Pressure profile separation of phenolic liquid compounds from cashew (Anacardium occidentale) shell with supercritical carbon dioxide and aspects of its phase equilibria

Cashew nut shell liquid (CNSL) can be separated from fragmented honeycombed cashew shell material without employing thermal techniques with a pressure profile method that uses supercritical carbon dioxide as solvent. In the method, materials are contacted with CO₂ at elevated pressure (ca. 30 MPa) for a given period of time (ca. 1 h) and then pressure is released before the separation process is begun. Using the method, extraction yields of CNSL of up to 10 times those obtained by usual supercritical fluid extraction were achieved. The CNSL obtained was clear with a yellow-light brown color. Analysis with liquid chromatography of the extracts contained approximately 50 mol% anacardic acids, 29 mol% cardols, and 21 mol% cardanols including mono-, di-, and tri-ene constituents. Phase equilibrium data of the extracted CNSL with CO₂ were measured at temperatures from 25 to 98 °C, at pressures from 1.7 to 11 MPa, and at CO₂ weight fractions from 0.0329 to 0.1139. Liquid-liquid-vapor equilibria occurs and the CO₂-CNSL system has an upper critical end point at 31.13 °C and 7.402 MPa. Liquid-vapor equilibrium data were used to develop a model for describing the solubility of the CO₂ in the CNSL phase. Based on calculations with the model, it was found that the temperature has different effects on the solubility depending on the pressure. At low pressures (<ca. 12 MPa), increasing temperature results in a decrease in CO₂ solubility in the CNSL phase. At pressures higher than 20 MPa, increasing temperature results in an increase in CO₂ solubility in the CNSL phase. Conditions that resulted in higher CO₂ solubilities in the CNSL gave high extraction yields. The extraction mechanism of pressure profile method seems to occur by (i) penetration of the CO₂ through the shell material, (ii) dissolution of the CO₂ into the CNSL, (iii) expansion and rupture of the shell matrix due to depressurization that increases mass transfer and phase contact area.