Introduction to Fractional Distillation

Overview

• Discussion on the principles of fractional distillation.
• Focused question: Can distillation achieve purities beyond those predicted by Raoult's, Dalton's, and ideal gas laws?

Key Concepts

• Previous Example: 50-50 mixture of toluene and benzene.
  • Expected vapor composition: 80 mole percent benzene.
• Challenge: Is a higher purity distillate possible?
  • Common response: Yes, through redistillation.
  • Result: Achieving distillate with 99-100% benzene is possible.

Simple Distillation Apparatus

• Schematic Description:
  • Boiling flask charged with 50 mol% benzene and toluene.
  • Vapor condenses in a separate region, leading to expected outcomes.
• Redistillation Process:
  • Attach collection flask to a second apparatus for further distillation.
  • New calculations yield a distillate of approximately 99-100% benzene.

Fractional Distillation

• Fractionating Column:
  • Added between boiling liquid and still head for enhanced purity.
  • Can be packed with solid materials for increased surface area.
• Process Explanation:
  • Vapor rises through the column, where it condenses and revaporizes, enriching the benzene concentration.
  • Expected outcomes: Higher purity distillate than simple distillation.

Behavior Inside the Fractionating Column

• Vapor Composition Changes:
  • Begins with 50 mole% liquid, vapor rises and condenses at different points in the column.
  • Near the boiling flask: approximately 80 mole% benzene.
  • Higher up: approximately 97 mole% benzene.
• Pure Benzene Collection:
  • At the top of the column, only pure benzene reaches the condenser.

Comparative Analysis

• Efficiency of Fractional Distillation:
  • Similar effect to multiple distillations done in a single run.
  • Example outcomes:
    • Single distillation: 80% pure distillate.
    • Second distillation: 97% pure.
    • Third distillation: essentially 100% pure.
• Theoretical Plates:
  • Efficiency reporting:
    • 97 mol% benzene = 2 theoretical plates.
    • 100% pure benzene = 3 theoretical plates.

Conclusion

• Fractional distillation offers a practical and efficient method to achieve high purity distillates compared to simple distillation.