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Fractional distillation, commonly referred to as distillation or simple distillation, is a common method for separating volatile compounds based on boiling points (BP). Some mixtures are not separable by distillation. In many of these instances, extractive distillation is employed. In this process, a high-boiling solvent is added to the feed, changing the relative volatilities of the feed mixture compounds. The solvent is then recovered in a separate distillation column and recycled.
Volatility, or the ease of converting a liquid into a gas, is a function of boiling point. If one substance boils at a higher temperature than another, then the lower boiling-point substance will tend to vaporize first and condense at a higher point in the column. Fractional distillation will not separate mixtures whose compounds have similar BPs, particularly if their chemical structures are similar.
Some liquid compounds form a special mixture called an azeotrope when they are present in certain ratios. Azeotropes are mixtures of usually two, but sometimes three, separate compounds that behave as though they were a single compound with a single boiling point. Azeotropes cannot be separated into their constituents by fractional distillation.
In the cases of similar volatilities of compounds or azeotropic mixtures, extractive distillation often becomes an economical and effective process choice. In an extractive distillation, a solvent enters into the distillation column above the feed point. The solvent used is less volatile than either of the compounds to be separated.
The design of an extractive distillation column is often straightforward. The solvent must have a higher boiling point than the least volatile substance. The solvent concentration throughout the column must be high enough to have widely separated volatilities but low enough that two phases are not formed. Two phases result in two layers of liquids, like oil and water, which will not dissolve in each other. The solvent may be introduced to the column as a vapor to avoid a sudden solvent concentration change at the feed point.
Examples of extractive distillation include the separation of toluene (BP 110.8oC) from iso-octane (BP 99.3oC) using phenol (BP 181.4oC) as the solvent. The vapor from the top of the column condenses as relatively pure iso-octane. The toluene-phenol mixture from the bottom of the column separates by simple distillation; the phenol returns to the first column, and the toluene is collected as a product. The phenol, being less volatile, enhanced the volatility of the iso-octane.