Differential Stagewise Extraction

With differential stagewise (discontinuous) extraction, the solvent Lg and mixture F are intensively mixed in a vessel (Fig. 6-11). The raffinate R and extract E are formed at the end of the extraction. The extract phase is continuously withdrawn, and then separated by distillation into the key component S and the recycled solvent L, for example. 

In comparison with simple single stage extraction, the continuous reflux of solvent from the extract phase results in improved extraction of the key component from the mixture. 

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Eq. (6-24) may be also applied to differential stagewise extraction operated as a stagewise cross-flow extraction process, with the number of extraction cycles tending to infinity (n oo). The extract phase may be withdrawn continuously and fresh solvent is fed. For TV, - oo. 

As single stage extraction, also differential stagewise with recycled solvent (simplest and in laboratory-scale most common variation)... 

The concept of a mass-transfer unit was developed many years ago to represent more rigorously what happens in a differential contactor rather than a stagewise contactor. For a straight operating line and a straight equilibrium line with an intercept of zero, the equation for calculating the number of mass-transfer units based on the overall raffinate phase N r is identical to the Kremser equation except for the denominator when the extraction factor is not equal to 1.0 [Eq. (15-23)]. 

A wide variety of extraction column forms are used in solvent extraction applications and many of these, such as rotary-disc contactors (RDC), Oldshue-Rushton columns, and sieve-plate column extractors, have rather distinct compartments and a geometry, which lends itself to an analysis of column performance in terms of a stagewise model. As the compositions of the phases do not come to equilibrium at any stage, however, the behaviour of the column is therefore basically differential in nature. 

At the prevailing high levels of dispersion normally encountered in such types of extraction columns, the behaviour of these essentially differential type contactors, however, can be represented by the use of a non-equilibrium stagewise model. 

The principle of the perfectly-mixed stirred tank has been discussed previously in Section 1.2.2, and this provides an essential building block for modelling applications. In this section, the concept is applied to tank type reactor systems and stagewise mass transfer applications, such that the resulting model equations often appear in the form of linked sets of first-order difference differential equations. Solution by digital simulation works well for small problems, in which the number of equations are relatively small and where the problem is not compounded by stiffness or by the need for iterative procedures. For these reasons, the dynamic modelling of the continuous distillation columns in this section is intended only as a demonstration of method, rather than as a realistic attempt at solution. For the solution of complex distillation and extraction problems, the reader is referred to commercial dynamic simulation packages. 

There followed a brief discussion of equipment for carrying out solvent extraction in industrial practice, both by stagewise and differential contact. Some of the first principles for the design of differential contactors were outlined and the part played by the efficiency of extraction in continuous equipment was discussed. Finally there was an outline of methods for the control of solvent loss which forms probably the most important environmental aspect of the application of solvent extraction. 

Equipment customarily used for physical liquid-liquid extraction, may also be employed for reactive systems. They may be operated batch or semibatchwise for small scale productions(in the form of the usual mechanically agitated vessels) and continuously for large scale productions. The latter may involve both stagewise and differential contactors and Table 2 shows the classification of them according to Laddha and Degaleesan (5). ... 

The increasing diversity in the applications of liquid extraction has led to a correspondingly diverse proliferation of extraction devices that continue to be developed. This chapter focuses on those fundamental principles of diiiusion, mass transfer, phase equilibrium, and solvent selection that provide a unifying basis for the entire operation. Design procedures for both stagewise and differential contactors alM receive considemtion, including packed and perforated plate columns and mixer-settlers. Some mechanically aided columns are discussed and an attempt is made to conqiare the performance of various equipment tteigns. 

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