Supercritical fluid is a state of matter between gas and liquid that is neither gas nor liquid. This substance can only exist when its temperature and pressure exceed the critical point. It can be seen that supercritical fluid refers to a fluid between gas and liquid with a temperature and pressure above its critical temperature (Tc) and critical pressure (pc).
Supercritical fluid has a high density, similar to that of liquid, and its viscosity is closer to that of gas. Therefore, supercritical fluid is an ideal extractant.
The principle of the supercritical CO2 fluid extraction (SFE) separation process is to use the relationship between the solubility of supercritical fluid and its density, that is, to use the influence of pressure and temperature on the solubility of supercritical fluid. In the supercritical state, the supercritical fluid is contacted with the substance to be separated, so that it can selectively extract the components of polarity, boiling point and molecular weight in sequence.
The process flow of supercritical CO2 fluid extraction is set according to different extraction objects and to achieve different extraction purposes.
SFE is a typical operation at high solvent feed ratio, high air velocity and low fluid viscosity. Therefore, measures that can increase the solvent diffusion coefficient, reduce the diffusion distance and eliminate diffusion barriers will increase the mass transfer rate, and the full application of the basic theory of fluid dynamics will help analyze the mass transfer process of SFE.
The influence of plant chemical structure on SCF extraction
When using SCF to extract natural plants, it is necessary to use the structure and chemical knowledge of plants. The main reasons are as follows.
① In order to improve the extraction efficiency, the knowledge of plant structure can be used to determine whether the raw materials need to be pretreated first, such as grinding, slicing, reducing particle size or destroying cell walls.
② To maximize the extraction rate, concentration and yield of the target component and minimize the extraction rate, concentration and yield of impurities, relevant natural drug chemistry knowledge needs to be used; different extraction and separation conditions can result in different compositions of the obtained extracts.
Supercritical CO2 extraction process of curcumin
Curcumin is divided into dedimethoxycurcumin, demethoxycurcumin and curcumin. The main extraction methods can be divided into organic solvent extraction and water extraction. The existing extraction methods are prone to degrading curcumin during the process of extracting and recovering solvents. Supercritical carbon dioxide extraction technology, as a new green extraction and separation technology, can effectively extract high-purity curcumin from turmeric.
After the turmeric raw material is crushed and sieved, it is placed in an extraction kettle for extraction. After the extraction is completed, the content of curcumin in the extracted turmeric residue is detected, and the extraction rate of curcumin is obtained by comparing it with the content of curcumin in the raw material.
Under the condition that other conditions remain unchanged, the effect of different raw material particle sizes on the extraction effect is studied: as the raw material particle size decreases, the efficiency of supercritical carbon dioxide extraction increases. When the particle size is reduced to 0.5mm, the extraction rate no longer increases, but the material will stick together due to the small particle size, thereby increasing the difficulty and cost of crushing. Therefore, the particle size of the raw material should be selected at 1mm.
Conclusion
The yield and composition of the extraction product are mainly determined by the following three processing conditions: raw material primary processing, extraction conditions and separation conditions. In industrial production, each product should optimize the parameters in the above conditions.
SFE is a typical operation at high solvent feed ratio, high air velocity and low fluid viscosity. It is generally believed that SFE-CO2 can selectively extract plant aromatic oils, esters, alcohols, aldehydes, terpenes, and light fractions in waxes and resins, while leaving heavy fractions, fatty acids, triglycerides, chlorophyll, pigments and other high molecular weight substances in waxes and resins in the extraction phase.
Compared with traditional liquid-liquid extraction, SFE-CO2 has better selectivity, and supercritical fluid extraction technology has broad application prospects in the food, chemical and pharmaceutical industries. With the development of industrialized SFE devices, the reliability, safety, rationality of operation and sufficient convenience of SFE devices will be more effectively guaranteed.