Supercritical fluids are a gas that is compressed and heated so that it shows properties of a liquid and a gas at the same time. Supercritical carbon dioxide can be used to dissolve materials involved in chemical reactions. In this way, it can replace organic solvents which are much less environmentally friendly and generate greater amounts of waste.

Thomas Swan have developed this technology and built a "world's first" site that uses supercritical carbon dioxide to replace less environmentally-friendly organic solvents in reactions including hydrogenation, Friedel-Crafts alkylations and etherification. The plant is the first full-scale facility using supercritical fluids and can produce up to 1,000 tonnes of specialist chemicals per year.

Swan’s general manager, Dai Hayward, says that the process may make chemical reactions possible that were previously too dirty or inneficient. “Supercritical fluids have demonstrated their potential to solve some of the process problems encountered in the chemical industry and, in partnership with Nottingham University, we have transformed the theory into a practical technology with considerable benefits."

Carbon dioxide is the most common gas used as a supercritical fluid but the use of propane, propene and butane is also being studied.