Agglomerate Particulate Fluidization Research Articles

Effect of high stirring speed on the agglomerate behaviors for cohesive SiO2 powders in gas fluidization

The effects of superficial gas velocity and mechanical stirring speed on the precise regulation of flow regimes for cohesive SiO2 powders (mean diameter is 16 μm) were experimentally investigated in a stirring-assisted fluidized bed. The results showed that compared with the agglomerates formed in the non-assisted fluidization of cohesive SiO2 powders, the introduction of mechanical stirring could effectively reduce the size of agglomerates and well disperse the agglomerates during fluidization. The best regulation range of agglomerate particulate fluidization can be achieved at 600 rpm when agglomerate sizes were reduced to below 200 μm. Further investigation based on the operational phase diagram revealed that transformations of flow regimes were dominated by both stirring speed and gas velocity. The stirring applied enlarges the operational range of agglomerate particulate fluidization (APF) with a delayed onset of bubbling for cohesive particles. However, the exorbitant speed increases the collision velocity and contact area between small agglomerates, which results in the formation of unstable agglomerates and the whirlpool of powder.

Improvement in cyclic CO2 capture performance and fluidization behavior of eggshell-derived CaCO3 particles modified with acetic acid used in calcium looping process

Although calcium-based materials are the most promising adsorbents used in calcium looping process for carbon dioxide removing, their CO2 capture capacity decaying besides poor fluidization, still are the important challenges. In the present investigation, eggshell as a cheap, easily available and unpolluted source of calcium carbonate was used for CO2 capturing in calcium looping process. Eggshell particles were treated with various volume concentrations of acetic acid to improve its sorption capacity. According to the TGA results after 20 carbonation/calcination cycles, the effective carbonation conversion of modified eggshell with 5%, 20%, 30% and 40%. v/v acetic acid was 21.33%, 24.26%, 25.97% and 28.97%, respectively, which is considerable compared to 20.54% for untreated eggshell. The effect of initial eggshell particle size on the adsorption behavior of final adsorbent was also investigated by using two different sizes including dp< 45 µm and dp> 320 µm. The results showed that the effective conversion of the adsorbent containing 40%. v/v acetic acid derived from small particle size eggshells was 9.32% higher than that from larger particle size eggshells. In terms of fluidization behavior, surprisingly the addition of acetic acid to the eggshell particles also increased the bed expansion ratio as 8% and 36.2% at gas velocities of 0.27 and 6.67 cm/s, respectively. Further improvement in the fluidity of eggshell modified with 40% acid was performed by manually mixing of SiO2 nanoparticles at different weight percentages. According to the results, adding 7.5 wt% SiO2 leaded to the homogeneous and agglomerate particulate fluidization.

Theoretical and experimental study on the fluidity performance of hard-to-fluidize carbon nanotubes-based CO2 capture sorbents

Carbon nanotubes-based materials have been identified as promising sorbents for efficient CO2 capture in fluidized beds, suffering from insufficient contact with CO2 for the high-level CO2 capture capacity. This study focuses on promoting the fluidizability of hard-to-fluidize pure and synthesized silica-coated amine-functionalized carbon nanotubes. The novel synthesized sorbent presents a superior sorption capacity of about 25 times higher than pure carbon nanotubes during 5 consecutive adsorption/regeneration cycles. The low-cost fluidizable-SiO2 nanoparticles are used as assistant material to improve the fluidity of carbon nanotubes-based sorbents. Results reveal that a minimum amount of 7.5 and 5 wt% SiO2 nanoparticles are required to achieve an agglomerate particulate fluidization behavior for pure and synthesized carbon nanotubes, respectively. Pure carbon nanotubes + 7.5 wt% SiO2 and synthesized carbon nanotubes + 5 wt% SiO2 indicates an agglomerate particulate fluidization characteristic, including the high-level bed expansion ratio, low minimum fluidization velocity (1.5 and 1.6 cms−1), high Richardson—Zaki n index (5.2 and 5.3 > 5), and low Π value (83.2 and 84.8 < 100, respectively). Chemical modification of carbon nanotubes causes not only enhanced CO2 uptake capacity but also decreases the required amount of silica additive to reach a homogeneous fluidization behavior for synthesized carbon nanotubes sorbent.

Mixing of nanoparticle agglomerates in fluidization using CFD-DEM at ABF and APF regimes

Mixing of nanoparticle agglomerates in a fluidized bed was investigated by CFD-DEM. A parallel program was used that simultaneously uses the computational resources of GPU and CPU on a single computer. The bed expansion obtained from simulation was compared with experimental and good agreement was observed. Bed expansion in agglomerate particulate fluidization (APF) was 2 to 5 times the initial bed height and final bed expansion in agglomerate bubbling fluidization (ABF) was 1 to 2 times the initial bed height by changing the gas velocity from 2 to 4 and 4 to 8cm/s for APF and ABF regimes respectively. Gas volume fraction distribution was investigated for both APF and ABF regimes, which showed a more uniform distribution in APF. Increasing the gas velocity led to a more homogenous in the APF bed but it did not change the homogeneity of the ABF bed. Besides, the axial and radial dispersion coefficients in ABF were greater than those in APF. Axial dispersion coefficient varied from 2×10−6 to 4×10−5m2/s. Lacey mixing index was investigated in both APF and ABF regimes and it showed that the quality of mixing in ABF was better than APF.

Fluidization Dynamics of Hydrophobic Nanosilica with Velocity Step Changes

Nanosilica is widely used in various applications, with its market expected to grow over USD 5 billion by 2025. The fluidized bed technology, owing to its intimate contact and efficient mixing of phases, is ideally suited for the large scale processing of powders. However, the bulk processing and dispersion of ultrafine nanosilica using the fluidized bed technology are critically affected by the interparticle forces, such that the hydrophilic nanosilica shows agglomerate bubbling fluidization (ABF), while the hydrophobic nanosilica undergoes agglomerate particulate fluidization (APF). This study carried out a detailed investigation into the fluidization hydrodynamic of the hydrophobic nanosilica by monitoring the region-wise dynamics of the fluidized bed subjected to a regular step change of fixed duration in the gas velocity. The gas flow was controlled using a mass controller operated with an analog output signal from a data acquisition system. The analog input data were acquired at the sampling rate of 100 Hz and analyzed in both time and temporal frequency domains. The effect of velocity transients on the bed dynamics was quickly mitigated and appeared as lower frequency events, especially in regions away from the distributor. Despite the apparent particulate nature of the fluidization, strong hysteresis was observed in both pressure drop and bed expansion. Moreover, the fully fluidized bed’s pressure drop was less than 75% of the theoretical value even though the bed appeared to free from non-homogeneities. Key fluidization parameters, e.g., minimum fluidization velocity (Umf) and the agglomerate size, were evaluated, which can be readily used in the large scale processing of nanosilica powders using fluidized bed technology.

Fluidization of Nanoparticle Agglomerates at Elevated Temperatures

The behavior of hydrophobic and hydrophilic nanoparticles was investigated. In a novel approach, the interparticle force (IPF) was altered by changing the temperature of the gas–solid fluidized bed in the range 25–110 °C. The hydrodynamic state of the bed was monitored by examining pressure fluctuations using coherent and incoherent analysis. Formation, coalescence, eruption, and mobility of bubbles in the fluidized bed were determined in addition to investigating the behavior of agglomerates. The diameter of the agglomerate was calculated from the incoherent output of the power spectral density of pressure fluctuations in the agglomerate particulate fluidization (APF) regime at various temperatures. No bubbles were detected in the bed of the silica nanopowder at low temperatures whereas small bubbles form at higher temperature. In contrast, hydrophilic titania nanopowder becomes fluidized in the ABF regime at lower temperatures and tends to fluidize at the APF regime with increasing bed temperature.

Experimental Investigation on Gas‐Solid Behavior in Fluidized Beds with Superfine Particles

AbstractFluidized beds are commercially utilized in gas‐solid contacting processes such as the fluid catalytic cracking (FCC) process. Attempts have been made for bubbling fluidized beds to enhance the gas exchange between the bubble phase and the emulsion phase as well as to increase bubble holdup. Low‐density superfine particles 5070S were selected with average diameters of about 10 μm. Unlike agglomerate particulate fluidization of superfine particles on which other researchers have reported, the smaller bubbles, whose diameter is less than 10 mm, uniformly dispersed in the whole bed. Additionally, the gas exchange rate with 5070S became larger than with the FCC particles. Such a unique gas‐solid behavior is probably due to the proper density difference between particle aggregate and fluidizing gas.

Fluidization behavior of binary iron-containing nanoparticle mixtures in a vibro-fluidized bed

The present work is focused on the fluidization behavior of different mixed binary nanoparticles (Fe2O3+SiO2, Fe2O3+TiO2, and Fe2O3+ZnO) under the influence of a vibrated field. The results reveal that the iron-containing nanoparticle mixtures with different mass fractions can achieve uniform fluidization in the initial bed height of 40mm, amplitude of 3.0mm, vibration frequency of 40Hz, and binary mixtures containing nano-SiO2 and nano-Fe2O3 have commendable fluidization behavior. Richardson–Zaki equation index indicates that the particulate dispersion degree of the different proportion mixtures increases as the mass fraction of APF (agglomerate particulate fluidization) nanoparticle increases. For a binary nanoparticle mixture containing different proportions of high content nano-SiO2 and nano-Fe2O3, n/4.65 is closer to 1 and the relative deviation is smaller than that containing a high content TiO2 and ZnO, indicating that the system has a good dispersibility.

An improved model for determining fractal structure of nano‐agglomerates

AbstractEffects of operating conditions on fractal structures of three nanoparticle agglomerates (SiO2, TiO2, and ZnO) in a vibro‐fluidized bed were investigated. An improved model is proposed by combining the fractal relation, Richardson‐Zaki equation, and mass balance. This model was used to predict substantial properties of nanoparticle agglomerates, such as fractal dimension and size of the agglomerate particulate fluidization. It was shown that with increasing vibration intensity, the fractal dimension of agglomerates decreases slightly, while the number of primary particles in the agglomerate decreases significantly. It was found that the fractal dimension of nanoparticle agglomerates is in the range of 2.63–2.78, and the number of primary particles in the agglomerate is in the order of 1010. Agglomerate size reaches a constant value at high vibration frequency. Calculation results indicated that the vibration frequency has a more important role than its amplitude in reducing agglomerate size. Minimum fluidization velocity was calculated by applying estimated agglomerate sizes, and it was found that the results were in close agreement with the experimental data.

Experimental study on the fluidization behaviors of the superfine particles

Abstract In this study, the fluidization behaviors of the superfine particles with the mean diameter of 5.181 μm were studied. The bed collapse experiment was conducted to study the fluidization quality and regime. The minimum fluidization velocity based on the agglomerate was determined. The agglomerate size distribution characteristic was studied by the 2D Particle Image Velocimetry (PIV). The agglomerate voidage was measured and discussed. The results showed that the micro-particles can also be fluidized in the agglomerate particulate fluidization regime like the nano-particles. The minimum fluidization velocity based on the agglomerate is 3.4 cm/s and much larger than that based on the primary particle. The agglomerate size has the multi-scale characteristic. The distribution of agglomerate diameter is in subject to the Gaussian distribution. And there exists the minimum agglomerate diameter under the effect of van der Waals cohesive force. The agglomerates in different sizes and shapes may have the similar particle arrangement structure. The measured averaged agglomerate voidage is 0.966. It may have an important effect on the interaction between the gas and agglomerate phases.

Fluidization of mixed SiO2 and ZnO nanoparticles by adding coarse particles

Fluidization behavior of binary mixed SiO2 and ZnO nanoparticles by adding 3 different fluid catalytic cracking (FCC) or Al2O3 coarse particles was studied. The Richardson–Zaki exponents of the single and binary mixtures were calculated by using linear regression. The results showed that the improvement of the fluidization behavior of mixed nanoparticles by adding Al2O3 coarse particles was not obvious. Fluidization behavior of mixed nanoparticles can be improved significantly by adding FCC coarse particles. However, the effect of adding FCC2 (61–90μm) and FCC3(38–61μm) coarse particles is better than that of adding FCC1(90–109μm) coarse particles. The order of degree of agglomerate particulate fluidization of beds by adding the same amount of FCC coarse particles is pure SiO2, 70% SiO2 and 30% ZnO, 50% SiO2 and 50% ZnO, 30% SiO2 and 70% ZnO and pure ZnO nanoparticles.

A high efficiency particulate air filter based on agglomerated carbon nanotube fluidized bed

Fluidized bed filtration is a promising technique to remove particles from gas stream. In this work, a fluidized bed filter with agglomerated carbon nanotubes (CNTs) as bed materials is employed to filtrate aerosol particles and its filtration performance is investigated. The aerosol filtration efficiency of fluidized bed filters is found to highly depend on the fluidization state of the filter media. Agglomerate particulate fluidization brings much higher efficiency than agglomerate bubbling fluidization (ABF). Due to the effect of bubbles and the abrasion of fluidizing media in ABF, there are a growing number of nanoparticles releasing from the bed with the increase of gas velocity. The filtration performance of fluidized bed filters with agglomerated CNTs as the media is compared with that of packed bed filters with the same media. It is found that the former provides much lower resistance and obvious higher quality factor than the latter. The filtration efficiency of fluidized bed filter increases with the increase of the static bed height. Our results show that agglomerated CNTs could be employed as media of the fluidized bed which can be used as the high efficiency particulate air filter.

Fluidization behavior of binary mixtures of nanoparticles in vibro-fluidized bed

The fluidization behavior of different mixed SiO2, TiO2 and/or ZnO nanoparticles under the application of vibrated fields of constant vibrated frequencies (40Hz) and amplitude (3.0mm) is studied. The single nanoparticles experiments show that SiO2 nanoparticles have a better fluidization quality than TiO2 and ZnO nanoparticles. For binary mixtures of the nanoparticles, the amount of SiO2 nanoparticles generally has a beneficial effect on the fluidization quality of the binary mixtures. Using the linear regression, the Richardson–Zaki exponents of three kinds single and their binary mixture of nanoparticles are calculated. The Richardson–Zaki analyses indicate that the particulate fluidization degree of mixed nanoparticles can be greatly improved in agglomerate particulate fluidization (APF) behavior.

Fluidization and mixing of nanoparticle agglomerates assisted via magnetic impaction

Mixing of nanopowders in an environmentally benign magnetically assisted fluidized bed (MAFB) system was studied. Examination of fluidization behavior of agglomerate particulate fluidization (APF; silica R974 or R972) and agglomerate bubbling fluidization (ABF; alumina or titania) nano-powders in un-assisted and MAFB systems confirmed previous results on decreased minimum fluidization velocity and increased bed expansion of APF and ABF powders due to magnetic assistance. APF and ABF powder mixtures behaved like APF powders with the bed expansions in between those of individual constituents. Unlike previous MAFB studies, fluidization as a function of time was studied to examine its influence on nano-mixing. With time, the bed expansion reduced, and reduction was faster as magnet-to-powder ratio increased from 0:1 to 5:1, although fluidization was sustained, confirmed via the pressure drop measurements. Reduction in bed expansion was attributed to change in the nature of nanoagglomerates, which showed increased density as a function of processing time, ruling out electrostatics or elutriation as major factors. Mixtures of silica (APF) and alumina (ABF), processed at various magnet-to-powder ratios, were characterized via statistical analysis from energy dispersive x-ray spectroscopy using field emission scanning electron microscope to compute homogeneity of mixing (HoM). Magnetic assistance improved the HoM as a function of time, and was strongly related to the product of number of magnets and time, similar to previous results in magnetically assisted impaction mixing (MAIM). The best achievable HoM was significantly better than unassisted fluidization and comparable to previous results for rapid expansion of high-pressure suspensions and MAIM.

Fluidisation behaviour of silica nanoparticles under horizontal vibration

In present study, fluidization behaviour of silica nanoparticles with primary size between 7 and 16nm under horizontal vibration was investigated. Fluidization characteristics were assessed by observing pressure drop, bed expansion and bed collapse behaviour at the vibration frequencies between 0 and 34 Hz. Horizontal vibration is demonstrated to be as effective as the vertical configuration reported previously in terms of fluidization improvement. In all cases, smooth agglomerate particulate fluidization was observed at vibrational frequencies greater than 16.7 Hz as long as the vibrational amplitude is set to be more than 1. This is much lower than previous reported value, and suggests that fluidization assisted with horizontal vibration is a promising technique for the large scale processing of nanomaterials (e.g. reacting, dispersing, or coating).

Fluidization enhancement of agglomerates of metal oxide nanopowders by microjets

AbstractThe quality of gas–solid fluidization of agglomerates of nanoparticles has been greatly enhanced by adding a secondary flow in the form of a high‐velocity jet produced by one or more micronozzles pointing vertically downward toward the distributor. The micronozzles produced a jet with sufficient velocity (hundreds of meters per second), turbulence, and shear to break‐up large nanoagglomerates, prevent channeling, curtail bubbling, and promote liquid‐like fluidization. For example, Aerosil R974, an agglomerate particulate fluidization (APF) type nanopowder, expanded up to 50 times its original bed height, and difficult to fluidize agglomerate bubbling fluidization (ABF) type nanopowders, such as Aeroxide TiO2 P25, were converted to APF type behavior, showing large bed expansions and homogeneous fluidization without bubbles. Microjet‐assisted nanofluidization was also found to improve solids motion and prevent powder packing in an internal, is easily scaled‐up, and can mix and blend different species of nanoparticles on the nanoscale. © 2009 American Institute of Chemical Engineers AIChE J, 2010

Bubbling Suppression in Fluidized Beds of Fine and Ultrafine Powders

Liquid-fluidized beds of noncohesive beads, gas-fluidized beds of noncohesive light beads, and gas-fluidized beds of fine particles are examples of systems exhibiting nonbubbling fluid-like fluidization. In contrast, stable fluid bubbles have been seen to develop continuously in liquid-fluidized beds of high-density beads, gas-fluidized beds of noncohesive beads, and gas-fluidized beds of fine particles at high gas velocities. On the other hand, for slightly cohesive particles bubbling is just restrained by permanent interparticle contacts in a solid-like fluidized bed. The disparity of behavior between these apparently disconnected systems has contributed to feed a longstanding controversy on the mechanism of bubbling suppression in fluidized beds that still persists. In this article, a description of fluidized beds of cohesive powders based on particle agglomeration due to attractive forces is demonstrated. The effects of fluid viscosity and particle size and density on the type of fluidization behavior are investigated. A main finding is that for fluidization of sufficiently fine particles and/or with high-viscosity gas the fluidized bed transits directly from a nonbubbling fluid-like state to elutriation. A review of empirical observations reported in the literature illustrates a Geldart's diagram modified to consider multistage agglomeration of nanoparticles. The proposed diagram serves to discriminate between the so-called agglomerate particulate fluidization (APF) behavior and agglomerate bubbling fluidization (ABF) behavior on the basis of primary particle size and density.

Fluidization of fine and ultrafine particles using nitrogen and neon as fluidizing gases

AbstractWhen fluidized by a gas, some agglomerated fine and ultrafine particles display a regime of uniform, nonbubbling fluidization known as agglomerate particulate fluidization (APF). The agglomeration of micrometric sized particles, or simple pre‐existing agglomerates in the case of nanoparticles, is governed by the balance between hydrodynamic shear forces and interparticle attractive forces. From this balance the theoretical scaling law Bog ∼ kD+2 has been derived, where Bog, the granular Bond number, is the ratio of the interparticle attractive force to particle weight, k is the ratio of agglomerate to particle size, and D is the fractal dimension. In the experimental program the behavior of gas‐fluidized beds of fine and ultrafine particles as affected by the use of neon and nitrogen as fluidizing gas is studied. The experimental results indicate that there is no relevant distinction between the sizes of agglomerates fluidized with the different gases as theoretically predicted. However, it is seen that the relatively small increment of gas viscosity opens up a new window of highly expanded agglomerate particulate fluidization (APF) behavior with a delayed onset of bubbling. For a sufficiently high‐gas viscosity, and/or smaller particle size, full suppression of the bubbling regime is observed. For nanoparticles exhibiting agglomerate bubbling fluidization (ABF) behavior, where bed expansion is small, and bubbling occurs soon after minimum fluidization, we also observe a delayed onset of bubbling when fluidizing with a gas of higher viscosity. © 2007 American Institute of Chemical Engineers AIChE J, 54: 86–103, 2008

Behavior of mixtures of nano-particles in magnetically assisted fluidized bed

Gas-fluidization of nano-particle mixtures in magnetically assisted fluidized bed (MAFB) has been investigated. Nano-particles undergoing fluidization exhibit two types of behavior: agglomerate particulate fluidization (APF) and agglomerate bubbling fluidization (ABF). Fluidization of sole SiO 2 or ZnO nano-particles exhibits APF and ABF, respectively, but mixtures of them can be fluidized stably and almost homogenously. Stable fluidization of nano-particle mixtures can be achieved by variations in both the field intensity and the initial mixture content. Simple analytical model is developed to predict the apparent terminal velocity of the mixture of APF and ABF nano-particles, and the calculated values are in agreement with the experimental data.

Gas fluidization characteristics of nanoparticle agglomerates

AbstractAn experimental study is conducted to determine the effect of different types of nanoparticles on the gas fluidization characteristics of nanoparticle agglomerates. Taking advantage of the extremely high porosity of the bed, optical techniques are used to visualize the flow behavior, as well as to measure the sizes of the fluidized nanoparticle agglomerates at the bed surface. Upon fluidizing 11 different nanoparticle materials, two types of nanoparticle fluidization behavior, agglomerate particulate fluidization (APF) and agglomerate bubbling fluidization (ABF), are observed and systematically investigated. A simple analytical model is developed to predict the agglomerate sizes for APF nanoparticles, and the results agree fairly well with the optical measurements. Using the Ergun equation, the experimentally measured pressure drop and bed height, and the average agglomerate size and voidage at minimum fluidization predicted by the model, the minimum fluidization velocities for APF nanoparticles are calculated and also agree well with the experimental values. Other important fluidization features such as bed expansion, bed pressure drop, and hysteresis effects, and the effects of the primary particle size and material properties are also described. © 2005 American Institute of Chemical Engineers AIChE J, 51: 426–439, 2005