Exhaust gas converter

The following, well-acceptable assumptions are applied in the presented models of automobile exhaust gas converters Ideal gas behavior and constant pressure are considered (system open to ambient atmosphere, very low pressure drop). Relatively low concentration of key reactants enables to approximate diffusion processes by the Fick s law and to assume negligible change in the number of moles caused by the reactions. Axial dispersion and heat conduction effects in the flowing gas can be neglected due to short residence times ( 0.1 s). The description of heat and mass transfer between bulk of flowing gas and catalytic washcoat is approximated by distributed transfer coefficients, calculated from suitable correlations (cf. Section III.C). All physical properties of gas (cp, p, p, X, Z>k) and solid phase heat capacity are evaluated in dependence on temperature. Effective heat conductivity, density and heat capacity are used for the entire solid phase, which consists of catalytic washcoat layer and monolith substrate (wall). 

Model equations are presented here for a general situation that is not specific to automobile exhaust gas converters and includes transient reactor behavior. The following assumptions are made ... 

Many modeling studies and experimental investigations have demonstrated that the intentional unsteady operation of reactors can profoundly influence conversion and/or selectivity. Several reviews were published [28.29,31,44—46]. The effects for automotive exhaust gas converters were recently discussed [30]. 

The influence of the modulation frequency, see Fig. 19, shows beneficial effects in the frequency range 0-10 Hz, which is largest for NO reduction, with a maximum around the typical frequency of 1 Hz as met in the practice of automotive exhaust gas converters. 

Modeling of monolith reactors from first principles presents a valuable tool in the design of such reactors and in the analysis of the underlying phenomena. The results presented show that the reactor behavior can be adequately described and understood by a combination of the reactor s transport characteristics and the intrinsic kinetics obtained with a laboratory reactor of another type. As such we can generalize monolith models to other reaction networks, e.g., extend the given description of the dynamic operation for combined CO oxidation and NO reduction in the automotive exhaust gas converter to include other reactions, like the oxidation of various hydrocarbons and of hydrogen. The availability, however, of a proper kinetic model is a definite prerequisite. 

Due to their special chemical and physical properties (especially in elemental form), the PGM find various applications both in industry and in the laboratory, including organometallic chemistry, coordination and supramolecular chemistry, biological and medicinal chemistry, surfaces-, materials-and crystal engineering, photo- and electrochemistry and catalysis and organic synthesis. Today, the most important applications are, of course, their use in the catalytic automobile exhaust gas converter (e.g. Shelef and McCabe 2000), followed by their use in the jewelry industry. 

From occupational studies it has been shown that the most significant health risk from Pt exposure is sensitization of the airways caused by soluble Pt compounds (Rosner and Merget 2000). Thus, for an assessment of the health effects of Pt it seems reasonable to distinguish between elemental Pt and halogenated Pt compounds. Pt is emitted from catalytic exhaust gas converters of cars in amounts which are in the ng km range, mainly as elemental Pt. The nanocrystalline Pt particles are attached to jm-sized aluminum oxide particles. Only very limited data are available from current studies showing that ultra-... 

In this paper, the ability of the developed reactor model to predict converter s performance has been evaluated against experimental data. The data is obtained from the full-scale tests and the performed European driving cycle vehicle tests. The focus is on the warm-up period of catalytic exhaust gas converters and especially on the prediction of catalyst light-off. 

Kangas, J., Ahola, J., Maunula, T., KorpijMrvi, J. Tanskanen, J. (2002) Automotive exhaust gas converter model for warm-up conditions. 17 International Symposium on Chemical Reaction Engineering, Hong Kong, China. 

In a catalytic exhaust gas converter (monolith catalyst), the following reactions take place on a Pt catalyst ... 

The lead alkyls and scavengers contained in fuels cause rapid poisoning ol exhaust gas catalytic converters. They are tolerated only in trace quantities in fuels for vehicles having that equipment. The officially allowed content is 0.013 g Pb/1, but the contents observed in actual practice are less than 0.005 g Pb/1. 

M ass Transfer. Exhaust gas catalytic treatment depends on the efficient contact of the exhaust gas and the catalyst. During the initial seconds after start of the engine, hot gases from the exhaust valve of the engine pass through the exhaust manifold and encounter the catalytic converter. Turbulent flow conditions (Reynolds numbers above 2000) exist in response to the exhaust stroke of each cylinder (about 6 to 25 times per second) times the number of cylinders. However, laminar flow conditions are reached a short (- 0.6 cm) distance after entering the cell passages of the honeycomb (5,49—52). 

The NO analyzer is based on the principles of chemiluminescence to determine continuously the NO concentration in the sample gas stream. The analyzer should contain a NOg-to-NO converter, which converts the nitrogen dioxide (NO9) in the sample gas to nitrogen oxide (NO). An NOg-to-NO converter is not necessary if data are presented to demonstrate that the NO9 portion of the exhaust gas is less than 5 percent of the total NO9 concentration. 

Some pellet beds are housed in circular or elliptical cylinders, where the exhaust gas flows in the axial direction. The radial flow converter is a... 

Emission control from heavy duty diesel engines in vehicles and stationary sources involves the use of ammonium to selectively reduce N O, from the exhaust gas. This NO removal system is called selective catalytic reduction by ammonium (NH3-SGR) and it is additionally used for the catalytic oxidation of GO and HGs.The ammonia primarily reacts in the SGR catalytic converter with NO2 to form nitrogen and water. Excess ammonia is converted to nitrogen and water on reaction with residual oxygen. As ammonia is a toxic substance, the actual reducing agent used in motor vehicle applications is urea. Urea is manufactured commercially and is both ground water compatible and chemically stable under ambient conditions [46]. 

At the heart of an automotive catalytic converter is a catalyzed monolith which consists of a large number of parallel channels in the flow direction whose walls are coated with a thin layer of catalyzed washcoat. The monolith catalyst brick is wrapped with mat, steel shell and insulation to minimize exhaust gas bypassing and heat loss to the surroundings. 

Our earlier converter modeling study [3] has shown that during the cold-start period (when a cold monolith converter is suddenly exposed to hot exhaust gas), the upstream section of the monolith is first heated up to the reaction temperatures by the hot exhaust, leading to converter lightoff, and that the reaction is confined to a small fraction of the total... 

Additionally, NO is reduced by H2 and by hydrocarbons. To enable the three reactions to proceed simultaneously - notice that the two first are oxidation reactions while the last is a reduction - the composition of the exhaust gas needs to be properly adjusted to an air-to-fuel ratio of 14.7 (Fig. 10.1). At higher oxygen content, the CO oxidation reaction consumes too much CO and hence NO conversion fails. If however, the oxygen content is too low, all of the NO is converted, but hydrocarbons and CO are not completely oxidized. An oxygen sensor (l-probe) is mounted in front of the catalyst to ensure the proper balance of fuel and air via a microprocessor-controlled injection system. 

Cant et al [21] focused their attention on the concentration of N20 in the automotive exhaust gas, which are rather low (14 ppm) but quite dependent on the air-to-fuel ratio. Typically 60-80% of NO is converted into N20 below the light-off temperature on Rh and then the selectivity drops at relatively high temperature 370°C [21,22] when the partial pressures of NO tends below lOTorr [22-25]. 

Unavoidable loss of gas is compensated via a feed valve for supplying virgin nitrogen into the circulation pipe. The exhaust gas of the process has to be bypassed for purification. After the separation of dust by a filter, the gas is heated to 400 °C for the catalytic combustion of the side products. The gas is then cooled down, and the excess oxygen is catalytically converted to water by using hydrogen. For economic reasons, the gas flow will recover the heat via a heat exchanger and then be cooled down by a gas cooler. 

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