3 key points of supercritical CO2 extraction.
What are the 3 key points of supercritical CO2 extraction?
What is the optimization raw material processing method for CO2 extraction process?
Raw material pretreatment
The pretreatment of raw materials includes crushing method (crushing, slicing, etc.), particle size, and raw material humidity limitation, etc.;
Different extraction materials require different pretreatment methods.
For example, when extracting fresh flavors such as ginger, garlic, onion, etc., only need to peel, cut or directly squeeze the juice before extraction; while the frankincense pistacia, cardamom, pepper, etc., need to be roasted and crushed. The main factors affecting the extraction effect in the pretreatment process are the water content and particle size of the material.
Influence of Water Content on Supercritical CO2 Extraction
The efficiency of solvent extraction of essential oils from plant tissues is essentially determined by the contact of the extraction solvent with plant cell membrane phospholipids and cytoplasmic bodies, the efficiency of mutual solubility and the mass transfer rate of essential oils through the cell wall through the solvent system.
To improve the extraction rate, the key is to destroy the continuous water film at the interface of cells and organelles, so that the extraction solvent can effectively contact and dissolve with the essential oil components, and form a continuous main mass transfer system with the solvent.
When the water content is too large, the content of polar compounds in the extract will increase, while the solubility of non-polar compounds will decrease;
When there is still a small amount of moisture in the dry powder material, SC-CO2 will bring the moisture to the vicinity of the outlet of the extraction tank, causing the material to agglomerate quickly under high pressure, which will affect the permeability and the extraction process.
The effect of raw material particle size on CO2 extraction process
Although the supercritical CO2 fluid has better mass transfer performance and faster diffusion speed, the control step that transfers the solute in the solid to the supercritical CO2 fluid phase is the diffusion rate of the solute in the solid, which depends on the solute. The size of the diffusion coefficient in the solid and the size of the solid.
Therefore, the particle size of the raw materials has a significant impact on the extraction process and efficiency.
Oleoresin is generally stored in plant cells. If it is not crushed, the resistance of the cell wall will slow down the extraction speed and reduce the extraction volume; after moderate crushing, it may increase the contact area between the solid and the solvent and the extraction channel, making supercritical CO2 Diffusion into the raw material organization as soon as possible, thereby increasing the extraction rate.
Too fine a particle size will aggravate the thermal effect of the supercritical CO2 fluid-solid interface and block the mesh.Although the damage to the plant cell wall is more complete at this time, it increases the bulk density of the raw materials and the permeability becomes worse, causing the CO2 to only flow along. The lines with low resistance pass through the material layer, forming many pinholes, which makes the extraction significantly uneven. At the same time, it is also possible to quickly form micro-dense lumps under pressure, which may affect the yield of essential oils, and at the same time, the pressure difference between the front and back of the extractor will increase sharply, making the extraction impossible.
The optimization raw material processing method: moisture content <5%, particle size between 40 mesh and 80 mesh.
What are the optimal CO2 extraction process parameters?
CO2 extraction process operating parameters mainly include extraction pressure and temperature, extraction time, ratio of solvent to material flow rate or solvent flow rate, etc.;
CO2 density determined by extraction pressure, temperature and both.
The density and dielectric constant of supercritical fluids are large, and the solubility of substances is also large, and they change rapidly with the changes of temperature and pressure. Therefore, the ability to dissolve certain substances is strong and selective, and the solvent and extract are easily separated at room temperature.
What is the optimization extraction pressure for CO2 extraction process?
The dissolution capacity of supercritical CO2 fluid is proportional to the density. Near the critical point, if the pressure changes slightly, its density will have a relatively large change.
Therefore, for many solid or liquid solutes, if the solute and the solvent are not mutually soluble indefinitely, the dissolving ability of the supercritical fluid and the pressure have a significant correlation.
Under different pressures, the range of extracts is different. When extracting low molecular weight essential oil components (aromatic components) under low pressure, as the pressure increases, the range of extractable substances expands, but the two are not linear. When the pressure increases to At a certain level, the dissolving power increases slowly.
At the same time, the pressure is limited by equipment investment, safety and production costs. Therefore, in actual production, the pressure should not be raised unrestrictedly just to increase the yield, and comprehensive indicators such as product resources and overall operating parameters should be considered.
Example of extraction pressure
For the extraction of seasonings and flavors, the parameters can be selected in a relatively wide range. For materials containing only high solubility substances, choosing a low pressure zone of 7.0 MPa to 12.0 MPa is conducive to selective separation.
In the research of these products, the pressure is mostly between 12.0MPa and 25.0MPa.
Terpenoids can achieve high solubility in CO2 at 9.1MPa～12.2MPa, while neutral grease generally needs more than 16.2MPa.
- In the SC-CO2 extraction of cardamom oil, when the pressure is between 10MPa and 60MPa, the yield is relatively stable, but the content of non-volatile components increases with the increase of pressure, and at the same time, the loss of volatile components increases. ;
- When the temperature is the same, the solubility of pepper essential oil in the low-pressure zone increases rapidly with the increase of pressure, and when it reaches a certain value, it tends to balance;
- While the extraction rate of piperine increases with the increase of extraction pressure within the test range, and the high pressure ( ≥30MPa) SC-CO2 can effectively extract various pigments and capsicum in pepper.
- For some strongly polar substances containing -OH, -COOH and benzene hydroxyl groups, higher extraction pressures are also required.
The optimization supercritical CO2 extraction pressure for plant essential oils: 8.0MPa～12.0MPa.
But in our actual application, the optimization extraction pressure range is: 10 MPa-35 MPa.The extraction of astaxanthin is an exception, which requires a pressure above 70 MPa.
What is the optimization temperature for co2 extraction process?
Under constant pressure, the solubility of supercritical fluid may increase, but not decrease. The different isothermal solubility lines intersect at a point, which is called the change point.
Above and below the change point, the solubility will change differently with the change of temperature. This is because:
- As the temperature rises, the thermal motion of molecules accelerates, the probability of collision with each other increases, and the chance of association increases;
- The increase in temperature increases the volatility and diffusion coefficient of the solute;
- As the temperature increases, the CO2 density decreases and the ability to carry substances decreases.
Therefore, the extraction rate depends on which state prevails at this temperature.
- When the pressure is high, the CO2 density is very large, the compressibility is small, the increase in the molecular distance caused by the heating and the weakening of the intermolecular force, and the acceleration of the molecular thermal motion and the increase in the probability of collision have little effect on the solubility.
- When it is low, the increase in vapor pressure of the solute caused by the temperature rise is insufficient to compensate for the decrease in the solubility of the CO2 fluid, so the overall effect leads to a decrease in the solute concentration in the supercritical fluid.
Optimal stress conditions
For a certain substance to be extracted, there is an optimal extraction temperature that balances the above two contradictions under the optimal pressure conditions.
When the temperature rises from 40℃ to 60℃, the main components, especially the volatile components, are significantly reduced due to the high temperature flash volatilization carried by CO2, and the high temperature extraction operation extract contains more water. The high temperature process may increase the volatility of water, but it is not conducive to the extraction of terpenes and oxygenated derivatives. If the extract is a heat-sensitive active ingredient, it is especially important to consider using a lower temperature for extraction.
In the study of extracting allicin, it was found that when the extraction temperature was 45℃, the chromatogram of the extract reflected many small peaks of decomposition products. The chromatograms of the extracts at 25°C and 36°C are similar to the solvent extraction method, and the chromatograms of the extracts did not reflect many small peaks of decomposition products.
Optimum CO2 extraction temperature: 35℃～50℃
Optimal extraction conditions
The research results so far show that the optimal extraction conditions are generally between 8.0MPa～12.0MPa and 35℃～50℃.
As we all know, under higher CO2 density (such as 40°C and pressure higher than 20.0MPa), SC-CO2 exhibits strong solubility and lower selectivity. In fact, the extraction rate increases under high-density CO2 Mainly due to the increase of surface wax and other undesirable ingredients.
Adjusting the solubility and selectivity of supercritical fluids is helpful to overcome technical difficulties such as one-time extraction and entrainment extraction of non-volatile components in separation.
Under the condition of CO2 density lower than 0.6g/cm3, it is possible to extract as much essential oil components as possible, and other non-volatile components except surface wax will not be extracted.
When the CO2 density is too high, such as higher than 0.85g/cm3, not only the extraction rate decreases, but also technical difficulties are caused by the increase in the extraction of waxes and triglycerides.
Separation operating parameters include separation pressure and temperature, phase separation requirements, and solvent recovery and treatment in the process. The supercritical fluid circulation time depends on the solute extraction capacity and separation coefficient of the supercritical fluid.
Separation pressure and separation temperature
After the extraction process, the density of the SCF must be reduced to selectively separate the extract in the separator. To implement this separation, there are generally three adjustment methods, constant pressure heating or constant temperature pressure reduction, or pressure reduction and heating. The optimized operating conditions must be obtained through specific experiments.
When the separation pressure is constant, as the temperature of the separation process increases, the ability of CO2 to carry substances decreases, and it is easy to separate the extracted substances, but the selective separation is poor, and it is not easy to obtain a purer single substance, and the final product is purified. The process is complicated and the loss is large, resulting in a low yield of the final product, and the higher the temperature, the more likely the volatile substances will be lost with CO2, which is also detrimental to the heat-sensitive components.
In order to obtain purer extracts, or products with more volatile components, and to protect heat-sensitive substances, it is necessary to control a more appropriate separation temperature.
As the working pressure of the separation decreases, the density of SC-CO2 changes, so that the extract that has been dissolved in it will be separated due to the decrease in pressure after entering the separation kettle, but as the working pressure decreases, the separation rate is Tend to balance.
The separation pressure is different, the chemical composition of the extract will also have a certain difference. For extracts with poor single-stage separation, two-stage or even multi-stage separation should be considered.
The SC-CO2 extraction and separation of fennel oil uses two-stage separation.Under different separation conditions, the yield of the two products depends on the pressure of the first separator, but the pressure adjustment of the first separator has a certain range. If it is too high, fats, lipids and pigments cannot be deposited in the first separator, which will cause the second separator to contain these three types of substances, thus losing the advantages of fractionation.
Therefore, reasonable adjustment of the process parameters of the separator is the key to achieve the purpose of separating different substances.
How to obtain the optimization operating parameters for supercritical CO2 extraction process
Laboratory feasibility studies usually use methods such as least squares method, single factor test or response surface method to study the influence of various factors on the extraction rate (or recovery rate) and selectivity of the target substance, so as to optimize the appropriate operating parameters.