A kettle reboiler is a heat exchanger used in a chemical distillation process. This shell-type exchanger has a large open boiling surface much like a kettle on a kitchen stove. Typically, it is the vessel at the bottom of a distillation column. The vessel receives condensate from the separation tray above it and returns vapor to provide the heat energy to the column. The design of a kettle reboiler allows for a large transfer of heat on a continuous basis.
A distillation column separates compounds based on their boiling points at the operating pressure of the column. The column consists of a number of separation stages distributed vertically. At each stage, the ratio of one component in the liquid phase and the same component in the gaseous phase will be in equilibrium and a function of the stage temperature and the column pressure. A stage may consist of a physical tray or a site on a continuous medium, such as in a column filled with glass beads.
In a multi-compound mixture, several side streams plus an overhead draw may be taken from the column. Each stream is concentrated in a desired compound or a small mixture of similar compounds. Heat travels up the column in the form of a hot vapor. Vapor condenses on a tray or stage, releasing heat to the liquid at that stage. Some compounds absorb heat, boil, and transfer heat upward.
Liquids not boiling will overflow the stage and fall to a lower level. As this process of equilibrium is established from stage to stage, the components are separated and drawn off. New feed is added at a stage based on the column design. Heat is continually added to the column via the kettle reboiler at the bottom.
Shell-and-tube heat exchangers work by passing two separate streams of fluid at different temperatures by each other in close contact. The tubes resemble a collection of straws within a close-fitting cylinder. The tubes connect via manifolds at the head and the rear of the bundle. The cooler fluid may be on either the shell or tube side. Heat will pass by conduction through the tube walls toward the cooler fluid.
A kettle reboiler differs from the typical shell-and-tube heat exchanger in that above the bundle of tubes there is a large open space. The process fluid is on the shell side, where it boils up the distillation column for separation. The tube side carries the hot fluid, consisting of steam or another higher-temperature process fluid. Costs of a kettle reboiler run 15% to 25% higher than an equivalent-size shell-and-tube heat exchanger due to the larger shell and increased port size.