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.     

HYBRID DRYING OF GRANULAR MATERIALS BY COMBINED RADIATIVE AND CONVECTIVE HEATING

As one of the basic steps of the R&D on hybrid drying by combined radiative and convective heating, drying experiments of a stagnant horizontal wet granular layer heated simultaneously by infrared lamps and hot air were conducted throughout a whole period of the drying. An opaque model, in which the incident radiation is assumed to be absorbed only on the surface of the layer, was introduced to explain experimental trends. Then, the effects of combined heating on drying characteristics of three kinds of samples, a silica sand, a brick and a brown coal were discussed by comparing the experimental data with the calculated results.

The calculated results were found to be in a close agreement with the experimental data obtained on the time behaviors of the drying rate and the temperature distribution in the- layer during not only the preheating and the constant rate periods but also the falling rate period. In the case of the coal layer, however, the falling drying rate different from that in ordinary convective drying had to be considered since it appeared that the radiative heating had an effect to enhance the falling drying rate of the coal owing to some interactions of the coal and water.     

氢氧化镁纳米棒的热分析干燥动力学研究

在不同干燥介质温度（333～513K）和不同物料床层厚度（0．5～7mm）的恒定干燥条件下,得到氢氧化镁纳米棒的干燥曲线和干燥速率曲线。采用热分析动力学技术对干燥动力学实验数据进行处理后,得到氢氧化镁纳米棒的干燥微分机理函数为f（1-MR）=2MR（-MR）^1/2,干燥积分机理函数为g（1-Mg）=（-lnMR）^1/2,干燥方程为Mn=exp[-（kt）^2],干燥速率方程为-dMR/dt=2kMR（-lnMR）^1/2,干燥速率常数为k=Aexp[-Ea／RT]=Aexp[-Ev（1＋CLL）／RT];指前因子A=10．741min^-1,界面蒸发活化能Ev=10．671kJ/mol,经验常数CL=100．000m^-1。     

Drying Enhancement of Clay Slab by Microwave Heating

The effect of internal heating by microwave on the drying behavior of a slab was studied. A wet sample of kaolin pressed into a slab was subjected in microwave irradiation of 2.45 GHz. The absorption of microwave energy into a wet slab can be expressed by a function of the moisture content and the pathway length, which is a similar form to Lambert-Beer's law. The drying behavior was compared among three modes: microwave irradiation, hot air heating and radiation heating in an oven. Microwave heating with a constant power resulted in breaking the sample when the internal temperature achieves at 373 K. However, if the power was controlled to maintain the temperature less than the boiling point of water, the drying succeeded without any crack generation until the completion with a significantly faster drying rate than in convective heating or in the oven. It is also noted that the transient behavior of the temperature is quite different from the conventional drying.     

HYBRID DRYING OF GRANULAR MATERIALS BY COMBINED RADIATIVE AND CONVECTIVE HEATING

ABSTRACT

As one of the basic steps of the R&D on hybrid drying by combined radiative and convective heating, drying experiments of a stagnant horizontal wet granular layer heated simultaneously by infrared lamps and hot air were conducted throughout a whole period of the drying. An opaque model, in which the incident radiation is assumed to be absorbed only on the surface of the layer, was introduced to explain experimental trends. Then, the effects of combined heating on drying characteristics of three kinds of samples, a silica sand, a brick and a brown coal were discussed by comparing the experimental data with the calculated results.

The calculated results were found to be in a close agreement with the experimental data obtained on the time behaviors of the drying rate and the temperature distribution in the- layer during not only the preheating and the constant rate periods but also the falling rate period. In the case of the coal layer, however, the falling drying rate different from that in ordinary convective drying had to be considered since it appeared that the radiative heating had an effect to enhance the falling drying rate of the coal owing to some interactions of the coal and water.     

不溶性硫磺高温稳定性测定方法的研究

介绍测定不溶性硫磺高温稳定性的两种方法(湿法和干法)并进行比较。湿法操作复杂,但物料受热均匀,结果较合理;干法操作简便,但少量物料温度不一致,分析结果偏低,但仍在可接受范围。两种方法都可满足工业应用的要求。     

SIMPLIFIED MODELLING FOR THE DRYING OF A NON HYGROSCOPIC CAPILLARY POROUS BODY USING A COMBINATION OF DIELECTRIC AND CONVECTIVE HEATING

The drying of non-hygroscopic, capillary porous materials with dielectric and convective heating is considered. Simplified models are used to examine the effect of a partially wetted surface or receding evaporation front on the temperature of the wet material. Physical mechanisms which can enhance the internal moisture flow, as compared with convective heating alone, are examined ualitatively.     

SIMPLIFIED MODELLING FOR THE DRYING OF A NON HYGROSCOPIC CAPILLARY POROUS BODY USING A COMBINATION OF DIELECTRIC AND CONVECTIVE HEATING

The drying of non-hygroscopic, capillary porous materials with dielectric and convective heating is considered. Simplified models are used to examine the effect of a partially wetted surface or receding evaporation front on the temperature of the wet material. Physical mechanisms which can enhance the internal moisture flow, as compared with convective heating alone, are examined ualitatively.     

DEHUMIDIFICATION DRYING OF HIGH-MOISTURE ROUGH RICE IN A RADIAL-FLOW, CIRCULAR BIN

An experimental radial-flow, circular bin dryer with a dehumidifier was fabricated and tested for rough rice drying. Results showed that high-moisture paddy could be dried to 14% moisture content (wet basis) over a 24 h drying span at an average drying rate of 0.5%/h. The open-cycle system's specific moisture extraction rate and the dehumidifier's coefficient of performance were within the usual range of values cited for similar conventional systems or equipment. The quality of the dehumidifier-dried and naturally dried paddy was similar with respect to minimum grain Assuring, The system was found to be a technically feasible, more energy-efficient and safer alternative to electrical resistance heating in the conditioning of ambient air for drying under humid tropical conditions.     

含湿多孔介质的干燥特性

基于含湿多孔介质的水分蒸发过程及其内部毛细管水分的蒸发特性，分析了含湿多孔介质在干燥过程中发生体积收缩的原因．讨论了多孔介质的物性和外部干燥条件对其体积收缩特性的影响。     

Kinetics of Drying of Fibre Materials and Fibre-Forming Polymers with Warming

Drying with heating of fibres, fibre materials, and fibre-forming polymers takes place with low-intensity energy feed, with an increase in the thickness of the material dried, or with a high moisture content of the drying agent. Heating of the material in the initial drying period is analyzed and the corresponding equation for the drying kinetics is obtained. The dependences for describing the kinetics of drying with heating are obtained for different materials and conditions. The general kinetic curve describes periods of heating and a constant and diminishing drying rate. Drying with first and second periods (instantaneous heating of the material) is obtained from the overall dependence as a special case. The dependences obtained are compared with the experimental data for different types of drying of fibres, fabrics, and fibre-forming polymers. The calculated drying parameters are found. The calculated curves are in good agreement with the experimental data in the working range. A method for calculating drying using the dependences obtained is described.     

盐酸法饲料级磷酸氢钙产品的干燥

对盐酸法饲料级磷酸氢钙生产中干燥过程存在的问题进行了分析 ,认为湿料中氯化钙的存在 ,使湿物料的水蒸气分压降低 ,传质推动力减小是造成干燥速率下降的主要原因。解决问题的办法是增加对湿物料的水洗 ;减压干燥 ,降低系统压力 ;采用逆流操作的干燥器     

碱式碳酸镁纳米花的热分析干燥动力学研究

以六水氯化镁和尿素为原料,采用均匀沉淀法制备出碱式碳酸镁纳米花。在不同干燥介质温度和不同物料床层厚度下,得到碱式碳酸镁纳米花的干燥曲线和干燥速率曲线。采用热分析动力学技术对干燥动力学实验数据进行处理后,得到碱式碳酸镁纳米花的干燥微分机理函数为f(1-MR)=6MR2/3(1-MR1/3)1/2,干燥积分机理函数为g(1-MR)=(1-MR1/3)1/2,干燥方程为MR=〔1-(kt)2〕3,干燥速率方程为-dMR/dt=6k2tMR2/3,干燥速率常数为k=Aexp〔-Ev(1+CLL)RT〕;指前因子A=4.0725min-1,界面蒸发活化能Ev=16.9334kJ/mol,经验常数CL=30.3135m-1。     

Numerical Analysis of Drying Characteristics of Wet Material Immersed in Fluidized Bed of Inert Particles

The drying characteristics of a relatively large wet material immersed in a fluidized bed of inert particles were examined theoretically. Calculations of heat transfer and evaporation of water on the material surface were included in a discrete element method–computational fluid dynamics (DEM-CFD) simulation. Only the preheating and constant drying rate periods were considered in the simulation.

Our simulation made it possible to calculate the relation between the behavior of the fluidizing particles and the drying characteristics. The drying rate increased with an increase in the number of fluidizing particles in contact with the material and the number of fluidizing particles changed with time; that is, the drying rate fluctuated. When the drying rate increased, the fluid temperature decreased and the vapor content of the fluid increased around the material; that is, the temperature and vapor content of the fluid were associated with the behavior of the fluidizing particles.     

氢氧化镁粉体的干燥动力学研究

采用以六水氯化镁为原料,氢氧化钠和氨水为混合沉淀剂的直接沉淀法,制备得到平均粒径约为2μm的氢氧化镁粉体。通过不同干燥介质温度和不同湿物料层厚度的干燥动力学实验,获得氢氧化镁粉体的干燥曲线和干燥速率曲线。采用Weibull函数模型,对实验数据进行拟合分析,得到氢氧化镁粉体的干燥动力学参数和干燥动力学方程,探讨干燥动力学参数的影响因素。研究结果表明,可以用Weibull函数模型来描述氢氧化镁粉体的干燥过程,尺度参数与干燥介质温度及物料层厚度有关,随着干燥介质温度的提高,或物料层厚度的降低,干燥速率增大,尺度参数减小,但形状参数变化不大。     

DEHUMIDIFICATION DRYING OF HIGH-MOISTURE ROUGH RICE IN A RADIAL-FLOW,CIRCULAR BIN

Abstract

An experimental radial-flow, circular bin dryer with a dehumidifier was fabricated and tested for rough rice drying. Results showed that high-moisture paddy could be dried to 14% moisture content (wet basis) over a 24 h drying span at an average drying rate of 0.5%/h. The open-cycle system's specific moisture extraction rate and the dehumidifier's coefficient of performance were within the usual range of values cited for similar conventional systems or equipment. The quality of the dehumidifier-dried and naturally dried paddy was similar with respect to minimum grain Assuring, The system was found to be a technically feasible, more energy-efficient and safer alternative to electrical resistance heating in the conditioning of ambient air for drying under humid tropical conditions.     

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.     

湿法炼锌泥的微波干燥实验

目前回转窑干燥物料的方法主要存在干燥时间长、环境污染严重、效率低等缺点,为了解决这一问题,开展了微波干燥湿法炼锌泥的实验研究,采用实验室自制的微波干燥设备,考察微波功率、干燥时间以及物料质量对样品脱水率的影响。研究结果表明:在一定条件下,脱水率随干燥时间延长而增加,随着微波功率的增加而提高,随物料质量增加而降低。实验在微波功率600W、物料质量40g、微波加热时间3min时,湿法炼锌泥的脱水率达到99.57%;FT-IR分析可知,干燥后波数1625cm^-1和3300cm^-1附近水分子的伸缩振动峰基本消失,说明微波干燥后物料基本不含水分;与常规干燥进行比较,微波干燥湿法炼锌泥具有干燥时间短、脱水效率高、清洁无污染的优点,是一种高效节能的新型干燥方法。     

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.     

棒状MgCO3·3H2O的干燥动力学研究

以六水氯化镁为原料,碳酸氢铵为沉淀剂,采用直接沉淀法,制备出长约为80μm,直径为5～10μm的棒状三水碳酸镁;通过不同热空气温度及不同湿物料层厚度下的干燥动力学实验,获得棒状三水碳酸镁的干燥曲线和干燥速率曲线。采用相变动力学理论,对干燥动力学实验数据进行拟合分析,得到棒状三水碳酸镁的干燥方程、干燥速率方程及干燥动力学参数。