What are the characteristics of supercritical CO2? The extraction with SC-CO2 at the service of production processes

From the data reported in the table, it can be seen that the density of the supercritical fluid reaches that of the liquid, in conditions of relatively low temperature (around 30° C) and high pressures. The ability to solubilize is therefore close to that of a liquid and, in some cases, even higher. Diffusivity stabilizes on intermediate values between liquid and gas. Finally, the viscosity is close to the values of the gaseous state. It follows that: the best transport properties of supercritical CO2 expressed by these data (better viscosity and diffusivity than that of a liquid) demonstrate how the speed with which supercritical CO2 is able to interact with the solute to be extracted is greater compared to that of an organic solvent not in supercritical conditions. Therefore, CO2 in the liquid/supercritical state has very low surface tension and viscosity values, combined with a high solvent power, in particular for low-polar or apolar compounds.

How supercritical CO2 works in a process:

• Liquid CO2 is compressed to pressure higher than 73 bar and temperature higher than 31° C. In these conditions CO2 became a supercritical fluid and passing thorough the material it solubilizes and extracts the compounds of interest.
• After the solubilization of the compounds of interest, it can be easily converted back to the gaseous state and eliminated without further procedures. In this way a finished product can be obtained without a trace of solvents.
• Gaseous CO2 is than condensed to liquid and recirculated in the extractor.
• With a Tc close to room temperature, the damages to the thermolabile substances are reduced and the final product will be more valid and safe.
• By saturating the extraction chamber in a short time, chemical and biological oxidative processes that could alter the matrix or the compounds to be extracted are inhibited.
• Being a low molecular weight gas, each matrix will have a high permeability for it, and therefore the extraction time will be shorter than the other solvents.

The processes using supercritical CO2 are founded on the specific properties of Carbon Dioxide, particularly on the possibility to vary its solvent power over a wide range: it is used as “good” solvents (extraction solvents, chromatography eluents, reaction media) when operating conditions leads to a high specific gravity (high pressure, temperature above the critical temperature), and it is later turned into compressed gases with very low solvent power (pressure below the critical pressure, temperature above the liquefaction temperature at this pressure) in order to perform fluid-solute separation.

It has to be emphasized that one of the main interest of supercritical CO2 is related to the ability to set very precisely its solvent power vis-à-vis different compounds by tuning pressure, temperature and co-solvent content: this permits to perform very selective fractionation of complex mixtures that cannot be resolved with classical organic solvents or by any other process. This is used either for sorting compounds belonging to the same chemical family but differing by their carbon numbers (i.e. fatty acids or oligomers/polymers), or of similar molecular mass but with slightly different polarities).

Moreover, it is possible to combine this “tunable” solvent power with selective means known in chemical engineering for completing difficult separations like:

Fractionation of liquid mixtures, high-performance multi-stage counter-current packed or stirred columns are preferred; to increase selectivity, a reflux of extract is performed either by operating a temperature gradient along the contactor on pilot-scale equipment (causing solvent power decrease and consequently precipitation of the less¬soluble compounds that reflux in liquid phase) or by an external reflux on large-scale equipment;

Multi-stage separation of the fluid-solute mixture through separators in series operated at decreasing pressures in order to fractionate the solute according to its affinity with CO2;

Combination of extraction or fractionation with selective adsorption of the solute mixture dissolved in the depressurized fluid onto a selective adsorbent;

Adsorption of the most volatile compounds of the solute in order to avoid recycling with the fluid and important losses of such compounds or selectivity decrease.