The role of steam as a medium for droplet crystallization

Mannitol and sodium chloride (NaCl) were spray dried using a counter current superheated spray dryer. Superheated steam was found to induce relatively high nucleation during the solidification of the sprayed droplets when compared to hot air. This allowed the production of spherical mannitol particles with very fine crystals at a lower temperature. In addition, superheated steam led to the formation of unique salt microspheres consisting of hollow hopperlike sodium chloride crystals. These unique particle morphologies were not observed in hot air spray drying. Further analysis revealed that higher droplet temperature during the constant rate drying period, under superheated steam conditions, led to the high nucleation phenomenon. Results from this work illustrate the potential of superheated steam as a useful medium for in situ crystallization control in spray dryers.