Optimization of fixed-bed methanation reactors: Safe and efficient operation under transient and steady-state conditions

[Display omitted] •A 1D dynamic model for packed-bed methanation reactors is developed.•Reactors with catalyst dilution profiles are simulated.•Besides methane yield and space velocity, dilution profile impacts start-up time.•Dilution profile optimized for steady state can lead to transient unsafety.•A method is proposed to optimize dilution profile while avoiding transient unsafety. Designing and optimizing packed-bed methanation reactors for power-to-gas leads to conflicting issues. Because of the high exothermicity of the reaction, it might be inefficient (low Gas Hourly Space Velocity), or unsafe (runaway, thermal degradation). Dynamically operating reactors are thus particularly difficult to design. In this paper, the behavior of a reactor with multiple catalyst-dilution zones is studied numerically. Simulations of a dynamic sequence (reactor startup) are carried out. The effect of catalyst dilution profile on steady state and transient efficiency is analyzed. We introduce a method to maximize GHSV, while guarantying safe operation during both transient and steady state operation. A proper catalyst dilution profile can increase the steady state efficiency and stability of the reactor. However, the results of the present study show that this dilution profile can have a negative impact on transient efficiency and stability.