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Supercritical carbon dioxide

Basics

In all contexts the term ´Supercritical´refers to a state, not to any specific compound. Being in a supercritical state merely means that the temperature and pressure of a substance are simultaneously greater than the corresponding critical values of that specific substance. The critical temperature of a compound or a mixture of compounds is the temperature above which that compound cannot be liquefied, no matter how much the pressure is raised. It is said that the compound is then in the single-phase region.

 

Different pure compounds may have very different critical temperatures. For example, the critical temperature of hydrogen is - 239,95 °C for oxygen it is - 118,6 °C, and for their reaction product, water, it is a huge + 374 °C.

 

Tehnical advantages

scCO2 is a non-toxic, non-flammable solvent

Easy recycling of carbon dioxide in extraction and in reactors

Easy separation of dissolved compounds from CO2 by pressure reduction

Low-energy carbon dioxide recirculation

Tunability of scCO2 density, polarity and solvent power with pressure

Low- and diminishing-cost solvent

Technical disadvantages

Necessity for high pressure, above 72 bars.

scCO2 is a solvent for hydrophobic or volatile compounds only

Industrial uses

Recent, large supercritical CO2 installations include an extraction plant for producing de-oiled lecithin. The plant was started in 2006 in China and consists of 2 x 1500 litres extraction vessels rated at 500 bars.

 

Another recent installation is a cork extraction plant in Spain with three 8300 litres extraction vessels rated for 150 bars.

 

Other large plants include 3x 3800 litres sesame oil purification plant in South Korea rated at 550 bars and a rice treatment plant of 3x5800 litres / 325 bars in Taiwan.

 

Another type of industrial applications of scCO2 is a multi-purpose, continuous reaction plant at Thomas Swan & Co in England. The plant is rated for 500 bars, 200 C with a capacity of treating 1000 metric tonnes of compounds per year.