Batch Distillation of a Non-Miscible Binary Mixture

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Consider a non-miscible binary mixture composed of benzene and water; this mixture presents a heteroazeotrope with a boiling temperature equal to 69.14°C and a benzene mole fraction approximately equal to 0.704. The NRTL (Non-Random Two-Liquid) model, developed by Renon and Prausnitz, is adequate in order to determine vapor-liquid equilibrium data.

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This Demonstration dynamically simulates a simple batch distillation of this mixture. The temperature of the distillate is plotted versus time for selected values of the initial composition in the still.

The still is charged with 1000 kmol of the initial mixture. The vapor rate is assumed constant and equal to 10 kmol/hr.

It is found that (1) the first product to exit the column is the heteroazeotrope, which, as far as distillation is concerned, behaves as a low-boiling pure component; (2) if the initial composition is very rich in water (respectively in benzene) then this latter component will exit the column once all the benzene (respectively water) has left the column in the form of a binary heteroazeotrope.

If the initial composition is equal to the heteroazeotropic composition, then the temperature remains constant at all times and is equal to the boiling temperature of the heteroazeotrope (i.e., 69.14°C).

In addition, for this kind of separation, a single stage (i.e., simple batch distillation without a column) is sufficient and only two fractions are obtained (i.e., there is a sharp transition between the fraction containing the heteroazeotrope and the fraction containing either water or benzene; there is no intermediate fraction).

Applications of this type of distillation (i.e., involving a non-miscible mixture where one component is water) is hydrodistillation and steam-distillation of natural aromatic products to obtain essential oils, such as hydrodistillation of bitter-orange flowers to obtain neroli.

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Contributed by: Housam Binous (March 2011)
Open content licensed under CC BY-NC-SA


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