Hydrodynamic of a Riser System with Liquid Injection: A Three-Phase CFD-DEM Approach

Riser reactors are frequently applied in the process industry for highly important catalytic processes. In this work, a CFD-DEM model for a riser with liquid injection was developed and experimentally validated. In the CFD-DEM approach the gas phase is treated as a continuous phase by solving the Navier Stokes and continuity equations whereas the liquid droplets and catalyst particles as discrete elements represented via the Newtonian equations of motion. Two phenomena related to the presence of droplets require special attention: i) the effect of liquid deposited on the particle surface on the restitution coefficient and ii) the spatial distribution of the liquid on the particle surface. The former is treated with an energy balance approach that computes an effective wet restitution coefficient considering both, viscous and capillary forces. The latter is handled by computing the surface fraction of each particle that is covered with liquid due to the collision with droplets. This fraction combined with probabilistic rules can predict if the collision between a given pair of particles results in the encounter of wet-wet, wet-dry or dry-dry surfaces. To validate the obtained model, a pseudo-2D lab-scale riser (1.59 m x 0.07 m x 0.006 m) filled with Geldart D particles (ρ = 2500 kg/m³, 𝜙_p =0.85 mm) is used with a nozzle that feeds the liquid with a velocity and average droplet size of approximately 11 m/s and 50 𝜇m, respectively. These conditions ensure a Weber number small enough to promote liquid coating and prevent splashing of the liquid after droplet-particle collisions. The system was simulated and compared with the experimentally obtained pressure profile (from the pressure sensors) and the flow field and porosity profile (obtained using PIV/DIA techniques). The main effect of the liquid injection on the solid fluxes and cluster formation were determined in this work.