颗粒尺度下混合催化剂床层中CO_(2)加氢反应体系数值模拟

Numerical simulation of CO_(2) hydrogenation system in mixed catalyst bed at particle scale

CO_(2)的有效转化对于实现“双碳”目标具有重要意义。柱形颗粒比异形颗粒具有更优良的热导性和更小的颗粒表面积,因此,为获得更优异的床层性能,采用离散元方法(DEM)对异形颗粒混合堆积床内的CO_(2)加氢反应体系进行颗粒尺度CFD模拟计算,探究不同堆积方式对床层多物理场分布、CO_(2)转化率及CH_(4)产率的影响。结果表明,催化剂内部存在扩散阻力,随着反应进行,颗粒内物质由分层分布渐变为均匀分布。随机混合床与常规床相比,随机混合床稳态前热点不易波动、稳态产率更高,而柱形床径向热场更均匀且热点更高。在四种规则混合床中,底部为4孔的两种催化剂床相较于底部为柱形的两种催化剂床,整体流场更均匀、高速区少、压降大、高温区占比大、稳态前热点不易波动;2层-底4孔催化剂床的稳态出口CO_(2)转化率和CH_(4)产率均最大;4层-底4孔催化剂床高温区占比较高,随着反应进行,高温区占比呈上升趋势,CO_(2)转化率和CH_(4)产率大幅下降。

The effective conversion of CO_(2)is important for achieving the goals of peak carbon dioxide emissions and carbon neutrality.Cylindrical particles have better thermal conductivity but smaller surface area when compared with heteromorphic particles,therefore,for better bed performance,the discrete element method(DEM)is used to perform particle-scale CFD simulations of a CO_(2)hydrogenation system in a mixed bed of heterogeneous particles,to investigate the effect of different stacking methods on the multi-physical field distribution,CO_(2)conversion and CH_(4) yield in the bed.The results show as follows:there is a diffusion resistance within the catalyst,as the reaction progress,the material within the particles changes from stratified distribution to uniform distribution.Compared to conventional beds,random mixed beds are less prone to hot spot fluctuations before steady state and has higher yields at steady state,while columnar bed have a more uniform radial heat field and higher hot spot.Of the four regular mixed beds,the two catalyst beds with 4 holes at bottom have more uniform overall flow fields,fewer high velocity zones,larger pressure drops,a larger proportion of high temperature zones and less fluctuation of hot spot before steady state than the two catalyst beds with columnar bottom.2-layer mixed stacking-bottom 4 holes catalyst bed has maximum steady-state exit CO_(2)conversion and CH_(4) yield.The 4-layer mixed stacking-bottom 4 holes catalyst bed has high percentage of the high temperature region,which tends to increase as the reaction progress,resulting in significant decrease in CO_(2)conversion and CH_(4) yield.