Multiple Steady States-Catalytic Converters

An automobile catalytic converter is also a continuous reactor ihdii expedites three different chemical reactions. First, it catalyzes the reduction of NO (with the concomitant oxidation of CO)  

The three-way catalyst for these reactions is a formulation of metal particles supported on a metal-oxide surface. In this section we will study the design and operation of a reactor to conduct reaction (6.108). 

The reaction comprises a series of elementary steps involving CO, NO, and adsorption sites, designated 5  

The double arrows indicate reversible reactions. Because these reversible adsorp-tion/desorption reactions (6.111) and (6.112) are much faster than the surface reaction (6.113), the reversible reactions are at equilibrium. The mechanism of reactions (6.111)-(6.113) leads to the rate expression 

Let us use a CSTR to catalytically reduce NO with CO, reaction (6.108). The catalytic converter in your car is not a CSTR it is a PFR. Our analysis will suggest one reason why it is not a CSTR. 

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Industrial fixed-bed catalytic reactors have a wide range of different configurations. The configuration of the reactor itself may give rise to multiplicity of the steady states when other sources alone are not sufficient to produce the phenomenon. Most well known is the case of catalytic reactors where the gas phase is in plug flow and all diffusional resistances are negligible, while the reaction is exothermic and is countercurrently cooled. One typical example for this is the TVA type ammonia converter [38-40]. 

Mathematical model of three-way catalytic converter (TWC) has been developed. It includes mass balances in the bulk gas, mass transfer to the porous catalyst, diffusion in the porous structure and simultaneous reactions described by a complex microkinetic scheme of 31 reaction steps for 8 gas components (CO, O2, C2H4, C2H2, NO, NO2, N2O and CO2) and a number of surface reaction intermediates. Enthalpy balances for the gas and solid phase are also included. The method of lines has been used for the transformation of a set of partial differential equations (PDEs) to a large and stiff system of ordinary differential equations (ODEs . Multiple steady and oscillatory states (simple and doubly-periodic) and complex spatiotemporal patterns have been found for a certain range of operation parameters. The methodology of studies of such systems with complex dynamic patterns is briefly introduced and the undesired behaviour of the used integrator is discussed. 

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