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The most promising extraction technology - supercritical carbon dioxide fluid extraction

Introduction: With the development of science and technology, supercritical fluid technology as a common technology is gradually infiltrating high-tech fields such as materials, biotechnology, and environmental pollution control, and is considered to be a "green, sustainable development technology." Supercritical carbon dioxide extraction technology is the most mature technology used since the research and development of supercritical fluid technology, and it is also the most widely studied technique in various fields. Carbon dioxide is one of the most suitable fluids for supercritical fluid extraction and is used in food, medicine, petroleum, and environmental protection industries.


(I) Definition of supercritical fluid

There are three phases in any one substance - gas phase, liquid phase, and solid phase. The point at which three phases are in equilibrium is called the triple point. The point where the liquid and gas phases are in equilibrium is called the critical point. The temperature and pressure at the critical point are called critical pressures. Different materials require different pressures and temperatures at their critical points. When the fluid's temperature and pressure are above its critical temperature and critical pressure, the fluid is said to be in a supercritical state. At this point, a slight pressurization of the gas in this state will not liquefy the gas, but the density of the supercritical fluid will increase significantly. It is almost comparable to the liquid and has liquid-like properties while preserving the performance of the gas but exhibiting several The special nature of this supercritical state calls itself the fourth state of matter.

(II) The basic principle of supercritical carbon dioxide fluid extraction

The supercritical fluid extraction separation process utilizes the relationship between the dissolving energy of a supercritical fluid and its density, ie, the effect of pressure and temperature on the dissolving power of the supercritical fluid. When the gas is in a supercritical state, the property becomes a liquid. The single phase between the gas and the gas has a density close to that of the liquid. The viscosity is higher than the gas but significantly lower than that of the liquid. The diffusion coefficient is 10-100 times that of the liquid and therefore has good permeability and strong dissolution of the material. Ability to extract certain ingredients from the material.

In the supercritical state, CO2 fluids have the dual characteristics of gas-liquid two-phase, which not only has a high diffusion coefficient and low viscosity comparable to that of gases, but also has a density and a good dissolving power that are close to the liquid. The physical and chemical properties of supercritical carbon dioxide fluids are very different from those of non-critical fluids and gases. Since the density is the dissolving power, the viscosity is the resistance of the fluid, and the diffusion coefficient is the main parameter of the mass transfer rate. Therefore, the special properties of the supercritical carbon dioxide fluid determine a series of important features of the supercritical carbon dioxide fluid extraction technology. The supercritical carbon dioxide fluid has a viscosity of one-hundredth of that of a liquid and has a self-diffusion coefficient one hundred times that of a liquid. Therefore, it has good mass transfer characteristics and can greatly reduce the time required for phase equilibrium, and is an ideal medium for efficient mass transfer. The rate of dissolved solutes much faster than liquids has a much greater dissolving and carrying capacity for solid materials than gases; has unusually large compressibility, and near the critical point, small changes in pressure and temperature cause CO2 fluids The density has undergone great changes, and its solvency can be regulated by simply changing the pressure and temperature of the CO2 fluid, improving the selectivity of the extraction. CO2 fluids and dissolved products can be separated by reducing the pressure of the system, eliminating the need to eliminate solvents.

(III) The unique advantages of carbon dioxide as a solvent for supercritical fluid extraction

1. Strong extraction ability and high extraction rate.

2. The size of the extraction capacity depends on the density of the fluid and ultimately depends on the temperature and pressure. Changing one of them or simultaneously changing them can change the solubility. The separation of multiple substances can be selectively performed to reduce the impurities and make the active ingredients. Highly enriched for easy quality control.

3. The critical temperature of supercritical CO2 fluid is low, the operating temperature is low, and the effective components can be well preserved without being destroyed, and secondary growth does not occur. Therefore, it is particularly suitable for the extraction of components that are sensitive to heat and easily oxidatively decomposed and destroyed. .

4. The extraction time is fast and the cycle is short. At the same time, it does not require steps such as concentration. Even if the entrainer is added, it can be removed by the separation function or simply concentrated.

5. Supercritical CO2 fluid also has anti-oxidation, sterilization and other effects, which helps to ensure and improve product quality.

6. The operating parameters of the supercritical CO2 fluid extraction process are easy to control. Therefore, the active ingredients and product quality are stable, and the process flow is simple and easy to operate. Save labor and large amounts of organic solvents and reduce pollution.

7. CO2 is cheap and easy to obtain, and it is economical compared with organic solvents.


(IV) Factors Affecting the Solubility of Supercritical Carbon Dioxide Fluids

a) Effect of pressure

The pressure is SC-CO: one of the most important parameters of the extraction process. When the extraction temperature is constant, the pressure increases and the density of the liquid increases. Near the critical pressure, small changes in pressure will cause a sharp change in density, and an increase in density will cause an increase in solubility. By adjusting the pressure and temperature, the solubility of the supercritical fluid can be controlled. This is a unique advantage of supercritical extraction.

b) Effect of extraction temperature

The extraction temperature is SC-CO: another important factor in the extraction process. There are two favorable and unfavorable trends in the influence of temperature on the solubility of supercritical fluids. On the one hand, as the temperature increases, the density of the supercritical fluid decreases, and its solubility decreases accordingly, resulting in a decrease in the amount of extraction; on the other hand, the temperature increases. The volatility of the extracted solute increases, which increases the concentration of the extract in the supercritical gas phase, thereby increasing the amount of extraction. So the solubility-temperature curve usually has the lowest point.

c) Effect of entrainer

According to the principle of “similar dissolution”, the solubility of CO to non-polar substances is relatively large, and the solubility of substances with strong polar groups such as OH and COOH is small. The addition of a certain amount of polar component (ie, entrainer) to SC-CO can significantly change the polarity of the supercritical system and better solve this problem. For example, Baysal et al. use SC-CO to extract carotene and lycopene from tomatoes when the entrainer ethanol mass fraction is 5%. The highest amount of extraction. Andrew et al. calculated that when using SC-CO to extract fused ring aromatic compounds such as fluorene, phenanthrene, etc., adding appropriate amount of water can effectively improve its solubility in the supercritical system, and confirmed by Sewram et al. However, the use of entrainers brings benefits as well as the separation and recovery of entrainers in the extract.

d) Influence of physical form

The material being extracted may be solid, liquid, or gas. Gas materials are generally adsorbed by the same adsorbent before extraction. A small number of liquid materials can be directly subjected to supercritical carbon dioxide extraction. Most of them still need to be adsorbed by solid adsorbents first.

e) Effect of granularity

The particle size of the raw material also has an important influence on the extraction efficiency. In general, the smaller the raw material particles, the shorter the solute transport path from the raw material to the supercritical fluid, and the greater the contact surface area with the supercritical fluid, the faster and more complete the extraction proceeds. However, the particle size should not be too small. Fine crushing can easily cause plugging of the filter at the outlet of the extractor, causing a huge pressure difference and damaging the equipment.


(V) Extraction method of supercritical carbon dioxide extraction process

Conventional extraction: This process is generally suitable for the extraction of non-polar fat-soluble substances, such as various oils and oils and essential oils, natural flavors and plants containing alcohols, aldehydes, etc.

With entrainment agent: This type of extraction process is to use carbon dioxide as the main solvent, while adding some other solvents to improve the solubility of the target substance to achieve effective extraction.

Extraction of liquid materials: The extraction of liquid materials is different from the extraction of solid materials. The main difference is that the extraction of solid materials is batch type and cannot be continuously fed, while the extraction of liquid materials can be continuously fed. Therefore, the liquid extraction is better than that of solid extraction. The container has a small volume, but the extraction efficiency is high. The materials suitable for liquids include residual oil, lubricating oil, edible oil, and Chinese herbal medicine aqueous solution.

Supercritical jet extraction process: This type of extraction process is generally applied to viscous materials, such as the removal of neutral oil from lecithin feedstocks, using a high pressure jet extraction process.


(VI) Characteristics of Supercritical Fluid Extraction

1. Extraction and separation are combined into one, when the carbon dioxide supercritical fluid saturated with dissolved substances flows through the separator, the CO2 and the extract rapidly separate into two phases (gas-liquid separation) due to the pressure drop, and the materials are immediately separated. In the phase change process, the solvent is not recovered, and the operation is convenient. Not only the extraction efficiency is high, but also the energy consumption is low and the cost is saved.

2. Both pressure and temperature can be used to adjust the extraction process parameters. Near the critical point, small 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 process flow is short and time-consuming is small. No pollution to the environment, the extraction fluid can be recycled, and the production process is truly green.

3. The extraction temperature is low, the critical temperature of CO2 is 31.265°C, and the critical pressure is 7.18MPa, which can effectively prevent the oxidation and emission of heat-sensitive components, completely preserve the biological activity, and can make the high boiling point, low volatility, easy pyrolysis. The material is extracted below its boiling temperature.

4. The critical CO2 fluid is normally gas and non-toxic. After separation from the extracted components, there is absolutely no solvent residue, effectively avoiding the residual solvent toxicity under traditional extraction conditions. It also prevents the extraction process from harming the human body and contaminating the environment. It is 100% natural.

5. The polarity of the supercritical fluid can be changed. A certain diffuser should be passed down. As long as the pressure is changed or a suitable entrainer is added, different polar substances can be extracted.


(VII) Application of supercritical carbon dioxide fluid extraction

1. The characteristics of supercritical CO2 extraction determine its application range is very broad. For example, in the pharmaceutical industry, it can be used for the extraction of active ingredients of Chinese herbal medicines, the purification of heat-sensitive biological products, and the separation of lipid mixtures; in the food industry, the extraction of hops, the extraction of pigments, etc. are in the perfumery industry, natural and synthetic. Refinement of spices; separation of mixtures in the chemical industry. Specific applications can be divided into the following areas:

2. Extraction of bioactive molecules from medicinal plants, alkaloid extraction and separation

3. Lipid lipids from different microorganisms, or for the recovery of lipids, or the removal of sugars and proteins

4. Extraction of anti-cancer substances from various plants, especially paclitaxel obtained from the bark and branches of yew;

5. Vitamins, mainly the extraction of vitamin E;

6. Purification of various active substances (natural or synthetic) to remove unwanted molecules (such as removing pesticides from vegetable extracts) or "slags" to obtain purified products; Or the processing of antioxidant extracts, such as basil, thyme, thyme, garlic, onion, chrysanthemum. Paprika, licorice and fennel seeds.


(VIII) Problems with Supercritical Fluid Extraction Technology

1. Inadequate Phase Equilibrium and Transfer Studies: At present, there are still few physical data on the annual extraction of supercritical fluids, and there is also a lack of basic thermodynamic models that can accurately calculate the supercritical fluid extraction process. Without stable basic data and theory, process design and economic estimates are very difficult, which seriously hampers the development of supercritical fluid extraction processes.

2. High-pressure equipment and pumps: Supercritical fluids need to operate at relatively high pressures, and the investment in compression equipment is relatively large. Technology

Equipment is still relatively weak.

In short, the application of supercritical carbon dioxide fluid extraction technology is very broad. With the gradual deepening of the mechanism and applied research of these physical enhanced extraction, the existing problems will be solved continuously and will certainly promote the further development of the extraction industry and extraction theory. Various methods for strengthening the extraction process of supercritical carbon dioxide will surely have broad application prospects, and will also have an impact on the industrial application of supercritical fluid extraction technology.


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