When relative volatilities vary, the equilibrium curve is a weak function of the reflux ratio because the operating temperature shows some dependence on the reflux ratio. In order to draw the equilibrium curve, first set the reflux ratio. Set vapor and liquid flow rates. Calculate the limiting compositions of the nonkeys using Eqs. (2.50) to (2.53). Determine the relative volatilities of the keys at several values of xe in the rectifying and stripping sections, by carrying out bubble-point calculations using the limiting nonkeys compositions. Plot points \xe,ye) to give the equilibrium curve.

Figure 2.18a is the equilibrium curve for Example 2.4. There are two equilibrium curves, one for the rectifying section and one for the stripping section. The discontinuity at the feed is due to the sudden change in composition, and therefore temperature, when passing from one section to another. Also, some heavy nonkeys are present above the feed and some light nonkeys below it, Hengstebeck (14,15) recommends drawing the equilibrium curve arbitrarily in the feed region (Fig. 2.18a).

<7-)ine. Hengstebeck (15) recommends the following equation to calculate q:

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