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.     

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.     

DYNAMIC SIMULATION AND CONTROL OF CONVEYOR-BELT DRYERS

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.     

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.     

Heat Pump Drying of Green Sweet Pepper

Thin-layer drying experiments under controlled conditions were conducted for green sweet pepper in heat pump dryer at 30, 35, and 40°C and hot air dryer at 45°C with relative humidities ranging from 19 to 55%. The moisture content of sweet pepper slices reduced exponentially with drying time. As the temperature increased, the drying curve exhibited a steeper slope, thus exhibiting an increase in drying rate. Drying of green sweet pepper took place mainly under the falling-rate period. The Page equation was found to be better than the Lewis equation to describe the thin-layer drying of green sweet pepper with higher coefficient of determination and lower root mean square error. Drying in heat pump dryer at 40°C took less time with higher drying rate and specific moisture extraction rate as compared to hot air drying at 45°C due to lower relative humidity of the drying air in a heat pump dryer though the drying air temperature was less. The retention of total chlorophyll content and ascorbic acid content was observed to be more in heat pump–dried samples with higher rehydration ratios and sensory scores. The quality parameters showed a declining trend with increase in drying air temperature from 30 to 45°C. Keeping in view the energy consumption and quality attributes of dehydrated products, it is proposed to dry green sweet pepper at 35°C in heat pump dryer.     

Simulation of Recirculating Circular Grain Dryer with Tempering Stage

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 Method for Determination of Porosity Change from Shrinkage Curves of Deformable Materials

The novel low-cost band thermodynamic dryer equipped with a solar collector, a parabolic focusing collector, a heat exchanger, screw fan, and a drying cabinet with a band was designed and tested. The maximum temperature in the solar collector reached 85°C, which was 55°C above the ambient temperature. The required drying time was 4.5 h, much reduced from the traditional solar drying time of 48 h. The final moisture content of the Roselle calyx was 12% w.b., which is the recommended storage moisture content. Measurements of ambient temperature and humidity, air temperature, and relative humidity inside the dryer as well as solids moisture loss-in-weight data are employed as a means to study the performance of the dryer. Solar drying was compared with conventional sun drying and heated air drying, using the following evaluation criteria: drying time, dried Roselle color, texture, taste, and production cost. For evaluation, a model-based Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) methodology was used. After the evaluation, the proposed continuous solar dryer was found to be better than conventional drying and heated air drying due to slower drying rate and better quality of the dried Roselle.     

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

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.     

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.     

Influence of drum inlet air conditions on drying process in a domestic tumble dryer

This study examines how the inlet air temperature, relative humidity, and flow rate influence the textile drying process in an open cycle tumble dryer. An experimental setup was prepared by connecting a domestic tumble dryer to an external system for controlled heating, humidification, and transport of air. Experiments were conducted by drying cotton textiles (8?kg dry mass) at different air inlet conditions. On the basis of measured data, correlations for determination of the total drying time, the moisture evaporation rate during the constant drying rate, and the area-mass transfer coefficient were developed. The process in the drum was modeled by using an established moisture evaporation model, based on sorption isotherms. A commonly used and a recently reported sorption isotherm for cotton were used with the model. Agreement between calculated and measured drying curves was better in case of the commonly used sorption isotherm, but final moisture content was better predicted by the recently reported sorption isotherm.     

Drying Kinetics of Tomato Slices in Vacuum Assisted Solar and Open Sun Drying Methods

A lab model vacuum-assisted solar dryer was developed to study the drying kinetics of tomato slices (4, 6, and 8 mm thicknesses) compared with open sun drying under the weather conditions of Montreal, Canada. The drying study showed that the time taken for drying of tomato slices of 4, 6, and 8 mm thicknesses from the initial moisture content of 94.0% to the final moisture content of around 11.5 ± 0.5% (w.b.) was 360, 480, and 600 min in vacuum-assisted solar dryer and 450, 600, and 750 min in open sun drying, respectively. During drying, it was observed that the temperature inside the vacuum chamber was increased to 48°C when the maximum ambient temperature was only 30°C. The quality of tomato slices dried under vacuum-assisted solar dryer was of superior quality in terms of color retention and rehydration ratio. The drying kinetics using thin-layer drying models and the influence of weather parameters such as ambient air temperature, relative humidity, solar insolation, and wind velocity on drying of tomato slices were evaluated.     

Effect of operating parameters on the drying performance of multicylinder paper machine dryer section

The drying performance of multicylinder dryer section in a paper machine was investigated under various operating parameters: Inlet paper solid content (48–50%), inlet paper temperature (45–50°C), supply air temperature (100–90°C), and exhaust air humidity (75–85?g H₂O/kg dry air). The variation in environmental conditions was also considered. In this study, an improved static model was utilized to study the influence of these operating parameters on paper drying. The model was constructed using sequential modeling approach based on the drying techniques of multicylinder dryer section of a paper machine. The calculated paper solid content leaving each paper drying module and energy use is in agreement with the measured results. The simulation results showed that higher paper solid content and temperature entering the dryer section, lower supply air temperature, and higher exhaust air humidity were favorable for drying performance within the studied range of these parameters.     

IMPROVING MDF FIBRE DRYING OPERATION BY APPLICATION OF A MATHEMATICAL MODEL

Fibre drying is an important process in production of medium density fibreboard (MDF) which consumes a large amount of energy, affects product quality and, without appropriate control, causes environmental concerns. Based on fundamental knowledge of wood fibre-water relationships and heat/mass transfer, a mathematical model has been developed to simulate the MDF fibre drying processes. The model is able to predict fibre moisture content, air temperature and air humidity along the dryer length. After validation against the measured air temperature and humidity, the model has been extended to include both fibre drying and fibre conditioning, the latter occurring in the dry fibre conveyers. Due to potential benefits in reducing emissions of volatile organic compounds (VOCs) and in improving panel quality, lower drying temperatures are more desirable than higher temperatures. However, in order to achieve the target moisture content after drying, a higher air velocity is needed or a second-stage dryer is added. The model was employed to determine the air velocity required and to assist in designing a second dryer for further drying and recovery of moist vapour and heat. A further study was undertaken to investigate fibre drying or fibre conditioning in the fibre conveyers and, once again, the fibre drying model was used to determine the air conditions.     

IMPROVING MDF FIBRE DRYING OPERATION BY APPLICATION OF A MATHEMATICAL MODEL

Fibre drying is an important process in production of medium density fibreboard (MDF) which consumes a large amount of energy, affects product quality and, without appropriate control, causes environmental concerns. Based on fundamental knowledge of wood fibre-water relationships and heat/mass transfer, a mathematical model has been developed to simulate the MDF fibre drying processes. The model is able to predict fibre moisture content, air temperature and air humidity along the dryer length. After validation against the measured air temperature and humidity, the model has been extended to include both fibre drying and fibre conditioning, the latter occurring in the dry fibre conveyers. Due to potential benefits in reducing emissions of volatile organic compounds (VOCs) and in improving panel quality, lower drying temperatures are more desirable than higher temperatures. However, in order to achieve the target moisture content after drying, a higher air velocity is needed or a second-stage dryer is added. The model was employed to determine the air velocity required and to assist in designing a second dryer for further drying and recovery of moist vapour and heat. A further study was undertaken to investigate fibre drying or fibre conditioning in the fibre conveyers and, once again, the fibre drying model was used to determine the air conditions.     

A Small-Scale Pneumatic Conveying Dryer of Rough Rice

This article studies the possibility of reducing the high initial moisture content of wet rough rice using a small-scale low-cost pneumatic conveying dryer as a first stage dryer. The parameters investigated are final moisture content, surface temperature of rough rice, head rice yield, drying rate, power consumption per unit mass of evaporated water, and physical characteristics of rice. Parametric effects of the following variables are examined: velocity of drying air from 20 to 30 m/s, feed rate of rough rice from 150 to 350 kg/h, initial moisture content from 22 to 26% (wet basis), and drying air temperature from 35 to 70°C. From the experimental results, it is found that this drying method can be used for fresh rough rice with an initial moisture content of over 24% (wet basis). The drying process is able to lead to very rapid drying without any grain quality problems such as cracks in the rice kernel. The moisture content can be reduced to approximately 18% (wet basis) or about 5–6% of the initial moisture content within 3–4 s. The optimal drying air temperature is in the range of 50 to 60°C. A comparison of pneumatic conveying drying data obtained from the present study with fluidized bed drying data reported in the open literature is also discussed.     

COMPUTER SIMULATION ON INTERMITTENT DRYING OF ROUGH RICE

This study applied a partial differential equation model with newly-developed thin layer equations to simulate batch re-circulating dryers under different drying conditions, which are combinations of four parameters: drying air temperature, drying air absolute humidity, drying period duration, and tempering period duration. The moisture change and the drying rate, which were of particular concern with respect to the simulated data, were investigated. Validation drying tests were carried out in a lab scale re-circulating rice dryer. Two sets of experiment were performed involving different drying parameters to simulate re-circulating rice dryers which are extensively used in Asian countries. Comparing these two experimental data with two simulated drying curves respectively, it revealed they are quite consist with each other under the same drying conditions. Drying air temperature, drying air humidity, drying period duration and tempering period duration significantly influenced the drying rate. Under the same drying condition, the tempering period duration effect was insignificant to the drying rate in drying zone as the drying air humidity or temperature increased. And, a higher initial moisture content obtained higher time and energy efficiency for the re-circulating rice dryers.     

Drying of Soybean Seeds in a Concurrent Moving Bed: Heat and Mass Transfer and Quality Analysis

The purpose of the present work is to study the simultaneous heat and mass transfer between air and soybean seeds in a concurrent moving bed dryer, based on the application of a two-phase model to the drying process. The numerical solution of the model is obtained by using a computational code based on BDF methods (Backwards Differentials Formulas). The experimental data of air humidity and temperature and of seed moisture content and temperature at the dryer outlet are compared to the simulated values, showing a good agreement. This work also analyzes the effect of the main process variables (drying air temperature, air relative humidity, air velocity and solids flow rate) on the soybean seeds quality during drying. Empirical equations fitted to the experimental data are proposed for predicting the soybean seed quality (germination, vigor and fissures) as a function of the investigated variables.     

A SIMPLIFIED MODEL FOR DETERMINATION OF MOISTURE DIFFUSIVITY OF DATE FROM EXPERIMENTAL DRYING CURVES

Experimental drying curves for Tunisia Deglet Nour dates were obtained in a laboratory dryer under different drying conditions The air temperature was varied from 30 to 69^°C, relative humidity from 11.6 to 47.1 % and air velocity from 0.9 to 2.7 m/s. A numerical method to obtain a solution of a diffusion equation in which the diffusivity depends upon temperature and moisture content has been proposed to investigate the moisture movement in a date by assuming the sample to be a homogenous infinite cylinder. To rind the fitting moisture and temperature dependent diffusivity, the calculated drying curves are compared with the observed drying curves and an empirical equation for the moisture diffusivity of the date has presented as a function of temperature and moisture. It has been shown that the moisture distribution in the date during drying can be obtained by using the empirical equation presented.     

Coupled Heat and Mass Transfer in a Solar Grain Dryer

This paper presents the analysis of a coupled heat and mass transfer process in a fixed-bed solar grain dryer. Measurements of moisture concentration and air humidity along with temperature measurements were carried out in a solar grain dryer located in Port Harcourt, Nigeria, at the latitude of 4.858°N and longitude of 8.372°E. The process was also modelled, mathematically, by a set of partial differential equations that were coupled within the grain and through the grain boundary with the hot drying air. A finite difference scheme was used to obtain the moisture concentration and air humidity, and temperature fields within the grain and drying air. There was good agreement between the theoretical and experimental results at specified Biot and Posnov numbers, and varying Fourier number. The effects of time, space, and key model parameters such as the Biot and Posnov numbers and the initial conditions of the grains and drying air were simulated and discussed. The results from this study can be used to specify the design parameters for solar grain dryers.     

Drying Kinetics of Some Building Materials

Abstract

Moisture is one of the most deteriorating factors of buildings. The deteriorating effect of moisture occurs mainly during the drying phase, not in the wetting phase. Environmental factors, such as air temperature, air humidity, and air velocity affect drying. An experimental air dryer of controlled drying air conditions was used to investigate the drying performance of 4 stone materials, 2 bricks, and 6 plasters. Drying kinetics was examined at 3 air temperatures, 5 air humidities, and 3 air velocities. A first-order kinetics model was obtained in which the drying time constant was a function of the drying conditions, and the equilibrium material moisture content was described by the modified Oswin equation. The parameters of the proposed model were found to be affected strongly by the material characteristics.