Raschig rings are cylinders made from ceramics, metal or plastic, and are used to increase efficiency in gas, petroleum and chemical refinery towers. The rings are packed randomly into the tower to increase the surface area available for chemical reactions. They are typically shaped like hollow cylinders, with a diameter equal to the length of the cylinder.
Friedrich Raschig invented the rings in the 1880s but did not patent them until the 1910s. He also invented or refined several key processes for making various organic chemicals. His Raschig rings would revolutionize the chemical industry, because the packing greatly improved the efficiency of existing manufacturing processes. Prior to his invention, early chemists filled tanks with broken glass bottles, pottery and even rocks to gain surface area.
Surface area for chemical processes is important, because without adequate mixing of gases and liquids, very little reaction will occur. If gas is bubbled into a tank of liquid, some mixing may occur, but a reaction can only occur where the gas and liquid meet, resulting in low efficiency. Horizontal plates with holes can be installed in a vertical tower, with liquid flowing from the top down, and gas from the bottom up. Tower trays will help, but reaction efficiency is only partially improved.
Packing a tower with randomly placed Raschig rings creates a very large surface area for reactions. This occurs because liquid will move in the thousands of channels formed by the rings, coating the inside and outside surfaces, and eventually flowing to the bottom. Gas entering at the bottom will flow through the same paths formed by the rings, and will have time to contact and react with the liquid.
Different materials are available for the rings, with the earliest being a ceramic, or heat-fired pottery, material. Ceramics were resistant to high temperatures, but were brittle and could easily break when loading the tower or if a pressure event caused the packed ceramic rings to be lifted and then dropped. In the 20th century, manufacturers developed Raschig rings made from metal, plastics and even carbon graphite for high temperature applications.
Manufacturers provide tables that show the packing characteristics for various ring sizes. Raschig rings contain an average surface area for a given volume of tower, and the tables show these values for different ring diameters and thicknesses. If weight is an issue, the loaded ring weight can be determined from the table and the volume of the tower.
Raschig rings are considered the first generation of engineered tower packing, and due to a relatively simple design were still widely used in the 21st century. Other packing designs were developed in the 20th century, including shapes such as saddles, coiled springs and cylinders with many holes or perforations. These shapes were all attempts to improve reaction times or reduce weight, and were useful for some types of chemical processes. Despite these improvements, a simple Raschig ring design provided excellent reaction efficiency at reasonable prices versus more exotic designs.