Automobile exhaust system

Air Pollution Control Device Meehanism or equipment that eleans emissions generated by a source (e.g., an incinerator, industrial smokestack or an automobile exhaust system) by removing pollutants that would otherwise be released to the atmosphere. 

The catalytic converters (Figure 16-17) built into automobile exhaust systems contain two types of heterogeneous catalysts, powdered noble metals and powdered transition metal oxides. They catalyze the oxidation of unbumed hydrocarbon fuel (reaction 1) and of partial combustion products such as carbon monoxide (reaction 2, shown in Figure 16-18). 

It is desirable to carry out these reactions in automobile exhaust systems. Carbon monoxide is very poisonous. The latter reaction is so slow that a mixture of CO and O2 gas at the exhaust temperature would remain unreacted for thousands of years in the absence of a catalyst Yet the addition of only a small amount of a solid, finely divided transition metal catalyst promotes the production of up to a mole of CO2 per minute. Because this reaction is a very simple but important one, it has been studied extensively by surface chemists. It is one of the best understood heterogeneously catalyzed reactions. The major features of the catalytic process are shown in Figure 16-18. 

This reaction is spontaneous. Catalysts do not shift the position of equilibrium. They favor neither consumption nor production of NO. They merely allow the system to reach equilibrium more rapidly. The time factor is very important because the NO stays in the automobile exhaust system for only a very short time. 

Catalytic converters in automobile exhaust systems reduce emissions of oxides of nitrogen. 

Paustenbach (USA) 2006 Removal of automobile exhaust systems Chrysotile asbestos Personal, bystander, and area airborne fiber concentrations (fibers/m )... 

Paustenbach, D. J., Madl, A. K., Donovan, E., Clark, K., Fehling, K., and Lee, T. C. (2006). Chrysotile asbestos exposure associated with removal of automobile exhaust systems (ca, 1945-1975) by mechanics Results of a simulation study. J Expo Sci Environ Epidemiol 16, 156-171. 

Petroleum products undergo catalytic hydrodesulfurization and re-forming processes. Catalysts in automobile exhaust systems convert pollutants (hydrocarbons, CO, NO) to... 

Catalysts for automobile exhaust systems are designed to remove environmental pollutants such as unburned hydrocarbons, CO formed from incomplete combustion and oxides of nitrogen. Three-way catalysts are based on Pt and Rh together with various additives that together perform a complex series of reactions, including removal of hydrocarbons by oxidation and steam re-forming (see above), and... 

Catalytic converters in automobile exhaust systems were developed to remove some of the carbon monoxide and unburned hydrocarbons from automobile exhaust. A catalyst is any substance that speeds a chemical reaction without being permanendy altered itself Some of the transition metals, such as platinum, palladium, iridium, and rhodium,... 

Lithium hydroxide s light weight and relatively low cost have recommended it over other alkali hydroxides, any of which could do the same job. Lithium hydroxide is water soluble, however, which is undesirable in situations where less care might be taken to keep it dry. The Toshiba company has been working on a lithium silicate absorption device that could turn out to be cost-effective for use in absorbing C02 from car exhaust. The difficulty is in making a device small enough to fit in a typical automobile exhaust system. 

Corrosion of automobile exhaust systems by direct reaction of the metal with high-temperature gases and by condensation of water and absorption of the oxides of sulfur and nitrogen to produce aqueous acid environments... 

Figure 7. Alumina used in automobile exhaust systems.

Coal can also be converted into gaseous and liquid low-sulfur fuels. The catalytic converter in an automobile exhaust system reduces NO to N2 and NH3, and in power plants, NO is removed from the hot stack gases with ammonia ... 

Macroscopic properties often influenee the performanee of solid eatalysts, which are used in reaetors that may simply be tubes packed with catalyst in the form of partieles—chosen beeause gases or liquids flow through a bed of them (usually continuously) with little resistanee (little pressure drop). Catalysts in the form of honeycombs (monoliths) are used in automobile exhaust systems so that a stream of reaetant gases flows with little resistanee through the channels and heat from the exothermie reaetions (e.g., CO oxidation to CO2) is rapidly removed. 

Heterogeneous catalysis is also utilized in the catalytic converters in automobile exhaust systems. The exhaust gases, containing compounds such as nitric oxide, carbon monoxide, and unburned hydrocarbons, are passed through a converter containing beads of solid catalyst (see Fig. 12.16). The catalyst promotes the conversion of carbon monoxide to carbon dioxide, hydrocarbons to carbon dioxide and water, and nitric oxide to nitrogen gas to lessen the environmental impact of the exhaust gases. However, this beneficial catalysis can, unfortunately, be accompanied by the unwanted catalysis of the oxidation of SO2 to SO3, which reacts with the moisture present to form sulfuric acid. 

Heterogeneous catalysis is also used in the catalytic converters of automobile exhaust systems. The exhaust gases, containing compounds such as nitric oxide, carbon monoxide, and unburned hydrocarbons, are passed through a converter containing beads of solid catalyst (see Fig. 15.18). The... 

Typical ceramic materials produced on a co-rotating twin screw extruder are for example catalyst carriers. They are commonly shaped into granules for use as bulk material in reactors in the chemical industry or into honeycombs for catalytic converters in automobiles exhaust systems (Fig. 12). After extrusion, the catalyst carriers are cut oversized in the lineal direction, dried and then cut to the proper length. Afterwards the binder is removed and the carriers are calcinated or sintered. Finally, to provide them with catalytic properties, they are impregnated with an active film in a bath [Fri76]. 

---