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.     

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.     

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.     

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.     

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

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)     

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 STOCHASTIC APPROACH TO THE DRYING OF SOLIDS IN A MULTI-STAGE FLUIDIZED BED

The residence times of solids flowing through a fluidized bed dryer exhibit dispersion about the mean. In this paper, expressions for the probability density functions of solids moisture content in the various stages of a multi-stage dryer are derived. A simple recurrence relationship for the moments of the distribution is also presented. The analysis is applied to the drying of cereal grains, and it is shown that the degree of drying increases with the number of stages in the dryer. Probability density functions of the moisture content are presented.     

A STOCHASTIC APPROACH TO THE DRYING OF SOLIDS IN A MULTI-STAGE FLUIDIZED BED

The residence times of solids flowing through a fluidized bed dryer exhibit dispersion about the mean. In this paper, expressions for the probability density functions of solids moisture content in the various stages of a multi-stage dryer are derived. A simple recurrence relationship for the moments of the distribution is also presented. The analysis is applied to the drying of cereal grains, and it is shown that the degree of drying increases with the number of stages in the dryer. Probability density functions of the moisture content are presented.     

MATHEMATICAL MODELING OF DIFFUSION-CONTROLLED PNEUMATIC-CONVEYING DRYERS

A mathematical model for a continuous pneumatic-conveying dryer has been developed for removing internally bound moisture from solid particulates. The dryer relies on a recirculating carrier gas stream for entrainment. Drying is carried out by injecting into the gas loop a fresh stream of conditioned drying gas while an equal amount of wet gas is vented out. Because pneumatic-conveying dryers usually employ huge gas velocities, the particulates are well dispersed in the gas. Therefore, for solids absent of surface moisture, the drying kinetics is controlled by intraparticle diffusion. The mode! developed based on the diffusion mechanism relates the moisture reduction in the solids to various process parameters (diffusiv-ity, partition coefficient, particle size, residence time, solids loading, drying gas usage, and carrier gas recirculating rate), and is fully predictive. Therefore, it can be used to study the effects of these variables. The model was compared with the plant data and found to match the data within ± 15%.     

SIMULATION OF FORESTRY BIOMASS DRYING IN A ROTARY DRYER

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.     

对撞流干燥的实验与理论研究

The experiments of one-stage semi-circular and two-stage semi-circular impinging stream drying as well as the vertical and semi-cricular combined impinging stream drying were carried out.The velocity distribution and the mean residence time of particles,and the influence of various factors on drying characteristics were studied.A mathematical model of granular material drying in a semi-circular impinging stream dryer was proposed,in which the flow characteristics as well as the heat and mass transfer mechanisms were considered.Reasonable numerical methods were used to solve the equations.Under various conditons,the calculated results of drying rate and moisture content versus time were obtained.The results indicate that constant drying rate period does not exist in a semi-circular impinging stream dryer.Appropriate semi-cricular stage number and curvature radius,flow-rate ratio,air velocity,and higher inlet air temperature should be used for enhancing the drying process.     

Modelling and Simulation of a Direct Contact Rotary Dryer

A mathematical model able to predict solid and drying gas temperature and moisture content axial profiles along a direct contact rotary dryer was developed. The study was focused on the drying kinetics based on phenomenological models. Two different drying mechanisms in the decreasing drying rate period were tested: proponional to the unbound moisture content and moisture diffusion inside the particle. Experimental data collected in a pilot-scale direct contact rotary dryer was used to validate the model. Soya and fish meals were used as drying material.     

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.     

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.     

Modelling and Simulation of a Direct Contact Rotary Dryer

ABSTRACT

A mathematical model able to predict solid and drying gas temperature and moisture content axial profiles along a direct contact rotary dryer was developed. The study was focused on the drying kinetics based on phenomenological models. Two different drying mechanisms in the decreasing drying rate period were tested: proponional to the unbound moisture content and moisture diffusion inside the particle. Experimental data collected in a pilot-scale direct contact rotary dryer was used to validate the model. Soya and fish meals were used as drying material.     

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.     

DRYING OF PULP AND PAPER SLUDGE IN A PULSED FLUID BED DRYER

Hydrodynamics and drying kinetics for the pulp and paper primary sludge dried in a pulsed fluid bed dryer with relocated air stream are presented. Batch experiments have indicated that drying of disintegrated sludge to the required 12% moisture content takes place during the first drying period at practically constant material temperature close to the wet bulb temperature with respect to the inlet air conditions. Equations were developed for pressure drop, minimum pulsed-fluidization velocity, dynamic bed height, and volumetric mass transfer coefficient. Continuous experiments under drying conditions determined from the average residence time concept have confirmed that transportation of disintegrated sludge along the dryer follows the plug flow model.     

CFD Modeling of Microwave-Assisted Fluidized Bed Drying of Moist Particles Using Two-Fluid Model

The drying behavior of moist spherical particles in a microwave-assisted fluidized bed dryer was simulated. The two-fluid Eulerian model incorporating the kinetic theory of granular flow was applied to simulate the gas–solid flow. The simulations were carried out using the commercial computational fluid dynamics (CFD) package Fluent 6.3.26. The effects of different levels of microwave power densities as well as initial gas temperature on the prediction of solids moisture content, gas temperature, and gas absolute humidity were investigated. The effect of microwaves was incorporated into calculations using a concatenated user-defined function (UDF). The simulation results were compared with experimental data obtained from drying of soybeans in a pilot-scale microwave-assisted fluidized bed dryer and reasonable agreement was found. The mean relative deviation for prediction of solids moisture content, gas temperature, and gas absolute humidity were less than 3, 10, and 5%, respectively. Further work is needed to validate the proposed model for large-scale plants.     

Efficient <Emphasis Type="Italic">in situ</Emphasis> drying of low rank coal in a pressurized down-flow flash dryer

Flash drying of low rank coal with synthesis gas was addressed by using a pressurized down-flow dryer. The proposed method is a potential approach to secure gaseous water that is required in coal processing by utilizing moisture in the low rank coal. The drying process was promoted by increasing the initial temperature of the synthesis gas as a drying medium and decreasing the particle size of the coal. The moisture removal rate of the coal using synthesis gas at 9 bars and 500 °C reached up to 97% within ten seconds. Although it is a higher temperature than that of fixed bed or moving bed dryer, outlet moisture laden synthesis gas had the low level of tar enough to be a feedstock of downstream catalytic process due to the short residence time in the dryer. The chemical composition changes of the coal during the drying resulted in reducing oxygen content to the atomic ratio of oxygen to carbon as 0.1 and enhancing its calorific value. Disappearance of hydroxyl functional group from the surface and physical reduction of the surface area of the coal decreased the moisture re-adsorption capacity, which could prevent the spontaneous combustion of the low rank coal.