Process-intensified waste valorization and environmentally friendly d-limonene extraction

Supercritical fluid extraction (SFE) is an innovative, clean, sustainable, and efficient technology to obtain essential oils and bioactive substances from plants and herbs. This work focused on the SFE of lemon essential oil using dehydrated and ground waste peels from a Protected Geographical Indication Italian cultivar. After the characterization of peels, both traditional hydro-distillation and SFE were applied to extract lemon essential oil. The most abundant compound in extract was d-limonene, equal to 82% in fruits harvested in November, 88% in December, and 80% in January. The effect of operating conditions on total extract and d-limonene yield was analyzed in a series of experiments at 35-50 degrees C and 12.5-20 MPa. The optimum conditions for limonene extraction were 15 MPa and 40 degrees C. The effect of ripening stage, particle size, and CO2 flow rate was also investigated and discussed. Moreover, theoretical and experimental pressure drop and bed void fraction during SFE were analyzed, showing a packing phenomenon. Sovova's approximate model was applied to analyze the experimental results and it successfully fitted the extraction kinetics of essential oil. SFE can successfully be integrated in an intensified green process of citrus peel valorization to produce high-value and quality essential oil, with no use of chemical solvents, downstream treatments, and long-time extractions for a greener environment.