Non-polar CO2 can only effectively extract non-polar lipophilic substances of starch derivatives, and selectivity. In order to improve the extraction ability and selectivity of target solutes by CO2, add an appropriate amount of non-polar or polar solvents to utilize co-solvents.
Is an effective way to extend the application range of wide supercritical CO2 extraction technology. Co-solvents are also called entrainers or modifiers.
The role of co-solvent
- The role of the entrainer has two main points:
- First, it can significantly increase the solubility of the separated components in supercritical fluids;
Secondly, the selectivity (or separation factor) of the solute can be significantly improved when an appropriate entrainer that plays a specific role with the solute is added.
Co-solvent can be divided into two categories:
One type is miscible supercritical solvents, where the smaller content is considered as an entrainer.For example, the separation of heavy oils often uses two or more C3 ~ C3 light hydrocarbon mixtures.The synergy between the solvents makes the mixing Solvents are better than single solvents.
The other type 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 entrained by the supercritical fluid. A mixed solvent of two phases, but the latter case is generally undesirable.
Co-solvent can affect the solubility and selectivity of solutes in supercritical gases in two ways:
- One is the density of the solvent;
- The second is the interaction between solute and entrainer molecules.
In general, the addition of a small amount of entrainer has little effect on the density of the solvent gas, and the determining factor affecting solubility and selectivity is the Van der Waals force between the entrainer and the solute molecules or the specific intermolecular between the entrainer and solute Role, such as the formation of hydrogen bonds and other various chemical forces. In addition, after adding an entrainer, the critical point of the mixed solvent will change accordingly.
Some commonly used solvents are often used as co-solvent of CO2 for research. They can form a homogeneous miscible state with CO2 under moderate pressure and room temperature conditions.
For example, when polar solvents with high solubility parameters, such as methanol, ethanol, and acetone, are used as co-solvent and added to CO2, not only the continuous tunability of the fluid solubility parameter δ can be maintained, but also the δ value of the mixed fluid is improved; Because a special molecular force (such as Lewis acid-base force, hydrogen bonding force, association force, etc.) may be formed between the polar solute and the polar co-solvent, the solubility and selectivity of the solute are enhanced.
Use and selection criteria of entrainer
There are two ways to add an entrainer:
- The first is to directly add the entrainer to the raw material, and then pass the supercritical fluid (such as CO2) and maintain a certain contact time under constant temperature and pressure conditions, which is called the static method
- The other is that the supercritical fluid and the entrainer are mixed by a high-pressure metering pump at a certain ratio and flow into the extraction device at a suitable flow rate to contact the raw materials.
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. The infiltration of the raw material by the entrainer is beneficial to the release of the component to be extracted; and when the solubility of the component to be extracted in the supercritical fluid is The main influencing factor is that the continuous method of dynamic method will produce better extraction effect than the static method. The combination of the two is also common in the literature.
The choice of entrainer should consider three aspects:
- First, in the extraction section, the interaction between the entrainer and the solute is required to improve the solubility and selectivity of the solute;
- Second, in the solvent separation section, the entrainer and the supercritical solvent should be able to be separated easily, and the entrainer and the target product should also be easily separated;
- The third is to consider that the residue of the entrainer will not cause product pollution. The specific selection can refer to the selection method of the extractant during the solution extraction, or consider from the aspects of solubility parameters, Lewis acid-base dissociation constant, and changes in the absorption spectrum after the action of the entrainer with the solute