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Understanding Simple Distillation Principles

Aug 29, 2024

Notes on Simple Distillation Lecture

Introduction

  • Discussion on fundamental concepts of simple distillation.
  • Importance of understanding general chemistry laws.

Key Laws in Distillation

1. Raoult's Law

  • Predicts vapor pressure of a liquid in a mixture.
  • Vapor pressure of component = Vapor pressure of pure component × Mole fraction in mixture.

2. Dalton's Law

  • Total pressure in a system = Sum of vapor pressures of each component.

3. Ideal Gas Law

  • Common form: PV = nRT.
  • Rearranged: PV/nT = R (constant for all gases).
  • Relates pressure of compound and moles to total pressure in the system.
  • Mole fraction = Partial pressure of A / Total pressure.

Application of Laws in Distillation

  • Example of a binary mixture of toluene and benzene.
  • Mixture Composition: 50 mole percent benzene in toluene.
  • Boiling Points:
    • Toluene: 300 torr (pure).
    • Benzene: 1200 torr (pure).
  • Calculated Partial Pressures (at boiling point):
    • Toluene: 150 torr (0.5 mole fraction).
    • Benzene: 600 torr (0.5 mole fraction).
  • Vapor Composition:
    • Vapor above the liquid: 80 mole percent benzene.

Simple Distillation Apparatus

Components

  1. Boiling Flask
    • Contains the liquid mixture.
  2. Three-Way Condenser (Still Head)
    • Diverts vapor from boiling flask to cooler area for condensation.
  3. West Condenser
    • Long tube with a water jacket for cooling.
    • Water flows in bottom and out top for effective cooling.
  4. Vacuum Adapter
    • Diverts condensed liquid to receiving flask.
    • Open to atmosphere to prevent pressure build-up.
  5. Receiving Flask
    • Placed over an ice bath to maintain liquid phase.

Operation of Simple Still

  • Heat mixture in boiling flask.
  • Vapor forms with increased concentration of benzene (80% vs. 50% in liquid).
  • Only about 1 in 5 molecules escaping are toluene.
  • Collect enriched benzene in the receiving flask.

Liquid Vapor Composition Plot

  • Used to predict efficiency of simple distillation.
  • Plots temperature vs. mole percent of one constituent (e.g., benzene).
  • Example:
    • Start with a mixture of 50 mole percent benzene.
    • Boiling leads to 80 mole percent vapor composition.
  • Connect calculated points to create a curve (liquid vapor composition plot).
  • Allows for easier determination of distillate composition during distillation.

Conclusion

  • Next discussion: Achieving 100% pure sample vs. 80% enriched sample.

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