Coarse Grain Particles: Fluidized Bed

Fluidization is the process through which solid particles are transformed into a fluid-like state through suspension in a gas or liquid. The fluidization of particles is part of many industrial processes, including biomass fuel production, fluid catalytic cracking (FCC) and fluidized bed gasifiers. The container that holds the fluidized bed is called a fluidized bed reactor.

This tutorial demonstrates a workflow for modeling fluidized beds using DEM technology that achieves a balance between the accuracy of the results and the speed of the simulation. In this simulation, heat and mass transfer are not considered. A simple cylindrical geometry is used to model the fluidized bed reactor. The bottom inlet boundary of the geometry reproduces the behavior of a gas distributor (such as a sparger), separating the plenum porous region of the reactor from the internal space occupied by fluidized particles.

The following diagram shows the geometry that is used in this tutorial:

Air flows into the reactor through a mass flow inlet at a specified rate of 1.49 kg/s, thus providing a constant superficial velocity, which is higher than the minimum fluidization velocity. The air leaves the domain through a pressure outlet. The inflow and outflow boundaries are impermeable to the solid particles. All solid particles are injected into the injection volume at the first time-step, thereby reducing the time it takes for them to settle in the reactor.

For flows involving a comparatively small number of particles, it is possible to formulate and solve governing equations for every particle. However, if the number of particles is large, a statistical approach is more practical. For this purpose, Simcenter STAR-CCM+ provides the Parcel Contact Coarse Grain Particle model, in which a reduced number of computational parcels statistically represents multiple smaller identical particles. This approach reduces the computation time of DEM simulations, in particular those with high loadings of small solid particles such as in fluidized bed applications. The linear cohesion model is also applied to simulate the inter-molecular attraction forces between particles or the effect of cohesion between wet particles. The model acts in tandem with the contact model, so when particles attach both the parcel contact coarse grain and linear cohesion models work simultaneously. The two-way coupling model is activated from the start of the simulation which allows the particle phase to exchange mass and momentum with the continous phase.