Further work on grid leakage in a pilot-plant size gas-fluidized bed

Substantial grid leakage in large industrial fluidized beds is highly undesirable because it can lead, among other things, to grid erosion or plugging and particle attrition.Experiments have been carried out to study the important factors that influence grid leakage. Among factors investigated were bed height, grid thickness and grid hole pressure drop fluctuations. Wave-breakers and cone-shaped grid holes were found to reduce leakage drastically. Selecting the proper grid thickness reduced the grid leakage flux by up to 4 orders of magnitude.Grid leakage was shown to be caused by pressure fluctuations due to bubble formation at the grid and sloshing in the fluidized bed. This led to a mathematical model for grid leakage which was verified experimentally. A very simple technique using pressure transducers could thus be used for the instantaneous monitoring of grid leakage in industrial fluidized beds.     

Direct Measurements of Instantaneous Solid Flux in a CFB Riser using a Novel Multifunctional Optical Fiber Probe

With a novel optical fiber probe that can measure instantaneous local particle velocity and solid concentration simultaneously, extensive experiments were conducted to study transient flow structures in a 15.1‐m long circulating fluidized bed (CFB) riser of 100 mm in diameter. This study analyzed the radial and axial distributions of solid concentration, particle velocity, and their variations with nine operating conditions and at six axial levels. Instantaneous local solid concentration and particle velocity were found to be well correlated at most of the radial positions. The detailed time evolution, axial and radial distribution of instantaneous solid flux, and the variation of solid flux with operating conditions were also investigated. The radial solid flux profile showed a flat shape with a maximum at near wall area under most operating conditions. The instantaneous solid flux was found to have a strong fluctuation at a radial position of r/R = 0.8 ~ 0.9.     

Searching for a mesh-independent sub-grid model for CFD simulation of gas-solid riser flows

This work aims to examine the effects of grid size in applying the two-fluid model (TFM), and thereby attempts to search for a mesh-independent sub-grid model for simulating gas-solid riser flows. To this end, we performed a series of TFM simulations over a periodic domain with various grid resolutions and drag closures. Of these drag models, EMMS/matrix model in its simplified version was chosen to be the focus of discussion. It was found that TFM simulation with a homogeneous drag model reaches its numerically asymptotic solution when the grid scale is as small as 10 times the particle diameter, but it still fails to capture the characteristic S-shaped axial voidage profile and highly over-predicts the solids flux. By comparison, EMMS/matrix model seems to reach a mesh-independent solution of the effect of sub-grid structures on the drag force, and predict successfully the axial voidage profile and the solids flux with even coarse grid. Therefore, the fine-grid TFM simulation is inadequate for gas-solid riser flows. We need sub-grid modeling of the heterogeneous structure.     

Wind Tunnel Studies of Particle Transport and Deposition in Turbulent Boundary Flows

In this study, particle transport and deposition were studied in a wind tunnel by measuring both the airflow turbulence characteristics and deposition of monodisperse uranine particles of 2.0-4.5 w m diameter on smooth, horizontal surfaces. Turbulence characteristics behind a 2.54 cm high rectangular bar were investigated for free stream velocities ranging from 3.3 m/s to 15.3 m/s. The well-developed boundary layer thickness was approximately four times the height of the rectangular bar at a distance of about 55 cm from the bar. Results of the wind tunnel experiments show the complex nature of deposition in turbulent flows due to the interactions between particles and turbulence. In general, the particle deposition flux is larger in the near wake region than in the far wake region. The particle deposition flux is roughly independent of the dimensionless particle relaxation time when the relaxation time is less than one, but increases rapidly as the relaxation time increases above one.     

气固流化床中颗粒的夹带和扬析特性

在内径0.5 m、总高6 m的流化床中对玻璃珠和白刚玉颗粒的夹带和扬析现象进行了实验研究,两种颗粒通过筛分配比得到不同粒径分布的实验物料,以0.25~0.76 m/s的速度分批进行流态化试验,考察了表观气速、床层物料粒径分布以及颗粒粒径对颗粒夹带和扬析速率的影响,得到了颗粒的扬析速率常数(Ki*)。实验结果表明,夹带量和扬析速率常数随着表观气速呈指数增加;扬析速率常数随着颗粒粒径减小先增加,达到一个临界粒径(dcrit)后,扬析速率常数会随着粒径减小趋于平缓或降低。提出了一组经验关联式,分别用于预测临界粒径两侧颗粒的扬析速率常数,关联式能很好地预测本工作实验条件下的数据,误差在30%以内,并且关联式能够对文献中的实验数据进行较好的预测,可以用于流化床装置的放大。     

High-Resolution Analysis of Particle Deposition and Resuspension in Turbulent Channel Flow

Particle deposition and resuspension during turbulent flow were investigated using a rectangular channel with glass side walls. Micrometer-sized alumina particles were used in the experiments. Particle behavior in the rectangular channel was observed through a high-speed microscope camera with a resolution of 0.3 μm and a speed of 87,600 fps, and particle deposition and resuspension fluxes were quantified using digital image analysis. The experimental results showed that particle resuspension was caused by the collision of airborne particles with those deposited on the surface. The resuspension flux was found to be correlated with the deposition flux. Furthermore, the average residence time between particle deposition and resuspension was several tens of milliseconds, which was very short but much longer than the contact time at the collision. Additionally, the residence time decreased as the particle diameter increased.

Copyright 2015 American Association for Aerosol Research     

Unsteady-State Permeate Flux of Crossflow Microfiltration

Abstract

In this study a membrane filtration cell was installed to investigate the variation of permeate flux with filtration time under various operating conditions including crossflow velocity, pressure drop, particle concentration, membrane pore size, particle size, pH, and electrolyte concentration. The dimensions of the filtration channel in the CFMF cell were 6 cm x 0.6 cm x 0.036 cm, and the flow of the suspension in the channel was controlled under the laminar flow region. Spherical polystyrene latex particles of 0.303, 0.606, and 1.020 μm were used as the suspension particles in the experiments. The density of the particles was 1.05 g/cm³. It was found that the unsteady-state permeate flux increased with an increase in particle size, membrane pore size, or crossflow velocity, but decreased with an increase in particle concentration or electrolyte concentration in the suspension. A mathematical model based on mass balance and hydrodynamic theory was developed in this study. In addition, the effect of cake growth and particle concentration decline during experiments on the permeate flux were also considered in this model. This model predicts satisfactorily the unsteady-state permeate flux of CFMF under various operating conditions.     

CHARACTERIZATION OF ATMOSPHERIC DRY DEPOSITED PARTICLES AT URBAN AND NON-URBAN LOCATIONS

Microscopic techniques were used to size particles collected on a deposition surface and to generate mass size distributions of deposited particles sampled from urban and non-urban locations. The volume shape factor was defined as a conversion factor between the projected area diameter and equivalent volume diameter of a particle and was used as an indication of the irregularity of the particle shape. The average volume shape factor of deposited particles at the urban location (1.61±0.21) was higher than the average at the non-urban locations (1.16±0.10). This suggests that particles are more irregular in the urban areas. Since non-urban areas have less larger particles in ambient air, depositional mass-size distributions at the urban location had a larger average peak (58 μm) and average mass median diameter (49 μm) of coarse particle mode of the distributions than they did at non-urban locations (averaged 33 and 27 μm). Evaluation of correlation coefficients between parameters (wind speed, deposition flux, peak diamater, mass median diameter) indicates that there are more airborne coarse particles at urban locations than at non-urban locations and this distribution plays an important role in dry deposition. By directly observing the deposited particles, it was found that particles larger than 10 μm diameter contributed to more than 90% (in mass) of the atmospheric dry deposition even when ambient coarse particle concentration is low at non-urban locations.     

Unsteady-State Permeate Flux of Crossflow Microfiltration: Effect of Particle Size Distribution

Abstract

A mathematical model based on a hydrodynamic theory and mass balance was developed for the prediction of the unsteady-state permeate flux in crossflow microfiltration under the influence of particle size distribution. Experiments were also conducted in a membrane filtration cell to verify this model. Spherical polystyrene latex particles of 0.303, 0.606, and 1.020 μm were used to make suspensions of various particle size distributions. The flow of the suspension in the channel of the filtration cell was controlled under the laminar flow region. It was found that the unsteady-state permeate flux increased as the mean particle size of the suspension was increased. Moreover, the model predicted satisfactorily the unsteady-state permeate flux under the effect of particle size distribution.     

膜技术在油田采出水处理中的应用研究

采用无机陶瓷膜处理设备处理宝浪油田采出水,研究结果表明:处理后的采出水完全能满足宝浪油田注水的水质要求。产出水中的悬浮物含量小于1mg/L。0.1μm膜处理后的采出水中悬浮物含量平均值为0.417mg/L,粒径中值小于1μm;0.2μm膜处理后的采出水中悬浮物含量平均值为0.57mg/L,粒径中值小于1.5μm;产出水中油含量低于检测限;膜清洗周期为85个小时;清洗方法简单;膜通量的恢复率为99.93%。     

管道周向励磁漏磁检测磁路设计

管道周向励磁漏磁检测技术是一种新的检测方法,对于检测和定量评定轴向导向缺陷具有潜在优势。磁化器磁路设计是漏磁检测需要解决的首要问题,采用磁路计算原理研究管道磁化磁路中永磁体长度、厚度和宽度改变时,管道表面缺陷产生的漏磁场的变化情况,得到了缺陷漏磁场随永磁体长度、厚度和宽度改变时的变化规律。此规律有助于从整体上把握永磁体磁路设计,改善磁化效果,提高漏磁检测灵敏度,进而为漏磁检测磁路的设计提供依据。     

Effect of cake layer structure on colloidal fouling in reverse osmosis membranes

A series of reverse osmosis (RO) membrane filtration experiments was performed systematically in order to investigate the effects of various hydrodynamic and physicochemical operational parameters on a cake layer formation in colloidal and particulate suspensions. Bench-scale fouling experiments with a thin-film composite RO membrane were performed at various combinations of trans-membrane pressure (TMP), cross-flow velocity (CFV), particle size, pH, and ionic strength. In this study, silica particles with two different mean diameters of 0.1 and 3.0 μm were used as model colloids. Membrane filtration experiments with colloidal suspensions under various hydrodynamic operating conditions resulted that more significant permeate flux decline was observed as TMP increased and CFV decreased, which was attributed to the higher accumulative mass of particles on the membrane surface. Results of fouling experiments under various physicochemical operating conditions demonstrated that the rate of flux decline decreased significantly with an increase of the ionic strength as well as particle size, while the flux decline rate did not vary when solution pH changed. The experimentally measured cake layer thickness increased with a decrease in particle size and solution ionic strength. Furthermore, the model estimation of cake layer thickness by using a cake filtration theory based on the hydraulic resistance of membrane and cake layer was performed under various ionic strength conditions. The primary model parameters including accumulated mass and specific cake resistance were calculated from the cake layer resistance. This result indicated that the formation of cake layer could be closely related with solution water chemistry. The model estimated cake layer thickness values were in good agreement with the experimentally measured values.     

气固流化床气泡发生频率的研究

在单孔二维气固流化床中(292mm×16mm)用高灵敏度电容探针研究气泡发生频率.以频谱分析仪分析气泡频率分布曲线.考察了一系列参数对气泡频率功率分布密度曲线的影响,其中包括颗粒直径(0.105—0.590mm),颗粒重度(590—2990kg/m~3),颗粒最小流化速度(0.0072—0.481m/s),床层初始高度(205—565mm),探针离孔口垂直距离,孔口气体流率(0.5—35×10~(-4)m~3/s)以及床层辅助流化气速(0—3倍最小流化速度)等.对于重度低的小颗粒流化床,单孔气泡发生频率符合Davidson和Harrison早先推导的模型.随着颗粒直径和重度的增大,实验数据与上述模型呈有规律的偏差.本文提出气体从形成中气泡的顶半球以最小流化速度值向乳浊相泄漏的模型,推导了气泡发生频率的基本方程.以本研究的泄漏模型,用数值计算方法在计算机上计算的气泡发生频率与实验数据相吻合.     

Performance of the High Yield Technology Inc. PM-100 In Situ Particle Flux Monitor

An in situ particle flux monitor (High Yield Technology, Inc., model PM-100) was evaluated using monodisperse polystyrene latex and oleic acid aerosols. A conventional laser particle counter with a lower size limit of 0.3 μm was used to obtain efficiency data. The counting efficiency of the monitor was determined experimentally as a function of particle size in the diameter range of 0.5–6.0 μm. Other operating characteristics that were studied include the effect on counting efficiency of particle velocity, spatial uniformity of the viewing plane, aerosol concentration, and discriminator setting. In addition, the application of the monitor in a vacuum environment was demonstrated. Results showed that the monitor counting efficiency decreased with particle size (from 4.7% at 6 μm to 0.1% at 0.5 μm). Also, the counting efficiency was found to be dependent on sampling velocity but was unaffected by particle concentration. Further tests showed that the monitor had limited sizing ability. Mie scattering analysis of the sensor geometry suggested that the sensor performance could be improved by modifying the sensor sampling geometry and laser beam configuration. Although the counting efficiency of the monitor is lower than that of a conventional laser particle counter, the small size of the sensor makes it a useful process-monitoring tool (see Borden et al., 1987).     

碳化硅陶瓷膜处理采油污水的中试试验

用0.02μm的多通道非对称性碳化硅陶瓷膜对采油污水进行了现场中试。在各种运行条件下,陶瓷膜出水SS<1.0 mg/L,油<10 mg/L,粒径中值<1.0μm。还考察了不同运行条件下陶瓷膜膜通量和跨膜压差的变化,以及强化混凝过滤对膜通量的影响。根据试验结果对陶瓷膜在采油污水处理中的进一步研究提出了建议,并对其工程应用前景进行了展望。     

mufiltration of polydispersed suspension by a membrane screen/hollow-fiber composite module

Membrane and hollow-fiber technologies have widely been used in pure water production, and are increasingly used in conventional water and wastewater treatment for solid—liquid separation. A major challenge to these applications is to prevent or reduce particle fouling so that the membrane or hollow-fiber modules can be operated at a higher permeation flux and over a longer period of time. In this study, a membrane screen/hollow-fiber composite module has been developed for the mufiltration of polydispersed suspensions. The module combines a cylindrical membrane of larger pores as a screen and a bundle of hollow fibers of smaller pores for polishing treatment. Both cross-flow and dead-end filtrations occur in the same module. Experiments were conducted at various concentrations with kaolin particles of size ranging from 0.5 μm to 10 μm. The results showed that the membrane screen effectively removed larger particles from the permeating flow and reduced solid loading to the hollow fibers. As a result, the composite module generated considerably higher permeation flux, as compared to a similar hollow-fiber module without the membrane screen. It was found that larger particle deposition was more significantly affected by the cross flow, and initially deposited particles contributed greater specific resistance to the permeating flow.     

Numerical Study of Particle Behaviour in Split-Flow Thin-Channel Fractionation Using the Discrete Element Method

Abstract

This study examines the usefulness of the discrete element method (DEM) for studying particle motion in SPLITT fractionation. The method was tested against the conventional SPLITT theory and published experimental data for particle sizes 7, 10, and 15 µm at various run conditions and good agreement was achieved. Illustrative studies presented in this paper show that particle collisions occur at concentrations as low as 0.05%(v/v); and particle trajectory deviates from theory more notably for larger particles, 15 µm diameter and greater. The finding suggests the DEM can be useful in SPLITT calculations for modeling the influence of particle-particle interactions.     

催化裂化装置烟气轮机积垢及其增厚机理

针对近年来炼油厂中出现的催化裂化烟气轮机结垢严重问题, 采用SEM(扫描电子显微镜)对催化剂平衡剂、烟机入口催化剂颗粒和垢样进行了微观形貌的分析, 结果表明, 烟机叶片上的结垢主要是由1～2 μm的催化剂颗粒组成。对催化剂黏附在叶片表面的机理进行研究, 分析黏附力对黏附作用的影响, 结果表明, 催化剂颗粒粒径越小, 越容易被黏附, 与SEM的结果吻合。通过CFD模拟烟机流道内速度场的分布, 验证了黏附力对结垢的影响。     

Effect of the annulus aeration on annulus leakage and particle circulation in a three-phase spout-fluid bed with a draft tube

The distribution of flow between the annulus and the draft tube sections of a liquid spout-fluid bed with a draft tube was analyzed in the presence of air flow through the annulus using a tracer response technique.The responses of the tracer injections prove that part of the flow originating from the annulus liquid inlet leaks into the draft tube when any amount of annulus liquid inlet flow is present and this annulus leakage increases when air flow is introduced into the annulus. This finding extends the recently acquired knowledge about the annulus leakage and cross flow in the entrance region below the draft tube in liquid spout-fluid beds to three phase spout-fluid beds with a draft tube where the annulus is aerated.The experimental system studied consisted of a semi-circular column of 80 mm diameter equipped with a semi-circular draft tube of 20 mm diameter. Liquid and air flows coexisted in the annulus. The liquid velocities were well above the minimum fluidization velocity and the particles were glass beads of 3 mm diameter.Experimentally determined values of the superficial annulus and draft tube liquid velocities from the tracer response analyses together with the experimental draft tube pressure gradients were used as input into Grbavcic et al.'s [Z.B. Grbavcic, R.V. Garic, D.V. Vukovic, Dz.E. Hadzismajlovic, H. Littman III, M.H. Morgan, S.Dj. Jovanovic, Hydrodynamic modeling of vertical liquid-solids flow. Powder Technol. 72, (1992)183-191] variational model for vertical non-accelerating liquid-solids flow to calculate the draft tube voidage and the particle mass flux assuming an air free draft tube. Results indicate that annulus aeration lowers the particle mass flux in this particular column geometry.     

Particle Size Control of Barium Titanate Prepared from Barium Titanyl Oxalate

Barium titanates formed from decomposition of barium titanyl oxalates (BT-oxalates) exhibit the same powder state as that of the Bt-oxalates, so that controlling the particle size and shape of the starting BT-oxalates is necessary. In the present study, BT-oxalates were precipitated from aqueous solution under various aging conditions: aging fluids, temperature, and time. The particle size of the spherical BT-oxalates was controlled at 0.4 μm by aging at 25°C for 3 h, and the diameter of the barium titanate formed was similar to that of the BT-oxalates.