Observation of 3‐D Structure of Bubbles in a Fluidized Catalyst Bed

The interface area between the bubble and emulsion phases in a fluidized catalyst bed is one of the important parameters used to analyze and design the fluidized bed reactor. We used a fast‐scanning X‐ray CT system to observe the bubble shape and structure. We then obtained the transient 2‐dimensional cross sectional gas‐phase distribution in a fluidized catalyst bed. Using image‐processing techniques, pseudo 3‐dimensional images of the bubbles were reconstructed. The bubble structure was studied based on the 3‐dimensional images and the previously obtained results in a 2‐dimensional fluidized bed. It was found that the bubble shape was not spherical but complicated, and that the bubbles ascending in a fluidized catalyst bed consisted of some smaller bubbles.     

Novel phenomenological discrete bubble model of freely bubbling dense gas–solid fluidized beds: Application to two‐dimensional beds

A phenomenological discrete bubble model has been developed for freely bubbling dense gas–solid fluidized beds and validated for a pseudo‐two‐dimensional fluidized bed. In this model, bubbles are treated as distinct elements and their trajectories are tracked by integrating Newton's equation of motion. The effect of bubble–bubble interactions was taken into account via a modification of the bubble velocity. The emulsion phase velocity was obtained as a superposition of the motion induced by individual bubbles, taking into account bubble–bubble interaction. This novel model predicts the bubble size evolution and the pattern of emulsion phase circulation satisfactorily. Moreover, the effects of the superficial gas velocity, bubble–bubble interactions, initial bubble diameter, and the bed aspect ratio have been carefully investigated. The simulation results indicate that bubble–bubble interactions have profound influence on both the bubble and emulsion phase characteristics. Furthermore, this novel model may become a valuable tool in the design and optimization of fluidized‐bed reactors. © 2012 American Institute of Chemical Engineers AIChE J, 2012     

Effects of probe numbers and arrangement on the measurement of charge distributions around a rising bubble in a two-dimensional fluidized bed

The charge density distribution around a rising bubble in a two-dimensional fluidized bed was measured using eight induction probes flush with the outside wall of the column and connected to charge amplifiers for recording induced charge signals as bubbles passed by. The charge distribution surrounding the bubble was reconstructed using an iterative linear back projection (LBP) algorithm by assuming that the bubble is symmetrical and the charge around the bubble remains constant as the bubble rises. The results showed that the emulsion phase far from the bubble in a two-dimensional fluidized bed of glass beads was charged negatively, and the charge density decreased gradually toward the bubble-dense phase interface, with a nearly zero charge density inside the bubble. The effects of the number of probes are also investigated. The reconstructed results using both simulated charge distribution and experimental data for different numbers of probes of the same probe diameter of 3 mm showed that increasing the number of probes can improve the reconstruction resolution. The addition of one or two more probes outside the bubble path also increased the reconstruction resolution in the region outside the bubble.     

Fluidized‐bed reactor modeling for polyethylene production

Gas‐phase technology for polyethylene production has been widely used by industries around the world. A good model for the reactor fluid dynamics is essential to properly set the operating conditions of the fluidized‐bed reactor. The fluidized‐bed model developed in this work is based on a steady‐state model, incorporating interactions between separate bubble, emulsion gas phase, and emulsion solid polymer particles. The model is capable not only of computing temperature and concentration gradients for bubble and emulsion phases, calculating polymer particle mean diameter throughout the bed and polyethylene production rate, but also of pinpointing the appearance of hot spots and polymer meltdown. The model differs from conventional well‐mixed fluidized‐bed models by assuming that the particles segregate within the bed according to size and weight differences. The model was validated using literature and patent data, presenting good representation of the behavior of the fluidized‐bed reactor used in ethylene polymerization. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 321–332, 2001     

The hydrogenolysis of n-Butane on a nickel on silica catalyst: II Fluidized bed studies

This study was undertaken to obtain experience in developing steady-state mathematical models of full-scale chemical reactors through the use of a combination of bench-scale experiments and actual plant operating data. In this case, the hydrogenolysis of n-butane on a nickel on silica gel catalyst was studied in a %inch diameter pilot plant fluidized bed reactor. The kinetic parameters in a mechanistic reaction model were obtained in a bench-scale integral reactor and these rate equations were included in a simple two-phase fluidized bed model. The parameter relating to interchange of gas between bubbles and emulsion was determined from the fluidized bed data. A comparison of the predicted and observed selectivities suggested a weakness in the kinetic model. This shortcoming was then partially corrected by using the fluidized bed data. In this way, a reactor model was achieved which was satisfactory for overall plant simulations.     

Hydrodynamic characteristics of gas–solid fluidization at high temperature

Effect of temperature on the hydrodynamics of bubbling gas–solid fluidized beds was investigated in this work. Experiments were carried out at different temperatures ranged of 25–600°C and different superficial gas velocities in the range of 0.17–0.78 m/s with sand particles. The time‐position trajectory of particles was obtained by the radioactive particle tracking technique at elevated temperature. These data were used for determination of some hydrodynamic parameters (mean velocity of upward and downward‐moving particles, jump frequency, cycle frequency, and axial/radial diffusivities) which are representative to solids mixing through the bed. It was shown that solids mixing and diffusivity of particles increases by increasing temperature up to around 300°C. However, these parameters decrease by further increasing the temperature to higher than 300°C. This could be attributed to the properties of bubble and emulsion phases. Results of this study indicated that the bubbles grow up to a maximum diameter by increasing the temperature up to 300°C, after which the bubbles become smaller. The results showed that due to the wall effect, there is no significant change in the mean velocity of downward‐moving clusters. In order to explain these trends, surface tension of emulsion between the rising bubble and the emulsion phase was introduced and evaluated in the bubbling fluidized bed. The results showed that surface tension between bubble and emulsion is increased by increasing temperature up to 300°C, however, after that it acts in oppositely.     

Gas transfer from bubbles in a fluidized bed to the dense phase — II. Experiments

An experimental check was made upon the theory given in Part I. Cracking catalyst was used as a solid and differently adsorbed tracer gases were used. In a two-dimensional fluidized bed bubbles were formed underneath a gauze cap, while solid flowed along the bubble at the corresponding bubble velocity. Tracer injections provided the value for the transfer coefficient. In three-dimensional beds of 18 and 90 cm dia. large traced gas bubbles were injected. Tracer concentration was detected at certain heights. From the decrease the transfer coefficient was calculated. In the 90 cm bed the transfer coefficient was also calculated from residence time distribution measurements when the dense phase was perfectly mixed.It shows, that the two-dimensional bubble confirms the theory. For three-dimensional bubbles the transfer is higher than theoretically predicted, especially when the dense phase is expanded.     

The Effect of Bed Temperature on Mass Transfer between the Bubble and Emulsion Phases in a Fluidized Bed

The rate of interphase mass transfer between the bubble and emulsion phases of a bubbling fluidized bed is of primary importance in all models for fluidized bed reactors. Many experimental studies have been reported, however, all these investigations have been carried out in fluidized beds operated at room temperature. In this work, the effect of the bed temperature on the interphase mass transfer is reported. Single bubbles containing argon – used as a tracer – were injected into an incipiently fluidized bed maintained at the required temperature. The change in argon concentration in the bubble was measured using a suction probe connected to a mass spectrometer. The effects of bed particle type and size, bubble size, and bed temperature on the mass transfer coefficient were examined experimentally. The interphase mass transfer coefficient was found to decrease with the increase in bed temperature and bubble size, and increase slightly with increase in particle size. Experimental data obtained in this study were compared with some frequently used correlations for estimation of the mass transfer coefficient.     

INTERACTION BETWEEN HORIZONTAL TUBES AND GAS BUBBLES IN A FLUIDIZED BED

The purpose of the work described here was to study the interaction between horizontal tubes and gas bubbles injected singly into an incipiently fluidized bed of alumina powder. The bed was observed directly with X-rays and the bubble-tube interactions recorded on cine film for subsequent analysis. Various geometrical arrangements of tubes were investigated and both the horizontal and vertical distances between tubes were varied. In this way tube arrangements that led to bubble break-up and the resulting effect on the volumetric flow of bubble gas were established. The results indicate that under some circumstances horizontal tubes cause split bubbles to recombine thus nullifying the beneficial effects of splitting. In general when a bubble is split by a tube the sum of the volumes of the daughter bubbles is less than the volume of the parent indicating the some of the original bubble gas leaks into the emulsion phase of the bed during the splitting process. Furthermore the amount of leakage increases as the number of daughter bubbles increases. These facts could be of importance in the design of fluidized-bed reactors.     

Critical comparison of hydrodynamic models for gas-solid fluidized beds—Part I : bubbling gas-solid fluidized beds operated with a jet

In the Eulerian approach to model gas-solid fluidized beds closures are required for the internal momentum transfer in the particulate phase. Firstly, two closure models, one semi-empirical model assuming a constant viscosity of the solid phase (CVM) and a second model based on the kinetic theory of granular flow (KTGF), have been compared in this part in their performance to describe bubble formation at a single orifice and the time-averaged porosity profiles in the bed using experimental data obtained for a pseudo two-dimensional fluidized bed operated with a jet in the center. Numerical simulations have shown that bubble growth at a nozzle with a jet is mainly determined by the drag experienced by the gas percolating through the compaction region around the bubble interface, which is not much influenced by particle-particle interactions, so that the KTGF and CVM give very similar predictions. However, this KTGF model does not account for the long term and multi particle-particle contacts (frictional stresses) and under-predicts the solid phase viscosity at the wall as well as around the bubble and therefore over-predicts the bed expansion. Therefore, in the later part of the paper, the bubble growth at a single orifice and the time-averaged porosity distribution in the bed predicted by the KTGF model with and without frictional stresses are compared with experimental data. The model predictions by the KTGF are improved significantly by the incorporation of frictional stresses, which are however strongly influenced by the empirical parameters in this model. In Part II the comparison of the CVM and KTGF with experimental results is extended to freely bubbling fluidized beds.     

Utilization of the pressure differential records from gas fluidized beds with internals for bubble parameters determination

Simulated pressure gradient records, based on the theoretical pressure field around a bubble in fluidized bed derived by Davidson, are analysed by statistical methods to determine significant bubble parameters. It is shown that from two records measured by two pairs of pressure differential probes located on common vertical axis the following can be computed: bubble velocity, bubble depth (diameter), vertical spacing of bubbles, bubbling frequency, distributions of bubble sizes and spacings, and local bubble phase fraction. Simulated and actual records of the passage of non-interacting single bubbles over four pairs of probes located in a fluidized bed with internals show good agreement.     

气固流化床内费托铁基催化剂的流化特性

为研究费托(Fischer?Tropsch, FT)催化剂在气固流化床内的流动过程,分析了催化剂的主要物性参数,在不同直径流化床内测量了各表观气速下FT催化剂的流动特性,并与广泛应用的流化催化裂化(Fluid Catalytic Cracking, FCC)催化剂的流态化行为进行了对比。结果表明,同为A类颗粒,相较于FCC催化剂,由于FT催化剂的休止角较小(约为FCC催化剂的75%),其临界流化速度较小、床层膨胀高度和气节高度较小;两种催化剂在流化床内流化过程基本相似,随表观气速增大依次出现膨胀、鼓泡、湍动等流型,但各流型转变时的临界速度差异较大。催化剂物性参数对流化特性影响较大,FT催化剂在各阶段流化过程均相对稳定,有利于催化剂在流化床内均匀分布,其气固接触效果优于FCC催化剂;不同催化剂床层高径比下气节高度变化的转折点与流型存在对应关系,可将气节高度随表观气速的变化关系作为判断湍动流化区内流型临界速度的依据。     

Modeling a fluidized bed reactor by integrating various scales: Pore,particle, and reactor

This work proposes a novel population-balance based model for a bubbling fluidized bed reactor. This model considers two continuum phases: bubble and emulsion. The evolution of the bubble size distribution was modeled using a population balance, considering both axial and radial motion. This sub-model involves a new mathematical form for the aggregation frequency, which predicts the migration of bubbles from the reactor wall toward the reactor center. Additionally, reacting particles were considered as a Lagrangian phase, which exchanges mass with emulsion phases. For each particle, the variation of the pore size distribution was also considered. The model presented here accurately predicted the experimental data for biochar gasification in a lab-scale bubbling fluidized bed reactor. Finally, the aggregation frequency is shown to serve as a scaling parameter.     

Investigation into the hydrodynamics of gas–solid fluidized beds using particle image velocimetry coupled with digital image analysis

The hydrodynamics of a freely bubbling, pseudo 2‐D fluidized bed has been investigated experimentally for different bed aspect ratios at different superficial gas velocities by using Particle Image Velocimetry (PIV) combined with Digital Image Analysis (DIA). Coupling of both non‐invasive measuring techniques allows us to obtain information on both the bubble behaviour and emulsion phase circulation patterns simultaneously. In particular, the combination of DIA with PIV allows to correct for the influence of particle raining through the roof of the bubbles on the time‐averaged emulsion phase velocity profiles.     

流化床反应器气固传热面积模型

研究了流化床反应器内发生气固反应时间的传热机理,认为气固间的传热面积包括两部分,即气泡内所含颗粒的表面积与一气泡和泡晕间的有效传热面积 ,据此首次导出了气固传热面积模型A＝u0-umf/22.26db0.5-umf[4.5unf ρgCpg/db 5.85(λgρgCpg)0.5g^0.25/db^1.25]LmS/a将该模型 应用于半间歇的裂化催化剂烧炭再生过程,与实验数据的比较表明,模型预测是可靠的。     

Flow patterns in a pilot plant-scale turbulent fluidized bed reactor: Concurrent application of tracers and fiber optic sensors

Hydrodynamics of a turbulent fluidized bed is studied by means of the concurrent application of fiber optic sensors and a helium tracer. It is observed that in the vicinity of the column wall there is a high bubble activity region. Low bubble activity and negative bubble velocities are reported for the dense phase near the column centre-line region. A temperature increase from 22 to 145°C results in a more homogeneous turbulent fluidized bed with smaller bubbles and more gas flowing through an expanded dense bed emulsion phase. Mass transfer coefficients between bubble-emulsion (k_be) and bubble-annulus (k_ba) are evaluated. The dominant mass transfer path was the one from the bubbles to the annular region with k_ba being several times greater than k_be.     

Two-phase modeling of a gas phase polyethylene fluidized bed reactor

A two-phase model is proposed for describing the behavior of a fluidized bed reactor used for polyethylene production. In the proposed model, the bed is divided into several sequential sections where flow of the gas is considered to be plug flow through the bubbles and perfectly mixed through the emulsion phase. Polymerization reactions occur not only in the emulsion phase but also in the bubble phase. Voidages of the emulsion and bubble phases are estimated from the dynamic two phase structure hydrodynamic model. The kinetic model employed in this study is based on the moment equations. The hydrodynamic and kinetic models are combined in order to develop a comprehensive model for gas-phase polyethylene reactor. The results of the model are compared with the experimental data in terms of molecular weight distribution and polydispersity of the produced polymer. A good agreement is observed between the model predictions and actual plant data. It has been shown that about 20% of the polymer is produced inside the bubble phase and as such cannot be neglected in modeling such reactors.     

流化床反应器塔型内构件的研究

为了改善气-固流化床反应器的流化质量,本文提出了一种新型内构件。首先在二维床中对新的塔型构件的行为进行了研究,并且与其他类型构件进行了对比。所拍摄的照片和电影显示了塔型构件对气泡的破碎能力和对气固两相接触的增进。 塔型构件在萘催化氧化制苯酐的流化床反应器中进行了中间试验。试验结果表明,与床内原来使用的水平挡板相比较,催化剂萘负荷从50—100提高到了100—160g/kg催化剂·h,从而使生产能力可提高50—100％。 试验结果用两相模型进行了关联。结果表明,采用塔型内件的流化床反应器有很高的泡相和乳相间的交换系数。     

气相法聚乙烯冷凝模式操作的模拟研究(Ⅰ) 反应器运行状态的分析

建立了气相法聚乙烯冷凝模式操作反应器的两相模型 .模型涉及气泡相和乳化相中的热量和质量守衡、乳相和泡相之间的热量传递和质量传递、乳相中的聚合反应以及乳相中粒子的停留时间分布等 .通过模型研究了常规操作和冷凝操作时操作变量和反应器运行状态变量之间的关系 .模型模拟结果与工业的常规操作和冷凝操作数据符合较好 .得到了冷凝操作时时空收率、低温区域、聚合物灰分等的变化规律以及催化剂特性对冷凝操作的影响规律 .提出了适合于冷凝操作的催化剂类型