3.2. Preliminary SFE tests with and without ultrasoundPreliminary SFE tests were performed on the dried and milledraw material. Next, the extraction bed was prepared as describedin Section 2.6.1. CO2flow rate was 2.77 × 10−4kg/s in all the exper-iments.Table 2 shows the extract yields (X0) obtained in the preliminarySFEs. A significant increase in X0is observed when ultrasound wasapplied, mainly at 15 MPa. The application of ultrasound can pro-duce a cavitation effect near the cell walls [12], and also the releaseof soluble material onto the sample surface, leading to higher yields.Therefore, the extract yields achieved by SFE-US are quite higherthan those of SFE without ultrasound at the same pressures andtemperatures.Fig. 2 shows SFE curves obtained at the conditions reported inTable 2, and the fittings obtained through the spline model. In SFEat 15 MPa, a clear increase in the extraction rate can be noted bythe slopes of the curves (Fig. 2A and B) at the first 50 min, when SFErate is constant and fluid phase convection is the main mass transfermechanism. Such differences are not observed at 20 MPa (Fig. 2C),where the curves with and without ultrasound practically coincide.Possibly, the pressure of 20 MPa is high enough to provide the sameeffects of ultrasound, in terms of release of extractable material.These observations are confirmed by the parameters calculatedwith the spline model, which are reported in Table 2. The extractionrate extractionrate at the first extraction period (Mcer) increased when ultrasoundwas applied in SFE at 15 MPa, and decreased at 20 MPa. It can benoted, indeed, that the extraction yield (Rcer) and the extract con-centration in CO2(Ycer) also were increased with the application ofultrasound at 15 MPa, but not at 20 MPa.On the other side, the slopes of the curves with and with-out ultrasound are close at the final part of extraction, which iscontrolled by diffusion. This indicates that ultrasound enhancesconvective mass transfer, and such enhancement may be achievedby releasing part of the extractable material to the solid surface,where it is directly accessible to CO2.