Chemical Engineering Thermodynamics – II

Types of Azeotropes

AZEOTROPES:-

An azeotrope or a constant boiling mixture is a mixture of two or more liquids whose proportions cannot be altered by simple distillation. 

This happens because, when an azeotrope is boiled, the vapour has the same proportions of constituents as the unboiled mixture.

Types of Azeotropes

Minimum Boiling Azeotropes

Each azeotrope has a characteristic boiling point. The boiling point of an azeotrope is less than the boiling point temperatures of any of its constituents is a Minimum boiling azeotrope.

The diagram on the right shows a positive azeotrope of hypothetical constituents, X and Y.

The bottom trace illustrates the boiling temperature of various compositions. Below the bottom trace, only the liquid phase is in equilibrium.

The top trace illustrates the vapour composition above the liquid at a given temperature. Above the top trace, only the vapour is in equilibrium. 

Between the two traces, liquid and vapor phases exist simultaneously in equilibrium: for example, heating a 25% X : 75% Y mixture to temperature AB would generate vapor of composition B over liquid of composition A.

The azeotrope is the point on the diagram where the two curves touch. The horizontal and vertical steps show the path of repeated distillations.

Point A is the boiling point of a nonazeotropic mixture. The vapor that separates at that temperature has composition B.

The shape of the curves requires that the vapor at B be richer in constituent X than the liquid at point A.

The vapor is physically separated from the VLE (vapor-liquid equilibrium) system and is cooled to point C, where it condenses.

The resulting liquid (point C) is now richer in X than it was at point A.

If the collected liquid is boiled again, it progresses to point D, and so on.

The stepwise progression shows how repeated distillation can never produce a distillate that is richer in constituent X than the azeotrope. 

EXAMPLE

Maximum Boiling Azeotropes

Each azeotrope has a characteristic boiling point. The boiling point of an azeotrope is more than the boiling point temperatures of any of its constituents is a Maximum boiling azeotrope.

The diagram on the right shows a negative azeotrope of hypothetical constituents, X and Y.

Again the bottom trace illustrates the boiling temperature at various compositions, and again, below the bottom trace the mixture must be entirely liquid phase. 

The top trace again illustrates the condensation temperature of various compositions, and again, above the top trace the mixture must be entirely vapor phase

The point, A, shown here is a boiling point with a composition chosen very near to the azeotrope.

The vapor is collected at the same temperature at point B. That vapor is cooled, condensed, and collected at point C.

Because this example is a negative azeotrope rather than a positive one, the distillate is farther from the azeotrope than the original liquid mixture at point A was.

So the distillate is poorer in constituent X and richer in constituent Y than the original mixture

 Because this process has removed a greater fraction of Y from the liquid than it had originally, the residue must be poorer in Y and richer in X after distillation than before.

If the point, A had been chosen to the right of the azeotrope rather than to the left, the distillate at point C would be farther to the right than A, which is to say that the distillate would be richer in X and poorer in Y than the original mixture.

So in this case too, the distillate moves away from the azeotrope and the residue moves toward it.

This is characteristic of negative azeotropes.

EXAMPLE

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