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.     

Fluidization and mixing of solids distributed in size and density

This article presents a study of the fluidization of solids dispersed in size and density simultaneously. Minimum and complete fluidization velocities of these solids are measured and compared with their U_mf distribution. Extensions of some correlations established for binary mixtures are also proposed. The axial mixing state of the beds near these two velocities is then evaluated by sampling, and expressed in terms of several indices: size index, density index and the combined U_mf index. In contrast with U_mf, the axial profile of the bed is mainly controlled by the density distribution of the materials. An index based on the pressure drop along the bed height gives an on-line indication of that segregation.     

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.     

Size segregation in gas-solid fluidized beds with continuous size distributions

Discrete-particle simulations of a gas-solid fluidized bed are used to investigate the species segregation (de-mixing) behavior of systems with continuous particle size distributions. Both Gaussian and lognormal distributions are investigated over a range of distribution widths, restitution and friction coefficients, and gas velocities. The results indicate that: (i) the average particle diameter decreases as the height within the bed increases, (ii) the level of segregation increases with an increase in the width of the particle size distribution, and (iii) segregation is attenuated as bubbling becomes more vigorous. Furthermore, the shape of the local size distribution (i.e., Gaussian or lognormal) is found to mimic that of the overall size distribution in most regions of the fluidized bed.     

Segregation and mixing effects in the riser of a circulating fluidized bed

Segregation and mixing effects of binary mixtures of particles having difference in sizes and densities were studied in 0.1016 m-diameter riser of a circulating fluidized bed at gas velocities between 2.01 and 4.681 m/s and solids circulation rate between 12.5 and 50 kg/m² s. Two groups of bed materials (three quartz sand-spent fcc catalyst mixtures with different initial mass % of sand and two coal-iron mixtures, one with almost same sizes but with different densities and the other having both different sizes and densities) were used. Using local axial mass % of heavier/coarser particles and their mean sizes the extent of segregation was evaluated. The influence of operating conditions like superficial gas velocity and solids circulation rate on segregation was examined and found that with their increase segregation effects generally tend to decrease and a uniform mixture conforming to initial composition of the mixture results. Using the data available in the literature and those of the present authors an empirical correlation to obtain the extent of segregation in CFBs has been proposed.     

Numerical study on microscopic mixing characteristics in fluidized beds via DEM

In this paper, discrete element method (DEM), combined with computational fluid dynamics (CFD), is used to investigate the micro-mixing process in fluidized beds (FBs) of uniform particles. With the aid of snapshots and adoption of Lacey and Ashton indexes, mixing evolvement for two cases, fluidized bed using horizontal distributor with even gas supply and fluidized bed using inclined distributor with uneven gas supply, is discussed in detail. Results indicate that the Ashton index appears to be more effective in assessing the mixing dynamics in this work. Further analyses illustrate that in the case of horizontal distributor incorporated with even gas supply, diffusive mixing pattern is predominant, since bubbles lateral motion is reduced in such a bed; whereas, there is a faster convective mixing process in a fluidized bed using inclined distributor with uneven gas feed, followed by shear mixing. Generally, localized air supply induces the density gradient of particle distribution in the bed, which is the basic agent of convective particle stream. The analyses are confirmed by the comparison of solid flux during the simulations of the two cases. In addition, the mixing mechanism and the mixing time scale agree well with published experimental results.     

A novel multigrid technique for Lagrangian modeling of fuel mixing in fluidized beds

This paper presents a novel Lagrangian approach to model fuel mixing in gas–solid fluidized beds. In the mixing process, fuel particles are considerably larger than the inert bed material and therefore, the present work proposes three grids to account for the difference in size between the fuel particles and inert solids. The information between the grids is exchanged using an algorithm presented in the paper. A statistical method has been developed to analyze the distribution of the fuel particles in the bed. The results for the preferential positions, velocity vectors and horizontal dispersion coefficients are compared with experimental data in a bed applying simplified scaling relationships for different operating conditions. The effects of initial bed height and inlet gas velocity on the fuel mixing are investigated.It is found that the proposed Lagrangian modeling can capture the complex pattern of the movement of the fuel particles, in spite of the large difference in diameter between inert and fuel particles.     

A study of the mixing of solids in gas-fluidized beds, using ultra-fast MRI

Magnetic resonance imaging has been used to measure the rate of axial mixing in a vertical direction of a small batch of poppy seeds suddenly added to the upper surface of a bed of sugar crystals fluidized by air.     

Numerical study of particle mixing in bubbling fluidized beds based on fractal and entropy analysis

The DEM–LES coupling method is used to study the mixing of mono-dispersed identical density particles in bubbling fluidized beds based on the fractal and entropy analysis. A dimensionless function is used to study the microscopic characteristics of mixing interface. A criterion for identification of the boundary of bubbles is proposed and used to investigate the effect of bubble on particle mixing characteristics. Moreover, the Shannon information entropy is used to evaluate the macroscopic level of mixing. It is found that both the mixing interface of particles and the boundary of bubbles in fluidized beds are fractal. The bubble boundary dimension decreases as the fluidization velocity increases. The fractal bubble boundary induces the inhomogeneous characteristics of mixing interfaces of particles. On the other hand, the radial distribution function indicates the universal and intrinsic characteristics of particle mixing, independent of the initial effects after a short segment of evolution. Moreover, the information entropy, which is defined based on the radial distribution function, increases as the fluidization velocity increases. The mean information entropy is a good indication and a credible evaluation on the macroscopic mixing levels under various operating conditions of the beds.     

Investigation of the solid flow between two fluidized beds connected by an orifice

Two adjacent fluidized beds separated by a wall can exchange solids through an orifice due to the local pressure oscillations on both sides of the orifice produced by passing bubbles. This flow of solids between the two fluidized bed reactors is sufficient for some practical applications where a highly integrated, simple and cost-effective arrangement might be required. Solid transfer rates through orifices of different geometries and dimensions have been measured in this work using a novel image analysis technique. The measured solid fluxes can reach up to in some experimental conditions. An empirical correlation has been proposed to fit the available experimental data as a function of key operating conditions.     

流化床内颗粒混合研究

分析了流化床内颗粒混合机理，综合了床内水平方向和垂直方向上颗粒混合，将床内颗粒混合过程分成两部分：一是向上运动的尾迹相和向下运动的乳化相之间的对流交换，二是乳化相内横向扩散。建立了二维的对流扩散模型，数值结果和实验数据吻合。     

Flow structure of the solids in gas-solid fluidized beds

Existence of clusters in dense fluidized beds was investigated by analyzing the time-position data of a tracer obtained in several radioactive particle tracking experiments. It was found that in the case of sand particles, more gas passes through the bed as bubbles with increasing the superficial gas velocity and in the case of FCC powder, flow of the gas through the bed as bubbles does not increase in the turbulent fluidization regime. Cluster diameters were estimated from their velocities and found that descending clusters are generally larger than ascending ones and the size of both increases with increasing the superficial gas velocity. Bubble velocities evaluated in this work are in good agreement with the correlations in the bubbling regime of the fluidization available in the literature.     

Artificial Neural Network approach to segregation characteristic of binary homogeneous mixtures in promoted gas-solid fluidized beds

Binary mixtures of size-different dolomites are fluidized in a bed where co-axial promoters are introduced. The segregation characteristic of jetsam particles has been determined for different mixtures in terms of the segregation distance by empirically correlating the results with the various system parameters viz. initial static bed height, height of a layer of particles above the bottom grid, superficial gas velocity and average particle size of the mixture with dimensional analysis for both the un-promoted and the promoted beds. Correlations have also been developed with the above system parameters from an Artificial Neural Network approach. Segregation distances for the promoted and un-promoted beds have been compared. The results through the correlations thus developed with the above system parameters from the ANN-approach and the findings with respect to the dimensional analysis approach have been compared.     

Particle dispersion in viscous three-phase inverse fluidized beds

Dispersion characteristics of low density fluidized particles such as polyethylene and polypropylene were investigated by using the stochastic method in three-phase inverse fluidized beds with viscous liquid medium ( in height). To establish the relationship between the pressure drop variation and the particle dispersion in test section, the histogram of pressure drop fluctuations were also measured and analyzed. Effects of operating variables such as gas and liquid velocities, liquid viscosity and media particle kind (density) on the fluctuating frequency, dispersion coefficient and exiting rate of media particles from the test section were determined. The fluctuating frequency and dispersion coefficient of particles increased with increasing gas or liquid velocity, but decreased considerably with increasing liquid viscosity in three-phase inverse fluidized beds. The dispersion coefficient of media particles of relatively higher density exhibited a value higher than that of lower density particles. The dispersion coefficients of particles were well correlated with operating variables in terms of dimensionless groups.     

Effect of particle shape on liquid-fluidized beds of binary (and ternary) solids mixtures: segregation vs. mixing

Experiments were carried out in water-fluidized binary (and ternary) mixtures of teflon spheres, discs and rods. All particles had the same volume, while the discs and rods had nearly the same sphericity. It is shown that segregation can occur by shape, with similar segregated and mixed zones as when binary mixtures of different size or density are fluidized. The model of Pruden and Epstein (1964; Stratification by size in particulate fluidisation and in hindered settling. Chemical Engineering Science 19, 696), in which the degree of segregation depends on the bulk density difference(s) of the corresponding monocomponent beds at the same liquid velocity, is vindicated qualitatively for each system, but sphericity is not sufficient as a single shape factor to yield a single quantitative correlation of the transitions between segregation patterns for the different systems. Segregation by shape of non-isometric particles appears to require higher reduced density differences than sizing of spheres, probably because of the greater bed instabilities generated by the non-isometric particles. Overall bed voidage is predicted well by the serial model of Epstein et al. (1981; Liquid fluidisation of binary particle mixture- I.Overall bed expansion. Chemical Engineering Science 36, 1803).     

Gas back-mixing studies in membrane assisted bubbling fluidized beds

Fluidized beds employing fine powders are finding increased application in the chemical and petrochemical industry because of their excellent mass and heat transfer characteristics. However, in fluidized bed chemical reactors axial gas back mixing can strongly decrease the conversion and selectivity. By insertion of membranes in fluidized beds large improvements in conversion and selectivity can be achieved, firstly by optimizing axial concentration profiles via distributive feeding of one of the reactants or selective withdrawal of one of the products, and secondly, by decreasing the effective axial dispersion via compartmentalization of the fluidized bed. Moreover, insertion of membrane bundles in a suitable configuration impedes bubble growth, thereby reducing reactant by-pass via rapidly rising large bubbles. In this work the influence of the presence and configuration of membrane bundles and the effect of gas addition via the membranes on the effective axial dispersion was studied experimentally.Steady state concentration profiles were measured where a CO₂-tracer was injected at different locations through a probe (point injection) or via the membranes (line injection) into a square fluidized bed containing glass particles (75-, 2550 kg/m³) fluidized with nitrogen distributed via a porous plate. Different bed configurations, viz. without internals, with vertical or horizontal membrane bundles were investigated and the effects of overall fluidization velocity and gas flow ratio of gas fed through the membrane bundles and the porous plate distributor were studied.Experimental results revealed that the insertion of vertical and horizontal membrane bundles decreases the effective axial dispersion considerably compared to a bed without internals. The point injection experiments indicated the importance of a non-uniform lateral emulsion phase velocity profile. The line injection experiments clearly pointed out the importance of bubble-to-emulsion phase mass transfer limitations. Gas addition through the membrane bundles decreases the effective axial gas dispersion enormously by almost annihilating the solids down flow along the walls and by decreasing the average bubble size and bubble fraction.     

Modeling of fuel mixing in fluidized bed combustors

This paper presents a three-dimensional model for fuel mixing in fluidized bed combustors. The model accounts for mixing patterns which were experimentally shown to govern mixing in risers with geometry and operational conditions representative for furnaces in fluidized bed combustors. The mixing process is modeled for three different solid phases in the furnace and the model, which includes the return leg, can be applied both under bubbling and circulating regimes. The semi-empirical basis of the model was previously validated in different large-scale fluidized bed combustors and is combined with a model for fuel particle conversion to obtain the fuel concentration field. Model results are compared with experimental data from the Chalmers 12 MW_th CFB combustor, yielding a reasonable agreement.     

Characterization of solids mixing patterns in bubbling fluidized beds

Behavior of the solid phase in fluidized beds was studied by a 2D CFD-DEM approach to obtain more information on the solid mixing and circulation. Hydrodynamic parameters, including solid diffusivity, and internal and gross circulations were considered in this study. To validate the simulation, time-position data obtained by the Radioactive Particle Tracking (RPT) technique were used. It was shown that the 2D model can satisfactorily predict the axial diffusivity, while the radial diffusivity calculated based on the model is an order of magnitude smaller than the experimental one in 3D. The influence of aspect ratio of the bed, type of distributor, and inlet gas velocity on solids mixing pattern were also studied. The solids flow pattern in the bed changed considerably by increasing the aspect ratio. Different solid circulations were captured by numerical model for the two types of distributors, namely porous and injection types. The results suggested that increasing the superficial gas velocity caused rigorous internal and gross circulations, which in return, improved solids mixing and decreased deviations from well mixed state.