Page 253 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

Further reading 253

There are two main classes of liquid-liquid equilibrium in extraction:
(i) One immiscible pair of species that produces the familiar envelope shown in Fig. 9.6 and is
discussed above. In the system shown, species A B and S B are completely miscible while
A S is partially miscible, and the mutual saturation limits are denoted by the envelope ends
(O,Q) on line AS. The more insoluble the liquids A and S, the closer will be the points to the
triangular apices. This nature of LLE is more common and is preferred for extraction with
solvent S. Solvents are often selected to get this. Nevertheless, the type can change with
temperature, and this needs to be considered.
(ii) Two pairs of immiscible species exist, and so the two-phase envelope crosses the triangular
diagram like a bridge. The ternary diagram drawn at 25 Cin Fig. 9.7 shows this nature e A B is

completely miscible in all proportions, but there is only partial miscibility between A S and
BeS. BeS becomes completely miscible at 3.5 C. At 50 C, the miscibility improves further, and

we note that the two-phase envelope shrinks to a single immiscible pair.

B B B

A S A S A S
25°C 35°C 50°C
FIGURE 9.7
Effect of temperature of equilibrium solubility on the ternary system.

Further reading
Treybal, R. E. (1980). Mass-transfer operations. McGraw-Hill Classic Textbook Reissue Series.
Reid, R. C., Prausnitz, J. M., & Poling, B. E. (1987). The properties of gases and liquids. New York: McGraw-Hill.
Prausnitz, J. M., Lichtenthaler, R. N., & de Azevedo, E. G. (1998). Molecular thermodynamics of fluid-phase
equilibria. Pearson Education.