Emission Control Catalyst Design

Examples of early designs are given elsewhere [20]. Ultimately, three different designs emerged for widespread application the bead catalyst reactor, the ceramic monolith reactor and the metal monolith reactor (Fig. 21). 

The bead catalytic reactor has been used in USA until about 1992 and is well described in the literature (Fig. 22). It consists of two sieve plates mounted in a more 

Bead porosity is the result of a compromise between mechanical requirements, the intraparticle mass transfer requirements (both with respect to the reactants which are to be converted, and also with respect to the poisoning elements), and physical requirements such as a low thermal mass of the converter system. It has been shown that the crushing strength decreases with increasing porosity, but also that the thermal mass is significantly reduced by increasing the porosity. 

Bead type Sieve fraction (mm) Average radius (mm) Weight per bead (mg) Volume per bead (pl) Geometrical surface area per bead (mm ) Bulk density of packed beads (kg ) 

The porosity of the beads used is the result of a lot of optimization, and is formed both by macropores with pore diameters exceeding 0.1 pm and by micropores with a pore diameter less than 20 nm. The micropores give the high BET surface area, whereas the macropores assure a high intraparticle mass transfer rate as well as a resistance against deactivation by poisoning. 

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Cooper, B. J., Renny, L. V., and White, R. J., Lead Poisoning of Automobile Emission Control Catalysts—Influence of Emission System and Catalyst Design Characteristics on the Poisoning Mechanism, Am. Chem. Soc.. Symp. Automot. Catal., Chicago Meet., 1975. 

Figure 21. Basic designs of emission control catalysts (beads, ceramic monoliths, metal monoliths).

Walker, A. (2012) Current and Future Trends in Catalyst-Based Emission Control System Design , presentation at the SAE Heavy-Duty Diesel Emission Control Symposium, September 2012, Gothenburg. 

The trends in the automobile emission standards for the USA and Eitrope since 1966 are given in Table 11.7. Compliance with the standards set has been made possible by the use of automobile emission control catalysts. These were first used in the US during 1975 and in Europe from 1993. As a resirlt of continuous improvement to design and manufacture, the catalysts have been able to conform with the increasing severity of the regulations. 

Engines are also designed to use either gasoline or methanol and any mixture thereof (132—136). Such a system utilizes the same fuel storage system, and is called a flexible fueled vehicle (EEV). The closed loop oxygen sensor and TWC catalyst system is perfect for the flexible fueled vehicle. Optimal emissions control requires a fuel sensor to detect the ratio of each fuel being metered at any time and to correct total fuel flow. 

The characterization of the flow in existing DPF materials has been assessed by experiments and macroscopic continuum flow in porous media approaches. However, when it comes to material design it is essential to employ flow simulation techniques in geometrically realistic representations of DPF porous media. Some first applications were introduced in Konstandopoulos (2003) and Muntean et al. (2003) and this line of research is especially important for the development of new filter materials, the optimization of catalyst deposition inside the porous wall and for the design of gradient-functional filter microstructures where multiple functionalities in terms of particle separation and catalyst distribution (for combined gas and particle emission control) can be exploited. 

Already these early results have shown the incompatibility of heavily leaded gasolines with catalysts designed to achieve a rather moderate goal of automotive emission control. The added stringent requirements of the... 

Beeckman, J. W and Hegedus, L. L., Design of Monolith Catalysts for Power Plant NO Emission Control. Paper No. 72e, AIChE Annual Meeting, Washington, D.C., Nov. 27-Dec. 2, 1988. 

S. H. Oh, Converter Modeling for Automotive Emission Control. In Computer-Aided Design of Catalysts (E.R. Becker and C.J. Pereira, eds.), Dekker, New York, 1995, pp. 259-296. 

J W Beeckman, L.L. Hegedus, Design of monolith catalysts for power plant NO, emission control, Ind. Eng. Chem. Res. 30.969 (1991). 

The fourth emission control concept is the oxidation catalyst. Secondary air is added to the exhaust gas to assure a lean composition, independent on the engine operation condition. The catalyst is designed to promote reactions between oxygen and both carbon monoxide and hydrocarbons, which can be removed to a high extent. However, nitrogen oxides cannot be removed in this manner. 

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