Figure 10. Comparison of crystal size distribution under different MACC and conventional cooling crystallization at different crystallization period. (a) 5 min after the initial nucleation (or seeding); (b)10 min after the initial nucleation (or seeding); (c) terminal products.
Furtherly, on line detective images revealed more detailed information on the diverse crystallization process occurred in the crystallizer (Figure 11). At 5 min after initial nucleation (or seeding), uniform crystals can be detected in MACC operation, which validated the effective auto-seeding function and controllable secondary nucleation in the crystallizer. While, for common CC, only limited number of tiny nucleus can be observed. At 10 min after initial nucleation (or seeding), obvious crystal growth with uniform growth rate can be detected in MACC operation compared to common CC (with seed and without seed). The spontaneous nucleation caused the crystallizer in conventional CC filled with the tiny crystals, and the two kinds of seed CC showed different degrees of secondary nucleation.
The terminal crystal product manufactured by MACC had a regular shape, smooth surface as desirable crystal habit, big particle size without defects. However, there were too many defects on the crystal surface and agglomeration among the terminal product of CC without seeding, and the purity was only 99.0 wt% or less (2nd pure grade for industrial application). It can be seen that the terminal products manufactured via PTFE MACC and PES MACC both had significantly superior crystal purity (>99.5 wt%, exceeding 1stpure grade for industrial application) and desire morphology compared to all the conventional cooling crystallization (with seed and no seed). PTFE MACC has the most concentrated crystal size distribution (CV=7.7, mean size>1.35 mm) and the best crystal morphology.