Structure of Crack in Thermally Dried Sludge Cake

We examined in this work the drying characteristics of wastewater sludge, considering the weight loss using an electronic balance and the morphology change of drying cake by an X-ray micro-computerized tomography scanner (micro-CT). The drying rates for sludge cake was 50-60% of the pure water test. The corresponding micro-CT images were scanned to explore the change in cake volume and development of internal crack over drying. The Otsu's method was applied to bilevel the scanned images, based on which the three-dimensional model for crack's internal structure was reconstructed. The cake porosity and the geometric factors of cracks, such as the compactness, the boundary fractal dimension, and the Sierpinski carpet fractal dimension, were evaluated. The shrinkage of cake volume and the development of internal crack occurred simultaneously. The skin layer formed by the former mechanism retards further drying. On the other hand, the internal cracks were shown to exhibit zigzagging with a noncircular cross section and fractal-like boundary, which should be able to enhance local drying. A possible role of change in crack structure on the rate of drying was discussed.     

Skin Layer on Thermally Dried Sludge Cake

This work utilized an X-ray micro-computerized tomography scanner (micro-CT) to investigate how skin layers—hard and relatively impermeable layers that hinder mass transfer—form and develop on wastewater sludge cake with (cake-O) or without artificial cracks (cake-N). A novel scheme that applies Otsu's algorithm twice was employed to determine the solid phase and skin layers in the scanned cake images. Initial cracks in a cake had significant impact on how the skin layer would form on the cake. A unique correlation between effective surface area to drying (not covered by a skin layer) and moisture content was identified. The advancement velocities of skin layers on drying cake were evaluated.     

Skin Layer on Thermally Dried Sludge Cake

This work utilized an X-ray micro-computerized tomography scanner (micro-CT) to investigate how skin layers—hard and relatively impermeable layers that hinder mass transfer—form and develop on wastewater sludge cake with (cake-O) or without artificial cracks (cake-N). A novel scheme that applies Otsu's algorithm twice was employed to determine the solid phase and skin layers in the scanned cake images. Initial cracks in a cake had significant impact on how the skin layer would form on the cake. A unique correlation between effective surface area to drying (not covered by a skin layer) and moisture content was identified. The advancement velocities of skin layers on drying cake were evaluated.     

Modeling convective drying of wet cake

This study analyzed convective drying of wastewater sludge cakes with three-dimensional cake structures probed using an X-ray micro-computerized tomography scanner (micro-CT), considering the development of cracks and cake morphology. The presence of artificial cracks on cake surface assist drying, but those occurred naturally cannot. The cake surface is noted far from saturation over drying. Moreover, the cracks transport easily moisture to cake surface, hence yielding high surface humidity (and rates) for drying. Comprehensive drying model has to incorporate real boundary conditions for success modeling.     

覆尘滤袋综合渗透率的分形求解

基于分形理论计算滤袋及滤饼构成的综合渗透率,描述其渗流特性。结合滤饼扫描电镜实验与图像处理技术分析滤饼孔隙结构;利用改进的毛细管模型近似模拟滤饼孔隙通道,根据流体动力学原理得到了滤饼渗透率的分形表达式,并由达西定律计算得覆尘滤袋综合渗透率,并用其对袋式除尘器流场压力分布进行数值模拟。结果表明,滤饼孔隙结构具有自相似特点。覆尘滤袋综合渗透率为(1.615~4.784)×10?12 m2,模拟所得的滤袋内外压差与实验结果的相对误差小于26%。覆尘滤袋综合渗透率计算方法合理,可较好地描述复合多孔介质的渗流特性。     

USE OF X-RAY MICROTOMOGRAPHY TO FOLLOW THE CONVECTIVE HEAT DRYING OF WASTEWATER SLUDGES

ABSTRACT

X-ray microtomography is proposed as a new tool to investigate the evolution of size, shape and texture of soft materials during a drying operation. This study is focused on the drying of mechanically dewatered sludges from a secondary wastewater treatment. The shrinkage phenomenon is shown to play a crucial role in the control of the drying process. The shrinkage curves are determined by analysing the shape and size of cross sectional microtomographic images of sludge extrudates at different levels of drying. The observation of drying and shrinkage curves allows us to determine 3 critical water content values, which define different drying zones where extragranular, intragranular or mixed limitations prevail. When drying is externally controlled, the decrease of the drying rate observed during experiments can be related to the reduction of the external area of the sample, i.e., to shrinkage. When drying is internally controlled, resistances inside the solid govern the process. Between these two extreme situations, the drying rate reduction is the result of both the external area decrease and the development of internal resistances limiting drying. A multizone model is proposed to describe quantitatively these observations. The analysis of the internal texture of the sludge extrudates reveals crack formation at the end of the drying process. The onset of crack formation is clearly related to the appearance of internal transfer limitations, i.e., humidity and temperature gradients inside the material.     

分形理论在有机玻璃有效弹性模量及裂纹聚合路径研究中的应用

运用分形理论获得了含多裂纹的有机玻璃的裂纹分形特征,并研究了分形维数与有效弹性模量及双裂纹聚合路径的关系。结果表明：有效弹性模量随分形维数的增大成比例减小趋势;双裂纹的聚合路径可以用分形维数判定。     

Measurement of Shrinkage and Cracks Associated to Convective Drying of Soft Materials by X-ray Microtomography

Abstract

Traditionally, the measurement of shrinkage occurring during drying is performed by destructive or poorly accurate techniques such as volume displacement methods. Cracks detection and quantification are realised either by destructive techniques or sophisticated but expensive nondestructive ones (NMR imaging). X-ray microtomography in combination with image analysis provides an accurate, nondestructive and easy to use technique to determine simultaneously shrinkage and crack extent. Results reported in this article concern drying of wastewater sludges whose management will become a real challenge in the years to come. These results show a clear relation between drying kinetics and crack development. This could be related to the development of internal diffusional limitations inducing moisture gradients and mechanical stresses leading to cracks formation.     

混杂钢纤维超高性能混凝土梁裂缝分形理论研究

混凝土梁在受弯过程中表面裂缝分布及演化的宏观特征可以反映其受弯性能,本文基于不同废旧轮胎钢纤维(WTSF)取代率的玄武岩纤维增强复合材料(BFRP)筋混杂钢纤维超高性能混凝土梁受弯试验结果,利用分形理论分析试验梁表面裂缝开展情况。结果表明,BFRP筋混杂钢纤维超高性能混凝土梁表面裂缝分布具有分形特征,满足自相似性,分形维数的变化区间为[0.892,1.064]。探讨了梁表面裂缝分形维数与施加荷载值、WTSF取代率、跨中挠度和最大裂缝宽度之间的关系,并分别拟合了WTSF取代率与完全破坏状态下全梁区和纯弯段分形维数的函数关系。分形维数与施加荷载值、跨中挠度和最大裂缝宽度均呈对数函数关系,完全破坏状态下,WTSF取代率变大会增大梁表面裂缝的分形维数,但总体来说不利影响并不明显,研究结果可为超高性能混凝土的工程实际应用提供参考。     

Fractal Characterization of Some Physical Properties of a Food Product under Various Drying Conditions

The present work was aimed at monitoring and studying the relationship between microstructural changes of a food product and its physical property changes during two different types of drying; i.e., conventional hot air drying and low-pressure superheated steam drying, using fractal analysis. The external changes of a model food product, viz. carrot, which were represented in terms of the percentage of shrinkage and the rehydration behavior, correlated well with its microstructural changes, which were represented by the rate of change of the fractal dimension of the microstructural images. The changes of physical properties and microstructure of carrot could be divided into two periods, which are periods of uniform and nonuniform deformation. The microstructural changes of the samples undergoing different drying techniques were quite different, however. Fractal dimension of carrot undergoing drying increased with drying time for both hot air drying and low-pressure superheated steam drying cases. Fractal dimension was found to be a good indicator of the microstructural changes of a product undergoing different drying techniques and conditions.     

Effects of Recycling-Sludge Operation on the Structure and Moisture Content of Floc in Water Treatment Plant

Abstract

Recycling-sludge operation is applied in the Feng-Yuan Water Treatment Facility to treat low turbidity source water. By employing fractal dimension analysis and the dilatometry technique, the effects of recycling-sludge operation on the structure and moisture content of sludge flocs are studied. As the results show, the recycling-sludge operation can decrease the average bound water content by nearly 13–24% and increase the effective floc density, thereby forming a compact structure with a high fractal dimension and low floc porosity. The results also indicate that a significant correlation arises between the fractal dimensions and the bound water content of the sludge flocs.     

颗粒群的体积分维数对过滤性能的影响

从理论上证明了颗粒群的体积分维数的物理含义,结果表明颗粒群的体积分维数可定量描述颗粒群的粒度分布宽度,准确反映了颗粒群填充三维空间的能力,并通过试验研究了颗粒群的体积分维数对滤饼孔隙率及比阻的影响规律。     

废弃纤维再生混凝土孔结构及碳化性能分形特征研究

以水灰比、再生骨料取代率、废弃纤维长度和体积掺量为设计变量,利用压汞试验及快速碳化试验,探讨了废弃纤维再生混凝土的孔结构、碳化性能的分形特征以及两者之间的关系.结果表明:废弃纤维再生混凝土孔结构具有显著的分形特征,废弃纤维的加入可阻止结构中有害孔的形成,改善废弃纤维再生混凝土内部的孔结构;纤维的加入可以提高再生混凝土的碳化性能,最优体积掺量为0.12%,碳化边界轮廓线的分形维数越小,对应的碳化深度越大;废弃纤维再生混凝土的碳化深度与孔隙体积分形维数之间存在相关性,随着孔体积分形维数减小碳化深度增大,根据孔隙体积分形维数来评价不同设计变量的废弃纤维再生混凝土碳化深度是可行的.     

Sewage Sludge Dewatering Process Using Liquefied Dimethyl Ether as Solid Fuel

This article describes the dewatering of sewage sludge using liquefied dimethyl ether (DME), which clarified the effect of sludge type and its particle size. Changes in the heating value of the dewatered sludge were also examined. In addition, the change in dewaterability upon reuse of the liquefied DME was investigated experimentally.

Regardless of the nature of the sludge, its dewatering behavior was similar. The amount of dewatered water could be increased by decreasing the diameter of the sludge cake. Complete drying of the sludge cake via DME dewatering resulted in retention of approximately 96% of the heating value (17.7 MJ/kg-sludge) of the original sludge cake dry base (18.4 MJ/kg db). The DME could be recovered after extraction, and reuse experiments showed that liquefied DME dewatering performance was unchanged after five reuses.     

Guest Editorial: Drying Technology—The Next 25 Years

This work aimed at studying the drying kinetics of yacon slices during vacuum drying and evaluating the effect of drying temperature, slice thickness, and citric acid concentration in the enzymatic inhibition solution on the fractal dimension and rehydration ratio of yacon slices. The drying kinetics was tentatively fitted to various thin-layer drying models, and the best fit was obtained using the modified Page model. The fit was validated after evaluating the values of coefficient of determination and parameters percentage error. The final product's fractal dimension, as calculated by the box counting method, was be between 1.5904 and 1.8358, which confirms that vacuum-dried yacon slices are fractals. Among the studied factors, drying temperature was found to negatively affect the fractal dimension and slice thickness was found to negatively affect the rehydration ratio. A good correlation was established between fractal dimension and moisture content, rehydration ratio, and hue angle, suggesting that the moisture, rehydration, and color measurements during yacon vacuum drying could be substituted or complemented by an image analysis.     

ESTIMATION OF THE EFFECT OF PRODUCT SHRINKAGE ON THE DRYING TIMES,HEAT INPUT AND CONDENSER LOAD OF THE PRIMARY AND SECONDARY DRYING STAGES OF THE LYOPHILIZATION PROCESS IN VIALS

ABSTRACT

The effects of cake shrinkage on the drying times and energy requirements of the primary and secondary drying stages of the freeze drying process involving the drying of a material in a vial, are estimated through the construction and solution of an extended unsteady state and spatially multidimensional model that accounts for the changes introduced by cake shrinkage on the internal and external mass and heat transfer mechanisms of the freeze drying     

USE OF X-RAY MICROTOMOGRAPHY TO FOLLOW THE CONVECTIVE HEAT DRYING OF WASTEWATER SLUDGES

X-ray microtomography is proposed as a new tool to investigate the evolution of size, shape and texture of soft materials during a drying operation. This study is focused on the drying of mechanically dewatered sludges from a secondary wastewater treatment. The shrinkage phenomenon is shown to play a crucial role in the control of the drying process. The shrinkage curves are determined by analysing the shape and size of cross sectional microtomographic images of sludge extrudates at different levels of drying. The observation of drying and shrinkage curves allows us to determine 3 critical water content values, which define different drying zones where extragranular, intragranular or mixed limitations prevail. When drying is externally controlled, the decrease of the drying rate observed during experiments can be related to the reduction of the external area of the sample, i.e., to shrinkage. When drying is internally controlled, resistances inside the solid govern the process. Between these two extreme situations, the drying rate reduction is the result of both the external area decrease and the development of internal resistances limiting drying. A multizone model is proposed to describe quantitatively these observations. The analysis of the internal texture of the sludge extrudates reveals crack formation at the end of the drying process. The onset of crack formation is clearly related to the appearance of internal transfer limitations, i.e., humidity and temperature gradients inside the material.     

Numerical Study on Moisture Transfer in Ultrasound-Assisted Convective Drying Process of Sludge

A coupled heat and moisture transfer model for ultrasound-assisted convective drying process of sludge was established. In this model, the permeable flow caused by acoustic pressure gradient in sludge was considered. The pore structure variety in sludge with ultrasonic irradiation was microscopically studied, and the pore size distribution of sludge was described by fractal geometry. Based on the fractal characterization, the physical properties of sludge including permeability, porosity, and tortuosity factor were determined, and the effective moisture diffusion coefficient of sludge under ultrasonic irradiation was also derived considering the effects of ultrasonic excitation energy and thermal effect on migration rate of water molecule. The effects of ultrasonic energy density and convective air temperature on convective drying process of sludge were numerically analyzed. The results showed that the ultrasonic irradiation changes the pore size distribution in sludge, the sludge flocs are dispersed, and the connectivity of pore structure is improved. Ultrasonic treatment is favorable to accelerating the moisture transport in the convective drying process of sludge, and the ultrasonic influence on moisture transport in sludge intensifies gradually with the increase of acoustic energy density from 0.2 to 0.6 W/ml. Furthermore, it can be also found that the enhancement effect of ultrasound on the average drying rate of sludge is more obvious at the connective air temperature of 65°C than that at 40°C under the uniform acoustic energy density and air velocity of 1.5 m/s.     

玉米干燥中应力裂纹的生成、扩展、检测和预防分析

针对谷物干燥过程中产生裂纹的类型和对粮食品质的影响作了概述。以玉米为例,就干燥工艺中热风温度、湿度、干燥速率、玉米初始含水率及玉米的种类、粒度、大小形状和内部结构以及干燥机的选型、冷却、缓苏等因素对玉米产生应力裂纹的影响进行了分析;介绍了裂纹产生的机理和检测方法,提出了减少裂纹的措施,并介绍了近年来该项技术的最新进展。     

固体推进剂SEM图像分形维数研究

以GAP(缩水甘油叠氮聚合物)、NEPE(硝酸酯增塑聚醚)和HTPB(端羟基聚丁二烯)推进剂为研究对象,对3种推进剂的扫描电镜(SEM)图像分形维数进行了研究.利用分形维数对固体推进剂裂纹定量表征以研究推进剂的力学性能,对阈值选取方法进行了分析,结果表明,利用全局阈值分割法选取图像适合的阈值可以对图像进行可靠的分割.固...