Fractional distillation separates volatile compounds based on boiling points, but some blends require extractive distillation. Azeotropes cannot be separated by fractional distillation. Extractive distillation uses a high-boiling solvent to change relative volatilities and separate compounds. The solvent must have a higher boiling point than the less volatile substance and be introduced as a vapor. Examples include separating toluene from iso-octane using phenol as a solvent.
Fractional distillation, commonly called distillation or simple distillation, is a common method of separating volatile compounds based on boiling points (BP). Some blends are not separable by distillation. In many of these cases, extractive distillation is employed. In this process, a high-boiling solvent is added to the feed, changing the relative volatilities of the compounds in the feed mix. 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, the lower boiling 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 if they were a single compound with a single boiling point. Azeotropes cannot be separated into their constituents by fractional distillation.
In case of similar volatilities of azeotropic compounds or mixtures, extractive distillation often becomes an economical and effective process choice. In an extractive distillation, a solvent enters 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 less volatile substance. The solvent concentration throughout the column must be high enough to have widely separated volatilities but low enough not to form two phases. Two phases result in two layers of liquids, such as oil and water, which do not dissolve in each other. The solvent can be introduced into the column as a vapor to avoid a sudden change in solvent concentration 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 a solvent. The vapor from the top of the column condenses as relatively pure isooctane. The toluene-phenol mixture from the bottom of the column is separated by simple distillation; the phenol returns to the first column and toluene is collected as product. Phenol, being less volatile, increased the volatility of iso-octane.
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