Profiles of temperature, concentration and supersaturation within atomized droplets during flash-crystallization

• Simulation of temperature and concentration profiles in flash-evaporating droplets.
• Buildup of supersaturation inside droplets is short compared to the time of flight.
• Introduced Flash number determines suitability of solute–solvent combinations.
• Induction time of solute is limiting for the flash-crystallization process.

Flash-crystallization is a suspension crystallization process to produce small, fine crystals (L50,3 =  20–90 μm) of substances which are well soluble. A hot and undersaturated solution is atomized into a low vacuum. The small droplets generated partially evaporate and cool down until thermal and mechanical equilibrium is reached. Consequently, they are highly supersaturated in a controlled manner and, thus, enable high nucleation rates. Experiments show that only certain solute–solvent combinations exhibit the high nucleation rates required and, thus, are suitable for flash-crystallization. We show that this observation is not caused solely by the buildup rate of supersaturation, but also by nucleation and growth kinetics. This is concluded by a comparison between the time of flight τf of the droplets in the crystallizer and the buildup time τb needed for the evolution of temperature and concentration profiles inside each droplet. The buildup time of supersaturation τb is negligibly short compared to the time of flight of the droplet. Thus, the driving force for nucleation and growth rate is present during virtually the entire time of flight, and therefore, slow nucleation and growth kinetics are the reason for the behavior observed. The model proposed is in good agreement with data measured.