Entrance effects at a multi-orifice distributor in gas-fluidised beds

The behaviour of multi-orifice distributors in gas-solids fluidised beds has been studied with particular regard to the height of the entrance effect and the mechanics of gas-solids flow in the region immediately above the distributor plate. A model is proposed to predict the height of the entrance effect for a given distributor and gas-solids system at various fluidising flow-rates, and good agreement has been found with experiment. Experiments have been carried out with (a) a two-dimensional air-fluidised bed using three sizes of sand particles (d_p: 137, 263, and 350 μm) and four distributors (orifice diameters: 0.001 m, 0.002 m; orifice spacings: 0.025 m, 0.05 m); and (b) a three-dimensional air-fluidised bed, 0.3 m square in cross-section, using 350 μm sand particles on a distributor with 0.003 m diameter orifices at a spacing of 0.04 m. The principal factors influencing the height of the entrance effect were found to be the incipient fluidising velocity, mean particle size, orifice spacing and gas flow-rate. The model has been used to estimate the minimum ratio of distributor pressure drop to bed pressure drop to bring about an even distribution of gas at the bottom of the bed.     

Gas distributor in fluidised beds

Satisfactory performance of fluidised bed reactors in terms of bed characteristics, chemical conversion and power requirement for pumping the fluidising medium is influenced by the design features of gas distributors. In industrial practice multi-orifice types of distributors are frequently used.This paper presents the results of experiments carried out in a 100-mm-diam. glass column using multi-orifice plate distributors. The effect of bed height and bed materials on the number of operating orifices is reported. The number of operating orifices is found to be a function of gas velocity and the ratio of pressure drops across the distributor and the bed.     

溢流管式多层气固流化床稳定操作气速范围的影响因素

在四段溢流管式多层流化床装置中研究了固体颗粒加料速率、分布板开孔率及孔径、溢流管内径、颗粒种类及粒径对多层流化床稳定操作气速范围的影响. 结果表明,多层流化床最小稳定操作气速(Umin)随颗粒加料速率、分布板开孔率及颗粒粒度增大而增大,随溢流管管径增大而减小,与分布板孔径大小无关;最大稳定操作气速(Umax)随分布板开孔率和孔径及颗粒粒度增大而增大,随溢流管管径增大而减小. 石英砂和煤颗粒的最大稳定操作气速均先随加料速率增加而增加,超过某一加料速率(表观颗粒速度约为1.0′10-4 m/s)后基本保持不变. 在相同固体加料速率下,石英砂的最大与最小稳定操作气速比(Umax/Umin)约是煤颗粒的3倍,表明石英砂比煤颗粒具有更大的操作弹性,可能是由于使用的褐煤含水量高、流动性差所致.     

The effect of bed materials on the solid/bubble motion in a fluidised bed

Gas fluidisation provides good mixing and contact of the gas and particle phases as well as good heat transfer. These attractive features are achieved by the high degree of bubble-induced particle circulation within the bed. Bubble and particle motion vary with bed materials and operating conditions, as investigated in the present study, by the use of the non-intrusive positron emission particle tracking (PEPT) technique. The selected materials were spherical polyethylene and glass particles.The data obtained by the PEPT technique are used to determine the particle velocities and circulation pattern. Bubble rise velocities and associated sizes can be inferred from the particle velocity data, since particles travel upwards mostly in the bubble wake. The results indicate that the flow structure and gas/solid motion within the fluidised beds were significantly different, even at the same value of the excess gas velocity, U-U_mf. The solid circulation pattern within the beds differ: if for glass beads, a typical UCDW-pattern existed (upwards in the centre of the bed, downwards near the wall), the pattern in the polyethylene bed is more complex combining a small zone of UWDC movement near the distributor and a typical UCDW-pattern higher up the bed. Transformed data demonstrate that at the same value of excess gas velocity, U-U_mf, the air bubbles in the polyethylene fluidised bed were smaller and rose more slowly than in the fluidised bed of glass beads, thus yielding a longer bubble residence time and improved gas/solid contact. For polyethylene beads, the size and rise velocity of air bubbles did not increase monotonically with vertical position in the bed as would be predicted by known empirical correlations, which however provide a fair fit for the glass beads data. Bubble sizes and solid circulation patterns are important parameters in the design of a fluidised bed reactor, and vary with the bed material used.     

细颗粒喷动床的流体力学特性

在直径186 mm的喷动床中考察了细颗粒(d_p＝0.241～0.874 mm)体系的流体力学性质及夹带和磨损特性.采用不同的喷嘴直径(D_i＝6～14 mm)和锥底顶角(θ＝45°～80°)对其操作状态进行了研究,发现当D_i/d_p<19～21时可以形成稳定喷动.随着气体速度的增加,床层依次出现固定床、稳定喷动、不良喷动和腾涌等4个流动区域.实验测量的最小喷动速度与Mathur-Gishler关联式的误差在±15%之内.采用γ射线扫描仪测量了固体密度分布,结果表明喷动区的固体密度随高度的增加而增加,环流区的密度比松堆密度大3%～10%.颗粒的磨损和夹带随着气速的增大而增大,在稳定有序的喷动状态下变得最小.     

Discharge of a fluidised bed of particles through an orifice

This paper describes measurements and modelling of the discharge of a pressurised bed of fluidised particles through a circular orifice. A simple one dimensional flow model is used to describe three stages of flow; radial convergence of flow towards the orifice, axial acceleration through the orifice, and continued acceleration of the free particle jet beyond the orifice. This model correctly predicts previously observed trends in the final velocity and voidage of particle jets discharging from low pressure beds. Experimental results reported here for discharge from higher pressure beds are also well predicted. Experimental results are described for discharge of particles with a mean diameter of 150 μm to atmosphere through 3 mm and 4 mm diameter orifices, from a fluidised bed at pressures up to 5 bar. For the larger orifice diameter, the solids mass flow rate increased with the square root of the pressure drop across the orifice, consistent with theory and observation described by other authors for lower pressure systems. For the smaller diameter orifice the solids flow rate had a dependency on the pressure drop to a power of 0.4. This is explained in terms of changes in the pressure profile through the orifice that occurs when the orifice diameter is small compared to the thickness of the orifice plate. Gas volumetric flow rates increased approximately linearly with pressure.     

Particle attrition mechanism with a sonic gas jet injected into a fluidized bed

High velocity gas jets in fluidized beds provide substantial particle attrition: they are used industrially to control the particle size in fluid bed cokers and to grind products such as toner, pharmaceutical or pigment powders. One method to control the size of the particles in the bed is to use an attrition nozzle, which injects high velocity gas and grinds the particles together. An important aspect of particle attrition is the understanding and modeling of the particle breakage mechanisms. The objective of this study is to develop a model to describe particle attrition when a sonic velocity gas jet is injected into a fluidized bed, and to verify the results using experimental data. The model predicts the particle size distribution of ground particles, the particle breakage frequency, and the proportion of original particles in the bed which were not ground. It was found that the particle breakage frequency can be used to predict the attrition results in different bed sizes. A correlation was also developed, which uses the attrition nozzle operating conditions such as gas density and equivalent speed of sound to predict the mass of particles broken per unit time.     

The influence of distributor structure on the solids distribution and flow development in circulating fluidized beds

The influence of distributor structure on solids distribution is studied in two riser circulating fluidized bed reactors with different distributor structures but similar diameters. Optic fibre probes were used for the measurement of local solids distribution. The axial and radial distribution of solids holdup in the riser with a multi‐tube distributor is more uniform than that with a multi‐orifice distributor. The radial profiles of particle velocity in the riser with the multi‐tube distributor are also more uniform than that with the multi‐orifice distributor. In the riser with the multi‐tube distributor, both gas and particles are distributed more uniformly across the section, so that the flow acceleration is much more uniform and faster. The flow development is much faster and the fully developed region is reached early for the riser with the multi‐tube distributor. The distributor design is an important factor for the design of circulating fluidized bed reactor.     

Experimental study on solid circulation in a multiple jet fluidized bed

Particle image velocimetry was used to investigate the evolution of multiple inlet gas jets located at the distributor base of a two‐dimensional fluidized bed setup. Results were used to estimate the solid circulation rate of the fluidized bed as well as particle‐entrainment into the individual jets. The effects of fluidization velocity, orifice diameter, orifice pitch, particle diameter, and particle density were studied. It was determined from this study that the solid circulation rate linearly increased with an increase in the fluidization velocity until the multiple jet system transitioned from isolated to an interacting system. In the interacting system of jets, the solid circulation increased with fluidization velocity but at a much lower rate. For multiple jet systems, this phenomenon may indicate the presence of an optimum operating condition with high circulation rate and low air input in the bed. © 2011 American Institute of Chemical Engineers AIChE J, 58: 3003–3015, 2012     

流化床与固定床耦合反应器中的颗粒磨损

在直径50mm的冷模流化床与固定床耦合反应器中,考察了活性炭颗粒在130～150℃及不同气速下的磨损情况,得到了不同气速下固定床中圆柱状颗粒的磨损率随时间的变化关系,同时分析了滞留在流化床、固定床及袋滤器中的细颗粒在不同气速下的粒径分布与质量分布.结果表明,颗粒在该耦合反应器中磨损严重,在0.212～0.424m/s气速下,固定床中颗粒质量损失可达3%～4%,流化床中颗粒平均粒径由200μm降至100μm以下.     

Influence of multiple gas inlet jets on fluidized bed hydrodynamics using Particle Image Velocimetry and Digital Image Analysis

A rectangular fluidized bed setup was developed to study the evolution of inlet gas jets located at the distributor. Experiments were conducted with varying distributor types and bed media to understand the motion of particles and jets in the grid-zone region of a fluidized bed. Particle Image Velocimetry and Digital Image Analysis were used to quantify the parameters that characterize these jets. A grid-zone phenomenological model was developed to compare these parameters with those available in the literature. It was determined from this study that jet penetration length behavior is consistently different for fluidization velocities below and above the minimum fluidization. For velocities above minimum fluidization, jet lengths were found to increase more rapidly with increase in orifice velocity than for operating conditions below minimum fluidization.     

THE EFFECTS OF DISTRIBUTOR DESIGN ON FLUIDIZED-BED HYDRODYNAMIC BEHAVIOR

should be addressed. The distributor was investigated for the purpose of design and scale up of large fluidized-bed combustors. Four orifice plates with different configurations were used to study the effect of distributor design on bubble formation and solid mixing. Experiments were carried out on a three-dimensional fluidized bed of 27.94 cm diameter and a two-dimensional bed with dimensions of 30.48cm ×1.27 cm. Motion pictures were used to study bubble formation and coalescence. Pressure profiles inside the three-dimensional bed were measured for several distributors to study bubble flow patterns, and tracer particles were used to study mixing patterns at various superficial velocities and particle sizes. The results show that the distributor plate with two-size orifices causes a non-uniform gas bubble flow inside the bed. This non-uniform gas bubble flow is associated with variations in local bed density and local voidage. Horizontal or radial solid circulation is also caused by this non-uniform gas bubble flow. The local bed density and voidage variations and the radial solid circulation cause the bubbles to move toward the area above the smaller orifices as the bubbles rise up and coalesce. This reduces the wall effect, and the bed is very uniformly fluidized when the two-size orifice plate with small holes in the center is employed.     

The choice of distributor to bed pressure drop ratio in gas fluidised beds

The choice of the gas distributor to bed pressure drop ratio for the stable operation of gas fluidised beds is determined by the type of gas distributor and by the characteristics of the bed material. A critical analysis regarding the choice of the ratio has been made and reasons for diverse views expressed in the literature have been offered. An equation to determine U_M, the superficial gas velocity at which all the orifices in a distributor become operative in a uniformly fluidised bed has been suggested, i.e.

where U_mf and U_t are the minimum fluidisation velocity and the terminal velocity respectively. The distributor to bed pressure drop ratio (ΔP_d/ΔP_b) can be calculated from U_mf and U_M using the equation

where the constant C equals 2.     

A circulating fluidised bed

Induced particle circulation was studied in a 0.3 m diam. air fluidised bed of sand with central draught tubes of 0.2 m and 0.15 m diam. and 0.6 m and 1.2 m in length. A “two-dimensional” bed, 0.3 m in width, of similar cross-section, was also used to study catalyst particle circulation. Superficial gas velocities of up to 0.4 m/s of air were supplied to the base of the draught tube to induce particle circulation rates in the annular downcomer of up to 400 kg/m² s. The circulation rate was shown to be affected by the gap height between the distributor and the draught tube, but was not affected by the draught tube length of height of bed above it. A model was developed to predict the circulation rate, assuming that the driving force for circulation was the density difference between draught tube and annulus and that energy was dissipated by particle shear at the walls. The theory is in reasonable agreement with the experimental results. A tentative model for predicting the shear stress at the wall of a flowing fluidised bed is presented.     

Uniformity of fluidization on a multi-orifice gas distributor

As the gas flow rate is increased through a bed of particles lying on a multi-orifice gas distributor, several gas pressure maxima arise in the plenum chamber which correspond to the occurrence of fluidization in different sections of the bed. The gas pressure at the distributor orifices may considerably exceed the quantiy ρ_sg(1 ? ?₀)H. A formula has been derived for the complete-fluidization index at which all caps come into play.     

煤沥青球气固流化磨损特性实验研究

固体颗粒的流化磨损是流态化技术重要的基础问题之一,气固流动过程中颗粒的磨损特性以及两种磨损机制的研究,对流态化技术的应用具有重要意义。针对煤沥青球设计可视化冷态流化实验系统,研究表观气速、初始粒径和高径比对颗粒流化磨损行为的影响,探讨颗粒流化磨损机理。结果表明：经过流化磨损后,仍在初始粒径范围内的煤沥青颗粒球形度增加,表面更光滑;流化磨损过程受到体相断裂和表面剥层两种磨损机制的共同作用：高速磨损阶段由表面剥层主导,低速磨损阶段表面剥层和体相断裂同时存在,稳态阶段再次由表面剥层主导;提高表观气速和高径比、降低初始粒径均会加剧流化磨损过程,流化数从2.7增加到3.9,体相断裂和表面剥层程度分别增加了3.6%和1.4%。     

Intelligent fitting of minimum spout‐fluidised velocity in spout‐fluidised bed

The experiments were carried on to study the minimum spout‐fluidised velocity in the spout‐fluidised bed. It was found that the minimum spout‐fluidised velocity increased with the rise of static bed height, spout nozzle diameter, particle density, particle diameter, fluidised gas velocity but decreased with the rise of carrier gas density. Based on the experiments, least square support vector machine (LS‐SVM) was established to predict the minimum spout‐fluidised velocity, and adaptive genetic algorithm and cross‐validation algorithm were used to determine the parameters in LS‐SVM. The prediction performance of LS‐SVM is better than that of the empirical correlations and neural network.     

An analysis of the distribution of flow between phases in a gas fluidised bed

A new and comprehensive theory is developed to describe the division of gas between the bubble and interstitial phases of a fluidised bed, something not satisfactorily predicted by existing theories. It is based on the hydrodynamic models of Davidson, Harrison and Murray and distinguishes between bubbles with and without clouds. It makes no assumptions about the value to be attributed to the average dense phase porosity but requires an expression relating it to permeability. An example of application is given using data obtained from a bed of silicon carbide particles of mean diameter 262 μm fluidised by air. In this case, the dense phase porosity and the interstitial gas velocity decrease with height. Near the distributor a large proportion of the bubbles are small, slow moving and therefore without clouds but this proportion decreases sharply with increase in height.     

A simple energy balance to determine heat transfer coefficients from low temperature measurements in a fluidised bed

A bench-scale fluidised bed (105 × 200 mm) was set-up for studying bed-to-gas and wall-to-bed heat transfer. Low temperature (17-200 °C) experiments were conducted at steady state avoiding excessive instrumentation and time. Compressed dry air at ambient temperature entered the bed through a distributor of a 200-mesh brass sieve and fluidised the single charge of alumina particles with a mean diameter of approximately 250 μm. The superficial gas velocity ranged from 0.085 to 0.412 m s^− 1. A simple model was developed based on steady state energy balances, i.e. equating the electrical power input separately to the rate of heat transfer from the heater walls to the bed and from the bed to the gas. The bed-to-gas heat transfer coefficient was calculated from the model equations. Inserting this value into the relevant heat transfer equations then extracted the wall-to-bed and bed-to-gas heat transfer coefficients. The agreement between the experimental and predicted values of temperatures validated the model. The latter may be successfully used to design fluidised beds for e.g. drying or combustion.     

Influence of hydrodynamic parameters on particle attrition during fluidization at high temperature

In a fluidized bed, attrition both increases the number of particles and reduces particle size, which may affect reactor performance, fluidizing properties, operating stability and operating costs. Most fluidized applications are conducted at high temperature, but in the past most attrition correlations were performed at room temperature, so the attrition rate at high temperature could not be predicted. In contrast, this study investigates the attrition rate of fluidized materials at high temperature. Silica sand was used as the bed material; the operating parameters included temperature, particle size, static bed height and gas velocity to assess the attrition rate. Then an appropriate correlation was developed by regression analysis to predict attrition rate at high temperature. Experimental results indicated that the attrition rate increases with increasing temperature. In addition, the particle attrition increased as average particle size decreased because the probability of collision increases with surface area. The attrition rate increased with increasing gas velocity because of increased kinetic stress of particle movement. The actual density and viscosity of air at specific fluidization temperature were modified and an Ar number was introduced to fit our experimental data. The experimental correction agrees with the experimental results, which can predict particle attrition rate at high temperatures.