Isothermal Drying of Pore Networks: Influence of Friction for Different Pore Structures

An existing network model for isothermal drying of capillary porous media is extended to account for viscosity in the liquid phase so that it is no longer restricted to structures with large pores. Modeling challenges and solution methods are presented in detail. The model is compared with a bundle of capillaries model of drying. Finally, simulation results for two-dimensional pore networks with mono-modal and bimodal pore structure are shown and discussed.     

Isothermal Drying of Pore Networks: Influence of Friction for Different Pore Structures

An existing network model for isothermal drying of capillary porous media is extended to account for viscosity in the liquid phase so that it is no longer restricted to structures with large pores. Modeling challenges and solution methods are presented in detail. The model is compared with a bundle of capillaries model of drying. Finally, simulation results for two-dimensional pore networks with mono-modal and bimodal pore structure are shown and discussed.     

Drying of Coarse Particles in a Vertical Pneumatic Conveyor

In the present work, one-dimensional two-phase continuum models were applied to simulate the pneumatic drying of porous alumina and solid glass particles. Pressure profiles, gas and solid temperature, and gas and solids moisture profiles were obtained in a 53.4-mm conveying tube. For both particles, maximum values of gas-to-particle heat transfer coefficients were obtained at air velocities close to the minimum pressure gradient velocity. Experimental temperature and moisture profiles of gas and solids were compared to simulated predictions, showing that models based on the two-phase flow approach fail to predict all the observed physical phenomena in simultaneous momentum, heat, and mass transfer for pneumatic drying of coarse particles. However, using adequate correlations and constitutive equations to predict interaction forces and transport parameters, it was possible to obtain good predictions of gas and solid temperature profiles and of moisture content.     

Fractal Theory on Drying: A Review

Fractal geometry has been widely used in various dried materials and drying processes. This review summarizes the related studies and identifies the opportunities for future investigation. The application of fractal concept on drying can be categorized into describing microscopic and macroscopic structure of material in drying with fractal geometry and theoretical models with fractal theory for drying mechanism. And also, the capillary and network models for drying of capillary porous media are discussed and a fractal tortuous capillary model for drying of capillary porous media is proposed. From the selected examples, it is clear that the fractal theory has many advantages for identifying the complex structure of products and investigating drying mechanism. At last, some comments are made for the current investigations and also some prospects for the future development of this field are pointed out.     

Analysis of Drying Kinetics and Moisture Distribution in Convective Textile Fabric Drying

The drying process of crude cotton fabric is analyzed under two main aspects: analysis of moisture distribution inside the textile sheet, and analysis of certain operational convective drying process variables. Experimental apparatus consisted of a drying chamber in which samples of pure cotton textile were suspended inside the drying chamber and exposed to a convective hot air flow. The influence of the operational variables on the drying process behavior was studied by two different ways with generalized drying curves. The behavior of moisture distribution profiles was compared to average moisture content of the textile fabric verifying whether average values were able to represent the textile moisture content during the drying process.     

石灰调质污泥恒温干燥特性及动力学模型研究

对添加石灰的市政污泥进行恒温干燥实验,研究在不同的石灰添加量和不同的温度条件下污泥干燥特性、有效水分扩散系数和干燥活化能。结果表明:高温干燥时,石灰添加量的增加对平均干燥速率以及有效扩散系数的提高影响显著,低温干燥时影响不显著;石灰的添加可以降低干燥活化能;基于Modified Page模型建立的通用干燥模型能准确地描述石灰调质污泥的干燥特性;模型方程预测值和实验值吻合,均方根误差为0.32%。     

Viscous stabilization of drying front: Three-dimensional pore network simulations

In this study, a recently developed pore network drying model [Metzger, T., Irawan, A. and Tsotsas, E., 2007, Isothermal drying of pore networks: influence of friction for different pore structures, Dry Technol, 25: 49–57], which accounts for liquid viscosity, is applied to three dimensions for the first time. Isothermal convective drying is simulated for a cubic network (25 × 25 × 50) with pore throats with a narrow radius distribution. The role of liquid viscosity is assessed by comparison with non-viscous drying of the same network. Simulation results are presented as phase distributions, saturation profiles and drying rate curves. In the viscous case, a stabilization of the drying front is observed. However, as the network dries out from the surface, the finite drying front gradually widens up and does not approach an asymptotic limit.     

Shrinkage Kinetics during Convective Drying of Selected Berries

Drying and shrinkage kinetics of gooseberry, black and red currants, cherry, and blackberry were studied using a fully automated experimental apparatus, designed for continuous monitoring of the material moisture content, its temperature, and surface area. Experimental drying curves were approximated with a modified exponential model. The volume and surface area of berries were determined from the pictures taken during drying and transformed into digital images. The procedure for calculation of the object-related pixels in a two-dimensional plane to the temporal three-dimensional volume and surface area of the berries was developed. The modified exponential model was proposed to correlate the volumetric and surface area shrinkage with moisture content. Volumetric shrinkage at the end of drying was maximal (0.3) for black currant and minimal (0.45) for gooseberry.     

A Three Dimensional Model for Heat and Mass Transfer in Convective Wood Drying

A three-dimensional model is proposed to describe the heat and mass transport process in drying of wood. The model is based on conservation of mass and energy and uses constant parameters obtained by comparing experimental data with numerical results. The model uses parameter values from literature. Experimental results obtained for temperature profiles during drying of a block of beech wood are compared with the model results. Satisfactory agreement is obtained over a range of drying air temperatures and velocities.     

IR Technique for Studies of Microwave Assisted Drying

This article examines the idea of using an infrared technique in obtaining temperature profiles in solid particles heated with microwaves. The geometry of a cylinder and a regular prism made from thermally and structurally different materials (potato and gypsum) were tested. A suitability of the IR photography for visualization and quantification of temperature distribution in heated samples has been confirmed. Symmetrical temperature profiles were detected in cylinders in contrast to non-symmetrical ones in prisms.     

IR Technique for Studies of Microwave Assisted Drying

This article examines the idea of using an infrared technique in obtaining temperature profiles in solid particles heated with microwaves. The geometry of a cylinder and a regular prism made from thermally and structurally different materials (potato and gypsum) were tested. A suitability of the IR photography for visualization and quantification of temperature distribution in heated samples has been confirmed. Symmetrical temperature profiles were detected in cylinders in contrast to non-symmetrical ones in prisms.     

A Moisture Transmembrane Transfer Model for Pore Network Simulation of Plant Materials Drying

The drying of plant materials with cellular tissue is often viewed as drying of porous media that is assumed to consist of cell cytoskeleton and intercellular space. Various approaches have been reported in the literature to describe heat and mass transfer during drying of such porous materials. However, the fact remains that the water in a cellular tissue is mostly intracellular and it should be driven out of the cells across cell membranes before transporting in cell gaps, as in a general porous media. In the present study, the transport process of moisture in a cellular tissue was analyzed. A mathematical model for moisture transport across the cell membrane was established, which was correlated to a self-developed, dual-scale pore network model (cell and pore network) for drying of plant materials. The relationship between mass volumetric flux and average intracellular moisture content was developed based on the microscopic images and the drying experiments.     

A Simple Data Processing Approach for Drying Kinetics Experiments

A simple mathematical approach is proposed to be applied to drying kinetics raw data processing. The data collected in a drying experiment of powder cork under constant air drying conditions served as case study to present the methodology. Two functions (linear and third-degree polynomial) were used to fit solid moisture content in the constant drying rate and the falling rate periods. The drying rate curve was obtained by differentiation and the time at which the drying rate period's transition occurs was determined iteratively until virtually continuous functions were achieved. The critical moisture content was easily identified and two falling drying rate periods were detected.

The powder cork moisture decrease was also used to test several semiempirical models available in the literature. The Logarithmic, Midilli, and Page Modified I models were the ones that revealed the best correlations performance. When the methodology proposed was applied using these models, the critical moisture content was underpredicted.

The effective moisture diffusivity and the activation energy were also obtained for powder cork after the proposed mathematical approach has been applied on the raw data obtained in experiments performed at different air drying temperatures.     

多孔介质干燥孔道网络模型的研究进展

建立在物质微观传输基础上的孔道网络干燥理论，通过完全离散化的方法在孔道等级上对干燥过程进行研究，描述了多孔介质内部结构参数对干燥过程的影响。介绍了建立孔道网络模型的原理和方法，阐述了基于单元体上孔道网络研究的内容及目的，综述了基于产品等级上孔道网络研究的最新进展，阐明了孔道网络模型方法对干燥理论研究的重要意义。指出，进一步提高网络模型中孔道的拓扑等价性、形状的不规则性及尺寸的相关性，探索网络构建新方法以及增加孔道网络信息量，是孔道网络干燥理论的主要发展方向，并应加强同分形、渗流理论的进一步结合。     

Fractal Pore Network Simulation on the Drying of Porous Media

Based on the knowledge of fractal geometry, physics of flow through porous media, and transport process principle, a fractal pore network model for the drying process of a natural porous body was established in this article. This model takes various factors into consideration, such as liquid-phase flow, vapor-phase diffusion, temperature gradient, and pore microstructure characteristic. The drying dynamics characteristics of potato slices were obtained by the simulation of a fractal pore network model. The simulation results of the fractal pore network model were contrasted with those of a regular one and the experimental data, respectively. The wet patches were observed on the potato slices during the drying experiments, and it was validated by the drying simulation. The results indicate that the drying kinetics from the fractal pore network model, as well as the distributions of moisture and temperature inside the porous body, are more consistent with that of the drying experiments than that of the regular one, and the throat size distribution in the pore network of the porous media has a notable influence on the drying process.     

多孔陶瓷孔径及其分布测定方法研究进展

孔径及其分布决定了多孔陶瓷的性能及应用,因此对其测定和定量表征非常重要。本文综述了多孔陶瓷孔径及孔径分布的常见测定方法,包括气泡法、压汞法、气体透过法、气体吸附法、气体渗透法、液-液法、悬浮液过滤法、X射线小角度散射法、核磁共振成像法、X射线断层扫描法以及电子显微镜图像分析法。比较了各种测试方法的优缺点,认为电子显微镜图像分析法是最直接有效的测定方法,并对多孔陶瓷的测试表征方法提出了展望。     

Modeling Drying Kinetics of Pistachio Nuts with Multilayer Feed-Forward Neural Network

Drying kinetics of pistachio nuts (Akbari v.) was simulated using a multilayer feed-forward neural network (MFNN). Experiments were performed at five drying air temperatures (ranging from 40 to 80°C) and four input air flow velocities (ranging from 0.5 to 2 m/s) with three replicates in a thin-layer dryer. Initial moisture content in all experiments was held at about 0.3 kg/kg d.b. To find the optimum model, various multilayer perceptron (MLP) topologies, having one and/or two hidden layers of neurons, were investigated and their prediction performances were evaluated. The (3-8-5-1)-MLP, namely, a network having eight neurons in the first hidden layer and five neurons in the second hidden layer resulted in the best-suited model estimating the moisture content of the pistachio nuts at all drying runs. For this topology, R² and MSE values were 0.9989 and 4.20E-06, respectively. A comparative study among MFNN and empirical models was also carried out. Among the empirical models, the logarithmic model, with MSE = 7.29E-6 and R² = 0.9982, gave better predictions than the others. However, the MFNN model performed better than the Lewis, Henderson and Pabis, two-term, and Page models and was marginally better than the logarithmic model.     

Study of Drying of a Porous Medium Using Gamma-Ray Attenuation Technique

In this work the drying of porous media is considered and modeled according to Luikov equations. The experimental apparatus consists of a cylindrical porous medium positioned vertically, with its bottom closed and maintained at a constant temperature. The top surface of this porous medium is in contact with a known flow rate of dry air at room temperature. Under these conditions, the transient axial temperature and moisture concentration profiles are measured. A noninvasive technique has been used to measure the moisture content, which is based on the attenuation of a horizontal beam of gamma-rays penetrating the porous medium. The measurement of this attenuation allows determining the water content of the sample. The mathematical model has been developed using two different approaches. In the first one, the one dimensional Luikov equations are applied considering all the physical parameters as constant. In the second approach, heat and mass balance equations of this process are the simple one dimensional transient diffusion equations, considering both diffusion coefficients as adjustable function of the moisture content. As shown, this second approach is superior to the first one.