Quantification of Solid Mixing in Bubbling Fluidized Beds via Two-Fluid Model Simulations

Mohammad-Sadegh Salehi,Stefan Radl,Maryam Askarishahi

doi:10.1021/acs.iecr.9b06343

Abstract

The influence of particle density and fluidization velocity on the mixing behavior of Geldart B particles was investigated in bubbling fluidized beds for various operating conditions using a two-fluid model approach. The simulation results proved that the predicted solid flow pattern for a wide range of Geldart B particles differs from particles of Geldart B/D classification at identical fluidization numbers. Quantitative analysis of the predicted solid velocity variances revealed that the solid axial dispersion rate is higher than the lateral one. In detail, a Peclet number of 1.54 and 3.92 was predicted for axial and lateral mixings, respectively. Also, the solid dispersion and diffusion coefficients were linearly correlated with the excess gas velocity for a range of particle properties. Finally, the characteristic global particle mixing times were computed. Thereby, the characteristic mixing time can be correlated with the particle Froude number, expanded bed height, and solid dispersion coefficient with high accuracy.

Highlights

Bubbling fluidized beds (BFBs) are widely used in chemical and petrochemical, pharmaceutical, energy, and power industries because of their excellent heat and mass transfer characteristics as well as fast solid and gas mixing.[1]
Despite the claim that fluidized beds (FBs) are well-mixed and controllable for heterogeneous and gas-phase reactions, there are more challenges related to (i) the optimum design and (ii) operation in some applications. Such challenges stem from segregation and/or poor mixing of solid and gas phases, for example, when FBs are applied to process biomass,[7] to coat particles,[8] to crack bitumen,[9] or to chemical looping reforming reactors involving extremely reactive particles.[10]
Essential to quantify the rate of mixing in FBs as it can provide valuable information, for example, to correctly design and locate fuel feeding ports in FB reactors.[11−13] a large enough mixing rate of solid particles is required for homogenous heating or drying in some processes

Summary

Introduction

Bubbling fluidized beds (BFBs) are widely used in chemical and petrochemical, pharmaceutical, energy, and power industries because of their excellent heat and mass transfer characteristics as well as fast solid and gas mixing.[1]. Despite the claim that fluidized beds (FBs) are well-mixed and controllable for heterogeneous and gas-phase reactions, there are more challenges related to (i) the optimum design and (ii) operation in some applications Such challenges stem from segregation and/or poor mixing of solid and gas phases, for example, when FBs are applied to process biomass,[7] to coat particles,[8] to crack bitumen,[9] or to chemical looping reforming reactors involving extremely reactive particles.[10] The reason is that in these fluid bed applications, the release of energy due to chemical reactions or evaporation as well as the addition of gases or liquids (e.g., via a spray) can be faster than the global mixing of solids. Avoiding hot-spot formation in highly exothermic reactive systems[14] necessitates extremely fast mixing in an FB

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Conclusion