流化连续造粒过程传热传质分析

根据连续造粒机造粒室内的温度分布状况 ,将流化连续造粒过程沿高度方向分成四个区域 ,并对各区域粉粒的传热传质进行了分析 ,得出了适合于连续造粒机的传热计算式 ,这对该类机型的研制开发具有一定的指导意义。     

流化振动连续造粒机中成粒规律

对流化振动连续造粒机内的流化机理进行了分析。得出流态化时床内粉料浓度的分布特征,对结构设计具有实际意义,对喷雾造粒过程中两种不同的颗粒成粒机理的分析。可以指导易溶颗粒及颗粒包衣的生产。     

螺旋振动流化干燥器

介绍了螺旋振动流化干燥器的结构，流程及干燥机理，论述了操作参数对干燥强度的影响，最后简述了该干燥器的特点和应用。     

影响喷雾流化干燥系统的诸因素分析

在实践的基础上，分析了影响喷雾流化干燥系统的因素，探讨了该系统主要设计参数的选择趋势。     

餐厨垃圾烘箱干燥和流化干燥过程动力学

在60—100℃条件下,选择烘箱和流化床分别对餐厨垃圾干燥过程的水分变化动力学特性进行研究,并运用4个模型进行数学拟合处理。结果表明:温度对餐厨垃圾干燥过程的影响显著,随着温度的增加,餐厨垃圾样品的含水率(质量分数)和水分比均快速下降,相同温度下,流化干燥达到平衡含水率所需时间远小于烘箱干燥;餐厨垃圾干燥过程的动力学特性可以用Page模型很好地进行模拟和预测;研究中所用的餐厨垃圾样品,其有效湿分扩散系数在烘箱干燥和流化干燥中分别为3.35×10-10—2.43×10-9,1.00×10-7—2.20×10-7m2/s;活化能分别为46.52,21.93 kJ/mol;餐厨垃圾流化干燥的效率比烘箱干燥要高得多。     

流化床干燥器运行过程中影响流化的因素分析

在化学工业中,湿分是以自由溶剂和（或）化合剂溶剂存于湿物料中的水分或溶剂。我们常常把借热能使湿物料中湿分气化,并将产生的蒸汽由惰性气体带走,从而得到含有一定含有湿分的固体产品的过程叫做干燥。而干燥由恒速干燥和降速干燥两个过程组成,它们影响着干燥速率的好坏。在正常的生产中,干燥器的应用非常广泛。但结块现象的出现严重的影响了车间的正常生产与产品的质量,因此流化行为的正常与否已成为干燥器运行过程好坏的重要指标。流化干燥是指干燥介质使固体颗粒在流化状态下进行干燥的过程。本文就以振动流化床为例,从流化床干燥器的运行过程出发。对流化床干燥器的工作原理、影响流化的因素及其解决方案进行分析,以期望提升流化率,提高生产效率。     

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.     

旋流振动流化床干燥性能的研究

文章以聚四氟乙烯粒子和蔗糖为物料研究了旋流振动流化床的流体力学及干燥特性,并与振动流化床的各种特性进行了对比。实验在一个干燥室直径为240 mm的小型圆筒振动流化床中进行,采用斜孔分布板使床内产生旋流。实验结果表明:在空气分布板开孔率相差不多的情况下,物料流化以前,旋流振动流化床的床层压降要大于普通振动流化床,一旦物料达到正常流化状态以后,2种振动流化床的床层压降相差不多;旋流振动流化床可以降低物料的临界含湿质量分数。     

A mathematical model for continuous fluidized bed drying

A mathematical model for continuous drying process in a fluidized bed-dryer is presented.To describe the heat and mass transfer in gas phase, a Kunii—Levenspiel type, three-phase model representing a dilute (bubble) phase, interstitial gaNumerical computation based on this model was done to study the effects of the process parameters as particle and bubble-size, gas velocity, inlet gas     

Development of a tribolelectric procedure for the measurement of mixing and drying in a vibrated fluidized bed

Fluidization of fine, pharmaceutical powders makes them easier to dry, coat and mix. Fine powders, however, are difficult to fluidize well with gas flow only. Vibration can often help achieve smooth fluidization at a lower gas flow. The objective of the present study was, thus, to develop reliable and quick experimental methods to characterize mixing and drying in vibrated fluidized beds of fine powders.Effective mixing is critical in many industrial applications and, in gas-solid fluidized beds, requires gas velocities greater than the minimum bubbling velocity (U_mb). There are a number of techniques available for determining U_mb. However, they often are impossible or impractical to use in an industrial application. A new measurement technique involving the use of triboelectric probes was developed. Signal characteristics obtained from sophisticated signal analysis were used to identify the minimum bubbling velocities. These predictions corresponded well with the values obtained from more traditional laboratory methods such as the bed pressure gradient.In a fluidized bed, particles hitting a metal probe will generate a small triboelectric current. Triboelectric probes are able to detect rapid changes in particle surface properties. Surface properties of the particles were modified by wetting the particles in a low shear mixer. This change was detected by triboelectric probes at various locations inserted throughout the bed. The water adsorbed on the particles will begin to evaporate when exposed to the gas stream and the surface properties of the particles will gradually return to their original dry state. The triboelectric probes were able to monitor this drying process. The effects of vibration amplitude on the mixing and drying rate of the bed were also determined.     

Model for continuous drying of solids in fluidized/spouted beds

A mathematical model is presented for predicting the dynamics of continuous drying of solids in fluidized and spouted beds (start-up period). The outlet solids moisture content \documentclass[article]\pagestyle[empty]\begin[document]$ \bar Q $ \end[document], the outlet humidity and the solids temperature are predicted as a function of time for both falling (internal diffusion controlled) and constant rate drying periods. It is shown that the initial moisture content of the solids in the bed, Q_d, is a critical variable. Three characteristic types of dynamic behaviour are identified depending upon the value of Q_d. For falling rate, a large Q_d results in unacceptably high \documentclass[article]\pagestyle[empty]\begin[document]$ \bar Q $ \end[document] in the initial periods whereas a small Q_d results in excessive temperature rise which may cause damage to solids. For constant rate, the responses of outlet humidity and solids temperature are independent of Q_d. Analytical expressions are derived for the time required to reach steady state, the solids moisture content, and temperature at steady state. The model should also be useful for control of dryers.     

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.     

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.     

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.     

Numerical simulation of particle motion in vibrated fluidized bed

The particle motion in vibrated fluidized bed was examined by using numerical simulation. A two-dimensional fluidized bed was used, and discrete element method (DEM) was used as for calculating the particle motion in vibrated fluidized bed. Geldart group B particle, which is regarded to have no effect of agglomerate, was used as the fluidizing particle. The vibration directions (vertical and horizontal) and vibration parameters (frequency and amplitude of vibration) were changed.In the case with vertical vibration, large bubbles caused by vibration gap (defined as the gap between particle bed and wall caused by vibration) appear in bed. When the vibration strength is high and the vibration frequency is low, it is difficult to fludize the particle bed in the case with horizontal vibration because the vibration gap acts like the channel. When the vibration frequency increases at the same vibration strength, the effect of vibration on the particle motion becomes small. The effect of vibration on the pressure loss in particle bed is large in the case with horizontal vibration.