Thermodynamics, extraction processes

The separation of components by liquid-liquid extraction depends primarily on the thermodynamic equilibrium partition of those components between the two liquid phases. Knowledge of these partition relationships is essential for selecting the ratio or extraction solvent to feed that enters an extraction process and for evaluating the mass-transfer rates or theoretical stage efficiencies achieved in process equipment. Since two liquid phases that are immiscible are used, the thermodynamic equilibrium involves considerable evaluation of nonideal solutions. In the simplest case a feed solvent F contains a solute that is to be transferred into an extraction solvent S. 

The various equilibria involved in the solvent-extraction process are expressed in terms of the following thermodynamic constants ... 

In this process, the two streams flow countercurrently through the column and undergo a continuous change in composition. At any location are in dynamic rather than thermodynamic equilibium. Such processes are frequently carried out in packed columns, in which the liquid (or one of the two liquids in the case of a liquid-liquid extraction process) wets die surface of the packing, thus increasing the interfacial area available for mass transfer and, in addition, promoting high film mass transfer coefficients within each phase. 

Design of extraction processes and equipment is based on mass transfer and thermodynamic data. Among such thermodynamic data, phase equilibrium data for mixtures, that is, the distribution of components between different phases, are among the most important. Equations for the calculations of phase equilibria can be used in process simulation programs like PROCESS and ASPEN. 

Ir(IV), Pt(IV), with the states from Rh(III) being termed inert. Thus, kinetic factors tend to be more important, and reactions that should be possible from thermodynamic considerations are less successful as a result. On the other hand, the presence of small amounts of a kinetically labile complex in the solution can completely alter the situation. This is made even more confusing in that the basic chemistry of some of the elements has not been fully investigated under the conditions in the leach solutions. Consequently, a solvent extraction process to separate the precious metals must cope with a wide range of complexes in different oxidation states, which vary, often in a poorly known fashion, both in kinetic and thermodynamic stability. Therefore, different approaches have been tried and different flow sheets produced. 

Numerical methods have been used in solvent extraction chemistry for treating experimental data for many years. As shown in Chapter 4 (see section 4.14.3), modeling of extraction processes in terms of a set of assumed equilibria with adjustable (best-fit) coefficients was widely used to identify and characterize the major species formed in these processes. The improvement of this approach, directed toward studying systems of greater diversity and complexity, including corrections for thermodynamic activity... 

An alternative to the injection method for importing enzymes into a microemulsion is the phase transfer method. In this method, a layer of an aqueous enzyme solution is located under a mixture of surfactant and oil. Upon gentle shaking, the enzyme is transferred into the reverse micelles of the hydrocarbon phase. Finally, the excess of water is removed and the hydrophobic substrates can be added. The main advantage of this method is that it ensures thermodynamically stable micro emulsions with maximum water concentrations. However, the method is very time consuming. The method is often applied in order to purify, concentrate or renaturate enzymes in the reverse micellar extraction process [54-58]. 

The distribution ratios (D s) for crown-ether-based extraction processes using conventional solvents depend on two major factors (1) the thermodynamic driving force for cafion complexation by a crown efher and (2) the solvation of fhe cafion and counfer anion by the organic solvent [1,4] the former factor is usually thermodynamically favored (see Equation 10.3). Difficulties in increasing the solvent-extraction efficiency of conventional solvent-extraction systems using crown ethers as extractants lie in the... 

From a thermodynamic perspective, the solvation of ionic species (see Equations 10.6 and 10.7), such as crown-ether complexes, NO3, and SO 4, in the ILs, should be much more favored thermodynamically than those of conventional solvent extractions (Equations 10.1 and 10.2). This is one of the key advantages of using ILs in separations involving ionic species. In this case, cationic crown-ether complexes and their counter anions are not expected to form ion pairs, but to be solvated separately by ionic species from the ILs. Therefore, the extraction process using crown ethers in ILs may not be an ion-pair extraction process. 

A special class of reservoir capacity known as extraction was introduced in Chapter 4. Thermodynamically, the process is no different from the features described above, but from a use point of view, the process represents a special kind of bottle. Instead of leakage via diffusion caused by a potential energy difference, the leak arises from a shift in the partition coefficient from a change in temperature or in the solvent environment. 

An interesting property of resins impregnated with oximes or oxines is that the selectivity of, for example, copper over iron(III), approaches that of a pure solvent-extraction process only when an inert solvent is present in the pores of the resin.396 Thus, in a /S-hydroxyoxime SIR, the selectivity for copper over iron(III) improved by a factor of 20 when the solvent perchloroethylene was introduced into the SIR, and by a factor of 700 in a similar resin impregnated with 8-hydroxy-quinoline.396 This is believed to be due to kinetic and thermodynamic restrictions in the extraction of iron(III), but not of copper, at an aqueous—organic boundary.396 397... 

Thus, if the inlet and outlet compositions are known, then VjO can be determined. For an extraction process, this corresponds to the required solvent/diluent flow ratio. The ratios O jF and V/F could be determined with the appropriate substitution in one of the component balances. IRemember these ratios are determined by mass balances around the conflol volume. Nothing has been stated about thermodynamic equilibrium within the control volume.]... 

The solubility parameter approach is a thermodynamically consistent theory and it has some links with other theories such as the van der Waals internal pressure concept, the Lennard-Jones pair potentials between molecules, and entropy of mixing concepts of the lattice theories. The solubility parameter concept has found wide use in industry for nonpolar solvents (i.e. solvent selection for polymer solutions and extraction processes) as well as in academic endeavor (thermodynamics of solutions), but it is unsuccessful for solutions where polar and especially hydrogen-bonding interactions are operating. 

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