Continuous Slug Flow Crystallization: Impact of Design and Operating Parameters on Product Quality

Abstract A slug flow crystallizer for cooling crystallization with focus on enhanced product quality control in terms of narrow particle size distribution (PSD) was designed and characterized. Emphasis was put on slug flow stability while maintaining a convex shape of slugs to make use of the advantage of a narrow residence time distribution. Measurements of the latter one showed plug-flow-like behavior for the liquid and solid phase independent of the operating conditions within the range investigated. A tube-in-tube temperature concept led to smooth cooling from 50 to 31 ° C avoiding supersaturation peaks. Seeded cooling crystallization of aqueous l- alanine solution successfully led to reproducible product PSDs avoiding secondary nucleation. Thereby the crystals’ median diameter was increased by 240  μ m. Within a residence time of 10.5 min only, a relative process yield of approximately 83% was achieved. To identify optimized operating conditions for narrow PSD and reduced agglomeration, the effect of total volume flow rate, slug length, and cooling rate on the PSD and agglomeration degree was quantified. A high flow rate (40 mL min − 1 ) and a high cooling rate (3.6 K min − 1 ) reduced agglomeration to its minimum and shifted the size distribution of single crystals without its broadening by in average 90  μ m.