Self-organized Flow Structure In Liquid–Solid Circulating Fluidized Bed And Fractal Modeling Reconstruction

A circulating fluidized bed has got the advantages of high processing capacity , good mass transfer performance,low axial back-mixing degree, simultaneous procedure of adsorption in the downcomer and desorption in the riser. However, during the operations, groups of particles tend to stay close and move as an ensemble, which leads to separation efficiency reduction. This phenomenon is called self-organization structure or coherent structure.Investigations of particle cluster are mainly in gas-solid system domestic and international documents, In order to recognize the affecting factors on particle cluster in liquid-solid system, superficial liquid velocity, liquid viscosity, particle density, particle size are involved in studies. Nevertheless, most of the studies are limited to expanded bed, such as: Wilhelm and Kwauk investigated lead particles fluidization; Fan studied the clustering and crushing behaviors of nylon particles and glass beads. Qiu Ou established the optimal viscosity model of particle aggregation. In circulating fluidization, Liang Wu-geng developed a core-annulus model to describe the radial nonuniform flow structure in the riser; Lan studied the axial holdup distribution in the riser. Few studies concern the particle aggregation in circulating fluidization, so this paper studied the particle flow structure and its affecting factors in the riser.Affecting factors on particle clusters such as Particle density and size, fluid viscosity and density, main and auxiliary flows were studied with the applications of CCD system. It indicated that most of the clusters were doublet and triplet, and big clusters were well aligned. Fractal dimension was studied for which could represent the phenomena of particle aggregation at a certain extent. Fractal dimension and number of clusters increase with the decrease of particle size while increase with the increase of particle density, liquid viscosity. However, Fractal dimension and number of clusters increase at first and decrease afterward with the increasing liquid flow.Based on experiments and fractal theory, a solid clustering behavior model in liquid-solid circulating fluidized bed was developed with Cantor set and Duan Yi-ran's fractal hypothesis of bubbles. The model simulated the effects of particle density and size, fluid viscosity and density on particle aggregation. It was found that theoretical model is in good agreement with the experimental results.