Mixing behaviors in an industrial-scale spout-fluid mixer by 3D CFD-TFM

A new industrial-scale spout-fluid mixer (H = 3.4 m and I.D. = 0.8 m) was proposed for mixing 18.75 t/h of biomass (ρ_b = 400 kg/m³, d_b = 2 mm) and 281.25 t/h of heat-carrier (ρ_c = 2600 kg/m³, d_c = 0.5 mm), and the corresponding three-dimensional computational fluid dynamics approach, facilitated with the Eulerian-Eulerian multiphase fluid model (CFD-TFM), was developed to simulate the three-phase mixing behaviors in the mixer. The performance of the mixer and effects of operating parameters were numerically investigated. It was found that the mixing effectiveness, M_e, can reach over 80% under the proposed industrial operational conditions (for example, a spouting gas flowrate Q_s = 0.067 kg/s and fluidizing gas flowrate Q_f = 0.035 kg/s), and the flow pattern in this case is a “jet in the fluidized bed with bubbling”. Increasing Q_s will increase the sizes of bubbles, as well as their generation frequency and rising velocity, which are conducive to the mixing of heat-carrier and biomass particles. However, a too large Q_s will affect the falling solid flows in the distribution region and transition region of the mixer. Conversely, increasing Q_f will aggravate the swaying and leaning of bubbles or jets towards the mixer walls, leading to a slight decrease of M_e.