Mechanically Agitated Columns

A number of conmeicurrem liquid-liquid ex miction devices that provide mechanically agitated internals are available. These devices can be divided into two general catagories (I) columns that contain rotating components and (2) columas that provide reciprocating, or vibrating, internals. 

These contactors provide corn pone ms that rotate around the vertical axis of the column (see Fig. 7.6-1). Generally, a rotational drive is provided at the top of the contactor. The rotational frequency can be ndjuated to increare the interfacial area and the rate of mass transfer between the two phrees. To reduce vertical mixing, baffles are often inserted within Ihe column and interspersed between the rotating internals. A number of designs are commercially available. 

FIGURE 7.6-1 Typical types of mechenically agitaied columns. (Reprinted from Ref. 45 by permission of McGraw-Hill,) 

The effect of column diameter on the HETS varies with the square rout3 of the column diameter 

Several sredies of the drop size distribution4 2 and axial mixing have also been reported for the Yortc-Scheibel column. The more recent designs4 10 are reported ro give even higher efficiencies. Commercial columas with diameters up ro 2.59 m (8.5 ft) have been operated successfully. 

There are several versions of diis column. The earliest model was introduced around 1946 and h was the first to enjoy wide commercial qiplication. h provides altematir compartments to aid dispersion with impellers and coalescence with a wire mesh (about 97% void space). Cavity and mass tiansfer data have been developed for columns with diameters from 25 to 300 mm and in three different liquid-liquid systems. The r rted column capacity depends on the system properties but varies from about 14.000 to 24,(XX)L/h (or 350-600 gal/h - fi. The Mutphtee-siage efficiency for a 12 in. diameter column can be correlated as 

FIGURE 7.6-1 Typical types of mechanically of McGraw-Hill.) 

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Fig. 15. Mechanically agitated columns (a) Scheibel column (b) rotating-disk contactor (RDC) (c) asymmetric rotating-disk (ARD) contactor (d) Oldshue-Rushton multiple-mixer column (e) Kuhni column and (f) reciprocating-plate column.

The liquid-phase mixing in a multistage mechanically agitated reactor is best correlated by Eq. (2.31) in the absence of gas flow and by Eq. (2.32) in the presence of gas flow. The mixing time can be estimated from the study of Paca et al. (1976). Experimental work is needed to estimate gas-phase back-mixing. The use of Eq. (2.36) for the calculation of the gas-liquid volumetric mass transfer coefficient in a multistage mechanically agitated column is recommended. 

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

For ELM applications, conventional solvent extraction contactors, e.g., mixer-settlers and mechanically agitated columns, have been generally employed. Recently, Raghuraman and Wiencek (63) investigated ELM extractions in a microporous hollow-fiber contactor. The hollow-fiber contactor may improve the efficiency of extraction by decreasing membrane swelling and leakage, particularly for poorly formulated ELMs with stability problems. 

Where columns with tower packings or sieve trays are used, the difference in densities between the liquid phases is the force causing flow, as well as dispersion of one phase into the other. However, mechanically agitated columns are equipped with a series of radial mixers driven by a vertical shaft, which produce the dispersion. Also, there is a hybrid device that uses radial mixers separated by wire mesh packing. 

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