Mass Transfer In Fluidized Beds Research Articles

Experimental investigation and modeling of the impact of random packings on mass transfer in fluidized beds

This study investigates the impact of two novel settings of bubbling fluidized beds equipped with random metal packings on the mass transfer. To do this, a comprehensive approach is applied that integrates experiments and modeling and explores the relevance of the different underlying mechanisms involved in interphase mass transfer in fluidized beds. Firstly, the mass transfer of water from moisturized silica gel particles to dry air is studied in both a packed-fluidized bed and a freely bubbling bed with no packing. The experimental set-up consists of a cylindrical bubbling fluidized-bed column with an inner diameter of 22 cm. Silica-gel particles with a mean particle diameter of 797 μm are used as bed material. The total bed amount ranges from 4 to 8 kg, while the fluidization number (F) varies between 1.7 and 2.3. Two types of packing, RMSR (stainless steel thread saddle rings) and Hiflow (stainless steel pall rings) are examined and compared to the reference case of a bubbling bed with no packing. The height of the packed section is maintained at 60 cm. The results show that, at all operating conditions, the use of packings enhances the amount of desorbed water in the fluidized bed. The increase is up to 17%, as compared to the bed without packing. The effect is believed to be inhibition of bubble growth in the packed-fluidized bed. To study this further, a mass-transfer model is introduced to analyze the different mass-transfer steps (intra-particle, particle surface to emulsion gas, and emulsion gas to bubble gas) in packed-fluidized beds compared to beds with no packing. TGA experiments are applied to describe the intra-particle mass transfer through a desorption kinetic model. Model analysis shows that the main resistance for mass transfer occurs across the bubble-emulsion boundary. The calculated value of the mass transfer coefficient for this, Kb, at reference conditions (6 kg of silica gel and F = 2.3) is 7.6e-5 s−1 with packings (in average) and 6.2e-5 s−1 without packings, i.e. a 23% improvement in the governing mass-transfer coefficient.

A priori error estimates for finite element approximations to transient convection-diffusion-reaction equations in fluidized beds

Abstract In this work, a priori error estimates for finite element approximations to the governing equations of heat and mass transfer in fluidized beds are derived. These equations are time dependent strongly coupled system of five semilinear convection-diffusion-reaction equations. The a priori error estimates for all the five variables are obtained for the error measured in L ∞(L 2) and L 2 ( E ) ${L}^{2}\left(\mathcal{E}\right)$ , E $\mathcal{E}$ is the energy norm.

A posteriori error estimates and an adaptive finite element solution for the system of unsteady convection-diffusion-reaction equations in fluidized beds

The a posteriori error estimates for finite element approximations to the governing equations of heat and mass transfer in fluidized beds are derived in this work. These are a system of five time dependent coupled nonlinear convection-diffusion-reaction equations. Based on the variational formulation, computable residual based a posteriori error estimates are obtained. The time discretization has been done using the implicit Euler method. The a posteriori error estimates for all the five variables are derived using the total residual and error indicators due to spatial discretization, time discretization and linearization. An adaptive finite element solution of these model equations is computed and the performance is illustrated.

Improved Gas–Solids Mass Transfer in Fluidized Beds: Confined Fluidization in Chemical-Looping Combustion

© 2019 American Chemical Society. Fluidized bed processes with reactive bed material have become increasingly popular as research topics, with applications such as chemical-looping technologies, oxygen carrier aided combustion, and fluidized bed gasification being extensively investigated. When used at commercial scale the performance of such processes may be limited not by gas-solid reactivity, but by mass transfer of reactants from bubbles to the emulsion phase. In an effort to break down the two-phase flow structure, and thereby increase the bubble-emulsion mass transfer coefficient, spherical packing material was added to a fluidized bed using ilmenite as bed material. Two types of packing were tested: expanded clay aggregate (ECA) and aluminum silicate balls (ASB). Both packings had a diameter of about 12 mm but drastically different bulk densities of 240 kg/m 3 and 1400 kg/m 3 , respectively. These were tested in chemical-looping-combustion batch experiments using a stainless-steel reactor with a diameter of 78 mm, with syngas or carbon monoxide as fuel at 915 °C. The lighter packing formed a floating plug while the heavier remained stationary at the reactor bottom. To compare the confined fluidized bed to a reference conventional one, a simple reaction model was implemented based on the experiments. It showed that in the confined fluidized bed the associated effective reaction rate constant increased by up to a factor of 2 for a given bed mass. Further, up to 4 times less oxygen carrier bed mass was needed to achieve the same gas conversion, at a lower total pressure drop. Experiments with only carbon monoxide showed similar gains when using aluminum silicate balls, indicating that catalysis of the water gas shift reaction was not the main factor for improved gas conversion. It can be concluded that the concept of confined fluidization has great potential to increase mass transfer in fluidized beds with active bed material.

Particle-Scale Study of Structural Transition of Solid Phase in Gas-Fluidized Beds

Bed structure is important to heat and mass transfer in fluidization. This study investigates solid structural transition for cohesive particles by the combined approach of computational fluid dynamics and discrete element method, supported by Voronoi tessellation. Macroscopic behaviors of fluidized beds are first discussed in connection with microscopic structural transitions. Then, the properties of Voronoi polyhedra are analyzed at both bed and particle scales. In addition, the correlation of coordination number and porosity is examined at a particle scale, and their combined probability distribution is analyzed to identify dominant structure of particles in different flow regimes. Finally, an index is proposed as the ratio of coordination number and porosity to differentiate microscopic states of a particle. These findings should be useful for the study of flow, heat, and mass transfer in fluidized beds and for the understanding of solid-like to fluid-like transition of granular materials.

Analogy between momentum and heat transfer in liquid–solid fluidized beds

Wall-to-bed heat transfer in particulate fluidized beds of spherical particles was studied. Experiments were performed using spherical glass particles of 0.80–2.98mm in diameter with water in a 25.4mm I.D. copper tube equipped with a steam jacket.Heat transfer data related to the fluid–particle interphase drag coefficient were obtained and compared with previous results for wall-to-bed mass transfer in fluidized beds [Bošković et al., Powder Technol., 79 (1994) 217].All the data for momentum, heat and mass transfer in particulate fluidized beds of spherical particles, showed the existence of an analogy among these three phenomena.

Three-dimensional numerical study of heat and mass transfer in fluidized beds with spray nozzle

The numerical computations of temperature and concentration distributions inside a fluidized bed with spray injection in three-dimensions are presented. A continuum model, based on rigorous mass and energy balance equations developed from Nagaiah et al., is used for the three-dimensional simulations. The three-dimensional model equation for nozzle spray is reformulated in comparison to Heinrich. For solving the non-linear partial differential equations with boundary conditions a finite element method is used for space discretization and an implicit Euler method is used for time discretization. The time-dependent behavior of the air humidity, air temperature, degree of wetting, liquid film temperature and particle temperature is presented using two different sets of experimental data. The presented numerical results are validated with the experimental results. Finally, the parallel numerical results are presented using the domain decomposition methods.

Wall-to-liquid mass transfer in fluidized beds and vertical transport of inert particles

Mass transfer coefficients in single phase flow, liquid fluidized beds and vertical hydraulic transport of spherical inert particles were studied experimentally using 40 mm and 25.4 mm diameter columns. The mass transfer data were obtained by studying the transfer of benzoic acid from a tube segment to water using the dissolution method. In all runs, the mass transfer rates were determined in the presence of spherical glass particles 1.2, 1.94 and 2.98 mm in diameter. The influence of different parameters, such as liquid velocity, particles size and voids on mass transfer in fluidized beds and hydraulic transport are presented. The data for mass transfer in all the investigated systems are shown using the Sherwood number (Sh) and mass transfer factor - Colburn factor (jD) - as a function of Reynolds number (Re) for the particles and for the column. The data for mass transfer in particulate fluidized beds and for vertical hydraulic transport of spherical particles were correlated by treating the flowing fluid-particle mixture as a pseudo fluid by introducing a modified mixture Reynolds number (Rem). A new correlation for the mass transfer factor in fluidized beds and in vertical hydraulic transport is proposed.

Mass transfer from the wall of a column to the fluid in a fluidized bed of inert spherical particles

Mass transfer in fluidized beds is an important operation for separation processes. Two effects can be achieved by using fluidized beds in mass transfer processes increasing interface area and relative movement between the phases. These effects are both desirable because they lead to greater process rates. This paper presents an experimental investigation regarding mass transfer from the wall of a column to the fluid in a fluidized bed of inert spherical particles. The experiments were conducted in column 40 mm in diameter with spherical particles 0,8-3 mm in diameter and water as one fluidizing fluid. The method of dissolution of benzoic acid was used to provide very low mass flux. The average wall-to-fluid mass transfer coefficients were determined for two systems: single-phase fluid flow and a fluidized bed of inert particles The measurements encompassed a Reynolds number range from 100-4000 for single-phase flow and 600-4000 in fluidized beds. The mass transfer coefficients for both systems were calculated from weight loss of benzoic acid. The effects of superficial liquid velocity and particle diameter on the mass transfer coefficient were investigated. It was found that mass transfer was more intensive in the fluidized bed in comparison with single phase flow. The best conditions for mass transfer were reached at a minimum fluidization velocity, when the mass transfer coefficient had the greatest value. The experimental data were correlated in the form: jd = f(Re), where jd is the dimensionless mass transfer factor and Re the Reynolds number.

Mass transfer in fluidized beds of inert particles. Part II : Effect of particle size and density

The mass transfer rate in fluidized beds of inert particles (FIB) is shown to be dependent on the electrolyte flow velocity and the intensity of particle collisions with the electrode. The influence of particle size and density on the ratio of the magnitude of these two influences on the mass transfer rate in a FIB was studied. Use of particle materials of varying density in an FIB permits variation of the two effects. The influence of collision currents prevails in FIBs of low density materials, and the influence of interstitial velocity is dominant in beds of high density material. The ratio of these factors also depends on the size of particles of the same density. With smaller particle size the influence of collision currents is greater. Smoothing of mass transfer maxima in beds of particles both of small and high density is explained. The results establish a basis for the selection of FIB materials for electrochemical processes.

Mass transfer in fluidized beds of inert particles. Part I: The role of collision currents in mass transfer to the electrode

The effect of particle-wall collision on the mass transfer rate mechanism in liquid fluidized bed electrochemical cells was studied. Collision frequencies and currents were measured at microelectrodes set in the bed wall. It is postulated that, at each particle–electrode collision, a specific microvolume of bulk concentration electrolyte is introduced into the near-electrode diffusion layer during particle movement towards the electrode causing an enhancement of the limiting diffusion current. Based on measurements made at microelectrodes calculations of the contribution of the particle collision mechanism to total mass transfer to a planar electrode are attempted and are in good agreement with experimental values.

Discrimination of fluid bed models and investigation of particle-to-gas mass transfer by means of drying experiments

Particle-to-gas mass transfer in fluidized beds has been investigated by drying experiments. To avoid the introduction of additional unknowns into the problem, the particle-side kinetics are described by the method of normalization. The normalized single particle drying curve has been derived by separate, direct measurement. The Sherwood number can be obtained by comparison with a heterogeneous fluid bed model. This procedure is demonstrated by measurements with aluminium silicate particles. The resulting Sherwood number is significantly lower than the respective single particle value. Model discrimination reveals that this effect can hardly be explained by bypassing or axial dispersion. Discrimination of this kind would not have been possible on the basis of traditional measurements with sublimation of compact material or drying of surface-wet particles.

Experimental methods and correlation of solid—liquid mass transfer in fluidized beds

Solid—liquid mass transfer in gas—liquid—solid fluidized beds is studied experimentally to clarify conflicting results in the literature which may be due to experimental procedure, equipment, or operating conditions. Particle tethering is found to affect measurement of the mass transfer, but the magnitude of the difference is a function of type of tether, column diameter, and flow regime. Effects of phase hold-ups and particle mixing are addressed, as well as liquid and gas superficial velocities. Correlation of the mass transfer coefficient is then discussed in light of the results of this study and the literature. In particular, correlations based on the energy dissipation model are reviewed in detail with respect to their qualitative and quantitative accuracy. A new correlation is then developed to account for the enhancement of the mass transfer due to the energy input to the system by the gas phase over that of the liquid—solid fluidized bed base case. The correlation accurately predicts the trends of the mass transfer with gas and liquid superficial velocities, particle and liquid properties, and phase hold-up.

Mass Transfer in Oil Sand Fluidized Beds—A Simplified Model

Experimental data obtained previously relating to the behavior of single spherical particles of oil sands in hot uniform velocity oxidizing gaseous streams were employed and extended to estimate in a preliminary fashion the extent of mass transfer from oil sand fragments in a fluidized bed. This has been done through employing experimental correlations published in the literature on fluidization. A simple closed-form analytical expression was derived for estimating the transient rates of mass transfer in fluidized beds of oil sands in terms of the main controlling parameters.

Transport phenomena in multi-particle systems—III. Active particle mass transfer in fluidized beds of inert particles

The detailed modelling of the fluidized-bed combustion of coal depends on the elucidation of the mass transfer processes around large active particles in fluidized beds of smaller inert particles. Experimental techniques based on naphthalene sublimation as well as carbon combustion have been used to obtain mass transfer data for this configuration. In previous modelling studies the active particle has been considered to reside only in the emulsion phase. In this paper, the effect of active-particle motion on mass transfer is considered. The active particle is assumed to reside alternately in the bubble and emulsion phases. By considering this particle motion in conjunction with the gas flow distribution, expressions are obtained for the appropriate Sherwood numbers in each phase [an extension of the work presented by Agarwal (1988, Chem. Engng Sci. 43, 2501–2510)]. Using the emulsion phase residence probability (Agarwal, 1987 Chem. Engng Sci. 42, 2481–2483), an expression for the overall Sherwood number is developed. The results of the model calculations are compared favourably with experimental data reported in the literature for fixed as well as fluidized beds.

THREE-PHASE FLUIDIZED BEDS WITH VISCOUS LIQUID: HYDRODYNAMICS AND MASS TRANSFER†

Hydrodynamics and gas/fiquid mass transfer in fluidized beds of glass spheres (3–8 mm diameter)were studied employing viscous aqueous solutions (16–53 mPas) Increasing liquid viscosity reduced the bubble disintegration capability of the particle beds. The most pronounced consequence was a strong decrease in the volumetric mass transfer coefficients (kLa) From a comparison of kLa in Newtonian and pseudoplastic liquids it is concluded that the effective shear rates in three-phase fluidized beds are higher than in bubble columns.

Maximum liquid-to-wall mass transfer in fluidized beds: energetic aspects

Maximum liquid-to-wall mass transfer in fluidized beds: energetic aspects

Mass transfer between the particles and the fluid in fluidized beds of large particles

Generalized empirical correlations available in the literature for particle to fluid mass transfer in fluidized beds are tested for their effectiveness in predicting the experimental mass transfer data for fluidized beds of large particles. Rowe's modification of Nelson and Galloway's theory of particle to fluid mass transfer in dense systems of fine particles is also re-examined and found to agree fairly well with large particle mass transfer data for liquid fluidized beds. A simplified correlation, which is shown to be an approximation of the Nelson-Galloway-Rowe asymptotic expression, is proposed for predicting the mass transfer rate.

Wärme‐ und Stoffübertragung in der Wirbelschicht

AbstractHeat and mass transfer in fluidized beds. Methods of calculation are presented which enable reasonably accurate prediction of heat and mass transfer coefficients in fluidized beds. Some well established equations already given in the literature could be used for estimating the range of existence of the fluidized bed and for calculating maximum fluidization velocities, bed expansion, and particle‐to‐fluid heat and mass transfer. However, a new model had to be developed for the wall‐to‐bed heat transfer from solid surfaces immersed in fluidized beds. This model makes use of some basic ideas adopted from the kinetic theory of gases in order to describe the mechanism of energy transfer through the moving particles. Predictions with this new model are in good agreement with most of the experimental observations, particularly regarding the effects of particle diameter, temperature, pressure, physical properties of gas and particles, and gas velocity.

Mass transfer between a flowing liquid and a wall or an immersed surface in fixed and fluidized beds

This paper proposes an empirical and semi-theoretical derivation of the correlations between the j M factor and the modified Reynolds number Re' for mass transfer between a wall or an immersed surface and a flowing liquid in fixed beds. The correlations are compared with empirical ones obtained by several authors and good agreement is observed. The qualitative influence of several parameters on the overall mass transfer coefficient is obtained from the theoretical approach. The analysis is extended to the case of mass transfer in fluidized beds. Here the analysis leads to approximate correlations for different hydrodynamic regimes. Good agreement is found between the correlations and published data for the corresponding flow regimes.