Investigation of the kinetics of fluidized bed spray agglomeration based on stochastic methods

Particle formation during fluidized bed spray agglomeration is modeled by a Monte Carlo method. The methodology is based on the micromechanisms occurring within the bed. The most important model parameters are identified as interparticle collision time, deposited droplet drying time and droplet addition time. It is found that a high number of collisions leads to a negligible role of the drying mechanism. In the real bed, however, the process is dependent on the gas inlet temperature. This indicates that the number of collisions relevant to agglomeration is relatively low. The accordance of the model with experimental results for variations of several process parameters demonstrates that the approach is a promising way to simulate the formation of agglomerates. In addition, the model is able to reproduce slower agglomeration at increased temperatures. This result is, for the first time, based on physical mechanisms rather than on the use of fitted agglomeration kernels. © 2011 American Institute of Chemical Engineers AIChE J, 2011     

Monte Carlo modeling of binder‐Less spray agglomeration in fluidized beds

Fluidized bed spray agglomeration is used in the industry to increase the particle size and to improve several properties, for example, bulk density, flowability, and dissolution behavior of particulate products. Usually, a binder liquid is sprayed on a particle bed. If amorphous materials are used, spraying of pure water may cause agglomeration due to glass transition at wet spots on the particle surface. As no process models covering binder‐less spray agglomeration currently exist, a model based on a Monte Carlo method is presented. In this method, the process is described by events and processes on the single particle scale. Additionally, agglomeration experiments in a lab‐scale fluidized bed using three different maltodextrins are presented. For each experiment, a simulation was performed. The simulation results are compared with the obtained experimental data. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3582–3594, 2018     

流化床反应器结块故障的声纹特征提取及监测技术

流化床反应器的物料结块故障不仅影响产品质量,严重的还会影响生产。为了监测流化床反应器的物料结块故障,提出了一种基于压电声波传感器和声纹特征提取的故障监测方法。在流化床外壁粘贴压电陶瓷声波传感器,采用长屏蔽电缆电荷传输和音频采样方式,监测流化床内物料撞击床壁的声波信号。分析了正常颗粒物料和物料结块情况下声波信号的时域波形、功率谱和声纹特征,重点比较了正常信号和故障信号声纹特征的稳定性和可区分度。通过提取声纹特征,运用神经网络模型实现了对物料结块故障的准确监测。用不同位置声波传感器的感测信号验证故障监测模型的结果验证了这种方法具有较高的时空域鲁棒性。用不同信号抽取率对原始信号进行了重采样,对重采样数据分别进行了声纹特征提取、监测模型的训练和检验,结果表明适当降低信号采样率不影响流化床物料结块的监测结果。为流化床物料结块故障监测问题提供了一种新的系统结构和实现方法。     

基于递归分析的喷雾气固流化床团聚状态识别

液体会影响喷雾气固流化中的颗粒循环模式,形成不同程度的颗粒团聚物,从而直接降低反应器的传热、传质效果。实验中向流化床喷入不同黏度的乙二醇溶液,来人工制造团聚现象。研究结果表明,喷入不同黏度的液体后,流化床内会形成不同的团聚结构,按照团聚物大小可分为四类：微团聚、成核团聚、黏结团聚以及糊状团聚。通过对不同工况中的压差波动信号进行递归分析,发现不同团聚状态下的递归图纹理结构有着明显的差异。与此同时,对50种流动条件下的递归特征量分布情况进行分析,以此来识别不同的团聚结构,整体识别率高达96.39%。结果表明：通过对压差信号的递归分析可以快速识别流化床中的团聚状态。     

Estimation of the dominant size enlargement mechanism in spray fluidized bed processes

This work deals with estimating the dominant size enlargement mechanism in spray fluidized beds. A new process model is presented, which consists of population balances and a heat- and mass-transfer model. New methods to incorporate the wet surface fraction and the Stokes criterion are proposed, which allow for the probability of wet collisions and the probability of successful wet collisions to be calculated. The product of these parameters, the probability of successful collisions, is linked to the dominant size enlargement mechanism. Simulation studies were performed to investigate the influence of inlet gas temperature, viscosity, droplet size, and contact angle on the probability of successful collisions. Further simulation results based on experiments available in literature suggest that exceeding a probability of successful collisions of 0.001 is sufficient for agglomeration to become dominant. Otherwise, layering will be the dominant size enlargement mechanism. Finally, regime maps of layering and agglomeration are constructed.     

Efficient synthesis of iron nanoparticles by self‐agglomeration in a fluidized bed

A two‐stage, fluidized reduction route is proposed to synthesize iron nanoparticles (NPs), with the aim of enhancing the quality of fluidization and preventing sintering activity. At both low and high temperatures, the degree of metallization η is approximately 80% due to the defluidization. Defluidization is mainly caused by the rapid sintering of the newly formed Fe NPs. The proposed two‐stage fluidization approach successfully resolves the defluidization problem through the self‐agglomeration of nanoparticles cultivated at low temperatures. These self‐agglomerated NPs showed an improved resistance to sintering at high temperatures. The high‐purity Fe NPs prepared by this approach exhibited excellent combustion activity, indicative of the potential as oxygen carriers in chemical looping combustion systems. © 2016 American Institute of Chemical Engineers AIChE J, 63: 459–468, 2017     

Stochastic simulation of agglomerate formation in fluidized bed spray drying: A micro-scale approach

A stochastic model that describes agglomerate growth during fluidized bed spray agglomeration is presented and numerically solved by constant volume Monte Carlo method. The methodology overcomes the difficulties of solving multivariate population balance equations and includes continuous binder addition and drying. Agglomerate formation is treated as a complex combination of consecutive and parallel micro-mechanisms. Due to the discrete nature of the approach, the individual role of the micro-mechanisms on the agglomeration behavior can be analyzed.The results suggest that the droplet capture mechanism governs the agglomeration speed while the maximum agglomerate diameter is ruled by the equilibrium reached between coalescence, rebound and breakage. The mechanism of deposited binder drying is found to play a negligible role on agglomerate formation because of an extremely rapid droplet consumption. The main process variables affecting each micro-mechanism have been identified showing that the liquid spraying rate affects directly the droplet capture mechanism whereas binder properties influence mainly the agglomeration and rebound interactions.The model presented in this study is able to predict qualitatively the experimentally observed response of the system as well as the general shape of the agglomerate size distribution under the variation of several process parameters, demonstrating the potential of the discrete micro-level approach.     

Monte Carlo modeling of fluidized bed coating and layering processes

A stochastic modeling approach based on a Monte Carlo method for fluidized bed layering and coating is presented. In this method, the process is described by droplet deposition on the particle surface, droplet drying and the formation of a solid layer due to drying. The model is able to provide information about the coating coverage (fraction of the particle surface covered with coating), the particle‐size distribution, and the layer thickness distribution of single particles. Analytical solutions for simplified test cases are used to validate the model theoretically. The simulation results are compared with experimental data on particle‐size distributions and layer thickness distributions of single particles coated in a lab‐scale fluidized bed. Good agreement between the simulation results and the measured data is observed. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2670–2680, 2016     

Monte Carlo simulations to determine coating uniformity in a Wurster fluidized bed coating process

This paper presents a coating model to predict the mass coating uniformity in a Wurster fluid bed coater using a Monte Carlo method. The velocity and voidage data obtained using imaging techniques on the same Wurster coater are used as inputs to the model. The semi-circular Wurster fluid bed used in this work was 22.9 cm in diameter. A batch of 3.6 kg tablets was used to conduct coating experiments and the coating weight gain distribution results were compared to predictions from the simulation. The model rigorously considers the sheltering effect of particles as they move in the spray zone. Good agreement was obtained when comparing the results with an analytical model.Spray shape and orientation of discretization were found to play an important role in predicting the coating uniformity. A simple spray experiment in a particle-free bed showed that the direction of spray material, in general, was vertically upward. Simulation results confirmed that an upward cylindrical spray model gives better agreement with experimental results compared to a solid cone spray model. Finally, the model was used to predict the changes in coating uniformity with bed operating conditions such as gas velocity and gap height. A wider coating distribution was found for the case with the lower gas velocity and gap height.     

Kinetics of fluidized bed spray agglomeration for compact and porous particles

The effect of the primary particle porosity during the formation of agglomerates in spray fluidized beds is presented in this study. The method is based on the single micro-interactions occurring within the fluidized bed such as inter-particle collisions, droplet spread on the particle surface, aging of the deposited droplets and particle coalescence. The porous character of the particles is expected to directly affect the aging process of the deposited binder layer by penetration into the pores of the substrate. The droplet penetration process is experimentally analyzed by single droplet deposition on spherical, porous alumina particles. The results indicate that the penetration process is mainly governed by the viscosity of the liquid and that at relatively low viscosities, droplet penetration is fast. For highly viscous liquids, the penetration velocity slows down and an additional mechanism, namely drying becomes important. A combined imbibition–drying model is developed and included into a comprehensive stochastic agglomeration model that allows the simulation of agglomerate formation in a batch process. Lab-scale agglomeration experiments with porous and non-porous particles are carried out in an attempt to validate the general tendencies predicted by the main agglomeration model. The results show that the agglomeration rate for porous particles is significantly reduced due to the losses of deposited droplets into the pores of the primary particles; this tendency is much more pronounced at low binder viscosities.     

Radioactive particle tracking in a lab‐scale conical fluidized bed dryer containing pharmaceutical granule

Radioactive particle tracking (RPT) has been used to study the motion of the particulate phase in a bench‐scale conical fluidized bed containing dried pharmaceutical granule. RPT revealed that there is a distinct circulation pattern of the granule with particles moving upwards at high velocities near the centre of the bed and falling slowly near the walls. There was also a localized region near the centre of the bed where particles moved downward rapidly. The particle size distribution (PSD) of the granule had an appreciable impact on particle motion with a wide PSD leading to larger fluctuations in particle velocity as well as poorer granule mixing.     

Radiation model of a TiO2‐coated,quartz wool,packed‐bed photocatalytic reactor

The radiation field of a packed‐bed photocatalytic reactor filled with quartz wool coated with titanium dioxide was modeled using the Monte Carlo technique and the following information: the radiation flux emitted by the lamps, the diameter size distribution of the quartz fiber cloth, the mass of quartz fibers and of TiO₂ that was immobilized on the fiber surface as well as the refractive index, and the spectral absorption coefficient of the materials of the system. Modeling predictions were validated with radiometer measurements of the transmitted radiation through the reactor, the root mean square error being <9.7%. Finally, by means of a parametric study, the validated model was used to analyze the effect of the design variables, such as the radii of the quartz fibers, thickness of the TiO₂ coatings, and amount of TiO₂‐coated quartz wool, on the distribution and nonuniformity of the radiative energy distribution inside the reactor. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

A two‐phase model for variable‐density fluidized bed reactors with generalized nonlinear kinetics

A model based on the classical two‐phase concept is developed for the simulation of variable‐density reaction with generalized nonlinear kinetics in a bubbling fluidized bed. The influence of reaction density parameter on the fluidodynamics and performance of the reactor for four general types of reactions was explored. The results show that the expansion factor has a significant effect on both fluidodynamic characteristics and reaction conversion. In all types of reactions, higher values of hydro‐dynamic variables were obtained when ? ≥ 0. Reaction conversion, however, dropped as the expansion factor increased. This trend was more pronounced for reaction orders higher than unity. This suggests that bubbling fluidized operations are probably not optimal and applicable for certain types of reactions. Comparative analysis between reaction type and implications for optimum fluidized bed reactor are discussed.     

新型流化床粉煤多元气化制合成气技术及应用

介绍了灰粘聚循环流化床粉煤气化的原理、工艺流程及工艺特点。该流化床底部设置了灰粘聚分离装置,在炉内形成中心高温区,使炉渣在中心高温区内粘聚成灰球,借助密度差异,有选择地使煤粉与灰球分离,从而降低灰渣的含碳量,提高了煤中碳的转化率。将该技术应用于合成氨生产中,每吨氨可增加综合经济效益295 14元。     

A novel dual circulating fluidized bed system for chemical looping processes

A fluidized bed system combining two circulating fluidized bed reactors is proposed and investigated for chemical looping combustion. Direct hydraulic communication of the two circulating fluidized bed reactors via a fluidized loop seal allows for high rates of global solids circulation and results in a stable solids distribution in the system. A 120 kW fuel power bench scale unit was designed, built, and operated. Experimental results are presented for natural gas as fuel using a nickel‐based oxygen carrier. No carbon was lost to the air reactor under any conditions operated. It is shown from fuel power variations that a turbulent/fast fluidized bed regime in the fuel reactor is advantageous. Despite the relatively low riser heights (air reactor: 4.1 m, fuel reactor: 3.0 m), high CH₄ conversion and CO₂ yield of up to 98% and 94%, respectively, can be reported for the material tested. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

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     

Characteristics of anthraquinone hydrogenation catalysts in a liquid‐solid fluidized bed

The fluidization characteristics of anthraquinone hydrogenation catalysts were investigated in a liquid–solid fluidized bed. The effects of the initial bed conditions such as particle size, bed depth‐to‐column diameter ratio and liquid density and viscosity on the fluidization behaviour, bed expansion and applicability of the Richardson–Zaki equation were studied. The results reveal a strong particle size effect on the Richardson–Zaki (R‐Z) expansion index which in general decreased as the particle diameter increased. One type of particles exhibited two distinct bed expansion behaviours, depending mainly on the bed depth‐to‐column diameter ratio, with an experimentally established boundary at . This behaviour could be attributed to increasing wall friction and a tendency to exhibit slugging. The dependence of the Richardson–Zaki exponent on the liquid dynamic viscosity confirms the classic result .     

Treatment of high‐level arsenic‐containing wastewater by fluidized bed crystallization process

Gallium arsenide technology has been widely applied in the communication and optical electronics industries. The process of chip manufacturing produces a stream of wastewater unique in its low flow rate and high arsenic concentration. Fluidized bed crystallization (FBC) technology combines the advantages of a fluidized bed reactor and crystallization. It is highly efficient with low capital and operational costs, while producing no sludge. The waste from the FBC is small in volume, high in crystal purity and recyclable. Jar tests were first performed to evaluate the precipitation of arsenic sulfide. Then a lab‐scale fluidized bed reactor was applied to screen critical operational parameters and the process was optimized to meet the wastewater discharge standard. The results obtained in this study confirmed that the FBC process is capable of treating wastewater containing high concentrations of arsenic, reducing the concentration to µg L^?1 levels. Sulfide dosage and operating pH are the two most significant parameters determining the residual arsenic concentration of the effluent, with optimum conditions of pH = 2 and S/As = 2.2 to meet the local discharge limit. Copyright © 2007 Society of Chemical Industry