基于细胞结构的植物物料干燥过程模拟及实验研究
发布时间：2018-09-13 11:49
【摘要】：高水分植物细胞物料的干燥过程中,物料失去的水分主要来源于封闭的细胞。现有模型和理论对细胞封闭结构在整个干燥过程中的变化以及细胞封闭结构在这个过程中扮演的角色缺乏清晰完整的描述。本文从亚细胞水分传输出发,建立了关于高水分植物细胞物料低温对流干燥的等温水分传输模型,模型中表观水分传输参数由组织微观参数算得。最后利用模型对干燥过程进行了研究。通过对低温对流干燥过程中细胞结构变化的分析,认为整个干燥过程中都可以把组织看成是由细胞构成的,并提出了对整个干燥过程都适用的模型细胞,模型细胞保留了真实细胞中必要的、参数可测的亚细胞结构。模型细胞构成了植物细胞组织概念模型。利用场发射冷冻电镜观察得到了马铃薯细胞的几何尺寸参数。给出了组织模型各部分水分状态和水分传输规律。为获得组织表观传输参数与微观参数之间的关系,建立了理想化立方体薄壁细胞串上细胞腔相和细胞壁相的水势传输方程。假设细胞腔内水势分布为线性,得到了水分从细胞中心到细胞壁传输的以及从细胞腔到细胞腔传输的包含细胞腔内扩散效应的水导系数。通过两相方程的尺度分析以及对细胞串上微观水分传输的模拟,得到了以下结论：薄壁细胞的结构特点使得细胞腔和细胞壁之间水势趋于平衡,干燥过程中细胞腔溶液水分扩散系数的减小会破坏局部平衡；当马铃薯细胞干基含水率不低于1时,细胞的细胞腔和细胞壁之间水势平衡,存在细胞腔到细胞腔的水分传输；干基含水率从1至0.32的过程中,局部平衡状态逐渐不再成立,细胞腔到细胞腔的水分传输终止。干燥过程中若细胞腔和细胞壁之间局部水势平衡,则细胞间隙和细胞也保持局部水势平衡。在局部平衡的条件下得到了组织表观水导系数和组织微观参数之间的关系模型。假设整个干燥过程中细胞腔、细胞壁和细胞间隙之间局部水势平衡,利用上文得到的组织表观水导系数模型建立了平板状物料干燥的一维等温水分传输和收缩模型。讨论了组织一维收缩效应,认为可以假设垂直于主流方向的截面上各相面积分数在干燥过程中保持不变,并获得了迂曲度系数和收缩系数的关系。把组织水分传输方程转化为由参考坐标描述的水分扩散方程,得到了等效扩散系数与组织收缩、组织水导系数以及组织水容的关系式。为验证组织水分传输模型的正确性,进行了热风温度为40℃的平板状马铃薯组织的干燥实验,并利用建立的扩散方程对干燥过程进行了模拟。模拟和实验结果表明：模型预测的干燥曲线在高水分段(平均含水率高于1)与实验吻合得较好,当组织平均含水率小于1.5时,模型预测的干燥速率明显比实验值要大。该结果表明假设整个干燥过程中细胞局部水势平衡使得模型高估了细胞腔到细胞腔水分传输存在的时间,进而高估了组织的干燥速率。模型预测表明：马铃薯干燥过程中物料内部的水分传输主要是液态扩散、跨膜传输和细胞壁中的毛细水分流动,细胞间隙中的蒸气扩散在高水分段可以忽略不计；影响干燥过程的主要微观参数是细胞各部分的水分传输参数,几何尺寸的影响相对来说较小。因此正确测定细胞各部分水分传输参数对揭示干燥过程中组织内水分传输机理至关重要。为完整描述整个干燥过程,需提出能考虑非局部平衡状态下水分传输的模型。
[Abstract]:In the drying process of high-moisture plant cell materials, the loss of water mainly comes from the closed cells. The present models and theories lack a clear and complete description of the changes of the closed cell structure and the role of the closed cell structure in the whole drying process. An isothermal moisture transfer model for high moisture plant cell materials during low temperature convective drying was established. The apparent moisture transfer parameters in the model were calculated from the microstructure parameters. Finally, the drying process was studied by using the model. Tissue is considered to be composed of cells, and a model cell suitable for the whole drying process is proposed. The model cell retains the necessary and measurable subcellular structure of the real cell. The model cell constitutes the conceptual model of plant cell tissue. The geometric size parameters of potato cells are obtained by field emission freezing electron microscopy. In order to obtain the relationship between the apparent transport parameters and the microscopic parameters of the tissue, the water potential transfer equations of the cell lumen and cell wall phases on the idealized cubic parenchyma cell string are established. Assuming the water potential distribution in the cell lumen is linear, the water flow from the cell center to the cell wall is obtained. The water conductivity of cell wall transport and that of cell wall transport from cell cavity to cell cavity including intracellular diffusion effect. Through the scale analysis of two-phase equation and the simulation of micro-water transport on cell string, the following conclusions are obtained: the structural characteristics of parenchyma cells make the water potential between cell cavity and cell wall tend to balance, and the drying process. When the water content in the cell lumen is not less than 1, the water potential between the cell lumen and cell wall is balanced and there is water transfer from the cell lumen to the cell lumen. The water transport in the cell lumen terminates. If the local water potential between the cell lumen and cell wall is balanced during drying, the cell gap and cell also maintain the local water potential equilibrium. A one-dimensional isothermal water transport and shrinkage model for plate material drying is established by using the model of apparent water conductivity obtained above. The effect of one-dimensional shrinkage of tissue is discussed. It is assumed that the area fraction of each phase on the cross-section perpendicular to the main direction remains unchanged during drying. The relationship between tissue water transfer equation and tissue shrinkage, tissue water conductivity coefficient and tissue water capacity was obtained by transforming tissue water transfer equation into water diffusion equation described by reference coordinates. The results of simulation and experiment show that the drying curve predicted by the model is in good agreement with the experiment in the high moisture section (the average moisture content is higher than 1). When the average moisture content of the tissue is less than 1.5, the drying rate predicted by the model is obviously higher than the actual drying rate. The results show that the model overestimates the time of water transport from the cell cavity to the cell cavity, and then overestimates the drying rate of the tissue. The model predicts that the water transport in the material during potato drying is mainly liquid diffusion, transmembrane transport and fineness. The capillary water flow in the cell wall and the vapor diffusion in the intercellular space can be neglected in the high water section; the main microscopic parameters affecting the drying process are the water transport parameters of the cell parts, and the geometric size has relatively small influence. In order to describe the whole drying process completely, it is necessary to propose a model which can consider the moisture transfer in non-local equilibrium state.
【学位授予单位】：中国农业大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：S375
本文编号：2241089
[Abstract]:In the drying process of high-moisture plant cell materials, the loss of water mainly comes from the closed cells. The present models and theories lack a clear and complete description of the changes of the closed cell structure and the role of the closed cell structure in the whole drying process. An isothermal moisture transfer model for high moisture plant cell materials during low temperature convective drying was established. The apparent moisture transfer parameters in the model were calculated from the microstructure parameters. Finally, the drying process was studied by using the model. Tissue is considered to be composed of cells, and a model cell suitable for the whole drying process is proposed. The model cell retains the necessary and measurable subcellular structure of the real cell. The model cell constitutes the conceptual model of plant cell tissue. The geometric size parameters of potato cells are obtained by field emission freezing electron microscopy. In order to obtain the relationship between the apparent transport parameters and the microscopic parameters of the tissue, the water potential transfer equations of the cell lumen and cell wall phases on the idealized cubic parenchyma cell string are established. Assuming the water potential distribution in the cell lumen is linear, the water flow from the cell center to the cell wall is obtained. The water conductivity of cell wall transport and that of cell wall transport from cell cavity to cell cavity including intracellular diffusion effect. Through the scale analysis of two-phase equation and the simulation of micro-water transport on cell string, the following conclusions are obtained: the structural characteristics of parenchyma cells make the water potential between cell cavity and cell wall tend to balance, and the drying process. When the water content in the cell lumen is not less than 1, the water potential between the cell lumen and cell wall is balanced and there is water transfer from the cell lumen to the cell lumen. The water transport in the cell lumen terminates. If the local water potential between the cell lumen and cell wall is balanced during drying, the cell gap and cell also maintain the local water potential equilibrium. A one-dimensional isothermal water transport and shrinkage model for plate material drying is established by using the model of apparent water conductivity obtained above. The effect of one-dimensional shrinkage of tissue is discussed. It is assumed that the area fraction of each phase on the cross-section perpendicular to the main direction remains unchanged during drying. The relationship between tissue water transfer equation and tissue shrinkage, tissue water conductivity coefficient and tissue water capacity was obtained by transforming tissue water transfer equation into water diffusion equation described by reference coordinates. The results of simulation and experiment show that the drying curve predicted by the model is in good agreement with the experiment in the high moisture section (the average moisture content is higher than 1). When the average moisture content of the tissue is less than 1.5, the drying rate predicted by the model is obviously higher than the actual drying rate. The results show that the model overestimates the time of water transport from the cell cavity to the cell cavity, and then overestimates the drying rate of the tissue. The model predicts that the water transport in the material during potato drying is mainly liquid diffusion, transmembrane transport and fineness. The capillary water flow in the cell wall and the vapor diffusion in the intercellular space can be neglected in the high water section; the main microscopic parameters affecting the drying process are the water transport parameters of the cell parts, and the geometric size has relatively small influence. In order to describe the whole drying process completely, it is necessary to propose a model which can consider the moisture transfer in non-local equilibrium state.
【学位授予单位】：中国农业大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：S375
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