基于连续方法的气固循环流化床全回路模拟及稳定性分析

相比对单个操作单元的模拟,气固循环流化床的全回路模拟能全面揭示各单元之间的联系、诊断操作突变等现象,对实际工业生产更具指导意义。本研究在连续介质模型结合颗粒动理论的框架下,对一套虚拟过程工程(VPE)的气固循环流化床装置进行了全回路模拟和稳定性分析。模拟发现了提升管中的颗粒浓度及压降发生大幅度的周期性震荡现象,两种完全不同的操作状态,即稀相输送和浓相输送,交替式地出现。为分析该现象产生的原因,考察了模型因素(主要是气固相间曳力)和操作因素(颗粒藏料量和提升管表观气速)对周期性震荡现象的影响。研究发现,将考虑非均匀结构影响的曳力替换成均匀曳力,仍不能消除周期震荡现象,其颗粒输送返回装置(Loop-seal)压头不足以保证颗粒从下降管平稳输送到提升管,而降低气速和增大藏料量都有利于颗粒循环输送的稳定性,防止“窜气”现象的发生。结合上述现象,进一步聚焦影响颗粒输送的关键点,即Loop-seal气动阀,采用引入虚拟阀门的方式提高Loop-seal输送管中的输送阻力,从而有效改进了全回路模拟的稳定性,其预测得到的提升管轴向压降分布与实验值基本吻合。

Full-loop simulation and stability analysis of a gas-solid circulating fluidized bed

The gas-solid fluidized bed has wide application in industry such as refinery, metallurgy, and ore calcination. In the practical operation, it is usually combined with other units like conveying pipe, cyclone, downer and valves to run in a full-loop way. Compared to simulation of a single operating unit, the full-loop simulation of the gas-solid circulating fluidized bed system can reveal interactions between different operating components and diagnose the sudden change of operation, thus being of greater importance in industrial operation. In this study, the full-loop simulation and stability analysis of a gas-solid circulating fluidized bed of virtual process engineering (VPE) are carried out under the framework of two fluid model and kinetic theory of granular flow. The simulation shows that there exists periodic fluctuation of solids volume fraction and pressure drop in the riser where two distinct fluidization states, i.e., dilute fluidization and dense fluidization, appear alternatively, because of the occurrence of gas bypassing. To figure out the underlying cause, the influence of model factors (mainly refer to gas-solid drag) and operating parameters (i.e., solid inventory and superficial velocity) on the periodic fluctuation phenomenon is numerically investigated. It is found that changing the drag model cannot eliminate the fluctuation, while reducing the gas velocity and increasing the solid inventory are conducive to the stability of particle circulating transportation and avoiding the occurrence of "gas bypassing" because of the increase in pressure drop of the Loop-seal. On this basis, the Loop-seal valve which is reported to be closely related to particle conveying is focused on. The simulation with adding a virtual valve in the middle of the inclined pipe is performed and shows that the resistance of particle conveying increases thus ensuring the enough pressure drop of the Loop-seal to operate the full-loop system steadily. The time-averaged axial profile of pressure drop predicted by the simulation agrees with the experimental data. This method is found to be helpful to improve the stability of the full-loop simulation.