摘要
聚乙烯流化床中,颗粒与颗粒之间、颗粒与壁面之间以及颗粒与气体之间存在反复碰撞、摩擦及分离,若流化介质为高绝缘性物质,则不可避免地会发生静电的产生与积累,产生的静电场会改变流化床内的流体行为,导致颗粒团聚、粘壁,形成死区和沟流等。当静电积累到一定程度,达到周围介质的击穿场强,还可能引起火花放电甚至爆炸。静电问题已成为长期困扰气相法聚乙烯流化床生产过程的突出技术难题。因此,研究聚乙烯气固流化床中静电产生机理以及静电特征具有非常重要的理论意义和实用价值。
论文针对上述问题,开展了以下四方面的工作:1.分析流化床中静电的产生机理;2.流化床中静电压分布特征的研究;3.基于静电压分布的流化床料位检测研究;4.基于流化床中静电测量的结片检测研究。具体的工作及成果如下:
1.针对气相流化床聚合反应器生产聚乙烯的特点,提出了流态化颗粒的双极带电理论。该理论主要包括三部分内容:流化床中的静电产生-累积模型、颗粒的双极带电机理以及流化床中粒径分布与静电压分布的关系。前者利用能带模型对流化床中静电的产生和累积机理作出了解释;颗粒的双极带电理论则通过建立颗粒有效功函数与颗粒粒径和介电常数之间的物理模型,证明同种材质的聚合物颗粒的有效功函数随粒径增大而减小,因此不同粒径颗粒在接触分离后所带电荷极性相反。当可转移电荷载体为电子或阴离子时,则大颗粒带正电,小颗粒带负电,当电荷载体为阳离子,则大颗粒带负电,小颗粒带正电;最后提出颗粒在流化床轴向上具有一定的粒度分布,因此流化床中的静电荷同样存在一定分布,其中的静电场为非均匀电场。
2.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”型分布,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。最后结合静电产生-累积模型和颗粒的双极带电机理对上述现象进行了合理解释。
3.基于静电压在料位处发生极性改变的特征,提出了一种流化床料位检测的新方法,该方法可实现床内静电压和料位高度的双重监控,能替代现有对人体具有致命伤害的γ射线,或可作为其它料位检测方法的补充。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4.08%,平均相对误差为2.02%,具有较高精度,符合工业生产中的要求。
4.应用自行开发的静电流在线测量系统,提出了一种基于静电流的流化床结片检测方法。针对聚乙烯流化床中正常流化状况下,以及壁面和分布板上存在结片时的故障情况,分别对不同轴向和径向位置处的静电流进行了测量。实验结果表明,在正常流化状况下,床内流化均匀,左右两侧静电流大小和波动情况基本一致,对称性良好。当存在壁面结片和分布板上非对称分布结片时,均在一定区域破坏了流化状况的均匀性。前者主要影响结片下方区域的流化状态,造成下方左右两侧各测点处的平均静电流和标准偏差的差别较大。后者则对距离分布板较近的两个测点影响较大,同样造成了左右两侧静电流大小和波动程度的差异增大。结片位于分布板中心时,对流化床中流动状况的对称性几乎没有影响,但与空白实验结果相比,相同高度、相同气速下的平均电流和标准偏差均增大。
Electrostatics can cause serious problems for fluidized beds, altering hydrodynamics, causing agglomeration, interfering with instrumentation, generating nuisance discharge and even creating the danger of explosion. The accumulated electrostatic charges on powder particles or plastic films inside large commercial fluidized bed reactors can interfere with their performance, leading to undesirable byproduct/waste. Despite these negative effect, the mechanism of charge generation and dissipation are still poorly understood. Although there have been some efforts to prevent electrostatic charges in fluidized beds, the mechanism of reducing electrostatic charge accumulation are still poorly understood.
Aiming at the problems mentioned above, four aspects work were carried out: 1. The generation mechanism of electrostatics in fluidized beds was analysed; 2. Experimental study on electrostatic potential profile in the fluidized beds; 3. Study on a new method measuring the bed level basing on the characteristics of electrostatic potential profile; 4. Sheeting detection research basing on the mearsurement of triboelectrical current in the fluidized bed. The main results in this article can be summarized as following:
1. The mechanism of static charge generation and electrostatic character inside gas-solid fluidized beds was analysed, and a theory on bipolar charging of fluidized particles was proposed, which consists of three parts: a physical model for charge generation and accumulation in fluidized beds, the principle on bipolar charging of different sizes particles and the relationship between particle size distribution and chage distribution in fluidized beds. The first one could interprete how to generate and accumulate static charges with band level model. By developing the physical model realating the effective work function of particles, particle diameter and relative permittivity, the principle on bipolar charging of different sizes particles showed that the effective work function of different sizes particles with similar materials would decrease with the increase of particle diameter, therefore, the polarity of the charge on the different sizes particles would be opposite after contacting each other and separating. If the transferable charge carriers were electrons or negative ions, the coarse particles would charge positively, and the small particles would charge negatively. While the transferable charge carriers were positive ions, the coarse particles would charge negatively, and the small particles would charge positively. Finally it was proposed that due to the axial particle size distribution in fluidized bed, there would be as well a static charge distribution in it and the electric field would be not uniform.
2. An on-line measurement technique, which consists of the electrostatic probe, signal transmitter and data acquisition systems, was developed to study the electrostatic characters in gas-solid fluidized beds. By measuring the electrostatic potential at different bed height, it was found that the electric field inside the bed was non-uniform. The axial profiles of potential were Z-shaped, and the voltage polarity would reverse near the interface between dense and dilute phases. The axial height 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 potential polarity would change significantly when there was a obvious interface between dense and dilute spaces, and the converse was not true. The experimental phenomena mentioned above could be explained plausibly combining the physical model for charge generation and accumulation with the principle on bipolar charging of different sizes particles.
3. A new method to determine bed level basing on the characteristics of static 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% through the method in this article. The novel electrostatic approach measuring the bed level in gas-solid fluidized beds could replace theγ-ray in future, and monitor electrostatic potential in the bed and bed level simultaneously.
4. A new method to detect sheeting basing on the triboelectric current was investigated preliminarily. The results showed that sheeting on the column wall and distributor would influence the mean values and standard deviationof triboelectric current. By detecting the triboelectric current at different radial and axial locations of the column, the position of sheeting could be found out approximately. Triboelectric current had sensible reflection on the sheetings both at wall and distributor and was promisingly to be a novel way to detect sheetings in the fluidized bed reactor.
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