# Supercritical carbon dioxide
**Supercritical carbon dioxide** (sCO₂) is carbon dioxide held above its critical temperature of 31.1 °C (304.13 K) and critical pressure of 7.38 MPa (7.3773 MPa by the NIST value), where it exists as a single supercritical phase with no boundary between liquid and gas[1](https://www.nist.gov/pml/sensor-science/fluid-metrology/database-thermophysical-properties-gases-used-semiconductor-4) • [2](https://www.sciencedirect.com/topics/chemistry/critical-pressure). In this state it combines a liquid-like density with gas-like transport properties, which makes it useful both as a tunable solvent in extraction, drying, and chromatography and as the working fluid of compact, high-efficiency power cycles[3](https://schneider.cheme.cmu.edu/Files/Perry%27s%20Chemical%20Engineers%27%20Handbook%207th%20Edition/22.%20Alternative%20Separation%20Processes.pdf) • [4](https://www.sciencedirect.com/science/article/pii/S0196890419309926).
| Key fact | Detail |
|---|---|
| Critical point | 31.1 °C (304.13 K) and 7.38 MPa (73.8 bar); NIST lists 7.3773 MPa[1](https://www.nist.gov/pml/sensor-science/fluid-metrology/database-thermophysical-properties-gases-used-semiconductor-4) • [5](https://pmc.ncbi.nlm.nih.gov/articles/PMC4206850/) |
| Fluid character | Liquid-like density with gas-like viscosity and diffusivity; self-diffusion coefficients of 10⁻³ to 10⁻⁵ cm²/s, up to about 100 times those of conventional liquids[3](https://schneider.cheme.cmu.edu/Files/Perry%27s%20Chemical%20Engineers%27%20Handbook%207th%20Edition/22.%20Alternative%20Separation%20Processes.pdf) |
| Tunable solvent strength | Density and solubility parameter vary widely with small changes in pressure or temperature near the critical point[3](https://schneider.cheme.cmu.edu/Files/Perry%27s%20Chemical%20Engineers%27%20Handbook%207th%20Edition/22.%20Alternative%20Separation%20Processes.pdf) • [5](https://pmc.ncbi.nlm.nih.gov/articles/PMC4206850/) |
| Power-cycle role | Working fluid of closed Brayton cycles with turbomachinery about 1/10 the size of an equivalent steam cycle[4](https://www.sciencedirect.com/science/article/pii/S0196890419309926) |
| Flagship extraction | Coffee decaffeination, the most widely cited industrial supercritical fluid extraction; first U.S. patent in 1974 to General Foods[6](https://www.govinfo.gov/content/pkg/GOVPUB-C13-a32a9a92c5b3bbad766268b2b1e322c9/pdf/GOVPUB-C13-a32a9a92c5b3bbad766268b2b1e322c9.pdf) |
| Carbon capture link | Direct-fired sCO2 cycles burn fuel with oxygen and yield a high-purity CO2 stream ready for use or storage without separate capture equipment[7](https://netl.doe.gov/node/7548) |
| Key hardware challenge | Compact heat exchangers with surface area density above 700 m²/m³, operating above 700 °C and at pressures up to 30 MPa[8](https://www.energy.gov/sco2-rd-focus-areas) |
## Physical behavior near the critical point
Above the critical temperature and pressure, carbon dioxide forms a single phase with no clear interface with liquid or gas[9](https://pmc.ncbi.nlm.nih.gov/articles/PMC12232899/). Its density approaches that of a conventional liquid while its transport properties stay closer to those of a gas: viscosity is orders of magnitude lower than at liquid-like conditions, and diffusion coefficients are far larger than in ordinary liquids[3](https://schneider.cheme.cmu.edu/Files/Perry%27s%20Chemical%20Engineers%27%20Handbook%207th%20Edition/22.%20Alternative%20Separation%20Processes.pdf).
The defining practical property is tunability. Because the fluid is highly compressible near the critical point, a small isothermal pressure change or small isobaric temperature change moves the solvent strength over a wide range, so a product can be dissolved at one condition and precipitated at another[3](https://schneider.cheme.cmu.edu/Files/Perry%27s%20Chemical%20Engineers%27%20Handbook%207th%20Edition/22.%20Alternative%20Separation%20Processes.pdf). Chemically, sCO₂ is a nonpolar medium with a large quadrupole moment; for nonpolar solutes it is a weaker solvent than n-hexane, and co-solvents such as small amounts of acetone or ethanol are often added to raise solvating power[3](https://schneider.cheme.cmu.edu/Files/Perry%27s%20Chemical%20Engineers%27%20Handbook%207th%20Edition/22.%20Alternative%20Separation%20Processes.pdf) • [5](https://pmc.ncbi.nlm.nih.gov/articles/PMC4206850/) • [6](https://www.govinfo.gov/content/pkg/GOVPUB-C13-a32a9a92c5b3bbad766268b2b1e322c9/pdf/GOVPUB-C13-a32a9a92c5b3bbad766268b2b1e322c9.pdf).
Thermophysical properties change drastically near the critical point, which complicates both measurement and design. Property calculations for cycle and process work are most widely performed with NIST REFPROP[10](https://doi.org/10.1016/j.applthermaleng.2020.116447).
## Power generation
The largest engineering effort around sCO₂ concerns closed Brayton cycles, in which the fluid is compressed near the critical point, heated by a source, expanded through a turbine, and cooled in a gas cooler before returning to compression[11](https://www.sandia.gov/research/publications/details/superficial-carbon-dioxide-brayton-cycle-energy-conversion-research-and-dev-2014-07-08/) • [12](https://link.springer.com/article/10.1007/s42452-020-2116-6). Compressing a fluid whose density is close to a liquid's cuts the pumping work dramatically, so the cycle's thermal efficiency improves well beyond what an ideal-gas [Brayton cycle](https://www.edgepedia.org/brayton-cycle) can reach[11](https://www.sandia.gov/research/publications/details/superficial-carbon-dioxide-brayton-cycle-energy-conversion-research-and-dev-2014-07-08/).
**Compactness is the headline benefit.** The high operating pressure and resulting low volumetric flow rate shrink the turbomachinery to roughly 1/10 the size of an equivalent steam cycle and permit compact heat exchanger technology[4](https://www.sciencedirect.com/science/article/pii/S0196890419309926). The heat-rejection exchanger and turbine can be orders of magnitude smaller than in a steam-Rankine system of the same output, and the higher rejection temperature allows direct air cooling, reducing water use[11](https://www.sandia.gov/research/publications/details/superficial-carbon-dioxide-brayton-cycle-energy-conversion-research-and-dev-2014-07-08/).
Candidate heat sources span nuclear reactors, concentrated solar power, fossil boilers, and industrial waste heat recovery in the 450–750 °C range[13](https://www.sciencedirect.com/science/article/pii/S0360544224010612) • [14](https://www.sciencedirect.com/science/article/pii/S1359431120339235). In direct-fired variants, fuel and oxygen are combusted so that the working fluid itself is a supercritical CO₂ and steam mixture; after cooling to remove water, the cycle delivers a high-purity CO₂ stream suitable for use or storage without energy-intensive capture and separation[7](https://netl.doe.gov/node/7548).
**Component performance couples tightly to cycle efficiency.** One analysis cited in the literature estimates that a drop in compressor efficiency from 90% to 80% would lower overall cycle efficiency by 2.0%, while the same drop in turbine efficiency would cost 4.4%[10](https://doi.org/10.1016/j.applthermaleng.2020.116447). The main compressor operates near the critical point, where properties are strongly non-ideal, so it may be called a pump or a compressor; the turbine and recompressor run farther from the critical point on a gas-like fluid[10](https://doi.org/10.1016/j.applthermaleng.2020.116447). Design challenges include bearings, seals, pressure containment, thermal management, and rotordynamics for compact, power-dense machines[15](https://www.sciencedirect.com/science/article/abs/pii/B9780081008041000074).
Experimental development has followed this logic. [Sandia National Laboratories](https://www.edgepedia.org/sandia-national-laboratories) began a laboratory-scale test program in 2006 with internal funds and U.S. Department of Energy Office of Nuclear Energy support, built a small Brayton loop to study compression near the critical point, and has since generated over 100 kWh, verified cycle performance, and operated the assembly in simple Brayton, waste-heat, and recompression configurations[11](https://www.sandia.gov/research/publications/details/superficial-carbon-dioxide-brayton-cycle-energy-conversion-research-and-dev-2014-07-08/) • [16](https://www.sandia.gov/research/publications/details/operation-and-analysis-of-a-supercritical-co2-brayton-cycle-2010-09-01/). Sandia's 2010 work also showed that a radial compressor designed for vapor-phase operation could pump liquid CO₂ and that the gas coolers could condense it[17](https://www.sandia.gov/app/uploads/sites/156/2021/09/LDRD_Annual_Report_FY10.pdf).
## Extraction and solvent applications
[Supercritical fluid extraction](https://www.edgepedia.org/supercritical-fluid-extraction) (SFE) uses the tunable solvent strength of sCO₂ to dissolve target compounds and then recover them by depressurization or temperature change, with the solvent recompressed and recycled[3](https://schneider.cheme.cmu.edu/Files/Perry%27s%20Chemical%20Engineers%27%20Handbook%207th%20Edition/22.%20Alternative%20Separation%20Processes.pdf). CO₂ dominates the field because it is non-toxic, non-flammable, non-corrosive, cheap, and available at high purity as a byproduct of fermentation, combustion, and ammonia synthesis[2](https://www.sciencedirect.com/topics/chemistry/critical-pressure) • [6](https://www.govinfo.gov/content/pkg/GOVPUB-C13-a32a9a92c5b3bbad766268b2b1e322c9/pdf/GOVPUB-C13-a32a9a92c5b3bbad766268b2b1e322c9.pdf).
**Coffee decaffeination is the classic case.** Original work by Kurt Zosel in Germany began in the early 1970s, and the first U.S. patent, obtained in 1974 by [General Foods](https://www.edgepedia.org/general-foods), described a three-step process using dry supercritical CO₂ at 29 MPa and 323 K to strip flavor and aroma oils, wet CO₂ to extract caffeine, and re-addition of the oils; later patents showed that wet supercritical CO₂ selectively extracts caffeine from raw beans, with recovery by pressure reduction, water scrubbing, or activated carbon adsorption[6](https://www.govinfo.gov/content/pkg/GOVPUB-C13-a32a9a92c5b3bbad766268b2b1e322c9/pdf/GOVPUB-C13-a32a9a92c5b3bbad766268b2b1e322c9.pdf). Hops extraction followed, driven by scrutiny of chlorinated solvents such as dichloromethane, with the first U.S. patent in 1978 and plants in Germany and the United States[6](https://www.govinfo.gov/content/pkg/GOVPUB-C13-a32a9a92c5b3bbad766268b2b1e322c9/pdf/GOVPUB-C13-a32a9a92c5b3bbad766268b2b1e322c9.pdf). Commercial applications also include tea [decaffeination](https://www.edgepedia.org/decaffeination), flavors from hops, cholesterol and fat from eggs, nicotine from tobacco, and acetone removal from antibiotics[3](https://schneider.cheme.cmu.edu/Files/Perry%27s%20Chemical%20Engineers%27%20Handbook%207th%20Edition/22.%20Alternative%20Separation%20Processes.pdf).
In pharmaceuticals, sCO₂-based particle formation is classified by the fluid's role: as a solvent in rapid expansion of supercritical solutions ([RESS](https://www.edgepedia.org/rapid-expansion-of-supercritical-solution)), as an antisolvent in methods such as SAS, GAS, ASES, and SEDS, and as a solute, co-solute, or co-solvent in processes such as PGSS[18](https://pubmed.ncbi.nlm.nih.gov/30026127/) • [19](https://pmc.ncbi.nlm.nih.gov/articles/PMC5849475/). These routes micronize or nanonize poorly water-soluble drugs to raise dissolution rates and bioavailability while avoiding organic solvent residues[18](https://pubmed.ncbi.nlm.nih.gov/30026127/) • [20](https://pmc.ncbi.nlm.nih.gov/articles/PMC6956038/).
## Drying, chromatography, and materials
**Aerogel production depends on sCO₂ drying.** Ambient evaporation of the liquid in a gel creates capillary forces that collapse the delicate pore network; supercritical drying removes the pore liquid with no liquid-gas interface, so no surface tension arises and the porous skeleton survives[9](https://pmc.ncbi.nlm.nih.gov/articles/PMC12232899/) • [21](https://pmc.ncbi.nlm.nih.gov/articles/PMC6523290). CO₂'s mild critical point (31.1 °C, 7.4 MPa) makes it the preferred drying fluid, and supercritical CO₂ itself had already been used in critical-point drying of microscopy specimens from the 1950s before entering aerogel production in the 1980s[9](https://pmc.ncbi.nlm.nih.gov/articles/PMC12232899/). The method preserves surface areas that can range from 2 to 1000 m²/g in dried aerogels[22](https://doi.org/10.1016%2Fj.nxmate.2024.100388).
In analytical chemistry, supercritical fluid chromatography (SFC) uses pressurized CO₂ as the major mobile-phase constituent, a technique that emerged in the late 1960s and has grown considerably with a new generation of commercial instruments; it is applied to biomolecules such as amino acids, peptides, and proteins, usually with polar stationary phases and co-solvent additions[23](https://chemistry-europe.onlinelibrary.wiley.com/doi/full/10.1002/ansa.202000151) • [24](https://www.sciencedirect.com/science/article/abs/pii/S0731708520316228). sCO₂ also serves as a medium for synthesizing nanostructured metal nanoparticles for biomass upgrading[25](https://arxiv.org/abs/2403.16751).
## Open engineering problems
The remaining barriers are concentrated in hardware and materials rather than in the cycle concept. Heat exchangers must combine very high surface area density (above 700 m²/m³) with compatibility at temperatures above 700 °C and stream-to-stream pressure differentials up to 30 MPa, with minimal leakage and scalable manufacturing[8](https://www.energy.gov/sco2-rd-focus-areas). Materials face oxidation, carburization, and erosion in hot, dense CO₂, and joining techniques must be revalidated for the environment[8](https://www.energy.gov/sco2-rd-focus-areas). Reviews also flag the scaling up of heat exchangers, rather than turbomachinery, as the limiting technology for some applications, since turbomachinery can draw on gas-turbine and steam-plant experience[12](https://link.springer.com/article/10.1007/s42452-020-2116-6). Cycle control is complicated by non-ideal-gas effects on turbomachinery behavior and by strong coupling between components[26](https://arxiv.org/abs/2108.12127).
## References
1. [Index of Semiconductor Process Gases - Carbon Dioxide, NIST](https://www.nist.gov/pml/sensor-science/fluid-metrology/database-thermophysical-properties-gases-used-semiconductor-4)
2. [Critical Pressure - an overview, ScienceDirect Topics](https://www.sciencedirect.com/topics/chemistry/critical-pressure)
3. [Supercritical Fluid Separation Processes, Perry's Chemical Engineers' Handbook, 7th ed., Ch. 22](https://schneider.cheme.cmu.edu/Files/Perry%27s%20Chemical%20Engineers%27%20Handbook%207th%20Edition/22.%20Alternative%20Separation%20Processes.pdf)
4. [Effects of technical progress on performance and application of supercritical carbon dioxide power cycle: A review, Energy Conversion and Management](https://www.sciencedirect.com/science/article/pii/S0196890419309926)
5. [Supercritical Carbon Dioxide and Its Potential as a Life-Sustaining Solvent in a Planetary Environment, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC4206850/)
6. [A review of supercritical fluid extraction, NBS Technical Note 1070](https://www.govinfo.gov/content/pkg/GOVPUB-C13-a32a9a92c5b3bbad766268b2b1e322c9/pdf/GOVPUB-C13-a32a9a92c5b3bbad766268b2b1e322c9.pdf)
7. [Supercritical CO2 power cycles, NETL](https://netl.doe.gov/node/7548)
8. [sCO2 R&D Focus Areas, U.S. Department of Energy](https://www.energy.gov/sco2-rd-focus-areas)
9. [Data-Driven Review on Aerogels and Polymeric Aerogels, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC12232899/)
10. [Review of supercritical CO2 technologies and systems for power generation, Applied Thermal Engineering](https://doi.org/10.1016/j.applthermaleng.2020.116447)
11. [Supercritical Carbon Dioxide Brayton Cycle Energy Conversion Research and Development Program, Sandia National Laboratories](https://www.sandia.gov/research/publications/details/superficial-carbon-dioxide-brayton-cycle-energy-conversion-research-and-dev-2014-07-08/)
12. [Review of supercritical carbon dioxide (sCO2) technologies for high-grade waste heat to power conversion, Discover Applied Sciences](https://link.springer.com/article/10.1007/s42452-020-2116-6)
13. [Research on compression process and compressors in supercritical carbon dioxide power cycle systems: A review, Energy](https://www.sciencedirect.com/science/article/pii/S0360544224010612)
14. [Review of supercritical CO2 technologies and systems for power generation, Applied Thermal Engineering](https://www.sciencedirect.com/science/article/pii/S1359431120339235)
15. [Turbomachinery, in Fundamentals and Applications of Supercritical Carbon Dioxide (sCO₂) Based Power Cycles, ScienceDirect](https://www.sciencedirect.com/science/article/abs/pii/B9780081008041000074)
16. [Operation and analysis of a supercritical CO2 Brayton cycle, Sandia National Laboratories](https://www.sandia.gov/research/publications/details/operation-and-analysis-of-a-supercritical-co2-brayton-cycle-2010-09-01/)
17. [Sandia National Laboratories 2010 LDRD Annual Report](https://www.sandia.gov/app/uploads/sites/156/2021/09/LDRD_Annual_Report_FY10.pdf)
18. [Supercritical carbon dioxide-based technologies for the production of drug nanoparticles/nanocrystals - A comprehensive review, Advanced Drug Delivery Reviews](https://pubmed.ncbi.nlm.nih.gov/30026127/)
19. [Supercritical Fluid Technology: An Emphasis on Drug Delivery and Related Biomedical Applications, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC5849475/)
20. [Using Supercritical Fluid Technology as a Green Alternative During the Preparation of Drug Delivery Systems, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC6956038/)
21. [Nanomaterials in Advanced, High-Performance Aerogel Composites: A Review, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC6523290)
22. [Recent advances in CO2 adsorption using aerogel materials: A review](https://doi.org/10.1016%2Fj.nxmate.2024.100388)
23. [Analytical challenges encountered and the potential of supercritical fluid chromatography: A perspective of five experts, Analytical Science Advances](https://chemistry-europe.onlinelibrary.wiley.com/doi/full/10.1002/ansa.202000151)
24. [Chromatographic analysis of biomolecules with pressurized carbon dioxide mobile phases - A review, Journal of Pharmaceutical and Biomedical Analysis](https://www.sciencedirect.com/science/article/abs/pii/S0731708520316228)
25. [Recent advances on CO2-assisted synthesis of metal nanoparticles for the upgrading of biomass-derived compounds, arXiv](https://arxiv.org/abs/2403.16751)
26. [Towards model predictive control of supercritical CO2 cycles, arXiv](https://arxiv.org/abs/2108.12127)
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*Topic: Encyclopedia › Physical world and mathematics › Chemistry › Elements and inorganic substances › Carbon oxides and carbon dioxide chemistry › Carbon dioxide substance chemistry › Supercritical carbon dioxide*
*Initially written Sep 17, 2026 · Reviewed: Sep 17, 2026; Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026*
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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license