以CFD-DEM模擬填充床內之二氧化碳再利用之合成反應

Abstract

本研究使用CFD-DEM模擬填充床反應器，先利用離散元素法(Discrete Element Method, DEM)自由落體生成虛擬填充床中的顆粒堆疊，後續耦合有限元素法計算流體力學(Computational Fluid Dynamics, CFD)。觸媒形狀影響填充床反應器的表現，本文選用圓球、圓柱、圓環或三葉柱狀觸媒顆粒進行填充床之堆疊，利用顆粒解析CFD來探討顆粒形狀對填充床內的影響，如氣體流態、壓降及溫度分布等，同時比較全域模式與局部切除兩模式的點接觸修飾表現，以提升有限元素法所需的網格品質。此外，本研究亦在填充床中耦合反應，第一部份是逆水煤氣轉化(RWGS)及費托(FT)整合反應，主要針對不同操作條件下對產物及反應器內的現象進行分析，第二部分是在殼管式反應器內進行二氧化碳甲烷化反應，探討不同殼層操作條件對轉化率之影響。 結果顯示使用局部切除1%修飾時，空隙度僅比真實空隙度少0.039%-0.231%，比較1 cm圓球於34.4 mm管中堆疊25 cm高的填充床兩端壓力降，在0.5 m/s-2 m/s空氣流速範圍內，CFD-DEM模擬預測的壓力降與實驗所測壓力降差異僅7.64%，當使用三葉柱狀顆粒填充時，填充床徑向空隙率變化最平緩、軸向速度分布最接近栓流，滯留時間分布也最窄，同時擁有最好的熱傳效果，且發生逆流的比例最低。 在RWGS和FT整合反應的部分，溫度為主要影響之因素，其次是滯留時間，而入口混合氣體各物質之比例影響最小，當管壁溫度上升時，能有效的提升二氧化碳轉化率及碳氫化合物之選擇率，同時能得到高烯烴/烷烴比。在二氧化碳甲烷化反應的部分，當殼層流動方向為逆流時，會有較高的二氧化碳轉化率，亦利用改變殼層入口溫度及氣體空速使內管達到適合反應之溫度，以得到高轉化率(80%左右)，殼層的操作條件會隨反應物進料溫度不同而改變，但其趨勢相同，殼層要操作在低溫及低流量或相對高溫及高流量來得到高二氧化碳轉化率。

The CFD-DEM simulation of packed bed reactors involves generating the packed bed structure by Discrete element method (DEM), followed by realizing the gas hydrodynamics using Computational Fluid Dynamics (CFD). The shape of the catalyst particles affects the performance of packed bed reactors. This study selects spherical, cylindrical, ring, or trilobe catalyst particles for the bed packing and utilizes particle-resolved CFD to discuss the influence of particle shape on various phenomena within the packed bed, such as gas flow field, pressure drop, and temperature distribution. In addition, this study also couples reactions within the packed bed. The first part focuses on the Reverse Water-Gas Shift (RWGS) and Fischer-Tropsch (FT) reactions, primarily analyzing the products and phenomena within the reactor under different operating conditions. The second part involves the carbon dioxide methanation reaction conducted in a shell-and-tube reactor, discussing the influence of different shell-side operating conditions on the conversion rate. In order to improve the mesh quality required by the finite element method, two particle-contact modification methods, the global gap method and the local cap method, are adopted in this research. When using the local cap method with the shrinking ratio of 1%, the predicted voidage is 0.039%-0.231% smaller than the voidage without contact-point modification. The average error between the predicted and the measured pressure drop in the superficial velocity range studied of 0.5 m/s-2 m/s is 7.64%, which is good enough for shape testing. When using trilobe particles for packing, the radial voidage shows the smoothest variations, the axial velocity distribution is closest to plug flow, and the gas residence time distribution is the narrowest. Additionally, it exhibits the best heat transfer performance and has the lowest backflow ratio. In the RWGS and FT reactions, temperature is the primary influencing factor, followed by residence time, while the ratio of different components in the inlet gas has the least impact. When the wall temperature increases, it effectively enhances the carbon dioxide conversion rate and the selectivity of hydrocarbon compounds, also get a higher ratio of olefins to paraffins. In the carbon dioxide methanation reaction, a higher carbon dioxide conversion rate is achieved when the shell-side flow is countercurrent. By adjusting the shell inlet temperature and gas hourly space velocity to attain the suitable reaction temperature in the inner tube, high conversion rates (around 80%) are obtained. The operating conditions of the shell side vary depending on the feed temperature of the reactants, but the trend remains the same. To achieve a high carbon dioxide conversion rate, the shell side should be operated at low temperature and low flow rate or relatively high temperature and high flow rate.