多階雙向流式流體化床之CFD-DEM模擬研究及應用於連續式揮發性有機物吸附

Abstract

本研究透過實驗及計算流體力學耦合離散元素法模擬(CFD-DEM)，分析多孔板式雙向流流體化床系統內部顆粒流態，並將系統應用於連續式揮發性有機物吸附。固體顆粒和揮發性有機物(Volatile organic compounds, VOCs)分別選擇瀝青基球型活性碳及甲乙酮(Methyl ethyl ketone, MEK)作為實驗材料。 流態探討中，以實驗觀察粒子於床內的滯留情況，歸類出四種形態：不成床、稀薄床、穩定床及溢流，並建立此多孔板的操作視窗(Operating window)。以CFD-DEM 模擬分析顆粒流態及掉落機制。對於顆粒流態，粒子於多孔板中心處上升，外圍處下降，具有對稱流態。對於粒子掉落機制，主要從多孔板外圍，氣體流速較慢的孔洞掉落。穩定床階段時以團聚大量掉落的 Dumping 機制為主，當由穩定床轉變為溢流後，Dumping 掉落機制逐漸衰微，改以零星碰撞方式造成粒子掉落。 連續式 VOCs 吸附中，固定 MEK 濃度 1200±100 ppm 進行研究。MEK 移除效率(MEK removal efficiency)主要與孔板上的滯留顆粒量成正相關。當固體進料速率為 0.16±0.005 kg/m2s 時，單階流化床在氣體流速 0.84 ~ 1.08 m/s 的 MEK 移除效率為 88 ~ 97%；二階流化床氣體流速為 1.00 m/s 時，移除效率可達到 99%以上。最後，活性碳經過三小時吸附後，達到約 45%的飽和吸附量，同時 MEK 出口濃度僅為入口濃度的 10%，顯示此系統應用於連續式吸附中的長時間高穩定吸附優勢。

This study employs experiments and computational fluid dynamics coupled with discrete element method (CFD-DEM) to analyze particle motion within a countercurrent fluidized bed fitted with perforated plate. Additionally, the system is applied to continuous adsorption of volatile organic compounds (VOCs). For the experimental materials, asphalt-based spherical activated carbon and methyl ethyl ketone (MEK) are selected as the representative absorbent particle and VOC, respectively. In the investigation of particle motion, the particle residence behavior within the bed was observed experimentally and categorized into four regimes: non-growth bed, dilute bed, stable bed, and flooding bed. Moreover, an operating window for the perforated plate fluidized bed system was established. CFD-DEM simulation was used to analyze the flow patterns and particle discharge mechanisms. Regarding the flow patterns, particles rise close to the center of the perforated plate and descend around the periphery, exhibiting a spouting flow pattern. As for the particle discharge mechanism, particles mainly exit through holes with lower gas velocities around the periphery of the perforated plate. During the stable bed regime, the dumping mechanism, characterized by the falling of large particle clusters, is predominant. As the system transfering from a stable bed regime to a flooding regime, the dumping mechanism gradually diminishes, and particles discharge primarily through sporadic collisions. In the continuous VOC adsorption study, the inlet MEK concentration was fixed at 1200±100 ppm. The MEK removal efficiency was positively correlated with the amount of particles retained on the perforated plate. With a particle feed rate of 0.16±0.005 kg/m2s, the MEK removal efficiency in a single-stage fluidized bed ranged from 88% to 97% at different gas velocities ranging from 0.84 to 1.08 m/s. In a two-stage fluidized bed with a gas velocity of 1.00 m/s, the removal efficiency could be higher than 99.0%. Finally, the activated carbon reached about 45% of its saturation adsorption capacity, while the MEK outlet concentration was only 10% of the inlet concentration after three hours of adsorption test. This demonstrates the advantage of this system for long-term, highly stable continuous adsorption applications.