficient increases, the solvent requirement for a given separation decreases. The apex of an equilateral triangular coordinate (in ternary liquid system) represents a pure component. In equilateral triangular coordinate a point on any side of the triangle represents a pure component of binary mixture as the interfacial tension of a liquid-liquid system decrease, the ease of Coalescence decreases for liquids of very small density difference and the most suitable extractor is a centrifugal extractor.
Podielniak extractor is a special type of centrifugal extractor. A spray tower is a differential contact extractor, Pulsing of a packed column extractor results in larger liquid-liquid interfacial area and higher mass transfer coefficient. For extraction of penicillin from nutrient broth, the most suitable extractor is a centrifugal extractor for liquid-liquid systems that form emulsions; the most suitable contactor is a centrifugal extractor. The udex process is a solvent extraction process in which benzene, toluene and xylene present in catalytic reformats are extracted with sulfur dioxide or diethyleneglycol. One of the solvent in the Duo-Sol process is propane.

Mercaptans present in light distillates are removed by extraction with aqueous caustic soda solution. In the hydrochloric acid route for the production of phosphoric acid, phosphoric acid is extracted from a mixture of aqueous phosphoric acid and calcium chloride solution by C5 – alcohols. In the Edeleanu process (one of the solvent extraction processes applied to light distillates in petroleum refineries) sulfur dioxide is used as a solvent. At the boiling point of sulfur dioxide, which is 14oF, the components which are miscible with sulfur dioxide are aromatic and unsaturated hydrocarbons. The operation which is employed for the removal of caffeine from coffee is solvent extraction. Extraction by supercritical CO2 is particularly important in food industry. The major application of the sulfolane extraction process is for the production of benzene, toluene and xylene of high purity from a mixture of aromatics and nonaromatics.