A kettle reboiler is a heat exchanger used in chemical distillation with a large boiling surface. It receives condensate from a separation tray and returns steam to supply thermal energy to the column. It differs from a shell and tube heat exchanger with a large open space above the tube bundle.
A kettle reboiler is a heat exchanger used in a chemical distillation process. This clamshell 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 the condensate from the separation tray above it and returns the steam to supply the thermal energy to the column. The design of a kettle reboiler allows for large heat transfer on a continuous basis.
A distillation column separates compounds based on their boiling points at the column’s operating pressure. The column consists of a series of vertically distributed separation stages. At each stage, the ratio of a component in the liquid phase to the same component in the gaseous phase will be in equilibrium and a function of stage temperature and column pressure. A stage may consist of a physical tray or a site on a continuous medium, such as a column filled with glass beads.
In a multi-compound mixture, several side streams plus a top draft may be taken from the column. Each stream is concentrated into a desired compound or a small mixture of similar compounds. The heat rises up the column in the form of hot steam. The vapor condenses on a tray or stage, releasing heat to the liquid in that stage. Some compounds absorb heat, boil and transfer heat upwards.
Non-boiling liquids will overflow the stage and fall to a lower level. As this equilibrium process is established from phase to phase, the components are separated and picked up. The new feed is added in one step based on the column design. Heat is continuously added to the column via the bottom kettle reboiler.
Shell and tube heat exchangers work by passing two separate streams of fluid at different temperatures from each other in close contact. The tubes look like a collection of straws inside a tight-fitting cylinder. The tubes connect via headers at the head and rear of the bundle. The coolant may be on the shell or tube side. The heat will pass by conduction through the pipe walls to the cooler fluid.
A kettle reboiler differs from the typical shell and tube heat exchanger in that there is a large open space above the tube bundle. The process fluid is on the side of the shell, where it boils the distillation column for separation. The side of the tube carries the hot fluid, consisting of steam or other higher temperature process fluid. The costs of a kettle reboiler are 15% to 25% higher than an equivalently sized shell and tube heat exchanger due to the larger shell and larger port size.
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