Supercritical fluid: Properties, Uses and Phase Diagram

A supercritical fluid (SCF) is a state of matter of a substance in which the temperature and pressure are above the critical point, resulting in properties between those of a gas and a liquid. When applied to the fluid, supercritical refers to a temperature and pressure above the critical point. Water (H₂O) and carbon dioxide (CO₂) are often used as supercritical fluids.
In other words, a gas or a liquid, heated and compressed above its critical temperature and pressure, become a supercritical fluid.

Supercritical fluid has many uses and applications. An important example is their use as solvents in extraction processes, such as decaffeination of coffee beans or the removal of fats from cocoa butter. Supercritical fluids can also be used as reactants in chemical reactions, such as polymerization. In addition, SCFs have been investigated for use in power generation, such as in coal gasification or combustion processes.

Occurrence

Supercritical fluids occur in the earth’s crust. Earth’s crust is driven by hydrothermal circulation when hot fluids in the crust start to move, flowing away in all directions which are termed supercritical fluids. For example, the creation of porphyry copper deposits or high-temperature seawater circulation. Hydrothermal vents are usually most common near mid-ocean ridges.

Chimney-like bodies of sulfide and sulfate minerals can emit fluids reaching temperatures of 400 °C. The precipitation of dissolved metals in the fluid leads to the fluid becoming a dark plume of smoke. Most of these vents reach subcritical conditions by the time they reach the seafloor. Turtle Pits and Beebee are the eruption sites in the Cayman Trough (a fault zone) that exhibited sustained supercritical forms as observed in the discharge vents.

Supercritical fluids also occur in planetary atmospheres. For example, Venus has 3.5% nitrogen and all the remaining part is composed of carbon dioxide (96.5%), a supercritical fluid. In this way, there is a pressure of 93 bars and a temperature is 735 °K on its surface. Above the critical temperature and critical pressure, the surface of this planet acts as a supercritical fluid.

Moreover, there are some planets in the solar system that are made up of gases like hydrogen and helium at various temperatures. When they reach critical conditions, they become supercritical fluids.

Properties of Supercritical fluid

Supercritical fluids exhibit both gaseous and liquid properties. A supercritical fluid behaves like gas because it can fill containers and like a liquid because it has comparable densities and solvating powers.
Supercritical fluids only occur when the critical temperature and pressure of a substance are reached.
Sometimes, even if the critical temperature or pressure exists, a supercritical fluid may either not exist at all or coexist with some other state of matter. For example, at high pressures, the freezing curve may ascend into the supercritical fluid range, enabling both solid and supercritical phases to coexist.
Unlike liquids and gases, supercritical fluids have no surface tension.
Depending on the pressure or temperature, such fluids can become more liquid or more gaseous at times.
Solubility typically increases with pressure, but the relationship with temperature is a bit more nuanced.
In supercritical fluids, solubility increases with the density of the fluid (at constant temperature).
It is true that the solubility of some substances increases with temperature, but the density will fall when the temperature reaches a critical point. Therefore, at or near the critical temperature, solubility typically decreases and then rises again.

Related topics

Properties of solids, liquids, and gases

Examples of Supercritical fluids

Some chemical compounds can be used as supercritical fluids, such as:

Carbon dioxide (CO₂)
Water (H₂O)
Methane (CH₄)
Ethane (C₂H₆)
Propane (C₃H₈)
Ethylene (C₂H₄)
Propylene (C₃H₆)
Methanol (CH₃OH)
Ethanol (C₂H₅OH)
Acetone (C₃H₆O)
Nitrous oxide (N₂O), etc

Phase Diagram showing Supercritical Fluid Phase

1. Pressure-temperature phase diagram of carbon dioxide (CO₂)

The pressure is plotted on y-axis and the temperature on x-axis. The curve obtained is called ‘the boiling curve’ which separates the gas and liquid phases from each other. This curve ends at a point called the critical point, where the gas and liquid phases diffuse into the supercritical fluid phase.

2. Density-pressure phase diagram of carbon dioxide (CO₂)

The density is plotted on y-axis and pressure on x-axis. Below the critical temperature (280 K), the pressure increases, which results in the compression of gas, condensing it into a denser liquid.

There are two phases in the equilibrium; one is a denser liquid, and the other is a low-density gas. As the temperature increases (300 °K = Critical temperature), the density of gas becomes higher as compared to the liquid. At the critical point (304.1 °K and 73.8 bar), both phases are converted into a single phase (fluid phase) and there is no more difference between the density of gas and liquid.

20 most common uses and industrial applications of supercritical fluids

Supercritical fluid extraction
Supercritical fluid chromatography
Supercritical fluid decomposition
Supercritical fluid deposition
Formation of nanoparticles
Pharmaceutical cocrystals
Supercritical drying
Impregnation and dyeing
Water electrolysis
Wastewater oxidation
Water hydrolysis
Water gasification
Power generation
Biodiesel production
Oil recovery
Antimicrobial
Geothermal system
Chemical reactions
Drying cleaning
Refrigeration

These applications have been explained below:

1. Supercritical fluid extraction

Because of the low viscosities and high diffusivities associated with supercritical fluids, their extraction is quick as compared to normal liquid extractions.

Alternative solvents to supercritical fluids may be dangerous, combustible, and hazardous to the environment to a considerably greater extent than water or carbon dioxide.

Controlling the density of the medium allows some selective extraction, and the extracted material is readily recovered by simply depressurizing, enabling the supercritical fluid to return to the gas phase and evaporate with little or no solvent residue.

The most frequently used supercritical solvent for extraction purposes is carbon dioxide.

2. Supercritical fluid chromatography

When employed on an analytical scale, supercritical fluid chromatography combines many of the benefits of gas chromatography (GC) and high-performance liquid chromatography (HPLC). In contrast to HPLC, it may be utilized with non-volatile, thermally labile analytes.

Such fluids work with the universal flame ionization detector and produce smaller peaks because of quick diffusion, except at some instances.

Although very useful, the benefits provided by Supercritical fluid chromatography (SFC) are still not adequate to replace the extensively used HPLC and GC.

3. Supercritical fluid decomposition

By gasifying biomass in supercritical water, biomass can easily be degraded. This kind of biomass gasification may be utilized to create hydrogen for use in fuel cells or hydrocarbon fuels for use in efficient combustion devices.

Due to steam reforming, water is a hydrogen-providing player in the overall process. In the latter scenario, hydrogen production can be significantly larger than the hydrogen content of biomass.

4. Supercritical fluid deposition

Functional nanostructured films and metal nanoparticles may be applied as surface coatings using supercritical fluids. When compared to vacuum systems used for chemical vapor deposition, the fluid has higher diffusivities and concentrations of the precursor, which enables deposition to take place in a regime.

Where the rate of reaction at the surface is constrained, resulting in stable and uniform interfacial growth. This is important for creating more potent electrical components, and the metal particles that are deposited in this manner. They also function as potent catalysts for electrochemical and chemical synthesis reactions.

Furthermore, because of the rapid precursor transport in solution, it is feasible to coat large surface-area particles. In the case of chemical vapor deposition, the depletion is close to the system outlet and is more prone to producing unstable interfacial growth characteristics like dendrites. The end result is the deposition of very thin and uniform coatings at rates that are substantially quicker than atomic layer deposition. This is the best alternative method for coating particles at this larger scale.

5. Formation of nanoparticles

In the pharmaceutical and other sectors, the production of microscopic or nanoparticles of a chemical with a limited size distribution is a crucial operation. By quickly exceeding a solute’s saturation point through dilution, depressurization, or a combination of these, supercritical fluids provide a number of strategies to accomplish this otherwise difficult task.

These processes take place more quickly in supercritical fluids than in liquids. This favors spinodal breakdown or nucleation over crystal formation and produces very tiny and regular-sized particles.

Recent research on supercritical fluids has demonstrated their capacity to shrink particles as small as 5 – 2000 nanometers.

6. Pharmaceutical cocrystals

Supercritical fluids serve as a new medium for the formation of pharmaceutical cocrystals. They are crystalline forms of active pharmaceutical ingredients. A new platform provided by supercritical fluid technology enables the single-step generation of particles that are challenging or even impossible to produce using conventional methods.

Due to the special characteristics of supercritical fluids, it is now possible to produce new, dried, pure cocrystals. For example, the atomization enhancement, anti-solvent effect, solvent power of carbon dioxide (CO₂), etc.

7. Supercritical drying

Supercritical drying is a technique for eliminating solvents without affecting surface tension.

Surface tension causes minuscule structures inside a solid to drag when a liquid dries, resulting in deformation and shrinkage. Since there is no surface tension when a fluid is supercritical, it may be withdrawn without causing distortion or shrinkage per se.

Aerogel production and the drying of fragile materials, including biological and archaeological samples for electron microscopy, etc require supercritical drying.

8. Impregnation and dyeing

In essence, impregnation is the opposite of extraction. A material is dissolved in a supercritical fluid, and the solution either deposits on or dissolves in a solid substrate after flowing through the substrate.

An example of impregnation is the dyeing process, which can easily be done on polymer fibers like polyester using dispersion (non-ionic) colors.

Numerous polymers also dissolve in carbon dioxide, which causes them to expand and speed up the diffusion process.

9. Supercritical water electrolysis

Supercritical water electrolysis lowers the overpotentials present in other electrolyzers by increasing the electrical effectiveness of the synthesis of oxygen and hydrogen. Thermodynamic barriers are lowered as the temperature rises, and kinetics are increased. The absence of oxygen or hydrogen bubbles on the electrodes prevents the formation of an insulating layer between the catalyst and water, lowering ohmic losses.

Moreover, Rapid mass transport is made possible by gas-like characteristics of such fluids.

10. Supercritical Wastewater oxidation

Hazardous waste is oxidized using supercritical water as a medium, which prevents the creation of dangerous combustion products. The oxidation reaction starts when molecular oxygen (or an oxidizing agent that releases oxygen upon breakdown, such as hydrogen peroxide) is dissolved in the supercritical water with the waste product to be oxidized.

11. Supercritical water hydrolysis

All biomass polysaccharides and their accompanying lignin may be converted into low-molecular-weight molecules using supercritical water contact. Supercritical water serves as a solvent, a source of bond-breaking thermal energy, a heat transfer agent, and a supply of hydrogen atoms.

In a second or less, all polysaccharides are transformed into simple sugars with a nearly quantitative yield. The lignin’s aliphatic inter-ring connections are likewise easily broken down into free radicals, which are then stabilized by hydrogen from the water. Short reaction periods do not impact the lignin’s aromatic rings, resulting in low molecular weight mixed phenols as the lignin-derived products.

12. Supercritical water gasification

Supercritical water gasification is a technique that uses supercritical water’s advantageous properties to transform aqueous biomass streams into clean water and gases like hydrogen (H₂), methane (CH₄), carbon dioxide (CO₂), carbon monoxide (CO), etc.

13. Power generation

The temperature differential between the heat source and sink determines a heat engine’s efficiency in the end (Carnot cycle). Elevating the operating temperature is necessary to increase power plant efficiency. This pushes the system towards supercritical conditions while using water as the working fluid.

With today’s technology, efficiency may be increased from a subcritical operation to about 39-45% of the operation.

Supercritical water reactors (SCWRs) are promising modern nuclear technologies that provide comparable thermal efficiency improvements.

Nuclear power facilities with supercritical cycles may employ carbon dioxide as well, with comparable efficiencies.

14. Biodiesel production

The process of transesterification converts vegetable oil from triglyceride to methyl ester and glycerol, resulting in biodiesel. Other than catalysts, supercritical methanol can be used for this purpose, which requires no catalyst, although it is frequently combined with caustic or acid catalysts.

This offers the advantages of permitting a greater variety and higher water content of feedstocks (cooking oil), avoiding the requirement for product washing to remove the catalyst, and making continuous process design simpler.

15. Oil recovery

In oil fields that are already developed, supercritical carbon dioxide (CO₂) is employed to improve oil recovery. Additionally, it is possible to use “clean coal technology” to combine improved recovery techniques with carbon sequestration.

The carbon dioxide (CO₂) is isolated from other exhaust gases, compressed to a supercritical condition, and then injected into geological storage or active oil fields to boost production.

16. Antimicrobial agent

High-pressure carbon dioxide (CO₂) has antibacterial qualities. Although its efficacy has been demonstrated for a variety of applications, the process of microbe inactivation is still not entirely understood.

17. Geothermal system

As a working fluid for geothermal systems, supercritical carbon dioxide (CO₂) has been investigated as an alternative to water.

18. Chemical reactions

Compared to traditional organic solvents, there are also substantial environmental advantages to using supercritical fluids.

The production of:

Polyethylene from supercritical ethene
Isopropyl alcohol from supercritical propane
2-butanol from supercritical butene
Ammonia from a supercritical mixture of nitrogen and hydrogen

are all examples of industrial syntheses carried out under supercritical circumstances.

The production of methanol and thermal (non-catalytic) oil cracking are two more processes that have been carried out industrially under supercritical circumstances.

19. Drying cleaning

Perchloroethylene (PERC) and other unpleasant dry-cleaning solvents can be replaced by supercritical carbon dioxide (SCD).

Sometimes, when supercritical carbon dioxide intercalates into buttons, the buttons snap or disintegrate when the SCD is depressurized.

Carbon dioxide-soluble detergents increase the solvent’s solvating capacity. To prevent harm to the buttons, carbon dioxide-based dry cleaning equipment employs liquid carbon dioxide, not supercritical CO₂.

20. Refrigeration

A transcritical cycle is employed in modern, CFC/HFC-free household heat pumps, where supercritical carbon dioxide is developing as a suitable high-temperature refrigerant. Supercritical carbon dioxide heat pumps are already being commercially commercialized in Asia. Some of the earliest commercially successful high-temperature household water heat pumps are the Japanese EcoCute systems.

Other Types of Fluids

Some other types of fluids (with some differences to that from supercritical fluids) are; compressed fluids, pressurized fluids, and subcritical fluids.

Compressed fluids have a higher density than liquid water and can be used to create high-pressure environments.
Pressurized fluids have a lower density than liquid water and can be used to create low-pressure environments.
Subcritical fluids have a density between that of liquid water and ice and can be used to create both high- and low-pressure environments.

Concepts Berg

Is CO₂ a supercritical fluid?

Carbon dioxide (CO₂) is a supercritical fluid under supercritical conditions.

Is water a supercritical fluid?

Water acts as supercritical fluid above its critical point.

What is a supercritical condition?

A supercritical condition is a condition at which a fluid acts as a supercritical fluid.

Is supercritical fluid a phase?

Yes, in a phase diagram, above the critical point it acts as a phase.

Is a supercritical fluid a state of matter?

Yes, it is a state of matter as it is an interconnected state between the liquid and the gas states.

References

Supercritical fluid (scimed.co.uk)
Application of supercritical fluid (wikipedia.org)
Properties of supercritical fluid (link.springer.com)