|The processes using SCF are founded on the specific properties of these fluids, particularly on the possibility to vary their solvent power over a wide range : They are used as “good” solvents (extraction solvents, chromatography eluents, reaction media) when operating conditions leads to a high specific gravity (high pressure, temperature near the critical temperature), and they are later turned into compressed gases with very low solvent power (pressure below the critical pressure, temperature over 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 SCF is related to the ability to set very precisely their 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 :
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.
- For 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 lesssoluble 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 the fluid ;
- Combination of extraction or fractionation with selective adsorption of the solute mixture dissolved in the depressurized fluid onto a selective adsorbent ;