Editing Distillation (section)

==Azeotropic process==
{{Main|Azeotropic distillation}}
Interactions between the components of the solution create properties unique to the solution, as most processes entail non-ideal 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 result in separation. For example, 95.6% [[ethanol]] (by mass) in water forms an azeotrope at 78.1&nbsp;°C.

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

[[Miscibility|Immiscible]] liquids, such as water and [[toluene]], easily form azeotropes. Commonly, these azeotropes are referred to as a low boiling azeotrope because the boiling point of the azeotrope is lower than the boiling point of either pure component. The temperature and composition of the azeotrope is easily predicted from the vapor pressure of the pure components, without use of Raoult's law. The azeotrope is easily broken in a distillation set-up by using a liquid–liquid separator (a decanter) to separate the two liquid layers that are condensed overhead. Only one of the two liquid layers is refluxed to the distillation set-up.

High boiling azeotropes, such as a 20 percent by weight mixture of [[hydrochloric acid]] in water, also exist. As implied by the name, the boiling point of the azeotrope is greater than the boiling point of either pure component.

===Breaking an azeotrope with unidirectional pressure manipulation===
The boiling points of components in an azeotrope overlap to form a band. By exposing an azeotrope to a vacuum or positive pressure, it is possible to bias the boiling point of one component away from the other by exploiting the differing vapor pressure curves of each; the curves may overlap at the azeotropic point, but are unlikely to remain identical further along the pressure axis to either side of the azeotropic point. When the bias is great enough, the two boiling points no longer overlap and so the azeotropic band disappears.

This method can remove the need to add other chemicals to a distillation, but it has two potential drawbacks.

Under negative pressure, power for a vacuum source is needed and the reduced boiling points of the distillates requires that the condenser be run cooler to prevent distillate vapors being lost to the vacuum source. Increased cooling demands will often require additional energy and possibly new equipment or a change of coolant.

Alternatively, if positive pressures are required, standard glassware can not be used, energy must be used for pressurization and there is a higher chance of side reactions occurring in the distillation, such as decomposition, due to the higher temperatures required to effect boiling.

A unidirectional distillation will rely on a pressure change in one direction, either positive or negative.

===Pressure-swing distillation===
{{further|Azeotrope#Pressure swing distillation}}

Pressure-swing distillation is essentially the same as the unidirectional distillation used to break azeotropic mixtures, but here both [[Azeotrope#Pressure swing distillation|positive and negative pressures may be employed]].

This improves the selectivity of the distillation and allows a chemist to optimize distillation by avoiding extremes of pressure and temperature that waste energy. This is particularly important in commercial applications.

One example of the application of pressure-swing distillation is during the industrial purification of [[ethyl acetate]] after its catalytic synthesis from [[ethanol]].