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.
Examples:
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