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.     

SOLIDS TRANSPORTATION MODEL OF AN INDUSTRIAL ROTARY DRYER

ABSTRACT

A complete simulation model has been developed for an industrial rotary dryer to account for the heat and mass exchange between the solids and the gas. This simulator is mainly composed of three models: solids transportation model, furnace model, and gas model. The solids transportation model is the modified Cholette-Cloutier model It consists of a series of interactive reservoirs which are subdivided into an active and a dead compartments to account for the characteristic extended tail of the residence time distribution (RTD) curves observed in industrial dryers.

To expand the validity of the model, experiments have been performed in an industrial rotary dryer to obtain RTD curves under different mineral concentrate and gas flow rates. This paper describes these experiments and presents the variation of the average residence time and model parameters as function of solids and gas flow rates.     

COMPARISON OF RESIDENCE TIME MODELS FOR CASCADING ROTARY DRYERS

ABSTRACT

The predictions of the models of Matchett and Baker (1988) Saeman and Mitchell (1954) and Friedman and Marshall (1949) for the solids residence time in rotary dryers have been compared with both pilot-scale and industrial-scale data. A countercurrent pilot-scale dryer of 0.2m diameter and 2m long has been used with air velocities up to 1.5 ms^?1 to measure the residence times of sorghum grain, The average discrepancy for the solids residence time between the predictions and the experiments that were carried out in the pilot-scale rotary dryer is — 10.4% Compared with the models of Friedman and Marshall (1949) and Saeman and Mitchell (1954) for the pilot-scale data obtained here, the Matchett and Baker model is more satisfactory for predicting the solids residence time in this pilot-scale dryer. It has also been found that the model of Matchett and Baker describes the industrial data of Saeman and Mitchell (1954) than the correlation of Friedman and Marshall (1949).     

INDIRECT THERMAL DRYING OF LIGNITE: DESIGN ASPECTS OF A ROTARY DRYER

ABSTRACT

An investigation of the thermal drying of lignite has been carried out, by using an indirect heat pilot rotary drum. The process aims at the production of dry lignite and clean steam as part of a gasification procedure. Both flighted and bare drum modes have been employed. Temperature profiles along the dryer length, the amount of evaporation (moisture conversion) and the solids residence time distribution (RTD) were measured. A non-isothermal model was tested under three different regimes of solids flow. Model integration, by taking account of experimental amount of evaporation at dryer exit and temperature profiles along the dryer length, has been utilized in the validation of drying kinetics and heat transfer correlations. Model predictions compare satisfactorily with the operating data of an indirect heat industrial lignite dryer. Overall heat transfer coefficients of the pilot rotary dryer were found to agree well with those reported for direct heat dryers.     

Heat Transfer in Food Cooling Applications by Ibrahim Dinter Taylor and Francis publishers, 1900 Frost Road,Suite 101, Bristol,PA 19007 1997, 399 pages

Abstract

Prediction of residence time in rotary dryers is useful for equipment design as well as for the right selection of operation conditions in order to obtain the optimal unit functioning.

An empirical relationship for residence time estimation that may be applied when the dryer slope is zero or different from it is presented in this work. It was derived from experimental testing carried out with biological origin particulated solids (fish and soya meal, sawdust) and from mineral origin (sand) in a pilot rotary dryer. The fitting degree of the experimental data to those predicted by reported equations was also established here.     

INFLUENCE OF SOLIDS MOISTURE CONTENT ON THE AVERAGE RESIDENCE TIME IN A ROTARY DRYER

The solids mean residence time in a rotary dryer is influenced by several variables such as dryer dimensions and solids characteristics. One of these characteristics, usually not taken into account in correlations proposed to estimate the mean residence time, is the solids feed moisture content. Although it is well known that the solids moisture content has a major impact on the ability of the solids to move along the rotary dryer, it does not enter as a parameter in available correlations. In this investigation, numerous experiments were performed in a pilot-scale rotary dryer to study the influence of solids moisture content and drying gas temperature on the mean residence time. Sand employed in cement makeup was used to perform these experiments. Results show that the mean residence time for a moisture content in the range of 8% to 12% is four times higher than for dry solids. The moisture content and the drying gas temperature influence significantly the shape of the residence time distribution curve.     

THE DEVELOPMENT AND VALIDATION OF A SYSTEM MODEL FOR A COUNTERCURRENT CASCADING ROTARY DRYER

ABSTRACT

An overall system model for a countercurrent rotary dryer has been developed with the ullimale aim of assessing controller pairings in these dryers. This model is based on heat and mass balances within dryer regions combined with two subsidiary models, one describing the equipment (which determines particle transport and heat transfer)and the other describing the behaviour of the material (the drying kinetics). Six partial differential equations have been set up to evaluate six state variables: solids moisture content, solids temperature, gas humidity, gas temperature, solids holdup and gas holdup as functions of time and rotary dryer length. A control-volume method has been used to reduce the six partial differential equations with respect to time and the length of the rotary dryer to six ordinary differential equations in time.

The drying model has been implemented in the SPEEDUP flowsheeting package (with FORTRAN subroutines) The model has been validated by fifteen experiments-in a pilot scale countercurrent-flow rotary dryer (0.2m in diameter and 2m in length)     

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.     

SOLIDS TRANSPORTATION MODEL OF AN INDUSTRIAL ROTARY DRYER

A complete simulation model has been developed for an industrial rotary dryer to account for the heat and mass exchange between the solids and the gas. This simulator is mainly composed of three models: solids transportation model, furnace model, and gas model. The solids transportation model is the modified Cholette-Cloutier model It consists of a series of interactive reservoirs which are subdivided into an active and a dead compartments to account for the characteristic extended tail of the residence time distribution (RTD) curves observed in industrial dryers.

To expand the validity of the model, experiments have been performed in an industrial rotary dryer to obtain RTD curves under different mineral concentrate and gas flow rates. This paper describes these experiments and presents the variation of the average residence time and model parameters as function of solids and gas flow rates.     

A model for solids transport in flighted rotary dryers based on physical considerations

The following paper outlines the development of empirically fitted and pseudo-physically derived compartment models of a flighted rotary dryer with counter current airflow processing solid material. Underloaded, overloaded and design-loaded dryers are considered. Four key parameters are estimated to fit the empirical model to industrial residence time distribution data and common empirical mean residence time/holdup correlations. Utilising the fundamental structure of the empirical model and physical and mechanical properties such as the dryer and flight geometry as well as solids material properties, the number of estimated parameters in the pseudo-physical model was reduced to 2. The experimental data required to characterise and validate the models is discussed. Optimisations to determine the model parameters were undertaken by comparison with an experimental residence time distribution curve for an industrial dryer processing sugar. Simulation of the model using gPROMS^® illustrates model performance. The potential to integrate the solids transport model and a full heat and mass transfer model is also discussed.     

SIMULATION OF FORESTRY BIOMASS DRYING IN A ROTARY DRYER

ABSTRACT

Drying of forestry biomass in a rotary dryer has been performed. The raw material used was Erica Arborea belonging to the ever-green, broad leaves ecosystem which covers Central Greece and other Mediterranean countries. The study was part of a project concerning a Greek biomass pyrolysis demonstration plant where drying of biomass is very important in the contribution to the global energy balance and product yields of pyrolysis.

The study includes two parts. First, the experimental part concerns the influence of air flowrate, temperature, rotation speed and inclination of a laboratory rotary dryer to biomass residence time and biomass outlet moisture content. The second part concerns the development of a mathematical model for biomass drying in a rotary dryer. Experimental measurements in a rotary dryer were compared to the data from the model, in order to check the validity of the model.     

INDIRECT THERMAL DRYING OF LIGNITE: DESIGN ASPECTS OF A ROTARY DRYER

An investigation of the thermal drying of lignite has been carried out, by using an indirect heat pilot rotary drum. The process aims at the production of dry lignite and clean steam as part of a gasification procedure. Both flighted and bare drum modes have been employed. Temperature profiles along the dryer length, the amount of evaporation (moisture conversion) and the solids residence time distribution (RTD) were measured. A non-isothermal model was tested under three different regimes of solids flow. Model integration, by taking account of experimental amount of evaporation at dryer exit and temperature profiles along the dryer length, has been utilized in the validation of drying kinetics and heat transfer correlations. Model predictions compare satisfactorily with the operating data of an indirect heat industrial lignite dryer. Overall heat transfer coefficients of the pilot rotary dryer were found to agree well with those reported for direct heat dryers.     

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.     

COMPARISON OF RESIDENCE TIME MODELS FOR CASCADING ROTARY DRYERS

The predictions of the models of Matchett and Baker (1988) Saeman and Mitchell (1954) and Friedman and Marshall (1949) for the solids residence time in rotary dryers have been compared with both pilot-scale and industrial-scale data. A countercurrent pilot-scale dryer of 0.2m diameter and 2m long has been used with air velocities up to 1.5 ms^-1 to measure the residence times of sorghum grain, The average discrepancy for the solids residence time between the predictions and the experiments that were carried out in the pilot-scale rotary dryer is — 10.4% Compared with the models of Friedman and Marshall (1949) and Saeman and Mitchell (1954) for the pilot-scale data obtained here, the Matchett and Baker model is more satisfactory for predicting the solids residence time in this pilot-scale dryer. It has also been found that the model of Matchett and Baker describes the industrial data of Saeman and Mitchell (1954) than the correlation of Friedman and Marshall (1949).     

变径炭素回转窑内物料平均停留时间的计算

在前人厚度变化公式基础上推导出适用于窑尾扩径和增设翻料装置的炭素回转窑内物料平均停留时间的计算公式,同时建立了实验室回转窑模型,进行了冷态实验。将物料停留时间的计算值与变径回转窑模型实验测试值作了比较。结果表明,公式计算值与实验测试值比较吻合,误差范围在-15％～12％内,并且计算中不需要引入修正系数;回转窑转速和斜度的增加均使物料平均停留时间缩短,而窑尾挡料圈高度的增加可以使平均停留时间缩短,也可以使平均停留时间延长。     

Hydrodynamic Characteristics of a Pilot-Scale Screw Conveyor Dryer

This article presents results of an experimental study on the flow characteristics of a pilot-size screw conveyor dryer (SCD). In particular, the effects of granular solids flow rate (15.2-206 kg/h) and screw speed (10.8-28 rpm) on the residence time distribution (RTD) were studied using sand as the model material. The RTD was measured using a dynamic step change in the solids flow rate. Parameters such as solids holdup, degree of fullness, mean residence time, and uniformity of the discharge flow were studied. The solids holdup and hence the degree of fullness was found to decrease with increase in the screw speed and decrease with the solids flow rate. The mean residence time was found to decrease with an increase in the screw speed, as expected. The screw speed and the solids flow rate strongly affected the discharge uniformity. An optimum value of the degree of fullness was observed with regard to the solids flow rate. Over the operating range examined, the solids flow pattern was close to plug flow, as indicated by high values of N and Pe number.     

Hydrodynamic Characteristics of a Pilot-Scale Screw Conveyor Dryer

This article presents results of an experimental study on the flow characteristics of a pilot-size screw conveyor dryer (SCD). In particular, the effects of granular solids flow rate (15.2–206 kg/h) and screw speed (10.8–28 rpm) on the residence time distribution (RTD) were studied using sand as the model material. The RTD was measured using a dynamic step change in the solids flow rate. Parameters such as solids holdup, degree of fullness, mean residence time, and uniformity of the discharge flow were studied. The solids holdup and hence the degree of fullness was found to decrease with increase in the screw speed and decrease with the solids flow rate. The mean residence time was found to decrease with an increase in the screw speed, as expected. The screw speed and the solids flow rate strongly affected the discharge uniformity. An optimum value of the degree of fullness was observed with regard to the solids flow rate. Over the operating range examined, the solids flow pattern was close to plug flow, as indicated by high values of N and Pe number.     

A Funtadamental Study on Particle Transport Through Rotary Dryers for Flight Design and System Optimisation

ABSTRACT

A model for particle transport in a flighted horizontal rotary dryer is developed in this paper. Mathematical principles applied to the current study are in the areas of differential calculus and analytical geomentry. In contrast to the conventional approaches which are either based on mpirical/semi-empirical correlations or obtained from the investieation of single particle trajectories, this paper develops rigorous mathematical analysis of the transport of bulk solids. A variety of important issues in rotary drying, such as axial flowrate of solids, retention time distribution and solid holdup are addressed and treated by using non-traditional methods. Since the model takes dimension, number and geometry of flights into account, it possesses the following two haracteristics : (1) it is not only useful in the study of rotary drying dynamics, but lso applicable to other processes employing flighted rotating cylinders (such as granulation drumsand crushers) and (2) based on the model, an optimal drum configuration can be designed by using optimisation techniques. The model can be incorporated within a distributed arameter dryer model developed previously to form a more rigorous integrated dynamic model. A heoretical foundation for optimal flight design by using the current model is explained.

A pilot scale perspex rotary dryer equipped with a video camera has been constructed and used for model validation. Raw sugar was handled in the experiments. Particle transport was observed and measured by using a flow visualisation technique supplemented with traditional sampling methods. A significant model quality improvement has been observed through a comparative study between the newly developed model and conventional ones.     

EXPERIMENTAL STUDY OF RESIDENCE TIME IN A DIRECT ROTARY DRYER

Prediction of residence time in rotary dryers is useful for equipment design as well as for the right selection of operation conditions in order to obtain the optimal unit functioning.

An empirical relationship for residence time estimation that may be applied when the dryer slope is zero or different from it is presented in this work. It was derived from experimental testing carried out with biological origin particulated solids (fish and soya meal, sawdust) and from mineral origin (sand) in a pilot rotary dryer. The fitting degree of the experimental data to those predicted by reported equations was also established here.     

Modeling of Sewage Sludge Flow in a Continuous Paddle Dryer

A model based on the theory of Markov chains has been developed to represent the residence time distribution (RTD) of municipal sewage sludge in a continuous paddle dryer. The flow of dry solids is described by a chain of n perfectly mixed cells, n corresponding to the number of paddles attached to the shaft. The transition probabilities between the cells are governed by two parameters: the parameter of internal recirculation, R, and the solids hold-up, Hu. In the absence of available correlation, both parameters are identified by fitting the model to experimental RTD data. The model demonstrates its ability to describe the sludge flow in a continuous lab-scale paddle dryer. A sensitivity analysis highlights that R is critical for the treatment uniformity while Hu controls the mean residence time and thus the final moisture content.