chemistry

Supercritical carbon dioxide

Supercritical carbon dioxide is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Supercritical carbon dioxide rather than just read about it. In short: Supercritical carbon dioxide (sCO2) is a fluid state of carbon dioxide where it is held at or above its critical temperature and critical pressure. Carbon dioxide usually behaves as a gas in air at standard temperature and pressure (STP), or as a solid called dry ice when cooled and/or pressurised sufficiently.

Key takeaways

Supercritical carbon dioxide belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Supercritical carbon dioxide to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Supercritical carbon dioxide from memory before moving on to harder problems.

Reference excerpt

Supercritical carbon dioxide (sCO2) is a fluid state of carbon dioxide where it is held at or above its critical temperature and critical pressure. Carbon dioxide usually behaves as a gas in air at standard temperature and pressure (STP), or as a solid called dry ice when cooled and/or pressurised sufficiently. If the temperature and pressure are both increased from STP to be at or above the critical point for carbon dioxide, it can adopt properties midway between a gas and a liquid. More specifically, it behaves as a supercritical fluid above its critical temperature (304.128 K, 30.9780 °C, 87.7604 °F) and critical pressure (7.3773 MPa, 72.808 atm, 1,070.0 psi, 73.773 bar), expanding to fill its container like a gas but with a density like that of a liquid. Supercritical CO2 is becoming an important commercial and industrial solvent due to its role in chemical extraction, in addition to its relatively low toxicity and environmental impact. The relatively low temperature of the process and the stability of CO2 also allows compounds to be extracted with little damage or denaturing. In addition, the solubility of many extracted compounds in CO2 varies with pressure, permitting selective extractions.

Applications

Solvent

Carbon dioxide is gaining popularity among coffee manufacturers looking to move away from classic decaffeinating solvents. sCO2 is forced through green coffee beans which are then sprayed with water at high pressure to remove the caffeine. The caffeine can then be isolated for resale (e.g., to pharmaceutical or beverage manufacturers) by passing the water through activated charcoal filters or by distillation, crystallization or reverse osmosis. Supercritical carbon dioxide is used to remove organochloride pesticides and metals from agricultural crops without adulterating the desired constituents from plant matter in the herbal supplement industry. Supercritical carbon dioxide can be used as a solvent in dry cleaning. Supercritical carbon dioxide is used as the extraction solvent for creation of essential oils and other herbal distillates. Its main advantages over solvents such as hexane and acetone in this process are that it is non-flammable and does not leave toxic residue. Furthermore, separation of the reaction components from the starting material is much simpler than with traditional organic solvents. The CO2 can evaporate into the air or be recycled by condensation into a recovery vessel. Its advantage over steam distillation is that it operates at a lower temperature, which can separate the plant waxes from the oils. In laboratories, sCO2 is used as an extraction solvent, for example for determining total recoverable hydrocarbons from soils, sediments, fly-ash, and other media, and determination of polycyclic aromatic hydrocarbons in soil and solid wastes. Supercritical fluid extraction has been used in determining hydrocarbon components in water. Processes that use sCO2 to produce micro and nano scale particles, often for pharmaceutical uses, are under development. The gas antisolvent process, rapid expansion of supercritical solutions, and supercritical antisolvent precipitation (as well as several related methods) process a variety of substances into particles. Due to its ability to selectively dissolve organic compounds and assist enzyme functioning, sCO2 has been suggested as a potential solvent to support biological activity on Venus- or super-Earth-type planets.

Manufactured products Environmentally beneficial, low-cost substitutes for rigid thermoplastic and fired ceramic are made using sCO2 as a chemical reagent. The sCO2 in these processes is reacted with the alkaline components of fully hardened hydraulic cement or gypsum plaster to form various carbonates. The primary byproduct is water. sCO2 is used in the foaming of polymers. Supercritical carbon dioxide can saturate the polymer with solvent. Upon depressurization and heating, the carbon dioxide rapidly expands, causing voids within the polymer matrix, i.e., creating a foam. Research is ongoing on microcellular foams. An electrochemical carboxylation of a para-isobutylbenzyl chloride to ibuprofen is promoted under sCO2.

Working fluid sCO2 is chemically stable, reliable, low-cost, non-flammable and readily available, making it a desirable candidate working fluid for transcritical cycles. Supercritical CO2 is used as the working fluid in domestic water heat pumps. Manufactured and widely used, heat pumps are available for domestic and business heating and cooling. While some of the more common domestic water heat pumps remove heat from the space in which they are located, such as a basement or garage, CO2 heat pump water heaters are typically located outside, where they remove heat from the outside air. sCO2 has also been considered as a working fluid for enhanced geothermal systems due to its aqueous and gas properties. sCO2 is also favorable for enhanced geothermal due to its high buoyancy, large expansivity and low viscosity.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Supercritical carbon dioxide

Start with the simplest possible case. Write down what Supercritical carbon dioxide claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Supercritical carbon dioxide before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Supercritical carbon dioxide ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Supercritical carbon dioxide

In research: Supercritical carbon dioxide appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Supercritical carbon dioxide in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Supercritical carbon dioxide is common in secondary-school and first-year university syllabi. It links to neighbouring topics Carbon dioxide, Gas technologies, Industrial gases, so understanding it makes those chapters shorter.
In everyday life: Look for Supercritical carbon dioxide outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Supercritical carbon dioxide in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Supercritical carbon dioxide means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Supercritical carbon dioxide out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Supercritical carbon dioxide in simple terms?

Why does Supercritical carbon dioxide matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Supercritical carbon dioxide?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Supercritical carbon dioxide.