引入气相对D类颗粒间歇卸料过程特性的影响

通过测量并分析引入气相前后D类颗粒间歇卸料过程中示踪颗粒的流动状态、压力分布和颗粒流率,发现床体内气固两相流动特性不仅随时间变化,还受到气速（正负压差）的影响。据此将卸料过程按时间分为3个阶段：初始蓄压（PS）阶段、稳定卸料（SD）阶段和非满管流（PP）阶段;并给出各卸料阶段不同正、负压差和重力条件下的气固流动特性。在时间较长、流场较为稳定的SD阶段,发现卸料口颗粒阻力是影响颗粒流率的关键参数,通过修正De Jong公式、Beverloo公式,依次建立卸料口颗粒阻力、D类颗粒卸料流率预测模型,与实验值吻合较好,有望为引入气相调控D类颗粒卸料流率的方法提供参考。     

The effects of particle and gas properties on the fluidization of Geldart A particles

We report on 3D computer simulations based on the soft-sphere discrete particle model (DPM) of Geldart A particles in a 3D gas-fluidized bed. The effects of particle and gas properties on the fluidization behavior of Geldart A particles are studied, with focus on the predictions of U_mf and U_mb, which are compared with the classical empirical correlations due to Abrahamsen and Geldart [1980. Powder Technology 26, 35-46]. It is found that the predicted minimum fluidization velocities are consistent with the correlation given by Abrahamsen and Geldart for all cases that we studied. The overshoot of the pressure drop near the minimum fluidization point is shown to be influenced by both particle-wall friction and the interparticle van der Waals forces. A qualitative agreement between the correlation and the simulation data for U_mb has been found for different particle-wall friction coefficients, interparticle van der Waals forces, particle densities, particle sizes, and gas densities. For fine particles with a diameter , a deviation has been found between the U_mb from simulation and the correlation. This may be due to the fact that the interparticle van der Waals forces are not incorporated in the simulations, where it is expected that they play an important role in this size range. The simulation results obtained for different gas viscosities, however, display a different trend when compared with the correlation. We found that with an increasing gas shear viscosity the U_mb experiences a minimum point near , while in the correlation the minimum bubbling velocity decreases monotonously for increasing μ_g.     

Why the two-fluid model fails to predict the bed expansion characteristics of Geldart A particles in gas-fluidized beds: A tentative answer

It is well known that two-fluid models (TFMs) can successfully predict the hydrodynamics of Geldart B and D particles. However, up to now, TFM have failed to accurately describe the hydrodynamics of Geldart A particles inside bubbling gas-fluidized beds: Researchers have reported that bed expansions are over-predicted by as much as 70%. In this work we show—for the first time—that TFM can predict the correct bed expansion, without any artificial modifications, provided that a sufficiently fine grid size and small time step is used. This suggests that the previously reported failure of TFM is mainly due to the lack of scale resolution, and that from a modeling point of view there is no fundamental difference between Geldart A particles and Geldart B and D particles.     

乙烯气相聚合流化床反应器内Geldart B类和Geldart D类颗粒流动特性的三维数值模拟

乙烯气相聚合流化床反应器的设计、操作和优化依赖于对聚合物颗粒粒径大小和分布、气泡运动特性及聚合反应状况的准确描述。采用Eulerian-Eulerian 双流体模型和群体平衡模型耦合方法对某乙烯气相聚合中试规模的工业流化床反应器分别处于常规聚合工艺(属Geldart B 类颗粒)和免造粒工艺(属Geldart D 类颗粒)时床体的气固流动特征以及不同颗粒类型对反应器操作状态和颗粒运动特性的影响进行了三维数值模拟研究。与传统聚乙烯生产工艺相比,免造粒工艺时的Geldart D 类聚合物颗粒更易聚集于气体入口处区域,而且会产生明显的旋涡并出现较大的气泡。研究结果可为免造粒聚乙烯生产工艺的工业推广应用提供参考。     

EMMS-based Eulerian simulation on the hydrodynamics of a bubbling fluidized bed with FCC particles

Although great progress has been made in modeling the bubbling fluidization of Geldart B and D particles using standard Eulerian approach, recent studies have shown that suitable sub-grid scale models should be introduced to improve the simulation on the hydrodynamics of Geldart A particles. In this study, the flow structures inside a bubbling fluidized bed of FCC particles are simulated in an Eulerian approach employing the energy minimization multi-scale (EMMS) model (Chemical Engineering Science, 2008, 63: 1553-1571) as the sub-grid scale model for effective inter-phase drag force, using an implicit cluster diameter expression. It was shown that the experimentally found axial and radial solid concentration profiles and radial particle velocity profiles can be well reproduced.     

Optimization of baker's yeast drying in industrial continuous fluidized bed dryer

Instant active dry baker's yeast is a well-known product widely used for leavening of bread, produced by fermentation, and usually dried by hot air to 94-96% dry matter content. Multi-stage fluidized bed drying process is a commercial effective method for yeast drying. In this work, optimum operating parameters of an industrial continuous fluidized bed dryer for the production of instant active dry yeast were investigated. The dryer contained four zones separated with moving weirs. The operating conditions such as temperature, loading rate of compressed yeast granules, and hot air humidity had direct effects on both yeast activity and viability. The most important factors that affected the quality of the product were loading rate and the operational temperature in each zone on the bed. Optimization was performed for three loading rates of the feed to the dryer, using response surface methodology for the experimental design. The most significant factor was shown to be the loading rate with mean fermentation activity values of 620, 652, and 646 cm³ CO₂/h for 300, 350, and 400 kg/h loading rates, respectively. The data analysis resulted in an optimal operating point at a loading rate of 350 kg/h and temperatures of zones 1, 2, 3, and 4 controlled at 33, 31, 31, and 29 °C, respectively. The best activity value was predicted as 668 ± 18 cm³ CO₂/h, and confirmation experiments resulted in 660 ± 10 cm³ CO₂/h. At the same operating point, the average viability of the cells was predicted as 74.8 ± 3.7% and confirmed as 76.4 ± 0.6%. Compared with the normal operating conditions at the plant, the optimization resulted in more than 12% and 27% improvement in the yeast activity and viability, respectively.     

D类颗粒节涌流态化的实验和数值模拟

应用双流体气固两相流模型,对均一粒径分布的Geldart D类颗粒的节涌流态化过程进行了模拟. 研究了流化过程中流化状态转变、床层膨胀比以及压力脉动功率谱,从Froude准数和颗粒动能的角度分析了模拟结果中的流化特性,并与实验数据进行了对比. 模拟结果准确预测了Geldart D类颗粒的节涌流态化特性,对床层膨胀高度和功率谱的预测与实验基本吻合. Froude准数和颗粒动能随时间的周期性脉动较好地反映了流化床内节涌流态化的气泡行为.     

Computational study of bubble coalescence/break-up behaviors and bubble size distribution in a 3-D pressurized bubbling gas-solid fluidized bed of Geldart A particles

A computational study was carried out on bubble dynamic behaviors and bubble size distributions in a pressurized lab-scale gas-solid fluidized bed of Geldart A particles. High-resolution 3-D numerical simulations were performed using the two-fluid model based on the kinetic theory of granular flow. A fine-grid, which is in the range of 3–4 particle diameters, was utilized in order to capture bubble structures explicitly without breaking down the continuum assumption for the solid phase. A novel bubble tracking scheme was developed in combination with a 3-D detection and tracking algorithm (MS3DATA) and applied to detect the bubble statistics, such as bubble size, location in each time frame and relative position between two adjacent time frames, from numerical simulations. The spatial coordinates and corresponding void fraction data were sampled at 100 Hz for data analyzing. The bubble coalescence/break-up frequencies and the daughter bubble size distribution were evaluated by using the new bubble tracking algorithm. The results showed that the bubble size distributed non-uniformly over cross-sections in the bed. The equilibrium bubble diameter due to bubble break-up and coalescence dynamics can be obtained, and the bubble rise velocity follows Davidson’s correlation closely. Good agreements were obtained between the computed results and that predicted by using the bubble break-up model proposed in our previous work. The computational bubble tracking method showed the potential of analyzing bubble motions and the coalescence and break-up characteristics based on time series data sets of void fraction maps obtained numerically and experimentally.     

An analytical solution of population balance equations for continuous fluidized bed drying

Continuous fluidized bed drying is widely used to remove moisture or solvents from granular materials. It is known that different residence times of the wet particles may lead to a distribution of product properties, e.g. different moistures. The prediction of such moisture distributions in fluidized bed dryers is of particular interest in industrial practice. In the present study, a simple analytical approach is introduced to calculate moisture distributions at the outlet of a continuous fluidized bed dryer. The model provides an analytical solution of the simple one-dimensional population balances. It will be contrasted with a traditional model approach based on averages and with experimental investigations conducted in a lab scale fluidized bed dryer under variation of the particle and the gas flow rate. Furthermore, the moisture distributions of the dried product were estimated by single particle measurements using nuclear magnetic resonance spectroscopy. It will be demonstrated that the developed analytical approach is capable to predict such moisture distributions for continuous drying processes.     

A numerical study of fluidization behavior of Geldart A particles using a discrete particle model

This paper reports on a numerical study of fluidization behavior of Geldart A particles by use of a 2D soft-sphere discrete particle model (DPM). Some typical features, including the homogeneous expansion, gross particle circulation in the absence of bubbles, and fast bubbles, can be clearly displayed if the interparticle van der Waals forces are relatively weak. An anisotropy of the velocity fluctuation of particles is found in both the homogeneous fluidization regime and the bubbling regime. The homogeneous fluidization is shown to represent a transition phase resulting from the competition of three kinds of basic interactions: the fluid-particle interaction, the particle-particle collisions (and particle-wall collisions) and the interparticle van der Waals forces. In the bubbling regime, however, the effect of the interparticle van der Waals forces vanishes and the fluid-particle interaction becomes the dominant factor determining the fluidization behavior of Geldart A particles. This is also evidenced by the comparisons of the particulate pressure with other theoretical and experimental results.     

Experimental study and simulation of vibrated fluidized bed drying

The paper addresses numerical simulation for the case of convective drying of seeds (fine-grained materials) in a vibrated fluidized bed, analyzing agreement between the numerical results and the results of corresponding experimental investigation. In the simulation model of unsteady simultaneous one-dimensional heat and mass transfer between gas phase and dried material during drying process it is assumed that the gas-solid interface is at thermodynamic equilibrium, while the drying rate (evaporated moisture flux) of the specific product is calculated by applying the concept of a “drying coefficient”. Mixing of the particles in the case of vibrated fluidized bed is taken into account by means of the diffusion term in the differential equations, using an effective particle diffusion coefficient. Model validation was done on the basis of the experimental data obtained with narrow fraction of poppy seeds characterized by mean equivalent particle diameter (d_S,d = 0.75 mm), re-wetted with required (calculated) amount of water up to the initial moisture content (X₀ = 0.54) for all experiments. Comparison of the drying kinetics, both experimental and numerical, has shown that higher gas (drying agent) temperatures, as well as velocities (flow-rates), induce faster drying. This effect is more pronounced for deeper beds, because of the larger amount of wet material to be dried using the same drying agent capacity. Bed temperature differences along the bed height, being significant inside the packed bed, are almost negligible in the vibrated fluidized bed, for the same drying conditions, due to mixing of particles. Residence time is shorter in the case of a vibrated fluidized bed drying compared to a packed bed drying.     

Hydrodynamics of gas-solid fluidized bed of disparately sized binary particles

The hydrodynamics of binary mixture of Geldart Group A and D particles in a turbulent fluidized bed were investigated by experiment and computational fluid dynamics (CFD) method in this paper. The results showed that at low gas velocity, the binary mixtures tend to segregate. At moderate gas velocity, they incline to mix well in the dense phase. Further increasing gas velocity, small particles are entrained and accumulate in the upper regime of the bed, and a segregation trend of the binary mixture appears again. At high gas velocities, segregation efficiency in the continuous classification process increases with increasing the gas velocity and mean residence time of the binary mixture, however, decreases with increasing the small particle content. A strong particle recirculation appears all over the dense phase of the bed, causing an approximately uniform solid composition in radial direction of the fluidized bed.     

Application of electrical capacitance tomography to the fluidized bed drying of pharmaceutical granule

A novel calibration technique that accounts for the presence of moisture has allowed for the application of electrical capacitance tomography (ECT) to a fluidized bed dryer containing wet pharmaceutical granule. A statistical attractor comparison test denoted as the S-statistic has been used to analyze both the reconstructed and non-reconstructed ECT images to determine radial variations in hydrodynamic behavior at an axial location 10.5 cm above the distributor. This analysis has shown that variations in the bed dynamics associated with the loss of moisture during the drying process are most intense near the walls of the dryer, while the central region is characterized by a more consistent dynamic behavior. Investigation of the voidage distribution in the individual tomograms over this period in the drying process shows that the changes in the S-statistic correspond to a reduction in area of the region involved in bubbling behavior. Tomograms from early in the drying process indicate a central core of gas channeling through the very wet granule. This is due to the high cohesive forces present at high moisture. At moisture contents below 5-wt%, near the end of drying, significant divergence in dynamic behavior is identified in the S-statistic. This divergence is associated with the appearance of asymmetric bubbling behavior in the individual tomograms.     

循环流化床中C类颗粒连续干燥的研究

循环流化床(CFB)作为一种新兴反应器,其结构简单、气固接触效率高、处理量大,成为气固二相干燥应用研究的新方向.文中在自建的循环流化床(内径0.104 m×高2.35 m)内,以玉米淀粉(dp=8 μm,ρp=800 kg/m3)为C类颗粒,进行了连续干燥.实验初步研究了进料速率、进风温度及气速等操作参数对淀粉平均停留...     

The bubbling behavior of cohesive particles in the 2D fluidized beds

The present work focuses on a fully statistical analysis of bubbling behavior in the two-dimensional (2D) fluidized beds with cohesive particles. Various significant bubble properties such as bubble size, rising velocity, aspect ratio, bed expansion and bubble hold-up, etc., were investigated. An equation for bubble diameter is developed, , and the observed bubbles are generally smaller than the ones generated in the beds with A or B type powders. Both the average bubble size and rising velocity initially increase with the elevation above the distributor and keep constant beyond certain heights. The bubbles exhibit oblong with the most density aspect ratio (β) equal to 0.7. In addition, the bubble rising velocity coefficient ranges from 0.8 to 1.5. Two core-annular flows form in the large diameter, shallow fluidized bed used in this experiment.     

Optimal quality control of baker's yeast drying in large scale batch fluidized bed

Optimal quality control of drying process of baker's yeast in large scale batch fluidized bed dryer is presented using neural network based models and modified genetic algorithm (GA). The objective of this study is to determine optimal conditions to maximize product quality while minimizing energy consumption. For this purpose, the drying process and quality models based on neural network with delay units are combined for predicting the dry matter, product temperature, change in dry matter and the quality loss while minimizing energy consumption and this model is then used for optimal quality control. A stochastic method based optimization structure is designed in order to solve the optimization problem whose the objective function is discontinuous, non-differentiable, complex and highly non-linear. The results obtained by optimal quality control based on modified GA showed that the performance of the existing industrial scale drying process was improved. The constructed optimal quality control structure is very convenient for the production process applications and may be applied without too much modification.     

Experimental data for code validation: Horizontal air jets in a semicircular fluidized bed of Geldart Group D particles

Experiments were conducted with 6 mm plastic beads (Geldart Group D) in a semi‐circular, gas‐fluidized bed with side jets. Attention was paid to particle characterization and bed measurements, making the resulting dataset ideal for CFD‐DEM validation and uncertainty quantification. The bed was operated slightly above and below the minimum fluidization velocity, with additional fluidization provided by one of two pairs of opposing jets located above the distributor near the flat, front face of the unit. Care is taken to report material properties and bed conditions with either measured distribution functions or uncertainty bounds. High‐speed video imaging and particle tracking velocimetry are used to extract bin‐averaged velocity profiles, which are used to extract jet penetration depths. The time‐averaged mean and standard deviation of the bed pressure drop is also reported. Finally, the lower jets are also inserted into the bed until the opposing jets merge to form a spout‐like pattern. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2351–2363, 2018     

洗衣粉悬浮液在惰性粒子流化床中干燥的研究

针对气体分布板开直孔的惰性粒子流化床,开展了洗衣粉悬浮液在床内的干燥性能研究。测定了流化床的床层压降曲线,考察了进料量、进风温度、进风速度及惰性粒子直径对于流化床传热性能的影响,且与气体分布板开斜孔的传热性能进行了初步比较。结果表明,流化床的床层压降主要是由惰性粒子的流化阻力所致;适当增加进料量和进风速度,或减小惰性粒子直径,以及将气体分布板的孔道由直孔改为斜孔,均可提高流化床的传热性能,但过高的进风温度则可能导致传热性能的下降。     

Experimental study on three-stage microwave-assisted fluidized bed drying of Shengli lump lignite

The drying characteristics of Shengli lump lignite in a three-stage microwave-assisted fluidized bed dryer (MFBD) are investigated in this study. The fluidized bed was operated with a continuous feed in the modes with one to three stages. The effects of microwave power and microwave time on the drying rate and fragmentation rate are studied. The drying rate of Shengli lump lignite increased with increasing microwave power. The fragmentation rate depended on microwave power, microwave time, and final moisture content. The fragmentation rate of dried lignite was in the range of 3–15%. Furthermore, an increase in fragmentation rate at a high microwave power correlates with an increase of the drying time. Pore size distribution in the lignite was determined by mercury intrusion porosimetry (MIP method). With increasing microwave power, total pore volume significantly decreased and average pore diameter was nearly unchanged.     

Analysis of the drying process of a biopolymer (poly-hydroxybutyrate) in rotating-pulsed fluidized bed

A rotating-pulsed fluidized bed (RPFB) dryer was employed to conduct the drying of poly-hydroxybutyrate (PHB) cohesive granules. Along the experiments, it was possible to identify, visually, 3 different dynamic regimes that were related with the temperature profile, the drying kinetics and the fluid dynamic behavior. The drying kinetics of PHB showed a short constant drying rate period followed by a decreasing drying rate period. The constant drying rate (N_c) and final moisture content (dry basis) were related to the rotation frequency (responsible for the pulsation effect), temperature and velocity of the inlet air. Furthermore, measurements of molecular mass (gel permeation chromatography analysis) and Carr Index (flowability test) on PHB samples were done before and after the drying. The RPFB dryer showed to be appropriate to dry the PHB granules, resulting in an excellent fluid dynamic behavior that provided uniform drying of the solid. The best conditions of drying were identified at 7 Hz of rotation frequency, 90 °C and 0.55 m/s of inlet air temperature and velocity. At these conditions the dried PHB reached final moisture content of 0.56% (wet basis) after 2 h of drying.