Extraction stage efficiency

The Murphree stage efficiencies are also in common use. In terms of the extract the Murphree extract stage efficiency Eme is defined as the ratio of the number of moles of solute actually transferred to the number which would be transferred if the final extract were in equilibrium with the actual final raffinate. Preservation of a material balance does not permit such final equilibrium concentrations actually to develop, but in terms of the concentrations shown in Fig. 2 this becomes... 

Eme Murphree extract stage efficiency, as a fraction, dimensionless... 

In the same article Treybal utilizes information on mass transfer from solids and heat transfer to spheres, to arrive at a procedure for making rough estimates of extraction stage efficiencies for baffled vessels agitated with flat-blade turbine impellers. Results from this calculation procedure are in general agreement with the experimental results of Flynn and Treybal and of Overcashier et al. 

In practice, when the extraction process is completed in the mixer and the two phases are separated in a settler, the two phases are often not at equilibrium with each other. In such a case, the end of the process is located at a point R (xjR, Xie). The extent of approach to equilibrium achieved by each phase is frequently estimated in the following way. The extract-phase final composition achieved is X( . However, if the raffinate phase leaving the mixer at a composition Xir were in equilibrium with this extract phase of composition Xjg, then this hypothetical extract composition would have a higher value, xj (corresponding to point C on the equilibrium curve in Figure 6.4.8(c)). An estimate of this deficiency is called the Murphree extract-stage efficiency, Eme-... 

The separation of components by liquid-liquid extraction depends primarily on the thermodynamic equilibrium partition of those components between the two liquid phases. Knowledge of these partition relationships is essential for selecting the ratio or extraction solvent to feed that enters an extraction process and for evaluating the mass-transfer rates or theoretical stage efficiencies achieved in process equipment. Since two liquid phases that are immiscible are used, the thermodynamic equilibrium involves considerable evaluation of nonideal solutions. In the simplest case a feed solvent F contains a solute that is to be transferred into an extraction solvent S. 

The overall stage efficiency of a staged extraction system is simply the number of theoretical stages divided by the number of actual stages times 100 [Eq. (15-28)]. 

Flynn and Treybal [Am. Inst. Chem. Eng. J., I,. 324 (1955)]. Continuous extraction of benzoic acid from toluene and kerosine into water baffled vessels, turbine agitators. Stage efficiency is correlated with agitator energy per unit of liquid treated. 

FIG. 15-47 Correlation of mixing-stage efficiency with power input and liquid flow rates. [Scheihel in Lo, Baird, Hanson, Handbook of Solvent Extraction, p. 428 John Wiley h- Sons, NY, 1983. Used with peimission.]... 

The actual stage can be a mixing vessel, as in a mixer-settler used for solvent extraction applications, or a plate of a distillation or gas absorption column. In order to allow for non-ideal conditions in which the compositions of the two exit streams do not achieve full equilibrium, an actual number of stages can be related to the number of theoretical stages, via the use of a stage-efficiency factor. 

E Activation energy of reaction (kJ kmol 3), or entrainer flowrate in azeotropic and extractive distillation (kg-s, kmol s-1), or extract flowrate in liquid-liquid extraction (kg s-1, kmol-s-1), or stage efficiency in separation (-)... 

Number of stages of extraction, scrubbing, and stripping Stage efficiency Flow capacity Droplet size... 

The flow sheet is similar to the zinc block diagram with two extraction stages, containing three parallel mixers followed by a settler, and two stripping stages with normal mixer-settler arrangement. The pH is monitored and the acidity controlled by the addition of NaOH in the two first mixers. The extraction efficiency for nickel is better than 98%. 

Should the extraction be continued until substantial equilibrium between the phases occurs, then the material balance equation (7) shows that the concentrations in the liquids move along line AB extended until the equilibrium curve is reached at C, giving rise to the ultimate equilibrium concentrations xe and ye. A fractional stage efficiency E may then logically be defined (T3) as the ratio of the number of moles N of solute actually transferred in an extraction to Ne, the moles which would be transferred should equilibrium be reached ... 

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