Extraction plait point

Capacity. This property refers to the loading of solute per weight of extraction solvent that can be achieved in an extrac t layer at the plait point in a Type I system or at the solubihty hmit in a Type II system. 

Thus the extractor column raffinate outlet rate and the solvent inlet rate are approximately equal. This is indeed the minimum solvent rate allowed, since a lower rate will overload the solvent, referencing the plait point. This rate will also set the required minimum extractor column diameter. For some refinery-type extraction operations, such as lube oil extractors, where relatively much larger solvent-raffinate rates apply, this method for determining minimum solvent rate is very economical and desirable. 

Density. The difference in density between the two liquid phases in equilibrium affects the countercurrent flow rates that can be achieved in extraction equipment as well as the coalescence rates. The density difference decreases to zero at a plait point, but in some systems it can become zero at an intermediate solute concentration (isopycnic, or twin-density, tie line) and can invert the phases at higher concentrations. Differential types of extractors cannot cross such a solute concentration, but mixer-settlers can. 

Figure 5.10. Equilibration paths during mass transfer in the system glycerol (1), water (2), and acetone (3) in a batch extraction cell. The point M is the mixture point and P represents the plait point. Experimental data correspond to Run C of Krishna et al. (1985).

Liquid extraction exhibits at least two common forms on the triangular diagram. Figure 7.3-2b shows both type 1 and type H systems, the former being characterized by the plait point. In type I systems, die tie lines linking equilibrium phases shrink to a point at the plait point, so that the two conjugate phases become identical. 

To apply the McCabe-Thiele method to extraction, the equilibrium data are shown on a rectangular graph, where the mass fraction of solute in the extract or V phase is plotted as the ordinate and the mass fraction of solute in the raffinate phase as the abscissa. For a type I system, the equilibrium line ends with equal compositions at the plait point. The use of only one concentration to characterize a ternary mixture may seem strange, but if the phases leaving a given stage are in equilibrium, only one concentration is needed to fix the compositions of both phases. 

To obtain data to construct tie lines, such as ER, it is necessary to make a mixture such as M (30% glycol, 40% water, 30% furfural), equilibrate it, and then chemically analyze the resulting extract and raffinate phases E and R (41.8% glycol, 10% water, 48.2% furfural and 11.5% glycol, 81.5% water, 7% furfural, respectively). At point P, called the plait point, the two liquid phases have identical compositions. Therefore, the tie lines converge to a point and the two phases become one phase. Tie-line data are given in Table 3.3 for this system. 

A ternary system well suited for solvent extraction is shown in Fig. 6.1-1. The feed consists of substance T (carrier) and a solute B, which has to be removed or recovered in pure form. Both substanees are mutually miscible. The solute B is also miscible with the solvent L whieh is often an organic compound. Substances T and L, however, must have a large miseibihty gap that narrows down with increasing content of solute and, eventually, diminishes at the critical point (or plait point). The boimdaiy of the two-phase region is called binodal curve. The dashed lines within the two-phase region, called tie lines (or conodes), describe the distribution equihbiium of solute B. The dashed-dotted line (eonjugation curve) allows the interpolation between given tie lines. 

Extract phase Tie lines Binodal curve Equilibrium curve Plait point 3X K = l... 

The binodal curve separates the single-phase region from the two-phase region. Any overall system composition (M) in that region will spontaneously split into two phases, i.e. the extract phase (E) and the raffinate phase (R). The compositions of the equilibrium conjugate phases lie on the curve on either end of the tie line that passes through the overall system composition. As the tie lines become shorter, the plait point is approached, at which concentration, only one liquid phase exists. 

In Figure 11-2, line ER connects the composition of an extract phase with the composition of a raffinate phase with which it is in equilibrium. Such lines are known as tie lines. These tie lines move vertically as the compositions of the two phases approach each other, until only a single phase exists (as shown in Figure 11-3). The point on curve JRPEK where a single liquid phase is formed is called the plait point (point P). The interfacial tension approaches zero as the plait point is approached. 

---