The supercritical CO2 extraction process is affected by many factors, including the properties of the extracted substances and the state of the supercritical CO2 fluid. In this article, we summarize 6 important factors that affect the efficiency of the CO2 extraction process. Pretreatment: water content, raw material particle size; Extraction pressure and temperature; Extraction time and CO2 flow.
The pretreatment process of the extracted material
Different CO2 extraction materials require different pretreatment methods. For example, when extracting fresh flavor substances such as ginger, garlic, onion, etc., it only needs to be peeled, sliced , or directly extracted after juice extraction; while frankincense, pistacia Chinensis, cardamom, peppercorns, etc., need to be roasted and crushed.
The main factors affecting the extraction effect in the pretreatment process are the moisture content and particle size of the material.
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, supercritical CO2 fluid will bring the moisture to the vicinity of the outlet of the extraction tank, causing the material to rapidly agglomerate under the action of high pressure, affecting the permeability and the extraction process.
Raw material particle size
The particle size of the raw material 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 amount. After moderate crushing, the contact area between the solid and the solvent and the extraction channel may be increased, so that the supercritical CO2 The fluid diffuses into the raw tissue as soon as possible, thereby increasing the extraction rate.
Too fine particle size will aggravate the thermal effect between the supercritical CO2 fluid and the solid interface and block the mesh. Although the damage to the plant cell wall is more complete at this time, the bulk density of the raw material is increased, and the permeability has deteriorated, so that CO2 only passes through the raw material layer along the line with low resistance, forming many pinholes, making the extraction significantly uneven.
At the same time, it is also possible to quickly form dense hard lumps under the action of pressure, which will affect the yield of essential oils in a light, and in serious cases, the pressure difference between the front and rear of the extraction vessel will increase sharply, making extraction impossible.
What are the correct moisture content and particle size?
Based on our 30+ years of experience working with CO2 extraction, the recommended parameters for material handling are:
- The moisture content is not more than 10%;
- The particle size of the raw material is between 40-60 mesh.
Extraction pressure and temperature
The extraction capacity of supercritical CO2 is proportional to the CO2 density, and the pressure and temperature of the CO2 fluid determine the CO2 density.
That is to say, the pressure and temperature of the CO2 fluid directly affect the CO2 extraction process.
How does CO2 density affect the CO2 extraction process?
- Extraction under high-density CO2 extracts surface wax, pigment, chlorophyll, and other unwanted components.
- Under the condition of CO2 density lower than 0.6g/cm3, it is possible to extract as many 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 does the extraction rate decrease but also the technical difficulty is caused by the increase in the extraction amount of waxes and triglycerides.
CO2 pressure and temperature
When the pressure is high, the density of CO2 is very large, and the compressibility is very small. The sum of the increase in molecular spacing and the weakening of intermolecular forces caused by heating, the acceleration of molecular thermal motion, and the increase in the probability of collision and binding have little effect on the solubility.
When the temperature is low, the increase in solute vapor pressure caused by heating is not enough to compensate for the decrease in the solubility of CO2 fluid, so the overall effect leads to a decrease in the concentration of solute in the supercritical fluid.
For a certain substance to be extracted, there is an optimal extraction temperature that balances the above two contradictions under the optimal pressure condition.
What are the commonly used pressures and temperatures for BIT?
The extraction pressure we usually use in extraction experiments is in the range of 150-250 bar, and the temperature is between 50-60 °C.
Extraction time and CO2 flow
The CO2 flow rate can significantly affect the supercritical extraction kinetics.
Lower CO2 flow rate, the extraction rate is not high;
When the flow rate of CO2 increases, the extraction rate increases accordingly, and the extraction time can be relatively shortened.
When the flow rate of CO2 is too large, the solute and solvent CO2 cannot fully interact, and the improvement of the product extraction rate is restricted, resulting in an increase in CO2 consumption and an increase in production costs.
Therefore, in the determination of the actual extraction process, it is necessary to comprehensively consider and select the appropriate CO2 flow rate through a series of experiments.
Under normal circumstances, when the flow rate is constant, at the beginning of the extraction, because the CO2 fluid and the solute have not reached a good contact, the extraction amount is small, and the extraction amount is small.
When the extraction time is prolonged, the mass transfer reaches a good state, and the extraction amount per unit time increases until it reaches its maximum value, and the corresponding time is the limit time of extraction. After that, the extraction rate gradually decreased due to the decrease in the content of the components to be separated in the extraction object.
Therefore, when determining the extraction time, the relationship between the energy consumption of the equipment and the extraction rate should be comprehensively considered. It is not necessary to choose the limit time and a single extraction method, but the best time and method that can balance the energy consumption of the system economically.
Based on the overview of supercritical CO2 extraction technology, this paper discusses many factors that affect extraction efficiency, such as pretreatment methods, extraction
The temperature, pressure, CO2 flow, and extraction time were systematically reviewed. Hope it can help readers better organize supercritical fluid extraction experiments and obtain the best extraction conditions for materials.