Distillate and Still Compositions for Batch Rectification of a Binary Mixture at a Very Large Reflux Ratio

Consider a binary mixture with constant relative volatility, , subject to a batch rectification. Initially the binary mixture in the still is equimolar. The rectifying section has stages. A very large reflux ratio is chosen so that the Fenske equation is valid. In such a case, the still and distillate compositions are given by:
and , where ξ is a warped time.
With this model, adding stages or increasing the relative volatility will produce the same effect. The Demonstration displays the mole fraction of the light component in the still (orange curve) and the distillate (blue curve) versus warped time.
Adding more stages to the rectifying section or increasing will lead to a much more sharply defined variation in the distillate composition. A desirable situation occurs where there is a cut or a distillation fraction. Indeed, the distillate is quite pure for a period of time, then drops sharply when the light component is depleted in the still.

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