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The effect of process parameters on the droplet drying process, which is crucial for tailor‐made particle designs in spray drying processes, was investigated. Because of the challenging observability in spray towers single droplet experiments were carried out using acoustic levitation. The influence of the air humidity on the particle and solid layer formation of single mannitol‐water droplets and temperature profiles of the complete drying process were experimentally determined. Also, the influence of the air humidity on the solid layer formation and the droplet temperature evolution was investigated.

Summary: Droplet polymerization of N-vinyl-2-pyrrolidone (NVP) and sodium acrylate (NaAA) was carried out in an acoustic levitator for different ambient temperatures and relative humidities. The resulting particle morphologies were compared to particles of two crystalline systems (mannitol and ammonium sulfate) and two disperse systems (silica and styrene -butyl acrylate dispersion), all obtained under similar drying conditions. NVP was found to form a higher amount of crystals with lower temperature and relative humidity, contrary to the behaviour observed for mannitol and ammonium sulfate. The processes of both, polymerization and drying of NaAA lead to similar morphologies at low temperatures and humidities, probably caused by the precipitation of NaAA during polymerization.

Summary: Acoustic levitation was investigated as a model for spray processes. The influence of different parameters on the drying process of aqueous polyvinylpyrrolidone (PVP) solutions was studied and compared to the evaporation of water. The adequacy of acoustic levitation as model for spray processes was demonstrated. Experiments with water and aqueous PVP solutions indicated no dependency of the droplet size on the drying process for droplets with a diameter between 300 mm and 1.5 mm. Particles dried in an acoustic levitator displayed good accordance of morphology with those obtained in a spray tower. Surprisingly the addition of PVP to water resulted in faster evaporation of the solvent. Mathematical models of single droplets within a spray process typically refer to spherically symmetric droplet geometries. The simulation of other morphologies and their evolution throughout the process is still very challenging. A new drying model based on a fully threedimensional meshfree approach is under development and shows good agreement to basic established models regarding the drying of a single droplet.