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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

  • 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


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AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.
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