EHD-ENHANCED DRYING WITH WIRE ELECTRODE

The enhancement of drying rate by electrostatic field (corona wind) has been experimentally evaluated in this study. The samples used are 3 mm and 6 mm glass beads saturated with water. Two sets of experiments were conducted, with and without cross-flow, at 1 kV increment from the corona threshold voltage until the occurrence of sparkover. For each case a companion experiment was carried out simultaneously under the same ambient conditions but without the application of electric field. The result of which is used as a basis in the evaluation of drying enhancement using electric field (with or without cross-flow). Each experiment lasted for at least 5 h. The weight loss of water as well as the ambient temperature and humidity were measured. For EHD drying without cross-flow, the enhancement in drying rate increases with the applied voltage. With the presence of cross-flow, the enhancement in drying rate is nearly independent of the applied voltage and is close to the result obtained by drying with cross-flow alone.     

EHD-Enhanced Drying with Multiple-Wire Electrode

Abstract

The enhancement of drying rate by corona discharge from multiple wire electrodes has been experimentally evaluated in this study. The corona wind produced by an array of three wire electrodes, which resembles a series of slot jets, impinges, and removes moisture from the surface of 6 mm glass beads saturated with water. Corona discharges with positive and negative polarities have been considered. Two sets of experiments have been conducted (one with the cross-flow of air and the other without), at 1 kV increment from the corona threshold voltage to a maximum voltage of 25 kV. To evaluate the drying enhancement, a companion experiment was carried out simultaneously for each case under the same ambient conditions but without the application of electric field. Each experiment lasted for at least 5 h. The weight loss of water as well as the ambient temperature and humidity were measured. The results are compared with those obtained with a single wire electrode to evaluate the effectiveness of a multiple-wire system.     

EHD-Enhanced Drying with Needle Electrode

Abstract

The enhancement of drying rate by corona discharge from a needle electrode has been experimentally evaluated in this study. The electrode produces corona wind, which resembles a round jet, impinges and removes moisture from the surface. Corona discharges with positive and negative polarities have been considered. The samples used are 3 and 6 mm glass beads saturated with water. Two sets of experiments have been conducted (one with the cross-flow of air and the other one without), at 1 kV increment from the corona threshold voltage until the occurrence of sparkover. For each case, a companion experiment was carried out simultaneously under the same ambient conditions but without the application of electric field. The result of which is used as a reference in the evaluation of drying enhancement using electric field. Each experiment lasted for at least 5 h. The weight loss of water as well as the ambient temperature and humidity were measured. The results are compared with those obtained with a wire electrode to determine the optimal configuration to be used in an EHD drying system.     

EHD-Enhanced Drying with Needle Electrode

The enhancement of drying rate by corona discharge from a needle electrode has been experimentally evaluated in this study. The electrode produces corona wind, which resembles a round jet, impinges and removes moisture from the surface. Corona discharges with positive and negative polarities have been considered. The samples used are 3 and 6 mm glass beads saturated with water. Two sets of experiments have been conducted (one with the cross-flow of air and the other one without), at 1 kV increment from the corona threshold voltage until the occurrence of sparkover. For each case, a companion experiment was carried out simultaneously under the same ambient conditions but without the application of electric field. The result of which is used as a reference in the evaluation of drying enhancement using electric field. Each experiment lasted for at least 5 h. The weight loss of water as well as the ambient temperature and humidity were measured. The results are compared with those obtained with a wire electrode to determine the optimal configuration to be used in an EHD drying system.     

EHD-Enhanced Drying with Multiple-Wire Electrode

The enhancement of drying rate by corona discharge from multiple wire electrodes has been experimentally evaluated in this study. The corona wind produced by an array of three wire electrodes, which resembles a series of slot jets, impinges, and removes moisture from the surface of 6 mm glass beads saturated with water. Corona discharges with positive and negative polarities have been considered. Two sets of experiments have been conducted (one with the cross-flow of air and the other without), at 1 kV increment from the corona threshold voltage to a maximum voltage of 25 kV. To evaluate the drying enhancement, a companion experiment was carried out simultaneously for each case under the same ambient conditions but without the application of electric field. Each experiment lasted for at least 5 h. The weight loss of water as well as the ambient temperature and humidity were measured. The results are compared with those obtained with a single wire electrode to evaluate the effectiveness of a multiple-wire system.     

EHD-Enhanced Water Evaporation

The enhancement of water evaporation by electrostatic field (corona wind) has been experimentally evaluated in this study. Two sets of experiments were conducted, with and without cross-flow, at an increment of 1 kV from the corona threshold voltage until the occurrence of sparkover. Two types of electrodes (wire and needle) were used. In addition, both positive and negative corona discharges were applied. The weight loss of water due to evaporation as well as the ambient temperature and humidity were measured. For each case a companion experiment was carried out simultaneously under the same ambient conditions but without the application of electric field. The result of which is used as a basis in the evaluation of the evaporation enhancement using electric field. Each experiment lasted for at least 5 h. With the application of electric field alone, the enhancement in the water evaporation rate increases with the applied voltage. With the introduction of cross-flow, the enhancement in the evaporation rate becomes nearly independent of the applied voltage and is close to the result obtained with cross-flow alone.     

EHD-Enhanced Water Evaporation from Partially Wetted Glass Beads with Auxiliary Heating from Below

Earlier studies have shown that an electric field in the form of corona wind can significantly enhance drying rate. It is particularly effective in the early stage of drying when the material is fully wetted. However, the effectiveness of corona wind in drying a partially wetted material, which is commonly believed decreasing, has not been critically examined. For the present study, enhancement of water evaporation from partially wetted glass beads by corona wind is experimentally evaluated. In addition, auxiliary heating provided by a thermofoil heater attached to the bottom surface of the sample container has been used as a possible means to maintain the effectiveness of corona wind. For the present study, a fine copper wire with a diameter of 0.5 mm is used as the emitting electrode. It is charged by direct current with negative polarity from the corona threshold voltage until the occurrence of sparkover. The water level inside the glass beads is below the external surface and is maintained constant in each set of experiments. For each case, a companion experiment is carried out simultaneously under the same ambient conditions but without the application of electric field or auxiliary heating, the result of which is used as a basis in the evaluation of the evaporation enhancement. The weight loss of water as well as the ambient temperature and humidity are continuously monitored. Each experiment lasted for at least 5 h. The results show that the electric field is effective in the enhancement of water evaporation from partially wetted glass beads, but its effectiveness diminishes when the water level in the glass beads recedes. By applying auxiliary heating, this shortcoming of EHD-enhanced drying can be overcome.     

DRYING WITH SUPERHEATED STEAM

The principle of drying with superheated steam is known for a long time already, and different designs have been worked out and partly realised. The principle of the steam drying described in this paper starts with superheated steam that is blown on to the products to be dried. The superheated steam transfers its sensible heat to the product and the water to be evaporated. The superheated steam acts both as heat source and as drying medium to take away the evaporated water. The majority of the superheated steam has to be recirculated and reheated. The excess steam from this process, corresponding to the evaporated water, can be used elsewhere in the process or plant, thus making an efficient energy recovery possible. Possible emissions from the drying process can be effectively removed by condensation. Drying by means of superheated steam impingement is one of the possible designs that are in progress now for the paper industry. Work is going on with respect to this type of drying for other materials, especially foodstuffs. At present experimental research at the laboratory steam dryer of TNO-MEP is carried out for vegetables, fries, herbs, cacao nuts, wheat, flour, etc. Besides the energy savings and environmental benefits, the oxygen free steam atmosphere and the higher product temperatures in the dryer appear to have positive effects on the product qualities. For foodstuffs combinations of steam drying with blanching, pasteurisation, sterilisation, etc. are possible and make the steam drying very attractive for food producers. Modelling of the processes in foodstuffs during steam drying is established, both on macro and micro scale. In this paper examples and results of research on steam drying for specific products will be presented.     

塑料的干燥及其设备

介绍塑料干燥的特点，方法和干燥设备的原理，结构及选用。     

SORPTION DRYING OF SOYBEAN SEEDS WITH SILICAL GEL

Seeds with high moisture content are easily attacked and damaged by microorganisms. It is important to reduce the moisture content to a safe level for storage. Soybean seeds, directly mixed with silica gel in different mass ratios, were dried in static beds at different environmental temperatures. The drying kinetics were compared and analyzed. A diffusion-type model was used to simulate the sorption drying process with success. Simple relations have been developed for sorption isotherms of both soybean and silica gel. A mass diffusivity equation for soybean drying was supplied in Arrhenius type.     

DRYING KINETICS DERIVED FROM DIFFUSION EQUATION WITH FLUX-TYPE BOUNDARY CONDITIONS

A novel approach to drying of both hygroscopic and non-hygroscopic capillary - porous materials is presented. The equations for drying kinetics are obtained, based on the analytical solution of the differential equation of diffusion with a boundary condition in the form of the flux. Theoretical drying curves generated form the model solution compare well with experimental literature data for drying of materials with different sorption properties exemplified by wool bobbins, brick slabs and corn kernels.     

DRYING CHARACTERISTICS OF OSMOTICALLY DEHYDRATED FRUITS COATED WITH SEMIPERMEABLE EDIBLE FILMS

This paper deals with key issues of the coating technology applied to plant materials prior to osmotic dehydration and convective drying. Coatings created with solutions of starch products and pectin were examined. It was shown that coating of foods to be dehydrated influences mass transfer in a plant tissue during osmotic pre-treatment as well as during convective drying. Each coating should be individually examined. The properties of coatings depend on their composition and the method used for the fabrication of a coating. Coating of foods with an artificial barrier on the surface may efficiently hinder the penetration of solute inside the food, not affecting much the rate of water removal. The coated plant materials had a greater water loss/solids gain ratio then the uncoated ones. Osmotic dehydration seems to be one of possible pre-treatment methods before drying. The conditions of convective drying are changed upon osmotic dehydration of plant materials. It was shown that changes in drying rate depend on the kind of coating substance. Bigger differences were observed at higher water content and these differences narrowed as water content in dried material decreased. The positive effect of coatings on the physical properties of dried fruits was found.     

OPTIMUM OPERATING CONDITIONS IN DRYING FOODSTUFFS WITH SUPERHEATED STEAM

It is inferred from experimental data that in drying foodstuffs with superheated steam, the initial drying rate has a direct effect on the rate at which the overall drying takes place. That is, the faster the initial drying rate, the shorter the overall drying time. This criterion is very convenient because at the beginning, water moistens the sample external surface so evaporation does not depend on internal sample characteristics, but only on external convective heat and mass transfer rates. Mass and energy balance equations are solved and the result converted into a general initial drying rate equation, in which all dryer characteristics are grouped into one dimensionless parameter. The initial drying rate equation is mathematically maximized and the optimum working conditions determined. The result shows that initial drying rate always increase with increases of either the superheated steam temperature or velocity, but once these two variables are fixed, there exists at least one “optimum” pressure at which the initial drying rate is a maximum. Finally, the initial drying rate and optimum condition equations are applied to three model dryers, a dryer for a flat sheet, a fixed bed dryer and a rotary dryer. In each case, numeric values are computed and plotted as drying rate versus pressure curves, in which the optimum drying rate is also included. Also presented is a chart to compare the optimum pressures as functions of temperature and steam velocity for the three dryers.     

DRYING KINETICS AND PARTICLE RESIDENCE TIME IN SPRAY DRYING

ABSTRACT

A unique experimental equipment for extensive trials on the spray drying kinetics and particles residence time involving “in situ” analysis of the properties of continuous and dispersed phases was designed, built, and tested. Advanced experimental techniques (including laser techniques) to determine current parameters of spray drying process (temperature, humidity, moisture content) and current structure of spray (particle size distribution, particle velocities, etc.) were employed. Full scale spray drying tests of baker's yeast and maltodextrin enabled identification of the effect of process parameters on drying kinetics and spray residence time in the tower. Quantitative relationship describing spray drying kinetics as a function of atomization ratio and drying agent temperature were determined. The experimental results proved that spray residence time was controlled by atomization ratio and airflow rate. Drying kinetics in spray drying process is presented for the first time in the literature.     

HOW TO USE FLUIDIZATION TO OBTAIN DRYING KINETICS COUPLED WITH QUALITY EVOLUTION

Abstract

This paper presents the main activities of the Gas Fluidization and Drying Team of the Chemical Engineering Laboratory at Toulouse. The emphasis has been put on the modeling of a fluidized bed dryer and its derivates such as a flotation fluidized bed, by the analysis of heat and mass transfer together with the physico-chemical transformations of the product during drying. Several studies have been performed on different solids (cereal and chemical products) to illustrate the effect of the operating conditions on dehydration and degradation kinetics The knowledge of heat and mass transfer coefficients in flotation fluidized bed allows to determine intrinsec drying and degradation kinetics. These kinetics can be used in any conductive or convective drier model.     

DRYING CHARACTERISTICS OF OSMOTICALLY DEHYDRATED FRUITS COATED WITH SEMIPERMEABLE EDIBLE FILMS

This paper deals with key issues of the coating technology applied to plant materials prior to osmotic dehydration and convective drying. Coatings created with solutions of starch products and pectin were examined. It was shown that coating of foods to be dehydrated influences mass transfer in a plant tissue during osmotic pre-treatment as well as during convective drying. Each coating should be individually examined. The properties of coatings depend on their composition and the method used for the fabrication of a coating. Coating of foods with an artificial barrier on the surface may efficiently hinder the penetration of solute inside the food, not affecting much the rate of water removal. The coated plant materials had a greater water loss/solids gain ratio then the uncoated ones. Osmotic dehydration seems to be one of possible pre-treatment methods before drying. The conditions of convective drying are changed upon osmotic dehydration of plant materials. It was shown that changes in drying rate depend on the kind of coating substance. Bigger differences were observed at higher water content and these differences narrowed as water content in dried material decreased. The positive effect of coatings on the physical properties of dried fruits was found.     

BATCH DRYING OF SHELLED CORN IN TWO-DIMENSIONAL SPOUTED BEDS WITH DRAFT PLATES

ABSTRACT

Geometrically similar spouted beds with draft plates were used to obtain the drying characteristics of freshly harvested shelled corn with 0.28 to 0.31 kg/kg initial moisture content at different air inlet temperatures and bed heights. Thermal equilibrium between air and grains was achieved at minimum spouting conditions. The drying kinetics of shelled corn in a drafted two-dimensional spouted bed was found to be of the "thin layer" type. Expressions for the model parameters in Page's equation accounting for the bed geometry, grain moisture content, and drying conditions were developed.     

等速干燥浅议

本文提出了干燥初期的数学模型，过程模拟方法以及用ＢＡＳＩＣ语言编制的计算框图。对等速干燥阶段开始的特征给出了定量的计算结果。     

A DYNAMIC MODEL WITH ON-LINE IDENTIFICATION FOR ROTARY SUGAR DRYING PROCESSES

A dynamic modelling methodology, which combines on-line variable estimation and parameter identification with physical laws to form an adaptive model for rotary sugar drying processes, is developed in this paper. In contrast to the conventional rate-based models using empirical transfer coefficients, the heat and mass transfer rates are estimated by using on-line measurements in the new model. Furthermore, a set of improved sectional solid transport equations with localized parameters is developed in this work to replace the global correlation for the computation of solid retention time. Since a number of key model variables and parameters are identified on-line using measurement data, the model is able to closely track the dynamic behaviour of rotary drying processes within a broad range of operational conditions. This adaptive model is validated against experimental data obtained from a pilot-scale rotary sugar dryer. The proposed modelling methodology can be easily incorporated into nonlinear model based control schemes to form a unified modelling and control framework.     

A DYNAMIC MODEL WITH ON-LINE IDENTIFICATION FOR ROTARY SUGAR DRYING PROCESSES

A dynamic modelling methodology, which combines on-line variable estimation and parameter identification with physical laws to form an adaptive model for rotary sugar drying processes, is developed in this paper. In contrast to the conventional rate-based models using empirical transfer coefficients, the heat and mass transfer rates are estimated by using on-line measurements in the new model. Furthermore, a set of improved sectional solid transport equations with localized parameters is developed in this work to replace the global correlation for the computation of solid retention time. Since a number of key model variables and parameters are identified on-line using measurement data, the model is able to closely track the dynamic behaviour of rotary drying processes within a broad range of operational conditions. This adaptive model is validated against experimental data obtained from a pilot-scale rotary sugar dryer. The proposed modelling methodology can be easily incorporated into nonlinear model based control schemes to form a unified modelling and control framework.