Optimal catalyst particle design for flexible fixed-bed CO2 methanation reactors

Power-to-Methane is a concept for energy storage harvested from renewable sources, for instance solar and wind power. A flexible operation of catalytic fixed-bed methanation reactors, according to the availability of energy, eliminates the need to store intermediate reactants in large buffer units. This requires reliable heat management, as the heat generation of the methanation reaction is a challenge even at steady-state conditions. For this purpose, many studies have been conducted on the reactor scale, but knowledge on how the catalyst particle design can influence reactor behavior is rather limited. Therefore, a heterogeneous reactor model is used in this study to investigate the influence of particle activity, permeability and heat conductivity on reactor performance. Subsequently, a nonlinear program is formulated and solved to find the optimal particle design depending on these three properties and their radial distribution in the catalyst particles. The designs include spherical particles with uniform properties (Case 1), particles with two uniform zones of different properties (Case 2) and particles with radially variable properties (Case 3). Optimization, subsequent sensitivity analyses and dynamic simulations reveal that reactors filled with particles of Case 2 should exhibit a particle core with high activity, which is surrounded by an inert, low-permeability shell (so-called ‘egg-yolk’ particles). In this way, the reactor shows a reduced parametric sensitivity and a high methane yield at the same time. In comparison to reactors filled with particles of Case 1, they also exhibit a favorable behavior for dynamic and flexible reactor operation. The benefit of further particle zones to increase reactor performance appears insignificant, as shown by the results of Case 3.