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• W406977324 abstract "Extraction is a process for the separation of one or more components through intimate contact with a second immiscible liquid called a solvent. If the components in the original solution distribute themselves differently between the two phases, separation will occur. Separation by extraction is based on this principle. When some of the original substances are solids, the process is called leaching. In a sense, the role of solvent in extraction is analogous to the role of enthalpy in distillation. The solvent-rich phase is called the extract, and the carrier-rich phase is called the raffinate. A high degree of separation may be achieved with several extraction stages in series, particularly in countercurrent flow.Processes of separation by extraction, distillation, crystallization, or adsorption sometimes are equally possible. Differences in solubility, and hence of separability by extraction, are associated with differences in chemical structure, whereas differences in vapor pressure are the basis of separation by distillation. Extraction often is effective at near-ambient temperatures, a valuable feature in the separation of thermally unstable natural mixtures or pharmaceutical substances such as penicillin.The simplest separation by extraction involves two immiscible liquids. One liquid is composed of the carrier and solute to be extracted. The second liquid is solvent. Equilibria in such cases are represented conveniently on triangular diagrams, either equilateral or right-angled, as for example on Figures 14.2 and 14.3. Equivalent representations on rectangular coordinates also are shown. Equilibria between any number of substances are representable in terms of activity coefficient correlations such as the UNIQUAC or NRTL. In theory, these correlations involve only parameters that are derivable from measurements on binary mixtures, but in practice the resulting accuracy may be poor and some multicomponent equilibrium measurements also should be used to find the parameters. Finding the parameters of these equations is a complex enough operation to require the use of a computer. An extensive compilation of equilibrium diagrams and UNIQUAC and NRTL parameters is that of Sorensen and Arlt (1979–1980). Extensive bibliographies have been compiled by Wisniak and Tamir (1980–1981).The highest degree of separation with a minimum of solvent is attained with a series of countercurrent stages. Such an assembly of mixing and separating equipment is represented in Figure 14.4(a), and more schematically in Figure 14.4(b). In the laboratory, the performance of a continuous countercurrent extractor can be simulated with a series of batch operations in separatory funnels, as in Figure 14.4(c). As the number of operations increases horizontally, the terminal concentrations E1 nd R3 approach asymptotically those obtained in continuous equipment. Various kinds of more sophisticated continuous equipment also are widely used in laboratories; some are described by Lo et al. (1983, pp. 497–506). Laboratory work is of particular importance for complex mixtures whose equilibrium relations are not known and for which stage requirements cannot be calculated.In mixer-separators the contact times can be made long enough for any desired approach to equilibrium, but 80–90% efficiencies are economically justifiable. If five stages are required to duplicate the performance of four equilibrium stages, the stage efficiency is 80%. Since mixer-separator assemblies take much floor space, they usually are employed in batteries of at most four or five units. A large variety of more compact equipment is being used. The simplest in concept are various kinds of tower arrangements. The relations between their dimensions, the operating conditions, and the equivalent number of stages are the key information.Calculations of the relations between the input and output amounts and compositions and the number of extraction stages are based on material balances and equilibrium relations. Knowledge of efficiencies and capacities of the equipment then is applied to find its actual size and configuration. Since extraction processes usually are performed under adiabatic and isothermal conditions, in this respect the design problem is simpler than for thermal separations where enthalpy balances also are involved. On the other hand, the design is complicated by the fact that extraction is feasible only of nonideal liquid mixtures. Consequently, the activity coefficient behaviors of two liquid phases must be taken into account or direct equilibrium data must be available. In countercurrent extraction, critical physical properties such as interfacial tension and viscosities can change dramatically through the extraction system. The variation in physical properties must be evaluated carefully." @default.
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• W406977324 date "2010-01-01" @default.
• W406977324 modified "2023-09-26" @default.
• W406977324 title "EXTRACTION AND LEACHING" @default.
• W406977324 doi "https://doi.org/10.1016/b978-0-12-372506-6.00004-6" @default.
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