EFFECT OF DISTRIBUTOR ON BUBBLE SIZE IN A CYLINDRICAL FLUIDIZED BED AT AN ELEVATED TEMPERATURE†

The effects of temperature and distributor on bubble diameter were investigated using a cylindrical fluidized bed of 147 mm in diameter. Three perforated distributors having different holes in diameter and the same ratio of holes to bed area were used. Eruption diameters of bubbles were measured using a high speed video-camera system under the following conditions: bed temperature = 300 and 600 K, bed particles = spherical glass beads of 272 μm in average size, excess gas velocity = 1-4 cm/s, and static bed height equals; 10-42 cm. The bubble diameter at 600 K was larger than that at 300 K. The difference became smaller with increasing the static bed height and with increasing the excess gas velocity. The distributor with larger holes gave larger bubbles. The effect of hole diameter of the distributor on the bubble diameter became insignificant with increasing the static bed height and with increasing the excess gas velocity.     

THE EFFECTS OF DISTRIBUTOR DESIGN ON FLUIDIZED-BED HYDRODYNAMIC BEHAVIOR

should be addressed. The distributor was investigated for the purpose of design and scale up of large fluidized-bed combustors. Four orifice plates with different configurations were used to study the effect of distributor design on bubble formation and solid mixing. Experiments were carried out on a three-dimensional fluidized bed of 27.94 cm diameter and a two-dimensional bed with dimensions of 30.48cm ×1.27 cm. Motion pictures were used to study bubble formation and coalescence. Pressure profiles inside the three-dimensional bed were measured for several distributors to study bubble flow patterns, and tracer particles were used to study mixing patterns at various superficial velocities and particle sizes. The results show that the distributor plate with two-size orifices causes a non-uniform gas bubble flow inside the bed. This non-uniform gas bubble flow is associated with variations in local bed density and local voidage. Horizontal or radial solid circulation is also caused by this non-uniform gas bubble flow. The local bed density and voidage variations and the radial solid circulation cause the bubbles to move toward the area above the smaller orifices as the bubbles rise up and coalesce. This reduces the wall effect, and the bed is very uniformly fluidized when the two-size orifice plate with small holes in the center is employed.     

The influence of distributor structure on the solids distribution and flow development in circulating fluidized beds

The influence of distributor structure on solids distribution is studied in two riser circulating fluidized bed reactors with different distributor structures but similar diameters. Optic fibre probes were used for the measurement of local solids distribution. The axial and radial distribution of solids holdup in the riser with a multi‐tube distributor is more uniform than that with a multi‐orifice distributor. The radial profiles of particle velocity in the riser with the multi‐tube distributor are also more uniform than that with the multi‐orifice distributor. In the riser with the multi‐tube distributor, both gas and particles are distributed more uniformly across the section, so that the flow acceleration is much more uniform and faster. The flow development is much faster and the fully developed region is reached early for the riser with the multi‐tube distributor. The distributor design is an important factor for the design of circulating fluidized bed reactor.     

Effect of Bed Diameter,Distributor and Inserts on Minimum Fluidization Velocity

The minimum fluidization velocity of beds has been determined experimentally in beds of 0.089 m and 0.29 m diameters, respectively. The particles studied had sizes ranging from 100 μm to 1 mm in diameter, and densities from 1128 to 11400 kg/m³. Three distributors were used in the experimental scheme, each perforated by holes of 0.8 mm in diameter but with varying hole densities, as well as a porous plate. It was found that the minimum fluidization velocity was affected by both the diameter and distributor used. The effect of vertical tubular inserts on the minimum fluidization velocity was investigated in the 0.29 m diameter bed. The experimental data in the large bed, using four distributors, were parameterized within experimental error.     

Influence of multiple gas inlet jets on fluidized bed hydrodynamics using Particle Image Velocimetry and Digital Image Analysis

A rectangular fluidized bed setup was developed to study the evolution of inlet gas jets located at the distributor. Experiments were conducted with varying distributor types and bed media to understand the motion of particles and jets in the grid-zone region of a fluidized bed. Particle Image Velocimetry and Digital Image Analysis were used to quantify the parameters that characterize these jets. A grid-zone phenomenological model was developed to compare these parameters with those available in the literature. It was determined from this study that jet penetration length behavior is consistently different for fluidization velocities below and above the minimum fluidization. For velocities above minimum fluidization, jet lengths were found to increase more rapidly with increase in orifice velocity than for operating conditions below minimum fluidization.     

射流流化床中锥形分布板对流动的影响

给出了具有锥形分布板的射流流化床中浓密气固两相流动的多相流体力学基本方程组. 采用二维正交曲线坐标并生成了数值网格,用改进的IPSA方法求解二维正交曲线坐标中的多相流基本方程组,并编制了大型通用程序,流场可视化使用Tecplot软件. 对于给定的模拟计算,计算结果与实验值吻合. 模拟计算中改变了锥形筛板的角度、射流管的直径、床层高度、分布板开孔率的分布、射流气速、床层表观气速等,通过模拟得到床内的流动图像,考察了射流高度及颗粒循环的影响.     

Entrance effects at a multi-orifice distributor in gas-fluidised beds

The behaviour of multi-orifice distributors in gas-solids fluidised beds has been studied with particular regard to the height of the entrance effect and the mechanics of gas-solids flow in the region immediately above the distributor plate. A model is proposed to predict the height of the entrance effect for a given distributor and gas-solids system at various fluidising flow-rates, and good agreement has been found with experiment. Experiments have been carried out with (a) a two-dimensional air-fluidised bed using three sizes of sand particles (d_p: 137, 263, and 350 μm) and four distributors (orifice diameters: 0.001 m, 0.002 m; orifice spacings: 0.025 m, 0.05 m); and (b) a three-dimensional air-fluidised bed, 0.3 m square in cross-section, using 350 μm sand particles on a distributor with 0.003 m diameter orifices at a spacing of 0.04 m. The principal factors influencing the height of the entrance effect were found to be the incipient fluidising velocity, mean particle size, orifice spacing and gas flow-rate. The model has been used to estimate the minimum ratio of distributor pressure drop to bed pressure drop to bring about an even distribution of gas at the bottom of the bed.     

Modeling of grid region heat transfer in a shallow gas-solid fluidized bed

Heat transfer coefficients between the bed and an immersed horizontal tube in the grid-region of a shallow gas-solid fluidized bed were experimentally and theoretically studied. Experiments were carried out in two fluidized bed columns with inside diameters of 88 and 137 mm, respectively. The fluidized particles tested were sand, limestone and glass beads. Experimental parameters also included particle size, superficial gas velocity, tube diameter, tube location and distributor design. A mechanistic model considering the contributions of jet phase, emulsion phase and dead phase was derived for estimating the grid-region heat transfer coefficients. Most of the model predictions were found to be within 25% of the experimentally observed data.     

IMPORTANCE OF DISPERSED SOLIDS IN BUBBLES FOR EXOTHERMIC REACTIONS IN FLUIDIZED BEDS

Ignition of activated carbon particles were measured in a vertical tube reactor of 4 cm ID, where single particles fell consecutively through a gas mixture containing oxygen.

A two dimensional fluidized bed reactor 24 cm wide, 51 cm high and 2.5 cm in thickness was used for visual observation through a wide front window 24 cm × 35 cm covered with a silica glass plate 1 cm thick. Activated carbon particles were fluidized incipientiy by air, and a gas mixture containing oxygen was injected upwards into the bed through a nozzle positioned 5 cm above the distributor, forming single bubbles intermittently.

It was observed that carbon particles dispersed in rising bubbles were ignited abruptly at emulsion phase temperatures above 550°C. Experimental findings from the fluidized bed were compared with those from the tube reactor, suggesting that the igniting conditions for particles dispersed in bubbles are nearly the same as for single particles falling in the tube reactor.     

IMPORTANCE OF DISPERSED SOLIDS IN BUBBLES FOR EXOTHERMIC REACTIONS IN FLUIDIZED BEDS

Ignition of activated carbon particles were measured in a vertical tube reactor of 4 cm ID, where single particles fell consecutively through a gas mixture containing oxygen.

A two dimensional fluidized bed reactor 24 cm wide, 51 cm high and 2.5 cm in thickness was used for visual observation through a wide front window 24 cm × 35 cm covered with a silica glass plate 1 cm thick. Activated carbon particles were fluidized incipientiy by air, and a gas mixture containing oxygen was injected upwards into the bed through a nozzle positioned 5 cm above the distributor, forming single bubbles intermittently.

It was observed that carbon particles dispersed in rising bubbles were ignited abruptly at emulsion phase temperatures above 550°C. Experimental findings from the fluidized bed were compared with those from the tube reactor, suggesting that the igniting conditions for particles dispersed in bubbles are nearly the same as for single particles falling in the tube reactor.     

Design Parameter Effects on Expansion in Fluidized Beds with Inserts

The effect of some design parameters on the expansion of particles has been studied in a fluidized bed of 300 mm diameter. Four distributors were examined; three perforated plates, each perforated by holes of 0.8 mm in diameter but different hole densities at 6 mm, 9 mm and 12 mm pitch, and a porous plastic distributor 17 mm thick. Particles of different materials in the Archimedes number range from 100 to 10⁵ were fluidized. The inserts were held vertically as arrays. All experimental data for four distributors were correlated within experimental error by the equation: whereU, U_mf, U₀ are the gas velocity, velocity at minimum fluidization and real or apparent terminal velocity, while e is the bed porosity and e_mf is the porosity at the condition of minimum fluidization. P is the hole pitch of perforated plate distributor in millimeters.     

THE ROLE OF GAS DISTRIBUTION IN FLUIDIZED BED CHEMICAL REACTOR DESIGN†

The design of fluid bed gas distributors may have a marked influence on the performance of a fluid bed reactor. The primary physical reason for this influence is that the distributor design influences the hydrodynamics and thus the gas/solid contacting pattern in the fluidized bed.

In the paper presented here the influence of distributor design on mass transfer and chemical reaction has been investigated systematically in fluid bed reactors with diameters of 0.2 and 1.0 meter. Coefficients of mass transfer between the bubble phase and the suspension phase were determined from chemical conversion and tracer gas residence time distribution measurements. In the experimental program the height of the fluidized bed was varied between 0.3 m and 0.9 m with superficial gas velocities in the range of 0.06 m/s to 0.30 m/s.

The comparison of the experimental results with a suitably modified and extended two-phase model yields quantitative relationships which allow to account for the influence of the gas distributor in the design of fluid bed chemical reactors.     

Effect of distributors on Gas—Solid Fluidization

The efficient operation of a fluidized bed is very much dependent upon distributor performance, which in turn depends on its design parameters. The work reported here deals with the characteristics of such distributors as are commonly employed in laboratories, pilot plant and large scale operations. Specifically a porous plate distributor, two bubble cap distributors of different geometries and four Johnson screen distibutors of different percent open area have been investigated in a 30.5 cm by 30.5 cm square fluidized bed as a function of air fluidizing velocity and bed height. The pressure drop data for all the distributors have been correlated by a single equation with two unknown constants. The ratio of distributor pressure drop to bed pressure drop is found to increase rapidly with increase in fluidization velocity. The bed expansion ratio is found to increase with increase in excess fluidization velocity and distributor pressure drop but decreases with increase in bed height or weight.     

THE ROLE OF GAS DISTRIBUTION IN FLUIDIZED BED CHEMICAL REACTOR DESIGN†

Abstract

The design of fluid bed gas distributors may have a marked influence on the performance of a fluid bed reactor. The primary physical reason for this influence is that the distributor design influences the hydrodynamics and thus the gas/solid contacting pattern in the fluidized bed.

In the paper presented here the influence of distributor design on mass transfer and chemical reaction has been investigated systematically in fluid bed reactors with diameters of 0.2 and 1.0 meter. Coefficients of mass transfer between the bubble phase and the suspension phase were determined from chemical conversion and tracer gas residence time distribution measurements. In the experimental program the height of the fluidized bed was varied between 0.3 m and 0.9 m with superficial gas velocities in the range of 0.06 m/s to 0.30 m/s.

The comparison of the experimental results with a suitably modified and extended two-phase model yields quantitative relationships which allow to account for the influence of the gas distributor in the design of fluid bed chemical reactors.     

Experimental study on solid circulation in a multiple jet fluidized bed

Particle image velocimetry was used to investigate the evolution of multiple inlet gas jets located at the distributor base of a two‐dimensional fluidized bed setup. Results were used to estimate the solid circulation rate of the fluidized bed as well as particle‐entrainment into the individual jets. The effects of fluidization velocity, orifice diameter, orifice pitch, particle diameter, and particle density were studied. It was determined from this study that the solid circulation rate linearly increased with an increase in the fluidization velocity until the multiple jet system transitioned from isolated to an interacting system. In the interacting system of jets, the solid circulation increased with fluidization velocity but at a much lower rate. For multiple jet systems, this phenomenon may indicate the presence of an optimum operating condition with high circulation rate and low air input in the bed. © 2011 American Institute of Chemical Engineers AIChE J, 58: 3003–3015, 2012     

Attrition of porous glass particles in a fluidised bed

Fluidisation is frequently accompanied by unwanted attrition of the bed material. This paper focuses on the mechanical aspects of fines creation by attrition in fluidised beds supported by multi-orifice distributor plates. The attrition rates of low-density porous glass particles were measured; these particles show abrasive wear behaviour rather than breakage. Positron emission particle tracking (PEPT) was used to follow particle motion in three dimensions within the fluidised bed. For a single orifice distributor with background fluidisation, the attrition rate increased exponentially with increasing orifice gas velocity. For a multi-orifice distributor, however, attrition rates were roughly proportional to excess gas velocity, except near to a critical ratio of particle to orifice diameter; as this ratio approached 2, attrition was observed to increase by an order of magnitude. A method is proposed for estimating attrition rates from a combination of small-scale experimental results and theoretical calculations of distributor jet entrainment rates.     

Evaluation of hydrodynamic behavior of the perforated gas distributor of industrial gas phase polymerization reactor using CFD-PBM coupled model

A 2D computational fluid dynamics (Eulerian–Eulerian) multiphase flow model coupled with a population balance model (CFD-PBM) was implemented to investigate the fluidization structure in terms of entrance region in an industrial-scale gas phase fluidized bed reactor. The simulation results were compared with the industrial data, and good agreement was observed. Two cases including perforated distributor and complete sparger were applied to examine the flow structure through the bed. The parametric sensitivity analysis of time step, number of node, drag coefficient, and specularity coefficient was carried out. It was found that the results were more sensitive to the drag model. The results showed that the entrance configuration has significant effect on the flow structure. While the dead zones are created in both corners of the distributors, the perforated distributor generates more startup bubbles, heterogeneous flow field, and better gas–solid interaction above the entrance region due to jet formation.     

The motion mechanism and characteristic of bubble in a pseudo-2D tapered fluidized bed

The different carbon nanotube (CNT) particles (^@A and ^@V) were bed materials in the pseudo-2D tapered fluidized bed (TFB) with/without a distributor. A detailed investigation of the motion mechanism of bubbles was carried out. The high-speed photography and image analysis techniques were used to study bubble characteristic and mixing behavior in the tapered angle of TFB without a distributor. The fractal analysis method was used to analyze the degree of particles movement. Results showed that an S-shaped motion trajectory of bubbles was captured in the bed of ^@V particles. The population of observational bubbles in the bed of ^@V particles was more than that of ^@A particles, and the bubble size was smaller in the bed of ^@V particles than that of ^@A particles. The motion mechanism of bubbles had been shown to be related to bed materials and initial bed height in terms of analysis and comparison of bubble diameter, bubble aspect ratio and bubble shape factor. Importantly, compared to the TFB with a distributor, the TFB without a distributor had been proved to be beneficial to the CNT fluidization according to the study of bubble characteristic and the degree of the particle movement. Additionally, it was found that the mixing behavior of ^@V particles was better than ^@A particles in the tapered angle of TFB without a distributor.     

布风方式对流化床混合特性的影响

通过将离散单元法同计算流体力学相结合,对流化床内物料混合过程进行了研究。给出了水平布风板均匀布风、倾斜布风板非均匀布风2种情况下的示踪颗粒场历变过程。模拟结果表明:瞬时颗粒场组图能够较为直观表征床内混合现象;其中,在均匀布风情况下,床内气泡横向运动受到限制,颗粒整体横向运动能力较弱,混合方式以扩散混合为主;而对于非均匀布风流化床,床内存在较大的横向颗粒浓度梯度,对流混和起主要作用,且混合速度较为迅速。     

RAINING OF PARTICLES FROM AN EMULSION-GAS INTERFACE IN A FLUIDIZED BED

This paper deals with the raining of particles from an interface between a dense fluidized phase and a gas phase with the fluidized phase uppermost. Such interfaces occur at the upper surfaces of gas bubbles and slugs in fluidized beds. Particle rain in these cases would enhance contact between gas and particles within the bubbles and slugs.

The rise velocities of single square-nosed slugs injected in incipiently fluidized beds of different diameters were measured. Relatively small columns of internal diameters of 0.0125, 0.019 and 0.0254 m were employed in the experiments; In such beds, square-nosed slugs are formed which span the entire cross-section of the beds and rise entirely due to raining of particles from their top surfaces. Since the upper surface of such slugs is flat, their motion can be analyzed using the one-dimensional hydrodynamic theory. Glass ballotini and sand of different sizes were used as bed particles. Comparison of theory and experiment has enabled the determination of the dimensionless gradient diffusivity characterizing the motion of particles induced by a gradient in the void fraction. The results confirm the scaling proposed by Batchelor (1988). The use of the calculated gradient diffusivity in the criterion for stability of a gas fluidized bed predicts the systems under consideration to be always unstable.