Multi-scale Simulation Of Reaction Process And Optimization Design Of Pore Structure In Methanation Catalyst

In the methanation process,the catalyst plays an important role.Exploring the reaction process in the catalyst particles from multiple scales will provide new ideas for the preparation of the catalyst and the strengthening of the methanation process.In this paper,based on previous experiments,using COMSOL Multiphysics simulation software,a single particle catalyst methanation reaction system model based on the continuum model was established.After verifying the effectiveness of the model,the heat and mass transfer process and reaction behavior in the catalyst particles of the methanation process were explored under different particle sizes,reaction temperatures,and reaction pressures.In view of the rapid reaction process of the methanation reaction and the significant influence of internal diffusion in the catalyst particles,this paper established a double-dispersed pore structure catalyst model containing macropores and mesopores,and compared the performance of monodisperse pore structure catalyst and the double-dispersed catalyst under different operating conditions（reaction temperature,inlet flow rate,hydrogen to carbon ratio）,the influence of particle’s pore structure(average pore size d_pore and porosityε)on the catalytic performance was also discussed.Finally,taking the catalyst particles in the fixed bed methanation reactor as the research object,a bed-particle dual-scale coupling model was established.The dual-scale coupling model also considered the heat and mass transfer behavior under the bed and the particle scale,and it can explore the effect of catalyst particles’microscopic pore structure（macropore average pore diameter d _M,mesopore average pore diameter d_m,macroporous porosityε_M and mesoporous porosityε_m）on the reactor performance（CO₂ conversion rate,CH₄selectivity and yield）at the bed scale.The optimization design of the catalyst pore structure under the simulated conditions was carried out with the goal of maximizing the methane yield.The results show that in the micro-scale study of single-particle,it is found that there is a large internal diffusion resistance in the methanation catalyst particles,the effective reaction area in the catalyst is the area of the normalized radius equal to 0.6to 1,and the overall utilization rate of the catalyst is low.The methanation reaction first occurs on the surface of the particles,releasing a large amount of heat of reaction,and the hot spots are mainly in the outer layer of the particles.As the reaction proceeds,the heat released is more likely to migrate to particle center,causing heat collection inside the particles.When the particle size increases from 2 mm to 6 mm,the internal diffusion resistance of the particles will increase,and the temperature gradient along the radius of the particles will also increase.Studies on the pore structure of the particles show that the double-dispersed pore structure embedded with macropores can improve the internal diffusion limit of the catalyst particles,thereby increasing the utilization rate of the internal area of the catalyst.At the same time,the embedding of macropores reduces the thermal resistance inside the particles.the heat collection phenomenon in the catalyst has also been improved.In the double-dispersed pore structure catalyst,there is a competition between diffusion limitation and reaction specific surface area.Since the embedding of macropores improves diffusion and also reduces the reaction specific surface area,the catalytic performance can only be improved when the positive effect brought about by the improvement of diffusion resistance is greater than the negative effect brought about by the loss of specific surface area.The bed-particle dual-scale coupling simulation found that in a fixed bed,the unoptimized intra-particle methanation is severely affected by diffusion limitation,the effective reaction area is about 20%of the outer layer of the particle,and the reaction rate at the center of the catalyst is low,And there is a large concentration gradient between the particle and the bed’s concentration;under the given operating conditions,the methane yield increases first and then decreases with the increasing of macroporous porosityε_M,and reaches the maximum whenε_M≈0.4;The methane yield fluctuates slightly with the macropore’s average pore diameter d _M,and tends to be stable in the interval of d _M≥120 nm;the smaller the average pore diameter d_m,the greater the methane yield,and when the d_m decreases to4.5 nm the rate of change slows down;through orthogonal experiments,for spherical particles with particle size d_p=5.4 mm,the pore structure that maximizes methane yield under simulated conditions is obtained:ε_M=0.4,ε_m=0.1,d _M=160 nm,d_m=4 nm,the final methane yield is about 83.2%,which is about 18%higher than the reference model.