From fluidisation to swirling: flow regime identification in a toroidal fluidised bed using imaging and pressure drop

Toroidal fluidised bed (TORBED) reactors have emerged as a promising option for superior heat and mass transfer in gas–solid contacting applications. This study investigates a small-scale, 3D-printed TORBED containing fine silica particles (90 µm) to perform a hydrodynamic analysis based on pressure drop measurements and high-speed camera imaging. Five bed regimes are identified through a combination of both measurement techniques: partially fluidised, fully fluidised and partially swirled, maldistributed, uniformly swirled, and entrainment. Four regimes were determined by pressure drop, whereas identification of maldistribution required visual observation. This highlights the utility of each of the analytical tools in the hydrodynamic study. Importantly, the determined minimum swirling velocity was significantly higher than the minimum fluidisation velocity, appearing more closely linked with the terminal velocity of these fine particles. Furthermore, the uniformly swirled regime was somewhat constrained by the particle size distribution. The removal of fine particles < 75 µm facilitated a broader range of operational conditions for the uniform swirling regime (loadings of 1–2.5 g and gas velocities of 0.25–0.4 m s⁻ 1 ). A bed loading of 2–2.5 g was the minimum requirement for uniform swirling to ensure the distributor was well-covered and prevent maldistribution/gas bypassing.