Water and alcohol form an azeotrope at atmospheric pressure. Basically, fractional distillation can only achieve a certain proof quality. Past about 94ethanol, 6water, distillation is not a viable option. In order to remove the water, we must use either a chemical that does not react with ethanol or use a dessicant. Calcium hydroxide has been suggested as a chemical method of drying ethanol. Another method is to add gasoline to the ethanol and redistill. Both of these options complicate the process and redistillation wastes energy. Corn grits and glycerin have been used to produce absolute ethanol as well, however, the far simplest method is to use a molecular sieve.
Zeolite is a synthetic aluminum-silica material. The surface of which is covered by pores of a certain size. Three angstrom and four angstrom zeolite are suitable for the dehydration of ethanol. The "pore size" of the zeolite gives the critical diameter of the molecules it can adsorb. Three and four angstrom zeolite are usually used as dessicants to remove carbon dioxide and water. The critical diameter of ethanol is 4.4 angstroms. Because of this, its molecules will not fit in the pigeon-holed surface of the zeolite. Water has a critical diameter of around 2.4 and is readily adsorbed. Zeolite is not consumed in the process. It can be regenerated by heating to boiling point of the substance it has adsorbed.