## 3 Results and Discussion
### Crystallization Study  
#### 3.3.1 Crystal Morphology  
The in-line PVM microscope showed two different structures precipitated in the feed tank. When 80-90% of the feed water was removed, needle-shaped crystals started to precipitate, followed by a precipitation of prismatic shaped crystals (Figure 5a,b, respectively). Both types of crystals were observed under all of the thermal and hydrodynamic conditions that were investigated, negating any correlation between the operating conditions and the type of crystals obtained. However, in all of the cases, needle-shaped crystals appeared before the prismatic crystals, perhaps due to the greater induction time of the latter compared to the former. The crystallization occurred fast when the precipitation point was reached; the needle-shaped structures grew especially faster. This could indicate that two different polymorph structures were precipitated. The same precipitation trend was observed in all of the experiments.  
Figure 4: (a) Flux and (b) conductivity of permeate as a function of recovery factor for the experiment conducted at 14.8 L/h, 21.6 L/h, and 30.7 L/h and feed temperatures of 47.7 \(\pm\) 0.3 \({}^{\circ}\)C, 47.2 \(\pm\) 0.3 \({}^{\circ}\)C, and 47.1 \(\pm\) 0.2 \({}^{\circ}\)C, respectively.  
The obtained crystals were also analyzed by using an optical microscope. Images from the optical microscope showed that the size of the prismatic crystalline structures increased with the temperature (Figure 6). The experiment at the lowest feed temperature precipitated the smallest prismatic crystals (\(\sim\)0.01 mm), while 56 \({}^{\circ}\)C precipitated the largest prismatic crystals (\(\sim\)0.08 mm). The size of the needle-shaped crystals did not change significantly with temperature (length of  
Figure 5: Pictures from an in-line PVM 19 microscope from experiment conducted at 56 \({}^{\circ}\)C and 21.6 L/h. (a) Needle-shaped crystalline structures, (b) needle and prismatic-shaped crystalline structures.  
-0.1 mm). Moreover, the prismatic crystals increased in size with a decrease in the flow rate (Figure 7), but there was no clear tendency for the needle-shaped crystals. The experiments carried out at 47\({}^{\circ}\)C and 14.8 L/h produced larger prismatic crystals and needle-shaped crystals than the other experiments. The needle-shaped crystals in the experiment at 47 \({}^{\circ}\)C and 30.7 L/h were also larger than those in the other experiments, and only a few small prismatic crystals were formed. The experiment at 47 \({}^{\circ}\)C and 21.6 L/h showed both prismatic and needle-structured crystals, but the needle-shaped crystals were smaller compared to the other experiments when the flow rate was changed.  
The images from SEM showed a sharp prismatic structure of the crystalline phases. Compared to the pictures from the optical microscope taken under wet conditions, the needle-shaped crystals decreased in amount. The SEM pictures at different magnifications can be seen in Figure 8. The same tendency of the crystal size was observed from the SEM images, i.e, an increasing size of the prismatic crystals with temperature, whereas no clear tendency was observed for the change in flow rate.  
Figure 6: Optical microscope images of the crystals precipitated in experiments carried out at a flow rate of 21.6 L/h. Magnification 10\(\times\). (**a**) 38 \({}^{\circ}\)C, (**b**) 47 \({}^{\circ}\)C, (**c**) 56 \({}^{\circ}\)C.  
Figure 7: Optical microscope images of the precipitated crystals in an experiment carried out at a feed temperature of 47 \({}^{\circ}\)C and flow rates of (**a**) 14.8 L/h, (**b**) 21.6 L/h, and (**c**) 30.7 L/h. Magnification in all of the cases was 1\(\times\).  
The crystalline phases from the previous described MD/MCr experiments were analyzed with XRD and ICP to determine the purity and composition of the crystals.