4.4 MACC and the conventional cooling crystallization (CC)
With the known thermal and interfacial property of PTFE and PES
membrane, MSZW for nucleation operation region, and nucleation kinetic
relevant parameters (Induction period and surface tension) with membrane
involved, we can furtherly investigate MACC with the other conventional
CC.
Six different cooling crystallizations modes were discussed, MACC with
PTFE hollow fiber membranes and PES hollow fiber membranes, common CC
without membranes (average cooling rate is 0.1
℃·min-1), rapid CC (average cooling rate is 0.5
℃·min-1), CC with 0.5 wt% seed and 1.5 wt% seed
addition. Except the rapid CC operation, the other operations have the
same pre-setting cooling curve after the nucleation period, the terminal
cooling temperature is also the same to ensure the same yield.
For the different nucleation inducing mechanism of the investigated
tests, the solution temperature fluctuation in the crystallizer during
nucleation period is an important evidence of the relevant controlling
performance. The detective temperature in the crystallizer were listed
in Figure 9. It can be seen from the figure that the solution
temperature fluctuation peak of the PES MACC reached its peak at an
early stage (around 0.20 ℃ at 100 s), which represented the latent heat
mainly released by nucleation. This value was slightly higher than that
of the PTFE MACC (0.08 ℃) with the similar membrane area. In addition,
the peak of PES MACC appeared earlier than that of PTFE MACC, which also
indicated that PES membrane is easier for induced heterogeneous
nucleation. In fact, a moderate, controllable nucleation is longing for
all the CC operation, and the smaller solution temperature fluctuation
peak induced by nucleation will benefit the following nucleation and
growth procedure. Herein, PTFE MACC possessed a better performance than
that of FES MACC.
As for artificial seeding CC, with optimized crystal seed amount and
seed timing, it is possible to maintain the solution temperature
fluctuation under a favorable range
(the
peak for 1.5 wt% seed CC is 0.02 ℃ and the peak for 0.5 wt% seed CC is
0.21 ℃, shown in Figure 9). While, the temperature fluctuation peak
appeared much more later than that of MACC operation (around 800 to 900
s after seeding, which is one order of magnitude longer than MACC). This
prolonged gap between the artificial seeding timing and bust nucleation
heat release may stand for the persistent secondary nucleation. Because
the artificial seeding introduced the exceeding amount of heterogeneous
nucleation interface all at once without well dispersion, thus, in the
crystallizer, the added seeds disperse with the stirring and will cause
the unwanted, exceeding nucleation. As an advantage of the MACC, by
adjusting the temperature and flow rate in tube side and shell side of
the membrane module, it is convenient to control the nucleation rate and
auto-seeding amount that transferred into the crystallizer.
Additionally, this membrane induced crystal seeds had uniform size
distribution and were predispersed evenly by the crystallization
solution.
As for spontaneous CC at different cooling rate, they both suffering the
explosive nucleation at a later time, and the temperature fluctuation in
the crystallizer increased significantly. With the increased cooling
rate, this explosive nucleation was also accelerated (from 0.70 ℃ to
1.50 ℃, correspondingly).