Azeotropic
solution is defined as such a solution in which the
components to be separated have nearly equal or very close boiling points. In
other words, their relative volatility is unity or near to that.
Another way of
expressing an azeotropic solution is that in an azeotropic solution, the
composition of vapor becomes equal to that of the liquid and no enrichment of
vapor occurs. It may be at the start of distillation process or during the
distillation operation.
This type of solution
cannot be separated by usual distillation method. It requires a special type of
distillation called “Azeotropic Distillation” to separate azeotropic solution.
Example:
If
a mixture of ethanol and water is distilled, the concentration of the alcohol
steadily increases until it reaches 96 percent by mass, when the composition of
the vapor equals that of the liquid, and no further enrichment occurs. This
mixture is now called an azeotrop.
In non-azeotropic
solutions, during distillation the vapor becomes steadily richer in the more
volatile component on successive plates. But, in azeotropic types of mixtures this
steady increase in concentration of more volatile component in vapor, either
does not takes plate, or it takes place so slowly that an uneconomic number of
plates is required.
Types
Of Azeotropic Distillation:
1. Minimum
Boiling Azeotropes
2. Maximum
boiling azeotropes
All that depends on “activity
coefficient”. Activity coefficient is similar to relative volatility concept, but
it is for non-ideal (real) systems. Activity coefficient approaches unity as
the liquid concentration approaches unity and the highest values of this
coefficient occurs as the concentration approaches zero.
1. Minimum
Boiling Azeotrops:
In this type of solution, components of the
azeotropic solution boils off, at a lower temperature, as compared if they were
ideal. Hence called minimum boiling azeotrops.
When the activity coefficient is greater than unity,
giving a positive deviation from Roult’s Law , the molecules of the components
in the mixture repel each other and exert a higher partial pressure than if
their behavior were ideal. This higher partial pressure is the indication of
lower solubility of components in each other. This leads to the formation of Minimum
boiling azeotrops.
2. Maximum
Boiling Azeotrops:
In this type of
solution, components of the azeotropic solution boils off, at a higher
temperature, as compared if they were ideal. Hence called maximum boiling
azeotrops.
When the activity
coefficient is less than unity, giving a negative deviation from Roult’s Law,
the molecules of the components in the mixture attract each other and exert a lower
partial pressure than if their behavior were ideal. This lower partial pressure
is the indication of higher solubility of components in each other. This leads to
the formation of Minimum boiling azeotrops.
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