The Process Simulation And Nozzle Design For Preparation Of Superfine Particles By Using Supercritical Fluids

Some recent research works and technical characteristics of producing superfine particles using supercritical fluid (SCF) technology have been summarized and analyzed in this paper. Rapid Expansion of Supercritical Solution (RESS) and Gas or Supercritical Anti-solvent Crystallization (GAS or SAS) are two promising technologies for microparticles production. As a popular new technology which relates to several science subjects, the theory and practice are now still in the researching and developing state. Few large-scale industrial appliances have been reported, however, the experimental research jobs were mainly about the effects of operation parameters on results and some results are significantly divergent. The model description and simulation research of its mechanism and process can not only explain experimental phenomenon, but also forecast experiment results, which is meaningful to experimental research and industrial application. At the same time how to increase the output of superfine particles is also a bottleneck problem which should be solved urgently. The work of designing the equipment and nozzle with deeply analysis and research will widen the applied field of this task and make a key step for solving this problem from the point of equipment. Based on these points, model simulation of superfine particles production and nozzle design by using supercritical fluid technology are two key points need to be researched in this paper.In order to guarantee the accuracy of simulation and design calculation, physical property of the fluid need to be calculated. By summarizing and comparing the calculation of state-parameters of supercritical CO₂ fluid phase, we determined what kind of state equation to be used to calculate state-parameters and as a result we would use Bender equation to calculate CO₂'s gas-phase density, velocity of sound and so on, and get state function relation from the R-K equation, and use P-R equation to calculate density of liquid-phase mixture. By comparing the applicability, advantage and disadvantage, the method and equations to calculate some physical performance parameters of pure CO₂ phase and liquid mixture have been given, which are viscosity, diffusivity coefficient, thermal conductivity coefficient, surface tension and so on.On the foundation, the model establishment and simulation calculation jobs on RESS flow process and aerosol solvent extraction system ( ASES ) process have been carried out.The essence of microparticle production using RESS process is making use of the solubility's pressure sensitivity of supercritical fluid around the critical area. The solubility of supercritical fluid is dramatically changed via the appropriate changing of pressure. When the supercritical fluid, which has dissolved solute, decompresses rapidly, strong mechanical scramble and huge supersaturation are achieved. The description of one-dimensional time-independent flow model included conical entrance segment, straight pipe for expansion and outlet for ultrasonic expansion. The distribution of temperature, pressure, density and speed of SCF from the process of flowing in nozzle were obtained. All of those are the state conditions which determine the ability to dissolve solute. The results of simulation showed: two dramatic changes about all state parameters occurred at outlet of conical segment (entrance of straight pipe )and exit of nozzle. According to the simulator, the pressure, temperature of entrance, nozzle diameter and nozzle straight length were considered. It has indicated some facts. High initial pressure was benefited to increasing expansion. In order to avoid condensation below the critical temperature (304.2K) of CO2, the initial temperature could not be too low, which would be decided by simulation results. In the laboratory device, about 0.1⁰.2mm of nozzle diameter could meet the expansion ratio requirement. As in pilot or industrial device, the diameter might be increased modestly, which was generally not to exceed 0.4mm. The ratio of nozzle length to diameter around 10-50 was suitable.The process of ASES is a kind of anti-solvent methods, and the essence is that the solvent, which is dissolved solute, sprays into SCF through nozzle and dispersed into superfine liquid drops. When the droplets contact with the SCF, the solute crystallizes to become microparticles because of supersaturation. According to the thermodynamics, hydrodynamics and process characteristic of anti-solvent and solvent system in crystallization vessel, the model equations of fluid flux, fluid components, droplet diameter, gas components and flux changing alone spraying height have been obtained. The changing orderliness of above parameters have been got by process-simulating research of CO2-acetone two phrases system. Based on this,the influencing disciplinarian of the parameters, including ratio of gas and liquid, nozzle diameter and the quality flux of solvent on process were researched. The crystalization quantity of solute was forecast. This calculating model and realizing method can be used in other anti-solvent—solvent systems. The simulation gave us some results. The mass transfer between solvent and anti-solvent was rapid at the first moment. The higher mass ratio of gas to liquid, the smaller nozzle diameter, the higher liquid mass flow, the more expansion of droplet could be achieved , that was to say, they were benefited to the mass transfer process.For the need of microparticle production, on the base of studying the technique and investigating the existing experiment equipments, a set of experimental equipment using supercritical fluid anti-solvent method to produce microparticles has been designed. It included the process design, control system design, heating system improvement design etc. for each main component we have chosen the types or machined outside. Approved by experiments, the small-scaled experiment device can meet the need of the microparticles production technique and experimental research.The nozzle, a key component of the supercritical fluid anti-solvent microparticles producing equipment, was specially designed and researched. By analyzing the atomization mechanism and the characteristics of different nozzle structures, air type was determined to be the main structure type of the nozzle. The craft calculation of spraying process of two-liquid-blend-outside nozzle and two-liquid-blend-inside nozzle were carried out. The new adjustable blend-outside and blend-inside nozzles were designed. All of them made use of the gap of two adjustable conic surfaces to jet the material, and change the normal fixed structure into loop-gap one. Blend-outside nozzle was designed for ASES method and blend-inside nozzle was designed for SEDS method. We have declared the patent of those two nozzles and manufactured the new blend-outside nozzle. The two kinds of nozzles can not only complete single solute microparticle production, but also realize superfine synthesized microparticle production, and have the characteristics of wide adaptability, easy adjustability, easy to realize scale-up production etc. It has an important meaning to the development of producing microparticles using supercritical fluids technology.To test the operational condition of nozzle and whole microparticle producing device and gain the initial technique condition of microparticle producing, the relatedexperimental research work was carried out. Under certain craft condition, the experiment to test the effect of heating system was done. By testing the temperature trend of nozzle jet area, it was to be proved that the water-bath heating system could satisfy the experimental craft condition requests and could keep the temperature of the nozzle within the scope of normal work condition. Blend-outside nozzle' microparticles producing effect also has been tested. Supercritical CO2 -acetone-hydroquinone system was applied to produce hydroquinone microparticles. The results indicate: the average microparticle diameter was 200 nm or so, which was about 1/10-1/30 of published data. At the meanwhile, high function could be achieved by the new type nozzle.