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Influence of hydrodynamic parameters on particle attrition during fluidization at high temperature 总被引:1,自引:0,他引:1
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. 相似文献
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The behaviour of several kinds of group B particles ranging from 100 μm to 600 μm was studied in a sound wave vibrated fluidized bed (SVFB). The fluidized bed consists of a transparent Plexiglas tube that is 54 mm i.d. × 1 m high. A speaker mounted at the top of the bed was supplied by a function generator with square waves and was used to generate the sound as the source of vibration of the fluidized bed. The influence of the particle size, density of particles and sphericity of particles on the minimum fluidization velocity, pressure fluctuations and bubble rise velocity in the SVFB was investigated. The minimum fluidization velocity decreased as the sound energy increased. When the sound energy was strong enough and greater than the critical power, the minimum fluidization velocity would approach the same value regardless of the degree of resonance (DOR) value if the particles were in spherical shape. For non-spherical shape particles the minimum fluidization velocity was the function of the DOR value if the power was greater than the critical power. For the middle particle size range, the standard deviation of pressure fluctuations in an SVFB became lower than the one without the effect of sound in high superficial gas velocity range, but the result was reverse for the low superficial velocity; for the large particle size range, the standard deviation of pressure fluctuations in an SVFB was larger than the one without the effect of sound. The sound could also reduce the bubble rise velocity in an SVFB. 相似文献
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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%。 相似文献
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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. 相似文献
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In this paper, a model on attrition of quartzite particles as an inert bed material in fluidized beds has been established on the particle-particle collision. For the convenience of describing the attrition of quartzite particles in fluidized beds, we chose the attrition rate constant (kARC) as one main characteristic parameter to develop the model.In order to verify the validity of the developed model, an attrition experiment of quartzite particles has been carried out in a lab-scale circulating fluidized bed. The predicted results from the population model were close to the experimental data as far as the engineering use is concerned. Finally, a sensitivity analysis was performed by using the developed model to examine effects of initial particle diameter, attrition time, and fluidization number on kARC. 相似文献
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Jason Wiens 《Powder Technology》2006,169(1):49-59
Hydrodynamics in a conical fluidized bed were studied using electrical capacitance tomography (ECT) for a bimodal and mono-disperse particle size distribution (PSD) of dry pharmaceutical granule. The bimodal PSD exhibited a continuous distribution with modes at 168 and 1288 μm and contained approximately 46% Geldart A, 32% Geldart B and 22% Geldart D particles by mass. The mono-disperse PSD had a mean particle size of 237 μm and contained approximately 71% Geldart A, 27% Geldart B, and 2% Geldart C particles by mass. The granule particle density was 830 kg/m3. Experiments were conducted at a static bed height of 0.16 m for gas superficial velocities ranging from 0.25 to 2.50 m/s for the mono-disperse PSD, and from 0.50 to 3.00 m/s for the bimodal PSD. These gas velocities covered both the bubbling and turbulent fluidization regimes. An ‘M’-shaped time-averaged radial voidage profile appeared upon transition from bubbling to turbulent fluidization. The ‘M’-shaped voidage profile was characterized by a dense region near the wall of the fluidized bed with decreasing solids concentration towards the centre. An increased solids concentration was observed in the middle of the bed. Frame-by-frame analysis of the images showed two predominant bubble types: spherical bubbles with particle penetration in the nose which created a core of particles that extended into, but not through, the bubble; and spherical bubbles. Penetrated bubbles, responsible for the ‘M’ profile, were a precursor to bubble splitting; which became increasingly prevalent in the turbulent regime. 相似文献
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This study describes the particle characteristics and fluidized hydrodynamics of peat granules. Peat granules, moistened with water, are a potential packing material in a gas–solid fluidized bed bioreactor used for treating air pollution. Information on the fluidization of wet peat granules is lacking. In order to advance this new type of bioreactor and to scale up its design for industrial use, fluidization studies of suitable packing material are required. Using abiotic experiments, three sizes of peat granules have been fluidized with air and fluidization characteristics were observed at different superficial gas velocities. Relative to other biomass particles, peat granules have a high particle density and sphericity, which contributes to favourable fluidization behaviour, without gas channelling. Fluidization experiments demonstrate that as the mean size of peat particles increased, minimum fluidization velocity increased. Increasing the moisture content of the peat granules resulted in a transition from bubbling bed fluidization to poor fluidization behaviour. Other types of moist biomass particles such as sawdust are difficult to fluidize and typically exhibit Geldart group C behaviour. In contrast, it was observed that wet peat granules could be fluidized in a bubbling bed regime, typical of group B particles. 相似文献
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A population model has been developed to simulate the size distribution of quartzite particles in a lab-scale fluidized bed. While quartzite particles as a kind of bed materials in the differential density circulating fluidized bed were loaded into the high temperature bed, the size distribution of quartzite particles would be changed due to the thermal fragmentation, which could significantly influence fluidization characteristics and heat transfer.With the purpose of comprehending the population of quartzite particles after thermal fragmentation, a fragmentation experiment of quartzite particles has been carried out in a lab-scale circulating fluidized bed. The results show that the fragmentation of particles mainly occurred on particles surface. Based on this experimental phenomenon, a mathematical population model was established to estimate the particle size distribution. The predicted value from the population model is close to the experimental value. 相似文献
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This study investigated the effects of high temperature and combustion conditions on the attrition of fluidized material in
a fluidized bed. Silica sand was fluidized in air at an atmospheric pressure between 873 K and 1,073 K. The operating parameters
evaluated in investigating the attrition rate of fluidized material included particle size, temperature and both combustion
and non-combustion conditions. Experimental results indicated that the total weight of attrition increased with increasing
temperature and decreased with increasing particle size. The attrition was higher during the initial fluidization period than
the later period, due to the loss of sharp corners and edges of the attrition particles. The initial and final attrition rates
during combustion were higher than those in the non-combustion condition, because the heat and thermal shock were produced
to increase attrition rate during incineration. Comparing the experimental data with previous correlations, that reveals a
significant level of error in the prediction results from existing correlations. This error may occur because the experimental
equations neglected the operating temperature and particle size. 相似文献
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The experiments were carried out in a fluidized bed of 56 mm in diameter and 1 600 mm in height to determine the fluidization characteristics of four sizes of printed circuit board plastic (PCBP) particles. It indicates that the fluidization characteristics of PCBP particles depend on the average size and particle type. 123 µm PCBP particles (1#), belonging to Geldart A group with strong viscous force, whose fluidization behaviours was similar to those of Geldart C, was difficult to fluidize. Whereas, 275 µm (2#), 354 µm (3#), and 423 µm (4#) PCBP particles, belonging to Geldart B, were fluidized smoothly. The bed collapsing process is composed of three stages: the bubble escaping stage, the sedimentation stage, and the solid consolidation stage. The collapsing process of 1# PCBP particle lasts 6 s or long. 2#, 3#, and 4# PCBP particles, Geldart group B particles, collapse process consists of the bubble escaping stage and the solid consolidation stage. The minimum fluidization velocities from modified Ergun Equation were agreement with experimental data for 2#, 3#, and 4# PCBP particles. 相似文献
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研究了稻草及不同温度热解半焦颗粒在内径100 mm、高1000 mm的有机玻璃流化床中的流化特性. 结果表明,稻草颗粒无法单独流化,而其热解半焦颗粒可单独流化;半焦颗粒的最小流化速度随粒径增大而增大,与床层高度无关,筛分粒径为0.45~0.9, 0.18~0.45, 0.125~0.18 mm的半焦颗粒的最小流化速度分别为0.19, 0.16, 0.14 m/s;300~550℃温度范围内稻草热解半焦颗粒的流化特性无明显区别;半焦与稻草颗粒混合流化时,稻草颗粒不大于20%(w)时床层有较好的流化质量,混合颗粒的最小流化速度都随混合颗粒中稻草含量增大而增大. 相似文献
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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. 相似文献
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Analyzing the attrition of Victorian brown coal during air and steam fluidized bed drying, the change in particle size distribution over a range of initial moisture contents (60% to 0%) and residence times (0 to 60 minutes) was determined. Dried at a temperature of 130°C with a fluidization velocity 0.55 m/s and an initial particle size of 0.5–1.2 mm, both fluidization mediums show a shift in the particle size distribution between three and four minutes of fluidization, with a decrease in mean particle size from 665 µm to around 560 µm. Using differential scanning calorimetry (DSC), the change in particle size has been attributed to the transition between bulk and non-freezable water (approximately 55% moisture loss) and can be linked to the removal of adhesion water, but not to fluidization effects. This is proved through the comparison of air fluidized bed drying, steam fluidized bed drying, and fixed bed drying—the fixed bed drying is being used to determine the particle size distribution as a function of drying. The results show the three drying methods produce similar particle size distributions, indicating that both fluidization and fluidization medium have no impact upon the particle size distribution at short residence times around ten minutes. The cumulative particle size distribution for air and steam fluidized bed dried coal has been modeled using the equation Pd = A2 + (A1 ? A2)/(1 + (d/x0)p), with the resultant equations predicting the effects of moisture content on the particle size distribution. Analyzing the effect of longer residence times of 30 and 60 minutes, the particle size distribution for steam fluidized bed dried coal remains the same, while air fluidized bed dried coal has a greater proportion of smaller particles. 相似文献