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The unique characteristics of gas-solids two-phase flow and fluidization in terms of the flow structures and the apparent behavior of particles and fluid-particle interactions are closely linked to physical properties of the particles, operating conditions and bed configurations. Fluidized beds behave quite differently when solid properties, gas velocities or vessel geometries are varied. An understanding of hydrodynamic changes and how they, in turn, influence the transfer and reaction characteristics of chemical and thermal operations by variations in gas-solid contact, residence time, solid circulation and mixing and gas distribution is very important for the proper design and scale-up of fluidized bed reactors. In this paper, rather than attempting a comprehensive survey, we concentrate on examining some important positive and negative impacts of particle sizes, bubbles, clusters and column walls on the physical and chemical aspects of chemical reactor performance from the engineering application point of view with the aim of forming an adequate concept for guiding the design of multiphase fluidized bed chemical reactors.

One unique phenomenon associated with particle size is that fluidized bed behavior does not always vary monotonically with changing the average particle size. Different behaviors of particles with difference sizes can be well understood by analyzing the relationship between particle size and various forces. For both fine and coarse particles, too narrow a distribution is generally not favorable for smooth fluidization. A too wide size distribution, on the other hand, may lead to particle segregation and high particle elutriation. Good fluidization performance can be established with a proper size distribution in which inter-particle cohesive forces are reduced by the lubricating effect of fine particles on coarse particles for Type A, B and D particles or by the spacing effect of coarse particles or aggregates for Type C powders.

Much emphasis has been paid to the negative impacts of bubbles, such as gas bypassing through bubbles, poor bubble-to-dense phase heat & mass transfer, bubble-induced large pressure fluctuations, process instabilities, catalyst attrition and equipment erosion, and high entrainment of particles induced by erupting bubbles at the bed surface. However, it should be noted that bubble motion and gas circulation through bubbles, together with the motion of particles in bubble wakes and clouds, contribute to good gas and solids mixing. The formation of clusters can be attributed to the movement of trailing particles into the low-pressure wake region of leading particles or clusters. On one hand, the existence of down-flowing clusters induces strong solid back-mixing and non-uniform radial distributions of particle velocities and holdups, which is undesirable for chemical reactions. On the other hand, the formation of clusters creates high solids holdups in the riser by inducing internal solids circulations, which are usually beneficial for increasing concentrations of solid catalysts or solid reactants.

Wall effects have widely been blamed for complicating the scale-up and design of fluidized-bed reactors. The decrease in wall friction with increasing the column diameter can significantly change the flow patterns and other important characteristics even under identical operating conditions with the same gas and particles. However, internals, which can be considered as a special wall, have been used to improve the fluidized bed reactor performance.

Generally, desirable and undesirable dual characteristics of interaction between particles and fluid are one of the important natures of multiphase flow. It is shown that there exists a critical balance between those positive and negative impacts. Good fluidization quality can always be achieved with a proper choice of right combinations of particle size and size distribution, bubble size and wall design to alleviate the negative impacts.

Meso-scale structures existing in the form of particle-rich clusters, streamers or strands in circulating fluidized beds, and of ascending bubble plumes and descending liquid-rich vortices in bubble columns and slurry-bed reactors, as commonly observed, have played an important role in the macro-scale behavior of particle-fluid systems. These meso-scale struc-tures span a wide range of length and time scales, and their origin, evolution and influence are still far from being well understood.

Recent decades have witnessed the emergence of computer simulation of particle-fluid systems based on computa-tional fluid dynamic (CFD) models. However, strictly speaking these models are far from mature and the complex nature of particle-fluid systems arising from the meso-scale structures has been posing great challenges to investigators. The reason may be that the current two-fluid models (TFM) are derived either from continuum mechanics by using different kinds of averaging techniques for the conservation equations of single-phase flow, or from the kinetic theory of gases in which the assumption of molecular chaos is employed, thereby losing sight of the meso-scale heterogeneity at the scale of computational cells and leading to inaccurate calculation of the interaction force between particles and fluids. For example, the overall drag force for particles in a cell is usually calculated from the empirical Wen & Yu/Ergun correlations, which should be suspected since these correlations were originally derived from homogeneous systems.

Schemes to solve this problem for gas-particles systems may be classified into four categories. First, one could capture the detailed meso-scale structure information at the cell scale by employing the so-called direct numerical simulation (DNS) (Hu, 1996), the pseudo-particle modeling (PPM) (Ge & Li, 2003), or the Lattice-Boltzmann method (LBM) to track the interface between gas and particles. Second, refinement of the computational meshes may reduce the heterogeneity to some extent and may be capable of capturing some meso-scale heterogeneity though there still exists some argument about the physical rationality of this approach such as the treatment of particle phase as a continuum while fining the meshes. Third, it is generally agreed that a cascade description, viz. extracting the closure correlations for TFM from microscopic simulations such as PPM and LBM (van der Hoef et al., 2004), can suggest a practical way to explore the multi-scale heterogeneity. Although the above three schemes are logical and fundamental, they are generally difficult to implement at present due to the complexity of the models or the enormous computational cost. The fourth scheme we adopted in this study is the so-called energy-minimization multi-scale (EMMS) model which seems to be a simple yet reasonable approach at the moment.

In the present approach, a "structure" model is established to describe the meso-scale heterogeneity through the definition of eight "structure parameters" and the resolution of structure involving a particle-rich dense cluster phase and a gas-rich dilute phase. Gas-solid interaction is also resolved into that between gas and particles inside both the dense cluster phase and the dilute phase, and that between the cluster phase and the dilute phase. This means that the drag force for the dense cluster phase includes two parts, namely, bypassing drag (k_i) and permeating drag (k_c) as depicted in Fig.1. We found that the absolute value of the difference (Δk) between k_c and k_i could be employed to evaluate the extent of the system heterogeneity. On the basis of this structure model, the average acceleration (a) induced by gas-solid interactions can be obtained, and then the average drag coefficient (β) for the two-fluid model can be calculated. Calculation results show that the computed value of β with the EMMS model is much less than that with the Wen & Yu/Ergun correlations, which is in reasonable agreement with conclusions derived from experiments. We further simplified this model by assuming that the dense phase voidage (∊_c) is a constant because the direct incorporation of this new model into the two-fluid model is difficult at present due to computational cost resulting from the iterative process and certain limitations of this model stemming from its original derivation from the global fluidized bed system. With the Ug and Gs for the global system as the input parameters, we can calculate β for each specified ∊ and thereby obtain the correlation of correction factor ω (∊) as a function of ∊. Implanting this correlation into each cell and employing the local slip velocity and voidage, β for each cell can thus be obtained.

The simulation is performed for a FCC riser by combining the two-fluid model with this EMMS-based drag model and the Wen & Yu/Ergun correlations respectively. Comparison of the simulation results shows that the EMMS-based drag model produces more reasonable results than the Wen & Yu/Ergun correlations. The former shows its improvement in predicting the solids entrainment rate, the meso-scale heterogeneous structure involving clusters or strands, and the radial and axial voidage distributions. For the latter, the simulated flow structure is homogeneous and no clusters are observed, and the predicted solid entrainment rate is too much larger than experimental measurements. We also employ different approaches to predict the occurrence of choking, indicating that this EMMS-based drag model has the ability to capture this important phenomenon in CFB systems. For the details, the interested reader is referred to the work of Li and Kwauk (1994), and Yang et al. (2003a; 2003b; 2004; 2005).

Simulations of the gas fluidization of a cohesive powder were performed using the Stokesian Dynamics method and an agglomeration-deagglomeration model to investigate methods of improving the fluidizability of fine powders. Three techniques (a) high gas velocity (b) vibration-assisted fluidization and (c) tapered fluidizer were used in the simulations which provided detailed information on the bed microscopy such as the motion of 100 particles in a fluidizing vessel along with the formation and destruction of cohesive bonds during collisions. While all three techniques were found to effectively improve the fluidizability of a strongly cohesive powder, we suggest a combination of high velocity fluidization assisted by external vibration of the fluidized bed to minimize entrainment of particles.

The hydrodynamic behavior of fine powders in jet-fluidized beds was studied numerically and experimentally. The starting point of numerical simulation was the generalized Navier-Stokes (N-S) equations for the gas and solids phases. The κ-ε turbulence model was used for high-speed gas jets in fluidized beds. Computation shows that a suitable turbulence model is necessary to obtain agreement between the simulation and literature experimental data for a high-speed gas jet. The model was applied to simulating the fluidization of fine powders in fluidized beds with an upward or a downward air jet. An empirical cohesion model was obtained by correlating the cohesive force between fine particles using a cohetester. The cohesion model was embedded into the two-fluid model to simulate the fluidization of fine powders in two-dimensional (2-D) beds. To study the fluidization behavior of fine and cohesive powders with a downward jet, experiments were performed in a 2-D bed. Agreement between the computed time-averaged porosity and measured data was obtained. With an upward jet in the bed center, the measured and computed porosities show a dilute central core, especially at very high jet velocities. Based on our experiments and computations, a downward jet located inside the bed is recommended to achieve better mixing and contacting of gas and solids.

Electrostatic charges are generated by particle-wall, particle-particle and particle-gas contacts in gas-solids transport lines and fluidized bed reactors. High particle charge densities can lead to particle agglomeration, particle segregation, fouling of reactor walls and internals, leading to undesirable by-product and premature shut-down of processing equipment. In this paper, the charge generation, dissipation and segregation mechanisms are examined based on literature data and recent experimental findings in our laboratory. The particle-wall contact charging is found to be the dominant charge generation mechanism for gas-solids pneumatic transport lines, while bipolar charging due to intimate particle-particle contact is believed to be the dominant charge generation mechanism in gas fluidized beds. Such a bipolar charging mechanism is also supported by the segregation patterns of charged particles in fluidized beds in which highly charged particles tend to concentrate in the bubble wake and drift region behind rising bubbles.

The influence of gas type (helium and argon) and bed temperature (77–473 K) on the fluidization behaviour of Geldart groups C and A particles was investigated. For both types of particles tested, i.e., Al₂O₃ (4.8 μm) and glass beads (39 μm), the fluidization quality in different gases shows the following priority sequence: Ar > He. In the same gaseous atmosphere, the particles when fluidized at an elevated temperature usually show larger bed voidages, higher bed pressure drops, and a lower u_mf for the group A powder, all indicating an enhancement in fluidization quality. Possible mechanisms governing the operations of gas type and temperature in influencing the fluidization behaviours of fine particles have been discussed with respect to the changes in both gas properties and interparticle forces (on the basis of the London-van der Waals theory). Gas viscosity (varying significantly with gas-type and temperature) proves to be the key parameter that influences the bed pressure drops and u_mf in fluidization of fine particles, while the interparticle forces (also varying with gas-type and temperature) may play an important role in fine-particle fluidization by affecting the expansion behaviour of the particle-bed.

A magnetofluidized bed consists of a bed of magnetizable particles subjected to a gas flow in the presence of an externally applied magnetic field. In the absence of magnetic field, there is a given gas velocity at which naturally cohesive fine particles can form a network of permanent interparticle contacts capable of sustaining small stresses. This gas velocity marks the jamming transition of the fluidized bed. For gas velocities above the jamming transition, the bed resembles a liquid. Below the jamming transition, the bed behaves as a weak solid and it has a nonvanishing tensile strength. In the absence of magnetic field, the tensile strength of the solidlike stabilized bed has its only origin in nonmagnetic attractive forces acting between particles. In the presence of a magnetic field, the gas velocity at the jamming transition and the tensile strength of the bed depend on the field strength as a consequence of the magnetostatic attraction induced between the magnetized particles. In this work we present experimental measurements on the jamming transition and tensile strength of magnetofluidized beds of linearly magnetizable fine powders. It is shown that powders with similar magnetic susceptibility but different strength of the nonmagnetic forces show a different response to the magnetic field. This finding can be explained by the influence of the nonmagnetic natural forces on the network of contacts. Thus, our experimental results reported in this paper reinforce the role of short-ranged interparticle contact forces on the behavior of the system, which contrasts with the usual modeling approach in which the magnetofluidized bed is viewed as a continuum medium and a fundamental assumption is that the fields can be averaged over large distances as compared with particle size.