What is the entrainer of supercritical CO2 extraction?
Non-polar CO2 can only effectively extract non-polar lipophilic substances with low molecular weight, and the selectivity is not high. The extracts are mostly mixtures.
In order to improve the extraction capacity and selectivity of CO2 to target solutes, adding an appropriate amount of non-polar or polar solvents as co-solvents (modifiers) is an effective way to broaden the application scope of supercritical CO2 extraction process.
Co-solvents are also called entrainers or modifiers, and we are collectively called entrainers.
The role of entrainers
Can significantly increase the solubility of the separated components in the supercritical fluid;
When a suitable entrainer that plays a specific role with the solute is added, the selectivity (or separation factor) of the solute can be significantly improved.
How many types of entrainers are there for supercritical fluid CO2 extraction?
Entrainers can be divided into two categories:
- One type is miscible supercritical solvents, of which the less content is regarded as an entrainer. For example, the separation of heavy oils often uses two or more C3~C5 light hydrocarbon mixtures, and the synergy between the solvents makes the mixing The solvent is better than a single solvent.
- The other is to add subcritical organic solvents to pure supercritical fluids. Depending on the amount of addition, they may form a single-phase miscible supercritical mixed fluid, or they may be part of the liquid phase entrained by the supercritical fluid. The two-phase mixed solvent, but generally do not want the latter situation.
Entrainers affect the solubility and selectivity of solutes
Entrainers can affect the solubility and selectivity of solutes in supercritical gases from two aspects
- One is the density of the solvent;
- The second is the interaction between the solute and the entrainer molecule.
Density of solvent
In general, the addition of a small amount of entrainer has little effect on the density of the solvent gas, and the decisive factor that affects the solubility and selectivity is the van der Waals force between the entrainer and the solute or the entrainer and the solute have a specific intermolecular relationship. Functions, such as the formation of hydrogen bonds and various other chemical forces.
In addition, after adding the entrainer, the critical point of the mixed solvent will change accordingly.
Interaction between solute and entrainer(cosolvent) molecules
Part of the solvent is often used as an entrainer of CO2 to be studied.
They can form a homogeneous miscible state with CO2 under moderate pressure and room temperature conditions.
When polar solvents with high solubility parameters such as methanol, ethanol, acetone, etc. are used as entrainers and added to CO2, not only the continuous adjustment of the solubility parameter δ of the fluid can be maintained, but the δ value of the mixed fluid is also improved;
Moreover, a certain special molecular force (such as Lewis acid-base force, hydrogen bonding force, associative force, etc.) may be formed between the polar solute and the polar co-solvent, thereby enhancing the solubility and selectivity of the solute.
What are the the use method and selection criteria of entrainer (cosolvent)?
There are two ways to add the entrainer (cosolvent).
The entrainer is directly added to the raw material, and then the supercritical fluid (such as CO2) is passed in and kept in contact for a certain time under constant temperature and pressure conditions, which is called the static method.
Studies have shown that when the binding effect of the raw material matrix on the solute is the first influencing factor, the static method is effective, and the infiltration of the entrainer on the raw material is conducive to the release of the components to be extracted;
The supercritical fluid and the entrainer are mixed in a certain proportion through a high-pressure metering pump and flow into the extraction device at an appropriate flow rate to contact the raw materials, which is called a dynamic method.
When the solubility of the components to be extracted in the supercritical fluid is the main influencing factor, the continuous addition method according to the dynamic method will produce a better extraction effect than the static method.
What should be considered when choosing an entrainer?
The choice of entrainer should consider three aspects:
- In the extraction section, the interaction between the entrainer and the solute is required to improve the solubility and selectivity of the solute;
- In the solvent separation section, the entrainer and the supercritical solvent should be easily separated, and the entrainer and the target product should also be easily separated;
- It should be considered that the residue of the entrainer will not cause product contamination. The specific selection can refer to the selection method of the extractant in the solution extraction process, or consider the solubility parameter, the Lewis acid-base dissociation constant, the change of the absorption spectrum after the action of the entrainer and the solute, etc.
Supercritical carbon dioxide extraction process using entrainer
The following figure shows a schematic diagram of a general pilot-scale supercritical extraction process flow, which includes a CO2 pressurization system, a CO2 circulation system, an entrainer system, 2 extractors and 1 separator.
It can be seen that in the study of the supercritical CO2 extraction process using the entrainer, the equipment required for the supercritical fluid extraction with CO2 as a single medium and the process parameters required to be studied are all necessary in the supercritical extraction using the entrainer. , The difference is that the device adds an entrainer addition system. When optimizing operating parameters, it is also necessary to investigate the type of entrainer, the method of addition, the amount of addition and the speed of adding the entrainer, etc., for the extraction rate (or recovery rate) and selectivity of the target substance. Influence.
The following takes the extraction of active ingredients from the Chinese herbal medicine Andrographis paniculata as an example to illustrate the advantages of supercritical CO2 extraction with entrainers. Studies have shown that the active ingredients in Andrographis paniculata are andrographolide and dehydroandrographolide, both of which are diterpene lactones.
But andrographolide and dehydroandrographolide are unstable when exposed to heat, and are easily destroyed in the traditional alcohol extraction process.
At present, the new supercritical CO2 extraction process developed in China solves the problems of thermal decomposition and long production cycle of the active ingredients during the traditional process of extraction and baking paste.
- The extraction rate of supercritical pure CO2 extraction is the lowest, which is only 1/10 of other extraction methods;
- When 95% ethanol is used as the entrainer and the operation is carried out according to the optimized process conditions (extraction pressure 25MPa, extraction temperature 40°C, extraction time 4h, and entrainer dosage is 1/2 of the raw material), the yield of Andrographis paniculata extract (8.3%), andrographolide content (19.79%), dehydrated andrographolide content (12.27%) are much higher than traditional extraction methods, and the product quality is stable, the efficacy is high, and all indicators are better than the original solvents used Law.
- Ultrasonic ethanol extraction method and ethanol cold extraction are fast and good, but the solvent processing volume is large, and the solvent loss is large.
The above example shows that the SCCO2 extraction process using the entrainer can broaden the application range of supercritical extraction, especially when the solubility of the extracted components in the supercritical solvent is small or highly selective extraction is required, the application of the entrainer is very effective. .
The SFE process using entrainer not only improves the solvent’s ability to extract solutes and reduces the amount of solvent required, but also greatly reduces the required pressure.
But the disadvantage is that the entrainer complicates the separation of solute and solvent, and requires special recovery systems, such as the use of evaporation and rectification units, which is far less simple than the process of using a single SCF.
It is necessary to consider the advantages and disadvantages to decide whether to use an entrainer for extraction.
Supercritical fluid extraction (SFE) is the process of separating one component (the extractant) from another (the matrix) using supercritical fluids as the extracting solvent.
Supercritical CO2 extraction (SCFE) is used particularly in the food, beverage, cosmetics and pharmaceutical industry for extracting natural substances, aromas, fats, oils, waxes, polymers, enzymes and colourants in their supercritical physical state.
CO2 is a natural and environmentally-friendly solvent which has advantages over synthetic and harmful media such as n-hexane when it comes to sustainability.
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