Wheat Drying Research Articles

Microwave drying and grinding characteristics of wheat ( Triticum aestivum)

Wheat samples of approximately 20 g each were dried in a domestic microwave oven for different time periods ranging from 15 to 150 s with different moisture contents ranging from 0.11 to 0.23 kg of water/kg of dry weight of solids. The samples showed an average moisture loss of 4.4–10.6×10 −4 kg of water/kg of dry weight of solids per second. The dried samples were ground in a domestic dry grinder, and the ground samples were subjected to sieve analysis to calculate the average final particle size. The grinding characteristics were evaluated by calculating the Bond’s work index. The microwave dried samples for 120 s were crisp and consumed less energy for grinding compared to the control samples. The same trend was maintained even when the wheat samples were dried in bulk by taking 1 kg of sample (initial moisture content of 0.11 dry weight basis) and dried for 15 min. The Bond’s work index for the bulk sample was 2.26 compared to 2.41 kW h kg −1 for the control of the equal moisture content. Studies were also made on the effect of the microwave power supply on drying and grinding, and this was found to have effect on the latter. These studies have all indicated that microwave drying of wheat samples before grinding helps reduce power consumption in due course in wheat milling industries. The microwave drying did not change the total protein content, but there were some functional changes in the protein which was evident from the gluten measurements.

Wheat drying kinetics. Diffusivities for sphere and ellipsoid by finite elements

A two-dimensional finite element model was used to analyze isothermal thin layer drying of wheat representing the grains as axisymmetric ellipsoids. The effective diffusion coefficient was estimated by minimizing the sum of squares of the residuals between numerically predicted and experimental moistures. Estimations done for drying air temperatures of 35°C, 50°C, 60°C and 70°C and initial grain moistures of 0.2694, 0.2396, 0.2133 and 0.1891 d.b., were correlated by means of Arrhenius-type functions. These correlations were compared to those previously obtained with the same data by assuming grains as spheres. The results, all in the order of 10 −11 m 2/ s , indicated that the diffusion coefficients found for ellipsoidal geometry were some 9–13% lower than the values fitted for spheres. The ratio of diffusivities for ellipsoids to those for spheres was in the order of the wheat sphericity. However, further theoretical and experimental studies are required to verify this observation.

Drying of particles in batch fluidized beds

For a batch fluidized bed dryer (BFBD), a mathematical model is presented. Based on this model the moisture content and temperature of solid particles and drying air are calculated by a numerical method. In order to evaluate the accuracy of the predicted results, an experimental setup is erected and some experimental data are obtained for drying of wheat in BFBD. Comparison of calculated data with experimental results shows a good agreement and so verifies the validity of the proposed model.

Thermal analysis of a fluidized bed drying process for crops. Part II: Experimental results and model verification

An experimental investigation was undertaken to provide key data for the drying of wheat, and to assess the validity of the mathematical model described in Part I, and test its accuracy. The results of numerical solutions are compared with experimental data for wheat grains. Wheat is one of the main agricultural products and has extensive application in drying systems and therefore the accuracy of results for this product is of primary concern. Furthermore, as the desorption isotherms of wheat material are well documented, it is possible to accurately simulate the drying processes. Wheat belongs to the hygroscopic materials group and has a very low-mass diffusivity. Usually, no constant rate period is observed in the course of drying of wheat in the moderate range of initial moisture content, and drying takes place in the falling rate mode. In this study, it was confirmed that immediately after the particles are exposed to the drying medium, the surface moisture of material transfers rapidly due to low external resistance to heat and mass transfer. However, later on the moisture removal rate decreases noticeably with time because of the high internal resistance to diffusion. In the experiments, temperature at different elevations, humidity of the exit air, bed pressure drop, fluidization velocity and moisture content of particles at different times are measured. Each experiment is repeated at three inlet air temperatures: 40, 49.5, and 60°C. Results of the model predictions show very good agreement with a large number of experimental data obtained in drying wheat. The agreement of model prediction and experimental data indicates that the model is fully predictable for drying of other materials in the fluidized bed with no restriction in the range of operational conditions such as the air temperature and the relative humidity. The model can be applied to the other type of processes such as spouted bed and fixed-bed drying with slight modification. It is found that the drying of grain materials is usually controlled by internal mass transfer parameters. The inlet air temperature has an important effect on the magnitude of drying rate while the gas velocity and bed hold-up do not show significant contribution to the drying rate. The initial moisture content of the bed materials can have an important effect on the drying rate depending on the physical properties of bed materials. Copyright © 2000 John Wiley & Sons, Ltd.

Thermal analysis of a fluidized bed drying process for crops. Part II: Experimental results and model verification

An experimental investigation was undertaken to provide key data for the drying of wheat, and to assess the validity of the mathematical model described in Part I, and test its accuracy. The results of numerical solutions are compared with experimental data for wheat grains. Wheat is one of the main agricultural products and has extensive application in drying systems and therefore the accuracy of results for this product is of primary concern. Furthermore, as the desorption isotherms of wheat material are well documented, it is possible to accurately simulate the drying processes. Wheat belongs to the hygroscopic materials group and has a very low-mass diffusivity. Usually, no constant rate period is observed in the course of drying of wheat in the moderate range of initial moisture content, and drying takes place in the falling rate mode. In this study, it was confirmed that immediately after the particles are exposed to the drying medium, the surface moisture of material transfers rapidly due to low external resistance to heat and mass transfer. However, later on the moisture removal rate decreases noticeably with time because of the high internal resistance to diffusion. In the experiments, temperature at different elevations, humidity of the exit air, bed pressure drop, fluidization velocity and moisture content of particles at different times are measured. Each experiment is repeated at three inlet air temperatures: 40, 49.5, and 60°C. Results of the model predictions show very good agreement with a large number of experimental data obtained in drying wheat. The agreement of model prediction and experimental data indicates that the model is fully predictable for drying of other materials in the fluidized bed with no restriction in the range of operational conditions such as the air temperature and the relative humidity. The model can be applied to the other type of processes such as spouted bed and fixed-bed drying with slight modification. It is found that the drying of grain materials is usually controlled by internal mass transfer parameters. The inlet air temperature has an important effect on the magnitude of drying rate while the gas velocity and bed hold-up do not show significant contribution to the drying rate. The initial moisture content of the bed materials can have an important effect on the drying rate depending on the physical properties of bed materials. Copyright © 2000 John Wiley & Sons, Ltd.

PROGRAMA COMPUTACIONAL PARA SIMULAÇÃO DE SECAGEM DE GRÃOS

A computer program was developed in Clipper/dbase IV language to simulate the drying of agricultural products (rice, coffee, beans, maize, soybean and wheat). The program was written containing alternatives to determine the psychometric properties of ambient air as function of the dry bulb temperature (Tdb) and wet bulb temperature (Twb), or dry bulb temperature (Tdb) and relative humidity (RH) of the air and also for the conditions of air heating from 1 to 150 o C. The program allows the simulation of drying in fixed-bed dryers using the Thompson model. For the case of maize it also permits the simulation by the Hukill model. The program is further able to simulate the drying of maize in cross flow and concurrent flow dryers. It is a very versatile program, which allows for the introduction of other grains, although requiring specific knowledge of the product equations. In this paper, the simulations for the drying of rice, coffee, beans, maize, white maize, soybean and wheat were tested, leading to the conclusion that the simulation models are adequate to simulate the values obtained experimentally by various authors for these six products, for drying air temperature varying from 25 to 80 o C.

Simulation of Recirculating Circular Grain Dryer with Tempering Stage

ABSTRACT The diiffusion model describing internal diiffusion of moisture within a grain kernel during drying and tempering stages was incorporated in the cross-flow drying model to simulate the recirculating circular grain dryer with drying and tempering stages. Experiments were conducted on an experimental prototype recirculating circular grain dryer for wheat and rough rice drying. The simulated grain temperature and moisture content were compared with the experimental data of drying wheat and rough rice, the maximum deviation of the outlet grain temperature was 5°C and the maximum deviation ofthe final grain moisture content was 0.3% w.b. The simulating program for recirculating circular grain dryer was used for analyzing the effects of structure parameters and hot air parameters on the dryer performance. Recommendations for design of the recirculating circular grain dryers are drawn from the experiments and simulation.

A Two-Dimensional Finite Element Model For Wheat Drying In A Novel Rotating Jet Spouted Bed

ABSTRACT Based on Liquid diffusion model a two-dimensional finite element simulator was developed for the drying of wheat. Simulations were carried out for the drying of a single wheat particle in a novel rotating jet spouted bed. The effects of the choice of the geometric model for the wheat kernel, the temperature and flow rate of the drying air on the moisture removal rate as well as kernel temperature were investigated. Numerical experiments showed that the accuracy of the model geometry chosen has great influence on the computed moisture removal rate partly because of the way the moisture published diffusivily correlations were obtained. Sample results are presented and discussed.

Fluidized Bed Drying of Large Particles

Batch drying of red spring wheat in a fluidized bed was performed. Effects of bed height, gas velocity, initial moisture content and air temperature on drying rate were investigated. Wheat was conditioned to simulate high moisture contents just after harvest. Heated air was the medium for moisture removal with thermocouples located along the height of the freeboard recording bed temperatures. Samples were removed at regular intervals during drying and their moisture contents determined. It was found that the rate of moisture removal was controlled by the internal rate of diffusion and hence slugging had no effect on the drying rate. The temperature distribution within the bed was uniform due to the high degree of mixing that occurs as wall slugs are formed with wheat. At lower bed temperatures the rate of drying slowed considerably. Drying times were considerably shorter than other drying apparatus.

Cross-Flow Drying of Wheat: A Simulation Program with A Diffusion-Based Deep-Bed Model and A Kinetic Equation for Viability Loss Estimation

ABSTRACT A mathematical model of heat and mass transfer for fixed beds was developed according to the modern theory of process simulation and standard laws of thermodynamics and transportphenomena. The mass transfer grain–air was predicted with simplified diffusional expressions together with an equation for the static equilibrium moisture content. Four differential equations were obtained for a grain layer and they were integrated along the bed depth and time with second

Cross-Flow Drying of Wheat. A Simulation Program with a Diffusion-Based Deep-Bed Model and a Kinetic Equation for Viability Loss Estimations.

ABSTRACT A mathematical model of heat and mass transfer for fixed beds was developed according to the modern theory of process simulation and standard laws of thermodynamics and transport phenomena. The mass transfer grain-air was predicted with simplified diffusional expressions together with an equation for the static equilibrium moisture content. Four differential equations were obtained for a grain layer and they were integrated along the bed depth and time with second and a fourth-order methods, respectively. The model was validated by comparing drying time predictions with experimental values, being the average error of 6%. The model was extended into a program for continuous cross-flow drying-cooling

Stress Cracking During High-Intensity Drying and its Effect on Grain Quality

ABSTRACT Convective and microwave-assisted thin-layer drying of wheat has been studied to identify the relationship between drying parameters and stress cracks affecting the grain quality. Internal cracks developed during drying were visualized by the soft X-ray photography and quantified by a binary index. Definite effects from drying conditions on stress cracks and further on the grain quality categorized in terms of the germination capacity, sprout length and mechanical strength have been found. This confirms suitability of the X-ray method of cracks detection for selection of optimum drying parameters.

Thin-layer drying and wetting of wheat

Abstract The research objectives were to determine accurate thin-layer drying and wetting equations for ‘Katepwa’ wheat. Equilibrium moisture content (EMC) equations for adsorption and desorption were also developed based on the experimental drying and wetting data. The modified Chung-Pfost equation predicted reasonably well the EMC for both adsorption and desorption of wheat. The range of test conditions were: temperature 7.6–35.1 °C; relative humidity 28–92%; air velocity 0.04–0.2 m s−1; and initial moisture content 9.2–16.7% (wet mass basis). Thin-layer drying of wheat was affected by air temperature, relative humidity, and initial moisture content while thin-layer wetting also slightly depended on air velocity. The semi-empirical equation of Page was more accurate than the theoretical diffusion equation with the diffusion coefficient dependent on temperature only.

Effect of preharvest applications of glyphosate on the drying, yield and quality of wheat

In experiments conducted from 1988 to 1990 at four locations in the Parkland zone of western Canada, the drydown of seed and foliage, seed yield, seed quality and baking quality of wheat (Triticum aestivum L.) following preharvest applications of glyphosate were compared with those following windrowing prior to harvest or direct cutting of the standing crop. Glyphosate was applied in late July to early September at rates of 0.45, 0.9 and 1.7 kg acid equivalent ha−1 to wheat with seed moisture contents ranging from 80 to 11%. When compared with a standing crop, all glyphosate treatments applied at seed moisture contents above 25% slightly enhanced the drydown of wheat seed and foliage. Treatments applied at seed moisture contents below 25% had no effect on either seed or foliage drydown. Seed yields were generally greater from plots to which glyphosate was applied than from windrowed control plots, although in one of five experiments the reverse trend occurred. Increasing the rate from 0.45 to 1.7 kg ha−1 had no effect on yield. Windrowing or applying glyphosate to wheat with a seed moisture content of 61% or more reduced yields more than windrowing or applying glyphosate at seed moisture contents below this level. In experiments where the wheat in both control and sprayed plots was direct cut at maturity, average yields decreased as the rate of glyphosate increased from 0 to 1.7 kg ha−1. However, applications at seed moisture of 40% or less, caused little or no yield loss. There was little or no difference in 1000-seed weight, sample density, seed germination and protein content from plots sprayed with glyphosate at seed moisture contents below 40% than from control plots windrowed at the same moisture content or direct cut at maturity. Baking quality was not affected by any of the glyphosate treatments in experiments where the wheat was harvested by direct cutting. Key words: Glyphosate, wheat, desiccation, wheat quality, Triticum aestivum

Modelling the Drying Kinetics of Maize in a Microwave Environment

Microwave drying of grains is fundamentally different from either convection or conduction drying. Drying of cereal grains appears to proceed mainly in the period when the drying rate is decreasing (in the falling rate period), a characteristic of internally controlled diffusion. However, the analytical solution to Fick's second law of diffusion for a homogeneous, isotropic sphere with constant initial and boundary conditions does not adequately describe the drying behaviour. In an attempt to overcome this problem, the moisture ratio was written in terms of the surface moisture content rather than the equilibrium value. The associated surface drying coefficient, as determined by an iterative technique, was found to be expressible as a linear function of the initial free moisture content of the grain. The resulting empirical model better described the observed drying kinetics. This approach also resulted in good fits to independent data from experiments on convective drying of rough rice, microwave drying of wheat and combined microwave-fluidized bed drying of wheat.

Heat production kinetics and the scope for thermal self-ignition in wheat drying

Heat production has been examined at various oxygen partial pressures in the oxidation and thermal destruction of grain products by the use of a DAK-1-2 differential microcalorimeter. Estimates have been made on the adiabatic induction periods and critical sizes of the grain product layers on a metal surface (boundary conditions of the first kind) in the absence of heat transfer from the other surface. It is found that thermal self-ignition will occur in degenerate mode when thermal destruction is decisive.

The selection of sorption isotherm equations for wheat based on the fitting of available data

Thirty-three sources of EMC/ERH data for wheat were identified, from which 1109 experimental points were extracted. Nineteen sources covered a range of temperatures. Five commonly cited EMC/ERH equations, Chen-Clayton, Modified-Chung-Pfost, Modified-Henderson, Modified-Oswin and Strohman-Yoerger, were compared for their ability to fit 29 sets of wheat data from these 19 sources. The Modified-Chung-Pfost and the Modified-Oswin equations gave the best fit for r.h. = f( M, T) and M = f(r.h., T) respectively. Both equations were then used to fit sub-sets of the data comparing adsorption, desorption, low temperature, hard, soft and durum varieties with the overall fit to the data. The adsorption and desorption equations gave quite distinct curves. The low temperature fit did not differ greatly from the overall. Amongst the varieties, the hard wheat at low temperature showed the greatest difference from the overall fit. The larger data set than previously available and the equations fitted provide a sound basis for future work on the drying and storage of wheat.

BULK VOLUME SHRINKAGE DURING DRYING of WHEAT and CANOLA

The bulk volume shrinkage of canola and wheat were measured for the temperature range of 20–80C and relative humidity range of 15–90%. the volume decreased exponentially with time as seed moisture content was reduced. For canola, an oilseed, shrinkage and moisture reduction were linearly correlated with a shrinkage coefficient of about 1.0. For wheat, a starchy grain, the relationship was also linear but the coefficient was greater than 1.3. the shrinkage coefficients for both wheat and canola did not show a correlation with drying temperature but varied linearly with relative humidity of the drying air.

Optimization of control systems for near-ambient drying of wheat under maritime and continental climates

Three important control systems for near-ambient wheat drying were optimized using historical weather data from the maritime climate of Waddington, England and the continental climate of Winnipeg, Canada. Statistical characteristics of output results were compared with statistical characteristics of input weather conditions. Results of the comparison were used to define the basis for developing the best possible control system for drying grain under the continental Canadian Prairie climate. Under the maritime climate the lowest cost of drying wheat was simulated using an optimized control system that varies mainly the amount of heat added to the air in response to the simulated progress of drying and weather conditions. Under the continental climate the optimized control system should vary the airflow rate in response to the drying potential of the air and the difference between the progress of drying and the progress of spoilage. This generalized control system is widely applicable and flexible because it continually readjusts the control set-points.

A Generalized Control-strategy for Near-ambient Drying of Wheat Under Canadian-prairie Climate

Computer simulation was used to explore options for varying airflow in the control of near-ambient drying of wheat in the Canadian prairies. Earlier work had identified that the best way to use ambient air for drying in the prairie climate was not to adjust air humidity by the addition of heat, but to take advantage of diurnal swings in humidity by making appropriate variations in airflow. Year-to-year variation in the general level of the climate affected the required base level of airflow.