3.2. Preliminary SFE tests with and without ultrasound
Preliminary 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 experiments. Table 2 shows the extract yields (X0) obtained in the preliminary SFEs. A significant increase in X0is observed when ultrasound was applied, mainly at 15 MPa. The application of ultrasound can produce a cavitation effect near the cell walls [12], and also the release of soluble material onto the sample surface, leading to higher yields.Therefore, the extract yields achieved by SFE-US are quite higher than those of SFE without ultrasound at the same pressures and temperatures.
Fig.2 shows SFE curves obtained at the conditions reported in Table 2, and the fittings obtained through the spline model. In SFEat 15 MPa, a clear increase in the extraction rate can be noted by the slopes of the curves (Fig. 2A and B) at the first 50 min, when SFE rate is constant and fluid phase convection is the main mass transfer mechanism. 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 same effects of ultrasound, in terms of release of extractable material.
These observations are confirmed by the parameters calculated with the spline model, which are reported in Table 2. The extraction rate extraction rate at the first extraction period (Mcer) increased when ultrasound was applied in SFE at 15 MPa, and decreased at 20 MPa. It can be noted, indeed, that the extraction yield (Rcer) and the extract concentration in CO2(Ycer) also were increased with the application of ultrasound at 15 MPa, but not at 20 MPa.
On the other side, the slopes of the curves with and without ultrasound are close at the final part of extraction, which is controlled by diffusion. This indicates that ultrasound enhances convective mass transfer, and such enhancement may be achieved by releasing part of the extractable material to the solid surface,where it is directly accessible to CO2.