A Simulation of Wet Pocket Lumber Drying

In general, wood containing wet pockets is difficult to dry and to ensure uniformity of moisture content at the end of the drying process. Large variations of final moisture content and severe case hardening are common problems associated with the drying of wet wood. In order to devise optimal strategies for drying wood containing wet pockets, it is necessary to understand its complex moisture movement mechanisms and therefore predict drying times and final moisture content. Sub-alpine fir dimension lumber was used in this research because of its inherent issues related to wet pockets.

A two-dimensional mathematical drying model for wood containing wet pockets was developed. An effective diffusion coefficient (D_eff) was utilized in the model and heat and mass transfer equations were solved using a control volume approach. The difficulties involved in the simulation of the drying process of wet pocket lumber are due to the differences in moisture content and physical properties between wet and normal wood. Thus, an adjustable D_eff based on the moisture content (for both below and above fiber saturation point) was used during the simulation.

Four drying runs involving green unsorted sub-alpine fir lumber were carried out in a 3-ft laboratory kiln and in an 8-ft pilot kiln. The results of the simulations were in agreement with the results obtained through the drying experiments.     

A Simulation of Wet Pocket Lumber Drying

In general, wood containing wet pockets is difficult to dry and to ensure uniformity of moisture content at the end of the drying process. Large variations of final moisture content and severe case hardening are common problems associated with the drying of wet wood. In order to devise optimal strategies for drying wood containing wet pockets, it is necessary to understand its complex moisture movement mechanisms and therefore predict drying times and final moisture content. Sub-alpine fir dimension lumber was used in this research because of its inherent issues related to wet pockets.

A two-dimensional mathematical drying model for wood containing wet pockets was developed. An effective diffusion coefficient (D _eff ) was utilized in the model and heat and mass transfer equations were solved using a control volume approach. The difficulties involved in the simulation of the drying process of wet pocket lumber are due to the differences in moisture content and physical properties between wet and normal wood. Thus, an adjustable D _eff based on the moisture content (for both below and above fiber saturation point) was used during the simulation.

Four drying runs involving green unsorted sub-alpine fir lumber were carried out in a 3-ft laboratory kiln and in an 8-ft pilot kiln. The results of the simulations were in agreement with the results obtained through the drying experiments.     

A Diffusional Model with a Moisture-Dependent Diffusion Coefficient

Many investigators point out that a more realistic diffusion model is obtained when the effective diffusivity is considered with both temperature and moisture content dependent. Two mathematical models to predict the drying curves of pineapple at different temperatures have been compared. The simulation provided by model I, where the effective diffusion coefficient was considered as a function of the temperature, was unsatisfactory as indicated by the mean relative error (%E) of 19.8 ± 2.4%. The diffusion equation was modified by including both the effect of the temperature and the local moisture content on the effective diffusion coefficient in model II, being the %E of the simulation of 4.5 ± 2.0%.     

A Diffusional Model with a Moisture-Dependent Diffusion Coefficient

Many investigators point out that a more realistic diffusion model is obtained when the effective diffusivity is considered with both temperature and moisture content dependent. Two mathematical models to predict the drying curves of pineapple at different temperatures have been compared. The simulation provided by model I, where the effective diffusion coefficient was considered as a function of the temperature, was unsatisfactory as indicated by the mean relative error (%E) of 19.8 ± 2.4%. The diffusion equation was modified by including both the effect of the temperature and the local moisture content on the effective diffusion coefficient in model II, being the %E of the simulation of 4.5 ± 2.0%.     

Variability in Transport Properties Regarding Drying Behavior for Blackbutt Timber in New South Wales

Variability is a key issue in the processing of many biological materials, in this case the drying of hardwood timber. This article reports the measurements of variability of the diffusion coefficient (a transport property), the initial moisture content, and the basic density that are relevant to the drying of blackbutt, Eucalyptus pilularis Sm, from northern New South Wales in Australia. The diffusion coefficient was quantified using a mathematical model solving Fick's second law of diffusion for mass transfer, and Fourier's law for heat transfer. The initial moisture content and the basic density were measured using experimental procedures. Specifically, within-tree and between-tree variations are reported. The coefficients of variation of the initial moisture contents and final moisture contents are 0.24 and 0.19, respectively, for within-tree variability. A similar result was found for the amount of between-tree variability. Compensating differences in the diffusion coefficients of the timber boards were a significant reason for the small dispersion of final moisture contents, despite the large variation in initial moisture contents.

An analysis of variance showed that some timber properties were affected by the board positions within trees and between trees. Circumferential and radial effects were significant for the within-tree variability of most transport properties. Moreover, principal components analysis suggested that timber boards with low densities have high initial moisture contents and high diffusion coefficients. A potential reason is that if there is less wood material per unit volume (lower density), then there is more space to be occupied by water (higher initial moisture content), and there is also less resistance to the diffusive transport of moisture (higher diffusion coefficients).     

Diffusive Model with Variable Effective Diffusivity Considering Shrinkage for Hot-Air Drying of Lightly Salted Grass Carp Fillets

A diffusion model has been developed for predicting the change of moisture content in lightly salted grass carp fillets under the consideration of shrinkage and variable volumetric effective diffusivity. The established relationship between moisture content, temperature, and effective diffusivity validated the effectiveness of considering the variable volumetric effective diffusivity. The appearance of shrinkage during drying and the shrinkage coefficient have been determined for better predicting the variation of moisture content during drying to provide a thorough theoretical basis for industrial processes.     

基于气固流态化原理的油页岩干燥动力学

为了考察气固流化床干燥器能否使油页岩含水质量分数达到要求,以柳树河油页岩颗粒为原料,研究进口气体温度和颗粒直径对油页岩干燥性能的影响,采用薄层干燥模型,对油页岩干燥实验数据进行模拟,确定油页岩干燥方程和干燥速率方程,建立油页岩干燥速率特征常数和有效扩散系数之间的关联式。研究结果表明:薄层干燥模型中修正Page模型Ⅰ适合描述油页岩的干燥过程;油页岩在流化床内干燥过程主要发生在降速干燥阶段,进口气体温度越高,油页岩颗粒直径越小,所需干燥时间越短,进口气体温度为350℃时,使2.4 mm油页岩含水质量分数低于5%,所需干燥时间为2.5 min。     

A New Variable Diffusion Drying Model for the Second Falling Rate Period of Paddy Dried in a Rapid Bin Dryer

The objectives of this article is to propose a new drying model for the second falling rate period known as the variable diffusion controlled period that follows after the first falling rate period and to propose a new method to determine the second critical moisture content that separates these two periods. Experimental work on paddy drying at minimum fluidization velocity was carried out in a rapid bin dryer. The effects of operating temperatures (60-120°C) and bed depths (2-6 cm) on the paddy drying characteristics were investigated. It was found that the normalized drying rate of paddy was proportional to the normalized moisture content in the first falling rate period but in the second falling rate period, the normalized drying rate of the material varies exponentially with the normalized moisture content. The different relationship between the normalized drying rate and the normalized moisture content in the first and second falling rate periods indicate that two different mechanism of moisture transport are at work. The new exponential model of the second falling rate period and the linear model of the first falling rate period were found to fit the experimental data very well. Derivation from variable diffusion equation shows that the linear model is the result of constant diffusion coefficient whereas the new exponential model is the result of linear diffusion coefficient. This also implies that the first falling rate period is a constant diffusion controlled period and the second falling rate period is a variable diffusion controlled period. In addition, drying kinetics data of a drying process that fits the exponential model over a very slow drying period will show that the drying process is under the effect of a linear diffusion coefficient. It was also found that the proposed new method to determine the second critical moisture content that distinguishes between the first and second falling rate periods by using a sudden change in the value of the drying rate gradient to a much lower value at that point is more rigorous and yet simpler than the method of determining the specific location of the receding drying boundary since it is based on the behavior of the actual drying kinetic data.     

Air drying and colour characteristics of chestnuts pre-submitted to osmotic dehydration with sodium chloride

Air drying kinetics of chestnuts (Castanea sativa Mill.) submitted previously to osmotic dehydration with sodium chloride solutions (22%, w/w, 25 °C, 8 h) were experimentally determined. Drying experiments were carried out at 45, 55 and 65 °C during 32 h. Before osmotic dehydration operation, shell and tegument tissue were carefully removed in all samples. Rough external surface was maintained (peeled samples) or also removed (cut chestnuts) in different tests in order to evaluate the effect of the presence of this layer in the mass transfer rate. A diffusional model involving shrinkage was employed to simulate the drying kinetics of the all studied systems and values of the effective coefficient of diffusion of water were obtained. These coefficients were successfully correlated with temperature by means of an exponential equation and were employed to evaluate the additional mass transfer resistances due to the presence of layers and by the acquisition of osmotic solute in cut and peeled samples. The experimental determination of local salt and moisture contents during drying showed the existence of a drying front for water that moves along from surface to the centre of the sample and that salt accumulates near surface but also achieve the centre during the osmotic dehydration. Additionally, the analyses of the surface colour of air dried samples indicated that osmotic dehydration with salt keeps the characteristics observed for non-treated cut samples and deteriorates the colour quality in the case of peeled samples.     

褐煤干燥特性的实验研究

为获得褐煤颗粒干燥器设计的基础数据,对宁夏褐煤进行了工业分析、热重分析,得到了褐煤水分、挥发分的质量分数,确定了宁夏褐煤干燥的温度范围.褐煤的热重分析表明:褐煤干燥温度应不高于250℃.在热重分析确定的温度范围内,采用薄层干燥方式进行了褐煤颗粒的等温干燥实验,获得了褐煤颗粒干燥速率随水分的变化关系,颗粒的干燥主要处于降...     

Variability in Transport Properties Regarding Drying Behavior for Blackbutt Timber in New South Wales

ABSTRACT

Variability is a key issue in the processing of many biological materials, in this case the drying of hardwood timber. This article reports the measurements of variability of the diffusion coefficient (a transport property), the initial moisture content, and the basic density that are relevant to the drying of blackbutt, Eucalyptus pilularis Sm, from northern New South Wales in Australia. The diffusion coefficient was quantified using a mathematical model solving Fick's second law of diffusion for mass transfer, and Fourier's law for heat transfer. The initial moisture content and the basic density were measured using experimental procedures. Specifically, within-tree and between-tree variations are reported. The coefficients of variation of the initial moisture contents and final moisture contents are 0.24 and 0.19, respectively, for within-tree variability. A similar result was found for the amount of between-tree variability. Compensating differences in the diffusion coefficients of the timber boards were a significant reason for the small dispersion of final moisture contents, despite the large variation in initial moisture contents.

An analysis of variance showed that some timber properties were affected by the board positions within trees and between trees. Circumferential and radial effects were significant for the within-tree variability of most transport properties. Moreover, principal components analysis suggested that timber boards with low densities have high initial moisture contents and high diffusion coefficients. A potential reason is that if there is less wood material per unit volume (lower density), then there is more space to be occupied by water (higher initial moisture content), and there is also less resistance to the diffusive transport of moisture (higher diffusion coefficients).     

A study on moisture diffusion in drying and drying shrinkage of concrete

It is the purpose of this paper to clarify the behavior of moisture existing in concrete as drying occurs, and the relationship between moisture loss and drying shrinkage. Assuming that the behavior of moisture existing in concrete when drying was governed by nonlinear diffusion equation, the diffusion coefficient was expressed as a function of the moisture content by the experiment. The surface factor was also determined by the experiment. The relationship between diffusion coefficient or surface factor and the water-cement ratio of concrete were given. It was shown that the shrinkage strain was closely related to moisture loss.     

Finite Element Simulation of Drying of Longan Fruit

A two-dimensional finite element model has been developed to simulate moisture diffusion in longan fruit during drying and moisture diffusivities of the components of longan fruit determined experimentally are used in this simulation. Shrinkage of the flesh of longan during drying is also taken into account. The finite element model is programmed in Compaq Visual FORTRAN version 6.5. This finite element model satisfactorily predicts the moisture diffusion during drying. Moisture contents in the different components in the longan fruit during drying are simulated. Moisture content profiles of the longan fruit are also predicted. Knowledge gained from this study will be useful in the understanding of the transport processes in the different components of the longan fruit.     

Modeling Shrinkage Response to Tensile Stresses in Wood Drying: I. Shrinkage-Moisture Interaction in Stress-Free Specimens

This article reports on the wood shrinkage during drying in relationship with the temperature and moisture content. All tests were performed perpendicular to the grain on small clear wood specimens of green Western hemlock while drying at 40, 60, and 80°C to 17, 11, and 5% final moisture contents. Overall, wood dimensional changes and moisture loss phenomena were successfully analyzed and interpolated. The shrinkage strain followed a nonlinear pattern with the moisture loss being the driving force and exhibited good correlation with the square value of moisture content in tangential, and linear moisture values could be used to describe shrinkage in radial direction. Both shrinkage intersection points and end of capillary water values increased with temperature; the distinction between the two values could not be made at all times. A nonlinear function containing two regression coefficients (α and β) was found to be a good interpolation of the moisture loss experimental data. Further analyses revealed that β is independent of both target moisture content and temperature, whereas α appears to be influenced by both variables. The correlation between shrinkage and moisture loss rate is intended to be used as a stress prediction tool.     

Water Diffusivity and Drying Kinetics of Air Drying of Figs

The aim of this study is to evaluate experimentally the effective diffusion coefficient and the drying kinetics of whole unpeeled figs (Ficus carica L. var. tsapela) in terms of drying conditions. Estimation of the effective diffusion coefficient was carried out employing Fick's law for unsteady diffusion, which was solved analytically and numerically. In both methods, shrinkage effect was considered. The results from the two methods were compared and presented together with the limited results from the literature. The estimated effective diffusion coefficient values by both methods were fitted to a modified Arrhenius equation. Finally, a model predicting drying kinetics was developed. The model's coefficients were associated to the experimental conditions.     

Application of Bi-S Drying Correlation to Microwave Drying Data of Pharmaceutical Powders

The moisture diffusivities and moisture transfer coefficients characterising the drying of pharmaceutical powders were determined using a correlation proposed by Dincer et al. (2002, Development of a new drying correlation for practical applications. International Journal of Energy Research 26, 245-251). Experimental moisture content data for lactose, Aspirin and Paracetamol samples dried under convective, microwave, combined convective-microwave and combined vacuum-microwave conditions were obtained. The drying coefficients and lag factors were determined from the experimental measurements and incorporated into the model. The mass transfer Bi numbers were found to be in the range 0.058 to 0.194, indicating the presence of finite internal and external resistances. Moisture diffusivity and diffusion coefficient values in the range 0.135 × 10^-9 to 102 × 10^-9 m² s^-1 and 0.067 × 10^-7 to 8.21 × 10^-7 ms^-1 respectively, were calculated. The predicted moisture profiles showed adequate agreement with the experimental observations, with the average error between experimental and predicted results being ± 15.9%.     

Distribution of water in ceramic green bodies during drying

In order to investigate drying mechanisms at different stages, the distribution of water within the ceramic green bodies at different scales has been examined. The experimental measurements, using a simple weighing technique and Magnetic Resonance Imaging (MRI), show that during the first stage of drying involving shrinkage the material is constituted of uniquely solid and water with no gradient in water content within the sample. Then, during the second stage of drying, significant differences of water content as a function of position appear. As a complement, at the grain scale, observations using environmental scanning electron microscopy were made giving useful information on the solid–liquid–gas interfaces in the near surface part of the green body. Finally, the gradients in the water distribution were exploited to make a simple estimate of the diffusion coefficient of water with its dependence on the moisture content.     

DENSITIES, SHRINKAGE AND POROSITY OF SOME VEGETABLES DURING AIR DRYING

Bulk density, particle density, shrinkage and porosity were experimentally determined at various moisture content during air drying for apple, carrot and potato cubes. A simple mathematical model was used to predict the above properties versus material moisture content. Four parameters were incorporated in the model: enclosed water density, dry solids' density, bulk density of dry solids, and volume-shrinkage coefficient. The model was fitted to experimental data satisfactorily, and the parameters were estimated. The influence of varying drying conditions was also investigated.