Modeling catalytic monoliths

Leclerc, J. P., and Schweich, D., Modelling catalytic monoliths for automobile emission control, in Chemical Reactor Technology for Environmentally Safe Reactors and Products (H. I. de Lasa, G. Dogu, and A. Ravella, Eds.), pp. 547-575. Kluwer Academic Publishers, Netherlands (1993). 

Leclerc J.P., and D. Schweich Modelling catalytic monoliths for automobile emission control, in "Chemical reactor technology for enviromnentally safe reactors and products", H.I. de Lasa et al. Ed., Kluwer Academic Pub., tlie Netherlands, p. 547-576, 1993. 

J.P. Leclerc, D.Schweich, Modeling Catalytic Monoliths For Automobile Emission Control, H.I. de Lasa et al. (eds.) Chemical Reactor for Enviromnentally Safe Reactors and Products, 547-576, (1993) Kluwer Academic Publishers. 

MODELING CATALYTIC MONOLITHS FOR AUTOMOBILE EMISSION CONTROL... 

Modeling Catalytic Monoliths for Automobile Emission Control... 

In this section the models employed for simulation of catalytic monolith reactor are discussed, focusing on effective description of heat and mass transfer phenomena in monolith channel. The number of different mathematical models developed for converters of automobile exhaust gases over the last decades is huge—cf., e.g. Heck et al. (1976), Young and Finlayson (1976), Oh and Cavendish (1982), Zygourakis and Aris (1983), Chen et al. (1988),... 

This methodology can be used for the calculation of local reaction rates and effectiveness factors in dependence on gas components concentrations, temperature and porous catalytic layer structure (cf. Fig. 9). The results can then be used as input values for simulations at a larger scale, e.g. the effective reaction rates averaged over the studied washcoat section can be employed as local reaction rates in the ID model of monolith channel. 

The solver is implemented in Fortran, using optimized treatment of diagonal-band matrices and analytical derivatives of reaction rates to minimize computation time. The software structure is modular, so that different reaction-kinetic modules for individual types of catalysts can be easily employed in the monolith channel model. The compiled converter models are then linked in the form of dynamic libraries into the common environment (ExACT) under Matlab/Simulink. Such combination enables fast and effective simulation of combined systems of catalytic monolith converters for automobile exhaust treatment. 

This intermediate scale affords a preliminary validation of the intrinsic kinetics determined on the basis of microreactor runs. For this purpose, the rate expressions must be incorporated into a transient two-phase mathematical model of monolith reactors, such as those described in Section III. In case a 2D (1D+ ID) model is adopted, predictive account is possible in principle also for internal diffusion of the reacting species within the porous washcoat or the catalytic walls of the honeycomb matrix. 

When a simple, fast and robust model with global kinetics is the aim, the reaction kinetics able to predict correctly the rate of CO, H2 and hydrocarbons oxidation under most conditions met in the DOC consist of semi-empirical, pseudo-steady state kinetic expressions based on Langmuir-Hinshelwood surface reaction mechanism (cf., e.g., Froment and Bischoff, 1990). Such rate laws were proposed for CO and C3H6 oxidation in Pt/y-Al203 catalytic mufflers in the presence of NO already by Voltz et al. (1973) and since then this type of kinetics has been successfully employed in many models of oxidation and three-way catalytic monolith converters... 

Sharma et al. (2005) developed a ID two-phase model for the analysis of periodic NOx storage and reduction by C3H6 in a catalytic monolith, based on a simplified kinetic scheme. They focused on the evaluation of temperature and reaction fronts along the monolith and their effect on NOx conversion. Kim et al. (2003) proposed a phenomenological control-oriented lean NOx trap model. 

Models of the catalytic monolithic. Paper presented at the Levich Conference, Oxford, 1978. 

D.J. Worth, S.T. Kolaczkowski, and A. Spence, Modelling channel interaction in a catalytic monolith reactor, Trans. ICheniE. 77 331 (1993). 

Reactor control models for monoliths require a more detailed study of the time scales of all occurring subprocesses, because of their dynamic character. Under dynamic circumstances, the rates of the individual elementary steps of a catalytic cycle, such as adsorption, surface reaction, and desorption, are not equal to each other anymore, since the time scales of the corresponding processes may differ by many orders of magnitude. Therefore, accumulation effects on the catalyst surface have to be taken into account as well, which demands that continuity equations for surface species be included in the model. Such aspects may even play a role in the steady state if the kinetics depend on rate-determining steps that change according to the concentration level of the reactants... 

Much effort has been made to study this light-off behavior of catalytic monolith. Oh and Cavendish studied the response of the monolith to a step increase in the feed stream temperature by using a onedimensional two-phase (gas and solid) model. They tracked the cross-sectional average temperature and concentration in each phase and used heat and mass transfer coefficients to describe interphase transport. The results indicated that the light-off occurs at the monolith entrance for a sufficiently high inlet exhaust temperature. For a lower inlet exhaust temperature, the light-off occurs in the downstream section, and the... 

The multiphase nature of the flow inside the monolith channels influences hydrodynamics and mass transfer. We briefly review the main aspects related with modeling of monolith reactors with Taylor flow. Particular attention is devoted to the consequences from using gas-liquid flow in contact with a catalytic surface as a reactor ... 

Groppi G, Belloli A, Tronconi E, Forzatti P. A comparison of lumped and distributed models of monolith catalytic combustors. Chemical Engineering Science 1995 50 2705-2715. 

Excellent summary lists of publications where the TAP technique is applied to different catalytic systems can be found in [79, 80]. These lists include the many types of catalytic materials that are studied including supported metals, mixed metal oxides, zeolites, metal particles, metals deposited on screens, catalytic monoliths, and nanoparticles or atoms deposited on microparticles, single crystals, and other model catalysts. 

Sari A, Safekordi A, Farhadpour FA Comparison and validation of plug and boundary layer flow models of monolithic reactors catalytic partial oxidation of methane on Rh coated monoliths, IntJ Chem React Eng 6, 2008, Article A73. 

Jirat J, KuWcek M, Marek M (1999) Mathematical modelling of catalytic monolithic reactors with storage of reaction components on the catalyst surface. Catalysis Today, 53, 583-596... 

Tischer S, Deutschmann O (2005) Recent advances in numerical modeling of catalytic monolith reactors. Catalysis Today, 105, 407-413... 

Almost all catalytic converters have to contend with the decay or poisoning of the catalyst In some form and the catalytic monolith Is no exception. Indeed this Is notorious In the automotive application where the catalytic converter must survive 50,000 miles of operation and still perform adequately. Although we shall use the kinetics of carbon monoxide oxidation over a platinum catalyst as an obvious and Important example, our main objective Is to develop a model which can handle any catalyst decay question and to point out the differences In two types of poisoning. Thus our study comes within the third main division of the subject as laid out by Butt (1 ) In 1972 not the mechanism or rate determination but the effect of deactivation on the operation of the reactor. 

T. C. Tien, J. S. T len, 1991, Catalytic ignition model in monolithic reactor with in-depth reaction, Aerothermodynamics in combustors lUTAM Symposium, National Taiwan Univ., Taipei, Selected Papers (A93-51626 22-25), 231-44. 

The partial oxidation of methane in catalytic monoliths at short contact-times is another example with several empty routes illustrating importance of thermodynamic consistency in selection of kinetic parameters. This reaction offers a promising route for the conversion of natural gas into more useful chemicals such as synthesis gas (syngas), a mixture of hydrogen and carbon monoxide. Syngas can subsequently be converted into methanol or higher hydrocarbons. The kinetic model for partial oxidation of methane on Rh includes 19 reversible reactions with six-gas phase species and 11 adsorbed species [5]. Presence of 19 steps, one balance equation (which relates coverage of surface species) and... 

For tigheat transfer mechanism in the channel, enhancing the upstream propagation of the reaction front. A numerical experiment was conducted to quantify the effect of radiation on the apparent thermal conductivity of the solid wall, by recomputing Case 15 and assuming an artificial material (cordierite ) for the microreactor wall, which had essentially the same properties with cordierite in Table 8.1 except for a higher thermal conductivity the value of was increased at 0.5 W/mK steps, until fig and fit assumed values close to the ones of Case 16. For = 5.5 W/mK, ignition and steady-state times for a microreactor without surface radiation, became essentially identical to the characteristic times of a microreactor with — 2.0 W/mK and a surface emissivity of 8 = 0.6 (see Case 17 in Table 8.2). This apparent increase in the thermal conductivity of the solid, however, should not be confused with numerical models employing effective wall thermal conductivity for catalytic monoliths [23]. 

Many elements of a mathematical model of the catalytic converter are available in the classical chemical reactor engineering literature. There are also many novel features in the automotive catalytic converter that need further analysis or even new formulations the transient analysis of catalytic beds, the shallow pellet bed, the monolith and the stacked and rolled screens, the negative order kinetics of CO oxidation over platinum,... 

There is a general trend toward structured packings and monoliths, particularly in demanding applications such as automotive catalytic converters. In principle, the steady-state performance of such reactors can be modeled using Equations (9.1) and (9.3). However, the parameter estimates in Figures 9.1 and 9.2 and Equations (9.6)-(9.7) were developed for random packings, and even the boundary condition of Equation (9.4) may be inappropriate for monoliths or structured packings. Also, at least for automotive catalytic converters. 

Metal monoliths show good thermal characteristics. A typical support with herringbone channels made from Fecralloy performed satisfactory in automotive applications [27]. Modeling showed that overall heat transfer was about 2 times higher than for conventional pellets [28,29]. Hence, there is potential for structured catalysts for gas-phase catalytic processes in multitubular reactors. 

In this paper, we first briefly describe both the single-channel 1-D model and the more comprehensive 3-D model, with particular emphasis on the comparison of the features included and their capabilities/limitations. We then discuss some examples of model applications to illustrate how the monolith models can be used to provide guidance in emission control system design and implementation. This will be followed by brief discussion of future research needs and directions in catalytic converter modeling, including the development of elementary reaction step-based kinetic models. 

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