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.     

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.     

Liquid-Solid mass transfer in a slurry bubble column and a gas-liquid-solid three-phase fluidized bed

Mass transfer coefficients between particles and liquids in a slurry bubble column and a three-phase fluidized bed containing small size particles were obtained with two mass transfer systems: (1) K⁺ –Na⁺ ion-exchange in cation-exchange resin bead beds, including anion-exchange resin beads as inert particles; (2) zinc dissolution by HCl in zinc-plated glass bead beds, and in beds of non-plated glass beads. Operating parameters were gas velocity, liquid velocity, particle diameter, and particle concentration. The dependence of mass transfer coefficients on these parameters is discussed from the viewpoint of the energy supplied into the systems. Correlations of the experimental data using dimensionless groups are compared to previous correlations.     

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.     

Mathematical modeling of mass transfer in a three-phase fluidized system

An algorithm and a program for calculating the driving force of the process of mass transfer and the mass fluxes of gas and liquid phases are developed on the basis of the fundamental two-parameter equation of convective diffusion taking into account the amplitude-frequency characteristics of wave packets. The algorithm and the program of engineering calculation of the processes of physical absorption and chemisorption are written.     

Heat transfer and bubble dynamics in a three-phase inverse fluidized bed

In this study, surface-to-bed heat transfer experiments were performed to gain insight on heat transfer and hydrodynamics in a three-phase inverse fluidized bed. Air, tap water or 0.5 wt.% aqueous ethanol, and polypropylene were, respectively, the gas, liquid and solid phases. The solid loading was varied from 0 to 30 vol.%, and the gas and liquid superficial velocities from 2 to 50 mm/s and 0 to 21 mm/s, respectively. Visual observations were associated with measured phase holdups and instantaneous heat transfer coefficients. Larger gas velocities lead to an increase in bubble size due to the transition to the coalesced bubble flow regime. The greater turbulence induced by the larger bubbles increases the average heat transfer coefficient. On the other hand, adding ethanol reduces the heat transfer coefficient. Solid concentrations up to ∼13 vol.% increase the average heat transfer coefficient whereas higher solid concentrations tend to lower it. The distribution of instantaneous heat transfer coefficient peak height is wider at higher gas and liquid velocities while the addition of a surfactant narrows it. Gas holdups and average heat transfer coefficients are both compared with existing correlations, which are then adjusted for a better fit.     

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.     

Development of a three-phase fluidized bed reactor with enhanced oxygen transfer

A three-phase fluidized bed reactor (TFBR) was developed in this study with the objective to achieve high rates of oxygen transfer from the gas to the liquid phase in the presence of fluidized solid particles. With 2.9 m height, 0.605 m diameter, and a short residence time of 8 h, the TFBR is particularly suitable for industrial applications such as aerobic biodegradation of high-strength wastewaters including refractory compounds. Experiments with tap water and air show that the TFBR enables complete fluidization. With the water and air superficial velocities in the respective ranges of 0.005–0.203 and 0.8–2.0 cm/s, the volumetric oxygen transfer coefficient is 2.3 × 10⁻² s⁻¹, which is higher than that obtained in similar experimental studies on oxygen transfer carried out in the past. These results suggest that the developed TFBR could be very effective in industrial applications where short hydraulic time and high gas-to-liquid mass transfer rates are desirable.     

漏斗型导流内构件对内循环三相流化床流体力学与传质特性的影响

通过增加新型内构件来改善内循环三相流化床的流体力学与传质特性,以实现化工、环保领导中追求高氧利用率的过程。针对此过程设计了3种不同结构参数的漏斗型导流内件并设置于导流筒顶端,分别测定反应器内气含率、液相混合时间、液体循环速度、体积氧传质系数的数据并分析其变化规律,以解析内件的作用机制。实验在有效体积39L,以空气为气相、水为液相、多孔泡沫颗粒为固相的反应器中进行,研究发现：漏斗型导流内件的设置使升流区气含率平均增大10%,体积氧传质系数kLa提高了15%,液相混合时间下降10%-25%;内件的设置可以改变液体循环速度,当表观气速<0.5cm/s时,液体循环速度加快,当表观气速>0.5 cm/s时,液体循环速度下降;此外,漏斗型导流内件的结构参数变化对流化床流体力学与传质特性有较大影响。结果表明,流化床内增加新型内构件并合理设置能够实现反应器效能的提高。     

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.     

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.     

An investigation on gas mixing in a fluidized bed

The radial dispersion of gas in a fluidized bed of 10 cm diameter, charged with resin beads, was investigated using a new strip staining technique with bromine as tracer. Paper strips containing KI, which were placed in tubes with capillary tips, provided stains which had previously been calibrated against air—bromine mixtures flowing under similar conditions in the bed. A similar trend of the concentration profiles related to the two-dimensional diffusion model was obtained.     

垂直管内三相流化床沸腾传热特性

研究了三相流化床沸腾传热的特性和影响传热系数的诸因素。在传热过程中，由于固体粒子的存在，强化了传热。以玻璃球粒子为固相的三相流化床沸腾传热系数，是相同条件下汽液两相流沸腾传热的二倍。以铜粒子为固相的三相流化床沸腾传热系数，是相同条件下汽液两相流沸腾传热系数的３倍。     

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.     

A loop three-phase fluidized bed reactor

An airlift three-phase reactor in which solid particles are fluidized in the riser by the recirculating liquid was developed. Three particle types of 1.7–8.6 mm in diam., porous and nonporous, spheres and cylinders, were studied in the water–air system. Little effect of the alumina porous particles (apparent density 1.47 × 10³ kg/m³) on the gas holdup and liquid superficial velocity was found. On the other hand, the heavier, nonporous ceramic particles (density 2.4 × 10³ kg/m³) had a significant effect on both design parameters. The axial distribution of particles was also a function of the particle density and liquid superficial velocity.     

Gas-liquid mass transfer in three-phase fluidized beds with viscous pseudoplastic liquids

Liquid phase volumetric mass transfer coefficients for oxygen were determined in three-phase fluidized beds of 8 mm glass spheres fluidized by a cocurrent flow of air and pseudoplastic polysaccharide solutions (carboxymethyl cellulose, xanthan). A semi-theoretical relation for the effective shear rate was suggested. The mass transfer coefficients could be correlated, together with literature data for particle diameters of 3 mm and 5 mm in other liquids, using the terminal velocity as the particle-specific property.     

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     

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.     

Heat transfer in three-phase fluidized beds

Heat transfer coefficients h have been measured in two-phase (water—air, water—glass beads) and three-phase (water—air—glass beads) fluidized beds. Experiments were performed over a wide range of liquid and gas flowrates in a 0.24 m diam. column fitted with an axially mounted cylindrical heater. Four solids were employed ranging in size from 0.5 to 5 mm.Typical maximum values of h in the three-phase, liquid—gas, liquid—solid and liquid beds were approximately 4800, 4300, 3800 and 1300 W/m²K respectively. In the three-phase beds h generally increased with liquid and gas velocity and with particle size. Correlations are presented to calculate h in the different beds.