For effective distillation, the proper reboiler technique must be in place due to the heat involved in the process. Reboilers provide heat at the bottom of industrial distillation columns. They can be regarded as heat-exchangers that provide sufficient energy to heat liquids at the bottom of a column to their boiling points. The steam from the boiling liquid is what provides the heat for vaporization. There are various types of reboilers, each with its advantages and disadvantages.
Forced Circulation Reboilers
Forced circulation reboilers can be used in distillation columns that are either vertical or horizontally installed. A forced circulation reboiler uses pumps to push the liquid at the bottom of the distillation column and is primarily used where heavily contaminated materials are expected in the final product. The advantages are that forced circulation reboilers cause minimal fouling and control the circulation of the liquid during distillation. However, there is a high cost involved because of the amount of power used during distillation, as well as the additional pumps and piping that come with this type of reboiler.
Kettle reboilers are reliable in that they can handle high vaporization of up to 80 percent and are easy to maintain. The liquid from the bottom of the tower flows through the tube bundle and exits as condensate. A restraining device (baffle) controls the liquid level over the bundle. However, the high level of vaporization makes it prone to fouling, and these reboilers are expensive due to their large shell size and maintenance.
Vertical Thermosyphon Reboilers
Vertical thermosyphon reboilers do not require pumping the liquid at the bottom of the distillation column into the reboiler. They also provide a simple, low-cost way of adding heat to the distillation process. The advantages of this type of reboiler are: less fouling, low maintenance costs, less space and piping required, and high heat transfer rates, thus less power is used during distillation. However, these reboilers have reliability issues, cannot be used where a large surface area is needed and can be troublesome in vacuum services.
Horizontal Thermosyphon Reboilers
The heating fluid circulates through the tubes, as vaporization takes place, in the shell side in this type of reboiler. When you compare this type of reboiler to vertical thermosyphon reboilers, it has a higher fouling tendency because the boiling fluid in the shell makes it difficult to clean. It also utilizes more plots and piping space, making it costly, and it also has issues with reliability. Its advantage is that it can be used in a large surface area where vertical thermosyphon reboilers have failed.