More about supercritical carbon dioxide.
Supercritical carbon dioxide is now well established as a solvent for use in extraction (figure 1). This is for a number of reasons. It can generally penetrate a solid sample faster than liquid solvents because of it's high diffusion rates, and can rapidly transport dissolved solutes from the sample matrix because of it's low viscosity. There are also of course less solvent residues present in the products.
Figure 1. Industrial scale scCO2 extraction plant.
One of the most well known examples is the pioneering work of Zosel, where supercritical carbon dioxide was used for the decaffination of coffee and tea, which is now an established industrial process. Since then, many other examples have appeared including the extraction of hops, natural products, high value pharmaceutical precursors, essential oils, and environmental pollutants. Other important commercial technologies are also emerging involving supercritical carbon dioxide, such as dry cleaning (visit HEC to learn more) and paint spraying.
Another main area in which supercritical carbon dioxide is important is polymerization. This was pioneered by DeSimeone at the University of North Carolina, and has since developed into an extensive area of research. Initial work concentrated on flouropolymer synthesis, exploiting the known affinity of supercritical carbon dioxide for organofluorine compounds. The main driving force for this was that the usual reaction solvents for such polymerizations were chlorofluorocarbons (CFC's) which are now known to be environmentally unacceptable with consequent supply problems. This work has led to recent reports of a large-scale speciality fluoropolymer manufacturing plant using supercritical carbon dioxide. An initial $40million development facility is under construction by Dupont in the USA, which, if successful, will lead to a $235million commercial scale facility.
The stages of carbon dioxide achieving supercriticality can be seen here:
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