CO2 exists as a supercritical fluid above 31°C and 7.38 MPa. Its density and viscosity can be modulated over quite a wide range by changing the pressure and temperature. The 'dense phase fluid' has properties close to those of a non-polar organic solvent, such as pentane or dichloromethane. The benefits derived from its use are well known today, so that its utilization is spreading in various industrial sectors. Environmental concerns about the use of organic solvents have also propelled research efforts towards the replacement of traditional organic solvents with sc-CO2. Such CO2 has been in use for a long time in many chemical applications, which include:
• Decaffeination of coffee beans (Anastas et al., 1996; Arakawa et al., 2001);
• Extraction of fragrances and essences from plants, or proteins (Wahbeh, 1997), or fatty acids and hydrocarbons from algae (Aresta et al., 2003d);
• Use as a solvent for reactions, crystallizations, prepartions of solid thermal-sensitive phamaceuticals having controlled size distribution (Van Ginneken and Weyten, 2003a);
• Catalysis, both homogeneous (Holmes et al., 2003) and heterogeneous (Poliakoff et al., 2003);
• Synthesis (Filardo et al., 2003a) and modification of polymers including per-fluoropolymers (Filardo et al., 2003b);
• As a mobile phase for supercritical fluid chromatography (SFC) (Van Ginneken and Weyten, 2003b);
• For dyeing (Penninger, 2002), dry-cleaning (DeSimone, 2001) and even nuclear waste treatment (RUCADI Project Final Report).
The most important feature of sc-CO2 use is that it can be easily recovered at the end of the process (by thermal decompression), recompressed and recycled. The market demand is growing and may reach several million tonnes per year in the short term. It is worth recalling here that for each tonne of organic solvent (global market of the
Table 7.3. Compounds extracted from algae.
Classes of products
Proteins Amino acids Lipids
Essences, fragrances and oil
Pigment, chlorophille, carotenoids Amines
Geraniol, formates, acetates, citronellol, nonanol, eucalyptol
Mono-, di-, trimethylamines, ethylamine, propylamine, isobutylamine Iodides, bromides, sulphates, nitrates
Food industry Food industry Biofuel, food industry, pharmaceutical industry Various industries
Pharmaceutical industry Pharmaceutical industry order of 35 Tg per year) replaced with sc-CO 2, there is a reduction of the CO2 emission of the order of 3 t due to the fact that waste solvents will not be burned.
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