Effect of the micro-flocculation stage on the flocculation/sedimentation process: The role of shear rate.
Effect of the micro-flocculation stage on the flocculation/sedimentation process: The role of shear rate.
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DOI：
10.1016/j.scitotenv.2018.03.286
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发表时间：
2018-08
期刊：
影响因子：
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通讯作者：
Zhenbei Wang;Jun Nan;Xiaoyu Ji;Yueming Yang
中科院分区：
文献类型：
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作者：
Zhenbei Wang;Jun Nan;Xiaoyu Ji;Yueming Yang
为了更好地理解絮体生长的连续过程，优化絮凝过程的控制，对絮凝过程中颗粒粒径分布（PSD）的变化规律及其分形特征进行了动态分析。结果表明，絮凝过程可分为三个阶段，微絮凝阶段、生长阶段和稳定（或破碎）阶段。微絮凝阶段是形成微絮体（大絮体的组成单元）的关键阶段，对絮凝沉降过程起着重要作用。结果表明，微絮凝阶段剪切速率的增大（11 s-1< G < 30 s-1）有利于微絮凝体的尺寸增大和结构致密化。随着剪切速率的进一步增大（30 s-1< G < 55 s-1），微絮体的平均粒径从13.61 μm逐渐减小到10.91 μm，而微絮体的二维分形维数从1.85逐渐增大到1.89。这说明微絮凝过程中剪切速率的进一步增大有利于形成结构更加紧密的小絮凝体。从最终絮体的性质来看，中等剪切速率（G = 30 s-1）有利于微絮体的形成，形成具有所需性质的最终絮体，进一步提高了整个工艺的处理效率。基于微絮凝阶段的动力学，提出了不同剪切速率下微絮体生长的概念模型，进一步揭示了微絮凝阶段不同剪切速率下最终絮体性质差异的原因。结合计算结果和模型分析，认为微絮凝阶段的剪切速率主要通过微絮体结构的控制来影响最终絮体的形成。本研究为提高固液两相流的效率提供了理论和实际工作的依据。
Dynamic analysis on the variation of particle size distribution (PSD) and the fractal characteristics of PSD (Df) were investigated to better understand the continuous procedure of the floc growth and optimize the control of flocculation process. It was found that the flocculation process could be divided into three stages, i.e., the micro-flocculation stage, the growth stage and the steady (or breakage) stage. As the stage which is crucial to the morphology of micro-flocs (the building blocks of large flocs), the micro-flocculation stage plays an important role on flocculation/sedimentation process. The results showed that an increase in shear rate (11 s−1< G < 30 s−1) during the micro-flocculation stage contributed to micro-flocs with larger size and more compact structure. As shear rate further increased (30 s−1< G < 55 s−1), the micro-floc average size gently decreased from 13.61 μm to 10.91 μm, whereas two-dimension fractal dimension of micro-flocs gradually increased from 1.85 to 1.89. This indicated that further increase of shear rate during the micro-flocculation was incline to the formation of smaller micro-flocs with more compact structure. According to the results of final floc properties, the moderate shear rate (G = 30 s−1) benefited to the micro-floc formation to form final flocs with desired properties, further improved the treatment efficiency in the whole process. Based on the kinetics in the micro-flocculation stage, a conceptual model was proposed to describe the micro-floc growth under different shear rates, further revealed the reason for the different properties of final flocs under various shear rate during the micro-flocculation stage. Combining the results with model, it was concluded that shear rate during the micro-flocculation stage mainly affected final flocs by the domination of micro-floc structure. This research gives indications both for theoretical and actual works to improve the efficiency in the solid/liquid process.