Pulsed gravity flow extractors

Mechanically Pulsed Column. The Karr column (Fig. 23) consists of perforated plates ganged on a common shaft which is oscillated by an external drive. The perforated area and hole size are much larger than in typical sieve plate operation. At high amplitude x frequency product in larger columns, the tendency for excessive backmixing can be curtailed by installation of some fixed baffles. 

Mixer-Settlers. The extraction function of bringing feed and solvent intimately together, and then allowing them to separate is frequently done in mixer-settlers. The functions may be done in separate vessels, or in different portions of the same vessels, or sequentially in the same vessel on a batch basis. As noted earlier in Fig. 3, the flows of feed and solvent can be countercurrent to each other through a series of mixer-settlers. 

Sizing of the mixer is based upon providing sufficient agitation and sufficient residence time to allow equilibrium to be approached, and thus will depend upon the flows to be processed as well as the physical properties of the two liquids. Since some extractions actually involve a chemical reaction, the time of contact can be very important. If, for reasons of improved mass transfer, it is desired to disperse the high flow phase, it may be necessary to recycle some of the low flow phase to keep an appropriate phase ratio in the mixer different than the feed flow ratio. 

The settler must provide a long enough quiescent residence time for the emulsion which is produced in the mixer to break, and a low enough lineal velocity for the two phases to become essentially free of entrainment. In some instances, coalescing material, such as wire mesh, may be installed to lessen entrainment, however, such material should be used with some caution because of the tendency for fouling by accumulation of foreign material. 

It is frequently possible to introduce one of the phases into the eye of the impeller, and thus be able to pump one entering fluid while the other flows by gravity from the next upstream and downstream stages, without the need for separate interstage pumps. 

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