Lecture 27: Introduction to Reboilers

Overview

• This is the 27th lecture in the course on Process Equipment Design.
• Focus topic: Reboilers, covered over the next four lectures.

Definition of Reboiler

• A reboiler is a heat exchanger used to generate vapor supplied to the bottom tray of a distillation column.
• It vaporizes a fraction of the bottom product exiting a distillation column.
• Key Functions:
  • Converts liquid into vapor.
  • Sends vapor back to the distillation column.

Reboiler vs. Vaporizer

• Reboiler: Converts a fraction of liquid to vapor.
• Vaporizer: Converts all feed to vapor.
• Both are similar in design and operation but differ in purpose.

Design and Operation of Reboilers

• Heating Medium: Typically steam, but any higher temperature stream can be used.
• Boiling Mechanism: Continuous boiling due to continuous liquid feed, which can enter multiple times.
• Circulation Types:
  • Boiling can happen in tube side or shell side.

Classification of Reboilers

• Choice depends on:
  • Nature of process fluid (viscosity, fouling tendency).
  • Operating pressure (vacuum or pressure).
  • Equipment layout and available headroom.
• Types of Reboilers:
  1. Kettle Reboiler
  2. Natural Circulation (Thermosyphon) Reboiler
  3. Forced Circulation Reboiler

Kettle Reboiler

• Structure: Horizontally mounted T-kettle shell with a tube bundle (U-tube or straight tube).
• Heating medium is merged in liquid, allowing vapor generation and liquid-vapor separation.
• Key Features:
  • Provides pure vapor to the distillation column.
  • Low circulation rates, making it reliable even under varying pressures.
  • More residence time can lead to scale formation.

Natural Circulation (Thermosyphon) Reboiler

• Structure: Boiling occurs in either shell side (horizontal) or tube side (vertical).
• Circulation is maintained by density differences between phases.
• Advantages:
  • Less residence time, reducing fouling issues.
  • Simplistic operation due to lack of pumps.
• Disadvantages:
  • No vapor-liquid separation; requires additional units if used as vaporizer.

Forced Circulation Reboiler

• Incorporates a pump for circulation of liquid and vapor.
• Can facilitate boiling on both shell and tube sides.

Internal Reboiler

• A tube bundle is directly inserted into the distillation column.
• Limited heat transfer area and can lead to operational issues due to froth formation.

Circulation Types: Recirculating vs. Once Through

• Recirculating: Liquid partially converted to vapor and recycled.
• Once Through: Liquid completely converted to vapor; only the vapor exits as product.

Heat Transfer Fundamentals in Reboilers

• Boiling Mechanism: Depends on temperature difference between heated surface and liquid.
• Boiling Curve: Affects heat transfer efficiency.
  • Regions include free convection, nucleate boiling, transition boiling, and film boiling.
• Heat Transfer Coefficients: Vary based on boiling type and mixture.
  • Convective boiling and nucleate boiling components are combined to find effective heat transfer coefficients.

Conclusion

• Next lecture will focus on the design of reboilers.