Vertical mixing of solids in a bubbling fluidized bed

Artikel i vetenskaplig tidskrift, 2027

This work aims at advancing the understanding of the vertical mixing of bulk solids under conditions relevant for hot operation of large-scale bubbling fluidized beds. A novel implementation of the magnetic solids tracing method is used to sample the transient local concentration of a solids tracer at different heights during batch experiments. The experimental set-up consists of a cold flow model (0.22 m i.d.) operated according to fluid-dynamical scaling laws (yielding the use of 125 μm bronze powder), which allows scale-up of the result data to a virtual large (0.82 m i.d.) bed of 470 μm sand particles operated at 700 °C. Results indicate that the description of solids vertical mixing cannot be simplified to a purely convective or dispersive process, but a combination of the two. Vertical solids convection acts faster than dispersion for all conditions tested ( u 0 /u mf = 1.5–2.5 and H b /d b = 0.5–1), with Pe L -number values ranging between within 1.1–4.6. Increased fluidization velocity enhances both convective and dispersive transport, while increased bed height – a parameter often overseen in previous literature – yields higher dispersion coefficients but lower solids velocity. On up-scaled hot basis, the solids vertical velocity ranges within 0.044–0.12 m/s and dispersion coefficient within 4.9–25 ∙ 10⁻³ m²/s.