Bed Of Particles Research Articles

A comparative study in 3D of bed particle layer characteristics in quartz and K-feldspar from fluidized bed combustion of woody biomass using X-ray microtomography

Bed particle layer and crack layer characteristics at different ages were studied for quartz and K-feldspar bed particles from a 30 MWth bubbling fluidized bed and a 90 MWth circulating fluidized bed, both using woody biomass as fuel. X-ray microtomography (XMT) was utilized to determine the bed particle layer distribution on the bed particles' surface. For each bed particle type, the average bed particle layer thickness as well as average volume fractions of the bed particle layer and crack layers to the entire bed particle volume were determined at three different bed particle ages by utilizing XMT analysis. Comparison of the two different bed particle types showed that K-feldspar retains a thinner bed particle layer in both conversion processes compared to quartz. Crack layers were observed extensively in quartz bed particles to the extent of 19.3 vol% and 32.1 vol% after 13 days in the BFB and the CFB, respectively, which could cause deposition of the bed particle fragments. On the contrary, K-feldspar has almost no tendency toward forming crack layers.

充填層の毛管力を利用したメタノールからの合成ガスの生成

Passive production of synthesis gas from liquid methanol using a capillary action in a packed bed of porous particles supporting a catalyst is investigated. Heating of an upper side of the packed tube while the bottom is immersed in liquid methanol causes upward fluid flow due to capillary action enhanced by evaporation and produces the synthesis gas in the high temperature region. For an increased diameter of the bed, the partial invasion of wetted region into the core of the dried region may cause a significant reduction in reaction yield. The radial distribution of temperature and its effect on the reaction are identified from the experiment. Un-uniformity caused in the radial direction is discussed based on the physical modeling of the capillary liquid flow toward the heated wall.

Effect of temperature on the minimum spouting velocity of heavy particles in conical spouted bed used for nuclear fuel coating

The minimum spouting velocity (Ums) of heavy particles in conical spouted bed at high temperatures are critical importance for nuclear fuel coating, which are different from that at room temperature. However, current researches on the Ums were concentrated on the Cold Mockup spouted bed. There were few studies about the Ums of heavy particles in conical spouted bed at high temperatures. In this study, the effect of temperature (373–1273 K) on the Ums is systematically investigated. A new Ums correlation with temperature, cone angle, static bed height, particle diameter and particle density is obtained:Ums=3.30×108∙dp1.63∙ρp0.57∙tanγ21.18∙H0Dc1.45∙T-1.44. The equation extends the application of the Ums at high temperatures and can be applied to determine the Ums value well under different temperature. Besides, the influence mechanism of temperature, cone angle, static bed height, particle diameter and particle density on the Ums has been discussed. This correlation is recommended for the determination of the Ums well at high temperatures and thus provides significant reference for producing coated fuel particles of high quality.

Residence Time Distribution Analysis of Drip-Irrigated Beds—The Effect of Material and Fluid Properties with Implications for Heap Leaching Practice

The quantitative effect of particle shape, porosity, wettability, particle size, and solution viscosity on the residence time distribution (RTD) profiles of non-reactive, steady-state, drip-irrigated ore beds characteristic of heap leaching systems is presented. Results were obtained using step-up tracer tests and allowed for the analysis of preferential flow behaviour within the systems. The key findings were as follows. Increased particle sphericity enhanced channelling in beds of smaller particles, but not for larger particle sizes. Higher particle wettability caused greater liquid dispersion during both initial wetting studies and at steady-state fluid flux. Higher porosity levels and the inclusion of fines in mixed sized beds resulted in longer average solute residence times, higher liquid hold-up, longer solution and tracer breakthrough times, and increases in drain-down moisture percentages. Increasing the irrigation fluid’s viscosity, reflective of the increase in ionic concentrations in leach solutions, reduced both the solution and tracer breakthrough times and increased dispersion with signs of more discontinuous or isolated fluid volumes at steady-state. These results highlighted the importance of the inclusion of fines in agglomerated beds to improve uniform wetting especially those with low to moderate particle porosities (<2.5 m2/g specific surface area). The viscosity results suggest that there may be changes in preferential flow extent, due to variations in viscosity owing to the increasing sulphate concentration within the liquid phase in heaps and with time.

Selection of Relative DEM Time Step for Modelling Fast Fluidized Bed of A-Type FCC Particles

In chemical production processes, the most suitable operation regime for A-Type powders such as typical FCC particles is high-speed fast fluidization owing to their uniquely advantageous properties. Discrete element method (DEM) for modelling fast fluidization with A-Type powders has rarely been reported. How to appropriately select the DEM time step and the stiffness coefficient is one of the most critical problems for stable and accurate calculation. This article mainly discusses the effect of the stiffness coefficient and DEM time step on simulations of A-type FCC particles. In order to describe the effect of both parameters and their complex interaction, a dimensionless relative DEM time step is introduced. In total, nine cases with different numbers of relative time steps are adopted for modelling a microfluidized bed of A-Type FCC particles, the regime of which is proved to be fast fluidization. Results show that three bifurcations occur in all the simulations. Only the moderate relative time step possesses the capability of modelling the process of particle collision and thus predicts the right flow regime with asymmetric and heterogeneous typical fast fluidized structures. When the relative time step increases to large rank, simulations predict untrue fluidization regimes with symmetric and homogeneous structures. Moreover, both over-large and over-low relative time steps cause excessive particle overlap and thus a divergence of simulation. The further optimization of moderate relative DEM time step in relation to real particle property is unidentifiable and is thus an outstanding issue. That the range of the moderate relative time step is limited indicates that the common soft-sphere model is poor at modelling fast fluidization of A-Type particles. It is suggested that possible future work should be focused on improving the simulation frame and employing the molecular dynamics model to more appropriately deal with particle contact.

Assessment of Shaft Gas Injection Technology for Blast Furnace Ironmaking

Shaft injection of reducing gas into ironmaking blast furnaces (BF) helps mitigate BF carbon footprint. However, its effectiveness relies on the interaction between shaft-injected gas (SIG) and other phases. This paper numerically studies shaft gas injection operations based on a 380-m3 industrial BF. A recently developed three-dimensional process model has been adopted to do so. This model is extended to track SIG and hearth-generated gas (HGG), define SIG penetration, and quantify the respective contributions of the two gases to BF performance. After validation, the model is applied to study the effects of three variables related to SIG penetration into the particle bed: SIG flow rate, shaft tuyere number, and horizontal cavity depth. The detailed analysis of flow and thermochemical behaviors shows that increasing the SIG flow rate increases the gas penetration and indirect reduction rate by SIG and lifts the cohesive zone. However, it impedes the indirect reduction reaction by HGG and increases the bed pressure. These effects collectively identify an optimum SIG flow rate. In addition, as the shaft tuyere number or horizontal cavity changes, the SIG penetration zone size changes oppositely in radial and circumferential directions, leading to similar total penetration zone sizes. Consequently, these two variables can limitedly improve BF performance, especially compared with the effect of SIG flow rate. The results suggest that the model offers convenience for exploring the shaft gas injection technology.

CFD-DEM study on the interaction between triboelectric charging and fluidization of particles in gas-solid fluidized beds

The triboelectric charging of particles has a substantial effect on the fluidization process and flow dynamics. Conversely, the fluidization state has an impact on the charge transfer of particles. In this study, we added models to the Computational Fluid Dynamic coupled Discrete Element Method (CFD-DEM) that could describe particle triboelectric charging and electrostatic interactions. According to numerical simulations, it was found for the first time that the behavior of bipolar charged particles undergoes a chaining stage and an agglomerating stage as the particle size decreases, where the interactions between triboelectric charging and fluidization are different. The effect of the wall is positively correlated to the ratio of the particle-wall collision rate to the total particle collision rate, and the fluidization conditions of particles affect the charging process of particles via this correlation. This work contributes to further understanding of the interaction between triboelectric charging and fluidization of particles and helps to optimize fluidized bed design and fluidization conditions.

FLUID DYNAMIC STUDY USING RTD IN A PACKED REACTOR AND EVALUATING FLOW PATTERN MODELS ON THE CHEMICAL CONVERSION. AN APPROACH TO REAL BEHAVIOR

In this work was done a fluid dynamic study in an experimental packed reactor using RTD exposing bimodal and modal behavior when it was varied the particle diameter and bed presenting flow channels, dead zones in the vessel and lateness of the material mixture in the equipment. Additionally, using flow pattern models was estimated the chemical conversion with greater approximation to the experimental value. This result could be use in the scaling stages of the process and minimize overdesigns.

Effect of inlet gas velocity on gas-solid fluidization characteristics in fluidized bed

Abstract In this article, the Eulerian–Eulerian TFM model is used to simulate the fluidization of the synthesis process of organosilicon monomers. A new method for analyzing the gas-solid fluidization characteristics is proposed by combining the gas-solid two-phase flow evolution formula with the parameters such as particle concentration and bed voidage. On this basis, the fluidization characteristics of silicon powder particles at constant velocity and variable velocity are compared, and the fluidization characteristics of silicon powder particles with different particle sizes under five sets of variable velocity are discussed. The simulation results show that compared with constant velocity, the mean bed voidage is 0.55 when silicon particles adopt variable velocity, which can not only keep silicon particles fully fluidized but also improve the problem of poor gas-solid contact. For silicon particles with particle diameters of 300.1–515 μm, variable velocity fluidization has the advantages of uniform bed distribution and sufficient gas-solid fluidization. In the five groups of variable velocity function, when the inlet gas velocity and time are the quadratic functions of the opening upward, the fluctuation of pressure fluctuation is small, and the maximum fluctuation range of particle solid phase distribution is only 0.13, indicating that the heat and mass transfer efficiency between silicon particles is better, the gas-solid mixing is sufficient, and the gas-solid fluidization quality is better.

Hydrodynamics and mass transfer around active particles in dense gas-fluidized beds

The hydrodynamics of bubbling fluidized beds of Geldart B granular solids operated at ambient temperature and at 500 °C are investigated by uncooled capacitance probes at different gas superficial velocities. Statistical analysis of the time-series of the local bed voidage generates Probability Density Functions (PDF) that display multimodal patterns. A remarkable bimodal character is apparent in the part of the PDF that refers to the emulsion phase when the gas superficial velocity is increased. It is inferred that two phases with different porosity co-exist in the emulsion phase: a lower voidage LV-phase and a higher voidage HV-phase. The link between the LV- and HV-phases as assessed from PDF of capacitance signals and the actual flow patterns induced in the emulsion phase by the bubbles is open to scrutiny.Along a different path, the influence of the bed hydrodynamics and expansion patterns of the emulsion phase on the gas mass transfer around a freely moving coarse active particle is investigated. To this end, mass transfer-limited reaction experiments are performed, consisting in catalytic oxidation of carbon monoxide at 450 °C over Pt-loaded γ-alumina spheres immersed in the fluidized bed. The mass transfer coefficient around the freely moving active particles is measured and worked out to calculate the particle Sherwood (Sh) number. Results obtained from reaction experiments confirm the close relationship between mass transfer and expansion patterns of the emulsion phase, and the need for proper hydrodynamic characterization of the bed. A Frӧssling-type equation, Sh=2·∊e+K·(Ree/∊e)1/2·Sc1/3, well correlates the results. Sh increases with fluidization gas velocity and a best fitting is obtained using the bed voidage of the HV-region of the emulsion phase in the correlation.

Thermal transport-porosity-microstructural characteristics: unpicking the relationship in ultra-porous α-Al2O3 powder

This study deals with the effective thermal conductivity of porous and ultra-porous unconnected solid phase media composed of α-alumina in air at low temperature (<500K), and at ambient pressure. A multi-scale model is developed, starting from the quantum scale, to describe the conductivity of the single crystal, passing through a microscopic scale to describe the particles (grain size), a mesoscopic scale to take into account the intraparticle porosity and a macroscopic scale with the interparticle porosity. The model is based on percolation theory to describe the strong decrease in conductivity with porosity in this type of medium. This translates into the breaking of contacts between particles past a certain porosity, the critical porosity pc. This original approach makes it possible to consider this microstructural factor, which accounts for the thermal contacts between particles. These parameters are theoretically deduced or measured for certain samples using an X-ray micro-tomograph (µCT). Combining all these parameters, our model is able to predict 99.7% of the thermal conductivity decay from the pure crystal to the particle bed. Our model proves to be more consistent than current existing models on the same subject. Its validation range extends over the entire porosity range from 0 to 1.

Modeling of radiation heat transfer in dense-bed flows of solids in indirectly heated rotary kilns

• modification and implementation of a DOM-radiation model which combines all required physical submodels to simulate heat radiation to dense beds of particles. • Full time and space resolved simulation of thermal conversion of wet moving bulk in a large-scale rotary kiln reactor. • Analytical validation of model and justification with laboratory-scale experiments. • Efficiency optimization of the numerical solver for industrial-scale and long-time simulations on parallel supercomputers. • Parameter studies for large-scale reactors to investigate conversion rate and product quality. This work presents further development and validation of the Discrete Ordinates Model for thermal radiation implemented in OpenFOAM® for application to packed beds of particles. The radiation model is an important part of a more comprehensive model for simulating the thermochemical conversion of the discrete phase (here for instance wet biomass particles). The comprehensive Eulerian-Lagrangian model is part of a three-dimensional, time-resolved simulation of the essential chemical and physical processes occurring within and in-between particles in a moving bed. For the thermal treatment of solid particles, convective and radiative heat transfer couple the energy exchange between the reactor wall, gas- and disperse phase. The original implementation of the finite volume Discrete Ordinate Model (fvDOM) present in OpenFOAM® is valid for a dilute particulate phase and neglects the effect of local opacity due to the existence of individual particles. In the present application, a dense-packed bed of the particulate phase exists in the reactor and, therefore, this direction-based radiation model is adjusted for a computational cell with arbitrary particle volume fractions. To validate the results with the present thermal radiation model, a simple test case is first presented, where a bed of particles is heated from the top of the computational domain. A second test case relates to an experimentally investigated laboratory-scale reactor. The results of the improved fvDOM are compared to the original implementation in OpenFOAM® and the more simple and computationally cheap P-1 radiation model. In the presence of a packed bed, the P-1 model largely overpredicts the radiative heat transfer while the original fvDOM underpredicts the heat flux by about 15 % for the first test case. The new improved model delivers results within 1% deviation at the expense of maximum of 10 % increase in the computational time. Large-scale parallel simulations of real setups are crucial tools to predict the efficiency of a process and improve the operational parameters. To achieve the pre-defined product quality at minimum cost, an example of the implemented radiation model in thermochemical conversion of wet biomass in a rotary drum is been given and the importance of the radiation heat transport to the bulk is highlighted.

Simulation study on the flow behavior of wet particles in the power-law liquid-solid fluidized bed

Simulation study on the flow behavior of wet particles in the power-law liquid-solid fluidized bed

Numerical Investigation of the Particle Dynamics in a Rotorgranulator Depending on the Properties of the Coating Liquid.

In the pharmaceutical industry, the coating of particles is a widely used technique to obtain desired surface modifications of the final product, e.g., controlled release of the active agents. The production of round, coated particles is particularly important, which is why fluidized bed rotor granulators (FBRG) are often used for this process. In this work, Computational Fluid Dynamics (CFD) coupled with the Discrete Element Method (DEM) is used to investigate the wet particle dynamics, depending on the properties of the coating liquid in a FBRG. The DEM contact model was extended by liquid bridge model to account for capillary and viscous forces during wet contact of particles. The influence of the relative contact velocity on the maximum length of the liquid bridge is also considered in the model. Five different cases were compared, in which the particles were initially wetted, and the liquid loading as well as the surface tension and viscosity of the liquid were changed. The results show that increasing viscosity leads to a denser particle bed and a significant decrease in particle rotational velocities and particle motion in the poloidal plane of the FBRG. Reducing the liquid loading and surface tension results in increased particle movement.

Magnetic properties of ilmenite used for oxygen carrier aided combustion

Oxygen carrier aided combustion is a combustion process that utilizes oxygen carrying particles in a fluidized bed to transport oxygen from oxygen-rich to oxygen-poor regions in the reactor. A commonly used oxygen-carrying material is ilmenite (FeTiO3) which is a naturally occurring mineral. At higher oxygen partial pressures ilmenite can react to pseudobrookite (Fe2TiO5) and thereby take up oxygen. Upon reduction of pseudobrookite in oxygen-lean locations the oxygen is released, which enhances the distribution of oxygen through the reactor.Ilmenite was used as bed material in an industrial 115 MWth circulating fluidized bed (CFB) boiler where recycled waste wood and wood chips were utilized as fuel. Bottom ash samples were extracted after one and two weeks and the samples were separated into two fractions by a magnetic separator. The magnetic fraction was expected to be enriched in iron-containing oxides and was therefore aimed to be recirculated into the boiler.The SEM-EDS analysis revealed that the non-magnetic fraction consists to the largest extent of feldspar (KAlSi3O8) particles. A significant amount of freshly introduced ilmenite particles was also classified as non-magnetic by the magnetic separator. Characteristic for these particles was a lack of ash layer, suggesting they had only recently been added to the system. In the magnetic fraction, several feldspar particles were found which were covered by a layer rich in Ca, Fe, Ti, and Si. Comparing the XRD analysis of the ash prior to magnetic separation with its magnetic fraction revealed a decrease of the peaks corresponding to feldspar. The removal of feldspar particles by magnetic separation was further corroborated by XRF analysis, where the concentration of K, Al and Si was significantly higher in the non-magnetic fraction, however, no changes were observed in the concentration of Fe. The present analyses shows that prolonged exposure time of ilmenite increases its magnetic susceptibility. Non-magnetic feldspar was shown to acquire significant magnetic susceptibility by formation of a surface layer containing Fe-rich attrition products from ilmenite.

CFD-DEM simulation of distribution and agglomeration characteristics of bendable chain-like biomass particles in a fluidized bed reactor

Biomass is gradually perceived as an important carbon neutral and renewable energy source. Generally, the pretreatment dehydration of non-spherical biomass particles is a crucial process in biomass production. The effectiveness of moisture removal, however, is directly affected by the fluidization characteristics of particles in drying facilities. In this article, the distribution and agglomeration characteristics of chain-like biomass particles in a fluidized bed were studied according to the coupled model through the computational fluid dynamics-discrete element method. Based on the special structural feature of filamentous biomass particles, a new three-dimensional model of bendable chain-like particle was established. The effects of gas superficial velocity and mass flow rate of biomass particle on the distribution and agglomeration characteristics of chain-like biomass particles were experimentally and numerically investigated. The results demonstrated that the chain-like biomass particles were more likely to agglomerate near the wall and corners due to wall effects, with a maximum of 0.11 % of the volume fraction. The distribution of particle concentration showed an inverted C-shape at the lower cross section. Also, the velocity of biomass particles presented as an open downward parabolic distribution in the radial direction and reached a maximum at the central region. The maximum cluster area of chain-like biomass particles was 73.43 cm2 under the condition of ug = 5.71 m/s, appearing in the corners and bottom of the fluidized bed. The band-like clusters transformed into irregular triangular clusters gradually with the increase of the gas velocity. As the particle mass flow rate increased, the particle clusters became larger and extended upward along the tube wall. The results will provide valuable information on the mixing processes of non-spherical particles, as well as guidance for the development and optimization of multi-phase drying plants.

Collision characteristics and breakage evolution of particles in fluidizing processes

Fluidized beds are widely used in chemical reactors. Particles in fluidized bed usually circulate between reactors, and therefore, particle attrition cannot be avoided. Catalyst particle attrition in the fluidized bed significantly shortens their lifespan and raises operating costs. This study established an irregular polyhedron model of the particles to acquire their motion and force characteristics by the Euler-Lagrangian method and then employed the Ab-T10 breakage model to predict the particle's breakage process. The collision frequency and stress characteristics of the three-dimensional surface of a single particle were explored, as well as the time-dependent variations in the collision frequency and impact power between particles. The breakage and elutriation processes of a single particle were observed. The investigation found that the velocity differential between particles was the main factor that caused impact power. The particles were typically crushed one to six times before becoming completely fine powder, and their size evolution can be summarized in three stages. The complete process of particle breakage was revealed by tracing the size evolution of a single particle. This work would contribute to a deeper understanding of particle collision behavior and breakage mechanisms in fluidized beds.

Penetration into a granular bed in the presence of upward gas flows

We present a numerical study on the penetration of spherical projectiles into a granular bed in the presence of upward gas flows. Due to the presence of interstitial fluid, the force chains between particles in the granular bed are weakened significantly, and this distinguishes the penetration behavior from that in the absence of fluid. An interesting phenomenon, namely granular jet, is observed during the penetration, and the mechanism for its formation and growth is attributed to the merging of granular vortices generated by the interaction between the intruder and primary particles. Moreover, both the final penetration depth and the maximum diameter of the crater are found to follow a power-law dependence with the impact velocity, and the maximum height reached by the granular jet tends to increase linearly as the impact velocity increases, agreeing well with the experimental results reported in the literature.

Experimental investigation of wall-to-bed heat transfer of carbon nanotubes in a fluidized bed

Wall-to-bed heat transfer characteristics of multi-walled carbon nanotubes (MWCNTs) were determined in a fluidized bed (0.15 m i.d. by 2.0 m height). Two types of CNTs with different nanotube shapes were used: entangled CNTs (ENCNTs) and vertically aligned CNTs (VACNTs). The flow regimes of MWCNTs were identified as fixed bed, partial fluidization, and complete fluidization from the pressure drop across the bed (ΔP) with gas velocity. In the partial fluidization regime, different gas channeling phenomena occurred depending on the nanotubes shape on the CNT particles, affecting the bed hydrodynamics. The instantaneous surface temperature (Tsi) of the heat transfer probe decreased and its fluctuation increased with the gas velocity proportional to the bed particle movement near the wall. Fluctuations in ΔP and Tsi at the wall had a strong correlation for all MWCNTs, and the wall-to-bed heat transfer was greatly influenced by the movement of particles induced by a preferential flow path of gas in the bed such as gas channeling and bubbles. Fluctuations in ΔP and Tsi of VACNTs were considerably lower than those of ENCNTs because of the formation of a chain-like network structure of the bed. The wall-to-bed heat transfer coefficient (hw) in the CNT fixed bed was considerably higher than that of comparable particles owing to their high conductivity. The average hw of the CNTs increased with gas velocity, and the rate of increase depended on the shape of the nanotubes. The hw of the VACNTs which form easily tangled agglomerates by many long nanotubes, was lower than that of the ENCNTs. Correlations for hw prediction were firstly developed for the partial and complete fluidization regimes. The predicted values well accorded with the experimental results, and the hw changes of the MWCNTs with gas velocity could be predicted well based on the proposed correlation and two-phase model of the bubbling fluidized bed.

Simulation and Property Characterization of Nanoparticle Thermal Conductivity for a Microscale Selective Laser Sintering System

Abstract Current additive manufacturing (AM) technologies are typically limited by the minimum feature sizes of the parts they can produce. This issue is addressed by the microscale selective laser sintering system (μ-SLS), which is capable of building parts with single micrometer resolutions. Despite the resolution of the system, the minimum feature sizes producible using the μ-SLS tool are limited by unwanted heat dissipation through the particle bed during the sintering process. To address this unwanted heat flow, a particle scale thermal model is needed to characterize the thermal conductivity of the nanoparticle bed during sintering and facilitate the prediction of heat affected zones. This would allow for the optimization of process parameters and a reduction in error for the final part. This paper presents a method for the determination of the effective thermal conductivity of copper nanoparticle beds in a μ-SLS system using finite element simulations performed in ansys. A phase field model (PFM) is used to track the geometric evolution of the particle groups within the particle bed during sintering. Computer aided design (CAD) models are extracted from the PFM output data at various time-steps, and steady-state thermal simulations are performed on each particle group. The full simulation developed in this work is scalable to particle groups with variable sizes and geometric arrangements. The particle thermal model results from this work are used to calculate the thermal conductivity of the copper nanoparticles as a function of the density of the particle group.