Multicompartment Columns

Sieve-tray towers are very effective, both with respect to liquid-handling capacity and extraction efficiency, particularly for systems of low interfacial tension which do not require mechanical agitation for good dispersion. The general assembly of plates and downspouts is much the same as for gas-liquid contact except that a weir is not required. Towers packed with the same random packing used for gas-liquid contact have also been used for liquid extractors however, mass-transfer rates are poor. It is recommended instead that sieve-tray towers be used for systems of low interfacial tension and mechanically agitated extractors for those of high interfacial tension (Treybal, 1980). 

A number of industrial centrifugal extractors have been available since 1944 when the Podbielniak (POD) extractor, with its short residence time, was successfully applied to penicillin extraction (Barson and Beyer, 1953). In the POD, several concentric sieve trays are arranged around a horizontal axis through which the two liquids flow countercurrently. Liquid inlet pressures of 4 to 7 atm are required to overcome pressure drop and centrifugal force. As many as five theoretical stages can be achieved in one unit. 

Because of the large number of important variables, an accurate estimation of column diameter for liquid-liquid contacting devices is more complex and less 

These values are scaled to commercial-size units by assuming that HETS increases with column diameter raised to an exponent, which varies from 0.2 to 0.4. 

Estimate the diameter and height of an RDC to extract acetone from a dilute toluene-acetone solution into water at 293 K. The flow rates for the dispersed organic and continuous aqueous phases are 12,250 and 11,340 kg/h, respectively. The density of the organic phase is 858 kg/m3 and that of the aqueous phase is 998 kg/m3. For the desired degree of separation, 12 equilibrium stages are required. 

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Solvent extraction systems often display nonlinear equilibrium behavior. Pratt (1983) and Pratt and Stevens (1991) have illustrated methods in which results of sufficient accuracy are obtained by representing the equilibrium curve by means of two or three straight segments for both continuous as well as multicompartment columns. Additional simple and shortcut procedures have been described by Von Stockar and Lu (1991). The extent of backmlxing In the continuous phase in Karr reciprocating-plate extraction columns (Figure 8.1.35(d)), widely used In the petroleum, pharmaceutical and hydro-metallurgical Industries, has been studied experimentally by Stella et al. (2006). 

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