Aggregation behavior of nanoparticles in fluidized beds

The fluidization behavior of fumed silica, zirconia, and iron oxide nanopowders was studied at atmospheric and reduced pressures. Using a high-speed laser imaging system, the characteristics of fluidized aggregates of nanoparticles were studied in real time. The effect of different particle interactions such as London-van der Waals, liquid bridging and electrostatic on different fluidization parameters was studied at atmospheric pressure. The reduction of interparticle forces resulted in a reduced aggregate size and minimum fluidization velocity (U_mf) and an increased bed expansion. Nanoparticles were also fluidized at reduced pressure (∼ 16 Pa) with vibration to study the effect of low pressure on the minimum fluidization velocity. Aggregate properties (size, density) instead of primary nanoparticle properties were found to govern the minimum fluidization velocity and expansion of the fluidized bed. An important consideration is the relative strength of intra-aggregate interparticle forces (forces within the aggregate holding nanoparticles together) to inter-aggregate interparticle forces (forces between aggregates). This relative strength may be inferred from the sphericity of the aggregates during fluidization.     

The onset of fluidization

A packed bed is stabilized by frictional forces between the particles; in a homogeneously fluidized bed these forces are usually absent. This distinction can be used to determine the minimum fluidization velocity. The disappearance of interparticle friction manifests itself in the angle of repose, which then becomes zero if no other interparticle forces exist. The experimental method is simple and leads to a very reproducible and easily interpretable value of U_mf? The value obtained with the angle of repose method agrees very closely with the value obtained via the well-known pressure drop method, if experiment and interpretation are carried out with care.     

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.     

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.     

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     

Transition to turbulent fluidization in a binary solids fluidized bed

This paper presents a study on the transition velocity from bubbling to turbulent fluidization in a binary solids fluidized bed. Experiments were carried out with two kinds of binary solids mixtures with FCC as fine particles and silica sands as coarse particles. The onset velocity to turbulent fluidization, U_c, determined by the measurement of pressure fluctuations, was found to increase with increasing the fraction of coarse/heavy solids. By introducing an equivalent particle diameter and an equivalent particle density, the results obtained in this study can properly be described by a general correlation of U_c proposed by Cai and co-workers (1989) for mono-density particles with relatively narrow size distribution.     

Minimum fluidization velocity in gas‐liquid‐solid minifluidized beds

The initial fluidization characteristics of gas‐liquid‐solid minifluidized beds (MFBs) were experimentally investigated based on the analyses of bed pressure drop and visual observations. The results show that U_Lmf in 3–5 mm MFBs can not be determined due to the extensive pressure drop fluctuations resulting from complex bubble behavior. For 8–10 mm MFBs, U_Lmf can be confirmed from both datum analyses of pressure drop and Hurst exponent at low superficial gas velocity. But at high superficial gas velocity, U_Lmf was not obtained because the turning point at which the flow regime changes from the packed bed to the fluidized bed disappeared, and the bed was in a half fluidization state. Complex bubble growth behavior resulting from the effect of properties of gas‐liquid mixture and bed walls plays an important role in the fluidization of solid particles and leads to the reduction of U_Lmf. An empirical correlation was suggested to predict U_Lmf in MFBs. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1940–1957, 2016     

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     

Design Parameter Effects on Expansion in Fluidized Beds with Inserts

The effect of some design parameters on the expansion of particles has been studied in a fluidized bed of 300 mm diameter. Four distributors were examined; three perforated plates, each perforated by holes of 0.8 mm in diameter but different hole densities at 6 mm, 9 mm and 12 mm pitch, and a porous plastic distributor 17 mm thick. Particles of different materials in the Archimedes number range from 100 to 10⁵ were fluidized. The inserts were held vertically as arrays. All experimental data for four distributors were correlated within experimental error by the equation: whereU, U_mf, U₀ are the gas velocity, velocity at minimum fluidization and real or apparent terminal velocity, while e is the bed porosity and e_mf is the porosity at the condition of minimum fluidization. P is the hole pitch of perforated plate distributor in millimeters.     

Photo-degradation characteristics of TCE (trichloroethylene) in an annulus fluidized bed photoreactor

The effects of gas velocity, annulus gap on solid (ε_s) and gas phase holdups (ε_g), UV light transmittance and photocatalytic reduction of TCE (trichloroethylene) over TiO₂/silica gel photocatalyst have been determined in an annulus fluidized bed photoreactor. The optimum TCE reduction efficiency exhibits at ε_s/ε_g of 0.48, annulus gap of 8 mm and UV light transmittance around 0.20. The most pronounced effects on TCE conversion are found to be gas velocity (U_g) and annular gap in the photoreactor.     

Granular temperature in a liquid fluidized bed as revealed by diffusing wave spectroscopy

We report granular temperature and solid fraction fields for a thin rectangular bed (20×200 mm cross-section and 500 mm high) of glass particles (mean diameter of 165 μm and density of 2500 kg/m³) fluidized by water for superficial velocities ranging from 0.05U_t, which is approximately double the minimum fluidization velocity, to 0.49U_t, where U_t is the particle terminal velocity estimated by fitting the Richardson-Zaki correlation to the bed expansion data. At superficial velocities below 0.336U_t, the solid fraction and granular temperature are uniform throughout the bed. At higher superficial velocities, the solid fraction tends to decrease with height above the distributor, whilst the granular temperature first increases to a maximum before decaying towards the top of the bed. Correlation of the mean granular temperature with the mean solid fraction and the local granular temperature with the local solid fraction both suggest that the granular temperature in the liquid fluidized bed can be described solely in terms of the solid fraction. The granular temperature increases monotonically with solid fraction to a maximum at φ≈0.18 where it then decreases monotonically as φ approaches the close-packed limit.     

Effects of pressure and type of gas on particle-particle interaction and the consequences for gas—solid fluidization behaviour

The fluidization behaviour of cracking catalyst has been studied up to pressures of 15 bar with different fluidization gases (Ar, N₂, H₂). A number of parameters of both the homogeneous and heterogeneous fluidized bed has been examined experimentally.The experimental results reveal that the minimum fluidization velocity (U_mf) is independent of the pressure. The bubble point velocity (U_bp) and the maximum bed expansion (H_bp) at this velocity increase with increasing pressure. This also holds for the dense phase voidage (ε_d) and the dense phase gas velocity (U_d) in the bubbling bed. The bubble size decreases drastically with increasing pressure. However, the above-mentioned parameters are also strongly dependent on the type of fluidization gas used.The cohesion constant of the powder was measured, using a tilting bed technique. The results reveal that the cohesion constant increases with increasing pressure. Analysis of the results of adsorption measurements of the different gases to the solid reveals for the adsorption as well as for the cohesion and for the beu expansion the same pressure dependence.It is believed that the gas adsorption influences the cohesion between the particles and hence the elasticity modulus introduced by Rietema and Mutsers [1,2]. The increasing elasticity modulus with increasing pressure also explains the increasing bed expansion with pressure.     

Effects of temperature on local two-phase flow structure in bubbling and turbulent fluidized beds of FCC particles

A novel high temperature optical fiber probe has been developed to study the effects of bed temperature on the local two-phase flow structure in a pilot scale fluidized bed of the FCC particles with bed temperatures ranging from 25°C to 420°C, covering both the bubbling and turbulent fluidization regimes. The results show that fluidization is enhanced and fluctuations of the local two-phase flow structure become more intense with increasing bed temperature. At constant superficial gas velocities, the averaged local particle concentration, the dense phase fraction and particle concentration in the dense phase decrease with increasing bed temperature, whereas both the frequency of the dilute/dense phase cycle and the ratio of the dilute phase duration to the dense phase duration increase. In addition, the effects of temperature on the dilute phase depend on superficial gas velocity. The conventional two-phase models fail to predict these changes of the local flow structure with temperature, which may be explained by the fact that the role of interparticle forces is neglected at different bed temperatures. Indeed, fluidization behaviors of the FCC particles tested increasingly shift from typical Geldart A towards B with increasing temperature due to a decrease of the interparticle attractive forces and a simultaneous increase of interparticle repulsive forces.     

Attribution of sewage sludge and sludge-based char in a fluidized bed reactor

Fluidized bed pyrolysis has been recognized as an innovative technology for sewage sludge treatment. The physical and attrition properties of sewage sludge are changed through the fluidized bed pyrolysis. The minimum fluidization velocities and attribution rate constants for sewage sludge and sludge based-char were obtained from pressure drop and attribution tests. As a result, sewage sludge with 20% moisture content and char were classified as Geldart B solids and the superficial gas velocity for bubbling fluidization was 0.2142-0.8755 m/s. In addition, attribution of the sewage sludge and char was more affected by particle size than by material type. The equations for the overall attrition rate constants are K _a × 10⁵ = 1.09U − 14.82 for sludge and ln k _a = 0.1(U−U _mf )− 13.63 for char, respectively.     

Synthesis of multiwalled carbon nanotubes on Al2O3 supported Ni catalysts in a fluidized‐bed

Multiwalled carbon nanotubes (MWNTs) were synthesized on Al₂O₃ supported Ni catalysts from C₂H₂ and C₂H₄ feedstocks in a fluidized bed. The influence of the ratio of superficial gas velocity to the minimum fluidization velocity (U/U_mf), feedstock type, the ratio of carbon in the total quantity of gas fed to the reactor, reaction temperature, the ratio of hydrogen to carbon in the feed gas, and nickel loading were all investigated. Significantly, the pressure drop across the fluidized‐bed increased as the reaction time increased for all experiments, due to the deposition of MWNTs on the catalyst particles. This resulted in substantial changes to the depth and structure of the fluidized bed as the reaction proceeded, significantly altering the bed hydrodynamics. TEM images of the bed materials showed that MWNTs, metal catalysts, and alumina supports were predominant in the product mixture, with some coiled carbon nanotubes as a by‐product. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

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.     

磷矿颗粒流态化特性的实验研究

在高1 m、内径42mm的流化床中,对粒径54-600 μm、密度2 252-2 665 kg/m3的磷矿颗粒的流态化特性进行实验研究.实验结果表明:磷矿颗粒粒径和密度对磷矿颗粒流态化行为有较大影响,床层膨胀比随着磷矿颗粒粒径的增大而逐渐减小.当磷矿颗粒属于Geldart B类颗粒时,流化较好;而当颗粒平均粒径为82 ...     

Linking micro‐scale predictions of capillary forces to macro‐scale fluidization experiments in humid environments

The effects of increasing relative humidity (RH) on fluidization/defluidization are investigated experimentally and understood via particle‐level predictions for the resulting capillary force. Experimentally, defluidization is found to be more sensitive to small changes in RH than fluidization. This sensitivity is captured by a new defluidization velocity U_df, which characterizes the curvature of the defluidization plot (pressure drop vs. velocity) observed between the fully‐fluidized (constant pressure drop) and packed‐bed (linear pressure drop dependence on velocity) states; this curvature is indicative of a partially‐fluidized state arising from humidity induced cohesion. Plots of U_df vs. RH reveal two key behaviors, namely U_df gradually increases with a relatively constant slope, followed by an abrupt increase at RH ～55%. Furthermore, the bed transitions from Group A to Group C behavior between RH of approximately 60–65%. From a physical standpoint, these macro‐scale trends are explained via a theory for capillary forces that, for the first time, incorporates measured values of particle surface roughness. Specifically, a model for the cohesive energy of rough surfaces in humid environments shows the same qualitative behavior as U_df vs. RH for RH <55%, unlike predictions of the cohesive force. Furthermore, the abrupt transition at RH ～60–65% is explained via the previously observed onset of liquid‐like water adsorption, rather than crystal/ice‐like adsorption, onto glass surfaces. © 2016 American Institute of Chemical Engineers AIChE J, 62: 3585–3597, 2016     

The onset velocity of a liquid–solid circulating fluidized bed

The critical transition velocity, U_cr, previously defined by Liang et al. [W.-G. Liang, S.-L. Zhang, J.-X. Zhu, Y. Jin, Z.-Q. Yu and Z.-W. Wang, Flow characteristics of the liquid–solid circulating fluidized bed, Powder Technol., 90 (1997) 95–102.] to demarcate the liquid–solid conventional and circulating fluidization regimes, was found to vary with the total solids inventory and the solids feeding system. In this work, an onset velocity for circulating fluidization regime, U_cf, is proposed to give the lowest U_cr value and to provide a convenient demarcation velocity that is independent of system geometry. This liquid velocity is obtained by measuring the time required to empty all particles in a batch operated fluidized bed under different liquid velocities. This method can be used for a wide range of particles and involves less influence of the operating conditions such as the solids inventory and the solids feeding system. Compared to the critical transition velocity, this newly defined onset velocity is a more intrinsic parameter, only dependent on the liquid and particle properties. Based on the experimental results obtained in this work and other published results, the influence of particle properties and equipment setup on the onset velocity is also discussed.     

Layer inversion and mixing of binary solids in two- and three-phase fluidized beds

Water fluidization in a 210 mm diameter semi-cylindrical acrylic column of a binary solids mixture of 3.2 mm polymer beads (ρ_s=1280 kg/m³) and 0.385 mm glass beads (ρ_s=2500 kg/m³) at superficial liquid velocities from 18.1 to 43.1 mm/s is shown to generate layer inversion at a superficial liquid velocity, U_L, of 33.1 mm/s. Introduction of air with a superficial velocity, U_g, of 1.92 mm/s yielded a layer inversion velocity at U_L=30.4 mm/s. The latter is explainable if it is assumed that the determinant of layer inversion is the interstitial liquid velocity and that therefore the main function of the gas in this respect is to occupy space.Mixing of the binary solids, as quantified by a mixing index applied to measured particle compositions at different levels of the fluidized bed, is shown to be greatest at the layer inversion velocity for liquid fluidization and, in general, to increase as co-current gas flow increases at a fixed value of U_L.