Numerical simulation of spray drying of hydroxyapatite nanoparticles

As a way of improving handling safety of nanoparticles for environmental applications on an industrial scale, we considered the consolidation of nanoparticles into micron-sized spray-dried agglomerates. Spray drying has recently been favored for manufacturing of nanoparticle agglomerates including hydroxyapatite-based composites. This study numerically investigated the drying mechanism of the slurry of hydroxyapatite nanoparticles in an industrial-scale spray dryer. The detailed model of drying kinetics of a slurry droplet was combined with the energy, mass, and momentum balances of drying air and droplets in the dryer chamber. The results were compared to drying of silica nanoparticles. The simulation analysis exposed dissimilarities between drying mechanisms of hydroxyapatite and silica slurry droplets. Hydroxyapatite droplets required longer drying times. Higher drying air temperature and lower humidity were detected in the chamber during drying of hydroxyapatite droplets. Significant temperature differences were observed between the surface and center of agglomerates. We also found lower rates of external heat and mass transfer between drying air and droplets, higher heat transfer resistance, and heat accumulation in the crust layer of hydroxyapatite droplets. We showed that the larger density, lower heat conductivity, and higher heat capacity of hydroxyapatite were responsible for the differences in drying mechanisms of hydroxyapatite and silica. The obtained results are significant for designing an industrial-scale spray dryer capable of producing nanoparticle agglomerates for environmental applications.