Extraction, partially miscible

Extraction is a process where one or more solutes are removed from a liquid by transferring the solute(s) into a second liquid phase. The second liquid phase, the solvent, is a mass separating agent that must be recovered later. The two liquid phases must be immiscible (that is, insoluble in each other) or partially immiscible. In this chapter we discuss extraction equipment, immiscible extraction, partially miscible extraction, and equipment design. The separation is based on different solubilities of the solute in the two phases. Since vaporization is not required, extraction can be done at low tenperature and is a gentle process suitable for unstable molecules such as proteins or DNA. 

Best ternary predictions are usually obtained for mixtures having a very broad two-phase region, i.e., where the two partially miscible liquids have only small mutual solubilities. Fortunately, this is the type of ternary that is most often used in commercial liquid-liquid extraction. 

In the isolation of organic compounds from aqueous solutions, use is frequently made of the fact that the solubility of many organic substances in water is considerably decreased by the presence of dissolved inorganic salts (sodium chloride, calcium chloride, ammonium sulphate, etc.). This is the so-called salting-out effect. A further advantage is that the solubility of partially miscible organic solvents, such as ether, is considerably less in the salt solution, thus reducing the loss of solvent in extractions. 

Robbins ( Oquid-Liquid Extraction, in Schweitzer, Handbook of Separation Techniques for Chemical Engineers, McGraw-Hill, New York, 1979, sec. 1.9) reported that most liquid-liquid extrac tion systems can be treated as having either (A) immiscible solvents, (B) partially miscible solvents with a low solute concentration in the extract, or (C) partially miscible solvents with a high solute concentration in the extract. 

In case B the solvents are partially miscible, and the miscibihty is nearly constant through the extractor. This frequently occurs when all solute concentrations are relatively low. The feed stream is assumed to dissolve extraction solvent only in the feed stage and to retain the same amount throughout the extractor. Likewise, the extraction solvent is assumed to dissolve feed solvent only in the raffinate stage. With these assumptions the primary extraction-solvent rate moving through the extractor is assumed to be S, and the primary feed-... 

Separation of two liquid phases, immiscible or partially miscible liquids, is a common requirement in the process industries. For example, in the unit operation of liquid-liquid extraction the liquid contacting step must be followed by a separation stage (Chapter 11, Section 11.16). It is also frequently necessary to separate small quantities of entrained water from process streams. The simplest form of equipment used to separate liquid phases is the gravity settling tank, the decanter. Various proprietary equipment is also used to promote coalescence and improve separation in difficult systems, or where emulsions are likely to form. Centrifugal separators are also used. 

Solutes have differing solubilities in different liqnids dne to variations in the strength of the interaction of solnte molecnles with those of the solvent. Thus, in a system of two immiscible or only partially miscible solvents, different solutes become unevenly distribnted between the two solvent phases, and as noted earlier, this is the basis for the solvent extraction technique. In this context, solvent almost invariably means organic solvent. This uneven distribution is illustrated in Fig. 1.3, which shows the extractability into a kerosene solution of the different metals that appear when stainless steel is dissolved in aqueous acid chloride solution. The metals Mo, Zn, and Fe(III) are easily extracted into the organic solvent mixture at low chloride ion concentration, and Cu, Co, Fe(ll), and Mn at intermediate concentration, while even at the highest chloride concentration in the system, Ni and Cr are poorly extracted. This is used industrially for separating the metals in super-alloy scrap in order to recover the most valuable ones. 

In an ideal case, an ionic liquid dissolves the catalyst and displays a partial miscibility with the reactants under reaction conditions (giving a relatively high reaction rate) and negligible miscibility with the product (giving enhanced selectivity and yield). At the termination of the reaction, the product can be removed by simple decantation without the need to extract the catalyst. This mode of operation eliminates heating and therefore results in reduced loss of catalyst by thermal decomposition (/). 

As an alternative to distillation, extraetion with a eo-solvent that is poorly mis-eible with the ionie liquid has often been used. There are many solvents that can be used to extract product from the ionic liquid phase, whether from a monophase reaction or from a partially miscible system. Typical solvents are alkanes and ethers (15). Supercritical CO2 (SCCO2) was recently shown to be a potential alternative solvent for extraction of organics from ionic liquids (22). CO2 has a remarkably high solubility in ionic liquids. The SCCO2 dissolves quite well in ionic liquids to facilitate extraction, but there is no appreciable ionic liquid solubilization in the CO2 phase in the supercritical state. As a result, pure products can be recovered. For example, about 0.5 mol fraction of CO2 was dissolved at 40°C and 50 bar pressure in [BMIMJPFe, but the total volume was only swelled by 10%. Therefore, supercritical CO2 may be applied to extract a wide variety of solutes from ionic liquids, without product contamination by the ionic liquid (29). 

Liquid-Liquid Extraction Principle. If a liquid solvent which is either immiscible or only partially miscible is mixed with a solution containing solute A, the solute will distribute between the two liquids until equilibrium is established. The solute s concentration in the two phases at equilibrium will depend on its relative affinity for the two solvents. Although... 

CH2Cl2 is partially miscible with ethylene glycol. For this reason, and because of the dark color of both phases, in the first extractions it is quite difficult to see a phase separation at this stage the difference in the viscosity of the solvents is quite useful for their separation. 

Several liquid phases coexist in a system when the solvents are not completely miscible. Liquid-liquid equilibrium properties are very useful in solvent extraction and in biotransformation or enzymatic syntheses in two-solvent systems. One speaks about liquid-liquid equilibrium in two cases (1) if the two solvents are not completely miscible, it is said that there is partial miscibility of the two solvents (2) if there is distribution of a compound in the two non-miscible solvents. 

In solvent extraction, the species to be separated is transferred between two immiscible or partially miscible phases, such as water and a nonpolar organic phase. To achieve sufficient solubility in the organic phase, the species must be in the form of a neutral, nonhydrated species. The transfer between phases is achieved by selectively complexing the species of interest causing its solubility in water to decrease with a concomitant increase in its solubility in the organic phase. 

Liquid-liquid extraction is based on partial miscibility of liquids. In the simplest extraction system, two compounds have to be separated. This can be done by extracting with a carefully selected solvent, in which one compound (solute) easily dissolves whereas the other (nonsolute) does not. The solvent has to be recovered... 

Partitioning of components between two immiscible or partially miscible phases is the basis of classical solvent extraction widely used in numerous separations of industrial interest. Extraction is mostly realized in systems with dispergation of one phase into the second phase. Dispergation could be one origin of problems in many systems of interest, like entrainment of organic solvent into aqueous raffinate, formation of stable, difficult-to-separate emulsions, and so on. To solve these problems new ways of contacting of liquids have been developed. An idea to perform separations in three-phase systems with a liquid membrane is relatively new. The first papers on supported liquid membranes (SLM) appeared in 1967 [1, 2] and the first patent on emulsion liquid membrane was issued in 1968 [3], If two miscible fluids are separated by a liquid, which is immiscible with them, but enables a mass transport between the fluids, a liquid membrane (LM) is formed. A liquid membrane enables transport of components between two fluids at different rates and in this way to perform separation. When all three phases are liquid this process is called pertraction (PT). In most processes with liquids membrane contact of phases is realized without dispergation of phases. 

Crude phosphoric acid is often black and contains dissolved metals and fluorine, and dissolved and colloidal organic compounds. Suspended solid impurities are usually removed by settling and solvent extraction (using a partially miscible solvent, such as n-butanol, /.so-butanol, or n-heptanol), or solvent precipitation is used to remove the dissolved impurities. The phosphoric acid is extracted, and the impurities are left behind. Back-extraction with water recovers the purified phosphoric acid. Solvent precipitation uses a completely miscible solvent plus alkalis or ammonia to precipitate the impurities as phosphate salts. After filtration, the solvent is separated by distillation and recycled. 

Liquid-liquid extraction is a separation unit used to transfer a substance from one liquid phase to another liquid phase. The unit exploits the difference in solubility between these partially miscible phases. 

Liquid extraction is a separation process in which a liquid feed solution is combined with a second solvent that is immiscible or nearly immiscible with the feed solvent, causing some (and ideally most) of the solute to transfer to the phase containing the second solvent. The distribution coefficient is the ratio of the solute mass fractions in the two phases at equilibrium. Its value determines how much solvent must be added to the feed solution to achieve a specified solute transfer. When the two solvents are partially miscible, a triangular phase diagram like that in Figure 6.6-1 simplifies balance calculations on extraction processes,... 

For liquid-liquid extraction, a selective solvent is required, which shows only a partial miscibility with the liquid stream to be separated. In Figure 2, a typical multistage counter-current liquid-liquid extraction process is shown for the separation of aliphatics from aromatics. In this process, with the help of the selective solvent (extractant) the desired components (aromatics) are extracted from the feed stream. Distillation is typically used for the recovery of the selective solvent from the extract and raffinate stream leaving the extraction column. 

The polarity of some l-alkyl-3-methylimidazolium ionic liquids has been probed using the solvatochomic dye Nile Red (Figure 1(d)), and found to be comparable to that of lower alcohols.31 As such, polar organic solvents like dichloromethane and diethylether are miscible with ionic liquids, solvents of low polarity show partial miscibility, and nonpolar solvents are essentially immiscible. Extractants such as crown ethers can be used to extract cations such as Na+, Cs+, and Sr2+ from ionic liquids.32... 

Industrial liquid-liquid extraction most often involves processing two immiscible or partially miscible liquids in the form of a dispersion of droplets of one liquid (the dispersed phase) suspended in the other liquid (the continuous phase). The dispersion will exhibit a distribution of drop diameters d, often characterized by the volume to surface area average diameter or Sauter mean drop diameter. The term emulsion generally refers to a liquid-liquid dispersion with a dispersed-phase mean drop diameter on the order of 1 pm or less. 

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