Microfluidic Approaches for the Controlled Synthesis of Particles
Simon Kuhn,
Associate Professor,
Katholieke Universiteit 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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