Distillation Towers : Major Design Considerations

Distillation is the process of separating components or substances from the feed mixture of different miscible liquids by vaporizing and condensing the elements at their relative boiling points (bp). It is an essential component of the chemical process industry.


Through this article, let’s discuss the operations of the distillation tower and design considerations for the piping layout.




Principle of distillation operation


It is one of the most challenging tasks for modeling and control. With higher boiling point liquids will condense first, followed by elements with lower boiling points. Fractionating columns are designed by creating zones of different temperatures along the tower's length, a substance with the lowest bp at the top and highest bp at the bottom. As the vapor rises in the column, they start losing heat and condense based on their respective boiling points.

Distillation Tower and Related Equipment


Distillation tower is not a piece of standalone equipment. It is connected to upstream and downstream equipment that together make up a comprehensive process system. The standard equipment includes Reboilers, Bottom Pumps, Reflux Condensers, Trim Cooler, Overhead Condensers, Reflux Drum and more.

Design Considerations for Piping Layout


  • When locating any column, the piping designer should consider piping interconnections between the column and its adjacent equipment such as pumps, condensers, reboilers and more. Documents such as P&ID, Process Vessel Sketch, Plot Plan, Piping & Plant layout specification are required to locate the column.
  • Once the column gets placed on the plot plan, then the related equipment is located.
  • Thermosyphon reboilers are usually placed near their associated towers.
  • Pumps shall be placed adjacent to the pipe racks. Air-cooled heat exchangers are typically mounted on the top of the pipe racks near the distillation tower by considering the required flexibility in vapor line.
  • The reflux drum and condensers are placed on a technological structure near the distillation tower.
  • The factors considered in determining tower elevation include - Net Positive Suction Head (NPSH), operator access, tower dimensions, maintenance access, minimum clearance, bottoms pipe diameter and tower foundation.
  • Industrial distillation is typically performed in tall towers that require a platform at various levels for easy access and proper maintenance.
  • Ladder length should not exceed 9 meters.
  • Platforms shall be provided for operational access to valves and instruments. The minimum width of the platform shall be 750mm.
  • The width of the manhole platform should be ≥ 900mm.
  • Column and vessel, which are grouped, should have their platforms at the same elevation. It helps in ease of operation and maintenance.
  • The column platforms must be designed in such a way that all the nozzles are easily approached from the platform.
  • Access manholes should be located on the top or in the cylindrical portion of the column. Their orientation shall be in conjunction with tray.
  • Interconnections shall be done wherever feasible after taking thermal expansions of towers into considerations.
  • The piping of the columns shall travel down the column radially on the pipe wayside. Avoid locating valves and flanges inside vessel skirts.
  • It is preferable to arrange vents, drains and utility connections to prevent unintentional or undetected leakage.

Hence, the process equipment design engineering team follows a stepwise procedure for modeling the distillation tower.