Abstract

Back-mixing is an operation that can be used to reinforce the texture of soft and pasty sludges to improve the drying effect. In this study, three mass fractions in a back-mixing operation (40%, 60%, and 70% on a dry basis) were chosen for convective sludge drying in a fixed bed. The structural characteristics including the total exchange surface, object volume, and shrinkage of the sludge bed were measured by X-ray macrotomography during the drying process. The average drying rates are 0.076 g/s (original sludge), 0.079 g/s (40% back-mixed sludge), 0.092 g/s (60% back-mixed sludge), and 0.099 g/s (70% back-mixed sludge), respectively. For the original sludge and three back-mixed sludges used, both the object volume and total exchange surface decrease in a quasi-linear manner with decreasing water content in the sludge. The final total exchange surface increases by 97.1% for the 70% back-mixed sludge compared with that of the original sludge. After drying, the shrinkage ratio of the object volume of the original sludge is 82.1%, however it is decreased to 50.8% when the 70% back-mixed sludge is used. The average convective mass transfer coefficient increases by 60% for the 70% back-mixed sludge compared with that of the original sludge. It was assumed for belt drying that, the lengths of the belt used for the 40% back-mixed sludge, 60% back-mixed sludge, and 70% back-mixed sludge will be nearly 0.96, 0.83, and 0.77 times, respectively, as long as that used for the original sludge.

摘要

返混是一种操作，可用于增强柔软和糊状污泥的质地，以提高干燥效果。在本研究中，在固定床中选择了三种回混操作的质量分数（40%、60% 和 70%，干基）用于对流污泥干燥。在干燥过程中，通过X射线宏观断层扫描测量了污泥床的总交换面、物体体积和收缩率等结构特征。平均干燥速率为 0.076 g/s（原污泥）、0.079 g/s（40% 返混污泥）、0.092 g/s（60% 返混污泥）和 0.099 g/s（70% 返混污泥）混合污泥），分别。对于所用的原始污泥和三种回混污泥，目标体积和总交换面均随着污泥中含水量的降低呈准线性下降。与原污泥相比，70%返混污泥的最终总交换面增加了97.1%。干燥后原污泥的目标体积收缩率为82.1%，而使用70%的回混污泥时，收缩率为50.8%。70%回混污泥的平均对流传质系数比原污泥增加了60%。假设皮带干燥用于40%反混污泥、60%反混污泥和70%反混污泥的皮带长度分别接近0.96、0.83和0.77倍，只要用于原污泥。然而，当使用 70% 的反混污泥时，它减少到 50.8%。70%回混污泥的平均对流传质系数比原污泥增加了60%。假设皮带干燥用于40%反混污泥、60%反混污泥和70%反混污泥的皮带长度分别接近0.96、0.83和0.77倍，只要用于原污泥。然而，当使用 70% 的反混污泥时，它减少到 50.8%。70%回混污泥的平均对流传质系数比原污泥增加了60%。假设皮带干燥用于40%反混污泥、60%反混污泥和70%反混污泥的皮带长度分别接近0.96、0.83和0.77倍，只要用于原污泥。