Mixer extraction cascades

Fig. 3.46 Computer simulation output for a five-stage mixer-settler cascade with entrainment. 3.3.1.10 Staged Extraction Columns...

Fig. 10.5 Flov/ sheet of three-stage countercurrent mixer-settler extraction cascade.

Liquid-Liquid Mixer Design Many different types of impellers are used for liquid-liquid extraction, including flat-blade and pitched-blade turbines, marine-type propellers, and special pump-mix impellers. With pump-mix designs, the impeller serves not only to mix the fluids, but also to move the fluids through the extraction stages of a mixer-settler cascade. The agitated vessel should be baffled if the vessel is operated with a gas-liquid surface, to avoid forming a vortex. As noted earlier in reference to Eq. (15-172), baffles are not needed if the vessel is operated with the liquid full [Weinstein and Treybal, AIChEJ., 19(2), pp. 304-312 (1973)]. 

Generally, the mass transfer between two liquid phases in liquid-liquid extraction is substantially slower than that for rectification processes. Small values of the tray efficiency factors therefore follow. With average extraction, tray efficiency factors range from 0.3-0.7 with sieve trays, and 0.3-0.6 with Koch cascade trays. Only in mixer-settler cascades and with centrifugal extractors is the actual tray efficiency almost the theoretical tray efficiency. 

Stagewise contact with controlled coalescence redispersion cycles Tray column Pulsed sieve column. Pulsed Mixer-Settler-cascade, Extraction tower with controlled cycle Scheibel column, ARD-Extractor, Leisibach column, Mixer-Settler cascade ... 

Other Metals. Because of the large number of chemical extractants available, virtually any metal can be extracted from its aqueous solution. In many cases extraction has been developed to form part of a viable process (275). A review of more recent developments in metal extraction including those for precious metals and rare earths is also available (262). In China a complex extraction process employing a cascade of 600 mixer—settlers has been developed to treat leach Hquor containing a mixture of rare earths (131). 

Normally, a number of stages of extraction work in a countercurrent cascade, with mixer-settlers. A generalized flowsheet for the Am ex process is shown in Figure 5.34. Stripping may be implemented by any one of the reagents, as indicated in the figure. Alternatively,... 

Wardius, D.S. and Hatton, T.A. (1985). A model for liquid membrane extraction with instantaneous reaction in cascaded mixers. Chem. Eng. Commun. 37, 159-71. 

Thermodynamic equilibrium between the two liquids determines the direction of mass transfer and the theoretical amount of compound(s) transferred in a given step. The rate of transfer depends on the level of agitation provided to the dispersion and the interfacial areas between the phases. After the extraction step is completed, separation of phases is (hopefully) rapid. As already indicated, the separated phases are often then sent countercurrent to another extraction unit [2]. Countercurrent cascades of mixers and settlers generally provide the most efficient use of solvent. 

TABLE 8.5-2. Comparison of Experimental and Calculated Stream Concentrations for Copper Extraction by Keiex 100 in a Three-Stage Countercurrent Cascade of Mixer-Setters... 

Acetic acid is to be extracted from a dilute aqueous solution with isopropyl ether at 298 K in a countercurrent cascade of mixer-settler units. In one of the units, the following conditions apply ... 

Hatton and coworkers have also analyzed processes involving ELMs. Using their advancing-front model as a basis, they have studied staged operations (10 1), continuous stirred tank reactors (105), and mixer cascades (106). One interesting aspect of their analysis is the effect of emulsion recycle. They analyzed the effect on extraction rate of recycling used emulsion and combining this with new emulsion. 

D12. We plan to recover acetic acid from water using 1-butanol as the solvent. Operation is at 26.7°C. The feed flow rate is 10.0 kmol/h of an aqueous solution that contains 0.0046 mole frac acetic acid. The entering solvent is pure and flows at 5.0 kmol/h. This operation will be done with three mixer-settlers arranged as a countercurrent cascade. Each mixer-settler can be assumed to be an equilibrium stage. Equilibrium data are available in Table 13-3. Find the exiting raffinate and extract mole fractions. 

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