CFD-DEM investigation of the gas-solid flow characteristics in a fluidized bed dryer

Wet granules drying in a fluidized bed is a complex process and the process involves heat and mass transfer between gas and solid phases as well as the moisture changes in the granules. CFD-DEM model was employed to investigate the wet granule flow characteristics, the heat and mass transfer process in a fluidized bed drying process. The proposed model considered the liquid bridge force varying with granule moisture content and was validated by experimental data. Due to the computational capability restriction, pseudo 2D model simulations were first employed for the whole drying process and then 3D model simulations were performed at certain moisture contents to investigate the flow dynamics in more detail. The effects of granule number and the changing liquid bridge force on the drying process were further analyzed. Granule agglomeration was generated in the drying process at high moisture content due to the liquid bridge force, which affects the fluctuations of pressure and bed height. Moreover, the heat and mass exchange between the gas and solid phases mainly occurred in the interface of bubble phase and granule phase. With the increase of bed height, the granule volume fraction decreased and the transfer coefficients between these two phases decreased.