(268e) Detailed Numerical Modeling of Catalytic Monolith Reactors

Catalytic monolith reactors have numerous applications in industrial processes and as technical devices. Due to their regular, hierarchical structure, they are also suitable model systems for experimental and numerical investigations. Although even for a model system, very complex interactions between chemical processes, transport of the fluid, and the solid structure have to be considered. Broad variations of time scales for each of these processes let detailed simulation become computationally challenging. The computer program DETCHEM-MONOLITH [1, 2] has been improved in order to be capable of simulating a large variety of monolithic reactors. It applies hierarchically arranged detailed models from an atomic scale up to reactor scale. The core is a library for the description of species properties based on atomistic models and for reactions among gas-phase and surface species based on elementary-step reaction mechanisms. Diffusion in washcoats can be accounted for either by solving a reaction-diffusion equation or by applying an effectiveness-factor model. Under a quasi stead-state assumption, the fluid flow in each channel is described by either a 1D plug-flow with transport limitation models or a 2D boundary-layer approach with detailed transport models. Individual channels are combined into a transient temperature model for the monolithic bulk. Inlet conditions varying in space or on the same time scale as the thermal response of the solid can be considered. Since the numerical simulation of the fluid flow is the most time consuming step, not every possible channel is simulated in detail. A new cluster-agglomeration algorithm is applied to choose representative channels. The channels can differ in size and surface properties. Moreover, the same code can also be applied to simulate counter-flow configurations. This new feature will be illustrated by a numerical simulation of a catalytic combustion monolith with spatially varying inlet conditions. If catalytic monolith reactors shall be applied as a combustion stage for gas turbines, one faces a problem: Today's catalysts will not resist the high temperatures if all the fuel is burnt catalytically. As a solution, the catalytic burner is only used as a first combustion stage. In order to avoid overheating, the monolith consists of alternating catalytic active and inactive channels. Such a system will also be simulated numerically. One of the most popular and widespread monolith reactors is the automotive catalytic converter. Recently, catalytic devices with oxygen or NOx storage components have received considerable attention. Whereas for the classic systems, the time scales of the catalytic processes are usually much smaller than the thermal response times of the reactor, we now also must consider storage reactions and heat balance simultaneously. Therefore DETCHEM-MONOLITH has also been extended by a storage module. Storage media, as for instance barium oxide, are treated as an additional surface. Besides the temperature, the storage capacities are calculated transiently. A separation of time scales between the fast catalytic reactions and the slow storage reactions is assumed. This method will be applied to simulate a storage catalyst with alternating inlet conditions.

References: [1] S.Tischer, C. Correa, O. Deutschmann; Catalysis Today 69 (2001) 57-62. [2] O.Deutschmann, S. Tischer et al.; DETCHEM 2.0, http://www.detchem.com