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Explore, use, make the most of supercritical fluids

What does "supercritical" mean ?

Any substance is characterized by a critical point which is obtained at specific conditions of pressure and temperature. When a compound is subjected to a pressure and a temperature higher than its critical point, the fluid is said to be " supercritical " .

In the supercritical region, the fluid exhibits particular proporties and has an intermediate behavior between that of a liquid and a gas. In particular, supercrical fluids (SCFs) possess liquid-like densities, gas-like viscosities and diffusities intermediate to that of a liquid and a gas.

As shown on the phase diagram, the gas-liquid equilibrium curve is interrupted at the critical point, providing a continuum of physico chemical properties.

The fluid is said "supercritical" when it is heated obove its crititcal temperature and compressed obove its cirical pressure.This particular behavior of substances was first observed in 1822 by French engineer and physicist, Charles Cagniard de La Tour in his famous cannon barrel experiment. It was then defined as supercritical fluid by Irish chemist, Thomas Andrews. The most widely used supercritcal fluids are CO2 and water.

What is supercritical CO2 ?

Carbon dioxide (CO2) is the most widely used supercritical fluid. This is because CO2 is cheap, chemically inert, non-toxic, non-flammable and readily available at high purities and at low costs. Besides, the critical point of CO2 is easily accessible (critical temperature 31°C and critical pressure 74 bar) allowing the fluid to be used at mild conditions of temperatures (40-60°C) without leaving harmful organic residues. Due to its interesting properties Supercritical CO2 can be described as a "green" solvent.

Click on the "Continue" button in the phase-diagram to see the full animation.

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What is subcritical and supercritical water ?

The critical point of water is much higher than that of CO2. However its applications are very promising and some are in the process of industrialization. Processes using subcritical and supercritical water are called hydrothermal processes.

Water, in the subcritical phase (pressure : 15 to 100 bar, temperature : 150 to 250°C) can solubilize hydrophobic compounds. Therefore, subcritcal water (sometimes referred to as hot compressed water)  can be used for the extraction of plant materials (polyphenols, tannins, terpenes ...). Oxidation in subcrtical water can also be carried out in order to achieve efficient waste processing. In this case, the process is called wet air oxidation (WAO).

In supercritical water ( pressure > 221 bar , temperature> 374°C ) organic compounds and gases become highly miscible and precipitation of inorganic compounds occurs. Oxidation reactions in supercrtiical water can also be performed. Applications include the treatment of harmful wastes and the synthesis of nanoparticles. These processes are called supercritical water oxidation (SCWO). 

Are there any other supercritical fluids ?

In addition to water and CO2 , compounds such as ethane, propane, methanol, ethanol are also used in their supercritical phase for certain applications (extraction, chemical reactions .. )

What's the point of using supercritical fluids ?

Depending on the fluid and equipment used, supercritical fluid processes can :

  • Raise technological barriers
  • Optimize your manufacturing processes
  • Provide a sustainable alternative to conventional production methods

Supercritical fluids can be used to perform the following processes :