Abstract:

An on-line measurement technique, which consisted of electrostatic probe, signal transmitter and data acquisition systems, was developed to study the electrostatic characteristics in gas-solid fluidized beds.By measuring the electrostatic potential at different bed heights, it was found that the electric field inside the bed was non-uniform.The axial profiles of electrostatic potential were Z-shaped, and the voltage polarity would reverse near the interface between dense and dilute phases.The heights where voltage polarity reversal happened would increase with the increase of gas velocity and static bed level.Comparing the fluidization of particles with different sizes, it was discovered that the polarity reversal would change significantly when there was an obvious interface between dense and dilute phases, and the converse was not true.Subsequently, the principle on bipolar charging of different sizes fluidizing particles was presented.Work function, surface structure, selective adsorption of contaminants and surface energy were all dependent on the particle size, which resulted in finer and coarser particles charged reversely, consequently influencing the electrostatic potential profile in fluidized beds.A new method to determine bed level based on the characteristics of electrostatic potential in fluidized beds was proposed.The results showed that bed level could be successfully predicted with the maximum relative error 4.08% and the mean relative error 2.02% with the method in this work.The novel electrostatic approach to measuring the bed level in gas-solid fluidized bed could replace the γ-ray method in the future, and could monitor electrostatic potential in the bed and bed level simultaneously.

Key words: 气固流化床, 静电压, 摩擦起电, 双极带电, 料位

摘要：

应用自行开发的静电压在线测量系统，对聚乙烯气固流化床中不同床高处的静电压进行测量，发现床内的静电场为一非均匀电场，静电压沿床层轴向呈“Z”形分布，且在床层稀密相分界面，即料位附近会发生极性的改变。实验同时发现，提高流化气速和静床高，稀密相分界面上升，极性改变位置也随之出现较明显的升高。而颗粒的粒径发生改变后，若颗粒在流化过程中存在较明显的料位，则极性改变现象明显，反之这种现象不会发生。在此基础上，提出了流态化颗粒的双极带电理论，指出不同粒径的颗粒在有效功函数、表面结构、吸附杂质的能力以及表面能等方面均存在差异，导致大小颗粒在接触带电中所带电荷极性相反，进而影响了整个流化床中的静电分布。最后，基于静电压的分布特征提出了一种确定流化床料位高度的新方法，实验结果表明，该方法计算得到的料位高度值与实测值之间的最大相对误差为4.08%，平均相对误差为2.02%，具有较高精度，能替代现有对人体具有致命伤害的γ射线，且可实现床内静电压和料位高度的双重监控。

关键词: 气固流化床, 静电压, 摩擦起电, 双极带电, 料位