Essential oils from seeds: solubility of limonene in supercritical CO2 and how it is affected by fatty oil

Fatty oil influence on the solubility of limonene in CO2 was investigated under pressures 8–12 MPa at 313.2 K, a temperature typically applied in supercritical fluid extraction of essential oils. Solubility in CO2 was measured using the dynamic method both for limonene and for the mixture of limonene and blackcurrant seed oil. In the whole range of pressures applied, the concentration of fatty oil in the vapour phase is negligible in comparison with the concentration of limonene. Limonene is distributed between the liquid phase rich in fatty oil and the vapour phase rich in CO2, and its equilibrium concentration in the latter decreases with the diminishing limonene-to-oil ratio in the saturator. There is a steep increase of the limonene partition coefficient with pressure between 8 and 10 MPa, near the critical pressure of the binary mixture of limonene and CO2. The observed behaviour of the three-component system was confirmed and explained by thermodynamic modelling. The thermodynamic model applied was the Soave–Redlich–Kwong cubic equation of state with either the one fluid linear van der Waals mixing rule or with the MHV2 mixing rule. Appropriate conditions for an efficient supercritical fluid extraction of essential oils from seeds follow from the results obtained. Extraction pressure should be approx. 20% larger than the critical pressure of the essential oil+CO2 binary mixture and rather tight packing of the ground seed in the bed should be applied.