Supercritical fluids (SCF) in general and supercritical carbon dioxide (CO2) in particular allow convenient and environmentally friendly extraction processes because of their liquid-like solvent properties and gas-like transport properties, that allow efficient and fast extraction processes, and complete elimination of solvent traces from extracts and treated substrates. High-pressure CO2 is an inexpensive gas that offers safe and selective supercritical fluids SCF extraction (SCFE) processes at near-environmental temperatures that can be use to recover high-value compounds in vegetable substrates.
This chapter reviews mass transfer and of phase equilibrium parameters that are required to design industrial SCFE processes for plant essential oils. Relevant mass transfer parameters include an external mass transfer coefficient and an effective diffusivity (D e), among others. Values of D e range from 102 to 105 times the binary diffusion of plant essential oils in CO2 which suggests pronounced limitations to mass transfer within the solid matrix during SCFE of plant essential oils. A relevant phase equilibrium parameter is the “operational” solubility of plant essential oils in high-pressure CO2, which depends markedly on system temperature and CO2 density, the amount of essential oils in the plant material, the interactions between the many constituents of the essential oils, and the interactions between the essential oil components and the solid matrix, all of which decrease solubility of the essential oil components as compared to their thermodynamic solubility in simple CO2-containing binary and ternary systems.