In chemistry, azeotropic distillation refers to a range of techniques used to break an azeotrope in distillation.

A common distillation with an azeotrope is the distillation of ethanol and water. Using normal distillation techniques, ethanol can only be purified to approximately 95% (hence the 95% (190 proof) strength of some commercially available grain alcohols).

Once at a 95/5% ethanol/water concentration, the activity coefficients of water and ethanol are the same, so the vapor from the boiling mixture is also 95/5%. Further distillation is therefore ineffective. Some uses require a higher percentage of alcohol, for example when used as a gasoline additive. The 95/5% azeotrope needs to be "broken" in order to refine further.

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.

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.

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 95.5% The 95.5% alcohol is then sent to a distillation column that is under a different pressure, one that pulls the azeotrope down, maybe to 93%. Since the mixture is already above the 93% 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 95%, then run over a molecular sieve which absorbs water from the mixture. The concentration is now above 95% and can be further distilled. The sieve is heated to remove the water and reused.