Mixers-settlers mechanical agitation

Equipment suitable for reactions between hquids is represented in Fig. 23-37. Almost invariably, one of the phases is aqueous with reactants distributed between phases for instance, NaOH in water at the start and an ester in the organic phase. Such reac tions can be carried out in any kind of equipment that is suitable for physical extraction, including mixer-settlers and towers of various kinds-, empty or packed, still or agitated, either phase dispersed, provided that adequate heat transfer can be incorporated. Mechanically agitated tanks are favored because the interfacial area can be made large, as much as 100 times that of spray towers, for instance. Power requirements for L/L mixing are normally about 5 hp/1,000 gal and tip speeds of turbine-type impellers are 4.6 to 6.1 i7i/s (15 to 20 ft/s). 

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. 

The archetypal, stagewise extraction device is the mixer-settler. This consists essentially of a well-mixed agitated vessel, in which the two liquid phases are mixed and brought into intimate contact to form a two phase dispersion, which then flows into the settler for the mechanical separation of the two liquid phases by continuous decantation. The settler, in its most basic form, consists of a large empty tank, provided with weirs to allow the separated phases to discharge. The dispersion entering the settler from the mixer forms an emulsion band, from which the dispersed phase droplets coalesce into the two separate liquid phases. The mixer must adequately disperse the two phases, and the hydrodynamic conditions within the mixer are usually such that a close approach to equilibrium is obtained within the mixer. The settler therefore contributes little mass transfer function to the overall extraction device. 

Mechanical agitation Group E Rotating disc contactor Oldshue-Rushton column Zeihl column Graesser contractor Group F Scheibel column Mixer-settlers... 

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.)...

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

More often a mechanical agitator is required to form the emulsion of the two immiscible liquids at each stage. One common design is the mixer-settler (shown above). The two liquid phases are fed to the tank where the mixer imparts mechanical energy and thus disperses one of the phases as fine droplets in the second phases (usually continuous). This state of mixtures is called an emulsion. 

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. 

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