(74e) The Role of Particle Friction in the Stabilization of Pulsed Gas-Solid Fluidized Beds. from Surface Waves to Structured Bubble Nucleation

Many operations in the process industry and the energy sector, e.g., catalytic reactors and dryers, rely on good contact between solid and gas phases. Fluidized beds in their various designs represent the means to disperse a solid phase and promote the transport of mass and heat with a surrounding fluid. The level of mixing and the circulation of the solids become critical factors in determining the transport properties and so the overall performance of a bed.

However, optimization and, particularly, scale-up remain a challenge when designing gas-solid fluidized systems. The instabilities arising from the coupling between the transport of momentum in the gas and the particles and the nonlinear behavior emerging from highly dissipative particle-particle interactions give rise to macroscopic phenomena that are difficult to predict and are highly dependent on particle level properties. Spouting, channeling, jetting, bubbling or expansion are some of these, often associated to the characteristic features of classic fluidization regimes. Over the decades, much work has focused on understanding their hydrodynamics and how the transport properties are affected or could be modified, but not much is known about how more regular and predictable flow structures may occur. This has been the focus of some efforts in recent years; one such way to induce structure in fluidized beds, is by pulsating the gas flow. This gives rise to surface patterns in vibrating and fluidized shallow granular layers [1, 2], and dynamic bubble patterns in quasi-2D gas-solid fluidized beds under certain conditions, whereby bubbles rearrange into a triangular tessellation pattern with a characteristic bubble size and wavelength[3, 4] . This contribution investigates the role of frictional contacts in the formation of such dynamic patterns. It studies the transition from the generation of a surface wave in a shallow granular layer to the nucleation of bubbles in alternate sites, as the thickness increases. We report experimental data in a quasi-2D pulsating bed along numerical studies under different flow conditions and particle friction factors. A comparison between a continuous and discrete formulation is used to display the role of friction in inducing the bubble nucleation at a critical bed height and in stabilizing the subsequent pattern. Both Eulerian-Eulerian (TFM) and Eulerian-Lagrangian (CFD-DEM) models succeed in predicting the formation of a surface wave in a shallow fluidized granular layer, which appears dominated by hydrostatic effects. However, as the height increases, the solid circulation leads to locally dense regions where particles interact by sustained multi-particle contacts and the effect of friction becomes dominant. The inherent difficulty of continuum fluid mechanical formulations to deal with correlated particle velocities and the resulting anisotropic distribution of contacts explains why the commonly used closures for the solid stresses cannot reproduce the rearrangement of bubbles in dense systems. On the contrary, we will show how the Eulerian-Lagrangian formulation explicitly solves the granular rheology and can track the boundary of the areas locked in the plastic regime, rendering the correct bubble size, wavelength and sequence in the location of the nucleation sites, up to a quantitative agreement with the experiments.

[1] F. Melo, P. Umbanhowar, H.L. Swinney, Transition to parametric wave patterns in a vertically oscillated granular layer, Phys. Rev. Lett. 72 (1994) 172.

[2] J. Bougie, K. Duckert, Continuum simulations of shocks and patterns in vertically oscillated granular layers, Phys. Rev. E 83 (2011) 011303.

[3] M.-O. Coppens, J.R.van Ommen, Structuring chaotic fluidized beds, Chem. Eng. J. 96 (2003) 117-124.

[4] M.-O. Coppens, M.A. Regelink, C.M. van den Bleek, Pulsation induced transition from chaos to periodically ordered patterns in fluidised beds, Proceedings of the 4th World Congress on Particle Technology, Sydney, 2002.