Technical principle of supercritical extraction
The relationship between the solubility of the supercritical fluid and its density, that is, the effect of pressure and temperature on the solubility of the supercritical fluid. In the supercritical state, the supercritical fluid is contacted with the substance to be separated, so that it selectively extracts the components of polarity, boiling point and molecular weight. Of course, the extract obtained corresponding to each pressure range cannot be single, but the conditions can be controlled to obtain the best proportion of mixed components, and then the supercritical fluid is changed into ordinary gas by means of decompression and heating, and the extracted substance is completely or basically precipitated, thus achieving the purpose of separation and purification. Therefore, the supercritical CO2 fluid extraction process is composed of extraction and separation processes. Supercritical CO2 refers to a compressible high-density fluid above the critical temperature and critical pressure (called the critical point). It is the fourth state other than the three states of gas, liquid and solid. Its intermolecular force is very small, similar to gas, but its density is very large, close to the wave body, so it has the duality of gas wave between gas and liquid, at the same time, it has high solubility of liquid and high fluidity of gas, higher mass transfer rate than ordinary liquid solvent, and the diffusion coefficient is between liquid and gas, with good permeability and no interphase effect.
Therefore, it helps to improve the extraction efficiency and can greatly save energy. The physicochemical properties of supercritical CO2 are very different from liquids and gases in the non-critical state. Since density is the solubility, viscosity is the fluid resistance, and diffusion coefficient is the main parameter of mass transfer rate, the special nature of supercritical CO02 determines that the supercritical CO2 extraction technology has a series of important characteristics. The viscosity of supercritical CO2 is percent of that of liquid, and the self-diffusion coefficient is 100 times that of liquid, so it has good mass transfer characteristics, can greatly shorten the time required for phase equilibrium, is an ideal medium for efficient mass transfer, has a much faster rate of dissolving solute than liquid, and has a much greater ability to dissolve and carry solid substances than gas. It has unusually great compressibility and is attached at the critical point, small changes in pressure and temperature will cause great changes in the density of CO2, so the solubility of CO2 can be adjusted by simply changing the temperature or pressure of the system to improve the selectivity of extraction: by reducing the body.
pressure to separate CO2 and dissolved products, eliminating the need to eliminate the solvent process. In the traditional separation method, solvent extraction is the use of solvent and the affinity between the solute (expressed in solubility) difference to achieve separation; distillation is the use of the solution of each component of the volatility (vapor pressure) to achieve separation. Supercritical CO2 extraction is separated by adjusting the pressure and temperature of CO2 to control the solubility and vapor pressure, so supercritical CO2 extraction combines the two functions and characteristics of solvent extraction and distillation, which determines that supercritical CO2 extraction has the advantages that traditional ordinary fluid extraction methods do not have: by adjusting the pressure and temperature to easily change the nature of the solvent and control its selectivity; appropriate selection of extraction conditions and solvents, can be operated at close to room temperature, can be applied to heat-sensitive substances; due to small viscosity, large diffusion coefficient, fast extraction speed; solute and solvent separation thoroughly and easily.