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).