Bubbles and slugs coexist in a fluidized bed with strong inter-particle forces. However, the effects bubbles and slugs impose over fluidizing stability, and chemical reaction efficiency are different. The present work distinguished bubbles and slugs with the aid of X-ray tomography, and the evolution of their properties with varying inter-particle force in a fluidized bed were studied. The results show that the gas-holding capacity of the emulsion phase improves as inter-particle forces initially increase. In this process, the bubble frequency decreases noticeably, and the bubble size slightly increases. Bubbles rise slower than theoretically predicted because inter-particle force reduces the particle flow. The frequency and the size of slugs increase with inter-particle forces. As forces initially increases, the slug size increases with measurement height. An opposite trend is observed when inter-particle forces continues to strengthen. Near the gas distributor, the slug rising velocity is independent of size. In contrast, large slugs have a higher velocity than small slugs near the bed surface. Finally, three slugging patterns are distinguished, and the influence they impose on particle flow is analyzed.

The increase of inter-particle cohesive force greatly changes the fluidization dynamics, finally leading to the partial or complete failure of fluidization. However, few studies concern such transition process. This paper investigates the fluidization dynamics of Geldart B particles with a wide-range of cohesive force by analyzing the in-bed pressure fluctuation signals. Combining the bubble information reported in Part I, the local and global fluidization dynamics under different cohesive forces were discussed. The results show that bulk bubble dynamics is weaken with the presence of inter-particle cohesive force. As the force increases, fluidization changes from multi-bubbling regime to single-bubble regime and the factor governing the pressure fluctuation changes from bubble formation to bubble eruption. When the cohesive force is strong, slugging appears near the bed surface, then gradually extends toward the bottom bed by capturing freely fluidizing particles, and finally develops into the whole-bed slugging. At this time, regular fluidization turns into an alternative process between whole-bed slugging and regular status, corresponding to two distinct peaks in power spectral density of pressure signals at 0.1 Hz and 1 Hz respectively. The size of gas slug decreases with the elevation of measurement height. Basically, any operations that promote bubble growth will also facilitate the appearance of whole-bed slugging under strong cohesive force. Reducing the static bed height is a preferable approach to weaken, or even avoid the defluidization of whole-bed slugging, without changing other operational parameters.

Due to the presence of inter-particle cohesive force, cohesive particles reveal totally different fluidization behaviors as compared to the non-cohesive system. This paper studies the fluidization dynamics of Geldart B particles with varying thermal-induced cohesive forces. Multi-source X-ray tomography was applied to reconstruct 3D temporal images of bubbles, based on which, various bubble properties were extracted. The results show that increasing cohesive force will decrease bubble number while increase bubble size, implying that the presence of cohesive force facilitates bubble coalescence. By examining the bubble size distribution, cohesive force is found to have no effect on the number of median bubbles but greatly influence small and large bubbles. When the cohesive force is strong, the bubbles grow to a considerable size similar with bed dimension, giving rise to slugging near bed surface. With the action of inter-particle cohesive force, particle slug gradually grows by capturing other freely fluidizing particles, finally inducing “whole-bed” slugging. The particle slug may rupture in the rising process, and the bed turns back to normal fluidization. In comparison to normal bubbles, the gas slug has much larger size but far smaller frequency. The rise velocity of gas slug is also very low due to the particle-wall friction and gas-solid momentum dissipation. Therefore, the averaged values of bubble properties dramatically changed as bed temperature exceeds 35 °C. When the temperature attains 45 °C, the cohesive force is so strong that the fluidization completely fails in terms of stable whole-bed slugging.

In many applications of fluidized reactors, the particles processed are cohesive. The presence of inter-particle cohesive force causes different fluidization dynamics compared to non-cohesive system. This paper studied the fluidization dynamics of cohesive Geldart B particles in a three dimensional fluidized bed. X-ray tomography and in-bed pressure fluctuation analysis were conducted to reveal bubble behaviors as well as overall fluidization status. The results show that increasing cohesive force facilitates bubble coalescence, leading to the growth of bubble size. When bubbles grow to similar dimensions to the column cross section, the particles above bubbles are lifted as a particle slug and the fluidization turns to slugging. As the cohesive force increases, the energy distribution of power spectral density of pressure signal changes from mono-modal shape to bi-modal shape where two peaks relate to slugging and normal fluidization respectively. At low static bed heights, no slugging takes place even under high cohesive forces. Therefore, to avoid slugging in the practical application, the static bed height should be kept as low as possible. With further increase in the static bed heights, bubbles keep growing by continuous coalescence and become considerable near the bed surface, which triggers slugging and causes an increase in slugging duration.

Nanoparticles form multi-stage agglomerates when they are fluidized: simple agglomerates (a few dozens of micrometres), and complex agglomerates (a few hundreds of micrometres). This paper studies nanoparticle fluidization by using adhesive CFD-DEM (Discrete Element Model) simulation, which includes an adhesive contact model having the ability to model the contact and bounce/stick between elastic-plastic and cohesive particles. The discrete elements represent the simple agglomerates of nanoparticles. The effects of particle density, fluidizing gas velocity, and van der Waals force on gas-solid flow and agglomerate behaviors are investigated. It is shown that the particles, representing simple agglomerates, form large agglomerates during fluidized process. As the agglomerates move around in the bed, they continuously break and recombine. The solid movement is evaluated by a dispersion coefficient and it decreases with increase in cohesion. By monitoring the evolution of contacts, the agglomerate breakage is visualized and compared for the different conditions. Increasing fluidizing gas velocity and reducing particle adhesion both increase agglomerate breakage rate. The structure parameters of agglomerates are further analyzed: averaged coordination number of agglomerates around 3.0, packing fraction around 0.2–0.3, and fractal dimension around 1.9–2.3, which can be used to indicate structures and formation mechanisms of agglomerates.

Nanoparticles are fluidized as agglomerates with hierarchical fractal structures. In this study, we model nanoparticle fluidization by assuming the simple agglomerates as the discrete element in an adhesive (Computational Fluid Dynamics—Discrete Element Modelling) CFD-DEM model. The simple agglomerates, which are the building blocks of the larger complex agglomerates, are represented by cohesive and plastic particles. It is shown that both the particle contact model and drag force interaction in the conventional CFD-DEM model need modification for properly simulating a fluidized bed of nanoparticle agglomerates. The model is tested for different cases, including the normal impact, angle of repose (AOR), and fluidization of nanoparticle agglomerates, represented by the particles with the equivalent material properties. It shows that increasing the particle adhesion increases the critical stick velocity, angle of repose, and leads from uniform fluidization to defluidization. The particle adhesion, bulk properties, and fluidization can be linked to each other by the current adhesive CFD-DEM model.