Distillation

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Image:Scotland Strathisla distillery.jpg Distillation is physical process for separating liquids through differences in their vapor pressures.

Known since antiquity, the concentration of alcohol by the application of heat to a fermented liquid mixture is perhaps the oldest form of distillation (see distilled beverages). However, the technique is now widely used for a variety of liquids in the chemical industry and in the production of petroleum products, among other fields, despite the fact it is energy-consuming.

The liquid mixture evaporates, such that the vapor has a composition determined by the chemical properties of the mixture. Distillation of a given component is possible, if the vapor has a higher proportion of the given component than the mixture. This is caused by the given component having a higher vapor pressure — and thus a lower boiling point — than the other components.

However, interactions between the components of the mixture can create properties unique to the mixture. Such interactions can result in an azeotrope. At an azeotrope, the mixture contains the given component in the same proportion as the vapor, so that evaporation does not change the purity, and distillation does not effect separation. For example, ethyl alcohol and water form an azeotrope of 95% at 78.2°C.

By the nature of the process, it is theoretically impossible to completely purify the components using distillation, as distillation only tends to purity, never reaching it. This is comparable to dilution, which never reaches purity. If ultra-pure products are the goal, then further chemical separation must be used.

The minimum in distillation is flash distillation, where either the temperature is rapidly increased or pressure reduced, and vapor and liquid fractions are thus obtained, which may be processed as such. The device used in distillation is referred to as a still and consists at a minimum of a reboiler (pot) in which the source material is heated, a condenser in which the heated vapor is cooled back to the liquid state, and a receiver in which the concentrated or purified liquid is collected.

The equipment may affect separation by one of two main methods. Firstly the vapours given off by the heated mixture may consist of two liquids with significantly different boiling points. Thus, the vapour that is given off is in the vast majority of one or the other liquid, which after condensation and collection effects the separation.

The second method (fractional distillation) is more effective at separating liquids with similar boiling points. This method relies upon a gradient of temperatures existing in the condenser stage of the equipment. Often in this technique, a vertical condenser, or column, is used. By extracting products that are liquid at different heights up the column, it is possible to extract liquids that have different boiling points. The greater the distance over which the temperature gradient in the condenser is applied leads to easier and more complete separation.

It is also possible to separate fractions by cooling, using differences in their freezing points. In the American vernacular, this was known as 'jacking'. The most popular drink produced by this process during Colonial times was applejack, which was fermented apple cider that was then frozen in the winter months, and for which the liquid (containing the most alcohol) would be poured off to make applejack, with the result of approximately doubling the alcohol content of the resulting beverage.

Many countries tax distilled alcohol, and preserve government income by legal restrictions on the use of a still.

Distillation was developed into its modern form with the invention of the alembic by Arab-Yemeni (Iranian-born) alchemist Jabir ibn Hayyan c. 800; he is also credited with the invention of numerous other chemical apparatus and processes that are still in use today. Chemists often use distillation in their work as a means of separating compounds or components. A distillation apparatus sometimes used by chemists is a rotary evaporator to distill (or evaporate) away solvent from a mixture.

See also

External links

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