The effects of particle and gas properties on the fluidization of Geldart A particles

We report on 3D computer simulations based on the soft-sphere discrete particle model (DPM) of Geldart A particles in a 3D gas-fluidized bed. The effects of particle and gas properties on the fluidization behavior of Geldart A particles are studied, with focus on the predictions of U_mf and U_mb, which are compared with the classical empirical correlations due to Abrahamsen and Geldart [1980. Powder Technology 26, 35-46]. It is found that the predicted minimum fluidization velocities are consistent with the correlation given by Abrahamsen and Geldart for all cases that we studied. The overshoot of the pressure drop near the minimum fluidization point is shown to be influenced by both particle-wall friction and the interparticle van der Waals forces. A qualitative agreement between the correlation and the simulation data for U_mb has been found for different particle-wall friction coefficients, interparticle van der Waals forces, particle densities, particle sizes, and gas densities. For fine particles with a diameter , a deviation has been found between the U_mb from simulation and the correlation. This may be due to the fact that the interparticle van der Waals forces are not incorporated in the simulations, where it is expected that they play an important role in this size range. The simulation results obtained for different gas viscosities, however, display a different trend when compared with the correlation. We found that with an increasing gas shear viscosity the U_mb experiences a minimum point near , while in the correlation the minimum bubbling velocity decreases monotonously for increasing μ_g.     

The rise of a buoyant sphere in a gas-fluidized bed

The rise velocity, V, of a single sphere, released in the bottom of a bed of sand fluidized by air, was measured: the sphere had a diameter of 9.0 or 13.2 mm; its density ranged from 900 to . These experiments with a single sphere used: (i) a bubbling bed, diameter 141 mm, with 1.05<U/U_mf<2.00, (ii) a slugging bed, diameter 24 mm, with 1.70<U/U_mf<3.20. Here U is the fluidizing velocity; U=U_mf at incipient fluidization. It was found that, for each sphere in a given bed, V=V_mf+C(U-U_mf): the constant C was up to 10 times larger for bubbling beds than slugging beds.The rise velocity at incipient fluidization, V_mf, is governed, for both types of bed, by the apparent viscosity of the incipiently fluidized bed. Therefore, Stokes's law was used to predict V_mf, but using an important modification: since each buoyant sphere appears to carry on its top a defluidized ‘hood’ of particles, Stokes's law was applied to the composite ‘particle’ consisting of the sphere plus its hood. Analysis of the measured V_mf then gave the volume of the hood, in agreement with direct measurements of it above a fixed cylinder in a two-dimensional bed. In addition, the analysis gave the apparent viscosity of the incipiently fluidized bed to be 0.66 Pa s, in excellent agreement with the estimate of Grace (Can. J. Chem. Eng. 48 (1970) 30) for similar sand.     

A new approach to the identification of transitions in fluidized beds

Earlier work with silica sand has indicated that in a system where a bed of particles is aerated at increasing superficial velocities, the von Neumann ratio, T, based on the bed pressure drop, may be useful in identifying both minimum fluidizing velocity, U_mf, and minimum bubbling velocity, U_mb. Plots of T^− 1 against superficial velocity exhibited significant change at velocities consistent with those where bed height first changed (onset of fluidization), the packed bed pressure drop underwent a transition from a monotonic linear function of superficial velocity to a steady value (onset of fluidization) and standard deviation of bed pressure drop rapidly increased (onset of bubbling). However, the suggestion that T might be a valid indicator of U_mf was supported by data for a single material only. In this paper additional data is presented that supports the suggestion that T might be useful in measuring U_mf, and also provides additional evidence for the potential utility of T in determining U_mb. In principle, this would allow a single campaign of pressure measurements to be used to identify both U_mf and U_mb.     

Behaviour of gas-fluidized beds of fine powders part I. Homogeneous expansion

The minimum bubbling velocity has been correlated from the results of experiments on 48 gas/solid systems and is a function of the density and viscosity of the fluidizing gas, the mean sieve size of the powder and the fraction of fines less than 45 μm. An improved equation is presented to predict U_mb/U_mf; this parameter also correlates well with the maximum non-bubbling bed expansion ratio H_mb/H_mf which is used to calculate the maximum dense phase voidage ?_mb. An equation based on the Carman-Kozeny theory can be used to predict bed voidages between incipient fluidization and bubbling. An accurate value of particle density is essential in these equations and a simple comparative method has been used to determine particle densities of fine porous powders.     

Mass transfer around freely moving active particles in the dense phase of a gas fluidized bed of inert particles

The mass transfer coefficient around freely moving active particles under bubbling/slugging fluidized bed conditions was measured in a lab-scale reactor. The technique used for the measurements consisted in the oxidation reaction of carbon monoxide at over one or few Pt catalyst spheres immersed in an inert bed of sand. It was shown that this technique is simple and accurate, and allows to overcome most of the difficulties and uncertainties associated with other available techniques. The experimental campaign was carried out by varying the fluidization velocity (0.15-0.90 m/s), the active particle size (1.0-10.0 mm) and the inert particle size (0.1-1.4 mm). Results were analyzed in terms of the particle Sherwood number. Experimental data showed that Sh is not influenced by the fluidization velocity and by a change of regime from bubbling to slugging, whereas it increases with a square root dependence with the minimum fluidization velocity and with the active particle size. These results strongly suggest that the active particles only reside in the dense phase and never enter the bubble/slug phase. Data were excellently fitted by a Frössling-type correlation:

Sh=2.0·ε_mf+K·(Re_mf/ε_mf)^1/2·Sc^1/3     

Determination of minimum fluidization velocity by pressure fluctuation measurement

Using the standard deviation of pressure fluctuations to find the minimum fluidization velocity, U_mf, avoids the need to de-fluidize the bed so U_mf, can be found for operational bubbling fluidized beds without disrupting the process provided only that the superficial velocity may be altered and that the bed remains in the bubbling fluidized state. This investigation has concentrated on two distinct aspects of the pressure fluctuation method for U_mf determination: (1) the minimum number of pressure measurements required to obtain reliable estimates of standard deviation has been identified as about 10000 and (2) pressure fluctuation measurements in the plenum below the gas distributor are suitable for U_mf determination so the problems of pressure probe clogging and erosion by bed particles may be avoided.     

Theoretical and experimental study on hydrodynamic characteristics of fluidization in air-sand conical beds

This work was aimed at modeling hydrodynamic characteristics of fluidization in conical beds using quartz sand as the inert bed material and air as the fluidizing agent. The minimum fluidization velocity, u_mf, and the minimum velocity of full fluidization, u_mff, were determined by Peng and Fan's models modified for conical fluidized bed. Meanwhile, the pressure drop across a bed, Δp (including Δp_max and Δp_mff corresponding to u_mf and u_mff, respectively), was predicted by using modified Ergun's equations for variable superficial air velocity at an air distributor, u₀. The predicted results were validated by experimental data for some operating conditions. Effects of the sand particle size, cone angle and static bed height on the fluidization pattern and hydrodynamic characteristics are discussed. With the proposed models, the Δp-u₀ diagram were obtained with rather high accuracy for the conical air-sand beds of 30-45^° cone angles and 20-30 cm static bed heights, when using 300- sand particles. For the predicted u_mf and u_mff, the relative computational errors were found to be within 20% for wide ranges of operating variables, whereas Δp_max and Δp_mff could be predicted with lower (10-15%) relative errors. With higher cone angles and/or bed heights, the computational accuracy was found to deteriorate.     

Fluidization of Group B particles with a rotating distributor

A novel rotating distributor fluidized bed is presented. The distributor is a rotating perforated plate, with 1% open-area ratio. This work evaluates the performance of this new design, considering pressure drop, Δp, and quality of fluidization. Bed fluidization was easily achieved with the proposed device, improving the solid mixing and the quality of fluidization.In order to examine the effect of the rotational speed of the distributor plate on the hydrodynamic behavior of the bed, minimum fluidization velocity, U_mf, and pressure fluctuations were analyzed. Experiments were conducted in the bubbling free regime in a 0.19 m i.d. fluidized bed, operating with Group B particles according to Geldart's classification. The pressure drop across the bed and the standard deviation of pressure fluctuations, σ_p, were used to find the minimum fluidization velocity, U_mf. A decrease in U_mf is observed when the rotational speed increases and a rise in the measured pressure drop was also found. Frequency analysis of pressure fluctuations shows that fluidization can be controlled by the adjustable rotational speed, at several excess gas velocities.Measurements with several initial static bed heights were taken, in order to analyze the influence of the initial bed mass inventory, over the effect of the distributor rotation on the bed hydrodynamics.     

Bubble behavior in corrugated‐wall bubbling fluidized beds—Experiments and CFD simulations

A new concept to harness bubble dynamics in bubbling fluidization of Geldart D particles was proposed. Various geometrical declinations of a cold‐prototype corrugated‐wall bubbling fluidized bed were compared at different flow rates (U_g) to conventional flat‐wall fluidized bed using high‐speed digital image analysis. Hydrodynamic studies were carried out to appraise the effect of triangular‐shaped wall corrugation on incipient fluidization, bubble coalescence (size and frequency), bubble rise velocity, and pressure drop. Bubble size and rise velocity in corrugated‐wall beds were appreciably lower, at given U_g/U_mb, than in flat‐wall beds with equal flow cross‐sectional areas and initial bed heights. The decrease (increase) in size (frequency) of bubbles during their rise was sustained by their periodic breakups while protruding through the necks between corrugated plates. Euler‐Euler transient full three‐dimensional computational fluid dynamic simulations helped shape an understanding of the impact of corrugation geometry on lowering the minimum bubbling fluidization and improving gas distribution. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Effective particle diameters for simulating fluidization of non‐spherical particles: CFD‐DEM models vs. MRI measurements

Computational fluid dynamics—discrete element method (CFD‐DEM) simulations were conducted and compared with magnetic resonance imaging (MRI) measurements (Boyce, Rice, and Ozel et al., Phys Rev Fluids. 2016;1(7):074201) of gas and particle motion in a three‐dimensional cylindrical bubbling fluidized bed. Experimental particles had a kidney‐bean‐like shape, while particles were simulated as being spherical; to account for non‐sphericity, “effective” diameters were introduced to calculate drag and void fraction, such that the void fraction at minimum fluidization (ε_mf) and the minimum fluidization velocity (U_mf) in the simulations matched experimental values. With the use of effective diameters, similar bubbling patterns were seen in experiments and simulations, and the simulation predictions matched measurements of average gas and particle velocity in bubbling and emulsion regions low in the bed. Simulations which did not employ effective diameters were found to produce vastly different bubbling patterns when different drag laws were used. Both MRI results and CFD‐DEM simulations agreed with classic analytical theory for gas flow and bubble motion in bubbling fluidized beds. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2555–2568, 2017     

Fluidization of fine powders with air in the particulate and the bubbling regions

Several fractions of a silica—alumina catalyst and an impregnated catalyst were fluidized with air to check published bubble point correlations and to determine the expansion of the dense phase in the bubbling region. The minimum bubbling velocities were fairly close to those reported by Geldart and others, but U_mb varied with only the 0.6 power of the particle size. The dense phase expansion was significant even at velocities much above U_mb and corresponded to relative velocities up to 2 – 3 times U_mf.     

Flow properties of homogeneously aerated,expanded emulsion phase of fine powders (quasi-solid emulsion phase viscosity)

The viscosity of quasi-solid emulsion phase (μ_e) of the “homogeneously aerated, expanded emulsion phase” (HAE emulsion phase) of fine powders within the gas velocity range of minimum fluidization (U_mf) and minimum bubbling (U_mb), was obtained at ambient and elevated temperatures by measuring the strain retardation time (τ) of the HAE emulsion phase in this study.The deformation coefficient (Y) was obtained by our previously developed experimental method [Powder Technol., 81 (1994) 177].We confirmed experimentally that the HAE emulsion phase behaves exactly as quasi-elastic body, by observing the emulsion phase volume expansion and contraction within the velocity range of U_mf and U_mb. It is to be noted that U_mb>U_mf. The volume element of the unit HAE emulsion phase could be expanded or contracted by the force balance between the excessive gas drag force (above the gravitational force) and the inter-particle force of the particles of the emulsion phase. This experimental evidence enabled us to decide using the visco-elastic rheological model of the Voigt–Kelvin model body (VK model body) in terms of mathematics to analyze our experimental data. The rheological parameters, i.e., the elastic deformation coefficient (Y) and quasi-solid viscosity (μ_e) of our experiments, were obtained by using the HAE emulsion phase.Using these experimental data together with the VK model body, we developed our experimental model equation including the measured strain retardation time (τ) to obtain the “quasi-solid viscosity” of the HAE emulsion phase (μ_e). The physical meaning of this viscosity of the HAE emulsion phase developed in this study is quite different from the “apparent quasi-liquid viscosity” defined in aggregative fluidized beds, under the condition of U_mf=U_mb.The interesting experimental data correlations of comprehensive rheological parameters (μ_e, Y, σ_f) of the HAE emulsion were obtained for fine powders at ambient and elevated temperatures.     

Étude hydrodynamique des lits fluidisés sous vide et sous haute température

After having analyzed the few literature results concerning fluidized beds hydrodynamics under reduced pressure, new theoretical elements are proposed which provide an estimation of the beginning and finishing fluidization velocities. Numerous experimental results(U_mfapp, U_mb, U_mp, ?_mf, ?_mb, ?_mp), obtained at 20 and 500°C, are then presented for several sub-atmospheric pressures. Finally, f& the first time, at leastto our knowledge, by high frequency recording pressure drops through the bed, the influence of pressure decreases on the hydrodynamics is accurately analyzed.     

The heat transfer coefficient between a particle and a bed (packed or fluidised) of much larger particles

The heat transfer coefficient has been measured for a heated phosphor-bronze sphere (diam. 2.0, 3.0 or 5.56 mm) added to a bed of larger particles, through which air at room temperature was passed. The bronze heat transfer sphere was attached to a very thin, flexible thermocouple and was heated in a flame to before being immersed in the bed. The cooling of the bronze sphere enabled the heat transfer coefficient, h, to be measured for a variety of U/U_mf, as well as diameters of both the particles in the bed and the heat transfer sphere. It was found that before the onset of fluidisation, h rose with U, but h reached a constant value for U?U_mf. These measurements indicate that in this situation (of a relatively small particle in a bed of larger particles) all the heat transfer is between the hot bronze sphere and the gas flowing over it. Consequently, a Nusselt number, based on the thermal conductivity of the gas, is easy to define and for U?U_mf (i.e. a packed bed), Nu is given by

     

Development of a tribolelectric procedure for the measurement of mixing and drying in a vibrated fluidized bed

Fluidization of fine, pharmaceutical powders makes them easier to dry, coat and mix. Fine powders, however, are difficult to fluidize well with gas flow only. Vibration can often help achieve smooth fluidization at a lower gas flow. The objective of the present study was, thus, to develop reliable and quick experimental methods to characterize mixing and drying in vibrated fluidized beds of fine powders.Effective mixing is critical in many industrial applications and, in gas-solid fluidized beds, requires gas velocities greater than the minimum bubbling velocity (U_mb). There are a number of techniques available for determining U_mb. However, they often are impossible or impractical to use in an industrial application. A new measurement technique involving the use of triboelectric probes was developed. Signal characteristics obtained from sophisticated signal analysis were used to identify the minimum bubbling velocities. These predictions corresponded well with the values obtained from more traditional laboratory methods such as the bed pressure gradient.In a fluidized bed, particles hitting a metal probe will generate a small triboelectric current. Triboelectric probes are able to detect rapid changes in particle surface properties. Surface properties of the particles were modified by wetting the particles in a low shear mixer. This change was detected by triboelectric probes at various locations inserted throughout the bed. The water adsorbed on the particles will begin to evaporate when exposed to the gas stream and the surface properties of the particles will gradually return to their original dry state. The triboelectric probes were able to monitor this drying process. The effects of vibration amplitude on the mixing and drying rate of the bed were also determined.     

Influence of carbon dioxide partial pressure and fluidization velocity on activated carbons prepared from scrap car tyre in a fluidized bed

The effect of carbon dioxide partial pressure and fluidization velocity on activated carbons produced by carbon dioxide activation of scrap car tyre rubber in a fluidized bed has been studied. The method consisted of carbonization at under nitrogen followed by activation at . Three types of activated carbons were produced using activated gas concentrations of 20, 60 and 100% carbon dioxide by volume, the rest nitrogen, at a constant fluidization velocity (0.0393 m/s) to investigate the influence of carbon dioxide partial pressure. Within the experimental setup and activation time of 4 h, it was observed that BET surface area and total pore volume increased with carbon dioxide partial pressure reaching and , respectively, for 100% activation with carbon dioxide. Three other types of activated carbons were produced using 100% carbon dioxide at two (0.0393 m/s), three (0.0589 m/s) and four (0.0786 m/s) times the minimum fluidization velocity (U_mf). The BET surface area and total pore volume were observed to increase with fluidization velocity (which can be viewed as an indicator of the intensity of mixing in the bed), reaching and , respectively, at four times the minimum fluidization velocity.     

Modified gas‐perturbed liquid model (GPLM) for minimum fluidization velocity in a two‐phase inverse fluidized bed reactor with Newtonian and non‐Newtonian systems

In the present investigation minimum fluidization velocity, U_mf, in a two‐phase inverse fluidized bed reactor is determined using low‐density polyethylene and polypropylene particles of different diameters (4,6 and 8 mm) by measuring pressure drop. In a glycerol system U_mf decreased gradually with increase in viscosity up to a value of 6.11 mPa s (60%) and on further increase there was a slight increase in U_mf. In the case of the glycerol system the U_mf was found to be higher when compared to water. In the non‐Newtonian system (carboxymethylcellulose), U_mf decreased with increase in concentration in the range of the present study. The U_mf was found to be lower when compared to water as liquid phase. The modified gas‐perturbed liquid model was used to predict the minimum fluidization liquid velocity (U_lmf) for Newtonian and non‐Newtonian systems. Copyright © 2006 Society of Chemical Industry     

Effect of elevated temperature and silica sand particle size on minimum fluidization velocity in an atmospheric bubbling fluidized bed

The impact of temperature and particle size on minimum fluidizing velocity was studied and analyzed in a small pilot scale of bubbling fluidized bed reactor. This study was devoted to providing some data about fluidization to the literature under high temperature conditions. The experiments were carried out to evaluate the minimum fluidizing velocity over a vast range of temperature levels from 20 °C to 850 °C using silica sand with a particle size of 300–425 μm, 425–500 μm, 500–600 μm, and 600–710 μm. Furthermore, the variation in the minimum fluidized voidage was determined experimentally at the same conditions. The experimental data revealed that the U_mf directly varied with particle size and inversely with temperature, while ε_mf increases slightly with temperature based on the measurements of height at incipient fluidization. However, for all particle sizes used in this test, temperatures above 700 °C has a marginal effect on U_mf. The results were compared with many empirical equations, and it was found that the experimental result is still in an acceptable range of empirical equations used. In which, our findings are not well predicted by the widely accepted correlations reported in the literature. Therefore, a new predicted equation has been developed that also accounts for the affecting of mean particle size in addition to other parameters. A good mean relative deviation of 5.473% between the experimental data and the predicted values was estimated from the correlation of the effective dimensionless group. Furthermore, the experimental work revealed that the minimum fluidizing velocity was not affected by the height of the bed even at high temperature.     

Study of transition velocities from bubbling to turbulent fluidization by statistic and wavelet multi-resolution analysis on absolute pressure fluctuations

Experiments were conducted in three circulating fluidized beds of high, high and high (with an expanded top section of 1.5 m high) at ambient condition. The spent fluid cracking catalyst (FCC), glass beads and sand particles were taken as bed materials. The transition velocities U_c and U_k were analyzed by means of traditional statistic analysis and multi-resolution analysis (MRA) of wavelet analysis on the absolute pressure fluctuation (APF) signals acquired at different axial positions in the beds. According to the standard deviation of APF, the effects of axial positions and static bed heights on U_c and U_k were systematically investigated. An appropriate measuring interval of relative axial position was recommended to identify U_k and two correlations calculated by regression of data in literatures and this work were proposed to predict U_c and U_k for absolute pressure measurement. By means of MRA of wavelet analysis, a redefined variable, homogeneous index HI, deduced from the energy of SF and LF subsignals, was successfully applied to determine the U_c and U_k and demarcated the dynamic behaviors of Geldart group A (spent FCC) and group B (sand particles and glass beads) particles in the circulating fluidized beds.