Assessment of fluidised bed ion exchange equipment

Continuous countercurrent fluidised bed ion exchange equipment has certain advantages over moving packed bed or fixed bed equipment. The treatment of high flow rate dilute waste streams containing particulate matter and the processing of uranium ore unfiltered pulps are considered in the application of fluidised bed systems. Methods of obtaining design data are outlined and techniques for calculating the performance of continuous counterflow systems are proposed for various situations.     

Solid-liquid mass transfer in a gas-liquid-solid fluidized bed

Solid-liquid mass transfer in co-current two- and three-phase fluidized beds of water, air and benzoic acid pellets is studied. An axial dispersion model is used to describe the liquid flow when evaluating the solid-liquid mass transfer. The axial concentration profile of benzoic acid in the liquid is compared to that obtained experimentally and is found to be accurate. Three-phase fluidized bed solid-liquid mass-transfer coefficients are higher than the corresponding two-phase bed coefficients. The mass-transfer coefficient increases with increasing gas rate and is independent of liquid rate over the entire range studied. The mass-transfer coefficient also appears to be dependent on particle size, but only at high gas rates. At low or zero gas rates, k is nearly independent of particle size. A generalized correlation is developed which accurately and conveniently predicts the mass transfer in both two- and three-phase fluidized beds. Comparison to the solid-liquid mass-transfer characteristics of slurry bubble columns is also performed.     

Interphase mass transfer in a gas fluidized bed

Mass transfer rates have been measured for bubbles containing ozone injected into an air-fluidized two-dimensional bed of particles. The overall mass transfer coefficient, corrected for unsteady bubble growth effects and with entrance effects eliminated, lies between predictions of models which assume that throughflow and diffusion are both additive and rate-controlling and those models which assume that mass transfer is governed by diffusion at the cloud boundary. The experimental value is consistent with a model which bases mass transfer purely on the throughflow predicted by Murray. Failure to account for bubble growth leads to drastic overestimation of the mass transfer coefficient.     

Interphase mass transfer in an aggregative fluidized bed

The experimental technique developed by Chavarie and Grace has been used to measure interphase mass transfer rates for bubbles in a two-dimensional fluidized bed with different sizes of particles from 90 to 39O μm. It is shown that the overall transfer rate, corrected for bubble growth and with negligible effects of adsorption, increases with particle diameter. The best model for the single bubble case assumes that transfer is controlled by diffusion and convection at the bubble interface, given by the penetration theory and by Murray's theory respectively. Agreement between this model and the data is best if corrections for bubble shape are applied and if it is assumed that there is no interaction between the two transfer mechanisms. Measurements of tracer concentrations as bubbles undergo coalescence indicate that there may be some enhancement of transfer due to bubble interaction, but this does not appear to be a major factor for the conditions studied.     

Heat and mass transfer characteristics in a gas-slurry-solid fluidized bed

The gas-slurry-solid fluidized bed is a unique operation where the upward flow of a liquid-solid suspension contacts with the concurrent up-flow of a gas, supporting a bed of coarser particles in a fluidized state. In the present study we measured the gas holdup, the coarse particle holdup, the cylinder-to-slurry heat transfer coefficient, and the cylinder-to-liquid mass transfer coefficient at controlled slurry concentrations. The slurry particles were sieved glass beads of 0.1 mm average diameter and their volumetric fraction was varied at 0, 0.01, 0.05 or 0.1. The slurry and the gas velocities were varied up to about 12 and 15 cm/s, respectively. The coarse particles fluidized were sieved glass beads of average diameters of 3.6 and 5.2 mm. The individual phase-holdup values were measured and served for use in correlating the heat and mass transfer coefficients. The heat and mass transfer coefficients in the slurry flow, gas-slurry transport bed, slurry-solid fluidized bed and gas-slurry-solid fluidized bed operations can be correlated well by dimensionless equations of a unified formula in terms of the Nusselt (Sherwood) number, the Prandtl (Schmidt) number and the specific power group including the energy dissipation rate per unit mass of slurry, with different numerical constants and exponent values, respectively, to the heat and mass transfer coefficients. The presence of an analogy between the heat and mass transfer from the vertically immersed cylinder in these slurry flow, gas-slurry transport bed and gas-slurry-solid fluidized bed systems is suggested.     

Investigation of hydrodynamics and mass transfer in a three-phase fluidized bed

On the basis of semiempirical turbulence theory and wave theory of the motion of liquid films, we have composed an engineering method for calculating the energetic, amplitude-frequency, and kinetic characteristics of phase interface, which are used in calculating the processes of absorption and chemosorption.     

Mass transfer in liquid fluidized beds of ion exchange particles

Mass transfer rate data in a shallow liquid fluidized bed of ion exchange resin particles have been obtained in the range 2 < Re_s, < 25. At low voidage, ε < 0.55–0.60, the rate of mass transfer is reduced and it is inferred that a fluidized bed tends to maintain an ordered axial structure. This is lost at high voidage due to increased flow perturbation caused by the distributor plate. A generalised correlation is given for fluidized, fixed and distended beds in the range 2 < Re_s, < 25.     

Enhancement of mass transfer in the fluidized bed electrode reactors

With the view of developing the fluidized bed electrode system, mass transfer coefficient, overpotential distribution, and copper degradation have been observed in this investigation. Particles whose diameters were one of 327, 388, 510, 548, 750, and 960 μm were fluidized by the 1,000 ppm copper sulfate electrolyte. This study used two types of the experimental reactor. One had 5x5.5 cm bed-dimension with various thickness in a rectangular side-by-side configuration; the other 3.2 cm bed-diameter with various height in a cylindrical flow-through configuration. Mass transfer coefficient increased with increasing particle diameter, and the optimum fluidization was obtained at the condition of bed porosity near 0.65. For processing a large fluidized bed reactor, the expansion of bed height at a distance between electrodes was found to be more effective than the enlargement of bed thickness between electrodes. By replacing a three-dimensional current-feeder with a plane feeder, degradation and residual concentration of copper ion in a batch recycling mode could be achieved to be higher than 99% and less than 5 mg/L, respectively.     

An investigation of mass transfer in a countercurrent three-phase fluidized bed

The rate of absorption with chemical reaction has been measured in a bed of solid spheres fluidized by upward flowing gas and irrigated by downward flowing liquid for different reactant concentrations in the liquid phase and different values of the liquid superficial velocity.Values of the effective inerfacial area per unit volume of a static bed, a, and the gas-film mass-transfer coefficient, k_g, are reported as functions of the liquid superficial velocity.     

Gas-liquid mass transfer in a two-stage draft tube gas-liquid-solid fluidized bed

Experiments were conducted to study the gas-liquid mass-transfer characteristics in a two-stage draft tube gas-liquid-solid fluidized bed (DTFB). The DTFB was operated at essentially zero liquid velocity and superficial gas velocities up to 250 m/h. The sodium sulphite oxidation method coupled with an analysis of the off-gas concentration was utilized to determine the mass-transfer coefficients in each of the two stages and the overall mass-transfer coefficient for the two-stage DTFB. The particles used in the study were glass beads of 300 and 600 μm diameter and activated carbon particles of 307 and 714 μm diameter. The overall mass-transfer coefficient for the two-stage DTFB was compared with that for the single stage or one-stage DTFB.     

Heat and mass transfer in apple cubes in a microwave-assisted fluidized bed drier

In the present study, a pilot scale microwave assisted fluidized-bed dryer was designed and used to dry apple cubes. A model was developed to describe heat and mass transfer in apple cubes during drying in a combined microwave-assisted fluidized-bed dryer. A numerical solution based on the finite difference method was used to develop the model for moisture distribution and temperature variation of samples. The model was validated using experimental data, including average moisture content, center and surface temperature at various air temperatures and microwave power densities.     

The effect of adsorption on mass transfer in fluidized bed catalytic reactors

Tracer gas residence time distributions (RTD) in a laboratory scale fluidized bed system have been measured for pulses of three different tracer gases (methane, ethane and propane) at different temperatures in the range 323 to 435 K. The fluidized solid was a commercial zeolite based FCC catalyst (CBZ‐2), and measurements were carried out in a superficial air velocity range of 0.01 to 0.04 m/s. The data were interpreted with two‐phase dense phase dispersion models for adsorptive tracers, available in the literature. In addition, modified models were considered by assuming a stationary dense phase and neglecting axial dispersion in this phase. Mean residence time, μ₁, and the variance of the residence time, σ², of RTD data were calculated for each experimental run. Applying the moment technique in the Laplace domain, the differential equations for all models considered were analytically solved. Mass transfer coefficients obtained from dynamic experiments were compared with the values estimated from the relations available in the literature. It was found that methods considering convective flux alone between the bubble and emulsion phases give closer values to the experimental ones than the methods also including the diffusive flux.     

Effect of ring promoter on ionic mass transfer in a fluidized bed

Mass transfer data were obtained at the wall of a batch fluidized bed in the presence of a ring promoter used for the reduction of ferricyanide ion. The ring promoter was a copper rod on which rings of uniform size were fixed at equal distances. In these studies, the cross‐sectional diameter of the ring, the spacing between the rings, the particle size and the void fraction were varied and the limiting current data were measured. It was observed that the mass transfer coefficients were augmented with increased particle diameter, decreased spacing between the rings, and increased cross‐sectional diameter of the ring. Copyright © 2003 Society of Chemical Industry     

Gas-Liquid mass transfer in a three-phase fluidized bed with floating bubble breakers

The effects of liquid and gas velocities, particle size and volume ratio of floating bubble breakers to solid particles (V_f/V_s) on both the volumetric mass transfer coefficient, k_la, and the gas-liquid interfacial area, a, have been determined in three-phase fluidized beds with floating bubble breakers. Beds having a volume ratio (V_f/V_s) of about 0.15 showed a maximum increase in both k_la and a of about 30% in comparison to that in the corresponding bed without floating bubble breakers. The volumetric mass transfer coefficient in three-phase fluidized beds with or without floating bubble breakers can be estimated from the surface renewal frequency of liquid microeddies and the particle size.     

Dynamics of cyclic fixed bed ion exchange an analysis based on a linear mass transfer model

Solutions for fixed bed ion exchange based on a linear mass transfer model (linear isotherm and linear driving force rate equation) and general initial and boundary conditions are presented. The linear model solutions are applied to the study of the cyclic operation of fixed bed ion exchange and the equations describing both the transient and cyclic fixed bed processes for co-current regeneration are derived. Examples which illustrate the simplicity of application of the analytical solutions are discussed.     

Continuous ion exchange in fluidized beds

A mathematical model has been developed for prediction of the performance of a continuous ion-exchange column, comprising a series of fluidized beds of resin, operated with periodic flows of both phases. Simple small-scale tests provide the data required in calculations. Experimental work was carried out on uranium extraction from unfiltered ore leach pulps and clarified liquors.     

流化床液固两相传质过程的模拟研究进展

液固两相流化床具有液固相接触效率高、传质和传热性能好、颗粒分布均匀等优点,已被广泛应用于众多工业过程中。然而,流化床中与传质过程耦合的颗粒流化的复杂非线性特征及其湍动特性,使得对传质过程特性的研究十分困难。且仅依靠实验观测和理论预测难以揭示多相流相互作用规律,无法获得全面和详细的速度场和浓度场分布情况。近年来,数值模拟的快速发展为深入探索流化床中液固两相流动行为及其与传质过程耦合问题提供了重要的途径。本文对流化床液固两相流动与传质过程模拟方法进行了综述,并对其未来研究趋势进行了展望。借助于计算传质学理论可以更精确地预测局部浓度的分布情况,进而可以深入分析液固两相流化床中的传质过程规律与传质特性,为液固两相流化床的设计和优化提供理论基础。