Automotive exhaust systems

A recently developed drying appHcation for zeoHtes is the prevention of corrosion in mufflers (52,55). Internal corrosion in mufflers is caused primarily by the condensation of water and acid as the system cools. A unique UOP zeoHte adsorption system takes advantage of the natural thermal cycling of an automotive exhaust system to desorb the water and acid precursors. 

An interesting and novel use of a soHd desiccant, the reduction of cold condensate corrosion in automotive exhaust systems, illustrates a hybrid closed—open system. Internal corrosion occurs in mufflers when the water vapor in the exhaust condenses after the engine is turned off and the muffler cools. Carbon dioxide dissolves in the condensate to form an acidic soup. In an essentially closed static drying step, an acid- and heat-resistant desiccant located in the muffler adsorbs water vapor from the exhaust gas as it cools to prevent formation of corrosive acidic condensate. When the engine is restarted, the system becomes open, and the desiccant is regenerated by the hot exhaust gas to be ready for the next cooldown step (19). 

Automotive Exhaust System Repair Tire Retreading and Repair Paint... 

The effect of alkali addition on the adsorption of NO on metal surfaces is of great importance due to the need of development of efficient catalysts for NO reduction in stationary and automotive exhaust systems. Similar to CO, NO always behaves as an electron acceptor in presence of alkalis. 

Potentialities of silver-based catalysts in automotive exhaust systems... 

The depth profiling studies suggest that two different processes govern the formation of automotive exhaust particles. The elemental surface predominance on large particles is attributed to the deposition of volatile Pb and S species (e.g. PbBrCl, SO2) onto the surfaces of refractory iron-containing particles in the automotive exhaust system (11,12). The iron-rich particles are probably derived from corrosion and ablation of the exhaust system. The smaller, more homogeneous particles may form by a nucleation process in which PbBrCl forms rather pure molten droplets when the exhaust system temperature falls below the saturation point (12). 

It should be noted that it is possible to produce fully stabilized bodies with much higher fracture strengths than listed here but this requires the use of fine particle size, chemically prepared powders (3). The use of this type of material involves a number of penalties both in cost and processability that may be prohibitive for a high volume automotive application. In addition to the type of partially stabilized body described here, two other basic types of partially stabilized bodies have been reported (4, ). Both are classified as transformation toughened partially stabilized zirconias and involve different processing techniques to obtain a body with various amounts of a metastable tetragonal phase. While the mechanical properties of these materials have been studied extensively, little has been reported about their electrical properties or their stability under the thermal, mechanical and chemical conditions of an automotive exhaust system. 

Since ferritic stainless steels contain more carbon than other classes, they are relatively harder to weld and shape than other varieties, which have historically limited their applications. However, since the 1960s, processes such as argon oxygen decarburization have resulted in steels with less carbon, allowing for smaller concentrations of chromium to be used. As a result, the price for ferritic stainless steel has dramatically dropped, and a number of applications now employ these materials - more than 2/3 of which include automotive exhaust systems. 

These conditions are abnormal in automotive exhaust systems, but can occur especially in the case of engine malfunctions, high load, and transient conditions. 

Benson M, Bennett CR, Harry JE, Patel MK and Cross M (2000) The recovery mechanism of platinum group metals from catalytic converters in spent automotive exhaust systems. Resources, conservation and recycling 31 1-7. 

Outlandish as it may seem, a new scheme has been proposed to turn automobiles into air purifiers, devouring the pollutants ozone and carbon monoxide. BASF, an Iselin, New Jersey, company that specializes in the manufacture of catalytic converters for automotive exhaust systems, has developed a catalyst that decomposes ozone to oxygen and converts carbon monoxide to carbon dioxide. BASF proposes to paint the catalyst on automobile... 

Some catalysts developed for this reaction are reasonably effective under the grueling conditions in automotive exhaust systems. Nevertheless, scientists and engineers are continuously searching for new materials that provide even more effective catalysis of the decomposition of nitrogen oxides. 

The use, since 1975, of catalytic converters in automobiles has resulted in increased exhaust gas temperatures, up to 870°C (1600°F), with metal temperatures up to 760 °C (1400 F), as well as increased concentrations of corrosive chemicals, such as sulfuric acid, in the exhaust gas stream. As a result, materials resistant to high-temperature oxidizing conditions, such as hot dip aluminum-coated type 409 stainless steel, are being used in automotive exhaust systems [35-37]. 

J. Douthett, Automotive Exhaust System Corrosion, in ASM Handbook, Vol. 13C, Corrosion Environments and Industries, ASM International, Materials Park, OH, 2006, pp. 519-520. 

Find et al. [25] developed a nickel-based catalyst for methane steam reforming. As material for the microstructured plates, AluchromY steel, which is an FeCrAl alloy, was applied. This alloy forms a thin layer of alumina on its surface, which is less than 1 tm thick. This layer was used as an adhesion interface for the catalyst, a method which is also used in automotive exhaust systems based on metallic monoliths. Its formation was achieved by thermal treatment of microstructured plates for 4h at 1000 °C. The catalyst itself was based on a nickel spinel (NiAl204), which stabUizes the catalyst structure. The sol-gel technique was then used to coat the plates with the catalyst slurry. Good catalyst adhesion was proven by mechanical stress and thermal shock tests. Catalyst testing was performed in packed beds at a S/C ratio of 3 and reaction temperatures between 527 and 750 °C. The feed was composed of 12.5 vol.% methane and 37.5 vol.% steam balance argon. At a reaction temperature of 700°C and 32 h space velocity, conversion dose to the thermodynamic equilibrium could be achieved. During 96 h of operation the catalyst showed no detectable deactivation, which was not the case for a commercial nickel catalyst serving as a base for comparison. 

As discussed in the "Chemistry at Work" box in Section 14.7, one of the goals of automotive catalytic converters is to achieve the rapid conversion of NO to N2 and O2 at the temperature of the exhaust gas. Some catalysts for this reaction have been developed that are reasonably effective under the grueling conditions found in automotive exhaust systems. Nevertheless, scientists and engineers are continually searching for new materials that provide even more effective catalysis of the decomposition of nitrogen oxides. 

Gaugh, R. R., "Corrosion of Materials for Automotive Exhaust System, Proceedings of the Automotive Corrosion and Prevention Conference, SAE P-250, Wairendale, PA, 1991. 

Figure 10.5 View into metallic monoliths produced by EMITEC (photograph courtesy of Emitec). the monolith and the reactor shell. It holds the monohth and prevents by-pass of gases through the gap [57]. A popular ceramic mat used in automotive exhaust systems is Interam produced by 3M [57j. It degrades at temperatures above 800 °C, and therefore a high temperature ceramic fibre material such as CC-Max from Unifrax must be used for monolithic reformer reactors, which are operated at higher temperature [57].

Catalytic gas combustion radiant heaters have generated substantial interest. The combustible air-gas mixture is introduced to the heater directly below a porous bed of catalyst that is similar to the catalyst used in automotive exhaust systems or camper heaters. Combustion and radiation occur at the catalyst surface. Catalytic gas systems are desired for their uniform surface temperature and low operating cost. Lack of temperature modulation is the major problem with gas combustion heaters. The catalytic gas system needs many gas lines and controls, as well as, an electric heater that must preheat the catalyst bed prior to initiating combustion. As a result, the initial installation cost is very high compared to the allelectric heating systems. 

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