Modeling of the SCR Reactor

Thermal sintering of SCR-DeNOx catalysts was studied by calcining at increasing temperature samples of a commercial catalyst and testing them under operating conditions of industrial relevance. Results were analyzed by means of a complete model of the SCR reactor. 

DeNOx - Scope of the model analysis was to evaluate on a quantitative basis the effective dependence of the intrinsic activity of the monoliths on the thermal sintering, and separate it from the contributions of inter-phase mass transfer and the effect of morphological modifications on intra-porous diffusion. When excess ammonia is present, as in the case of the experiments herein analyzed, then the Ealy-Rideal kinetic expression which is contained in the model of the SCR reactor reduces to a first order dependence on NO concentration under such conditions, an unique adaptive parameter, kc, accounts for the DeNOx intrinsic activity. Estimation of kc for the three calcined catalysts was obtained by fitting the model to each set of experiments. Input data included the operating conditions, the geometrical... 

In their model of the SCR reactor (107), Baiker and co-workers have included a LH rate expression for ammonia combustion accounting also for the dependence on oxygen concentration. 

A simple ID model of the SCR reactor based on equation 30 was found in close agreement with rigorous, but more involved, multidimensional models. The same ID model proved also successful in reproducing published data concerning the effects of flow rate and channel size on NO reduction in commercial square-channelled honeycomb catalysts, such effects being directly associated with the role of gas-solid mass transfer (113). 

Therefore, there is a strong motivation to develop a dynamic model of the SCR monolithic reactor suitable for extended temperature operation and to study the fast SCR reaction in view of future possible applications. In the following, we will focus on these two issues. 

It has been demonstrated that kg can be estimated by analogy with the Graetz-Nusselt problem governing heat transfer to a fiuid in a duct with constant wall temperature (SH= Nut) [30] and that the axial concentration profiles of NO and of N H 3 provided by the 1D model are equivalent and almost superimposed with those of a rigorous multidimensional model of the SCR monolith reactor in the case of square channels and of ER kinetics, which must be introduced to comply with industrial conditions for steady-state applications characterized by substoichiometric NH3 NO feed ratio, that is, a[Pg.401]

Equation 20 and 21 have been successfully used by many authors for steady-state modeling of the SCR monolith reactor operating with NH3/NO < 1 and > 1, respectively (62,67,92). 

Also, the study of the reaction kinetics under unsteady-state conditions (i) may provide relevant information concerning the mechanism of the reaction and (ii) allow to decouple the study of the kinetics of the reactants adsorption-desorption from that of the surface reaction under representative conditions (95,97,102). For all the above-mentioned applications, reliable engineering analysis and reactor modeling calls for a dynamic kinetic model of the SCR reaction. 

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. 

In a companion paper (117), the dynamic model of Reference (116) has been completed with account of the SO2 oxidation reaction. For this purpose transient SO2 conversion data were collected over a commercial V-W/Ti02 honeycomb catalyst during SO2 oxidation experiments, involving step changes in temperature, area velocity, and feed composition (SO2, O2, H2O and NH3) with respect to typical DeNOx conditions. Characteristic times of the system response were of a few hours, and peculiar SO3 emission peaks were noted upon step increments of reaction temperature and H2O feed content. All the data could be successfully fitted by a dynamic kinetic model based on the assumption that buildup-depletion of surface sulfate species is rate controlling (see eg. Figure 17). Finally, it was shown that the dynamic model of the SCR monolith reactor in Reference... 

Monolith reactor model—For validation purposes, the kinetic models of the SCR catalyst [6] and of the PGM catalyst [18] were used to simulate catalytic activity runs over honeycomb monoliths coated with the SCR and the PGM component, respectively, of the studied ASC system. In the case of the SCR catalyst, the kinetics were implemented in a heterogeneous dynamic ID -I- ID model of a single monolith channel, accounting both for external (gas-solid) and internal (intra-porous) mass transfer resistances [12, 25, 26]. Model simulations... 

A simple isothermal pseudo-homogeneous, single-channel, ID model is typically adopted to model a monolith SCR reactor [27, 30, 38, 40-50], which implies uniform conditions over the entire cross-section of the monolith catalysts and accounts only... 

Tronconi et al. [46] developed a fully transient two-phase 1D + 1D mathematical model of an SCR honeycomb monolith reactor, where the intrinsic kinetics determined over the powdered SCR catalyst were incorporated, and which also accounts for intra-porous diffusion within the catalyst substrate. Accordingly, the model is able to simulate both coated and bulk extruded catalysts. The model was validated successfully against laboratory data obtained over SCR monolith catalyst samples during transients associated with start-up (ammonia injection), shut-down (ammonia... 

Figure 13.9 Validation of the dynamic model ofthe monolith SCR reactor during ESC and ETC tests. All concentrations are normalized by the respective maximum inlet valueduringthe test cycle. Dotted black lines, inlet values solid black lines, outlet measurements gray lines, outlet simulations. Adapted from ref. [62].

As outlined in the section Effects of the Operating Variables, the approach to design and analysis of monolith SCR reactors customarily adopted in the technical literature has been based on simple pseudo-homogeneous models accovmt-ing only for axial concentration gradients. The effects of inter- and intraphase mass transfer limitations were lumped into effective pseudo-first-order rate constants, such as in equation 14, which were specific for each type of catalyst. Such constants actually varied not only with temperature, but included dependences on the gas flow velocity, on the monolith channel geometry, and on the catalyst pore structure as well. 

The model of Reference (67) was later applied to evaluate the performance of an SCR catalyst with proprietary composition (124). Koebel and Elsener also compared, on a fully predictive basis, a similar model to experimental data of NO conversion and NH3 slip obtained on a diesel engine test stand (125). In this case, while the model was shown to describe qualitatively the performance of the SCR monolithic reactor, specifically with reference to the NO conversion versus NH3 slip relationship, an exact quantitative match was found impossible. According to the authors, the reasons for the discrepancies may include unaccovmted kinetic effects of the contaminants present in the diesel exhaust gases, vmcertainties due both to the extrapolation of the kinetic parameters and to the measurement of the intraporous diffusivities, and the excessive simplification involved in the assumption of a pure Langmuir isotherm for NH3 adsorption. 

Progress has been made towards the development of monolith reactor models that predict SCR performance under both steady state and transient operation. Guth-enke et al. [33]. provided a thorough review of SCR reactors. Most of the earlier work in this area was done for the more established Vanadia-based catalysts and involved the use of global kinetic models [81-83]. More recent works by Nova et al. provided detailed transient model for the SCR reaction system on Vanadia-based catalyst [8, 45]. Olsson and coworkers developed both global and detailed kinetic models for NH3-SCR reactions on Cu-ZSM-5 catalysts [14, 15, 49]. More recent works have communicated models for NH3-SCR reactions on Fe-zeoUte catalysts [25, 57, 76, 84]. 

Another modeling of the reactor volume was performed by Seo et al. [80] with a special insight to the formation of N2O. In the NSR-SCR coupling, it is important to minimize (or annihilate) both ammonia and N2O in the aftertreatment exhaust gas. This means that ammonia should be used to reduce NOx (or be oxidized to N2) while N2O, if formed, should be destroyed in the catalytic system. The NSR catalyst was composed of Pt/Pd/Rh/Ba/Ce/Zr on AI2O3 (relative... 

For the design of a SCR reactor, we need an appropriate reactor model, and the differential equations have to be solved based on the parameters of the intrinsic and effective kinetics. 

The kinetic parameters estimated by the experimental data obtained frmn the honeycomb reactor along with the packed bed flow reactor as listed in Table 1 reveal that all the kinetic parameters estimated from both reactors are similar to each other. This indicates that the honeycomb reactor model developed in the present study can directly employ intrinsic kinetic parameters estimated from the kinetic study over the packed-bed flow reactor. It will significantly reduce the efibrt for predicting the performance of monolith and estimating the parameters for the design of the commercial SCR reactor along with the reaction kinetics. 

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