Supercritical fluid
[1] It can effuse through porous solids like a gas, overcoming the mass transfer limitations that slow liquid transport through such materials.
[2] SCFs are used as a substitute for organic solvents in a range of industrial and laboratory processes, most commonly carbon dioxide for decaffeination and water for steam boilers for power generation.
Some substances are soluble in the supercritical state of a solvent (e.g. carbon dioxide) but insoluble in the gaseous or liquid state—or vice versa.
This can be used to extract a substance and transport it elsewhere in solution before depositing it in the desired place by allowing or inducing a phase transition in the solvent.
†Source: International Association for Properties of Water and Steam (IAPWS)[4] Table 2 shows density, diffusivity and viscosity for typical liquids, gases and supercritical fluids.
At very high pressures, an SCF can be compressed into a solid because the melting curve extends to the right of the critical point in the P/T phase diagram.
In 1822, Baron Charles Cagniard de la Tour discovered the critical point of a substance in his famous cannon barrel experiments.
Supercritical fluids have found application in a variety of fields, ranging from the extraction of floral fragrance from flowers to applications in food science such as creating decaffeinated coffee, functional food ingredients, pharmaceuticals, cosmetics, polymers, powders, bio- and functional materials, nano-systems, natural products, biotechnology, fossil and bio-fuels, microelectronics, energy and environment.
Much of the excitement and interest of the past decade is due to the enormous progress made in increasing the power of relevant experimental tools.
The development of new experimental methods and improvement of existing ones continues to play an important role in this field, with recent research focusing on dynamic properties of fluids.
These fluids are thought to reach supercritical conditions under a number of different settings, such as in the formation of porphyry copper deposits or high temperature circulation of seawater in the sea floor.
[13] The interior atmospheres of the Solar System's four giant planets are composed mainly of hydrogen and helium at temperatures well above their critical points.
Alternative solvents to supercritical fluids may be poisonous, flammable or an environmental hazard to a much larger extent than water or carbon dioxide are.
In practice, the advantages offered by SFC have not been sufficient to displace the widely used HPLC and GC, except in a few cases such as chiral separations and analysis of high-molecular-weight hydrocarbons.
[24] Other reactions were, in the past, performed industrially in supercritical conditions, including the synthesis of methanol and thermal (non-catalytic) oil cracking.
Supercritical fluids provide a number of ways of achieving this by rapidly exceeding the saturation point of a solute by dilution, depressurization or a combination of these.
These processes occur faster in supercritical fluids than in liquids, promoting nucleation or spinodal decomposition over crystal growth and yielding very small and regularly sized particles.
Supercritical fluid technology offers a new platform that allows a single-step generation of particles that are difficult or even impossible to obtain by traditional techniques.
The generation of pure and dried new cocrystals (crystalline molecular complexes comprising the API and one or more conformers in the crystal lattice) can be achieved due to unique properties of SCFs by using different supercritical fluid properties: supercritical CO2 solvent power, anti-solvent effect and its atomization enhancement.
Electrolysis of water in a supercritical state, reduces the overpotentials found in other electrolysers, thereby improving the electrical efficiency of the production of oxygen and hydrogen.
The supercritical water, acts as a solvent, a supplier of bond-breaking thermal energy, a heat transfer agent and as a source of hydrogen atoms.
The aliphatic inter-ring linkages of lignin are also readily cleaved into free radicals that are stabilized by hydrogen originating from the water.
The aromatic rings of the lignin are unaffected under short reaction times so that the lignin-derived products are low molecular weight mixed phenols.
Supercritical carbon dioxide is also proposed as a working fluid, which would have the advantage of lower critical pressure than water, but issues with corrosion are not yet fully solved.
Supercritical water reactors (SCWRs) are proposed advanced nuclear systems that offer similar thermal efficiency gains.
[33] Conversion of vegetable oil to biodiesel is via a transesterification reaction, where a triglyceride is converted to the methyl esters (of the fatty acids) plus glycerol.
This has the advantage of allowing a greater range and water content of feedstocks (in particular, used cooking oil), the product does not need to be washed to remove catalyst, and is easier to design as a continuous process.
The CO2 is separated from other flue gases, compressed to the supercritical state, and injected into geological storage, possibly into existing oil fields to improve yields.
Supercritical carbon dioxide is also emerging as a useful high-temperature refrigerant, being used in new, CFC/HFC-free domestic heat pumps making use of the transcritical cycle.
[40] These systems are undergoing continuous development with supercritical carbon dioxide heat pumps already being successfully marketed in Asia.
