Relative volatility

The dimensionless relative volatility describes the ratio of the volatilities of the components in a binary mixture . ${\displaystyle \alpha }$

definition

For two-substance mixtures, the relative volatility α is defined as:

${\displaystyle \alpha :={\frac {K_{1}}{K_{2}}}}$

with the K-factor

${\displaystyle K_{i}={\frac {y_{i}}{x_{i}}}}$

With

• y: mole fraction of a component in the steam
• x: mole fraction of a component in the liquid .

Therefore the relative volatility can also be written as:

${\displaystyle \Rightarrow \alpha ={\frac {y_{1}/x_{1}}{y_{2}/x_{2}}}={\frac {y_{1}*x_{2}}{y_{2}*x_{1}}}}$

meaning

The relative volatility is an important parameter in the separation of substances :

• high relative volatilities permit easy separation of substances by distillation or rectification
• a relative volatility of 1 describes an azeotrope which is caused by the two o. g. Procedure can no longer be separated.

example

The mixture of chloroform and ethanol shows a pronounced azeotrope at approx. 84 mol percent chloroform and p = 101.325 kPa;
here:

• if the vapor-liquid equilibrium is at a minimum,
  

  Vapor-liquid equilibrium
  ( isobaric )
• cross the K-factors (K ₁ = K ₂ )
  

  K-factors
• the relative volatility α = 1.
  

  Relative volatility

It can be seen that the K-factors and the relative volatility are derived from the vapor-liquid equilibrium.

See also

• Separation factor
• Fenske equation