Attrition strength of different coated agglomerates

The goal of this paper is to test the strength against attrition of coated particles. What is the influence of different coating materials on different particles? To answer this question, two different core materials are coated with four different water-soluble polymers. The core materials are micro-crystalline cellulose (MCC) and sodium benzoate granules in the size range between 850 and . The coated granules are attrition tested in the repeated impact tester (RIT). During attrition testing the uncoated MCC granules do not show any attrition. When the MCC granules are coated, all the attrition observed is due to the failure of the coating. When the sodium benzoate (Purox-S) granules are attrition tested, significant attrition is observed. Coating the sodium benzoate granules with the polymers increases the strength against attrition. During attrition three typical failure modes are distinguished viz. the peeling mechanism, the erosion mechanism and the layer fatigue mechanism. For the four different coatings different attrition mechanisms are observed indicating that the polymer mechanical properties play an important role. For example, a higher molecular mass for poly(ethylene glycol) increases the mechanical stability of the granules. It is now important to study the mechanical polymer properties in more detail, to explain further the observed failure modes. With the RIT it is possible to select the optimal settings in the coating process, to optimize the coating material or composition and finally to optimize the coating thickness.     

Breakage patterns of agglomerates

The experimental information available in the literature regarding the patterns of breakage of agglomerate materials is scarce, particularly in dynamic loading. The primary objective of this paper is to present our findings on the breakage patterns of the agglomerates and the interparticle bond. A high-speed digital video imaging technique is used here to gain an insight into the impact behaviour of individual agglomerates against a target plate. Several breakage patterns are observed. Agglomerates may suffer localised damage only, with the disintegration of the damaged zone into very fine debris, or localised damage combined with fracture. The frequency of occurrence of these patterns depends on the impact velocity and agglomerate structure. The pattern of breakage affects significantly the size distribution of the impact product. An investigation of the breakage of individual interparticle bonds is also presented. Two forms of failure are observed, internal (cohesive) and interfacial (adhesive) failure. The morphology of the fractured surface depends greatly on the type of breakage. Internal breakage shows irregular surfaces due to crack jumping, whereas interfacial failure produces clean, smooth fracture surfaces. These observations should provide the necessary foundation for the development of a fundamental model of agglomerate breakage.     

Tensile strength of agglomerates

Firstly, this paper gives a review of tensile strength of agglomerates; i.e. a discussion of the most important theoretical considerations, the various methods of measurement, a comparison of measuring techniques by experimental results and a brief look at deformation behaviour under unidirectional loading. In the second part new developments are dealt with.For the tensile strength of agglomerates in which forces are transmitted at points of contact, a new relationship is shown in which the displacements due to individual forces are taken into consideration. Experiments in which not only the tensile strength but also the force distribution are measured demonstrate the necessity of the new approach when short-range forces are present. However, the force—displacement characteristics of the attractive forces need not to be taken into account when long-range forces — e.g forces due to suitability large liquid bridges — act. The attractive forces may then be linearly superposed.The measurement of the tensile strength of agglomerates is discussed in detail. In particular the problematic nature of measurements using the split plate apparatus is pointed out, and a new method of interpreting results is suggested. The applicability of this method is tested by comparable experiments.     

Porosity measurement of fragile agglomerates

Agglomeration processes in the food and pharmaceutical industries frequently produce porous, brittle agglomerates intended for redispersion in liquids (a.k.a. “instant powders”). Typical products have a porosity ranging from =0.4 to =0.8 and a particle size between 0.2 and 2 mm. The agglomerate porosity is an important product property, as it has a major influence on the wetting behaviour. Due to their high porosity and comparatively weak internal bondings, instant agglomerates are fragile.

Compared to the wide range of currently available particle sizing methods, few porosity-measuring methods exist, most of which are not applicable to instant agglomerates. For controlling the porosity of instant agglomerates produced in our laboratory, we used individual particle weighing and sizing. A balance with 1 μg of precision allowed reliable weighing of individual agglomerates as small as 300 μm diameter. The volume of the particles was determined using a long-distance microscope with CCD camera attached to a computer running public domain image analysis software.

Porosity measurements of agglomerated skim and whole milk powder, agglomerated pectin, agglomerated flour/maltodextrin mixture, and agglomerated rice starch/maltodextrin mixtures were carried out. The results of our measurements show, for the majority of the samples, an approximately constant porosity over a wide particle size range, with a tendency towards lower porosity for smaller particles.     

Attrition of high-density polyethylenes

An annular shear cell has been deployed to explore the influence of polymer structure on bulk attrition for various grades of high-density polyethylene, these grades differing in molecular weight and degree of chain branching. Decreased bulk attrition of coarse material was found, provided that the molecular weight was high. Polymer branching also served to reduce bulk attrition at low shear strain. The process of attrition caused fine products to be pressed into the clefts of large particles in addition to the general formation of free fine materials. Particles of intermediate size were made by fragmentation. The kinetics of attrition were not first order; an empirical formulation due to Gwyn was more satisfactory.     

Hydrodesulphurization of char

Treatment of coal char with acid removes ash constituents, primarily calcium and iron compounds. Removal of these constituents reduces much of the back reaction of char with hydrogen sulphide, produced during the treatment of char with hydrogen at elevated temperatures, thus reducing the hydrogen-to-char ratio needed to effect desulphurization. The combination of an acid wash and the use of a char prepared by flash pyrolysis gives a hydrodesulphurization reaction which has a favourable ‘equilibrium’ and fast kinetics¹.     

Co-gasification behavior of meat and bone meal char and coal char

The co-gasification behavior of meat and bone meal (MBM) char and two types of coal (Jincheng anthracite (JC) and Huolinhe lignite (HLH)) char was investigated using a thermogravimetric analyzer (TGA). The effects of coal type, mineral matter in MBM, gasification temperatures and contacting conditions between MBM char and coal char on the gasification behavior were studied. The results show that the gasification behavior of MBM char and HLH char can be well described by ash diffusion controlled shrinking core model, while that of JC char can be described by chemical reaction controlled shrinking core model. The co-gasification rate of MBM/JC chars at 950 °C is approximately 1.5 times faster than that calculated from independent behavior. The mineral matter in MBM may play as a catalyst during co-gasification. However, the analogous effect observed in the blends of HLH/MBM chars is smaller, suggesting that the coal types play a great role. Furthermore, as the gasification temperature increased from 850 to 1000 °C, the maximum synergistic effect is observed at 900 °C. The lower temperature is not conducive to transferring the mineral matters of MBM to the coal char, while the higher temperature makes Na and Ca react with minerals of coal, leading to a loss of catalytic activity.     

Attrition mill operating characteristics

The attrition mill is a device for mechanically reducing solid particle size by intense agitation of a slurry of material being milled and coarse milling media. For example, in 10 hours of milling, specific surfaces of 40 and 25 m²/g were obtained for alumina and barite, corresponding to 38 and 56 nm equivalent spherical diameter, respectively. Size reduction rates for relatively coarse particles were first-order and increased linearly with power input to the mill. Optimum milling medium concentration corresponded to medium particles moving a distance of approximately 0.7 of their diameter before collision with another such particle. Power characteristics of the attrition mill were essentially the same as those of a radial flow turbine mixer. Laminar flow became disrupted at N_Re ≈ 200, while turbulent flow was established at N_Re > 8000. Slurries of fine powders exhibited the same linear power-average density dependence as single-phase liquids. However, a different dependence was observed with large particles.     

Studies in char gasification—IV: Multi-stage char gasifier

A detailed analysis of a gasifier with two and three stages is made. It is shown that a multi-stage gasifier is superior to a single stage gasifier from the point of view of net carbon conversion as well as methane mole fraction in the product gases.     

Characteristics of airborne fractal-like agglomerates of carbon nanotubes

Airborne carbon nanotubes (CNTs) pose a health threat at workplaces and play more and more important roles in toxicity studies, yet data and model on their detailed characteristics are lacking. Thus we investigated aerosolized multiwalled CNTs (MWCNTs) by tandem measurement of the mass and mobility size in order to characterize the MWCNT agglomerates. The results revealed a fractal-like relationship between the mass and mobility size in the range 50–500 nm indicating quite compact agglomerate structure, and an effective density in the range of 0.51–0.83 g/cm³. With the tube diameters and intrinsic density, the tube length in an agglomerate was determined reliably from the mass. We developed a model to compute the porosity and geometrical outer diameter of the agglomerates and derived the fractal relation between the mass and the outer diameter with a fractal dimension of 2.6, which agreed well with fractal dimensions of bulk CNT assemblies determined by other analytical methods. The effective density based on the outer diameter was in the range of 0.11–0.35 g/cm³ and decreased with the increasing agglomerate size. Electron macroscopic images of the aerosolized MWCNTs provided comparable data for the outer diameter and fractal dimension.     

酒糟焦与无烟煤焦共气化特性的研究

利用固定床反应器研究了酒糟焦与无烟煤焦在CO_2气氛下的共气化反应性。采用扫描电镜和X射线能谱分析手段,考察了焦样的孔隙结构和表面元素组成。结果表明:酒糟焦与无烟煤焦共气化过程中,除气化剂CO_2外,尾气主要产生H_2和CO;共气化过程中具有明显的协同作用,气化温度和酒糟焦掺混比是影响共气化反应的主要因素,共气化反应活性随气化温度的升高和酒糟焦掺混比的增加而增大;酒糟含有的高挥发分在气化过程中产生大量蜂窝状的孔隙结构,酒糟焦和无烟煤焦中分别含有的K元素和Ca元素对共气化反应起到的催化作用,是共气化反应活性提高的主要原因。     

Effect of char gasification reaction order on bounding solutions for char combustion

The rate of oxidation and surface temperature of a char particle during combustion is determined by the combined action of reactions of carbon with oxygen and CO₂ at the surface and carbon monoxide with O₂ in the gas phase. Limiting values for the rates and temperature were determined by Amundson et al. assuming that the CO₂ and CCO₂ reactions were first order with respect to the gas reactant concentration. Results are presented here for the cases in which the CO₂ reaction is zeroth order and the CCO₂ reaction is both first and zero order to bound the reaction orders found in practice. Allowance for zeroth order kinetics is found to have a profound effect on the shape of the bounding solutions for burning rates and temperatures, most notably by showing that diffusion-limited combustion can occur at much lower temperatur than would be predicted using first order kinetics.     

Deformation behaviour of agglomerates under tensile stress

A new instrument has been developed in which the stress—strain behaviour of agglomerates can be measured in uniaxial tension. The instrument permits tension to be measured under predetermined stresses, and further enables the tensile stress—strain behaviour to be determined in the agglomerate in the unstable region.Agglomerates with different bonding mechanisms were studied. In the case where agglomerates are held together by liquid bridges, a model has been developed which permits the stress—strain behaviour to be predicted. This is confirmed by experiments. For agglomerates with a high degree of liquid saturation, the tensile stress—strain behaviour can be predicted from the capillary pressure relation. The tensile stress—strain curves of dry agglomerates and of agglomerates with solid bridges between particles are shown and compared. From the different stress—strain characteristics, the physical nature of the bonds can be elucidated.     

Sintering behavior of bimodal iron nanopowder agglomerates

The most important issue in the processing of nanoscale metal powders is whether the metal nanopowder can be fully consolidated into ultra-fine- or nano-grained powder metallurgy parts by pressureless sintering. This paper focuses on the sintering behavior of bimodal iron (Fe) nanopowder agglomerates by considering their microstructure and densification kinetics. During the sintering, bimodal Fe nanopowder compacts underwent discontinuous shrinkage behavior until they neared full density. Three contributions to the sintering mechanisms, asymmetric sintering, densification enhancement, and grain growth inhibition, are presented in relation to the effect of bimodal nanopowder structure. Smaller nanoparticles in the bimodal nanopowders, which are predominantly present at the boundaries and interstitial spaces of larger nanoparticles, are responsible for the three mechanisms stated above. This result is strongly supported by the apparent activation energy values ranging from 48.2 to 90.6 kJ/mol, which correspond to the energy for grain-boundary diffusion in Fe. The experimental results of this study show that bimodal nanopowder agglomerates can be used to produce full density nano-grained powder metallurgical parts by pressureless sintering.     

Measurement and simulation of soot agglomerates and aggregates

Soot clusters produced by partial oxidation of acetylene show three distinct phases - primary particles, agglomerates of the primary particles, and aggregates of the agglomerates. The dominant mode of soot formation can be controlled by the concentration of acetylene. Comparison of optical microscopy with inertial measurements indicate that the aggregates are not fractal (f_HB = 3) but porous with a density of 0.094. Those simulations for the smallest aerodynamic diameter suggest a transition from aggregates which are fractal to agglomerates. Algorithms based on generalized Menger sponges are developed to simulate the transition from agglomerates to aggregates.     

The strength of agglomerates bound by salt bridges

The correlation between the drying behavior and the tensile strength of agglomerates bound by salt bridges in the dry state was investigated. The pellets examined had been prepared by compacting limestone powders, which were premoistened with a salt solution, in a manually operated die press. The percentage liquid saturation of the moist agglomerate, the concentration of the salt solution and the drying temperature were altered. The influence of the surface incrustation on the drying process could be examined by measuring the temperatures within the cylindrical pellet at different radial positions during drying and by determining the drying rate curves. The tensile strength of the dried pellets, which was usually measured after removing the salt crust, depends on the quantity and the local distribution of the crystallized salt, and the strength of the salt bridges. The latter depends on the crystal structure which is influenced by the drying rate.     

超细颗粒聚团流化的临界流化速度

在内径50 mm的流化床实验台上,测量SiO₂、Al₂O₃和TiO₂ 3种超细颗粒原生粒径从30 nm增加到5 μm的临界流化速度（U_mf）,并以Geldart A类颗粒（粒径45 μm）为参照。结果表明：3种超细颗粒的U_mf随粒径的变化规律一致,随原生粒径从30 nm增加到5 μm,U_mf逐渐增大;当颗粒粒径增加到45 μm,U_mf大幅度减小,其与原生粒径为30和200 nm时接近。对于不同材料,U_mf由大至小的顺序依次为TiO₂、Al₂O₃、SiO₂。粉体安息角测量表明：对于同种材料颗粒,原生粒径对超细颗粒的U_mf和安息角的影响规律一致,即5 μm超细颗粒的安息角最大。聚团尺寸模型计算表明：稳定流化时,聚团尺寸随原生粒径的变化趋势以及随不同材料的变化趋势均与U_mf的变化趋势一致。研究结果为超细颗粒流化临界速度预测研究奠定了基础。