气固顺逆重力场流态化特征分析

Analyzing on Fluidization Characters of Gas-Solid Concurrent/Countercurrent with Gravity Field

气固并行顺重力场与逆重力场流动形成了迥然不同的流态化机制 :下行床中 ,局部颗粒的聚集会使局部颗粒及气体速度增大 ,而局部气体速度的增大又会破坏颗粒的聚集 ;提升管中因气固逆重力场流动 ,颗粒的聚集会使局部气体及颗粒速度降低 ,而这种降低又会加重颗粒的聚集。与提升管相比 ,下行床具有气固速度和颗粒含率径向分布均匀和气固停留时间短以及返混小等特点 。

The fluidization mechanism of gas solid concurrent with gravity field is disparate with that of gas solid countercurrent with gravity field. In both the downer and the riser, higher local solid concentration results in the reduction of drag coefficient. In the downer, reduction of the drag force will increase local particle and gas velocity, which in turn reduce the particla aggregation. However, in the riser, reduction of drag coefficient will decrease the local particle and gas velocity, which in turn increases the tendency of the particle aggregation. Compared with the riser, the downer have several distinct advantages such as short gas solids resident time, more uniform distribution of gas solids velocity and solids content, and reduced axial backmixing. The condition of gas solid contact in the downer is more close to plug flow.