(I) definition of supercritical fluid

There are three phases in any substance - gas, liquid and solid. The point where three phases coexist in equilibrium is called the point of three phases. The point of equilibrium between liquid and gas is called the critical point. The temperature and pressure at the critical point are called critical pressures. Different substances require different pressures and temperatures at their critical points.

In Supercritical fluid (SCF) technology, SCF refers to the fluid whose temperature and pressure are both higher than the critical point, such as carbon dioxide, ammonia, ethylene, propane, propylene, water, etc. A state which is above the critical temperature and pressure and close to the critical point is called a supercritical state. In a supercritical state, the gas-liquid phase is so similar that it cannot be distinguished, so it is called SCF.

At present, carbon dioxide is the most studied supercritical fluid, which is more commonly used because of its advantages of non-toxic, non-combustion, non-reaction to most materials and low price. In the supercritical state, the CO2 fluid has the dual characteristics of gas-liquid phase, which not only has the same high diffusion coefficient and low viscosity as the gas, but also has the similar density as the liquid and good solubility of the substance. Its density is very sensitive to temperature and pressure changes and is proportional to solubility within a certain pressure range, so the solubility of a substance can be changed by controlling temperature and pressure.

(ii) basic principle of supercritical fluid extraction

Supercritical fluid extraction and separation process is conducted by using the relation between the solubility of supercritical fluid and its density, that is, the effect of pressure and temperature on the solubility of supercritical fluid. When the gas is in the supercritical state, it becomes a single phase state between the liquid and the gas. It has similar density to the liquid. Although the viscosity is higher than the gas, it is obviously lower than the liquid. Therefore, it has good permeability and strong solubility to materials and can extract some components from materials.

In the supercritical state, the supercritical fluid contacts with the substance to be separated, so that it selectively extracts the components of polarity, boiling point and molecular weight in turn. In addition, the density and permittivity of supercritical fluid increase with the pressure of closed system and the polarity increases. Corresponding to various pressure range, of course, the extracts obtained can't be a single, but can be by controlling the conditions suitable for mixing ingredients, and then with the help of decompression, heating method of supercritical fluid into normal gas, separating out completely or by automatic extraction substance is basic to achieve the purpose of separation and purification, and the extraction, separation of the two that is the basic principle of supercritical fluid extraction separation. Solubility of supercritical CO2

Under supercritical state, the solubility of CO2 to different solutes varies greatly, which is closely related to the polarity of solutes, boiling point and molecular weight. Generally speaking, the following laws are followed:

1. Lipophilic, low boiling point components can be extracted at low pressure (104Pa), such as volatile oil, hydrocarbon, ester, etc. 2. The more polar groups a compound has, the harder it is to extract. 3. The higher the molecular weight of the compound, the more difficult to extract.

Supercritical CO2 has become a commonly used extractant. It has the following characteristics:

1. The CO2 critical temperature is 31.1 ℃, critical pressure is 7.2 MPa, the critical conditions, easy to achieve. 2. The chemical property of CO2 is inert, colorless, odorless, non-toxic and safe.

3. Cheap, pure and easy to get.

Therefore, CO2 is particularly suitable for extraction of active ingredients of natural products.

(iv) principle and general situation of supercritical extraction device

Supercritical extraction (sfe) is an advanced separation technology in modern chemical engineering. Supercritical extraction means that the solvent in the extraction cylinder contacts with the extracted substance under high pressure and appropriate temperature, the solute diffuses into the solvent, and then changes the operating conditions in the separator so that the dissolved substance can be separated to achieve the purpose of separation [2]. In recent years, the supercritical benzene-extraction technology has achieved rapid development at home and abroad, and has been industrialized in the fields of hops, spices, Chinese herbal medicine, oil and fat, petrochemical industry, food and health care [3]. Since CO2 medium is selected as supercritical extractant, supercritical equipment has the following characteristics: it has a wide operating range and is easy to adjust. Is selective, can control pressure and temperature, targeted extraction of the required ingredients. With the temperature low and extraction at close to room temperature, it is particularly suitable for thermo-sensitive ingredients. The extraction process removes the possibility of oxygen oxidation and light reactions, and the extract retains its natural flavor. In one step from extraction to separation, CO2 after extraction does not remain on the extract. CO2 is non - toxic, tasteless, non - burning, cheap and easy to get, and reusable. Extraction speed is fast. Supercritical fluid extraction device (5) of the main structure and technological process of supercritical extraction device consists of the following parts: the purity is 99% or greater of CO2 gas cylinders, refrigeration unit, temperature control system, pressure control system, safety protection device, carrying agent tank, filter, mixer, heat exchangers, storage tank, plunger pump cylinder, separator, and extraction distillation column, electric control cabinet, valve, pipe fittings and shelving, etc.

Supercritical foaming equipment