Analysis of the Particle Reynolds Number in a Gas-Solid Down-Flow Reactive System Using Computational Fluid Dynamics

This study examines how the particle Reynolds number evolves in a gas-solid down-flow reactive system, using fluid catalytic cracking of gasoil in a downer reactor as case study. In heterogeneous catalytic processes such as fluid catalytic cracking (FCC), in which particles act as catalyst, the contact between gas and particles determines the overall performance. This contact depends on the turbulent conditions around the particles. Depending on the flow conditions, the gas layer around particles may behave differently, particularly when chemical reactions (kinetics) are present. This study explores how the particle Reynolds number (Re_p) characterizes the hydrodynamics in a gas-solid down-flow reactive system.

The particle Reynolds number is a dimensionless variable that relates the physical properties of the carrier fluid and the particles. It gives information about the flow conditions around the particle under specific conditions of velocity. The present work used computational fluid dynamics (CFD) coupled with Large Eddy Simulation (LES) to conduct the multiphase flow simulations. The hydrodynamic behavior of the system was modeled under an Euler-Lagrange framework. 200000 particles were injected to a three-dimensional down-flow system to perform the hydrodynamic study. Mono-dispersed, 70-micron size particles with a particle density of 1500 kg/m³ were used for the simulations that were conducted with and without chemical kinetics. The catalytic cracking mechanism proposed by Gianetto et al., 1994 was employed for the simulations of the reactive system.

In the simulations in absence of chemical reactions, three different zones were observed along the longitudinal coordinate in the down-flow system: in the first one, near the entrance, the Re_p was high (>1) due to the large slip velocity between gas and particles. A second zone was observed as the particles were accelerated by the action of the drag force and gravity. In this zone, the slip velocities reach similar values and Re_p decreases, and becomes less than 1, a value that indicates the existence of creeping flow regime around the particles. As particles velocity increases, Re_p increases again, passing to a third zone. The first and third zone operate in an intermediate or transition regime around the particle between laminar and turbulent flow as Re_p is higher than 1.

The three different zones, above described, were also observed in the simulations including chemical reactions. The variations on Re_p of the first and second zone were more pronounced in comparison to the non-reactive system. The expansive effect produced by the action of cracking molecules increases the gas velocity, generating higher slip velocities between gas and solids in the first zone of the reactor. In consequence, higher Re_p numbers were obtained. The results indicate that the presence of creeping flow regime is reduced when the catalytic cracking reactions are included. As the accelerating effect is stronger, the extent of the second zone where Re_p is lower than 1, becomes shorter. The present study allows to have a guide on how flow regimes around particles evolve in a down-flow reactive system.