Particles Transfer Behavior Between Adjacent Bed Layers In A Multistage Fluidized Bed With Downcomer

The multi-stage fluidized bed is a multi-phase device developed from the single stage fluidized bed,which is widely used in chemical engineering,metallurgy,energy,and environmental protection.The multi-stage fluidized bed with downcomers is the most widely used type among the multi-stage fluidized devices.The downcomer is the main place through which particle transfers between the layers in the device.In this way,the stability of particle flow and transfer in the downcomer becomes the main factor that affects the stability of the device and limits the stable operation range.However,because the downcomer is usually located inside the device,it is difficult to observe and detect the particle flow behavior in it.In this way,the need for more convenient research methods becomes necessary.To solve the problem,this paper targeted on the particle flow in the downcomer.Firstly,we developed a single downcomer device which makes the detection and observation possible.Based on visualization and pressure drop detection,the control and evolution mechanism of the particle flow in the downcomer was revealed.Subsequently,based on the needs of industrial applications,a new type of overflow assisted downcomer was proposed.The influence of the operating parameters and the structural parameters on the particle flow rate and the control law of the mixed particle flow were investigated.Finally,the CFD simulation of a typical unit in the multi-stage fluidized bed was employed.Through the analysis of the internal flow field and concentration field,the influence of operating conditions on the fluidization state of the bed and the particle flow in the downcomer were discussed.The results of this paper show that the design of a simplified experimental equipment based on the pressure drop balance and the CFD simulation are preferred method to study the particle flow characteristics in the downcomer.The main content and results are as follows.(1)Based on the pressure drop balance theory,the simplified single downcomer device was designed and built.Based on the imaging methods and pressure drop detection,solid flow patterns in the downcomer were clarified and the particle flow control mechanisms between the stable moving bed and the bubbling moving bed were figured out.The results showed that,under the experimental conditions of this study,there were three types of flow patterns existed in the downcomer,which were the stable moving bed,the bubbling moving bed and the slugging bed.Among them,the stable moving bed and the bubbling moving bed can be distinguished by the spectrum characteristics of pressure pulsation due to the periodic difference.Under the slugging bed,the particle flow became instable,and the relative standard deviation of pressure drop increased sharply.According to the above characteristics,flow pattern division diagrams were drawn,with pressure drop as the abscissa and the particle flow rate as the ordinate.The diagrams showed that as the orifice diameter increased,the instability of particle flow increased,and the gas blow-by area started to exist.For the stable moving bed,the particle flow rate was controlled by the superficial gas velocity in the downcomer,while for the bubbling moving bed,the particle flow rate was controlled by the cavities and bubble behaviors near the orifice.(2)A new overflow-cup assisted downcomer was designed.Based on the visualization method,the influence of the blow-back gas flow rate and the relative distance between the downcomer and the overflow-cup on the particle flow rate was studied.The result showed that,for the quartz sand,with the increase of the blow-back gas flow rate,the particle flow rate increased first and then basically unchanged.While with the increase of the relative distance between the downcomer and the overflow-cup,the particle flow rate increased exponentially until the flow limit.For mixed particles of cohesive calcium fluoride and quartz sand.At the lower addition ratio of cohesive particle,calcium fluoride particles had a lubricating effect on the particle flow in the downcomer,which increased the feeding capacity of the mixed particles.While at the higher addition ratio of cohesive particles,the particle cohesion hindered the particle flow.In this paper,the optimal addition ratio of cohesive particle is 5%.(3)By using the porous media model to simulate the gas distributer,the CFD simulation of a typical structural unit of the multi-stage fluidized bed was developed.Through the analysis of the flow field and concentration field,the influence of operating gas velocity on the fluidization state of adjacent layers and particle flow in the downcomer was studied.The result showed that the particle bed in the downcomer had a buffering effect,which made the particles quickly dispersed after entering the lower bed and ensured the homogeneity of solid phase in the lower bed.With the increase of operating gas velocity,the flow rate of gas short-circuited into the downcomer from the lower bed increased,resulting in a decrease in the solid concentration of dense phase zone,an increase in concentration gradient,and an increase in the length of dense phase zone.Meanwhile,the length of transition zone increased,bubbles and gas embolism started to appear in the dense phase zone.With the increase of operating gas velocity,due to the enhanced blocking effect of the ascending airflow in the downcomer,the intermittent solid flow from the upper to the downcomer became more difficult,and the particle accumulation occurred above the downcomer.The change of particle flow state and the increasing difficulty of intermittent particle flow from the upper bed to the downcomer may be the possible reasons for the instability of particle overflow.