Monday, 20 October 2014

Azeotropic Solution, It’s Role In Distillation and It’s Types

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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