Pressure Drop and Gas Holdup Studies in a Spout-Fluid Bed

Spout-fluid beds are used for a variety of processes involving particulate solids. They are employed where the particle agglomeration, dead zones, and sticking of particles to the vessel are the common problems in conventional spouted beds. Applications involved are granulation, coating, drying, combustion, and gasification. In this study, experimental studies have been carried out in a cylindrical Perspex column (0.094 m internal diameter and 1.217 m height) using glass beads and air. The effects of initial bed loading, spout velocity, and background (fluidization) velocity on pressure drop and gas holdup have been investigated. It is found that the minimum spout-fluidizing velocity increases with increase in initial bed loading. The pressure drop and gas holdup increase with increasing bed loading. In spout-fluid bed condition, at a constant spout velocity, as the background gas velocity increases, the gas holdup increases, and it is found to be high for smaller bed loading and is low for larger bed loading at higher velocities. The fountain height increases as spouting velocity increases and it decreases with initial bed loading. The total velocity required to fluidize the particles in spout fluidization is lower in comparison to spouted beds and fluidized beds.     

Experimental and numerical studies on a bubble-induced inverse gas-liquid-solids fluidized bed

Hydrodynamics in a newly invented bubble-induced inverse gas–liquid-solids three-phase fluidized bed has been studied via both experimental and numerical methods. With experiments in a 3.0 m column of 0.153 m in diameter, four fluidization regimes including a fixed bed regime, a bed expansion regime, a complete fluidization regime, and a freeboard regime have been identified with the increase in the superficial gas velocity. A three-phase Eulerian-Eulerian CFD model was developed to simulate the hydrodynamics in the inverse three-phase fluidized bed and the simulation results have a good agreement with the experimental data. The effects of the particle property and solids loading on the transitions across the flow regimes were numerically studied. A higher solids loading and/or a larger particle density are reported to contribute to an easier fluidization and a faster flow development to the complete fluidization regime. The radial flow structure becomes less uniform with increased inner circulation of the liquid after introducing more bubbles into the column.     

Entrainment of Heterogeneous Particles from Gas-Fluidized Bed

The entrainment of heterogeneous particles in a gas-fluidized bed for particle mixtures of categories A and B according to the Geldart classification was conducted in this study. The experiments were carried out in an acrylic column of 0.092 m diameter. The distributors were perforated plates of up to 5.9% free area fraction. Measurements of pressure gradients were made using 24 pressure taps. Glass beads with particle diameters from 60 to 400 μm were used. The mixtures were described using the Rosin-Rammler-Bennet granulometric distribution model. The transport disengaging height (TDH) heights were obtained using the Geldart methodology. A 2⁵ experimental design was applied relating the dispersion index, mean diameter, solid mass, superficial gas velocity, and free area fraction of the distributor to obtain the TDH heights. Slugging and transition for turbulence regimes characterized the behaviors of fluidized beds. The results showed that TDH heights for heterogeneous particles were dependent on the solid mass and superficial gas velocity.     

Rheological behavior of aqueous suspensions of hydroxyapatite (HAP)

The aim of this study is to characterize the influence of organic dispersants on the stability of hydroxyapatite (HAP) dispersions, using viscosity measurements, electrokinetic sonic amplitude (ESA) measurements and adsorption isotherms. Colloidal stability of aqueous suspensions of HAP as a function of solids loading and of the type and concentration of deflocculant has been studied. The interaction between particles and dispersant molecules was characterized. Adsorption measurements of the amount of dispersants on the particles surface, viscosity and electrophoretic measurements as a function of the dispersant concentration are correlated. Suspension parameters have been adjusted in order to obtain a high solids loading slurry with a plastic viscosity which will allow its use in colloidal processing.     

The impact of vertical internals array on the key hydrodynamic parameters in a gas-solid fluidized bed using an advance optical fiber probe

The effect of a circular configuration of intense vertical immersed tubes on the hydrodynamic parameters has been investigated in a gas-solid fluidized bed of 0.14?m inside diameter. The experiments were performed using glass beads solid particles of 365?μm average particle size, with a solid density of 2500?kg/m³ (Geldart B). An advanced optical fiber probe technique was used to study the behavior of six essential local hydrodynamic parameters (i.e., local solids holdup, particles velocity, bubble rise velocity, bubble frequency, and bubble mean chord length) in the presence of vertical immersed tubes. The experimental measurements were carried out at six radial positions and three axial heights, which represent the three key zones of the bed: near the distributor plate, the middle of the fluidizing bed, and near the freeboard of the column. Furthermore, four superficial gas velocities (u/u_mf?=?1.6, 1.76, 1.96, and 2.14) were employed to study the effect of operating conditions. The experimental results demonstrated that the vertical internals had a significant effect on all the studied local hydrodynamic characteristics such that when using internals, both the solids holdup and bubble mean chord length decreased, while the particles velocity, bubble rise velocity, and bubble frequency increased. The measured values of averaged bubble rise velocities and averaged bubble chord lengths at different axial heights and superficial gas velocities have been compared with most used correlations available in the literature. It was found that the measured values are in good agreement with values calculated using predicted correlation for the case without vertical internals. While, the absolute percentage relative error between the measured and calculated values of these two hydrodynamic parameters indicate large differences for the case of vertical internals.     

Discrete element simulation of dynamic behaviour of partially saturated sand

The discrete element method (DEM) together with the finite element method (FEM) in LS-DYNA was employed to investigate the dynamic behaviour of sand under impact loading. In this approach, the partially saturated sand was modelled in DEM with capillary forces being taken into account through an implicit capillary contact model, while other solids were simulated using FEM. A slump test was first performed with dry sand to calibrate the contact parameters in DEM. Low velocity impact tests were then conducted to investigate the effect of water saturation on the shape and height of sand piles after impact, and to validate the simulations. It was found in the experiments that an increasing water saturation (in the range between 10 and 30 %) affected the height of sand pile for a given drop height due to an increasing cohesion between particles. The simulations captured the experimental ejecta patterns and sand pile height. Finally, a low confinement split Hopkinson pressure bar test from earlier literature was modelled; the DEM–FEM simulations could reproduce the trends of experimentally observed stress–strain curves of partially saturated sand under high strain rate loading, indicating that it was feasible to model dynamic behaviour of dry and wet sand with low saturation (<20 %) in LS-DYNA; however, a number of questions remain open about the effect of grain shape, grain crushing and viscosity.     

Entrainment of particles and gas induced by draft fan over the particles bed

The draft fan is used to generate a controlled transportation of particles to enhance entrainment of gas and particles from the particles bed. Present investigations show the entrainment behavior of particles induced by an axial 4-blade draft fan hovering over the particles bed. The distributions of velocities and volume fractions of gas and particles are simulated using Euler-Euler two-fluid model (TFM) with kinetic theory of granular flow (KTGF) at different hovering heights and rotational speeds of the draft fan. The dense region with high solids volume fraction and low particles velocity and the dilute region with low solids volume fraction and high particles velocity exist beneath the draft fan along hovering heights. The entrainment of particles increases with the decrease of hovering height and increase of rotational speed of the draft fan. Present numerical simulations confirm that the gas-solid TFM with the kinetic theory of granular flow and multiple reference frame model can be effectively applied to analysis for entrainment of particles induced by draft fan.     

Study of the distribution of solids in a counterflow baffled gas-suspension chamber by the beta-ray method

The distribution of solids over the height and cross section of a counterflow baffled gas-suspension chamber has been investigated by the beta-ray method. The particles are decelerated by introducing spiral mesh baffles.     

Influence of Fines Concentration on Pneumatic Transport Capacity of Alumina

The presence of fine particles in bulk solids has been known to affect the pneumatic transport characteristics. However, there is still no tool available to predict the exact nature of the variation of transport capacity with change in fines concentration in a bulk material. This article presents experimental findings on the nature of the variation of pneumatic transport capacity for alumina for various sub-45 μm particle concentrations. It has been shown that the nature of the variation of the transport capacity depends upon the solids loading ratio and, consequently, the amount of air used for pneumatic transportation. Above a solids loading ratio of 35, alumina transport capacity first decreased with increase in fines concentration and then increased beyond a certain value of fines concentration. For a solids loading ratio less than 35, such a trend was not seen, but a trend of reaching a peak value of transport capacity at increasing fines concentration was seen.     

The effect of processing parameters on the properties of γ-alumina washcoats deposited on ceramic honeycombs

The coating of cordierite honeycomb specimens with -alumina slurries for the preparation of washcoats for automotive applications was investigated. The dependence of slurry viscosity on factors such as the solids content, the pH and the particle size distribution of the powder used, was determined. Slurry viscosity was correlated to the loading percentage achieved, as well as to the quality of the washcoat in terms of homogeneity and reproducibility. It was found than an adequate solids content in the slurry is necessary for the achievement of satisfactory loading per impregnation. When, though, the particle size of the powder employed is of colloidal dimensions, high solids content leads to extremely high viscosity values. Adjustment of the slurry viscosity is therefore necessary and this was achieved with the use of either HCl or ammonium poly-methacrylate, an organic polyelectrolyte. Optimum loading conditions were achieved when the slurry viscosity lied between 50–150 mPa·s. For a specific solids content, the organic polyelectrolyte led to lower viscosity slurries and resulted in better reproducibility of the loading percentage. With the use of ammonium poly-methacrylate, slurries of fine particles, up to 40 wt% solids content could be handled, resulting in reproducible loading percentages of the order of 15 wt%     

PRESSURE CHANGES AT A SUDDEN EXPANSION IN GAS-SOLID FLOWS

There is a pressure recovery when a two-phase mixture passes through a pipe enlargement. This paper describes an experimental project for the determination of this pressure recovery in air-solids systems. The solids used are medium size wax and lucite particles. Two pipe diameter ratios were investigated, 0.63 and 0.76. The Reynolds numbers for the experiments varied between 45000 and 72000 and the loading ratios were low to intermediate (up to 6), it was observed that the major part of the pressure recovery in these mixtures was due to the air alone and that only a small part of it (up to 25%) was due to the deceleration of solid particles.     

Heat absorption characteristics of gas in a solar fluidized bed receiver with carbon nanotubes

Carbon nanotubes (CNTs) have been proposed as new candidate particles to enhance the utilization efficiency of solar energy in solar fluidized bed receiver (SFBR) for solar air heating in low- and mid-temperature ranges. Heat absorption characteristics of the CNTs have been determined in a SFBR (50 mm i.d. X 150 mm high). Two types of experimental particles were used which consisted of multi-walled CNTs with different nanotube shapes, such as entangled CNTs (ENCNTs) and vertically aligned CNTs (VACNTs). The particle dynamics and heat absorption characteristics of CNTs were studied and compared with those of silicon carbide (SiC), a conventional particle. CNTs showed lower pressure fluctuation with relatively uniform particle behavior in the freeboard compared to SiC. The outlet gas temperatures of the receiver with CNTs were higher than those inside the fluidized bed above 0.10 m/s of gas velocity. The temperature increment of gas per irradiance (ΔT/I_DNI) decreased with increasing gas velocity. VACNTs, which are characterized by the coexistence of aggregates and nanotubes in the freeboard, showed a higher value of ΔT/I_DNI than ENCNTs for the same gas velocity. The relative heat absorption temperature (T*) decreased with increasing gas velocity, and dropped below 1.0 at the solid holdup of 0.04, indicating that the freeboard region’s contribution to the receiver’s heat absorption increased. VACNTs and ENCNTs showed maximum thermal efficiencies of 26.7 % and 30.5 % at gas velocities of 0.12 and 0.16 m/s, respectively, which was 33 % higher than that of SiC. Considering the particle properties and particle dynamics, the obtained thermal efficiencies in the present and previous studies were correlated with the Reynolds, Archimedes and Prandtl numbers and the ratios of the specific heat capacities of the particles to the gas.     

About the Features of Transient to Steady State Deformation of Solids

The features of transient to steady state deformation of solids are theoretically investigated.Modeling of various types of loading was carried out by the Movable Cellular Automata method.A stress state of material at the stage of transient to a steady state is shown to be essentially non-uniform, that may in its turn result in stable structures in velocity field of particles of the material. It may also influence development of deformation at the further stages.     

快速床提升管内颗粒的扩散行为研究

在直径１００ｍ ｍ ×３ｍ 的实验台上,利用热粒子为示踪剂,研究快速床提升管内的颗粒的扩散行为。在不同的径向位置测得反映扩散行为的温度响应曲线,提出二维扩散模型来描述颗粒的轴、径向扩散。通过最小二乘拟和估计轴向扩散系统Ｄａ 和径向扩散系数Ｄｒ。实验发现,Ｄａ 随气体表面风速的增加而增加,Ｄｒ 则基本保持不变。静止床高的增加使Ｄａ、Ｄｒ 略有增加。     

Modeling of Particle Concentration Distribution in a Fluidized Bed by Means of the Theory of Markov Chains

A one-dimensional mathematical model of particulate solids fluidization based on the theory of Markov chains is proposed. Transition probabilities that form the matrix and control the migration of particles over the bed are subdivided into two types, convection and diffusion. The convection transition probabilities depend on the local particle concentration in cells of the chain and mainly define the bed expansion at certain hydrodynamic conditions. The diffusion transition probabilities form particle concentration distribution over the bed height. On the basis of the model, the heat exchange between gas and particulate solids is described. The batch fluidization model is generalized to the case of continuous fluidization that allows predicting a connection between throughput and hold-up, as well as particle residence time distribution in a bed. Some aspects of the approach are verified experimentally.     

直接凝固注模成型技术

直接凝固注模成型是瑞士苏黎世联邦高等工业学院L.J.Gauckler实验室发明的一项新的成型技术,具有素坯密度高、密度均匀、坯体收缩和形变极小等优点,特别适用于大尺寸、复杂形状的陶瓷部件的成型,有广阔的应用前景．本文着重介绍了直接凝固注模成型的基本原理和技术关键．     

Forming processes of strontium barium niobate ceramics

In forming strontium barium niobate (SBN) ceramics, two methods (pressure filtration and slip casting) were employed to investigate the consolidation behaviour. The zeta potentials were measured to understand the interparticle forces of SBN powders. It was found that the zeta potentials of SBN powders were negative above pH 2.2. Several experiments have been conducted to investigate the effect of pH on the rheological behaviour of SBN slurries with 20 vol% solids loading. The rheological behaviour of the slurries SBN with 20 vol% solids loading at pH 11.5 is shear thinning. It is suggested that the increase of the flow rate of the fluid might have the advantages to enhance the packing density and prevent fine particles from clogging in pressure filtration and slip casting. Two different moulds i.e. plaster and alumina have been used to investigate the effect of pore morphology of the moulds on the cake microstructures. A uniform microstructure of cast cake was formed for using an alumina mould and significant contamination was observed in using a plaster mould.     

Artificial Neural Network Approach to Segregation Characteristic of Binary Heterogeneous Mixtures in Promoted Gas-Solid Fluidized Beds

Binary mixtures of particles of the same size but of different densities are fluidized in a 15 cm diameter column with a perforated plate distributor and two coaxial promoters. In the present work an attempt has been made to study the fluidization and the segregation characteristic of density-variant solids of the same size in terms of segregation distance. The dimensionless segregation distance has been correlated with other dimensionless groups relating to various system parameters: ratio of the density of jetsam particles to that of flotsam, initial static bed height, height of layer of particles above the bottom grid, superficial gas velocity, and average density of the mixture on the basis of the dimensional analysis approach for both un-promoted and promoted beds. Correlations have also been developed with the above system parameters by using an artificial neural network approach for different types of fluidized beds, and the findings with respect to both approaches have been compared with each other. The values of segregation distance for promoted beds have also been compared with those for the un-promoted bed in this work.     

Spalling in zirconia-reinforced ceramics

Analysis of spalling cracks in zirconia-reinforced ceramics shows that the presence of dilatantly transforming particles can give rise to significant strengthening, even for cracks that are close to the surface with respect to the height of the transformed region. The inherently mixed-mode loading on the crack tip results in an asymmetrical zone shape and makes toughening-strengthening phenomena more sensitive to material parameters than the usually studied symmetrical loading configurations.