![]() ![]() Furthermore, interfacial tension between the two phases should not be so low that subsequent phase disengagement becomes difficult and the density difference between the phases should be large enough to maintain countercurrent flow of the phases under the influence of gravity. Thus not only should the solvent be selective for the solute being extracted but it should also possess other desirable features such as low cost, low solubility in the feed-phase and good recoverability as well as being noncorrosive and noninflammable. No single criterion can be used to assess the suitability of a solvent for a particular application and the final choice is invariably a compromise between competing requirements. This nomenclature is unique to liquid-liquid extraction processes and will be used from hereon. The equipment may be comprised of either discrete mixers and settlers or some form of column contactor in which the feed and solvent phases flow countercurrently by virtue of the density difference between the phases.įinal settling or phase separation is achieved under gravity at one end of the column by allowing an adequate settling volume for complete phase disengagement.Īny one extraction operation gives rise to two product streams: the extracted feed solution, more usually termed the raffinate phase, and the solvent containing extracted solute termed the extract phase. On an industrial scale, the extraction operation more usually involves more than one extraction stage and is normally carried out on a continuous basis. This is analagous to the laboratory procedure employing a separating funnel. The simplest extraction operation is single-contact batch extraction in which the initial feed solution is agitated with a suitable solvent, allowed to separate into two phases after which the solvent containing the extracted solute is decanted. In general, extraction is applied when the materials to be extracted are heat-sensitive or nonvolatile and when distillation would be inappropriate because components are close-boiling, have poor relative volatilities or form azeotropes. The petroleum industry takes advantage of this characteristic of the process and has used extraction to separate, for example, aromatic hydrocarbons from paraffin hydrocarbons of the same boiling range using solvents such as liquified sulfur dioxide, furfural and diethylene glycol. This property is frequently characteristic of the chemical type so that entire classes of compounds may be extracted if desired. Whereas distillation affects a separation by utilizing the differing volatilities of the components of a mixture, liquid-liquid extraction makes use of the different extent to which the components can partition into a second immiscible solvent. Since then many other processes have been developed by the petroleum, chemical, metallurgical, nuclear, pharmaceutical and food processing industries. The first commercially-successful liquid-liquid extraction operation was developed for the petroleum industry in 1909 when Edeleanu’s process was employed for the removal of aromatic hydrocarbons from kerosene, using liquid sulfur dioxide as solvent. Solvent extraction is an old, established process and together with distillation constitute the two most important industrial separation procedures. In practical terms, however, many solutes may be present in the initial solution and die extracting ‘solvent’ may be a mixture of solvents designed to be selective for one or more solutes, depending upon their chemical type. In its simplest form, this involves the extraction of a solute from a binary solution by bringing it into contact with a second immiscible solvent in which the solute is soluble. Liquid-liquid (or solvent) extraction is a countercurrent separation process for isolating the constituents of a liquid mixture.
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