Radial Model for Particle Formation from the rapid expansion of supercritical solutions

A numerical model for the rapid expansion of supercritical-fluid solutions (RESS) is presented. Considering the complete expansion of the fluid into a vacuum, the process is modeled as inviscid, radial flow. The particulate material is described by the moments of the particle size distribution by numerically integrating the moments with the equations describing an inviscid flow of a non-ideal solution. By varying the initial process conditions, we examine the trends due to changes in specific operational parameters for typical RESS solutions. These calculations show that in the limit of a very rapid expansion with no precipitation in the nozzle itself, an increase in solute concentration produces a large increase in the particle size of the solids produced. Increasing the initial temperature causes a decrease in the calculated mean particle size, while an increase in pressure produces larger particles. However, both temperature and pressure have a much smaller effect on the calculated particle sizes than does concentration. The calculations demonstrate that small particles with a narrow size distribution can be achieved by using as low a solute concentration as possible and operating at relatively low preexpansion pressures and high preexpansion temperatures.