Centrifuge extractors

The first differential centrifugal extractor to be used in industry was the PodbieHiiak extractor which was introduced in the 1950s (209,210) and can be regarded as a perforated-plate column wrapped around a rotor shaft. Rotation creates a centrifugal force which results in a great reduction in the equivalent height and contact time that would be needed in a conventional perforated-plate column. 

A new countercurrent continuous centrifugal extractor developed in the former USSR (214) has the feature that mechanical seals are replaced by Hquid seals with the result that operation and maintenance are simplified the mechanical seals are an operating weak point in most centrifugal extractors. The operating units range between 400 and 1200 mm in diameter, and a capacity of 70 m /h has been reported in service. The extractors have been appHed in coke-oven refining (see Coal conversion processes), erythromycin production, lube oil refining, etc. 

Antibiotics. Solvent extraction is an important step in the recovery of many antibiotics (qv) such as penicillin [1406-05-9] streptomycin [57-92-17, novobiocin [303-81-1J, bacitracin [1405-87-4] erythromycin, and the cephalosporins. A good example is in the manufacture of penicillin (242) by a batchwise fermentation. Amyl acetate [628-63-7] or -butyl acetate [123-86-4] is used as the extraction solvent for the filtered fermentation broth. The penicillin is first extracted into the solvent from the broth at pH 2.0 to 2.5 and the extract treated with a buffet solution (pH 6) to obtain a penicillin-rich solution. Then the pH is again lowered and the penicillin is re-extracted into the solvent to yield a pure concentrated solution. Because penicillin degrades rapidly at low pH, it is necessary to perform the initial extraction as rapidly as possible for this reason centrifugal extractors are generally used. 

General Characteristics Countercurrent flow is maintained by virtue of the difference in densities of the hquids and either the force of gravity (vertical towers) or centrifugal force (centrifugal extractors). Only one of the liquids may be pumped through the equipment at any desired velocity. Tne maximum velocity for the second is then fixed if it is attempted to exceed this limit, the second hquid will be rejected and the extractor will be flooded. 

Wiped film stills in place of continuous still pots —Centrifugal extractors in place of extraction columns —Flash dryers in place of tray dryers —Continuous reactors in place of batch —Plug flow reactors in place of CFSTRs —Continuous in-line mixers in place of mixing vessels... 

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. 

Various types of proprietary centrifugal extractors are also used. 

Contaminated toluene is fed to a water wash system shown in Figure 5-16. The toluene is pumped from a 50-gal drum into a countercurrent centrifugal extractor. The extractor... 

Countercurrent contactors, design of, 10 757 Countercurrent continuous centrifugal extractor, 10 781... 

Discrete photodiodes, 19 152-153 Discrete stage centrifugal extractors,... 

If separation is difficult in a mixer-settler unit, a centrifugal extractor may be used in which the mixing and the separation stages are contained in the same unit which operates as a differential contactor. 

Figure 13.41. Working principle of Alfa-Laval centrifugal extractor...

In addition to the rotating columns previously described, there are a number of other designs for centrifugal extractors, many originally developed for the separation of radioactive wastes in nuclear processes (see Chapter 12). They are both of the mixer-settler type, as discussed in section 9.3.3, and of the rotating column types. 

Fig. 9.10 (a) Robatel four-stage centrifugal extractor. The heavy and the light liquids... 

Fig. 9.14 Flooding point diagram of centrifugal extractors (see Fig. 9.9). Curve 1 marks the lower limit, when the rotor is filled with heavy phase curve 2 marks an upper hmit, when the rotor is filled with light phase curve 3 marks the limit of the total throughput of both phases.

Fig. 9.19 The residence times of heavy droplets in a centrifugal extractor (see Fig. 9.9) with three concentric perforated sheets. Extremely short residence times are a main attribute of centrifugal extractors.

Fig. 9.21 The mass transfer performance of a centrifugal extractor (see Fig. 9.9) with four concentric sheets (stages). This is dependent on the radial path with the field force as parameter n, number of stages h, height of a stage r , radial drop path in a stage r,-, inactive radial distance (thickness of perforated sheet and of coalesced layer in a stage). (From Ref. 9.)...

These phenomena of surface energy all are time dependent. The shorter the contact time of the phases, the more difficult it is for them to develop. Therefore, flow and mass transfer in centrifugal extractors are hardly affected. 

Preselection of a suitable extractor type on the basis of available information and testing at pilot scale Establishment of suitable operating conditions as, for example, the pulse intensity of pulsed extractors or the speed of rotation of centrifugal extractors Determination of the flooding point... 

Several tests using countercurrent separation in mixer-settler or centrifugal extractors with simulated and genuine high-level waste showed that the recovery of An(III, IV, VI) is quantitative. The back extraction of An(III) was complete. These early flow sheets were not designed to strip U(VI) and Pu(IV). The distribution ratios of these ions at low acidities are lower than those measured with CMPO and one can guess that the stripping... 

Fig. 12.13 Schematic drawing for the hot test of the DIAMEX process in a 16-stage centrifugal extractor battery. (From Ref. 53.)...

Fig. 14.13 Centrifugal extractor arrangement with five CENTREK EC-320 units. 

As explained in the Introduction, conventional countercurrent extractors are frequently ineffectual because of the small difference in specific gravity which exists in most two-liquid systems. The centrifugal extractors overcome this problem by increasing the effective specific gravity difference with the help of centrifugal force. There have been many designs proposed, but only those which have had important commercial acceptance will be mentioned here. 

Eisenlohr (El) has described a three-stage centrifugal extractor based on an early design of Coutor (C6) and arranged in a vertical position, normally operated at about 3,800 r.p.m. The maximum capacity of the... 

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