Separation of Azeotropes and Liquids of Similar Volatility

Separation of Azeotropes and Liquids of Similar Volatility

Systems that form azeotropes cannot be separated by fractional distillation, although in some cases, the formation of the azeotrope can be precluded by changing the distillation pressure. Problems of separation are also found with mixtures of liquids with similar volatility. Separation of these systems can be facilitated by adding a third component. If this component forms one or more azeotropes with the original components of the mixture, the process is called azeotropic distillation. The addition of a relatively nonvolatile component, which alters the relative volatility of the original components, gives a process known as extractive distillation.

In the azeotropic distillation of minimum-boiling binary mixtures, the third component forms either a new binary azeotrope of lower boiling point or a ternary azeotrope of lower boiling point containing the original components in different proportions. The newly formed azeotrope must be easily separated after distillation. The process is illustrated by the dehydration of alcohol with benzene. The binary azeotrope of ethyl alcohol and water boils at 351.15 K, the ternary azeotrope of benzene, water, and alcohol boils at 337.8 K, and the binary azeotrope of benzene and alcohol boils at 341 K. Distillation of the alcohol-water azeotrope with benzene yields the ternary azeotrope that separates on con-densation to give two layers, one of which contains almost all the water. In a batch process, the column would then give the benzene alcohol azeotrope, leaving anhydrous alcohol in the still. In a continuous process, the various stages would each be performed on a different column.

Extractive distillation is illustrated by the separation of benzene and cyclohexane by adding phenol. The relative volatility of the original components is modified so that cyclohexane is recovered as the distillate, leaving a mixture of phenol and benzene, which is passed to a second column for separation. The phenol, which is added to the top of the column, appears to aid separation by preferentially dissolving benzene during its passage downward. This leads to the term extractive distillation.

FIGURE 10.8 Large-scale molecular still.