The electrification mechanism in a fluidized bed

On the basis of numerous data available in the literature, as well as according to the experiments which we have carried out, a hypothesis is proposed for the mechanism of static electrification in a fluidized bed.     

Multi-fluid modelling of hydrodynamics in a dual circulating fluidized bed

In this work, a multi-fluid model based on the Eulerian-Eulerian framework is used to study the gas-solid hydrodynamics, such as solid distribution, particle motion and solid velocity, in a three-dimensional (3D) dual circulating fluidized bed (DCFB). The influence of four different drag force models, including two classic models, i.e. Gidaspow, EMMS drag model and two recent drag models, i.e. Rong and Tang drag model, on hydrodynamics in DCFB are assessed. Numerical results show that the characteristics of solid distribution and velocity in different sections are distinct. For qualitative analysis, all the drag models can predict a reasonable radial solid distribution and pressure distribution, but only the EMMS, Rong and Tang drag model can capture the phenomenon of dense solid concentration in the low part. For quantitative analysis, the solid circulating rate predicted by the EMMS drag model is the closest to the experimental value while the Gidaspow drag model shows the most significant deviation. The overall assessments confirm that the drag model selection has a significant influence on the simulations of gas-solid flow in DCFBs. This study sheds lights on the design and optimization of fluidized bed apparatuses.     

Pressure pulsation mechanism in a nonuniform fluidized bed

A model is proposed for pressure oscillations in a fluidized bed. A formula is obtained for the oscillation frequency and calculated values are compared with experimental data.     

Characterization of granular silicon,powders, and agglomerates from a fluidized bed reactor

Growth of polycrystalline silicon from fluidized bed reactors (FBR) produces two general types of silicon products: granular material (diameters on the order of mm) and homogeneously nucleated material often called nanopowder (diameters in the range 10–100 nm). Nanopowder particles tend to be amorphous and have a spherical morphology with an average particle diameter of ~80 nm. Granular material is generally spherical, highly twinned, polycrystalline with crystallite sizes that can reach 200 nm, and includes regions of porosity. The porosity is ~1–4 volume percent, and only the smallest pores exhibit evidence of amorphous regions along the pore surface. The amount of nanopowder that agglomerates on the granular material has been identified using transmission electron microscopy, but agglomeration plays only a minor role in the overall growth process. Therefore, it is proposed that the primary mechanism for granular formation in commercial FBR is chemical vapor deposition, and the pores are associated with nanopowder agglomeration and incomplete sintering.     

Parametric study of silicon carbide coatings deposited in a fluidized bed

Silicon carbide coatings have been deposited on microspheres in a fluidized bed by the reaction of methyltrichlorosilane and hydrogen. The principal variables of reactant gas composition and temperature were correlated with the morphology, the microstructure and the density of the coating. The effect of the coating rate on these characteristics was shown to be slight. The optimum gas composition for highly dense coatings was about 5 vol. % or less methyltrichlorosilane in H₂. The amount of grown-in porosity increased as the gas composition increased above 5 vol. % methyltrichlorosilane in H₂. The optimum temperature for highly dense coatings was in the range 1475–1675 °C. Lower temperatures resulted in silicon-rich deposits, while higher temperatures caused unacceptable porosity.     

The mechanism of heat transfer of particles in a fluidized bed

The relationship between the contact and convective-conductive components of the heat flux in the heating of particles in a fluidized bed is examined.     

On the mechanism of pressure pulsation in a bubbling fluidized bed

The mechanism of pressure pulsation in fluidized-bed apparatus having a gas-distribution grid of high hydrodynamic resistance or a subgrid chamber with volume much smaller than that of the bed is considered. The results of calculation and measurement of extremal pressures at different levels of the fluidized bed are given.Ural State Technical University, Ekaterinburg. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 66, No. 1, pp. 34–37, January, 1994.     

Optimization and characterization of direct coating for ibuprofen particles using a composite fluidized bed

Ibuprofen particles (mean particle size, 27 μm and melting point, 76 °C) as core materials were directly coated with a water-soluble polymer. The primary particles were preserved using a composite fluidized bed with a dispersing mechanism at the bottom of the fluidized bed apparatus. Coated primary particles were obtained under the following 3 conditions: (1) Setting the spray air flow rate at 10 L/min from the initial to 2% coating, (2) adding the low-viscosity water-soluble polymer macrogol 6000 to the hypromellose coating solution, and (3) changing the spray air flow rate to 15 L/min from 2% coating. The particles obtained were confirmed to be coated primary particles by scanning electron microscopy of their cross sections prepared by the cryo-focused ion beam method. The dissolution test showed a marked improvement in the solubility of ibuprofen from the coated primary particles compared with that of a physical mixture. In conclusion, the optimization of the direct coating process made it possible to undertake primary particle coating of a raw material that has a low melting point and a particle size of not more than 50 μm. Primary particle coating contributes to improvements in the physicochemical properties of drugs.     

CFD modelling and simulation of an industrial scale continuous fluidized bed roaster

In the present work, computational fluid dynamics (CFD) based modelling of an industrial scale continuous fluidised bed roaster (FBR) has been carried out to study its performance at different operating conditions, so that the sulphide-sulphur content in the product is within 0.4% at the designed feed rate of 39.75 DMT/h. Eulerian-Eulerian multiphase model, considering four granular phases and one gas phase has been implemented to investigate the velocity and mass fraction profile of the particles in the FBR. The heat and species mass balance calculations have been performed external to CFD, by dividing the roaster into several sections. The conversion of ZnS to ZnO at various sections of the roaster has been estimated using reaction kinetics under isothermal condition (1203 K). The heat liberated and possible temperature rise at each section was predicted based on the heat of reaction and sensible heat of the solid and gaseous products. The CFD model was validated with the plant data for a feed rate of 36.5 DMT/h, air flow rate of 65,000 Nm³/h and O₂ content of 21%. The proposed model predicted the sulphide-sulphur content in the product to be 0.4% for the designed feed rate of 39.75 DMT/h, when the O₂ content in the inlet air was increased to 25%.     

Particle dynamics in a multi-staged fluidized bed: Particle transport behavior on micro-scale by discrete particle modelling

This work studies the particle exchange rates in horizontal fluidized beds equipped with different weir designs between compartments. These particle exchange rates provide information on the axial dispersion of the solid material within the process. For this purpose discrete particle modelling (DPM) was used to determine the particle exchange on microscopic level. This method uses a coupled CFD-DEM approach to observe particle dynamics in a fluid field. The model was validated against exchange rates in a lab-scale setup as determined by Particle Tracking Velocimetry (PTV) with very good quantitative agreement, showing the suitability of the method for the evaluation of weir designs. Simulations were performed for different weir designs and under variation of the hold-up mass, the feed rate and gas velocity to predict their transport behavior in a pilot-scale 3D horizontal fluidized bed. The results indicate that the solids transport behavior is strongly dependent on the used weir design and the main driving force for the particle transport that can be influenced by the process conditions. The installation of weirs between two compartments induces a transport resistance, while the base type without the installation of a weir between the two chambers represents the fastest possibility for mixing the particles of a two-compartment system. It has been observed that the general trend shows higher particle recirculation rates for the overflow weir and base configuration (no weir), whereas the underflow and sideflow weir applications improve the solids transport through the horizontal fluidized bed.     

Investigation of gas-solid flow characteristics in a novel internal fluidized bed combustor by experiment and CPFD simulation

Based on the coal self-preheating combustion technology, this research proposed a novel internal fluidized bed combustor (IFBC) with an internal separator for stable preheating of fuel. In order to verify feasibility and operation stability of IFBC, cold experiment, electrical capacitance tomography (ECT) and computational particle fluid dynamic (CPFD) simulation were performed in a laboratory-scale IFBC. The effects of superficial air velocity (U_g) and return valve structure on the operation and gas-solid flow characteristics were investigated. The results revealed that the CPFD prediction agreed well with the experiment values. The pressure balance curve presented an “8″ shape distribution, and the particle volume fraction (PVF) showed ‘core-annular’ distribution features. With the increase of U_g, the PVF in the standpipe increased, and the discharge pattern of the return valve changed from continuous discharge to intermittent discharge, and the solid circulation flux showed a trend of increasing first and then decreasing. With the decrease of the outlet opening of return valve (Φ), the gas–solid flow behavior in standpipe experienced a transition from gas leakage, stabilizing material seal, and blocking state. For U_g = 2 m/s, Φ = 50 %, an effective solid seal in the return valve was established and IFBC has a stable circulation and operation.     

Kinetics of the removal of liquid from capillaryporous bodies in a fluidized bed under nonisotherma l conditions

A theory of mass transfer in capillary-porous bodies is proposed which allows for thermogradient transfer of a bound substance in liquid form. The results obtained are used to calculate the process of drying of ceramic articles in a fluidized bed.Notation t, T temperature - a_m coefficient of moisture conductivity - , , density, viscosity, and surface tension of liquid - h thickness of liquid film - n disjoining pressure - A Hamaker constant - coefficient of external mass exchange - ₁ coefficient of heat transfer - coefficient of thermal conductivity - Fo Fourier number - volumetric moisture content of porous body - W_s surface moisture content of body - n=n₁+n₂ porosity - r pore radius in the particles - F= F₁+ F₂ surface area of porous body - R₁, R₂ radii of wide and narrow capillaries - moisture content - x_o, x_oe coordinates of menisci in wide capillaries and at end of first stage, respectively - P_o, P_oe Pressures at surface of body and at end of first stage, respectively Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 39, No. 1, pp. 11–18, July, 1980.     

Spectra of thermal fluctuations in a fluidized bed

We have studied the spectra of temperature fluctuations and the heat-transfer coefficient in a disperse medium, and have compared the results with experimental data.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 47, No. 1, pp. 116–124, July, 1984.     

Combustion of a polydisperse fuel in a fluidized bed

A method is presented for calculating the combustion of a polydisperse fuel inside a fluidized bed and above the bed.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 53, No. 1, pp. 70–77, July, 1987.     

Boriding of nickel in a fluidized bed reactor

Heat treatments of alloys in fluidized bed reactors have been carried out for more than 25 years. Recently, this technology has been used for surface engineering applications in the deposition of hard and/or corrosion resistant layers. In the present paper we used FBT to deposit boride coatings on nickel metal. The coatings were examined by means of optical microscopy, X-rays diffraction and Vickers microhardness in terms of the coating’s morphology, thickness, hardness and phase formation. The coating’s tribological properties were evaluated under dry wear. The as-produced coatings are characterized by good uniformity and it was found that only Ni₃B (space group Pnma) was formed during the treatment. Furthermore, the boride layer improved the tribological properties of nickel.     

Kinetics and mechanism of reaction between silicon carbide and silica

The kinetics and mechanism of reaction between SiO₂ and SiC are studied. The reaction rate is shown to be limited by the carbon diffusion from the SiC bulk to the SiC/SiO₂ interface, where carbon reacts with the SiO and oxygen resulting from SiO₂ decomposition. Equations are presented which describe the time variation of SiC conversion and the temperature variation of the carbon diffusivity in SiC.     

Free-space and external heat-transfer fluctuations in a fluidized bed

Experimental results are used to show that fluctuations in the free space near the wall have little effect on the local heat-transfer coefficients.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 30, No. 6, pp. 986–995, June, 1976.