Supercritical carbon dioxide

from Wikipedia, the free encyclopedia
Phase diagram of carbon dioxide (not to scale)
Carbon dioxide at the critical point

Supercritical carbon dioxide (also supercritical carbon dioxide or scCO 2 , from English supercritical ) is carbon dioxide in a fluid state above its critical temperature and pressure .

properties

The properties of carbon dioxide in the supercritical aggregate state are between those of gas and liquid . They thus differ significantly from the properties under standard conditions . Supercritical carbon dioxide is just as dense as a liquid, but has the same viscosity as a gas.

Supercritical CO 2 arises when the pressure and temperature are above the critical point P c for carbon dioxide, i.e. at a temperature of more than 304.13  K (30.980  ° C ) and at a pressure of over 7.375  MPa (73.75  bar ). The critical molar volume is 94 cm 3 · mol −1 .

The solvent properties strongly depend on the density, which can be adjusted over a relatively wide range. A higher density increases the solubility of most substances.

use

Due to its easy availability, non-toxicity and easy separation from the dissolved substances, supercritical carbon dioxide is a promising solvent for a wide range of applications.

Thanks to its fluidity, supercritical carbon dioxide is an extremely agile substance. Thus, it can penetrate into other substances and is used in industry as extracting agent or distraction agent (z. B. for decaffeination of coffee ) is used because it has a high solubility. The progress of the process can be tracked online using NIR technology , for example. In 2007, a fifth of all decaffeinated coffees were made by extraction with supercritical carbon dioxide. However, it is also used for a variety of other industrial processes as a foaming agent, coolant and solvent .

With supercritical CO 2 , organic preparations can be dried very gently (e.g. for scanning electron microscopy ). The samples are first cross-linked, the water is gradually exchanged for a solvent (usually acetone ) and the acetone is discharged with supercritical CO 2 . As a result of this procedure, the structures are largely retained and only a few artifacts occur. The process is called critical point drying or supercritical drying .

Recent studies have shown that supercritical carbon dioxide is an effective alternative for the sterilization of biological material and medical instruments. The process takes place under mild conditions and maintains the morphology and protein profile of dead vaccines . On a laboratory scale, supercritical carbon dioxide has been used as an extractant in the determination of polycyclic aromatic hydrocarbons in soils and solid waste and in the determination of extractable hydrocarbons in sediments, fly ash, water and other media.

It has been used for more than 30 years in oil exploration through enhanced oil recovery , as supercritical carbon dioxide can significantly increase the discharge of oil from reservoirs. Even with the proposed transfer of carbon dioxide underground ( sequestration ), the injected gas will in many cases be in a supercritical state due to the pressure and temperature conditions.

Even in areas of active volcanoes , it must be assumed that carbon dioxide is in a supercritical state at depths of 1 km or more. The use of supercritical carbon dioxide is conceivable for the geothermal use of dry rocks at depths of 4 km or more. Here supercritical carbon dioxide has advantages over water, which result in an overall 30% better energy balance .

Supercritical carbon dioxide has also been used in commercial cleaning for private customers since 2006, but the number of collection points was limited to a few dozen throughout Germany at the end of 2007. Because the solvent carbon dioxide enables a much more environmentally friendly chemical cleaning, there is the Blue Angel (Jury Umweltzeichen, RAL-UZ 126).

According to the journal Industrial & Engineering Chemistry Research , physicist Nabil Lawandy , who teaches at Brown University , USA, has a patent on the cleaning of banknotes with supercritical carbon dioxide so that the banknotes can be reused instead of being destroyed by shredding. Two G8 central banks would currently (2014) start testing the system.

Another approach is its use as a medium in turbines for storing and generating electricity .

Use in analytics

Supercritical carbon dioxide is used as the mobile phase in supercritical fluid chromatography .

literature

Web links

Individual evidence

  1. ↑ Peter W. Atkins, Julio De Paula: Short textbook physical chemistry . 4th edition. Wiley-VCH, Weinheim 2006, ISBN 978-3-527-31807-0 , pp. 216 .
  2. ↑ David R. Lide (Ed.): CRC Handbook of Chemistry and Physics . 89th edition. (Internet version: 2009), CRC Press / Taylor and Francis, Boca Raton, FL, Critical Constants, pp. 6-44.
  3. ↑ Measuring cell saves energy: Use in the extraction of natural substances and cleaning . www.analytica-world.com. Retrieved November 23, 2009.
  4. ↑ Heike Scharnhop: Application of high-speed countercurrent chromatography for fractionation and isolation of coffee constituents . Cuvillier Verlag, Göttingen 2007, ISBN 978-3-86727-417-3 , p. 11 ( limited preview in Google book search - also dissertation, TU Braunschweig).
  5. ↑ Björn Thorleif Berger: Supercritical carbon dioxide as a reaction medium for dispersion polymerization. Dissertation University of Mainz, 2000, urn : nbn: de: hebis: 77-764 .
  6. ↑ Angela White, David Burns, Tim W. Christensen: Effective terminal sterilization using supercritical carbon dioxide . In: Journal of Biotechnology . tape 123 , no. 4 , 2006, p. 504-515 , doi : 10.1016 / j.jbiotec.2005.12.033 ( PDF ).
  7. ↑ Valérie Camel: Recent extraction techniques for solid matrices – supercritical fluid extraction, pressurized fluid extraction and microwave-assisted extraction: their potential and pitfalls. In: The Analyst . Volume 126, pp. 1182-1193, doi: 10.1039 / B008243K .
  8. ↑ Carbon dioxide cleaning service, RAL-UZ 126. Accessed on August 15, 2018 .
  9. ↑ Christoph Behrens: Researchers invent machine for money laundering. In: Süddeutsche.de. 4th January 2014.
  10. ↑ David Talbot: Mini-turbine is supposed to supply an entire city. In: Heise Online . April 28, 2016.