Catalytic monolith

The strong intra-phase diffusion limitations are accounted for by the following equations for diffusion-reaction of the reactants in the catalytic monolith wall (Equation 13.21) with the appropriate boundary conditions (Equation 13.22) ... 

In additional experiments, a second catalytic monolith was added immediately after the first monolith. Although tiie residence time was doubled in these experiments, neither the water-gas shift reaction (2) or the steam reforming reaction (1) was found to significantly improve the reaction conversion and selectivity. From these data, it is apparent that the primal hurdle to achieving the perfect reactor operation involves the selective oxidation of CH4 to H2 and CO only. If CO2 and H2O are formed, the amount of available O2 is obviously reduced accordingly. From stoichiometry, this results in unreacted CH4 in the product gases since the reforming reaction is too slow to consume this metiiane at these short residence times. Thus, the only way to improve Sh2 and Sep at these short residence times is to maximize the partial oxidation reaction selectivity. 

From these experiments, one can see that the direct partial oxidation of CH4 to synthesis gas over catalytic monoliths is governed by a combination of transport and luetic effects, with the transport of gas phase species governed by the catalyst geometry and flow velocity and the lanetics determined by the nature of the catalyst and the reactor temperature. Under the conditions utilized here, the direct oxidation... 

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),... 

Transfer coefficients in catalytic monolith for automotive applications typically exhibit a maximum at the channel inlet and then decrease relatively fast (within the length of several millimeters) to the limit values for fully developed concentration and temperature profiles in laminar flow. Proper heat and mass transfer coefficients are important for correct prediction of cold-start behavior and catalyst light-off. The basic issue is to obtain accurate asymptotic Nu and Sh numbers for particular shape of the channel and washcoat layer (Hayes et al., 2004 Ramanathan et al., 2003). Even if different correlations provide different kc and profiles at the inlet region of the monolith, these differences usually have minor influence on the computed outlet values of concentrations and temperature under typical operating conditions. 

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. 

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. 

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). 

There are many chemically reacting flow situations in which a reactive stream flows interior to a channel or duct. Two such examples are illustrated in Figs. 1.4 and 1.6, which consider flow in a catalytic-combustion monolith [28,156,168,259,322] and in the channels of a solid-oxide fuel cell. Other examples include the catalytic converters in automobiles. Certainly there are many industrial chemical processes that involve reactive flow tubular reactors. Innovative new short-contact-time processes use flow in catalytic monoliths to convert raw hydrocarbons to higher-value chemical feedstocks [37,99,100,173,184,436, 447]. Certain types of chemical-vapor-deposition reactors use a channel to direct flow over a wafer where a thin film is grown or deposited [219]. Flow reactors used in the laboratory to study gas-phase chemical kinetics usually strive to achieve plug-flow conditions and to minimize wall-chemistry effects. Nevertheless, boundary-layer simulations can be used to verify the flow condition or to account for non-ideal behavior [147]. 

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

Monoliths exhibit a large flexibility in operation. They are well suited for optimal semibatch, batch, continuous, and transient processing. Catalytic conversion can be combined with in situ separation, catalytic reactions can be combined, heat integration is possible, and all lead to process intensification. In the short term, catalytic monoliths will be applied to replace trickle-bed reactor and slurry-phase... 

Fig. 1. The multiple scales in the catalytic monolith reactor (a) catalytic monolith (10 cm), (b) channel with catalyst washcoat on the walls (1mm), (c) SEM image of the washcoat layer (10 pm), (d) TEM image of meso-porous y-Al203 with dispersed Pt (200 nm).

The macro-porosity emacro and the correlation function corresponding to the macro-pore size distribution of the washcoat were evaluated from the SEM images of a typical three-way catalytic monolith, cf. Fig. 25. The reconstructed medium is represented by a 3D matrix and exhibits the same porosity and correlation function (distribution of macro-pores) as the original porous catalyst. It contains the information about the phase at each discretization point— either gas (macro-pore) or solid (meso-porous Pt/y-Al203 particle). In the first approximation, no difference is made between y-Al203 and Ce02 support, and the catalytic sites of only one type (Pt) are considered with uniform distribution. 

Fig. 4. Schematic diagram illustrating the different length scales in a catalytic monolith reactor.

More complex reactors, like packed-bed reactors or catalytic monoliths, consist of many physically separated scales, with complex nonlinear interactions between the processes occurring at these scales. Figure 3 illustrates scale separation in a packed-bed reactor. The four length (and time) scales present in the system are the reactor, catalyst particle, pore scale, and molecular scale. The typical orders of magnitude of these four length scales are as follows reactor, lm catalyst particle, 10 2m (1cm) macropore scale, 1pm (10 6m) micro-pore/molecular scale, 10 A (10 9m). The corresponding time scales also vary widely. While the residence time in the reactor varies between 1 and 1000 s, the intraparticle diffusion time is of the order of 0.1s and is 10 5s inside the pores. The time scale associated with molecular phenomenon like adsorption is typically less than a microsecond and could be as small as a nanosecond. 

Figure 4 is a picture of another reactor with widely varying scales—a catalytic monolith. Like the tubular reactor, the monolith itself has two intrinsic length scales, the radius and the length, which are typically 10-20 cm and between 30 and 50 cm, respectively. The monolith cross-section has a honeycomb structure... 

FIGURE 20 Most important routes in synthesis of catalytic monoliths (56,57). 

J. Villermaux, and D. Schweich, Is the catalytic monolith reactor well suited to environmentally benign processing Ind. Eng. Chem. Res. 35 3025 (1994). 

R.E. Hayes and S.T. Kolaczkowski, Mass and heat transfer effects in catalytic monolithic reactors, Chem. Eng. Sci. 49 3587 (1994). 

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

C.J. Pereira. J.E. Kubsh, and L.L. Hegedus, Computer-aided design of catalytic monoliths for automobile emission control, Chenu Eng. Sci. 43 2087 (1988). 

Table 3 lists some data on internally finned monoliths of the types depicted in Fig. 8, with relative instead of absolute values for the dimensions. It can be seen that for the relative wail thickness chosen, the fractional catalyst volumes (in the case of incorporated catalytic monoliths) are of the same order of magnitude (about 0.6) as in conventional packings of particulate catalysts. 

Schwiedernoch, R., Tischer, S., Correa, C., Deutschmann, O. Experimental and numerical study on the transient behavior of partial oxidation of methane in a catalytic, monolith. Chem. Eng. Sci. 2003, 58, 633-42. 

Methane reactants within noncatalytic channels remain unburned and they must be combusted in a fully catalytic stage or in a homogenous flame. A fully catalytic monolith would cause the catalyst to reach adiabatic temperatures and to deactivate. Thus, the only practical option is to complete the combustion in a homogenous combustion process. 

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... 

S. T. Gulati, Effects of Cell Geometry on Thermal Shock Resistance of Catalytic Monoliths, SAE 75071. 

Most of the work cited above has dealt with treating the soot in some way before doing the combustion experiments. We wish to report experiments conducted on soot from a diesel vehicle which has been deposited onto catalytic monolithic substrates. This sooted substrate is then placed in a laboratory apparatus where a synthetic gas mixture flows over the sample, and the soot combustion is monitored as a function of temperature. The laboratory set up simulates regeneration conditions on a vehicle. Using this technique we have been able to obtain kinetic information about the oxidation of soot and gaseous products. Comparisons of base metal and noble metal catalysts were also conducted and are reported. It is intended that this work will help elucidate the mechanism involved in the catalytic combustion of soot which should help in developing improved catalytic materials. 

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