Convective drying of wastewater sludges and sawdust/sludge mixtures was studied. The first part of this work was an experimental study performed in a cross-flow convective dryer using 500 g of wet material extruded through a disk with circular dies of 12 mm. The results showed that the sawdust addition has a positive impact on the drying process from a mass ratio of 2/8, on a dry basis, with observed drying rates higher than the original sludge. The second part of this work consisted of developing a drying model in order to identify the internal diffusion coefficient and convective mass transfer coefficient from the experimental data. A comparison was made between fitted drying curves, well represented by the Newton’s model, and the analytical solutions of the diffusion equation applied to a finite cylinder. Variations of dimensional characteristics such as the volume and exchange surface of the sample bed were obtained by X-ray tomography. This technique allowed confirm that shrinkage, which is an important phenomenon occurring during sludge and sawdust/mixture drying, must be taken into account. The results showed that both the internal diffusion coefficient and convective mass transfer coefficient were affected by mixing and sawdust addition. The internal diffusion coefficient changed from 7.77×10-9 m2/s for the original sludge to 7.01×10-9 m2/s for the mixed sludge and then increased to 8.35×10-9 m2/s for the mixture of a mass ratio of 4/6. The convective mass transfer coefficient changed from 9.70×10-8 m/s for the original sludge to 8.67×10-8 m/s for the mixed sludge and then increased to 12.09×10-8 m/s for the mixture of a mass ratio of 4/6. These results confirmed that sawdust addition was beneficial to the sludge drying process as the mass transfer efficiency between the air and material increased. Reinforcing the texture of sludge by adding sawdust can increase the drying rate and decrease the drying time, and then the heat energy supply will be reduced significantly. The study also showed that neglecting shrinkage phenomenon resulted in an overestimation for the internal diffusion coefficient for the convective drying of sludges and sawdust/sludge mixtures.
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机译：对废水污泥和锯末/污泥混合物的对流干燥进行了研究。这项工作的第一部分是在横流对流干燥器中进行的实验研究，使用了500 g湿物料通过带有12 mm圆形模具的圆盘挤出。结果表明，添加木屑对干燥过程有积极影响，以干基计，质量比为2/8，观察到的干燥速率高于原始污泥。这项工作的第二部分包括建立干燥模型，以便从实验数据中识别内部扩散系数和对流传质系数。比较了用牛顿模型很好地表示的拟合干燥曲线与应用于有限圆柱体的扩散方程的解析解。通过X射线断层扫描获得尺寸特征的变化，例如样品床的体积和交换表面。这项技术可以确认必须考虑到收缩，收缩是污泥和锯末/混合物干燥过程中发生的重要现象。结果表明，内部扩散系数和对流传质系数均受混合和添加木屑的影响。内部扩散系数从原始污泥的7.77×10-9 m2 / s变为混合污泥的7.01×10-9 m2 / s，然后以质量比的混合物增加到8.35×10-9 m2 / s 4/6。对流传质系数从原始污泥的9.70×10-8 m / s变为混合污泥的8.67×10-8 m / s，然后对于质量混合的物质，对流传质系数增加到12.09×10-8 m / s比为4/6。这些结果证实，随着空气和物料之间的传质效率的提高，添加木屑有利于污泥的干燥过程。通过添加木屑来增强污泥的质地，可以提高干燥速度，减少干燥时间，从而显着减少热能的供应。研究还表明，忽略收缩现象导致对流干燥污泥和木屑/污泥混合物的内部扩散系数被高估。
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