It is generally believed that the parameters that affect supercritical CO2 extraction mainly include extraction pressure, extraction temperature, extraction time, CO2 flow rate, physical properties of the extraction raw materials (mainly water content and crushing particle size) and entrainers also have an impact on the extraction rate.
In the research process, supercritical CO2 extraction is usually compared with other methods such as Soxhlet extraction to confirm the effectiveness and repeatability of this method.
The extraction pressure is one of the most important process parameters for supercritical CO2 extraction.
The solubility curves of different raw materials under different supercritical conditions show that the solubility of extracts in supercritical CO2 is closely related to the density of supercritical CO2, and the extraction pressure is an important parameter that changes the solubility of supercritical fluids. This solubility The relationship with extraction pressure forms the basis of the supercritical CO2 fluid extraction process.
By changing the extraction pressure, the density of the supercritical fluid can be changed, the mass transfer distance can be changed, and the mass transfer efficiency between the solute and the solvent can be changed, thereby increasing or reducing its ability to dissolve the substance. As the supercritical extraction pressure increases, the solubility of the extract generally rises sharply.
Supercritical CO2 density
When the extraction temperature and CO2 flow are constant, the extraction pressure increases, the density of supercritical CO2 increases, the distance between molecules decreases, and the molecular motion increases, and the interaction between internal molecules increases sharply, making it closer to the internal molecules of the oil. According to the principle of similar compatibility, the solubility of vegetable oils in CO2 increases.
However, the relationship between the two is not linear. When the extraction pressure increases to a certain level, the solubility of vegetable oils in CO2 increases slowly, and there is a problem of “maximum solubility” extraction pressure.
And the extraction pressure is too high, compressing the raw materials into blocks, which is not conducive to extraction, and the color of the extracted oil becomes darker (CO2 under high pressure also extracts part of the pigment in the raw material).
In addition, considering that high pressure will increase equipment investment and operating costs, and affect the purity of grease, the extraction pressure is not as high as possible.
The influence of acid value and iodine value
In addition, the change of extraction pressure also affects the acid value and iodine value of vegetable oils.
The experiment of extracting corn germ oil with supercritical CO2 density showed that the acid value of corn germ oil decreased with the increase of extraction pressure, and the acid value also decreased with the increase of separation pressure.
The use of low-pressure extraction in the initial stage to separate a part of the free fatty acids first can greatly reduce the acid value of corn germ oil. This segmented extraction method can improve selectivity and has been widely used in the separation of free fatty acids.
Optimal pressure selection
Generally, the determination of the optimal extraction pressure requires comprehensive consideration of various factors such as the properties of raw materials, the ability to dissolve grease, the selectivity of leaching, product quality, and equipment investment. Based on literature data, the extraction pressure of vegetable oils should generally be between 20 and 30 MPa.