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What are Supercritical fluids ?

In physics, common states of matter are solid, liquid and gas. Some other states can exist at extreme conditions, such as "supercritical state" when a fluid iplaced at a temperature and pressure above its critical point.

Under this state, the supercritical fluid exhibit interesting properties coupling some properties of a liquid (high density) and some of a gas (high diffusivity, low viscosity).

The two most widely used supercritical fluids are carbon dioxide (CO2) and water (H2O). Both fluids are readily available and can be used in a pressurized form to replace organic solvents or serve as alternative environment-friendly processes.

supercritical co2 phase diagram

Carbon dioxide (CO2)

CO2 is the most commonly used supercritical solvent. Produced in excess by industry at high purity, it is inexpensive, non-toxic, non-flammable and has a near ambient critical temperature (31°C). Critical pressure is reached at 74 bar.

High density coupled with diffusivity gives to the supercritical CO2 a very good and interesting solvent property.

property of co2 according to its state


Supercritical fluid and high pressure systems enable the development of sustainable and environment-friendly processes and products.

Supercritical processing is widespread across industries including natural products, bio- and advanced-materials, textile manufacturing and others.

The most well-known example is the extraction of caffeine from coffee beans using supercritical CO2, which has been used commercially for many years at a large scale. Today supercritical fluids are efficiently used and developed by Extratex for many applications:

  • Food and Nutraceuticals
  • Perfumes and Cosmetics
  • Pharmaceuticals
  • Textiles
  • Electronics
  • Aerogels, Ceramic and innovative Materials
  • Waste treatment and Waste valorisation
  • Oil and Gas industries
application area of supercritical co2

Supercritical fluid technology is applicable anywhere when the objectives are :

  • to produce higher performant and economical products
  • to use more environment-friendly and sustainable processes
  • to reduce solvent consumption and VOC emissions

Supercritical fluids based processes include extraction, impregnation, particle formation, formulation, sterilization, cleaning and chemical reactions among others. In all cases, the supercritical fluid is used as an alternative to traditional organic liquid solvents.The most widely used supercritical fluids are CO2 and water but some processes (extraction, reactions) involove the use of supercritical methanol, ethanol, propane, ethane mainly.

Some examples of processes well known by Extratex are listed below.

The principle consists in circulating supercritical CO2, through the raw material (e.g. natural plants), and depressurize the mixture to recover the extract. Indeed, after depressurization the CO2 is released in gaseous form (re-usable) and lose its solvant property leading to condensation of the extract into liquid or solid form.

supercritical CO2 extraction layout

Process makes it possible to :

  • Isolate and selectively extract with precision a molecule or a compound
  • Reach easily the core of a material without damaging the material
  • Extract a product / molecule at low temperature, preserving its integrity and shelf-life

Moreover, the low temperatures used (generally 40°C to 60°C) make it possible to preserve the chemical integrity of the thermolabile molecules (sensitive to heat). The extracts obtained, whatever their purpose, are different from those obtained by the so-called conventional methods (extraction by solvent, liquids or hydrodistillation) with an often increased quality.

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CBD oil extract

Supercritical fractionation is equivalent to solid extraction, being performed on liquid material and on continuous way (using counter-current percolation).

It is used to extract, purify or enrich compounds on the raw liquid, when these compounds have a different solubility in supercritical CO2 at a certain condition of pressure and temperature.

This operation can also be carried out at the end of a supercritical CO2 extraction.

Process makes it possible to :

  • Enrich a product with an interesting compound (ex: odorous compounds from rhum).
  • Separate compounds with different solubility into supercritical CO2 (without polarity).
supercritical CO2 fractionation column

Supercritical CO2 can be used to solubilize compounds to be eliminated from a matrix / substate. Its high diffusivity property enables to easyly penetrate in a complex porous matrix. Its selectivity enables to select compounds to be eliminated..

Examples of industrialized processes:

  • Cleaning of textiles, porous bones, complex metallic parts
  • Purifications of polymers (extration of residual oligomers), pharmaceutical powders
  • Debinding of metallic or ceramic parts / tools

Supercritical CO2 is also interesting to replace toxic or forbidden solvents such as chlorinated solvents.

supercritical CO2 cleaning bone

Supercritical impregnation means to use CO2 to bring an interesting molecules into a pr matrix (solid in general). This use the high diffusivity power of supercritical CO2 to reach the very deep of the solid , added by its high "mass transfer" power on soluble compounds.

Some advantage of impregnation with supercritical CO2 :

  • Due to its high diffusivity, it percolates through dense matrix with low porosity.
  • Due to its high density, it "bring with him" more product than a gas.
  • Impregnate colorant into textiles (no effluent, less toxic), textile dyeing.
  • Impregnate wood to strengthen its structure (ex: impregnation with furfural) or to color it in a deeper way.
  • Soak active compounds into a matrix (oligomers, catalyst, etc...).
supercritical CO2 impregnation with wood

Supercritical drying processes rely on the extraction of water or organic solvents using supercritical CO2.

The aerogel material is a "super-insulated" material, with a nano-porous matrix giving him a high insulation property.

This material is initially prepared as a gel into a liquid alcohol environment, that need to be dried after an aging phase.

The drying method / process is a key point to reach the best insulation parameters (thermal conductivity).

Here is the interest of supercritical CO2 :

  • It consist of replacing the organic solvent by CO2 by bypassing its critical point, without any evaporation, avoiding to create surface tension during liquid / gas phase transition.
  • The entire operating protocol allows the drying to proceed continuously and quickly (quicker than atmospheric drying).

Process makes it possible to :

  • Develop new product with a low density, a high open porosity, nanostructured and a very low thermal conductivity.
  • Make aerogel of various shapes and sizes, either monolithic or in grain, in powder or in a composite matrix.
  • Develop new functionalized products.
  • To circumvent problems associated with capillary forces appearing during a traditional drying with phase transition.

Some kind of process can also be used for the lyophilization of food products.

Particle design and dry formulation can be applied using supercritical CO2 as a solvent media to atomise API and exipient or anti-solvent media to dry conventional solvent from API during atomisation.

This can be used to control the shape of molecules or encapsulate them within an excipient (in order to enhance bio-availability, avoid use of basic solvent, etc...).

RESS and SAS principles

In the RESS process, the CO2 is used to extract a molecule at a certain supercritical condition, and spray it into an atomisation vessel at lower pressure. This phenomenon lead to the precipitation or recrystallisation of the material into micro or nano particules.

In the case that the molecule or API is not soluble into supercritical CO2, it is firstly solubilized into an organic solvent (itself soluble into sc CO2). Then the solution is spayed in an atomisation vessel through a co-axail spraying needle where it is put in contact with CO2 that will separate the molecule from the organic solvent, leading to precipitation. This is called "anti-solvent" process (SAS).

supercritical CO2 impregnation with wood

Process makes it possible to :

  • Crystallize a polymer or an active pharmaceutical ingredients (API), handly soluble in organic solvents.
  • Control the shape of the particles amorphous, spheric, crystalline.
  • Reach incredibly fine particle size (up to nanometer size).
  • Formulate powder that cannot be mechanically crushed for safety (explosive) or physicochemical reasons (softening ,elasticity, reagglomeration, ...).
supercritical CO2 impregnation with wood

PGSS principle

The Particles from Gas Saturated Solution (PGSS) process is a particle formation technique based on the interactions of supercritical carbon dioxide with low-melting piont molecules or polymers. A mixture of products (e.g. API + exipient) one saturated and melted with CO2 before being quickly expanded in atmospheric pressure, releasing gaseous CO2 and creating the particles (e.g. API encapsulated by the exipient).

Process makes it possible to :

  • Forming single crystals.
  • Form hollow or solid fibrils from polymers.
  • Formulating pharmaceutical active ingredients.
  • Micro-encapsulation.
  • Making virus coatings.
  • Obtain controlled polymorphic forms (stable, pure).
  • Work at low temperature with a green solvent.
supercritical CO2 impregnation with wood

Conventional chemical reactions (synthesis, biocatalysis, oxydation, etc...) can be enhanced by using supercritical CO2 as a solvent or media of the reaction, bringing some advantages on certain cases (better solubility, high diffusivity, better heat transfer, inerting, etc...).

Various processes and supercritical fluids are used depending on the product to be synthesized.

For example :

  • Supercritical CO2 Polymerization.
  • Enzymatic biocatalysis in supercritical environment.
  • Hydrogenation by CO2 or supercritical propane.
  • Hydrothermal oxidation or supercritical water.

Process makes it possible to :

  • Use supercritical fluid as a solvent or media for the reaction.
  • Increase the speed of the reaction.
  • Bring the heat or catalyst to enhance the reaction.
  • Perform polymerisation reaction.
  • Mix different solvents/products together.

The critical point of water is much higher than that of CO2. However its applications are very promising and some are in the process of industrialization. Processes using subcritical and supercritical water are called hydrothermal processes.

Subcritical water

Water, in the subcritical phase (pressure : 15 to 200 bar, temperature : 150 to 250°C) can solubilize hydrophobic compounds. Therefore, subcritcal water (sometimes referred to as hot compressed water) can be used for the extraction of plant materials (polyphenols, tannins, terpenes ...) directly on the wet raw material.

Supercritical water

In supercritical water ( pressure > 221 bar , temperature> 374°C ) organic compounds and gases become highly miscible and precipitation of inorganic compounds occurs. Oxidation reactions in supercrtiical water can also be performed. Applications include the treatment of harmful wastes and the synthesis of nanoparticles.

Makes it possible to :

  • Oxidize waste (without smoke or organic residue) using supercritical water oxidation process (SCOW).
  • Gasily organic compounds (waste, biomass, etc...).
  • Liquefy organic compounds (waste, biomass, etc...) under wet conditions and subcritical conditions.
  • Synthetise/Hydrolyse compounds within chemical reaction or catalyse.
supercritical CO2 impregnation with wood