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Types of gas fluidization
Abstract
The behaviour of solids fluidized by gases falls into four clearly recognizable groups, characterized by density difference (ϱs–ϱf) and mean particle size. The most easily recognizable features of the groups are: powders in group A exhibit dense phase expansion after minimum fluidization and prior to the commenment of bubbling; those in group B bubble at the minimum fluidization velocity; those in group C are difficult to fluidize at all and those in group D can form stable spouted beds. A numerical criterion which distinguishes between groups A and B has been devised and agrees well with published data. Generalizations concerning powders within a group can be made with reasonable confidence but conclusions drawn from observations made on a powder in one group should not in general be used to predict the behaviour of a powder in another group.
... Such systems can be found at many length-scales, from biological matter, to colloidal suspensions, emulsions, up to traffic flow or even star-forming regions. In granular media, agitation can counteract their dissipative nature by a constant energy input [1,2]. Transforming static granular solids into dynamic granular fluids, such constant agitation maintains a non-zero mean kinetic energy per particle, i.e., a finite granular temperature, T [3]. ...
... Experiments.-We measure the steady-state rheology of air-fluidized glass particles (glass bulk density ρ p = 2.5 g cm −3 , diameter d ∈ [150-200] µm, sample mass M = 170 g, Geldart group B [1]), and vary the fluidization gas flow velocity, u. The single fluidization control parameter is u: the bed expands with increasing u, which lowers the global packing fraction, φ (see Fig. 1 inset) and, at the same time, increases agitation. ...
Agitated granular media have a rich rheology: they exhibit Newtonian behavior at low shear rate and density, develop a yield stress at high density, and cross over to Bagnoldian shear thickening when sheared rapidly -- making them challenging to encompass in one theoretical framework. We measure the rheology of air-fluidized glass particles, spanning five orders of magnitude in shear rate, and show that all rheological regimes can be delineated by two dimensionless numbers. We propose a constitutive relation that captures all flow behaviors, qualitatively and quantitatively, in one unified framework.
... The solid amine sorbent (DRC-I) used in this study is a high spherical particle with an average particle size of 572 mm and a particle density of 716 kg/m 3 , so it is categorized in Geldart's group B (Geldart, 1973). More details can be seen in our previous study (Xu et al., 2023;Xu, Zhang, Peng, Yang, et al., 2024;Xu, Zhang, Peng, Yang, et al., 2024). ...
... It is commonly known that U mf marks the transition from fixed bed (FB) to bubbling fluidized bed (BFB) for Geldart B particles (Geldart, 1973), which is commonly determined by the variation of DP of the bed. As exhibited in Fig. 4(a), the DP between measuring P 1 and P 4 firstly increase linearly with U g when the sorbent bed is fixed, and then levelled off irrespective of further increase in U g when the sorbent bed is fully fluidized. ...
Fluidization technology has been used in CO2 capture processes, the successful design and operation of the heat exchangers involved in this process require much information on the bed-to-wall heat transfer of the sorbent particles in fluidized states. In this study, the bed-to-wall heat transfer coefficient (h) of a solid amine sorbent was measured by a heat transfer probe in a large-scale circulating fluidized bed cold model unit, where full spectrum of fluidization regimes can be realized. The corresponding hydrodynamic signals were also studied by pressure sensors and optical fiber probes to further explain the newly discovered phenomenon. The results show that in a dense bed, due to the counterbalanced effect of time fraction of packet and packet renewal frequency, h of the Geldart B particle reaches a peak within the bubbling fluidized regime, and the radial distribution of h are opposite in bubbling and turbulent fluidized regimes. In a fast fluidization regime, gas convection becomes the dominant factor affecting h when the solids holdup is low enough. Correlations were provided or recommended to guide the design of heat exchangers in the fluidized bed CO2 capture processes.
... In accordance with the Geldart diagram [32], inert ABS particles can be classified as Group B, owing to their spherical diameter of 2.7 ± 0.3 mm and a solid density of 1077.1 ± 4.6 kg/m 3 . The fluidization of these particles is characterized by the formation of bubbles and a densified appearance, where gas bubbles form at the base of the bed, followed by subsequent expansion and collapse. ...
This study utilized passive acoustic emissions from a fluidized bed containing spherical inert ABS particles, captured by an external piezoelectric microphone, to monitor fluidization agitation intensity. Acoustic signals were recorded during fluidization profiles achieved under air velocities ranging from 0.5 to 3.0 m/s and during the drying of water or maltodextrin aqueous solution (1:5 w/w) introduced as droplets. Analyzing audio features like waveforms, the Discrete Fourier Transform (DFT), and Mel Frequency Cepstral Coefficients (MFCCs) revealed changes corresponding to the agitation intensity of the particles. The MFCC coefficients were input into a three-layer artificial neural network (ANN) to predict fluidization dynamics based on air velocity, liquid flow rate, and drying time. The ANN efficiently learned from the data, achieving high predictive accuracy (R2 > 0.8) after 15 epochs of training, showcasing the robustness of MFCC coefficients for modeling. This approach highlights that the application of passive acoustic signals and neural networks allows for real-time monitoring of fluidization behavior during drying processes.
... In gas-solid fluidized beds, a dense medium of suitable particle size is required to form a stable fluidized state and uniform density distribution in order to achieve efficient separation. Geldart B particle magnetite powder is generally used as the dense medium in coal separation [36]. In order to achieve the separation effect, reduce processing costs, and make the dense medium easy to recycle, wide-particle-size magnetite powder is generally used as the dense medium, with particle sizes mainly distributed between 0.074 and 0.3 mm and an average particle size of 225.37 µm. ...
Gas–solid separation fluidized bed is an efficient and clean coal separation technology with a good separation effect for coal particles. However, there is a lack of systematic research on the complex motion behavior of the feeding particles in gas–solid separation fluidized beds. In this study, the separation kinetics of non-spherical single feeding particles in the fluidized bed are examined. The particle sphericity coefficient Φ is introduced to characterize the morphology of irregular coal particles, and the drag coefficient for the feeding particles is modified to verify the suitability of the non-spherical particle drag model for gas–solid fluidized bed separation. After optimization and correction, a ρS.sus (the bed density when single feeding particles are suspended in the gas–solid separation fluidized bed) prediction model is obtained. When the prediction accuracy of the ρS.sus prediction model is 90%, the confidence degree is 85.72%. This ρS.sus of the single non-spherical feeding particle prediction model highlights a direction for improving the separation effect, provides a theoretical basis for the industrialization of gas–solid fluidized bed, and promotes the process of dry fluidized separation.
... Particle classification given by Geldart[33]. ...
In recent years, the fast growth of computational power has allowed the application of computational fluid dynamics (CFD) in a wide range of areas of interest, such as gas–solid unit operations. In this context, the multiphase particle-in-cell (MP-PIC) method appears as an option to represent fluid–particle and particle–particle interactions, avoiding the complexity of tracking each particle and the high computational cost derived from this. The MP-PIC method can represent the particles as a group with the same characteristics, allowing the simulation of gas–solid systems at different scales. To achieve this, the particle–particle interactions are simplified using the solid stress tensor to represent them; this does not require explicit expressions. This approach has a low computational cost, allowing the simulation of industrial cases using just workstations. This paper provides a review of the literature on the solid stress tensor and its commercial and non-commercial applications, including its historical and mathematical development in the description of particle–particle interactions. In addition, to consolidate the knowledge and advancing understanding in this crucial aspect of multiphase flow simulations, this review identifies the current challenges and opportunities for future research in multiphase systems based on the solid stress tensor. In addition, this review identifies the current challenges and opportunities for future research in multiphase systems based on the solid stress tensor.
... Details on the microstructural attributes of the powders are illustrated in Fig. 3, and their physical properties are summarized in Table 1. These properties include the characteristic diameters of the particle size distribution, pycnometric density (ρ p ), fluidization classifications according to Geldart (Geldart, 1973), and minimum fluidization velocities (v mf ) calculated using Grace's correlations (Grace, 1986). These correlations, which compile extensive experimental data, provide valuable insights into fluidization behavior. ...
Within the intricate world of granular materials, the behavior of grain assemblies presents complexities characterized by nonlinear and inelastic phenomena, which seamlessly link the microscopic grain scale to the macroscopic bulk scale. A key challenge in understanding the mechanics of granular materials lies in establishing connections between these microscopic grain properties and their macroscopic flow behavior. This study delves into vibration-induced densification, a phenomenon relevant across various technological domains in powder processing and manufacturing. Specifically, we explore the vibrational conditions that induce compaction and decompaction under vertical vibration, employing a particle damper across industrial powders, including glass beads, joint filler, wheat flour, and pharmaceutical excipients. The experiments involve controlling the vibration wave by adjusting parameters such as frequency and amplitude while measuring and recording the acceleration and force signals. Our findings reveal a significant correlation between the force required to decompact the powder bed and the attractive forces between grains. This correlation facilitates the determination of a dimensionless granular number Ad, offering insights into the contact force network at a macroscopic level and its relation to flow indices. By proposing this experimental approach, we provide a straightforward method to unveil the intricate relationship between local particle interactions and the overarching mechanical behavior of granular materials, contributing to advancements in understanding and predicting powder flow behavior.
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... The reactor column used for the cold flow model process (29-34°C) uses acrylic material with a total reactor height 1.70 m and hot flow model (200-550°C) with different fluidized bed reactors (total reactor height 1.04 m). The particle feedstock used is oil palm frond biomass with the size of 420 µm (Group B) contains most materials in the mean size ranges 40µm<dsv<500µm [28]; mass 200 gr; solid density ( ) 253.134 kg/m 3 , bulk density ( ) 100.201 kg/m 3 , voidage (ε OPF ) 0,604, and silica sand sphericity: round sand 0.86, sharp sand 0.66 [14], mass 200 gr; solid density ( ) 1262.720 kg/m 3 , bulk density ( ) 257.543 kg/m 3 , voidage (ε sand ) 0.796. ...
In designing a fluidized bed reactor, the gas distribution plate plays a pivotal role. It ensures proper functioning of the gas-solid fluidized bed. This study investigated two types of distributor plates: perforated and bubble caps, with sintered and triangular configurations. The aim was to compare the pressure drop when air flowed through these distributors. The bed material comprised oil palm frond particles (420 µm in size, 200 gr, bulk density of 100.201 kg/m³) and silica sand, with different column diameters (2 inch, 4 inch, and 6 inch). Observations were made on pressure drops under both cold and hot flow model conditions. The pressure drops across both the material bed (ΔpB) and the distributor plate (ΔpD) were influenced by several factors, including distributor plate configuration, number of bubble caps, and bed material density. Additionally, the pressure drops across the material bed decreased as its mass decreased due to increased gas flow velocity. In hot operating conditions, the bubble cap configuration facilitated easier particle movement due to increased pressure, thus requiring less pressure. Both triangular and sintered bubble cap configurations with the largest number of holes and optimal pitch distance to the distributor surface area were found to be effective.
... The sizes of the spheres varied (Figure 1c), and agglomerated crystalline substances were attached to their surfaces ( Figure 1d). Geldart suggests that when the particle size of fly ash is <20 µm, the interparticle forces outweigh gravity, resulting in a certain degree of aggregation and adhesion [32]. Davidovits found that fly ash undergoes partial dissolution-aggregation hydration reactions under high air humidity conditions. ...
Arsenic contamination of various environmental components poses a serious threat to human and animal health. Soil As contamination is particularly hazardous, as soil is a vital pathway to the food chain. We conducted experiments on soil from a typical pharmaceutical and chemical industry relocation site in Hubei Province, focusing on modification using fly ash through mechanical and chemical mechanisms. We subjected varying proportions of lime, ferrous sulfate, and fly ash to mechanical ball milling and used these mixtures to perform remediation of arsenic-contaminated soil and site restoration. Our findings are as follows: in soil culture experiments, the As stabilization efficiency reached 90% within 90 days with ferrous salt-modified fly ash. In actual site restoration, As-stabilization efficiency exceeded 95% across different soil depths within 30 days, demonstrating significant stabilization effects. Optimal modified dosages were determined as 2% ferrous sulfate and 2% fly ash. After stabilization, As in the soil primarily existed in amorphous iron-aluminum oxide-bound (F3) and crystalline iron-aluminum oxide-bound (F3 + F4) and residual (F5) states. Fluctuations in the moisture content and pH mainly activated F3 and F4, transitioning them into exchangeable (F1) and surface-adsorbed (F2) states. Arsenic leaching was predominantly associated with the F1 form. Fly ash-based restoration technology demonstrates promising capabilities in waste treatment and pollution control, offering significant potential for widespread application.
... It is worth noting that lower characteristic mean diameters could probably have been obtained by a more energetic air dispersion in the laser granulometry device. Knowing that the skeleton density of pure alumina is 3.9 g cm −3 , this powder belongs to Geldart's group C (cohesive powder), meaning that it could be difficult to fluidize conventionally [29]. Its untapped bulk density is close to 2 g cm −3 , as deduced from bed mass and height measurements in the fluidization devices. ...
Uniformly coating micronic particles with metals is of main interest for a broad range of applications. This study demonstrates the feasibility of depositing pure copper on the surface of micronic alumina particles by the fluidized bed chemical vapor deposition process from the cheap and nontoxic copper acetylacetonate precursor. Thanks to the development of a preconditioning protocol, a complete fluidization of the particles organized as porous agglomerates was reached. The coating of the individual particles was favored by using conditions involving low deposition rates. The influence of key operating parameters on the process behavior and on the characteristics of the deposit was studied. The deposited copper was of cubic crystal structure without carbon nor oxide contamination.
... Moreno-Atanasio et al. [9] investigated the effect of particle elasticity on the fluidization behavior of adhesive particles in a fluidized bed and found that, for a high value of surface energy, the system with a smaller stiffness is difficult to fluidize in contrast to the case of a larger stiffness. Kobayashi et al. [10] applied a reduced spring constant model to a DEM-CFD simulation of the fluidization behavior of Group A particles in the Geldart classification [11]. A validation experiment was conducted using spherical glass particles with a diameter of 60 μm. ...
When the DEM simulations are performed, the particle stiffness of the Discrete Element Method (DEM) is often reduced to reduce the computational cost. The Dynamic Adhesion/Cohesion Force Model (DAFM/DCFM) was proposed for adhesive/cohesive particles to make the effect of adhesion/cohesion force on the collisional motion of a particle with a reduced particle stiffness equivalent to that of the original system. This study validates the applicability of DCFM to the contact-dominated regime by means of a series of DEM simulations of two-dimensional simple shear flow of cohesive spherical particles. The results demonstrate that DCFM accurately represents the cohesive nature of the original system with respect to the fluid-to-solid phase transition of granular medium due to a reduction in shear rate, as well as the shear stress of the granular medium. It is demonstrated that the bond-breaking model proposed in the present study describes well the validity of DCFM in the contact-dominated regime.
... Finally, it has to be pointed out that the drag correlations cited above do not account for inter-particle forces (such as cohesion [148]), which may lead, for example, to the formation of agglomerates (not to be confused with clusters arising from a hydrodynamic instability [28,29]). Cohesion commonly gets relevant for rather small particles, which are generally referred to Geldart type A and C particles [149,150], since it commonly scales linearly with the particle diameter while the drag force scales with the particle's cross-section [148]. Particularly, cohesion lowers the expansion of the bed due to the larger effective diameter of the agglomerates (figure 4). ...
In this review paper, we provide a brief overview of the recent advances in the continuum modeling of gas–particle flows. First, we focus on the kinetic theory-based two-fluid models, which have become a valuable tool to investigate small-scale moderately dense turbulent gas–particle flows. Second, the continuum description is quite restrictive with respect to the maximum grid spacing, and large-scale simulations usually employ coarse mesh resolutions to keep the analyses practicable. Such coarse-graining inevitably neglects the small unresolved scales, which requires additional modeling. Here, filtered two-fluid models have been applied successfully to a variety gas–solid flow problems. Finally, we give a condensed outline about future research challenges for the continuum modeling of gas–particle flows.
... Complementing this, other references focus on factors affecting flowability and powder handling, measurement techniques, and practical implications for the food industry (Fitzpatrick, 2005(Fitzpatrick, , 2013. Research on gas fluidization principles, although not exclusively food-focused, provides valuable insights for handling and processing food powders, optimising fluidized bed operations, and assessing powder flowability (Geldart, 1973). In relation to the morphology of particles, accurate descriptions have been derived from visual observation and computer-assisted image analysis (Russ, 1994;Russ & Neal, 2018). ...
The size and morphology of particles impact the quality and functionality of food in derived particulate systems. This review presents an extensive bibliometric analysis targeting food science and technology. Through keyword compilation and bibliographic coupling analysis, the research topics identified are as follows: stability and functionality of food systems, ultrasound‐assisted processes, food oral processing and digestion, starch properties and modifications, and spray drying and microencapsulation. This comprehensive overview details the primary methods employed in particulate systems and highlights the influence of particles in the design of food systems.
... [31][32][33] The detachment of API particles adhered to the carrier occurs when shear forces exceed the adhesion forces established between the API and the carrier. 34,35 The mechanisms involved in the movement of powder particles, with high and low fine content, 31 usually have different fluidization properties, which can be included in the Geldart's classification 36,37 that is based on mean particle sizes (d p ) and density differences between the particles and the gas (q pq g , where q p is particle density and q g is the fluidizing gas density). This classification is defined into four classes in reducing size: D (dense or spoutable); B (bubbly-ready or sandy-like); A (aeratable); and C (cohesive). ...
In contemporary times, there has been a rise in the utilization of dry powder inhalers (DPIs) in the management of pulmonary and systemic diseases. These devices underwent a swift advancement in terms of both the equipment utilized and the formulation process. In this review, the carrier physicochemical characteristics that influence DPI performance are discussed, focusing its shape, morphology, size distribution, texture, aerodynamic diameter, density, moisture, adhesive and detachment forces between particles, fine carrier particles, and dry powder aerosolization. To promote the deposition of the active principal ingredient deep within the pulmonary system, advancements have been made in enhancing these factors and surface properties through the application of novel technologies that encompass particle engineering. So far, the most used carrier is lactose showing some advantages and disadvantages, but other substances and systems are being studied with the intention of replacing it. The final objective of this review is to analyze the physicochemical and mechanical characteristics of the different carriers or new delivery systems used in DPI formulations, whether already on the market or still under investigation. [Figure: see text].
... The cohesive forces and thus the tendency to agglomeration effects can be reduced by a larger particle size [20]. Granulation is a method that increases the size of particles through shaping. ...
One solution to reduce CO2 emissions and advance climate protection in the heat sector is the utilization of thermochemical energy storages (TCES) based on the material Ca(OH)2 to store waste heat or surplus electricity. In contrast to powdered materials, the material in granular form has a lower tendency to be affected by agglomeration effects and provides higher energy density and thermal conductivity. This results in enhanced durability, stability, and efficiency of the storage, making the storage material Ca(OH)2 in granular form more suitable for TCES applications. This work investigates the performance of a chemically unmodified granular sample over 15 storage cycles in a test fixed bed reactor and evaluates whether its performance and durability are already sufficient for potential applications without further modification. In addition, different granule samples with different diameters were also cycled to determine whether the selection of certain diameter sizes can improve the stability of the granules. Furthermore, different methods were used in the pre- and post-analyses to assess the stability of the samples. The comparison of three tested granular samples with different diameters reveals that the sample with a diameter of 2–4 mm has the highest durability and reactor performance. Despite a decrease in mechanical strength, this configuration could be suitable for applications with a small number of storage cycles, such as seasonal storages.
... Circulating fluidised beds (CFB) enable continuous operation as they circulate the solid particles in and out of the bed without a recognised freeboard area. BFBs are less complex than their counterparts but are more suited to small-scale storage, with CFPs offering better mixing and thermal throughput at 5000-7000 kW/m 2 compared to 1200-1600 kW/m 2 for BFBs [62]. However, significant drawbacks of CFPs include the resultant cost of their complexity in addition to particle attrition and abrasion. ...
Thermochemical energy storage (TCES) presents a promising method for energy storage due to its high storage density and capacity for long-term storage. A combination of TCES and district heating networks exhibits an appealing alternative to natural gas boilers, particularly through the utilisation of industrial waste heat to achieve the UK government’s target of Net Zero by 2050. The most pivotal aspects of TCES design are the selected materials, reactor configuration, and heat transfer efficiency. Among the array of potential reactors, the fluidised bed emerges as a novel solution due to its ability to bypass traditional design limitations; the fluidised nature of these reactors provides high heat transfer coefficients, improved mixing and uniformity, and greater fluid-particle contact. This research endeavours to assess the enhancement of thermochemical fluidised bed systems through material characterisation and development techniques, alongside the optimisation of heat transfer. The analysis underscores the appeal of calcium and magnesium hydroxides for TCES, particularly when providing a buffer between medium-grade waste heat supply and district heat demand. Enhancement techniques such as doping and nanomaterial/composite coating are also explored, which are found to improve agglomeration, flowability, and operating conditions of the hydroxide systems. Furthermore, the optimisation of heat transfer prompted an evaluation of heat exchanger configurations and heat transfer fluids. Helical coil heat exchangers are predominantly favoured over alternative configurations, while various heat transfer fluids are considered advantageous depending on TCES material selection. In particular, water and synthetic liquids are compared according to their thermal efficiencies and performances at elevated operating temperatures.
This article also discusses the demonstration of a fluidized bed dryer, its principles, application, and other processing. This has led to the development of technology, processes, and equipment that not only reduce time but also increase output. A fluidized bed dryer significantly reduces drying time compared with a vacuum dryer or tray dryer. In the pharmaceutical industry, fluidized bed dryers are typically used for pellet drying, coating, andgranulation. Fluidized bed drying is very well known to yield high heat and mass transfer and is hence adopted in many industrial drying processes, particularly pharmacy products. In this paper, we show the construction and working of the fluidized bed dryer. It works on the drying principle. It has several advantages and disadvantages.
The importance of tryptophan in both human and animal nutrition continues to drive its demand and market growth. To enhance the usability of tryptophan products, such as flowability, bulk density, and mixing efficiency, the fluidized bed granulation process plays a critical role in the industrial production of tryptophan from microbial fermentation. This process involves a simplified method of evaporation and granulation to efficiently produce commercial granular products. This study conducted a solid flow analysis for scaling up fluidized bed granulators in the manufacturing of calcium–tryptophan granules, focusing on the solid flow characteristics, the flow regime within the granulator, and the granulator's design considerations. The scale-up case for calcium–tryptophan granules revealed that the bubbling fluidization flow regime occurred similarly at both pilot and plant scales. Then, the transport disengaging height to minimize fine particle loss inside the fluidized bed granulator and the entrainment rate to secure the capacity of entrained particle recovery equipment such as cyclones or bag filters were determined to obtain basic design data for scale-up of the fluidized bed granulation process.
Thesis (Ph. D.)--University of Washington
As the result of an experimental study of the fluidization of light solids by gases under various pressures and the fluidization of heavy solids by water, a general family of curves has been produced relating a generalized fluid velocity to a bed porosity function for a series of values of a parameter P involving the fluid and solid properties only. This correlation satisfies both liquid- and gas-fluidised beds and indicates that there is no fundamental difference in mechanism between a liquid-fluidized bed and a gas-fluidized bed. It also enables direct comparison to be made between flow through a fluidized bed and flow through a fixed bed of similar porosity. By treating an aggregatively-fluidized bed as a two-phase system and making reasonable assumptions about the rise of bubbles the general bed expansion correlation is used to produce general correlations of mean bubble velocities, frequencies, concentrations and volumes in terms of the mean bed porosity and the parameter P. These correlations predict that a gas system normally has a comparatively small number of large bubbles and appears to fluidize aggregatively. A liquid system may have a large number of small bubbles and appear to fluidize particulately although certain liquid-solid systems will fluidize aggregatively. The values of the bubble characteristics for both liquid and gas systems are in approximate agreement with those observed. These correlations can also be used to predict the fraction of fluid passing through the bed in bubble form and hence gives assistance to the estimation of the efficiency of the fluidized bed as a chemical reactor. The effect of gas pressure on this efficiency is also discussed.
Gas flow through a fluidized bed is normally streamline and the pattern of flow depends on the bubbles within the bed and their velocities relative to gas. When the gas velocity for fluidization is low compared with the bubble velocity (as in beds of fine particles) a region of gas circulation or a “cloud” as it is called, is formed around the bubble.This paper describes experimental observations of the shape and size of gas clouds as they form around bubbles in two-dimensional fluidized beds of spherical and irregular particles. Exchange of gas between the cloud and interstitial phases is shown to occur by a process of “cloud shedding,” and the behaviour of clouds as their parent bubbles split and coalesce is described. The observations of cloud shape are compared with the theoretical predictions of Davidson, Jackson and Murray and from this an estimate of the cloud shape in real three-dimensional beds is made.RésuméL'écoulement du gaz à travers un lit fluidisé se produit ordinairement en ligne droite et sa disposition dépend des bulles au sein du lit et de leur vitesse relative par rapport au gaz. Quand la vitesse du gaz de fluidisation est basse comparée à celle de la bulle (comme dans les lits de fines particules) il se forme autour de la bulle une région de circulation gazeuse que l′on appelle “nuage”.Cet article décrit des observations expérimentales sur la forme et la dimension de nuages gazeux qui se forment autour les bulles dans des lits fluidisés, à deux dimensions, de particules sphériques ou irrégulières. L'auteur montre que l'échange de gaz entre le nuage et la phase intersticielle se produit par un procédé de “cloud shedding” et décrit le comportement des nuages quand leurs bulles associées se divisent et coalescent. Il compare les observations sur la forme des nuages avec les prédictions théoriques de Davidson, Jackson et Murray et en déduit une prévision de la forme des nuages dans les lits réels à trois dimensions.
The fluidisation characteristics of particles which are of such a size and density that the velocity of gas in the dense phase exceeds the rising velocity of the bubbles have been studied.Spherical particles in the range 1–2 mm were fluidised in two- and three-dimensional equipment. Minimum fluidisation velocities measured in the 3D equipment at 200° C and 350°C were found to agree reasonably well with those predicted using an equation based on Ergun's equation for packed beds.Measurements were made in the 2D bed of bubble shapes, sizes, velocities and concentrations, as well as of bed expansion and solid mixing rates. The distribution of gas between the dense and bubble phases was found to be significantly different from predictions based on the two-phase theory of fluidisation.
Measurements of bubble velocities in freely bubbling beds have shown that the velocities of individual bubbles are influenced more markedly by the disposition of surrounding bubbles than by the size of the individual bubble itself. Bubbles present in high concentration rise at much greater velocities than isolated bubbles of the same volume.The apparent anomalies reported by Pyle and Harrison for the expansion of bubbling beds have been explained in terms of bubble velocities in excess of slug velocities.RésuméLes calculs des vélocités des bulles dans une couche à bouillonnement libre ont montré que les vélocités des bulles individuelles sont davantage influencées par la disposition des bulles environnantes que par le grosseur de la bulle elle-même volume. Les bulles présentes dans des concentrations élevées s élévent plus rapidement que des bulles isolées de même volume. Les anomalies apparentes rapportées par Pyle et Harrison pour l'expansion de couches bouillonnantes ont été expliquées en terme de veólocités des bulles en excès des veĺocités en bloc.ZusammenfassungMessungen der Blasengeschwindigkeiten in frei durchperlten Betten haben erwiesen, dass die Geschwindigkeit der Einzelblasen stärker durch die Verteilung der umgebenden Blasen als durch die Grösse der Einzelblasen selbst beeinflusst wird. In hohen Konzentrationen anwesende Blasen perlen wesentlich schneller als isolierte Blasen des gleichen Volumens.
A method is described to determine quantitatively the change of the heat conductivity in an electric field parallel to the temperature gradient. Measurements were performed on NF3, CHCl3, C2H5Cl, CH3CN and C2H5CN. While the heat conductivity of NF3, CHCl3 and C2H5Cl decreases in an electric field, an increase of the conductivity has been found for CH3CN and C2H5CN.
A critical examination of the results of previous workers together with new experimental work carried out in a 30.8-cm-diam. bed by the author has shown the fluidisation behaviour of many powders to be independent both of mean particle size and—more significantly—of particle size distribution. In particular, mean bubble size is found to depend only upon the gas distributor, the distance above the distributor and the excess gas velocity U-U0. An equation relating these variables gives good agreement with published data on bubble sizes observed in larger beds fitted with commercial type distributors.
The apparent viscosity of air-fluidized beds of spherical particles has been measured with a torsion pendulum viscometer especially designed to minimize disturbance of the bed. Beds were made of closely sized uniform spheres or of binary mixtures of these. Sphere diameters ranged from 125 to 305 microns. Superficial air velocities ranged up to 10 cm. per second. A general viscosity correlation for uniform beds of this particle density has been obtained. The measurements tend to indicate Newtonian behavior, in which case the observations would be of a true kinematic viscosity. Evidence that fluidized beds can assume certain crystal-like structures has been found.
By examining the equations for unsteady motions of solid particles and fluid through a bed of uniform solids concentration it is found that, owing to the inertia of the solid and fluid, a small disturbance in the mixture density will grow exponentially as it moves upward through the bed. A uniform density distribution is therefore unstable and a fluidized bed cannot be expected to be free of small irregularities, which may grow into "bubbles." The rate of growth of the waves of solids concentration depends on the wavelength of the disturbance, on the void fraction, and on the mean fluidizing velocity. The growth rate is generally greater the shorter the wavelength, but growth of the shortest waves is inhibited by the bed's resistance to shear. As a result, there is a wave that grows faster than any other; its wavelength is presumably related to the initial size of the "bubbles" that form spontaneously at all points in the bed.
The transition from homogeneous to heterogeneous fluidization is described by the occurrence of shock waves. Shock waves will arise in a fluidized bed when the rising velocity of a porosity fluctuation exceeds the longitudinal propagation velocity of an equilibrium disturbance. An expression for the latter velocity has been derived via the elastic properties of a fluidized system. The elasticity modulus of this system has been calculated from a drag force-interparticle distance relation. It is shown that along these lines a satisfactory criterion for the transition between homogeneous and heterogeneous fluidization has been obtained.
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