Rectangular Fluidized Bed Research Articles

Gas dispersion in a rectangular bubbling fluidized bed

Horizontal gas dispersion within a bubbling fluidized bed was investigated using response surface methodology (RSM). All the experiments were conducted in a rectangular fluidized bed cold model with the cross-section of 0.2 m × 0.4 m and 1.5 m in height. Carbon dioxide used as the tracer gas was injected continuously into the center of the bed by a point source. The concentrations of tracer gas were detected by a Nondispersive Infrared gas analyzer (ETG IMA 3000B). Glass beads with three mean sizes of 360, 460 and 545 μm were used as bed material. The coefficient of horizontal gas dispersion and the kurtosis coefficient were adopted for description of horizontal gas mixing in a bubbling gas–solid flow regime. The coefficient of horizontal gas dispersion, D r was well correlated with operating parameters and particle property: ( U − U mf)/ U mf, f d and d p. The results show that the horizontal gas dispersion coefficient increases with superficial gas velocity (3.5 U mf < U < 6.5 U mf) and particle size (360 μm < d p < 460 μm). The distributor design is also a significant factor for gas dispersion. Also it was found that the higher the gas dispersion coefficient the lower the coefficient of kurtosis. The relationship between D r and K was well correlated.

Effects of using two- versus three-dimensional computational modeling of fluidized beds: Part I, hydrodynamics

Simulations of fluidized beds are performed to study and determine the effect on the use of coordinate systems and geometrical configurations to model fluidized bed reactors. Computational fluid dynamics is employed for an Eulerian–Eulerian model, which represents each phase as an interspersed continuum. The transport equation for granular temperature is solved and a hyperbolic tangent function is used to provide a smooth transition between the plastic and viscous regimes for the solid phase. The aim of the present work is to show the range of validity for employing simulations based on a 2D Cartesian coordinate system to approximate both cylindrical and rectangular fluidized beds. Three different fluidization regimes, bubbling, slugging and turbulent regimes, are investigated and the results of 2D and 3D simulations are presented for both cylindrical and rectangular domains. The results demonstrate that a 2D Cartesian system can be used to successfully simulate and predict a bubbling regime. However, caution must be exercised when using 2D Cartesian coordinates for other fluidized regimes. A budget analysis that explains all the differences in detail is presented in Part II [N. Xie, F. Battaglia, S. Pannala, Effects of Using Two-Versus Three-Dimensional Computational Modeling of Fluidized Beds: Part II, budget analysis, 182 (1) (2007) 14] to complement the hydrodynamic theory of this paper.

Single jet fluidized beds: Experiments and CFD simulations with glass and polypropylene particles

Understanding hydrodynamics of bubbling fluidized beds is crucial in proper design and scale up of these beds. CFD models have shown promise in gaining this understanding. In order to generate confidence in CFD models, predicted time averaged and dynamical characteristics of the bubbling fluidized beds need to be validated against experimental data. This paper describes such studies with rectangular fluidized beds operated with a central jet. Digital image analysis and analysis of wall pressure fluctuations were used for this characterization. Fluidization of two types of particles, glass and polypropylene (PP) was studied at two different initial bed heights of H / D = 1 and 2 with three central jet velocities 5, 10 and 20 m/s. Time averaged as well as dynamical characteristics were studied. The Eulerian–Eulerian two fluid model based on kinetic theory of granular flows was used to simulate these experiments. The predicted results were compared with the experimental data and previously published correlations. Although, there is agreement with experimental data in some aspects, complete agreement was not found. The presented experimental data and comparison with CFD predictions will provide useful basis for further work on understanding bubbling fluidized beds.

A study on the behavior of bubbles and solids in bubbling fluidized beds

In present work, a novel bubble distribution model, based on the population balance of bubbles, is proposed incorporating with the Navier–Stokes equation for solid flow in order to predict the solid mixing and circulation as well as the bubble behavior in bubbling fluidized beds. This work is comprised of three sections: The first section is “estimation of solid flow pattern in bubbling fluidized bed”. In this section, we compare the computed values of bubble diameter and solid velocities with experimental values in cylindrical bubbling fluidized bed. The second section is “estimation of solid flow pattern in rectangular fluidized beds”. This section is divided into two parts. In the first part, we apply the simulation model to bubbling rectangular fluidized bed, which is expressed by the rectangular coordinate system in place of the cylindrical coordinate. In the second part, we compute the solid behavior in beds equipped with immersed vertical tubes. Third section is “solid mixing in bubbling fluidized beds”. This section is also divided into two parts. In the first part, we investigate experimentally the solid mixing characteristic in bubbling fluidized beds using the tracer particle. In the second part, we incorporate the simulation model with a solid dispersion model for predicting the solid mixing in bubbling fluidized beds.

Continuous separation of particles by fluidized beds

For a non-mixable binary solid system, the particles will segregate along the axis in a fluidized bed. The continuous separation of a binary mixture was investigated using a cold model in a rectangular fluidized bed. The optimum operating conditions for the separation of a mixture containing two different-sized glass beads was studied experimentally. Extractive fluidized classification, which is analogous to liquid-liquid extraction, was investigated for a mixable binary system. This process is quite effective as a dry process for coal cleaning.

Gas-solids circulating fluidization in a packed bed

Gas-solid-solid circulating fluidized bed is a novel gas-solid contactor which offers interesting heat and mass transfer properties. In this type of equipment, large particles (often used as the catalyst) are fixed in the bed forming a packing section while fine powders (often used as the absorbent or heat carrier) are carried by the gas and flow through the packing. In this paper, the pressure gradient and powder hold-up in the packing have been measured in a rectangular fluidized bed. A theoretical analysis for the prediction of pressure drop, which is caused by the powder hold-up and the friction between gas and packing and between powders and packing, has been proposed. The predicted results agree well with the experimental results.

A measure of the degree of inhomogeneity in a distribution and its application in characterising the particle circulation in a fluidized bed

A quantitative method to evaluate variational processes such as particle circulation in a fluidized bed is presented. It involves the calculation of H, an index of the degree of inhomogeneity in the tracer circulation in standard measure. The position of a given particle in a rectangular fluidized bed was followed using Positron Emission Particle Tracking. This technique is able to locate a labelled solid and construct a three-dimensional trajectory of its movement. The degree of inhomogeneity in the tracer distribution was then calculated from the observations. The values of H for the tracer movement when the bed is operated with a differential air supply across the distributor to stimulate gross solids circulation, suggest that there are relatively large regions of the bed in which the presence of the tracer is much less frequent than elsewhere. This lack of homogeneity in the tracer particle circulation is consistent with visual observation of particle circulation in the investigated bed. Increase in H with increasing length of the duration of the test is consistent with a stable circulation pattern. H affords a quantitative measure of how the fluidized bed parameters influence the particles movement.

Fluidization in tapered vessels

Methods for the prediction of bed height and pressure drop in tapered air-fluidized beds are proposed. A comparison was made with measurements in slugging fluidized beds in the slow bubble regime over a wide range of apex angle from 0° (rectangular) to 45°. The bed height and pressure drop are well predicted by each of the proposed methods using the same constant values of bubble rise velocity and fluctuation ratio as for the rectangular fluidized bed with the same distributor geometry.

Hydrodynamics of fluidization: Fast-bubble simulation in a two-dimensional fluidized bed

The hydrodynamics of a ‘fast bubble’ in a two-dimensional rectangular fluidized bed is studied numerically. The fast bubble (which moves faster than the fluidizing velocity) is produced by injecting gas through a jet for a short time into a bed at minimum fluidization. The numerical computations are in good agreement with the classical theories of Davidson, Jackson, and Murray and with experimental data, thus enhancing confidence in the computer modeling of the hydrodynamics of fluidization.

Mechanism of hydraulic transport of a packed bed at the start-up of a three-phase fluidized bed

The phenomenon of hydraulic transport of a packed bed is observed to occur at the start-up of three-phase fluidized beds operating under high gas hold up conditions. This phenomenon, which deviates from the conventional slugging flow dictated by the column size, may lead to a total or partial entrainment of the particles during the start-up of a fluidized bed reactor system. In this study, flow visualization techniques are used to investigate the bubble dynamics underneath the packed bed at the instant when the liquid flow is introduced. Cylindrical hydrotreating catalysts and 1.5 mm glass beads are used in a rectangular fluidized bed. An aqueous surfactant solution of 0.5 wt. % t-pentanol is employed to yield high gas hold up fluidization conditions. The experimental results indicate that, at a continuous gas flow rate prior to the liquid flow start-up, the small bubbles are attached to the surface of the particles. Consequently, at the start-up, the liquid flow is impeded and the small bubbles accumulate beneath the packed bed, thus forming a stable foam layer and a foam-particle aggregate at the bottom of the bed. The constriction of the liquid flow through the foam layer and foam-particle aggregate results in the formation of a stable moving packed bed.

Porosity distributions in a fluidized bed with an immersed obstacle

AbstractErosion in bubbling fluidized‐bed combustors is a serious issue that may affect their reliability and economics. Available evidence suggests that the key to understanding this erosion is detailed knowledge of the coupled and complex phenomena of solids circulation and bubble motion. A thin transparent “two‐dimensional” rectangular fluidized bed with an obstacle served as a rough model for a fluidized‐bed combustor. This model was studied experimentally and computationally using two hydrodynamic equation sets. The computed hydrodynamic results agree reasonably well with experimental data. Bubble frequencies and sizes compare well with those obtained from analyzing a high‐speed motion picture frame‐by‐frame. Time‐averaged porosities computed from both models agree with time‐averaged porosity distributions measured with a gamma‐ray densitometer. The principal diferences between the data and the computations in this paper are due to asymmetries present in the experiment and to the simplified solids rheology used in the hydrodynamic models.

Heat transfer around a horizontal tube in freeboard region of fluidized beds

AbstractAn experimental investigation was carried out to measure local and average heat transfer coefficients for horizontal tubes located in freeboard region of air fluidized beds. Tests were carried out at room temperature and atmospheric pressure in a rectangular fluidized bed, with mean particle diameters of 275 to 850 μm.Both local and average heat transfer coefficients were found to vary with particle diameter, flow rate, static bed depth, and elevation in the freeboard region.

Charge transfer rate in liquid-solid and gas-liquid-solid fluidized bed electrodes.

Copper deposition from acidic aqueous solutions containing copper ion was studied in rectangular fluidized bed electrodes for liquid-solid and gas-liquid-solid systems. The effective specific resistance of particle phase, ρm, was determined by comparison of the experimental potential profile with the theoretical one. The value of ρm decreased with increase in gas velocity, and the gas flow increased the total cell current by 10-30% at a given cell voltage in the range of 15 to 30% bed expansion. No explicit effects of fluidized bed height and particle diameter on ρm were observed. The overall effective specific resistance of bed, ρb, was measured with an alternating current bridge circuit, and the relationship between ρm and ρb is discussed. The charge transfer in the particle phase was analysed by the parallel effective resistance model based on the charge conductive mechanism and the collision mechanism.

Solid Mixing in Liquid Fluidization

Concerning the subjects of the radial mixing of particles in the liquid fluidization, Eq. (10) has been theoretically derived to represent the mass of tracer particles displaced in lateral direction in a given time under batch-unsteady state.The particles (2032 mesh) of marble, fire brick and lime stone were fluidized with water at 20°C, in the rectangular fluidized bed shown in Fig. 3, and the values of Erp were evaluated from Eq. (10). Experimental results are given in Table 2, and the plotting as shown in Fig. 4 has led to the expression of Erp as in Eq. (15).In order to determine the values of longitudinal dispersion coefficient of solid particles, Ezp, on the basis of Eq. (17) which represents residence-time curve of tracer particles, the experimental work was carried out on the fludization of solid particles, given in Table 1, in water (20°C) in a glass tube (I. D. 7.1cm). From the experimental results, Fig. 6 was obtained and the expression of Ezp was given as in Eq. (24).Magnitudes of the transient radial-and longitudinal-mixing of solid particles were theoretically derived as in Eqs. (28) and (32), respectively. The expression of the mixing efficiency of solid particles, Mp, is given by Eq. (34). As the result it has been concluded that the values of Mp increase in accordance with the increase in the values of Rem and θh.