Abstract:

Computational fluid dynamics (CFD) method was employed to study the effect of a novel internal-hollow cross disk (HCD) on flow and cracking in coil by coupling flow and energy equations with cracking reaction kinetics. Simulation results implied that geometrical structure change of inner wall surface by HCD embedment in cracking coil re-distributed patterns of flow field and strengthened radial velocity at reasonable pressure loss. The resulting longitude vortex disrupted flow boundary layer and improved near-wall turbulence, which in turn reduced thermal resistance and enhanced homogeneity of temperature distribution. Compared to regular cracking coils, the coil with HCD modification increased C₃H₈ conversion by 7.24%, olefin selectivity by 3.67%, and overall olefin yield by 6.94% which C₂H₄ yield had a slight decrease of 0.87% while C₃H₆ yield had a notable rise of 16.50%. Moreover, radial velocity from longitudinal vortex was found to promote exchange of near-wall high-temperature fluid to central low-temperature fluid with maximum temperature difference between fluids of 0.7℃. At the outlet, concentration of coking component of C₆ and higher, was found decreased by 28.33% in coil with HCD than that in counterpart, indicating that HCD introduction could prevent near-wall high temperature and over-cracking and inhibit occurrence of coking. HCD strengthening mechanisms on heat and mass transfer were quantitatively analyzed and an overall evaluation was performed with consideration of pressure loss and performance improvement.

Key words: cracking coil, flow structure, reactive turbulence, computational fluid dynamics

摘要：

通过计算流体力学（CFD）的方法，将丙烷裂解反应动力学与流动方程、能量方程耦合，考察了在普通裂解炉管中加装中空立交盘（hollow cross-disk，HCD）内构件对管内流动及裂解反应的影响。结果发现，HCD内构件通过壁面几何形状变化重布了流场结构，以合理的压力损失为代价产生径向速度，并诱导产生纵向涡剪切破坏边界层，强化了流体的湍动程度，降低热阻，提高了温度分布均匀性。相比于普通炉管，加入中空立交盘后，裂解管丙烷转化率提高7.24%，烯烃选择性提高3.67%，乙烯收率降低0.87%，但丙烯收率大幅上升16.50%，烯烃总收率上升6.94%。此外发现，纵向涡产生的径向流动促进了近壁区高温流体和管中心区相对低温流体的换位，流体温度最高下降了0.7℃；与普通炉管相比，新型裂解管出口处重组分浓度下降了28.33%，说明加入中空立交盘可防止近壁面高温区域过度裂解，有助于抑制结焦。在此基础上，结合模拟所得的场分布数据，定量分析了HCD强化传热和传质的机理，并就阻力损失和强化效果做出综合评价。

关键词: 裂解炉管, 流场结构, 反应流, 计算流体力学