Phase equilibrium extraction processes

The extraction and back-extraction steps take place consecutively, connected by the concentration of the metal-extractant complex species in the organic phase. The description of the back-extraction process is carried out using similar equations to those used in the extraction process. The equilibrium of the interfacial reaction between the organic complex species and the back-extraction agent is applied in this case. 

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 liquid-liquid extraction process is based on the specific distribution of dissolved components between two immiscible fluids, for instance, between aqueous and organic liquids. The process refers to a mass exchange processes in which the mass transport of component (j) from phase (1) to phase (2) by means of convection or molecular diffusion acts to achieve the chemical potential (p) equilibrium (134) ... 

In processing, it is frequently necessary to separate a mixture into its components and, in a physical process, differences in a particular property are exploited as the basis for the separation process. Thus, fractional distillation depends on differences in volatility. gas absorption on differences in solubility of the gases in a selective absorbent and, similarly, liquid-liquid extraction is based on on the selectivity of an immiscible liquid solvent for one of the constituents. The rate at which the process takes place is dependent both on the driving force (concentration difference) and on the mass transfer resistance. In most of these applications, mass transfer takes place across a phase boundary where the concentrations on either side of the interface are related by the phase equilibrium relationship. Where a chemical reaction takes place during the course of the mass transfer process, the overall transfer rate depends on both the chemical kinetics of the reaction and on the mass transfer resistance, and it is important to understand the relative significance of these two factors in any practical application. 

A second extraction is considered, and in that case the weight of the solute left in the aqueous phase is taken to be m2. Like those while on first extraction, the equilibrium concentration in aqueous phase is equal to m2/ Va and the equilibrium concentration in organic phase is equal to (m1 - m2)/V0. The expression for D in the second extraction process carried out is as follows ... 

In many practical situations solute A may dissociate, polymerize or form complexes with some other component of the sample or interact with one of the solvents. In these circumstances the value of KD does not reflect the overall distribution of the solute between the two phases as it refers only to the distributing species. Analytically, the total amount of solute present in each phase at equilibrium is of prime importance, and the extraction process is therefore better discussed in terms of the distribution ratio D where... 

Extraction is an equilibrium process, and therefore a finite amount of solute might be in both phases, necessitating other processing steps or manipulation of the chemical equilibria. 

Solvent extraction deals with the transport of chemical substances from one phase into another one, the chemical kinetics of this process, and the final equilibrium distribution of the substances between the two phases. Such transport and distribution processes are the motors that make life in biological systems possible. Fundamental studies of such solvent extraction processes contribute to the better understanding of all processes in nature. Here, only the lack of imagination stands in the way of important new scientific discoveries. 

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. 

Solvent extraction processes usually run at ambient pressures and temperatures. If higher pressures are applied, it is mostly because a higher extraction temperature is required when equilibrium or mass transfer conditions are more favorable at an elevated temperature. Distillation, on the other hand, is usually carried out at higher temperatures and ambient pressures. To avoid thermal degradation, the pressure sometimes has to be lowered below ambient pressure. Distillation is based on the differences in vapor pressures of the components to be separated, whereas solvent extraction utilizes the differences in intermolecular interactions in the liquid phase. 

The term chromatography now embraces a variety of processes that are based on the differential distribution of the components in a chemical mixture between two phases. The difference between extraction processes involving a single equilibrium of two bulk phases and chromatography is... 

Matte-slag-gas reactions in Cu-Fe-Ni sulphide ores. Sulphide ores are a major source of Cu, Ni and precious metals. A basic principle of the extraction processes is to blow air into the molten sulphide in order to oxidise (1) S, which forms a gas and (2) Fe, which forms predominantly FeO and then partitions to a slag phase which covers the matte. A key element in the recovery of the metals is the solidification of the matte which separates into a sulphur-rich matte and metal-rich liquid. This process may occur under non-equilibrium conditions with precious metals concentrating in the last metallic liquid. 

No other solvent extraction process other than the CO2 technology allows such a strong influence on loading, phase equilibrium, and selectivity. Unfortunately, the solubility of extracted substances in CO2 is relatively low, compared with the usual solvents which give absolute miscibility with the extracted valuable materials in most cases. The determination of solubility and solvent ratios is therefore important for the economy of the process. 

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. 

Equilibrium and selectivity constitute important aspects of reactive and nonreactive extraction processes. Another important factor is the reaction kinetics, which has to be reasonably fast. Most RE processes are close to equilibrium in less than five minutes. Many ion exchangers need reaction times of less than one minute, and thus diffusion of the solute complex in the organic phase is the rate-determining step. 

Treybal, in his book Liquid Extraction [1], works equilibrium material balances with triangular coordinates. The most unique and simple way to show three-phase equilibrium is a triangular diagram (Fig. 7.1), which is used for extraction unit operation in cumene synthesis plants [2], In this process benzene liquid is used as the solvent to extract acetic acid (the solute) from the liquid water phase (the feed-raffinate). The curve D,S,P,F,M is the equilibrium curve. Note that every point inside the triangle has some amount of each of the three components. Points A,... 

The potential of supercritical extraction, a separation process in which a gas above its critical temperature is used as a solvent, has been widely recognized in the recent years. The first proposed applications have involved mainly compounds of low volatility, and processes that utilize supercritical fluids for the separation of solids from natural matrices (such as caffeine from coffee beans) are already in industrial operation. The use of supercritical fluids for separation of liquid mixtures, although of wider applicability, has been less well studied as the minimum number of components for any such separation is three (the solvent, and a binary mixture of components to be separated). The experimental study of phase equilibrium in ternary mixtures at high pressures is complicated and theoretical methods to correlate the observed phase behavior are lacking. 

Processes for supercritical extraction of oils have been described in numerous literature references, including Paulaitis et al. ( ), Ely and Baker (2), Gerard (3.), Stahl et al. (4K and Robey and Sunder (5). The literature lacks detailed phase equilibrium data on multicomponent essential oils with supercritical solvents in the proximity of the solvent critical temperature. 

Americium is separated from plutonium by a liquid-liquid extraction process involving immiscible molten salt and molten plutonium metal phases. The molten salt extraction process is based upon equilibrium partitioning (by oxidation-reduction reactions) of americium and plutonium between the molten chloride salt and molten plutonium metal phases. 

Two unit operations are used in the equilibration of the salt and metal phases (1) intermixing of salt and metal, and (2) disengagement of salt and metal. Because this is a batch extraction, both operations (intermixing and disengagement of phases) occur sequentially in the same vessel. For practical operation of the molten salt extraction process, attainment of equilibrium or near-equilibrium conditions (when the value of F approaches 1) in a relatively short period of time is essential. 

Martin and Synge (3) introduced the important concept of theoretical plates into chromatography. Their concept was derived from partition theory and random statistics, and was related to similar ideas developed for extraction and fractional distillation. They supposed that the column could be divided into a number of sections called theoretical plates, and that solutes (dissolved compounds) could be expected to achieve equilibrium between the two phases (mobile and stationary) that exist within each plate. The chromatographic process, like an extraction process, can be visualized to occur when mobile phase (solvent) is transferred to the next plate, where a new equilibrium is established. Theoretical plate numbers of 1000 or more are common for HPLC columns, which means that 1000 separate equilibria must be established to obtain the same degree of separation by solvent... 

Reactive distillation is in theory a simpler process than extractive distillation, but it has yet to be demonstrated experimentally. There are two key differences between reactive and extractive distillation. First, unlike the extractive process, the HI, azeotrope is not broken, so the composition in both the liquid and vapor phases is the same. Second, the reactive process must be conducted under pressure. Figure 4.7 shows a schematic of the reactive distillation flow sheet, and the processing conditions are listed in table 4.4. In this process, azeotropic HI, is distilled inside a pressurized reactive column and the HI gas within the HI vapor stream is decomposed catalytically, resulting in a gas mixture of HI, Ij, H2, and H2O. To accomplish this, the HI feed from Section I is first heated to 262°C from 120°C and is then fed into the reactive column. At the bottom of the column, the HI is brought to a boil at around 310°C, and this boiling HI vapor results in an equilibrium vapor pressure of 750 psi inside the distillation column. 

The treatment of partition equilibrium was further generalized to the cases in the presence of ion-pair formation [19] and ion-ionophore complex formation [21]. An important corollary of this theory of partition equilibrium based on standard ion transfer potentials of single ions is to give a new interpretation to liquid extraction processes. Kakutani et al. analyzed the extraction of anions with tris(l,10-phenan-throline) iron(II) cation from the aqueous phase to nitrobenzene [22], which demonstrated the effectiveness of the theory and gave a theoretical backbone for ion-pair extraction from an electrochemical point of view. 

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,... 

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