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Raschig rings are pieces of tube, approximately equal in length and diameter, used in large numbers as a
packed bed In chemical processing, a packed bed is a hollow tube, pipe, or other vessel that is filled with a packing material. The packing can be randomly filled with small objects like Raschig rings or else it can be a specifically designed structure ...
within columns for
distillation 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 he ...
s and other
chemical engineering Chemical engineering is an engineering field which deals with the study of operation and design of chemical plants as well as methods of improving production. Chemical engineers develop economical commercial processes to convert raw materials in ...
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 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 ...
, who patented them in 1914.


Use

They form what is known as random packing, and enabled Raschig to perform distillations of much greater efficiency than his competitors using
fractional distillation Fractional distillation is the separation of a mixture into its component parts, or fractions. Chemical compounds are separated by heating them to a temperature at which one or more fractions of the mixture will vaporize. It uses 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 Reboilers are heat exchangers typically used to provide heat to the bottom of industrial distillation columns. They boil the liquid from the bottom of a distillation column to generate vapors which are returned to the column to drive the distillatio ...
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
biofilm A biofilm comprises any syntrophic consortium of microorganisms in which cells stick to each other and often also to a surface. These adherent cells become embedded within a slimy extracellular matrix that is composed of extracellular po ...
s in biological reactors. Raschig rings made from
borosilicate glass Borosilicate glass is a type of glass with silica and boron trioxide as the main glass-forming constituents. Borosilicate glasses are known for having very low coefficients of thermal expansion (≈3 × 10−6 K−1 at 20 °C), m ...
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.


Developments

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.


Pall-Ring

The Pall-Ring, commonly spelled as Pall ring, developed by Wilhelm Pfannmüller of
BASF BASF SE () is a German multinational chemical company and the largest chemical producer in the world. Its headquarters is located in Ludwigshafen, Germany. The BASF Group comprises subsidiaries and joint ventures in more than 80 countries ...
during the WWII, 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.


Super ring

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.


Białecki ring

This design, developed by the same Pfanmüller as the Pall-Ring and first patented by him in 1944, is mistakenly named after the Polish chemical engineer from
Kraków Kraków (), or Cracow, is the second-largest and one of the oldest cities in Poland. Situated on the Vistula, Vistula River in Lesser Poland Voivodeship, the city dates back to the seventh century. Kraków was the official capital of Poland un ...
Zbigniew Białecki. Like the Pall rings, they are an improved version of Raschig rings. The rings may be injection moulded of plastics or press-formed from metal sheet without welding.
Specific surface area Specific surface area (SSA) is a property of solids defined as the total surface area of a material per unit of mass, (with units of m2/kg or m2/g) or solid or bulk volume (units of m2/m3 or m−1). It is a physical value that can be used to dete ...
of filling ranges between 60 and 440 m2/m3. Advantages of Białecki rings: * two or three times lower fluid flow resistance than Raschig rings, * two or three times higher bandwidth, * disperse the liquid evenly over the entire cross-section of the column, regardless of the method of supply, even with point-type liquid supply, * liquid retention ("hold-up") in relation to other fillings is negligible - the liquid flows down quickly after the column stops working, * resolving power at a constant level regardless of column load, * the mass transfer coefficient increases with the size of the ring dimensions, unlike the other fillings used so far, * minimal overgrow (do not cover with sediment), * a rigid construction, which allows weight reduction of up to 20% compared to other filling rings.


See also

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Fractionating column A fractionating column or fractional column is an essential item used in the distillation of liquid mixtures to separate the mixture into its component parts, or fractions, based on the differences in volatilities. Fractionating columns are used in ...
*
Dixon rings 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 surfac ...
*
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. Ra ...
*
Stripping (chemistry) Stripping is a physical separation process where one or more components are removed from a liquid stream by a vapor stream. In industrial applications the liquid and vapor streams can have co-current or countercurrent flows. Stripping is usually ...


References

{{reflist Distillation Industrial processes