Comparison of solids conveying mechanisms in fluidized bed systems – Alternatives to riser

Journal article, 2025

The macroscopic convective transport of solids in fluidized bed systems is vital for efficient heat and mass transfer in solids-looping and solids-throughput processes. While risers are a conventional setup to promote solids convection, emerging applications demand higher solids flux and more compact, energy-efficient alternatives. This study investigates five solids conveying mechanisms in a fluid-dynamically down-scaled cold flow model using Geldart B-type solids. The system features a closed circulation loop, interconnecting a bubbling bed with a conveying module that induces net horizontal solids crossflow. Each conveying strategy is driven by a distinct mechanism: (i) Free solids splashing relies on bubble bursts at the bed surface to eject particles; (ii) Confined solids splashing amplifies this effect within a narrow geometry through turbulent interactions to enhance particle lift; (iii) Slugging induces transport via the rise and collapse of gas slugs in vertical ducts; (iv) Solids entrainment achieves lift by elutriating particles from the dense bed through high gas velocities; and (v) Directed gas injection imparts lateral momentum via angled nozzles. The solids flow rate is measured using magnetic solids tracing technique. Conveying efficiency is assessed by comparing energy imparted to solids with energy input from fluidization gas. Free solids splashing and directed gas injection achieve the highest upscaled transport rates (5 × 10⁻²–2 × 10³ kg/m²⋅s) at gas velocities of 1.9–4.3 m/s, outperforming conventional risers. Free solids splashing also offers the highest energy efficiency, while slugging and directed gas injection offer intermediate performance. Confined solids splashing and solids entrainment show the lowest efficiency.