Explain the process of Hydrodistillation and Solvent Extraction. Why CO2 SCFE is preferred over these methods?

Hydrodistillation and solvent extraction are some of the methods that serve as the alternatives for the supercritical fluid extraction (SCFE) process. These methods, however, come with their own set of limitations and are therefore falling out of favour.

All distillation processes including hydrodistillation utilize heat to achieve the desired separation. Different molecules have diverse boiling points. At a certain temperature, one particular molecule will boil and separate from the mixture. Later, the same molecule is condensed. Since each molecule has a unique boiling point, the condensate obtained will be the pure molecule. The only problem is that heat is used for boiling off the molecule. If the molecule is heat sensitive, its chemical composition will change at high temperatures. This is precisely the challenge with hydrodistillation – it cannot be used to isolate heat sensitive compounds.

Solvent extraction uses solvents to first dissolve the target molecule from the raw material. Thereafter, the solvent is separated from the target molecule via evaporation or purging. The solvent does not completely disassociate from the target molecule i.e. it leaves behind residues which affect the purity of the extracted target molecule.

In contrast to these methods, carbon dioxide supercritical fluid extraction (CO2 SCFE) is carried out at relatively low pressures and temperatures. This is possible because the critical temperature of CO2 is 31.1 deg-Celsius, which is approximately equal to the room temperature. Furthermore, the critical pressure of CO2 is 73.9 bar, which is not very high. As a result, there is no thermal distortion of the target molecule.

Moreover, the separation action of CO2 SCFE is based on the differences in the solvent power of supercritical carbon dioxide (sCO2) at various pressures. It dissolves the target  molecule at high pressure. When pressure is reduced, the target molecule separates out from sCO2. The pressure selected for extraction is such that sCO2 dissolves little or no part of molecules other than the target molecule. This means, the separated molecule is largely pure.

Earlier, solvents such as hexane and acetone were regularly used to extract astaxanthin from microalgae. Astaxanthin is used for making nutraceuticals. Green alga (Haematococcus pluviales) contains astaxanthin as well as astaxanthin combined with lipids. Acetone cannot distinguish between these two components. Therefore, astaxanthin extracted using acetone has very low yield. But because CO2 SCFE is selective in its approach, it provides higher yields when used to isolate astaxanthin from green alga.

 Another example of the limitations of solvent extraction is the use of hexane to extract seed oil, a kind of essential oil. Hexane does not completely disassociate from seed oil and the residues decrease the purity of the extracted seed oil. The process also employs some degree of heat which opens the door for thermal degeneration of seed oil. CO2 SCFE, on the contrary, is carried out at lower temperatures and pressures. And, as mentioned, it is highly selective in its approach. Both these factors work in its favour and allow it to isolate the target molecule in a largely pure form.