Polydisperse, Segregating Fluidized Beds: From Detailed, Small-Scale Investigations to Fast, Large-Scale Simulations

Fluidized beds with polydisperse particle size distributions can exhibit segregation, which might even lead to undesired, partial defluidization. Corresponding numerical simulations like CFD-DEM are challenging because of the distinct time scales involved: Grain collisions take place within microseconds whereas meso- and macroscopic mass transport happens within seconds. Finally, segregation might evolve over the course of minutes. Therefore, CFD-DEM simulations covering such durations are only feasible for small test cases below the spatial scales of many experiments or of industrial reactors. Nevertheless, such small-scale investigations can provide relevant insights. We studied the influence of the gas inlet profile varying between homogeneous aeration, a central spout and inflow at the sides of the bed for fixed total mass flux. Switching between the different profiles allowed us to control segregation in a simple fashion that may be easily implemented in experimental setups without the need to adapt the overall gas flow.
Furthermore, we discuss how fast, long-term simulations of larger scales may be carried out. We employ our data-assisted time-extrapolation technique recurrence CFD (rCFD), which provides a numerically cheap approximation for recurrent, fast degrees of freedom (such as the particle velocity field in a fluidized bed). On top of the resulting long time series of flow fields, passive or weakly coupled transport processes can be investigated in an extremely efficient way. Based on preliminary results, we are optimistic that rCFD simulations can be used to picture the long-term evolution of polydisperse fluidized beds, e.g. to understand the impact of continuous granulation, progressing segregation and the impact of counter-strategies like switching between different inlet profiles. Ultimately, real-time capable rCFD simulations might be used to monitor the behavior of an industrial reactor and help to control it.