Distillation

Laboratory distillation set-up using a Liebig condenser, without a fractionating column. Nowadays, electrical heating is preferred.

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.

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 by retort

Distillation was developed into its modern form with the invention of the alembic by Islamic 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.

As alchemy evolved into the science of chemistry, vessels called retorts became used for distillations. Both alembics and retorts are forms of glassware with long necks pointing to the side at a downward angle which acted as air-cooled condensers to condense the distillate and let it drip downward for collection.

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. This does not occur even in an idealized system. Idealized models of distillation are essentially governed by Raoult's law and Dalton's law.

Raoult's law assumes that a component contributes to the total vapor pressure of the mixture in proportion to its fraction of the mixture and its vapor pressure when pure. For component A,

P_A = X_AP_A°

where X_A denotes the mole fraction of A and P_A° denotes the vapor pressure of pure A. If a component changes another's vapor pressure, or the volatility of a component is dependent on its fraction, the law will fail.

Dalton's law states that the total vapor pressure is the sum of the vapor pressures of each individual component in the mixture.

P_total = Σ P_i, for components i = A, B, C, ...

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. Notice that a given mixture has one boiling point, when the components are mutually soluble.

The idealized model is accurate in the case of chemically similar liquids, such as benzene and toluene. In other cases, severe deviations from Raoult's law and Dalton's law are observed, most famously in the mixture of ethanol and water. Although there are computational methods that can be used to estimate the behavior of a mixture of arbitrary components, the only way to obtain accurate vapor-liquid equilibrium data is by measurement.

By the nature of the process, it is theoretically impossible to completely purify the components by 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 applied.

Laborarory scale distillation

The device used in distillation, sometimes referred to as a still, consists at a minimum of a reboiler or pot in which the source material is heated, a condenser in which the heated vapour is cooled back to the liquid state, and a receiver in which the concentrated or purified liquid, called the distillate, is collected. Several laboratory scale techniques for distillation exist (see also Distillation Types)

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), 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. 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.

Short path distillation

Short path distillation is a distillation technique that involves the distillate traveling a short distance, often only a few centimeters. A classic example would be a distillation involving the distillate traveling from one glass bulb to another, without the need for a condenser separating the two chambers. The Kugelrohr is a short path distillation apparatus which is commercially available.

Vacuum distillation

Dimethylsulfoxide usually boils at 189 °C. Under a vacuum, it distills off into the receiver at only 70 °C.

Some compounds have very high boiling points. To boil such compounds, it is often better to lower the pressure at which such compounds are boiled instead of increasing the temperature. Once the 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 referred to as vacuum distillation and it is commonly found in the laboratory in the form of the rotary evaporator.

This technique is also very useful for compounds which boil beyond their decomposition temperature at atmospheric pressure and which would therefore be decomposed by any attempt to boil them under atmospheric pressure.

File:Perkin triangle distillation apparatus.jpg

Perkin Triangle Distillation Setup
T1-T3: Teflon taps, V/G: Vacuum/Gas-line; Cf: Cold Finger, Tm: Thermometer,
Vc: Vigreux-column, S: Still, D: Distillate

Air-sensitive vacuum distillation

Some compounds have high boiling points as well as being air sensitive. A simple vacuum distillation system as exemplified above can be used, whereby the vacuum is replaced with an inert gas after the distillation is complete. However, this is a less satisfactory system if one desires to collect fractions under a reduced pressure. To do this a "pig" adaptor can be added to the end of the condenser, or for better results or for very air sensitive compounds a Perkin triangle apparatus can be used.

The Perkin triangle, has means via a series of glass or teflon taps to allows fractions to be isolated from the rest of the still, without the main body of the distillation being removed from either the vacuum or heat source, and thus can remain in a state of reflux. To do this, the sample is first isolated from the vacuum by means of the taps, the vacuum over the sample is then replaced with an inert gas (such as nitrogen or argon) and can then be stoppered and removed. A fresh collection vessel can then be added to the system, evacuated and linked back into the distillation system via the taps to collect a second fraction, and so on, until all fractions have been collected.

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.

Azeotropic distillation

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.

Rotary evaporation

In rotary evaporation a vacuum distillation apparatus is used to remove bulk solvents from a sample. Typically the vacuum is generated by a water aspirator or a membrane pump.

Kugelrohr distillation

In a kugelrohr distillation a short path distillation apparatus is typically used to distill high boiling (> 300 °C) compounds. The apparatus consists of an oven in which the compound to be distilled is placed, a receiving portion which is outside of the oven, and a means of rotating the sample. The vacuum is normally generated by using a high vacuum pump..

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 refineries

Industrial 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 and are often called the bottoms. 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.

Distillation in food processing

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.

See also

• Batch Distillation
• Pervaporation

External links

• Extractive Distillation
• Alcohol distillation
• Homedistiller.org - The mother of all home distilling information websites
• Alcohol Wiki at Homedistiller.org
• Essential and Fragrance Oils Distillation

Gallery

100px	Chemistry on its beginnings used retorts as laboratory equipment exclusively for distillation processes.
	A simple set-up to distill dry and oxygen-free toluene similar to the previous image.
100px	A rotary evaporator is able to distill solvents more quickly at lower temperatures through the use of a vacuum.
	Distillation using semi-microscale apparatus. The jointless design eliminates the need to fit pieces together. The pear-shaped flask allows the last drop of residue to be removed, compared with a similarly-sized round-bottom flask The small holdup volume prevents losses. A pig is used to channel the various distillates into three receiving flasks. If necessary the distillation can be carried out under vacuum using the vacuum adapter at the pig.