Substances exist in three forms: gas, liquid, and solid, and they can be converted at different pressures and temperatures.
When the temperature is higher than a certain value, any large pressure cannot convert the pure substance from the gas phase to the liquid phase.
Critical temperature Tc
The temperature at this time is called the critical temperature Tc; and at the critical temperature, the gas can be liquefied.
Critical pressure Pc
The lowest pressure is called the critical pressure Pc.
When the temperature of the substance is higher than the critical temperature and the pressure is greater than the critical pressure, the substance is in a supercritical state.
In the pressure map, the region above the critical temperature and the critical pressure is called the supercritical region.
If the fluid is heated or compressed to its critical temperature (Tc) and critical pressure (Pc) or above, the gas is Pressurization, the gas will not liquefy, but the density increases, has similar liquid properties, but also retains the gas properties, this state of the fluid is called supercritical fluid.
Characteristics of supercritical fluids
The density of supercritical fluids is similar to that of liquids.
The solubility of solute is greater than that below the critical point, but the viscosity is close to that of gas. The self-diffusion coefficient is usually more than 100 times that of ordinary liquids, and there is almost no surface tension.
Therefore, many of the physical properties of SCF are between gas and liquid, resulting in good quality and heat transfer properties (diffusion, viscosity, heat transfer, heat capacity). In addition, the temperature and pressure are slightly changed near the critical point, and the density usually changes greatly.
The use of supercritical fluids can change the solubility of the solute. Supercritical fluids not only have a high gas diffusion capacity, but also have a strong liquid solvency.
Table1.Phase physical properties
|Phase||Density (g/ml)||Diffusion coefficient (cm2/s)||Viscosity (g/cm.s)|
|Supercritical fluid (SCF)||0.3～0.9||10-4～10-3||10-4～10-3|
Therefore, the supercritical fluid has good fluidity and transmission performance and can be used as a solvent to replace the traditional organic solvent. Supercritical carbon dioxide is currently the most widely studied fluid and has been applied to industrial production.
Supercritical CO2 extraction technology
In the supercritical state, the supercritical fluid is brought into contact with the material to be separated to selectively extract the components of polarity size, boiling point height and molecular weight in order.
At the same time, the density, polarity, and dielectric constant of the supercritical fluid increase with the pressure of the closed system. Stepwise extraction of the components of different polarities can be performed by the pressure boost of a predetermined program. Of course, the extracts obtained for each pressure range cannot be single, but the optimal proportion of the mixed components can be obtained by controlling the conditions, and then the supercritical fluid can be converted into a common gas or liquid by decompressing and raising and lowering the temperature. The extracted material is completely precipitated, so as to achieve the purpose of separation and purification, and the two processes of extraction and separation are integrated. This is the basic principle of supercritical fluid extraction and separation.
Supercritical CO2 fluid
Supercritical CO2 is the most widely used, in addition to its own characteristics – the critical temperature of 31.265 °C, close to room temperature, easy to reach beyond the critical state, as a supercritical fluid, it also has many other excellent properties:
Natural and environmentally friendly
Supercritical CO2 fluid is a colorless, odorless, non-toxic gas under normal conditions.
After separation from the extracted components, there is no solvent residue, which can effectively avoid the solvent toxicity residue under the traditional solvent extraction conditions.
It also prevents the extraction process from harming the human body and contaminating the environment.
It is a natural and environmentally friendly extraction technology.
Low extraction temperature
The low extraction temperature, the critical temperature of CO2is 31.265°C, and the critical pressure is 72.9 atm, which can effectively prevent the oxidation, emission and reaction of heat-sensitive components, and completely preserve the biological activity of the bio-mass;
Meanwhile, it can also bring high boiling point. Low volatility, easily pyrolytic substances are extracted below their boiling point.
CO2 is cheap, high in purity, easy to obtain, and recycled in the production process, thereby reducing costs;
High extraction efficiency
When the extraction and separation are combined into one, when the CO2supercritical fluid containing solubilized material flows through the separator, the CO2and the extract quickly recover into two separate phases (gas-liquid separation) due to the pressure drop, and immediately separate.
There is no material phase change process, no solvent recovery, easy operation; not only high extraction efficiency, but also less energy consumption, cost savings, and in line with the trend of environmental protection and energy conservation.
Easy extraction, pressure and temperature can be used to adjust the extraction process parameters.
Near the critical point, slight changes in temperature and pressure will cause significant changes in the density of CO2, which will cause the solubility of the substance to be extracted to change. The purpose of extraction can be achieved by controlling the temperature or pressure.
The pressure is fixed, the temperature can be separated by changing the temperature; otherwise, the temperature is fixed, and the pressure is reduced to separate the extract; therefore, the technical process is short, time is less, and the area is small.
At the same time, it is truly friendly to the environment, and the CO2of the extracting fluid can be recycled without being used. Emission of waste carbon dioxide results in a greenhouse effect. Become a truly “green” production process.
Wide range of options is available
The polarity of the supercritical fluid can be changed. Under certain temperature conditions, as long as the pressure is changed or a suitable entrainer is added, substances of different polarities can be extracted, and a wide range of options is available.
Characteristics of supercritical carbon dioxide fluids
From the point of view of the properties of supercritical carbon dioxide fluids, it has some characteristics when used in extraction:
High extraction speed and liquid extraction, especially suitable for the separation and extraction of solid materials; operating at near normal temperature, energy consumption is lower than the general distillation Hair, suitable for the separation of heat-sensitive substances and easily oxidized substances; fast heat transfer rate, easy temperature control; suitable for the separation of volatile substances. Based on the advantages and characteristics of supercritical carbon dioxide extraction technology, it has been widely used in many fields.
Application of supercritical CO2 extraction
As early as 100 years ago, supercritical fluids were discovered for Hannay and Hogarth. However, due to technical and equipment reasons, Pilat and Gadlewicz had the idea of extracting “macromolecular compounds” using liquefied gas until the 1930s.
In 1954 Zosol confirmed experimentally that CO2 supercritical extraction can extract oil from oil.
Until the late 1970s, after Stahl and others in Germany first made breakthroughs in the research of high-voltage experimental devices, the new extraction and separation technology research and application of SFE has made gratifying substantive progress.
Now supercritical CO2 extraction technology has been applied to various fields such as chemical, food, medicine, and biological active substance extraction.
Application in Chemical Industry
The use of supercritical carbon dioxide instead of current organic solvent dyeing technology has many advantages for environmental protection, wastewater treatment and manufacturing costs.
Due to the supercritical carbon dioxide fluid, the basic characteristics are closer to the gas, so for the replacement of organic liquids, polyester fiber dyeing technology process, there will be no waste problems, while reducing water consumption and waste the amount of material, increase production, reduce energy consumption.
The United States DuPont company in North Carolina, invested 40 million US dollars to invest in the new research plant, the most attention, the main research direction is to use supercritical carbon dioxide as a reaction solution to produce fluoropolymers.
In addition, the following chemical industries have also begun to use supercritical carbon dioxide extraction technology to reduce the production of pollutants in the production process:
- De-oiling of petroleum residue oil;
- Recovery of crude oil, regeneration of lubricants;
- Separation of hydrocarbons, Extraction of coal liquefaction oil;
- Treatment of waste liquids containing hard-to-decompose substances.
Application in the food field
In the field of foods, supercritical carbon dioxide extraction is mainly used for the extraction of vegetable fats (soybean oil, castor oil, palm oil, cocoa butter, corn oil, rice bran oil, wheat germ oil, etc.);
- Animal fats (fish oil, liver oil, various Extraction of aquaculture oils;
- Degreasing of food ingredients (rice, noodles, eggs);
- Separation and purification of lipid mixtures (glycerides, fatty acids, lecithins, etc.);
- Decolorization and deodorization of fats and oils;
- Supercritical treatment with enzymes Exchange;
- Extraction of plant pigments and natural aroma components;
- Caffeine removal from coffee and black tea;
- Extraction of hops;
- Manufacture of soft drinks;
- Concentration of fermented alcohol.
- In the United States, the use of supercritical carbon dioxide extraction to extract soybean oil was successful. The quality of the product was greatly improved and there was no pollution problem.
- At present, oils can be extracted from sunflower seeds, safflower seeds, peanuts, wheat germ, palm, and cocoa beans using supercritical carbon dioxide. The proposed lipids contain neutral lipids, low phosphorus content, low coloration, and no odor. taste. This method has higher recovery than the traditional press method, and there is no problem of solvent separation of the solvent method.
Application in the field of medicine
Supercritical carbon dioxide extraction technology to extract natural drugs
Deng Qihuan and other use of supercritical carbon dioxide, a special alcoholic substances as entrainers, extraction of ginkgo biloba flavonoids and ginkgo lactone. The extraction rate of this method is 3.4%, which is 2 times higher than the 1% of the solvent method. The process is short, the extraction batch operation time is 11 times shorter than the solvent method, and the working efficiency is improved.
The quality of the effective components of Ginkgo biloba leaves (the content of Ginkgo flavonoids is 28%, ginkgo lactone content of 7.2%), are higher than the internationally recognized quality standards, there is no organic solvent residues and heavy metal residues.
The application of supercritical carbon dioxide extraction technology in the synthesis of chiral drugs
Liu Yan et al. used supercritical enzyme-catalyzed reactions for the synthesis and resolution of chiral compounds. M. mucorase lipase was used as a catalyst, and ibuprofen and propanol were used as substrates for synthesis and resolution to obtain 90% or more of S-type propyl ibuprofen. The stereoselectivity of the product can be controlled by manipulation of supercritical conditions.
Application of Bioactive Substance Extraction
Lucien et al. reported that the use of SFE-CO2 technology to extract and concentrate EPA and DHA in sardine oil can increase EPA and DHA from the original 17% to 12% to 58% and 67%, respectively.
Emanuele and Maria used SFE-CO2 to extract egg yolk phosphatides from dried egg yolk. Under certain conditions, 67 grams of egg yolk phospholipids can be extracted from 100 grams of sample. Compared with the traditional solvent method, the purity and extraction rate increase, the product color is good.
Baysal et al. extracted β-carotene from tomato rot, using 5% ethanol as the entrainer, and the extraction rate was 50% under certain conditions.
Li Xin et al. extracted the β-carotene from spirulina using petroleum ether and SFE-CO2 respectively. The results showed that SFE-CO2 technology has the advantages of high efficiency, high speed, simple process, and pure color.
In other applications
In addition to the above-mentioned categories of application areas, supercritical carbon dioxide extraction technology has also been applied to many other fields. Such as cosmetics industry, preparation of ultra-fine particles, analysis of pesticide residues, and precision instrument cleaning.
More promising applications are under development. With the continuous deepening of research on supercritical fluid extraction technology and continuous improvement of technology, the application of supercritical fluid extraction technology will be more extensive in the future.