Advantages and disadvantages, applications, processes, and influencing factors of supercritical CO2 fluid extraction technology

2021-07-21


Supercritical fluid extraction (SFE) refers to the process of extracting and separating specific components from liquids or solids by utilizing the high solubility of supercritical fluids in the supercritical state and low solubility in non supercritical states. Below, let's learn about the advantages and disadvantages, applications, processes, and influencing factors of supercritical CO2 fluid extraction technology with the editor of Xianji Network.
Any pure substance, with changes in temperature and pressure, will correspondingly present in three states: solid, liquid, and gas, known as the three states of matter. The temperature and pressure values of the transformation between the three states are called the triple point. Every substance with stable chemical properties has an inherent critical point, that is, the density of the substance at this temperature and pressure is close to its density when it is in the liquid state, but it retains the characteristics of gas. When the temperature and pressure exceed the critical point, the substance enters a supercritical state. The substance in the supercritical state is neither a gas nor a liquid state, called a supercritical fluid.
Advantages and disadvantages, applications, processes, and influencing factors of supercritical CO2 fluid extraction technology
Although there are many substances that can be used as supercritical fluids, only a very few solvents meet the requirements in terms of affordability, low critical temperature and pressure, low latent heat of evaporation, safety and environmental protection. The substances with a critical temperature within 0-100 ℃ and a critical pressure within 2-10MPa and a low latent heat of evaporation include carbon dioxide (TC31.1 ℃, Pc7.15MPa, latent heat of evaporation 25.25kJ/mol) and propane (TC96.8 ℃, Pc4.12MPa, latent heat of evaporation 15.1kJ/mol). Considering factors such as affordability and safety, carbon dioxide is considered a suitable supercritical fluid for extraction.
The density of supercritical fluids is very sensitive to changes in temperature and pressure, and its solubility is proportional to its density within a certain pressure range. Therefore, the solubility of substances in supercritical fluids can be changed by controlling temperature and pressure. Especially near the critical point, small changes in temperature and pressure can cause several orders of magnitude sudden changes in solute solubility, which provides a research basis for the feasibility of SFE technology.
Supercritical CO2 extraction process and its influencing factors
The process flow of supercritical CO2 extraction is set up according to different extraction objects and to complete different work tasks. The diagram shows a relatively simple process flow, which is mainly divided into an extraction section and a separation section. The extraction section refers to the process of solute transfer from the raw material to the carbon dioxide fluid, while the separation section refers to the separation of solute and carbon dioxide, as well as the separation between different solutes. In the actual operation process of supercritical carbon dioxide extraction, there are many factors that affect it, leading us to adopt different extraction processes. These impacts mainly include:
(1) The influence of extraction pressure
Extraction pressure is one of the important parameters of SFE. When the extraction temperature is constant, the pressure increases, the fluid density increases, the solvent strength increases, and the solubility of the solvent increases. The extraction pressure varies greatly for different substances.
(2) Effect of extraction temperature
The influence of temperature on the solubility of supercritical fluids is quite complex. Under a certain pressure, increasing the temperature increases the volatility of the extracted substance, which increases the concentration of the extracted substance in the supercritical gas phase, thereby increasing the extraction amount; On the other hand, as the temperature increases, the density of the supercritical fluid decreases, resulting in a decrease in solubility and a decrease in extraction volume. Therefore, when selecting the extraction temperature, these two factors should be considered comprehensively.
(3) The impact of raw material particle size
The particle size of raw materials can affect the extraction recovery rate. Reducing the particle size of raw materials can increase the contact area between solids and solvents, thereby increasing 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 blockage of the filter screen at the outlet of the extractor.
(4) Impact of CO2 flow rate
The change of CO2 flow rate has two effects on Supercritical fluid extraction. If the flow rate of CO2 is too high, it will cause an increase in the flow rate of CO2 in the extractor, shorten the CO2 residence time, and reduce the contact time with the extracted substance, which is not conducive to improving the extraction rate. On the other hand, an increase in CO2 flow rate can increase the mass transfer driving force of the extraction process, correspondingly increasing the mass transfer coefficient and accelerating the mass transfer rate, thereby improving the extraction ability of SFE. Therefore, reasonable selection of CO2 flow rate is also crucial in SFE.
(5) Selection of entraining agents
The solvents used in supercritical fluid extraction are mostly non polar or weakly polar, with higher solubility for lipophilic substances and lower solubility for substances with higher polarity. To address this issue, adding a certain amount of polar components (i.e. entrainers) to pure supercritical CO2 can significantly change the polarity of the supercritical CO2 fluid and broaden its applicability.
Advantages and disadvantages of supercritical CO2 extraction technology
Supercritical CO2, as a widely used fluid in extraction technology today, is not only safe, non-toxic, and inexpensive, but also has the following significant advantages compared to conventional extraction methods:
(1) The operation is close to room temperature (34-39 ℃), and the entire extraction process is shrouded in CO2 gas, effectively preventing the oxidation and degradation of thermosensitive substances. It can fully retain biological activity and extract substances with high boiling points, low volatility, and easy pyrolysis at temperatures far below their boiling points;
(2) It is a green extraction method that does not contain organic solvents throughout the entire process, so the extract has no solvent residue, thereby preventing the presence of harmful substances to human health and environmental pollution during the extraction process, ensuring 100% purity and naturalness;
(3) Extraction and separation are combined into one. When the CO2 fluid rich in dissolved substances enters the separator, the pressure or temperature is adjusted to quickly form a two-phase (gas-liquid separation) and immediately separate the CO2 and extract. This not only improves the extraction efficiency but also reduces energy consumption, improving production efficiency and reducing cost;
(4) CO2 is an inert gas that does not undergo chemical reactions during the extraction process and is a non combustible gas. It is odorless, odorless, non-toxic, and highly safe;
(5) Strong extraction ability and high extraction rate; Fast extraction time and short production cycle.
(6) Both pressure and temperature can become parameters for adjusting the extraction process. The purpose of extraction can be achieved by changing the extraction temperature and pressure, while the separation purpose can be achieved by changing the separation pressure or temperature. Therefore, the process is simple and easy to master, and the extraction speed is fast.
(7) Supercritical CO2 also has antioxidant and sterilization effects, which is beneficial for ensuring and improving product quality; Of course, supercritical CO2 extraction technology is not a universal extraction technology, and like any other technology, it also has its applicable scope. We need to confirm whether it is suitable for use based on its ability to dissolve different substances.
Generally speaking, the solubility of different substances in supercritical CO2 follows the following rules:
(a) Lipophilic and low boiling components can be extracted below 10MPa, such as aroma components in natural plants and fruits.
(b) When there is a significant difference in relative volatility or polarity between the components in the mixture, the mixture can be fractionated under different pressures.
(c) The introduction of strong polar groups (- OH, - COOH) makes extraction difficult. Within the range of benzene derivatives, substances with three hydroxyl phenols, as well as compounds with one carboxyl group and two hydroxyl groups, can still be extracted, but compounds with one carbonyl group and more than three hydroxyl groups cannot be extracted.
(d) Stronger polar substances, such as sugars, are difficult to extract below 40MPa.
(e) The higher the relative molecular weight of a compound, the more difficult it is to extract.
Application of Supercritical CO2 Extraction Technology in Plant Extracts
In recent years, with the gradual introduction of various natural concept cosmetics, the consumption demand for personal care products containing natural ingredients has continued to grow, and the position of plant extracts in cosmetics has become increasingly important. This also requires further optimization of various plant extracts in terms of safety and efficacy. Supercritical CO2 extraction technology not only completes deacidification, decolorization, and deodorization in one go in the extractor, but also uses CO2 as an extractant to ensure pollution-free, residue free, and pure natural characteristics. Compared with traditional extraction methods, SFE has significant advantages in the purity, yield, active ingredient content, color, aroma, and other aspects of plant extracts and essential oil extraction applications.
(1) In terms of purity of extract products: Magnolia officinalis is an important traditional Chinese medicinal herb in China, with the main active ingredients being magnolol and magnolol. Due to its significant antibacterial and antioxidant effects, it can be used as an effective ingredient in food, health products, pharmaceuticals, and daily chemical products. Traditional extraction processes have drawbacks such as low yield, low purity, and deep color. The extract of Magnolia officinalis obtained by Luo Andong et al. using supercritical CO2 extraction, after simple refinement, the purity of its active ingredients can reach over 98% [4]. At present, plant extracts extracted using supercritical CO2, such as Polygonum cuspidatum extract, Fructus cnidii extract, and paeonol, have a purity of over 98% after simple refinement. The industrial production of these extract products is quite mature.
(2) In terms of improving extract yield: The traditional Chinese medicine licorice is the root and rhizome of perennial leguminous herbaceous plants. Its extracted components, glycyrrhetinic acid and licorice flavonoids, are the preferred functional ingredients in cosmetics. However, the high prices of these two products make it necessary for engineers to replace these two products with other raw materials in some cosmetics. Comparing supercritical CO2 extraction technology with traditional extraction methods, it was found that the yield of glycyrrhetinic acid extracted by supercritical CO2 extraction technology is 8-12 times higher than that of traditional extraction methods [5], and the extraction rate of flavonoids in licorice is also increased by 1-1.5 times [6,7], laying a solid foundation for the large-scale application of these two raw materials.
(3) In terms of removing odors from plant extracts: Spirulina contains rich proteins and various bioactive ingredients, which have good skincare ability in cosmetics. It not only has good moisturizing and protective effects, but also has the ability to clear and inhibit free radicals, which can play a role in wrinkle prevention, sun protection, radiation resistance, freckle removal, and anti-aging [8]. Spirulina extracted using traditional methods has a heavy fishy odor, which reduces its active ingredients after deodorization, thereby limiting its further processing development. The combination of supercritical CO2 extraction technology and other extraction technologies can effectively solve the problem of removing its fishy odor without reducing its active ingredients [9,10].
(4) Extraction of effective ingredients and active substances in vegetable oil: Lv Xuebin and others extracted pomegranate seed oil by supercritical technology, and the content of pomegranate acid was more than 80% [11]. This result shows that the pomegranate seed oil extracted by supercritical CO2 contains rich conjugated unsaturated fat fatty acids. The oil has the advantages of no solvent residue, light color, high nutritional value, and meets the needs of food, medicine, health care, cosmetics and other industries for high-quality oil. The effective components of Ligusticum chuanxiong oil and Angelica sinensis oil extracted by supercritical CO2 technology can reach up to 70% ligustilide content [12].
(5) In terms of volatile oil (essential oil) extraction: The solvent extraction method is generally used for essential oil extraction, but the application of traditional extraction methods can cause some unstable aroma components to deteriorate due to heat, solvent residue, and loss of low boiling point aroma components, thereby affecting the aroma of the product. Non toxic and residue free supercritical CO2 extraction can extract essential oils and special aroma components simultaneously under room temperature operating conditions. Due to the low boiling point, small molecular weight, and small polarity of volatile oils, they have good solubility in supercritical CO2 fluids, making them suitable for supercritical CO2 fluid extraction. The yield of jasmine essential oil extracted by Zhang Jingcheng et al. through supercritical CO2 extraction is higher than that of traditional extraction methods, and the aroma has a natural jasmine flower fragrance, which is fresher than regular products, with stronger natural sense and transparency.
(6) Extraction of natural pigments: With the increasing attention paid to the unsafe nature of synthetic pigments, the types of synthetic pigments used in various countries around the world are decreasing. Natural pigments are not only safe to use, but also have certain nutritional value and are deeply loved by consumers. Taking chili red pigment as an example, using supercritical CO2 can remove the odor and residual solvents in chili oleoresin. At the same time, chili pigment can be divided into red and yellow pigments with less loss, which has obvious advantages compared to traditional deliciousness processes. Supercritical fluid CO2 extraction technology can also separate other natural pigments, such as lycopene, cocoa pigment, and β- Carotenoids et al. [14].
(7) Extraction of natural antioxidants: Rosemary extract extracted from rosemary leaves is an efficient natural antioxidant with higher antioxidant effects than VC, VE, tea polyphenols, as well as chemical antioxidants BHA and BHT. Due to its natural plant origin, it can meet consumers' needs for clean labeling and natural safety. Luo Andong et al. extracted rosemary extract by supercritical CO2 technology [15]. Its effective ingredients (carnosic acid, rosmarinic acid) are high in content, with the functions of antioxidant, moisturizing, freckle removing, and skin repairing. It is an excellent additive in cosmetics.
Supercritical CO2 extraction technology is not only widely used in the cosmetics industry, but also has significant applications in other industries, such as the processing of various natural antibacterial or antioxidant extracts, the extraction of hops, and the extraction of pigments in the food industry; In the pharmaceutical industry, the extraction of artemisinin, a new anti malaria drug, and the extraction of alkaloids.
Prospects for Supercritical CO2 Fluid Extraction Technology
At present, the application of SFE technology is being comprehensively carried out both domestically and internationally. Developed countries such as Japan and the United States are in a leading position, and new research results are constantly emerging in various fields. Foreign SFE equipment has become large-scale, and supercritical chromatography analyzers have also been introduced in Japan. In order to obtain higher purity and high added value products, research on supercritical countercurrent extraction and split flow extraction is increasing.
The above is an introduction to the advantages and disadvantages, application, process and influencing factors of supercritical CO2 fluid extraction technology. As a new chemical separation technology, supercritical CO2 extraction is more expensive than the traditional steam distillation method and organic solvent extraction method in terms of operating cost, although it faces the problem of large one-time investment in equipment in the application process, because it is pure, safe, active and not easy to be decomposed by heat Due to its strong stability and high extraction rate, it has become a high-tech extraction and separation method with considerable development potential in the daily chemical, food, and pharmaceutical industries.

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