High-pressure equilibrium of menthol + CO2

Data on menthol + CO2 phase behaviour and menthol solubility in dense CO2 are needed to optimise supercritical extraction of peppermint oil. The paper presents novel experimental data for the phase diagram of (−)-menthol + CO2 binary system in the temperature range 5–40 °C and pressures up to 8.2 MPa, including menthol melting point depression, as well as data on menthol solubility in dense CO2 in the temperature range 30–60 °C and pressure range 6.6–14.4 MPa. The system exhibits quadruple point from which four three phase loci originate; the point was located at 4.32 MPa and 9.9 ± 0.1 °C. The solubility data obtained in this work confirm previous results of Maier and Stephan. Both the solubility of menthol and the pressure–temperature diagram of the system menthol + CO2 were modelled with the Soave–Redlich–Kwong equation of state and good agreement with the experimental data was achieved.