Solids mixing and segregation in binary fluidized beds - Defining spatiotemporal patterns using a novel magnetic solids tracing technique

Journal article, 2026

This study investigates the vertical mixing and segregation patterns of a lean solids phase in a bubbling fluidized bed. A novel measurement method is introduced that allows for online acquisition of the vertical distribution of the lean solids phase using magnetic solids tracing (MST) coils arranged along a pin-probe. This method enables spatiotemporal profiling of the lean solids phase, based on which the mixing indices and transitional velocities between the mixing and segregation states can be calculated, as well as the concentrations in the frequency domain. Experiments are conducted under fluid-dynamically downscaled conditions resembling a large-scale (1.33 m(2) in cross-section) unit fluidizing sand (Geldart B) and biomass particles (lean solids phase) at 700 degrees C, typical of thermochemical conversion processes. The impacts of fluidization velocity, bed height, and lean phase loading are analyzed. Increasing the fluidization velocity enhanced mixing, whereas lean phase loadings above similar to 10% (vol) led to surface layering, which dampened the bubble eruptions and reduced particle mobility. Taller beds promoted deeper penetration of the lean phase into the dense bed and reduced stratification, while shallow beds and high loadings favored segregation, highlighting the critical roles of these parameters in the mixing behavior. Transitions between the segregated and mixed states were modulated by the bed height and fluidization velocity, suggesting a relationship with the characteristics of the bubble phase. However, mixing of the lean phase occurred at a lower characteristic frequency compared to the dominant frequency of the bed dynamics, indicating that although bubbles induce solids mixing, not all bubbles contribute effectively.