Raschig rings are pieces of tube, approximately equal in length and diameter, used in large numbers as a packed bed within columns for distillations and other chemical engineering processes. They are usually ceramic or metal and provide a large surface area within the volume of the column for interaction between liquid and gas or vapour. Raschig rings are named after their inventor, the German chemist Friedrich Raschig.
In a distillation column, the reflux or condensed vapour runs down the column, covering the surfaces of the rings, while vapour from the reboiler goes up the column. As the vapour and liquid pass each other countercurrently in a small space, they tend towards equilibrium. Thus, less volatile material tends to go downwards, and more volatile material upwards.
They are also used for devices where gas and liquid are put in contact for purposes of gas absorption, stripping or chemical reaction, and as a support for biofilms in biological reactors.
Raschig rings made from borosilicate glass are sometimes employed in the handling of nuclear materials, where they are used inside vessels and tanks containing solutions of fissile material, for example solutions of enriched uranyl nitrate, acting as neutron absorbers and preventing a potential criticality accident.
Given the success of the simply-shaped Raschig ring, there have been many other forms developed in an attempt to either improve upon it, or to avoid patents for particular designs.
The Pall ring attempts to increase the useful aspects of packing, by giving an increased number of edges to disrupt flow, whilst also reducing the volume taken up by the ring packing medium itself. Rather than using a solid-walled tube, the Pall ring resembles an open basket structure of thin bars. These form both a tube and also a radial structure of cross bars. Pall rings may be injection moulded of plastics, moulded of ceramics or press-formed from metal sheet.
The Raschig 'Super Ring' represents a further development of the same concepts behind the Pall ring. It optimises the production of turbulent film-type flows and prevents the formation of drops. The 'rings' no longer resemble rings but are pressed from metal sheet in the form of wave shapes of narrow strips. Super rings appeared in 1995 and have been developed through several improved generations since.
- Andrea Sella (2008). "Raschig's Rings". Chemistry World 5 (9): 83.
- Oak Ridge Associated Universities Raschig Rings for Criticality Control (1980s)
- "Pall Ring" (PDF). Raschig-Jaeger Technologies.
- "Raschig Super-Ring" (PDF). Raschig-Jaeger Technologies.
- Schultes, M. (January 2003). "Raschig Super-Ring: A New Fourth Generation Packing Offers New Advantages". Chemical Engineering Research and Design 81 (1): 48–57. doi:10.1205/026387603321158186.