Modelling Diffusion-Limited Drying Behaviour in a Batch Fluidized Bed Dryer

ABSTRACT

Fluidized bed dryers are often used to extract water from granular materials. When the drying process is mainly limited by the resistance against water transport inside the particle the drying behaviour is said to be diffusion-limited. In the literature there are several models that predict this drying process with very diverging results. In this study a model is set up to arrive at a better prediction for this drying process. The heat and mass transfer in the granular material and the drying air is described. The resulting equations are solved numerically. The model must be extended to incorporate the heat capacity of the dryer.     

Modelling Diffusion-Limited Drying Behaviour in a Batch Fluidized Bed Dryer

Fluidized bed dryers are often used to extract water from granular materials. When the drying process is mainly limited by the resistance against water transport inside the particle the drying behaviour is said to be diffusion-limited. In the literature there are several models that predict this drying process with very diverging results. In this study a model is set up to arrive at a better prediction for this drying process. The heat and mass transfer in the granular material and the drying air is described. The resulting equations are solved numerically. The model must be extended to incorporate the heat capacity of the dryer.     

分批模拟连续带液操作流化床的传热研究

通过分批模拟连续干燥实验,建立了相应的热量和质量传递模型;对流化床带液操作时气体与粒子之间的传热系数,粒子之间的传热系数进行了研究,得到了带液操作流化床中流化气速、平均粒径和湿粒子含率对传热系数的影响关联式。对理解气相法聚乙烯冷凝模式操作中的传热提供了帮助。     

Modeling Asd Optimization of Fluidized Bed and Rotary Dryers

Fluidized bed and rotary dryers are two popular types of dryers utilized in almost every area of dryine. In this study, both dryers are analysed regarding their design and operational performance, so that they could be compared in a straightforward way. The mathematical models describing the convective drying process were evaluated for both types of dryers. Design procedures aiming at the determination of optimum sizing and operational characteristics for each type involved were carried out by appropriately optimizing the total annual cost of each structure for a given production capacity. Both dryer types were compared explicitely by evaluating optimum configurations for a wide range of production capacity values. Once the dryer sizing parameters are defined, its operational performance can be evaluated by comparing the optimum operational cost versus production capacity for predefined optimum designed structures. Rotary dryers turn out to be rather expensive compared to fluidized bed dryers, regarding design. On operational grounds, however, it is the other way around, due to the understandably favored heat transfer achieved in rotarv dryers. Characteristic examples covering a wide range of materials - from food products to inorganic minerals -are included in order to demonstrate the performance of each process as well as the effectiveness of the proposed approach.     

Modeling Asd Optimization of Fluidized Bed and Rotary Dryers

ABSTRACT

Fluidized bed and rotary dryers are two popular types of dryers utilized in almost every area of dryine. In this study, both dryers are analysed regarding their design and operational performance, so that they could be compared in a straightforward way. The mathematical models describing the convective drying process were evaluated for both types of dryers. Design procedures aiming at the determination of optimum sizing and operational characteristics for each type involved were carried out by appropriately optimizing the total annual cost of each structure for a given production capacity. Both dryer types were compared explicitely by evaluating optimum configurations for a wide range of production capacity values. Once the dryer sizing parameters are defined, its operational performance can be evaluated by comparing the optimum operational cost versus production capacity for predefined optimum designed structures. Rotary dryers turn out to be rather expensive compared to fluidized bed dryers, regarding design. On operational grounds, however, it is the other way around, due to the understandably favored heat transfer achieved in rotarv dryers. Characteristic examples covering a wide range of materials - from food products to inorganic minerals -are included in order to demonstrate the performance of each process as well as the effectiveness of the proposed approach.     

Robust Simulation of Counterflow Moving Bed Dryers

Abstract

In this work, a robust and accurate strategy for simulating counterflow moving bed dryers is presented. A complete pseudo-transient phenomenological model for the process has been developed and the variable time is used as a continuation parameter. The desired solution is the final steady-state reached by the model. The code DASSL is used to perform the integration of the differential algebraic equation system and consistent initialization is achieved via a direct technique. The computational effort is surprisingly small comparing to those reported in the literature. Reasonable concordance between simulated and experimental results has been observed even though the mathematical model contains no adjustable parameters. Convergence problems have not been observed for all tested operating conditions, including those pointed by other authors as particularly difficult. The mathematical tool built is capable of making accurate predictions about the behavior of the equipment in affordable computing time and hence this constitutes an invaluable tool to study counterflow moving bed drying.     

Robust Simulation of Counterflow Moving Bed Dryers

In this work, a robust and accurate strategy for simulating counterflow moving bed dryers is presented. A complete pseudo-transient phenomenological model for the process has been developed and the variable time is used as a continuation parameter. The desired solution is the final steady-state reached by the model. The code DASSL is used to perform the integration of the differential algebraic equation system and consistent initialization is achieved via a direct technique. The computational effort is surprisingly small comparing to those reported in the literature. Reasonable concordance between simulated and experimental results has been observed even though the mathematical model contains no adjustable parameters. Convergence problems have not been observed for all tested operating conditions, including those pointed by other authors as particularly difficult. The mathematical tool built is capable of making accurate predictions about the behavior of the equipment in affordable computing time and hence this constitutes an invaluable tool to study counterflow moving bed drying.     

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.     

A Learning Process Simulator for Fluidized Bed Dryers Application to Bentonite Drying

A fluid bed dryer simulator was developed under Excel 5 wlth Visual Basic for Applications environment The simulator iS based on a mathematical model describing heat and mass transfer in the dryer. The total model incorporates empirical models for the Drying Constant and the Residence Time. These empirical models are crucial in the total model efficiency. Thus a procedure for updating the parameters of the empirical models is provided. This procedure constitutes the 'learnhg' property of the simulator. Two databases are Supplied. The first contains laboratory drying data and it is used for tuning the Drying Constant empirical model. The second contains industrial drying data from the real operation of the dryer, and it is used for tuning the Residence Time empirical model. The experience from the industrial application of the simulator proved that the simulator is a powerful tool for flexible operation of an industrial dryer. This paper is presents the total mathematical model of the dryer, the learning concept, and the databases, including useful information concerning the drying kinetics of bentonite. A simulator outline is presented and typical capabilities and uses are briefly described. A case study for flexible operation of an industrial dryer is discussed.     

A Learning Process Simulator for Fluidized Bed Dryers Application to Bentonite Drying

ABSTRACT

A fluid bed dryer simulator was developed under Excel 5 wlth Visual Basic for Applications environment The simulator iS based on a mathematical model describing heat and mass transfer in the dryer. The total model incorporates empirical models for the Drying Constant and the Residence Time. These empirical models are crucial in the total model efficiency. Thus a procedure for updating the parameters of the empirical models is provided. This procedure constitutes the ‘learnhg’ property of the simulator. Two databases are Supplied. The first contains laboratory drying data and it is used for tuning the Drying Constant empirical model. The second contains industrial drying data from the real operation of the dryer, and it is used for tuning the Residence Time empirical model. The experience from the industrial application of the simulator proved that the simulator is a powerful tool for flexible operation of an industrial dryer. This paper is presents the total mathematical model of the dryer, the learning concept, and the databases, including useful information concerning the drying kinetics of bentonite. A simulator outline is presented and typical capabilities and uses are briefly described. A case study for flexible operation of an industrial dryer is discussed.     

Batch Drying Kinetics in a Two-Zone Bubbling Fluidized Bed

This article deals with investigation and modeling of batch drying process of solids in fluidized bed apparatus. There has been used model of fluidized bed drying, which consists two zones: emulsion zone and bubbling zone with taking into consideration the presence of solid particles in the bubbles. The results of theoretical expectations that arise from simulation calculations have been verified with experimental data obtained with the use of fluidized bed dryer 0.225 m in diameter. A drying process of silica gel, sand, and ammonium sulfate has been tested. To verify the model, the concept of a generalized drying curve has also been employed.     

Modular Simulation of Fluidized Bed Reactors

Simulation of chemical processes involving nonideal reactors is essential for process design, optimization, control and scale‐up. Various industrial process simulation programs are available for chemical process simulation. Most of these programs are being developed based on the sequential modular approach. They contain only standard ideal reactors but provide no module for nonideal reactors, e.g., fluidized bed reactors. In this study, a new model is developed for the simulation of fluidized bed reactors by sequential modular approach. In the proposed model the bed is divided into several serial sections and the flow of the gas is considered as plug flow through the bubbles and perfectly mixed through the emulsion phase. In order to simulate the performance of these reactors, the hydrodynamic and reaction submodels should be integrated together in the medium and facilities provided by industrial simulators to obtain a simulation model. The performance of the proposed simulation model is tested against the experimental data reported in the literature for various gas‐solid systems and a wide range of superficial gas velocities. It is shown that this model provides acceptable results in predicting the performance of the fluidized bed reactors. The results of this study can easily be used by industrial simulators to enhance their abilities to simulate the fluidized bed reactor properly.     

液固流化床内床层塌落动态过程的数值模拟

在理论分析液固流化床内床层塌落过程的基础上,对水和玻璃球体系的床层动态塌落行为进行了多相流数值模拟,详细考察了液体速度突然减小后床层表面和分隔界面随时间的变化规律,模拟结果与经典模型预测值相吻合,表明多相流计算流体动力学模拟可以很好地预测液固流化床的床层动态塌落行为。     

Measurement Techniques to Monitor and Control Fluidization Quality in Fluidized Bed Dryers: A Review

Fluidized bed dryers (FBD) are commonly employed in many industries to dry particulate solids. FBDs provide good solids mixing, high rates of heat and mass transfer, and relative ease of material handling. For efficient operation, it is important to be able to monitor and control the fluidization regime, particle size distribution (PSD), moisture content, and bulk density as well as product chemical properties. This review provides an overview of the trends in the application of different experimental techniques to monitor and control the hydrodynamic conditions of FBDs which influence the particle physiochemical properties. This review covers a wide range of measurement techniques, including infrared moisture sensor (IR), near infrared (NIR) spectroscopy, analysis of pressure fluctuations, optical imaging techniques, acoustic emission (AE), electrical capacitance tomography (ECT), spatial filter velocimetry (SFV), Raman spectroscopy, focused beam reflectance measurement (FBRM), microwave resonance technology (MRT), triboelectric probes, positron emission particle tracking (PEPT), and some novel techniques for monitoring and control of FBDs. The present review summarizes the use of the diverse techniques and outlines their merits and limitations. Prospects for future research in this area are also identified. The measurement techniques can be used for research, development, and operation of fluidized bed equipment used in non-drying applications as well.     

快速流化床的整体模拟

工业应用中的快速流化床一般不能直接对固相速度进行调控,属于B类操作.本文采用一维双流体模型与存料量方程相结合的方法可以计算B类操作时的上升管内具有浓、稀两相特征的轴向空隙率分布,模拟与实验结果较为吻合.     

Mathematical Model for Batch Drying in an Inert Medium Fluidized Bed

A modified three-phase model is proposed for batch drying of fine powders in an inert medium fluidized bed. The overall heat and mass transfer coefficients between the interstitial gas and solid phases have been determined by the proposed surface-stripping model in which the Biot number is a governing parameter. The effects of gas velocity, inlet gas temperature and mass ratio of starch to inert particles on the drying characteristics of starch in a 0.083 m ID × 0.80 m high medium fluidized bed have been determined. Based on the proposed model, the internal resistance of mass transfer at the powder is equal to the external resistance. The model predicts well the bed temperature, humidity of outlet gas, moisture content of solid particles, heat and mass transfer in an inert medium fluidized bed.     

Control of Spouted Bed Dryers

During the last 8 years, the Drying Center at Universidade Federal de São Carlos has been investigating the control problems of drying process of pasty materials associated with the spouted bed dryer. Starting with structures involving PI controller, we also implemented advanced MPC control techniques and real time optimization (this last one based on simulations only). Different control structures were implemented in a pilot spouted bed dryer where the behavior of the system was tested for water and homogenized chicken eggs. Based on promising results, the perspective is to use an adequate control system in a spouted bed scale‐up.     

Monitoring the Moisture Content of Solids in Fluidized Bed Dryers by Analysis of Pressure Fluctuations

Perusing the hydrodynamic changes of fluidized bed dryer is important for online monitoring of the drying process. The present study investigates the drying process of wetted rice particles. Air at ambient conditions with superficial velocity of 1 ms^?1 was used for drying. Absolute pressure fluctuations were measured to monitor the fluidization status of the dryer. Fast Fourier transform, discrete wavelet transform, and statistical analyses of detailed signals were employed to evaluate the fluidization quality in the bubbling regime. Pressure fluctuations were decomposed by the wavelet transform to 10 subsignals. It was shown that the energy of subsignals is more sensitive to moisture changes than other studied parameters. Specifically, the energy of the subsignals corresponding to the macrostructure (large bubbles) can be used for determining the moisture content of the solids during the drying process. This method can be used for online monitoring of drying processes in a wide range of processing conditions in fluidized beds.