Figure 21. Raining bucket contactor.
There is little, if any, control of droplet size, but the raining bucket contactor is the only one that disperses each phase in the other. If the flow ratio differs greatly from unity, backmixing of the low flow phase can be serious, and line out with changed operating conditions can take a long time.
Liquid Pulsed Columns. The liquid in a packed or perforated plate column may be pulsed to promote better mass transfer (Fig. 22). If a sieve plate column is pulsed, downcomers are no longer required. Pulsing can be caused by a piston pump or by air pulsing external to the column. Frequencies are generally 1 to 3 Hz and amplitude up to 20 mm. Drop size is dependent upon the product of amplitude times frequency. As this product is increased, the smaller diameter drops so produced lead to more holdup and better mass transfer, but to a fall off in capacity. Eventually, at a high enough amplitude x frequency product, backmixing increases to the extent that efficiency also begins to diminish.
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.
Figure 23. Karr reciprocating plate column.
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.
Many of the commercial extraction processes encountered in the pharmaceutical industry involve systems which emulsify readily and are exceedingly difficult to separate cleanly. Stability of the solute may also be a factor, and rapid separation may be required to prevent degradation and loss of the product. Centrifugal extractors fill an important niche for just such problems.
The most common centrifugal extractor is the Podbielniak® Contactor, as shown in cutaway view in Fig. 24. Essentially it is a sieve plate column that has been wrapped around a shaft and spun to create a multigravitational force to do both the redispersion and the separation. All fluids enter and leave through shaft passageways and mechanical seals.
The performance of centrifugal extractors has been described by Todd and Davies in general detail'91 and specifically for pharmaceutical use.1101 The primary benefits of centrifugal extractors accrue from their compactness and superior clarifying capabilities. Solvent inventory can be held to a minimum. Centrifugal extractors are also particularly appropriate handling high phase ratios, as the low flow phase can be kept continuous without much backmixing, thereby allowing the large flow fluid to be dispersed to provide more mass transfer area.
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