Supercritical fluids based processes include extraction, impregnation, particle formation, formulation, sterilization, cleaning and chemical reactions among others. In all cases, the supercritical fluid is used as an alternative to traditional organic liquid solvents. The most widely used supercritical fluids are CO2 (Tc = 31°C, Pc = 74 bar) and water (Tc = 374°C, Pc = 221 bar) but some processes (extraction, reactions) involve the use of supercritical methanol, ethanol, propane, ethane mainly. Supercritical CO2 processes are the most widely spread as these are exempt of the operations of elimination of solvent residues, operations generally needed when the solvent used is an organic compound.
Some examples of proceseses carried out under supercritical and subcritical conditions are listed below:
Extraction
The principle of the extraction of solids using supercritical CO2
relies on the strong variation of the solvation power of CO2
which occurs by simple variation the operating conditions
(temperature and pressure). This allows to selectively extract
molecules according to their chemical nature. Very weakly polar, CO2
turns out to be an excellent solvent of nonpolar or small polar
molecules in supercritical conditions. Once the desired compound is
dissolved in supercritical CO2, the pure extract can
easily be obtained by simple depressurization. This results in the
separation of CO2, which becomes gas again, from the
extract, which is recovered in solid or liquid form. In theory, any
porous solid material (plants, plastics, wood, can be treated by
supercritical fluid extraction to recover valuable compounds (oils,
fragrances, pigments …) or undesired substances (pollutants,
residual solvents …).
Subcritical water extraction can also be
carried out to extract hydrophobic compounds (polyphenols, terpenes,
tannins …) from various plant materials (fruits, wood, algae …).
Fractionation
Based on the principle of supercritical CO2 extraction of solids, the extraction (fractionation) of liquids can also be performed in order achieve high levels of purification. Examples of liquids concerned by this process include plant extracts, vegetable oils, fish oil, polymers …
Impregnation
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 …
Powder formation
This refers to micro and nanoparticle generation, crystallization, precipitation, micronization of inorganic, organic, pharmaceutical, and polymeric materials. There are two main ways of precipitating micro and nano-particles either using supercritical fluid as solvent, the RESS technique (Rapid Expansion of Supercritical Solutions); or using it as anti-solvent, the SAS technique (Supercritical Anti-Solvent). Encapsulation of various substances and polymers can also be performed using the PGSS (Particles from Gas Saturated Solutions) process.
Deposition
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.
Chemical reactions
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 inter phase 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, oxidation, esterification and etherfication reactions among others.
Sterilization
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.
Drying
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. 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.
Cleaning
In the cleaning process, the use of supercritical CO2 with or without the addition of specific surfactants avoids the use of solvents such as trichloroethylene. For environmental reasons, this toxic solvent is 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 …
Chromatography
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 chromatography methods.