Solids behaviour in a dilute gas-solid two-phase mixture flowing through monolith channels

This paper reports, for the first time, the solids behaviour in a dilute gas-solid two-phase mixture flowing through monolith channels. The non-intrusive positron emission particle tracking (PEPT) technique was used in the work, which allowed investigation of three-dimensional solids motion at the single suspended particle level. Processing of the PEPT data gave solids velocity and occupancy in the monolith channels. The results showed a non-uniform radial distribution of both the solids velocity and concentration. The highest axial solids velocity occurred in monolith channels located in the central part of the column, whereas the highest solids concentration took place at a position approximately 0.7 times the column radius. The axial distribution of the axial solids velocity showed an entrance region with a length of approximately 33 times the hydrodynamic diameter of a monolith channel under the conditions of this work. Analysis of the PEPT data also gave distributions of particle residence time and tortuosity in terms of solids motion. The distributions were approximately Gaussian-type with the tortuosity distribution more skewed toward the right hand side. The peak residence time and tortuosity decreased with increasing superficial gas velocity and the distributions were broadened at lower superficial gas velocities. The results of this work also provided a possible explanation to our previously observed early laminar-to-turbulent flow transition in monolith channels.     

Solids mixing in a fluidized bed riser

The solids mixing in a riser with a height of 10 m and 0.186 m inner diameter was investigated by using pneumatic phosphor tracer technique. Considering the shielding effect of the bed material on the light emitted from the phosphor tracer particle, a modified method for the phosphor tracer measurement is proposed. And then the curves of particle residence time distribution were obtained. The experimental results show that the particle diffusion mechanism can be explained by the dispersions of dispersed particles and particle clusters in the axial direction, and as well the core-annulus nonuniform distribution of the solids fraction in the radial direction of the riser. Moreover, based on the experimental results, a two-dimensional dispersion model was established to predict the solids axial and radial diffusion. Furthermore, the effects of superficial gas velocity and solids circulating flux on the axial and radial Peclet number of the particles were discussed; two empirical correlation formulas about the axial and the radial Peclet numbers were given; the calculated values agree well with the experimental results.     

不同粒度分布高硫焦颗粒在强混燃烧器内的流动特性

利用一套新型强混燃烧器,对3种不同粒度分布的高硫焦颗粒的流动特性进行了较为详尽的考察. 反应器主要由预混段和提升段组成,两部分结构尺寸分别为f240 mm′3000 mm和f70 mm′9000 mm. 实验操作条件为：提升管表观气速8~18 m/s,颗粒循环速率60~200 kg/(m2×s). 通过对3种不同粒度分布的颗粒在强混燃烧器内颗粒固含率、颗粒速度轴径向分布的测定,发现随掺入细颗粒比例的增加,截面平均颗粒速度降低;在较粗的高硫焦颗粒中适当掺入细粉,可使床层内颗粒固含率增加,返混加剧,停留时间延长,有利于高硫焦的充分燃烧.     

Fluidization behavior in a circulating slugging fluidized bed reactor. Part I: Residence time and residence time distribution of polyethylene solids

Square nosed slugging fluidization behavior in a circulating fluidized bed riser using a polyethylene powder with a very wide particle size distribution was studied. In square nosed slugging fluidization the extent of mixing of particles of different size depends on the riser diameter, gas velocity, hold up and solids flux in the riser. Depending on the operating conditions the particle residence time distribution of a riser in the slugging fluidization regime can vary from that of a plug flow reactor to that of a well-mixed system.Higher gas velocities cause shorter particle residence times because of a significant decrease in the hold-up of particles in the riser at higher gas velocities. A higher solids flux also shortens the average residence time. Both influences have been quantified for a given polyethylene-air system.Residence time and residence time distribution were determined for different particle size and the influence of gas velocity, solids flux, hold up and riser diameter was studied. When comparing data from segregation and residence time experiments it is clear that segregation data can predict the spread in residence time as a function of overall residence time, particle size and gas velocity. The differential velocity between small and large particles found in the segregation experiments can predict the spread in residence time as found in the residence time distribution experiments with a powder with a broad particle size distribution. Raining of particles through the slugs was studied as a function of plug length, gas velocity and pulse length. It was found that raining is not the determining mechanism for segregation of particles.     

大型快速流化床内颗粒浓度和速度分布的实验研究

采用压力巡检仪和光纤测量仪,对直径300 mm的快速流化床反应器内气固两相流动特性进行了研究,考察了操作条件对快速床轴、径向催化剂颗粒浓度、颗粒速度、筛分分布等的影响. 结果表明,当操作气速提高到2.0~2.6 m/s,相应的催化剂循环强度在60~160 kg/(m2×s),床层密度可保持在50~650 kg/m3;催化剂颗粒浓度在径向上呈中心低、边壁高的不均匀分布,轴向上各径向位置在颗粒加速区逐渐降低、在充分发展区趋于稳定、随表观气速增大或催化剂循环强度减小而减小,且径向均匀性变好,在r/R<0.7的中心区域趋于一致;颗粒速度在径向上呈中心高、边壁低的抛物线形分布,且随操作气速增大或催化剂循环强度增大而更加明显.     

Behaviour of Cohesive Powders in a Powder‐Particle Spouted Bed

Behaviour of cohesive powders in a powder‐particle spouted Bed (PPSB) has been investigated under several operating variables and solids properties. The elutriation rate constant based on the hold‐up of fine powders in the bed decreased with a decrease in the size of fine powders, and with an increase in the size of coarse particles under a constant superficial gas velocity. This finding is quite different from the elutriation phenomena of particles more than 100 µm in size. Moreover, the mean residence time of fine powders increased with a decrease in the superficial gas velocity and the size of fine powders, and decreased with a decrease in the size of coarse particles.     

Discrete particle simulation of solids motion in a gas–solid fluidized bed

The solids motion in a gas–solid fluidized bed was investigated via discrete particle simulation. The motion of individual particles in a uniform particle system and a binary particle system was monitored by the solution of the Newton's second law of motion. The force acting on each particle consists of the contact force between particles and the force exerted by the surrounding fluid. The contact force is modeled by using the analogy of spring, dash-pot and friction slider. The flow field of gas was predicted by the Navier–Stokes equation. The solids distribution is non-uniform in the bed, which is very diluted near the center but high near the wall. It was also found that there is a single solids circulation cell in the fluidized bed with ascending at the center and descending near the wall. This finding agrees with the experimental results obtained by Moslemian. The effects of the operating conditions, such as superficial gas velocity, particle size, and column size on the solids movement, were investigated. In the fluidized bed containing uniform particles better solids mixing was found in the larger bed containing smaller size particles and operated at higher superficial gas velocity. In the system containing binary particles, it was shown that under suitable conditions the particles in a fluidized bed could be made mixable or non-mixable depending on the ratios of particle sizes and densities. Better mixing of binary particles was found in the system containing particles with less different densities and closer sizes. These results were found to follow the mixing and segregation criteria obtained experimentally by Tanaka et al.     

Detailed measurements of flow structure inside a dense gas–solids fluidized bed

Detailed local flow structures are investigated in bubbling and turbulent fluidized bed with FCC particles. The operating conditions ranges from 0.06 to 1.4 m/s. Extensive experiments are carried out using a newly developed optical fiber probe system, which can measure the solids concentration and velocity at multi-points. The results reveal that with increasing U_g, local solids concentrations go through three evolution stages, reflecting a gradual regime transition process. Under all operating conditions, upflowing and descending particles co-exist at all measuring locations. The upflowing particle velocity is strong function of both superficial gas velocity and spatial position. However, the descending particle velocity mainly depends on superficial gas velocity. The bed radial symmetry and the effects of static bed height on the local flow structures are also investigated.     

柱形流化床传热特性的数值模拟

对Shedid等搭建的圆柱体流化床采用欧拉?欧拉法进行三维数值模拟,考察了颗粒球形度、表观进气速度和床料初始堆积高度对流化床内垂直加热壁面与流动床料之间对流传热特性的影响,采用有效导热系数分别计算气相和固相的对流传热系数。结果表明,随表观进气速度增大,流化床内颗粒物料湍流运动加剧,加热壁面平均温度和流体平均温度下降,壁面流体间传热平均温度差减小,壁面流体间对流传热系数增大;随初始床料高度增加,流化床内颗粒与加热壁面的接触面积增大,导致固相平均对流传热系数增大。     

循环流化床提升管中颗粒速度的径向分布及其沿轴向的发展

采用5光纤速度探头对f100mm?5.1m循环床提升管8个高度截面上11个径向位置的局部颗粒速度进行了实验测试,并采用径向不均匀指数(RNI)对颗粒速度径向分布的不均匀性及其沿轴向的变化进行了定量描述。研究结果表明:在高气速、高颗粒循环量操作时,操作条件对颗粒上升速度和下降速度的径向分布的影响在加速段和充分发展段呈现出不同的规律;颗粒上升速度和下降速度沿轴向的变化在核心区和边壁区也表现出不同的趋势。当颗粒循环速率大于200 kgm-2s-1时,颗粒的加速段长度大大延长,以至于大于提升管的高度(15.1m)。颗粒速度径向分布的不均匀性沿轴向是逐渐增大的,并且与截面平均颗粒速度存在很强的相关性。     

Flow of a gas-solid two-phase mixture through a packed bed

This paper is concerned with an upward co-current flow of a gas-solid two-phase mixture through a packed bed, a system employed in a number of industrial processes. Experimental work was carried out by using glass balls for packed bed, and both glass beads and FCC as suspended particles. The effects of solids loading and gas velocity on the pressure drop as well as the static and dynamic solid hold-ups within packed bed were examined. Experimental results showed different behaviour of the FCC from glass beads. At a given gas velocity, pressure drop increased approximately linearly with solids loading with a slope for FCC much higher than that for glass beads. The static hold-up of glass beads was much lower than corresponding dynamic hold-up at a given gas velocity, and it did not seem to change much with solids loading under the conditions of this work. At a given gas velocity, the static hold-up of FCC, however, was found to be comparable with the corresponding dynamic hold-up. An analysis was conducted on the pressure drop using a modified version of the Ergun equation by taking into account the effects of suspended particles on the viscosity and density, as well as the gravitational force. It was found that the modified Ergun equation agreed well with the experimental results of both this work and those reported in the literature. Effort was also made to develop relationships for the dynamic hold-up and the interaction coefficient between the suspended and the packed particles, the so-called solid-phase friction factor in the literature. The dynamic hold-up was found to increase with an increase in the product of velocity ratio of the solid to gas phases and the square root of the diameter ratio of the suspended to packed particles, whereas the interaction coefficient increased in general with increasing Froude number but with significant scattering.     

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.     

Effect of louver baffles on hydrodynamics and gas mixing in a fluidized bed of FCC particles

The effect of louver baffles on the particle concentration profiles, pressure fluctuations, bed expansion, and gas mixing of a fluidized bed was investigated in a transparent 2-D column of cross-section 500×30 mm and height 6 m over a broad range of operating conditions covering both the bubbling and turbulent flow regimes. Visual observations, pressure fluctuations and steady gas tracer experiments showed that louver baffles can break bubbles, as indicted by the lower amplitudes and higher mean frequencies of differential pressure fluctuations, but they were only effective for superficial gas velocities <∼0.7 m/s for the FCC particles considered in this study. The ability of louver baffles to break bubbles reached a maximum near the onset of the turbulent flow regime. A gas cushion of low particle concentration appeared below the louver baffle, and its height increased with increasing superficial gas velocity, indicating increasing suppression of solids backmixing. Internal emulsion circulation was promoted above the louver baffle, causing an uneven distribution of gas flow. The addition of louver baffles reduced the upstream tracer gas concentrations by 80-90%, indicating a significant decrease in the backmixing fluxes of both gas and solids across the baffle layer. The tracer gas concentrations above the louver baffles increased resulting from the promoted emulsion circulation by louver baffles.     

DEM MODELING AND SIMULATION OF A DOWN-FLOW CIRCULATING FLUIDIZED BED

A mathematical model based on the distinct element method (DEM) was developed to investigate the hydrodynamics in a gas-solid down-flow circulating fluidized bed reactor (downer). The models consist of the equations of particle motion and fluid motion. The contact force is calculated by using the analogy of a spring, dashpot, and friction slider. Simulation results show that the radial solids holdup and particle velocity profiles are uniform in the core region. Near the wall, the solids holdup is higher with lower particle velocity. An increase in the particle size decreases the solids holdup and increases the particle velocity. The solids holdup decreases with superficial gas velocity but increases with solids circulation rate. Particle velocity increases with gas velocity and solids circulation rate. The solids holdup and particle velocity are almost uniform along the height of the downer except near the distributor. The hydrodynamic behavior from this simulation showed trends similar to those of the experimental results. The results obtained from this model fit better with the experimental results than Kimm's and Bolkan's models do.     

Studies on solid mean residence time in a three-stage gas-solid fluidized bed with downcomer

Fluidized bed reactors behave as a continuously stirred tank reactor having wide residence time of solids, which is not desirable if a homogeneous product is required. The multi-stage fluidized bed reactors narrow the solids residence time, making it useful for various operations. A three-stage fluidized reactor was designed, fabricated and operated under stable operating condition to investigate the mean particle residence time in the system. The materials taken for the study were lime and sand. In the particle residence time experiments, the results revealed that at a particular solids velocity, mean residence time decreased with increase in gas velocity and increased with decrease in gas velocity. Based on the data, a correlation has been presented for predicting mean residence time.     

大颗粒流化床中颗粒磨蚀的实验研究

为了进一步丰富和发展大颗粒流态化理论,促进其在水泥煅烧领域的应用,实验通过1个三维流化床试验台研究了大颗粒流化床中颗粒的破损方式为磨蚀,即颗粒在流化过程中表面磨碎后生成细粉,颗粒自身的粒径逐渐变小。实验表明,颗粒粒径、风速以及流化时间对大颗粒流化床中颗粒的磨蚀影响较大,静床高对磨蚀几乎没有影响。最后提出了大颗粒流化床合适的控制参数:粒径范围4—7 mm,表观风速1.3um f—1.6um f,静床高H0/D<2。     

高密度液固循环流化床流动特性研究及数值模拟

在φ80 mm×8000 mm的液固循环流化床提升管中,利用实心玻璃珠和常温水,采用实验和数值模拟相结合的方法对高密度液固循环流化床的流动特性进行了考察。实验发现,高密度液固提升管中,颗粒固含率和颗粒速度径向分布均为抛物线分布,轴向平均固含率分布总体上为下浓上稀的波动形式分布,颗粒在提升管中的流动表现出加速-减速-再加速直至充分发展的过程。这种分布特征与较高颗粒浓度、较高表观液速和颗粒循环速率及喷管式液体分布器的影响有关。液固提升管中固体颗粒的停留时间分布曲线均为尖而窄、较对称且没有明显拖尾的单峰分布,这表明颗粒基本是以弥散颗粒形式存在,颗粒停留时间分布较为均匀。通过将数值模拟结果与实验结果进行比较发现,模拟值与实验值吻合较好,说明所建立的数学模型较为合理,进一步通过数值模拟实验对颗粒密度和颗粒粒径对流动特性的影响规律进行了考察。     

竖直管气固鼓泡流化床传热机理的CPFD模拟

针对细颗粒气固鼓泡流化床中床料与竖直传热管壁面间的传热行为,在前期实验的基础上,采用计算颗粒流体力学(CPFD)方法从颗粒在传热壁面更新的角度,深入分析了传热特性与壁面气固流动行为之间的关联性。结果表明,模拟得到的传热管壁面颗粒更新通量和基于颗粒团更新模型的颗粒团平均停留时间均能很好解释实验测得的传热系数变化规律,这证实颗粒团更新是影响传热过程的控制性因素。模拟还发现随加热管从床层中心向边壁的移动,加热管周向方向上颗粒更新通量和传热系数的不均匀性都呈增大趋势。随着表观气速的增大,气泡行为导致床层颗粒内循环流率增大,这是导致颗粒团在加热管壁面上的更新频率增大以及床层与壁面间传热系数增大的根源。     

Monitoring the particle-wall contact in a gas fluidized bed by RPT

Particle-wall contact behavior of the solids in a gas-solid fluidized bed was experimentally studied using the radioactive particle tracking (RPT) technique in which the position of a radioactive tracer is monitored when moving freely in the bed. The solids were sand particles, fluidized by air at room temperature and atmospheric pressure at various superficial velocities, covering both bubbling and turbulent regimes of fluidization. The motion of individual particles near the wall of the bed was studied based on the position of the tracer. The contact time, contact distance and contact frequency of the particles at the wall were evaluated. It was found that the distribution functions of these three parameters become wider by increasing the superficial gas velocity. Axial profiles of contact time and contact distance were also studied in this work. Axial profiles of the overall heat transfer coefficient in the fluidized bed were estimated based on the formulas reported in the literature and the experimental particle-wall contact time evaluated in the present study. Based on such profiles, in order to benefit from the maximum heat transfer coefficient along the bed, it is recommended to place the heat exchanging surface in the middle of the bed, i.e., not very close to the gas distributor as well as far from the top of the dense bed.