Microfluidic Approaches for the Controlled Synthesis of Particles
Simon Kuhn,
Professor, Department of Chemical Engineering,
KU Leuven
Microfluidic flow reactors offer several advantages compared to conventional batch reactors, such as improved control, increased performance, and enhanced process safety. The integration of crystallization in these devices has remained difficult, as crystals tend to clog the flow channels. Both active (with an externally applied force, e.g. ultrasound) and passive methods (without external forces, e.g. multiphase flow) have been proposed to tackle this issue. Many crystallization processes rely on the addition of seeds to induce secondary nucleation. Seeding in continuous microfluidic reactors is rarely done, as the seeds are continuously flushed out and the clogging susceptibility increases. This contribution presents a seeded microfluidic nucleation section which can be used for continuous cooling crystallization. The performance of the off-line continuous seeding platform is established via the seed delivery efficiency, a measure for the seed transport through the seeding module, for constant and oscillatory flows. Second, the yields of seeded and unseeded crystallization are evaluated in the presence and absence of microbubbles. A statistically significant increase in the net yield was obtained when comparing unseeded and seeded crystallization, which can be attributed to the increased nucleation rates because of secondary nucleation. It is shown that also in the presence of seeds, the addition of microbubbles increases the productivity. Advancing to zeolite synthesis, a seeded milli-fluidic crystallizer is developed, which also features low frequency ultrasound integration, which is known to speed up crystallization kinetics during the synthesis thanks to the formation and collapse of cavitation bubbles, creation of local hotspots, intense mixing and enhanced dissolution of the amorphous species, which shorten induction time and increase growth rate. This system enables to compare the synthesis conditions in batch, silent and sonicated, and in continuous, silent and sonicated, in terms of residence time, crystallinity, and solid yield. Secondly, the change in the product characteristics, such as morphology, crystal size and crystal size distribution, pore size and its distribution in the framework is characterized. This work provides valuable insights into the design and operation of continuous crystallization processes.
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