Distillation is a method of separation of substances based on differences in their volatilities.

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 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.

An analogous method of purification using freezing instead of evaporation is called freeze distillation. It is not distillation, and does not produce products equivalent to distillation. This process is used in the production of ice beer and ice wine to increase ethanol and sugar content, respectively.

History
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.

The design of the alembic has served as inspiration for some modern micro-scale distillation apparatus such as the Hickman stillhead.

Theory
It is a common misconception that in a solution, each component boils at its normal boiling point - the vapors of each component will collect separately and purely. In reality, distillation is essentially governed by Raoult's law and Dalton's law. Raoult's law states that the vapor pressure (PA) due to a volatile component of the system is

PA = XAPA°
where XA denotes the mole fraction of A and PA° denotes the vapor pressure of pure A.

Dalton's law states that the total vapor pressure pressure is the sum of the vapor pressures of each individual component in the mixture. Vapor pressures increase with heat. When a multi-component system is heated, the vapor pressure of each component will rise, causing the total vapor pressure to rise in turn. When the total vapor pressure reaches the ambient pressure, boiling occurs and liquid turns to gas throughout the bulk of the solution.

By the nature of the process, it is theoretically impossible to completely purify the components using distillation, as distillation only tends to approach 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.

Simple distillation
In simple distillation, all the hot vapors produced are immediately channeled into a condenser which cools and condenses the vapors. Thus, the distillate will not be pure - its composition will be identical to the composition of the vapors at the given temperature and pressure, and can be computed from Raoult's law.

As a result, simple distillation is usually used only to separate liquids whose boiling points differ greatly (rule of thumb is 25 °C)[2], or to separate liquids from involatile solids. For these cases, the vapor pressures of the components are usually sufficiently different that Raoult's law may be neglected due to the insignificant contribution of the less volatile component. In this case, the distillate may be sufficiently pure for its intended purpose.

Fractional distillation
For many cases, the boiling points of the components in the mixture will be sufficiently close that Raoult's law must be taken into consideration. Thus, fractional distillation must be used in order to separate the components well by repeated vaporization-condensation cycles within a packed fractionating column.

As the solution to be purified is heated, its vapors rise to the fractionating column. As it rises, it cools, condensing on the condenser walls and the surfaces of the packing material. Here, the condensate continues to be heated by the rising hot vapors; it vaporizes once more. However, the composition of the fresh vapors are determined once again by Raoult's law. Each vaporization-condensation cycle (called a theoretical plate) will yield a purer solution of the more volatile component[3]. In reality, each cycle at a given temperature does not occur at exactly the same position in the fractionating column; theoretical plate is thus a concept rather than an accurate description.

More theoretical plates lead to better separations. A spinning band distillation system uses a spinning band of Teflon or metal to force the rising vapors into close contact with the descending condensate, increasing the number of theoretical plates[4].

Special cases: azeotropes
Interactions between the components of the solution create properties unique to the solution, as most processes entail nonideal mixtures, where Raoult's law does not hold. Such interactions can result in a constant-boiling azeotrope which behaves as if it were a pure compound (i.e., boils at a single temperature instead of a range). 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.

If the azeotrope is not considered sufficiently pure for use, there exist some techniques to break the azeotrope to give a pure distillate. This set of techniques are known as azeotropic distillation. Some techniques achieve this by "jumping" over the azeotropic composition (by adding an additional component to create a new azeotrope, or by varying the pressure). Others work by chemically or physically remove or sequester the impurity. For example, to purify ethanol beyond 95 %, a drying agent or a desiccant such as potassium carbonate can be added to convert the soluble water into insoluble water of crystallization. Molecular sieves are often used for this purpose as well.

Vacuum distillation
Some compounds have very high boiling points. To boil the compound, it may be easier to lower the ambient pressure instead of increasing the temperature. Once the ambient pressure is lowered to the vapor pressure of the compound (at the given temperature), boiling and the rest of the distillation process can commence. This technique is known as vacuum distillation; it is commonly found in the laboratory in the form of the rotary evaporator.

This technique is also useful for compounds which boil beyond their decomposition temperature at normal pressure.

Steam distillation
Like vacuum distillation, steam distillation is a method for distilling compounds which are heat-sensitive. It is often used in perfumery to extract essential oils from flowers.

This process involves using bubbling steam through a heated mixture of the raw material. By Raoult's law, some of the target compound will vaporize (in accordance with its partial pressure). The vapor mixture is cooled and condensed, usually yielding a layer of oil and a layer of water.

Reactive distillation
The process of reactive distillation involves using the reaction vessel as the still. In this process, the product is usually significantly lower-boiling than its reactants. As the product is formed from the reactants, it is vaporized and removed from the reaction mixture.

This technique is useful for driving equilibrium reactions such as acid-catalyzed esterification.

RCOOH + R'OH ↔ RCOOR' + H2O
The reactants, usually carboxylic acids and alcohols, are often high-boiling due to hydrogen bonds; the product, the ester, is usually more volatile. By continually removing the ester product, by le Chatelier's principle, the reaction will proceed to completion.

This technique is an example of a continuous vs. a batch process; advantages include less downtime to charge the reaction vessel with starting material, and less workup.

Destructive distillation
Destructive distillation involves the strong heating of solids (often organic material) in the absence of oxygen (to prevent combustion) to evaporate various high-boiling liquids, as well as thermolysis products. The gases evolved are cooled and condensed as in normal distillation.

The destructive distillation of wood to give methanol is the root of its common name - wood alcohol.

Applications

Distilled beverages
Carbohydrate-containing plant materials are allowed to ferment, producing a dilute solution of ethanol in the process. Spirits such as whiskey and rum are prepared by distilling these dilute solutions of ethanol. As shown in Theory, other components than ethanol are collected in the condensate, including water, esters, and other alcohols which account for the flavor of the beverage.

Laboratory chemistry
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 remove solvents from a solution. A Kugelrohr apparatus is used to distill high boiling (> 300 °C) compounds. Such temperatures are not possible to achieve using ordinary heating mantles or oil baths.

Industrial distillation
The most widely used industrial applications of continuous, steady-state fractional distillation are in petroleum refineries, petrochemical plants and natural gas processing plants.

Typical distillation towers in oil refineriesIndustrial distillation 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. When the process feed has a diverse composition, as in distilling crude oil, liquid outlets at intervals up the column 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 a more complete separation of products.

Design and operation of a distillation column depends on the feed and desired products. Given a simple, binary component feed, analytical methods such as the McCabe-Thiele method can be used. For a multi-component feed, simulation models are used both for design and operation. Moreover, the efficiencies of the vapor-liquid contact devices (referred to as "plates") used in distillation columns are typically lower than that of a theoretical 100% efficient equilibrium stage. Hence, a distillation column needs more plates than the number of theoretical vapor-liquid equilibrium stages.