POSSIBILITY OF CLAY DAMAGE DURING DRYING

The convective drying of plate made of clay is considered in the paper. The model of drying with material coefficients dependent on the moisture content and the temperature is presented. The material is assumed to be brittle-elastic (linear elasticity with moisture dependent material constants). The strength of the material estimated by bending test, is moisture dependent. Numerical simulation of drying processes is done. Through the comparison of the stresses arising during drying with the strength of the material the possibility of dried body damage is shown. The range of constant drying conditions in which damage is expected is specified.     

Mechanische Eigenschaften zyklisch be‐ und entfeuchteter Zeolithgranulate

The production of zeolite granules varies in the amounts of binder content and different drying/activation temperatures. Here, the influence of moisture content and cyclic moisture loading and unloading on the mechanical properties like the strength of three different types of zeolite granules has been studied by uniaxial compression tests. The force‐displacement curves and scanning electron microscopy pictures of the granule and fracture surfaces show that with increasing number of moistening‐drying cycles, a continuous decrease in the strength of granules take place due to non‐regenerative damage of the structure.     

Triaxial mechanical behaviour of mortar: Effects of drying

The analysis of concrete structure durability is based on the investigation of the material long-term behaviour. Such behaviour is influenced by mechanical, hydrous and thermal actions applied to structures. The main purpose of this study concerns the characterisation of the coupled effects between drying shrinkage and damage for a cementitious material. An experimental study on a normalised mortar (European norm) is then presented to characterise the damage effect, induced by drying and desiccation shrinkage on the multiaxial compressive behaviour. Triaxial compression tests are carried out at different times of drying. The observed increase in deviatoric strength and decrease of Young's modulus and Poisson's ratio are related to the loss in mass of specimens. These results are commented through the damage processes of material because the drying phenomenon causes microcracking by exceeding tensile strength. This microcracking will have a strong influence on the damage process of the material and then on its failure behaviour. Furthermore, the effect of drying leads to an increase of the capillary suction into the mortar, hence, to an increase of the specimen strength. Such couplings have to be taken into account in a reliable modelling.     

Change of mechanical properties related to starch gelatinization and moisture content of rice kernel during fluidized bed drying

Rice fissuring during the drying process is a major problem affecting rice quality. To alleviate this critical issue, it is necessary to understand the change of mechanical properties and the drying kinetics of paddy during drying. The objective of this work is therefore to study the drying characteristics and changes of mechanical properties, i.e., breaking force (F), ultimate tensile strength (UTS), and apparent modulus of elasticity (AMOE) during fluidized bed drying. Suphanburi 1 paddy variety with three initial moisture contents (Mi) of 29.5, 30.2, and 42.8% dry basis was used as the raw material, which was dried at drying air temperatures (Ta) of 110, 130, and 150?°C. A three-point bending method was used for testing the mechanical properties with a texture analyzer. The experimental results showed that the breaking force and the ultimate tensile strength of paddy during drying were more strengthened with higher drying temperatures and higher initial moisture content while its apparent modulus of elasticity was changed only with the moisture content. However, both operating parameters positively affected the apparent modulus of elasticity when evaluated at a 16% dry basis. The maximum changes in F, UTS, and AMOE concerning the initial moisture content were 25.1, 25.2, and 19.5%, respectively. Besides, the maximum changes in F, UTS, and AMOE concerning drying temperatures during drying were 14.2, 14.3, and 13.5%, respectively. The improvement of the mechanical properties could be attributed to the starch gelatinization of which the degree was higher in cases of higher initial moisture content and higher drying temperatures. The empirical models of ultimate tensile strength and apparent modulus of elasticity were developed and related to intermediate moisture content and the degree of starch gelatinization.     

Early-age elastic properties of cement-based materials as a function of decreasing moisture content

The moisture content is of particular relevance in cement-based materials, as it has a strong impact on their fundamental material properties. For example, it directly affects their strength and elastic properties, which in turn are closely related to volumetric deformations and cracking susceptibility. This paper investigates the influence of the decreasing moisture content on the elastic properties at early-ages, when the material properties are still developing simultaneously to the drying process. Mortar mixtures containing either Portland cement or cement blended with slag were specifically designed to halt the hydration at predefined stages without altering the microstructure or promoting further hydration during drying (equivalent systems). The elastic modulus of the equivalent mortars as a function of the moisture content is measured through resonant ultrasound spectroscopy. At early age the elastic modulus remained constant during drying, while at later ages a steady reduction was observed as a function of the decreasing relative humidity.     

干化污泥型煤抗压强度的实验研究

利用微波干燥脱水污泥制备型煤粘结剂,与粉煤混合压制成型煤。通过正交试验考察成型压力、干化污泥含水率及添加比例对型煤抗压强度的影响。结果表明：微波干化的污泥型煤较粘土型煤的抗压强度大;成型压力、干化污泥含水率及添加比例均为型煤抗压强度的显著因素,各因素优化范围：成型压力为25～30MPa,干化污泥水分为40％左右,干化污泥添加比例为20％～30％。     

Influence of uncertainty in heat–moisture transport properties on convective drying of porous materials by numerical modelling

The influence of uncertainties in heat–moisture transport properties, due to measurement errors and material heterogeneity, on the numerical simulation results of convective drying of two capillary-saturated porous materials is investigated by a transport-property parameter analysis (TP-PA), based on the Monte Carlo method. Here, the heat–air–moisture transfer model is evaluated many times, each time using a random set of one or multiple input parameters (i.e. transport properties), by which a stochastic model output is generated. The propagation of these transport-property uncertainties to the hygrothermal behaviour of the drying system is evaluated by statistical analysis of the model simulation output. The spread on the hygrothermal response is found to be strongly material dependent and is related to the dominant mode of moisture transport in the material, i.e. liquid or vapour transport. The TP-PA results clearly indicate that uncertainties in the heat–moisture transport properties can lead to significant differences in drying behaviour predictions, where differences of the total drying time with respect to its mean value up to 200% are found for the materials considered. Therefore, numerical modelling of heat and moisture transport in porous materials should preferably include a quantification of the propagation of these uncertainties, for example by means of the proposed transport-property parameter analysis. Such analysis, however, additionally requires detailed (a priori) experimental material characterisation to determine realistic uncertainty ranges.     

Damage analysis of microwave‐dried materials

The aim of this article is to analyze the drying‐induced stresses and destruction of ceramic‐like materials during their microwave drying. The kinetics of microwave drying at various microwave power levels are determined experimentally and numerically, and the distributions of temperature and moisture content in the tested samples are visualized with infrared camera and presented graphically. The experiments are performed on kaolin‐clay samples, where destruction provoked by microwave drying is visualized on photographs taken with the photo camera and microscope. The thermo‐hydro‐mechanical model of drying elaborated by authors is used to determine the distribution of stress components and to calculate the effective stress required to induce the failure of material. The effective stress is formulated with the use of the energetic criterion, and the material spots prone to damage are predicted numerically. The numerically predicted spots are compared with the experimentally appointed places of material damage and a good adherence of the numerical predictions with experiments is confirmed. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Einfluss der feuchtigkeit auf die druckfestigkeit dampfgehärteten gasbetons

The report includes the evaluation of the literature available as well as own tests referring to the relationship between the compressive strength and the moisture content of the autoclaved cellular concrete. According to this, independent from the bulk density of the cellular concrete between the related compressive strength after drying at 105°C and the moisture content in the material, the equation (4) is valid. Based on the practical moisture content and the described relationship the compressive strength of the cellular concrete should be determined at moisture contents of ? 3 Vol. -%.     

EFFECT OF DRYING METHOD ON SHRINKAGE AND POROSITY

The effect of drying method on bulk density, particle density, specific volume and porosity of banana, apple, carrot and potato at various moisture contents was investigated, using a large set of experimental measurements. Samples were dehydrated with five different drying methods: conventional, vacuum, microwave, freeze and osmotic drying. A simple mathematical model was used In order to correlate the above properties with the material moisture content. Four parameters with physical meaning were incorporated in the model: the enclosed water density p_w, the dry solid density p_s, the dry solid bulk density p_bo and the volume shrinkage coefficient β'. The effect of drying method on the examined properties was taken into account through its effect on the corresponding parameters. Only, dry solid bulk density was dependent on both material and drying method. Freeze dried materials developed the highest porosity, whereas the lowest one was obtained using conventional air drying.     

Estimation of the Effect of Shape and Temperature on Drying Kinetics Using MLP

The kinetics of drying is described in the article using artificial neural networks (multilayer perceptron MLP). Drying curves for vegetables are possible to obtain theoretically on the basis of the equations of mass transfer in a porous material. A key role in these equations is played by the effective coefficient of water diffusion in the form of liquid, vapor or jointly as liquid and vapor. The diffusion coefficient which depends both on moisture content in the material and temperature should be determined experimentally. The drying kinetic curve in this article is treated as a time series dependent on the state of material prior to drying and on the constant K characterizing process parameters such as drying temperature and describing the material, e.g., its shape and moisture content. Constant K characterizes the analyzed material from the drying point of view because it contains a diffusion coefficient and depends on the shape of material. The following materials were analyzed: beetroot and potato dried in the form of cubes, slices, chips, and strips. Experiments were carried out in a laboratory oven dryer at process temperature 50, 60, 70, 90, and 106°C.     

NONLINEAR FINITE ELEMENT ANALYSIS OF COUPLED HEAT AND MASS TRANSFER PROBLEMS WITH AN APPLICATION TO TIMBER DRYING*

A finite element formulation and solution of a set of nonlinear coupled heat and mass transfer equations for porous capillary media is presented. The model considers temperature and moisture dependent material properties and can accomodate diffusion of moisture as either a liquid or a vapor. Application was made to drying of timber and predicted results agreed well with the experimental data.     

NONLINEAR FINITE ELEMENT ANALYSIS OF COUPLED HEAT AND MASS TRANSFER PROBLEMS WITH AN APPLICATION TO TIMBER DRYING*

A finite element formulation and solution of a set of nonlinear coupled heat and mass transfer equations for porous capillary media is presented. The model considers temperature and moisture dependent material properties and can accomodate diffusion of moisture as either a liquid or a vapor. Application was made to drying of timber and predicted results agreed well with the experimental data.     

A Simulation Study on Convection and Microwave Drying of Different Food Products

It is now well recognized that matching the external drying condition with the drying kinetics of a material can lead to substantial savings of energy and in the case of heat-sensitive products, even to higher quality product. In this work, the effect of convection and microwave heat input and other product parameters on the batch drying characteristics of model materials, potato and carrot slabs, whose thermo-physical data are readily available in the literature, was modeled using a one dimensional liquid diffusion model. The influence of various thermo-physical properties of the product in drying of heat-sensitive materials was quantitatively assessed. Heat of wetting, temperature and moisture dependent effective diffusivity and thermal conductivity are considered in this model. The effect of moisture diffusivity on drying using convection and a microwave field is simulated in view of the interest in predicting the drying performance by simplified method. Conditions under which the drying rate is controlled by the external drying conditions and the internal thermo-physical properties of the product are computed and discussed.     

A review of some of the main results of a research on the ageing phenomena of concrete: Effect of moisture conditions on strength,shrinkage and creep of mature concrete

“Dry strengths” were about two thirds higher than “wet strengths”. An increase (a maximum) in compressive strength was obtained in the region of moisture condensation in pores. Very dry drying conditions may cause strength reduction. The shrinkage can be divided into three different parts: very strong between RH 100… 50%, minor or negligible between RH 50…20%, strong between 20… 0%. The creep is greatly dependent on moisture conditions. Carbonation increases creep.     

Optimal control of convective drying of saturated porous materials

Numerical simulations of optimal control applied to saturated capillary‐porous materials subjected to convective drying are presented. The optimization process is concerned with such drying parameters as drying rate, energy consumption, and product quality. The thermo‐hydro‐mechanical model of drying is developed to describe the kinetics of drying and to determine the drying‐induced stresses which are responsible for damage of dried products. The effective and the admissible stresses are defined and used to formulate the Huber‐von Mises–Hencky strength criterion enabling assessment of possible material damage. The method of genetic algorithm is used for operation with drying conditions in such a way as to ensure minimum energy consumption and to get the effective stress less than the strength of dried material, and thus, to preserve a good quality of dried products at possibly high drying rate. Numerically simulated optimal drying processes are illustrated on the examples of finite dimensions of kaolin‐clay cylinders subjected to convective drying. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4846–4857, 2013     

Oil quality from hydraulic pressed soybeans

Summary HYDRAULIC pressed soybean oil quality is affected a great deal by the moisture of the beans at the time of milling. At moistures above 12 to 13% the oil quality deteriorates sharply. A great improvement in the oil from wet or even moisture damaged soybeans can be made by drying the beans in below 12% before milling. Even badly deteriorated (sample grade) beans have been vastly improved. Blending of wet and dry soybeans to an average moisture below 12% does not seem to yield as good quality oil as drying wet beans to a corresponding moisture. Normal variations in other soybean characteristics determining bean grade have lesser effects on soybean oil quality. Some data on the effect of bean damage, splits, off-color varieties, and foreign material are included in this paper. Hydraulic oil mills operating on soybeans will be able to make large improvements in oil quality by drying whole wet beans at the mill and by removing foreign material from the beans.     

A Simulation Study on Convection and Microwave Drying of Different Food Products

Abstract

It is now well recognized that matching the external drying condition with the drying kinetics of a material can lead to substantial savings of energy and in the case of heat-sensitive products, even to higher quality product. In this work, the effect of convection and microwave heat input and other product parameters on the batch drying characteristics of model materials, potato and carrot slabs, whose thermo-physical data are readily available in the literature, was modeled using a one dimensional liquid diffusion model. The influence of various thermo-physical properties of the product in drying of heat-sensitive materials was quantitatively assessed. Heat of wetting, temperature and moisture dependent effective diffusivity and thermal conductivity are considered in this model. The effect of moisture diffusivity on drying using convection and a microwave field is simulated in view of the interest in predicting the drying performance by simplified method. Conditions under which the drying rate is controlled by the external drying conditions and the internal thermo-physical properties of the product are computed and discussed.     

Stress and Failure Prediction for the Drying, Tempering, and Cooling of Extruded Durum Semolina

This work presents a method to predict the stress and breakage that is caused by the drying of hygros-copic materials. Stresses were predicted for a viscoclasic cylinder with the properties of extruded durum semolina, or pasta noodles. The stresses were calculated as functions of the transient moisture and tem-perature gradients in the material which were predicted for the combined processes of drying, tempering,and cooling. The time and radial position of failure were predicted based on failure data for extruded semolina.

Isotherm data for extruded durum semolina were obtained for temperatures from 40 to 60°C and for relative humidities from 75 to 95%. The results were fit with a modified form of Henderson's equation.Thermal conductivities were measured for temperatures from 30 to 50°C and a moisture range of 12 to 27% (dry basis).

A drying model based on the principles of irreversible thermodynamics; (Fortes, 1978; Fortes and Okos, 1981a, 1981b) was used to successfully predict drying curves for a range of experimental conditions. Transient moisture and temperature profiles were calculated numerically, and a receding evapora-tion front was predicted to exist. Drying was predicted to be a coupled liquid, vapor, and heat transport phenomena.

The drying data were used in a stress analysis of a Maxwell viscoelastic cylinder to predict trends in stress development under various contiitions of combined drying, tempering, and cooling. High temperature-high humidity drying, HTHH, (lOO°C, 65% RH) was compared with low temperature-low humidity drying, LTLH, (53°C, 13% RH). The HTHH drying offered definite advantages in terms of reduced product breakage susceptibility. The reasons for those advantages were increased failure strength and a decreased moisture gradient at the end of drying. In a five-stage drying process, the cooling stage was shown to have a significant impact on the predicted levels of stress and on the strength of the extruded material. Analysis of the model suggested that gradual temperature and humidity transitions from stage to stage in multistage processes were important to product quality.