1. Overview

As the temperature and pressure of the environment change, there are three phases of any substance-gas phase, liquid phase, solid phase, and the point where the three phases coexist in equilibrium is called the triple point. The point at which the liquid and gas phases are in equilibrium is called the critical point. The temperature and pressure at the critical point are called critical temperature and critical pressure. Different substances have different pressures and temperatures at the critical point. SuperCritical fluid (SCF) refers to the fluid whose temperature and pressure are higher than its critical point. The supercritical fluids commonly used to prepare include carbon dioxide, ammonia, ethylene, propane, propylene, water, etc. When the object is in a supercritical state, it is called supercritical fluid because the nature of the gas-liquid two-phase is very similar, so it cannot be clearly separated.
2. The characteristics of supercritical fluid Supercritical fluid has similar gas diffusivity and liquid solubility, and at the same time has the characteristics of low viscosity and low surface tension, which enables supercritical fluid to quickly penetrate into the micro-pore material. Therefore, the extraction rate is faster and more effective than that of liquid when used for extraction, especially the solubility can be changed with temperature, the extraction and separation process of supercritical fluid with pressure and polarity changes is carried out by using the relationship between the solubility of supercritical fluid and its density, that is, the influence of pressure and temperature on the solubility of supercritical fluid. When the substance is in a supercritical state, it becomes a single phase state with properties between liquid and gas, and has a density similar to that of liquid. Although the viscosity is higher than that of gas, it is obviously lower than that of liquid, the diffusion coefficient is 10 to 100 times that of the liquid, so it has good permeability and strong solubility to the material, and can extract some components from the material. In the supercritical state, the supercritical fluid is contacted with the substance to be separated, so that the components with polarity, boiling point and molecular weight are selectively extracted in sequence. At the same time, the density, polarity and dielectric constant of the supercritical fluid increase with the increase of the pressure of the closed system. The components with different polarities can be extracted step by step by using the pressure rise of the predetermined program. Of course, the extract obtained corresponding to each pressure range cannot be single, but the best proportion of mixed components can be obtained by controlling the conditions, and then the supercritical fluid can be changed into ordinary gas or liquid by means of decompression and temperature rise and fall, and the extracted substance is automatically and completely precipitated, thus achieving the purpose of separation and purification, and integrating the two processes of extraction and separation. This is the basic principle of supercritical fluid extraction and separation.
3. Common supercritical fluids
In principle, any substance should be able to become supercritical, but the critical pressure of some substances is determined by the phase density pc (q/cm3) viscosity (Pas) diffusion coefficient (cm2/s)
Gas 10-3 10-5 10-1
Supercritical fluid 0.1~0.5 10-4~10-5 10-3
Liquid 10-3 10-3 10-5