Image processing of computed tomography scanned poly-dispersed beds for computational fluid dynamic studies

Achieving our emission reduction goals requires the bulk production of carbon-neutral fuels and chemicals, which are catalytically produced through heterogeneous fixed bed chemical reactors. To optimise and scale-up these reactors, accurate and validated Computational Fluid Dynamics (CFD) models are crucial. Of especial importance to CFD simulations is the accurate depiction of the 3D bed structure used during the experimental setup. A direct one-to-one coupling between experiments and simulations can be achieved by scanning the experimental bed using computed tomography and reconstructing the scanned images as a 3D geometry for CFD simulations. However, processing of the scanned images is necessary to minimise highly coarse features that could impact the overall mesh size. A highly poly-dispersed lab-scale fixed bed reactor, previously scanned and analysed, is processed using various image-processing operations. Depending on the number and the crudeness of the processing operations, the bed is progressively deformed, which impacts both its porosity and its interparticle pore connectivity. The impact of image-processing becomes more evident when the hydrodynamic behaviour, i.e., X-, Y-, and Z-velocity and static pressure, of the beds is explored. CFD simulations revealed highly heterogeneous flow profiles, with the maximum velocity reached being 16-times higher than the average superficial velocity within the bed. Moreover, small modifications in local topological features introduce significant changes to the flow profiles, while the 3D pore interconnectivity was seen to play an equally important role as the interparticle porosity. A particle size study revealed that large particles form less interconnected networks with higher pore volumes, which significantly reduce the flow velocity and the pressure drop experienced by the flow. The generated results yield key insights towards a deeper understanding of the behaviour of fixed bed chemical reactors, highly valuable for catalyst and reactor engineering.     

Mechanochemical synthesis of nanoparticles

The results of recent investigation of the mechanochemical synthesis of inorganic nanoparticles are reviewed. It was demonstrated that, by selecting suitable chemical reaction paths, stoichiometry of starting materials and milling conditions, mechanochemical processing can be used to synthesise a wide range of nanocrystalline particles dispersed within a soluble salt matrix. Selective removal of the matrix phase by washing the resulting powder with appropriate solvents can yield nanoparticles of the desired phase. This technique has been shown to have advantages over other methods of producing nanoparticles in terms of low cost, small particle sizes, low agglomeration, narrow size distributions and uniformity of crystal structure and morphology.     

Bed expansion ratio of mono-sized and binary mixtures in fluidized,spouted, and spout-fluid beds

Studies on bed expansion ratio were carried out in fluidized, spouted, and spout-fluid beds. A single column has been used to compare the characteristics of fluidized, spouted, and spout-fluid beds. Experiments were carried out using air and glass beads under fluidized, spouted, and spout-fluid bed conditions separately to study the effect of gas velocity, bed mass, and particle size on bed expansion ratio. Glass beads of different sizes (0.75, 1.2, 1.7, and 3.075?mm) have been used as solid bed material. Bed expansion ratio was determined for mono-size particles and binary mixtures (different diameter ratios and composition). It was found that the bed expansion ratio decreases with increase in bed mass for only spouting condition and spout-fluidization conditions. The bed expansion ratio increases with increase in bed mass for only fluidization condition.     

Effect of fluidised bed parameters on quenching of steel sections

Abstract

The effect of fluidised bed parameters on quenching efficiency was examined on two steels for various section sizes. Heat transfer coefficients were determined and the microstructure of some sections was examined. It was found that the heat removal rate by the fluidised bed lies between those of air and oil quenching and a martensitic structure was observed in quenched sections. The fluidising velocity affects greatly the heat transfer coefficient h between the bed and the cooled section. The value of h increases with the velocity to a maximum and then decreases. Higher values of h are obtained in beds of fine particles and wide particle size distribution. The bed/cooled section diameter ratio has an important effect on the heat removal rate. The treated section should be immersed well below the bed surface.

MST/658     

The yield stress of jammed magnetofluidized beds

By means of a magnetic field externally imposed, fluidized beds of magnetizable particles may experience a transition from a fluidlike unstable to a solidlike stable state. In our work, measurements have been taken of the gas velocity and particle volume fraction at the jamming transition as well as of the tensile yield stress of the stabilized bed subjected to a small consolidation. The influence of diverse physical parameters such as initialization mode, magnetic field orientation, average particle size and size polydispersion, are analyzed. Noninvasive visualization of the bed structure has revealed that magnetic stabilization is determined by the formation of particle chains. Due to the enhancement of the interparticle attractive force with field strength and particle size, the transition to stability takes place at higher gas velocities as the magnitude of these parameters is increased. The magnetic yield stress of magnetofluidized beds of naturally aggregated particles because of a large presence of fines is significantly larger than that corresponding to naturally nonaggregated particles. Moreover, the jamming transition occurs at larger gas velocities (or smaller field strengths) in the former case since the agglomerates behave magnetically as large effective particles. The effect of the magnetic field on the yield stress ia only relevant when it is applied during initialization of the bed in the bubbling regime and particles are free to move and restructure in chains. Measurements of the yield stress are presented when the applied magnetic field is oriented either in the vertical or horizontal direction (B co-flow and B cross-flow modes, respectively). The variation of the magnetic yield stress with particle size was found to be dependent on the field direction. This can be justified by the dependence of the interparticle magnetic force on the chain average angle with the field, which is affected by particle size as the stabilized bed is subjected to small consolidations.     

Bed expansion and fluctuation in cylindrical gas solid fluidized beds with stirred promoters

A profound and fundamental understanding of bed dynamics such as bed expansion ratio and bed fluctuation ratio of irregular particles of binary mixtures has been made in a cylindrical fluidized column for gas solid systems, resulting in a predictive model for fluidized beds. In the present work attempt has been made to study the effect of various system parameters (viz. rotational speed of the promoter, initial static bed height, superficial velocity of the fluidizing medium, particle size and density) on the bed dynamics through experimentation. The correlations for the bed dynamics have been developed on the basis of dimensional analysis. It was observed that the calculated values of bed dynamics agree well with the experimental values in most of the cases.     

Towards the large-scale synthesis of carbon nanotubes in fluidised beds

Carbon nanotubes (CNTs) are a form of crystalline carbon with extraordinary properties, making them valuable in a broad range of applications. However, the lack of suitable large-scale manufacturing techniques, which we define as being of the order 10000 tonnes per annum, continues to inhibit their widespread use. Of the three established synthesis methods for CNTs: (i) chemical vapour deposition (CVD), (ii) laser ablation, and (iii) arc discharge, CVD techniques show the greatest promise for economically viable, large-scale synthesis. In particular, the fluidised bed CVD (FBCVD) technique, where the CVD reaction occurs within a fluidised bed of catalyst particles, has the potential to produce high quality CNTs, inexpensively, in large quantities. In this work we report on the development of a catalytic chemical vapour deposition process, using batch fluidised bed reactors, for the synthesis of straight and spiral carbon nanotubes at pilot scale (up to 1 kg/hr). We believe this to be the first report of the synthesis of spiral carbon nanotubes using fluidised bed CCVD. Iron, nickel and cobalt transition metal catalysts supported on non-porous alumina substrates were fluidised in a mixture of nitrogen, hydrogen and ethylene at temperatures between 550 and 800 degrees C for between 15 and 90 minutes. Nanotube yield was inferred from thermogravimetric analysis and the quality and size of the CNTs from transmission electron microscopy. Conflicting information in the literature about the influence of synthesis parameters on CNT properties suggests that further investigation is necessary to understand the synthesis process at a fundamental level, i.e., independent of reactor design and operation.     

Rate of CO2 adsorbent attrition induced by gas jets on perforated plate distributors in bubbling fluidized beds

Particle attrition is a major challenge when handling bulk solid materials with fluidized beds due to its ability to cause particle loss. Herein, the particle attrition induced by the gas jets on a perforated plate distributor in a bubbling fluidized bed was investigated for CO₂ adsorbent particles. An attrition tube, which used air as the fluidizing gas, was used as the fluidized bed. At a constant fluidizing velocity, the initial static bed height and orifice gas velocity were considered as variables. It was confirmed that abrasion dominated the particle attrition. The trend indicating the change in the maximum size of the particles (d_pm,a) formed by attrition followed that of the attrition rate (i.e., the formation rate of fine particles via attrition). A new stirring factor that combined the model developed by Werther and Xi with the original stirring factor adequately explained the effect of the static bed height on both the attrition rate and d_pm,a when the initial static bed height was greater than the length of the orifice gas jet that penetrated the bed. The attrition rate increased linearly with the new stirring factor. However, d_pm,a increased exponentially with the new stirring factor. Relationships were successfully proposed to enable the estimation of the attrition rate and d_pm,a for the CO₂ adsorbent particles. This study provided the evidence indicating the significance of the effect of bed height on particle attrition induced by the gas jet on the distributor. Moreover, proper models for correlating the attrition rate and the maximum size of the fine particles formed by attrition in the bubbling fluidized bed were provided.     

An Introduction to Generalized Linear Models

Particle size is commonly used to determine quality and predict performance of particle systems. We consider particle size distributions inferred from a material sample using a fixed number of sieves with progressively smaller size openings, where the weight of the particles in each size interval is measured. In this article, we propose Bayes analyses for data from particle sieving studies based on parsimoniously parameterized multivariate normal approximate models for vectors of log weight fraction ratios. Additionally, we observe that the basic approach extends directly to modeling mixture contexts, which provides model flexibility and is a very natural extension when physical mixtures of materials with fundamentally different particle sizes are encountered. We also consider hierarchical modeling, where a single process produces lots of particles and the data available are (replicated) weight fraction vectors from different lots. Supplementary materials for this article are available online.     

Particle scale study on heat transfer of gas–solid spout fluidized bed with hot gas injection

ABSTRACT

In this paper, the heat transfer characteristics of a 2D gas–solid spout fluidized bed with a hot gas jet are investigated using computational fluid dynamics-discrete element method. The initial temperature of the background gas and particles in the spouted bed was set to 300?K. The particle temperature distribution after injection of 500?K gas from the bottom, center of the bed, is presented. The simulation results indicate well heat transfer behavior in the bed. Then, statistical analysis is conducted to investigate the influence of inlet gas velocity and particle thermal conductivity on the heat transfer at particle scale in detail. The results indicate that the particle mean temperature and convective heat transfer coefficient (HTC) linearly increase with the increase in inlet gas velocity, while the conductive HTC and the uniformity of particle temperature distribution are dominated by the particle thermal conductivity. The conductive and convective heat transfer play different roles in the spout fluidized bed. These results should be useful for the further research in such flow pattern and the optimization of operating such spouted fluidized beds.     

Incipient motion of gravel and coal beds

An experimental study on incipient motion of gravel and coal beds under unidirectional steady-uniform flow is presented. Experiments were carried out in a flume with various sizes of gravel and coal samples. The critical bed shear stresses for the experimental runs determined using side-wall correction show considerable disagreement with the standard curves. The characteristic parameters affecting the incipient motion of particles in rough-turbulent regime, identified based on physical reasoning and dimensional analysis, are the Shields parameter, particle Froude number, non-dimensional particle diameter and non-dimensional flow depth. Equations of critical bed shear stress for the initial movement of gravel and coal beds were obtained using experimental data. The method of application of critical bed shear stress equations is also mentioned.     

Effect of particle size on gas holdup in three-phase reactors

Gas holdups were measured in a batch three-phase cocurrent column in which glass beads ranging from 17 to 5000μm were suspended up to 20 vol %. The effect of particle size on gas holdup was found to be different in three types of reactors such as the gas-sparged slurry reactors, three-phase bubble columns and three-phase fluidized beds. An increase of particle size reduced gas holdup in three-phase bubble columns, while raising it in gas-sparged slurry reactors and three-phase fluidized beds. The maximum and minimum gas holdup were observed respectively for particle size of about 88–250μm and 500μm, but the values of particle size were dependent on solid content and gas velocity. This paper is dedicated to Dr L K Doraiswamy on his sixtieth birthday.     

Detecting Sudden Changes in Fluidization by Wall Vibration

Vibrations of fluidized bed walls reflect the nonlinear characteristics of bed hydrodynamics in gas–solid fluidized beds. Experiments were carried out in a lab-scale, two-dimensional fluidized bed operated at ambient conditions for three particle sizes, various gas velocities, three aspect ratios, and different probe locations. The S-statistic method, which is, in fact, the comparison of attractors of two dynamic signals in the state space, was used to determine de-fluidization condition in the bed. Different scenarios were tested to evaluate whether this method is able to detect changes in the hydrodynamics fluidized bed based on the bed vibrations, including change in the bed mass, particle size, and gas velocity. The results showed that this method is capable of detecting the de-fluidization state in the bed as a result of changes in gas velocity, particle size, and bed mass. However, an important factor is the location of probe, which can dramatically affect the capability of this method for detecting the de-fluidization state.     

Electrical resistance of a fluidized bed of graphite particles

Experimental data on the electrical resistivity of fluidized beds of graphite particles are given. The dependence of the resistivity on the filtration velocity, particle size, temperature of the bed, and position of the electrodes is determined.     

Preparation and evaluation of high drug content particles

To determine how to prepare high drug content particles using a Wurster fluidized bed to determine realizing the miniaturization of solid dosage forms, aspirin was selected as the model drug and granulated without any additive. In this study, the emphasis was on evaluating the key operation factors of airflow rate and atomizing flow volume. The properties of the resulting particles, such as the average diameter, particle strength, appearance, and compressibility using different airflow rates and atomizing flow volumes, were investigated. Furthermore, detailed optimization of the operation conditions was conducted by artificial neural network (ANN) analysis. The relationship between the controlling factors (powder supplied, concentration of spray liquid, the amount of consumed spray liquid, and spray rate) and the response variables (product yield, median diameter, angle of repose, and degradation of aspirin) was investigated after evaluating the airflow rate and atomizing flow volume effects. The resulting granules under optimum operation conditions showed excellent physicochemical properties such as particle size uniformity, flowability, and compressibility.     

Comparative discrete element modelling of a vibratory sieving process with spherical and rounded polyhedron particles

Current spherical particle usage in discrete element modelling (DEM) is not able to accurately reflect the particle shape effect in some specific industrial applications. This study specifically investigated the effect of particle shape in discrete element modelling of a vibratory sieving process, with the focus on comparing results from spherical and non-spherical modelling methods. The particle size distribution of an iron ore material was initially obtained experimentally through vibratory sieving tests. An identical process was replicated in DEM with both spheres and non-spherical particles, and resulting particle size distributions were subsequently compared against the experimental results. A rounded polyhedron shape was utilised to calibrate and generate non-spherical particles based on a 2D particle shape characterisation process. Modelling results suggested that the rounded polyhedron method was able to accurately reflect the particle-sieve contacts without excessive rolling resistance tuning, which was required by the spheres.     

Characterization and photo-chemical applications of nano-ZnO prepared by wet chemical and thermal decomposition methods

Nano-crystalline ZnO particles were synthesized using two different routes: soft-wet and dry methods. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to identify the particles structures and morphologies, while X-ray diffraction (XRD) was used for verifying the particles crystal structure. The thermal stabilities of the particles were examined through thermal gravimetric analysis technique and their surface areas were calculated using BET method. Moreover, the photocatalytic activities were evaluated using UV–vis spectroscopy and photoluminescence (PL) characterization. The results showed that all the prepared ZnO samples possess a hexagonal wurtzite structure with high purity. Different particle sizes and morphologies of spheres, rods and wires were obtained depending on the preparation method used. Particle sizes obtained by the dry method are smaller than that found by the wet chemical method. The effects of both particle size and morphology on each of surface as well as optical properties, photocatalytic activity, dye/ZnO solar cell efficiency and biological activity have been studied and discussed.     

BUBBLE CHARACTERISTICS IN THREE PHASE FLUIDIZED BEDS OF FLOATING BUBBLE BREAKER

The local bubble phase holdup and vertical bubble length in three phase fluidized beds and the beds of floating bubble breaker have been studied in a 15.2 cm-ID pyrex glass column

The effects of liquid velocity (1-9 cm/s), gas velocity (2-12 cm/s), particle size (1-6 mm) and the volume ratio of the floating bubble breaker to solid particles (0-30 %) on bubble properties at the different bed heights have been determined

The bubble phase holdup increased with gas velocity, volume ratio of floating bubble breaker to solids and particle size along the bed height but decreased with liquid velocity. The vertical bubble length increased with gas velocity along the bed height but decreased with liquid velocity, particle size and the volume ratio of floating bubble breaker to solid particles

The local bubble phase holdup and the vertical bubble length have been correlated with the experimental variables as well as dimension less groups of Froude and Weber numbers.     

Evolution of mesoscale structure in fluidized beds with non-spherical dry and wet particles

Fluidized beds with non-spherical dry and wet particles are widely used in industrial processes, and the mesoscale structure in the bed has an important influence. In this study, CFD-DEM simulations are performed to evaluate the flow behaviors and mesoscale structure in fluidized beds with non-spherical dry and wet particles. The accuracy of the model is validated by comparison with the results of the particle image velocimetry experiment. The force distributions at bubble boundaries are analyzed to explain the influence mechanism of different shapes of bubbles in non-spherical dry and wet particle systems. The factor analysis indicates the interaction of particle shape and viscous liquid on the translational and rotational kinetic energy of particles. When the bed height is low, as the particle aspect ratio increases, the bubble equivalent diameter gradually increases. In addition, as the liquid viscosity increases, the particle and bubble granular temperature gradually decrease, indicating the reduction of particle velocity fluctuate and the decrease of turbulent kinetic energy of bubble. These findings have guiding significance for the fluidization of non-spherical dry and wet particles and can be used to optimize related industrial processes.     

Simulation of particle bed breakage by slow compression and impact using a DEM particle replacement model

The present work introduces a particle replacement model implemented in the commercial software EDEM to describe breakage of particles. Several model parameters were initially estimated on the basis of single-particle breakage tests on iron ore pellets. The model was then used to simulate breakage of particle beds by both slow compression and impact. Model predictions were compared to experiments in terms of compressive force versus packing density, breakage probability of the particles versus compressive force applied to the bed, and the product size distribution in compression and impact. The model showed the expected trends as well as some agreement with the measured product size distributions both from confined and unconfined stressing conditions of the bed of particles, being a simple and effective approach to describe breakage in systems where particles are stressed as assemblies.