In chemistry, azeotropic distillation is any of a range of techniques used to break an azeotrope in distillation. // Introduction Chemistry is a large field encompassing many subdisciplines that often overlap with significant portions of other sciences. ... An azeotrope is a liquid mixture of two or more components which has a unique constant boiling point. ... Strathisla whisky distillery in Keith, Scotland Distillation is a means of separating liquids through differences in their vapor pressures. ...

A common distillation with an azeotrope is the distillation of ethanol and water. Using normal distillation techniques, ethanol can only be purified to approximately 96% (hence the 96% (192 proof) strength of some commercially available grain alcohols). Ethanol, also known as ethyl alcohol or grain alcohol, is a flammable, colorless chemical compound, one of the alcohols that is most often found in alcoholic beverages. ... Water has the chemical formula H2O, meaning that one molecule of water is composed of two hydrogen atoms and one oxygen atom. ... Alcoholic proof is a measure of how much ethanol is in an alcoholic beverage, and is approximately twice the percentage of alcohol by volume (ABV, the unit that is commonly used today). ... Ethyl alcohol, also known as ethanol or grain alcohol, is a flammable, colorless chemical compound, one of the alcohols that is most often found in alcoholic beverages. ...

Once at a 96/4% ethanol/water concentration, the activity coefficients of water and ethanol are the same, so the vapor from the boiling mixture is also 96/4%. Further distillation is therefore ineffective. Some uses require a higher percentage of alcohol, for example when used as a gasoline additive. The 96/4% azeotrope needs to be "broken" in order to refine further. The Activity coefficient for chemicals in a mixture is an indicator of what the concentration of that chemical will be in a vapor of the mixture. ...

One method is the addition of an "MSA", a material separation agent. The addition of benzene to the mixture changes the molecular interactions and eliminates ("breaks") the azeotrope. The drawback is that another separation is needed to remove the benzene. Benzene, also known as C6H6, PhH, and benzol, is an organic chemical compound which is a colorless and flammable liquid with a pleasant, sweet smell. ...

Another method, pressure swing distillation, relies on the fact that an azeotrope is pressure dependent. It also depends on the knowledge that an azeotrope is not a range of concentrations that can not be distilled, but the point at which activity coefficients 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 ethanol. Pressure (symbol: p) quantifies the intensity of a force acting on a surface in a direction perpendicular to that surface. ...

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 95% azeotrope, the distillation will not get "stuck" at that point and the ethanol can be distilled to whatever concentration is needed.

For the distillation of ethanol for gasoline addition, the most common means of breaking the distillation is the use of molecular sieves. Ethanol is distilled to 96%, then run over a molecular sieve which absorbs 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. Gasoline, as it is known in North America, or petrol (abbreviated from petroleum spirit), in many Commonwealth countries (sometimes also called motor spirit) is a petroleum-derived liquid mixture consisting primarily of hydrocarbons, used as fuel in internal combustion engines. ... 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. ...