Aggregates in fluidized beds: the effect of bonding angles on fluidization

TL;DR

The paper addresses deterioration of fluidization in very-narrow fluidized beds caused by particle bonding. It experimentally analyzes planar trios of spheres bonded at angles $\alpha$ across a range of flow velocities in a 25.4 mm tube, identifying six distinct macroscopic regimes. A regime map in $\alpha$ and $U/U_{if}$ and a bed-height scaling $H/H_{if} = c (U/U_{if})^{\gamma}$ quantify transitions, with $\alpha = 90^\circ$ maximizing fluidization. The results yield a design-guiding framework for maintaining fluidization in cohesive particle systems, with relevance to reactors and wastewater treatment, and provide insight into plug formation, clogging, and granular-temperature dynamics expressed as $\langle \theta \rangle$.

Abstract

Fluidized beds consist of solid particles suspended in a tube by an ascending fluid. In liquids, it is not rare that particles adhere to each other, decreasing the solid-liquid contact area and the ratio between the tube and grain diameters, deteriorating fluidization. We inquire into this problem by carrying out experiments with trios of spheres fluidized by water flows, the spheres being glued in predefined angles. In our tests, we used a 25.4-mm-ID (internal diameter) tube and 5.95-mm-diameter spheres, and we varied the angle of trios within 60$^{\circ}$ and 180$^\circ$ and water velocities within 0.027 and 0.210 m/s. Due to the small ratio between the diameters of the tube and spheres (approximately 4.3), the bed is prone to the formation of plugs and clogs. Our experiments show that elutriation, fluidization with plugs, glass transitions (amorphous static structures), packed beds, clogging, and a transitional clogged-plug regime can appear in the bed, depending on the bonding angles and water velocities. We report the relations between the bed height, bonding angles, and flow velocity, and show that they correlate with the granular temperature. We also show that an angle of 90$^\circ$ maximizes fluidization for a given fluid velocity, and we propose a regime map that organizes the different patterns based on the bonding angle and flow velocity. The proposed map can serve as a guide for selecting the fluid velocities in order to keep the bed fluidized at all times, helping in the design and operation of fluidized beds.

Figures (9)

• Figure 1: (a) Layout of the experimental setup. (b) Angles $\alpha$ used in the trios. (c) snapshot of the test section (with the visualization box), showing one example of fluidized bed.
• Figure 2: Snapshots of the bed placed side by side: (a) packed bed ($\alpha$$=$ 120$^\circ$, $U$$=$ 0.055 m/s); (b) glass transition ($\alpha$$=$ 150$^\circ$, $U$$=$ 0.099 m/s); (c) clogging ($\alpha$$=$ 120$^\circ$, $U$$=$ 0.121 m/s); (d) clogged-plug ($\alpha$$=$ 90$^\circ$, $U$$=$ 0.110 m/s); (e) fluidized ($\alpha$$=$ 120$^\circ$, $U$$=$ 0.137 m/s); (f) elutriation ($\alpha$$=$ 150$^\circ$, $U$$=$ 0,192 m/s). The time interval between each snapshot is 7 s in panels a-c and 0.7 s in panels d-f. (multimedia available online)
• Figure 3: Regime map in the $\alpha$ -- $U/U_{if}$ space. The symbols are listed in the figure key, and the curves separating the patterns were drawn using SVM (support vector machine).
• Figure 4: Histograms showing the distributions of bed heights $H/H_{if}$ measured for the different types of trios (bonding angles $\alpha$ are indicated in the key of figures).
• Figure 5: Bed height $H$ versus superficial velocity $U$ in (a) dimensional form, and (b) dimensionless form (using $H_{if}$ and $U_{if}$). In panel (a), the experimental data are represented by filled symbols (listed in the figure key). For each $\alpha$, a power law $H \propto U^\gamma$ was fitted to best match the experimental data points, the horizontal lines indicating $H_{if}$, and the inset showing the exponent $\gamma$. Panel (b) displays the data in log scales, with the fitting curve being accurately described by a power law ($c$$=$ 1.07, $\gamma$$=$ 2.1, and coefficient of determination $R^2 = 0.9982$).