Extraction differential contactor

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)]. 

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. 

Although the most useful extraction process is with countercurrent flow in a multistage battery, other modes have some application. Calculations may be performed analytically or graphically. On flowsketches like those of Example 14.1 and elsewhere, a single box represents an extraction stage that may be made up of an individual mixer and separator. The performance of differential contactors such as packed or spray towers is commonly described as the height equivalent to a theoretical stage (HETS) in ft or m. 

Countercurrent extraction (Fig. 15-5) is an extraction scheme in which the extraction solvent enters the stage or end of the extraction farthest from where the feed F enters and the two phases pass coun-tercurrently to each other. The objective is to transfer one or more components from the feed solution F into the extract E. When a staged contactor is used, the two phases are mixed with droplets of one phase suspended in the other, but the phases are separated before leaving each stage. When a differential contactor is used, one of the phases can remain dispersed as droplets throughout the contactor as the phases pass countercurrently to each other. The dispersed phase is then allowed to coalesce at the end of the device before being discharged. 

Liquid-liquid extraction columns may be designed in three different ways (1) as a collection of equilibrium stages, (2) as a continuous differential contactor with mass transfer, or (3) using purely kinetic models. The first two methods are more commonly used (particularly the first) and, when correctly and carefully performed, they give essentially the same results. The latter method, design... 

With both staged equipment and differential contactors, availability oradequate phase-equilibrium models and rate expressions would allow application of existing correlations and simulation algorithm). For example. knowledge of metal-extraction kinetics in terms of interfacial species concentrations conld be combined with correlations of film mass transfer coefficients in a particular type of equipment to obtain the inlerfacial flux as a fuuction of bulk concentrations. Correlations or separate measurements of inierfacial area and an estimate of dispersion characteristics would allow calculation of extraction performance as a... 

Difference point in extraction calculations Differential contactors Diffuse wave Diffusion... 

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. 

Liquid liquid extraction and leaching use different equipment, even though the analysis of this equipment can be similar. Liquid-liquid extraction can be accomplished either in differential contactors or in staged extractors (Godfrey and Slater, 1995). The differential contactors are analyzed in ways that parallel the analysis of gas absorption the staged extractors depend heavily on ideas developed for distillation. In both cases, an enormous variety of equipment is used, with specific apparatus often being optimized for particular separations. 

Fig. 14.2-1. Four important types of extraction equipment. Types (a) - (c) are differential contactors, described in the same manner as gas absorption. Type (d), a three-stage mixer-setder, depends on stages, as does distillation.

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. 

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