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Chemical plants use distillation towers, vertical columns with trays set at specific distances, to separate useful chemicals or fuels from raw unprocessed streams. Most distillation systems require heat to create a vapor stream that enters the tower. A reboiler is a heated tank that boils the unprocessed fluids, where the vapors are sent to the tower. Distillation occurs when vapors and liquids flow in opposite directions in the column and a product exits with a higher concentration of the preferred chemical than the unprocessed fluid.
Steam is normally the heat source for chemical distillation. It is non-flammable, easy to produce in large amounts, and can be delivered over long distances in insulated pipes. Boiling the unprocessed stream in the reboiler provides the heat source that drives the distillation process, which requires vapor and liquid to operate correctly. The vapor formed from boiling the liquid feed is sent to the tower, where it mixes with liquid flowing down from the top of the tower.
Liquid collecting at the bottom of the tower flows by gravity or is pumped back to the reboiler, where the steam heat boils it. Distillation continues with new unprocessed fluid being added continuously to the reboiler, and desired product exiting the top of the distillation column. Adding or removing heat, or varying the flow of unprocessed fluid changes the composition of the desired product. When the heat input and feed flows result in a desired product stream leaving the tower, the distillation system is balanced and is said to be at steady state.
Reboilers can be manufactured in a variety of designs, depending on the operating specifications of the distillation system. A heater can be placed directly in the bottom of the column, called a stab-in heater, which will boil any liquid collected in the tower. Although simple in design, stab-in systems can be more difficult to control than other types. In addition, any maintenance of the unit requires the entire distillation column to be empty and shut down.
Kettle reboilers are a separate tank that has feed, vapor and tower return liquid connections. Steam passes through the inside of coiled tubes inside the kettle, with the process liquid and vapor outside the tubes. A liquid level controller monitors the level of liquid in the tank and prevents low levels from exposing the tubes, which could cause overheating. Temperature controls adjust the steam flow to the coils, which results in different amounts of product vapor entering the tower.
A thermosiphon reboiler uses a steam heating tank, with the product feed passing through tubes placed vertically in the tank. As the product is heated and boiled, the pressure of the vapor forces itself out of the top of the tank, and pulls new liquid in at the bottom. A flow of liquid and vapor is created from the boiling process alone, and no pumps are used. Thermosiphons require careful control systems to prevent overheating the product or creating too much vapor, but can be useful for some distillation processes.
Pumped reboilers use liquid pumps to feed the product and liquid returning from the bottom of the column. The design is often a kettle design, but space limits or other reasons may require the kettle to be further away from the column, or at a level where gravity cannot be used. Controls are needed both for the steam flow and for the liquid flows to the kettle.
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