微波技术在种子干燥方面的研究进展

干燥是种子加工的重要环节，微波干燥技术具有独特的优点和特点。近十几年来，国内外在种子微波干燥方面的研究取得了不少成果，而有关这方面的综述却未见报道。本文在阐述微波干燥机理和特点基础上，就近年来国内外微波技术在种子干燥方面的研究进展作一综述，并探讨微波技术在种子干燥方面的应用前景。     

薄层干燥技术的研究进展

简要介绍了各种类型的薄层干燥方程,并对薄层干燥技术在农产品、水产品、中药材及其他产品干燥特性研究中的应用进行综合评述。     

基于薄层干燥模型的褐煤干燥动力学研究

在不同颗粒直径和不同的干燥介质温度下,对褐煤进行干燥动力学实验研究。得到了褐煤的干燥曲线和干燥速率曲线,采用薄层干燥模型对实验数据进行模拟,得到了褐煤的干燥方程和干燥速率方程。提出了褐煤干燥速率常数的经验公式k=A exp[（-E_v（1+C_dlnd））/（RT）],其中指前因子A=5.819min^-1,界面蒸发活化能E_v=21347.5KJ/mol,经验常数C_d=0.0409m^-1,干燥时间指数n=1.516。     

基于薄层干燥模型的碱式碳酸镁纳米花干燥动力学研究

在不同干燥介质温度和不同物料床层厚度下,对碱式碳酸镁纳米花进行干燥动力学实验,得到其干燥曲线和干燥速率曲线。采用薄层干燥模型对所得干燥动力学实验数据进行数学处理,得到碱式碳酸镁纳米花的干燥方程为M_R=exp[-(kt)~n],干燥速率方程为-(dM_R)/(dt)=knM_R(-lnM_R)~((n-1)/n),干燥速率常数k=A exp[(-E_v(1+C_LL))/(RT)],干燥时间指数n=1.738,界面蒸发活化能E_v=16.521kJ/mol,指前因子A=7.214min~(-1),经验常数C_L=29.900 m~(-1)。     

油页岩微波干燥过程分析及模拟

以柳树河油页岩为原料,分别在100℃热风温度和不同的微波功率的干燥条件下进行试验;用Weibull分布函数对油页岩的干燥曲线进行拟合分析,结合尺度参数估算水分有效扩散系数。结果表明:加速干燥阶段脱除的是油页岩颗粒的表面水;前期存在预热过程,温度升高,水分析出很少;随后干燥速率显著增大。恒速阶段析出的也是表面水,受物理脱附作用的影响;功率越大,恒速段时间越短。降速第一阶段主要是大孔隙中水分的脱除,降速第二阶段主要是中孔和微孔中水分的汽化。临界水分比随功率的增加而升高。Weibull分布函数准确模拟了油页岩微波干燥曲线;尺度参数α值随功率增加而减小,功率大于550 W后减小幅度降低;微波干燥的形状参数β1,即升速段出现在干燥前期;估算的水分有效扩散系数随功率增加而增大。微波干燥和热风干燥时相比,油页岩颗粒形态并没有发生显著变化。     

聚烯烃织物背胶干燥特性及干燥模型研究

研究了聚烯烃织物背胶在不同干燥温度和风速时的干燥曲线、干燥速率曲线、水分有效扩散系数(Deff)以及干燥活化能(Ea),建立了干燥的数学模型。研究结果表明:聚烯烃织物背胶的整个干燥过程属于降速干燥,干燥温度和风速的升高都有利于缩短干燥时间;Page模型能较好描述聚烯烃织物背胶的干燥过程,并且Deff为(2.46~4.15)×10-8m2/s,Ea为26.95 kJ/mol。     

氢氧化铝薄层干燥的节能方法探讨

氢氧化铝的洞道式薄层干燥在生产中有重要应用，但要消耗大量的能量。对氢氧化铝的干燥的基础特性进行了研究，并着重探讨了变温干燥的节能方法。试验结果表明：氢氧化铝的洞道式薄层干燥主要由水分内部迁移控制，强化其干燥速率应着重从改善物料状况着手；并采用冷热风交替的干燥方法将取得较好的节能效果。这些工作对改进生产中的氢氧化铝的干燥方法具有一定的指导意义。     

府谷半焦干燥特性研究

针对炼焦工艺及熄焦方式不同导致焦炭含水率波动较大问题,以府谷半焦为原料,进行半焦干燥实验研究,考察了粒径、干燥温度、有无鼓风、堆积厚度等因素对半焦干燥速率的影响。结果表明,在恒定的干燥条件下,粒径越小,干燥速率越快。若将半焦粒径从50 mm减小到10 mm,则焦炭含水率干燥至1%所需干燥时间由57 min降至31 min;干燥温度降低,干燥初始阶段的调整期延长,干燥速率迅速下降,水脱除量急剧减少。因此,为保证较快的干燥速率,干燥温度应控制在200℃以上为宜;增加鼓风,降低堆积厚度有助于提高干燥速率,通过鼓风方式干燥可将脱水率提高23%。     

鸡西油页岩非等温干燥特性分析

以鸡西油页岩为原料,利用热重分析仪进行了3种不同升温速率的干燥试验。结果表明:干燥速率随干基含水率的变化曲线分成3个阶段;对于含水率wX>wXA阶段,是表面水蒸发干燥过程,主要受物理脱附作用影响;wXA>wX>wXB阶段,失去的水分是较大毛细孔内的水,主要受油页岩样品体积收缩和内部毛细孔浓缩共同作用影响;wXB>wX>0阶段,这一阶段的主要是小毛细孔和中孔水分的蒸发。有效水分扩散系数有3段不同变化趋势,开始急剧增长,随之增长平缓,最后下降;在干燥的末端出现一个明显的突变点,该点之后有效水分扩散率有一个明显的激增。干燥速率随着升温速率的增加而增加;同时随着升温速率的增加水分有效扩散系数升高和降低趋势更加明显。     

Microwave-Assisted Thin Layer Drying of Wheat

Drying characteristics of Canada Western Red Spring (CWRS) wheat were studied using a domestic microwave convective oven. The effects of microwave power level, grain bed thickness, and initial grain moisture on the drying kinetics were investigated. Wheat samples with initial moisture levels of 0.18 to 0.29 kg water/kg of dry matter were dried for different drying periods of 180 to 360 s. The moisture loss data were recorded at regular short intervals. Then moisture loss data were fitted to various models (Page equation, modified drying equation, and Midilli equation) to study the drying kinetics of wheat. The results showed that wheat moisture loss increased with increasing microwave power level. A mathematical model was developed by coupling mass and energy balances, resulting in a system of non-linear equations. The predicted moisture loss data from the developed model were compared by fitting to experimental microwave data that were in good agreement.     

Thin Layer Drying Models for Osmotically Pre-dried Young Coconut

Thin layer convection drying was performed on osmotically pre-dried young coconut, strips, both thin and thick. A drying air temperature range of 50-70°C and an airflow of 0.25 m s^-1 was used to dry samples soaked in three sugar solution concentrations (40, 50, and 60°B) during the osmotic drying phase, with the convection drying alone serving as control. An analysis of variance (ANOVA) revealed that sugar concentration and thickness significantly affected osmotic drying rates as shown by their final moisture contents. While the drying air temperature and slab thickness significantly affected the average drying rate and the sugar concentration was an insignificant factor during convective drying phase. Effective diffusivity of water during hot air drying varied from 1.71 to 5.51 × 10^-10 m²s^-1 over the temperature range investigated, with energy of activation equal to 1173.0 kJ/kg. Three mathematical models available in the literature were fitted to the experimental data, with the Page model giving better predictions than the single or double term exponential model. The temperature dependence of the diffusivity coefficients was satisfactorily described by a simple Arrhenius type relationship.     

城市污泥与木屑混合薄层干燥实验及动力学分析

角度研究了比,添加木通过对纯污泥和添加木屑的污泥进行薄层干燥对比实验,分别从混合比例、薄层厚度、干燥温度、风速木屑添加对污泥干燥特性的影响,并引入薄层模型对其干燥过程进行模拟。结果表明：与纯污泥干燥相屑后污泥干燥速率明显加快,且木屑添加比例越大、薄层越薄、干燥温度越高干燥速率越快,风速对干燥速率的影响不大;Wang—singh模型能很好的描述两种污泥薄层干燥,利用费克第二扩散定律导出的无限平板公式求出纯污泥和添加木屑的污泥在温度120℃～170℃时的有效扩散系数分别为6．13×10-6m／s～1．11×10-5m／s、1．07×10-5m／s～1．67×10-5m／s;由阿伦尼乌斯方程分别求得活化能为Es=16．67kJ／mol、Es=12．97kJ／mol。     

Experimental Validation of a Mathematical Model for the Batch Drying of Corn Grains

ABSTRACT

Yellow dented corn samples were dryed i n the laboratory with heated air i n a batch fluidized bed dryer at three different operation conditions. The evolution o f the temperature nnd moisture content were predicted with both, a model proposed in the literature that neglects the position dependence o f the diffusion coefficient and a modified version o f i t that takes into account the above mentioned functionality. Both models were solved by means of collocation i n finite elements. The consideration o f the diffusivity as a position function leads to better agreement with the experimental results.     

Determination of Moisture Diffusivity of Lentil Seed During Drying

The mechanism of drying and the diffusion of water in green (fresh), red and whole lentils (4.20-4.32 mm diameter and 2.20-2.30 mm thickness) were successfully interpreted and modelled by using Fick's law. The moisture content of fresh green lentil was 55 % and moisture content of the red and whole lentils was increased to 39-40 % and drying temperature (45-60°C) was varied, but the velocity and the humidity of the drying air was kept constant.

The effective diffusuvity was estimated from drying rate curves and expressed by an Arrhenius relation.     

Numerical Modeling of Moisture and Temperature Distribution within a Rectangular Bagasse Layer Undergoing Drying

In this work, a numerical two-dimensional model using the finite volume method, which predicts the temperature and moisture distribution of the moist rectangular bagasse layer undergoing drying, is developed. During the drying process, variable heat and mass transfer coefficients are considered. The flow fields are numerically predicted using a commercial CFD package, Fluent. The temperature and moisture distributions under transient conditions are obtained, which determine both heat and moisture transport inside the material. The validation of the model is carried out by comparing the predicted mean moisture content values with those obtained experimentally. The comparison of the numerical and experimental result shows good agreement up to 8%.     

A Numerical Study of Transient Deformation and Stress Behavior of a Clay Slab During Drying

A transient three-dimensional analysis was carried out on internal strain-stress as well as heat and the moisture transfer in a ceramic slab during drying. A model was developed to analyze viscoelastic behavior, heat conduction and moisture diffusion. The basic equations were solved by the finite element method. The effects of several dimensionless parameters are discussed to find an optimum drying process and a precise design of molds in ceramic production. The stress and the gradient of moisture content were influenced significantly by the Biot or Lewis number. When the moisture diffusion is enhanced or the drying is controlled well so as to form only gentle gradients of moisture content in the slab, the maximum tensile stress can be reduced. Nonuniform drying results in the develoment of warp and increase in the maximum tensile stress. The drying characteristics were not appreciably influenced by shrinkage.     

Evaluation of Parameter Values For a High-Temperature Drying Simulation Model Using Direct Drying Experiments

ABSTRACT

An existing drying simulation model for softwood heartwood is revised lrom a theoretical point of view and is completed by extracting all the essential material parameters from moisture and temperature profiles measured in direct drying experiments. Some additional experimental results are presented to validate the qualitative characteristics of the model. The resulting computational model shows good agreement with experiments in the drying temperature range 45-120°C. Special attention is paid to the evolution of moisture gradient during drying, as the next step of the simulation project will be using the result as an input data for a computational stress model.