Cohesivity, Formation of Particle Clusters, and Blanket Settling Velocity in a Fluidized Bed

This study describes the formation, particle clustering, settling mechanism, and settling velocity ($V_s$) in a fluidized bed reactor using laboratory and field experiments based on its cohesivity, which is expressed as a sludge cohesion coefficient ($SCC$). At a constant upflow, solid loading leads to particle interactions that result in an increase in the floc size. The blanket thickens and reaches a steady state, at which point the blanket height remains static and $V_s$ reaches two to four times higher upflow velocity. During the steady state, $SCC$ increases with time ($0.6–0.8\mathrm{mm}/\mathrm{s}$) and $V_s$ decreases, whereas no significant variation in sludge volume ($SV$) is observed. Comparing the nondimensional parameter $V_s/SCC$ with the voidage ($1-SV$), the Reynolds number ($\mathsf{R}$) of the blanket is 3. At a high $\mathsf{R}$, hydrodynamic forces tend to overcome the adhesive forces. Turbulent conditions created by larger and faster settling clusters lead to their destruction, resulting in the formation of smaller subclusters. This increases the interstitial area between the clusters and reduces $V_s$. Blanket cohesivity increases due to the short-ranged cohesive forces between particles.