Enhanced Product Quality Control through Separation of Crystallization Phenomena in a Four-Stage MSMPR Cascade

Abstract

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.

Nucleation in continuous flow cooling sonocrystallization for coiled capillary crystallizers

“…For a continuous stirred tank, which is especially constructed for nucleation of the system used, it is known from the literature [10] that nucleation takes place from subcooling of about 2 K (46°C to 44°C, or supersaturation of 1.02) with an induction time of 1.68 min. It should also be considered that for continuously operated crystallizers, not only the MZW can be considered, but also the residence time.…”

“…In addition, high yield, high purity, and specific particle morphology are often required for sufficient product quality. To avoid the preparation of seed crystals some concepts have already been developed to integrate the nucleation into the crystallization process [9][10][11][12][13][14].…”

Equipment and Separation Units for Flow Chemistry Applications and Process Development

“…In general, continuous crystallization is performed in two basic apparatuses, namely, on the one hand in the continuously stirred tank (CST), especially the mixed-suspension, mixedproduct-removal (MSPMR) [26][27][28][29][30], and on the other hand in the plug flow (PF) [31][32][33][34][35][36][37][38]. There are many variants of both types, which are described in literature [39,40].…”

Enhancing continuous reactive crystallization of lithium carbonate in multistage mixed suspension mixed product removal crystallizers with pulsed ultrasound

“…Mixed-suspension mixed-product-removal (MSMPR) crystallizers are considered as the most commonly used continuous crystallizers because they are easier to convert from batch-to-batch operation and lower maintenance cost [5] , [6] . More importantly, single stage and multistage MSMPR crystallizers can provide sufficient time for nucleation and growth, which is appropriate for processes with slower conversion, such as reactive crystallization [7] .…”