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EXTRACTION AND LEACHING Such an assembly of mixing and separating equipment is xtraction is a process for the separation of one or represented in Figure 14.4(a), and more schematically in more components through intimate contact with a Figure 14.4(b). In the laboratory, the performance of a second immiscible liquid called a solvent. If the continuous countercurrent extractor can be simulated with a components in the original solution distribute series of batch operations in separatory funnels, as in Figure themselves differently between the two phases, separation 14.4(c). As the number of operations increases horizontally, will occur. Separation by extraction is based on this the terminal concentrations E1 and R3 approach principle. When some of the original substances are solids, the process is called leaching. In a sense, the role of solvent asymptotically those obtained in continuous equipment. in extraction is analogous to the role of enthalpy in Various kinds of more sophisticated continuous equipment distillation. The solvent-rich phase is called the extract, and also are widely used in laboratories; some are described by the carrier-rich phase is called the raffinate. A high degree of Lo et al. (1983, pp. 497–506). Laboratory work is of particular separation may be achieved with several extraction stages in importance for complex mixtures whose equilibrium series, particularly in countercurrent flow. relations are not known and for which stage requirements Processes of separation by extraction, distillation, cannot be calculated. crystallization, or adsorption sometimes are equally In mixer-separators the contact times can be made long possible. Differences in solubility, and hence of separability enough for any desired approach to equilibrium, but 80–90% by extraction, are associated with differences in chemical efficiencies are economically justifiable. If five stages are http://www.download-it.org/learning-resources.php?promoCode=&partnerID=&content=story&storyID=19976 structure, whereas differences in vapor pressure are the required to duplicate the performance of four equilibrium basis of separation by distillation. Extraction often is stages, the stage efficiency is 80%. Since mixer-separator effective at near-ambient temperatures, a valuable feature in assemblies take much floor space, they usually are the separation of thermally unstable natural mixtures or employed in batteries of at most four or five units. A large pharmaceutical substances such as penicillin. variety of more compact equipment is being used. The The simplest separation by extraction involves two simplest in concept are various kinds of tower arrangements. immiscible liquids. One liquid is composed of the carrier and The relations between their dimensions, the operating solute to be extracted. The second liquid is solvent. conditions, and the equivalent number of stages are the key Equilibria in such cases are represented conveniently on information. triangular diagrams, either equilateral or right-angled, as for Calculations of the relations between the input and example on Figures 14.2 and 14.3. Equivalent output amounts and compositions and the number of representations on rectangular coordinates also are shown. extraction stages are based on material balances and Equilibria between any number of substances are equilibrium relations. Knowledge of efficiencies and representable in terms of activity coefficient correlations capacities of the equipment then is applied to find its actual such as the UNIQUAC or NRTL. In theory, these correlations size and configuration. Since extraction processes usually involve only parameters that are derivable from are performed under adiabatic and isothermal conditions, measurements on binary mixtures, but in practice the in this respect the design problem is simpler than for resulting accuracy may be poor and some multicomponent thermal separations where enthalpy balances also are equilibrium measurements also should be used to find the involved. On the other hand, the design is complicated by parameters. Finding the parameters of these equations is a the fact that extraction is feasible only of nonideal liquid complex enough operation to require the use of a computer. mixtures. Consequently, the activity coefficient behaviors of An extensive compilation of equilibrium diagrams and two liquid phases must be taken into account or direct UNIQUAC and NRTL parameters is that of Sorensen and Arlt equilibrium data must be available. In countercurrent (1979–1980). Extensive bibliographies have been compiled extraction, critical physical properties such as interfacial by Wisniak and Tamir (1980–1981). tension and viscosities can change dramatically through the The highest degree of separation with a minimum of extraction system. The variation in physical properties must solvent is attained with a series of countercurrent stages. be evaluated carefully.
E
be the continuous phase; the type of equipment used may determine which phase is to be dispersed in the other phase. In general, distillation is used to purify liquid mixtures. However, liquid extraction should be considered when the mixture involves a:
14.1. INTRODUCTION The simplest extraction system is made up of three components: the solute (material to be extracted); the carrier, or nonsolute portion of the feed; and the solvent, which should have a low solubility in the carrier. Figure 14.1 illustrates a countercurrent extraction with a light-phase solvent. The diagram can be inverted for a heavy-phase solvent. The carrier-rich liquid leaving the extractor is referred to as the raffinate phase and the solvent-rich liquid leaving the extractor is the extract phase. The solvent may be the dispersed phase or it may
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481 Copyright ß 2010 Elsevier Inc. All rights reserved. DOI: 10.1016/B978-0-12-372506-6.00014-9
Low relative volatility (< 1:3) Removal of a nonvolatile component High heat of vaporization Thermally-sensitive components Dilute concentrations
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482 EXTRACTION AND LEACHING S+B (Extract)
A+B (Feed)
B
A - Carrier B - Soute S - Solvent
reliable method for predicting the interfacial tension. It should be noted that impurities and minor components can change the interfacial tension significantly. A high density difference promotes phase settling and potentially higher throughputs. High viscosities restrict throughput if the viscous phase is continuous, and result in poor diffusion and low mass transfer coefficients. The liquid molecular diffusion coefficient has a strong dependence on viscosity. In some applications, increasing the operating temperature may enhance the extractor performance. 14.2. EQUILIBRIUM RELATIONS
On a ternary equilibrium diagram like that of Figure 14.2, the limits of mutual solubilities are marked by the binodal curve and the compositions of phases in equilibrium by tielines. The region within the dome is two-phase and that outside is one-phase. The most S A common systems are those with one pair (Type I, Figure 14.2) and (Solvent) (Raffinate) two pairs (Type II, Figure 14.5) of partially miscible substances. Figure 14.1. Solvent extraction process For instance, of the approximately 800 sets of data collected and analyzed by Sorensen and Arlt (1979) and Arlt et al. (1987), 75% are Type I and 20% are Type II. The remaining small percentage of http://www.download-it.org/learning-resources.php?promoCode=&partnerID=&content=story&storyID=19976 Liquid extraction is utilized by a wide variety of industries. systems exhibit a considerable variety of behaviors, a few of which Applications include the recovery of aromatics, decaffeination of appear in Figure 14.5. As some of these examples show, the effect of coffee, recovery of homogeneous catalysts, manufacture of peniciltemperature on phase behavior of liquids often is very pronounced. lin, recovery of uranium and plutonium, lubricating oil extraction, Both equilateral and right triangular diagrams have the propphenol removal from aqueous wastewater, and extraction of acids erty that the compositions of mixtures of all proportions of two from aqueous streams. New applications or refinements of solvent mixtures appear on the straight line connecting the original mixextraction processes continue to be developed. tures. Moreover, the relative amounts of the original mixtures Extraction is treated as an equilibrium-stage process. Ideally, corresponding to an overall composition may be found from ratios the development of a new extraction process includes the following of line segments. Thus, on the figure of Example 14.2, the amounts steps: of extract and raffinate corresponding to an overall composition M are in the ratio E1 =RN ¼ MRN =E1 M. . Select solvent. Experimental data on only 28 quaternary systems were found . Obtain physical properties, including phase equilibria. by Sorensen and Arlt (1979) and Arlt et al. (1987), and none of . Obtain material balance. more complex systems, although a few scattered measurements do . Obtain required equilibrium stages. appear in the literature. Graphical representation of quaternary . Develop preliminary design of contactor. systems is possible but awkward, so that their behavior usually is . Obtain pilot data and stage efficiency. analyzed with equations. To a limited degree of accuracy, the phase . Compare pilot data with preliminary design. behavior of complex mixtures can be predicted from measurements . Determine effects of scale-up and recycle. on binary mixtures, and considerably better when some ternary . Obtain final design of system, including the contactor. measurements also are available. The data are correlated as activity coefficients by means of the UNIQUAC or NRTL equations. The The ideal solvent would be easily recovered from the extract. basic principle of application is that at equilibrium the activity of For example, if distillation is the method of recovery, the solventeach component is the same in both phases. In terms of activity solute mixture should have a high relative volatility, low heat of coefficients this condition is for component i, vaporization of the solute, and a high equilibrium distribution coefficient. A high distribution coefficient will translate to a low gi xi ¼ gi xi , (14:1) solvent requirement and a low extract rate fed to the solvent recovery column. These factors will minimize the capital and operating where designates the second phase. This may be rearranged into a costs associated with the distillation system. In addition to the relation of distributions of compositions between the phases, recovery aspects, the solvent should have a high selectivity (ratio of distribution coefficients), be immiscible with the carrier, have a xi ¼ (gi =gi )xi ¼ Ki xi , (14:2) low viscosity, and have a high density difference (compared to the carrier) and a moderately low interfacial tension. where Ki is the distribution coefficient. The activity coefficients are The critical physical properties that affect extractor performfunctions of the composition of the mixture and the temperature. ance include phase equilibria, interfacial tension, viscosities, denApplications to the calculation of stage requirements for extraction sities, and diffusion coefficients. In many extraction applications, are described later. these properties may change significantly with changes in chemical The distribution coefficient Ki is the ratio of activity coefficoncentration. It is important that the effect of chemical concencients and may be estimated from binary infinite dilution coeffitration on these physical properties be understood. cient data. The required equilibrium stages and solvent-to-feed ratio is determined by the phase equilibria, as discussed in Section 14.2. g1 The interfacial tension will affect the ease in creating drop size and Ki ¼ i,1 (14:3) gi interfacial area for mass transfer. Jufu et al. (1986) provide a
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14.2. EQUILIBRIUM RELATIONS
483
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Figure 14.2. Equilibria in a ternary system, type 1, with one pair of partially miscible liquids; A ¼ 1-hexene, B ¼ tetramethylene sulfone, C ¼ benzene, at 508C (R.M. De Fre, thesis, Gent, 1976). (a) Equilateral triangular plot; point P is at 20% A, 10% B, and 70% C. (b) Right triangular plot with tielines and tieline locus, the amount of A can be read off along the perpendicular to the hypotenuse or by difference. (c) Rectangular coordinate plot with tieline correlation below, also called Janecke and solvent-free coordinates.
Binary interaction parameters (Aij ) and infinite dilution activity coefficients are available for a wide variety of binary pairs. Therefore the ratio of the solute infinite dilution coefficient in solvent-rich phase to that of the second phase ( ) will provide an estimate of the equilibrium distribution coefficient. The method can provide a reasonable estimate of the distribution coefficient for dilute cases. n o RT ln g1 i,j ¼ Ai,j Ki ¼
g1 eAij =RT i ,1 ¼ A gi e ij =RT
(14:4) (14:5)
See Example 14.1 Extraction behavior of highly complex mixtures usually can be known only from experiment. The simplest equipment for that purpose is the separatory funnel, but complex operations can be simulated with proper procedures, for instance, as in Figure 14.4(c). Elaborate automatic laboratory equipment is in use. One of them employs a 10,000–25,000 rpm mixer with a residence time of 0.3–5.0 sec, followed by a highly efficient centrifuge and two
chromatographs for analysis of the two phases (Lo et al., 1983, pp. 507). Compositions of petroleum mixtures sometimes are represented adequately in terms of some physical property. Three examples appear in Figure 14.6. Straight line combining of mixtures still is valid on such diagrams. Basically, compositions of phases in equilibrium are indicated with tielines. For convenience of interpolation and to reduce the clutter, however, various kinds of tieline loci may be constructed, usually as loci of intersections of projections from the two ends of the tielines. In Figure 14.2 the projections are parallel to the base and to the hypotenuse, whereas in Figures 14.3 and 14.7 they are horizontal and vertical. Several tieline correlations in equation form have been proposed, of which three may be presented. They are expressed in weight fractions identified with these subscripts: CA solute C in diluent phase A CS solute C in solvent phase S SS solvent S in solvent phase S AA diluent A in diluent phase A AS diluent A in solvent phase S SA solvent S in diluent phase A.
Chapter extract
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