Supercritical fluid extraction (SFE) is a technology that uses supercritical fluid as an extractant to separate one component (extract) from another component (matrix).
The basic principle of supercritical fluid extraction is to dissolve the required chemical components with supercritical fluid under conditions above the critical temperature and critical pressure, and then reduce the pressure of the fluid solution or increase the temperature of the fluid solution so that the solute dissolved in the supercritical fluid precipitates due to its reduced density and solubility, thereby achieving the extraction of specific solutes.
Supercritical fluid is a high-density fluid above the critical temperature and critical pressure. It is neither a gas nor a liquid, and its properties are between those of a gas and a liquid, and it is characterized by having solvent properties. When the fluid is in a supercritical state, its density is close to that of a liquid, and changes significantly with changes in fluid pressure and temperature, while the solubility of the solute in the supercritical fluid increases with the increase in the density of the supercritical fluid. Supercritical extraction is carried out using this property of supercritical fluid.
There are many substances that can be used as extractants in supercritical extraction, but when extracting natural products by supercritical extraction, carbon dioxide (CO2) is generally used as the extractant. Because the critical temperature of CO2 (31°C) is close to room temperature, it has less damage to volatile or physiologically active substances. At the same time, CO2 is safe and non-toxic, and the extraction and separation can be completed in one go without residue, which is suitable for the extraction of food and drugs.
Five major aspects to consider in the design of supercritical extraction kettle
The extraction kettle is one of the keys to the development of supercritical extraction technology. Different extraction kettles need to be selected for materials of different forms. It can be seen that the extraction kettle should be designed according to the original data and process requirements provided by the design task.
Supercritical CO2 fluid extraction kettles generally work under a high pressure of 32Mpa (sometimes higher), and they need to be disassembled frequently. They are high-pressure pressure vessels, and their strength design is the difficulty and key to the strength design of the entire supercritical extraction equipment.
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 need to be determined according to the use conditions.
Design of extraction kettle head
Most of the heads of high-pressure vessels are flat covers, especially those with small diameters. 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 exchanger
The calculation of heat exchanger as a pressure vessel is the same as that of general vessels, but there is also a strength calculation unique to heat exchanger, including the calculation of temperature difference stress caused 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 calculated. Since the calculation process is relatively complicated and the standardization committee has recommended calculation software, you can directly use the software for calculation when necessary. Please check the relevant information.
Conclusion
The extraction kettle is one of the keys to the development of supercritical extraction technology. It has the characteristics of high strength, strong rigidity, good stability and durability, and good quick-opening sealing effect. Therefore, the design of supercritical extraction kettle should be strictly carried out according to the process requirements.