Azeotropes and pure components in azeotropic distillation (chemical knowledge)
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In the triple-component system, at atmospheric pressure, the boiling points are ethanol at 78.3°C, water at 100°C, and benzene at 80.1°C. Azeotropic distillation is used for separation. I’m not quite understanding why water and benzene come out of the top of the tower while ethanol remains at the bottom of the tower; those who know the principle could please explain it? Thank you
Azeotropic distillation is a separation method that achieves separation by adjusting pressure and temperature to equalize the boiling points of three or more components. In the triple-point analysis, the azeotrope is the component with the lowest boiling point in the mixture, which is ethanol. During azeotropic distillation, the three components are separated in the column according to their relative volatilities. Due to their higher relative volatility, water and benzene are carried away from the top of the tower along with the vapor, while ethanol, having lower relative volatility, remains at the bottom of the tower. However, since the boiling points of water and benzene are similar, azeotropic distillation is required to effectively separate them. If azeotropic distillation is not carried out and only ordinary distillation is used, it may be impossible to completely separate them. .
Can it be understood that after azeotrope formation, during the exchange process in the tower, methanol is cooled down, while water and benzene exit as a gas at the top of the tower?
The ethanol-benzene-water ternary system can form various azeotropes. In order of decreasing azeotrope temperature, they are: the ethanol-benzene-water ternary azeotrope at 64.9°C, the ethanol-benzene binary azeotrope at 68.2°C, the benzene-water binary azeotrope at 69.3°C, and the ethanol-water binary azeotrope at 78.1°C. The azeotopic boiling points of these azeotropes are all below the normal boiling point of ethanol, which is 78.3°C; unless the contents of benzene and water in the feed to the tower are relatively low, it is not possible to achieve a high recovery rate of ethanol at the tower bottom using conventional azeotropic distillation methods.
In a ternary system, it is necessary to use distillation residue curve diagrams to identify the relevant points within the distillation region corresponding to the desired feed composition; the material moves along the distillation curve until it reaches the minimum azeotrope； Under the conditions you described, within the distillation fraction where the composition of the feed is determined by the amount of azeotrope added, ethanol is the stable component with the highest boiling point, while the azeotrope of benzene and water is the component with the lower boiling point.