Adaptation of Temperature Profiles in CO ₂ Methanation Reactors by an Appropriate Selection of Catalyst and Dilution Agent

The control of temperature in CO₂ methanation reactors is a challenging task due to the high exothermicity of the reaction and the high reaction rate observed when feeding pure reactants. This study analyses a new concept for moderating the reaction exothermicity by controlled dilution of a Ni-based catalyst using materials with different thermal conductivities. The simple decrease in the concentration of the catalyst active phase is not sufficient to control the temperature in the reactor because of the parametric sensitivity of the reaction, which means that a certain threshold of active phase exists, above which the reaction becomes so fast to cause the formation of a pronounced reaction hotspot and below which the reaction rate is too low to achieve high conversion. Therefore, the range of catalyst active phase concentrations that enables the reactor to remain active while maintaining a temperature below 550 °C using an externally cooled reactor is too narrow for practical applications. To achieve reasonable temperature control and a sufficient CO₂ conversion, the catalyst could be mixed with an inert solid material that can enhance the axial heat dispersion, so as to induce a good distribution of the heat release over the axial coordinate. This would reduce the localized load to the cooling system and spread it over a larger surface. The conductivity of different standard inert materials was evaluated, showing that Al₂O₃ and ZnO are the best options for application in CO₂ methanation. It was then observed that when the reaction rate decreases due to the approach to thermodynamic equilibrium, the axial dispersion of heat should be reduced to enhance the reactor performance, thanks to a fast temperature reduction. This results in a clear trade-off relationship between the reactor length, to achieve a given conversion, and the hotspot temperature. To break this trade-off, it would be necessary to modify the catalyst dilution characteristic over the axial coordinate, allowing for at least two different catalyst dilution zones.