# High-Pressure Technologies for the Recovery of Bioactive Molecules from Agro-Industrial Waste
## 2 Presurized Liquid Extraction Processes
(3) a system cooling the fluid from oven temperature to room temperature or even lower; (4) an on/off pressure needle valve located after the cooler to maintain pressure during static extraction; (5) a N\({}_{2}\) flushing system, located between the high-pressure pump and the extraction system. The dynamic pressurized liquid extraction system is improved on the basis of the static pressurized extraction system, as shown in Figure 3b: (1) the selector valve connected to the N\({}_{2}\) flow is replaced by the inlet valve, because in dynamic mode, there is no need to purge the system; (2) the extraction vessel; (3) a restrictor connected to the outlet of the cooler (in place of the on/off valve) is used to maintain the constant pressure in the system [27]. Other configurations of the pressure-assisted process are also proposed in the literature. Extractor Naviglio is based on the application of pressure (about 8-9 bar) and decompressions up to atmospheric values, creating a suction effect and a mechanical transport of target compounds from the solid to the solvent phase. The system is composed of two cylinders in which extraction is performed, with the cylinders communicating through a tube on which an electro-valve is intercepted. A pneumatic system working with compressed air moves two pistons to provide a pressure rise during a static stage, during which pressure is held constant for a while; this improves the penetration of the solvent within the solid and a dynamic stage, when the pistons are removed from the equilibrium position and alternately work generating negative pressure gradient between the inside and the outside of the solid matrix and promoting the dragging effect of the extraction [30].This system is designed to work at room or low temperatures, reducing the thermal stress for thermo-sensitive compounds [31]. Moreover, high-pressure and temperature-assisted extraction (HPTE), similar to PLE, exploits the effects of pressure to work at a high temperature for the mass transfer improvements, but a different setup of the extraction cell is employed. Indeed, extraction is carried  
Figure 3: PLE system adapted from [27; 28; 29]. (**a**) Static mode; (**b**) Dynamic mode. SR, solvent reservoir; FI, flow indicator; PI, pressure indicator; SV, selecting valve; EC, extraction cell; C, cooler system; PV, pressure valve; INV, inlet needle valve; OV, outlet needle valve; R, restrictor. out in a stainless-steel vessel and a stirrer equipped by impellers provides an excellent and homogeneous mixing action [32]. The extractor can work up to 350 \({}^{\circ}\)C and 200 bar, with the two variables monitored by a thermocouple and a pressure gauge, respectively. The thermal sensor is connected to a temperature that maintains the variable at the desired set point value by an external electrical jacket. Before the extraction, an inert atmosphere is created inside the extraction vessel by bubbling inert gases for air removal with the aim of limiting degradation reactions of thermo-sensitive compounds. In HPTE, temperature and pressure are strictly related since the second factor corresponds to the vapor pressure of the solvent at the set point temperature [33]. Manosonication-assisted extraction is an example of pressure-assisted process coupled with other extraction aid, particularly the cavitation effects induced by ultrasounds. The system involves a double-jacket reaction chamber, a sensor, and PID controller for temperature control, while an ultrasonic generator and nitrogen gas controlled by a manometer was supplied to adjust the amount of external pressure applied to the reaction chamber through an inlet fitting [34]. Some authors use a combination of static and dynamic [11; 35; 36] layout to optimize the extraction process. Both modes have their own advantages and disadvantages. The most significant negative feature of dynamic pressurized extraction is that the solute in the extract is diluted, which requires a concentration step as post-processing.