# Exploring Convective Drying Behavior of Hydroxide Sludges Through Micro-Drying Systems
## 3 Results and Discussion  
### Drying Kinetics  
The drying process was controlled by repeatedly measuring the sample weight over a period of approximately 2 h, under different drying conditions. Figure 5 shows the variation of the sludge mass during the drying time at different temperatures for each air velocity studied. From the 3 graphics it's possible to say that higher temperatures have a bigger mass loss over the time, as expected. The drying time difference between the lowest and highest air speed is near to 100%; meaning that, sludge dried two time faster.  
The mentioned above can be see easily in Figure 5 which shows the moisture content versus time under varying temperature (70-110 \({}^{\circ}\)C) and air velocity (1-3 m/s). At higher temperature (e.g., 110 \({}^{\circ}\)C), moisture content decreases more rapidly over time due to the enhanced evaporation rate and increased internal moisture diffusion. The curves for lower temperatures (e.g., 70 \({}^{\circ}\)C) exhibit a slower decrease in moisture content, indicating a prolonged drying process.  
Similarly, higher air velocities (e.g., 3 m/s) further accelerate the moisture removal by reducing the thickness of the boundary layer around the material and improving convective  
Figure 5: Moisture content versus time.  
mass transfer. Lower air velocities (e.g., 1 m/s) result in a slower drying process as the driving force for moisture evaporation is less effective.  
Figure 6 shows the drying rate as a function of sample water content. As shown, a drying curve can be divided into three parts, a preliminary heating zone, where solid temperature and drying speed increase and gradually reach their maximum value, and a relatively narrow plateau, where solid temperature and drying rate are maintained at their maximum.  
The maximum mass flux and the drying rate both increase with the superficial air velocity, similarly to the mass transfer coefficient. Moisture is maintained keeping the driving force for mass transfer constant. However, the external solid surface begins to decrease due to shrinkage. As we can see in this figure, the moisture content of approximately 4.5 kgwater/kggs, the mass flux of the drying rate is no longer constant, as illustrated in Figure 6. This implies that the driving force starts to decrease. Consequently, the moisture at the air-solid interface decreases due to limitations in internal heat and/or mass transfer. Shrinkage of the solid continues to affect the external surface. Therefore, the evolution of the drying rate is represented by the combined effects of shrinkage and limitations in internal transfer.  
Figures 6 and 7 illustrate the impact of temperature and air velocity on the drying flux and rate. As shown, increasing the temperature enhances the drying flux and rate due to  
Figure 6: Drying rate versus moisture content.  
a stronger vapor pressure gradient, which accelerates moisture evaporation and internal diffusion. Similarly, higher air velocity improves the removal of moisture from the surface by reducing the boundary layer resistance, maintaining a steady drying rate. These results highlight the combined effect of these parameters, where optimal temperature and velocity significantly improve drying efficiency.  
The findings of this study demonstrate that increasing air temperature and velocity significantly enhances drying flux while reducing drying time, which aligns with the observations of [17]. However, while their study focused primarily on overall drying kinetics, it did not provide insights into the structural evolution of the sludge during drying. In contrast, our approach reveals a direct correlation between shrinkage dynamics and moisture content evolution, offering a more detailed mechanistic perspective on the drying process.