Distillation is a process that separates liquids in a mixture based on differences in their boiling points. Mixtures called azeotropes, however, cannot be separated by simple distillation and hence require other techniques.
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Ethanol forms an azeotrope with water once the ethanol is about 96 per cent of the mixture by volume, so simple distillation cannot produce ethanol that is more than 96 per cent pure. Heterogenous azeotropic distillation, extractive distillation and pressure-reduction distillation are all techniques to remove the remaining water and produce pure or near-pure ethanol, also called absolute ethanol.
In the heterogenous azeotropic distillation method, cyclohexane, pentane or benzene is added to the water-ethanol mix; the mixture of the three compounds forms a new azeotrope that evaporates more readily than the water-ethanol azeotrope, enabling distillers to remove most of the remaining water. Reducing air pressure to very low levels also changes boiling points of ethanol and water and makes it possible to separate the two through further distillation. In extractive distillation, inorganic salts or other compounds are added to change the relative boiling points of the ethanol and water to make further distillation possible.
Pressure-reduction distillation is still too expensive for widespread commercial use, although it may be a practical technique in the lab. Heterogenous azeotropic distillation may leave behind a residue of benzene or cyclohexane, which makes the ethanol potentially dangerous for human consumption; ethanol produced by this process is used for fuel or industrial processes.
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