- Also called:
- constant boiling-point mixture or constant-boiling mixture
- Related Topics:
- mixture
azeotrope
What is an azeotrope?
Why are azeotropic mixtures unable to be separated by simple distillation?
What are the two types of azeotropes based on boiling points?
What is Raoultâs law?
What methods are used to separate azeotropic mixtures?
azeotrope, in chemistry, a mixture of liquids that has a constant boiling point at a given pressure because the vapor has the same composition as the liquid mixture. The boiling point of an azeotropic mixture may be higher or lower than that of any of its components. Because boiling the azeotrope does not alter the relative concentration of its constituent liquids, the components of the solution cannot be separated by simple distillationâunlike non-azeotropic ideal mixtures, in which the more-volatile component separates first on boiling. Methods such as adding a third component to change the volatility of the azeotrope or using two distillation columns operating at different pressures are used to separate the constituents of the mixture. Azeotropes are used in chemical synthesis, refrigeration, distillation, and solvent recycling.
Types
Azeotropes are categorized into two types based on their boiling points. An azeotrope that has a boiling point higher than the boiling point of its constituents is called a negative, or maximum-boiling, azeotrope. These solutions show strong negative deviation from Raoultâs law. One example is a nitric acid (HNOâ) solution composed of 32 percent water and 68 percent nitric acid. This waterânitric acid mixture is an azeotrope and boils at 120.5 °C (248.9 °F), even though the individual components have lower boiling points: water boils at 100 °C (212 °F), nitric acid at 83 °C (181 °F).
At constant temperature, the partial pressure of a component in a liquid mixture is proportional to its mole fraction in that mixture; that is, each component exerts a pressure that depends directly on the number of its molecules present. Formulated in the late 19th century by the French chemist François-Marie Raoult, this law is important in understanding the behavior of solutions and mixtures.
Raoultâs law is mathematically expressed as
,
where Psolution is the vapor pressure of the solution, Xsolvent is the mole fraction of the solvent, and P0solvent is the vapor pressure of the pure solvent.
The second type is a positive, or minimum-boiling, azeotrope, which has a boiling point that is lower than that of its constituents. These solutions show strong positive deviation from Raoultâs law. A well-known example of a positive azeotrope is a solution that is 95.57 percent ethanol and 4.43 percent water, which boils at 77.95 °C (172.31 °F). This is lower than the boiling point of ethanol, 78 °C (173 °F), as well as that of water.
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Depending on whether the constituents of the solution mix uniformly, an azeotropic mixture may be homogeneous (when the components are completely miscible) or heterogeneous (when the components are immiscible). Azeotropes are also classified on the basis of the number of their componentsâbinary, when the mixture has two components, and ternary, when it has three.
Separation of azeotropic mixtures
Alternatives to conventional distillation for separating azeotropic mixtures include using entrainers, pressure-swing distillation, and pervaporation. When added to an azeotropic mixture, substances called entrainers, or azeotrope breakers, interact differently with the mixture components, often forming an azeotrope with one of them. This alters the relative volatility of the mixtureâs components and disrupts the equilibrium of the mixture, ultimately allowing the components to separate. For instance, benzene is used as an entrainer in the azeotropic distillation of bioethanolâan azeotropic mixture of ethanol and water. Benzene forms a low-boiling azeotrope with the water and ethanol; this ternary azeotrope can be distilled to carry away the bulk of the water, leaving behind purer ethanol that is nearly free of water.
When the azeotropic mixture is pressure sensitive, pressure-swing distillation is used for separation. In this technique, the azeotrope is passed through two columns maintained at different pressuresâone high, one low. This method alters the relative volatility of the mixture, which loses its azeotropic behavior; its components can then be separated by distillation. When pressure-swing distillation is operating continuously, the distillate of the second column is fed back into the first for uninterrupted separation and maximum output.
Pervaporationâa portmanteau of the terms permeation and evaporationâuses selective membranes to separate azeotropes. Specific components of the mixture pass through a nonporous or semi-porous membrane and evaporate on the other side. This occurs because of a pressure difference on the other side of the membrane. The membrane is specially designed to let one component pass through more easily than the other. The vapor is collected separately and condensed.
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