Ethanol is subsequently separated from the mash by distillation, in which the components of a solution (in this case, water and ethanol) are separated by differences in boiling point (or individual vapor pressure). Separation is technically limited by the fact that ethanol and water form an azeotrope, or a constant boiling solution, of 95.63 wt% alcohol and 4.37 wt% water. This azeotrope is a minimum boiling mixture (or a positive azeotrope), for which the boiling temperature of the azeotrope is lower than that of the individual pure components, that is, water and ethanol. The minimum boiling tempera­ture at the azeotropic concentration is 78.2°C, whereas the normal boiling points of ethanol and water are 78.4°C and 100°C, respectively.

The 5%, more precisely 4.37 wt%, water cannot be separated by conven­tional distillation, because the minimum boiling temperature is attain­able at the azeotropic concentration, not at the pure ethanol concentration. Therefore, production of pure, water-free (anhydrous) ethanol requires an additional unit operation step following distillation. Dehydration, a relatively complex step in ethanol fuel production, is accomplished in one of two methods. The first method uses a third liquid, most commonly benzene, which is added to the ethanol/water mixture. This third com­ponent changes the boiling characteristics of the solution (now a ternary system instead of a binary system), allowing separation of anhydrous ethanol. In other words, this third component is used to break the azeo­trope, thereby enabling conventional distillation to achieve the desired goal of separation. This type of distillation is also called azeotropic dis­tillation, because the operation separates mixtures that form azeotropes. The second method employs molecular sieves that selectively absorb water based on the molecular size difference between water and ethanol. Molecular sieves are crystalline metal aluminosilicates having a three­dimensional interconnecting network of silica and alumina tetrahedra. Molecular sieves have long been known for their drying capacity (even to 90°C). There are different forms of molecular sieves that are based on the dimension of effective pore opening, and they include 3A, 4A, 5A, and 13X. Commercial molecular sieves are typically available in powder, bead, granule, or extrudate forms.