Monolith Multiple steady states

While we have for heat and mass transfer in a porous catalyst explicit relations for parameters giving rise to multiple steady states, there is nothing similar developed for the monolithic catalysts so far. Hence we are forced to investigate, for particular conditions, the region of multiple steady states numerically. 

It should be emphasized that Oh and Cavendish assumed that the reactions only occur on the surface of the channel wall. This assumption is less realistic for a layer of washcoat (typically y-alumina) dispersed with catalyst applied on to the wall surface. Ramanathan, Balakotaiah, and West showed that the diffusion in the washcoat has a profound influence on the light-off behavior of a monolith converter. They derived an analytical light-off criterion based on a onedimensional two-phase model with position-dependent heat and mass transfer coefficients. The derivation of this criterion is based on the two key assumptions a positive exponential approximation (i.e., the Frank-Kameneskii approximation) and negligible reactant consumption in the fluid phase. The light-off is defined as the occurrence of multiple steady states with the attainment of the ignited steady state. Here, we discuss only the results of their analysis, without going into the details of their derivation. 

Multiple Steady States - Another important problem in modelling monolith reactions for exothermic reactions is the occurrence of multiple steady state solutions (MSSS). It has been shown, by comparing lumped with distributed models, that a one-dimensional model may give false solutions corresponding to higher multiple steady state solutions. 

The results of simulations of TWC model with microkinetics and diffusion resistance within the washcoat enable the interpretation of the dynamics of surface coverages and overal reactions and can serve for the improvement of the washcoat design. It has been found that not only multiple steady states (hysteresis) but also various types of periodic and complex spatiotemporal concentration patterns can exist in the monolith. Thorough analysis of bifurcations and transitions among existing patterns is numerically demanding task due to dimension of the problem. 

Figure 5 Product compositions and surface temperatures (upstream and downstream ends) for a 7-mm-Iong, 12-wt%-Pt extruded monolith (400 cells/in. ) at 2 5-sIpm total flow. The H2 and CO sclectivities are lower than those reported in previous experiments because the reactor was not insulated. A heterogeneous steady-state reaction is maintained for rich (> 12%) compositions, while at leaner compositions, a combination of heterogeneous and homogeneous reactions (a blue flame) is observed, with multiple ignited steady-states for compositions between 10 and 12% CH4. (From Ref. 7.)...

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