Batch Rectification of a Partially Miscible Binary Mixture

Consider a partially miscible binary mixture composed of -butanol and water; this mixture presents a heteroazeotrope with a boiling temperature of 93.49°C and a water mole fraction of approximately 0.2285. The Non-Random Two-Liquid (NRTL) model developed by Renon and Prausnitz is adequate in order to determine vapor-liquid equilibrium data.
This Demonstration performs the dynamic simulation of a batch rectification of this mixture. The temperature is plotted versus time for selected values of the initial composition, reflux ratio, and number of theoretical stages.
The still is charged with 1000 kmol of the initial mixture. The vapor rate is assumed constant at 10 kmol/hr. The molar hold-up of all stages and the condenser are 5 and 20 kmol, respectively.
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 -butanol) then this component will exit the column once all -butanol (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 equal to the boiling temperature of the heteroazeotrope (i.e., 93.49°C).
In addition, the separation is not affected very much when one chooses large reflux ratios and/or a large number of theoretical stages. This feature is due to the fact that the binary mixture is partially miscible.


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