# Convective-Microwave-IR Hybrid Drying of Kaolin Clay--Kinetics of Process
## 3 Results and Discussion  
### Kinetics of Process and Quality of Products  
In the first part of research, pure convective drying was performed. The drying kinetics and the appearance of the sample after drying are presented in Figure 2. The characteristic periods of the convective process, namely, preheating (Figure 2a), the constant drying rate period CDRP (2) and falling drying rate period FDRP (3), were observed. At the beginning, the material heats up to the so-called wet bulb temperature, losing a negligible part of moisture. The drying rate increases up to ca. 0.4 g/min. When the temperature of the surface reached the temperature of the wet bulb, the first period of drying started. The characteristic of this period is the constant temperature of the material being dried and the almost constant rate of moisture evaporation (linear change in moisture content over time). The constant temperature of the material, despite the continuous supply of thermal energy, is the result of the high moisture content and effective transport from the inside to its surface. It is often assumed that the surface of the material is covered with a liquid film during this period, and the drying process can be considered as evaporation from the free surface. Due to the constant value of the heat of evaporation, the rate of drying is determined by the amount of energy supplied to the surface and factors of external resistance related to the removal of evaporated moisture from the surface of the material to the core of the drying agent. As drying proceeds and the material's moisture content decreases, the first dry patches appear on the body's surface, and its temperature begins to rise. This is due to a decrease in the efficiency of moisture transport inside the material. Because of its small amount, it is no longer able to reach the surface/evaporation front so quickly, which causes heating of the surface. When the temperature of the material begins to increase, the second drying period begins, where the rate of the process diminishes and is limited by internal resistance: diffusion of moisture from the inside of the material to the surface/evaporation front (nonlinear change of moisture content over time). The transition point occurs after crossing the so-called critical moisture content. Taking into account the temperature of the material, the transition point from the CDRP to the FDRP was observed at a moisture content of approximately 0.15. If the drying rate curve is analyzed, it may be stated that its value is not constant and decreases slightly throughout the first period of drying. However, this trend is completely different from the character of the curve for the heating period and the second drying period and can thus be assumed to be constant. A maximum drying rate was observed during CDRP (ca. 0.46 g/min). The drying time was very long (360 min). The quality of the dried samples was very poor, with the upper surface of the sample cracked significantly (Figure 2b). In the second schedule of drying, pure convective drying was enhanced with microwave energy. Microwave radiation is a very efficient source of energy and heats the dried body in its entire volume, not only from the surface. The moisture evaporates not only on the surface of the body but also inside it. As vapor diffusion is much easier than liquid diffusion, the moisture transport is more efficient, and drying is faster. In addition, the increase in pressure inside the material, caused by the production of vapor, can also lead to nonevaporated moisture being pushed to the surface--the so-called plug flow--which further speeds up the process of drying. All these phenomena accelerate the drying and positively affect the kinetics. The kinetic curves (Figure 3a) clearly indicate a positive influence of the microwaves on the rate of the process. It increased nearly three times and reached a maximal value of ca. 1.2 g/min, which resulted in significant shortening of the drying time to 110 min.