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Abstract Rapid magnetic resonance imaging was used to take snapshots of solids volume fraction and particle velocity fields in an incipiently fluidized bed with single bubbles injected under both dry and wet conditions. Under wet conditions, cohesive liquid bridges formed between particles, altering the flow such that bubbles became flatter as compared to dry cases, and in some cases, the bubbles split or disintegrated entirely. The alteration in bubble behavior can be attributed to liquid bridging causing an increased heterogeneity in the roof of the bubble, which decreases the drag force on particles in the roof such that the gas flow through the roof can no longer support the weight of the particles in the roof. Quantification of the change in bubble behavior shows that increasing liquid loading has a much stronger effect on bubble behavior than increasing liquid viscosity, and liquid bridging affects small bubbles more than large bubbles. Graphical abstract Unlabelled Image Highlights • Rapid magnetic resonance imaging was used to image single bubbles injected in a wet fluidized bed. • Under wet conditions bubbles became flatter as compared to dry cases • Liquid bridging causes an increased heterogeneity in the roof of the bubble. [ABSTRACT FROM AUTHOR]

Wet fluidization occurs when small amounts of liquid are injected into a fluidized bed, causing liquid bridges to form between particles. These liquid bridges create complex interparticle forces with cohesion and dissipation due to surface tension and viscosity, respectively. Liquid bridging leads to significant changes in hydrodynamics, causing agglomeration of particles and in some cases defluidization. This article reviews prior work to understand wet fluidization, including microscale studies of individual liquid bridges and experimental and modeling techniques to understand liquid transport and changes in hydrodynamics in fluidized beds. The various effects on fluidization which have been observed are discussed, including efforts to map behavior into regimes based on liquid and operating parameters. This article concludes with an outlook on the future of wet fluidization research, highlighting major open questions in the field and important future studies. [ABSTRACT FROM AUTHOR]

Magnetic resonance imaging is used to generate snapshots of particle concentration and velocity fields in gas–solid fluidized beds into which small amounts of liquid are injected. Three regimes of bed behavior (stationary, channeling, and bubbling) are mapped based on superficial velocity and liquid loading. Images are analyzed to determine quantitatively the number of bubbles, the bubble diameter, bed height, and the distribution of particle speeds under different wetting conditions. The cohesion and dissipation provided by liquid bridges cause an increase in the minimum fluidization velocity and a decrease in the number of bubbles and fast particles in the bed. Changes in liquid loading alter hydrodynamics to a greater extent than changes in surface tension or viscosity. Keeping U/Umf at a constant value of 1.5 produced fairly similar hydrodynamics across different wetting conditions. The detailed results presented provide an important dataset for assessment of the validity of assumptions in computational models. © 2017 American Institute of Chemical Engineers AIChE J, 64: 2958–2971, 2018 [ABSTRACT FROM AUTHOR]

Gas-solid fluidization involving small amounts of liquid is simulated using a CFD-DEM model. The model tracks the amount of liquid on each particle and wall element and incorporates finite rates of liquid transfer between particles and pendular liquid bridges which form between two particles as well as between a particle and a wall element. Viscous and capillary forces due to these bridges are modeled. Fluidization-defluidization curves show that minimum fluidization velocity and defluidized bed height increase with Bond number (Bo), the ratio of surface tension to gravitational forces, due to cohesion and inhomogeneous flow structures. Under fluidized conditions, hydrodynamics and liquid bridging behavior change dramatically with increasing Bo, and to a lesser extent with capillary number, the ratio of viscous to surface tension forces. Bed fluidity is kept relatively constant across wetting conditions when one maintains a constant ratio of superficial velocity to minimum fluidization velocity under wet conditions. © 2017 American Institute of Chemical Engineers AIChE J, 63: 5290-5302, 2017 [ABSTRACT FROM AUTHOR]

Significance Using CFD-DEM simulations, a wet agglomerate of particles was placed in a void region of a dry vigorously fluidized bed to understand how wet agglomerates grow or breakup and how liquid spreads when agglomerates interact with dry fluidized particles. In the CFD-DEM model, cohesive and viscous forces arising from liquid bridges between particles were modeled, as well as a finite rate of liquid bridge filling. The liquid properties were varied between different simulations to vary Bond number (surface tension forces/gravitational forces) and Capillary number (viscous forces/surface tension forces) in the system. Resulting agglomerate behavior was divided into regimes of (i) the agglomerate breaking up, (ii) the agglomerate retaining its initial form, but not growing, and (iii) the agglomerate retaining its initial form and growing. Regimes were mapped based on Bo and Ca. Implications of agglomerate behavior on spreading of liquid to initially dry particles were investigated. This article identifies a new way to map agglomerate growth and breakup behavior based on Bo and Ca. In modeling both liquid forces and a finite rate of liquid transfer, it identifies the complex influence viscosity has on agglomeration by strengthening liquid bridges while slowing their formation. Viewing Ca as the ratio of bridge formation time to particle collision and separation time capture why agglomerates with high Ca struggle to grow. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2520-2527, 2017 [ABSTRACT FROM AUTHOR]

The formation of a denser and more viscous secondary liquid phase may impact the fluid dynamic behaviour of industrial ebullated bed reactors such as hydroprocessors. This study investigates the effects of particle size, shape and material on the global fluid dynamic behaviour of gas-liquid-liquid-solid fluidized beds subject to particle agglomeration. Ebullated bed experiments were carried out in a 152.4 mm diameter column at atmospheric conditions with biodiesel as the continuous liquid, 5 wt.% of glycerol as the denser and more viscous dispersed liquid, and nitrogen. Glass spheres with diameters of 4 and 1.5 mm were compared to aluminum cylinders with equivalent volume to surface area ratios, where the sphericity of both larger and smaller cylinders was approximately 0.8. In a liquid-solid fluidized bed, the previous particles were in the intermediate settling flow regime (0.2 < ReLT∞ < 500) in biodiesel; nonetheless, coalescing and dispersed bubble flow regimes were obtained with the smaller and larger particles, respectively, at the introduction of gas. Liquid-liquid-solid fluidized bed results established that particle size, shape and material had considerable impacts on agglomeration behaviour. In the gas-liquid-liquid-solid ebullated bed, the 1.5 mm glass beads transitioned from coalesced to dispersed bubble flow due to increased particle inertia from agglomeration. Larger glass beads experienced a reduced bed expansion due to agglomeration since the bubble flow regime remained constant. The studied aluminum cylinders did not agglomerate to the same extent as the glass beads due to differing material wetting properties, where negligible clustering occurred with the larger cylinders and an axial agglomerate size distribution was observed with the smaller cylinders. Preliminary experiments in a slurry bubble column using 100 to 150 µm glass beads were inoperable at a relatively low glycerol concentration of 0.7 wt.% due to considerable sedimentation on the distributor. Interparticle forces relevant to gas-liquid-liquid-solid fluidized beds are discussed, with an emphasis on the relation between fluid and particle properties with respect to attractive forces due to liquid bridging. [ABSTRACT FROM AUTHOR]

Abstract: The influence of moisture content on the fluidization behaviour of placebo pharmaceutical granule has been studied in a 14cm diameter cylindrical fluidized bed column. The dry granule has a mean diameter of and exhibits a bimodal size distribution with modes of 169 and . Bed pressure drop profiles and tapped density measurements were generated for granule moisture contents between 5 and 30wt%, which corresponds to typical final and initial moisture contents experienced during drying. At high moisture contents, the wet granule exhibits Geldart C type powder behaviour as channelling and defluidization exist. As the moisture content is reduced, the granule fluidity improves and demonstrates behaviour characteristic of Geldart B powders. The changing fluidization behaviour was quantified using parameters such as the full support velocity, full support bed voidage and Hausner ratio. These parameters were found to increase significantly above granule moisture contents of 10wt%. The increase in the Hausner ratio suggests that the interparticle force load in the bed increases. This change in interparticle force load is responsible for the increase in the full support velocity and bed voidage. [Copyright &y& Elsevier]

Particle–particle pulloff adherence forces were measured as a function of temperature in the ice/n-decane/ice and tetrahydrofuran (THF) hydrate/n-decane/THF hydrate systems using a newly developed micromechanical testing technique. Experiments using ∼200 μm radius particles were performed at atmospheric pressure over the temperature range 263–275 K. The ice and hydrate particles displayed very similar behavior. While the measured adherence forces had significant variation, the shapes of the cumulative force distribution curves were similar among the different sets of experiments. The measured adherence forces distributions shifted to lower force values as the temperature was decreased from the solid melting temperature. The observed forces and trends were explained by the capillary cohesion of rough surfaces, with the capillary bridging liquid being stabilized below its freezing point by the negative curvature of the bridging liquid/n-decane interface. [Copyright &y& Elsevier]

In the present paper, micro-scale contact induced by liquid bridging is studied numerically via FEM with consideration of coupling effect between the liquid bridging and the elastic deformation of the half space. The variations of the liquid-bridging force and contact area as functions of a dimensionless parameter χ (measurement of relative stiffness of the contact bodies) and the filling angle ψ (measurement of liquid volume) are obtained and discussed. The FEM numerical solution is compared with two approximate analytical solutions based on Hertzian contact mechanics. It is found that the dimensionless liquid bridging force and contact area obtained via the approximate solutions are much smaller than the values obtained via FEM under certain conditions. The error inhabited in the approximate solution is due to the coupling effect between the liquid-bridging force and the elastic deformation of the half space. [Copyright &y& Elsevier]

To analyze the phenomenon of the increase in cohesive force between fine particles under elevated temperature conditions, a new split-type tensile-strength tester of powder beds for high temperature was developed. Fused silica glass with low thermal expansion coefficient was used as the material for the suspended cell of a commercial split-type tensile-strength measuring device at room temperature. The powder samples used were pure fine silica and two kinds of fly ash powders collected in a pulverized coal (PC) combustion and a pressurized fluidized coal combustion systems. Under conditions of relatively low temperatures below 1000 K, the adhesive force of all powders increased gradually in proportion to the temperature. Rapid increases of tensile strength of both fly ash powder beds were observed in the high-temperature range (above 1100 K). Based on the results of thermo-mechanical analysis (TMA), it was estimated that a viscous flow sintering mechanism controlled this rapid increase phenomenon of adhesive behavior of ash powders at high-temperature conditions above 1100 K. On the other hand, using FT-IR analysis, it was demonstrated that the increase of the van der Waals force with the change of surface molecular structure controlled the stickiness of pure silica and both ash powders at relatively low-temperature conditions below 1000 K. [Copyright &y& Elsevier]