Supercritical fluid (SCF) refers to the object in its critical temperature (Tc) and critical pressure (Pc) above the state, to the state of the gas pressure, the gas will not be liquefied, but only the density increases, with a similar nature of the liquid, while still retaining the properties of the gas.
The supercritical fluid has the advantages of both gas and liquid, its density is close to that of the liquid, strong solubility, and viscosity is similar to the gas, diffusion coefficient is much larger than that of the general liquid, which is favorable for mass transfer. In addition, supercritical fluid has zero surface tension, it is easy to penetrate and diffuse into the micropores of the extracted material. Therefore, the supercritical fluid has good dissolution and mass transfer characteristics, and can quickly reach mass transfer equilibrium with the extracted material to realize the effective separation of substances.
Principle of supercritical fluid extraction and separation
Supercritical fluid extraction and separation process is the use of its solubility and density of the relationship between the use of pressure and temperature on the supercritical fluid solubility and the impact of the process. In the supercritical state, the fluid and the material to be separated in contact, so that it is selective to the size of the polarity, boiling point and molecular mass size of the different components of the extraction in turn. Then with the help of decompression, temperature method to make supercritical fluid into ordinary gas, the extracted material is automatically completely or basically precipitated, so as to achieve the purpose of separation and purification, and will be extracted and separated from the two processes into one.
The solvent of supercritical fluid extraction
Whether the supercritical fluid extraction process can effectively separate the product or remove impurities, the key is to extract the solvent used in the extraction must have good selectivity. At present, there are many types of supercritical fluids, mainly carbon dioxide, water, toluene, methanol, ethylene, ethane, propane, acetone and ammonia, etc. In recent years, the use of carbon dioxide supercritical fluid extraction is still mainly used. In recent years, the use of carbon dioxide supercritical fluid is still the main majority, because the critical state of carbon dioxide is easy to reach, its critical temperature (Tc = 30.98 ℃) is close to room temperature, the critical pressure (Pc = 7.377 MPa) is not too high, has a very good diffusion properties, low surface tension, and non-toxic, odorless, non-flammable, inexpensive, easy to refine and so on, these characteristics of heat-sensitive and easy to oxidize natural products are more attractive. These characteristics are more attractive to heat-sensitive oxidizable natural products
The main features of supercritical fluid extraction
Supercritical fluid technology in the extraction and distillation process, as an alternative to conventional separation methods, there are many potential applications. Its advantageous features are:
(1) the use of SFE is the cleanest extraction method, due to the whole process without organic solvents, so the extract is absolutely no residual solvent substances, thus preventing the presence of harmful substances in the extraction process and pollution of the environment, to ensure that 100% natural;
(2) extraction and separation into one, when saturated with the solute of CO2 (2) extraction and separation in one, when the saturated dissolved CO2 fluid into the separator, due to the drop in pressure or temperature changes, so that the CO2 and the extract quickly become two phases (gas-liquid separation) and immediately separated, not only the high efficiency of the extraction and less energy consumption, improve productivity and reduce the cost of costs;
(3) supercritical extraction can be close to the room temperature (35 ~ 40 ℃) and the CO2 gas shrouded in the extraction, effectively preventing The oxidation and dissipation of heat-sensitive substances can be effectively prevented.
(4) CO2 is an inactive gas, no chemical reaction occurs in the extraction process, and is a non-flammable gas, tasteless, odorless, non-toxic, and very good safety;
(5) CO2 gas is inexpensive, high purity, easy to produce, and in the production of the gas can be reused in the recycling process, which effectively reduces the cost;
(6) pressure and temperature can be adjusted to the extraction process. Both can be adjusted to the extraction process parameters, by changing the temperature and pressure to achieve the purpose of extraction, pressure fixed by changing the temperature can also be the same material separation; on the contrary, the temperature is fixed, by reducing the pressure to make the separation of extractables, so the process is simple and easy to master, and the extraction rate is fast.
The main factors affecting the supercritical fluid extraction process
(1) the effect of extraction pressure
Extraction pressure is one of the most important parameters of SFE, the extraction temperature is a certain amount of pressure increases, fluid density increases, the solvent strength, the solubility of the solvent increases. For different substances, the extraction pressure is very different.
(2) The effect of extraction temperature
Temperature on the supercritical fluid solubility is more complex, under a certain pressure, the temperature is increased volatility of the extracted material increases, which increases the concentration of the extracted material in the supercritical gas phase, thus increasing the amount of extraction; but on the other hand, the temperature increases, the density of the supercritical fluid decreases, which reduces the solubility of the chemical components, resulting in a decrease in the number of extractions. On the other hand, the temperature increases, the density of supercritical fluid decreases, and thus the solubility of chemical components decreases, resulting in the decrease of extraction number. Therefore, these two factors should be considered when selecting the extraction temperature.
(3) The effect of extraction particle size
Particle size can affect the extraction recovery rate, reduce the sample size, can increase the contact area between the solid and the solvent, so as to increase the extraction rate. However, if the particle size is too small or too fine, it will not only seriously block the sieve, but also cause the clogging of the filter at the outlet of extractor.
(4) The effect of CO2 flow rate
The change of CO2 flow rate has two effects on the supercritical extraction. Too much CO2 flow rate will cause the CO2 flow rate in the extractor to increase, shorten the residence time of CO2, and reduce the contact time with the extracted material, which is not conducive to the improvement of the extraction rate. On the other hand, an increase in the flow rate of CO2 increases the mass transfer driving force of the extraction process, which correspondingly increases the mass transfer coefficient and accelerates the mass transfer rate, thus improving the extraction capacity of SFE. Therefore, the rational selection of CO2 flow rate in SFE is also very important.
The process of supercritical fluid extraction is a combination of two stages: extraction and separation. According to the different separation methods, the supercritical extraction process can be categorized into: isothermal, isobaric and adsorption methods, as shown in Figure 2.
3.1 Isothermal Variable Pressure Extraction Process
Under the isothermal conditions, the extracted phase is depressurized, expanded, and the solute is separated, the solvent CO2 is pressurized by the compressor and then returned to the extraction tank, and the solute is separated by the separator and taken out from the bottom. The solvent CO2 is pressurized by the compressor and returned to the extraction tank, and the solute is separated by the separator and removed from the bottom. This cycle results in a separated extract. The process is easy to operate, more widely used, but energy consumption is higher.
3.2 Isobaric extraction process
Under isobaric conditions, the extraction phase is heated, the solute is separated, and the solvent CO2 is cooled and returned to the extraction tank. The process can be operated only with a circulating pump, the compression power is less, but need to use heating steam and cooling water.
3.3 Adsorption and extraction process
The solutes in the extracted phase are adsorbed by the adsorbent in the separation tank and the solvent CO2 is returned to the extraction tank. The adsorption-extraction process is suitable for extracting impurities, and the residue left in the extractor is the purified product.
Among them, the first two processes are mainly used to extract the solutes in the phase for the desired refined product, and the third process is often used for the removal of impurities or hazardous components in the extracted product.
The supercritical fluid has many different physicochemical properties from general liquid solvents, the supercritical fluid-based extraction technology has the advantages of traditional extraction technology can not be compared, in recent years, the research and application of supercritical fluid extraction technology from the basic data, process to the experimental equipment and so on, there is a relatively rapid development.
However, due to the lack of thorough understanding of the supercritical fluid itself, the chemical reaction, mass transfer theory and the reaction of the thermodynamics of the nature of the problem to be in-depth, and supercritical fluid extraction and separation technology requires high-pressure devices, and thus the requirements of the process equipment is often also higher, the need to have a larger investment and other reasons for the objective existence of the current large-scale practical application of supercritical fluid. There are many problems that need to be further solved.
At present, the international supercritical fluid extraction and granulation technology research and application is in the ascendant, technology development and application scope includes: extraction (extraction), separation (separation), cleaning (cleaning), coating (coating), immersion (impregnation), particle formation (particle formation) and reaction. particle formation and reaction. Germany, Japan and the United States has been in a leading position in medicine, chemical industry, food, light industry, environmental protection and other aspects of the research results continue to come out, industrialized large-scale supercritical fluid equipment 5000L ~ 10,000L scale, Japan has successfully developed a supercritical chromatography analyzer, and Taiwan has five king food companies use supercritical carbon dioxide extraction technology for rice pesticide residues and heavy metal extraction and removal. The company also uses supercritical carbon dioxide extraction technology to extract and remove pesticide residues and heavy metals from rice.
At present, the focus of international research on supercritical fluid extraction has been shifted, in order to obtain higher purity of high value-added products, the supercritical fluid countercurrent extraction and fractional extraction research is increasing. The research on the reaction under supercritical conditions has become the focus, especially the reaction under supercritical water and supercritical carbon dioxide, which has been more emphasized. Supercritical fluid technology is applied to a wider range of fields, in addition to the extraction of natural products, organic synthesis, but also environmental protection, materials processing, paint printing and dyeing, biotechnology and medicine, etc.; about the basic theory of supercritical fluid technology research has been strengthened, the international trend of these worthy of our attention.
Because supercritical carbon dioxide extraction technology can reuse carbon dioxide after extraction, to minimize the pollution of the environment, so the future of traditional industry if the supercritical carbon dioxide as the main solvent, that now we have the only earth, will be able to get relief.
In the 21st century, the chemical industry, pharmaceutical industry and other industries must adjust their own industrial structure and product structure, research and development of cleaner production and green industry of new processes and new technologies. Supercritical fluid technology is the rapid development of such a new technology in the past 30 years. We should recognize the importance of research and application of supercritical fluid technology from this strategic height, formulate research plans, increase investment, strengthen the basic and applied research on this technology, so that it can really be used in industrialized production, for the benefit of mankind, for the benefit of society.