A modeling study on reaction and diffusion in MTO process over SAPO-34 zeolites

In this work, a modeling approach based on the dual-cycle mechanism and Maxwell-Stefan diffusion theory is developed to investigate the reaction and diffusion in methanol-to-olefins (MTO) process over SAPO-34 zeolites. In this work, the effect of coke formation on diffusion and adsorption in SAPO-34 zeolites were taken into consideration. The diffusivities and adsorption isotherms of the molecules of reactant and main product were derived by experimental measurements of the uptake rate of guest molecules. This model was used to study MTO reaction over SAPO-34 zeolites with different crystal size. It is shown that the model can capture the main features of MTO reaction as obtained by experiments, for instance, the different roles of the olefins-base cycle and aromatic-base cycle at different stage of the reaction. This model can predict the evolution of activated coke and non-activated coke during the reaction process. In particular, the simulation results directly show that the formation of non-activated coke can cause the pore blocking inside SAPO-34 zeolites and constrain the diffusion of different guest molecules to various extend. This is essential for us to understand the effect diffusion on catalyst lifetime and product selectivity in MTO reaction over SAPO-34 zeolites.