Reboiler Theory

The transfer of heat to and from process fluids is an essential part of most chemical processes. A reboiler is a heat exchanger that is used to generate the vapor supplied to the bottom tray of a distillation column. The liquid from the bottom of the column is partially vaporized in the exchanger, which is usually of the shell-and-tube type. The
heating medium is most often condensing steam, but commercial heat-transfer fluids and other process streams are also used. Boiling takes place either in the tubes or in the shell, depending on the type of reboiler
The following design guidelines should be considered when the selection of a reboiler type is made.

1. Fouling -Tube-side is easier to clean than shell-side.
2. Corrosion – corrosion or process cleanliness may dictate the use of expensive alloys; therefore, these fluids are placed inside tubes in order to save the cost of an alloy shell.
3. Pressure – high pressure fluids are placed on tube side to avoid the expense of thick walled shells. For very low pressures (vacuum) other factors involved in the selection of reboiler type determines the tube-side fluid.
4. Temperatures – very hot fluids are placed inside tube to reduce shell costs. The lower stress limits at high temperatures affect shell design the same as high pressures.
5. Heating medium requirements may be more important than the boiling liquid requirements.
6. Boiling fluid characteristics: Temperature sensitive liquids require low holdup design. Boiling range and mixture concentration together with available ΔT affect circulation requirements to avoid stagnation. Foaming can be better handled inside tubes.
7. Temperature difference and type of boiling (film or nucleate) affects the selection.
8. Space constraints; e.g., if head room is limited then vertical units would be inappropriate or the limitation of space for internal reboilers.
9. Enhanced surfaces are suitable only for some types.