Operating Ranges of Coupled–Decoupled Counter-current Downer and Riser Reactors

The counter-current downers have the potential to combine the hydrodynamic characteristics of co-current risers and downers with less back-mixing and improved solid holdup. However, flooding may occur when particles suspend or reverse the flowing direction under increasing superficial gas velocity or solid mass flux. Here, we evaluate transported bed configurations by coupling the counter-current downer with a riser reactor to take advantage of the flooding behaviors. We analyze the theoretical hydrodynamics in risers and co- and counter-current downers from particle mechanics to cluster development. By validation with experimental results from the literature, we determine the proper simulation strategy for counter-current downers using computational particle fluid dynamics by replacing the particle size with empirically calculated cluster size. We investigate the effects of superficial gas velocity and solid mass flux with Geldart group A particles until beyond the flooding point of the counter-current downer. The coupled riser–counter-current downer reactor configuration offers more uniform axial and dynamic radial solid distribution while keeping a relatively high solid holdup to better utilize the reactor volume for enhanced gas–solid contact. The fluidization regime diagram by the Richardson–Zaki equation fails to capture the counter-current operation, so we provide a separate graph to mark the limitation of the coupled and decoupled riser and counter-current downer reactor configurations.