12 13 14 15 C

Stage number Ifrom bottom)

Figure 2.17 Temperature profile in multicomponent distillation, Example 2.4. (Ftom C. J. King, Separation Processes, 2d ed., Copyright O by McGraw-Hill, Inc. Reprinted by permission.)

Limiting compositions. In order to apply Eqs. (2.46) to (2.49), the limiting compositions of the nonkeys are determined. The limiting composition of each nonkey is the composition of the component in the zone where it has constant mole fraction.

Estimation of the limiting nonkey composition is based on Jenny's approximation (20). Accordingly, Klk, the equilibrium ratio of the light key, is equal to L'fV in the stripping section zone of constant heavy nonkey mole fraction. Similarly, Khk, the equilibrium ratio of the heavy key, is equal to LfV in the rectifying section zone of constant light nonkey mole fraction. Based on this approximation, the limiting compositions of each nonkey are [detailed derivation is elsewhere (7,15)].

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