Extractor agitated column

The earliest large-scale continuous industrial extraction equipment consisted of mixer—settlers and open-spray columns. The vertical stacking of a series of mixer—settlers was a feature of a patented column in 1935 (96) in which countercurrent flow occurred because of density difference between the phases, avoiding the necessity for interstage pumping. This was a precursor of the agitated column contactors which have been developed and commercialized since the late 1940s. There are several texts (1,2,6,97—98) and reviews (99—100) available that describe the various types of extractors. 

The RE process proceeds in three major types of equipment mixer-settler systems, column extractors, and centrifugal extractors. Countercurrent column extractors can be further subdivided into nonagitated nonproprietary columns and agitated proprietary extractors. Agitating the liquid-liquid system breaks up droplets and increases the interfacial area to improve the mass transfer and column efficiency. Various forms of energy input are used, e.g., rotation of propellers, impellers, and discs pulsation, vibration, and ultrasonic devices and centrifugal devices. 

A = Unagitated Columns B = Mechanically Agitated Columns C = Mixer - Settlers D = Centrifugal Extractors... 

Figures 6-41 to 6-43, provide information on operating ranges, loading limits, and separation efficiencies of selected agitated columns. Respective data are also given for certain substance systems with a certain mass transfer direction and apparatus geometry. Transformation to full-scale apparatus is only possible based on experimental scale-up knowledge (see, for example, Eq. (6-56)). Optimized dimensions of experimental extractors cannot usually be transferred to technical extractors, though the enlargement in an axial direction is generally smaller than in a radial direction.

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]). 

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 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). 

With liquid feed solutions, however, it is possible to work in a manner analogous to traditional solvent extraction. Pressurized columns can be of the packed-bed type or agitated by magnetic stirrers. Because of the efforts of pilot plant tests, much of the scale-up work has to be carried out in laboratory extractors. From solubility measurements, it is possible to determine parameters in thermodynamic models (e.g., equations of state), which can be used for the simulation of large-scale applications. 

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... 

Figure 11 Agitated extractors. Left to right RDC, Karr, Oldshue-Rushton, Scheibel, Kuhni extraction columns. 

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.)...

To complete sizing the Karr column requires sizing the electric motor. The size of the electric motor to disperse one of the phases is small. Walas [51] states that a 1.5 hp (1.12 kW) motor is sufficient to agitate a Karr extractor 30 in (0.762... 

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. 

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