Introduction to Distillation Columns

Overview

• Purpose of the lecture: Introduce the concept of distillation columns and their practical uses.

Key Concepts

• Distillation Column: A unit operation used to separate mixtures based on differences in boiling points.
• Components:
  • Reactant A: Boiling point of 100°C
  • Product P: Boiling point of 50°C
  • Product P is referred to as the lighter component due to lower boiling point and higher tendency to vaporize.

Operation of Distillation Column

• Feed Stream (F): Enters the distillation column at a feed plate.

Bottom Tray

• Heavy Component Concentration: At the bottom tray, the heavier component (Reactant A) is concentrated due to descending liquid phase.
• Heating Mechanism:
  • Uses a reboiler (e.g., copper coil with electric current or combustion).
  • Heat causes vaporization, resulting in vapor (V') that ascends the column with higher concentrations of the lighter component (Product P).

Vapor-Liquid Equilibrium

• Equilibrium at Each Tray: Sufficient residence time allows phases to reach equilibrium, separating lighter and heavier components based on boiling points.
• Ascent in Column: Each tray is progressively cooler, promoting the heavier component to condense back to liquid and allowing for better separation.

Thermal Dynamics

• Temperature Gradient: Essential for mass transfer.
  • Inefficiency: Same temperature across trays indicates poor performance.

  • Expected temperature gradient in binary systems:

    [ \text{Temperature Gradient} = \frac{100°C - 50°C}{10} = 5°C \text{ per tray} ]

Distillation Process

• As vapor ascends the column and cools, it reaches another vapor-liquid equilibrium at each tray.
• Condenser: At the top of the column, vapor condenses, yielding a liquid rich in Product P.
• Reflux Ratio: Important parameter to optimize separation.

Final Notes

• Distillate (D):
  • Should be at least equal to the flow rate of the lighter component in the feed to avoid product loss.
  • Accumulating the product in the bottom stream reduces conversion efficiency.
• Reboiler Duty: To double the feed rate, the heat supplied in the reboiler must also be doubled.

Conclusion

• Understanding distillation columns is critical for chemical engineers.
• Emphasis on practical efficiency and optimization of operations.