Rapid expansion of supercritical solution for microcrystals preparation
The rapid expansion of supercritical solution (RESS) prepares microcrystals (ultrafine particles) using the relationship between the solubility of solutes in the supercritical fluid and the density of the supercritical fluid.
The solution rapidly expands from the supercritical fluid state to a low-pressure, low-temperature gas state, and forms a mechanical disturbance transmitted at the speed of sound.The solution quickly reaches a high degree of supersaturation, so that the solute forms a large number of crystal nuclei in an instant, and in a short time Within a period of time, the growth of crystal nuclei is completed, and under uniform and fast nucleation conditions, a large number of tiny particles with a narrow and uniform size distribution are generated.
The RESS process diagram shows that the process is as follows: after the solvent liquid in the storage tank 1 is pressurized by the high-pressure pump 2, it enters the high-pressure dissolution kettle 4 containing solute, and becomes a solute saturated solution at a preset temperature. Then the saturated solution is sprayed into the low-pressure crystallization kettle 7 through the nozzle 6, and the particles nucleate and crystallize out.
1—storage tank; 2—high pressure pump; 3,5,8-three-way valve; 4—high pressure dissolution kettle; 6—nozzle; 7—low pressure crystallization kettle
The distinguishing feature of RESS is the extremely high supersaturation produced by the mechanical disturbance of rapid advancement and rapid depressurization.
The former makes the particle size uniform and the particle size distribution is narrow, the latter makes the particle size very small.
In addition, RESS can take advantage of the sensitivity of supercritical fluids to changes in temperature and pressure, and can significantly change the solvation ability of supercritical fluids by changing temperature and pressure.
Another advantage of the RESS process is the high purity of the crystals obtained, because the supercritical fluid used in this process is usually a gas under normal conditions, and there is very little solvent residue in the obtained product.
Maston et al. conducted a detailed study of the RESS process and concluded that it can be divided into the following three stagesSubsonic velocity expansion in the approximate adiabatic zone; supersonic free expansion in the isentropic zone; ultrafine particles are precipitated by the interaction between the jet and the gas in the expansion zone.
Factors affecting the shape and size of the particles
During the operation of RESS, the main factors affecting the shape and size of the particles are: the nature and composition of the raw materials, operating temperature, pressure drop, nozzle structure, etc., where the nozzle of the RES process is a key component that determines the fluid expansion characteristics and ultimately determines the product morphology and quality .
The research shows that: using the porous sintered plate nozzle can produce ultrafine particles with a smaller particle size than when using the capillary nozzle, and the average particle size of the powder decreases as the pore size of the sintered plate decreases.
Further research on the RESS process
Meziani et al. conducted further research on the RESS process, replacing supercritical solution expansion and decompression to gaseous state with rapid expansion of supercritical solution into liquid solvent (RESOLV).
This method is a derivative technology of the RESS process. Because the supercritical solution is sprayed in a liquid environment, the growth time of particles in the settler can be shortened, and the growth of particles in the liquid phase solvent can be prevented, thereby expanding the application range of RESS.
For example, combining the nucleation process of solid particles with compounds in a liquid solvent can also increase the chemical reaction steps to obtain nanoparticles with the desired composition. Sun and Rollins et al. dissolved Cd(NO3)2 and Pb(NO3)2 in supercritical ammonia, respectively, and sprayed it quickly into Na2S ethanol solution using RESOLV technology to obtain ultrafine particles with smaller particle size. Average particle sizeIt is 3.3nm CdS and PbS nanoparticles.
Operate of RESS method
The RESS method is easy to operate and the process is simple. It is an earlier technology in the field of supercritical fluids. Its theoretical and practical research has made some progress.
At present, the research on the RESS process is mostly limited to the influence of single or several influencing factors on the process.
The current theoretical research can only give a preliminary interpretation of the experimental results, and it is difficult to accurately describe the process parameters (such as fluid concentration before expansion, before and after expansion) The state parameters of the fluid, the shape and size of the nozzle, etc.), the physicochemical properties of the solute and SCF (Supercritical CO2 fluid), and the phase behavior of the two affect the final particle morphology.
In addition, the application of RESS also has some limitations, such as high requirements on equipment and operations, high production costs, low production continuity, and poor product reproducibility, which seriously restrict the application and promotion of this technology.
Now we have begun to use electronic computers to perform hydrodynamic calculations (CFD) of the RESS process in order to have a deeper understanding of the nature of the process.
The RESS process is generally carried out at a mild temperature below 80°C, so it is very suitable for heat-sensitive substances, and the process does not require the use of organic solvents, and has good safety.
The extremely high supersaturation produced by the fast-moving mechanical disturbance and rapid depressurization makes the prepared particles small in size and narrow in distribution, which can be controlled by process parameters such as temperature, pressure, nozzle size and fluid ejection speed adjust to adjust the size of the particles.
The crystallization process can be achieved only by changing the pressure of the system, without the need to add other substances, which avoids the contamination of the product by other impurities;
This process does not use a large amount of organic solvents, which greatly reduces the energy consumption required for wastewater discharge and solvent recovery; supercritical The fluid can be recycled only by recompressing, which greatly simplifies the process flow.
There are many application systems in the RESS process. Commonly used are inorganic supercritical water system and organic matter-supercritical carbon dioxide system. The former is mostly used in the preparation of silicon-containing ultrafine powders such as ceramics, and the latter is mostly used in the preparation of biochemical preparations including drug particles or Bao Di.
However, because particles are easily aggregated in a supersonic free jet, needle-like particles are often obtained instead of shapes.
The most commonly used solvents in this process are carbon dioxide, propane, acetone, etc., only non-polar substances and a few volatile polar substances such as ethanol can use this method, because polar substances such as various acids and salts are in supercritical fluids The solubility in is very poor, so it is generally not possible to use the RESS method to prepare microparticles.