Attrition of limestones by impact loading in fluidized beds: The influence of reaction conditions

The extent of attrition associated with impact loading was studied for five different limestones pre-processed in fluidized bed under different reaction conditions. The experimental procedure was based on the measurement of the amount and the particle size distribution of the debris generated upon impact of sorbent samples against a target at velocities between 10 and 45 m/s. The effect of calcination, sulfation and calcination/re-carbonation on impact damage was assessed. Fragmentation by impact loading of the limestones was significant and increased with the impact velocity. Lime samples displayed the largest propensity to undergo impact damage, followed by sulfated, re-carbonated and raw limestones. Fragmentation of the sulfated samples followed a pattern typical of the failure of brittle materials. On the other hand, the behaviour of lime samples better conformed to a disintegration failure mode, with extensive generation of very fine fragments. Raw limestone and re-carbonated lime samples followed either of the two patterns depending on the sorbent nature. The extent of particle fragmentation increased after multiple impacts, but the incremental amount of fragments generated upon one impact decreased with the number of successive impacts.     

Limestone fragmentation and attrition during fluidized bed oxyfiring

Attrition/fragmentation of limestone under simulated fluidized bed oxyfiring conditions was investigated by means of an experimental protocol that had been previously developed for characterization of attrition/fragmentation of sorbents in air-blown atmospheric fluidized bed combustors. The protocol was based on the use of different and mutually complementary techniques. The extent and pattern of attrition by surface wear in the dense phase of a fluidized bed were assessed in experiments carried out with a bench scale fluidized bed combustor under simulated oxyfiring conditions. Sorbent samples generated during simulated oxyfiring tests were further characterized from the standpoint of fragmentation upon high velocity impact by means of a purposely designed particle impactor. Results showed that under calcination-hindered conditions attrition and fragmentation patterns are much different from those occurring under air-blown atmospheric combustion conditions. Noteworthy, attrition/fragmentation enhanced particle sulfation by continuously regenerating the exposed particle surface.     

Flue gas desulfurization under simulated oxyfiring fluidized bed combustion conditions: The influence of limestone attrition and fragmentation

Flue gas desulfurization by means of limestone injection under simulated fluidized bed oxyfiring conditions was investigated, with a particular focus on particle attrition and fragmentation phenomena. An experimental protocol was applied, based on the use of complementary techniques that had been previously developed for the characterization of attrition of sorbents in air-blown atmospheric fluidized bed combustors. The extent and pattern of limestone attrition by surface wear in the dense phase of a fluidized bed were assessed in bench scale fluidized bed experiments under simulated oxyfiring conditions. Sorbent samples generated during the oxyfiring tests were further characterized from the standpoint of fragmentation upon high velocity impact by means of a particle impactor. The experimental results were compared with those previously obtained with the same limestone under air-blown atmospheric fluidized bed combustion conditions. The profound differences in the attrition and fragmentation extents and patterns associated with oxyfiring as compared to air-blown atmospheric combustion and the role played by the different attrition/fragmentation paths were highlighted. In particular, it was noted that attrition could effectively enhance particle sulfation under oxyfiring conditions by continuously disclosing unconverted calcium to the sulfur-bearing atmosphere.     

Study of limestone particle impact attrition

Impact attrition of limestone particles was investigated at temperatures from 25 to and 1 atm pressure. Impacts changed the particle size distribution and mean particle diameter significantly for conveying gas velocities of 20-100 m/s. With increasing temperature less attrition occurred due to a decrease in particle impact velocity and an increase in the threshold particle impact velocity. The activation energy for impact attrition was . The mean limestone particle diameter decreased with increasing number of impacts and increasing impact velocity. Two empirical equations give good agreement with the experiments. Based on the experimental observations and correlations, an impact mechanism is suggested, where the area of new surface generated is proportional to the total kinetic energy consumed, to the number of impact cycles and an exponential decrease with temperature. When particles break, each particle generally splits into 2-3 daughter particles. The threshold particle velocities for breaking limestone particles were found to be at , similar to the reported literature values.     

煤沥青球气固流化磨损特性实验研究

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%。     

撞击加载下炸药晶体的破碎特征

分别对AP、HMX、RDX晶体进行落锤撞击加载,用环境扫描电镜观察了3种晶体的破碎特点。实验结果显示:3种晶体虽然都出现剪切带,但在破碎特征方面有明显不同,其中AP以晶体的局部脆性断裂为主,晶体的破碎程度最小,且加载后仍呈颗粒状;HMX和RDX以晶体的塑性变形和破碎为主,且加载后多呈现"薄片"结构;加载后RDX晶体比HMX易分散,但HMX晶体的破碎程度更严重。研究认为:晶体的撞击加载是力-热转化过程,晶体内部热点形成与晶体的力学强度有关,认为这一转换过程与晶体的分子组成和热反应机理有关。在低速撞击加载下,具有较强分子稳定性的AP晶体破碎程度最小,且不易形成热点。对于RDX和HMX晶体,"薄片"结构的形成原因除晶体破碎外,内部还可能存在相变、转晶等变化。     

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.     

Modelling the impaction of a micron particle with a powdery layer

The interaction of an incoming micron particle with already deposited particles is an important factor in particulate fouling of heat exchangers. A numerical model was developed based on the discrete element method to simulate this interaction. The contact forces between the colliding particles are based on the concept of contact mechanics, which takes plastic deformation of particles into consideration. The numerical model predicts the critical sticking and removal velocities, which are important parameters in determining the fouling rate of heat exchangers. Very detailed information of the bed dynamics can be extracted from the numerical model. It appears that the time required for a particle to be ejected out of a bed of particles due to an incident particle impact is proportional to the interacting particles diameter and to the square root of the number of bed layers. The maximum indentation in an incident particle hitting a bed of particles is proven theoretically and numerically to be directly proportional to the velocity and diameter of the incident particle if plastic deformation occurs. Experiments were carried out in a vacuumed column to validate the numerical model. In the experiments, incident particles dropped onto a bed of particles and the sticking, bouncing and removal behaviour were measured as a function of the incident particle impact speed. Both the numerics and the experiments showed that there are velocity regimes at which the incident particle sticks, bounces off or removes particles from the bed of particles. The regimes overlap due to the impact angle effect. The numerical model predictions regarding the critical sticking and removal velocities are in agreement with the measured values.     

颗粒流化磨损研究进展

介绍了近年来流化磨损测试设备、流化磨损机理以及流化磨损动力学模型等3个方面的研究进展;通过比较单颗粒测试体系和多颗粒测试体系,阐明了多颗粒测试体系更接近工业流化磨损过程,并且介绍了实验室流化床测试设备的发展;概括了颗粒流化磨损的两种典型机理：表面磨损和体相断裂。综述了现有的流化磨损动力学模型,指出了流化磨损时变规律是研究颗粒流化磨损的基础,目前的时变规律模型是分段函数的形式,未能把流化磨损的起始阶段和平衡阶段统一起来;其他磨损模型致力于描述流化气速和流化床结构与磨损速率的关系。指出今后需在时变规律、颗粒性质和鼓泡特征等方面加强对流化磨损的研究,以满足完善流化磨损机理和开发高耐磨损性颗粒材料的需要。     

Breakage behaviour of enzyme granules in a repeated impact test

In many industries, handling or processing of relatively fragile particles takes place and predictions are required whether a significant proportion of the particles will be damaged. These processes have been designed and controlled solely on the basis of particle size and shape. Another parameter that needs to be introduced is particle strength. The stringent environmental laws demand improved particle mechanical quality, which has given rise to the need for a more accurate and fundamental particle strength measurement and its application in modelling and control of particulate processes. Particles need to show good resistance against static and dynamic loads.

The present paper deals with the study of breakage behaviour of different enzyme granules subjected to repeated impacts using a new instrument developed at the Delft University of Technology. The impact test involves bombarding the particles against a flat target repeatedly. The main feature of this new test is its ability to impact a large number of particles against a flat target repeatedly, and generate extremely reproducible results. Testing a large number of particles has the advantage of producing statistically correct results. The repeated impacts provide information on the breakage behaviour of the particles based on their history. In the new impact test enzyme granules can undergo very low impact velocities of the order of 5 m s⁻¹. These low impact velocities lead to attrition and chipping of the granules.

The current paper presents preliminary results on the breakage behaviour of the new impact test and its basic advantages over already existing tests. Furthermore, experiments were performed on enzyme granules, and the breakage mechanisms determined, depending on the change in size and shape of the particles.     

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.     

Limestone particle attrition and size distribution in a small circulating fluidized bed

Limestone particle attrition was investigated in a small circulating fluidized bed reactor at temperatures from 25 to 850 °C, 1 atm pressure and superficial gas velocities from 4.8 to 6.2 m/s. The effects of operating time, superficial gas velocity and temperature were studied with fresh limestone. No calcination or sulfation occurred at temperatures ?580 °C, whereas calcination and sulfation affected attrition at 850 °C. Increasing the temperature (while maintaining the same superficial gas velocity) reduced attrition if there was negligible calcination. Attrition was high initially, but after ∼24 h, the rate of mass change became constant. The ratio of initial mean particle diameter to that at later times increased linearly with time and with (U_g − U_mf)², while decreasing exponentially with temperature, with an activation energy for fresh limestone of −4.3 kJ/mol. The attrition followed Rittinger’s surface theory [Beke B. Comminution. Budapest: Akademiai Kiado, 1964; Ray YC, Jiang TS, Wen CY. Particle attrition phenomena in a fluidized bed. Powder Technol 1987a; 49:193-206]. The change of surface area of limestone particles was proportional to the total excess kinetic energy consumed and to the total attrition time, whereas the change of surface area decreased exponentially with increasing temperature. At 850 °C, the attrition rate of calcined lime was highest, whereas the attrition rate was lowest for sulfated particles. When online impact attrition was introduced, the attrition rate was about an order of magnitude higher than without impacts.     

An experimental study on the primary fragmentation and attrition of limestones in a fluidized bed

In this paper, an experimental study on the primary fragmentation and attrition of 5 limestones in a fluidized bed was conducted. The intensity of fragmentation and attrition were measured in the same apparatus but at different fluidizing velocities. It was found that the averaged size of the particles decreased by about 10-20% during the fragmentation process. The important factors for particle comminution include limestone types, heating rate, calcination condition and ambient CO₂ concentration. Fragmentation mainly occurred in the first a few minutes in the fluidized bed and it was more intense than that in the muffle furnace at the same temperature. The original size effect was ambiguous, depending on the limestone type. The comminution caused by attrition mainly occurred during calcination process rather than sulphation process. The sulphation process was fragmentation and attrition resisted. The attrition rate of sulphate was similar to that of lime in trend, decaying exponentially with time, but was one-magnitude-order smaller than that of lime. Present experimental results indicate that fragmentation mechanism of the limestone is dominated by CO₂ release instead of thermal stress.     

Visualization of Impact Damage in Ceramics Using the Edge-On Impact Technique

Projectile impact generates severe fragmentation in ceramics which propagates at high velocities and precedes the penetration of the projectile. The high-speed photographic technique of the Edge-On Impact (EOI) has been developed at the Ernst-Mach-Institute (EMI) in order to visualize dynamic fracture in brittle materials. In a typical EOI test the projectile hits one edge of a specimen and fracture propagation is observed during the first 20 us after impact by means of a Cranz-Schardin highspeed camera. EOI tests allow a characterization of different ceramics by the macroscopic fracture patterns, single crack velocities, and crack front velocities (damage velocities). The phenomenology of damage propagation in several ceramics and a ceramic-metal composite is discussed. The EOI technique is useful for the evaluation of damage models for brittle materials because it enables a direct comparison of model predictions to experimental data obtained during the impact process.     

Distinct element analysis of attrition of granular solids under shear deformation

The attrition of spherical particles in a periodic cell under shear straining is analysed by distinct element method (DEM). The damage mechanisms of surface wear and fragmentation are under consideration. The side crushing strength (SCS) of single particles is used as the criterion for fragmentation. For surface damage, the abrasive wear by the formation of lateral cracks during shearing is calculated using a model of chipping for the semi-brittle failure mode. The contributions to attrition by surface wear and particle fragmentation are therefore decoupled and quantified. The simulation results are then compared with the experimental data reported previously.The contribution of fragmentation to the overall attrition is found to be dominant over the full range of strains tested in this work in agreement with the experimental data. The extent of attrition predicted by the simulations compares generally well with the experimental data except at the lowest normal stress and at high shear strains. For the former, the reason for the poor agreement is unclear. For the latter, the difference is attributed to the presence of fragments and debris as they affect the contact force distribution and this feature has not been simulated. The parametric sensitivity analyses indicate that the simulation results are sensitive to particle properties such as Young's modulus and the coefficient of friction. Therefore, careful material characterisation is necessary in order to get reliable predictions. The effect of the shear strain rate on particle breakage has also been examined by simulation. The results indicate that the rate of attrition is not sensitive to strain rates normally used for shear cell tests, i.e., of order of .     

流化床与固定床耦合反应器中的颗粒磨损

在直径50mm的冷模流化床与固定床耦合反应器中,考察了活性炭颗粒在130～150℃及不同气速下的磨损情况,得到了不同气速下固定床中圆柱状颗粒的磨损率随时间的变化关系,同时分析了滞留在流化床、固定床及袋滤器中的细颗粒在不同气速下的粒径分布与质量分布.结果表明,颗粒在该耦合反应器中磨损严重,在0.212～0.424m/s气速下,固定床中颗粒质量损失可达3%～4%,流化床中颗粒平均粒径由200μm降至100μm以下.     

Agglomerate breakage and adhesion upon impact with complex-shaped particles

Understanding the breakage and adhesion of an agglomerate upon collision with a target particle is a primary step to fathom the adhesive mixing process. While the effect of several variables, such as collision velocity and particle interface energy, on collision behavior has been explored, the effects of target particle morphology have yet to be revealed. In this work, we generate three-dimensional particles with controllable shape and texture using Fourier harmonics and, using the discrete element method, we examine the collision of an agglomerate that impacts each target particle. Results show that the agglomerate breakage depends on the local curvature in the impact zone. We observe that the asperity and elongation factors of the target particle largely contribute to the extent of the deposition of fine particles and the size and number of generated fragments after impact, respectively. These results reveal the large potential error when approximating real particles as smooth spheres in fragmentation studies.     

Impact of a single crystal in solution, on an immersed target, in conditions which simulate impact attrition in crystallizers

In industrial crystallization, attrition due to particle–crystallizer contacts is a primary source of generation of fragments, which greatly affects operating conditions, and the resulting size-distribution and morphology. The objective of this work is to contribute to the understanding of the attrition propensity of crystals in suspension, under conditions of impact at velocities which typically occur in crystallizers. Two types of parameters were studied: those related to the operating conditions (impact velocity, number of impacts, type of target) and those related to the properties of the crystals and of the solution which they are in. The rate of attrition increases with the velocity of impact and depends on the orientation and on the material of the target. An attempt to correlate the data to the impact energy is made.     

Numerical modelling of the breakage of loose agglomerates of fine particles

This paper presents a numerical study of the breakage of loose agglomerates based on the discrete element method. Agglomerates of fine mannitol particles were impacted with a target wall at different velocities and angles. It was observed that the agglomerates on impact experienced large plastic deformation before disintegrating into small fragments. The velocity field of the agglomerates showed a clear shear zone during the impacts. The final breakage pattern was characterised by the damage ratio of agglomerates and the size distribution of fragments. While increasing impact velocity improves agglomerate breakage, a 45-degree impact angle provides the maximum breakage for a given velocity. The analysis of impact energy exerted from the wall indicated that impact energy in both normal and tangential directions should be considered to characterise the effects of impact velocity and angle.