The influence of stress and attrition on crystal size distribution

An important parameter influencing the crystal size distribution in mass crystallization from solution is the mechanical stress exerted on crystals in the crystallizer. This contribution presents the study of the influence of mechanical stress and attrition of the system potassium nitrate-water in an FC-crystallizer and in various draft-tube crystallizers, fitted with different types of impellers. The intensity of stress is a newly defined variable which is used to describe the level of stress in crystallizers. The reduction of crystal size by attrition is described by the linear attrition rate. The influence of impeller design and crystal hold-up on crystal size distribution and scale-up rules is discussed.     

The Influence of a Constructional Solution of the Crystallizer with a Jet Pump on Crystal Attrition

The research results concerning attrition of sodium chloride crystals in two types of jet pump crystallizers are presented. It was demonstrated that in the crystallizer with a jet pump crystals undergo considerable smaller attrition compared to the one equipped with agitator and draft tube (circulation profile). The influence of the initial mean crystal size, crystal volumetric concentration in the suspension and the residence time of the suspension in the crystallizer on the degree of solid phase destruction was determined and quantified in the form of empirical equations. The DTM MSCPR construction demonstrates the least destruction capabilities.     

Prediction of attrition in agitated particle beds

The majority of pharmaceutical powders produced through crystallisation are dried in agitated dryers. The rotation of the impeller causes shear deformation of the bed, which enhances the drying rate, but also leads to particle breakage. A method of predicting the extent of breakage occurring due to agitation is described and applied for Paracetamol in a small-scale dryer. The distributions of stresses and strains in the bed are estimated using the Distinct Element Method (DEM). The information obtained here is then coupled with the measured attrition of Paracetamol in an annular shear cell in order to predict the attrition in the agitated bed. The experiments are carried out on dry material so as to establish purely the effect of stresses and strains on attrition, whilst keeping moisture content and temperature constant.The shear cell provides uniform condition for stresses and strains so that the breakage taking place under relatively well-defined conditions is quantified. In contrast, the prevailing shear stresses and strains in the agitated bed have wide distributions, as little shearing takes place near the impeller shaft, whilst there are considerable shearing stresses near the impeller tip. Therefore, the bed is divided into a number of segments for which the extent of attrition can be evaluated for each segment, based on the shear cell data. A good quantitative agreement is found between the predictions and experimental results obtained for the attrition of Paracetamol in the small scale dryer. The resulting prediction also suggests that, for a given number of impeller rotations, the extent of breakage is independent of impeller speed in the range tested (20–78 rpm). This is expected as the prevailing strain rates are too low for the inertial effects to be dominating and the shear stresses are independent of shear rates within the range investigated. The attrition prediction suggest that over half of the attrition occurs in the bottom third of the bed, with increased attrition at greater radial distances. The attrition is also predicted to occur predominantly within the region extending from 30° in front of to 30° behind the impeller.     

Nanocrystalline titanium powders by high energy attrition milling

The present investigation deals with the synthesis of nanocrystalline titanium powders by the high energy attrition milling of micron sized titanium powder in an inert atmosphere. Titanium powders of about 12 μm particle size were subjected to high energy attrition milling in an argon atmosphere. Selecting suitable milling parameters, nanosize (< 100 nm) titanium powders were obtained after 15 h of milling. An average particle size of 35 nm was obtained at 30 h of milling after which the particle size stabilized with continuation of milling to 75 h. The powders after milling for various durations were characterized by XRD, SEM, TEM, ICP-AES and DTA and these results are reported and discussed.     

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.     

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.     

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.     

Impact attrition of particulate solids. Part 2: Experimental work

Single particle impact testing has been carried out to evaluate the attrition by chipping of ionic single crystals of MgO, NaCl and KCl, used as model materials having a semi-brittle failure mode. The results have verified the theoretical predictions presented in Part 1. The dimensionless attrition propensity parameter, η=ρv²lH/K²_c, is shown to account for the dependence of the material loss per impact on the material properties, impact velocity and particle size. The ‘chipping’ mechanism of impact attrition is further confirmed by the observations of the impact damage by the use of scanning electron microscopy. In addition, the influence of rigidity of target materials on attrition rate is also addressed.     

Two-dimensional population balance modeling for shape dependent crystal attrition

Many suspension crystallization processes can be described by growth and secondary nucleation. The prevailing mechanism for secondary nucleation in industrial processes is attrition caused by crystal-impeller collisions. The understanding and prediction of attrition rates is of fundamental importance for product and process design.Attrition of a crystal is affected by the size of the crystals and, as experimental evidence reveals, by their shape. For crystals with the same mass, the attrition rate is significantly larger if crystals have distinct and sharp corners, than if they were spherical. Because of the associated modeling and computational effort, shape dependent behavior has mostly been disregarded in crystallization process modeling. In this work, a two-dimensional population balance model is formulated. The inner variables are the size and the shape of the crystals. Consequently, the model accounts for size and shape dependent process behavior. In order to close the model equation shape modification function are introduced. The reinforcement function specifies the increase in attrition resistance by rounding the sharp corners and increasing material strength. The face attrition ratio represents the differences of material removal from sharp corners and flat faces.The sensitivity of the results with respect to these shape modification functions is investigated. The results show that the model is capable of reflecting shape dependent attrition behavior in a physically meaningful way. To fit experimental data, mainly the parameters of the shape modification function need to be tuned.     

A population balance approach to describing bulk attrition

Attrition arising from mechanical damage during processing has been studied in annular shear cells, these having the ability to vary readily the testing stress and shear strain. A population balance approach has been deployed to analyse the evolution of the size distribution with strain which bases its arguments on the kinetic theory of grinding. This incorporates the ideas of a selection function and a breakage function. The extent of attrition is determined in terms of three parameters, one related to the selection function, one to the breakage function, and additionally one which allows for the balance of fracture and abrasion. The product size distribution of the Gaudin-Schuhmann form is consistent with experimental findings from annular shear cells. Whatever reasonable physical assumptions are made about the form of the selection and breakage functions, and of the balance between fracture and abrasion, particle size distributions of the same form arise. From comparisons with several materials, the index in the breakage function is consistent with the particle fracture pattern observed experimentally.     

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.     

Analysis of granule breakage in a rotary mixing drum: Experimental study and distinct element analysis

Rotary drums are commonly used in particulate solid industries for mixing, coating and reactions. The process is often accompanied by undesirable breakage of granules. For this reason, a scaled-down version is sometimes used as an attrition testing device. In this work, the attrition of granules inside a rotary drum at 18, 35 and 52 rpm drum rotation speeds for 4000 cycles is studied. The granules used in this study have been produced by extrusion and spheronisation with a size range of 500 to 1000 μm. The rotary drum has an internal diameter of 0.39 m, axial length of 0.3 m and a single baffle. The extent of breakage is quantified by sieving out fine debris which is two sieve sizes smaller than the feed particles. To relate the extent of breakage in the drum to granule characteristics, single granule impact tests have been performed on one type of granule at several velocities. The effects of particle size and impact velocity are analysed and a power-law relationship is fitted between impact velocity and single granule breakage. This information is then used to simulate granule breakage in a rotary drum by Distinct Element Method (DEM). The drum is simulated for 5 rotations at the rotational speeds stated above and the breakage rate is extrapolated to 4000 cycles where it is compared to experimental results obtained. The trends for particle breakage in both experiments (determined by sieving) and extrapolated DEM simulations are in agreement however the orders of magnitudes are different. The comparison shows that the extent of breakage obtained from extrapolated simulations is overestimated at drum speed of 35 and 52 rpm and underestimated at 18 rpm. There is close agreement between experiments and extrapolated DEM simulations for particle breakage at 18 rpm only after 4000. Furthermore, the effect of air drag on the attrition of granules by impact at a drum rotation speed of 52 rpm is investigated, where it is found to significantly reduce the breakage results.     

Influence of hydrodynamic parameters on particle attrition during fluidization at high temperature

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.     

Entwicklung von Kristallisatoren

Development of crystallizers . The control of the crystallization kinetics and the spatial variation of the crystal mass are of decisive importance for the operation of industrial crystallizers. The influence of micro- and macro-attrition is demonstrated with particular reference to collisions with impeller blades and to breakage of crystals between impeller and draft tube. Spatial variations of supersaturation and crystal mass within the crystallizer have been shown to influence crystal size distribution. These variations are described by physical and mathematical models. Crystal size distributions measured in both pilot and industrial crystallizers show that simulation with the proposed models is very reliable. A model approach provides valuable support for the development of industrial crystallizers and also for the development of control using microprocessors.     

Investigation and mathematical modeling of attrition

A mathematical model is developed to describe attrition, which is the main source of nuclei in suspension crystallization. The model allows one to calculate the mass median size of the crystals obtained and the particle size distribution function. The effect of the suspension volumetric flow rate, the geometric sizes of a impingement device, and the initial size of a seed on the size of the crystals obtained is revealed. The model is applicable to describing the contact mechanism of nucleation in impact of crystals with a stirrer, pump blades, etc. To study attrition on a laboratory scale, a set of experiments was carried out with the ammonium sulfate crystal system under various conditions.     

Factors affecting methanol photocatalytic oxidation and catalyst attrition in a fluidized-bed reactor

A resolution IV fractional factorial experimental design explored the effects of seven factors on both the methanol photocatalytic oxidation (PCO) rate and the catalyst particle size distribution using a fluidized-bed reactor. The seven factors were as follows: calcination temperature, calcination time, grinding order, particle size, vibration amplitude, carrier gas humidity, and fluidization velocity. Decreasing calcination temperature from 726 to 623 K increased the activity of TiO₂/Al₂O₃ catalysts for methanol PCO. Attrition during fluidization liberated small TiO₂ particles from the bulk catalyst and the rate of attrition increased with gas velocity. Attrition was the primary cause of catalyst elutriation and not the presence of fine particles initially present in the bed from catalyst preparation. Increasing humidity caused agglomeration of fine particles, which reduced the amount of catalyst carryover. Removal of fines from the catalyst bed prior to fluidization caused an increase in catalyst attrition until the amount of fines present in the bed was similar to that of a bed in which fines were not removed.     

Impact attrition of brittle structured particles at low velocities: rigorous use of a laboratory vibrational impact tester

Brittle particle impact attrition was measured over three orders of magnitude of impact velocity by use of both single- and multiple-impact testers. The multiple-impact tester was a resonant cantilever impactor with dynamic amplitude control and small-gap cavities, designed to ensure operation near the transition between the bouncing and resonant impact regimes. For this impactor, a novel technique using a trajectory simulation was developed to elucidate average impact velocities, effective particle restitution behavior, and average losses per impact from a set of nominal attrition rates (loss/time). This allowed direct comparison of data from the single-impact and multiple-impact testers.Results were obtained for three brittle, porous pharmaceutical particles with significantly-different, well-characterized internal structures. Results reveal a relatively unexplored mode of attrition that is distinguished by lack of gross fragmentation at low velocities, with a steep velocity dependence. This regime is expected for any brittle particle for which simple chipping is not seen—such as rounded solid particles and many agglomerates. At higher impact velocity, gross particle fragmentation is observed. The transition between these attrition regimes appears connected to particle structure, such as the size of attrition-resistant primary particles in an agglomerate or the point at which dominant flaws (that lead to fragmentation) are no longer critically active. The ranking of particles according to damage in high-velocity impacts was not predictive of damage in low-velocity impacts, because the particle attrition did not necessarily exhibit the same velocity-dependence in the two regimes. Such differences are critical for predicting performance in operations such as pneumatic conveying or fluidized beds, respectively.     

Effects of high temperature and combustion on fluidized material attrition in a fluidized bed

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.     

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%。     

Sorbent attrition in a carbonation/calcination pilot plant for capturing CO2 from flue gases

There is increasing interest in CO₂ looping cycles that involve the repeated calcination and carbonation of the sorbent as a way to capture CO₂ from flue gases during the carbonation step and the generation of a pure stream of CO₂ in the oxyfired calcination step. In particular, attrition of the material in these interconnected fluidized bed reactors is a problem of general concern. Attrition of limestone derived materials has been studied in fluidized bed systems by numerous authors. In this work, we have investigated the attrition of two limestones used in a system of two interconnected circulating fluidized bed reactors operating in continuous mode as carbonation and calciner reactors. We observed a rapid initial attrition of both limestones during the calcination step which was then followed by a highly stable period (up to 140 h of added circulation for one of the limestones) during which particle size changes were negligible. This is consistent with previous observations of attrition in other systems that employ these materials. However, a comparison of the attrition model constants with the data reported in the literature showed the two limestones to be particularly fragile during the initial calcination and the first few hours of circulation. Thus, a careful choice of limestone based on its attrition properties must be taken into account in designing future carbonate looping systems.