FUNDAMENTALS OF DRYING TECHNIQUES

The samples of sliced and mashed apples were freeze-dried by controlling their surface temperatures over the usual pressure range of commercial operations. The surface of sliced samples could not be maintained at above 10°C in order to prevent the frozen layer from melting, while that of mashed samples was allowed to heat up to 70°C.

Thermal conductivities and permeabilities were determined by applying the uniformly-retreating-ice front model to the dried layer of the samples undergoing freeze-drying. The values of permeability for the mashed samples were found to depend on the ice-crystallization time during freezing. The results indicated that the drying rate of sliced samples was limited by the transfer rate of water vapor flowing through the dried layer. A cellular structural model is proposed for predicting the permeability of the dried layer, based on the resistance of the cell membrane to molecular transfer of water vapor.     

TRANSPORT PROPERTIES OF CELLULAR FOOD MATERIALS UNDERGOING FREEZE-DRYING

The samples of sliced and mashed apples were freeze-dried by controlling their surface temperatures over the usual pressure range of commercial operations. The surface of sliced samples could not be maintained at above 10°C in order to prevent the frozen layer from melting, while that of mashed samples was allowed to heat up to 70°C.

Thermal conductivities and permeabilities were determined by applying the uniformly-retreating-ice front model to the dried layer of the samples undergoing freeze-drying. The values of permeability for the mashed samples were found to depend on the ice-crystallization time during freezing. The results indicated that the drying rate of sliced samples was limited by the transfer rate of water vapor flowing through the dried layer. A cellular structural model is proposed for predicting the permeability of the dried layer, based on the resistance of the cell membrane to molecular transfer of water vapor.     

MEASUREMENT OF TRANSPORT PROPERTIES FOR THE DRIED LAYER OF COFFEE SOLUTION UNDERGOING FREEZE DRYING

A mathematical model has developed to determine the thermal conductivity and permeability for the dried layer of liquid sample undergoing sublimation dehydration. A microcomputer-based automatic measurement system has developed for the data acquisition as well as determination of these transport properties applying the drying data to the model. Aqueous solutions of 29-45 % soluble coffee solid were freeze dried under drying conditions used in commercial operations.

Thermal conductivity decreased in proportion to the porosity of the dried layer, and its temperature and pressure dependances were not appeared. The permeability increased with increasing the porosity, pressure and temperature of the dried layer. The results indicated that in commercial operations the solute concentration is one of the critical processing factors since this factor decisively governs the structure of a solute matrix formed during freezing of coffee solutions and the transport properties mainly depend upon the nature of this structure during drying.     

Numerical and analytical modeling of solid–liquid expression from soft plant materials

A physical model of solid–liquid expression from liquid containing plant materials is presented in one‐dimensional (1‐D) formulation. The layer of sliced cellular material is conceptualized as a double porosity system with extraparticle and intraparticle networks for liquid flow. Filtration‐consolidation equations with corresponding initial and boundary conditions were formulated for both extraparticle and intraparticle networks. It was supposed for the sliced plant material that the extraparticle network forms the first porosity with low‐storage capacity, while the intraparticle network forms a second porosity with high storage capacity. Computational modeling of pressure profiles in macro‐ and micropores versus time for different layer sections was done for real plant material (sugar beet tissue) with two different compressibility‐permeability characteristics corresponding to different degrees of tissue disruption. Results demonstrate the delayed pressure drop in the intraparticle network and retardation of consolidation kinetics for the less destroyed plant tissue. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4762–4771, 2013     

Modelling relationships between permeability and cement paste pore microstructures

Pore size distribution and the interconnectivity of cement paste pores have a strong influence on permeability. A computer network model has been developed to simulate flow through the pore microstructure cement paste. Network models have been successfully applied to predicting permeabilities of geological materials including rocks and soils but have not been specifically developed in the area of cement and concrete. The network model presented in this paper is used to obtain further insight into how pore size distributions of cement paste microstructures influences permeability. The model accounts for variations in pore microstructure by using a two-dimensional network of tubes with different log-normal size distributions.     

Gas permeability and electrical conductivity of structural concretes: Impact of pore structure and pore saturation

This paper investigates the gas permeability and the electrical conductivity of structural concretes under different pore saturations. The gas permeability was measured by CemBureau device and the electrical conductivity by alternating current method. The pore structure was characterized by mercury intrusion porosimetry (MIP) and gravimetric methods. The Archie's law is used to interpret the tortuosity of the pore structure. The impact of pore saturation is evaluated through the Van Genuchten-Mualem (VGM) and Kozeny's models. The results show that (1) the global correlation between the gas permeability in dried state and the electrical conductivity in saturated sate is weak; (2) both VGM and Kozeny's models can describe the relative permeability/conductivity, but the VGM model gives more consistent exponents for permeability and conductivity; (3) as the pore gas and liquid phases are both percolated, the gas permeability is correlated to the electrical conductivity for arbitrary pore saturation.     

Correlations between pore structure parameters and gas permeability of corundum porous materials

Corundum porous materials with different contents of calcium hexaluminate formed in situ were prepared using pure calcium aluminate cement as the calcium source. The surface fractal dimensions of the porous materials were calculated based on the experimental data of mercury intrusion. Correlations between pore structural parameters and the permeability coefficients k₁ and k₂ of the porous materials were then studied based on the grey system theory. The results showed that pores in the corundum porous materials have great fractal characteristics. The surface fractal dimension was a significant pore structural parameter that reflected the complexity of pore shape, pore surface, and pore-size distribution, which had the maximum correlation coefficient with the permeability of this type of porous materials. The apparent porosity and pore-size distribution had relatively high correlation coefficients to the permeability as well. Increasing the apparent porosity and the volume percentage of larger pores, and decreasing the volume percentage of smaller pores all benefited the permeability of the porous materials. In addition, the mean pore size and median pore size showed lower correlation coefficients to the permeability—especially for porous materials with a wide pore-size distribution.     

Fabrication and characterisation of cellular alumina articles produced via radiation curable dispersions

A general and versatile method for the production of cellular materials from radiation curable solvent-free colloidal ceramic dispersions containing pore formers has been developed. By this technique cellular ceramic articles with a precisely controlled porosity, cell size and shape are obtained for compositions containing solid pore formers. Monolithic bulk samples are obtained by thermal curing, whereas thin films and multi-layered articles are advantageously produced by UV curing. In this work the influence of three different spherical pore former types, PE, PS and PMMA, on the processing and final properties of the porous materials using alumina as model material is studied. The effect of pore former type and concentration on rheology, curing behaviour, debinding and sintering steps as well as thermal conductivity and mechanical strength of the sintered cellular materials is presented. It is also shown that the choice of pore former type modifies the sintering behaviour and resulting properties.     

Development of a porosity prediction model based on shrinkage velocity and glass transition temperature

Abstract

Pore formation and evolution is a common physical phenomenon observed in food materials during different dehydration processes. This change affects heat and mass transfer process and many quality attributes of dried product. Many mathematical models ranging from emperical to classical models proposed in the literature for predicting porosity during drying of food materials. Classical model is in its infancy as the required materials properties during drying are not avaiable for the material charecterisation. Empirical and semi-empirical models are reasonably well developed in establishing relationships between pore evolution and moisture content and determining experimental based coefficients. However, there are no simplistic models that considered process conditions and material properties together to predict the porosity. The purpose of this work is to develop a simplistic theoretical model for pore formation taking both process parameters and changing material properties during drying into consideration. A new “shrinkage velocity” approach has been introduced and the model has been developed based on this shrinkage velocity taking into account the main factors that influence the porosity including the glass transition temperature. Experimental results show good agreement with simulated results and thus validated the model. This study is expected to enhance the current understanding of pore formation of deformable materials during drying.     

探讨物料工业干燥动态特性的新方法

针对小质量物料干燥动态过程恒速率干燥阶段和降速率干燥阶段，提出了用偏差活化能的方法来避免寻找该两个干燥阶段临界湿分点的困难，并从物理化学的概念推导出了理论动态数学模型，建立了物料干燥动态特性实验台，以实验数据验证模拟结果，效果较好。研究结果表明，利用物料干燥过程偏差活化能的变化特性来研究物料的工业干燥动态特性和干燥机理是可行的。     

The structure and permeability of the disordered layer prepared by freeze-casting

Asymmetric ceramic membranes with disordered layer prepared by directional freeze-casting can be used as filter materials. However, there are few research achievements on the structural and permeability of disordered layers. Here, we obtained alumina asymmetric ceramic membranes with different disordered layer by adjusting the amount of alumina in the ceramic slurry. The results show that there are two kinds of structures in the disordered layer: spherical micropores and interconnected meshy pores, which lead to the change of airflow state. With the increase of the alumina content, the pore size of the disordered layer decreases and the network structure becomes complex, resulting in the decrease of the velocity of the inertial flow. The filter resistance of disordered layer increases obviously, but its proportion in the total filter resistance increases first and then decreases. The permeability is related to the pore size, specific surface area and channel complexity of the membrane.     

Separation and Purification of Polyhydroxyalkanoates from Newly Isolated Comamonas sp. EB172 by Simple Digestion with Sodium Hydroxide

A simple, mild, and effective process for the recovery of intracellular polyhydroxyalkanoate from a newly isolated gram-negative wild-type bacteria Comamonas sp. EB172 was developed using sodium hydroxide. Various parameters such as sodium hydroxide concentration, digestion time, and temperature were examined for their effect on polyhydroxyalkanoate recovery. The results showed that polyhydroxyalkanoate with 88.6% purity and 96.8% recovery yield were obtained by incubating the dried cells with 0.05 M sodium hydroxide at 4°C for 1 h, followed by purification steps using ethanol and water. Removal of non-polymeric cellular materials from the Comamonas sp. EB172 was increased under alkaline solution as a result of enhanced cell wall permeability. In addition, the presence of glycerol in the polymer suspension proved that saponification of the lipid layer in the bacterial cell wall occurred due to sodium hydroxide reaction.     

Finite element simulation for coffee (Coffea arabica) drying

This paper describes moisture diffusivity, shrinkage, equilibrium moisture content and finite element simulated drying of coffee. The moisture diffusivities in different components of parchment coffee were determined by minimizing the sum of square of deviations between the predicted and the experimental values of moisture contents during thin layer drying under controlled conditions of drying air temperature and relative humidity. The drying of coffee bean and parchment was conducted in thin layers at a temperature of 40, 50 and 60 °C with relative humidity in the range of 14–25%. The mean diffusivity values of coffee bean and parchment are related to the temperatures and are expressed by Arrhenius-type equations. The moisture diffusivities of parchment are lower than those of the coffee bean. The shrinkage of coffee bean derived from experiments is expressed as a function of moisture reduction. GAB model is sufficient for the prediction for sorption isotherm of parchment coffee and the parameters of the GAB model are a function of temperature and it is expressed by Arrhenius-type equations. Sensory evaluation of the coffee dried at 40, 50 and 60 °C shows that the overall acceptance by coffee cupping test and the concentration of caffeine is within the acceptable limit. A two-dimensional finite element model was developed for simulate moisture diffusion during drying process of parchment coffee. The finite element model was programmed in Compaq Visual FORTRAN version 6.5. The model simulates the moisture contents in different components of parchment coffee well and it provides a better understanding of the transport processes in the different components of the parchment coffee.     

辐射传热冷冻干燥速率的简捷估算

对Sheng 和Peck的辐射传热冷冻干燥模型进行简化，将该模型中升华阶段物料干燥层的误差函数温度分布近似为线性分布。计算表明，其结果与实验值及Sheng和Peck模型的预测值很接近。这种冷冻干燥过程中物料含水率和温度分布的简捷估算方法，在工程应用中十分方便。     

Compressive characteristics of cellular solids produced using vacuum-microwave, freeze, vacuum and hot air dehydration methods

Dried cellular solids were produced using different hydrocolloids such as locust bean gum, low methoxy pectin, methyl cellulose and tapioca starch. They were dried to less than 5% (w.b) moisture content using freeze-drying, vacuum drying, vacuum microwave drying or air-drying methods. The dry cellular solids were subjected to uniaxial compression using a Texture analyzer to study the compressive characteristics. True stress–strain relationship curves were developed for the dry cellular solids produced by different drying methods. Hencky’s strain was calculated for true strain. Comparisons of samples dried by different drying methods were done in terms of their compressive characteristics at various water activities. No matter the type of drying, the dried materials were brittle at low water activity, plastic at medium water activity and elastomeric at higher water activity levels. Due to non-uniformity in air-dried samples and more closed pores in vacuum dried samples as well as less mechanical strength, these two were considered inferior for production of strong elastomeric foams. Microwave vacuum dried foams were mechanically the strongest. All the microwave vacuum dried samples were close in their Young’s modulus. Increases in microwave power did not make any appreciable changes in pore structures although higher microwave power levels resulted in faster drying.     

Air permeability of electrospun polyacrylonitrile nanoweb

Ultrafine fibers were spun from polyacrylonitrile (PAN) solution in N,N‐dimethylformamide using a homemade electrospinning setup. Fibers with diameter ranging from 80 to 340 nm were obtained. Fiber size and fiber size distribution were investigated for various concentration, applied voltage, and tip‐to‐collector distance using image analysis. The diameters of the electrospun fibers increase when increasing the solution concentration and decrease slightly when increasing the voltage and needle tip‐to‐collector distance. Porosity and air permeability are vital properties in applications of electrospun nanofibrous structures. In this study, effects of process parameters on the porosity and air permeability of electrospun nanoweb were investigated as well. Results of statistical analysis showed that solution concentration and applied voltage have significant influences on pore diameters. It was concluded that nanofiber diameter played an important role on the diameter of pores formed by the intersections of nanofibers. A more realistic understanding of porosity was obtained and a quantitative relationship between nanoweb parameters and its air permeability was established by regression analysis. Two separate models were constructed for predicting air permeability in relation to process parameters. Optimization of electrospinning process for producing nanoweb with desirable air permeability is well achieved by these models. The models presented in this study are of high importance for their ability to predict the air permeability of PAN nanoweb both by process or structure parameters. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

冷冻干燥过程相迁移和相分布的孔尺度网络模型与模拟

构建了51×51二维孔-喉网络模型对冷冻干燥过程的升华干燥阶段进行模拟。与传统的连续介质模型相比,孔网络模型的特征是具有跟踪干燥过程中物料内部的干燥前沿和相分布的能力。采用网络模型预测了牛肉和火鸡肉的干燥曲线,并模拟了不同冻结速率的火鸡肉在干燥过程中形成的相分布。讨论了模型的计算特性,并分析了孔径分布对相分布特性的影响。结果表明：网络模型可较好地预测升华干燥阶段,可在孔尺度上揭示干燥过程的动力学机理,将为准确地判断升华干燥与解析干燥的转变点提供理论计算基础。     

Structural and Physical Properties of GDL and GDL/BPP Combinations and their Influence on PEMFC Performance

The change in the structural and physical properties of the components assembled in a fuel cell stack, when being compressed, is important for performance evaluation. The physical properties of Gas Diffusion Layer (GDL) materials, such as thickness, through plane resistivity and gas permeability and pore size data are presented as a function of compressive force. The data obtained are correlated with fuel cell performance data. Beyond the materials and components specific properties the behaviour of combinations of BPP and GDL materials, which are manufactured by SGL Technologies GmbH, are evaluated and presented. Through plane resistance of a GDL‐BPP‐GDL sandwich is evaluated for varied compression forces and materials permutations.     

DRYING CHARACTERISTICS AND PRODUCT QUALITY OF OKRA

ABSTRACT

Effects of size (whole and sliced), pre-treatment (blanching in water and 0.5% NaCl solution at 95° C) and temperature of drying air (40, 55 and 70° C) on the drying characteristics and quality of okra were studied. Estimation of the drying rate established that the drying of okra takes place under the falling rate period. Page's model was found to adequately describe the drying behavior of okra over a wide range of drying conditions used in the study. The coefficients of Page's model were correlated with air temperature and it was found that the dependence of the rate constant on the drying air temperature can be described using the Arrhenius law. The quality of the dried product was found to be best when okra was sliced and blanched at 95° C in 0.5% NaCl solution for 5 min and then dried at 55° C.     

Water and air permeability of needlepunch material

Use of the pore volume as a structural parameter of needlepunch materials in comparison to use of the porosity coefficient for this purpose results in simpler models for describing their permeability. Use of the permeability coefficient for assessing the filtration rate of media in needlepunch materials eliminates the restrictions that arise in using the air or water permeability for this purpose. The permeability of needlepunch materials is a function of the tortuosity of the pores and to a lesser degree, the pore size. Translated from Khimicheskie Volokna, No. 5, pp. 47–49, September–October, 2008.