Multi-scale Simulation Study On Multi-tubular Fixed-bed Reactor For Methanation Of Syngas

The gas-solid heterogeneous catalytic process is a complex process,which involves adsorption,diffusion,surface reaction and many other steps.The multi-scale simulation,which starts from the elementary reaction at the micro electronic scale,covering the micro reaction kinetics,the diffusion and mass transfer at the particle scale and the fluid mass transfer and heat transfer at the reactor scale,has become a new method for the design of catalytic reactors.Methanation of syngas is a strong exothermic reaction and complex flow,heat transfer,reaction and diffusion are involved inside reactor.In present study,a multi-scale simulation method was developed to study the laws of flow,heat transfer and reaction-diffusion in multi-tubular fixed bed reactor for methanation of syngas,which extended from the micro electronic scale to the molecular scale,particle scale and macro reactor scale.The method is helpful to realize the design and development of a new-type methanation multi-tubular fixed bed reactor.The characteristics of bed structure and flow,heat transfer inside fixed bed were systematically studied.Discrete element method（DEM）was used to simulate the packing processes of catalyst particles and the geometry models of packing structure for fixed bed were obtained.The results showed that the characteristics of bed structure are closely related to particle shape,size,diameter of reaction tube and particle packing rate.Based on the actual bed structure,DEM method gives the local void fraction distribution of bed structure accurately.The local flow and heat transfer process of the bed is closely related to the structure characteristics of the bed.Computational fluid dynamics（CFD）method was used to simulate the heat transfer process in a fixed bed.Based on CFD simulation results,the parameters of the empirical formulas of pressure drop and the model of pseudo-homogeneous heat transfer for fixed bed were improved.The micro-scale reactions of syngas methanation on catalyst surface were systematically studied.Through density functional theory（DFT）method,the key steps of CO₂hydrogenation in network of syngas methanation on crystal surface Ni₃Fe（211）-AB were improved.The stable configuration of adsorption state,the transition state and energy barrier of the elemental reaction process were obtained,and the kinetic parameters of the elemental reaction network were calculated.The mean field approximation（MF）was used to calculate the improved methanation elemental reaction network,and the macroscopic reaction performance of the methanation process on the crystal surface of Ni₃Fe（211）-AB was obtained.The reaction-diffusion process inside particle and the local flow,heat transfer and mass transfer inside fixed bed at particle scale were studied systematically.By coupling MF and CFD,the single particle model of methanation was constructed to describe the effect of internal diffusion restriction quantitatively.On this basis,the coupled model of single particle reaction-diffusion process and flow,heat transfer was established,and the distribution laws of temperature and surface concentration inside particles were found.Based on the bed structure obtained by DEM,the local velocity,temperature and concentration distribution of bed under different tube diameters and particle shapes were predicted by CFD method.Based on the simulation results of each scale in the above,the multi-scale model of multi-tubular fixed bed reactor for methanation of syngas was constructed,and the influence of process conditions and bed structure on reaction process was studied.The results showed that the multi-tubular fixed bed reactor has higher catalytic performance when compared with the existing adiabatic reactor,and the reaction heat can be effectively removed to inhibit the rise of bed temperature.The thermodynamic limit of reaction can be avoid and the catalyst life can be improved.