Supercritical fluids: the superheroes of green chemistry

Supercritical fluids are a class of solvents, alternative to organic solvents, for a more sustainable chemical industry.

Supercritical fluids started to gain attention from the research community in the early 1980s. They were the first real alternative to hazardous organic solvents, and a way to design sustainable chemical engineering processes. Since then, the interest in supercritical fluids has grown exponentially, as have their applications. But what are supercritical fluids?

A supercritical fluid is any substance above its critical point, a point that is at the end of the liquid-vapor equilibrium line. Above this temperate and pressure amazing things start to occur. At the critical point, the density of the liquid becomes equal to the density of the gas, meaning that it is no longer possible to distinguish between the liquid and the gas phases.

The substance is now in the supercritical region, retaining the best of the liquid and the gas world. In the supercritical region, the fluid has a liquid-like density and a gas-like viscosity. It is because of these properties that supercritical fluids were firstly termed ‘dense gases’. The most common solvent used as a supercritical fluid is carbon dioxide. CO2 is inert, non-flammable, with low toxicity. Moreover, its critical point at 73.9 bar and 31.1°C is easily achieved without compromising thermally labile substances.

The liquid-like density of fluids in the supercritical region allows them to dissolve solutes as a liquid solvent would normally do, but on the other hand, the gas-like viscosity allows the fluid to have a high diffusivity constant. These characteristics make these fluids ideal for the extraction of valuable compounds from solid matrices. More specifically, supercritical carbon dioxide (scCO2) is widely used for the extraction of hydrophobic compounds, such as essential oils, ω-3/6 fatty acids, carotenoids, waxes, etc.

Downstream processing

One of the main factors that affect chemical production costs is downstream processing. In extraction processes using organic solvents, it is necessary to evaporate and recover the solvent using energy intensive unit operations. However, achieving a completely solvent-free product is almost impossible as there are always traces of the solvent that remain in the extract. Additionally, if the solvent is toxic or harmful to the environment, as is the case of most organic solvents, its application in the food and pharmaceutical areas is compromised.

With scCO2, the recovering of the extract is easy. After the extraction process, whereby scCO2 passes through a solid matrix dissolving the molecules of interest, scCO2 is depressurised below the critical point to its gas phase. As a gas, CO2 is a poor solvent, and the solutes are completely precipitated. Furthermore, as a gas, CO2 evaporates totally from the extract, thus producing a solvent-free extract. The gaseous CO2 is then recovered and re-pressurised for a new extraction cycle.

Extraction and fractionation

There is also another amazing characteristic of fluids in the supercritical region: while passing through the equilibrium line, an abrupt change in density occurs. For example, when passing from the liquid to the gas phase, the density of a fluid can decrease tenfold, whereas if the fluid remains in the liquid or gas states, changes in pressure and temperature result in small changes in fluid density. However, in the supercritical region, changes in pressure and temperature give rise to a continuous, and noticeable, change in density.

Because density in strongly connected with solvent power, the higher the density, the higher the capability of scCO2 to dissolve compounds. On the other hand, at lower densities, although solvent power decreases, scCO2 is more selective. This unique characteristic is widely used for the fractionation of products with similar polarities and vapor pressures.

Fractionation of liquid mixtures by supercritical fluids is being considered as an alternative to more conventional processes, such as distillation and organic solvent extraction, and it is mainly carried out in columns equipped with contacting devices, such as trays or packing. The fact that the extraction solvent can be regarded as ‘green’ makes this technology attractive when fractionation and/or recovery of valuable compounds (e.g. nutraceuticals) from liquid matrices is involved.

Due to its innocuous properties and relatively low critical temperature, scCO2 gained special attention from the food industry. The most widely known industrial application of scCO2 technology is the decaffeination of coffee beans. Contrary to any other decaffeination process, scCO2 is highly selective towards caffeine, meaning that the aroma profile of the coffee is maintained.

Another important industrial application of supercritical fluids is the removal of the so called ‘cork taint’. Wines that remain bottled for long periods of time occasionally acquire a cork taste, ruining the wine quality. This cork taste is imparted by the compound trichloroanisole (TCA), which is present in some corks and slowly diffuses through the material until it reaches the wine. This is a million-dollar problem because it affects mainly expensive wines.

The only completely effective method for the removal of this compound without affecting the physical characteristics of the cork is the extraction with scCO2. scCO2 is able to diffuse into the cork and extract TCA below the required limit of 0.5 ppt.

Other applications

But there are many other applications for supercritical extraction in the food industry, such as deodorisation of olive oil, sterilisation and extraction of vitamins, oils, carotenoids, alkaloids, pigments, among other valuable products.

Although extraction is the most established application for supercritical fluids, there are many other applications where the characteristics of supercritical fluids are an advantage.

scCO2 has been used as a solvent for enzymatic reactions, significantly reducing diffusional limitations and increasing reaction rates. One example is the kinetic resolution of secondary alcohols with interest for the pharmaceutical industry. While conventional methods require intensive reaction and purification steps, using scCO2 and by manipulating pressure and temperature only, it is possible to have a one-step reaction/extraction/purification process.

Encapsulation

Encapsulation is also an important area of application of supercritical fluids. This process is of special interest for the food and pharmaceutical industries. scCO2 can dissolve important compounds, such as aromas, essential oils, vitamins or antioxidants for the food industry, as well as anti-inflammatory or antibiotic drugs for the pharmaceutical industry. By behaving as a gas, scCO2 is a carrier that can diffuse easily into any porous solid matrix. Following the impregnation step, depressurisation brings about a decrease in the solubility of the molecule of interest that precipitates inside the matrix.

Moreover, in the depressurisation step, CO2 expands, creating a highly interconnected porous matrix. These characteristics can be advantageously exploited in drug delivery, food preservation, and active compound stabilisation. Many processes were developed not only to encapsulate active molecules, but also to generate nano/micro-particles with high surface area. Amongst these processes, the most common are supercritical anti-solvent (SAS), particles from gas-saturated solutions (PGSS), and rapid expansion of supercritical solution (RESS).

The same principles apply to the use of scCO2 in the polymer and plastics industry.

Depending on the pressure and temperature applied in the scCO2 permeation step, and also on the depressurisation rate, different products can be obtained, from highly interconnected porous matrices with small pore sizes, to large pore size matrices. The swelling of the polymer creates a plasticising effect, bringing about a decrease in the glass transition temperature of the polymer. In practical terms, the polymer becomes more ductile.

Industrial applications

Supercritical fluid technology is an established technology that has been researched for many years, but only recently has it seen a boom in industrial applications. The technology has reached a maturity level that now makes it a viable alternative for many industrial processes. These include the decaffeination of coffee beans (Mawwell, Evonik Industries), flavor/aroma extraction (Evonik Industries), supercritical dying (Adidas/Nike), polymer processing (Ceapro), cork cleaning (Diam Corchos), sterilisation (Pfizer), fractionation of fish oil omega-3 fatty acids (Solutex) and leather tanning (ECO2).

The development of new processes with supercritical fluids is one of the focus areas of the Biocatalysis & Bioenergy group headed by Professor Susana Barreiros, Professor Pedro Simões, and Dr Alexandre Paiva.

Alexandre Paiva
LAQV@REQUIMTE
Chemistry Department
Faculdade de Ciências e Tecnologia
Universidade Nova de Lisboa
+35 1 212949681
alexandre.paiva@fct.unl.pt
Tweet @FCTNOVA
https://sites.fct.unl.pt/biocatalysis_and_bioenergy/

This article will appear in SciTech Europa Quarterly issue 30, which will be published in March, 2019

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