Bubble behavior in corrugated‐wall bubbling fluidized beds—Experiments and CFD simulations

A new concept to harness bubble dynamics in bubbling fluidization of Geldart D particles was proposed. Various geometrical declinations of a cold‐prototype corrugated‐wall bubbling fluidized bed were compared at different flow rates (U_g) to conventional flat‐wall fluidized bed using high‐speed digital image analysis. Hydrodynamic studies were carried out to appraise the effect of triangular‐shaped wall corrugation on incipient fluidization, bubble coalescence (size and frequency), bubble rise velocity, and pressure drop. Bubble size and rise velocity in corrugated‐wall beds were appreciably lower, at given U_g/U_mb, than in flat‐wall beds with equal flow cross‐sectional areas and initial bed heights. The decrease (increase) in size (frequency) of bubbles during their rise was sustained by their periodic breakups while protruding through the necks between corrugated plates. Euler‐Euler transient full three‐dimensional computational fluid dynamic simulations helped shape an understanding of the impact of corrugation geometry on lowering the minimum bubbling fluidization and improving gas distribution. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Distributor effects near the bottom region of turbulent fluidized beds

The distributor plate effects on the hydrodynamic characteristics of turbulent fluidized beds are investigated by obtaining measurements of pressure and radial voidage profiles in a column diameter of 0.29 m with Group A particles using bubble bubble-cap or perforated plate distributors. Distributor pressure drop measurements between the two distributors are compared with the theoretical estimations while the influence of the mass inventory is studied. Comparison is established for the transition velocity from bubbling to turbulent regime, U_c, deduced from the pressure fluctuations in the bed using gauge pressure measurements. The effect of the distributor on the flow structure near the bottom region of the bed is studied using differential and gauge pressure transducers located at different axial positions along the bed. The radial voidage profile in the bed is also measured using optical fiber probes, which provide local measurements of the voidage at different heights above the distributor. The distributor plate has a significant effect on the bed hydrodynamics. Owing to the jetting caused by the perforated plate distributor, earlier onset of the transition to the turbulent fluidization flow regime was observed. Moreover, increased carry over for the perforated plate compared with the bubble caps has been confirmed. The results have highlighted the influence of the distributor plate on the fluidized bed hydrodynamics which has consequences in terms of comparing experimental and simulation results between different distributor plates.     

INVESTIGATIONS OF THE BUBBLE PHASE BEHAVIOR IN A HIGH TEMPERATURE FLUIDIZED BED REACTOR WITH COALCOMBUSTION

The properties of bubbles in a bench-scale fluidized bed reactor, 30 cm in diameter, during coal combustion were determined by means of a newly developed cooled bubble probe and a data processing system at temperatures up to 850°C and fluidization indexes up to 10 in axial and radial positions in the bed. The fluidization index above 4 and the temperature have only slight effect on the bubble properties. Their variation along the height above the gas distributor is dominant.

Oxygen and Co₂-concentration profiles were measured in the bed and in the freeboard, and the o₂-profiles were calculated by means of measured bubble data and bubble models. A comparison of measured and calculated o₂-profiles indicates that the mass transfer rates between the emulsion and bubble phases are larger than the ones calculated by the models.     

Fluidization of Group B particles with a rotating distributor

A novel rotating distributor fluidized bed is presented. The distributor is a rotating perforated plate, with 1% open-area ratio. This work evaluates the performance of this new design, considering pressure drop, Δp, and quality of fluidization. Bed fluidization was easily achieved with the proposed device, improving the solid mixing and the quality of fluidization.In order to examine the effect of the rotational speed of the distributor plate on the hydrodynamic behavior of the bed, minimum fluidization velocity, U_mf, and pressure fluctuations were analyzed. Experiments were conducted in the bubbling free regime in a 0.19 m i.d. fluidized bed, operating with Group B particles according to Geldart's classification. The pressure drop across the bed and the standard deviation of pressure fluctuations, σ_p, were used to find the minimum fluidization velocity, U_mf. A decrease in U_mf is observed when the rotational speed increases and a rise in the measured pressure drop was also found. Frequency analysis of pressure fluctuations shows that fluidization can be controlled by the adjustable rotational speed, at several excess gas velocities.Measurements with several initial static bed heights were taken, in order to analyze the influence of the initial bed mass inventory, over the effect of the distributor rotation on the bed hydrodynamics.     

Combustion of liquid petroleum gas in a fluidized bed furnace with a jetting-mixing distributor

Liquid petroleum gas (LPG) fluidized beds have potential applications in metal heating or workpiece heat treatments. The combustion of LPG and the controls of the atmosphere inside the bed and the bed temperature are very concerned. The combustion of LPG has been investigated in a pilot-scale bubbling fluidized bed with a jetting-mixing nozzle distributor and hollow corundum sphere particles of 0.867-1.212 mm in diameter and 386-870 kg/m³ in bulk density at 800-1100°C. Experiments were carried out for fuel-rich mixtures to explore the possibility to obtain mild oxidizing, non-oxidizing or reducing atmosphere in the bed. Air factor (the ratio of the volume of air actually fed into the bed to that in a stoichiometric mixture) is in between 0.3 and 1.0 and U/U_mf 1.3-3.0. The distributor brings LPG and air into an intense contact sufficient to permit in-bed combustion without backfire problems. The experimental results show that the fluidized bed furnace offers excellent thermal uniformity and temperature control. The size of the combustion zone is usually larger than that of the temperature variation zone. Particle properties, initial bed height, air factor and U/U_mf all affect the bed temperature profile, whereas only the air factor and U/U_mf have significant effects on the combustion in the bed. The bed temperature can be adjusted by separate or combined adjusting of air factor and U/U_mf.     

Determination of minimum fluidization velocity by the statistical analysis of pressure fluctuations in a gas—solid fluidized bed

The pressure fluctuations in a gas—solid fluidized bed were measured by a differential pressure transducer (DPT). It was found from this study that the measuring height on the axis of a bed was a very important factor for the determination of minimum fluidization velocity from the linear relationship between standard deviation and air velocity. That is, the linearity between two parameters is strongly dependent on the distance above the distributor, because the pressure fluctuations are influenced by the jet formed at the distributor and the bubble coalescence, which depend on particle size and air velocity.     

Interpretation of the hydrodynamic behaviour in a conical fluidized bed dryer

Wet batches of placebo pharmaceutical granule were dried at inlet superficial gas velocities of 0.64 and 1.3 m/s in a Glatt GPCG-1 fluidized bed. Using pressure fluctuation analysis, the hydrodynamic behaviour indicates a transition from a multiple bubbling regime to a coalescence dominated regime as drying proceeds. The transitional fluidization behaviour is linked to the physical mechanisms associated with the constant and falling rate periods of drying porous materials. Excess surface moisture present during the constant rate period increases interparticle forces through liquid bridging. These liquid bridges stabilize the bed structure which limits bubble formation in the bed. Once the falling rate period is reached, the liquid bridges cannot be maintained and bubble coalescence increases. The resulting bubbling bed hydrodynamics can be explained using the simple two-phase model proposed by Toomey and Johnstone [1952. Gas fluidization of solid particles. Chemical Engineering Progress 48, 220-226] using the full support velocity and bed voidage characteristics of the granule at varying moisture contents.     

The effect of distributor rotation on gas mixing in a fluidized bed

The influence of distributor rotation on gas dispersion in a 15 cm diameter fluidized bed was investigated at four rotational speeds. Gas concentrations were determined at various bed depths and radial positions, according to a recently developed method. The concentration profiles were compared with those of a two-dimensional diffusion model. Rotation of the distributor plate caused an increase in the radial dispersion of the gas essentially at fluidization velocities within a range of u_mf.     

Discrete particle simulations of an electric-field enhanced fluidized bed

Reducing the size of gas bubbles can significantly improve the performance of gas-solid fluidized reactors. However, such a control of bubbles is difficult to realize without measures that either use a lot of energy or deteriorate the fluidization behavior. In this paper, we present the results of discrete particle simulations of an electric-field enhanced fluidized bed, and compare these results to experimental data.The simulations show a significant effect on the size of bubbles, both with horizontal and vertical electric-fields applied. When the field strength is increased to values higher than those used in the experiments, the particles are found to form strings in the direction of the electric field. At very high field strengths, defluidization is observed, consistent with the experiments.Through the analysis of the bubble behavior, it is concluded that moderate strength electric fields distribute gas more evenly at the bottom of the bed. As the bubbles rise through the bed, the coalescence rate is lower because of the guiding paths, or resistance, the particles form due to the field. This results in a smaller average bubble size in the higher region of the bed. The simulations presented here show how and why the electric fields reduce bubble size in electric-field enhanced fluidized beds.     

Bubble characteristics in shallow bubble column reactors

The bubble characteristics have been investigated in an air–water bubble column with shallow bed heights. The effect of bed height, location and the presence of solids on the bubble size, bubble rise velocity and overall and sectional gas holdup are studied over a range of superficial gas velocities. Optimal shallow bed operation relies on the combined entrance and exit effects at the distributor and the liquid bed surface. The gas holdup is found to decrease with an increase in H/D ratio but the effect is diminishing at high H/D ratios. A H/D ratio of 2–4 is found to be suitable for shallow bed operation. The presence of solids causes the formation of larger bubbles at the distributor and the effect is diminishing as the gas velocity is increased.     

EFFECT OF STATIC LIQUID HEIGHT ON GAS-LIQUID MASS TRANSFER IN A DRAFT-TUBE BUBBLE COLUMN AND THREE-PHASE FLUIDIZED BED

Experiments are performed under batch-liquid operating conditions to investigate the effect of static liquid height on the gas-liquid mass transfer coefficient (K_La) in a draft-tube bubble column (DTBC) and a draft-tube three-phase fluidized bed (DTFB). In addition, the effects of column diameter, gas-distributor, and draft-tube diameter are studied. The results indicate that for a given system with a porous plate gas-distributor at low superficial gas velocities (<70 m/hr), increasing static liquid height decreases K_La. At high gas velocities, K_La is independent of the static liquid height. For systems with a perforated gas-distributor, there is no effect of static liquid height on K_La. The formation of small dispersed bubbles at low gas velocities in the porous plate distributor system accounts for the considerably high K_La values and the observed effect of liquid height. On the other hand, the formation of large spherical-cap bubbles and the bubble coalescence at high gas velocities reduce the performance of the porous plate distributor system to that of the perforated one.     

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.     

Identification of stable operating ranges of a counter-current multistage fluidized bed reactor with downcomer

This paper describes the stable operating range for a multistage gas–solid fluidized bed reactor and the influence of downcomer and aspect ratio on stable and uniform gas–solid fluidization. A three-stage counter-current fluidized bed reactor was designed, fabricated and operated and range of stable operation for lime and sand particles has been determined. In addition to that a comprehensive discussion on dynamics of downcomer has been made to ensure smooth transfer of solids from one stage to another. A theoretical model has been developed to predict the solid height in downcomer and tested for experimental conditions. The influence of aspect ratio on distributor to bed ratio (P_p/P_b) was investigated for the reactor for uniform fluidization.     

A novel technique for particle tracking in cold 2-dimensional fluidized beds—simulating fuel dispersion

This paper presents a novel technique for particle tracking in 2-dimensional fluidized beds operated under ambient conditions. The method is applied to study the mixing mechanisms of fuel particles in fluidized beds and is based on tracking a phosphorescent tracer particle by means of video recording with subsequent digital image analysis. From this, concentration, velocity and dispersion fields of the tracer particle can be obtained with high accuracy. Although the method is restricted to 2-dimensional, it can be applied under flow conditions qualitatively resembling a fluidized-bed combustor. Thus, the experiments cover ranges of bed heights, gas velocities and fuel-to-bed material density and size ratios typical for fluidized-bed combustors. Also, several fluidization regimes (bubbling, turbulent, circulating and pneumatic) are included in the runs.A pattern found in all runs is that the mixing pattern of the tracer (fuel) solids is structured in horizontally aligned vortexes induced by the bubble flow. The main bubble paths always give a low concentration of tracer solids and with the tracer moving upwards, while the downflow of tracer particles in the dense bottom bed is found to take place in zones with low bubble density and at the sidewalls. The amount of bed material (bed height) has a strong influence on the bottom bed dynamics (development and coalescence of bubbles) and, consequently, on the solids mixing process. Local dispersion coefficients reach maximum values around the locations of bubble eruptions, while, in the presence of a dense bottom bed, an increase in fluidization velocity or amount of bed material enhances dispersion. Dispersion is found to be larger in the vertical than in the horizontal direction, confirming the critical character of lateral fuel dispersion in fluidized-bed combustors of large cross section.     

Effect of air distributor on the fluidization characteristics in conical gas fluidized beds

The effect of an air distributor on the fluidization characteristics of 1 mm glass beads has been determined in a conical gas fluidized bed (0.1 m-inlet diameter and 0.6 m in height) with an apex angle of 20‡. To determine the effect of distributor geometry, five different perforated distributors were employed (the opening fraction of 0.009–0.037, different hole size, and number). The differential bed pressure drop increases with increasing gas velocity, and it goes from zero to a maximum value with increasing or decreasing gas velocity. From the differential bed pressure drop profiles with the distributors having different opening fractions, demarcation velocities of the minimum and maximum velocities of the partial fluidization, full fluidization, partial defluidization and the full defluidization are determined. Also, bubble frequencies in the conical gas fluidized beds were measured by an optical probe. In the conical bed, the gas velocity at which the maximum bed pressure drop attained increases with increasing the opening fraction of distributors.     

高密度浓相流化床中气泡的兼并与分裂特性

利用先进的高速动态分析系统对二维床中气泡的行为进行了研究,通过对所拍摄图象的分析处理．得到了不同介质流化床内形成的气泡形状、大小、聚并及分裂的基本规律和特点．实验研究表明．气泡的兼并主要是两气泡问的合并、被合并气泡总是从气泡的尾涡区曳入气泡;气泡分裂主要发生在操作气速较大或大气泡中,是由于其顶部粒子流(或“剪切流”)的侵入造成的;操作气速较低,粒度、密度较大粒子形成的流化床更易于造成气泡的湮灭。     

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.     

Magnetic resonance imaging of gas–solid fluidization with liquid bridging

Magnetic resonance imaging is used to generate snapshots of particle concentration and velocity fields in gas–solid fluidized beds into which small amounts of liquid are injected. Three regimes of bed behavior (stationary, channeling, and bubbling) are mapped based on superficial velocity and liquid loading. Images are analyzed to determine quantitatively the number of bubbles, the bubble diameter, bed height, and the distribution of particle speeds under different wetting conditions. The cohesion and dissipation provided by liquid bridges cause an increase in the minimum fluidization velocity and a decrease in the number of bubbles and fast particles in the bed. Changes in liquid loading alter hydrodynamics to a greater extent than changes in surface tension or viscosity. Keeping U/U_mf at a constant value of 1.5 produced fairly similar hydrodynamics across different wetting conditions. The detailed results presented provide an important dataset for assessment of the validity of assumptions in computational models. © 2017 American Institute of Chemical Engineers AIChE J, 64: 2958–2971, 2018     

The effect of bed materials on the solid/bubble motion in a fluidised bed

Gas fluidisation provides good mixing and contact of the gas and particle phases as well as good heat transfer. These attractive features are achieved by the high degree of bubble-induced particle circulation within the bed. Bubble and particle motion vary with bed materials and operating conditions, as investigated in the present study, by the use of the non-intrusive positron emission particle tracking (PEPT) technique. The selected materials were spherical polyethylene and glass particles.The data obtained by the PEPT technique are used to determine the particle velocities and circulation pattern. Bubble rise velocities and associated sizes can be inferred from the particle velocity data, since particles travel upwards mostly in the bubble wake. The results indicate that the flow structure and gas/solid motion within the fluidised beds were significantly different, even at the same value of the excess gas velocity, U-U_mf. The solid circulation pattern within the beds differ: if for glass beads, a typical UCDW-pattern existed (upwards in the centre of the bed, downwards near the wall), the pattern in the polyethylene bed is more complex combining a small zone of UWDC movement near the distributor and a typical UCDW-pattern higher up the bed. Transformed data demonstrate that at the same value of excess gas velocity, U-U_mf, the air bubbles in the polyethylene fluidised bed were smaller and rose more slowly than in the fluidised bed of glass beads, thus yielding a longer bubble residence time and improved gas/solid contact. For polyethylene beads, the size and rise velocity of air bubbles did not increase monotonically with vertical position in the bed as would be predicted by known empirical correlations, which however provide a fair fit for the glass beads data. Bubble sizes and solid circulation patterns are important parameters in the design of a fluidised bed reactor, and vary with the bed material used.     

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.