The Elementary Theory of LLE

FIGURE 11.4 Solubility-temperature diagram for dipropyl amine and water [1, p. 434]. The squares and circles are the experimental data points. The horizontal axis extends only to 80%. 

So far this chapter has presented a description of the most common (and interesting and important) types of LLE. There are ample LLE data in the literature, and the experiments to determine LLE (of nontoxic liquids at modest T and P) need not be terribly expensive or difficult. So we could simply assume that we will look up or measure any LLE data we need. Current process design computer programs can estimate such LLE data from theory with fair-to-good accuracy much more quickly and cheaply than we could find it in the library or the laboratory. This section shows the fundamentals of that theory and some simplified (hand or spreadsheet calculable) versions of the more complex algorithms in those computer programs. 

The fundamental relation for any LLE is the same as that for VLE, or for any phase equilibrium, that for any of the species present at equilibrium the fugacity must be the same for that species in all of the phases present. 

FIGURE 11.6 Effect of temperature on solubility for the system n-hexane (H), methylcyclopentane (M), and analine (A). Increasing the temperature reduces the size of the two-phase region. (From Sender, J. D., and E. J. Henley. Separation Process Principles. 1998, New York Wiley, p. 438. Reprinted by permission of John Wiley Sons, Inc.) 

---