Equipment for Solvent Extraction

Three examples of static colttrtms operated coimtereirrrently are depicted in Fig. 6.3-1. The spray colunm does not have any irrtemals. The dispersed phase (droplet phase) is moving countereurrently to the corrtinuous phase. Spray colrrmns can only be used if the density difference between the two phases is higher than 150 kg/m and if the required separation efficiency is low. Hence, spray colurrms are seldomly used in extraction. 

The static packed column, shown in Fig. 6.3-1, is more efficient with respect to mass transfer. Its design is similar to that of gas/hquid systems (e.g., absorption and distillation). In contrast, the design of a static tray column, used for solvent extraction, is completely different from the corresponding column for gas/liquid service. The sieve trays have very small hole diameters (2-4 mm) and a very small free area 2-4%) (Fig. 6.3-1). 

Pulsed columns, depicted in Fig. 6.3-2, are frequently used in solvent extraction. The design of pulsed packed colunms is identical with the design of static packed columns. Just the two-phase liquid hold-up is periodically vertically moved with the frequency /. Typically, the pulsation height is about 0.8-1.2 cm. Very common is a pulsation intensity of a / 1-2.5 cm/s. The pulsation effects a decrease of droplet size and, in turn, an increase of interfacial mass transfer rates. 

Pulsed sieve tray columns differ significantly from static sieve tray colunms as they do not have any down comers. The light and the heavy phase pass periodically through the same holes. Furthermore, pulsed tray columns have a much larger relative free area of up to 0.25 with hole diameters of 2-A mm. 

A modification of a pulsed tray column is the Karr column shown at the right-hand side of Fig. 6.3-2. By an eccentric gear at the top of the column all trays (arranged as package) are periodically moved up and down (reciprocating trays). The relative 

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