Relative Volatility And It's Significance in Separation Process

     Relative Volatility of a liquid is the ratio of volatility of the two components.

                 Relative Volatility = volatility of component A / volatility of component B

     Where, "volatility" of a component A is the ratio of partial pressure (P_A) of that component in the vapor mixture to mole fraction of that component in liquid mixture (x_A). Therefore,

Relative volatility = P_Ax_B / P_Bx_B

     Using Dalton's Law of Partial pressure (click here), we can write it in the vapor phase mole fraction and liquid phase mole fraction. Subsitute Py_A for P_A and Py_B for P_B :

Relative Volatility = y_Ax_B/y_Bx_A

     From the definition of the volatility of a component, it is seen that for an ideal system the volatility is numerically equal to the vapor pressure of the pure component (P_A*), i.e,

Relative Volatility = P_A*/ P_B*

Significance of Volatility in Separation Process:

                                                                      For separation to be achieved, relative volatility must not equal 1. Considering the more volatile component (in the numenator), as relative volatility increases above unity, y increases and the separation becomes much easier.