Raschig ring is the earliest artificial filler used in industry. Usually made of ceramic or metal sheet, the height is equal to the diameter, and the commonly used size is 25mm~dao75mm (some as small as 6mm, as large as 150mm). The ceramic wall thickness is 2.5mm~9.5mm, and the metal ring is 0.8mm~1.6mm. When the strength permits, the ring wall should be as thin as possible. Although the mass transfer performance of Raschig ring is not ideal, it is still adopted by some factories due to its simple structure, easy manufacture and low price.