Why are Raschig rings used?

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, metal or glass and provide a large surface area within the volume of the column for interaction between liquid and gas vapours. Raschig rings are named after their inventor, German chemist Friedrich Raschig.

They form what is known as random packing, and enabled Raschig to perform distillations of much greater efficiency than his competitors using fractional distillation columns with trays.

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. They are used inside vessels and tanks containing solutions of fissile material, for example solutions of enriched uranyl nitrate. There they act as neutron absorbers to prevent a criticality accident.

Given the success of the Raschig ring, there have been other forms developed to either improve upon it, or to avoid patents for particular designs.