In chemical processing, a packed bed is a hollow tube, pipe, or vessel that is filled with packing material. The packing can be randomly filled or else it can be a specifically designed structured packing. The major purpose of a packed bed is to improve contact between two phases in a chemical or similar process. Packed beds can be used in a chemical reactor, a distillation process, or a scrubber, but packed beds have also been used to store heat in chemical plants. In this case, hot gases are allowed to escape through a vessel that is packed with a refractory material until the packing is hot. Air or other cool gas is then fed back to the plant through the hotbed, thereby pre-heating the air or gas feed. There are two types of packing arrangements.
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1. Structure Packing
Structured packing towers offer more surface area and have a lower pressure drop than packed towers do. Structured packing can be manufactured from corrugated sheets of perforated embossed metal, plastic, or wire gauze. The result is a very open honeycomb structure with vertical flow channels giving a relatively high surface area to volume but with a very low resistance to flow. The surfaces have been chosen to maximize liquid spreading. These characteristics tend to show significant performance benefits in low pressure and low flow rates per cross-sectional area of the column.
2. Random Packing
Random Packed towers are constructed by using a variety of metal or nonmetal materials, including ceramics or plastics. These materials provide the surface area for the distillation process. The Raschig ring was the first packing material but newer shapes provide low-pressure drop and high surface area per unit volume.