FCC Particles Research Articles

Solids dispersion in high density circulating fluidized beds

Transient local solid tracer concentration distributions were measured by a novel, simple and accurate measurement system with fast response to obtain axial solids dispersion in the riser of a circulating fluidized bed operated at solids fluxes up to 450 kg/m2s and gas velocities up to 8 m/s. Phosphorescent-coated FCC particles were used as the tracer which were activated by high intensity UV light and injected into the return leg of high-density circulating fluidized bed unit. A radially non-uniform axial dispersion model was utilized to determine axial solids dispersion coefficients, and the results were interpreted with the help of hydrodynamic data obtained in the same column. Comparison of the axial dispersion coefficients at different operating conditions revealed that dispersion decreases when solids circulation flux increases to 450 kg/m2s together with the increase in gas velocity to 8 m/s. Low solids dispersion coefficients (Daz < 0.1) at high solids circulation fluxes and high velocities resulted from the disappearance of the net downward flow of particles at the riser walls, signifying the existence of the dense suspension upflow regime.

Heat Treatment Induced Structural Transformations and High-Temperature Oxidation Behavior of Al21Co22Cr22Fe13Ni22 High-Entropy Coatings Produced by Non-Vacuum Electron Beam Cladding

In this study, Al21Co22Cr22Fe13Ni22 high-entropy coatings were produced on steel substrates by non-vacuum electron beam cladding of Co, Cr, and NiAl powders. The high-temperature oxidation behavior of the coatings was studied by holding the specimens at 900 °C in air. The microstructure and phase constitution of the samples were studied both in the as-cladded state and after the heat treatment. The microstructure was characterized using light microscopy (LM) and scanning electron microscopy (SEM). Synchrotron X-ray diffraction (SXRD) and energy-dispersive X-ray spectroscopy (EDX) were used to study the phase constitution of the coatings and the “coating-substrate” interface. The coating consisted of disordered bcc (A2), ordered bcc (B2), and disordered fcc (A1) phases. Annealing the coatings for 50 h at 900 °C led to the formation of fcc precipitates in the bcc dendritic grains and a mixture of fcc and σ-phase particles in the interdendritic regions. Needle-like nanosized B2-precipitates were formed due to annealing in the fcc grains at the coating/substrate interface. The microhardness at the top of the as-cladded coating was 585 HV and gradually decreased towards the substrate. A more uniform distribution of the microhardness was obtained after the annealing. Its average value was 441 HV. Rhomboid Cr2O3, needle-like Al2O3, and spinels of a different morphology were found on the surface of the samples after oxidation at 900 °C.

Improvement of Relative DEM Time Step Range in Fast Fluidization Simulation of Type-A FCC Particles

In many process engineering fields, gas-particle fluidized beds are widely used. In fluidized bed research, the discrete element method, or DEM, has been a powerful tool for design and operation purposes. However, with the use of Type-A powders, fluid catalytic cracking or FCC particles being classical cases, they have hardly been reported in DEM simulations of fast fluidization. This study paid close attention to the suitable selection of a stiffness constant and a DEM time step. To reflect their respective effects and complicated interactions, a so-called relative DEM time step was defined. The drag coefficient was correlated using the energy-minimization multi-scale (EMMS) approach to reasonably calculate the gas–solid interaction. Six representative cases with different relative time step values were chosen to simulate a micro-fluidized bed of Type-A FCC powders. The results showed that DEM employing EMMS-based drag force was able to greatly enlarge the suitable range of relative time steps in a fast fluidization simulation of Type-A powders. In addition, the typical macro flow structures of fast fluidization was successfully captured: axially dilute in the top and dense in the bottom, and radially dilute in the core and dense near the wall. Moreover, the distinct gas–solid backmixing, which is considered one of the most important pieces of evidence for the determination of fast fluidization regimes, was modeled. It was indicated that the EMMS-based drag model attenuated the overestimated drag force so that the soft-sphere contact model would be able to more appropriately deal with particle collision, and thus improve the suitable relative DEM time step range.

(Fe-Ni)-based glassy alloy containing Nb and Cu with excellent soft magnetic properties

Magnetic properties of amorphous Fe38Ni38B12P5Si3Nb3Cu1 alloy ribbons have been investigated in as-spun and annealed states with the aim of developing an ultrasoft magnetic material which is expected to be used in advanced information data devices. A small amount of simultaneous addition of Cu and Nb to a FeNi-metalloid amorphous alloy system caused the change in crystallization process from eutectic type to the primary one. After annealing at the temperature just below the glass transition temperature (Tg), the glassy alloy ribbon exhibit excellent soft magnetic properties, very low coercive force of 1 A/m and extremely high effective permeability of 7.5·104 at 1 kHz which have not been obtained for any kinds of Fe- and Ni-based amorphous alloys, in spite of relatively low saturation magnetic flux density of 0.86 T. The further increase in annealing temperature causes the precipitation of nanoscale fcc particles with a size of about 7 nm including a high density of planar faults and the resulting nanostructure alloy also exhibits good soft magnetic properties with low Hc below 2 A/m. The formation of nanostructure fcc-(Fe,Ni) + glass soft magnetic alloys is also novel. The syntheses of the new Fe-Ni-based glassy and nanocrystalline [fcc(Fe,Ni) + glass] alloy ribbons with ultrasoft magnetic properties are encouraging as a new type of energy saving-type soft magnetic material.

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.

Influence of oxygen on performance of multi-principal element alloy as braze filler for Ni-base alloys

In manufacturing of multi-principal element alloys (MPEAs), the casting process is often a primary point of oxygen introduction to the material system, which was demonstrated by the laboratory scale button arc-melting process. Oxygen introduction raises concerns for the mechanical performance of these alloys if employed as structural materials alone or in engineering applications, such as serving as filler materials to enable joining of similar or dissimilar alloy pairs that are conventionally considered difficult to join. In this work, oxide inclusions in an as-cast off-equiatomic MnFeCoNiCu MPEA and Ni-base alloy braze joints made with this MPEA were evaluated by synchrotron x-ray diffraction mapping and electron microscopy. MnO was found to be prevalent throughout the cast MPEA and was chemically reduced during brazing by trace Al from the base material, Ni-base Alloy 600. Incomplete reaction of the MnO with Al, and some alternative reaction with Cr, left a mixture of MnO, Al2O3, and Cr2O3 in the as-brazed joint, with the oxides concentrated near the braze centerline due to directional solidification. In the braze joints, the non-oxide constituent phases detected were an FCC matrix and particles of Cr23C6, Cr7C3, and TiN which are all native to Alloy 600. The evolution of oxides within the brazed joints at elevated service temperatures of 600–800 °C was also evaluated. While MnO and Al2O3 were stable during high-temperature service, Cr2O3 particles grew to several hundred microns, with dissolved oxygen in the MPEA providing a significant source for oxide growth. Comparing tensile performance of the as-brazed condition with post-service conditions showed that the evolution of oxide particles contributed to an increase in strength and decrease in ductility of the brazed joints. Evaluating particle morphologies among fracture surfaces of individual specimens demonstrated that large Cr2O3 particles and continuous cluster networks of Al2O3 were the features most detrimental to total elongation. Hence, unmitigated evolution of oxygen-containing species during brazing and service conditions is detrimental to the ductility of the brazed joints, which highlights the importance of oxygen control in initial manufacturing of MPEAs for engineering applications.

Low temperature in situ immobilization of nanoscale fcc and hcp polymorphic nickel particles in polymer-derived Si-C-O-N(H) to promote electrocatalytic water oxidation in alkaline media.

We synthesized nickel (Ni) nanoparticles (NPs) in a high specific surface area (SSA) p-block element-containing inorganic compound prepared via the polymer-derived ceramics (PDC) route to dispatch the obtained nanocomposite towards oxygen evolution reaction (OER). The in situ formation of Ni NPs in an amorphous silicon carboxynitride (Si-C-O-N(H)) matrix is allowed by the reactive blending of a polysilazane, NiCl2 and DMF followed by the subsequent thermolysis of the Ni : organosilicon polymer coordination complex at a temperature as low as 500 °C in flowing argon. The final nanocomposite displays a BET SSA as high as 311 m2 g-1 while the structure of the NPs corresponds to face-centred cubic (fcc) Ni along with interstitial-atom free (IAF) hexagonal close-packed (hcp) Ni as revealed by XRD. A closer look into the compound through FEG-SEM microscopy confirms the formation of pure metallic Ni, while HR-TEM imaging reveals the occurrence of Ni particles featuring a fcc phase and surrounded by carbon layers; thus, forming core-shell structures, along with Ni NPs in an IAF hcp phase. By considering that this newly synthesized material contains only Ni without doping (e.g., Fe) with a low mass loading (0.15 mg cm-2), it shows promising OER performances with an overpotential as low as 360 mV at 10 mA cm-2 according to the high SSA matrix, the presence of the IAF hcp Ni NPs and the development of core-shell structures. Given the simplicity, the flexibility, and the low cost of the proposed synthesis approach, this work opens the doors towards a new family of very active and stable high SSA nanocomposites made by the PDC route containing well dispersed and accessible non-noble transition metals for electrocatalysis applications.

Effect of heat treatment on microstructure and mechanical properties of Al1.2CoCrFeNi2.1 high-entropy alloy fabricated by powder plasma arc additive manufacturing

Powder plasma arc additive manufacturing (PPA-AM) is a promising method for fabricating high-entropy alloys (HEAs). However, the characteristics of thermal cycling during the PPA-AM process and the mechanism of subsequent heat treatment on the microstructure and mechanical properties are unclear. In this study, an Al1.2CoCrFeNi2.1 HEA with dual-phase (FCC + B2) was fabricated via PPA-AM. The microstructure and mechanical properties of the as-built and heat-treated samples were explored. The crystallographic orientations of the FCC and B2 phases were also determined. Because the alloy in the bottom region was affected by thermal cycling, B2 particles precipitated in the FCC phase, and FCC particles precipitated in the B2 phase. During the tensile test, the phase boundary between B2 matrix and FCC particles produced cracks preferentially. These cracks reduced the strength and elongation of the alloy. After heat treatment at 1200 °C, the FCC and B2 precipitates formed globular shapes inside the B2 and FCC grain matrix, respectively. The alloy showed a uniform structure and the ductility of the alloy was improved significantly. This work elucidates the mechanisms of thermal cycling and heat treatment on the microstructure and mechanical properties of the alloy.

An image processing method for feature extraction and dynamic tracking of particle clusters in CFBs

A new image processing method based on the high-speed camera is proposed to identify, locate, and track clusters. The instantaneous characteristic parameters of particle clusters in the riser of the circulating fluidized bed (CFB) can be acquired, such as solids holdup, vertical velocity, lateral displacement, aspect ratio and near-circularity. Experiments were carried out with glass bead particles, river sand particles and FCC particles. The time series of images of gas–solid flow in a CFB riser with a 100 mm × 25 mm cross-section and 3.2 m in length were obtained using high-speed cameras. The k-means++ clustering algorithm is utilized to identify the clusters, centroid is applied to locate the clusters, and the cross-correlation algorithm is employed to track the specific clusters and number them to get the instantaneous characteristic parameters. The results illustrate that the shapes of clusters in the center area are closest to circle, moving upwards at a uniform speed, while the clusters in the side-wall area are mostly elongated or long chain-like, moving slowly downwards. In the transition area, the clusters are more complex, moving upwards at a constant speed, and having large lateral displacement. The results show that the image processing method used in this study is successful in acquiring the dynamic and structural parameters of the clusters simultaneously.

CPFD simulation of cluster effect on mass transfer and reaction in downer with FCC particles

In this work, a cluster-based mass transfer and reaction model was proposed according to large number of simulation results by conducting ozone decomposition inside spherical cluster, which is more realistic compared with the conventional homogeneous description of mass transfer and chemical reaction process. Then, this model was coupled with three-dimensional Computational Particle Fluid Dynamics (CPFD) approach to simulate the ozone decomposition in pilot-scale gas-solids downer. According to the comparison between simulation results and experiment measurements, it was found that the model sufficiently considering the clustering effects on the hydrodynamics and chemical reaction predicted better ozone concentration distribution than the model based on homogeneous assumption. Meanwhile, the comprehensive model with consideration of cluster could accurately predict the gas-solids flow behaviors and reaction characteristics with different particle properties under different operating conditions. Finally, the effects of operating conditions such as superficial gas velocity and solids circulating flux on the hydrodynamics and reaction characteristics inside downer were fully analyzed on the basis of the accurate model predictions.

Microstructure evolution of a multi-track AlCoCrFeNi high entropy alloy coating fabricated by laser cladding

In order to understand the microstructure evolution characteristics of AlCoCrFeNi high entropy alloy (HEA) coating at high environmental temperatures, a laser cladded AlCoCrFeNi HEA coating is heated to 700 °C, 900 °C and 1100 °C respectively, holding for 30 min and cooled in air. Microstructure and micro-hardness of the treated coatings are investigated, in both interface and coating areas. In interface area, FCC blocks exist on the boundary of interface and coating in untreated specimen, besides the BCC single phase in coating. The primary FCC blocks remain in 700 °C treated coating, while they disappear in 900 °C and 1100 °C treated coatings. However, new FCC particles are formed instead near interface. In coating area, BCC single phase is acquired in untreated and 700 °C treated coatings. In 900 °C and 1100 °C treated coatings, lots of FCC and σ (Cr-rich carbide) particles are distributed mainly on grain boundaries of BCC phase matrix and spinodal decomposition occurs to BCC phase. Micro-hardness of 700 °C treated coating is slightly higher than that of untreated coating due to the age hardening effect, while the micro-hardness of 900 °C and 1100 °C treated coatings are much lower than that in untreated and 700 °C treated coatings due to the formation of soft FCC phase. Micro-hardness of 1100 °C treated coating is slightly lower than that of 900 °C treated coating due to the coarsening of σ particles and spinodal decomposed structures.

Comparisons of particle clustering phenomenon between gas-solids high-density and low-density circulating fluidized bed risers via numerical study

The clustering phenomenon in gas-solids high-density (Gs ≥ 300 kg/m2s) and low-density circulating fluidized bed (CFB) risers for FCC particles are compared through CFD studies using a cluster-based drag model. Large clouds of clusters can be clearly identified from the contours of local solids holdup distribution from CFD results, which correspond well to experimental observations. The size, shape, and distribution of the cluster clouds, as well as their dynamic properties including the varying velocity and appearance between the HDCFB and LDCFB risers were extracted and further compared. A cluster phase with higher average solids holdup has been distinguished from the dilute solids suspension phase based on the solids holdup's probability density distribution (PDD). Correlations between the cluster phase solids holdup with its fraction or with the overall bed density were proposed. The fraction and the average solids holdup of the dense cluster phase increases with the increase in the overall bed density.

Cluster Identification by a k-means Algorithm-Assisted Imaging Method in a Laboratory-Scale Circulating Fluidized Bed

Particle clusters for FCC particles in a gas–solid circulating fluidized bed with a 12.4 m high riser and a 5 m high downer were identified from the images of the gas–solid flow by a k-means machin...

Direct comparison of CFD-DEM simulation and experimental measurement of Geldart A particles in a micro-fluidized bed

Although CFD-DEM simulation of Geldart B and D particles has been experimentally validated extensively, its experimental validation for simulating the hydrodynamics of Geldart A particles has been seldom reported, due to the fact that even the particle number in a laboratory fluidized bed is too much to be tracked individually. In this study, one-to-one comparison between experimental and CFD-DEM study of a miniaturized gas-solid fluidized bed (micro-fluidized bed) containing FCC particles was performed. It was shown that the bed expansion characteristics in bubbling fluidization regime, the mean voidage of emulsion phase as well as the bed collapse process can be predicted accurately by CFD-DEM method. The results not only provide experimental validation of CFD-DEM method for studying the heterogeneous dynamics of Geldart A particles in bubbling fluidized beds, but also prove that CFD-DEM method is an ideal and powerful tool for exploring the behavior of micro-fluidized beds.

Calibration of solids concentration optical fibre probes with solids‐polymer blocks

AbstractOwing to the determinant effects of the local distribution of solids concentration on transport phenomena, particularly in gas‐solids flows in such processing units as gas‐solid fluidized beds and pneumatic conveying lines, its accurate measurement has received considerable attention. Despite the widespread applications of optical fibre probes for the measurement of local solids concentration, these instruments may create measurement uncertainties due to their structural designs and challenges in their accurate calibrations. Several calibration methods have been proposed in the literature, while they need to be further improved. In the present study, we calibrated in‐house, fabricated, solids concentration mixed parallel optical fibre probes by two sets of solids‐polymer blocks covering solids volume fractions in the range of 0‐0.30. The uniform blocks were produced by a twin‐screw extruder and an injection moulding machine. FCC particles, 86 μm in diameter, formed the dispersed solids in the blocks, while poly styrene and poly methyl methacrylate were separately acting as the transparent medium. They helped us investigate the effects of the refractive index of the medium and the probe tip design on the calibration curve. The blind distance (zone) in front of the adopted probes was determined, at about 0.70 mm, upon measuring their reflective curves. Two glass windows with 1.00 and 1.60 mm thicknesses were separately installed at the tip of a probe to study their effects on the calibration curve. Experimental results indicated that in the absence of a glass window, the probes' voltage readings varied nonlinearly with solids concentration, the voltage readings were higher for blocks with a higher refractive index of the transparent medium, while the normalized calibration curves were similar for both sets of solids‐polymer blocks. In addition, the extent of the uniform distribution of illuminating and reflecting fibre strands at the tip of a mixed parallel optical fibre probe can change the degree of nonlinearity of its calibration curve. The application of glass windows thicker than the blind distance decreased the degree of nonlinearity of the calibration curve, yet did not completely eliminate it.

Effect of structure parameters on forces acting on baffles used in gas–solids fluidized beds

Effects of some important structural parameters, i.e. slat pitch, and layout position, on dynamic forces acting on the baffles were examined in the fluidized bed of FCC particles operating under different superficial gas velocities. The experimental baffles were made of multiple inclined slats. We found that the forces acting on the baffles decreased significantly with reducing pitch between the slats. For the baffles with a small slat pitch, the forces acting on the baffles increased slightly and then decreased with increasing superficial gas velocity, which is very different from the measured results of a single slat or tube immersed in fluidized beds. The different results are greatly related to the appearance of the “gas cushion” beneath the baffles, whose height increases with increasing superficial gas velocity. On the other hand, a region with stronger particle circulation induced by the inclined slat array was observed in the experiments. The slat near the wall and located below the region of downward-flowing particles was found to be subjected to the severest forces. Therefore, the slats located in similar locations of industrial baffles are suggested to be reinforced to increase their structural strength.

Crystal orientation and grain boundary effects on plastic deformation of FCC particles under high velocity impacts

Material plastic behavior during deformation is affected by several parameters such as atomic structure, crystalline orientation, grain boundaries, impurities etc. In this study, molecular dynamic simulations were performed to provide a detailed understanding of the effects of grain orientation and grain boundaries on plastic deformation of FCC particles under high velocity impacts. Two single-crystal particles with different crystalline orientations, two simple polycrystal particles (bicrystal, and tricrystal) with inner walls parallel and perpendicular to the shock wave, and a polycrystal particle with 64 random grains were modeled. The results showed that the deformation behavior and microstructural evolution depended directly on their initial microstructure of the particles, whereas the resulting shear strain pattern was the same for all the particles. Phenomena such as recovery, crystallization, recrystallization, grain growth, twining, and grain rotation were observed during the deformation of the particles. In addition, it was shown that heterogeneous nucleation of Shockley partial dislocations was the main mechanism at the onset of the deformation. Another important deformation mechanism was solid-state amorphization due to the densification of the dislocation network in certain regions of the particles, also associated with thermal softening effects.

Intracellular Fate of Hydrophobic Nanocrystal Self‐Assemblies in Tumor Cells

AbstractControl of interactions between nanomaterials and cells remains a biomedical challenge. A strategy is proposed to modulate the intralysosomal distribution of nanoparticles through the design of 3D suprastructures built by hydrophilic nanocrystals (NCs) coated with alkyl chains. The intracellular fate of two water‐dispersible architectures of self‐assembled hydrophobic magnetic NCs: hollow deformable shells (colloidosomes) or solid fcc particles (supraballs) is compared. These two self‐assemblies display increased cellular uptake by tumor cells compared to dispersions of the water‐soluble NC building blocks. Moreover, the self‐assembly structures increase the NCs density in lysosomes and close to the lysosome membrane. Importantly, the structural organization of NCs in colloidosomes and supraballs are maintained in lysosomes up to 8 days after internalization, whereas initially dispersed hydrophilic NCs are randomly aggregated. Supraballs and colloidosomes are differently sensed by cells due to their different architectures and mechanical properties. Flexible and soft colloidosomes deform and spread along the biological membranes. In contrast, the more rigid supraballs remain spherical. By subjecting the internalized suprastructures to a magnetic field, they both align and form long chains. Overall, it is highlighted that the mechanical and topological properties of the self‐assemblies direct their intracellular fate allowing the control intralysosomal density, ordering, and localization of NCs.

Hierarchical microstructure strengthening in a single crystal high entropy superalloy

A hierarchical microstructure strengthened high entropy superalloy (HESA) with superior cost specific yield strength from room temperature up to 1,023 K is presented. By phase transformation pathway through metastability, HESA possesses a hierarchical microstructure containing a dispersion of nano size disordered FCC particles inside ordered L12 precipitates that are within the FCC matrix. The average tensile yield strength of HESA from room temperature to 1,023 K could be 120 MPa higher than that of advanced single crystal superalloy, while HESA could still exhibit an elongation greater than 20%. Furthermore, the cost specific yield strength of HESA can be 8 times that of some superalloys. A template for lighter, stronger, cheaper, and more ductile high temperature alloy is proposed.

3D CPFD simulation of gas-solids flow in the high-density downer with FCC particles

The gas-solids flow in high-density downers with FCC particles was simulated by Computational Particle Fluid Dynamics (CPFD) approach. A drag model was proposed based on the equivalent diameter of cluster with the analysis of force balance on particle cluster. A global sensitivity analysis method was used to analyze the influencing factors of cluster size. The drag model was incorporated into the three-dimensional (3D) CPFD approach to simulate the gas-solids flow in two different downers operating at various conditions. 3D CPFD simulation provided detailed trajectories and distributions of solids. Good agreements of the predicted solids holdup and velocity with experimental data were achieved at both low and high-density conditions, which verified the rationality and applicability of the present drag model considering the characteristics of particle cluster in the downer. Finally, the flow and clustering behavior in the high-density downer were adequately analyzed and compared with that in low-density downer.