Stability analysis and CFD validation of a new fluid-particle interaction force for mono-component gas-solid fluidized beds

A new closure relationship for the fluid-particle interaction force has been developed for the CFD modelling of gas fluidized beds. As a first step in verifying the applicability of the new closure, the mono-dimensional version of the equations of change for the fluid and solid phases has been examined analytically. Here, linear stability analysis theory has been applied to the continuity and momentum equations in the face of small fluctuations in the voidage or particle velocity. The effort leads to the development of a new stability criterion for homogenous gas fluidized beds with better predictive capability than the Foscolo and Gibilaro [P. Foscolo, L. Gibilaro, A fully predictive criterion for the transition between particulate and aggregate fluidisation, Chem. Eng. Sci. 39 (1984) 1667-1675.] criterion. This was attributed to the better predictive ability of the dynamic wave velocity developed in this work and which is expressed as a function of the local concentration of the particles. CFD validation of the drag force closure equation is also carried out. Here predictions of bed height and bed voidage obtained from the CFD simulation of a Geldart [D. Geldart, Types of gas fluidization, Powder Technol. 7 (1973) 285.] Group A material FCC catalyst (70 μm) were used to validate the model. Results from simulations showed a very good agreement with predictions from the Richardson-Zaki [J. Richardson, W. Zaki, Sedimentation and fluidization, Trans. Inst. Chem. Eng. 32 (1954) 35.] equation. In the final part of the paper, CFD simulations of three Group B industrial powders are presented comparing the model predictions obtained for the case where the solid stress tensor is at first neglected and subsequently included in the model via the standard granular kinetic theory, results revealed that the collisional stresses do not dominate the macroscopic fluid bed predictions for bubbling dense gas-solid systems.     

Nanoparticle fluidization and Geldart's classification

The behaviour of gas-particle interaction in a fluidized bed depends strongly on the size of the particles being fluidized. Fluidization characteristics of macro-sized particles, from several tens of microns to several millimetres, are well described by the Geldart [1973. Types of gas fluidization, Powder Technology 7, 285] classification. There is evidence that fluidization characteristics of nano-sized particles may be analysed by analogies to those of macro-sized particles. The aim of this paper is to look at whether the mechanism resulting in cohesive behaviour for macro-sized particles may be extended to nano-sized particles. Degussa Aerosil R974 powder, with a primary particle size of 12 nm, was fluidized using nitrogen in a cylindrical vessel, 50-mm-id and 900 mm in height. Characteristics of incipient fluidization are analysed in relation to variations in the initial packed bed conditions. Bed collapse experiments were performed and the results are used for assessing fluidization characteristics of the particles. It was found that nano-sized particles possess characteristics of both Group A and Group C of Geldart classification. Analysis shows that fluidization characteristics of nano-sized particles may be bridged to those of macro-sized particles through particle aggregation.     

宽筛分硅粉颗粒的流化特性

Fluidization characteristics of silicon particle system are studied by the pressure fluctuation method.The existence of fine particles in the system can improve fluidization. Silicon particles with a wide size distribution,preferably with some fines, behave as Group A particles according to Geldart classification, although the system belongs to Group B actually. The system is also approved to be suitable for organochlorosilane monomer production using a fluidized bed reactor. Experimental data obtained in this work are important for the design and operation of commercial fluidized bed reactors for the production of organochlorosilane monomers.     

Fluidization Characteristics of Silicon Particles with a Wide Size Distribution

Fluidization characteristics of silicon particle system are studied by the pressure fluctuation method. The existence of fine particles in the system can improve fluidization. Silicon particles with a wide size distribution, preferably with some fines, behave as Group A particles according to Geldart classification, although the system belongs to Group B actually. The system is also approved to be suitable for organochlorosilane monomer production using a fluidized bed reactor. Experimental data obtained in this work are important for the design and operation of commercial fluidized bed reactors for the production of organochlorosilane monomers.     

Onset of an innovative gasless fluidized bed—comparative study on the fluidization of fine powders in a rotating drum and a traditional fluidized bed

Geldart group C powders were found to be fluidized in rotating drums without requiring any external fluidizing gas. As a result, a rotating drum was proposed as a new gasless fluidized bed in contrast to a traditional fluidized bed, leading to a considerable amount of energy savings. In addition, the fluidization qualities of a series of Geldart group C powders were found to be further improved with the assistance of drum rotation because of the shearing movement among particles that eliminates channeling and cracks and possibly also breaks agglomerates. There is potential for the new gasless fluidized bed to replace some traditional fluidized beds where the fluidizing gas is not used as a reactant.In the gasless fluidized bed, a boundary layer of compacted powder adjacent to the drum wall was observed. The powder in this layer is carried up to the freeboard and then falls back to the powder bed, forming a powder circulation in the drum. The circulating powder leads to a circulation of internal gas in the drum, which essentially acts as fluidizing gas to realize the fluidization of Geldart C powders in the drum. In contrast to the fluidization of Geldart C powders, Geldart groups B and D powders show cascading and cataracting motions instead in the rotating drum due to their requirement of higher fluidization gas velocities. Geldart group A powders experience a transition of powder behavior between Geldart group B–D powders and C powders.     

Effect of internal on phase holdups of a three-phase fluidized bed

Individual phase holdups are important dynamic parameters in the designing of three-phase fluidized bed systems. The system chosen for the present study is nitrogen as gaseous phase, an electrolyte as liquid phase and glass balls as solid phase. The gas holdup was obtained from pressure drop measurements. The obstruction area of flow path was calculated by considering all the geometric parameters of the composite promoter for evaluating the actual velocity of the fluids through the test section. It is found that the presence of composite promoter has not shown any effect on pressure drop in three-phase fluidized beds. The bed porosity data fitted well with Richardson–Zaki equation with an exponent of 2.53. The infinite dilution velocities were increased significantly because of the presence of promoter. The data on gas holdup, liquid holdup and bed porosity were in good agreement with correlations reported earlier [S.D. Kim, C.G.J. Baker, M.A. Bergougnou, Phase holdup characteristics of three phase fluidized beds, Can. J. Chem. Eng. 53 (1975) 134–139; P. Dakshinamurthy, V. Subrahmanyam, K. Veerabhadra Rao, Indian Chem. Eng. 16 (1974) 3; W.Y. Soung, Bed expansion in three-phase fluidization, Ind. Eng. Chem. Proc. Des. Dev. 17 (1978) 33; S.R. Bloxom, J.M. Costa, J. Herranz, G.L. MacWilliam, S.R. Roth, Determination and correlation of hydrodynamic variables in three-phase fluidized bed, ORNL/MIT-219, Oak Ridge National Lab. (1975)].     

The measurement of powder flow properties with a mechanically stirred aerated bed

This paper re-examines a set of experimental data published by Bruni et al., 2007a, Bruni et al., 2007b [Bruni, G., Barletta, D., Poletto, M., Lettieri, P., 2007a. A rheological model for the flowability of aerated fine powders. Chem. Eng. Sci. 62, 397–407; Bruni, G., Lettieri, P., Newton, D., Barletta, D., 2007b. An investigation of the effect of the interparticle forces on the fluidization behaviour of fine powders linked with rheological studies. Chem. Eng. Sci. 62, 387–396] carried out on a mechanically stirred fluid-bed rheometer (msFBR), which was developed to study the rheology of aerated and fluidized powders. The use of aeration below fluidization allowed to carry out experiments with powders at very low consolidation levels. Two mathematical models, based on the Janssen approach to evaluate stresses in powder containers, were developed in order to relate the torque measurements in the Fluidized Bed Rheometer to the flow properties of the powders measured with standard powder flow testers. Results indicate that the models were able to satisfactorily predict the torque measured by the msFBR. The larger complexity of the Walker (1966) [Walker, D.M., 1966. An approximate theory for pressures and arching in hoppers, Chem. Eng. Sci. 21, 975–997] and Walters (1973) [Walters, J.K., 1973. A theoretical analysis of stresses in silos with vertical walls, Chem. Eng. Sci. 28, 13–21] stress analysis adopted in one of the two models did not introduce significant improvements in the evaluation of the stress distribution to justify its use. A procedure for the inverse application of the model was developed and applied to estimate the powder flow properties starting from msFBR data. The application of this procedure provided good results in terms of effective angle of internal friction and is promising for the ability of the system to explore powder flow at very low consolidation states.     

The use of peat granules in a fluidized bed bioreactor

This study describes the particle characteristics and fluidized hydrodynamics of peat granules. Peat granules, moistened with water, are a potential packing material in a gas–solid fluidized bed bioreactor used for treating air pollution. Information on the fluidization of wet peat granules is lacking. In order to advance this new type of bioreactor and to scale up its design for industrial use, fluidization studies of suitable packing material are required. Using abiotic experiments, three sizes of peat granules have been fluidized with air and fluidization characteristics were observed at different superficial gas velocities. Relative to other biomass particles, peat granules have a high particle density and sphericity, which contributes to favourable fluidization behaviour, without gas channelling. Fluidization experiments demonstrate that as the mean size of peat particles increased, minimum fluidization velocity increased. Increasing the moisture content of the peat granules resulted in a transition from bubbling bed fluidization to poor fluidization behaviour. Other types of moist biomass particles such as sawdust are difficult to fluidize and typically exhibit Geldart group C behaviour. In contrast, it was observed that wet peat granules could be fluidized in a bubbling bed regime, typical of group B particles.     

Variable-gas-density fluidized bed reactor model for catalytic processes

A generalized fluidized bed reactor model which covers the three fluidization flow regimes most commonly encountered in industry (bubbling, turbulent and fast fluidization) is proposed. The model is based on probabilistic averaging shown previously (Thompson, Bi, & Grace, Chem. Eng. Sci. 54 (1999) 2175; Grace, Abba, Bi, & Thompson, Can. J. Chem. Eng. 77 (1999) 305) to be applicable over a range of superficial gas velocities. In this paper, we extend the model to cases where the volumetric gas flow changes appreciably due to variations in molar flow, pressure and temperature. For the air-based oxy-chlorination process as a case study, it is shown that the volume change affects both the hydrodynamics and the reactor performance. Because the reactions are rapid, almost complete conversion of ethylene is attained immediately above the distributor resulting in an ∼25% reduction in volumetric flowrate. Using the probabilistic averaging technique, the model tracks the probability of being in the bubbling, turbulent and fast fluidization regimes along the reactor height. The impacts of temperature and pressure variations are also examined. The variable density model gives predictions which compare well with commercial data; ignoring density variations leads to significant underprediction.     

Prediction of pressure fluctuations and minimum fluidization velocity of binary mixtures of geldart group b particles in bubbling fluidized beds

A simple method was proposed to find the pressure fluctuations of binary systems of Geldart Group B particles under bubbling fluidized bed conditions. The pressure fluctuations of binary systems could be predicted from the pressure fluctuations of the individual particles component which comprised the binary systems for completely mixed and partially mixed systems. The predicted pressure fluctuations could be used to calculate the minimum fluidization velocity of the binary systems. The predicted and experimental values of pressure fluctuations and the minimum fluidization velocity of binary systems were in fairly good agreement.     

Investigation of Hydrodynamics of High‐Temperature Fluidized Beds by Pressure Fluctuations

Hydrodynamics of a gas‐solid fluidized bed at elevated temperatures was investigated by analyzing pressure fluctuations in time and frequency domains. Sand particles were fluidized with air at various bed temperatures. At a constant gas velocity, the standard deviation, power spectrum density function, and wide‐band energy of pressure fluctuations reach a maximum at 300 °C. Increasing the temperature to this value causes larger bubble sizes and after the bubbles reach their maximum size, they break into smaller bubbles. The Archimedes number decreases with higher temperature and the type of fluidization becomes closer to that of Geldart A boundary at this maximum temperature. Based on estimation of the drag force acting on the emulsion phase, it was concluded that 300 °C was a transition temperature at which the drag force reaches a minimum due to a significant change of interparticle and hydrodynamic forces.     

声场高温流化床流化特性

在内径50 mm、高1000 mm的声场高温鼓泡流化床中,研究Geldart A, B两类颗粒的流化特性,考察了床层温度、声波频率及声压级对流化床最小流化速度的影响. 结果表明,引入声场后,颗粒的最小流化速度随温度升高而下降;固定温度及频率,最小流化速度随声压级增大而减小;固定声压级与温度,颗粒最小流化速度随声波频率增大先减小后增大,存在一个最佳频率范围. 对床内压力波动信号进行分析,得出声场影响高温流化床流化质量的判据：当声压大于110 dB、频率在100~200 Hz范围内时压力波动偏差与最小流化速度值最小.     

Minimum spouting velocity for the pyrolysis of scrap tyres with sand in conical spouted beds

The effect of temperature (in the 298-973 K range) on minimum spouting velocity has been studied in conical spouted beds made up of mixtures of tyre and sand particles, for different sizes of tyre particles and for different tyre/sand ratios. The good fit to the experimental results confirms the validity of the correlations already proposed by Olazar et al. [M. Olazar, M.J. San José, A.T. Aguayo, J.M. Arandes, J. Bilbao, Stable operation conditions for gas-solid contact regimes in conical spouted beds, Ind. Eng. Chem. Res. 31 (1992) 1784-1791] for room temperature, which is due to the fact that they take into account the density and viscosity of the gas. For fine particles, the effect of density prevails on the effect of viscosity and the equations proposed by Bai et al. [D. Bai, Y. Masuda, N. Nakagawa, K. Kato, Hydrodynamic behavior of a binary solids fluidized bed, J. Chem. Eng. Jpn. 29 (1996) 211-216] are suitable for the calculation of the average particle size and density, in order for them to be used in the calculation of minimum spouting velocity. For coarse particles, the effect of gas viscosity is insignificant compared to the effect of density and the equation of Goossens et al. [W.R.A. Goossens, G.L. Dumont, G.L. Spaepen, Fluidization of binary mixtures in the laminar flow region, Chem. Eng. Prog. Symp. Ser. 67 (1971) 38-45] is suitable for the calculation of the average particle size required for ascertaining minimum spouting velocity.     

气-固微型流化床压降特性及最小流化速度的实验研究

在内径3~20 mm的4个气?固微型流化床中,分别考察了A类和B类两种类型颗粒的流化特性,同时研究了床几何结构、操作条件、物相性质等各因素对其最小流化速度的影响。结果表明,气?固微型流化床中的床层压降特性与颗粒类型密切相关,不同的流动状态下两种类型颗粒的流动特性存在显著地差异。在固定床阶段,与B类颗粒相比,A类颗粒与壁面间的相互作用更强,导致实验压降值偏离计算值更大;在流化床阶段,较大颗粒粒径和密度的B类颗粒在床层内表现出了更高的气泡聚并和破裂程度,加剧了颗粒间的碰撞,增加了能量损失,从而形成了较高的实验压降。气?固微型流化床的最小流化速度除了与操作条件和物相性质有关外,床内径与静态床层高度对其也会产生显著影响。随着床径减小及静态床高增加,最小流化速度逐渐增加。综合考察各影响的因素,提出了适用于实验考察范围内预测微型流化床最小流化速度的经验关联式。     

On Geldart Group A behaviour in fluidized beds with and without cohesive interparticle forces: A DEM study

This paper presents a 2D soft sphere discrete element method (DEM) simulation study of fluidization behaviour of Geldart Group A powders and Geldart Group B powders in the presence of externally imposed cohesive interparticle force. In the presence of externally imposed cohesive interparticle force, beds of Group B powders exhibit some of the characteristics of Group A powders (homogeneous bed expansion and pressure drop hysteresis), but show other characteristics (e.g. bed collapse) which are quite different. The homogeneous bed expansion for Group B particles with imposed cohesive interparticle force is found to follow the Richardson-Zaki relationship, with the exponent n being independent of the magnitude of interparticle force and with the degree of expansion increasing with the magnitude of interparticle force. Group A particles show homogeneous bed expansion even in the absence of cohesive interparticle force.     

磷矿颗粒流态化特性的实验研究

在高1 m、内径42mm的流化床中,对粒径54-600 μm、密度2 252-2 665 kg/m3的磷矿颗粒的流态化特性进行实验研究.实验结果表明:磷矿颗粒粒径和密度对磷矿颗粒流态化行为有较大影响,床层膨胀比随着磷矿颗粒粒径的增大而逐渐减小.当磷矿颗粒属于Geldart B类颗粒时,流化较好;而当颗粒平均粒径为82 ...     

几种典型铁矿粉的流化特性

利用内径150 mm的D型有机玻璃流化床模型,对澳矿、巴西矿、北方矿和钒钛矿典型铁矿粉的流化特性进行了实验研究,获得了其流化特性曲线、初始流化速度和最大床层压降,并将初始流化速度的实测值和理论计算值进行了比较分析. 结果表明,矿粉粒度是影响其流化特性的最主要因素,粒径越大,床层所需要的初始流化速度越大,实测值和理论估算值基本相符;粒度小于0.125 mm钒钛矿流动性较差,在流化过程中易出现沟流现象;粒度范围较宽的矿粉,完全流态化时,细矿粉随气流夹带逸出明显;在粒度相同的情况下,几种不同的铁矿粉的开始流化速度接近,而床层压降有较大差异,巴西矿的床层压降明显大于其他三种铁矿粉. 最大床层压降的最小值均出现在粒度为0.25~0.425 mm,为铁矿粉流态化还原过程中较适宜的粒度范围.     

Hydrodynamics of gas fluidized beds with mixture of group D and B particles

The dynamics of a gas‐solid fluidized bed containing Geldart Group D particles mixed with a small proportion of Geldart Group B particles are investigated using pressure fluctuations data. Time series analysis, using a variety of nonlinear dynamics tools, shows that the slugging present with Group D particles can be suppressed by the addition of a small proportion of Group B particles. The power spectra and the auto‐correlation function are used for a preliminary evaluation of dominant slug frequencies. It is shown that the bed fluidized with the mixture of Group D and B particles behaves in a less periodic manner and is dominated by more random bubble motion. On the other hand, the correlation integral is used to analyze the chaotic behaviour of the flow, through evaluation of the fractal structure of the reconstructed attractors. It is shown that the bed fluidized with Group D particles is characterized by a combination of steady slug motion and irregular particles motion. The behaviour of the fluidized bed of Group D/B particles mixture is characterized by a single correlation dimension for a wide range of fluidization velocities.     

气固搅拌流化床压力脉动的小波分析

在内径188 mm、静床高400 mm的搅拌流化床中,采用Geldart D类颗粒为实验物料,通过小波分析研究了不同气速和搅拌桨转速下搅拌流化床的压力脉动行为.实验发现,搅拌桨的转动作用促使在普通流化床中不易散式流态化的D类颗粒形成了散式流态化.随着气速的增加,第1尺度的小波能量特征值在某一个气速范围内发生急剧变化,进而提出了将该气速范围的下限和上限分别定义为临界鼓泡速度和充分鼓泡速度的判据.随搅拌转速的增加,散式流态化的气速操作范围线性增加.在鼓泡流态化状态下,气速是流化床气泡行为的主导因素,搅拌桨转速的增加对气泡产生的频率无明显影响但可使气泡的直径变小.     

回眸与展望流态化科学与技术

流态化作为一门具有科学内涵的学科始于20世纪40年代,但对于尚没有流态化命题下应用流态化技术的生活和生产活动,如淘米和扬谷,早已存在。此类活动的文字记载出现于16世纪。近代流态化理论以郭慕孙提出的广义流态化理论和无气泡气固接触理论以及Davidson提出的气泡理论为代表,而近代流态化技术的工业应用则以煤的气化和石油的催化裂化为代表。当前流态化的理论研究主要集中于流化床内由不同尺度的颗粒、液滴、气泡、聚团的时空不均匀分布所形成的不均匀结构的预测与优化调控理论与方法的研究,不均匀结构与传递和反应的关系理论的研究以及流态化床的计算机模拟与放大研究。当前流态化技术的工业应用范围已涵盖化工、冶金、能源、材料、环境等领域。在我国,由于经济快速发展,流态化技术应用尤为活跃。采用由流化床局部构效关系模型与两流体模型相结合的计算机模拟方法是解决流态化反应器过程优化调控和设备放大问题的有效途径。该方法面临许多挑战,需要付出艰苦的努力。目前中国工业正处于调整产业结构,淘汰落后产能,实现节能减排、清洁生产的转型期。在许多工业流程中,用高效节能的流态化床反应器替代低效高能耗的回转窑、固定床、移动床反应器正逢其时。