Azeotropes are mixtures of liquids that boil at a constant temperature and composition, behaving like pure components during distillation. They're crucial in vapor-liquid equilibrium because they can't be separated by simple distillation, posing challenges in industrial processes.
Understanding azeotropes is key to designing effective separation techniques. This section covers types of azeotropes, their properties, and separation methods like azeotropic distillation, extractive distillation, and pressure-swing distillation, which are essential for overcoming azeotropic limitations in industry.
Types of Azeotropes
Definition and Characteristics of Azeotropes
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Azeotrope
Mixture of two or more liquids that boils at a constant temperature and composition
Vapor and liquid compositions are the same at the azeotropic point
Behaves like a pure component during boiling and condensation
Cannot be separated by simple distillation (ethanol-water azeotrope)
Classification Based on Boiling Point Deviation
Positive azeotrope
Exhibits a maximum boiling point compared to the pure components
Boiling point is higher than that of either pure component at the same pressure
Vapor pressure of the mixture is lower than expected from Raoult's law (ethanol-water, nitric acid-water)
Negative azeotrope
Exhibits a minimum boiling point compared to the pure components
Boiling point is lower than that of either pure component at the same pressure
Vapor pressure of the mixture is higher than expected from Raoult's law (acetone-chloroform, methanol-benzene)
Classification Based on Phase Behavior
Homogeneous azeotrope
Azeotropic mixture forms a single liquid phase
Components are miscible in all proportions (ethanol-water, acetone-methanol)
Heterogeneous azeotrope
Azeotropic mixture forms two or more liquid phases
Components have limited miscibility (water-toluene, water-butyl acetate)
Decantation can be used to separate the phases after condensation
Azeotropic Properties
Composition and Boiling Point
Azeotropic composition
Specific mole fractions or mass fractions of components at which the azeotrope occurs
Depends on the system pressure and temperature
Can be determined experimentally or predicted using vapor-liquid equilibrium models (NRTL, UNIQUAC)
Azeotropic composition shifts with changes in pressure (ethanol-water: 95.6% ethanol at 1 atm)
Vapor-Liquid Equilibrium Behavior
Azeotropic systems exhibit non-ideal vapor-liquid equilibrium behavior
Deviations from Raoult's law due to intermolecular interactions between components
Activity coefficients are used to account for non-ideality in thermodynamic models
Azeotropic point represents a singular point in the vapor-liquid equilibrium diagram
Vapor and liquid compositions are equal at the azeotropic point
Tie lines converge at the azeotropic point in xy and Txy diagrams
Azeotropic Separation Techniques
Azeotropic Distillation
Azeotropic distillation
Separation technique that uses an entrainer to alter the relative volatility of components
Entrainer forms a new azeotrope with one or more components, facilitating separation
Distillate or bottoms product contains the desired component (ethanol dehydration using benzene)
Entrainer selection is crucial for effective azeotropic distillation
Should form a heterogeneous azeotrope with one of the components
Entrainer should be easily separable from the desired product
Commonly used entrainers include benzene, cyclohexane, and pentane
Extractive distillation
Separation technique that uses a high-boiling solvent to alter the relative volatility of components
Solvent interacts differently with the components, enhancing their separation
Solvent is introduced at a higher stage in the distillation column (ethanol dehydration using ethylene glycol)
Solvent selection criteria for extractive distillation
High boiling point to facilitate recovery and recycling
Selective interaction with one of the components
Low volatility to minimize solvent loss in the distillate
Examples of solvents: ethylene glycol, glycerol, and ionic liquids
Pressure-Swing Distillation
Pressure-swing distillation
Separation technique that exploits the pressure dependence of azeotropic composition
Two distillation columns operating at different pressures are used
Azeotropic composition shifts, allowing for separation of components (acetone-methanol separation)
Advantages of pressure-swing distillation
Avoids the use of additional components (entrainers or solvents)
Reduced energy consumption compared to other azeotropic separation techniques
Suitable for heat-sensitive components due to lower operating temperatures