vif xf y - x where subscript i represents the initial condition and f the final condition of the liquid in the still pot. Integration limits have been reversed to obtain a positive integral. If equilibrium is assumed between liquid and vapor, the right-hand side of Eq. (13-135) may be evaluated by plotting 1/(y - x) versus x and measuring the area under the curve between limits x, and Xf. If the mixture is a binary system for which relative volatility a is constant or if an average value that will serve for the range considered can be found, then the relationship that defines relative volatility,

can be substituted into Eq. (13-135) and a direct integration can be made:

1 ln |
Xf(1 |
- x,) |
+ ln |
1 - x, |

a - 1 |
U(1 |
- xf) |

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