Azeotrope

Encyclopedia : A : AZ : AZE : Azeotrope

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An azeotrope is a mixture of two or more compounds (molecules) which at a specific composition (ratio of compounds) maintains vapor and liquid phases which are in equilibrium and identical in composition. Due to the uniformity of liquid and vapor, chemical composition of the azeotrope cannot be changed by simple boiling (distillation). This constant composition will (almost always) occur at a minimum or maximum temperature over the composition space. The exception for binary mixtures is the rare double azeotrope. The exception for three or more components is the intermediate boiling azeotrope (saddle azeotrope). In a fake example, if there are 3 parts of liquid x to every 4 parts of liquid y, then you could not separate the 2 by boiling them because of the rates at which they boil.

In order to enrich a mixture past the azeotrope (or "break" the azeotrope), one may use several techniques:

1. Use azeotropic distillation, by adding in a third component which will generate a lower boiling heterogenous azeotrope that can be broken by phase separating the two immiscible liquids and decanting.
2. Use extractive distillation where a third compound is added near the top of a distillation column that reduces the volatility of one compound over another. For example, if you wanted to separate water from a type of alchohol by boiling, but they were at their azeotropic mixture, if you added salt to the mixture that would raise the boiling point of the water so you could boil more of the alchohol out of it.
3. Use liquid-liquid extraction to separate the compounds by segregating them into two different liquid phases.
4. Use a membrane to separate them, as in pervaporation.
5. For azeotropes that change composition when pressure is changed, you can use pressure swing distillation by operating two distillation columns at different pressures, and recycle material between these two distillation columns. One column will remove a pure component (two-component systems) in the bottom of one distillation column, the other will remove a pure component in the bottom of a second distillation column, and the azeotrope (of two different compositions) will be fed to the other column.

The word azeotrope comes from the Greek "zein tropos", or "constant boiling". An azeotrope is said to be positive if the constant boiling point is at a temperature maximum, and negative when the boiling point is at a temperature minimum. The vast majority of azeotropes are minimum boiling. All liquid mixtures which are immiscible and which form azeotropes are minimum boiling .

Examples of azeotropes

• nitric acid (68%) / water, boils at 120.5°C at 1 atm
• perchloric acid (28.4%) / water, boils at 203°C (negative azeotrope)
• hydrofluoric acid (35.6%) / water, boils at 111.35°C (negative azeotrope)
• ethanol (96%) / water, boils at 78.2°C
• sulfuric acid (98.3%) / water, boils at 100°C
• acetone / methanol / chloroform form an intermediate boiling azeotrope
• diethyl ether (33%) / halothane (66%) a mixture once commonly used in anaesthesia.
• benzene / hexafluorobenzene forms a double binary azeotrope.

See also

• Eutectic

External links

• [Azeotrope Databank]
• Update in Anaesthesia: The halothane/ether azeotrope - A reconsideration [link]
• [Azeotrope] defined with a limerick.

 

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