Extension of supercritical carbon dioxide extraction technology
In order to further improve the extraction ability and selectivity of target solutes by CO2, researchers have studied various ways to expand the supercritical CO2 extraction technology, so that the application range of the supercritical extraction technology has been expanded.
Use physical fields (ultrasonic, high-voltage pulsed electric fields, etc.) to enhance supercritical extraction
The extraction effect of transitional supercritical CO2 extraction has been significantly improved by physical field plus energy enhancement technology, such as Qiu Taiqiu and so on.
Combination of SFE technology with other separation and purification technologies
At present, the separation and purification technologies combined with SFE technology include: rectification, molecular distillation, adsorption, membrane separation, and crystallization.
① The combination of SFE technology and rectification technology 2 is the continuous process with sideline extraction in the separation process.Extractive distillation column, using the different relative volatility of components in the liquid phase, so that gas-liquid mixtures can be vaporized and condensed multiple times, and high-purity volatile components can be obtained in the gas phase. Volatile components can be selectively separated or concentrated, thereby improving the purity of the target product.
② The combination of SFE technology and molecular distillation technology 31 means that the separation process uses molecular distillation technology.Due to the low distillation pressure, short heating time, and high degree of separation, this technology is particularly suitable for high boiling point, thermal sensitivity, and easy oxide-based separations.
③The combination of SFE technology and adsorption technologyTarget ingredients or impurities to achieve the purpose of improving the purity of the target product.This technology is mostly used for small amounts of impuritiesGet rid of.
④ The combination of SFE technology and membrane separation technology: 3) That is, the separation process uses membrane separation technology (such as ultra-Filtration, nanofiltration, reverse osmosis, etc.) to selectively separate or concentrate supercritical extracts and improve the purity of the target product.
⑤Combination of SFE technology and crystallization technology 3,37 The supercritical extract is further purified by crystallization technology to obtain high purity products
Supercritical CO2 extraction with complexing agent
For example, supercritical extraction is applied to spent fuel aftertreatment and nuclear waste treatment (U and lanthanides and actinides, such as Pu, etc.), using metal ions first with organic complexing agents such as tributyl phosphate, TBP- (HNO) complexes to form neutral metal complexes (or chelates) that are easily soluble in supercritical fluids, thereby enabling supercritical CO2 to extract metal ions
Supercritical CO2 microemulsion system for extraction of polar compounds with large molecular weight
For polar compounds, such as proteins and sugars, which are still difficult to extract with the entrainer, supercritical microemulsion extraction can be used, that is, using appropriate surfactants to form supercritical fluid-in-water microemulsion systems. For example, Johnston et al. Found that perfluoropolyether ammonium carbonate can form a good microemulsion in supercritical CO2, and greatly increase the solubility of water in the system.The system can dissolve bovine serum protein with a molecular weight of 67,000 Opens up a new way to extract high molecular weight, poorly soluble drugs in supercritical CO2
Obviously, the above-mentioned multiple ways to strengthen and expand the supercritical CO2 extraction technology can expand the application range of SFE technology, and their specific application examples will be further explained in another article. Here, only the four typical processes (see the figure below) for the first industrialized supercritical CO2 removal of caffeine from coffee beans are introduced, indicating that supercritical extraction can be skillfully combined and integrated with a variety of chemical separation technologies to achieve Efficient green extraction process
Figure 37 (a) shows the semi-continuous operation process. The unroasted green coffee beans are impregnated with a certain amount of water in advance, and then placed in a high-pressure container and passed through 70 ~ 90 ° C and 16 ~ 22MPa CO2 for extraction, and the CO2 is recycled. The coffee beans must be moist because the caffeine in the coffee beans and the chlorogenic acid in the coffee beans exist in a chemically connected manner. The solubility in water is several orders of magnitude smaller; the addition of water can free caffeine, diffuse into the supercritical fluid phase, and dissolve in supercritical CO2. The caffeine-rich CO then enters the water washing tower and is rinsed with water. The caffeine is absorbed by water and separated from CO2. Some scholars have measured the distribution coefficient of caffeine in CO2 and water at 80 ° C and 31.0MPa (0.03 ~ 0.04 Mass basis), this partition coefficient is very small, which is disadvantageous when extracting caffeine with CO2, but it is very convenient to elute caffeine in CO2 with water. This is the basis for the washing process. This water is degassed and enters a distillation column to recover caffeine. CO2 from the cleaning deviceThe top escapes and is recycled. The literature shows that after the above processing of raw coffee beans, the caffeine in the coffee beans can be reduced to less than 0.02%, and the aroma is not lost.
Figure 3-7 Four processes for extracting caffeine from coffee using supercritical CO2
The extraction conditions in Figure 87 (b) are the same as those in Figure 317 (), except that the water washing tower was replaced with an activated carbon adsorber.The CO2 + H2O + caffeine mixture discharged from the top of the extractor enters the top of the adsorber. When passing through the activated carbon bed, caffeine is adsorbed by the activated carbon, and CO2 returns to the extractor. The activated carbon adsorbing caffeine is periodically discharged, and then managed to separate the caffeine from the activated carbon.
Figure 7 (C) is a mixture of green coffee beans and activated carbon with water, and then loaded into the extractor.Under the conditions of 90 ℃ and 2NXPa, the treatment of CO2 for 5h can reduce the caffeine content in coffee beans to 0.08%. Then, discharge all solid materials from the extractor, separate the activated carbon from the coffee beans with a vibrating screen, and then try to separate the caffeine from the activated carbon
Figure 3-7 (d) is taken from patent 42 of Katz et al.
All issues are considered together to make it an advanced process. Pre-soaked so that the green coffee beans contain a certain amount of water, and then added to the caffeine extraction tower 2 through the line, and decaffeinated with SCCO2 from the CO2 gas source 1. The supercritical CO2 fluid with caffeine leaves from the upper part of the extraction tower, enters the bottom of the water spray tower 3, and sprays water from the upper part of the tower to absorb the caffeine; the caffeine-rich water flows from the bottom of the tower After entering the reverse osmosis device 4, the concentrated aqueous solution is discharged from the reverse osmosis device 4. The water separated from the reverse osmosis unit is combined with fresh make-up water and returned to the upper part of the water spray tower. The CO2 discharged from the top of the water spray tower is combined with the CO2 from the CO2 gas source and circulated into the caffeine extraction tower through a pipeline. In this process, solid materials enter the extraction column intermittently and come into contact with a continuous gas stream. In the water spray tower, the liquid and supercritical fluid are in continuous contact in countercurrent. The so-called semi-continuous process refers to the intermittent addition of coffee beans to the extraction towerTo go, but during the feeding process, CO2 is circulated and continuously flowed, the feeding is performed under a pressure load, and the decaffeination process is also realized under continuous conditions
From the above discussion, it can be seen that even the same product can have different design paths for supercritical extraction processes.The above process design reflects the reasonable combination and integration of supercritical extraction and multiple chemical separation technologies, which not only improves The process flow improves product quality, reduces costs, and makes the process more environmentally friendly.
It is also worth noting that in order to further broaden the application range of supercritical extraction, in recent years, some scholars have also used other solvents, such as low-molecular hydrocarbon solvents, water, etc. as the extraction medium to conduct research on supercritical extraction. Among them, subcritical water has been successfully applied in the extraction of organic pollutants in environmental samples, extraction of active ingredients in natural products, and pre-analytical processing4. It is still the most widely used and important extractant in supercritical extraction.