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Relative volatility $\alpha$

 In this post, the relative volatility is represented as $\alpha$. This parameter accounts for the feasibility of separation in a mixture. For a binary mixture, this is defined as,

$\alpha_{A,B}=\dfrac{K_A}{K_B}=\dfrac{y_A/x_A}{y_B/x_B}=\dfrac{y_A/x_A}{\left( 1-y_A \right)/\left( 1-x_A \right)}$        Eq. (01)

Some cases can be devised:

  • For $\alpha{A,B}=1.0$ the mixture separation is not possible. As $\alpha_{A,B}$ gets closer to 1.0, the separation becomes more difficult.
  • For $\alpha_{A,B}=0$ there is practically no mixture. This is, the components are practically split.

The parameters $K_A$ and $K_B$ give a feeling of the tendency of the component of interest A to vaporize. As you can see in Eq. (01), the factor $K$ is defined as,

$K_A=\dfrac{\text{Mole fraction of component A in vapor phase}}{\text{Mole fraction of component A in liquid phase}}$        Eq. (02)

An interesting interpretation of Eq. (02) is given by Kister H.Z. in his book Distillation design. He says: "If the $K$-value is high, the component tends to concentrate in the vapor; if low it tends to concentrate in the liquid.".

This is the end of the post. I hope you find it useful.


Any question? Write in the comments and I shall try to help.

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