Stochastic Modelling of Grain Moisture Content.in Near-Ambient Drying

A stochastic analytical model was developed to describe the mass transfer in a thin layer of grain ventilated with unheated air. As an excitation to the drying system, a stochastic analytical model of variations in ambient air temperature and relative humidity during the drying period, based on a sine function. was used. Mean. carrelation function, variance and the standard deviation were determined for main moisture content treated as a random process. The stochastic models of variations in air temperature and relative humidity were fined to the observed historical data. The standard deviation and m ean of grain moisture content calculated from the developed analytical model were compared to changes in mean and the standard deviation obtained from the numerical deterministic model of mass transfer and hourly weather data from 17 year.     

Stochastic Analysis of Grain Drying with Unheated Air Under Two Different Climates

A stochastic analytical model of mass transfer was used to analyze processes of drying thin layers of grain ventilated with unheated air. As an excitation to the drying system, stochastic analytical models of variations in input air parameters during the post-harvest period of drying were used. Stochastic models were based on historical weather data of two different climates: the maritime climate of England, and the climate that is the mixture of continental and maritime climates. Changes in time of the mean and the standard deviation values of grain moisture content during drying were compared with statistical characteristics of input air parameters.     

Stochastic Analysis of Grain Drying with Unheated Air Under Two Different Climates

A stochastic analytical model of mass transfer was used to analyze processes of drying thin layers of grain ventilated with unheated air. As an excitation to the drying system, stochastic analytical models of variations in input air parameters during the post-harvest period of drying were used. Stochastic models were based on historical weather data of two different climates: the maritime climate of England, and the climate that is the mixture of continental and maritime climates. Changes in time of the mean and the standard deviation values of grain moisture content during drying were compared with statistical characteristics of input air parameters.     

STOCHASTIC MODELING OF GRAIN TEMPERATURE IN NEAR-AMBENT DRYING

Stochastic analytical model was developed to describe the heat transfer in a thin layer of grain ventilated with ambient air. As an excitation to the drying system, a stochastic analytical model of ambient air temperature variations during the period of drying, based on a sine function, was used. Mean, correlation function, variance and the standard deviation were determined for air and grain temperature, both treated as random processes. The stochastic model of air temperature variation was fitted to the observed historical data. The effect of the standard deviation of air temperature of the three different climates (Canadian in Winnipeg, Polish in Poman and English in Weddington) on the standard deviation of grain temperature is shown.     

DYNAMIC SHRINKAGE AND VARIABLE PARAMETERS IN BAKKER-ARKEMA'S MATHEMATICAL SIMULATION OF WHEAT AND CANOLA DRYING

Abstract

The bulk volume shrinkage was linearly related to instantaneous grain moisture reduction. The shrinkage coefficient was found to be primarily a function of air relative humidity. The Bakker-Arkema's bulk grain drying model was modified to incorporate the shrinkage equation. A correction factor for thin layer drying equation was also introduced to reduce deviation between the experimental and simulated moisture content. The Bakker-Arkema model showed significant improvement due to these modifications.     

Simulation and Validation of Microwave-Assisted Fluidized Bed Drying of Soybeans

A general mathematical model of heat and mass transfer was developed to simulate the microwave-assisted fluidized bed drying of bulk grain. The model was solved using the well-known Runge-Kutta-Gill method. The model is capable of predicting the moisture content of soybean as well as the drying air parameters (i.e., drying air temperature and moisture content) during drying. The values of mean relative deviation (MRD) were less than 8 and 10% for prediction of grain moisture content and outlet air parameters, respectively, which reflects an acceptable accuracy. In comparison with conventional fluidized bed drying of soybean, microwave-assisted fluidized bed drying led to 83.39–98.07% savings in drying time and 82.07–95.22% savings in specific energy consumption when reducing soybean moisture content from 18.32 to 12% (db).     

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.     

DYNAMIC SHRINKAGE AND VARIABLE PARAMETERS IN BAKKER-ARKEMA'S MATHEMATICAL SIMULATION OF WHEAT AND CANOLA DRYING

The bulk volume shrinkage was linearly related to instantaneous grain moisture reduction. The shrinkage coefficient was found to be primarily a function of air relative humidity. The Bakker-Arkema's bulk grain drying model was modified to incorporate the shrinkage equation. A correction factor for thin layer drying equation was also introduced to reduce deviation between the experimental and simulated moisture content. The Bakker-Arkema model showed significant improvement due to these modifications.     

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.     

A COMPARTMENTAL MODEL OF THIN-LAYER DRYING KINETICS OF ROUGH RICE

ABSTRACT

Rough rice at about 21% (wet basis) was dried at various conditions of temperatures and evaporating capacities of air. The influence of both parameters on drying rate has been studied. At high temperatures, high drying rates can be achieved with low evaporating capacities. In addition, desorption isotherms of rough rice were measured at 35, 60 and 85°C and the experimental isotherms data were fitted using a modified Pfost equation.

A compartmental model was developed to simulate the grain moisture content. Heat and mass transfer coefficients were optimized using a Nelder & Mead method. Internal mass transfer coefficient was written as an exponential function of the average moisture content and temperature of the grain and the external mass transfer coefficient as a function of air temperature. The compartmental approach predicts very well the average moisture content with a mean error of about 5% in static and dynamic conditions.     

THIN-LAYER DRYING CHARACTERISTICS OF ROUGH RICE AT LOW AND HIGH TEMPERATURES

Abstract

Thin-layer drying characteristics of rough rice were determined at temperature ranging from 11.8 to 51 °C and for relative humidities ranging from 37.1% to 91.3%, with initial moisture contents in the range of 24.7 to 41.6% dry basis. An oven, a self contain air conditioning unit, recently developed in Japan, was used for this experiments. With this apparatus, very smooth drying curves were obtained. The data of sample weight, and dry and wet bulb temperatures of the drying air were recorded continuously throughout the drying period for each test. The drying process was terminated when the moisture content change in 24 h was less than 0.2 % d.b. (weight change was less than 0.05 g). The final points were recorded as the dynamic equilibrium moisture contents.

The drying data were than fitted to the Page model. The model gave a very good fit for the moisture content with an average standard error of 0.294 % d.b? Both the drying parameters, K and N, are function of drying air temperature and relative humidity. The effect of variable initial moisture content was also described effectively by the empirical Page model. The drying time employed had a large effect on the K and N values. The results presented here, over typically 5 day drying, will be useful in the long term moisture transfer process occurring during ventilated storage.     

PERFORMANCE ANALYSIS OF A DOUBLE-DECK CONVEYOR DRYER - A COMPUTATIONAL APPROACH

ABSTRACT

This paper illustrates the use of numerical simulation models for evaluating the performance of a moving bed dryer. A finite-volume method is employed in developing a steady state, two-dimensional numerical model for a double-deck conveyor dryer. Using this numerical model, variations in the product moisture content and temperature along the length and across the height of the product beds are predicted. Similarly, the resulting variations in the temperature and relative humidity of the drying air are predicted in the entire two-dimensional domain of a dryer. Effect of air-to-product mass flow ratio and product residence lime on the average moisture content of the outgoing product are also evaluated for three different drying air temperatures.     

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.     

Moisture Changes in Grain During Aeration Under Warm Humid Conditions

ABSTRACT

Two mathematical models (an equilibrium model and a combined model) were evaluated, which described the moisture and heat transfer in low temperature drying and aeration. The predicted moisture contents from both models were compared with the experimental data. Comparisons indicated that the combined model was more accurate than the equilibrium model. The combined model is based on the idea that the partial pressure difference is the driving force in moisture transfer. A series of simulation was performed using the combined model to evaluate the effect of air temperature, air relative humidity, and the temperature difference between grain and air on the moisture changes in stored rough rice. The simulation results proved the concept of using the partial pressure difference to describe the moisture transfer in stored grain. A minimum of 5.6°C in     

A COMPARTMENTAL MODEL OF THIN-LAYER DRYING KINETICS OF ROUGH RICE

Rough rice at about 21% (wet basis) was dried at various conditions of temperatures and evaporating capacities of air. The influence of both parameters on drying rate has been studied. At high temperatures, high drying rates can be achieved with low evaporating capacities. In addition, desorption isotherms of rough rice were measured at 35, 60 and 85°C and the experimental isotherms data were fitted using a modified Pfost equation.

A compartmental model was developed to simulate the grain moisture content. Heat and mass transfer coefficients were optimized using a Nelder & Mead method. Internal mass transfer coefficient was written as an exponential function of the average moisture content and temperature of the grain and the external mass transfer coefficient as a function of air temperature. The compartmental approach predicts very well the average moisture content with a mean error of about 5% in static and dynamic conditions.     

THIN-LAYER DRYING CHARACTERISTICS OF ROUGH RICE AT LOW AND HIGH TEMPERATURES

Thin-layer drying characteristics of rough rice were determined at temperature ranging from 11.8 to 51 °C and for relative humidities ranging from 37.1% to 91.3%, with initial moisture contents in the range of 24.7 to 41.6% dry basis. An oven, a self contain air conditioning unit, recently developed in Japan, was used for this experiments. With this apparatus, very smooth drying curves were obtained. The data of sample weight, and dry and wet bulb temperatures of the drying air were recorded continuously throughout the drying period for each test. The drying process was terminated when the moisture content change in 24 h was less than 0.2 % d.b. (weight change was less than 0.05 g). The final points were recorded as the dynamic equilibrium moisture contents.

The drying data were than fitted to the Page model. The model gave a very good fit for the moisture content with an average standard error of 0.294 % d.b¨ Both the drying parameters, K and N, are function of drying air temperature and relative humidity. The effect of variable initial moisture content was also described effectively by the empirical Page model. The drying time employed had a large effect on the K and N values. The results presented here, over typically 5 day drying, will be useful in the long term moisture transfer process occurring during ventilated storage.     

THIN LAYER AND DEEP BED DRYING OF LONG GRAIN ROUGH RICE

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.     

CORN QUALITY AFTER DELYING BY FLUIDIZATION TECHNIQUE AT HIGH TEMPERATURE

ABSTRACT

Corn quality, in terms of stress crack, breakage and colour, after drying by fluidization technique was investigated. Drying air temperatures for this study were 150, 170 and 200°C. Experimental results showed that drying corn of 43% (d.b.) initial moisture content to 16% (d.b.) final moisture content (approximate 9 minutes) with drying air temperature up to 170°C (inlet air relative humidity less than 5%) could be done without significant loss of quality, i.e. no breakage of corn kernel, stress crack less than 12% and just small change of colour of grain surface. Empirical equations describing evolution of multiple stress crack and grain surface colour were developed. Results obtained from the equations are in good agreement with experimental ones.     

Assessment of the initial moisture content on soybean drying kinetics and transport properties

Several researchers have developed studies to obtain a mathematical model able to describe grain drying kinetics. However, most of these studies neglect the effect of grain initial moisture content on drying curves. In this study, we assessed the dependence of drying curves and mass transfer coefficients on this initial moisture, air temperature, and its velocity by measuring grain mass losses within time on a tray dryer. Mathematical models were adjusted and results indicated that initial grain moisture content has significant influence on drying curves and mass transfer coefficients.     

DYNAMIC SIMULATION AND CONTROL OF CONVEYOR-BELT DRYERS

Abstract

The dynamic behavior of conveyor-belt dryers involving externally controlled heat and mass transfer phenomena has been studied via digital simulation. The investigation concerned an industrial dryer used for the moisture removal from wet raisins. The dryer consisted of three drying chambers and a cooling section, all involving the same conveyor belt. For each chamber, perfect temperature control was assumed for the drying air temperature, while its humidity was left uncontrolled. The effect of material temperature and moisture content at the entrance of the dryer and the drying air temperature on material temperature and moisture content at the exit of the dryer and the corresponding drying air humidity, have been explored by step forcing the disturbance and manipulated variables in the non-linear dryer model simulator. Results showed that material moisture content at the exit of the dryer is greatly affected by material moisture content at the entrance as well as by the drying air temperature. Reliable transfer functions for each process module were obtained by fitting several transfer function models on the simulated data using a least-squares approach. It was found that when input material moisture content could be instantly measured, the system responded slowly enough so that excellent control could be achieved for material moisture content at the exit of each chamber. In this case a Pi-feedback cascade temperature controller was used. When a 15 sec delay measuring sensor was introduced, poor performance was observed. A simplified lead-lag feedforward controller, added to the system, in conjunction with the primary Pi-feedback cascade controller, resulted in good control performance of the delay sensor system.