Characterization of Solids Mixing in Binary Fluidized Beds

Licentiate thesis, 2026

Fluidization is a key technology for handling particulate systems, facilitating enhanced mixing, heat transfer, and mass transfer in gas–solid contact processes. In bubbling fluidized beds used for thermochemical conversion, where multiple solid phases coexist, typically involving a dense bulk solids phase and a dilute lean solids phase, the process performance is strongly governed by the extent of solid–solid mixing. This dependence has driven sustained research efforts focused on understanding the fluid dynamics of bubbling fluidized beds containing heterogeneous solid phases. Much of the existing experimental literature on solids mixing consists of studies under ambient (cold) conditions using the same solids representative of industrial hot applications but fluidized with readily available gases (such as air). This practice neglects the strong influence of temperature-dependent fluid properties on bed hydrodynamics, thereby limiting the validity of such experimental findings. Further, the inability to accurately measure the distribution of solids phases and their flow patterns in a space and time-resolved fashion has constrained the achievement of a mechanistic understanding of mixing in these heterogeneous systems.

This work addresses these gaps by developing a magnetic solids tracing technique for spatiotemporal characterization of axial solids transport and by experimentally evaluating solids mixing under industrially relevant conditions. The study systematically examines the influence on lean solids mixing for varying operating parameters, including fluidization velocity, bed height, lean solids loading, and different types of lean solids undergoing conversion and releasing gas.

The results demonstrate that neglecting the effect of temperature on fluid properties in cold experiments leads to substantial overestimation of bubble size, bed dynamics, and the extent of solids mixing, underscoring the value of fluid-dynamic scaling to capture realistic solids interactions. When hydrodynamic similarity is preserved by accounting for temperature effects, the results indicate that mixing is enhanced with increasing fluidization velocity and bed height, while high lean phase loadings (>10 %vol) promote segregation due to suppressed bubble-driven circulation. Axial transport is dominated by bubble dynamics rather than buoyancy, with mixing of lean solids occurring at lower characteristic frequencies than those associated with bubbles, indicating that not all the bubbles effectively contribute to mixing. The gas release from lean solids introduces an additional transport mechanism. At low fluidization velocities, the gas release modifies the axial distribution of lean solids by generating a localized and transient reduction in solids concentration around the particle, reducing inter-particle interactions between the lean solid particle and the surrounding bulk solids suspension. This reduces the effective drag and friction opposing the particle motion, allowing lean solids to respond differently to their relative density. While at higher fluidization velocities, the gas release from lean solids enhances the lateral dispersion of the particle by up to 40%.

By combining fluid dynamic scaling, novel magnetic tracing technique, and contemplation of effects of gas release from lean solids, this work advances the mechanistic understanding of solids mixing in fluidized beds and delivers novel information about mixing under industrially relevant conditions.