Factors affecting mass transfer in supercritical extraction
It is generally believed that solid solutes are physically, chemically or mechanically immobilized on a porous substrate.
Many scholars believe that for supercritical extraction of natural products, the following four steps can be used to describe the mass transfer process:
① Supercritical fluid diffuses into the microporous structure of the natural matrix;
② The extracted components solvate with the supercritical fluid in the natural matrix;
③ The solute dissolved in the supercritical fluid diffuses with the supercritical fluid through the porous matrix to the flowing supercritical fluid body;
④ Mass transfer between the extract and the main body of the supercritical fluid in the fluid extraction zone
Which of the above 4 steps is a control step depends on the type and magnitude of the solute to be extracted, the matrix, and the force existing between the solute and the matrix to be extracted.Because supercritical fluids have higher diffusion coefficients, and generally the solubility of high-boiling-point solutes in supercritical fluids is very low, the above mentioned ③ is often a control step
The main factors affecting the extraction rate of supercritical fluid include extraction pressure, extraction temperature, extraction time, solvent-to-material flow ratio and solvent flow rate.From a mass transfer perspective:
① Increasing the temperature can increase the vapor pressure of the solute, which is conducive to improving its volatility and diffusivity and providing the thermal energy necessary for the solute to be extracted to overcome its kinetic energy barrier during dissociation. However, increasing the temperature will also reduce the density of the supercritical fluid and reduce its extraction capacity, and the excessively high extraction temperature will also cause the degradation of the heat-sensitive substances. Factor constraints.
② An increase in pressure at a constant temperature will increase the density of the supercritical fluid, thereby increasing the extraction capacity of the supercritical fluid, but the pressure is limited by the equipment conditions.
③ Properly increasing the flow rate and the ratio of solvent to raw material of the supercritical fluid will increase the mass transfer rate, but too fast a flow rate will make the residence time of the extraction solvent too short, resulting in a reduction in the contact time with the extract.
④ Supercritical extraction is a typical operation at high solvent / feed ratio, high space velocity and low viscosity. Any measure that can increase the solvent diffusion coefficient, reduce the diffusion distance and eliminate the diffusion barrier will increase the mass transfer rate.