Xd Xs

The right-hand side of this equation may be integrated by plotting 1/(xd — xs) against xs. This enables the ratio of the initial to final quantity in the still to be found for any desired change in xs, and hence the amount of distillate . The heat to be supplied to provide the reflux is QR = and hence the reboil heat required per mole of product may be compared with that from the first method.

Example 11.12

A mixture of ethyl alcohol and water with 0.55 mole fraction of alcohol is distilled to give a top product of 0.75 mole fraction of alcohol. The column has four ideal plates and the distillation is stopped when the reflux ratio has to be increased beyond 4.0.

What is the amount of distillate obtained, and the heat required per kmol of product?


For various values of R the corresponding values of the intercept 0 and the concentration in the still xs are calculated. Values of xs are found as shown in Figure 11.35 for the two values of R of 0.85 and 4. The amount of product is then found from equation 11.98. Thus, for R = 4:

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