On Multiple Nucleation Bursts During Solution Crystallization in Pure and Impure Solvent

International audienceIt is widely accepted that during batch cooling crystallization processes, uncontrolled cooling can produce multiple nucleation events which have detrimental effects on the crystal size distribution (CSD). Controlling the CSD therefore requires mastering nucleation during the whole batch development: this is the starting point of many studies devoted to temperature control during industrial crystallization operations. When nucleation phenomena take place throughout the batch process, the time allocated to every crystal to grow is widespread, which enlarges the width of the final CSD. Successive nucleation phenomena are referred below to as multiple nucleation bursts (MNBs). Oscillatory supersaturation dynamics (OSD) are sometimes assumed to accompany the trend of MNBs, and it is suggested in the following that MNBs should not be misread as oscillatory supersaturation profiles. Experimental results obtained during the crystallization of ammonium oxalate in the presence of impurities (NiSO4) are presented to support and illustrate OSDs crystallizations which, even though they are often presented as a basic pattern of uncontrolled batch crystallization systems, were never observed before. A population balance equations (PBEs) simulation model based on Kubota-Mullin's pinning mechanism is developed to represent the observed oscillations of supersaturation. The latter are shown to be rather likely when inhibiting impurities are present in solution, but it is suggested that oscillations of supersaturation are unlikely in pure solvents. These latter two results shed some new light on the need to control temperature during batch cooling crystallization processes