Supercritical fluid (SF) refers to a fluid whose temperature and pressure exceed the critical temperature (Tc) and critical pressure (pc) of the substance. The density of supercritical fluid is very close to that of liquid, and it also has the diffusion properties of gas. In the supercritical state, the interface between gas and liquid disappears, the surface tension is zero, and the reaction rate and thermal conductivity show peak values. Among the compounds that can be used as supercritical fluids, CO2 is the most ideal supercritical fluid for extraction due to its stable properties, low cost and easy availability, non-toxic and odorless, no pollution to the environment and products, and low critical temperature (31.3 ℃) and critical pressure (7.15 MPa). Table 1 lists the properties of some available supercritical fluids. Supercritical carbon dioxide extraction technology combines the characteristics of distillation and liquid-liquid extraction. During the extraction process, by controlling the temperature and pressure of the carbon dioxide fluid to be higher than its critical temperature and critical pressure respectively, carbon dioxide has the dual characteristics of gas and liquid, and has a large solubility and good mass transfer performance, so that the effective components in the target substance can be extracted. When the temperature and pressure return to room temperature, the solubility of carbon dioxide decreases, and the effective components dissolved in the supercritical fluid are separated from the gaseous carbon dioxide, thereby achieving the purpose of extraction and separation.
The carbon dioxide gas in the storage tank is condensed and liquefied into liquid by the condenser, and then the pressure is increased to the pressure required for extraction by the high-pressure pump, and then the temperature is adjusted to exceed its critical temperature by the heat exchanger to make it a supercritical carbon dioxide fluid. The carbon dioxide fluid enters from the bottom of the extraction kettle as a solvent, fully contacts with the target substance, and extracts the required chemical components. The high-pressure carbon dioxide fluid dissolved with solute is decompressed through the throttle valve, and is turned into gas by the temperature regulator when passing through the decomposition kettle. At this time, the solubility of carbon dioxide for the solute is reduced, and the solute is separated from the gas and settles to the bottom of the decomposition kettle. The carbon dioxide gas is condensed again into carbon dioxide liquid through the condenser for recycling. The whole process mainly includes four stages: compression, extraction, decompression and separation of carbon dioxide. The carbon dioxide fluid is continuously circulated between the extraction kettle and the decomposition kettle, thereby extracting and separating the desired chemical components from the raw materials.