McCabe and Thiele (1925) developed a graphical solution method based on Lewis’ method and the observation that the operating Eqs. (4-12b) and (4-14) plot as straight lines (the *operating lines*) on a y-x diagram. On this graph the equilibrium relationship can be solved from the y-x equilibrium curve and the mass balances from the operating lines.

To illustrate, consider a typical design problem for a binary distillation column such as the one illustrated in Figure 3-8. We will assume that equilibrium data are available at the operating pressure of the column. These data are plotted as shown in Figure 4-4. At the top of the column is a total condenser. As noted in Chapter 3 in Eq. (3-7), this means ...

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