Relative volatility

Relative volatility is a measure comparing the vapor pressures of the components in a liquid mixture of chemicals.

This quantity is widely used in designing large industrial distillation processes.

Relative volatilities are used in the design of all types of distillation processes as well as other separation or absorption processes that involve the contacting of vapor and liquid phases in a series of equilibrium stages.

Relative volatilities are not used in separation or absorption processes that involve components reacting with each other (for example, the absorption of gaseous carbon dioxide in aqueous solutions of sodium hydroxide).

For a liquid mixture of two components (called a binary mixture) at a given temperature and pressure, the relative volatility is defined as When their liquid concentrations are equal, more volatile components have higher vapor pressures than less volatile components.

= 1 and separation of the two by distillation would be impossible under the given conditions because the compositions of the liquid and the vapor phase are the same (azeotrope).

When a binary mixture is distilled, complete separation of the two components is rarely achieved.

For example, some intermediate products in an oil refinery are multi-component liquid mixtures that may contain alkane, alkene and alkyne hydrocarbons—ranging from methane, having one carbon atom, to decanes having ten carbon atoms.

Thus, for the distillation of any multi-component mixture, the relative volatility is often defined as Large-scale industrial distillation is rarely undertaken if the relative volatility is less than 1.05.

have been correlated empirically or theoretically in terms of temperature, pressure and phase compositions in the form of equations, tables or graph such as the well-known DePriester charts.

values are widely used in the design of large-scale distillation columns for distilling multi-component mixtures in oil refineries, petrochemical and chemical plants, natural gas processing plants and other industries.