Mixing equilibrium in two-density fluidized beds by DEM

Interaction of fluid and granular flows in dense two-phase systems is responsible for the significantly different behavior of units used in the chemical industry such as fluidized beds. The momentum exchange phenomena involved during gas fluidization of a binary mixture of solids differing in density is such that the continuous mixing action of the fluid flowing upwards counteracts the natural tendency of the two (fluidized) solids to segregate with the heavier component fully settling at the bottom of the bed. In the present work the complex hydrodynamics of two-density gas-fluidized beds is studied by means of a DEM-CFD computational approach, combining the discrete element method (DEM) and a solution of the locally averaged equations of motion (CFD). The model is first validated against experimental data and then used to investigate the role of gas velocity versus density ratio of the two components in determining the distribution of the components in the system. It is shown first that a unique equilibrium composition profile is reached independent of the initial arrangements of the solids. Then, numerical simulations are used to find the equilibrium conditions of mixing/segregation as a function of the gas velocity in excess of the minimum fluidization velocity of the heavier component and as a function of the density ratio of the two solid species. A mixing map on the gas velocity-density ratio plane is finally reconstructed by plotting iso-mixing lines that shows quantitatively how conditions ranging from full mixing to fully segregated components are obtained.