Supercritical Fluids: what are they?

A supercritical fluid is one that exists above its critical temperature and is pressurised above its critical pressure (the equilibrium vapour pressure at the critical temperature). More simply this means that no matter how much pressure is applied to it, a supercritical fluid will not condense to a liquid. All supercritical fluids are high-density gases because they fill the container like a gas but their density is like that of a liquid. In particular the critical points for water, 374 deg C and 220 bar, indicate that SC water is hot and highly pressurised. At significantly greater temperatures SC water becomes corrosive and turns into an oxidant that supplies oxygen and produces hydrogen in a manner similar to the water-gas shift reaction.

SC water has solvent properties that are the reverse of normal water. Molecules of SC water form non-polar clusters, which Wayne Davies speculated resembled a benzene ring. Current thinking is that clusters occur but the majority adopt a linear form with n of up to 20 while the ring structures are in the minority. A 1992 report on SCW treatment of coal to produce useful liqud fuels, speculated on a benzene-like structure for SCW. It can be downloaded as a PDF.

Likewise, other SC fluids react more aggressively than do the normal liquid or vapour. SC methanol can transesterify vegetable oil to make fatty acid methyl ester (biodiesel) without a catalyst. It can convert wood into liquid fuels and can depolymerise plastics such as PET.

Generalised phase diagram of substance showing region of supercritical state (left), and the regions of commercial usefulness that includes the subcritical state (right)