Flow through baffles
Extractor Robatel Extractor
Podbielniak Extractor Alfa Laval Extractor
Packed Column. (Fig. 18b.) Interphase contact can be improved in the spray column by providing extensive surface for coalescence and redispersion. This surface is provided with packing which provides surface while maintaining a large open area for flow, such as Raschig rings, Berl saddles, and variants thereof. There is some loss in capacity because of the cross section occupied by the packing, but this is more than offset by the gain in improved mass transfer and lessening of continuous phase backmixing.
Packing should be chosen that preferentially is wetted by the continuous phase to discourage formation of rivulets of the dispersed phase bypassing through the column. In large diameter columns, redistribution trays should be installed to overcome potential channeling. Smaller packing size is generally more efficient, but restricts flow more, and is more prone to fouling by trapping solids. Eckert summarizes design criteria for the selection of packing for packed columns.
Perforated Plate Column. (Fig. 18c.) Sieve trays can be placed in the spray column to cause coalescence and redispersion of the dispersed phase. The trays can be designed to permit flow of both phases through the same perforations, but such trays generally have a quite narrow operating range. Generally, some sort of downcomer (or upcomer) is provided to allow a separate path for the continuous phase and one-way flow of the dispersed phase through the perforations. The density difference between the two phases and the height of coalesced phase provide the driving force for redispersion through the orifices.
In contrast with vapor-liquid columns, tray efficiencies are very low (5 to 30%) in liquid-liquid systems. The trays do limit continuous phase backmixing as well as provide drop redispersion, but at the expense of reduced capacity.
Provision of a shaft through the extraction column allows for repeated redispersion of the drops via various impellers located along the shaft. A variety of industrial equipment is available, with the differences being in the design of the impellers on the shaft for dispersion, and stators in the column for baffling and coalescence. Stirred columns offer the operator increased flexibility in operation by independent control over the dispersion process.
RDC Column. The rotating disc contactor (Fig. 19) provides for redispersion by a series of discs along the shaft, combined with a series of fixed stators. Vortices are formed in each compartment, and the shear of the fluid against the rotor or stator causes the drop breakup. In many instances, performance can be predicted from first principles, relating drop size to the energy input, and calculating slip velocity and mass transfer coefficients based on that diameter and the physical properties of the system (see Strand, Olney & Ackerman).
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