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.     

Experimental and theoretical investigation of shelled corn drying in a microwave-assisted fluidized bed dryer using Artificial Neural Network

Drying characteristics of shelled corn (Zea mays L) with an initial moisture content of 26% dry basis (db) was studied in a fluidized bed dryer assisted by microwave heating. Four air temperatures (30, 40, 50 and 60 °C) and five microwave powers (180, 360, 540, 720 and 900 W) were studied. Several experiments were conducted to obtain data for sample moisture content versus drying time. The results showed that increasing the drying air temperature resulted in up to 5% decrease in drying time while in the microwave-assisted fluidized bed system, the drying time decreased dramatically up to 50% at a given and corresponding drying air temperature at each microwave energy level. As a result, addition of microwave energy to the fluidized bed drying is recommended to enhance the drying rate of shelled corn. Furthermore, in the present study, the application of Artificial Neural Network (ANN) for predicting the drying time (output parameter for ANN modeling) was investigated. Microwave power, drying air temperature and grain moisture content were considered as input parameters for the model. An ANN model with 170 neurons was selected for studying the influence of transfer functions and training algorithms. The results revealed that a network with the Tansig (hyperbolic tangent sigmoid) transfer function and trainrp (Resilient back propagation) back propagation algorithm made the most accurate predictions for the shelled corn drying system. The effects of uncertainties in output experimental data and ANN prediction values on root mean square error (RMSE) were studied by introducing small random errors within a range of ±5%.     

AMARANTH GRAIN DRYING IN A FLUIDIZED BED

ABSTRACT

An empirical equation as a function of drying time and temperature was developed to calculate the moisture ratio required Tor balch fluidized bed dryers design for amaranth grain drying

The relative deviations of the moisture ratio values calculated with the proposed equation with regard to the experimental ones ranged between 0.014 and 0.095 for a drying air temperature between 60 and 100 ° C, and a grain initial moisture content between 23.5 and 16.6 % wb.     

Combined Microwave/Fluidized Bed Drying of Fresh Peppercorns

Abstract

A fluidized bed dryer (FBD) and a combined microwave/fluidized bed dryer (CMFD) are used to dry the fresh ripe peppercorns. The average moisture content vs. elapsed drying time, and drying rate vs. average moisture content are experimentally investigated. It is found that the microwave field from the CMFD can increase the potential of the conventional fluidized bed drying. The drying rates of both dryers are dependent on the inlet air temperature and velocity. For the CMFD, the effects of the air velocity on the drying rate are found to be opposite to our previous results tested with white pepper seeds i.e., the drying rates of the fresh ripe peppercorns decreased with increasing air velocity. By using a CMFD, the drying time required to reach the desired moisture content can be reduced to 80–90% of the drying time required for a FBD at the same drying air temperature and velocity. The color of the product dried by a CMFD is also attractive: it becomes flaming yellow, instead of black as obtained from a FBD. The physical structure of the peppercorn, before and after the drying process is also investigated by a metallurgical macroscope and an image analyzer. Different from drying by a FBD, the external form and matter of the white pepper seed are still maintained, even after passing through the drying process.     

Effect of Fluidized Bed Drying Temperature on Various Quality Attributes of Paddy

Abstract

As reported by many researchers, it was found that fluidized bed paddy drying using high drying air temperatures of over 100°C affected the head rice yield and whiteness of dried rice. However, only a few studies on fluidized bed paddy drying with drying air temperatures below 100°C were so far reported. The main objective of this work was therefore to study the effect of fluidized bed drying air temperature on various quality parameters of Suphanburi 1 and Pathumthani 1 Indica rice. Paddy was dried from the initial moisture contents of 25.0, 28.8, and 32.5% dry basis to 22.5 ± 1.2% dry basis using inlet drying air temperatures between 40 and 150°C at 10°C/step. After fluidized bed drying, paddy was tempered and followed by ambient air aeration until its final moisture content was reduced to 16.3 ± 0.5% dry basis. The results showed that the head rice yield of Suphanburi 1 was significantly related to the inlet drying temperature and initial moisture content whilst there was no significant relationship between the head rice yield, drying temperature and initial moisture content for Pathumthani 1. The whiteness of the two rice varieties was slightly decreased with increase in drying air temperature and initial moisture content. It was also found that the hardness of both cooked rice varieties exhibited insignificant difference (p < 0.05) comparing to rewetted rice, which was gently dried by ambient air aeration in thin layer. The thermal analysis by DSC also showed that partial gelatinization occurred during drying at higher temperatures. Using inlet drying air temperatures in the range of 40–150°C therefore did not affected the quality of cooked rice and paddy. The milling quality of paddy was also well maintained.     

Combined Microwave/Fluidized Bed Drying of Fresh Peppercorns

A fluidized bed dryer (FBD) and a combined microwave/fluidized bed dryer (CMFD) are used to dry the fresh ripe peppercorns. The average moisture content vs. elapsed drying time, and drying rate vs. average moisture content are experimentally investigated. It is found that the microwave field from the CMFD can increase the potential of the conventional fluidized bed drying. The drying rates of both dryers are dependent on the inlet air temperature and velocity. For the CMFD, the effects of the air velocity on the drying rate are found to be opposite to our previous results tested with white pepper seeds i.e., the drying rates of the fresh ripe peppercorns decreased with increasing air velocity. By using a CMFD, the drying time required to reach the desired moisture content can be reduced to 80-90% of the drying time required for a FBD at the same drying air temperature and velocity. The color of the product dried by a CMFD is also attractive: it becomes flaming yellow, instead of black as obtained from a FBD. The physical structure of the peppercorn, before and after the drying process is also investigated by a metallurgical macroscope and an image analyzer. Different from drying by a FBD, the external form and matter of the white pepper seed are still maintained, even after passing through the drying process.     

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.     

DRYTNG OF PEPPER SEEDS USING A COMBINED MICROWAVE/FLUIDIZED BED DRYER

Abstract

The drying rates curves in terms of moisture content versus elapsed drying time for white pepper seeds were obtained experimentally using a fluidized bed and a combined microwave/fluidized bed. The combined microwave/fluidized bed employs a microwave field to assist convective drying. For both procedures, the drying rates were found to be dependent on the inlet air temperature and velocity. Significantly improved drying rates were achieved utilized a combined microwave/fluidized bed drying compared with a conventional fluidized bed.     

Prediction of Moisture Content Evolution in a Grain Batch Dryer Based on On–line Temperature Measurements

ABSTRACT

The time evolution of the moisture content in a grain batch fluidized bed dryer is estimated by means of an on–line non–linear estimator (Marquardt–Levenberg algorithm). The inputs to the estimation algorithm are on–line temperature measurements and the output is the surface moisture content. The surface moisture content is used for predicting the grain moisture content through the solution of a single ordinary differential equation that combines the moisture and energy balances over the dryer. In this way the drying curves are obtained through incorporating in a very simple model easily obtainable physical information of the process.     

Drying Characteristics of Coriander Seed Particles in a Reduced Pressure Superheated Steam Fluidized Bed

Drying characteristics of coriander seed particles were experimentally analyzed in a reduced pressure superheated steam fluidized bed. The typical moisture gain, reported in some other studies during the warm-up period of the process, was reduced in most of the cases by supplying additional heat into the column. The experimental results demonstrated that the drying rate increases and the equilibrium moisture content decreases by increasing the operating temperature. However, variation of the operating pressure (40–67 kPa) and the superficial steam velocity (2.3–4.0 m/s) did not present significant effects on the moisture contents. The degree of superheating was found to be the most important parameter for the process. The experiments also showed that the equilibrium moisture content decreases upon increasing the degree of superheating. Finally, employing a reduced pressure superheated steam fluidized bed appears as an option to carry out drying processes at relatively lower temperatures.     

Attrition of Victorian brown coal during drying in a fluidized bed

Analyzing the attrition of Victorian brown coal during air and steam fluidized bed drying, the change in particle size distribution over a range of initial moisture contents (60% to 0%) and residence times (0 to 60 minutes) was determined. Dried at a temperature of 130°C with a fluidization velocity 0.55 m/s and an initial particle size of 0.5–1.2 mm, both fluidization mediums show a shift in the particle size distribution between three and four minutes of fluidization, with a decrease in mean particle size from 665 µm to around 560 µm. Using differential scanning calorimetry (DSC), the change in particle size has been attributed to the transition between bulk and non-freezable water (approximately 55% moisture loss) and can be linked to the removal of adhesion water, but not to fluidization effects. This is proved through the comparison of air fluidized bed drying, steam fluidized bed drying, and fixed bed drying—the fixed bed drying is being used to determine the particle size distribution as a function of drying. The results show the three drying methods produce similar particle size distributions, indicating that both fluidization and fluidization medium have no impact upon the particle size distribution at short residence times around ten minutes. The cumulative particle size distribution for air and steam fluidized bed dried coal has been modeled using the equation P_d = A₂ + (A₁ ? A₂)/(1 + (d/x₀)^p), with the resultant equations predicting the effects of moisture content on the particle size distribution. Analyzing the effect of longer residence times of 30 and 60 minutes, the particle size distribution for steam fluidized bed dried coal remains the same, while air fluidized bed dried coal has a greater proportion of smaller particles.     

Assessment of new in-bin drying and storage technology for soybean seed

On-farm, in-bin drying and storage of soybean in environments with unconditioned air often result in repeated drying and rewetting of the grains which may have adverse effects on quality metrics; if done using natural air, as recommended for soybean destined to the seed market, the in-bin drying and storage method require operation at well-defined local weather-dependent strategies to maintain the seed quality. This study simulated in-bin drying and storage of soybeans. Different fan control options and drying strategies were used to assess performance in terms of drying duration to target final moisture content (MC), percent over drying, energy expenditure, and drying cost. Fan operation included running the fan continuously, only at night, only during the day, at a set window of equilibrium MC (EMC) of natural air, and set EMC window with supplemental heating of ambient air as an option (EMC-H). Drying and storage performance were tested for soybean at initial moisture content (IMC) (16–22%, wet basis), air flow rate (1.04–5.0?m³?min^?1 [air] t^?1 [soybean]), and harvesting start dates (August 15 to November 15). Simulation model was validated using a bench-scale pressure drop system filled with soybeans with IMC of 22% wet basis. The result shows that fan control strategies, air flow rates, harvest date, and initial MC of the soybeans significantly (P?χ² was 0.88.     

An Experimental Study of Wheat Drying in Thin Layer and Mathematical Simulation of a Fixed-Bed Convective Dryer

Drying of wheat (Algerian cultivar: Hadba03) in thin layers was studied and mass flux phenomenon was used to characterize the thin-layer drying process. Thin-layer drying of wheat was determined for drying air temperature range of 40–60°C, relative humidity of drying air from 10 to 30%, air velocity of 0.7 m/s, and initial grain moisture from 26 to 31% (dry basis). Equilibrium moisture content of wheat was determined using desorption isotherms obtained from the thin-layer drying data. An equilibrium model for a stationary deep bed with drying air moving vertically upward was developed using mass and energy balance between grain and drying air in the bed and drying air characteristics obtained from thin-layer drying experiments. The developed model was validated by drying wheat in a laboratory dryer using different drying air temperatures and initial moisture contents.     

Stationary versus Fluidized-Bed Drying of High-Moisture Paddy with Rest Period

Drying of high-moisture paddy was examined experimentally under stationary and fluidized bed with and without intervening rest periods. Introduction of a rest period between first and second stage of drying improved drying rate and lowered the energy requirement and increased head rice yield. Fluidization further improved the overall drying process. A single-term drying diffusion equation was suitably fitted to first, rest, and second stage drying data of fluidized and stationary bed by applying nonlinear regression method and effective diffusion coefficients were evaluated. During the period of rest stage, paddy grain released a considerable amount of moisture as an effect of residual grain temperature. An appropriate moisture ratio at which resting should start and the length of resting were evaluated by measuring changes in relative humidity in the headspace of mass of paddy and also from the diffusion coefficient values obtained from the experimental drying data. Resting duration between 75 and 90 min at moisture ratio around 0.715 was found suitable for overall good performance in both fluidized and stationary bed drying. A considerable amount of energy (21–44%) can be saved by providing a rest period from 30 to 120 min between the two stages of drying. Fluidization further reduces (≈ 50% against continuous drying under stationary bed) the energy requirement. No significant difference was found in head rice yield obtained from fluidized and stationary bed experiments, though discontinuing drying by providing intervening rest periods considerably improved the percentage head rice when compared with the results from continuous drying.     

Ultrasound-assisted fluidized bed drying of paddy: Energy consumption and rice quality aspects

Several studies have been conducted on equipping conventional fluidized bed with some technologies to increase drying efficiency and its performance. The objective of this study was to investigate the influence of high-power ultrasound (HPU) on fluidized bed drying of paddy in terms of drying kinetics, grain quality (percentage of cracked kernels and bending strength of grain kernels), and specific energy consumption (SEC). To decrease the initial moisture content of paddy from 26.5?±?0.5% (kg/kg, d.b) to the final moisture content of 13?±?0.5% (kg/kg, d.b), the experiments were conducted in a factorial design at three levels of ultrasound power densities (11.1, 14.6, and 18.7?kW/m³), four levels of frequencies (20, 25, 28, and 30?kHz), and three levels of drying air temperatures (30, 40, and 50°C). Application of HPU in conjunction with conventional fluidized bed drying led in 23% decrease in drying time as well as improvement in grain quality, in terms of percentage of cracked kernels and bending strengths. In addition, SEC reduced approximately by 22%, as HPU applied at selected drying condition.     

Continuous fluidized bed drying: Residence time distribution characterization and effluent moisture content prediction

The mixing and drying behavior in a continuous fluidized bed dryer were investigated experimentally by characterizing the residence time distribution (RTD) and incorporating a micromixing model together with the drying kinetics obtained from batch drying. The RTD of the dryer was modeled using a tank-in-series model. It was found that a high initial material loading and a low material flow rate resulted in a reduced peak height and broaded peak width of the RTD curve. To predict the continuous dryer effluent moisture content, we combined: (a) the drying kinetics as determined in a batch fluidized bed dryer, (b) the RTD model, and (c) micromixing models—segregation and maximum mixedness models. It was found that the segregation model overpredicted the effluent moisture content by up to 5% for the cases we have studied while the maximum mixedness model gave a good prediction of the effluent moisture content.     

DRYING CHARACTERISTICS OF CORN IN FLUIDIZED BED DRYER

A batch fluidized bed dryer was carried out for corn drying. Drying characteristics of corn were investigated The experimental results indicated that moisture transfer inside a corn kernel was controlled by internal diffusion by the following conditions : inlet hot air temperatures of 120 - 200 °C, superficial air velocities of 2.2- 4 m/s, bed depths of 4 - 12 cm, fraction of air recycled of 0.5 -0.9 and initial moisture content of corn of 43 % dry-basis. The Wang and Sing equation could describe in accordance with the results. Inlet hot air temperature and specific air flow rate were independent variables for drying constant model in the Wang and Singh equation.     

Drying Behavior of Carrots Dried in a Spout–Fluidized Bed Dryer

The effect of water blanching treatment and the inlet air temperature on drying kinetics as well as the quality attributes of carrot cubes dried in a spout–fluidized bed dryer at 60, 70, 80, and 90°C were analyzed. The material shrinkage and the rehydration potential were calculated to assess the changes in quality of dried carrots. It was found that the value of the air velocity during the drying of carrot cubes in a spout–fluidized bed dryer should be related to the moisture content of the carrot particles. A high value of air velocity at the beginning of the drying cycle and a lower value for the later stages were also required. The linear equation was correlated to the data of shrinkage of raw and blanched carrots. Blanching significantly influenced the coefficients in the shrinkage model derived for drying of carrot cubes in a spout–fluidized bed dryer, while drying temperature did not influence the shrinkage of carrot particles. The intensity of heat and mass transfer during spout–fluidized drying of carrot cubes was dependent on the drying temperature. A correlation was developed to calculate the values of effective moisture diffusivity of dried carrot cubes as a function of the moisture content and temperature of the material. It was observed that for any given time of rehydration, both the moisture content and the rehydration ratio calculated for samples dried at 60°C were higher than for samples dried at temperatures of 60, 70, 80, and 90°C.     

DRYING CHARACTERISTICS OF CORN IN FLUIDIZED BED DRYER

ABSTRACT

A batch fluidized bed dryer was carried out for corn drying. Drying characteristics of corn were investigated The experimental results indicated that moisture transfer inside a corn kernel was controlled by internal diffusion by the following conditions : inlet hot air temperatures of 120 - 200 °C, superficial air velocities of 2.2- 4 m/s, bed depths of 4 - 12 cm, fraction of air recycled of 0.5 -0.9 and initial moisture content of corn of 43 % dry-basis. The Wang and Sing equation could describe in accordance with the results. Inlet hot air temperature and specific air flow rate were independent variables for drying constant model in the Wang and Singh equation.     

THIN LAYER AND DEEP BED DRYING OF LONG GRAIN ROUGH RICE

ABSTRACT

The paper presents new data for thin-layer drying characteristics of Thai long grain rough rice measured under various conditions of drying air temperature (35 to 60?°C), drying air relative humidity (30 to 70 % ) and the initial moisture content of rough rice (20 to 40 % dry basis). Empirical equations were developed using the instantaneous weight, the weight loss and drying time, with temperature, relative humidity and initial moisture content of rough rice as the independent variables. A computer program was developed to simulate the deep-bed drying process. The thin-layer drying equation developed before was used in the computer simulation. Experimental data from the fixed bed dryer were compared with the results from the calculation.