Simulation And Experiment Study On Plant Materials Drying Based On The Cellular Structure

In the drying of plant cellular materials with high moisture content, the dehydrated moisture of the tissue mainly comes from the closed cells. The existing models and theories cannot account for the change of the closed cell structure in the whole drying process, so cannot describe the roles of the closed cell structure on the moisture transport process, clearly. In this paper, based on the subcellular moisture transport laws, we constructed an isothermal moisture transfer model for the low temperature drying of plant cellular material with high moisture content. In the model, the apparent moisture transport parameter is calculated from the microscale parameters. At the end, the drying process is studied by the moisture transfer model.After analyzing the changes of the closed cellular structures, such as the cell membrane and cell wall, we proposed that the plant parenchyma can be viewed as a composite of cells in the entire term of convective drying, and proposed a model cell that preserves proper subcellular structures, whose parameters can be determined by experiments. The geometric parameters of potato cells were obtained by using a field emission cryo scanning electron microscopy. The water status and the moisture transport law in each subcellular compartment of the model cell were given.To obtain the relationship of the apparent transport parameter to the microscale parameters, the water potential transport equations for the cell cavity phase and cell wall phase of a file of idealized cubic cells were constructed, respectively. By assuming the linear water potential distribution in cell cavities, the hydraulic conductivities from cell center to the cell wall and from cell cavity to cell cavity were obtained, respectively. Both of the conductivities include the diffusion effect in cell cavities. Through the scale analysis of the potential transport equations and the microscale simulations of the water potential transport in a cell file, the following results were obtained:the structure characteristic of parenchyma cell makes the water potential of a cell cavity approaching to the water potential of the cell wall, however, the decrease of the diffusion coefficient in the cell cavity in drying will break the local potential equilibrium down. When the moisture content of a potato cell is higher than 1 (d.b.), the cell cavity and cell wall are in a local potential equilibrium; when the moisture content changes from 1 to 0.32, the local potential equilibrium breaks down gradually. When the local potential equilibrium holds, there is moisture transport for cell cavity to cell cavity; or the moisture transport from cell cavity to cell cavity ceases to exist. If the cell cavity and cell wall are in a local potential equilibrium, the intercellular air space is also in a local potential equilibrium with the cell phase. Under the local potential equilibrium condition, the relationship between the apparent hydraulic conductivity of tissue and the microscale parameters of tissue was obtained.Assuming the cell cavity phase, cell wall phase and intercellular space in tissue in a local potential equilibrium state in the whole drying process, we constructed the moisture transport equation for the isothermal drying of plant cellular tissue based on the apparent hydraulic conductivity obtained previously. After discussing the influence of the tissue shrinkage on the area fractions and tuotorsities, we suppose the area fraction of each phase on the cross section perpendicular to the principal flux remain constant and obtained the relationship of the tuotorsities to the shrinkage coefficient. We obtained the diffusion form moisture transport equation by transforming the moisture transport equation from a spatial description to a referential description. The effective diffusion coefficient is related to the tissue shrinkage coefficient, tissue hydraulic conductivity and tissue moisture capacity.To validate the moisture transport model, the convective drying experiments of potato slab under 40℃ are conducted. The drying process is simulated by using the constructed model. The results show the prediction of the moisture transport model agrees well with the experimental drying curves when the moisture content is higher than 1 (d.b). However, when the average moisture content is lower than 1.5, the discrepancy of the predicted drying rate from the experimental drying rate increases with the moisture content decrease. These phenomena show the assumption that assumes the local potential equilibrium in tissue during the whole drying process makes the model overestimating the existing time of the cell cavity to cell cavity moisture, and overestimating the drying rate. The prediction results show the main moisture transport mechanism in potato drying is liquid diffusion, transmembrane transport and the capillary transport in the cell wall, and the vapor transport in the intercellular can be neglected when the tissue moisture is high. The main parameters that influence the drying process is the transport properties of the cell components, the influences of the geometry parameters is relatively small. So it’s critical to measure the precise values of the transport parameters for revealing the microscale moisture transport mechanism, correctly. To describe the whole drying process, the model that can account for the moisture transport under local non-equilibrium state.