下行床弧面锥体气固分离装置的分离效率实验

以FCC颗粒为物料，实验研究了在气固并流下行循环流化床(f=37 mm, H=5 m)中，气固两相分离装置的结构、颗粒循环量、表观操作气速对分离器气固分离效率的影响. 结果表明，在气固并流下行系统中，采用弧面锥体气固分离装置，内加导流板，在气速为1~5 m/s，颗粒循环速率20~90 kg/(m2.s)条件下，可使气固分离效率达到99%以上，压力损失小于500 Pa.     

旋流气固分离器内气粒两相运动特性及分离效率

基于旋转气固多相流复杂的运动特征,结合旋流气固分离器的特点,同时考虑颗粒间的碰撞与并聚和喷水加湿对颗粒间并聚及碰撞、颗粒运动特性的影响,并考虑颗粒对气相的作用构建了描述旋流气固分离器内湍流气固多相流的三维时均方程组.模型封闭采用κ-ε/ RNG模型,数值模拟了旋流气固分离器内气固两相流场、固相运动特性及分离效率,提出了合理的旋流气固分离器分离效率计算关联式.     

旋风分离器内气固两相流动特性的模拟研究

通过数值模拟对旋风分离器内的气固两相流动进行研究。采用RSM湍流模型和DPM两相流模型分析旋风分离器内的气固两相流动特性。旋风分离器内的气流切向速度呈中心准强制涡、外侧准自由涡的双涡分布,分界面大约在0.8倍排气管半径处;气流轴向速度呈中心上行、外侧下行的双行流分布,分界面即零速包络面大约与排气管直径一致;小粒径颗粒的运动具有随机性,粒径大于7μm的颗粒可以完全被捕集分离;流动的非轴对称特性和顶灰环对气固分离不利,应给予重视。     

旋流分离-颗粒床耦合气固分离装备旋流场静压分布

针对一种集旋风分离器和内置颗粒床优势于一体的新型耦合气固分离装备在无灰负荷及固定床操作条件入口环形空间、分离空间和灰斗内静压场进行研究。结果表明,静压沿周向为非对称分布,轴向为非均匀分布,径向则呈中心低两侧高分布;且存在着静压分布的降压区(0?~180?,以入口处为0?)和增压区(180?~360?);装备中心的负压及旋流作用在轴向高度H=6.41D(D为旋风壳体内径)以下对静压的影响不再显著;内置颗粒床外壁附近存在“滞留层”,有利于提高装备的分离性能。根据实验数据给出了静压周向分布和静压轴向分布的经验公式。     

高入口浓度下PV型旋风分离器的分离效率计算

基于Muschelknautz 分离模型,以PV型旋风分离器为对象,针对高入口浓度的分离效率的计算,将旋风分离器分离空间的气固分离过程划分为2个区域,提出了串级分离模型。当入口浓度大于临界入口浓度时,旋风分离器内有器壁附近的颗粒支配区和中心区域的气体支配区。颗粒支配区内颗粒速度大于气体速度,颗粒夹带气体沿器壁螺旋下行进入灰斗被全部捕集,形成了颗粒的一级分离;气体支配区内气体速度大于颗粒速度,气体携带颗粒做旋转运动进行离心分离过程,形成了颗粒的二级分离。旋风分离器总的气固分离过程是一级分离和二级分离的叠加。通过高入口浓度的实验对串级分离模型进行了验证,基于串级分离模型给出的PV型旋风分离器的分离效率与实测值较吻合。研究表明旋风分离器临界入口浓度对总效率的计算影响较大。串级分离计算模型包含了结构参数和气、固相物性等参数,具有很好的通用性,可以满足PV型旋风分离器的工程计算和设计要求。     

双进气道旋风分离器内不同粒径颗粒流动特性

采用改进的RNG k-ε湍流模型和欧拉多相流模型,对一种单入口双进气道旋风分离器内的气固多相紊流过程进行数值模拟。计算得到旋风分离器内不同粒径颗粒速度和浓度分布规律,结果表明:大粒径颗粒比小粒径颗粒轴向速度分布更平坦,切向速度峰值位置和外准自由涡区也越向壁面靠近;与普通单入口旋风分离器相比,相同处理量时,此种旋风分离器内速度和不同粒径颗粒浓度分布轴对称性更好,大粒径颗粒切向速度峰值位置外移更明显,筒体段颗粒有更向壁面浓集的趋势,锥体段不同轴向位置处中心旋流区双进气道的颗粒浓度低于单进气道的。小粒径颗粒捕集能力增强,有助于提高分离器分离效率,减少不稳定流动导致结焦的颗粒源供给,从流动角度保证了抗结焦和长周期稳定操作。     

一种新型气固分离器内两相流动的数值模拟

采用数值模拟的方法对带有切向开缝中心管式气固分离器的气固两相流进行了研究.计算中对气相采用了雷诺应力模型,对颗粒相采用了Lagrange坐标系下的随机轨道模型.结果表明:气相流场总体特征为拱形空间内气流以切向速度为主绕排气管做旋转运动,切向气速随径向位置的增加而减小.大部分气体通过开缝进入中心排气管而从中心管排出,少部分气体由排气管下方空间返回入口区.颗粒相的引入对气相流场有显著的影响,分离器拱形空间内同一径向位置的气体切向速度明显降低.颗粒由于自身惯性向边壁运动的趋势改变了气相流场分布,使气体更易向中心管内运动,从而达到较高的分离效率.计算得到的分离效率与实测值吻合较好,证明了数学模型的合理性,为进一步优化分离器结构提供了可靠依据.     

喷嘴收缩角度对射流反应器内颗粒浓度分布及发展的影响

为了强化气固射流反应器内颗粒弥散和气固混合性能,采用光纤探针对不同喷嘴收缩射流颗粒浓度分布规律及其发展特性进行了研究;考察了射流速度和颗粒负载率对颗粒浓度分布、质量、动量和回流通量的影响。结果表明,喷嘴收缩效应使颗粒向射流轴线汇聚形成局部浓相区,收缩角的增大使浓相区向喷嘴靠近。回流卷吸和壁面效应的共同作用使颗粒浓度沿径向呈“多段式”分布特征。颗粒在射流轴线的汇聚和边壁区的富集降低了颗粒分布的均匀性。在射流近场,颗粒的质量和动量通量主要发生在射流剪切层;随剪切层的发展其通量峰值向边壁移动,而近壁区的颗粒回流使质量和动量通量显著增加。     

超短接触旋流反应器混合腔内气固混合特性的数值模拟

采用欧拉双流体模型对超短接触旋流反应器内的气固两相流场进行了数值模拟,主要研究了混合腔内固相的体积分数分布情况。计算结果表明：混合腔内的气流在切向进气的作用下得到了一定的混合加速效果,切向的高速射流有效地缩短了气固停留时间,保证短接触反应效果。通过对两种不同混合腔结构反应器的对比计算发现,在相同入口速度条件下结构2（切向进气管位于混合腔顶部）较结构1（切向进气管位于混合腔下部）气固停留时间短,由于切向气流的迅速作用,增加了混合腔内的湍动强度,使催化剂颗粒迅速有效扩散、增强气固接触效果而更有利于催化裂化反应的进行,更易实现短接触操作要求。计算结果与实验测量结果的比较表明模型能有效地描述超短接触旋流反应器内气固两相流动形态。     

细颗粒物在旋风分离器内流动特性

为了研究操作条件和颗粒特征对细粉体颗粒在旋风分离器中流动状态的影响,文中采用欧拉-拉格朗日方法对细颗粒在旋风分离器内的停留时间进行模拟计算。探究入口气速、粉体粒径和固气比对粉体在旋风分离器内的停留时间分布密度函数、平均停留时间和停留时间量纲一方差的影响。结果表明：粉体颗粒在旋风分离器内部的停留时间分布密度函数总体呈正态分布;当入口气速u=10 m/s时,粉体颗粒平均停留时间为0.40 s,停留时间量纲一方差为0.11,此时粉体颗粒平均停留时间最长,颗粒运动状态更接近平推流;在模拟工况范围内,随着粉体粒径增大,停留时间量纲一方差增大,颗粒在旋风分离器内的返混程度加剧;颗粒粒径和固气比变化对平均停留时间影响较小。     

分解炉内气固两相流动特性的数值模拟

采用Eulerian—Eulerian气固两相双流体模型、大涡模拟方法模拟气相湍流流动、颗粒动力学理论模拟颗粒相流动,数值模拟分解炉内气固两相流体的动力特性。用小波分析方法研究分解炉内气固两相湍流特性。在分解炉中心区域形成高浓度-高速度的上升颗粒流、在壁面区域形成高浓度、低速度的下降颗粒流,构成颗粒的内循环流动。     

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.     

Removing 20 nm Particles Using a Supersonic Argon Particle Beam Generated with a Contoured Laval Nozzle

A cryogenic particle beam is an effective means of removing nano-sized contaminant particles, but the particle beam generated with a simple-hole nozzle has not been successful in removing particles smaller than 30 nm. Based on molecular dynamics (MD) simulation results that smaller cryogenic particles moving at a higher velocity are more effective in removing contaminant particles in the 10 nm range, a contoured Laval nozzle of a particular expansion angle and length was used in this study, instead of the simple-hole nozzle, to generate particle beams of high intensity and controlled size moving at high velocities. A variety of particle size and velocity were obtained by controlling the stagnation pressure/temperature and the back pressure, and using Laval nozzles with differing throat sizes and expansion angles. The new particle beams could remove almost completely a variety of ceramic and Cu particles, down to 20 nm size range, on a flat surface or in trenches.     

High velocity attrition nozzles in fluidized beds

Gas-solid fluidized beds are used in many industrial applications such as polyethylene production, drying, coating, granulation, fluid catalytic cracking and fluid coking. For some industrial applications, controlling the size distribution of the particles in a fluid bed is extremely important in order to avoid poor fluidization. One method to control the size of the particles in the bed is to use attrition nozzles, which inject high velocity gas jets into the bed creating high shear regions and grinding particles together. The objective of this study was to test different high velocity attrition nozzles and operating conditions in order to determine the effects of fluidization velocity, nozzle size, nozzle geometry, bed material and attrition gas properties on the grinding efficiency. Samples of solids were taken from the bed and analyzed before and after each injection and a grinding efficiency was defined as the new surface area created per mass of attrition gas used. An empirical correlation was also developed to estimate the grinding efficiency, and its predictions were validated using the experimental data. Large diameter nozzles with a Laval nozzle geometry, operating at high upstream pressures and high fluidization velocities, resulted in the highest grinding efficiencies. Gas properties, such as speed of sound and density, had a significant impact on the grinding efficiency.     

Prediction for particle removal efficiency of a reverse jet scrubber

Numerical computation was conducted to predict the collection performance of a reverse jet scrubber for polydisperse particles. The particle size distribution of polydisperse particles was represented by a lognormal function, and the continuous evolution of the particle size distribution in a reverse jet scrubber is taken into account with the first three moment equations. Numerical results were compared with the analytic results using average relative velocity in all zones and experimental results.

In a reverse jet scrubber, the impaction is the main particle collection mechanism because of high relative velocity and short collection time. The particle collection by impaction increases with an increase in particle size, and geometric mean diameter and geometric standard deviation decrease as time goes on. High droplet velocity and gas velocity increase the particle collection efficiency, and the small droplet size also increases the collection efficiency because smaller droplet size provides broader surface area. The packing density is a factor affecting particle collection efficiency in a scrubbing process. The dense packing density also provides large surface area and leads to high collection efficiency.     

负压差立管内气固两相流的流态特性及分析

对于出口插入密相床的立管，管内气固两相的流动特点是下行的颗粒速度大于气体速度和颗粒的逆压差流动，颗粒下行是一个减速运动过程. 管内的气固两相流的流态有两种形式，当GsGsc时，流态是浓相输送流态，气流下行. 两种流态可以互相转变，主要取决于颗粒质量流率的大小. 负压差立管的流态变化与气固两相之间滑落速度和轴向压力的变化密切相关，滑落速度随颗粒质量流率的增加逐渐减小，而轴向压力则逐渐增大以平衡立管的负压差.     

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.     

Removal of 10-nm contaminant particles from Si wafers using CO2 bullet particles

ABSTRACT: Removal of nanometer-sized contaminant particles (CPs) from substrates is essential in successful fabrication of nanoscale devices. The particle beam technique that uses nanometer-sized bullet particles (BPs) moving at supersonic velocity was improved by operating it at room temperature to achieve higher velocity and size uniformity of BPs and was successfully used to remove CPs as small as 10 nm. CO2 BPs were generated by gas-phase nucleation and growth in a supersonic nozzle; appropriate size and velocity of the BPs were obtained by optimizing the nozzle contours and CO2/He mixture fraction. Cleaning efficiency greater than 95% was attained. BP velocity was the most important parameter affecting removal of CPs in the 10-nm size range. Compared to cryogenic Ar or N2 particles, CO2 BPs were more uniform in size and had higher velocity and, therefore, cleaned CPs more effectively.     

Single-Fiber Interception Efficiency for Elliptical Fibers

Synthetic fibers with non-circular cross-sections are used in air filters. These fibers may offer performance advantages over traditional fibers with circular cross sections because they have more surface per unit volume of fiber upon which particles can collect. Starting with a solution for the velocity field around fibers with elliptical cross-sections, expressions for predicting the single-fiber efficiency for particle collection by the interception mechanism were developed for elliptical fibers. The interception efficiency predictions depend on filter solidity, fiber properties such as size, aspect ratio, and orientation of the cross section relative to the incoming flow, and particle diameter. The expressions demonstrate that single fiber interception efficiency for elliptical fibers generally increases with increasing particle diameter, increasing solidity, increasing aspect ratio, and as the major axis of the ellipse becomes more perpendicular to the incoming air flow.     

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.