Reboilers are heat exchangers typically used to provide heat to the bottom of industrial distillation columns. They boil the liquid from the bottom of a distillation column to generate vapors which are returned to the column to drive the distillation separation.
Proper reboiler operation is vital to effective distillation. In a typical classical distillation column, all the vapor driving the separation comes from the reboiler. The reboiler receives a liquid stream from the column bottom and may partially or completely vaporize that stream. Steam usually provides the heat required for the vaporization. The most critical element of reboiler design is the selection of the proper type of reboiler for a specific service. Most reboilers are of the shell and tube heat exchanger type and normally steam is used as the heat source in such reboilers.
- Kettle reboilers. Kettle reboilers are very simple and reliable. In this reboiler type, steam flows through the tube bundle and exits as condensate. The liquid from the bottom of the tower, commonly called the bottoms, flows through the shell side.
- Thermosyphon reboilers. These do not require pumping of the column bottoms liquid into the reboiler. Natural circulation is obtained by using the density difference between the reboiler inlet column bottoms liquid and the reboiler outlet liquid-vapor mixture to provide sufficient liquid head to deliver the tower bottoms into the reboiler. Thermosyphon reboilers (also known as calandrias) are more complex than kettle reboilers and require more attention from the plant operators.
- Fired reboilers. Fired heaters (furnaces) may be used as a distillation column reboiler. A pump is required to circulate the column bottoms through the heat transfer tubes in the furnace’s convection and radiant sections. The heat source for the fired heater reboiler may be either fuel gas or fuel oil. Coal would rarely, if ever, be used as the fuel for a fired heater reboiler.
- Forced circulation reboilers. This type of reboiler uses a pump to circulate the column bottoms liquid through the reboilers. It should be noted steam is not the only heat source that can be used. Any fluid stream at a high enough temperature could be used for any of the manyshell and tube heat exchanger reboiler types.