Hydrodynamics of a gas-driven gas-liquid-solid spouted bed with a draft tube

A cylindrical gas-liquid-solid spouted bed, driven exclusively by gas flow, has been developed with a high potential for use in biochemical processes, such as a biological wastewater treatment. A plexiglass column with a 152 mm inner diameter was used in combination with a 53 mm inner diameter plexiglass draft tube. Three particle types were studied with densities ranging from 1044 kg/m³-1485 kg/m³ and average particle sizes ranging from 0.7-2.5 mm. Four flow regimes were observed when increasing the gas velocity, including fixed bed, semispouted bed, full spouted bed, and internal circulating fluidized bed. The transition gas velocities between those regimes were experimentally measured and termed as minimum spouting velocity, full spouting velocity, and minimum circulating velocity, respectively. A measurement of the downward particle flux in the annulus was used to identify the minimum spouting velocity, while the particle velocity and dense phase retraction in the annulus were monitored for the full spouting and minimum circulating velocities. All regime transition velocities increased with more dense particles and longer draft tubes. The minimum spouting velocity and full spouting velocity were not affected when varying the nozzle-tube gap, while the minimum circulating velocity increased with longer nozzle-tube gaps. Experiments without a draft tube were carried, though the spouting stability was significantly reduced without the draft tube.     

Fluid flow pattern and solids circulation rate in a liquid phase spout-fluid bed with draft tube

The fluid flow pattern and solids circulation rate in a liquid phase spout-fluid bed with a draft tube were studied in a semi-circular column 196 mm in diameter equipped with a semi-circular draft tube 34.5 mm in diameter using water and spherical glass particles 1.20, 1.94 and 2.98 mm in diameter. Both the solids circulation rate and fluid distribution between the draft tube and the annulus are influenced by the location of the draft tube, the relative magnitude of the inlet spouting and annulus flowrates, and the total fluid flowrate entering the bed. A one-dimensional steady state model of vertical non-accelerating liquid-solids flow predicts the solids mass flux in the draft tube with a mean deviation of 12.3% when experimental values of the axial pressure gradients in the draft tube and annulus are specified. A simple correlation for the fountain height is also presented.     

导向管直径对喷动床流动特性影响的计算颗粒流体力学数值模拟

导向管喷动床是较为常见的一种喷动床改进床型,通过阻断喷动区与环隙区气固接触来提高颗粒循环的规律性与稳定性。本文采用计算颗粒流体力学（CPFD）方法对于直径150mm的柱锥式导向管喷动床进行了数值模拟研究,考察了导向管直径对于喷动床内颗粒流动特性的影响,从环隙区死区分布、颗粒速度分布、固体循环量等方面分析了具有不同直径导向管喷动床的运行状态。结果表明,加入导向管在减少床内死区的同时也降低了运行时的固体循环量,对于本次采用的喷动床结构尺寸与运行参数,只有在导向管直径为40～60mm时才能保证床内具有良好喷动状态,综合考虑各因素,选用直径50～55mm的导向管最为合适。对于具有类似结构与运行条件的柱锥喷动床,导向管直径可考虑选为无导向管运行时喷动区直径的1.2～1.375倍。     

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.     

Gas spouting hydrodynamics of fine particles

Hydrodynamic measurements for the spouting of fine particles were obtained in a conical‐based cylindrical bed of diameter 0.186 m, inlet orifice diameters of 6 to 14 mm, and included cone angles of 45 to 80 degrees. Four different types of fine particles with diameters ranging from 0.286 to 0.861 mm were investigated. Four different flow regimes—fixed bed, stable spouting, unstable spouting and slugging—were identified and mapped. The termination of the spouting of fine particles was caused by choking rather than by fluidization of particles in the upper bed. Stable spouting could be achieved when the ratio of the orifice diameter to mean particle diameter was less than 20. The measured minimum spouting velocities were within ±15% of those predicted by the Mathur—Gishler equation. Solids density measurements obtained by a gamma ray densitometer showed that the solid density in the spout increases with bed height and in the annulus is 3% to 10% higher than in a loosely packed bed.     

Hydrodynamics of spout-fluid beds

Experiments were carried out in a half-column incorporating auxiliary flow, introduced through up to five slots along a 60° conical base, in addition to a central flow of air. The column had a diameter of 0.15 m and inlet diameters of either 19 or 25 mm. Three different types of particle were investigated, all with mean particle sizes in the 2 – 4 mm range. Four different flow regimes — fixed bed, spouting with aeration, spout-fluidization and jet in a fluidized bed — were identified and mapped. The minimum total gas flow required for spouting with aeration and for spout-fluidization was always greater than that corresponding to minimum spouting. Gas percolation through the annulus increased by as much as 50% as the proportion of auxiliary flow was increased for a given total gas flow rate. Solids circulation was increased somewhat by addition of auxiliary flow for deep beds, but a decrease occurred for shallow beds. The overall bed pressure drop under minimum spouting with aeration conditions increased linearly with auxiliary flow. On the other hand, the fountain height decreased as the fraction of auxiliary flow was increased for a fixed total gas flow, and increased with auxiliary flow for a fixed central gas flow. The empirical correlation of McNab predicted the average spout diameter well if the sum of the central and auxiliary flows was used in the correlation.     

双喷嘴矩形喷动床流动性能实验研究

在120 mm×240 mm的双喷嘴矩形不锈钢床内,对新型双喷嘴矩形导流管喷动床的最小喷动速度和喷动高度进行了研究,考察了喷动气速、粒径、静床层高度、导流管直径、导流管安装位置对最小喷动速度和喷动高度的影响。结果表明:最小喷动速度随颗粒直径、导流管直径、导喷距的增大而增大,随静床层高度的增大而减小;喷动高度随喷动气速的增大而增大,随导流管直径的增大而减小,受静床层高度和导喷距的影响不大,并得出了最小喷动速度的关联式。     

纳米TiO_2颗粒在声场导向管喷动流化床中的流化特性

在内径120 mm的半圆柱型声场导向管喷动流化床中,以平均粒径290 nm的TiO_2颗粒为原料,高速空气射流为喷动气,考察了操作条件、声参数(频率和声压)对纳米颗粒在声场导向管喷流床中的流态化特性的影响。结果表明:声波可以有效抑制沟流,改善环隙流化质量,防止射流旁路,从而促使粉体稳定循环,加快循环速率;同时声波可以显著地降低纳米TiO_2颗粒的最小喷动速度,声波频率一定时,最小喷动速度随声压的增加而减小;声压一定时,最小喷动速度在声波频率为80 Hz时达到最小值,低于或者高于80 Hz,最小喷动速度都会增大。     

Spouting Enhancement by Addition of Small Quantities of Liquid to Gas‐Spouted Beds

The addition of liquid water to beds of polystyrene particles exceeding 3 mm in diameter enhanced spouting by causing a decrease in the minimum spouting velocity. While the degree of enhancement is quantitatively comparable to what we reported previously for fluidized beds of large light hydrophobic particles, the mechanism is quite different. Enhancement of spouting takes place because the liquid congregates at the spout‐annulus interface, forming an effective nearly‐impermeable draft tube which then increases the velocity of the gas up the spout.     

Annulus leakage and distribution of the fluid flow in a liquid spout-fluid bed with a draft tube

The distribution of flow between the annulus and the draft tube sections in a liquid phase spout-fluid bed with a draft tube was studied in a flat-based semi-circular column of diameter equipped with a semi-circular draft tube of diameter at superficial fluid velocities well above the minimum fluidization velocity. The particles used were glass spheres of diameter, and the spouting medium was tap water. A sodium chloride solution was injected into the reactor and the response recorded to determine the flow distribution between the draft tube and the annulus sections. The responses of the tracer injections prove that part of the flow originating from the annulus inlet leaks into the draft tube when any amount of annulus inlet flow is present. This finding makes a significant contribution to the understanding of the entrance region below a draft tube which has been modeled as a spout-fluid bed where fluid enters the annulus region based on the pressure distribution at the spout-annulus interface while the flow is in the radial direction from the spout into the annulus. This work shows that there is cross flow in the entrance region resulting in fluid exchange between the streams originating from the spouting inlet nozzle and the annulus inlet flow.The amount of leakage is found to increase with increasing inlet flow rates. An empirical correlation is developed to predict the fraction of the leakage of the annulus inlet flow. The superficial fluid velocity through the draft tube is found to vary linearly with the total flow rate through the bed under the experimental conditions studied. The pressure distributions in the entrance region favor the leakage of fluid from the annulus inlet flow into the draft tube.     

Pressure drop and flowrate characteristics of a liquid phase spout-fluid bed at the minimum spout-fluid flowrate

Data on the minimum spout-fluid flowrates and the pressure drop in the annulus at that flowrate are presented as a function of bed height and nozzle diameter for a bed spout-fluidized with water. The column was 62.6 mm. in diameter and contained 3.09 mm. glass particles. The particle Reynolds number at the minimum fluidizing velocity was 91.6. A plot of the annular vs. the nozzle flowrate shows that the minimum spout-fluid flowrates fall on a straight line between the minimum fluidizing and minimum spouting flowrates. The annular pressure drop is explained using an extension of the theory of Mamuro and Hattori.     

Pressure and flow characteristics of a gas phase spout-fluid bed and the minimum spout-fluid condition

Spoutfluidization is a new technique for solid-fluid contact which aims at incorporating the advantages of spouted bed and fluidized bed technique. The characteristics of physical state of the bed with the variation of the variables which include flow of fluid, particle diameter, orifice diameter, bed height, are studied in this investigation. Experimental study of minimum spoutfluidizing velocity using glass beads with mean particle diameters from 0.254 to 0.600 mm has been carried out in a 90 mm glass column with three spouting inlet orifice sizes at different bed heights. Phase diagrams indicate that the minimum spout-fluid flow rate in a gas-solid system may be a point property for a given bed. A correlation is presented in which the standard deviation of experimental from calculated minimum spoutfluidizing velocity is within 5%.     

狭缝式矩型喷动床中多粒度颗粒体系的最小喷动速度

在150 mm×50 mm×1100 mm的矩形喷动床中,采用宽度为2, 4, 6 mm 的3种狭缝式气体分布板,研究了单一粒度组成和多粒度组成玻璃珠的最小喷动速度. 实验证明,矩形喷动床的最小喷动速度与物料的粒度和组成有关. 给出了最小喷动速度与颗粒粒径和粒度组成的关联式,作出了多粒度组成颗粒体系最小喷动速度的相图.     

水平辅助气对喷动床流动特性影响的研究

在上部内径为 195mm柱状、下部为 6 0°锥体的喷动床中水平引入辅助气 ,考察了水平辅助气对流动特性的影响。实验结果发现 ,辅助气的水平引入比竖直或法向引入可以更有效地抑制喷动气体向环流区扩散 ,降低最小喷动气速。在总气速一定的条件下 ,增大辅助气速的比例 ,可以降低喷动区空隙率而使颗粒浓度增大 ,提高颗粒的循环流动量。相比而言 ,喷动气速对喷动区颗粒的流动速度影响较大 ,而水平辅助气速对环流区颗粒的流动速度影响较大     

Spouted bed hydrodynamics in a 0.91 m diameter vessel

Hydrodynamic measurements were obtained in a flat-based half-cylindrical column of diameter 0.91 m and inlet orifice diameters of 76 to 114 mm. Beds of 3.5 to 6.7 mm diameter particles with static depths of 0.53 to 1.83 m were spouted with air. In agreement with measurements by earlier workers in smaller columns, it was found necessary to operate with inlet orifice diameters less than about 30 times the mean particle diameter in order to be able to achieve stable spouting. Correlations for minimum spouting velocity developed on small vessels generally gave poor predictions for the large diameter vessel employed in this work and failed to predict the observed dependence of U_ms on the static bed height. Substantial dead regions where particles were stagnant were observed in the lower outer portion of the vessel. Other aspects of behaviour studied, including spout diameters and shapes, fountain heights, pressure profiles and gas velocities in the annulus, were qualitatively similar to those in smaller columns, although equations developed for the smaller vessels did not always provide accurate predictions.     

Numerical analysis of spouted-bed hydrodynamics

Some of the basic concerns in the analysis of spouted-bed hydrodynamics are the flow pattern and the pressure field of the fluid and the spout shape. These are closely related with other design concerns such as spoutability, minimum spouting velocity and maximum spoutable bed depth. A computer program was developed in order to determine the flow pattern, pressure distribution in the fluid phase and the spout shape for a given operational condition. The computer program relies upon a two region model with a spout and annulus element for an axisymmetric bed. The paper presents some results for grain drying beds and compares them with the existing experimental data qualitatively.     

喷动床中气体停留时间分布的研究

In a spouted bed of 80mm in ID and 1700mm in height, the gas residence time distributions at different radial positions in both spout and annular area were measured with five different kinds of particles as spouting material, air as spouting gas, and hydrogen as tracer. The effects of superficial gas velocity, operating pressure, particle size and its category on gas residence time distribution were discussed. It was found that the gas velocity profile in spout was more uniform than that in annulus. It could be concluded that the gas flow in the spout could be treated as a plug-flow, while that in the annulus inhibited a strong non-ideal flow behavior. Increasing the superficial gas velocity and decreasing the operating pressure, the particle density and its size gave rise to spouting disturbance, thus the measured tracer concentrations vs. time curves fluctuated. The variances of residence time distribution curves could be taken as a measure of the gas fluctuation degree.     

Hydrodynamics of a spouted bed with an impermeable draft tube for binary particle systems

A spouted bed with an impermeable draft tube was employed to obtain fundamental data of binary mixtures of glass beads for both the operating conditions and the design factors. These data were compared with those for the coarser particle system only. From this view point, minimum spouting velocity, pressure drop, hold-up of solid particles within a draft tube, upward gas flow rate within the annulus and solids circulation rate were determined by changing the total gas flow rate and mass fraction of finer particles as operating parameters and by changing distance of entrainment zone and draft tube diameter as geometric parameters.     

Numerical simulation and experimental study of fluid-particle flows in a spouted bed

In this work, the Eulerian-Eulerian multiphase model is used in the computational simulation of fluid dynamics of spouted beds with two different geometries: conical-cylindrical and conical. For the conical-cylindrical spouted bed, the simulated results of radial velocities of particles with a 1.41 mm diameter along bed heights in the range of 0.022 to 0.318 m are compared with experimental values obtained by He et al. [Y.L. He, C.J. Lim, J.R. Grace and, J.X. Zhu, Measurements of Voidage Profiles in Spouted Beds, Canadian Journal of Chemical Engineering, 72 (1994), 229-234], and show a good agreement. The influence of static bed height on the characteristic curve is assessed through simulations using different airflow rates. The respective minimum spouting velocities are compared with experimental values and with values obtained through empirical correlations reported in the literature. The results of the CFD simulations show a deviation of 3.8% when compared with the experimental data, which is less than the aforementioned correlations. The stages of transition from the condition of static bed to spouting bed are presented through the simulation of solids volume fraction distribution and the radial profile of voidage in the spouting region. The characteristic curve and minimum spouting conditions for a simulated conical bed, with glass particles of 6 mm diameter are compared with the experimental results showing deviations of 12.1% for the pressure drop and 5.6% for the minimum spouting velocity.     

Numerical Simulations of the Effect of Conical Dimension on the Hydrodynamic Behaviour in Spouted Beds

The flow behaviours of gas‐solids were predicted by means of a hydrodynamic model of dense gas‐solid flow in spouted beds. Constitutive equations describing the particulate solids pressure and viscosity were implemented into a hydrodynamic simulation computer program. The effect of operating conditions (inclined angle and gas spouting velocity) on particle velocity and concentration in the spout, annulus and fountain regions were numerical studied. Both vertical and horizontal particle velocities increased with increasing spouting gas velocity. The diameter of the spout increases with decreasing the inclination angle. As the inclination angle is set greater than 60°, the spout cross‐section starts becoming bottlenecked, limiting the upwards flow of solids.