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).