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What is a Supercritical Fluid?

A supercritical fluid (SCF) is a highly compressed fluid that combines the properties of gases and liquids. They are created by increasing temperature and pressure beyond a substance’s critical point. SCFs form the basis of clean technology across multiple industries as alternative solvents for extracting natural products, chemicals and other substances.

SCFs create fluids with densities equivalent to that of liquids and diffusion and viscosities exhibited by gases.
These properties create SCFs that can be manipulated for use in industrial processes by diffusing through solids like a gas, while dissolving materials like a liquid. Carbon dioxide and water are the most commonly used supercritical fluids in industry.

What is supercritical CO2?

Many substances can achieve supercritical conditions but CO2 has the most accessible critical temperature, 31°C, and pressure, 74 bar. These properties, combined with its non-toxic, non-flammable, chemically inert and low-cost nature, make it one of the most versatile green solvents available.

This means you can use supercritical CO2 fluid at mild temperature conditions without producing harmful organic residues.

For most solutes, supercritical CO2’s solvent capability is similar to that of light hydrocarbons, such as hexanes and pentanes.
In addition to extraction, SCFs can be used to control the particle size of solid materials. This can be achieved by dissolving a substrate in the supercritical phase, then rapidly expanding the contents through depressurization, when the dissolved materials quickly precipitate. This rapid precipitation prevents material agglomeration or crystal growth, creating finely divided solids in the nano scale. This particular process is known as RESS (rapid expansion of supercritical solutions).

SCFs also mix readily with gases, such as nitrogen or hydrogen, leading to much higher concentrations of dissolved gases, compared to the effects achieved using solvents. This can be useful for synthetic alterations such as hydrogenations.

What are the main processes that use supercritical fluids?

Extraction
The main advantage of supercritical CO2 over organic solvents in extraction processes is its high diffusion and the ability to selectively adjust the properties to extract different components.

Subsequent depressurisation and vaporisation of the CO2 releases the extracted components, expanding the supercritical CO2 to a gas.

This high pressure gas can then be condensed back to a liquid and recycled back into the system.

Impregnation
Supercritical fluid impregnation involves delivering the active substance onto an inert surface. Substrates dissolved in CO2 pass over the solid and impregnate it with the active substance. This enables the saturation of solid substances with active compounds. Examples include textile dyeing, impregnating wood and tanning leather.

Purification
SCF replaces organic solvents using supercritical CO2 as the mobile phase in chromatography. SCF chromatography achieves better resolution than its liquid equivalent and is ideal in the separation of racemic mixtures of enantiomers.

Chemical reactions
Many reactants and products are highly soluble in supercritical fluids, increasing reaction rates. Applications include biomass conversion, fuel processing, various forms of catalysis and polymerisation.

Drying
Due to an absence of surface tension in SCFs, you can use them to dissolve compounds into the supercritical phase without collapsing complex networks found in highly porous materials. This leaves a dried, solvent-free product with an unaltered porous structure.

The process is similar to freeze drying but instead of phase change from a solid to gas, supercritical drying involves a phase change from supercritical solution to a gas. This process was first adapted for the drying of aerogels.

1. Food and Drink

The use of supercritical fluids as extraction solvents for organic food products has been long established. Applications in the food and drink industry include coffee decaffeination, production of natural colourants, milk sterilisation and deodorising fish oils.

2. Pharmaceutical

Supercritical fluids help in the processing of various plant extracts used in the pharmaceutical industry: the most well recognised is the extraction of CBD and THC from hemp and cannabis.

They are also used in the micronisation of active pharmaceutical ingredients to improve drug solubility by control of the particle size.

3. Cosmetics

The cosmetic industry uses natural antioxidants in many of its products. Supercritical CO2 enables manufacturers to extract these antioxidants from a wide variety of fruit and vegetables.

These antioxidants include tocopherols (vitamin E), polyphenols and carotenoids. Supercritical fluids also extract floral fragrances from flowers.

4. Plastics and Polymers

Diffusing supercritical CO2 in polymers changes their structure and, therefore, their physio-chemical properties. Supercritical CO2 is a good plasticising agent that reduces the glass transition temperature. This improves both flexibility and active substance penetration in entangled polymer chains, allowing greater quantitative application in the matrix. Supercritical CO2 is also used to purify polymers from residual solvents and to synthesise them with composites.

5. Textiles

You can dye textiles using supercritical CO2 as an alternative solvent, and the same principles apply to tanning leather. In both applications, supercritical fluids provide solutions that are both economical and environment-friendly.

Similarly, the use of supercritical CO2 in dry cleaning makes for a green alternative to chlorinated solvents.

What are the benefits of supercritical fluids?

As an alternative to organic solvents, supercritical fluids can contribute to a greener chemical industry and support environment-friendly industrial processes.

They provide the means for designing sustainable chemical engineering processes. Supercritical CO2 can be used to remove contaminants from the environment, such as pesticides from soil. Supercritical fluids also have financial benefits. For instance, extraction and downstream processing using organic solvents contributes to production costs.

Solvent evaporation and solute recovery add unwelcome steps, which are imperfect and may have to be repeated.

Supercritical fluid extraction processes use highly compressed gases such as CO2, meaning recovery is much easier, safer and more efficient.

What instrumentation is needed for supercritical fluid extractions?

Supercritical fluid extraction is performed by compressing CO2 to high pressures and pumping it through a fixed bed of substrate, dissolving and depleting soluble components. The loaded solvent is then passed through a separator which precipitates the solutes by adjusting the temperature and pressure before de-pressurisation and vaporisation of CO2.  The CO2 can then be recondensed and collected as liquid CO2 for later use.

The instrumentation used in a typical SFE system includes:

  • CO2 (or similar) pumpIdeal for high pressure, supercritical fluids, and pulseless flow applications.
  • Co-solvent pump: Adding small amounts of solvent aids in the extraction of more polar compounds.
  • Cooling heat exchanger: Cools and liquefies the CO2, ensuring it’s a liquid before it enters the pump for maximum efficiency.
  • Flow meter: Measures the liquefied CO2 input to the pump which can be used to more accurate control the pump flow rate.
  • Preheater: Heats the high pressure liquid CO2 changing into a supercritical fluid before entering the vessel.
  • Extraction vessel: The soluble compounds are extracted from the substrate bed here, typically a milled biomass.
  • Automated back pressure regulator (BPR): Regulates pressure in the extraction vessel as the SCF flows through to optimise solubility.
  • Collection vessels with manual BPR: A series of collection vessels can be used to create a CO2 density gradient that allows the separation of different classification of compounds as a result of the extraction.

Recycler: Stores high pressure CO2 gas from the extraction process for re-use

Are you interested in using supercritical fluids?

Supercritical fluids provide versatile, green alternatives in key industrial processes and applications.

If you want to find out more about leaving organic solvents behind for a green alternative, visit SciMed’s supercritical fluid products section or make an enquiry with one of our expert product managers.

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