Phase hold-up and critical fluidization velocity in a three-phase inverse fluidized bed

We studied the hydrodynamic characteristics of a three-phase inverse fluidized bed made of a transparent acrylic column of 0.115 m inner diameter and 2 m heights. Air, water and polyethylene particles were used as the gas, liquid and solid phase, respectively. We used both hydrophobic low density polyethylene (LDPE) and hydrophilic LDPE as solid phase, and distilled water as liquid phase, and filtered air as gas phase. The LDPE was chemically treated by chlorosulfonic acid to change the surface property from hydrophobic to hydrophilic. We tried to solely investigate the effect of the surface hydrophilicity of polymeric particles on the phase holdup and the critical fluidization velocity of three-phase inverse fluidization. Thus, we measured the static pressure and eventually observed critical fluidization velocity. Critical fluidization velocity became smaller in case of using MDPE hydrophobic particles than LDPE hydrophilic particles. This was thought to be due to the retardation of rising bubbles near hydrophobic particles and, subsequently, the increase of gas hold-up.     

The fluidization process of binary mixtures of solids: Development of the approach based on the fluidization velocity interval

The main characteristic of the fluidization process of particulate mixtures is the interaction between bed suspension and segregation-remixing dynamics of their components.On addressing the fluidization properties of binary beds of solids differing either in particle density or diameter, the paper shows how interpretations based on definition of a minimum fluidization velocity of the mixture can lead to erroneous conclusions about the influence played by system variables on their behaviour. An alternative method of investigation is followed which takes into consideration the existence of a finite velocity interval, bounded by the “initial” and “final fluidization velocity” of the mixture, along which the whole process of fluidization has place. The results of a wide series of experiments demonstrate that this alternative approach allows recognizing the independent variables of binary fluidization; they also highlight the differences of behaviour between density- and size-segregating beds.     

Voidage characteristics and prediction of bed expansion in liquid-solid inverse fluidized bed

Studies on voidage fluctuations, axial voidage profile and bed expansion are carried out by measuring the local void fraction using particles of wide ranging characteristics in liquid-solid inverse fluidized bed. The quality of fluidization is elucidated by the local voidage fluctuations. The RMS voidage fluctuation depicts a maximum with respect to average bed void fraction and increases with increase in Archimedes number. The fluidization quality has been quantified using average normalized RMS voidage fluctuation in terms of Transition number. The axial void fraction is almost uniform throughout the bed except for particles with size distribution. All the literature and present experimental data on bed expansion are unified in terms of Richardson and Zaki equation using experimental terminal velocities. A new correlation is proposed for predicting the wall effect corrected experimental terminal velocities, as a substitute for standard drag equation. The bed expansion data are also predicted using the drift flux model.     

Prediction of minimum bubbling velocity, fluidization index and range of particulate fluidization for gas-solid fluidization in cylindrical and non-cylindrical beds

A uniform fluidization exists between minimum fluidization velocity and minimum bubbling velocity. Experimental investigations have been carried out for determination of minimum bubbling velocity and fluidization index for non-spherical particles in cylindrical and non-cylindrical beds. In the present paper equations have been developed for the prediction of minimum bubbling velocity for gas-solid fluidization in cylindrical and non-cylindrical (viz. semi-cylindrical, hexagonal and square) beds for non-spherical particles fluidized by air at ambient conditions. A fairly good agreement has been obtained between calculated and experimental values. Based on the experimental data it is concluded that under similar operating conditions minimum bubbling velocity and the fluidization index are maximum in case of either semi-cylindrical conduit or hexagonal conduit for most of the operating conditions and minimum in case of square one. It is further observed that the range of uniform (particulate) fluidization is maximum in case of semi-cylindrical bed for identical operating conditions.     

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.     

Prediction of minimum fluidization velocity for vibrated fluidized bed

The prediction of minimum fluidization velocity for vibrated fluidized bed was performed. The Geldart group A and C particles were used as the fluidizing particles. The method based on Ergun equation was used to predict the minimum fluidization velocity. The calculated results were compared with the experimental data.The calculated results of minimum fluidization velocity are in good agreement with experimental data for Geldart group A particles. For group C particles, the difference between the calculated results and experimental data is large because of the formation of agglomerates. In this case, the determination of agglomerate diameter is considered to be necessary to predict the minimum fluidization velocity.     

Bed height and material density effects on fluidized bed hydrodynamics

Characterizing the hydrodynamics of a fluidized bed is of vital importance to understanding the behavior of this multiphase flow system. Minimum fluidization velocity and gas holdup are two of these key characteristics. Experimental studies addressing the effects of bed height and material density on the minimum fluidization velocity and gas holdup were carried out in this study using a 10.2 cm diameter cylindrical fluidized bed. Three different Geldart type-B particles were tested: glass beads, ground walnut shell, and ground corncob, with material densities of 2600, 1300, and 1000 kg/m³, respectively. The particle size range was selected to be the same for all three materials and corresponded to 500–600 μm. In this study, five different bed height-to-diameter ratios were investigated: H/D=0.5, 1, 1.5, 2, and 3. Minimum fluidization velocity was determined for each H/D ratio using pressure drop measurements. Local time-average gas holdup was determined using non-invasive X-ray computed tomography imaging. Results show that minimum fluidization velocity is not affected by the change in bed height. However, as the material density increased, the minimum fluidization velocity increased. Finally, local time-average gas holdup values revealed that bed hydrodynamics were similar for all bed heights, but differed when the material density was changed.     

Effects of acoustic vibration on nano and sub-micron powders fluidization

Fluidization of nano and sub-micron powders with and without acoustic vibration was investigated. The effects of sound pressure level and frequency were studied. Loudspeakers located under the distributor plate were used as the sound source to disintegrate larger agglomerates concentrated at the bottom of the bed. Nanoparticles showed fluid-like behavior similar to Geldart's A group and application of sound vibration improved their fluidization quality. Submicron particles were hard to fluidize and their fluidization quality was partially improved by sound excitation. Bed compaction, caused by rearranging of the agglomerates, was observed for submicron particles at low gas velocities while the bed was fixed. Nanoparticles did not experience any bed compaction. Sound vibration led to a decrease in minimum fluidization velocity and an increase in bed pressure drop and bed expansion for both types of particles. The fluidization quality of both particles increased at low frequencies, while the reverse was observed at higher frequencies. Fluidization of these particles was improved by increasing sound pressure level. There was a critical sound pressure level of 110 dB, below which the effect of sound vibration was insignificant. A novel technique was employed to find the apparent minimum fluidization velocity from pressure drop signals.     

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.     

Bubbling fluidization of biomass and sand binary mixtures: Minimum fluidization velocity and particle segregation

Understanding the minimum fluidization velocity of biomass and sand mixtures is fundamental to ensuring the optimal performance of fluidized beds in a thermo-conversional process, such as fast pyrolysis. The present work aimed to determine the minimum fluidization velocity of binary mixtures using the characteristic diagram of pressure drop in the bed and to develop an experimental correlation for the minimum fluidization velocity of biomass and sand mixtures. Three types of biomass (sweet sorghum bagasse, waste tobacco and soybean hulls) and four sands with different sizes were investigated. The results showed that the fluid dynamic behavior of binary mixtures is directly related to the biomass size and shape. For sweet sorghum bagasse (more irregular particles), higher biomass percentages led to lower minimum fluidization velocities, which differed from the behaviors observed for waste tobacco and soybean hulls. The diameter ratio inert/biomass effectively influenced the segregation, with a higher ratio causing more pronounced bed segregation. A good fluidization regime (with little segregation) for biomass and sand mixtures was obtained using the smallest sand (d₅₀ = 0.35). Considering the studied operating conditions, the proposed correlation can be used satisfactorily to predict the minimum fluidization velocities for mixtures of biomass and sand in fluidized beds.     

Simulation and experimental studies on fluidization properties in a pressurized jetting fluidized bed

The influence of pressure on the bubble size and average bed voidage has been investigated experimentally and computationally in a circular three-dimensional cold-flow model of pressurized jetting fluidized bed of 0.2 m i.d. and 0.6 m in height with a central jet and a conical distributor, which roughly stands for the ash-agglomerating fluidized bed coal gasifier. The pressurized average bed voidage and bubble size in the jetting fluidized bed were investigated by using electrical capacitance tomography (ECT) technique. The time-averaged cross-sectional solids concentration distribution in the fluidized bed was recorded. The influence of pressure on the size of bubble and the average bed voidage in a pressurized fluidized bed was studied. Both experimental and theoretical results clearly indicate that there is, at the lower pressure, a small initial increase in bubble size decided by voidage and then a decrease with a further increase in pressure, which proves the conclusion of Cai et.al. [P. Cai, M. Schiavetti, G. De Michele, G.C. Grazzini, M. Miccio, Quantitative estimation of bubble size in PFBC, Powder Technology 80 (1994) 99-109]. At higher pressure, bubbles become smaller and smaller because of splitting. The average bed voidage increases gradually with the pressure at the same gas velocity. However, there is a disagreement between the experimental results and simulation results in the average bed voidage at the higher gas velocity, especially at the higher pressure. It suggests that the increase in density of gas with pressure may result in the drag increase and the drag model needs to be improved and revised at higher pressure.     

Effect of air distributor on the fluidization characteristics in conical gas fluidized beds

The effect of an air distributor on the fluidization characteristics of 1 mm glass beads has been determined in a conical gas fluidized bed (0.1 m-inlet diameter and 0.6 m in height) with an apex angle of 20‡. To determine the effect of distributor geometry, five different perforated distributors were employed (the opening fraction of 0.009–0.037, different hole size, and number). The differential bed pressure drop increases with increasing gas velocity, and it goes from zero to a maximum value with increasing or decreasing gas velocity. From the differential bed pressure drop profiles with the distributors having different opening fractions, demarcation velocities of the minimum and maximum velocities of the partial fluidization, full fluidization, partial defluidization and the full defluidization are determined. Also, bubble frequencies in the conical gas fluidized beds were measured by an optical probe. In the conical bed, the gas velocity at which the maximum bed pressure drop attained increases with increasing the opening fraction of distributors.     

Fundamental particle fluidization behavior and handling of nano-particles in a rotating fluidized bed

The fluidization behavior of the three kinds of nano-particles (TiO₂, SiO₂, Al₂O₃) was analyzed in a rotating fluidized bed (RFB). Bed pressure drop, minimum fluidization velocity, bed expansion, entrainment and particle mixing characteristics under various centrifugal accelerations were experimentally investigated. The effects of centrifugal acceleration on agglomerate size and density were analyzed based on a Richardson-Zaki approach coupled with a fractal model.The bed pressure drop behavior showed almost similar to that of A or B-particles of Geldart's classification. Dimensionless particle bed height became smaller when the centrifugal acceleration was larger. Size of agglomerate decreased and its density increased with an increase in centrifugal acceleration. The agglomerate size in the RFB showed smaller than that in other types of fluidized bed system such as vibration and magnetic field as well as in a conventional fluidized bed, and the agglomerate density became larger. Particle entrainment became smaller in the case of the higher centrifugal acceleration. These results confirmed that the RFB can reduce the size of a nano-particle agglomerate and fluidize nano-particles at high gas velocity without any significant entrainment. The RFB is thus expected as more effective gas-solid fluidization system for handling of a large amount of nano-particles than other types of fluidized bed.     

CFD simulation of solids residence time distribution in a multi-compartment fluidized bed

The present work focuses on a numerical investigation of the solids residence time distribution(RTD)and the fluidized structure of a multi-compartment fluidized bed,in which the flow pattern is proved to be close to plug flow by using computational fluid dynamics(CFD)simulations.With the fluidizing gas velocity or the bed outlet height rising,the solids flow out of bed more quickly with a wider spread of residence time and a larger RTD variance(σ2).It is just the heterogeneous fluidized structure that being more prominent with the bed height increasing induces the widely non-uniform RTD.The division of the individual internal circulation into double ones improves the flow pattern to be close to plug flow.     

Why the two-fluid model fails to predict the bed expansion characteristics of Geldart A particles in gas-fluidized beds: A tentative answer

It is well known that two-fluid models (TFMs) can successfully predict the hydrodynamics of Geldart B and D particles. However, up to now, TFM have failed to accurately describe the hydrodynamics of Geldart A particles inside bubbling gas-fluidized beds: Researchers have reported that bed expansions are over-predicted by as much as 70%. In this work we show—for the first time—that TFM can predict the correct bed expansion, without any artificial modifications, provided that a sufficiently fine grid size and small time step is used. This suggests that the previously reported failure of TFM is mainly due to the lack of scale resolution, and that from a modeling point of view there is no fundamental difference between Geldart A particles and Geldart B and D particles.     

An experimental study of non-newtonian fluid flow in fluidized beds: minimum fluidization velocity and bed expansion characteristics

In this work, new experimental measurements of the minimum fluidization velocity and velocity-voidage characteristics are reported for a variety of liquid-particle systems in glass columns of two different diameters. Three types of liquids, namely, Newtonian, visco-inelastic, and visco-elastic fluids, were used to fluidize the beds of glass particles of four different sizes (1.27–15.8 mm). The results obtained with Newtonian liquids conform to the expected behaviour. The applicability of a variety of equations has been examined with a view to predicting the values of the minimum fluidization velocity and fluidization index for non-Newtonian systems. The experimental results reported herein embrace the following ranges of conditions: 1.27 < D_p < 15.8 mm; D_T = 50.8 and 101.6 mm, and 0.382 n 1.00.     

Prediction of minimum fluidization velocity for fine tailings materials

The suitability of several minimum fluidization velocity correlations has been investigated for fine zinc slime, iron ore tailings, both pre and post hydro-cyclone uranium tailings, and a relatively coarse grade of fly ash. It has been found that most of the published correlations significantly underestimate the value of minimum fluidization velocity for the four different tailings materials. Predictions due to Van Heerden et al. (1951) [45], Noda et al. (1986) [76], Coltters and Rivas (2004) [39], and Xu and Zhu (2009) [93], agree reasonably with the experimental data, whereas the fly ash, which belongs to Geldart Group B materials, has shown a large deviation. Thus these correlations could be used for the fine tailings materials that comprise a variety of constituents and possess a degree of cohesiveness. A modification of Coltters and Rivas correlation has also been suggested to assess the combined effect of particle size and density on minimum fluidization velocity.     

Computational fluid dynamics of a circulating fluidized bed under various fluidization conditions

A computational fluid dynamics (CFD) model was developed to simulate the hydrodynamics of gas-solid flow in a circulating fluidized bed (CFB) riser at various fluidization conditions using the Eulerian-Granular multiphase model. The model was evaluated comprehensively by comparing its predictions with experimental results reported for a CFB riser operating at various solid mass fluxes and superficial gas velocities. The model was capable of predicting the main features of the complex gas-solids flow, including the cluster formation of the solid phase along the walls, for different operating conditions. The model also predicted the coexistence of up-flow in the lower regions and downward flow in the upper regions at the wall of the riser for high gas velocity and solid mass flux, as reported in the literature. The predicted solid volume fraction and axial particle velocity were in good agreement with the experimental data within the high density fast fluidization regime. However, the model showed some discrepancy in predicting the gas-solid flow behavior in the riser operating in dense suspension up-flow and low density fast fluidization regimes.     

Fluidization of moist sawdust in binary particle systems in a gas–solid fluidized bed

The fluidized behavior of binary mixtures of moist sawdust and glass spheres has been investigated. The sawdust alone was observed to fluidize poorly, with extensive channelling occurring. The addition of 0.322 and 0.516 mm glass spheres to the fluidized bed of sawdust improved the fluidization characteristics. The mixtures of sawdust and 0.322 mm spheres were completely mixed when fluidized. Mixtures of sawdust and 0.516 mm spheres were either partially or completely mixed, depending upon gas velocity in the fluidized bed. As the moisture content of the sawdust was increased, the minimum fluidization velocity of the binary mixture also increased. There was an upper limit to the moisture content of the sawdust at which fluidization could be achieved. When the moisture content of the sawdust exceeded 33 and 54 wt% on a dry basis, agglomeration and channelling occurred in the mixtures of sawdust and glass spheres, with sizes 0.322 and 0.516 mm, respectively. The moisture likely contributes to interparticle liquid bridging forces. Binary mixtures of larger 0.777 and 1.042 mm glass spheres and up to 82% moisture sawdust did not readily agglomerate, but the two components completely segregated during fluidization.