Monolith unsteady-state models

We can extend the hyperbolic model to cases in which the solute diffuses in more than one phase. A common case is that of a monolith channel in which the flow is laminar and the walls are coated with a washcoat layer into which the solute can diffuse (Fig. 4). The complete model for a non-reacting solute here is described by the convection-diffusion equation for the fluid phase coupled with the unsteady-state diffusion equation in the solid phase with continuity of concentration and flux at the fluid-solid interface. Transverse averaging of such a model gives the following hyperbolic model for the cup-mixing concentration in the fluid phase ... 

In this chapter, modeling of monolith reactors will be considered from a first-principles point of view, preceded by a discussion of the typical phenomena in monoliths that should be taken into account. General model equations will be presented and subsequently simplified, depending on the subprocesses that should be described by a model. A main lead will be the time scales at which these subprocesses occur. If they are all small, the process operates in the steady state, and all time-dependent behavior can be discarded. Unsteady-state behavior is to be considered if the model should include the time scale of reactor startup or if deactivation of the catalyst versus time-on-stream has to be addressed. A description of fully dynamic reactor operation, as met when cycling of the feed is applied, requires that all elementary steps of a kinetic model with their corresponding time scales are incorporated in the reactor model. 

Unsteady-state operation of a monolith was modeled to study the light-off behavior of catalytic mufflers as well as the deactivation of the catalyst. 

Unsteady Models of Monolith SCR Reactors. As pointed out already in the section Unsteady-State Kinetics of the SCR Reaction, the growing interest in the dynamic behavior of DeNO systems has originated in recent years a number of studies related to transient operation of monolith SCR reactors. Mathematical modeling appears to be particularly useful for the analysis and development of unsteady SCR processes. 

Ciardelli C et al (2004) SCR-DeNOx for diesel engine exhaust aftertreatment unsteady-state kinetic study and monolith reactor modeling. Chemical Engineering Science, 59 5301-5309... 

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