3.2.2. Drying rate curves and effects of acoustic field
Figs. 8 and 9 showed the evolution of the drying rate curves
during the drying process of lignite assisted by sound waves. As
seen in Figs. 8 and 9, all curves have two stages. The constant rate
period becomes shorter and drying takes place mostly at falling
rate period. This was due to the fact that drying rate reduces with
time or with the reduction of moisture content. The drying rate
rapidly reaches its maximum value after the first period of drying;