Mathematical Modeling the Drying of Poplar Wood Particles in a Closed-Loop Triple Pass Rotary Dryer

Closed-loop drying systems are an attractive alternative to conventional drying systems because they provide a wide range of potential advantages. Consequently, type of drying process is attracting increased interest. Rotary drying of wood particles can be assumed as an incorporated process involving fluid–solid interactions and simultaneous heat and mass transfer within and between the particles. Understanding these mechanisms during rotary drying processes may result in determination of the optimum drying parameters and improved dryer design. In this study, due to the complexity and nonlinearity of the momentum, heat, and mass transfer equations, a computerized mathematical model of a closed-loop triple-pass concurrent rotary dryer was developed to simulate the drying behavior of poplar wood particles within the dryer drums. Wood particle moisture content and temperature, drying air temperature, and drying air humidity ratio along the drums lengths can be simulated using this model. The model presented in this work has been shown to successfully predict the steady-state behavior of a concurrent rotary dryer and can be used to analyze the effects of various drying process parameters on the performance of the closed-loop triple-pass rotary dryer to determine the optimum drying parameters. The model was also used to simulate the performance of industrial closed-loop rotary dryers under various operating conditions.     

A SIMPLE DYNAMIC MODEL FOR SOLID TRANSPORT IN ROTARY DRYERS

The solid particle movement in a rotary drum plays an important role in drying processes. The solid distribution in the drum affects the amount of contact surface between the solid and the gas. The retention time of solids influences the time particles can stay in contact with the gas in order to transfer heat and mass. Any heat and mass transfer model for a solid particle dryer must be able to predict solid flowrate and solid hold-up. There have been several reports in the literature regarding the modelling aspects of solid transport in dryers. If the model is developed for model-based control, it must be simple and yet represent dynamics of the system accurately. This paper addresses solid motion modelling and the effects of different variables involved in solid transport phenomena. Sugar drying process is the case study in this work. A steady state semi-empirical model was modified to predict solid hold-up and flowrate in rotary dryers. This model was incorporated into a heat and mass transfer model ;o predict solid moisture and temperature for inferential and model-based control purposes. Results of several experiments that have been used to investigate dynamics of the system in terms of solid motion and to validate the model are also presented. The approach advocated in this paper is directly applicable to the transport of other solids in rotary drum equipment and can thus be regarded as a generalized model.     

A SIMPLE DYNAMIC MODEL FOR SOLID TRANSPORT IN ROTARY DRYERS

ABSTRACT

The solid particle movement in a rotary drum plays an important role in drying processes. The solid distribution in the drum affects the amount of contact surface between the solid and the gas. The retention time of solids influences the time particles can stay in contact with the gas in order to transfer heat and mass. Any heat and mass transfer model for a solid particle dryer must be able to predict solid flowrate and solid hold-up. There have been several reports in the literature regarding the modelling aspects of solid transport in dryers. If the model is developed for model-based control, it must be simple and yet represent dynamics of the system accurately. This paper addresses solid motion modelling and the effects of different variables involved in solid transport phenomena. Sugar drying process is the case study in this work. A steady state semi-empirical model was modified to predict solid hold-up and flowrate in rotary dryers. This model was incorporated into a heat and mass transfer model ;o predict solid moisture and temperature for inferential and model-based control purposes. Results of several experiments that have been used to investigate dynamics of the system in terms of solid motion and to validate the model are also presented. The approach advocated in this paper is directly applicable to the transport of other solids in rotary drum equipment and can thus be regarded as a generalized model.     

Modeling and simulation of drying characteristics on flexible filamentous particles in rotary dryers

Experiments were conducted to demonstrate the effects of the drum wall temperature on the heat and mass transfer in rotary dryers. The drying characteristics of flexible filamentous particles in rotary dryers were further explored. In addition, the inlet and outlet temperatures and moisture contents of granular particles were measured. As a result, the good agreement between the simulations and experiments verified the rationale and feasibility of the numerical method. Therefore, the approach was adopted to evaluate the temperature and moisture content of wet granular particles in a rotary dryer in different conditions, for instance, drum wall temperature and rotational speed. The results revealed that the higher drum wall temperature led to hotter particles with lower outlet moisture content. Conversely, the higher rotational speed resulted in cooler particles with higher outlet moisture content due to the decrease of residence time in the rotary dryer.     

Further Theoretical Studies on Rotary Drying Processes Represented By Distributed Systems

Mathematical tools for studying panicle transpon in rotary drying and cooling processes are developed in this paper. In contrast to conventional approaches aimed at deriving empirical or xmi-empirical correlations, a rigorous mathematical analysis which employs dilferential calculus and analytical geometry is emphasis4 in the current research. These developments allow accurale computations of solid flowrate, retention time and particle holdup in rotary dryers with arbilrary flight configurations. Consequently, optimal dryer configuration design in terms of drum dimension, flight number and geometry can be achieved through a better understanding of the mathematical insight of rotary drum performance.

Techniques developed using this method are applied to the distributed parameter model eslablished earlier by the authors (Wang el al., 1993) to replace out-dated correlations for the determination of retention Lime and solid holdup. As a result of the new developments, the distributed parameter approach to the dynamics of rotary drying processes becomes more general and more reliable.     

Modeling of Fertilizer Drying in Roto-Aerated and Conventional Rotary Dryers

Conventional rotary dryers are equipped with flights placed parallel along the length of the shell to promote a rain of solids across the dryer section. In the roto-aerated dryer the hot air flows through the particles that run on the bottom of the drum through a series of mini-pipes and there is no cascading. This study analyzed heat and mass transfer modeling between the air and the fertilizer particles in conventional rotary and roto-aerated dryers, as well as the simulation results with the experimental data. A good agreement between the simulated and experimental results was obtained for the two rotary dryer configurations analyzed.     

Further Theoretical Studies on Rotary Drying Processes Represented By Distributed Systems

ABSTRACT

Mathematical tools for studying panicle transpon in rotary drying and cooling processes are developed in this paper. In contrast to conventional approaches aimed at deriving empirical or xmi-empirical correlations, a rigorous mathematical analysis which employs dilferential calculus and analytical geometry is emphasis4 in the current research. These developments allow accurale computations of solid flowrate, retention time and particle holdup in rotary dryers with arbilrary flight configurations. Consequently, optimal dryer configuration design in terms of drum dimension, flight number and geometry can be achieved through a better understanding of the mathematical insight of rotary drum performance.

Techniques developed using this method are applied to the distributed parameter model eslablished earlier by the authors (Wang el al., 1993) to replace out-dated correlations for the determination of retention Lime and solid holdup. As a result of the new developments, the distributed parameter approach to the dynamics of rotary drying processes becomes more general and more reliable.     

Aerodynamic Separation and Fractional Drying of Alfalfa Leaves and Stems—A Review and New Concept

This article examines the state-of-the art on aerodynamic separation and drying of leaves and stems. Relevant aerodynamic and drying characteristics of alfalfa leaves and stems, important in the design and functional performance evaluation of appropriate drying and separation systems, are presented. General features and design parameters of rotary drum dryers are discussed. A new efficient approach to combined drying and separation in a rotary drum dryer is described in which fresh or pre-wilted alfalfa mixture is dried at a moderate temperature, and in the same operation the dry leaf fraction is aerodynamically separated from the stem fraction. Preliminary test data obtained from the dryer indicated that the separated product stream had comparatively high leaf purity, confirming the feasibility of the new approach.     

A physical description of solids transport in flighted rotary dryers

In this paper, a solids transport model for flighted rotary dryers is presented. Emphasis is placed on combining statistical and mechanistic modelling efforts to create a physically motivated compartment model involving pairs of perfectly mixed tank reactors linked in a series arrangement. Here, each tank pair, or cell, aims to physically describe a finite slice of a case study industrial rotary sugar dryer, and is hence governed primarily by flight geometry and dryer operational variables such as rotational speed and dryer inclination. Solids flow paths are structured to properly represent the different modes of transport in the rotary dryer, and values of transport coefficients are based on calculated rates of rotational and axial flows. A solids dispersion variable is used to correlate the model residence time distribution (RTD) prediction with available data from a tracer study conducted during industrial operation of a full-scale raw sugar dryer. RTD results from the model show intuitive responses to variations in solids feed rate, rotational speed and drum inclination.     

Aerodynamic Separation and Fractional Drying of Alfalfa Leaves and Stems—A Review and New Concept

Abstract

This article examines the state-of-the art on aerodynamic separation and drying of leaves and stems. Relevant aerodynamic and drying characteristics of alfalfa leaves and stems, important in the design and functional performance evaluation of appropriate drying and separation systems, are presented. General features and design parameters of rotary drum dryers are discussed. A new efficient approach to combined drying and separation in a rotary drum dryer is described in which fresh or pre-wilted alfalfa mixture is dried at a moderate temperature, and in the same operation the dry leaf fraction is aerodynamically separated from the stem fraction. Preliminary test data obtained from the dryer indicated that the separated product stream had comparatively high leaf purity, confirming the feasibility of the new approach.     

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.     

几种常用干燥设备的简捷计算方法

为简化干燥设备的设计步骤,根据工业生产实践和经验公式,提出了流化床、振动流化床、内热流化床、惰性粒子流化床、离心喷雾干燥、压力喷雾干燥、旋转闪蒸干燥及气流干燥8种常用干燥设备的主体尺寸的简捷设计方法,并介绍了所有干燥设备都适用的基础计算。所述简捷计算采用了许多来自生产实践的经验数据,对设计工业应用的干燥器有重要的指导作用。除了设备主体尺寸之外,尚有诸多结构尺寸需要确定,设计者需根据自己的经验或参考更为详尽的资料才能完成。     

THE DESIGN OF FLIGHTS IN CASCADING ROTARY DRYERS

In designing cascading rotary dryers, it is important to size the flights correctly in order to ensure that they can adequately accommodate the contents of the drum. A set of equations has been derived to calculate the solids holdup on angular and extended-circular flights in such dryers. Three examples of the practical use of these equations are presented. These include estimation of the design holdup of the dryer, selection of the number of flights, and exploration of the flight cascade characteristics.     

THE DESIGN OF FLIGHTS IN CASCADING ROTARY DRYERS

In designing cascading rotary dryers, it is important to size the flights correctly in order to ensure that they can adequately accommodate the contents of the drum. A set of equations has been derived to calculate the solids holdup on angular and extended-circular flights in such dryers. Three examples of the practical use of these equations are presented. These include estimation of the design holdup of the dryer, selection of the number of flights, and exploration of the flight cascade characteristics.     

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.     

DETERMINATION OF RESIDENCE TIME OF GRAINS IN DRUM DRYER

On the basis of the theoretical analysts of the forces applied to grains and moving locus of grains in rotary drum dryer, residence time and contact heating time of grains in the dryer were calculated. The values of calculation conformed to the measured values. These results supplied reliable foundation in theory and practice for further studying and improving the dryer. They also provided designers with references to construct similar type of dryers.     

Dynamic Characteristics of Solids Transportation in Rotary Dryers

An original method was proposed for the determination of the mean residence time in a continuous dryer, based on the step-change in the solids feed rate. The method has been validated through experiments performed in a pilot-scale rotary dryer. The effect of the solids flow rate, gas flow rate, dryer rotation speed, and dryer slope was quantified. Several design correlations to predict the residence time in rotary dryers were critically evaluated, and a new, more accurate correlation was derived.     

Dynamic Characteristics of Solids Transportation in Rotary Dryers

Abstract

An original method was proposed for the determination of the mean residence time in a continuous dryer, based on the step-change in the solids feed rate. The method has been validated through experiments performed in a pilot-scale rotary dryer. The effect of the solids flow rate, gas flow rate, dryer rotation speed, and dryer slope was quantified. Several design correlations to predict the residence time in rotary dryers were critically evaluated, and a new, more accurate correlation was derived.     

DETERMINATION OF RESIDENCE TIME OF GRAINS IN DRUM DRYER

Abstract

On the basis of the theoretical analysts of the forces applied to grains and moving locus of grains in rotary drum dryer, residence time and contact heating time of grains in the dryer were calculated. The values of calculation conformed to the measured values. These results supplied reliable foundation in theory and practice for further studying and improving the dryer. They also provided designers with references to construct similar type of dryers.