Dixon rings are a form of random packing used in chemical processing. They consist of a stainless steel mesh formed into a ring with a central divider, and are intended to be packed randomly into a packed column. Dixon rings provide a large

surface area The surface area of a solid object is a measure of the total area that the surface of the object occupies. The mathematical definition of surface area in the presence of curved surfaces is considerably more involved than the definition of arc ...

and low

pressure drop Pressure drop is defined as the difference in total pressure between two points of a fluid carrying network. A pressure drop occurs when frictional forces, caused by the resistance to flow, act on a fluid as it flows through the tube. The main de ...

while maintaining a high

mass transfer Mass transfer is the net movement of mass from one location (usually meaning stream, phase, fraction or component) to another. Mass transfer occurs in many processes, such as absorption, evaporation, drying, precipitation, membrane filtration, ...

rate, making them useful for

distillations Distillation, or classical distillation, is the process of separating the components or substances from a liquid mixture by using selective boiling and condensation, usually inside an apparatus known as a still. Dry distillation is the heat ...

and many other applications.

Background

Packed columns

Packed columns are used in a range of industries to allow intimate contact between two immiscible fluids which can be liquid/liquid or liquid/gas. The fluids are passed through in a

countercurrent flow Countercurrent exchange is a mechanism occurring in nature and mimicked in industry and engineering, in which there is a crossover of some property, usually heat or some chemical, between two flowing bodies flowing in opposite directions to each ...

through a column.

Random column packing

Random column packing Random column packing is the practice of packing a distillation column with randomly fitting filtration material in order to optimize surface area over which reactants can interact while minimizing the complexity of construction of such columns. Ran ...

used to characterize the maximum volume fraction of a solid object obtained when they are packed randomly. This method of packing has been used since the early 1820s; the types of packing used were originally made out of glass spheres. However, in 1850 they were replaced by a more porous pumice stone and pieces of coke. In the early 20th century

Friedrich Raschig Friedrich August Raschig (also called Fritz Raschig) (8 June 1863 – 4 February 1928) was a German chemist and politician. He was born in Brandenburg an der Havel. After he received his PhD in 1884 from the University of Berlin for his work with ...

realized the importance of a high void fraction and having the internal surface of the packing media take part in the mass transfer. He designed the

Raschig ring 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 la ...

, which was more effective than previous forms of random packing and became very popular. Raschig rings are usually built from ceramic or metal and provided a large

within the column for interaction between liquid and gas vapors.

The development of the Dixon ring

In 1943 Dr Olaf George Dixon of ICI applied for a patent of a new product for column

distillation Distillation, or classical distillation, is the process of separation process, separating the components or substances from a liquid mixture by using selective boiling and condensation, usually inside an apparatus known as a still. Dry distilla ...

. He used

stainless steel Stainless steel is an alloy of iron that is resistant to rusting and corrosion. It contains at least 11% chromium and may contain elements such as carbon, other nonmetals and metals to obtain other desired properties. Stainless steel's corros ...

mesh instead of sheet steel in the

Lessing ring Random column packing is the practice of packing a distillation column with randomly fitting filtration material in order to optimize surface area over which reactants can interact while minimizing the complexity of construction of such columns. Ran ...

in order to improve the

of the packed column (in fact, they were called "wire gauze Lessing rings" in a 1949 publication). High performance was achieved between radioactive and non-radioactive materials, and despite high cost they are still used in the nuclear industry e. g. for water detritiation.

Application

Dixon rings are used for mainly for laboratory distillation applications.

Performance principles

The enhanced performance of the Dixon ring is based on liquid surface tension: when the mesh is wet its

increases greatly, with an accompanying increase in the rate of

. Dixon rings require pre-wetting (flow of liquid over the packed bed prior to starting the reaction flow). While this increases batch processing startup time, the increased performance of the Dixon ring overcomes this.

References

{{reflist Chemical equipment Distillation