Liquefied gases as alternative solvents for green extraction of natural products

The first known use of liquefied gases as a solvent for extracting natural products dates back to 1930 in the USA, with a process patented by E. R. EBENEZER for extracting oils from oilseeds. In France, liquefied gases were first used in 1940 to extract fragrant compounds from fragile raw materials, particularly flowers.

N-butane was the first liquefied gas to be used, as its boiling point (- 1°C) means that it can be easily liquefied at low pressure to extract lipophilic molecules, but without leaving any trace in the final product once evaporated. Since then, new liquefied gases have been tested and new applications have emerged, mainly linked to the extraction of vegetable oils and fragrant extracts. Although effective and leaving no trace in the final extract, they have been abandoned for economic reasons in favor of other petrochemical liquid organic solvents (hexane, benzene, toluene, dichloromethane...), which are simpler to use industrially.

However, the ingredients market's growing interest in high-quality products of natural origin, combined with the tightening of regulations aimed at protecting our environment but above all the health of operators and consumers, have prompted manufacturers to look for alternatives more in line with the principles of green chemistry. Academic and industrial research has led to the discovery of several alternative solvents, but none of them has yet succeeded in replacing hexane in industry. The most serious alternative is the use of supercritical CO ₂ extraction processes, which can effectively extract lipophilic compounds at generally moderate temperatures (30-70°C) and without leaving traces in the extract. Nevertheless, the pressures required to reach the supercritical state (between 100 and 1,000 bar, depending on the process temperature) limit its use to very high value-added products, due to the high investment cost for an industrial installation. This hindrance to the development of supercritical CO ₂ processes has enabled liquefied gases to become highly credible alternatives to existing solvents, since they too enable lipophilic compounds to be extracted at room temperature, without leaving traces in the final product, with low energy consumption and moderate pressure (1-10 bar) that facilitates its implementation on an industrial scale.

In 2018, there are very few studies dealing with the use of liquefied gases as extraction solvents on an industrial scale. In order to give readers a better overall understanding of the process, this article presents :

• the main types of liquefied gases and why they are potentially good solvents;

• ...