Mass Transfer: Horizontal Fluidized Bed Dryer

Horizontal Fluidized Bed Simulation for Soil Drying in ANSYS Fluent

Introduction

Solid particles, when sufficiently small, can behave like a fluid under vertical flow conditions—a phenomenon known as fluidization. By applying an upward flow to a bed of fine particles, the drag force exerted by the fluid on the particles can balance the gravitational force acting on them, causing the particles to levitate and exhibit fluid-like behavior. This phenomenon is widely used in industrial applications, including increasing residence time for burning waste biofuels and supporting catalytic processes in petroleum production. This project simulates a horizontal fluidized bed designed to dry incoming soil containing moisture. Soil enters the bed at a mass flow rate of 1 kg/s with a water mole fraction of 0.1, while air flows in from the bottom at 1.7 m/s and 393 K, driving evaporation of the moisture contained within the soil as it passes through the fluidized bed.

Geometry and Mesh

The geometry is two-dimensional, with a fluidizer height of 0.5 m and a length of 2 m. It was designed in SpaceClaim and meshed using ANSYS Meshing with a structured mesh totaling 22,000 elements.

Methodology

The simulation was solved as unsteady, with gravity activated to capture its effect on the soil particles and the energy equation enabled to resolve thermal behavior. The Eulerian multiphase approach was used to model the two-phase system, with each phase containing two species defined through the species transport model. The primary phase is a gas composed of air and water vapor, while the secondary phase is granular, with a particle size of 0.0018 m, containing soil and liquid water as its constituent species. Mass transfer between liquid water and vapor was captured using the evaporation-condensation method with the Lee model. Viscous effects were modeled using the standard k-ε model with standard wall functions, chosen for its robustness and relatively low computational cost.

Results and Conclusion

Contours of temperature and species mass concentration illustrate the progressive evaporation of soil moisture within the fluidized bed, showing a decrease in liquid water concentration accompanied by a corresponding increase in vapor concentration. The air temperature drops to the saturation temperature of water by the outlet, while the soil temperature remains essentially constant throughout the bed, indicating that the heat lost by the gas phase is consumed by the evaporation process rather than by heating the soil. The gas-phase pressure drop across the fluidized bed was calculated as 284.8 Pa. Key quantitative results are summarized below:

These results confirm that the horizontal fluidized bed effectively facilitates moisture removal from the soil through convective drying, with the coupled heat and mass transfer behavior consistent with the physical expectations of an evaporation-driven fluidization process.