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.
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.
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:
Supercritical fluid technology is applicable anywhere when the objectives are :
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.
Process makes it possible to :
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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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 :
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:
Supercritical CO2 is also interesting to replace toxic or forbidden solvents such as chlorinated solvents.
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 :
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).
Reaction
Gel in basket
Aging gel
Aerogel ready to be drying
Here is the interest of supercritical CO2 :
Process makes it possible to :
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...).
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).
Process makes it possible to :
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 :
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 :
Process makes it possible to :
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.
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.
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 :
