Ln fx

where Wf = moles in still at end

W0 = moles originally changed to still

Typically the right-hand side of Eq. (5.12) is evaluated by graphical integration, using the assumption that the vapor and liquid phases are in equilibrium.

If the system to be separated is ideal or can be assumed to be ideal, relative volatility [Eq. (5.6)] is constant, and Eq. (5.7) can be used di

rectly to compute equilibrium compositions. If the system is not ideal, experimental vapor-liquid equilibrium data must be available (see Chap. 2).

Assuming the relative volatility of a benzene-toluene mixture is 2.90, the vapor-liquid equilibrium compositions can be calculated as shown in Table 5.1. The resultant curve is plotted in Fig. 5.1a.

table 5.1 Equilibrium Vapor-Liquid Compositions, Benzene-Toluene System, 14.7 psla_

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