In countercurrent extraction

Clearly, it would be preferable from the point of view of the purity of the product if there were high concentrations of A in both the extract and the raffinate of a stage. This can be achieved in countercurrent extraction, which allows both high recovery and the achievement of high product purity when properly designed. 

The characteristic velocity k is a function of droplet size, density difference, viscosity, etc. Thus, the holdup tends to increase either as the superficial flow velocities Uc and Ud are increased or as the characteristic velocity is reduced (e.g., by increasing agitation). A point is eventually reached where the increase in holdup becomes unstable (typically when = 0.3-0.4). This phenomenon is known as flooding, and it imposes a limit on the flow rates and agitation levels that can be used in countercurrent extraction processes. 

Interaction of Density, Viscosity and Interfacial Tension in Countercurrent Extraction with Near-Critical Fluids... 

A new potentially exciting development in this area of extractions concerns the use of different reversed micellar systems in countercurrent extractions of different rare earth metals. A mathematical model was developed in order to help optimize the different parameters of this new mode of extraction (364). This should facilitate the further development and utilization of this approach to metal ion separations. 

The oil extracted, the residual oil, and the rate of extraction are all independent of the concentration of oil in the solvent that is, there can be no advantage in countercurrent extraction. 

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. 

FIGURE 11.21 Strategy for determining the number of theoretical stages in countercurrent extraction. 

Figure 11-2 is a diagram of the component parts needed for an analysis. There are two main instrument systems in countercurrent extraction (1) the hydrostatic equilibrium system and (2) the hydrodynamic equilibrium system. In the hydrostatic equilibrium system (HSES), a stationary coil is placed in a horizontal position. In the hydrodynamic equilibrium system (HDES), the coil rotates around its own axis These will be discussed individually in more detail, but first, what happens when two immiscible liquids are placed in a tube and the tube rotated slowly will be examined. 

Resolution is a measure of how well two components are separated. In later chapters, you will learn that it involves primarily three factors k, the capacity factor, a, the selectivity, and N, the number of plates. In countercurrent extraction, another term is involved, Sf, the fraction of stationary phase in the column. Equation 11-1 shows how these are related. Equation 11-2 is easier to use in practice and gives nearly the same results. 

COUNTERCURRENT EXTRACTION OF TYPE II SYSTEMS USING REFLUX. Just as in distillation, reflux can be used in countercurrent extraction to improve the separation of the components in the feed, This method is especially effective in treating type II systems, because with a center-feed cascade and the use of reflux, the two feed components can be separated into nearly pure products. 

Supercritical fluid extraction (SFE) is a separation technique that uses sc-fluids as separating solvents. Supercritical fluids can replace other solvents in many purification procedures, even in countercurrent extraction. In synthetic chemistry, SFE can be an alternative to conventional methods for purification/isolation of complex products, for example pharmaceuticals, nutraceuticals and vitamins [12, 18j. Since SFE is still quite a young discipline, physical properties and basic parameters for many interesting compounds and mixtures are not yet known (in contrast to classical methods like distillations). Therefore, it must be pointed out that for all applications of sc-fluids the phase equilibria have to be determined properly. Unfortunately, for many technical or industrial applications of procedures based on supercritical fluids, the basic parameters are often not yet known. For industrial implementation, scale-up, miniplant, or pilot plant activities, it is absolutely necessary to have information about phase behaviour, solubility, energy balances and... 

Figure 7. Distribution coefficients in countercurrent extraction of catalytic reformate at 27°C...

A7. What situation in countercurrent extraction is superficially analogous to total reflux in distillation How does it differ ... 

Fig. 6-17. Operating diagram for constant flow rates of carrier and solvent in countercurrent extraction.

With known overall mass transfer coefficients kd and /tg, specific volumetric interface area Og and axial dispersion coefficient, the solution gives the actual concentration profile of the key component in the column. In [6.26], methods to measure the longitudinal mixing in countercurrent extraction columns are described and approaches to calculate the Bodenstein number and the axial dispersion coefficient for common extractor designs are given. 

In countercurrent extraction columns the settling time and separation area are calculated through the product of Ag- g (Ag density difference, g gravitational acceleration). For centrifugal ectractors, g is replaced by the centrifugal acceleration b = r a>. b may be a multiple of g depending on the rotor diameter r (distance of the fluid element from the axis of rotation), the... 

Many of the fundamental equations of countercurrent gas absorption and of rectification are the same or similar to those used in countercurrent extraction. Because of the frequently high solubility of the two liquid phases in each other, the equilibrium relationships in extraction are more complicated than in absorption and distillation. 

EXAMPLE 12.7-2. Number of Stages in Countercurrent Extraction Pure isopropyl ether of 450 kg/h is being used to extract an aqueous solution of 150 kg/h with 30 wt % acetic acid (y4) by c ountercurrent multistage extraction. The exit acid concentration in the aqueous phase is 10 wt %. Calculate the number of stages required. 

Ans. (a) Minimum solvent flow rate 1630 kg/h (b) 7.5 stages Number of Stages in Countercurrent Extraction. Repeat Example 12.7-2 but use an exit acid concentration in the aqueous phase of 4.0 wt %. 

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