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SUPERCRITICAL FLUIDS: CARBON DIOXIDE
A supercritical
fluid is a substance under pressure above its critical temperature. Under these
conditions the distinction between gases and liquids does not apply and the
substance can only be described as fluid.
The photograph on
the left shows a substance below its critical temperature existing as a liquid
with the gas or vapour above it. As the temperature is raised, the liquid
density falls due to expansion and the gas density rises as more of the
substance evaporates. The densities approach each other and the meniscus
between the two phases becomes less distinct, as shown in the middle
photograph. Eventually, at the critical point, the densities become identical,
as shown on the right. Other properties also become identical and the
distinction between liquid and gas disappears, like the meniscus. The substance
is now a supercritical fluid.
Supercritical
fluids have properties intermediate between those of gases and liquids,
controlled by the pressure, and these may be optimum for some processes. Carbon
dioxide is available as a convenient supercritical fluid substance, which
offers environmental advantages, as it can replace organic solvents, reducing
pollution and avoiding solvent residues in products. The carbon dioxide used is
a by-product of other processes and does not add to global warming.
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