A four-stage cascade of MSMPR crystallizers
has been designed and set up for the production of l-alanine
crystals to achieve an enhanced control of product quality in terms
of crystal size and size distribution. The cascade is characterized
by a local separation of the crystallization phenomena nucleation
and crystal growth as well as by the fact that no pumps were used
to transfer the suspension from one vessel to the next. Instead, a
unique vessel design was chosen in which lateral and vertical overflow
tubes were installed, which enabled a gentle suspension transfer by
gravity. Controlled induction of nucleation was achieved by gassing
with synthetic air, leading to crystals with a median diameter of
35 μm. These crystals grew about 85 μm over the entire
cascade, whereby agglomeration and breakage were mostly avoided. To
reach a steady-state operation within the first three stages, a startup
phase of approximately 6.5 h was necessary. The continuous operation
of the entire MSMPR cascade could only be maintained for over 1 h,
underlining the complexity and uncertainty involved in the crystallization
process.
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