How To Make Essential Oils

The rest of the aromatics, not technically considered essential oils, are obtained using solvents: hexane, dimethyl ether, or supercritical carbon dioxide. With hexane and ether, the residue is typically less than 10 ppm (parts per million), which is fairly negligible. The least toxic of these solvents, though, is carbon dioxide (CO2)—as in, the stuff you just exhaled.

In this method, which is gaining popularity, the solvent, CO2, is placed in a chamber with the plant material. The chamber is then put under extreme pressure (100 to 200 times normal atmospheric pressure), at approximately 85 degrees Fahrenheit. This combination of mildly increased temperature and hugely increased pressure puts the CO2 into a "supercritical" state. This is a nerdy concept, but it’s where the CO2 is part liquid and part gas (like a dense fog), which allows aromatic components of the plant to dissolve in the CO2 fog. The supercritical CO2, now containing extracted aromatic compounds, is separated from the remaining raw plant material, and then returned to normal atmospheric pressure, where carbon dioxide can only exist as a gas. When it changes back to gas, it leaves behind ONLY the extracted aromatic part of the plant. There is literally zero residue of the solvent in the remaining CO2 extract because the solvent has changed physically from a supercritical state to a pure gas. It’s a very cool trick, and a very expensive one, so its utility is still fairly limited.

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https://www.mindbodygreen.com/articles/how-essential-oils-are-made