Exhaust streams, automobile

Tests were run on a small ejtperimental reactor used for decomposing nitrogen oxides in an automobile exhaust stream. In one series of tests, a nitrogen stream containing various concentrations of NOj was fed to a reactor and the kinetic data obtained are shown in Figure P5-11. Each point represents one complete run. The reactor operates essentially as an isothermal backmix reactor (CSTR). What can you deduce abmit the apparent order of the reaction over the temperature range studied ... 

Another example is the solid catalyst used to reduce the emission of pollutants such as unburned hydrocarbons, carbon monoxide, and nitrogen oxides in the exhaust streams of automobile engines (Fig. 18.16). A catalytic converter is designed to simultaneously oxidize hydrocarbons and CO through the reactions... 

Maintaining the continued efficiency of all three reactions in a three-way catalytic converter is a delicate matter. It requires control of such factors as the O2 supply pressure and the order in which the reactants reach the catalyst Modern automobile engines use microcomputer chips, based on an O2 sensor in the exhaust stream, to control air valves. 

Catalytic combustion applications can be classified as either primary or secondary pollution control, that is, emissions prevention or emissions clean-up. The most common example of catalytic combustion for emissions clean-up is the catalytic converter in the exhaust system of automobiles. Catalytic combustion is also increasingly used for the removal of volatile organic compounds (VOCs) from industrial exhaust streams. The use of catalytic combustion in exhaust gas clean-up is discussed in other sections of this Handbook this section deals only with primary control applications. 

Solutions have not been limited to water, however. Atmospheric pollution from automobiles has decreased dramatically in the last 25 years since the development and deployment of the catalytic converter (Box 2-1). Modern three-way catalysts can simultaneously reduce the concentrations of carbon monoxide, hydrocarbons, and nitrogen oxides in the automobile s exhaust stream. Also, new methods for remediation of contaminated soils have been provided by the selection of unique plants and microbes for this purpose. 

One measure of the efficiency of a work-producing process is the actual work obtained (regardless of the actual final temperamre and pressure of the exit stream, which could be, for example, the exhaust of an automobile) compared with the maximum work that could have been obtained if the process was reversible, and the exhaust streams had been at ambient conditions, that is... 

In this chapter, we used a simplified equation for the combustion of gasoline. But the products generated in the operation of real automobile engines include various oxides of nitrogen, and it is desirable to remove these compounds from the exhaust stream rather than release them into the atmosphere. Currently this is done by the catalytic converter, but other technologies are being explored. One possible method involves the use of isocyanic acid, HNCO, which reacts with NO2 to produce N2, CO2, and H2O, as shown in the equation below ... 

Use the web to determine the range of partial pressures that oxygen sensors must measure in the exhaust manifold of automobile engines. Why is it important for an engineer to know the amount of oxygen in an exhaust stream ... 

Suppose that the exhaust stream of an automobile has a flow rate of 2.55 L/s at 655 K and contains a partial pressure of NO of 12.4 torr. What total mass of urea is necessary to react completely with the NO formed during 8.0 hours of driving ... 

Steady streams of 0 vapor or SO2 could be introduced before the preheater to simulate automobile exhaust. When 0 was added to the feedstream (we used a liquid chromatographic pump), it was condensed out before the gas entered the vent line. However, no water- or SO -containing feedstreams were used in the examples shown in this paper. 

The major route of formation of these nitro compounds is via the reaction of VOCs with the NO arising from hot flue gases, such as automobile exhaust gases and gas streams used for drying food stuffs, etc. In these combustion systems the aliphatics can react with nitro compounds or arenes to produce nitro-PAH and nitroarenes. Some of the NO produced are thus converted into C-nitroso compounds. The interactions and reaction chemistry of these compounds is complex and difficult to interpret. 

The catalysts were evaluated by exposure to a simulated automobile exhaust gas stream composed of 0.2% isopentane, 2% carbon monoxide, 4% oxygen and a balance of nitrogen. The temperature required to oxidize the isopentane and carbon monoxide was used to compare catalyst performance. The chromium-promoted catalyst oxidized isopentane at the lowest temperature, and a mixed chromium/copper-promoted catalyst proved the most efficient for oxidizing carbon monoxide and isopentane. It is interesting to note that the test rig used a stationary engine with 21 pounds of catalyst. Although the catalyst was very effective it is difficult to envisage uranium oxide catalysts employed for emission control of mobile sources. 

This competitive adsorption drives the platinum deeper into the extrudates, and when sufficient HCl is added, the PtCl / adsorption reaction is moderated to such an extent that a reasonably homogeneous distribution is obtained. One can also add acids that adsorb even more strongly than chloroplatinic acid, e.g. oxalic or citric acid, with which Pt profiles such as shown in Fig. 10.6 can be prepared. Profile (b) can be useful when a strongly adsorbing poison is present in the stream to be treated, e.g. Pb in automobile exhaust gas, while profiles (c) and (d) could be advantageous e.g. in diffusion-limited consecutive reactions. 

Corrosive gas samples will often react with the container. Sulfur dioxide, for example, is troublesome. In automobile exhaust, SO2 is also lost by dissolving in condensed water vapor from the exhaust. In such cases, it is best to analyze the gas by a stream process. 

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. 

The concentrations of lead and sulfur on the catalysts characterized in Figures 5 and 6 were 0.2 and 2-4 wt % respectively. These levels correspond to about 3000-5000 miles with 1975 model year automobile fuel (0.03-0.05 g Pb/gal). The lead was impregnated onto the fresh catalyst using lead nitrate in order to obtain more nearly solely chemical poisoning effects rather than the pore mouth blocking effects of lead oxide from exhaust gas. After impregnation, the leaded catalyst was heat treated 4.5 hrs at 482°C. For the sulfur, the catalyst was treated at 482°C in an air stream containing 5% steam and 1% S02. 

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]. 

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