Hongzhong Li

Fluidization of fine particles

Experiments on the fluidization of fine particles (Geldart group C) with mean sizes 0.01-18.1 mu m and densities 100-8600 kg/m(3) were conducted. Experimental results show that the process of fluidizing fine particles usually involves plugging, channeling, disrupting, and agglomerating. When fluidized, the entities fluidized generally consist of particle agglomerates varying in size from the largest at the bottom of the bed (some even defluidized) to the smallest at the top (some even unassociated to discrete particles). Best to fluidize are the agglomerates which have reached a uniform equilibrium size after repeated solids circulation. Lowering agglomerate density proves to be an effective measure for improving the fluidization quality of fine particles. The fluidizing behavior of fine-particle agglomerates, compared with that of normal discrete particles, is illustrated diagrammatically. The large amount of factual findings poses a challenging problem for analysis, and even possible quantitative correlation, between particle properties and fluidization behavior

Cluster structure in a circulating fluidized bed

Abstract Micrographs in a 10-m high and 90-mm ID fluidized catalytic cracking (FCC) catalyst—air fast fluidized bed were taken by a video camera provided with an optical fiber probe inserted radially into the bed to study cluster size and shape in gas—solid suspension. A computer aided (CA) particle image processing system was used to analyze images which were obtained under different superficial gas velocities and solid circulating rates at different radial positions. Cluster size and its probability density distribution were investigated and a simple model was established. A roundness factor was put forward to describe the shape of the cluster. We also discovered that clusters assembled by particles show evident fractal character both in ‘shape’ and ‘fragmentation’. The fractal dimension of clusters at different flow patterns was found to be a function of position.

Wavelet analysis of pressure fluctuation signals in a bubbling fluidized bed

Pressure fluctuations of fluidized beds have been used to evaluate the fluidization quality. In bubbling fluidized beds, the bed behavior is characterized by bubbling. The information indicated by the pressure fluctuation signals can be applied to describe those behaviors. The signals representing the characteristics of bubbling can be separated from original signals through discrete Wavelet analysis. Considering the principles of Wavelet, the Scale 4 detail signals can reveal the bubble behaviors in a fluidized bed. The peak frequency of the Scale 4 detail signal stands for the bubbling frequency and the peak amplitude for the bubble size.

Micro-visualization of clusters in a fast fluidized bed

Micrographs in a 10-m high by 90-mm ID FCC-air fast fluidized bed were taken by using a video camera provided with a special probe, which consists of a set of lenses and an optical fiber flashlight transmitter, inserted radially into the bed. These micrographs, taken at different radial and axial positions, can distinguish two phases in the fast fluidized bed: a dispersed phase and a cluster phase. In the dispersed phase, the solid particles are noted to be present essentially individually, while in the cluster phase the solid particles agglomerate with one another and are enmeshed in the dispersed phase. The shape of clusters is in general irregular, and their size, highly variable. It seems that the clusters transform from strands at the center of the bed into spheres near the wall. The radial distribution of cluster concentration corresponds to that of solids suspension density.

Effects of adding different size particles on fluidization of cohesive particles

The fluidization of SiC cohesive particles of different sizes shows that (1) the fluidized characteristics is greatly affected by the size of particles, and the smaller the size of particles, the bigger the interparticle force, the worse the fluidized behavior; (2) SiC of average size larger than 10 mu m can be fluidized by increasing gas velocity, but SiC (5 mu m) cannot be fluidized. Particle agglomeration number Ae, which can be used to describe the fluidized behavior of particles is proposed based on interaction between the agglomerate body and the outer-most particles cohered. The calculation of particle agglomeration number Ae shows that cohesive particles can be fluidized if particle agglomeration number Ae less than or equal to 40 000. SiC5 may be fluidized with the method of adding particles, and optimum additive amount can be calculated by particle agglomeration number Ae

Aggregative and particulate fluidization—The two extremes of a continuous spectrum

Solid particles belonging to Geldart Group A, B and D were fluidized in liquids of changing viscosity and in CO2 under ambient to supercritical conditions. Local bed voidage signals were collected and processed in conjunction with global expansion characteristics to give quantitative indexes for evaluating fluidization quality. Results showed that the fluidized state changes progressively from aggregative to particulate when fluidized separately by gas, supercritical fluid and liquids, indicating an infinite number of intermediate states existing between aggregative and particulate fluidization. A discrimination number Dn is proposed to describe the transition from particulate to aggregative. A heterogeneity index δ and a nonideality index fh are proposed to quantify, respectively, the local and the global characteristics of the intermediate states.

Force balance modelling for agglomerating fluidization of cohesive particles

To estimate the size of agglomerates formed in fluidized beds of cohesive particles, a model of force balance is developed. The average agglomerate size estimated by this model is in reasonable agreement with experimental data. An agglomeration criterion is proposed by analyzing the model. Theoretical analysis shows that higher superficial gas velocity and fluid density, lower particle cohesion, and the collision between agglomerates are advantageous to the agglomerate fluidization of cohesive particles

Studies on the inclined jet penetration length in a gas—solid fluidized bed

An experimental investigation was performed in a 314x25 mm two-dimensional gas-solid fluidized bed with a vertical jet in the center and an inclined jet at the side of the bed, Miller or two kinds of silica sand were used as bed materials in the experiment. The development of the inclined jet was recorded by a video camera. A semi-theoretical correlation was obtained which predicted a jet penetration length which differed from the actual value by at most 25%. When the jet inclination angle was small, by neglecting the influence of the jet position, a simplified equation could be obtained, which differed from the experimental data by at most 40%. A two-phase model was used to simulate macroscopic gas-solid flow in the fluidized beds. The model equations were solved by the improved implicit multifield (IMF) method which can be used for both low and high speed fluid flow. A general-purpose computer program based on the two-phase model was developed. The mechanism of the formation of jets was analyzed by numerical simulation. The influences of jet velocity, nozzle diameter, nozzle inclination angle and nozzle position on the inclined jet penetration length were examined and compared with experimental data. The difference between the horizontal and vertical jets is discussed.

Fluidization of CaCO3 and Fe2O3 particle mixtures in a transverse rotating magnetic field

Under the action of a transverse rotating magnetic field, the fluidization behaviors of the Fe2O3 and CaCO3 particle mixtures were studied and the motion behaviors of the ferromagnetic particles of Fe, Fe2O3, and Fe3O4 were observed. The experiments showed that those ferromagnetic particles, which were placed in a beaker, exhibited four types of motion behaviors (vibrating, forming rotating chains, moving around the wall, and keeping still). The conclusion can be drawn by theoretical analysis of forces and energies that these motions could bring many benefits for fluidization of Geldart Type-C particles. The experiments demonstrated that the fluidization quality of Geldart Type-C particles could be improved by addition of ferromagnetic particles under the external field. It was found that the rotating frequency, field intensity, and volume fraction of magnetic particles were the important factors acting on the fluidization. When field intensity, volume magnetic particle fraction, and rotating frequency were 795.5 A/m, 10% and 5 Hz, respectively, the fluidization was in its optimal condition. The transverse rotating magnetic field affected the incipient fluidization velocity slightly for the mixture of 10% Fe2O3 and 90% CaCO3, but heavily for the mixture of 30% Fe2O3 and 70% CaCO3

Numerical simulation and verification of a gas-solid jet fluidized bed

From the Navier-Stokes equations and the Newtonian equations, which describe gas flow and the movement of a single particle respectively, a two-phase model describing gas-solid flow macroscopically in fluidized beds was derived by using volume averaging and the parameters of the model were determined. This general model can be reduced to several forms of simpler models reported in the literature. An improved interphase slip algorithm (IPSA) method based on finite volume approach was used in the solution of the model equations. The influences of the velocity of the central jet, height of the bed and superficial gas velocity in the bed on jet penetration height were obtained by numerical simulation. The computed flow patterns of the binary mixture were also obtained. The theoretical prediction was verified against the experiments made on a two-dimensional jet fluidized bed with both unitary and binary component materials. Under the operating conditions of an industrial ash-agglomerating coal gasifier, the arithmetical mean based upon particle number was introduced to calculate the mean diameter of the binary components. Thus the simulation of the binary components was simplified. All the computed jet penetration heights under various conditions were in good agreement with experimental and literature data.