Heat and mass transfer in fixed bed drying of non-deformable porous particles

A drying model for non-deformable porous spherical particles in a fixed bed passed by a drying agent was developed. The model was based on coupled moisture and heat balance equations, where the moisture molar flux evaporated from a pore was estimated according to the diffusion through a stagnant gas layer. Model equations included various process factors, e.g., characteristic parameters of porous particle (diameter, porosity, and pore tortuosity), fixed bed particles (void fraction, height, initial moisture content, and initial temperature), and drying agent (superficial velocity, input humidity, and initial temperature). Particle porosity and pore tortuosity were selected as adjustable parameters. An experimental study on air drying of fixed bed activated carbon was conducted and the adjustable parameters were fitted based on experimental data. Values of 0.28 ± 0.04 m3/m3 for particle porosity and 1.25 ± 0.15 for pore tortuosity were obtained under studied operating conditions. The drying was faster with an increase in air superficial velocity (0.04-0.42 m/s) and initial air temperature (25-45 °C). The model predicted accurately different experimental data and it could be used in designing and optimizing the convective drying of fixed bed porous particles.