What Is a Supercritical Fluid?
A state where the pressure and temperature of a substance exceed its critical pressure (P c ) and critical temperature (T c ) at the same time, or in other words, the relative pressure (P / P c ) and the relative temperature (T / T c ) of a substance are greater than 1 at the same time The state is called the supercritical state of the substance.
- There are three phase states of any kind of substance-
- According to the ideal gas state equation PV = nRT, at a certain temperature, the volume V of a certain amount of ideal gas is inversely proportional to its pressure P. If P is used as the ordinate and V is plotted as the abscissa, an inverse curve can be obtained, called
Supercritical state definition
- Supercritical fluid extraction (SPE) is a new extraction technology. Supercritical fluid extraction technology uses gas under supercritical conditions as an extractant to extract certain components from liquids or solids and separate them. Technology.
- A gas under supercritical conditions, also known as a supercritical fluid (SF), is a substance that exists above the critical temperature (T c ) and critical pressure (P c ) in the form of a fluid. Usually there are carbon dioxide (CO 2 ), nitrogen (N 2 ), dinitrogen oxide (N 2 O), ethylene (C 2 H 4 ), trifluoromethane (CHF 3 ) and the like. [3]
Basic Principles of Supercritical State
- Basic principle of supercritical fluid extraction: When the gas is in a supercritical state, it becomes a single phase state between liquid and gas, with a density similar to that of liquid, although the viscosity is higher than that of gas but significantly lower than that of liquid, and the diffusion coefficient It is 10 to 100 times of liquid, so it has better permeability and strong dissolving ability to the material, and can extract some components in the material. And the density and dielectric constant of the supercritical fluid increase with the increase of the pressure of the closed system, and the polarity increases. The components of different polarities can be extracted by using the program pressure increase. After the extraction is completed, the temperature or pressure of the system is changed so that the supercritical fluid becomes ordinary gas and escapes, and the extracted components in the material can be completely or substantially completely precipitated to achieve the purpose of extraction and separation.
- Four of the states of a substance (solid, liquid, gaseous, and supercritical) (and other states) change with its temperature and pressure. Taking CO 2 as an example, at the three-phase point of CO 2 , the temperature T (tr) of the coexistence of solid, liquid, and gas phases is -56.4 ° C (217K), and the pressure P (tr) is 5.2 × 10 5 Pa. The vapor pressure line of CO 2 ends at the critical point C (T c = 31.3 ° C, P c = 7.38 × 10 6 Pa, c = 0.47 g / cm 3 ). Above the critical point, the liquid-gas interface disappears and becomes a supercritical fluid (SF). The diffusion coefficient of SF (10 -1 10 -4 cm 2 / s) is an order of magnitude higher than that of general liquids (10 -2 10 -5 cm 2 / s), and its viscosity (10 -2 10 -4 N · s / m 2 ) is an order of magnitude lower than ordinary liquids (10 -1 10 -3 N · s / m 2 ). Compared with liquid-liquid extraction systems, SF systems have faster mass transfer and extraction speeds. Therefore, it can effectively penetrate into the space of the solid sample for extraction and separation. The density of SF changes with temperature and pressure, resulting in a change in its solubility parameter. At lower densities, the solubility parameter of SF-CO 2 is close to hexane; at higher densities, it can be close to chloroform. Therefore, the density (temperature and pressure) of SF can be controlled to obtain the required solvent strength. This ability allows SF to arbitrarily change the strength of the solvent to suit different solutes. Generally speaking, SF can effectively dissolve non-polar solids, and it can also perform selective extraction according to the polarity of the solute, which is widely used in the field of separation and analytical chemistry.
- CO 2 has a lower critical temperature and pressure, is cheap, non-toxic, and has low activity. Therefore, SF-CO 2 is often used to extract non-polar and slightly polar substances.
- In the supercritical state, the fluid has the dual characteristics of gas-liquid two-phase, not only has a high diffusion coefficient and low viscosity equivalent to that of gas, but also has a density similar to that of liquid and a good ability to dissolve substances. Its density is very sensitive to changes in temperature and pressure, and is proportional to the solubility in a certain pressure range, so the solubility of a substance can be changed by controlling the temperature and pressure. Supercritical fluids have been used in the extraction, synthesis, analysis and processing of drugs. [3]