Extraction mechanically agitated columns

For ELM applications, conventional solvent extraction contactors, e.g., mixer-settlers and mechanically agitated columns, have been generally employed. Recently, Raghuraman and Wiencek (63) investigated ELM extractions in a microporous hollow-fiber contactor. The hollow-fiber contactor may improve the efficiency of extraction by decreasing membrane swelling and leakage, particularly for poorly formulated ELMs with stability problems. 

Extractors with mechanical agitation, such as mixer-settlers, Kuhni columns, York-Schiebel columns, etc., should be avoided as much as possible. Up to seven theoretical stages packed extraction columns can be conveniently adopted. Sieve-plate extractors can be used up to 20 stages. When a very efficient extraction has to be carried out with expensive solutes, and for reasons of material stability and requirements of low expensive product inventory, we may have to use centrifugal extractors or hollow-fibre extractors. 

Extraction columns can be further sub-divided according to the method used to promote contact between the phases packed, plate, mechanically agitated, or pulsed columns. Various types of proprietary centrifugal extractors are also used. 

Liquid-liquid extraction can take place in a column with various internals to foster contact between the dispersed and continuous liquid phases. Internals can include sieve trays, baffle trays and packing, as well as mechanical agitation and pulsing of the liquid. 

Countercurrent columns with additional kinetic energy input have found a broad range of industrial applications [42-48]. Examples of extraction towers with energy input are pulsed towers, pulsed packed columns and pulsed perforated-plate towers. A number of units with some form of mechanical agitation are also used (Karr column, Scheibel column, Oldshue-Rushton column, Ktihni column, RZE extractor, RDC and ARD extractor, Graesser contactor). 

A second portion of the extract ( 25 mg) was dissolved as completely as possible in 5 ml -hexane using sonication and mechanical agitation. This solution was loaded onto a liquid-solid chromatography column layered from the bottom to top with activated copper (for sulfur removal), deactivated alumina (5% water), and activated silica gel (Grade 62 and 923). The column was eluted with n-hexane followed by 30% DCM in n-hexane. 

A number of countercurrent liquid-liquid extraction devices that provide mechanically agitated internals are available. These devices can be divided into two general catagories (1) columns that contain rotating components and (2) columns that provide reciprocating, or vibrating, internals. 

The mechanism of transfer of solute from one phase to the second is one of molecular and eddy diffusion and the concepts of phase equilibrium, interfacial area, and surface renewal are all similar in principle to those met in distillation and absorption, even though, in liquid-liquid extraction, dispersion is effected by mechanical means including pumping and agitation, except in standard packed columns. 

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