Attrition Behavior Research Articles

Evaluation of a highly reactive and sulfur resistant synthetic Fe-based oxygen carrier for CLC using gaseous fuels

Because of its low cost and environmental compatibility, Fe-based oxygen carriers are now being considered as an attractive option for chemical-looping combustion (CLC) applications. In this work, a synthetic Fe-based oxygen carrier made by impregnation using Al2O3 as support was evaluated with respect to gas combustion in a 500 Wth CLC continuous unit using CH4 as fuel during long-term operation. Furthermore, a study on the fate of sulfur during CH4 combustion was carried out with this oxygen carrier.During the experimental tests, it was observed that an increase in the oxygen carrier-to-fuel ratio (φ) produces an increase in the combustion efficiency, reaching full gas combustion at φ values 2 1.5. In addition, the presence of sulfur in the fuel gas did not affect to the combustion efficiency, independently of the amount of sulfur in the gas stream in the range of 700 to 2000 vppm H2S. However, the attrition of this oxygen carrier was higher than other materials used in CLC.Therefore, it can be concluded that the synthetic Fe-based oxygen carrier developed in this work was highly reactive and sulfur resistant and it can be considered as a suitable oxygen carrier for CLC with gaseous fuels that contain sulfur when the attrition behavior was improved.

Modified lime-based pellet sorbents for high-temperature CO2 capture: Reactivity and attrition behavior

Modified lime-based pellets have been developed as potential regenerable high-temperature CO2 sorbents using calcium aluminate cement binders to enhance pellet strength. A mechanical pelletizer was used for granulation of the powdered materials, namely quick lime and hydrated lime, produced from Graymont limestone with the addition of spray water. The CO2 carrying capacity of both the pellet sorbents and the parent limestone was tested in a thermogravimetric analyzer (TGA) at 800°C with repeated calcination/carbonation cycles. It was found that the CO2 carrying capacity of the pelletized sorbent was higher than that of the parent limestone, and the stability over multiple cycles was improved when cement was added to the pellets. The attrition resistance of these pellets was examined using a bubbling fluidized bed (50mm ID). The particle size distribution (PSD) of both the calcined pellets and limestone was determined before and after 2h attrition tests which were performed using air as the fluidizing gas at room temperature and at 800°C. The results of attrition tests showed that after fluidization for 2h, particle size distribution changed such that the average particle diameter (d50) of the sample always decreased, but for the pellet sorbents, high-temperature fluidization did not result in significantly more pronounced attrition.

The attrition behaviour of oxygen-carriers under inert and reacting conditions

The attrition of two potential oxygen-carriers for chemical-looping, 100wt% mechanically-mixed, unsupported iron oxide (400–600μm diameter) and 25wt% copper oxide impregnated on alumina (600–900μm diameter), has been studied. The rates of attrition of batches of these particles whilst they were being fluidised and subjected to successive cycles of reduction and oxidation were determined by measuring the rate of production of fine particles elutriated from the bed, as well as progressive changes in the distribution of particle sizes retained in the bed. The ability of the particles to withstand impacts was also investigated by examining the degree of fragmentation of 1g of reacted particles of known size on projecting them at a target at various velocities. It was found that the mechanical strength of the iron oxide particles deteriorated significantly after repeated cycles of oxidation and reduction. Thus, the rate of elutriation increased ∼35-fold between the 1st and 10th cycle. At an impact velocity of 38m/s, the amount of fragmentation in the impact test, viz. mass fraction of particles after impact having a size less than that before impact, increased from ∼2.3wt% (fresh particles) to 98wt% after the 10th cycle. The CuO particles, in comparison, were able to withstand repeated reaction: no signs of increased rates of elutriation or fragmentation were observed over ten cycles. These results highlight the importance of selecting a durable support for oxygen-carriers.

A social network analysis of student retention using archival data

This study attempts to determine if a relationship exists between first-to-second-year retention and social network variables for a cohort of first-year students at a small liberal arts college. The social network is reconstructed using not survey data as is most common, but rather using archival data from a student information system. Each student is given a retention score and an attrition score based on the behavior of their immediate relationships in the network. Those scores are then entered into a logistic regression that includes tradition background and performance variables that are traditionally significantly related to retention. Students’ friends’ retention and attrition behaviors are found to have a greater impact on retention that any background or performance variable.

Toward a desktop attrition tester; validation with dilute phase pneumatic conveying

A new attrition tester has been developed which on a bench scale measures the attrition behavior of granules. Two different types of granules are tested viz. sodium benzoate and ammonium nitrate fertilizer granules. Attrition in the repeated impact tester is compared to attrition during dilute phase pneumatic conveying. Typical impact velocities in the new tester vary between 1m/s and 5m/s, during dilute phase pneumatic conveying typical air velocities are in the range of 20–40m/s. To compensate for the velocity difference the number of collisions is increased. Experiments show an initial similarity between transferred kinetic energy and the attrition of the granules independent of the impact velocity. This is explained by the removal of surface asperities of the initial granules. The initial similarity region includes 12 bends at 40m/s. In practice often not more than 6–10 bends are used for pneumatic conveying. The new attrition tester therefore seems a promising desktop tool for testing attrition during for example pneumatic conveying. But to be more conclusive more measurements with different types of particles are required.

Devolatilization and Attrition Behavior of Fuel Pellets during Fluidized-Bed Gasification

Devolatilization, fragmentation, and attrition phenomena occurring to fuel pellets during fluidized-bed gasification have been studied. Three pelletized fuels, one based on wood and the other two based on a mixture of wood and coal, were characterized in a lab-scale fluidized-bed reactor under gasification, inert, and combustion conditions, for comparison. Similar and relatively long devolatilization times were observed for the three types of pellets. Pellet breakage by primary fragmentation upon devolatilization appeared to be rather limited for all fuels, indicating that fuel pelletization gives sufficient mechanical strength to the particles. On the contrary, secondary fragmentation and attrition by abrasion of char particles during gasification were extensive, suggesting a gasification-assisted attrition enhancement effect. This mechanism, associated with the low reactivity of the generated fines, made the loss of carbon by fine elutriation during char gasification much more significant than that found under combustion conditions. Larger carbon losses were associated with fuel pellets with a lower reactivity.

Investigation of clay content and sintering temperature on attrition resistance of highly porous diatomite based material

Diatomite based granulates with a diameter of 500 μm were produced by extrusion with the addition of 16, 28 and 48 vol.% kaolin and organic binder in order to optimise the flow behaviour. The influences of clay content and sintering temperature on the microstructure and attrition resistance were investigated. Bimodal pore size distribution was obtained by mercury intrusion porosimetry at 800 °C. Pore sizes smaller than 250 nm were assumed to originate from the diatomite porous structure and the larger ones mainly from the interparticle porosity. Meso and macropores of diatoms tend to close at 1100 °C and due to coalescence at higher temperature a unimodal macro pore size distribution occurs. An increase of the pore size in temperature range between 1100 and 1300 °C was associated with a different sintering behaviour between diatomite and clay. Additionally α-cristobalite crystallization was observed at sintering temperatures between 1100 and 1300 °C. Attrition resistance was determined by a grinding test and a new analysis method to evaluate different attrition behaviour materials. The new analysis method uses the absolute value of the exponent obtained from attrition resistance behaviour versus the number of revolutions. High clay content in diatomite based materials and high sintering temperature lead to an increase of attrition resistance of the material due to phase transformations of the materials into mullite and cristobalite.

Formation of a Ca12Al14O33 Nanolayer and Its Effect on the Attrition Behavior of CO2-Adsorbent Microspheres Composed of CaO Nanoparticles

The attrition behavior of microspheres composed of CaO nanoparticles that were used as high-temperature CO2-reactive adsorbents was investigated. The nano-CaO/Al2O3 microsphere adsorbents were prepared by a spray technique from a slurry containing a precursor of CaCO3 nanoparticles and an aluminum oxide gel. A mechanism of formation of the layer of nano-Ca12Al14O33 was proposed and optimized, with calcination temperatures ranging from 900 to 1000 °C and Ca/Al molar ratios between 2.3 and 3.5. The attrition behavior of the adsorbent was investigated in detail using the air jet method to measure fine loss, as well as with scanning electron microscopy and a particle size analyzer to examine the changes in surface morphology and particle size distribution. The attrition studies showed that complete formation of a Ca12Al14O33 nanoscale framework under calcination temperatures ranging from 900 to 1000 °C and Ca/Al molar ratios between 2.3 and 3.5 resulted in improved resistance to attrition and increased durabi...

Attrition behavior of fine particles in a fluidized bed with bimodal particles: Influence of particle density and size ratio

To process the solid particulates in fluidized bed and slurry phase reactors, attrition is an inevitable consequence and is therefore one of the preliminary parameters for the catalyst design. In this paper, the mechanical degradation propensity of the zeolite catalysts (particles) was investigated in a bimodal distribution environment using a Gas Jet Attrition — ASTM standard fluidized bed test (D-5757). The experimentation was conducted in order to explore parameters affecting attrition phenomena in a bimodal fluidization. In a bimodal fluidization system, two different types of particles are co-fluidized isothermally. The air jet attrition index (AJI) showed distinct increases in the attrition rate of small particles in a bimodal fluidization environment under standard operating conditions, in comparison with single particle. A series of experiments were conducted using particles of various sizes, with large particles of different densities and sizes. Experimental results suggest that the relative density and particle size ratio have a significant influence on attrition behavior during co-fluidization. Therefore a generalized relationship has been drawn using Gwyn constants; those defined material properties of small particles. Moreover, distinct attrition incremental phenomenon was observed during co-fluidization owing to the change in collision pattern and impact, which was associated with relative particle density and size ratios.

Repeated impacts tests and nanoindentation as complementary tools for mechanical characterization of polymer‐coated particles

AbstractPolymer‐coated particles have been produced by applying two grades of hydroxypropyl methylcellulose and one grade of polyvinyl alcohol onto sodium benzoate Purox‐S core particles by using top‐spray fluid bed coater and then stored under two different conditions, namely ambient conditions (23°C, 55% RH) and in the freezer (−18°C, 25% RH). Surface morphology has been firstly analyzed using scanning electron microscope and atomic force microscope. Resistance to attrition and viscoelastic properties have been measured by repeated impact tester (RIT) and nanoindentation, respectively. The resistance to attrition have been discussed as function of type of coating materials, breakage mechanisms, and storage temperature, and then compared with uncoated Purox‐S. The storage conditions is not influencing the morphology, whereas is strongly affecting the resistance to attrition. Coated particles stored at −18°C were found to be more resistant to attrition than ones stored at room conditions. Such differences negligible at low energies (low numbers of impacts) increase as soon as the number of impacts and the energy rise. The improvement in the resistance to attrition was related to the plasticizing effect of water content. Displacement, storage modulus (E′), loss modulus (E″), and damping factor (tan δ) have been measured for all coated particles. Quasistatic and dynamic nanoindentation were found to agree very well with each other. Comparison of the nanoindentation results and the RIT results showed that tan δ, as measure of viscoelasticity and flexibility of the polymeric coating material, is related to the attrition behavior. It was found, in fact, that a higher tan δ gives more resistance to attrition. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Does Attrition Behavior Help Explain the Duration of Interstate Wars? A Game Theoretic and Empirical Analysis

Does attrition behavior, defined as waiting for the other side to give in despite the costs of delay, help explain the duration of interstate wars? To answer this question we develop a war and bargaining model that integrates a dynamic progression of the war and allows the rivals to make offers at any time of their choosing while they fight. The model predicts that, in equilibrium, states choose to fight without making significant offers in the hope that the other side will give in to outstanding demands. The model also predicts that each side’s flow cost of war increases with the probability that the other side gives in to its demands. This is the testable consequence of attrition behavior. Our statistical analysis suggests that both challenger and defender include attrition behavior in their conduct of warfare, and that this behavioral element significantly affects the duration of interstate wars.

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.

Factors contributing to attrition behavior in diabetes self-management programs: a mixed method approach.

BackgroundDiabetes self-management education is a critical component in diabetes care. Despite worldwide efforts to develop efficacious DSME programs, high attrition rates are often reported in clinical practice. The objective of this study was to examine factors that may contribute to attrition behavior in diabetes self-management programs.MethodsWe conducted telephone interviews with individuals who had Type 2 diabetes (n = 267) and attended a diabetes education centre. Multivariable logistic regression was performed to identify factors associated with attrition behavior. Forty-four percent of participants (n = 118) withdrew prematurely from the program and were asked an open-ended question regarding their discontinuation of services. We used content analysis to code and generate themes, which were then organized under the Behavioral Model of Health Service Utilization.ResultsWorking full and part-time, being over 65 years of age, having a regular primary care physician or fewer diabetes symptoms were contributing factors to attrition behaviour in our multivariable logistic regression. The most common reasons given by participants for attrition from the program were conflict between their work schedules and the centre's hours of operation, patients' confidence in their own knowledge and ability when managing their diabetes, apathy towards diabetes education, distance to the centre, forgetfulness, regular physician consultation, low perceived seriousness of diabetes, and lack of familiarity with the centre and its services. There was considerable overlap between our quantitative and qualitative results.ConclusionReducing attrition behaviour requires a range of strategies targeted towards delivering convenient and accessible services, familiarizing individuals with these services, increasing communication between centres and their patients, and creating better partnerships between centres and primary care physicians.

Model-based analysis of the effect of particle and impact geometry on attrition of brittle material

Secondary nucleation by attrition frequently occurs in industrial crystallizers. By collisions with the impeller or with the crystallizer walls, small fragments of larger crystals get chipped off, which serve as secondary nuclei. Gahn and Mersmann ( Chem. Eng. Sci., 54(9), 1999, Part A: 1273–1282, Part B: 1283–1292) presented a mechanistic modeling approach to predict the attrition behavior of crystals on the basis of measurable material properties. Their model is based on several assumptions to facilitate an analytical derivation. One of these assumptions is that the crystal corner is hit by a target, such that the target surface is perpendicular to the central axis of the corner. In this contribution their principal modeling approach is extended to a more general formulation allowing impact angle and opening angle of the corner to be variable. The results show that there is a large dependence of the resulting attrition volume on the geometric conditions. The model is evaluated for platonic solids with fictitious material properties to investigate the general effect of geometry. For a selection of crystalline species (ammonium sulphate, potassium nitrate, potash alum, tartaric acid, citric acid), detailed crystal shapes have been used to predict the attrition volume for defined single crystal impact experiments. Comparing the model predictions to the experimental results showed that predictiveness can be improved by considering a detailed impact and crystal geometry.

Attrition behavior by micro-hardness parameters in suspension-crystallization processes

We measured the elastic modulus and fracture strength at the micro-level for various types of crystal generated by an MSMPR crystallizer. The attrition behavior was determined from the fracture energy and attrition coefficient, which were calculated from micro-hardness properties. The different values observed for each tested crystal size revealed that crystal size was an important factor in attrition behavior. Therefore, we could predict the amount of attrition through the fracture energy. Future work will involve applying the estimation method in a scaled-up suspension crystallizer.

Diffusion Modeling of Bulk Granular Attrition

A simple empirical model for bulk granular attrition is proposed and investigated in this paper. We attempt to model the attrition process that occurs in various types of experimental systems reported in the literature with a diffusion-type equation. The proposed empirical model is found to reproduce much of the experimentally observed behavior and our own numerical simulation results. This may suggest similarities in statistical characteristics or other fundamental properties between the process of bulk granular attrition and diffusion of material. A comparison of the model with the well-established Gwyn correlation may also provide insights on why such a power-law-type correlation has been generally successful in describing granular attrition behavior.

Extent and Determinants of Panel Attrition in the European Community Household Panel

The aim of the paper is to analyse the extent and determinants of panel attrition in the European Community Household Panel (ECHP). The fact that, after five waves, in some countries the response rate has declined to about 50%, leads to concerns about the representativeness of the remaining participants. We find the extent and determinants of panel attrition to reveal high variability across countries as well as for different waves within one country. Differences were also found when comparing attrition behaviour across different surveys running parallel in the same countries, as was the case for Germany and the United Kingdom (UK). Response rates are found to depend strongly on whether households moved during the sample period and whether the interviewer in the sample period changed. Compared to these two influences, all other characteristics are of minor importance. Despite these different attrition rates, neither is the analysis of income biased, nor is the ranking of national results disturbed.

Transfer of fracture mechanical concepts to bulk solids attrition in pneumatic conveying

In the present project, attrition in pneumatic conveying—as in fracture mechanics and comminution—is regarded as a result of a process function (stress conditions) and a material function (influence of material properties). The first part of the paper describes the results of the determination of the stress conditions, where special emphasis is put on the numerical simulation of dilute-phase conveying. In the second part, results of single-particle tests are discussed. They simulate the two basic stress modes: impact and friction. The results show that the attrition behavior of the polymers tested changes between both stress modes. It is currently evaluated which material properties are suitable to describe this experimentally observed behavior.

Identification of material specific attrition mechanisms for polymers in dilute phase pneumatic conveying

In the approach presented, attrition of bulk solids in pneumatic conveying is regarded as a result of a process function (stress conditions) and a material function (influence of material properties). For dilute phase conveying the process function was determined by employing computational fluid dynamics (CFD). Opposed to what was expected initially, the numerical simulations indicate that sliding friction apparently is of importance in dilute phase conveying. This conclusion can be drawn from the low calculated impact angles in a pipe bend ( r B/ D n=5; D n=80 mm) which lie between 5 and 35°. Consequently, the tangential impact velocity components ranging from 33 to 44 m/s are considerably higher than the normal ones (5–25 m/s). These results were at first endorsed by observations made in experiments to determine the material function. The relative attrition behavior of four different polypropylenes (PP) under pure sliding friction conditions closely resembles that observed in attrition experiments carried out in a pipe bend of the above geometry, while differences are observed for normal impact conditions. But further experiments with polymethylmethacrylates (PMMA) and polystyrenes (PS) showed that here the attrition behavior encountered under normal impact conditions is similar to that observed in the pipe bend. With dynamic mechanical analysis (DMA) it was possible to show that PP on one hand and PMMA and PS on the other are attrited by different mechanisms and that the glass transition temperature of the polymers is a key factor in the determination of the prevailing attrition mechanism.

Breakage of sodium benzoate granules under repeated impact conditions

In many industries, handling or processing of relatively fragile particles takes place and predictions are required whether a significant proportion of the particles is damaged. Particles need to show good resistance against static and dynamic loads. The present paper deals with the study of particle resistance to dynamic loads (mainly the attrition behaviour) using a new instrument developed at the Delft University of Technology. The attrition test involves bombarding the particles against a flat target repeatedly, using a crank-slide mechanism. The new equipment generates extremely large number of collisions for each particle, resulting in reproducible generation of damage. The velocity is controlled by changing the frequency and amplitude of oscillation. The particles encounter many collisions on each spot on the particle surface during the test operation. The change in the particle size distribution at different intervals gives a measure of the damage caused to the particles. The damage has been characterised by estimating the specific rate of breakage, which is correlated to the frequency and amplitude of oscillation.