Drying characteristics of a draft tube spouted bed

Using the results of earlier work on the flow pattern of gas and solids in a draft-tube spouted bed, the drying characteristics of such a spouted bed are discussed. Paddy (unpeeled rice) was dried in a 30 cm bed with a 5.5 cm draft tube and temperatures and moisture content of solid and gas phase were measured as a function of drying time at various points in the system. It appears that the (constant) drying rate is determined by the heat transfer in the recirculation zone (just above the air inlet) and in the draft tube. After the short and rapid heating in the tube, the temperature and moisture gradients in the particle equilibrate in the annulus. It is concluded that a draft-tube spouted bed is a promising dryer design for heat-sensitive particles with slow intra-particle mass transfer coefficients.     

Determination of PVC powder drying kinetics at particle scale: Experimental study and modeling

An original experimental method is used to determine drying kinetic at particle scale. The particle scale kinetics was obtained by immersion of a small mass of wet polyvinyl chloride (PVC) particles (cake) in a batch dense fluidized bed containing inert hot particles (glass bead). The results are summarized here and prove clearly that the PVC drying is controlled by a competition between internal and external transfers. The drying kinetic was described by a particle scale model taking into account the convective–diffusive (mass transfer) and the convective–evaporative (heat transfer) phenomena. To validate this model with the experimental data, the experimental fluidized bed dryer is modeled following two different approaches: a perfect stirred reactor model and a 3D numerical simulation using the multiphase flow code NEPTUNE_CFD. The aim of this 3D simulation is to simulate the phenomena occurring, at local scale, in a dense fluidized bed dryer and to show the limitations of the perfect stirred reactor model.     

Two-Fluid, Two-Dimensional Model for Pneumatic Drying

Pneumatic drying is a widely used process in the chemical industries and includes simultaneous conveying and heat and mass transfer between the particles and the heat gas. The increase in the use of this unit operation requires reliable mathematical models to predict processes in the industrial facilities. In the present study a Two-Fluid model has been used for modeling the flow of particulate materials through pneumatic dryer. The model was solved for a two-dimensional steady-state condition and considering axial and radial profiles for the flow variables. A two-stage drying process was implemented. In the first drying stage, heat transfer controls evaporation from the saturated outer surface of the particle to the surrounding gas. At the second stage, the particles were assumed to have a wet core and a dry outer crust; the evaporation process of the liquid from a particle is assumed to be governed by diffusion through the particle crust and by convection into the gas medium. As evaporation proceeds, the wet core shrinks while the particle dries. The numerical procedure includes discretization of calculation domain into torus-shaped final volumes, solving conservation equations by implementation of the SIMPLE (Semi-Implicit Method for Pressure-Linked Equations) algorithm and controls over coupling of phases by IPSA (Interphase Slip Algorithm). The developed model was applied to simulate a drying process of wet PVC particles in a large-scale pneumatic dryer and to a drying process of wet sand in a laboratory-scale pneumatic dryer. The numerical solutions are compared successfully with the results of independent numerical and experimental investigations. Following the model validation, the two-dimensional distributions of the flow characteristics were examined.     

Two-Fluid,Two-Dimensional Model for Pneumatic Drying

Abstract

Pneumatic drying is a widely used process in the chemical industries and includes simultaneous conveying and heat and mass transfer between the particles and the heat gas. The increase in the use of this unit operation requires reliable mathematical models to predict processes in the industrial facilities. In the present study a Two-Fluid model has been used for modeling the flow of particulate materials through pneumatic dryer. The model was solved for a two-dimensional steady-state condition and considering axial and radial profiles for the flow variables. A two-stage drying process was implemented. In the first drying stage, heat transfer controls evaporation from the saturated outer surface of the particle to the surrounding gas. At the second stage, the particles were assumed to have a wet core and a dry outer crust; the evaporation process of the liquid from a particle is assumed to be governed by diffusion through the particle crust and by convection into the gas medium. As evaporation proceeds, the wet core shrinks while the particle dries. The numerical procedure includes discretization of calculation domain into torus-shaped final volumes, solving conservation equations by implementation of the SIMPLE (Semi-Implicit Method for Pressure-Linked Equations) algorithm and controls over coupling of phases by IPSA (Interphase Slip Algorithm). The developed model was applied to simulate a drying process of wet PVC particles in a large-scale pneumatic dryer and to a drying process of wet sand in a laboratory-scale pneumatic dryer. The numerical solutions are compared successfully with the results of independent numerical and experimental investigations. Following the model validation, the two-dimensional distributions of the flow characteristics were examined.     

TWO-FLUID MODEL FOR PNEUMATIC DRYING OF PARTICULATE MATERIALS

The Two-Fluid model has been used for modeling the flow of particulate materials through pneumatic dryer. The model was solved for a one-dimensional steady-state condition and was applied to the drying process of wet PVC particles in a large-scale pneumatic dryer and to the drying process of wet sand in a laboratory-scale pneumatic dryer. A two-stage drying process was implemented. In the first drying stage, heat transfer controls evaporation from the saturated outer surface of the particle to the surrounding gas. At the second stage, the particles were assumed to have a wet core and a dry outer crust; the evaporation process of the liquid from a particle assumed to be governed by diffusion through the particle crust and by convection into the gas medium. As evaporation proceeds, the wet core shrinks while the particle dries. The drying process is assumed to stop when the moisture content of a particle falls to a predefined value or when the particle riches the exit of the pneumatic dryer. Our developed model was solved numerically and two operating conditions, adiabatic and given pneumatic dryer wall temperature, were simulated. Comparison between the prediction of the numerical models of Rocha and DryPak, (Pakowski, 1996), which were presented by Silva and Correa (1998), with the prediction of our numerical simulation reviled better agreements with DryPak then with the models of Rocha. The results of the developed model were also compared with experimental results of Baeyens et al. (1995) and Rocha.     

TWO-FLUID MODEL FOR PNEUMATIC DRYING OF PARTICULATE MATERIALS

The Two-Fluid model has been used for modeling the flow of particulate materials through pneumatic dryer. The model was solved for a one-dimensional steady-state condition and was applied to the drying process of wet PVC particles in a large-scale pneumatic dryer and to the drying process of wet sand in a laboratory-scale pneumatic dryer. A two-stage drying process was implemented. In the first drying stage, heat transfer controls evaporation from the saturated outer surface of the particle to the surrounding gas. At the second stage, the particles were assumed to have a wet core and a dry outer crust; the evaporation process of the liquid from a particle assumed to be governed by diffusion through the particle crust and by convection into the gas medium. As evaporation proceeds, the wet core shrinks while the particle dries. The drying process is assumed to stop when the moisture content of a particle falls to a predefined value or when the particle riches the exit of the pneumatic dryer. Our developed model was solved numerically and two operating conditions, adiabatic and given pneumatic dryer wall temperature, were simulated. Comparison between the prediction of the numerical models of Rocha and DryPak, (Pakowski, 1996), which were presented by Silva and Correa (1998), with the prediction of our numerical simulation reviled better agreements with DryPak then with the models of Rocha. The results of the developed model were also compared with experimental results of Baeyens et al. (1995) and Rocha.     

Experimental and numerical analysis of oil shale drying in fluidized bed

The main objective of this work was to experimentally and numerically investigate the Liu Shu River oil shale drying by the means of flue gas in a fluidized bed dryer. Several experiments were performed under different temperatures conditions. The moisture content of oil shale was measured during the experiments. The two-stage drying model was incorporated in computational fluid dynamics (CFD) package FLUENT via user-defined functions (UDF) and utilized for simulation of heat and mass transfer of oil shale drying in the fluidized bed dryer. The simulation results for solid moisture content agreed well with experimental data. The effects of the temperature and velocity of flue gas, initial bed height, and the particle size on the drying characteristics were predicted and analyzed. It is shown that the gas temperature and velocity are the important parameters in the whole drying process. The particle size has more obvious influence in the falling drying period than the constant drying period. The temperatures of gas and solid phases were monitored. It is shown that the so-called “near gas distributor zone” is the most effective heat transfer zone, which agrees well with the calculated value. The system quickly reached thermal equilibrium, characterizing a nearly isothermal bed. The developed model provides a very good demonstration to describe the oil shale drying in the fluidized bed dryer, and may provide important information for design, optimization of operation conditions.     

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.     

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.     

Modelling and simulation of heat and mass transfer in a packed bed of solid particles having high diffusion resistance

The heat and mass transfer processes between gas phase and solid particles are studied in a fixed bed throughflow dryer. The particles have a moderate size but high heat and mass transfer resistances. To describe the drying process, simple cell model approach is used. The transport resistances of the particles are taken into account by imbedding a Luikov-type distributed parameter model into each cell.

The relationships of this model to the cell model with lumped variables for the particles, as well as to the continuous two phase model are analysed. Numerical investigations show that moisture diffusion taking place inside the particles can control the drying process even at moderate Biot numbers.     

Online measurement and control of solids moisture in fluidised bed dryers

Currently, two main methods are used to take online measurement of the solids moisture in fluidised bed dryers, namely microwave resonance and near infrared spectroscopy. In this paper, a new online approach to solids moisture measurement of batch fluidised bed dryers by electrical capacitance tomography (ECT) is presented for the first time. Based on online measurement of solids moisture, it is possible to implement feedback control and process optimisation of batch fluidised bed drying processes, aiming to increase the operation efficiency and to improve product quality. A twin-plane ECT sensor with eight electrodes in each plane is mounted in the bottom of a glass fluidisation chamber. From the adjacent electrode pairs, the water content of the solids is estimated based on the correlation between the moisture content and the permittivity value. To reduce measurement error, the effect of temperature on moisture measurement is compensated. The fluidisation velocity is estimated by a semi-empirical function based on the measured water content. The acquired information is sent to a controller to adjust the air flow rate of the fluidised bed dryer. To validate the moisture measurement by ECT, a mathematical model has been developed, based on the measured temperature and relative humidity of the outlet air. The Landweber iteration method is applied to reconstruct images. The averaged solids concentration along the radial direction at different fluidisation conditions is given and compared with results by the linear back-projection (LBP) method. Results from batch drying processes with online measurement and feedback control are given and compared with no feedback control. To compare the operation efficiency, the thermal efficiency is considered and the results show the possibility of online control and optimisation of the fluidised bed drying processes, based on online measurement of solids moisture by ECT. Some challenges and future work are discussed.     

Comparison of Moving Bed Dryers of Solids Operating in Parallel and Counterflow Modes

Abstract

The drying rates in moving bed dryers are compared. The gas and the solids to be dried are in parallel flow or counterflow. A new simplified method to simulate the drying in parallel and counterflow moving beds is developed. This model is based on the solution of arbitrary experimental or theoretical drying rate Equations of single solid particles (or thin-layer drying rate equation) coupled with heat and mass conservation Equations of the dryer. The solution is presented in an integral form of the drying equation showing the relation between time or location in the dryer and degree of drying. The method allows rapid calculation of the moisture, vapor mass fraction, and temperature distributions along the dryer in drying with moist air or steam. The model is demonstrated by using a result based on the receding front evaporation model as the specific thin-layer drying equation in the moving bed model. Wood chips are chosen as an example of the substance to be dried, but the method applies also for other medium, if the dependence of the drying rate on moisture and ambient temperature and humidity (thin layer drying rate) is known. The size of the dryer needed to reach the same degree of drying operating in the parallel mode is much greater than that of counterflow type, when the drying medium is air or flue gases. The reason for the poorer drying in parallel flow is mainly the unfavorable distribution of the evaporation temperature. In steam drying, the difference in the size is not so great, since the evaporation takes place approximately at constant temperature.     

Drying granular solids in fluidized bed—I: Description on basis of mass and heat transfer coefficients

In this paper on drying of wet solids in a fluidised bed a model is presented which describes the mass transfer from the solids to a rising bubble for the case that there is no diffusion limitation inside the solids. Two contributions are accounted for: the mass transfer from the dense phase across the cloud boundary and the mass transfer from the solids which are passing the cloud during the rise of the bubble. A similar model is presented for the heat exchange between the bed and the rising bubbles.     

Comparison of Moving Bed Dryers of Solids Operating in Parallel and Counterflow Modes

The drying rates in moving bed dryers are compared. The gas and the solids to be dried are in parallel flow or counterflow. A new simplified method to simulate the drying in parallel and counterflow moving beds is developed. This model is based on the solution of arbitrary experimental or theoretical drying rate Equations of single solid particles (or thin-layer drying rate equation) coupled with heat and mass conservation Equations of the dryer. The solution is presented in an integral form of the drying equation showing the relation between time or location in the dryer and degree of drying. The method allows rapid calculation of the moisture, vapor mass fraction, and temperature distributions along the dryer in drying with moist air or steam. The model is demonstrated by using a result based on the receding front evaporation model as the specific thin-layer drying equation in the moving bed model. Wood chips are chosen as an example of the substance to be dried, but the method applies also for other medium, if the dependence of the drying rate on moisture and ambient temperature and humidity (thin layer drying rate) is known. The size of the dryer needed to reach the same degree of drying operating in the parallel mode is much greater than that of counterflow type, when the drying medium is air or flue gases. The reason for the poorer drying in parallel flow is mainly the unfavorable distribution of the evaporation temperature. In steam drying, the difference in the size is not so great, since the evaporation takes place approximately at constant temperature.     

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.     

Generalized drying curves for porous solids

The purpose of this study is to obtain generalized drying curves which can be used in the prediction of drying times of porous solids in the falling-rate period. The drying model introduced assumes that the movement of moisture in the solid body is by capillary motion, and that the major resistance to heat and mass transfer lies on the boundary layer at the solid surface. It has been applied to the drying of gypsum particles of various sizes in a rotary dryer at different feed rates and gas temperatures. The results show that the deviation between the experimental data and the ones predicted from theory is very small. The deviation increases at low moisture concentrations which may indicate that internal resistance to moisture flow starts to dominate the drying phenomena.     

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.     

SIMULATION OF TRANSPORT DYNAMICS IN FLUIDIZED-BED DRYERS

A mathematical model for predicting three-dimensional, two-phase flow, heat and mass transfer inside fluidized-bed dryers has been developed. The model consists of the full set of partial-differential equations that describe the conservation of mass, momentum and energy for both phases inside the dryer, and is coupled with correlations concerning interphase momentum-, heat-, and mass-transfer.

It is shown that the model can predict the most important engineering aspects of a fluidized-bed dryer including pressure drop, particle holdup, temperature distribution in both phases as well as drying efficiency all over the fluidized-bed. Plug-flow conditions are predicted for the gas phase, while back-mixing is predicted for the particles.

The effect of particle mass-flow-rate on fluidized-bed dryer performance is evaluated. It is shown that the lower the particle mass flow-rate, the more intense the horizontal moisture gradients, while the higher the particle rate the more uniform the moisture distribution throughout the bed.     

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%.