Simulation of carbon deposition behaviors of bi-disperse pore catalytic particles in methane cracking
Objective When methane contacts with the catalyst,cokeis easily deposited on the catalyst surface.Carbon behaviors on the catalyst surface can cause a decrease in the intrinsic activity,leading to a reduction in the reaction rate.At the same time,car-bon deposition can also cause the evolution of catalyst pore structure,further affecting the efficiency of mass transfer and reaction.However,few studies about the relationship between dynamic evolution of pore structure and reaction deactivation can be available.In this work,considering the dynamic evolution of pore structure caused by carbon deposition,a particle-scale model is estab-lished.The impact of carbon deposition on the performance of monodisperse pore and bidisperse catalysts is analyzed,which pro-vides a theoretical foundation for the optimization design of porous catalyst.Methods In this paper,the catalyst wasfirst regarded as theporous media.Based on the transport model of porous media,a particle-scale model coupling mass transfer and reaction activitywas constructed,where the decay of reaction performance and the dynamic evolution of pore structure caused by carbon deposition were considered.Secondly,the simulation of methane cracking was carried out.The evolution of pore structure and diffusion-reaction performance of monodisperse and bidisperse catalysts were evaluated.Finally,the catalysts were packed in a fixed bed,and the reaction performance and carbon deposition distribution of bidisperse catalysts were analyzed.Results and Discussion Basedontheparticle-scale model coupling the mass transfer andreactivity,the instantaneous variations of hydrogen concentration inside the catalyst are shown in Fig.3.Compared to the monodisperse particle,the bidisperse particle has a lower diffusion resistance,leading to enhanced uniformity of hydrogen concentration distribution.The instantaneous varia-tions of carbon deposition and reaction rate are shown in Fig.4 and Fig.5.The maximum carbon deposition mass of bidisperse particleis also significantly increased,and carbon deposition continue to occur at around 10 000 s.In contrast,the decay rate of monodisperse particle is faster.Fig.6 shows the instantaneous change of effective diffusion rate.The effective diffusion rate of the bidisperse particle is always higher than that of themonodisperse particle.As the carbon deposition proceeds,the diffusion rate distribution of thebidisperse particle is also less inhomogeneous than that of themonodisperse particle.This indicates that the introduction of macropores in the catalyst promotes the gas diffusion from the shell to the core,and inhibits the catalyst deactiva-tion.Fig.7 shows the instantaneous contour plot of velocity and coke deposition.When the gas flows through the pore channels,the region with locally high velocity is formed,resulting in ahigh coke deposition area.Fig.8 shows the radial distribution of bed porosity,activity factor and coke accumulation in a fixed bed reactor.The wall effect leads to an increase in local porosity,enhancing the local flow velocity and producing more coke.Conclusion In this paper,carbon deposition behaviors of porous catalyst particles in the methane cracking are numerically inves-tigated based on a particle-scale model coupling with the reaction kinetics model.The results reveal that coke deactivation moves from the surface of porous particles to the inner core.In the early stage of deactivation,the reaction rate in the shell region is relatively high.Ascoke is continuously generated,the reaction rate in the core region dominates,with opposite reaction ratesbe-tween the shell and the core region.In contrast to the monodisperse catalyst,the bidisperse particle has a stronger coke resist-ance.Monodisperse porous particles tend to become inactive after about 5 000 s,while bi-disperse porous particles continue to react after about 10 000 s.In addition,a high mass transfer coefficient near the walls ofthefixed-bed reactor leads tomore severe deactivation of catalyst.
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