Relative Volatility

What is this formula?

Relative volatility measures how easily one component can be separated from another by distillation. It compares the vapor-liquid equilibrium behavior of two components and is one of the most important parameters in separation process design.

When to use it

Use this formula when designing distillation columns, evaluating separation difficulty, selecting solvents, or estimating the number of equilibrium stages required.

Example

For a binary mixture, component A has an equilibrium ratio KA = 2.4 and component B has an equilibrium ratio KB = 0.8.

Formula:

α = KA / KB

Substitution:

α = 2.4 / 0.8

Result:

α = 3.0

Component A is significantly more volatile than component B, making separation by distillation relatively easy.

Applications

- Distillation column design

- Vapor-liquid equilibrium analysis

- Petrochemical separations

- Solvent recovery systems

- Chemical process simulation

Note

Relative volatility is generally not constant and often varies with temperature, pressure, and composition. Values greater than about 1.5 typically indicate that distillation is feasible, while values close to 1 indicate difficult separations requiring many stages or alternative technologies.