Nanoparticles Agglomerate In Vibrated Fluidized Bed Fluidized

Nano-particles, belonging to Geldart's group C patricles or cohesive powders, are easy to aggregate into fluidized agglomerates during fluidization, which results in factually agglomerates of cohesive powders. Usually, the fluidized agglomerates are in wide size distribution. Measurements of agglomerate properties and study on the mechanism for particle agglomerating behavior have been one of the hot spots in the field of fluidization.The behavior and agglomerate sizes of different cohesive SiO2, TiO2 and ZnO nano-particles are investigated in the conventional fluidized bed (CFB) and vibro-fluidized bed (VFB). It was found that slugs and channels were formed firstly under low superficial gas velocity due to the specific physical and surficial properties of particles. With increasing the superficial gas velocity, slugs and channels vanished, whilst the bubbling in bed was intensified obviously, and there existed a severe degree of the size-segregation of the agglomerates throughout the whole bed, namely, the agglomerates sizes were small in the upper zone and large in the lower zone. Because of the severe entrainment, the fluidization quality was poor. The experimental results showed that slugging and channeling of bed disappeared, the minimum fluidization velocity and the measured agglomerate sizes were decreased, and the fluidization quality was significantly improved due to introduction of vibration energy. The effects of vibration parameters on three nano-particles agglomerate sizes are systematically investigated in VFB. The nano-particles agglomerates were formed during fluidization after a period of fluidization time. Vibration wave propagation was hampered by powers under higher original bed height of nano-particles, and shallow bed contributed to the absorption of vibration energy and better agglomerates fluidization quality which could be obtained at ho/D=0.8 or 1 for nanoparticles, while ratio of height to diameter had little effect on agglomerates sizes. Meanwhile, mechanical vibration had complex effect on agglomerate size. The average agglomerate size is changed from 300μm to around 100μm when the vibration is applied. The effect of vibration on the agglomeration in vibro-fluidized beds of nano-particles depends on the critical vibration frequency corresponding to a minimum agglomerate size, and the critical point thresholds linking to A=3 mm of SiO2, TiO2, ZnO were 10 Hz,15 Hz,15 Hz, respectively. It can help to suppress formation of agglomerates due to the additional vibration energy, and it can also prefer to the coalescence of the agglomerates due to the enhanced contacting probability between particles and/or agglomerates. However, the overall trend of agglomerate sizes decreased with increasing vibration intensity. In this context, the effect of the vibration frequency on agglomerate sizes is more obvious than that of the vibration amplitude. Both the experimental and theoretical results show that a higher gas velocity leads to a smaller agglomerate size.An energy balance model for the prediction of agglomerate sizes has also been established on the basis of energy balance of the agglomerate collision energy, the effective energy arising from vibration wave, energy generated by hydrodynamics shear and cohesive energy in VFB. The agglomerate sizes predicted is in good agreement with the experimental data for three nano-particles by the vibration excitation. Based on vibration mechanism, agglomeration and deagglomerates in VFB were also studied.