Modeling and simulation of monolith reactors
Keller, Paul Victor
Doctor of Philosophy
Monolith reactors are studied with models and simulations, with particular attention given to the catalytic converter. It is found that previously noted differences between one- and two-dimensional monolith reactor models are largely due to artifacts introduced into the two-dimensional models through the boundary conditions at the reactor entrance. These artifacts can be avoided by modeling a pre-entrance region. Difficulties have been encountered in previous studies which attempt to match two-dimensional catalytic combuster models to data from experimental systems, which are essentially three-dimensional in character. These difficulties can be resolved with a new approach to choosing parameters for two-dimensional models. Considering that the procedure for choosing parameters for one-dimensional model approximations to either two- or three-dimensional systems is well understood, it is recommended that parameters be chosen for two-dimensional models such that one-dimensional model approximations are preserved. In a related development, it is recommended that the merits of various passage shapes be evaluated by comparing reactors with identical one-dimensional approximation. Using this basis for comparison the inherent merits of various geometries are found to be far less than had previously been judged. An elementary analysis of a monolith reactor indicated that there is an optimal heat transfer coefficient at each point in the reactor. If a reactor is designed with an optimal heat transfer coefficient profile, the required amount of catalyst could be only one fourth of the minimum for any uniform transport rate profile. This result is not altered when detailed models and complex kinetics are considered. A simple way to approach this optimum is to use a graded cell monolith. Additional improvements may be achieved with complex geometries such as reactor passages with lips at the front. Finally, the usual conception that with increasing temperature, density, residence time, and conversion go down, is inaccurate. While density is decreasing, diffusivity is increasing at twice the rate. Higher temperatures lead to higher transport rates which usually means greater conversion. Incidental to this study it is found that the there is little radial convective transport in the neighborhood of a reaction zone.
Automotive engineering; Chemical engineering