RADIAL MODEL FOR PARTICLE FORMATION FROM THE RAPID EXPANSION OF SUPERCRITICAL SOLUTIONS

A numerical model for the rapid expansion of supercritical-fluid solut ions (RESS) is presented, Considering the complete expansion of the fl uid into a vacuum, the process is modeled as inviscid, radial flow. Th e particulate material is described by the moments of the particle siz e distribution by numerically integrating the moments with the equatio ns describing an inviscid flow of a non-ideal solution. By varying the initial process conditions, we examine the trends due to changes in s pecific operational parameters for typical RESS solutions. These calcu lations show that in the limit of a very rapid expansion with no preci pitation in the nozzle itself, an increase in solute concentration pro duces a large increase in the particle size of the solids produced. In creasing the initial temperature causes a decrease in the calculated m ean particle size, while an increase in pressure produces larger parti cles. However, both temperature and pressure have a much smaller effec t on the calculated particle sizes than does concentration. The calcul ations demonstrate that small particles with a narrow size distributio n can be achieved by using as low a solute concentration as possible a nd operating at relatively low preexpansion pressures and high preexpa nsion temperatures.