What is supercritical fluid?
According to the difference in temperature and pressure, the substance presents state changes such as liquid, gas, and solid. Gas molecules have the greatest kinetic energy and the greatest penetration. When the temperature drops and the pressure rises, the gas will condense into liquid, and the liquid molecules The distance between them is greatly reduced, and the density and solubility are increased.
For a specific substance, when it is in a state above its critical temperature and critical pressure when pressure is applied to the gas in this state, the gas will not be liquefied, but its density will increase, and it has properties similar to a liquid state while retaining the properties of the gas. This state of the fluid is called a supercritical fluid.
The above figure is the pressure-temperature phase diagram of pure substances. The critical point C in the figure refers to the point where the gas-liquid interface disappears when the gas-liquid equilibrium line in the phase diagram extends to high temperatures. The corresponding temperature and pressure here are critical temperature and critical pressure. The shaded part in the figure is the supercritical region. The density of the supercritical fluid is hundreds of times that of gas, which is equivalent to that of liquid; its viscosity is close to gas, but compared with liquid, it is 2 orders of magnitude smaller; its diffusion coefficient is between gas and liquid: about 1% of gas, 10 to 100 times larger than liquid, has good solubility and mass transfer properties.
Five stages of development history
- In 1822, Cagniard reported the critical phenomenon of matter for the first time;
- In 1869, Andrew measured the critical parameters of carbon dioxide;
- In 1879, Hanny et al. discovered that supercritical fluids can dissolve solids, providing a basis for the application of supercritical fluids.
- In the 1960s, supercritical fluid extraction technology began to be applied, and related research work continued to deepen to achieve breakthroughs in theory and technology.
- In the past 30 years, supercritical fluid technology has been greatly developed due to the promulgation of environmental protection regulations.
Types of supercritical fluids
Supercritical fluids that can be used as extraction solvents and reactive fluids must be selected according to their respective characteristics and adaptability. Many types of substances can be used as supercritical fluids, and the critical parameters of more than 1,000 substances can be determined at present.
For some reasons, it is necessary to consider a series of factors such as solubility, selectivity, critical point data, and the possibility of chemical reactions. Although many fluids can be used as supercritical extraction solvents, only a dozen are actually used, mainly CO2, H₂O, propane, etc.
8 Conditions that Supercritical Fluids Used as Extractants Should Possess
- Chemically stable, non-corrosive to equipment, and does not react with extracts;
- The critical temperature is close to the normal temperature or operating temperature, and should not be too high or too low;
- The operating temperature should be lower than the decomposition temperature of the extracted solute;
- Low critical pressure to save power costs;
- High selectivity, capable of selectively extracting target substances;
- The strong dissolving ability of the extracted solute, good mass transfer performance;
- The critical point of the solvent is lower than that of the extracted substance, so it is easy to separate;
- Sources are plentiful and prices are reasonable.
Three characteristics of supercritical fluid CO2
Commonly used supercritical fluid CO2 has the following characteristics:
- The critical temperature of CO2 is 31.06°C, and supercritical fluid technology operation can be realized near room temperature to save energy consumption, so the requirements for equipment are relatively low;
- The supercritical CO2 fluid has a high density, a strong dissolving ability for most solutes, and a high mass transfer rate, while the solubility of water in the CO2 phase minimally, which is conducive to extraction with near-critical or supercritical CO2 Separation of organic aqueous solutions;
- CO2 also has the characteristics of being non-flammable, cheap, non-toxic, with good chemical stability, and easy to separate from the extraction product.
Because of the above characteristics, supercritical fluid CO2 is currently the most widely studied supercritical fluid used as an extraction agent.