Various mechanisms have been proposed to explain the nonphotochemical laser-induced nucleation (NPLIN). Identifying the dominant mechanism requires addressing a large set of experimental parameters with a statistically significant number of samples, forced by the stochastic nature of nucleation. In this study, with aqueous KCl system, we focus on the nucleation probability as a function of laser wavelength, laser intensity, and sample supersaturation, whereas the influence of filtration and the laser-induced radiation pressure on NPLIN activity is also studied. To account for the nucleation stochasticity, we used 80-100 samples. The NPLIN probability showed an increase with increasing laser intensity. The results are different from the previous report, as a supersaturation independent intensity threshold is not observed. No dependence of the NPLIN probability on the laser wavelength (355, 532, and 1064 nm) was observed. Filtration of samples reduced the nucleation probability suggesting a pronounced role of impurities on NPLIN. The magnitude and the propagation velocity of the laser-induced radiation pressure were quantified using a pressure sensor under laser intensities ranging from 0.5 to 80 MW/cm². No correlation was found between the radiation pressure and NPLIN at our unfocused laser beam intensities ruling out the radiation pressure as a possible cause for nucleation.

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The principal objective of the research focuses on the intensification of the batch and continuous crystallization processes through enhanced nucleation control, proper plug flow conditions in continuous tubular crystallizers and development of advanced image analysis based PAT tool for process monitoring. Nucleation control is addressed through manipulation of the number of crystals in the crystallizer; by either controlling the rate of nuclei formation or through dissolution of the excess nuclei to limit the nucleation overshoot or through continuous seeding in case of flow crystallizer to suppress nucleation in the tubes. The following topics are addressed: 1. The efficiency of the Direct Nucleation Control(DNC) strategy using microwave heating. 2. Induction of high nucleation rates at low supersaturation by the application of laser or ultrasound energy. 3. Combination of the ultrasound assisted internal seed generation in the continuous tubular crystallizer, under plug flow conditions. 4. Characterization of nucleation and the crystal properties through development of in-situ imaging based PAT technology. @en

In this study, an in situ imaging system has been analysed to characterize the crystal size, the shape and the number of particles during a continuous crystallization process in a Continuous Oscillatory Baffled Crystallizer (COBC). Two image analysis approaches were examined for particle characterization in the suspension containing both small nuclei and larger grown crystals (nonspherical and irregular in shape). The pattern matching approach, in which the particles are approximated to be spherical, did result in an overestimation of the size. Alternatively, a segmentation-based algorithm resulted in reliable crystal size and shape characteristics. The laser diffraction analysis in comparison to the image analysis overestimated the particle sizes due to the agglomeration of particles upon filtration and drying. The trend in the particle counts during the start of crystallization process, including nucleation, determined by the image analysis probe was comparable with the one measured by FBRM, highlighting the potential of in situ imaging for process monitoring.

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The use of a continuously operated oscillatory flow baffled crystallizer (COBC) has been promoted as a promising alternative crystallizer design for continuous crystallization because of the claim, based on dispersion of liquid, that plug flow can be achieved. Plug flow can lead to uniformity in product quality, if good control over nucleation and the growth of crystals is also achieved. In this study a residence time distribution (RTD) analysis was made for both homogeneous (methylene blue-water) and heterogeneous tracer system (melamine-water). In literature it is proposed, on the basis of homogeneous tracer experiments only, that the velocity ratio ΨΨ (the ratio between the oscillatory velocity and the superficial velocity of the imposed flow) is sufficient to identify optimal operating conditions for plug flow in COBC. Multiple combinations of amplitude and frequency result in the desired ΨΨ value. Our results show that operating at high amplitudes increases dispersion, reducing the plug flow like mixing. Thus ΨΨ alone is not sufficient for optimizing the mixing. Our study for the first time compares dispersion of homogenous and heterogeneous tracer in the commercially available DN15 system, addressing the knowledge gap in handling solids in COBC. Comparable responses were obtained with both the tracer systems for changes in the oscillatory flow variables. Operation at relative low amplitudes was optimal to obtain plug flow like behavior, even with 10% (w/w) slurry with no problems of the particles settling. The optimal operating condition for minimal dispersion was clearly different for the homogenous and the heterogeneous system.@en

The control of nucleation in crystallization processes is a challenging task due to the often lacking knowledge on the process kinetics. Inflexible (predetermined) control strategies fail to grow the nucleated crystals to the desired quality because of the variability in the process conditions, disturbances, and the stochastic nature of crystal nucleation. Previously, the concept of microwave assisted direct nucleation control (DNC) was demonstrated in a laboratory setup to control the crystal size distribution in a batch crystallization process by manipulating the number of particles in the system. Rapid temperature cycling was used to manipulate the super(under)saturation and hence the number of crystals. The rapid heating response achieved with the microwave heating improved the DNC control efficiency, resulting in halving of the batch time. As an extension, this work presents a novel design in which the microwave applicator is integrated in the crystallizer, hence avoiding the external loop though the microwaves oven. DNC implemented in the 4 L unseeded crystallizer, at various count set points, resulted in strong efficiency enhancement of DNC, when compared to the performance with a slow responding system. The demonstrated crystallizer design is a basis for extending the enhanced process control opportunity to other applications.

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