Question

During the extraction of D-Limonene from orange rinds usinf liquid CO2, what state of matter of carbon dioxide is doing the “extraction”? How does this process occur?

Answer 1

To understand the phase of CO₂ during the extraction of D-limonene from orange rinds, you would need to consider the phase diagram of CO₂ which is as follows:

Notice that at atmospheric pressure and above -78.5 ^oC, carbon dioxide sublimes from a solid to a gaseous state. However, if one can achieve sufficiently high pressure (here, 5.11 atm), at temperature -56.4 ^oC (or near it), phase transition from solid to liquid CO₂ occurs. This liquid carbon dioxide is used to extract the organic compound from your solid sample. D-limonene is a liquid and is solvated by liquid carbon dioxide.

To do this, you would normally need to increase the pressure of the setup. This is usually done by packing the sample in solid CO_2, sealing it and heating it. If the seal holds, it would liquify the carbon dioxide at the triple point (the point at which all phases coexist, marked by 5.11 atm and -56.4 ^oC in the phase diagram). This then 'extracts' the natural product you want to isolate.