CFD Simulation On The Injection Flow Field In The Supercritical Anti-solvent Process

Supercritical anti-solvent (SAS) technique is one of the most promisingmicronization methods. The SAS precipitator plays an important role duringgranulation, where nucleation and particles growth occur. It is beneficial to reduce thetime to achieve super saturation for the solvent jetted into the SAS precipitator. It isnecessary to enhance the mixing of antisolvent and the solution in order to obtainuniform nano-particles. Furthermore, eddies existing in the precipitator could causecollision for particles formed. This could damage the particle morphology andtherefore should be avoided.The position of the CO2entrance relatied to solution nozzle is one of the mainfactors that influenced the flow field in the precipitator. In this dissertation,Computational Fluid Dynamics (CFD) method was employed to investigate the flowfield in the precipitator. The calculated results showed that the closer of CO2entranceto solution spray nozzle and the lower position of the CO2inlet position relative to thenozzle, the better flow field could be obtained which was good for the mixing of thesolution and anti-solution near the nozzle. When CO2entrance was on the top of theprecipitator, the optimum angle of CO2was45°.In this paper, the diameter of the precipitator and the CO2inlet were optimizedwith CFD. The CO2entrance was kept on the top of the precipitator with an angle of45°during optimizing the diameter of the precipitator and the CO2inlet. Thecalculated results implied that high fluxes of CO2can strengthen the fluid turbulencenear the nozzle exit and promote the mixing of solution and anti-solvent. This couldfinally help to increase the nucleation rate and decrease the particle size as well.Furthermore, the increasing of the CO2fluxes can reduce the eddies and make itshrinked to the top of the precipitator. This can promote particles moving ahead inmanner of pseudo plug-flow. In other words, this can also contribute to reduce thepossibility for particle collision and therefore, make the particle morphology as wellas particle size more uniform. According to the CFD simulation results on CO2inletdiameter, the decrease of CO2inlet diameter made two different effects on theprecipitation process. On one hand, this caused the increasing of the incidence rate and consequently increased the incident momentum, which promoted the mixing,strengthen the fluid turbulence and speed up the nucleation rate in the solution jetzone near the nozzle exit, and finally benefit to obtain smaller particles; on the otherhand, it shrinked the collision interface of the two stream of CO2, which was notconducive to promote the mixing.This dissertation investigated the effects of precipitator structure on the flowfield in the SAS process based on the CFD simulation. The results and conclusion ofthis study will be helpful for the optimization and engineering amplification of theprecipitator.