# Convective-Microwave-IR Hybrid Drying of Kaolin Clay--Kinetics of Process
## 3 Results and Discussion
### Kinetics of Process and Quality of Products
Unfortunately, the high drying rate negatively influenced the quality of the material. The samples were cracked not only on their upper surface but also on their lateral surface (Figure 3b). This was caused by the vapor pressure inside the sample. Due to the poor quality of the samples obtained in the previous schedule, in the next schedule of CV + MW\({}_{60}\), the duration of microwave exposure was shortened to the first 60 min of the process. This was aimed at utilizing the most effective period of convective microwave drying (increasing and maintaining a high drying rate) and limiting exposure to microwaves when the efficiency of the process decreased (drop in drying rate), as observed in Figure 3a. One can see that the shortening of microwave exposure negatively affected the drying kinetics (Figure 4a). Although the maximum drying rate in this process attained a value similar to one observed previously (1.1 g/min), after switching off the magnetron, it started to decrease rapidly, which led to a very long drying time (approx. 300 min). A positive observation is that the quality of the samples improved slightly compared to previous programs (Figure 4b). The cracks were smaller, and the surface was more uniform. Figure 2: Dry basis moisture content (black line), sample surface (red dashed line) and drying rate (blue cross) for schedule No 1—CV (**a**) and appearance of dried sample (**b**). In schedule 4, convective drying was enhanced with microwaves and also with infrared radiation (IR) for the first 40 min of drying. Infrared radiation is a very effective source of heat and can be delivered to the material almost lossless. Its use in program 4 was aimed at accelerating the evaporation of moisture from the liquid film present on the surface and maximally shortening the first drying period. As can be seen in Figure 5a, the use of three different energy sources had a very positive effect on the kinetics of the process. The drying rate increased rapidly in the first stage to approximately 1.6 g/min, resulting in a significant drop in moisture content. Unfortunately, after 40 min and turning off both the microwaves and infrared, the drying rate began to drop sharply, and finally, the total drying time was similar to that observed in the previous program (approx. 300 min). The temperature curve clearly indicates that extending the time of exposure to microwave and infrared radiation could adversely affect the quality of the material and even lead to its bursting. Such effective heat sources caused very fast heating of the sample surface to almost 80 \({}^{\circ}\)C. The quality of the product was very good, as there were no visible cracks on the surface of the samples (Figure 5b). Figure 4: **Figure 3**. Dry basis moisture content (black line), sample surface (red dashed line) and drying rate (blue cross) for schedule No 2—CV-MW (**a**) and appearance of dried sample (**b**). In the next schedule, convective drying was enhanced only with IR for the first 80 min of the process. This IR enhancement duration results from preliminary research and is assumed to coincide with the first drying period. During this period, the surface of the material is covered with a liquid film, which can be effectively evaporated using IR radiation. As a result, it was expected to shorten the first drying period. As can be seen, the kinetic curves did not differ significantly from the previous schedule (Figure 6a). The maximum drying rate was lower compared to CV-MW and CV + (MW \(-\) IR)\({}_{40}\) (ca. 0.9 g/min) and dropped instantly after the IR was turned off. This resulted in a long drying time of ca. 300 min. Furthermore, at the end of the strengthening period, the temperature of the sample surface increased rapidly to approximately 80 \({}^{\circ}\)C, and after turning off the IR heater, it dropped to a value of approximately 56 \({}^{\circ}\)C just as quickly. The rapid change in temperature caused drying stresses and numerous cracks on the surface of the sample (Figure 6b). However, the quality of the product was acceptable.