Stochastic Modeling of Fluidized Bed Agglomeration: Determination of Particle Moisture Content

The present study describes the theoretical modeling of particle formation by agglomeration in fluidized beds. The model is mathematically solved by applying a stochastic Monte Carlo method. It takes into account the complete droplet history (predrying during flight, spreading, solidification and penetration after deposition on a particle) so that thermal effects such as the increase in fluidization gas mass flow or gas temperature have a physically based impact on process kinetics. Contradictions between measured and simulated particle moisture contents were found. Thus, we present a modified drying model and compare results with experimental data.     

An Evaluation of Factors Influencing the Energy-Efficient Operation of Well-Mixed Fluidized Bed Dryers

This article describes the results of calculations of specific energy consumption, E_s, performed on a well-mixed fluidized bed dryer simulator. Exhaust air temperature-humidity loci required to yield a specified outlet moisture content were also determined. Most of the calculations related to solids whose drying rate was gas-film controlled. Six model drying curves were employed to examine the effects of drying rate and hygroscopicity in addition to the normal operating parameters. The results indicated that E_s was highest for slow-drying hygroscopic solids and lowest for fast-drying, non-hygroscopic solids. Specific energy consumption increased with decreasing bed temperature and outlet moisture content and with increasing heat loss but was independent of solids loading and airflow rate. For both the aforementioned solids and a much slower drying material (wheat), there was close agreement between the zero heat loss data and a single theoretical curve approximating the performance of an ideal adiabatic dryer. Distinct differences between the behavior of well-mixed and plug flow fluidized bed dryers are reported.     

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.     

Daubechies小波对流化床压力波动的分解研究

由于小波具有紧支集的正交特性，作为一种信号处理方法已引起了越来越多的关注和重视。基于床层压力波动信号的小波分析已用于流态化的研究。但由于在采用多大紧支集的Daubechies小波时具有一定的随意性，对相同的信号，应用不同紧支集的Daubechies小波可能会得到不同的结果。为了了解小波分析的误差，本文对流化床不同测量位置的压力波动信号用不同紧支集的Daubechies小波族在1-9尺度下进行分解和重构，对比重构信号和原始信号发现，紧支集为[-1，2]的Dau2小波用于压力波动信号分析时误差最小，为压力波动Daubechies小波分析的最优小波。而紧支集为[-9，10]的Daul0小波分解误差最大，其次为紧支集为[-4，5]的Dau5小波。     

脉冲鼓泡流化床压力波动特性的模拟

在分析脉冲气流作用下均匀鼓泡流化床的波动结构和波动机理的基础上,建立了反映其压力波动特性的二维离散单元模型。根据脉冲鼓泡流化床的特点,提出了模型离散单元的划分方法,确定了系统的模型参数。采用直接数值积分的方法,对床层的压力波动进行了模拟研究,计算与实验结果基本上一致。     

Volumetric Heat Transfer Coefficient in Fluidized‐Bed Dryers

In literature, there are several Nu=f(Re) equations to specify the heat transfer coefficient between solids and the drying gas, but these equations differ significantly because of the inaccuracy of determining the contact surface between the two phases. A pilot‐plant fluidized‐bed dryer is developed to study the heat and mass transfer phenomena during the drying process. A volumetric heat transfer coefficient is applied for modeling fluidized‐bed dryers. A modified Nusselt number is defined to compare the experimental results and those of the literature. The modified Nu'=f(Re) equation exhibits a proper correlation between the results of the experiments in the literature and those of our experiments.     

Prediction of Physicochemical Properties of Raspberry Dried by Microwave-Assisted Fluidized Bed Dryer Using Artificial Neural Network

The main target of this research is to dry raspberries in a microwave-assisted fluidized bed dryer. Artificial neural network (ANN) modeling was used in order to evaluate and predict the physicochemical properties of this fruit. In this research, the effects of five variables—microwave power (0, 300, and 600 W), temperature (55, 70, and 85°C), air flow rate (15, 20, and 25 m/s), starting time of microwave input (from the moment when the moisture content decreased until 334, 400, and 466 g water/g dry matter), and amount of loaded material (50, 100, and 150 g)—on nine outputs—drying time, rehydration capacity, density, porosity, hardness, water activity, phenolic compounds content, anthocyanins content, and the antioxidant activity of dried raspberries—were studied. A feed-forward multilayered perceptron trained by back-propagation algorithms for five independent variables was developed to predict these nine outputs. The optimal configuration of the neural network model had a hidden layer with nine neurons. The predictive ability of the ANN was compared using a separate data set of 52 unseen experiments based on root mean square error (RMSE), mean absolute error (MAE), and coefficient of determination (R ²) for each output parameter. The optimum model was able to predict the nine output parameters with a coefficient of determination higher than 0.92. The results indicated that the experimental and ANN-predicted data sets were in good agreement, so it is feasible to use an ANN to predict the physicochemical properties of dried black raspberry.     

循环流化床提升管压力瞬时波动研究

在φ0.4m×9.1m循环流化床提升管中采集了分布板以上不同轴向高度的气体压力瞬时波动信号,并利用统计方法和功率谱进行分析。结果表明,在相同操作气速条件下,固体颗粒循环速率越大,在同一轴向位置,压力平均值越大;在相同操作条件下,压力标准偏差随轴向高度的增加而减小;功率谱谱图中存在一个明显主频。     

液固两相外循环流化床压力波动信号的统计及频谱分析

压力波动信号是表征流化床内流体运动特性的重要信息. 为了更好地了解液固外循环流化床内流体流动特征,在液固外循环流化床中,对床体壁面压力波动信号进行了时域、频域及自相关性分析. 结果表明,沿床体稳定流化段上的压力波动特征相似,流体流动和颗粒运动所引发的压力波动能量频带分别集中在0~10和30~40 Hz之间,压力波动的概率密度近似呈正态分布,液固两相外循环流化床中的压力波动信号介于周期信号和随机信号之间.     

Effect of Humidity on Drying of Porous Materials in Fluidized Bed under Reduced Pressure

For fluidized bed drying under a reduced pressure, the effect of the humidity of the drying gas on the drying characteristics of porous materials immersed in the bed was examined experimentally and theoretically. The temperature at the sample center increased with the humidity at relatively high pressures in the drying chamber (101.3 and 50 kPa), and the degree of the increment in the temperature with the humidity increased with the chamber pressure. The effect of the humidity on the temperature at the sample center and the drying time was insignificant at a relatively low chamber pressure (20 kPa).     

Modeling of Moisture and Temperature Changes of Wheat during Drying in a Triangular Spouted Bed Dryer

A mathematical model of temperature and wheat moisture content distribution inside a triangular spouted bed dryer was developed. The model is based on analysis of heat and mass transfer inside the dryer. In addition to that, an empirical bulk density model has been developed for wheat and included in the drying simulation. A laboratory-scale triangular spouted bed (TSB) dryer was used to dry wheat grain to validate the model. The dryer was divided into three sections, namely spouting, downcomer, and fountain. A series of drying runs were conducted to record moisture and temperature profile. There were two distinct regions observed during wheat drying. A constant rate period was observed during the initial drying stage and the falling rate period took place at the later drying stage. Initial moisture content and operating drying temperature governed the timing of transition from constant rate period to falling rate period. The model can be used to accurately predict the moisture content of wheat during drying. The temperature prediction inside the TSB dryer was less accurate, especially at high temperatures due to heat losses in the experimental dryer. Further studies are needed to improve the accuracy of this model, especially with regard to the temperature prediction.     

Modeling of Moisture and Temperature Changes of Wheat during Drying in a Triangular Spouted Bed Dryer

A mathematical model of temperature and wheat moisture content distribution inside a triangular spouted bed dryer was developed. The model is based on analysis of heat and mass transfer inside the dryer. In addition to that, an empirical bulk density model has been developed for wheat and included in the drying simulation. A laboratory-scale triangular spouted bed (TSB) dryer was used to dry wheat grain to validate the model. The dryer was divided into three sections, namely spouting, downcomer, and fountain. A series of drying runs were conducted to record moisture and temperature profile. There were two distinct regions observed during wheat drying. A constant rate period was observed during the initial drying stage and the falling rate period took place at the later drying stage. Initial moisture content and operating drying temperature governed the timing of transition from constant rate period to falling rate period. The model can be used to accurately predict the moisture content of wheat during drying. The temperature prediction inside the TSB dryer was less accurate, especially at high temperatures due to heat losses in the experimental dryer. Further studies are needed to improve the accuracy of this model, especially with regard to the temperature prediction.     

Drying Characteristics of Coriander Seed Particles in a Reduced Pressure Superheated Steam Fluidized Bed

Drying characteristics of coriander seed particles were experimentally analyzed in a reduced pressure superheated steam fluidized bed. The typical moisture gain, reported in some other studies during the warm-up period of the process, was reduced in most of the cases by supplying additional heat into the column. The experimental results demonstrated that the drying rate increases and the equilibrium moisture content decreases by increasing the operating temperature. However, variation of the operating pressure (40–67 kPa) and the superficial steam velocity (2.3–4.0 m/s) did not present significant effects on the moisture contents. The degree of superheating was found to be the most important parameter for the process. The experiments also showed that the equilibrium moisture content decreases upon increasing the degree of superheating. Finally, employing a reduced pressure superheated steam fluidized bed appears as an option to carry out drying processes at relatively lower temperatures.     

Solids Holdup of High Flux Circulating Fluidized Bed at Elevated Pressure

Experimental studies on the solids holdup of a high‐flux circulating fluidized bed (HFCFB) at an operating pressure up to 0.5 MPa were carried out. The effects of operating pressure, solids mass flux and superficial gas velocity on the solids holdup distribution were systematically tested. It was found that the solids holdup at elevated pressure increases with increasing solids mass flux but decreases with increasing superficial gas velocity, which is similar to the trends at atmospheric condition. As a result, the condition of a high‐density circulating fluidized bed (HDCFB), i.e., solids holdup in everywhere of the riser is larger than 10 %) is easier to be achieved at elevated pressure than in a HDCFB operated at atmospheric pressure. In the current work, the condition of a full HDCFB with Geldart group B particles has been achieved successfully at 0.5 MPa.     

Drying of Suspensions in a Fluidized Bed of Inert Particles Under Reduced Pressure

A drying method in which a heat-sensitive material is immersed in a fluidized bed under reduced pressure was used to continuously obtain dispersed, dry, fine powders of that material from a dilute suspension or solution at a low temperature with a high drying rate. The mass velocities of the drying gas, sample flow rate, and chamber pressure were varied, and the effects of these variations on the corresponding drying characteristics were examined.

The fluidization state of a fluidized bed of inert particles strongly affects the drying rate in the bed and has a greater effect than the chamber pressure on the corresponding drying characteristics. In other words, it is important to maintain a vigorous fluidization state to achieve a high drying rate. Although the maximum drying rate is independent of the chamber pressure, it can be achieved at a low mass velocity of the drying gas under reduced pressure. That is, at a low chamber pressure a high drying rate can be achieved, even at a relatively low mass velocity of the drying gas. The bed temperature at the time of drying is strongly influenced by the drying rate and decreases linearly with an increase in the maximum drying rate when the sample flow rate is equal to the drying rate.     

流化床耦合磁场除高氟废水可行性分析研究

赣南地区稀土金属矿产资源丰富,与此同时该地区大部分钽铌、钨和部分稀土冶炼企业产生了大量废水,废水中含有浓度极高的氟化物、多种重金属离子以及其他污染物质,含氟量一般在1.2~3.2 g/L,超标120~320倍,对环境造成很大危害。通过曾平关于流化床耦合磁场的基本原理和近几年来在流化床耦合磁场方向上的进展研究,我们发现流化床耦合磁场技术可以用在该行业用来除高氟废水并使产物资源化。以稀有稀土金属冶炼行业为例进行了分析。     

Residence Time Distribution of Solids in a Fluidized Bed

Experimental investigation on RTD of solids is carried out in a single‐stage fluidized bed provided with an internal, using uniformly sized particles and a binary solid mixture, varying gas flow rate, solids rate, bed height, dilution and the bed geometry. The effect of these variables on first and second moments as well as on F‐curves has been determined. Using a binary solid mixture or an internal inside the bed is found to reduce backmixing of solids. The data is fitted to FTEM and the values of N obtained were compared for different variables.     

GZQ型振动流化床干燥机在双氰胺干燥上的应用

介绍了ＧＺＱ振动流化床干燥机的性能、优点及该机在大氰胺干燥应用中取得的成效。     

Drying of Pepper Seed Particles in a Superheated Steam Fluidized Bed Operating at Reduced Pressure

A series of drying experiments was performed in a reduced-pressure superheated steam fluidized bed, employing pepper seed particles and some novel data were obtained. Experiments were carried out using different chamber pressures (40–67 kPa), temperatures (90–122°C), steam velocities (2.35–4.10 m/s), and mass flow rates (0.0049–0.0134 kg/s). In the majority of the experiments, the moisture gain observed in some other studies in the warm-up period of the process was prevented through some supplementary heat provided to the column. The drying rate was found to be increasing by operating temperature; however, it was not affected much by the superficial gas velocity and the operating pressure. Nevertheless, the reduced pressure operation increases the degree of superheating that appears as the most important parameter of the process. The experimental results showed that the equilibrium moisture content decreases by the increasing degree of superheating. On the other hand, the critical moisture content assumes higher values for the greater degrees of superheating. It was concluded that a relatively lower temperature process can be achieved through a reduced-pressure superheated steam fluidized bed.