基于收缩特性分析的叶丝快速对流干燥动力学模型

为准确表征叶丝脱水过程中的干燥动力学特性,分析了烤烟和白肋烟两种叶丝在下行床快速对流干燥中的孔隙结构变化特征,对叶丝干燥收缩过程进行了数学模型拟合,建立了考虑收缩形变的叶丝干燥过程水分扩散模型,并对模型进行了验证。结果表明:①随着干燥过程中两种叶丝内孔容积的减小,叶丝中孔径大于0.5μm的孔容比例呈降低趋势,而孔径小于0.05μm的孔容比例呈升高趋势;在不同干燥阶段两种叶丝孔径分布的分形维数介于2.45~2.71之间,表明其孔隙结构具有分形特征;②线性叠加式收缩模型能够较好地描述叶丝干燥过程中的收缩现象,烤烟叶丝和白肋烟叶丝体积比V/V₀和含水率比X/X₀的线性相关系数均大于0.99,采用该收缩模型对基于Fick第二定律的水分扩散模型进行修正,实验数据的拟合精度从修正前的0.9069提升到修正后的0.9580;③采用修正的水分扩散模型描述叶丝快速干燥动力学发现,考虑了干燥过程中的叶丝收缩现象后,得到的叶丝水分有效扩散系数降低,表明叶丝干燥过程中的体积收缩不利于传质过程。 

Kinetic Model for Rapid Convective Drying of Cut Strips Based on Shrinkage Property Analysis

In order to accurately characterize the kinetic characteristics of cut strips in the process of dehydration,the variation of pore structure in cut strips of flue-cured and burley tobaccos during rapid convective drying in a downflow bed was analyzed.The process of cut tobacco drying and shrinking was fitted by a mathematic model,and the model for moisture diffusion during drying taking into account the shrinking deformation was developed and verified.The results showed that:1) With the decrease of pore volume of cut strips of flue-cured and burley tobaccos during drying,the proportion of volume of pores of>0.5 μm in size decreased,while that of<0.05 μm increased.The fractal dimension of the two tobaccos ranged from 2.45 to 2.71 at different drying stages,which indicated that their pore structure possessed fractal characteristics.2) The linear superposition model described the shrinkage of cut strips during drying pretty well,the linear correlation coefficients for the volume ratio(V/V₀) and moisture content ratio(X/X₀) between cut strips of the two tobaccos were higher than 0.99.The shrinkage model was used to revise the moisture diffusion model based on Fick's second law,so that the fitting accuracy of experimental data increased from 0.906 9 to 0.958 0.3) From the revised moisture diffusion model,it was found that the effective diffusivity of moisture was overestimated when the shrinkage of cut strips during drying was neglected,which indicated that the shrinkage of cut strips impacted a negative influence on mass transfer process during drying.