The Flow Behavior Of Irregular Cohesive Particles In The Spout-Fluid Bed With A Draft Tube

Draft tube spout-fluid beds have been widely used in drying, coating and granulating processes with the advantage of regular particle motion and wide range of particle size. As the increasing applications of these special spout-fluid beds, describing and regulating the gas-solid flows within the reactor internals become hot researches in the fields of materials and chemical engineering. However, the difficult problems in these processes involved the flow behavior of irregular particles and the beds operated under high temperatures. There are currently little informations concerning the complex flow characteristics of irregular particles. The present work is devoted to revealing complex gas-solid flow regions in a conical-cylindric spout-fluid bed with draft tube by experimental and simulated approaches. The complex gas-solid flow characteristics at elevated temperature were described in details. Based on the gas-solid flow characteristics and principle, the measures to improve the spout-fluidization quality of irregular particle were proposed.In the spout-fluid bed with a draft tube, six flow patterns were identified by pressure drop fluctuations and digital images:fixed bed, spouting in draft tube, jet-in-fluidized flow, spouting with aeration, spout-fluidization, unstable spouting. The typical flow pattern images, fluctuation of pressure drop and pressure drop spectrum signals were presented for classifying the flow patterns. Gas-solid flows within the reactor internals showed certain regularity with the change of operation conditions. Elevating operation temperature and decreasing stable bed height would decrease the gas velocity ranges of spout-fluidization. The gas velocity ranges of unstable spouting were enlarged with the increase of temperature. Promoting the entrainment height and draft tube diameter would decrease the gas velocity ranges of internal spouting.In the draft tube spout-fluid bed, the temperature is one of the key factors exerting a strong effect on the hydrodynamic performance. In the case of low fluidizing gas velocity (Uf< 0.010 m/s), the spouting gas flow rate transferring into the annulus region was increased with temperature. Therefore, the minimum spouting velocity (Ums) increased with temperature at a lower fluidizing gas velocity. When the fluidizing gas velocity exceeded by 0.020 m/s, high temperature promoted the gas flow rate transferring from the fluidizing gas into the draft tube, which resulting in a decrease in minimum spouting velocity. As the increasing in temperature, the minimum spout fluidizing velocity (Umsf) were decreased. Forecasting equations for the values of Ums and UmSf were proposed based on the large experimental data, while these forecasted values were confirmed by our research results and the previous publications.The irregular cohesive particle:non-metal material from waste printed circuit boards (NPCB) showed poor flowability and channeling, which restrained the stable spouting in the spout-fluid bed. The quality of stable spouting was improved when above 40 wt.%PP particles added in the NPCB/PP mixtures. The mechanism was that the PP particle accelerated the movement of NPCB. Meanwhile, the less density differences between NPCB and PP particle decreased the segregating of mixtures. The minimum spouting velocity decreased with the increase in gas fluidization velocity and ratio of NPCB particle in NPCB/PP mixtures. Two flow patterns of unstable spouting and unstable spouting fluidization were observed over a large range of gas velocity according to the flow pattern images, while the ranges of gas velocity in the two flow patterns enlarged with the increase in mass fraction of NPCB within the NPCB/PP mixtures.Gas-solid flow behaviors in the draft tube spout-fluid beds were simulated based on the two-fluid model (TFM). The solid volume fraction in the beds at different times indicated the formation and development of the flow patterns. Gas-solid flow within the entrainment zone at the bottom of the draft tube was clearly displayed by the simulated images. Spouting gas had significant effects on solid volume fraction in the draft tube. Particle concentrations in central areas of draft tube increased with the axial height, and this was similar to the experimental results measured by PV-6A. Solid volume fraction under beds decreased with the spouting and fluidizing gas velocities, whereas increased with spouting gas velocity upon draft tube. TFM provided a theory support for these reactor designs since the very well forecasting in gas-solid fluidization in spout-fluid bed with a draft tube.