Multiscale Simulation Of The Methanation Process In A Fixed Bed Reactor

Coal gasification is a very important part of China's energy strategy.In this context,methanation,as the core technology of coal gasification,has always been a research hotspot in related fields.Therefore,it is very significant to understand the reaction-mass transfer-heat transfer and other transfer characteristics at different scales in methanation reactors.However,this process involves the multi-scale coupling of macroscopic,mesoscopic and microscopic,which makes it difficult to explore internal laws of the process.In order to carefully understand the rules at various scales in this process and guide the optimization of relative reactor more effectively,this paper set about studying the multi-scale effects in the methanation fixed bed with COMSOL multiphysics.Generally,there are two kinds of multiscale methods:the concurrentmulti-scale methods and the hierarchical multiscale methods.In this study,several multi-scale models of a fixed bed reactor were established based on a real fixed-bed reactor and the above two multi-scale methods.In the hierarchical model,the catalyst bed model is established on mesoscopic scale.At the microscale,an additional dimension has been constructed by software to describe the chemical processes within individual catalyst particles.In the concurrent model,the catalyst particle domain and fluid domain are divided by software to describe the chemical reaction inside the catalyst particle and the flow and mixing of materials in the bed.According to the characteristics of different models,the two-dimensional concurrent multiscale model and the there-dimensional hierarchical multiscale method was selected to simulate the methanation process in a fixed bed reactor from a multiscale perspective.The results show that:?1?on the mesoscale,from the bed inlet to the outlet,the concentration of reactant CO gradually decreases,but the concentration of product CH₄gradually increases.At the 50th second of the reaction,the temperature increases first and then decreases and the maximum is 557.06 K.At the same time,in the radial direction of the bed,all substances present uneven.For the reactant CO,its concentration is positively correlated with the gap size between catalyst particles.For the product,CH₄,there's a negative correlation with the gap size.The temperature distribution in the radial direction of the bed is uniform,but fluctuates obviously at the entrance of the bed.?2?on the microscale,the reactant CO presents a state of low concentration in the center of particles and high concentration on the surface of particles.And the product,CH₄,is just opposite to the CO.At the same time,the temperature distribution inside the catalyst particles is very uniform,showing an isothermal state.?3?at different scales,with the decrease of catalyst particle diameter,at the same bed height,the concentration of reactant CO in the main flow of the bed decreases gradually,and the concentration of product CH₄ increases gradually.At the same time,for one substance,the larger the diameter of the catalyst particle is,the greater the concentration difference within the particle is.And the small-size catalyst particle can effectively improve the methanation reaction rate and the load utilization rate of the catalyst particle on the bed scale.