Packed towers, extraction efficiency

As the potentialities of liquid extraction as a separation method were developed, the need for efficient, continuously operated, multistage equipment became apparent. It was natural therefore to turn to devices which had been so successful in other similar fluid-contacting operations, such as the bubble-tray tower and the packed tower of distillation. These devices have proved to be disappointing in liquid-extraction service, however for example, bubble-tray towers provide tray efficiencies in liquid-extraction operations of less than 5% (S7), and conventional packed towers show heights of transfer units of 10 to 20 ft. or more (T3). 

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). 

It certainly is apparent that today s countercurrent packed extraction tower is not the ultimate in contacting and separating equipment. These huge towers, 40 to 80 feet in height, are often equivalent to only 1.5 to 4 theoretical stages. The desirability of a more compact, economical, efficient device is self-evident. 

Updated and revised ihroughom, this standard reference describes recent advances in the design of tower packings and the commercial application of packed columns such as absorption, stripping, distillation, and extraction. It reflects current changes to improve energy efficiency and reduce the environmental impact of processing operations. 

---