Simulation of Wood Particle Motion Through a Concurrent Triple-Pass Rotary Dryer

The retention time of solids in a drum is an important parameter for the design of rotary dryers, since it directly influences the mass and heat transfer rates. If it is too short, the wood particles do not become adequately dried. If it is too long, they become over-dried. Therefore, having an appropriate retention time is useful in terms of both energy and plant capacity. Wood particle mean retention time in a rotary dryer is affected by several variables, such as dryer dimensions, solid characteristics, and operational parameters. The purpose of this work was to simulate the effects of some wood particle characteristics and operational parameters on the mean retention time, drum holdup, and velocity of the wood particles during drying in a pilot-scale, closed-loop, triple-pass rotary dryer by means of a computer code. The simulation results of wood particle motion can be used for modeling, design, and optimization of closed-loop, triple-pass rotary dryers.     

FLIGHT DESIGN I N ROTARY DRYERS

The effectiveness of the rotary drum drying process depends primarily on the contact between the cascading particles and the drying gases within the drum. This paper considers first the factors which contribute to the cascade pattern and which influence the design of the lifting flights which distribute the particles in the top half of the drum. A generalised calculation design procedure for flights is developed and described in detail for the case of Equal Horizontal Distribution (EHD) flights. It is reasoned that the EHD flights, which have an equal distribution of particles across the horizontal diameter of the rotary drum dryer, give the optimum distribution in the context of the heat and mass transfer operations of the rotary drying process. An industrial design which approximates the optimum flight design is suggested.     

MATHEMATICAL MODELLING OF MDF FIBRE DRYING: DRYING OPTIMISATION

In the production of MDF, wet resinated fibre must be dried to its target moisture content, normally 9 to 11%, before compaction into a board by hot pressing. Fibre drying can be interpreted as an incorporated process involving gas-solid two phase-flow, inter-component transfer, and heat and mass transfer within the fibre. Based on these mechanisms, a mathematical model has been developed to simulate the MDF fibre drying process. From the model, fibre moisture content, air temperature and air humidity along the dryer length can be predicted and factors affecting the drying rate examined. The model can be employed to optimise drying conditions and to evaluate improvements in dryer design. A case study of drying improvement in reduction of dryer emissions and heat consumption is given to demonstrate the potential application of the developed dryer model.     

MATHEMATICAL MODELLING OF MDF FIBRE DRYING: DRYING OPTIMISATION

ABSTRACT

In the production of MDF, wet resinated fibre must be dried to its target moisture content, normally 9 to 11%, before compaction into a board by hot pressing. Fibre drying can be interpreted as an incorporated process involving gas-solid two phase-flow, inter-component transfer, and heat and mass transfer within the fibre. Based on these mechanisms, a mathematical model has been developed to simulate the MDF fibre drying process. From the model, fibre moisture content, air temperature and air humidity along the dryer length can be predicted and factors affecting the drying rate examined. The model can be employed to optimise drying conditions and to evaluate improvements in dryer design. A case study of drying improvement in reduction of dryer emissions and heat consumption is given to demonstrate the potential application of the developed dryer model.     

FLIGHT DESIGN I N ROTARY DRYERS

ABSTRACT

The effectiveness of the rotary drum drying process depends primarily on the contact between the cascading particles and the drying gases within the drum. This paper considers first the factors which contribute to the cascade pattern and which influence the design of the lifting flights which distribute the particles in the top half of the drum. A generalised calculation design procedure for flights is developed and described in detail for the case of Equal Horizontal Distribution (EHD) flights. It is reasoned that the EHD flights, which have an equal distribution of particles across the horizontal diameter of the rotary drum dryer, give the optimum distribution in the context of the heat and mass transfer operations of the rotary drying process. An industrial design which approximates the optimum flight design is suggested.     

STEAM DRYING OF WOOD CHIPS IN PNEUMATIC CONVEYING DRYERS

A model for a pneumatic conveying dryer is presented. Although the main emphasis is put on superheated steam drying of wood chips, it can be used for other porous materials as well

The model includes a comprehensive two-dimensional model for the drying of single wood chips which accounts for the main physical mechanisms occurring in wood during drying. The external drying conditions in a pneumatic conveying dryer were calculated by applying the mass, heat and momentum equations for each incremental step in dryer length. A plug flow assumption was made for the dryer model and the single particle and dryer models were solved in an iterative manner. The non-spherical nature of wood chips were accounted for by measuring the drag and heat transfer coefficients

Model calculations illustrate the complex interactions between steam, particles and walls which occur in a flash dryer. The drying rate varies in a very complex manner through the dryer. The internal resistance to mass transfer becomes very important in The drying of less permeable wood species such as spruce. Two effects were observed as the particle size was increased: firstly the heat transfer rate decreased, and secondly the residence time increased. To some extent, these effects compensate for each other, however, the net result is that larger chips have a higher final moisture content.     

IMPROVING MDF FIBRE DRYING OPERATION BY APPLICATION OF A MATHEMATICAL MODEL

Fibre drying is an important process in production of medium density fibreboard (MDF) which consumes a large amount of energy, affects product quality and, without appropriate control, causes environmental concerns. Based on fundamental knowledge of wood fibre-water relationships and heat/mass transfer, a mathematical model has been developed to simulate the MDF fibre drying processes. The model is able to predict fibre moisture content, air temperature and air humidity along the dryer length. After validation against the measured air temperature and humidity, the model has been extended to include both fibre drying and fibre conditioning, the latter occurring in the dry fibre conveyers. Due to potential benefits in reducing emissions of volatile organic compounds (VOCs) and in improving panel quality, lower drying temperatures are more desirable than higher temperatures. However, in order to achieve the target moisture content after drying, a higher air velocity is needed or a second-stage dryer is added. The model was employed to determine the air velocity required and to assist in designing a second dryer for further drying and recovery of moist vapour and heat. A further study was undertaken to investigate fibre drying or fibre conditioning in the fibre conveyers and, once again, the fibre drying model was used to determine the air conditions.     

IMPROVING MDF FIBRE DRYING OPERATION BY APPLICATION OF A MATHEMATICAL MODEL

Fibre drying is an important process in production of medium density fibreboard (MDF) which consumes a large amount of energy, affects product quality and, without appropriate control, causes environmental concerns. Based on fundamental knowledge of wood fibre-water relationships and heat/mass transfer, a mathematical model has been developed to simulate the MDF fibre drying processes. The model is able to predict fibre moisture content, air temperature and air humidity along the dryer length. After validation against the measured air temperature and humidity, the model has been extended to include both fibre drying and fibre conditioning, the latter occurring in the dry fibre conveyers. Due to potential benefits in reducing emissions of volatile organic compounds (VOCs) and in improving panel quality, lower drying temperatures are more desirable than higher temperatures. However, in order to achieve the target moisture content after drying, a higher air velocity is needed or a second-stage dryer is added. The model was employed to determine the air velocity required and to assist in designing a second dryer for further drying and recovery of moist vapour and heat. A further study was undertaken to investigate fibre drying or fibre conditioning in the fibre conveyers and, once again, the fibre drying model was used to determine the air conditions.     

STEAM DRYING OF WOOD CHIPS IN PNEUMATIC CONVEYING DRYERS

ABSTRACT

A model for a pneumatic conveying dryer is presented. Although the main emphasis is put on superheated steam drying of wood chips, it can be used for other porous materials as well

The model includes a comprehensive two-dimensional model for the drying of single wood chips which accounts for the main physical mechanisms occurring in wood during drying. The external drying conditions in a pneumatic conveying dryer were calculated by applying the mass, heat and momentum equations for each incremental step in dryer length. A plug flow assumption was made for the dryer model and the single particle and dryer models were solved in an iterative manner. The non-spherical nature of wood chips were accounted for by measuring the drag and heat transfer coefficients

Model calculations illustrate the complex interactions between steam, particles and walls which occur in a flash dryer. The drying rate varies in a very complex manner through the dryer. The internal resistance to mass transfer becomes very important in The drying of less permeable wood species such as spruce. Two effects were observed as the particle size was increased: firstly the heat transfer rate decreased, and secondly the residence time increased. To some extent, these effects compensate for each other, however, the net result is that larger chips have a higher final moisture content.     

A DISTRIBUTED PARAMETER APPROACH TO THE DYNAMICS OF ROTARY DRYING PROCESSES

A nonequilibrium distributed parameter model for rotary drying and cooling processes described by a set of partial differitial equations with nonlinear algebraic constraints is developed in this work. These equations arise from the multi-phase heat and mass balances on a typical rotary dryer. A computational algorithm is devekped by employing a polynonial approximation ( orthogonal collocation) with a glotal splinc technique leading to a differential-algebraic equation ( DAE) system. The numerical solution is carried out by using a standard DAE solver.

The two- phase-flow heat transfer coelficient is computed by introducing a correction factor to the commonly accepted correlations. Since interaction between the falling particles are considered in the correction factor,the results are more reliable than those computed by assuming that heat transfer between a single falling particle and the drying air is unaffected by other particles. The heat transfer computations can be further justified via a study on the analogies between heat and mass transfer.

The general model devloped in this work is mathematically more ritorous yet more flexible that the lumped parameter models established by one of the authors (Douglas et al., (1993)). The three major assumptions of an equilibrium operation, perfect mixing and constant drying raic, are removed in the distributed parameter model.

The simulation results are compared with the operational data from an industrial sugar dryer and predictions from earlier models. The model and algorithm successfully predict the steady state behaviour of rotary dryers and collers. The generalized model can be applied to fertilizer drying processes in which the assumption of constant drying rate is no longer valid and the existing dynamic models are not applicable.     

Mathematical Modeling of Rotary Drying of Woody Biomass

Woody biomass in the form of forestry industry residues has been recognized as a promising resource for renewable energy and liquid fuels. Drying of the woody biomass is one of the key operations in development of the energy conversion technologies. Rotary drying is an effective method due to the enhanced contact between the solids and the drying medium (hot air). In this work, a mathematical model was developed to simulate the drying of the woody biomass as chips in a rotary dryer, based on energy and mass balance and transfer, experimental drying kinetics of the wood chips, and using literature correlations for the residence time. A new correlation between the theoretical maximum drying rate and the actual constant drying rate for the wood chips was obtained from the drying experiments, which was incorporated in the drying model. The model was applied both for cocurrent and countercurrent rotary dryers, and the simulation results are consistent with the observed trend. However, the accuracy of the model needs to be further investigated through experimental validation of the residence time correlation.     

Mathematical Modeling and Simulation of Drying Using Two Industrial Concurrent and Countercurrent Rotary Dryers for Ammonium Nitrate

Drying of ammonium nitrate (AN) is accomplished in the Shiraz Petrochemical Complex (SPC) using a concurrent rotary dryer following a countercurrent rotary dryer. A mathematical model for these rotary dryers including heat and mass transfer was developed. The model was checked against industrial-scale data, which showed a good agreement. The average absolute deviation of the simulation results compared to the industrial data for the concurrent dryer was 4.0% for solids moisture, 1.3% for solids temperature, and 1.8% for air temperature and for the countercurrent dryer it was 9.0% for the solids moisture, 2.0% for solids temperature, and 4.6% for air temperature. These simulation results reveal that for outlet solid moisture, inlet AN moisture, and air temperature as well as the outlet temperature of product, the inlet solid and air temperature have major effects for both concurrent and countercurrent flow.     

Superheated Steam Drying of Single Wood Particles: Modeling and Comparative Study with Hot Air Drying

A deterministic model is developed to describe the superheated steam drying process of single wood particles. A comparison between calculated data and experimental observations infers that the moisture‐dependent effective diffusivity is suitable to be used for beechwood material drying. To reduce the computational cost of the deterministic drying model, a semi‐empirical model is proposed within the framework of a reaction engineering approach (REA). The validity of the proposed model is checked by comparing against experimental data from literature. The experimental drying behavior may fairly be reflected by the reduced model. Due to the simplicity and predictive ability of the REA model, this semi‐empirical model can be implemented to describe heat and mass transfer between a population of single particles and a drying agent in dryer models.     

A DISTRIBUTED PARAMETER APPROACH TO THE DYNAMICS OF ROTARY DRYING PROCESSES

ABSTRACT

A nonequilibrium distributed parameter model for rotary drying and cooling processes described by a set of partial differitial equations with nonlinear algebraic constraints is developed in this work. These equations arise from the multi–phase heat and mass balances on a typical rotary dryer. A computational algorithm is devekped by employing a polynonial approximation ( orthogonal collocation) with a glotal splinc technique leading to a differential–algebraic equation ( DAE) system. The numerical solution is carried out by using a standard DAE solver.

The two– phase–flow heat transfer coelficient is computed by introducing a correction factor to the commonly accepted correlations. Since interaction between the falling particles are considered in the correction factor,the results are more reliable than those computed by assuming that heat transfer between a single falling particle and the drying air is unaffected by other particles. The heat transfer computations can be further justified via a study on the analogies between heat and mass transfer.

The general model devloped in this work is mathematically more ritorous yet more flexible that the lumped parameter models established by one of the authors (Douglas et al., (1993)). The three major assumptions of an equilibrium operation, perfect mixing and constant drying raic, are removed in the distributed parameter model.

The simulation results are compared with the operational data from an industrial sugar dryer and predictions from earlier models. The model and algorithm successfully predict the steady state behaviour of rotary dryers and collers. The generalized model can be applied to fertilizer drying processes in which the assumption of constant drying rate is no longer valid and the existing dynamic models are not applicable.     

Modeling the Drying of Grass Seeds (Brachiaria brizantha) in Fluidized Beds

A modified three-phase model is developed to simulate the drying of Brachiaria brizantha in fluidized beds. In this new model, the constitutive equation of drying kinetics is formulated including both the constant rate and the falling rate mechanisms; the seed shrinkage is taken into account during all drying operation and the transition between bubbling to slugging regime is delineated for estimating the bubble velocity and size. Such modifications improve the mathematical model to better simulate the drying of coarse particles in fluidized beds. The best estimation of the five adjustable model parameters, which are required to define heat and mass transfer mechanisms between interstitial gas and seed particles and to specify the heat loss from dryer walls to ambient air, is attained by incorporating an optimization routine into the computer model program. Having been specially designed to supply data for this model, experiments are performed in a bath laboratory-scale fluidized bed. Additional data are generated to validate the model and program routines. Results show a good agreement between simulated and experimental data, validating the approach used to describe drying kinetics and particle shrinkage.     

The Drying of Sludge in a Cyclone Dryer Using Computational Fluid Dynamics

A control volume-based technique implemented in FLUENT (ANSYS Inc., Canonsburg, PA) computational fluid dynamics (CFD) package was applied along with the kinetic theory of granular flow (KTGF) to simulate the flow pattern and heat and mass transfer processes for sludge material in a large-scale cyclone dryer. The drying characteristics of sludge at the dryer inlet were obtained from a previous study on the drying of sludge in a large-scale pneumatic dryer. User-defined subroutines were added to extend FLUENT's capability to account for mixture properties and to simulate the constant and falling rate drying periods. The convective heat and mass transfer coefficients were modeled using published correlations for Nusselt and Sherwood numbers. Sensitivity analysis was conducted to determine the effect of gas-phase velocity and temperature on the final product outcome. Numerical predictions for the multiphase flow hydrodynamics showed a highly diluted region in the dryer core and a higher concentration of particles close to the wall region, an indication of nonuniform distribution of particles at a cross-sectional area. The numerical predictions for the hydrodynamic profiles qualitatively depicted the flow behavior natural to these designs. The work demonstrated the successful application of CFD in the design stage of a combined pneumatic-cyclone dryer model.     

Effects of Drying Parameters on Heat Transfer during Drying of Fermented Ground Cassava in a Rotary Dryer

The effects of drying parameters on heat transfer during drying of fermented ground cassava in a rotary dryer were studied. The fermented ground cassava was dried in a bench-scale rotary dryer at different inlet air temperatures, inlet air velocities, relative humidities, feed rates, drum drive speeds, and feed drive speeds. It is shown that inlet air temperature, inlet air velocity, and feed rate have significant effects on the specific heat transfer coefficient and heat load in the material. Models that predict the specific heat transfer coefficient as a function of inlet air temperature and inlet air velocity and the heat load as a function of inlet air temperature, inlet air velocity, and feed rate are also presented. Predictions of the models are compared with experimental data and good agreement is obtained.     

Modeling the Fractional Drying and Aerodynamic Separation of Alfalfa into Leaves and Stems in a Rotary Dryer

A computerized mathematical model was developed to predict fractional drying and aerodynamic separation of alfalfa into leaves and stems in one process in a rotary dryer. Aerodynamic separation was characterized by separation efficiency or the total amount of the desired component (leaf or stem) recovered, relative to the amount entering the process, and by purity of leaf or stem component collected at exit ports of the dryer. The model development assumed 100% separation efficiency and purity. The model was validated by comparing model predicted results with measured experimental and field test data obtained from a small industrial rotary dryer and a full-scale industrial dryer. Changes in leaf, stem, and drying gas moisture contents and temperatures were measured and predicted by the model under various drying conditions. The model-predicted results agreed well with measured data. The model was also used to simulate the performance of industrial rotary dryers under various operating conditions. The model can be used to determine the optimum drying and aerodynamic separation parameters. It can also be used to design and redesign new and existing industrial rotary dryers in order to combine drying and aerodynamic separation into one process.     

Modeling the Fractional Drying and Aerodynamic Separation of Alfalfa into Leaves and Stems in a Rotary Dryer

A computerized mathematical model was developed to predict fractional drying and aerodynamic separation of alfalfa into leaves and stems in one process in a rotary dryer. Aerodynamic separation was characterized by separation efficiency or the total amount of the desired component (leaf or stem) recovered, relative to the amount entering the process, and by purity of leaf or stem component collected at exit ports of the dryer. The model development assumed 100% separation efficiency and purity. The model was validated by comparing model predicted results with measured experimental and field test data obtained from a small industrial rotary dryer and a full-scale industrial dryer. Changes in leaf, stem, and drying gas moisture contents and temperatures were measured and predicted by the model under various drying conditions. The model-predicted results agreed well with measured data. The model was also used to simulate the performance of industrial rotary dryers under various operating conditions. The model can be used to determine the optimum drying and aerodynamic separation parameters. It can also be used to design and redesign new and existing industrial rotary dryers in order to combine drying and aerodynamic separation into one process.     

Heat and Mass Transfer in an Agitated Contact-Convective Heated Dryer

For the investigation of the drying process of a pharmaceutical fermentation waste and for determining specific heat and mass transfer coefficients an agitated contact-convective heated dryer was constructed. This dryer is also suitable for drying of other granular solids with high moisture content. Hence we investigated the drying of a by-product from bio-ethanol production, as well. The pilot-plant agitated dryer makes possible continuous measurement and data-acquisition. Data-acquisition of heated wall temperature, inlet and outlet air temperatures and humidity, mass reduction of the material makes possible the determination of transfer coefficients by the heat and mass balance of the dryer. The measured heat and mass transfer coefficients serve as proper input parameters for the simulation calculations.