Modeling of Moisture and Temperature Changes of Wheat during Drying in a Triangular Spouted Bed Dryer

A mathematical model of temperature and wheat moisture content distribution inside a triangular spouted bed dryer was developed. The model is based on analysis of heat and mass transfer inside the dryer. In addition to that, an empirical bulk density model has been developed for wheat and included in the drying simulation. A laboratory-scale triangular spouted bed (TSB) dryer was used to dry wheat grain to validate the model. The dryer was divided into three sections, namely spouting, downcomer, and fountain. A series of drying runs were conducted to record moisture and temperature profile. There were two distinct regions observed during wheat drying. A constant rate period was observed during the initial drying stage and the falling rate period took place at the later drying stage. Initial moisture content and operating drying temperature governed the timing of transition from constant rate period to falling rate period. The model can be used to accurately predict the moisture content of wheat during drying. The temperature prediction inside the TSB dryer was less accurate, especially at high temperatures due to heat losses in the experimental dryer. Further studies are needed to improve the accuracy of this model, especially with regard to the temperature prediction.     

DYNAMIC ANALYSIS OF CONTINUOUS WELL-MIXED FLUIDIZED/SPOUTED BED DRYERS

A diffusion-based mathematical model is presented for the prediction of the dynamics of drying in continuous well-mixed fluidized/spouted beds. Numerical techniques are used to solve the model equations. The outlet solids moisture content, the outlet air humidity and solids temperature are predicted as a function of time for the falling rate drying period. The model is helpful in describing the drying process during the startup periods and in studying open loop behavior of drying process. The model is also useful in designing control system for fluidized/spouted bed dryers.     

A MATHEMATICAL MODEL FOR CONTINUOUS DRYING OF GRAINS IN A SPOUTED BED DRYER

ABSTRACT

A mathematical model for a continuous spouted bed dryer has been presented to predict moisture content, air and grain temperatures as well as energy consumption. To better understand the interactive influence of processes in each region of the spouted bed, solution schemes for the spout and downcomer were treated separately. The behavior of dryer was investigated experimentally and found that the dryer behaved differently from an ideal plug flow. The drying rate as simulated by the model is almost constant during grain movement in the dryer. Absence of airflow in the downcomer leads to a tempering process that takes place in the downcomer while intense heat and mass transfer occurs mainly in the spout due to the high airflow rate there. Furthermore, by considering the predicted grain temperature history as one of the indicators of product quality, one can, in principle, design appropriate successive processes in a continuous spouted bed dryer to minimize product damage.     

A MATHEMATICAL MODEL FOR CONTINUOUS DRYING OF GRAINS IN A SPOUTED BED DRYER

A mathematical model for a continuous spouted bed dryer has been presented to predict moisture content, air and grain temperatures as well as energy consumption. To better understand the interactive influence of processes in each region of the spouted bed, solution schemes for the spout and downcomer were treated separately. The behavior of dryer was investigated experimentally and found that the dryer behaved differently from an ideal plug flow. The drying rate as simulated by the model is almost constant during grain movement in the dryer. Absence of airflow in the downcomer leads to a tempering process that takes place in the downcomer while intense heat and mass transfer occurs mainly in the spout due to the high airflow rate there. Furthermore, by considering the predicted grain temperature history as one of the indicators of product quality, one can, in principle, design appropriate successive processes in a continuous spouted bed dryer to minimize product damage.     

A Quasi-stationary Approach to Drying Kinetics of Fluidized Particulate Materials

A modified quasi-stationary method has been proposed to describe drying kinetics of particulate materials dried in convective dryers with active hydrodynamic regimes. Both our own results and literature data were used. These include sliced celery, cranberry, diced carrot, wheat and polystyrene granules dried in four types of dryers: pulsed fluid bed dryer, fluid bed dryer with a mixer, spouted bed dryer with a draft tube, and vortex dryer. The method was validated by comparing experimental data with results of modeling in terms of a reduced moisture content and material temperature. A new form of the generalized drying curve has been proposed with the reduced time and the index of hydrodynamic intensity as parameters. The equations developed can be used to calculate the total drying time and determine the temporal variation of the moisture content and material temperature.     

Corn,Rice, and Wheat Seed Drying by Two-Stage Concept

Corn, rice, and wheat seeds with an initial moisture content (IMC) of 20–25% wb were dried to moisture content below 18% wb at 40–80°C in a fluidized bed dryer (FBD) and spouted bed dryer (SBD) and the seeds with IMC 18% wb were dried to below 14% wb at air temperatures 18–30°C and relative humidity 60–70% by an in-store dryer (ISD). As a result, it appears that a two-stage drying concept is feasible in drying high-moisture-content seeds due to the high germination rate of dried seeds. Nonetheless, the drying temperature must be carefully selected. A drying temperature of 40°C was clearly safe for all samples, whereas more than 90% of wheat seeds still germinated after drying at 60°C in FBD. Furthermore, drying seeds with IMC 18% wb by ISD was safe under specified drying conditions.     

Drying of industrial sludge waste in a conical spouted bed dryer. Effect of air temperature and air velocity

To determine the performance of a conical spouted bed dryer for the drying of sludge waste, an experimental study of drying in a spouted bed regime was performed under different experimental conditions. The drying performance was determined based on the time evolution of solid moisture content, and the influence of operating conditions (inlet air temperature, air flow rate, and bed mass) on the drying rate of sludge waste in spouted beds of a conical geometry was analyzed.     

SPOUTED AND SPOUT-FLUIDIZED BEDS FOR GRAM DRYING

The objectives of this work are to analyze the drying performance of conical-cylindrical spouted bed (CSB) dryers for three different grains (rice, corn and wheat), and to compare the drying efficiency of CSB dryers with that of spout-fluid bed (SFB) dryers. A PC-program was developed for: (I) -optimization of the CSB dryer dimensions; (2) -simulation of drying grains in the optimized CSB dryer (including start-up period); and, (3) -analysis of the drying performance in a similar SFB dryer.

The liquid diffusion model is used to describe the falling rate drying period. Semi-empirical correlations available in the literature as well as information obtained in the authors' laboratory for spouted and spout-fluidized beds of grains are used to describe the aerodynamic parameters.

The results are presented in terms of the size of the dryer, energy consumption, air handling requirement, drying characteristics etc for different drying conditions. The drying effeciency in a CSB is compared with that in a similar SFB for different grain feed rates and drying temperatures.     

SPOUTED AND SPOUT-FLUIDIZED BEDS FOR GRAM DRYING

The objectives of this work are to analyze the drying performance of conical-cylindrical spouted bed (CSB) dryers for three different grains (rice, corn and wheat), and to compare the drying efficiency of CSB dryers with that of spout-fluid bed (SFB) dryers. A PC-program was developed for: (I) -optimization of the CSB dryer dimensions; (2) -simulation of drying grains in the optimized CSB dryer (including start-up period); and, (3) -analysis of the drying performance in a similar SFB dryer.

The liquid diffusion model is used to describe the falling rate drying period. Semi-empirical correlations available in the literature as well as information obtained in the authors' laboratory for spouted and spout-fluidized beds of grains are used to describe the aerodynamic parameters.

The results are presented in terms of the size of the dryer, energy consumption, air handling requirement, drying characteristics etc for different drying conditions. The drying effeciency in a CSB is compared with that in a similar SFB for different grain feed rates and drying temperatures.     

Experimental Study and Mathematical Modeling of Green Pea Drying in a Spouted Bed

In this study, green pea drying is investigated experimentally in a laboratory-scale spouted bed dryer. A mathematical model is also developed to investigate the effect of operating conditions on the performance of the system. The effect of operating parameters such as inlet air temperature, particle size, and flow rate of the drying air on the performance of the dryer are studied experimentally. In order to build the process model, it is necessary to analyze the transport in both solid and gas phases. A complete set of equations with no adjustable parameters is derived for existing zones in the spouted bed dryer in order to predict variations in the temperature and moisture content of the solid and gas phases with time for batch drying conditions. Model results are compared with corresponding experimental data. Agreement between the model results and experimental data is good.     

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.     

COMBINED MICROWAVE AND SPOUTED BED DRYING OF DICED APPLES: EFFECT OF DRYING CONDITIONS ON DRYING KINETICS AND PRODUCT TEMPERATURE

ABSTRACT

The influence of microwave power (0 to 8.0 W/g, dry basis) and hot air temperature (25°C to 95 °C) on drying rate and product temperature of diced apples (from 31 % to 5% moisture content, dry basis) in a laboratory microwave and spouted-bed combined dryer was investigated. Product temperature initially increased sharply to a plateau about 12 to 15°C above the spouted bed air temperature at a microwave input power 6.4 W/g. This temperature remained almost constant thereafter. Uniform microwave heating was achieved as evidenced by uniform product color and product temperature. Drying rates increased with increasing spouted-bed air temperature or microwave power level, But higher microwave power caused more darkening of the product. Drying of the diced apples in the microwave and spouted bed drying system exhibited two falling rates periods. The influence of air temperature on effective moisture diffusivity followed an Arrhenius type equation. The activation energies were 23.7 kJ/mol and 26.7 kJ/mol for the first and second falling rate periods, respectively.     

COMBINED MICROWAVE AND SPOUTED BED DRYING OF DICED APPLES: EFFECT OF DRYING CONDITIONS ON DRYING KINETICS AND PRODUCT TEMPERATURE

The influence of microwave power (0 to 8.0 W/g, dry basis) and hot air temperature (25°C to 95 °C) on drying rate and product temperature of diced apples (from 31 % to 5% moisture content, dry basis) in a laboratory microwave and spouted-bed combined dryer was investigated. Product temperature initially increased sharply to a plateau about 12 to 15°C above the spouted bed air temperature at a microwave input power 6.4 W/g. This temperature remained almost constant thereafter. Uniform microwave heating was achieved as evidenced by uniform product color and product temperature. Drying rates increased with increasing spouted-bed air temperature or microwave power level, But higher microwave power caused more darkening of the product. Drying of the diced apples in the microwave and spouted bed drying system exhibited two falling rates periods. The influence of air temperature on effective moisture diffusivity followed an Arrhenius type equation. The activation energies were 23.7 kJ/mol and 26.7 kJ/mol for the first and second falling rate periods, respectively.     

A TWO-REGION MATHEMATICAL MODEL FOR BATCH DRYING OF GRAINS IN A TWO-DIMENSIONAL SPOUTED BED

A two-region non-equilibrium model of a two-dimensional spouted bed dryer has been developed. The model, based on heat and mass transfer interactions between the spout and downcomer regions, predicts the air and grain temperatures and moisture content throughout each elementary thin layer of grain in the spouted bed. The model was validated with the experimental results on batch drying of corn. Drying and heating processes are shown to occur in both the spout and downcomer regions. Overall, the agreement between experimental and simulated results is satisfactory.     

A TWO-REGION MATHEMATICAL MODEL FOR BATCH DRYING OF GRAINS IN A TWO-DIMENSIONAL SPOUTED BED

A two-region non-equilibrium model of a two-dimensional spouted bed dryer has been developed. The model, based on heat and mass transfer interactions between the spout and downcomer regions, predicts the air and grain temperatures and moisture content throughout each elementary thin layer of grain in the spouted bed. The model was validated with the experimental results on batch drying of corn. Drying and heating processes are shown to occur in both the spout and downcomer regions. Overall, the agreement between experimental and simulated results is satisfactory.     

Diffusion-Controlled Batch Drying of Particles in a Novel Rotating Jet Annular Spouted Bed

Results are presented for batch drying of rewetted wheat (used as model particles) in a newly developed annular spouted bed in which the drying air jet rotates slowly thus spouting bed intermittently. For particles drying in the falling rate period intermittent spouting is shown to result in significant energy savings with small increase in the drying time. A finite element simulation is employed to evaluate the effective moisture diffusivity of the test material which is nonspherical in geometry. Further, advantages of variable spouting and heating of the spouting air are demonstrated by a parametric study of the simulation model.     

Study of model application for drying of pulp fruit in spouted bed with intermittent feeding and accumulation

In the present study, we applied and improved a model to describe the behavior of a spouted bed dryer with intermittent feeding for suspension drying, considering suspension accumulation inside the device. This model is important in macroscopic heat and mass balances, represented by a system of ordinary differential balances, implemented and resolved numerically using Fortran routines, in which the influence of process variables on drying dynamics were assessed. The results obtained in modeling and mathematical simulations were compared with experimental data, indicating that the model fits the process well, based on outlet air temperature and moisture.     

Diffusion-Controlled Batch Drying of Particles in a Novel Rotating Jet Annular Spouted Bed

Abstract

Results are presented for batch drying of rewetted wheat (used as model particles) in a newly developed annular spouted bed in which the drying air jet rotates slowly thus spouting bed intermittently. For particles drying in the falling rate period intermittent spouting is shown to result in significant energy savings with small increase in the drying time. A finite element simulation is employed to evaluate the effective moisture diffusivity of the test material which is nonspherical in geometry. Further, advantages of variable spouting and heating of the spouting air are demonstrated by a parametric study of the simulation model.     

An Evaluation of Factors Influencing the Energy-Efficient Operation of Well-Mixed Fluidized Bed Dryers

This article describes the results of calculations of specific energy consumption, E_s, performed on a well-mixed fluidized bed dryer simulator. Exhaust air temperature-humidity loci required to yield a specified outlet moisture content were also determined. Most of the calculations related to solids whose drying rate was gas-film controlled. Six model drying curves were employed to examine the effects of drying rate and hygroscopicity in addition to the normal operating parameters. The results indicated that E_s was highest for slow-drying hygroscopic solids and lowest for fast-drying, non-hygroscopic solids. Specific energy consumption increased with decreasing bed temperature and outlet moisture content and with increasing heat loss but was independent of solids loading and airflow rate. For both the aforementioned solids and a much slower drying material (wheat), there was close agreement between the zero heat loss data and a single theoretical curve approximating the performance of an ideal adiabatic dryer. Distinct differences between the behavior of well-mixed and plug flow fluidized bed dryers are reported.     

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.