Azeotrope

An azeotrope is a liquid mixture of two or more components which has a unique constant boiling point. This boiling point may be higher or lower than the boiling points of the constituent liquids, and the liquid retains the same composition as it is boiled. As a consequence, the vapor has the same composition as the liquid and simple distillation will not separate the constituents as it would with most liquid mixtures. Technically speaking, perfect azeotropic behavior occurs only at a precise temperature and pressure, but many mixtures are near-perfect azeotropes across a wide range of conditions.

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.

Examples of azeotropes :

• nitric acid (68.4%) / water, boils at 122°C
• perchloric acid (28.4%) / water, boils at 203°C (negative azeotrope)
• fluorhydric acid (35.6%) / water, boils at 111.35°C (negative azeotrope)
• ethanol (95%) / water, boils at 78.2°C
• sulphuric acid (98.3%) /water, boils at 330°C

ADVANCED THEORY:

   There is another interesting phenomena that has been observed. Some azeotropic systems exhibit what is called "Multiplicity" i.e. Multiple steady states.

For example the ternary system Methanol - Toluene - MethylButyrate exhibit this Multiplicity.

For the same conditions of input such as Molar feed rate, composition, Reflux rate, Distillate flow rate and Column temperature profiles; we can observe different product compositions, Temperature profile and internal flow rates.

We can have for example 3 steady states: 2 Stable steady states and 1 unstable steady state.

See Also

• Eutectic