Abstract:

Numerical simulation based on COMSOL Multiphysics was conducted to understand the heat transfer dynamics and reaction control mechanism over methanation catalyst particles in size of about 100 μm under conditions of transport bed. The high heat transfer efficiency in transport bed makes the catalyst particle into the bed quickly approach its steady state at about 0.1 s, a really very short transient period. For the catalyst particle at steady state there are temperatures sharing little difference among particle center, particle surface and gas flow bulk. However, it was clear that the temperature is still gradually lower from the center to surface of the particle, and on particle surface its temperature is higher than that in gas bulk. This clarifies that the exothermic heat of methanation reaction first heats catalyst particle, and the temperature-raised particle then reaches a balance of heat transfer with its surrounding gas. Calculating the radial profiles of temperature, gas components and reaction rate inside the catalyst particle clarified that for methanation at higher gas bulk temperature and elevated pressure (2 MPa here) the mass transfer into particles is quicker and the CO consumption rate becomes gradually lower from the center to surface of the particle. On the contrary, for atmospheric methanation at lower gas velocity, the reaction rate was conversely lower at the particle center. It demonstrates that the methanation reaction is subject to kinetic control for the former but to mass transfer for the latter.

Key words: transport bed, methanation, coal-to-SNG, numerical simulation, heat transfer, dynamic behavior, reaction mechanism

摘要：

采用数值模拟对输送床甲烷化的粒径100 μm级单颗粒催化剂的反应与热传递行为进行了研究，揭示了反应热在单颗粒催化剂上的动态传递规律和典型条件下的反应控制机制。在输送床反应器中，热传递效率高，进入反应器的单个催化剂颗粒温度一般在0.1 s左右即可达到稳态。稳态的100 μm级催化剂颗粒表面、中心、流体之间的温差很小，但催化剂颗粒的径向温度表现为由表面向中心逐渐升高的分布，证明了甲烷化反应热升高了催化剂颗粒温度，建立了温度升高的催化剂颗粒与反应气氛之间的热传递平衡。模拟甲烷化反应速率与组分气体在催化剂颗粒内的分布，揭示了在加压和较高气速反应条件下，反应物向催化剂颗粒的扩散加快，催化剂颗粒的局部甲烷化反应受动力学控制，反应速率由中心向表面逐步减低。反之，常压与低气速条件使得催化剂颗粒的甲烷化反应受气体扩散控制，反应速率自表面向中心逐渐降低。

关键词: 输送床, 甲烷化, 煤制天然气, 数值模拟, 传热, 动态行为, 反应机制