# Convective-Microwave-IR Hybrid Drying of Kaolin Clay--Kinetics of Process
## 2 Materials and Methods
### Drying Procedures  
All drying tests were carried out in a laboratory hybrid dryer, whose scheme and view are presented in Figure 1. This apparatus allows simultaneous drying using three different techniques:  
* convective--hot air;
* microwave;
* infrared. The hot air for convective drying is provided by the air heating system (6) consisting of the electric heater (P = 2 kW) and fan (P = 1 kW). A microwave with a frequency of 2450 MHz is produced by the water-cooled magnetron (7), which works in continuous wave (CW) mode. The power of the microwaves is adjusted by the plate current from 100 to 500 W (effective power). The IR electric heater (3), located about 150 mm above the ceramic pan, supplies the IR radiation directly to the sample surface. The power of the IR may be set to 100 and 250 W. A balance (8), type WPS 2100/C/1 (precision 0.01 g), produced by Radway (Radom, Poland), was used to measure the reduction in the sample mass during the process. The temperature of the sample surface was measured using a pyrometer (4), MI model (precision 1 \({}^{\circ}\)C), produced by Raytek (Wilmington, NC, USA) and placed in the corner of the dryer chamber. The pyrometer is a non-contacting device, which intercepts and measures the thermal radiation emitted by the material. Air velocity was measured with the use of a hot wire anemometer, model CTV 100 (precision 0.1 m/s), produced by KIMO (Toronto, ON, Canada). The relative humidity (RH) of the air used for drying was measured with a humidity and temperature transmitter, model Hygrotest 600 DHT-20/120, produced by Testo (Titisee Neustadt, Germany). The air RH ranged between 20 and 22%, that is, 21% on average. The electrical network analyzer, model MPR53S, produced by Entes (Istanbul, Turkey), was used to measure the energy consumption (EC) during each process (precision 0.1 kWh). The drying process was controlled by the industrial PLC controller (2). All the measured parameters were recorded throughout the whole process and stored on a standard personal computer (1) equipped with data acquisition software. The software controlling and collecting the research data was provided by the dryer manufacturer, Ertec-Poland (Wroclaw, Poland). The program was written in C++ and enables the setting of process parameters, such as the temperature and rate of air, the microwave power, the duration of the IR power, drying phase, the end condition of the \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline
**Component** & **SiO2** & **TiO2** & **Al2O3** & **Fe2O3** & **MgO** & **CaO** & **Na2O** & **K2O** & **P2O5** & **SO3** & **Sum** \\ \hline \% mass & 53.3 & 0.5 & 42.5 & 0.4 & 0.1 & 0.1 & 2.0 & 0.6 & 0.2 & 0.1 & 99.8 \\ \hline \end{tabular}
\end{table} 
Table 1: Composition of the KOC kaolin clay [19]. Figure 1: Schematic and photo of the hybrid dryer: 1—computer, 2—controller, 3—IR heater, 4—pyrometer, 5—drying chamber, 6—air heating system, 7—microwave generation system, 8—balance, A, B, C—temperature, velocity and RH of air measurement points. drying phase and data sampling frequency. It also conducts a fully automatic acquisition of experimental data, such as sample temperature and mass, temperature, flow rate, drying air humidity and reflected power of the microwaves. During the research, 7 different drying schedules were tested experimentally. The following settings of drying parameters were chosen:  
* air temperature T\({}_{\text{a}}\) = 85 \({}^{\circ}\)C;
* air flow velocity \(\text{v}_{\text{a}}\) = 1.2 m/s;
* microwave power P\({}_{\text{MW}}\) = 100 W (if used);
* infrared power P\({}_{\text{IR}}\) = 250 W (if used).