Distillation

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Image:Scotland Strathisla distillery.jpg Distillation is a method of separation of substances based on differences in their vapor pressures.

Known since antiquity, the concentration of alcohol by the application of heat to a fermented liquid solution is perhaps the oldest form of distillation, in the course of producing 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.

The liquid solution evaporates, such that the vapor has a composition determined by the chemical properties of the solution. Distillation of a given component is possible, if the vapor has a higher proportion of the given component than the solution. 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 solution can create properties unique to the solution. Such interactions can result in an azeotrope. At an azeotrope, the solution 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. Image:Retort-in-operation-early-chemistry.PNG 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 evaporation, 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 or 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 solution 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. The most widely used industrial applications of continuous, steady-state fractional distillation are in petroleum refineries, petrochemical plants and natural gas processing plants. Image:ShellMartinez.jpg Industrial distillation Template:Ref label Template:Ref label is typically performed in large, vertical cylindrical columns known as distillation towers or distillation columns with diameters ranging from about 65 centimeters to 6 meters and heights ranging from about 6 meters to 60 meters or more. The distillation towers have liquid outlets at intervals up the column which allow for the withdrawal of different fractions or products having different boiling points or boiling ranges. The "lightest" products (those with the lowest boiling point) exit from the top of the columns and the "heaviest" products (those with the highest boiling point) exit from the bottom of the column. Large-scale industrial towers also use reflux to achieve more complete separation of products.

Distillation was developed into its modern form with the invention of the alembic by Persian 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. Image:Simple chem distillation.PNG Chemists often use distillation in their work as a means of separating compounds or components. See at right a diagram of a simple distillation set-up without a fractionating column often used by chemists. A distillation apparatus sometimes used by chemists is a rotary evaporator to distill (or evaporate) away solvent from a solution.

An analogous method with freezing instead of evaporation is called freeze distillation. It is not distillation, and does not produce products equivalent to distillation.