The objective of extraction is to remove one constituent from a solid or liquid by means of a liquid solvent. These techniques fall into two categories: the first is used to dissolve soluble matter from its mixture with an insoluble solid and it’s called leaching or solid extraction and the second is used to separate two miscible liquids by the use of a solvent that preferentially dissolves one of them and it’s called liquid extraction.
2.Filed of application
Extraction is the most important liquid-liquid separation process used in industrial chemistry. It is used mainly where other separation methods or direct distillation are ineffective or too expensive. Its typical uses include:
● Separation of components with similar boiling points (e.g. separating aromatics from hydrocarbons)
● Separation of high boilers from aqueous solution
● Separation of mixtures with high boiling points
● Separation of temperature sensitive compounds
● Separation of azeotropic mixtures (e.g. extraction of acetic acid from aqueous media)
● Separation of pollutants for the minimization of waste water streams.
3. Description Of Techniques, Methods And Equipment
In order to extract a substance, an extraction solvent must be added to form a second liquid phase solution. Generally the desired substance is then separated from the solvent by distillation and the solvent is recycled. Sometimes the selective action of the solvent is used in combination with distillation (extractive distillation or azeotropic distillation), for example in the manufacture of very pure, light aromatics. Extraction solvents like dimethyl sulfoxide, morphonils, sulfolane and diethylene glycol are wiely used in the production of aromatics. N-methyl-pyrrolidone, dimethylformamide and acetonitrile are also important solvents, especially for the extraction and separation of butenes and butadienes. Various types of mixre-settlers, centrifugal extractors and columns are used as extraction apparatus. All of them add the light phase at the bottom of the column, and the heavy phase is removed from the top. The density difference causes the light phase to rise through the heavy phase and affect the mass transfer between the two phases.