Supercritical fluid (SCF) means that when an object is higher than its critical temperature (Tc) and critical pressure (Pc), the gas will not liquefy, but its density will increase, and it has liquid-like properties while retaining the properties of gas.
Supercritical fluid has the advantages of both gas and liquid, its density is close to that of liquid and its solubility is strong, while its viscosity is close to that of gas, and its diffusion coefficient is much larger than that of ordinary liquid, which is beneficial to mass transfer. In addition, the surface tension of supercritical fluid is zero, and it is easy to permeate and diffuse into the micropores of the extracted material. Therefore, supercritical fluid has good dissolution and mass transfer characteristics, which can quickly achieve mass transfer balance with the extracted substances and realize effective separation of substances.
Principle of supercritical fluid extraction and separation
The process of supercritical fluid extraction and separation is based on the relationship between its solubility and density, that is, the influence of pressure and temperature on the solubility of supercritical fluid. In the supercritical state, the fluid contacts with the substance to be separated, so as to sequentially and selectively extract components with different polarities, boiling points and molecular weights. Then the supercritical fluid is changed into ordinary gas by depressurizing and heating, and the extracted substance automatically precipitates completely or basically, so as to achieve the purpose of separation and purification, and the two components of extraction and separation are separated.
Supercritical fluid extraction solvent
Whether supercritical fluid extraction can effectively separate products or remove impurities depends on the good selectivity of solvents used in extraction. At present, there are many kinds of supercritical fluids studied, mainly including carbon dioxide, water, toluene, methanol, ethylene, ethane, propane, acetone, ammonia and so on. In recent years, carbon dioxide supercritical fluid is mainly used, because the critical state of carbon dioxide is easy to reach. Its critical temperature (Tc=30.98℃) is close to room temperature, and its critical pressure (Pc=7.377 MPa) is not high. It has good diffusion performance, low surface tension, non-toxic, tasteless, nonflammable, low price and easy to refine. These characteristics are more attractive to heat-sensitive and easily oxidized natural products.
Main characteristics of supercritical fluid extraction
As a substitute for traditional separation methods, supercritical fluid technology has many potential application prospects in extraction and distillation. Its advantages are as follows:
(1) SFE is the cleanest extraction method. Because no organic solvent is used in the whole process, there is no residual solvent in the extract, which avoids the existence of harmful substances to human body and environmental pollution during the extraction process and ensures the pure nature of 100%;
(2) Integration of extraction and separation. When CO2 fluid saturated with solute enters the separator, CO2 and extraction liquid quickly become two phases (gas-liquid separation), and are immediately separated due to pressure drop or temperature change, which not only has high extraction efficiency, but also consumes less energy, thus improving production efficiency and reducing cost;
(3) Supercritical extraction can be carried out near room temperature (35 ~ 40℃) under the cover of CO2 gas, which effectively prevents the oxidation and escape of heat-sensitive substances.
(4)CO2 is an inert gas, which does not undergo chemical reaction during the extraction process, and belongs to nonflammable gas, which is odorless, odorless, nontoxic and very safe;
(5)CO2 gas is cheap, high in purity, easy to prepare, and can be reused in production, thus effectively reducing the cost;
(6) Both pressure and temperature can be used as parameters to adjust the extraction process. The purpose of extraction can be achieved by changing the temperature and pressure, and substances can be separated by changing the temperature when the pressure is fixed; On the contrary, the process of fixing the temperature and separating the extract under reduced pressure is simple and easy to master, and the extraction speed is fast.
Main influencing factors of supercritical fluid extraction process
(1) Effect of extraction pressure
Extraction pressure is one of the most important parameters of SFE. When the extraction temperature is constant, the solubility of solvent increases with the increase of pressure, fluid density and solvent strength, and the extraction pressure of different substances varies greatly.
(2) Influence of extraction temperature
The influence of temperature on the solubility of supercritical fluid is complicated. Under a certain pressure, the volatility of the extracted substance increases with the increase of temperature, which increases the concentration of the extracted substance in the supercritical gas phase, thus increasing the extraction amount. On the other hand, with the increase of temperature, the density of supercritical fluid decreases, which reduces the solubility of chemical components and leads to the decrease of extraction number. Therefore, these two factors should be considered comprehensively when selecting extraction temperature.
(3) Effect of extraction granularity
Particle size can affect the extraction recovery rate, reduce the sample particle size and increase the contact area between solid and solvent, thus improving the extraction speed. However, if the particle size is too small or too fine, it will not only seriously block the sieve holes, but also cause the blockage of the filter screen at the outlet of the extractor.
(4) Influence of 4)CO2 flow rate
The change of CO2 flow rate has two effects on supercritical extraction. If the CO2 flow rate is too large, the CO2 flow rate in the extractor will increase, the CO2 residence time will be shortened, and the contact time with the extracted material will be shortened, which is not conducive to the improvement of extraction rate. On the other hand, the increase of CO2 flow will increase the driving force of mass transfer in the extraction process, correspondingly increase the mass transfer coefficient and speed up the mass transfer rate, thus improving the extraction capacity of SFE. Therefore, it is also necessary to reasonably select CO2 flow in SFE.
Supercritical fluid extraction process includes two stages: extraction and separation. According to different separation methods, the supercritical fluid extraction process can be divided into isothermal method, isobaric method and adsorption method, as shown in Figure 2.
3. 1 isothermal pressure swing extraction process
Under isothermal conditions, the extraction phase is decompressed and expanded, and the solute is separated. The solvent CO2 is pressurized by the compressor and then returned to the extraction tank. Solute is separated by separator and taken out from the bottom. Separate extracts are obtained by this cycle. The process is simple to operate and widely used, but it consumes a lot of energy.
3.2 Isobaric and Variable Temperature Extraction Process
Under the condition of constant pressure, the extraction phase is heated, the solute is separated, and the solvent CO2 is cooled and returned to the extraction tank. The process only needs circulating pump to run, and the compression power is small, but it needs to heat steam and cooling water.
3.3 Adsorption Extraction Process
The solute in the extraction phase is adsorbed by the adsorbent in the separation tank, and the solvent CO2 returns to the extraction tank. Adsorption extraction process is suitable for removing impurities by extraction, and the residue left in the extractor is the purified product.
Among them, the first two processes are mainly used to extract the solute in the phase state as the required refined product, and the third process is often used to remove impurities or harmful components from the extracted product.
Supercritical fluid has many physical and chemical characteristics different from ordinary liquid solvents, and the extraction technology based on supercritical fluid has incomparable advantages over traditional extraction technology. In recent years, the research and application of supercritical fluid extraction technology has developed rapidly from basic data, technological process and experimental equipment.
However, due to the lack of thorough understanding of supercritical fluid itself, the research on its chemical reaction, mass transfer theory and thermodynamic essence in the reaction needs to be deepened, and the extraction and separation technology of supercritical fluid requires high-pressure equipment, so the requirements for process equipment are often high and large investment is needed. Therefore, there are still many problems to be solved in the large-scale practical application of supercritical fluid.
At present, the research and application of supercritical fluid extraction granulation technology is in the ascendant in the world, and the application scope of technology development includes: extraction, separation, cleaning, coating, impregnation, particle forming, reaction and so on. Germany, Japan and the United States have always been in the leading position, and have made research achievements in medicine, chemical industry, food, light industry and environmental protection. It's coming out continuously. The scale of industrialized large-scale supercritical fluid equipment is 5000 L ~ 10000 L. The Japanese successfully developed the supercritical chromatography analyzer, and five food companies in Taiwan Province Province also used supercritical carbon dioxide for extraction.
At present, the research focus of supercritical fluid extraction has shifted around the world. In order to obtain high purity and high added value products, there are more and more researches on supercritical fluid countercurrent extraction and fractional extraction. The research on reactions under supercritical conditions has become a hot spot, especially various reactions under supercritical water and supercritical carbon dioxide. The application of supercritical fluid technology is more extensive, including environmental protection, material processing, paint printing and dyeing, etc. The basic theoretical research of supercritical fluid technology has been strengthened, and these international trends deserve our attention.
Because supercritical carbon dioxide extraction technology can reuse the extracted carbon dioxide and minimize environmental pollution, if supercritical carbon dioxide can be used as the main solvent of traditional industries in the future, then our only earth can breathe a sigh of relief now.
In 2 1 century, chemical industry and pharmaceutical industry must research and develop new technologies for clean production and green industry by adjusting their own industrial structure and product structure. Supercritical fluid technology is a new technology developed rapidly in recent 30 years. From this strategic height, we should understand the importance of research and popularization of supercritical fluid technology, make research plans, increase investment, strengthen the basic and applied research of this technology, and make it really used in industrial production and bring into play benefits.