The Numerical Simulation Of Basic Hydrodynamics Problem About The Microparticle Preparation By Supercritical Fluid

Micro-particle preparation technologies, especially those based on supercritical fluid technology are one of the most important research fields about material science and chemical engineering. It is the researcher's purpose to obtain the small micro-particles with a uniform distribution. At present several supercritical fluid technologies have been proposed to prepare the micro-particles. Among these, the rapid expansion of supercritical solution (RESS) process, the supercritical antisolvent (SAS) process and the supercritical assisted-atomization (SAA) process are very typical. While, a key part i.e. a nozzle with the inner diameter of 200μm is used in above three technologies to prepare the microparticels successfully. So it is important to simulate the flow fileds inside and outside of the nozzle of supercritical fluid to predict and optimize the processes.In order to ensure the accuracy of simulation and calculation on the rapid expansion process, physical parameters of the supercritical CO₂ and ethanol are calculated and compared. The equation of state used in this paper is determined and the calculation method of the viscosity, thermal conductivity coefficient and the thermal capacity of the carbon dioxide and ethanol are obtained.Toward the basic hydrodynamics behavior of the rapid expansion process of the pure CO₂ through the nozzle, the rapid expansion mathematical model of the supercritical fluid being sprayed into the free space through a nozzle is established. Based on the simulation conditions, the rapid expansion process in the calculation region is numerically simulated by the commercial calculation software FLUENT, the evolutional curves of the pressure, temperature, density and velocity along with the axis are obtained and the influences of the operating parameters, such as pre-expansion pressure, the temperature and the nozzle diameter on the expansion process are investigated.SAA is a recently proposed micronization technology based on the supercritical fluid to prepare the micro-particles of solid. The most important point of this method is that this process is based on the high velocity of the air stream to break the solvent into very small droplets and the microparticles are obtained. With the ethanol as model material used generally in the SAA process, the model of air assisted atomization through a nozzle is made in this paper. The atomization process is simulated and calculation by the commercial software FLUENT, the variation rules of the diameter, the velocity and the temperature of the droplets in the precipitator are obtained and the influences of the operating parameters, such as the inlet pressure, the inlet temperature and flow rate of the solvent on the diameter and size distribution of the droplets are investigated.Supercritical fluid micro-particle preparation technologies have more advantages than the conventional micronization ones. However, the smaller nozzle inner diameter limited the yield of the produce greatly and then the application and promotion of these processes. In this paper, a new-style nozzle used in the supercritical fluid micro-particle technology is designed which is characterized the ring shape space composed by inner core and shell. The basic hydrodynamics behavior of the rapid expansion process through this kind of nozzle has been simulated. The results show that the same expansion effects as the single-hole nozzle are obtained while the yield of produce increases greatly.