The development of ultra-fine particles, especially nano-scale particles, has become a hot field in the current high and new technology. It has been widely used in the fields of materials, chemical industry, light industry, metallurgy, electronics, and biomedicine. Supercritical fluids are used to prepare fine particles. The technology of microparticles can process and prepare particles, microspheres, microcapsules, porous materials, liposomes and other fine materials.
Supercritical fluid micronization technology
There are many methods for the preparation of ultrafine particles. As a high-tech, supercritical fluid deposition technology can more accurately control the crystallization process, produce fine particles with a small average particle size, and control the size distribution of their particles. .
There are two supercritical fluid deposition techniques: RESS (rapid expansion of supercritical solution) method and GAS (gas anti-solvent crystallization) method. The processes derived from GAS method include SAS, ASES (PCA), SEDS, etc., this equipment The following experiments can be carried out: GAS method, ASES method (ie PCA method), SEDS method.
When the high-pressure gas dissolves into the solution phase containing the solute, the solvent in it expands, thus reducing the solubility of the solute in it, leading to the precipitation of the solute.
The working principle and influencing factors of the SAS method are almost the same as those of GAS, except that supercritical carbon dioxide is injected into the solution instead of gaseous CO2
The ASES process is the aerosol solvent extraction process, also known as the PCA process. The GAS process sprays gaseous CO2 into the solution. The ASES process is the opposite of the process, which sprays the solution into supercritical carbon dioxide.
The crystallization kettle is pre-filled with CO2 fluid at a certain pressure. After the pressure and temperature of the crystallization kettle reach a stable state and form a supercritical state, a solution of a certain concentration is sprayed into the crystallization kettle through a nozzle to form fine droplets.
Because high-pressure CO2 and organic solvents can be miscible with each other, on the one hand, CO2 quickly dissolves into the droplet, causing it to swell, and the dissolving ability of the solvent is rapidly reduced. On the other hand, the organic solvent in the droplet also quickly dissolves into the high-pressure CO2. A very high degree of supersaturation is formed in a very short time, so that the solute is precipitated quickly, and extremely fine and uniform particles are formed. If a small amount of CO2 is added to the organic solution in advance, porous particles will be generated after spraying. The formed particles are filtered on the bottom filter of the crystallization kettle, and the mixed fluid of carbon dioxide and solvent flows out of the crystallizer and enters the decompression kettle for gas-liquid separation.
In the ASES process, the operating parameters that affect the particle morphology, particle size and particle size distribution are temperature, pressure and initial concentration of the solution. Like the RESS process, the nozzle structure and size have an important impact on the results.
Difference between the ASES process and the GAS process
In the ASES process, particle formation and drying proceed at the same time, and the residual organic solvent content in the formed particles is small. The crystallization and precipitation process of the GAS process all occur in the liquid phase, and it takes a long time to wash to obtain dry solid particles. The operating pressure of the GAS process is much lower than that of the RESS process, about 10 MPa.
The SEDS process (precipitation process of supercritical fluid enhanced solution dispersion) is to spray supercritical CO2, a solute-containing aqueous solution and an organic solvent into the crystallizer together, and undergo a brief contact and mixing process.
Since the organic solvent is much higher than the flow rate of the solute aqueous solution, the solute is precipitated quickly after the two contact and mix, and the flow rate of high-pressure CO2 is much higher than the flow rate of the organic solvent, so that the solution and the organic solvent are immediately washed away by the high-speed jet CO2 after mixing. As liquid droplets, at the same time a small amount of organic solvent and water in the droplets quickly dissolve in the CO2, so that the precipitated solute particles can be quickly dried.
- Mold working pressure: 30MPa
- Working temperature: room temperature～150℃
- Crystallizer volume: 1000mL
- Working pressure of separator: 10MPa
- Separator volume: 1000mL
- Cooling capacity: 3300kcal/h
- Power supply: AC380V 10kW
Supercritical CO2 micronization equipment consists of the following parts: CO2 supply system, solute-containing solution feeding system, organic solvent feeding system, crystallizer, pressure relief device, temperature control system (heated by thermostat air bath), safety protection System composition.
CO2 supply system
It is composed of CO2 cylinder, purifier, refrigeration system, CO2 plunger pump, buffer tank, preheating coil and corresponding pipe valves.
The purifier has a volume of 200mL, a design pressure of 8MPa, and is filled with molecular sieve or silica gel, which can filter impurities and moisture. The internal filter mesh is 400 meshes (equivalent to 38μm).
Used to refrigerate gaseous CO2 into liquid CO2. It is convenient for high-pressure injection and stable supercharging, with a cooling capacity of 3300kcal/h, air-cooled water circulation system, and silent operation.
CO2 plunger pump
The discharge pressure is 32MPa, the flow rate is 10L/h, and the three-plunger frequency conversion adjustment can reasonably reduce the impact of high-pressure input pulses and ensure product quality.
It is mainly used to buffer CO2 before the crystallizer to reduce the impact of pulse. The volume is 500mL, and the working pressure is 32MPa.
It is mainly used for preheating CO2 before the crystallizer and placed in a thermostat.
Solution feeding system containing solute
It is composed of glass cup, plunger pump and corresponding pipe valves. Among them, the plunger pump: the flow rate is 400mL/h, and the discharge pressure is 32MPa.
Organic solvent feed system
It is composed of glass cup, plunger pump and corresponding pipe valve parts, which is specially designed for SEDS process. Among them, the plunger pump: the flow rate is 4L/h, and the discharge pressure is 32MPa.
The volume is 1L, and the inlet is connected with a nozzle. The nozzle is made of stainless steel and is perforated by laser. The ratio of length to inner diameter is 15:1. The inner diameter of the nozzle is 60μm, 80μm, 100μm, and 120μm can be selected according to the needs of the experiment.
Single-head nozzle is equipped inside the crystallizer. The single-head nozzle can be exchanged for nozzles with different fine inner diameters. The single-head nozzle structure is mainly used in the GAS method and the ASES method (that is, the PCA method).
The volume is 1000mL, the design pressure is 10MPa, and it is mainly used for gas-liquid separation. The structure is slender, which is convenient for separation and recovery of liquid solvents.
The temperature is controlled by a temperature controller, a heater and a circulating fan are installed in the thermostat, the heat exchange adopts convection and radiation heat transfer, and the working rooms are all stainless steel inner tanks.
Safety protection system
In order to ensure the safety of the system, we designed a safety protection system, which is mainly composed of an electric contact pressure gauge and a safety valve. The electric contact pressure gauge controls the pressure of the pressure source (plunger pump). The upper limit pressure can be set to avoid overpressure. . In addition, a safety valve is installed on the mold to implement overflow protection.
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