Distillation Equipment Selection and Column Design

Criteria for Selecting Distillation Equipment

• Pressure Drop: Packed towers have a lower pressure drop (1/5th to 1/8th of tray towers), making them suitable for high vacuum distillation.
• Tower Diameter: Packed towers are preferred for very small diameters (<150 mm) due to fabrication difficulties with trays, while tray towers are used for large diameters to ensure proper liquid distribution.
• Liquid-Vapor Ratio: Packed towers are suitable for high L/V ratios, while tray towers are better for very low L/V ratios due to minimum wetting rate requirements.
• Corrosive Systems: Packed towers with plastic or reinforced plastic packings are more economical than tray towers requiring special metal alloys (e.g., Hastelloy-B for methyl iodide-acetic acid separation).
• Side Product Draw-off: Tray towers are preferred when side products need to be withdrawn, such as in refinery distillation columns.

Design Process of a Distillation Column

1. Component Selection: For multicomponent mixtures, choose two key components; for binary mixtures, identify the more and less volatile components.
2. Operating Pressure and VLE Data: Decide the column’s operating pressure and obtain vapor-liquid equilibrium (VLE) data at that pressure.
3. Separation Requirements and Material Balance: Define product compositions (distillate and residue) and perform a complete material balance.
4. Reflux Ratio and Stages: Determine the minimum reflux ratio (Rm), optimum reflux ratio (R), and number of theoretical stages required.
5. Tower Selection: Choose between a tray tower or packed tower, and select tray type or packing type accordingly.
6. Tower Dimensions: Calculate tower diameter and pressure drop.
7. Efficiency and Packing Height: Determine tray efficiency and actual number of trays (for tray towers) or HETP (Height Equivalent to a Theoretical Plate) and packing height (for packed towers).
8. Construction and Insulation: Select materials for all components and design appropriate insulation.
9. Condensers and Reboilers: Design both the condenser and reboiler.
10. Control and Mechanical Design: Decide control methodology, line sizes, nozzle locations, and carry out mechanical design, including column supports.