Heat and mass transfer implications of droplet injection in a riser system: a CFD-DEM approach

In risers, important catalytic processes occur which benefit from the intensive heat and mass transfer in this type of reactor. Typically the reactants are introduced in the liquid phase in the form of small droplets which rapidly evaporate. In this work the effect of this evaporation on the heat and mass transfer is studied using a computational fluid dynamics-discrete element method approach, treating the gas as a continuous phase and the particles and injected liquid as discrete elements. The closures for particle-droplet interaction were obtained from advanced DNS computations reported in literature. The simulations are validated with experiments conducted using air, glass beads and water in a pseudo-2D riser with dimensions 1.57 m x 0.07 m x 0.0065 m. The results indicate that under the experimental conditions (fast fluidization regime and temperatures between 20°C and 150°C), evaporation primarily occurs from the liquid deposited on the particles surface. Significant radial gradients in water vapor concentration prevail, with high concentrations near the wall close to the nozzle. The overall thermal behavior, including asymmetries in the radial profiles, is driven by the strong solids circulation in the bottom section of the riser.