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. In effect, it indicates the ease or difficulty of using distillation to separate the more volatile components from the less volatile components in a mixture. By convention, relative volatility is usually denoted as α {displaystyle alpha } . 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. In effect, it indicates the ease or difficulty of using distillation to separate the more volatile components from the less volatile components in a mixture. By convention, relative volatility is usually denoted as α {displaystyle alpha } . 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. Thus, a K {displaystyle K} value (= y / x {displaystyle y/x} ) for a more volatile component is larger than a K {displaystyle K} value for a less volatile component. That means that α {displaystyle alpha } ≥ 1 since the larger K {displaystyle K} value of the more volatile component is in the numerator and the smaller K {displaystyle K} of the less volatile component is in the denominator. α {displaystyle alpha } is a unitless quantity. When the volatilities of both key components are equal, α {displaystyle alpha } = 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). As the value of α {displaystyle alpha } increases above 1, separation by distillation becomes progressively easier.