Fabrication of fine powders by RESS with a clearance nozzle

Manufacture of fine particles is one of the major supercritical fluids (SCF) applications. Nozzle design plays an important role to obtain micro-particles in the rapid expansion supercritical solution (RESS), particles generation from gas saturated solutions (PGSS) and other supercritical anti-solvent (SAS) techniques like the aerosol solvent extraction system (ASES) and the solution enhanced dispersion by supercritical fluids (SEDS). A clearance nozzle has been developed so that spray from a narrow exit in size of a few microns can be realized. In the RESS demonstration experiments using the clearance nozzle, comparison of the micronization effects between laser-drilled orifice nozzles and the clearance nozzle was made. It was found that the particle size could be remarkably reduced to sub-micrometers by using the clearance nozzle. The morphology and crystal patterns of the particles sampled out of the nozzle or collected in aqueous solutions were observed. A possible micronization mechanism was proposed based on the experimental findings. It was found that the nozzle size as well as particle collecting method both had great impact on the final crystal forms. Surface control seems necessary to prevent formation of huge crystals during the particle collection process.