## 3 Undercooled Xylitol Crystallization Induced by Bubbling
### Bubbles Observation into Molten Xylitol  
Numerous experiments have been performed in order to get an overall view of the crystallization process by bubbling. Most of them were carried out in reactor n\({}^{\circ}\)1. The air flow is still of 80 L\(\cdot\)h\({}^{-1}\) and the starting temperature of the melt (To) varies from 30 \({}^{\circ}\)C to 75 \({}^{\circ}\)C. The snapshots in Figure 3 show the different stages of bubbling from the side whereas the top views are shown in figure 4. The initiation and the progress of the crystallization in the experiment carried out at 50 \({}^{\circ}\)C (i.e., undercooling degree \(\Delta\)T = 45 \({}^{\circ}\)C) is the same whatever the applied initial thermal conditions.  
The crystallization process follows the next steps:  
* Bubbles are formed continuously at the submerged orifice. First they grow, detach from the orifice and rise in the melt up to the liquid free-surface. The bubbles generation is periodic and induces an oscillatory motion of the liquid, with a major circulation cell which fills the entire column. Snapshots in Figure 5 illustrates the beat of the free-surface of the liquid.
* At the free-surface of the liquid, the bubbles burst in a very short time and generate smaller bubbles (see Figure 6 (colored circles)). Part of these smaller bubbles are caught by the fluid and dragged by the flow cell as shown in Figure 6. As it can be seen in Figure 7, the small bubbles are then dispersed within the liquid and their amount increases with time.
* After an average stirring time of 5 min of the mixture \"bubbles/Xylitol\", Xylitol starts to crystallize. Crystallization begins on the surface of the bubble, at the interface air/Xylitol, then it progresses in the outward direction forming a spherical solid shell.  
Figure 5: Oscillation (up and down motion) of the free-surface of the liquid with time produced by the periodic bubbling.  
Figure 6: Pictures of the initial stages of bubbling when the small bubbles created at the free surface are captured by the liquid and entrained in its movement. Bubbles dragged by the liquid flow are highlighted by yellow and red circles.  
Different stages of the crystallization process from the bubbling in the undercooled melt to a quite advanced stage of the crystallization process are presented in Figure 8a. Numerous spherical solid white particles can be observed in suspension.  
Six of them with a diameter inferior to 0.5 mm have been extracted as can be seen in Figure 8b. Magnified pictures of the cross-section of two particles showing the presence of a hole in the center of each sphere are presented in Figure 8c. The hole corresponds to the air bubble on which crystallization has been initiated.  
An air injection allows the occurrence of air bubbles and of a flow cell into the molten Xylitol both observed for reactors 1 and 2. These phenomena participate to the formation and the spreading of crystallized Xylitol particles.  
The bursting of the primitive air bubbles at the free liquid surface with production of small bubbles acting as nucleation sites appear, indeed, as a necessary condition for the crystallization of undercooled Xylitol.  
The efficiency of the crystallization process depends thus on (i) the number of the produced nucleation sites; as well as (ii) the extent and the homogeneity of their dispersion within the melt.  
Next section is consequently dedicated to the preliminary thermal study of the previous described experiments but the glass beaker filled with undercooled Xylitol is, this time, thermally insulated.