The liquid outlet from bottom of a distillation column is close to its initial boiling point, due to near equilibrium conditions on each of the distillation column trays. This liquid is mostly made of the heavy component from the mixture at the distillation column inlet. But some amount of the light component also remains dissolved in this liquid. To recover this light component in its vapor form and thus also to improve purity of the heavy component in bottoms outlet, this liquid stream is sent to a reboiler.
This reboiler, which is essentially a heat exchanger, heats up the liquid. A reboiler may use steam, fire, another hot liquid etc. to provide heat and depending on the heating medium used there are different types of reboilers namely - kettle type, fired, thermosiphon reboilers etc.
Being close to the initial boiling point, the light component is readily vaporized. These vapors are recycled back to the distillation column and introduced to the column at the last stage from where they flow to the top of the column.
Most of the heavy component remains as a liquid in a reboiler, which can be taken out as a product of continuous distillation process. This type of reboiler is known as partial reboiler, as the liquid is only partially vaporized. Heavy bottoms outlet is close to its boiling point. If in a reboiler, the bottoms liquid is completely vaporized, that reboiler is known as total reboiler.