Navigating reactor safety in catalytic microchannel reactors.

摘要

High temperature catalytic reactions are being intensely studied since many decades due to their large industrial potential, such as in pyrolysis, total oxidation (i.e. combustion) and partial oxidation of hydrocarbons. The reactions are characterized by extreme reaction temperatures (T > 1000°C) where homogeneous (i.e. non-catalytic gas phase) reactions can occur in parallel to catalytic reactions. This occurrence of homogeneous reactions is typically an undesired feature, since it complicates the understanding of reaction mechanisms, leads to selectivity losses, and often poses a safety hazard due to potentially explosive behavior [1]. Since free surfaces tend to bind radical species, eventually lead to a quenching of gas-phase reactions.; Microreactors, i.e. chemical reactors with characteristic dimensions in the sub-millimeter range, hold great promise for fundamental studies of existing processes offering small thermal inertia, high heat and mass transport rates, compactness etc. Due to their large surface-to-volume ratio, microreactors can be expected to suppress undesirable gas phase reactions and thus form safe reactor configurations for highly explosive processes.; In the present study, we numerically investigate the reactive flow of H2/air mixtures in a microchannel to gain insights into the reason for the absence of explosion observed in previous experiments [2, 3]. The H2 oxidation reaction is chosen as model reaction due to its high exothermicity and wide flammability range. It also constitutes an important sub-set of reactions in hydrocarbon oxidation.; In a two-dimensional boundary layer numerical model, we used coupled mechanisms with detailed elementary-step kinetics for gas-phase and catalytic surface reactions. The influence of different wall materials, reactor dimension, feed conditions and reaction pressure on the coupling of heterogeneous and homogeneous reaction pathways in the microreactor was studied. The results demonstrate that the attainability of 'intrinsic safety' in microchannel reactors is strongly dependent on a complex interplay between homogeneous and heterogeneous reaction pathways in the individual reaction system. In particular, it is found that intrinsic reactor safety breaks down at sufficiently high reactor pressure. Generalized equations for the current reaction systems are derived.; As an outlook, other industrially relevant reaction systems, i.e. CO oxidation and NOx formation, are preliminary investigated with respect to the effect of heterogeneous-homogeneous interactions and radical quenching in particular, on the behaviour of these reaction systems.