The principle of supercritical fluid impregnation consists of a scan of a porous solid material (polymers, wood, textiles...) by a supercritical (mainly CO2) in which the active substance to impregnate is previously dissolved. This step is followed by a relaxation that causes the passage of the CO2 in the gaseous state, leaving the "target" material impregnated with the active substance. Therefore, the impregnation of a solid matrix with an active compound is easy achievable using supercritical fluids based technologies. Example of the use of supercritcal impregnation include the dyeing of textile, the tanning of leather, wood impregnation by fongicides ...
Conducting chemical reactions at supercritical conditions affords opportunities to manipulate the reaction environment (solvent properties) by increasing pressure to enhance the solubilities of reactants and products, to eliminate interphase transport limitations thus increasing reaction rates, and to integrate reaction and separation unit operations. Supercritical conditions may be advantageous for reactions involued in fuels processing, biomass conversion, biocatalysis, homogeneous and heterogeneous catalysis, environmental control, polymerization, materials synthesis and chemical synthesis. Examples of chemical reactions carried out at an industrial scale include hydrogenation, oxydation, esterification and etherfication reactions among others.
In the cleaning process, the use of supercritical CO2 with or without the addition of specific surfactants or cosolvent avoids the use of solvents such as trichloroethylene or perclorethylene. For environmental reasons, these toxic solvents are subject to many health and regulatory limitations, leaving the place to "green" solvents. Although only hydrocarbon solvents are nowadays able to meet the market’s demand for alternative solutions to chlorinated solvents, the use of CO2 under pressure or supercritical CO2 appears as a particularly interesting industrial alternative to chlorinated solvents. This process is industrialized for the dry-cleaning of textiles, the cleaning of mechanical spare parts ...
Supercritical drying processes rely on the extraction of water and other solvents using supercritical CO2. The absence of surface tension allows the supercritical fluid to be removed without distortion. The material to be dried is put in a tank where it is subdued to a scan by CO2 at a temperature above its critical temperature and then at a pressure above its critical pressure. A relaxation is made to free the vapor to get the solid skeleton of the gel (aerogel). Such processes are used to make aerogels but also for the lyophilization (freeze-drying) of biological and food matrices and the dry-cleaning of clothes (as a replacement for chlorinated solvenst) in an environmentally-friendly way.
Supercritical fluids have demonstrated the ability to inactivate bateria, fungi, yeasts and viruses thus providing an efficient mean for the sterilization of food and medical devices. The mechanism of micro-organism inactivation are not fully understood yet but some evidence show that this may be caused by cell wall alteration due to a strong interaction of the fluid with the lipids and the inactivation of key-enzymes resulting from pH decrease inside the cell.
Chemical Fluid Deposition (CFD) involves the chemical reduction of organometallic compounds in supercritical fluids to yield high purity deposits. Typically, the reaction is initiated upon the addition of H2 or other reducing agent. The advantages of CFD over conventional deposition techniques are a consequence of the unique properties of supercritical fluids, thus promoting infiltration into complex geometries and minimizing mass transfer limitations common to liquid phase reductions. High purity metal films including Pt, Pd, Au, Rh, Ni, Cu, Al have been deposited by CFD from supercritical fluids CO2 using appropriate precursors.
Supercritical Fluid Chromatography (SFC) is used for the analysis and purification of low to moderate molecular weight, thermally labile molecules and the the separation of chiral compounds. Principles are similar to those of high performance liquid chromatography (HPLC), however SFC typically utilizes carbon dioxide as the mobile phase. Therefore the technique is more versatile, exhibits better resolution and faster analysis times than conventional liquid chromatographic methods.
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