Discrete particle simulations of an electric-field enhanced fluidized bed

Reducing the size of gas bubbles can significantly improve the performance of gas-solid fluidized reactors. However, such a control of bubbles is difficult to realize without measures that either use a lot of energy or deteriorate the fluidization behavior. In this paper, we present the results of discrete particle simulations of an electric-field enhanced fluidized bed, and compare these results to experimental data.The simulations show a significant effect on the size of bubbles, both with horizontal and vertical electric-fields applied. When the field strength is increased to values higher than those used in the experiments, the particles are found to form strings in the direction of the electric field. At very high field strengths, defluidization is observed, consistent with the experiments.Through the analysis of the bubble behavior, it is concluded that moderate strength electric fields distribute gas more evenly at the bottom of the bed. As the bubbles rise through the bed, the coalescence rate is lower because of the guiding paths, or resistance, the particles form due to the field. This results in a smaller average bubble size in the higher region of the bed. The simulations presented here show how and why the electric fields reduce bubble size in electric-field enhanced fluidized beds.     

Effects of Particle Shape and Size Distribution on Sorption and Flow Performance in Electrically Stabilized Expanded Beds

Abstract

Use of the electrically stabilized expanded bed is an approach to the improvement of the performance of processes in beds of solid sorbents. Particle motion in a fluidized bed of nonconducting particles such as molecular sieves is halted when the bed is “frozen” by a strong electric field imposed across the bed. This allows sorption performance to approach that of fixed beds while maintaining the low pressure drops which—are characteristic of fluidized beds. Beds with axial and radial electrode geometries function equally well in the sorption of carbon dioxide (C0₂2) from nitrogen (N₂). The effects of particle shape and particle size distribution on bed stabilization are presented. Particle shape was varied from spherical to the irregular forms that are characteristic of crushed particles. The sorption performance was not appreciably different with particles having various shapes.     

The effect of electrophoretic convection for the separation of solute in the chromatography column (I)

A theoretical model has been derived in an electrophoretic packed column where an electric potential is applied to a column in the axial direction. The effect of electrophoretic convection in gel particles packed in the column significantly contributes to the separation of large polyelectrolytes because the conformation of polyelectrolyte quickly orients in the field direction. The dependence of the transport in the gel particle upon field intensity and molecular size aids in understanding the transport of polyelectrolyte in the packed column, since the convective velocity of polyelectrolyte is accelerated inside a porous gel particle. There are few convection studies of large poly-electrolyte in a column packed with porous gel particles under an electric field for the separation. Convective-diffusive transport of a large polyelectrolyte is analyzed using Peclet number described by electrophoretic mobility and diffusion coefficient measured experimentally. The separation of two different polyelectrolytes in the packed column is performed using a value ofPe _f/Pe_g of individual polyelectrolyte by molecular size and an electric field. The purpose of this paper is to study the separation of solute from a mixture in the column using the physicochemical properties in the gel particle which are measured experimentally.     

Electrically Stabilized Expanded Beds for Sorption Separations

Abstract

Recent studies have shown that when high electric fields are imposed on expanded fluidized beds, motion of the particles can cease. This provides the potential for fixed bed operations with the void fraction sufficiently large that only moderate pressure drops are required, even with small particles. This paper explores the dielectric forces that inhibit particle motion in alternating-current electric fields and the potential and limitations of the concept for adsorption operations. The dielectric constants of the particles and the fluid, the size of the particles, the density and viscosity of the fluid, and the imposed electric field gradient are the variables of most importance to bed stabilization. In adsorption tests, the expanded-stabilized beds perform essentially as well as densely packed fixed beds but with only a fraction of the pressure drop.     

Removal of Lead in a Fixed-Bed Column Packed with Activated Carbon and Crab Shell

ABSTRACT

Crab shell particles (Protunus trituberculatus) and activated carbon (Norit 0,8 SUPRA) were used as packing materials in a fixed-bed column. When 1 g crab shell was added in a column packed with 10 g activated carbon, breakthrough occurred at 1500 bed volumes as compared to 380 bed volumes for 10 g activated carbon only. The addition of crab shell particles into an activated carbon column resulted in an increased uptake of lead. The dramatic improvement might be attributed to an increase in and OH^? available for binding lead. From the results of SEM, XRD, and FT-IR analyses, the major mechanism of lead removal was based on dissolution of CaCO₃ in the crab shell followed by precipitation of Pb₃(CO₃)₂(OH)2(s) on the surface of activated carbon. The lead uptake increased twofold when the influent lead concentration was increased from 10 to 50 mg/L.     

Microfibrous entrapped small particle adsorbents for high efficiency heterogeneous contacting

Hexane adsorption breakthrough tests on activated carbon particles (ACP) were used to demonstrate and understand the anomalous high efficiency of novel microstructured heterogeneous contacting systems: Microfibrous Entrapped Sorbents (MFES). MFES consist of small particles (0.05–0.30 mm) of adsorbent entrapped in micron-diameter fibers. They are characterized by small structural dimensions and uniform structures of high voidages. These materials are in the form of flexible sheets about 0.5–2.0 mm thick and can be conveniently stacked to create a required bed height. Experimental breakthrough curves were obtained from packed beds of various particle sizes (0.15–0.84 mm) and bed dilutions (with inerts) and microfibrous entrapped carbon particles (0.18–0.25 mm) of various void fractions (62.5–85%). The performance of the various bed configurations was evaluated based on their breakthrough times and percentage adsorbent utilizations. The rate of adsorption and hence the performance of packed beds improved with a decrease in particle size and also with an increase in bed dilution level. MFES clearly outperformed packed beds of similar particle size. A mathematical model accounting for axial molecular diffusion and intrabed flow maldistribution (or channeling) was used to explain these experimental results. In accordance with literature findings, the negative effect of the axial dispersion due to flow maldistributions on the transport rates in packed beds were evident in low Re regime (Re < 10). These shortcomings of packed beds were significantly minimized by high voidages and uniform flow distributions inherently present within MFES, thus leading to higher fluid–solid contacting efficiency.     

Spontaneous inter-particle percolation: A kinematic simulation study

Spontaneous inter-particle percolation is a mechanism of particle segregation which arises when a particle falls through the voids among large particles due to gravity or other applied force, the size difference being such that this can occur without the need for applying strain. It is a phenomenon arising in some packed bed processing operations and in some dispersers for particles. Here the mechanism is modelled by computer simulation. The model uses particle sizes, a coefficient of restitution and also, to cater for less elastic materials, a coefficient of friction. This yields percolation velocities quantitatively consistent with prior laboratory studies, showing the correct dependence on coefficient of restitution and particle size. The simulation clarifies the dependence of radial dispersion on packing height. The radial distributions are well described only at higher coefficients of restitution, pointing to the need for a better means of describing collisions between percolating and packed particles.     

Evaluation of flow properties of toner powder using conical rotor

In order to accurately evaluate the dynamic flow properties of toner powder, a new rotary shearing tester with a conical rotor was developed. This instrument was equipped with an automatic pressing system to compress toner powder. The tester could simultaneously measure torque and compression load during the intrusion and rotation of the conical rotor into the same packed toner powder. The optimum rotational speed and intrusion rate of the conical rotor for the characterization of the flow properties of toner powder were discussed based on test results; their values were calculated as 0.017 s^− 1 and 0.083 mm/s, respectively. The torque of toner powder changed in proportion to the cube of the depth of intrusion in the toner powder bed. The surfaces of toner powder samples prepared from polymer resin and carbon pigment particles were coated with fine particles (SiO₂, TiO₂) under a different condition. The flow characteristics of toner powder with a different particle surface were evaluated based on the relationship between the shearing torque and the void fraction of packed toner. In the present case, the Rumpf model was applied to estimate the shearing force H at the contact point between two particles of toner powder. The value of H for toner powder with a rough particle surface, which was covered with fine particles (SiO₂, TiO₂), was 41 nN, while that for toner powder with a smooth particle surface, which was not covered with fine particles, was 357 nN. Further the effects of the particle shape of the toner on the torque of toner powder after compression under the same conditions were investigated. The torque of toner powder decreased with an increase in circularity.     

Electric field phenomena in fluidized and fixed beds

Stabilization of a bed of dielectric particles against fluidization by an electric field (≥ 10³ volts/cm) is described. Glass bead and silica gel particle beds have been observed to behave as packed beds with flow rates (and pressure drops) of the fluidizing gas up to 15 times the normal incipient fluidization rate. The pressure drop at the breakup of this fixed bed was dependent on the second power of voltage, the particular bed material, and geometry of the system. Under suitable conditions 100% bed expansion without diffusive particle motion or bubble formation was obtained using silica gel particles. Comparison with iron particle bed-magnetic field effects are presented. Surface polarization charge effects are the simplest explanation of the phenomena. Several of the possible applications are suggested, such as precipitation enhancement in an aerosol filter or as a new tool for investigating aggregative fluidization.     

The Separation of Polyelectrolyte Using an Electric Field in a Packed Column

Abstract

The effect of electrophoretic convection in a packed column is presented. When an electric field is applied, the conformation of polyelectrolyte quickly orients in the field direction. The convective velocity of polyelectrolyte inside a porous gel particle is accelerated. The dependence of the transport in the gel particle upon field intensity and molecular size aids in understanding the transport of polyelectrolyte in the packed column. To date, few dynamic studies of polyelectrolyte in a porous gel particle have been attempted for the separation of polyelectrolyte in a packed column. A large polyelectrolyte like DNA will separate due to conformation changes in the presence of the electric field. Convective-diffusive transport of DNA is analyzed by physical properties measured experimentally, such as the diffusion coefficient, the electrophoretic convection, and the gel porosity. The purpose of this study is to show how the variation of physicochemical properties in the gel particle affects the separation of DNA from a mixture in a packed column. A theoretical model using the characteristic method is used to calculate the separation point in a packed column.     

Granular filtration for airborne particles: Correlation between experiments and models

Granular filtration has been widely used for liquid filtration and hot gas filtration, but less is known for the filtration of airborne particles, especially the ultrafine ones, at the room conditions. A cylindrical packed bed was designed and tested for the filtration of particles in the range of about 10 nm to 15 µm in diameter at different configurations and kinetic conditions. Three sizes of uniform glass beads (2, 4, and 6 mm in diameter) were tested as the filtration media each at three media thicknesses (H = 2.5, 7.6, and 12.7 cm), and at two airflow rates (50 and 65 liters per minute). The filtration efficiencies were the lowest for particles between 0.1 and 1 µm in diameter. The particle filtration efficiency decreased with the increase in the granule size and the airflow rate, but a thicker bed corresponded to higher filtration efficiency. The experimental results showed much higher efficiency than existing models can predict, therefore, an empirical model using least square method is reported.     

Simulation of micro- and macro-transport in a packed bed of porous adsorbents by lattice Boltzmann methods

In this study we report 3D simulation of concentration profiles in a fixed bed packed with spherical porous adsorbents using lattice Boltzmann methods (LBMs). The lattice models have been developed to investigate evolution of concentration profiles due to inter- and intra-particle mass transport in an adsorber having small tube-to-sphere diameter (d_t/d_p) ratios. The multi-scaling feature of LBMs permits full 3D simulation of concentration profiles both in the bed voids and within the pores of the adsorbents without using any empirical correlations or without making 1D or 2D approximations that are usually made in traditional numerical methods. The model simulation is carried out for varying packing arrangements and small to large pore diffusivities. The simulation results show significant concentration gradients for small pore diffusivities and large particle size, which must be considered in predicting breakthrough and adsorption times for a tubular adsorber having d_t/d_p<10. The model predicted breakthrough curves are validated with the experimental data obtained by tomography technique in a tubular adsorber packed with zeolite particles.     

A relationship between maximum packing of particles and particle size

Experimental data indicate that the volume fraction of particles in a packed bed (i.e. maximum packing) depends on particle size. One explanation for this is based on the idea that particle adhesion is the primary factor. In this paper, however, it is shown that entrainment and immobilization of liquid by the particles can also account for the facts.     

Separation of Colloidal Particles from Nonaqueous Media by Cross-Flow Electrofiltration

Abstract

Charge characteristics of particles in aqueous or nonaqueous slurries are known to play an important role in solids-liquid separation processes. We have been conducting a fundamental study on filtration of colloidal particles suspended in nonaqueous media, such as coal and tar sand slurries based on their charge characteristics. This paper presents results of such a study involving cross-flow electrofiltration of nonaqueous slurries. Data are reported for α-Al₂O₃ particles suspended in tetralin. The effects of feed rate, driving pressure and electrical field strength on the filtration rate, total deposition rate on the central electrode, and the efficiency of the filter are presented.

The outlet slurry concentrations were measured with a specially built X-ray densitometer. These data are analyzed by a mathematical model using a Graetz-type analysis. The rate of deposition was found to be determined mainly by the electric field. The sludge flow near the central electrode significantly affected the efficiency of separation.     

Fluidization Characteristics of SiO2 Nanoparticles in an Acoustic Fluidized

Under the action of an acoustic field, the fluidization behavior of 5–10 nm SiO₂ nanoparticles, with and without surface modification, was investigated. In a packed bed, the sound wave energy has a significant influence on the compact ratio of the bed. Experimental results indicated that the bed of nanoparticle agglomerates can be fluidized smoothly with the assistance of an acoustic field, and the minimum fluidization velocity is initially reduced dramatically with increasing sound frequency and then rises with increasing sound frequency. Under the same experimental conditions, the minimum fluidization velocity of 5–10 nm SiO₂ nanoparticles is greater than that of 5–10 nm SiO₂ nanoparticles with surface modification. The collapse of the bed demonstrates that SiO₂ nanoparticles, surface modified using organic compound, have longer minimum collapse times than SiO₂ nanoparticles.     

Evaluating the Mobility of Nanorods in Electric Fields

The mobility of a nonspherical particle is a function of both particle shape and orientation. In turn, the higher magnitude of electric field causes nonspherical particles to align more along the field direction, increasing their mobility or decreasing their mobility diameter. In previous works, Li et al. developed a general theory for the orientation-averaged mobility and the dynamic shape factor applicable to any axially symmetric particles in an electric field, and applied it to the specific cases of nanowires and doublets of spheres. In this work, the theory for a nanowire is compared with experimental results of gold nanorods with known shape determined by TEM images. We compare the experimental measured mobility sizes with the theoretical predicted mobility in the continuum, free molecular, and the transition regime. The mobility size shift trends in the electric fields based on our model, expressed both in the free molecular regime and in the transition regime, are in good agreement with the experimental results. For rods of dimension: width d_r = 17 nm and length L_r = 270 nm, where one length scale is smaller than the mean free path and one larger, the results clearly show that the flow regime of a slender rod is mostly controlled by the diameter of the rod (i.e., the smallest dimension). In this case, the free molecule transport properties best represented our nanorod. Combining both theory and experiment we show how, by evaluating the mobility as a function of applied electric field, we can extract both rod length and diameter.

Copyright 2013 American Association for Aerosol Research     

Effect of bed void volume on pressure vacuum swing adsorption for air separation

The effects of a poorly packed bed on the pressure vacuum swing adsorption (PVSA) process were investigated experimentally and theoretically by a five-step two-bed PVSA system. At first, the adsorption dynamics of a zeolite LiX bed for air separation (78 mol% N₂, 21 mol% O₂ and 1 mol% Ar) was studied at various adsorption pressures and flow rates. In breakthrough results, the effect of adsorption pressure on variations in bed temperature was greater than that of the feed flow rate. A combined roll-up of Ar and O₂ by N₂ propagation was observed and the roll-up plateau reached about 4 mol%. The fluid dynamic behavior of the poorly packed bed was simulated at each step in the PVSA process. The pressure and velocity profiles in the non-isobaric steps were clearly different from those of a normally packed bed. The two-bed PVSA process using one poorly packed bed with additional 1% void volume in feed end of bed could produce a purity of 92.3mol% O₂ from air, which was almost 1% purity lower than the PVSA with normal two beds. Even small asymmetry between beds, due to poor bed packing, could greatly reduce the product purity in the PVSA process.     

Computational study of forced convective heat transfer in structured packed beds with spherical or ellipsoidal particles

Randomly packed bed reactors are widely used in chemical process industries, because of their low cost and ease of use compared to other packing methods. However, the pressure drops in such packed beds are usually much higher than those in other packed beds, and the overall heat transfer performances may be greatly lowered. In order to reduce the pressure drops and improve the overall heat transfer performances of packed beds, structured packed beds are considered to be promising choices. In this paper, the flow and heat transfer inside small pores of some novel structured packed beds are numerically studied, where the packed beds with ellipsoidal or non-uniform spherical particles are investigated for the first time and some new transport phenomena are obtained. Three-dimensional Navier–Stokes equations and RNG k–ε turbulence model with scalable wall function are adopted for present computations. The effects of packing form and particle shape are studied in detail and the flow and heat transfer performances in uniform and non-uniform packed beds are also compared with each other. Firstly, it is found that, with proper selection of packing form and particle shape, the pressure drops in structured packed beds can be greatly reduced and the overall heat transfer performances will be improved. The traditional correlations of flow and heat transfer extracted from randomly packings are found to overpredict the pressure drops and Nusselt number for all these structured packings, and new correlations of flow and heat transfer are obtained. Secondly, it is also revealed that, both the effects of packing form and particle shape are significant on the flow and heat transfer in structured packed beds. With the same particle shape (sphere), the overall heat transfer efficiency of simple cubic (SC) packing is the highest. With the same packing form, such as face center cubic (FCC) packing, the overall heat transfer performance of long ellipsoidal particle model is the best. Furthermore, with the same particle shape and packing form, such as body center cubic (BCC) packing with spheres, the overall heat transfer performance of uniform packing model is higher than that of non-uniform packing model. The models and results presented in this paper would be useful for the optimum design of packed bed reactors.     

DRYING OF AIR IN SILICA GEL PACKED COLUMNS

Silica gels that were made humidity indicating by impregnation of CoCl₂ were used for air drying in isothermal packed column. The effects of type of silica gel, packing height, particle size and air flow rate on breakthrough curves were studied. The breakthrough curves predicted by Rasmuson and Neretniek's analytical solution were in good agreement with experimental data.     

Measuring ultrafine aerosols by direct photoionization and charge capture in continuous flow

Direct ultraviolet (UV) photoionization enables electrical charging of aerosol nanoparticles without relying on the collision of particles and ions. In this work, a low-strength electric field is applied during particle photoionization to capture charge as it is photoemitted from the particles in continuous flow, yielding a novel electrical current measurement. As in conventional photocharging-based measurement devices, a distinct electrical current from the remaining photocharged particles is also measured downstream. The two distinct measured currents are proportional to the total photoelectrically active area of the particles. A three-dimensional numerical model for particle and ion (dis)charging and transport is evaluated by comparing simulations of integrated electric currents with those from charged soot particles and ions in an experimental photoionization chamber. The model and experiment show good quantitative agreement for a single empirical constant, K_cI, over a range of particle sizes and concentrations providing confidence in the theoretical equations and numerical method used.

Copyright © 2018 American Association for Aerosol Research