基于静电测量的聚乙烯气固流化床结片检测

Detection of sheeting in polyethylene gas-solid fluidized bed by measuring electrostatic current

以聚乙烯气固流化床内无结片正常流化状况为参照,对发生黏壁结片、黏分布板非对称结片和黏分布板对称结片等3种典型结片时不同径向和轴向位置的静电流(EC)进行了测量,发现静电流对结片的灵敏度较高,并据此提出了一种基于静电流的新型结片检测方法,可通过静电流信号平均值(MV)和标准偏差(STD)的改变判断结片的位置.研究发现,在正常流化状况下,床内流化状况均匀,静电流径向对称性良好.而黏壁结片和黏分布板非对称结片的发生均在一定区域内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,后者则对距离分布板较近的两个测点影响较大,两者均造成有结片侧和无结片侧静电流大小和波动程度的差异增大.黏分布板对称结片对流化床中流动状况的对称性几乎没有影响,但与无结片时的实验结果相比,相同高度、相同气速下的平均电流和标准偏差均增大.

Electrostatic current （EC） signals at different positions in the polyethylene gas-solid fluidized bed with sheeting were investigated respectively with the reference to the non-sheeting fluidization condition. Results showed that EC was sensitive to the different kinds of sheeting, including sheeting on column wall, asymmetrical and symmetrical sheeting on distributor. Then a new method to detect sheeting was proposed based on EC. The method estimated the position of sheeting through changes of EC＇s mean value （MV） and standard deviation （STD）. Results showed that EC＇s MV and STD in the bed without sheeting were axial symmetrical, while the formation of sheeting on the column wall and asymmetrical sheeting on the distributor destroyed the symmetry of EC in a certain region. The former primarily influenced the fluidization condition of the zone under the sheeting, while the latter significantly affected the zone near the distributor, both of which resulted in the greater differences of EC＇s MV and STD between the regions of the fluidized bed without and with sheeting. Symmetrical sheeting on the distributor had little influence on the symmetry of the fluidization condition, but EC＇s MV and STD were both greater than the results from the bed without sheeting.