連續式雙向流流體化床中顆粒乾燥之CFD-DEM模擬

Abstract

本研究使用CFD-DEM模擬連續式雙向流體化床，顆粒自系統上方進入系統，空氣則從下方反向進入，形成雙向流(Counter-current flow)乾燥程序。分配板使用開孔度0.37、孔徑5.5 mm的多孔板作，可使粒徑1.6 mm、含水量0.25的濕氧化鋁顆粒於板上累積形成懸浮床同時持續排出系統，故分配板同時扮演分散氣體及排出顆粒的角色。研究中探討了系統在進料空氣溫度25、40、60℃及流速3.18、3.30、3.41 m/s時的床內流態及熱質傳等現象，同時設計了三種多孔板嘗試分析其影響。 模擬結果中，各操作條件下平衡後的床態皆呈現穩定床(Stable bed)，符合實驗所示的操作視窗(Operating window)，平衡床體量隨進料溫度上升些微降低，但隨流速增加明顯上升，流速3.41 m/s時床體量約其他條件時的1.5倍，內部固體流態則大致無顯著差別。床中顆粒穩態的乾燥速率及溫度隨溫度明顯上升，但受流速影響較小，主要的乾燥區域落在靠近孔板約2 cm及氣泡附近，這些區域亦呈現較高的Nu、Sh及氣體流速，顯示較好的熱質傳效率。各條件下以進出口氣體含水率計算的乾燥效率(Drying efficiency)約落在90~95%，代表床中良好的氣固接觸。以模擬結果預測之產物含水量，除了在流速3.41 m/s的操作條件下，實驗與模擬誤差小於8%，且隨溫度的上升，產物含水量明顯下降，但對於流速則無顯著影響。 對於固定操作條件不同孔板的模擬結果，內部固體流態不因孔徑大小及開孔度分布有明顯差別，但對顆粒的排出有顯著影響，進而影響到平衡床體量，使用外圈小孔的設計，其床體量提升最大至約2倍。床體內顆粒的乾燥及溫度變化無明顯差異，但開孔度均勻時計算的乾燥效率約92%，不均勻的情況則略降至約90%。

In the present study, a continuous counter-current fluidized bed is simulated by the CFD-DEM method. A perforated plate with an open ratio of 0.37 and a hole diameter of 5.50 mm is used as the distributor. Wet alumina particles of 1.6 mm diameter are fed from the top of the column, and the suspension bed is formed above the distributor with particles discharging through the holes at the same time. Drying air of temperatures 25, 40, 60℃ and rates 3.18, 3.30, 3.41 m/s is input from the bottom of the column, forming a counter-current flow drying system. The steady state of the bed, solid flow pattern, and heat/mass transfer under different operating conditions are analyzed. In addition, different perforated plates with the same open ratio are designed to see the effect on the system. The result shows that a stable bed is formed under each operating condition, which agrees with the operating window of the experiment. The solid hold-up decreases slightly with drying air temperature but significantly increases with flow rate, where the solid hold-up of flow rate of 3.41 m/s is approximately 1.5 times the other operating conditions. The solid flow pattern does not show remarkable differences for different operating conditions. Particle inside the bed shows a significant increase in drying rate and temperature at steady state as drying air temperature increase, but a small difference as the flow rate increase. It is shown that the effective drying region is at about 2 cm bottom of the bed and also in the bubbling. High Nu, Sh numbers and flow rates indicate efficient heat and mass transfer in these regions. The drying efficiency (DE) calculated by specific humidity of the gas at the inlet and outlet ranges from 90~95%, indicating well contact between gas and solid. The predicted final water contents of particles show errors of less than 8% except for the case of flow rate 3.41 m/s. The final water content reduces as the drying air temperature decrease. For different designed plates under fixed operating conditions, a similar solid flow pattern is observed, but the discharge of particles is affected by the distribution of hole size and open ratio, which in turn results in different solid hold-ups. The plate designed with small holes on the outer ring shows the largest solid hold-up which gives about 2 times the case using the original plate. Although the drying rate and temperature of particles don’t show significant differences for different plate designs, the calculated drying efficiency is approximately 92% for design with uniform open ratio, but slightly decrease to 90% for nonuniform design.