Microwave Drying of Porous Materials

Experimental results on microwave drying of the porous particles exposed to air stream at 40°C are presented. The temperature and moisture distribution inside a particle were measured for gypsum spheres of 9, 18, 28, and 38 mm. The mass reduction was monitored during the drying process. The rate of drying and changes in temperature and moisture profiles for different drying conditions were analyzed and compared with the ones for convective drying.     

Microwave Drying of Porous Materials

Abstract

Experimental results on microwave drying of the porous particles exposed to air stream at 40°C are presented. The temperature and moisture distribution inside a particle were measured for gypsum spheres of 9, 18, 28, and 38 mm. The mass reduction was monitored during the drying process. The rate of drying and changes in temperature and moisture profiles for different drying conditions were analyzed and compared with the ones for convective drying.     

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.     

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.     

Three-Dimensional Numerical Modeling of Heat and Moisture Transfer in Natural Rubber Sheet Drying Process

The effects of heat and moisture transfer on rubber sheet drying were investigated by 3-D modeling using computational fluid dynamics (CFD). Complexities arose due to the inclusion of moisture transport. Local convective heat transfer coefficients for the sheets were predicted and then used to determine the mass transfer coefficients using the Chilton–Colburn analogy. Deviations between experimental and simulated temperatures were observed to range from 4.0 to 7.7°C. Temperature and moisture content of the rubber sheets were accurately predicted, and agreement between the experimental and simulated results was acceptable. This is useful for the design of an efficient rubber sheet drying chamber.     

Drying with Internal Vaporisation : Introducing the Concept of Identity Drying Card (IDC)

ABSTRACT

Different drying configurations (convective drying with moist air and superheated steam, microwave drying and vacuum drying) on different materials (isotropic and anisotropic) were experimentally studied in order to model and visualise the evolution of internal pressure and temperature. To be able to do so, in addition to measuring the average moisture, a method which can determine internal-local pressure and temperature simultanously by using specially designed sensors was developed. In combination with the experiments, the numerical code TRANSPORE has been used to simulate drying processes. A less comprehensive but more comprehensible analytical model was also provided to facilitate the better understanding of internal phenomena. Based on the results gained from measurement and numerical analysis, the dynamic distribution and development of local temperature and pressure inside seasoned medium are coupled together by a temperature-pressure graph, which is herewith called “Identity Drying Card” (IDC), a new concept initialid in the paper. By using IDC, the internal profile of temperature and pressure, the dominant transport properties (penncability and difisivity), the mechanism of transport (diffusion, convection or both) and the phase transitions during drying can be visualised. More specifically, the amount of dry air, the moisture content in the hygroscopic rcgion or the danger due to internal mechanical loads of handled materials can be figured out with the aid of IDC.     

玻璃纤维纺织型浸润剂烘干工艺研究

研究了玻璃纤维纺织型浸润剂的烘干工艺。试验了原丝不同自然存放条件、热风烘干的温度与时间等烘干工艺对玻璃纤维原丝含水率、可燃物含量、强度、浸润剂成膜效果与迁移分布,以及退并合股纱的纺织工艺情况和强度与耐磨性能等理化指标的影响。     

Measurement of Absorbed Power of Glass Particle Layer on Moisture Content and Drying Rate by Combined Convective and Microwave Drying

Dependency of absorbed power by microwave on the local moisture content in a glass particle layer was measured with a new method; that is, heating the wet layer. The heating experiment was performed using a laboratory-scale combined convective and microwave heater/dryer that was manufactured by modifying a domestic microwave oven at 2.45 GHz. The measured result was strongly dependent on the local moisture content and showed a maximum and a minimum within the measured range of the moisture content. This dependency can be explained by the assumption that moisture in the wet layer behaves as a mass of the free water. The combined drying rate of the wet layer measured with the heater/dryer was simulated with both the power dependency and the experimental convective-only drying rate. Power dependency on temperature is as important as the moisture content in the simulation. Simulated results agree very well with experimental ones.     

Effective Diffusivity and Evaporative Cooling in Convective Drying of Food Material

This article presents mathematical models to simulate coupled heat and mass transfer during convective drying of food materials using three different effective diffusivities: shrinkage dependent, temperature dependent, and the average of those two. Engineering simulation software COMSOL Multiphysics was utilized to simulate the model in 2D and 3D. The simulation results were compared with experimental data. It is found that the temperature-dependent effective diffusivity model predicts the moisture content more accurately at the initial stage of the drying, whereas the shrinkage-dependent effective diffusivity model is better for the final stage of the drying. The model with shrinkage-dependent effective diffusivity shows evaporative cooling phenomena at the initial stage of drying. This phenomenon was investigated and explained. Three-dimensional temperature and moisture profiles show that even when the surface is dry, the inside of the sample may still contain a large amount of moisture. Therefore, the drying process should be dealt with carefully; otherwise, microbial spoilage may start from the center of the dried food. A parametric investigation was conducted after validation of the model.     

Ultrasonic Convective Drying Kinetics of Clipfish During the Initial Drying Period

A hybrid drying system for high-intensity airborne ultrasound applied in convective drying was investigated for the drying of salted codfish (clipfish). Convective drying with ultrasonic assistance at 10, 20, and 30°C was compared to the same process without ultrasound. The Weibull model was used to model and investigate the drying behavior, and the effective diffusion in Fick's law was determined. The ultrasound decreased the drying time more at lower drying temperatures. The drying time was reduced by over 90% at a drying temperature of 10°C. For an industrial drying process at a temperature of 20°C, the drying time was reduced by 32.2%. The ultrasonic, convective drying of clipfish at a temperature of 20°C was faster than the same process without ultrasound at 30°C. The investigations showed a thermal effect for all products when ultrasound was applied. The specific moisture extraction ratio (SMER) in the investigated system was improved by 0.2 kg_water kWh^?1. The heat transfer coefficient in the system used was increased by 32.6% for a heating process in a separate investigation, whereas for a cooling process no increased heat transfer coefficient was determined. The thermal effect might (at least partially) explain the faster drying of ultrasonic-assisted convective drying. The results obtained demonstrate the potential of airborne ultrasound in convective drying with regard to drying time, energy consumption, and product quality. Documentation of the thermal effect should be included in future R&D on this topic.     

Testing New In-Kiln Meter for Monitoring Lumber Moisture Content during Drying

The objective of this study was to test a new in-kiln sensor for monitoring lumber moisture content during industrial drying. The theoretical foundation of the technology was already known, because it is based on electrical conductivity, but the mechanism of implementation was new and required validation. For this reason, the technology was compared with two other widely used methods for assessing lumber moisture content, namely, the oven-drying and electrical capacitance methods. The tests were performed in a 120-m³ industrial kiln operated by a sawmill in the eighth region of Chile, and the results showed that the average moisture content at the end of drying was satisfactorily determined by the new in-line sensor. As predicted by theory, the sensor was not able to accurately measure moisture content above 25%, but it was still able to provide the equivalent of a drying curve for monitoring of the drying process.     

The Change of Body Temperature During Convective Drying of Cube-Shaped Apple Pieces

When using the convective drying method, the mass transfer between drying air and moisture diffusion of the material is very important. The moisture moves inside the material because of the volume change caused by the increased temperature. This movement is additionally affected by the texture of the material. According to the research dealing with colloid capillary porous texture of vegetables and fruit, the humidity migration inside the material occurs in both fluid and steam condition at the same time. This migration is stimulated by the heating and decreasing humidity along with the flow. In this research, winter varieties of apples (Jonathan, Golden Delicious, and Idared) were studied by heating of a 20 × 20 × 20 piece of apple with a thermocouple in it. On the basis of the measurements it can be stated that for the fruits with a high percentage of moisture (75–90%), when making the drying condition, the moisture gradient is influenced by the fractured cells of the cut area. It was found that the variety of apple is very important in relation to the heating and water loss gradient. According to the results, the wet volume change due to the heating highly influences the water loss. The models that describe the temperature of the material have a connection with the water loss.     

The Change of Body Temperature During Convective Drying of Cube-Shaped Apple Pieces

When using the convective drying method, the mass transfer between drying air and moisture diffusion of the material is very important. The moisture moves inside the material because of the volume change caused by the increased temperature. This movement is additionally affected by the texture of the material. According to the research dealing with colloid capillary porous texture of vegetables and fruit, the humidity migration inside the material occurs in both fluid and steam condition at the same time. This migration is stimulated by the heating and decreasing humidity along with the flow. In this research, winter varieties of apples (Jonathan, Golden Delicious, and Idared) were studied by heating of a 20 × 20 × 20 piece of apple with a thermocouple in it. On the basis of the measurements it can be stated that for the fruits with a high percentage of moisture (75-90%), when making the drying condition, the moisture gradient is influenced by the fractured cells of the cut area. It was found that the variety of apple is very important in relation to the heating and water loss gradient. According to the results, the wet volume change due to the heating highly influences the water loss. The models that describe the temperature of the material have a connection with the water loss.     

Probabilistic Analysis of Timber Drying Schedules

Timber drying schedules are primarily chosen to achieve a certain final moisture content accompanied by a minimum amount of board degrade. However the schedule adopted also influences the dispersion in moisture content between boards at the end of drying. A simple double set point schedule consisting of two distinct and sequential equilibrium moisture contents is selected to illustrate the concept. Theoretical expressions that predict mean and standard deviation in board moisture content vs. time are developed. The predictions of these equations are compared with the output of a Monte Carlo model of timber drying and with experimental measurements from a laboratory kiln. The advantages of a double set point over a single set point schedule are explained and an optimum double set point schedule is determined. The behavior of a commercial drying schedule is analyzed and its performance with respect to the optimum schedule is quantified. Finally some inherent characteristics of the variability in timber drying systems are outlined.     

Probabilistic Analysis of Timber Drying Schedules

Abstract

Timber drying schedules are primarily chosen to achieve a certain final moisture content accompanied by a minimum amount of board degrade. However the schedule adopted also influences the dispersion in moisture content between boards at the end of drying. A simple double set point schedule consisting of two distinct and sequential equilibrium moisture contents is selected to illustrate the concept. Theoretical expressions that predict mean and standard deviation in board moisture content vs. time are developed. The predictions of these equations are compared with the output of a Monte Carlo model of timber drying and with experimental measurements from a laboratory kiln. The advantages of a double set point over a single set point schedule are explained and an optimum double set point schedule is determined. The behavior of a commercial drying schedule is analyzed and its performance with respect to the optimum schedule is quantified. Finally some inherent characteristics of the variability in timber drying systems are outlined.     

MIMO Control of Conveyor-Belt Drying Chambers

The development of MlMO control systems for individual conveyor-belt drying chambers was studied for the case of industrial units used for the moisture removal from wet raisins. The process was considered to be a 2×2 open-loop system in which material moisture content and temperature at the exit of the drying chamber were to be controlled. Its dynamic behavior was investigated via digital simulation of the corresponding process mathematical model which involves 6 state variables. The effect of several manipulated and load variables on process outputs was explored by examining the corresponding responces obtained when the input variables were step forced into the non-linear dryer model simulator around the operational point studied. Reliable transfer functions for each interaction module were produced, based on simulated data and process basic constants. The best control configuration was selected bv deriving the RGA values of each one that was based on its frequency information. The propsed scheme, used steam and fresh air flowrates as manipulated variables. This scheme was found to imply insignificant interactions between material moisture content and temperature loops. The suggested pairing was material moisture content controlled by steam flowrate and fresh air flowrate controlling material temperature. Single-loop PI-feedback controllers were installed in each loop and tuned by Ziegler-Nichols techniques. The closed-loop system performance was examined by suitably introdcuing step changes to both set-points as well as to process disturbances. The overall control system performance proved to be quite satisfactory.     

Non-destructive Measurement of Moisture Distribution in Wood during Drying Using Digital X-ray Microscopy

The objective of this study was to develop a nondestructive method by which moisture distribution in wood during drying could be predicted. A newly developed digital X-ray microscope was used to measure the moisture content of wood and its accuracy and resolution was evaluated compared to the classic oven-dry method.

Small green wood specimens of Sugi (Cryptomeria japonica D. Don) were cut and dried under constant temperature and humidity. As the weight was decreasing during drying, X-ray microscope images of cross section were obtained. From these digital images and specimen weight, the moisture content during drying was measured by the two methods. After the shrinkage of the specimen was canceled, the standard error achieved finally was about 1% moisture content within the experimental range. As the image was divided into small subimages, the clear moisture distribution can be seen. It was found that the image divided into 32 × 32 subimages in each size of 0.625 × 0.625 mm might be valid to determine the moisture distribution, and that the drying rate in early wood is larger than in late wood.     

Optimizing Drying Conditions for the Microwave Vacuum Drying of Enzymes

The aim of this study was the methodological use of experimental planning for the optimization of microwave vacuum drying of enzymes using α-amylase as a model. A factorial in star designwas used to optimize the microwave vacuum–drying process, and the variables were power output and vacuum pressure. The material dehydrated by this technique was analyzed with regard to its enzymatic activity, water activity, and moisture content. Response surface methodology was used to estimate the main effects of vacuum pressure and power on the enzymatic and water activities. The experimental in star design revealed that microwave vacuum drying is influenced mainly by power. The dehydrated product showed high enzymatic activity and low water activity.     

MIMO Control of Conveyor-Belt Drying Chambers

ABSTRACT

The development of MlMO control systems for individual conveyor-belt drying chambers was studied for the case of industrial units used for the moisture removal from wet raisins. The process was considered to be a 2×2 open-loop system in which material moisture content and temperature at the exit of the drying chamber were to be controlled. Its dynamic behavior was investigated via digital simulation of the corresponding process mathematical model which involves 6 state variables. The effect of several manipulated and load variables on process outputs was explored by examining the corresponding responces obtained when the input variables were step forced into the non-linear dryer model simulator around the operational point studied. Reliable transfer functions for each interaction module were produced, based on simulated data and process basic constants. The best control configuration was selected bv deriving the RGA values of each one that was based on its frequency information. The propsed scheme, used steam and fresh air flowrates as manipulated variables. This scheme was found to imply insignificant interactions between material moisture content and temperature loops. The suggested pairing was material moisture content controlled by steam flowrate and fresh air flowrate controlling material temperature. Single-loop PI-feedback controllers were installed in each loop and tuned by Ziegler-Nichols techniques. The closed-loop system performance was examined by suitably introdcuing step changes to both set-points as well as to process disturbances. The overall control system performance proved to be quite satisfactory.     

The Comparison of Two Models of Convective Drying of Shrinking Materials Using Apple Tissue as an Example

Modeling of drying of capillary-porous materials is a mathematically complex problem. It takes into consideration simultaneous heat and mass transfer inside the material with physicochemical properties changing during the drying process. Modeling of the process mentioned above consists of describing the heat and mass transfer balances by means of differential equations. Moisture diffusion coefficient as a function of moisture content and temperature of the material is a crucial parameter that controls the process. An additional problem occurs when moving boundary of the shrinking material is taken into account. In the present work, the identification of diffusion coefficient as a function of moisture content and temperature on the basis of two different models is shown. The two models include the Pakowski model (defined in the stationary coordinates) and the Kechaou model (defined in moving coordinates). Experimental data necessary to verify the models were obtained on the basis of series of tests for different boundary conditions performed on an apple tissue. During the drying process, samples of apple undergo significant volumetric shrinkage. In this article, the comparison of the two models describing the convective drying process of shrinking material is presented together with the comparison of the identified moisture diffusion coefficient.