Reflux Policy for a Batch Distillation Operation with a Constant Distillate Composition

In order to achieve a constant distillate composition for a batch distillation operation, one has to continuously vary the reflux ratio. This Demonstration presents the reflux policy needed to achieve the user-specified constant distillate purity. The still is initially fed with a binary mixture for which a constant relative volatility () adequately describes the vapor-liquid equilibrium (VLE) data. The feed composition is 30% mole fraction light component. The still's composition varies with warped time, , as follows:
where is the warped time, is the initial still composition, and is the desired constant distillate composition.
VLE data is given by .
Finally, the reflux ratio is given by the following approximation:
.
This Demonstration plots the reflux ratio and the still composition as a function of warped time, . In addition, a parametric plot shows the behavior of the reflux ratio versus the still composition.
It is obvious that as the warped time increases, the still composition goes to zero (the light component in the still is depleted). Thus, in order to achieve constant distillate compositions, the reflux ratio has to be increased (up to excessively high values) and the cost of the batch distillation operation becomes very high.

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For more information, see:
M. F. Doherty and M. F. Malone, Conceptual Design of Distillation Systems, New York: McGraw-Hill, 2001.
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