Mixer-Settler Cascades

Benzoic acid is to be continuously extracted from a dilute solution in water using toluene as solvent in a series of mixer-settler vessels operated in countercurrent flow. The flow rates of the feed and solvent are 1.89 and 2.84 m3/min, respectively. The residence time in each mixer is 2 min. Estimate  

To achieve a high stage efficiency (between 90 and 100%) it is necessary to provide fairly vigorous agitation. Based on the work of Skelland and Ramsay (1987), a minimum impeller rate of rotation Qmin is required for complete and uniform dispersion of one liquid into another. For a flat-blade impeller in a baffled vessel of the type discussed above, this minimum rotation rate can be estimated from 

Ap = absolute value of the difference in density between the liquids a= interfacial tension between the liquid phases D - fractional holdup in the tank of the dispersed liquid phase pM= two-phase mixture density, given by 

The viscosity of the mixture given by equation (7-36) can exceed that of either constituent (Treybal, 1980). 

The agitator power, P, can be estimated from an empirical correlation in terms of a power number, Po, which depends on an impeller Reynolds number, Re, where 

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Treybal Tower Treybal [U.S. Patent 3,325,255, 1967) Chem. Eng. Prog., 60(5), 77 (1964)] adapted a mixer-settler cascade in tower form in which the liquids are setued between stages. 

Figure 3.45. The separate mixer and settler compartments for stage n of a mixer-settler cascade. 

Figure 3.49. Multistage mixer-settler cascade with entrainment backmixing. M and m refers to mixer and S and s to settler stagers are 1 to N.

Figure 3.51. Computer simulation output for a five stage mixer settler cascade with entrainment.

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)]. 

At the high thorium and nitric acid concentrations used in this flow sheet, two solvent phases may form, one rich in thorium and TBP and the other, lean. Callow [C2] states that formation of the two solvent phases does not interfere with operation of a mixer-settler cascade, whereas difficulty would be experienced with a pulse column. Conditions at which a... 

Other devices in commercial use include a mixer-settler cascade in column form invented by R. Treybal, and pulsed sieve or plate columns with a... 

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 ... 

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