Supercritical CO2 fluid extraction (SFE) is carried out by utilizing the relationship between the solubility of supercritical fluid and its density, that is, the influence of pressure and temperature on the solubility of supercritical fluid. In the supercritical state, the supercritical fluid is contacted with the substance to be separated, so that it can selectively extract the components of polarity, boiling point and molecular weight in sequence.
The characteristics of supercritical CO2 extraction determine that its application range is very broad. For example, in the pharmaceutical industry, it can be used for the extraction of effective ingredients of Chinese herbal medicines and the separation of lipid mixtures; in the food industry, hops extraction, pigment extraction, etc.; in the spice industry, the refining of natural and synthetic spices; separation of mixtures in the chemical industry, etc.
Design requirements for supercritical CO2 fluid extraction kettle
Supercritical CO2 fluid extraction kettle generally works under a high pressure of 32Mpa (sometimes higher), and needs to be disassembled frequently. It is a high-pressure pressure vessel. Its strength design is the difficulty and key to the strength design of the entire supercritical extraction equipment.
When the container shell is opened, if the hole diameter exceeds the maximum hole diameter allowed without additional reinforcement, the reinforcement area must be calculated as required.
Calculation of wall thickness of extraction kettle
In small and medium-sized supercritical CO2 fluid extraction equipment, fatigue analysis is usually not performed because the volume of small and medium-sized equipment is small and the wall thickness of supercritical CO2 fluid extraction kettle is also small. When calculating the strength, the safety factor can be increased to deal with it, so that only the strength problem can be considered.
Before calculating the wall thickness, the relevant design parameters of the supercritical CO2 fluid extraction kettle must be determined according to the use conditions.
Design of extraction kettle head
Most of the heads of high-pressure vessels are flat covers, especially small-diameter high-pressure vessels. Since flat covers are easy to manufacture and suitable for a variety of seals, flat end covers can be selected.
Design of extraction kettle cover threads
Considering that the threads of the supercritical CO2 fluid extraction kettle cover mainly bear single-phase axial force, serrated threads can be selected.
Calculation of supports
Supercritical fluid extraction equipment generally uses skirt supports, which need to be strength-checked.
Calculation of shell-and-tube heat exchangers
The calculation of heat exchangers as pressure vessels is the same as that of general vessels, but there are also strength calculations unique to heat exchangers, including the calculation of temperature difference stress generated by the temperature difference between the tube wall and the shell, the calculation of tube sheet thickness, and the calculation of tube pull-off force; when the pull-off force is too large, the expansion joint must be selected.
Conclusion
Supercritical CO2 fluid extraction kettle is one of the keys to the development of supercritical extraction technology. It needs to solve the problems of machinery, heat exchange, fluid transportation and safety assurance caused by the process under high pressure (generally 8-35MPa or higher). It should be safe, easy to operate, versatile, and can run continuously while meeting the production purpose. According to the regulations on pressure vessels, it belongs to high-pressure equipment, and different extraction kettles should be selected for different forms of materials.