Figure 9. Comparison of solution temperature fluctuation in the crystallizer during nucleation period (PTFE MACC, PES MACC, seed CC with different seed amount and common CC with different cooling rate were listed).
CSD analysis results confirmed the advantages of MACC on the nucleation control and crystal growth (shown in Figure 10). At the initial period (Figure 10a), MACC and seed CC had the similar CSD, which illustrated the auto-seeding function of MACC. As expected, CC without seed had the widest CSD due to the uncontrollable spontaneous nucleation. While, with the nucleation occurred and crystal growth launched the competition under diverse supercooling degree, the different seeding mechanism began to present significantly different impact on the subsequent crystallization procedure. For the artificial seeding CC, the drawbacks of exceeding heterogeneous seeds interface introduced into the solution system abruptly became manifest. The exceeding secondary nucleation induced by the solid seed leading to the wider and wider CSD in the solution system (Figure 10b), which is an inherent problem that nucleation and growth competition in the same crystallization devices. While, for PTFE and PES MACC, illustrated in Figure 1, the stable nucleation and seed generation were realized in the membrane module, the feed flow suspended with uniform seeds transferred from membrane module to crystallizer effectively isolated the primary nucleation and crystal growth from space-time aspect. This new nucleation induced and seeding mechanism essentially decouple the nucleation and crystal growth, which then provided a feasible approach for controlling crystallization with high space-time accuracy. This new mechanism of MACC at the nucleation period had a profound and lasting effect on the CSD of terminal products (Figure 10c).