What is supercritical carbon dioxide extraction separation technology? What is its basic principle?

- Sep 08, 2019-

● Supercritical fluid extraction, also known as supercritical extraction, pressure fluid extraction, supercritical gas extraction. It uses a high-pressure, high-density supercritical fluid as a solvent to extract the required components from liquid or solid, and then uses solvent, extracted and extracted by means of temperature rise, pressure reduction or both and absorption (adsorption). Component separation.

 

● As early as 1897, the concept of supercritical extraction was recognized. At that time, the compressed gas in the supercritical state was found to have a special dissolution effect on the solid. For example, at a temperature higher than the critical point, the metal halide can be dissolved in ethanol or carbon tetrachloride, and can be precipitated when the pressure is lowered. But it was not until the 1960s that research on its industrial applications began. At present, supercritical carbon dioxide extraction has become a new type of extraction and separation technology, which is widely used in the production of food, medicine, chemical, energy, flavor and fragrance industries.

supercritical carbon dioxide extraction

 

● 1. Principle of supercritical extraction

When the temperature and pressure of the liquid is at its critical state.

Figure 1 is a typical pressure-temperature diagram of a pure fluid. In the figure, AT represents the sublimation curve of the gas-solid equilibrium, BT represents the melting curve of the liquid-solid equilibrium, CT represents the vapor pressure curve of the saturated liquid of the gas-liquid equilibrium, and the point T is the coexistence of the gas-liquid-solid three phase. Phase point. According to the phase rate, when the gas-liquid-solid phase of the pure substance coexists, the degree of freedom in determining the state of the system is zero, that is, each pure substance has its own determined triple point. The pure substance is heated along the gas-liquid saturation line. When the C in the figure is reached, the gas-liquid interface disappears, the properties of the system become uniform, and are no longer divided into gas and liquid, and the point C is called the critical point. The critical temperature and pressure corresponding to this point are referred to as critical temperature T0 and critical pressure P0, respectively. The shaded areas above the critical temperature and critical pressure in the figure belong to the supercritical fluid state.

 

In this state, it is neither completely the same as the general gas phase nor liquid phase, so it is called a supercritical fluid. The supercritical fluid has the characteristics of gas and liquid phase. It has high permeability and low viscosity comparable to gas, and has similar density to liquid and excellent solubility to substances. This solvency can be continuously changed as the system parameters change, so that the solubility of the components and the selectivity of the extraction can be conveniently adjusted by changing the temperature and pressure of the system. Utilizing the above characteristics, the supercritical carbon dioxide extraction technology is mainly divided into two major types of principle processes, namely, a constant temperature pressure reduction process and a constant pressure temperature rising process. The former extract phase is depressurized and the latter extract phase is heated. Both of the supercritical fluids lose their ability to dissolve the solute, and the purpose of separating the solute to recover the solvent is achieved. The solvent is recycled after being pressurized or cooled. Solvents suitable for supercritical fluid extraction are ethylene, carbon dioxide, ethane, propane, ammonia, n-heptane, toluene, etc., and industrial carbon dioxide is a commonly used solvent. The critical temperature is 31.5 ° C and the critical pressure is 7.38 MPa. As a supercritical extraction fluid, it has many unique features, such as the critical point is easy to reach, generally does not react with the extract, colorless, odorless, non-toxic, odorless, safe to use, non-flammable, easy to remove, It is easy to recycle, low in price, no pollution to the environment, and has antibacterial effect. Therefore, it is widely used in light industry, food, medicine and other fields.

 

1.1 Supercritical fluid pressure-density relationship

Figure 2 plots the comparative pressure of carbon dioxide versus the contrast density. The shaded portion of the figure is the actual operating area of the supercritical extraction. It can be seen that in the region slightly above the critical point temperature, a small change in pressure will cause a large change in density. By utilizing this property, it is possible to extract and separate the desired components under high-density conditions, and then to separate the components extracted by the solvent by slightly raising or lowering the pressure.

Figure 2 pure carbon dioxide contrast pressure-contrast density curve

 Supercritical fluid pressure-density relationship

1.2 Basic properties of supercritical fluids

Density, viscosity and self-diffusion coefficient are the three basic properties of supercritical fluids. Table 1 compares these three basic properties of supercritical fluids and gases and liquids at normal temperature and pressure. It can be seen that the density of the supercritical fluid is close to that of the liquid, the viscosity is close to that of the gas, and the self-diffusion coefficient is between the gas and the liquid, which is about 100 times larger than the liquid, which means that the supercritical fluid has a similarity to the liquid solvent. The solvency and simultaneous mass transfer rate during supercritical extraction will be much greater than the solvent extraction rate at the same stage and will soon reach the extraction equilibrium.

 

1.3 The solubility properties of supercritical fluids

The solubility properties of supercritical fluids can be closely related to their density. Generally, there is a relationship between the solubility of a substance in a supercritical fluid and the density of a supercritical fluid, that is, lnC=klnp+m

Where k is a positive number, ie the solubility of a substance in a supercritical fluid increases as the density of the supercritical fluid increases. The solubility of different materials in supercritical carbon dioxide is shown in FIG.

Figure 3. Dissolved density of different substances in carbon dioxide

 Dissolved density of different substances in carbon dioxide

1.4 Typical Process of Supercritical Extraction

The supercritical extraction process consists mainly of two phases: the extraction phase and the separation phase. In the extraction phase, the supercritical fluid extracts the desired composition from the forgiveness; in the separation phase, by changing a parameter, the extracted component is separated from the supercritical fluid set and the extractant is cycled. According to the different separation methods, the supercritical extraction process can be divided into three categories, namely isothermal transformation process, etc.

The pressure variable temperature process and the isothermal isobaric adsorption process are as follows.

 

1.4.1 Constant temperature depressurization process 

The difference in supercritical fluid extraction ability under different pressures is used to separate the solute from the supercritical fluid by changing the pressure. By isothermal is meant that the temperature of the fluid in the extractor and separator is substantially the same. This is the most convenient one, as shown in Figure 4. First, the extractant is compressed

The machine reaches the supercritical state, and then the supercritical fluid enters the extractor and is mixed with the raw material for supercritical extraction. The supercritical fluid extracted by the solute is subjected to pressure drop after the pressure reducing valve, the density is lowered, the solubility is decreased, and the solute and the solvent are separated. Separated in the device. The extractant is then brought to a supercritical state by compression and the above extraction-separation step is repeated until a predetermined extraction rate is reached.

 isotherm pressure diagram

1-extractant 2-expansion valve 3-separation tank 4-compressor

Figure 4 isotherm pressure diagram

1.4.2 The constant pressure heating process

Use the difference in solubility of the substance in the supercritical fluid at different temperatures, and the solute is separated from the supercritical fluid by changing the temperature. By isobarty is meant that the pressure of the fluid in the extractor and separator is substantially the same. As shown in Fig. 5, the solute-extracted supercritical fluid is heated and heated to separate the solute from the solvent, and the solute is taken out under the separator, and the extractant is recycled after being compressed and tempered.

 Isobaric temperature rise diagram

Fig. 5 Isobaric temperature rise diagram


1.4.3 The isothermal isobaric adsorption process

 Place an adsorbent that adsorbs only the solute and does not adsorb the extractant in the separator. The solute is separated from the extractant by being adsorbed in the separator, and the extractant is recycled after being compressed, as shown in Fig. 6. Shown.

 Isothermal isobaric adsorption diagram

1-extractor 2-absorbent 3-separation tank 4-pump

Fig. 6 Isothermal isobaric adsorption diagram


1.5 Characteristics of supercritical extraction

As mentioned above, supercritical extraction has outstanding advantages in terms of dissolving power, transfer performance and solvent recovery, mainly in the following aspects.


1.5.1 Since the density of the supercritical fluid is close to that of the liquid, the supercritical fluid has the same solubility as the liquid solvent, and at the same time it maintains the transfer characteristics of the gas, thereby having a higher mass transfer rate than the liquid solvent extraction. , can achieve the extraction balance faster.

1.5.2 Because near the critical point, small changes in pressure and temperature will cause supercritical fluid tightness

Degree change, which causes a change in its solubility, so the solute and solvent are easily separated and can be extracted after extraction.

1.5.3 The supercritical extraction process has the dual characteristics of extraction and rectification, and it is possible to separate some difficult to separate substances.

1.5.4 Because supercritical extraction generally uses substances that are chemically stable, non-toxic and non-corrosive, and whose critical temperature is neither too high nor too low, as an extractant, it will not cause pollution of the extract, and can be used in medicine, food and other industries. It is especially suitable for the separation or purification of heat sensitive and oxidizable substances.


The disadvantage of supercritical extraction is that the equipment and operation are carried out under high pressure, and the one-time investment of the equipment is relatively high. In addition, the research on supercritical fluid extraction started late. At present, the research on the thermodynamics and mass transfer process of supercritical extraction is far less mature than the traditional separation technology, which needs further research.

 

● 2. Process of supercritical carbon dioxide extraction technology

2.1 Process

The CO2 released from the cylinder is liquefied through the gas purifier into the liquid tank (generally the liquefaction temperature is about 0 to 5 °C, and it is cooled by Freon); then it is pumped into the extraction tank by the liquid pump through preheating and purifier, and decompressed. Afterwards, due to the decrease in CO2 solubility, the extract and CO2 separation extract are discharged from the bottom of the separation tank, and the CO2 is discharged from the upper part of the separation tank through the purifier into the liquefaction tank.

 

2.2 Process Flow Chart


 Process flow chart of supercritical carbon dioxide extraction

Figure 7 Process flow chart of supercritical carbon dioxide extraction


● 3. Characteristics of supercritical carbon dioxide extraction technology

Supercritical carbon dioxide extraction technology has the following main features compared to commonly used unit operations such as distillation, extraction, and absorption:

 

3.1 High extraction yield and good product quality

    The density of supercritical carbon dioxide is close to that of gas, much smaller than liquid, and its diffusion coefficient is about 100 times larger than that of liquid. Supercritical fluids are superior in properties and performance compared to liquids. Therefore, the supercritical carbon dioxide extraction has a shorter phase equilibrium time than the usual liquid-liquid extraction, and the extraction rate is high, and the purity of the product can be improved.

 

3.2 Suitable for separating substances containing heat sensitive components and physiologically active substances

    The separation of a substance containing a heat sensitive component or a physiologically active substance by a general distillation method easily causes separation, polymerization, and even coking of the heat sensitive substance, and destroys the physiological activity of the substance. Although a vacuum distillation method can be employed, the reduction in temperature by pressure reduction is limited, and there is still a great limitation for separating high-boiling heat-sensitive substances. The supercritical carbon dioxide extraction process, although the pressure is relatively high, generally 20 MPa to 50 MPa, but can be operated at a lower temperature, generally slightly higher than its critical temperature of 31.5 ° C, not only does not destroy the molecular structure, but also maintain color The fragrance and taste are not degraded. For example, the egg yolk lecithin widely used in the medicine and health food industries can achieve satisfactory results by using 55 ° C and 38 MPa, which not only maintains the biological activity of lecithin, but also improves the extraction yield and product purity.

 

3.3 Saving energy

    In the supercritical extraction process, including extraction and separation, there is often no phase change process. Even if some processes have a phase change process, the phase change heat is small near the critical point. In the usual distillation operation, a large amount of heat energy must be supplied to the distillation column, and only a small portion of the heat supplied can be utilized, and most of it is carried away by the condenser of the overhead condenser. If liquid condensation is used, the separation and concentration of the bulk and solvent are often distilled or evaporated, which also consumes a large amount of heat. In contrast, the energy saving effect of supercritical extraction is significant.

 

3.4 Protecting the environment

In recent years, research on supercritical extraction technology has received increasing attention. As people's awareness of protection increases, government agencies in some countries have imposed strict restrictions on waste discharged from production. Encourage people to explore ways to avoid or reduce environmental pollution, and supercritical carbon dioxide extraction technology has the potential to reduce environmental pollution to a certain extent, and become a way to protect the environment and treat three wastes.

 

● 4. Application fields of supercritical carbon dioxide extraction technology

4.1 The perfume industry is mainly used for the extraction of natural flavors and the purification of synthetic perfumes. Essential oils in plants are unstable and susceptible to heat deterioration or volatilization, so supercritical carbon dioxide extraction with low operating temperatures is an ideal alternative to traditional steam distillation and organic solvent extraction. Moreover, essential oils have a high solubility in supercritical carbon dioxide fluids and are completely compatible with liquid carbon dioxide. Therefore, the supercritical carbon dioxide extraction of essential oils can be almost quantitative. Essential oils that can be extracted at present, such as crocodile oil, night primrose oil, rose oil, mastic oil, pepper oil, clove oil, etc., have higher purity and higher extraction rate than conventional solvents.

 

4.2 Pharmaceutical industry The pharmaceutical industry is widely involved in the extraction of pharmaceutical ingredients from plants and animals. The analysis of medicinal ingredients and the concentration and refining of crude products, especially the recent emergence of new biochemical drugs, the purification, drying, granulation, and sustained release pills have provided new topics for the chemical industry. Therefore, the characteristics, research and application of supercritical carbon dioxide extraction are very active. Supercritical carbon dioxide extraction technology is the most effective way to obtain vitamin E. The obtained product is non-toxic, has no organic solvent residue, and its biological activity is 2 to 3 times that of the synthesis method. In addition, the supercritical carbon dioxide extraction technology has also achieved remarkable effects in the processes of concentration, purification, and solvent removal of drugs such as antibiotics.

 

4.3 Food industry Almost all oils can be extracted by supercritical carbon dioxide extraction technology. For example, corn germ oil, colza oil, natural food color red pepper, ginger oil, lecithin, etc., which are highly praised by the international nutrition community. Continuous high-pressure extraction of rapeseed oil with a mixture of supercritical carbon dioxide and propane provides excellent refining and erucic acid. Supercritical carbon dioxide extraction technology extracts allicin, which is used in food additives, antibacterial injections, cosmetics, etc., and finds a value-added application method for garlic produced in Shandong and other regions.



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