Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 120

Azeotropic distillation is the use of a third component to separate two close-boiling components by means of the formation of an azeotropic mixture between one of the original components and the third component to increase the difference in the boiling points and facilitates separation by distillation.

All compounds have definite boiling temperatures, but a mixture of chemically dissimilar compounds sometimes causes one or both of the components to boil at a temperature other than that expected. For example, benzene boils at 80oC (176oF), but if it is mixed with hexane, it distills at 69oC (156oF). A mixture that boils at a temperature lower than the boiling point of either of the components is called an azeotropic mixture.

Chemical bonding between the components of the mixture creates properties unique to the mixture. If the system forms azeotropes, as in a benzene and cyclohexane system, a different problem arises - the azeotropic composition limit the separation, and for a better separation, this azeotrope must be bypassed in some way. At the azeotropic point, the mixture contains the given component in the same proportion as the vapor, so that evaporation does not change the purity, and distillation does not affect separation. For example, ethyl alcohol and water form an azeotrope (azeotropic mixture) at 78.2°C (172.8°F).

If the separation of individual components from petroleum itself or from petroleum products is required, there are means by which this can be accomplished. For example, when a constant-boiling mixture of hydrocarbons contains components whose vapor pressure is affected differently by the addition of, say, a non-hydrocarbon compound, distillation of the hydrocarbon mixture in the presence of non-hydrocarbon additive may facilitate separation of the hydrocarbon components.

In general, the non-hydrocarbon additive is a polar organic compound and should also have the ability to form a binary minimum constant-boiling (or azeotropic) mixture with each of the hydrocarbons. Thus, it is often possible to separate compounds that have close boiling points by means of azeotropic distillation.

See also: Azeotrope, Distillation.