Azeotropic distillation
Encyclopedia
In chemistry
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....

, azeotropic distillation is any of a range of techniques used to break an azeotrope
Azeotrope
An azeotrope is a mixture of two or more liquids in such a ratio that its composition cannot be changed by simple distillation. This occurs because, when an azeotrope is boiled, the resulting vapor has the same ratio of constituents as the original mixture....

 in distillation
Distillation
Distillation is a method of separating mixtures based on differences in volatilities of components in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction....

. In chemical engineering
Chemical engineering
Chemical engineering is the branch of engineering that deals with physical science , and life sciences with mathematics and economics, to the process of converting raw materials or chemicals into more useful or valuable forms...

, azeotropic distillation usually refers to the specific technique of adding another component to generate a new, lower-boiling azeotrope that is heterogeneous (e.g. producing two, immiscible liquid phases), such as the example below with the addition of benzene to water and ethanol. This practice of adding an entrainer which forms a separate phase is a specific sub-set of (industrial) azeotropic distillation methods, or combination thereof. In some senses, adding an entrainer is similar to extractive distillation
Extractive distillation
Extractive distillation is defined as distillation in the presence of a miscible, high boiling, relatively non-volatile component, the solvent, that forms no azeotrope with the other components in the mixture. The method is used for mixtures having a low value of relative volatility, nearing unity...

.

Material Separation Agent

The addition of a Material Separation Agent, such as benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....

 to an ethanol/water mixture, changes the molecular interactions and eliminates the azeotrope. Added in the liquid phase, the new component can alter the activity coefficient of various compounds in different ways thus altering a mixture's relative volatility. Greater deviations from Raoult's law make it easier to have significant changes in relative volatility with the addition of another component. In azeotropic distillation the volatility of the added component is the same as the mixture, and a new azeotrope is formed with one or more of the components based on differences in polarity. If the material separation agent is selected to form azeotropes with more than one component in the feed then it is referred to as an entrainer. The added entrainer should be recovered by distillation, decantation, or another separation method and returned near the top of the original column.

Example - distillation of ethanol/water

A common historical example of azeotropic distillation is its use in dehydrating ethanol
Ethanol
Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...

 and water mixtures. For this, a near azeotropic mixture is sent to the final column where azeotropic distillation takes place. Several entrainers can be used for this specific process: benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....

, pentane
Pentane
Pentane is an organic compound with the formula C5H12 — that is, an alkane with five carbon atoms. The term may refer to any of three structural isomers, or to a mixture of them: in the IUPAC nomenclature, however, pentane means exclusively the n-pentane isomer; the other two being called...

, cyclohexane
Cyclohexane
Cyclohexane is a cycloalkane with the molecular formula C6H12. Cyclohexane is used as a nonpolar solvent for the chemical industry, and also as a raw material for the industrial production of adipic acid and caprolactam, both of which being intermediates used in the production of nylon...

, hexane
Hexane
Hexane is a hydrocarbon with the chemical formula C6H14; that is, an alkane with six carbon atoms.The term may refer to any of four other structural isomers with that formula, or to a mixture of them. In the IUPAC nomenclature, however, hexane is the unbranched isomer ; the other four structures...

, heptane
Heptane
n-Heptane is the straight-chain alkane with the chemical formula H3C5CH3 or C7H16. When used as a test fuel component in anti-knock test engines, a 100% heptane fuel is the zero point of the octane rating scale...

, isooctane, acetone
Acetone
Acetone is the organic compound with the formula 2CO, a colorless, mobile, flammable liquid, the simplest example of the ketones.Acetone is miscible with water and serves as an important solvent in its own right, typically as the solvent of choice for cleaning purposes in the laboratory...

, and diethyl ether
Diethyl ether
Diethyl ether, also known as ethyl ether, simply ether, or ethoxyethane, is an organic compound in the ether class with the formula . It is a colorless, highly volatile flammable liquid with a characteristic odor...

 are all options as the mixture. Of these benzene and cyclohexane have been used the most extensively. However, because benzene has been discovered to be a carcinogenic compound, its use has declined. While this method was the standard for dehydrating ethanol in the past, it has lost favor due to the high capital and energy costs associated with it.
Another favourable method and less toxic than using benzene to azeotrope water, is to utilise toluene instead.

Pressure-swing distillation

Another method, pressure-swing distillation, relies on the fact that an azeotrope is pressure dependent. An azeotrope is not a range of concentrations that cannot be distilled, but the point at which the activity coefficient
Activity coefficient
An activity coefficient is a factor used in thermodynamics to account for deviations from ideal behaviour in a mixture of chemical substances. In an ideal mixture, the interactions between each pair of chemical species are the same and, as a result, properties of the mixtures can be expressed...

s of the distillates are crossing one another. If the azeotrope can be "jumped over", distillation can continue, although because the activity coefficients have crossed, the water will boil out of the remaining ethanol, rather than the ethanol out of the water as at lower concentrations.

To "jump" the azeotrope, the azeotrope can be moved by altering the pressure. Typically, pressure will be set such that the azeotrope will be closer to 100% concentration. For ethanol, that may be 97%. Ethanol can now be distilled up to 97%. It will actually be distilled to something slightly less, like 96.5%. The 96.5% alcohol is then sent to a distillation column that is under a different pressure, one that pulls the azeotrope down, maybe to 96%. Since the mixture is already above the 96% azeotrope, the distillation will not get "stuck" at that point and the ethanol can be distilled to whatever concentration is needed.

Molecular sieves

For the distillation of ethanol for gasoline
Gasoline
Gasoline , or petrol , is a toxic, translucent, petroleum-derived liquid that is primarily used as a fuel in internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of petroleum, enhanced with a variety of additives. Some gasolines also contain...

 addition, the most common means of breaking the azeotrope is the use of molecular sieve
Molecular sieve
A molecular sieve is a material containing tiny pores of a precise and uniform size that is used as an adsorbent for gases and liquids.Molecules small enough to pass through the pores are adsorbed while larger molecules are not. It is different from a common filter in that it operates on a...

s. Ethanol is distilled to 96%, then run over a molecular sieve which adsorbs water from the mixture. The concentration is now above 96% and can be further distilled. The sieve is heated to remove the water and reused.
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