Distillation towers separate chemicals or fuels from raw streams using heat and steam. Reboilers boil raw fluids to create steam, which is sent to the tower. Liquid and vapor flow in opposite directions, with the desired product exiting at the top. Reboilers come in various designs, including plug-in, kettle, thermosyphon, and pumped. Controls are required for steam and liquid flows.
Chemical plants use distillation towers, vertical columns with trays set at specific distances, to separate useful chemicals or fuels from the raw, unprocessed streams. Most distillation systems require heat to create a flow of steam into the tower. A reboiler is a heated tank that boils raw 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 quantities and can be delivered over long distances in insulated pipes. The boiling of the unprocessed stream in the reboiler provides the heat source that drives the distillation process, which requires steam and liquid to function properly. The vapor formed from the boiling of the liquid feed is sent to the tower, where it mixes with the liquid flowing down from the top of the tower.
The liquid that collects at the bottom of the tower either flows by gravity or is pumped back to the reboiler, where the heat from the steam boils it. The distillation continues with the continuous addition of new unprocessed fluid to the reboiler and the desired product exiting the top of the distillation column. Adding or removing heat or changing the raw fluid flow changes the composition of the desired product. When the heat input and feed streams result in a flow of desired product leaving the tower, the distillation system is balanced and said to be at steady state.
Reboilers can be manufactured in a variety of designs, depending on the operational specifications of the distillation system. A heater can be placed directly at the bottom of the column, called a plug-in heater, which will boil any liquid collected in the tower. While simple in design, stab-in systems can be more difficult to control than other types. Additionally, any maintenance of the unit requires the entire distillation column to be emptied and shut down.
Kettle reboilers are a separate tank equipped with feed, steam and tower liquid connections. The steam passes through the inside of the coiled tubes into the kettle, with the process liquid and steam outside the tubes. A liquid level controller monitors the liquid level in the tank and prevents low levels from exposing the pipes, which could cause overheating. Temperature controls regulate the flow of steam to the coils, which results in different amounts of product steam entering the tower.
A thermosyphon reboiler uses a steam heating tank, with the product feed passing through pipes placed vertically in the tank. As the product is heated and boiled, the pressure of the steam pushes out the top of the tank and draws new liquid into the bottom. A stream of liquid and vapor is created by the boiling process alone and no pumps are used. Thermosyphons require careful control systems to avoid overheating the product or creating too much steam, but they can be useful for some distillation processes.
Pumped reboilers use liquid pumps to feed product and return liquid from the bottom of the column. The design is often a kettle design, but space limitations 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 required for both steam flow and liquid flows to the kettle.
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