Interaction of catalytic chemistry and transport inside and around porous catalyst pellets for CO2 methanation under enforced dynamic operation

点击翻译按钮获取中文摘要

Future production of fuels and chemicals will face fluctuating energy and raw materials supply. In process applications like power-to-X, catalytic reactors will more often be subjected to dynamic boundary conditions such as start-up, shutdown or load changes than current reactors running on fossil fuels and energy. The dynamic hydrogenation of carbon oxides to methane (methanation) is one such example for which catalytic reactors will have to be operated dynamically. A profound understanding of physical and chemical processes is needed on all relevant length and time scales in order to design this dynamic process properly. While much theoretical and experimental work has been devoted in the past to catalytic processes on the nano-scale (catalyst and active site dynamics) and on the macro-scale (reactor dynamics under transient conditions), comparably few studies focused on dynamic effects on the meso-scale (inter- and intraparticle dynamics). In particular, the interplay between diffusion and reaction inside a single catalytic pellet coupled with the surrounding flow field under enforced transient operation is poorly understood. Therefore, we hypothesize that only with a combined approach uniting detailed operando experiments and detailed CFD modeling, we are able to understand the dynamic methanation process on the pellet level. Based on these detailed single pellet insights, we can conclude on spatiotemporal patterns occurring in packed beds. We therefore study an isolated single pellet with defined dynamic feed conditions, both with experiments and with CFD models. With the capillary sampling technique, we quantify the dynamic local temperature and concentration profiles inside the pellet and in the boundary gas layer over a wide range of operating and perturbation conditions. In addition, IR thermography quantifies the surface temperature of the pellet. With this large data set, we develop and validate a transient single pellet CFD model coupling the surrounding gas flow with a three-dimensional reaction-diffusion model inside the pellet. Furthermore, the experimental and CFD setup is extended to an array of pellets mimicking a core section of a packed bed. This bridges our single pellet findings with industrially operating methanation fixed-bed reactors. Finally, we apply system identification tools to describe the dynamic pellet response under perturbation.