In this study, we report on experiments designed to deconvolute the particle breakage kinetics and mechanism from the parallel phenomena (growth-dissolution, agglomeration) in attrition

The analysis, development, and implementation of novel complex industrial crystallization processes requires kinetic knowledge of not only crystal growth and nucleation but also breakage, dissolution, and agglomeration processes. In this work, the crystal growth, dissolution, and agglomeration kinetics of sodium chlorate (NaClO3) in aqueous solutions are estimated via seeded batch experiments with in-line particle size distribution measurements. Contrary to previous works, the growth/dissolution kinetics are expressed in terms of the fundamental driving force of crystallization calculated from the activity of supersaturated solutions. The activity-based driving force is roughly 2-fold higher than the commonly used representation, which assumes ideal solutions. By fitting experimental desupersaturation data to mechanistic and empirical growth models, we show that the growth of sodium chlorate is surface integration controlled and is best described by a two-dimensional birth and spread surface nucleation mechanism. The dissolution of sodium chlorate crystals is diffusion controlled and is ∼4 times faster than the growth at an equal initial driving force. Particle agglomeration is substantial in the early stages of crystallization experiments, likely due to a strong increase in the particle number due to initial breeding secondary nucleation upon seeding. The agglomeration rate constant increases with supersaturation and decreases at higher energy dissipation rate (by increased agitation) due to a strong decrease in the efficiency of interparticle collisions. Seeding with material of different sizes does not influence the agglomeration rate constant, although substantial amount of small particles was present in all seeding materials.

Industrial synthesis of enantiopure compounds is nowadays heavily based on the separation of racemates through crystallization processes. Although the application of ultrasound in solution crystallization processes (sonocrystallization) has become a promising emerging technology, offering several benefits (e.g. reduction of the induction time and narrowing of the metastable zone width, control over the product size, shape and polymorphic modification), little attention has been paid so far to the effects of ultrasound on chiral crystallization processes. Several recent studies have reported on the application of acoustic energy to crystallization processes that separate enantiomers, ranging from classical (diastereomeric) resolution and preferential crystallization to new and emerging processes such as attrition-enhanced deracemization (Viedma ripening). A variety of interesting effects have been observed, which include among others, enhanced crystallization yield with higher enantiomeric purity crystals, spontaneous mirror symmetry breaking crystallization, formation of metastable conglomerate crystals and enhanced deracemization rates. The objective of this review is to provide an overview of the effects of ultrasound on chiral crystallization and outline several aspects of interest in this emerging field.

It has been recently observed that a suspension of conglomerate crystals of both chiralities in contact with a solution where racemization takes place can undergo symmetry breaking due to periodic fluctuations in temperature. The interplay between the mechanisms behind this process is still a matter of debate, although there is consensus that growth/dissolution of crystals and racemization in the solution phase play a crucial role. In this work, we present a population balance model (PBM), based on temperature-size dependent solubility, which simulates a system of enantiomeric crystals of both chiralities subjected to Ostwald ripening under periodic temperature fluctuations, in the presence of a liquid-phase racemization reaction. Our simulations reveal that for a racemic system with small initial size asymmetries between the enantiomers complete deracemization is achieved due to temperature fluctuations, whereas isothermal Ostwald ripening only leads to partial enantiomeric enrichment. This implies that size-temperature dependence of solubility is an essential mechanism in temperature cycling-enhanced deracemization. Although enantiopurity is achieved with only growth, dissolution and temperature cycling, other mechanisms, such as breakage, could also be included. While our model qualitatively reproduces many of the experimental observations in temperature cycling-enhanced deracemization, the autocatalytic nature of the process is not captured and, similarly to Viedma ripening, some sort of chiral feedback mechanism (e.g., agglomeration or secondary nucleation) seems to be required to fully describe the process dynamics. The results presented here offer further insight into temperature cycling-enhanced deracemization and help pinpoint the influence of the various mechanisms in the process.