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Macro level mixing - Physical processing

Macro-level mixing topics covered here describe how mixing affects the dispersion of the two phases, which are most often aqueous and organic. Dispersion topics here include drop size, drop size distribution, creation of fines, entrainment, drop break-up mechanisms, phase continuity, flooding, dispersion stability, and operating ranges for stabile dispersions. Emulsions and the process of emulsification are described here, too. Both stabile dispersions and emulsions can often be described as a single phase and studies from Fluid Motion and Miscible Liquids can apply here when using the appropriate physical parameter averages. Reactors include the batch reactor and continuous flow reactors such as column extractors and mixer-settlers. A common application of liquid-liquid dispersion mixing is the purification of pharmaceutical intermediates. Another application is hydrometallurgy. In the mining industry, solvent extraction finds applications in the production of 99.999% copper, nickel, zinc, uranium, vanadium, and rare earths by dispersing the pregnant acidic leach solution into an organic fluid largely consisting of kerosene. Often the limiting, controlling step is just making a stabile dispersion, not the liquid-liquid mass transfer or extraction of desired products.

When two liquids are mixed and form two different phases, the difficulty is in trying to keep the macro level of mixing as uniform as possible. It is important to size a mixer so that enough energy and shear is supplied to maintain a dispersion without phase separation or puddles (high local concentration) of either liquid. It is equally important not to apply too much energy and shear, so that a large amount of fines is not produced or that an emulsion is created, unless that is desired. Often the reason for mixing two liquid phases is for liquid-liquid extraction and mass transfer, but often that is not the limiting case. Creating and maintaining the dispersion is often the controlling factor.


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