Azeotropic distillation

In chemistry, azeotropic distillation^[1] is any of a range of techniques used to break an azeotrope in distillation. In chemical engineering, azeotropic distillation usually refers to the specific technique of adding another component to generate a new lower-boiling azeotrope that is heterogeneous, such as the example below with the addition of benzene to water and ethanol.

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Example - distillation of ethanol/water

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).

Once at a 96/4% ethanol/water concentration 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.

Material separation agent

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.

Pressure-swing distillation

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 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

Main article: molecular sieve

For the distillation of ethanol for gasoline addition, the most common means of breaking the azeotrope 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.

See also

• Azeotrope
• Theoretical plate

References

1. ^ Kister, Henry Z. (1992). Distillation Design, 1st Edition, McGraw-Hill. ISBN 0-07-034909-6.