Pulsed extractor columns

FIG. 23-38 Efficiency and capacity range of small-diameter extractors, 50 to 150 mm diameter. Acetone extracted from water with toluene as the disperse phase, V /V = 1.5. Code AC = agitated cell PPC = pulsed packed column PST = pulsed sieve tray RDC = rotating disk contactor PC = packed column MS = mixer-settler ST = sieve tray. (Stichlmair, Chem. Ing. Tech. 52(3), 253-255 [1980]). 

The previous sections have pointed out that mathematical models of the processes must be proved by experiments, or adapted to experimental results with the aid of pilot extractors. For economic reasons, pilot extractors are usually much smaller than large-scale industrial apparatus. Pulsed pilot columns, for example, have a diameter between 30 and 250 mm, whereas industrial-size columns are up to 2500 mm and more in size. Thus, the question arises of whether or not the calculations or pilot experiments may be used for the dimensions of large-scale apparatus. This is a general problem for engineers. 

Fig. 26. Mechanically agitated industrial contactors, (a) mixer-settler (b) rotating-disk column (c) mixco column (d) asymmetric rotating-disk column (e) pulsed packed column (f) Podbielniak centrifugal extractor. (Reprinted from Doraiswamy, L. K and Sharma, M. M., Heterogeneous Reactions Analysis, Examples and Reactor Design, Vols. I and 2, 1984, John Wiley and Sons.)...

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). 

Fig. 6.3-4 Typical performance data of ten different extractor designs for the system tolulene/acetone/water. MS mixer-settler, SE static sieve tray column, PC static packed packed column, RDC rotating disc contactor, PSE pulsed sieve tray column, PPC pulsed packed column, RZE agitated cell extractors...

Extraction apparatus with continuous phase contact Spray column Packed column Pulsed packed column Rotating disc contactor, Oldshue-Rushton column, Graesser-Contactor, Kuhni column Podbiehiiak- Extractor, Luigi-Westfalia- Extractor, De Laval-Extractor... 

Stagewise contact with controlled coalescence redispersion cycles Tray column Pulsed sieve column. Pulsed Mixer-Settler-cascade, Extraction tower with controlled cycle Scheibel column, ARD-Extractor, Leisibach column, Mixer-Settler cascade ... 

There are many areas of appHcation for liquid/liquid extractors, where non-pulsed packed columns are preferred, e.g. for the separation of corrosive products, which must... 

Extraction can be performed in stirred tanks if the process proceeds fast and separation of phases is ea.sy, but column extractors are most commonly used. The column can be filled with a particulate material. The liquids flow countercurrently whereby the flow can be uniform or pulsed. Reciprocated and rotary agitators are often used to enhance mass transfer. An example of the latter type is shown in Fig. 7.2-13 (asymmetric rotating disk (ARD) extractor). 

The need to use multiple extraction to achieve efficient extraction required the development of new types of continuously working extractors, especially mixer-settlers and pulsed columns, which were suitable for remotely controlled operations. These new extractors could be built for continuous flow and in multiple stages, allowing very efficient isolation of substances in high yield. A good example is the production of rare earth elements in >99.999% purity in ton amounts by mixer-settler batteries containing hundreds of stages. These topics will be further developed in Chapters 6 and 7. 

Pulsing means that either the whole liquid content of a sieve tray column is continually pushed up and down by a piston that moves to and fro, or the whole plate package is moved up and down [3]. Figure 9.5 illustrates the two extractor constructions schematically. They show about the same efficiency... 

The method of calculation introduced in this chapter not only allows an exact determination of the column diameter for nonpulsed sieve tray columns, but also allows a good estimation of the diameters of pulsed and stirred extractors. For the latter, however, more exact specific equations exist for the flooding point, see for example [1,4]. 

If a rapid, reciprocating motion of relatively short amplitude is applied to the liquid contents of an extraction column, the column is said to be pulsed. The agitation thus provided has been found to give improved extraction rates. Devices of this sort were suggested as early as 1935 (VI), but it was not until the atomic energy programs in the United States and Great Britain developed a need for efficient extractors of low... 

Several methods have been used to produce the pulse action required in these columns. A reciprocating, plunger pump, from which the valves have been removed, may be connected to the extractor as at (a), Fig. 12, with a direct liquid connection between the column and the pump piston. Such an arrangement is perhaps mechanically most reliable, al-... 

The arrangement used is shown in Fig. 12(a). The horizontal plates are pierced with holes of small diameter (approximately J -in. diameter), whose total cross section may be 20 to 25% that of the column. No downspouts are provided, as for ordinary perforated-plate extractors, and both liquids must pass through the same holes. The plate spacing is usually small, 2 in. in large-diameter columns, for example. Columns of this sort may be pulsed at amplitudes of 1 in. and frequencies up to 150 cycles/min., depending upon the circumstances. 

In the tests carried out in centrifugal extractors, the extraction and recovery of cesium higher than 99.99% were obtained on simulated effluents, with a very good coherence between calculated flowsheets and experimental results.102 Tests confirmed the feasibility of the implementation of the cesium process in pulsed columns, the latter representing the most adapted contactors for the industrial implementation to overcome the drawback due to the presence of solid matter in waste to be treated. 

For some applications, instead of pulsed sieve-plate columns, vibrating-plate extractors have been recommended (Rod, 1976). Rod has pointed out some advantages of this type of contactor for the oximation of cyclohexanene. 

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