PGSS-drying: Mechanisms and modeling

Micronization of polyethylene glycol from aqueous solutions has been successfully performed with particles from gas-saturated solutions (PGSS)–drying process, producing spherical PEG particles with average particle size of 10 μm and residual water content below 1 wt%. Based on experimental results, an analysis of the fundamentals of the process has been developed, discussing mass and energy balances, phase equilibrium conditions, mass transfer rates and atomization mechanisms. Some discrepancies between experimentally observed moisture concentration in powder and calculations based on the mass balance and the phase equilibrium have been observed, which have been attributed to the kinetic evolution of pressure and temperature along the expansion path. The static mixer used to saturate the solution with CO2 has been analyzed with phase equilibrium and mass transfer calculations, concluding that a significant fraction of water is extracted to the gas phase already in the static mixer, and high CO2 concentrations are achieved in the liquid due to the high solubility of CO2 in PEG. All experimental trends of variation of particle size with process parameters can be explained considering a flash-boiling atomization mechanism dependant on the concentration of CO2 in the solution after the static mixer.