The Automotive Catalytic Converter

the mass transfer limited case is nearly temperature independent, and the pore diffusion limited case has an activation energy E/2. 

The largest market for chemical reactors by far is the automotive catalytic converter (ACC), both in number of reactors in existence (many million sold per year) and in amount of reactants processed (mUhons of tons per year). There are 50 mtUion automotive catalytic converters operating (or at least existing) throughout the world, and everyone owns one if he or she has a car less than 10 years old. 

The engine is basically a batch reactor [what is the reaction time ] in which the combustion reaction 

Thus the engine of an automobile produces approximately 5% of various hydrocarbons, 1% of CO, and 0.1% of NO. These gases are major pollutants in urban areas, and clean air regulations require that they be removed to approximately 0.2, 0.05, and 0.03%, respectively. 

The purpose of the converter is to reduce unbumed hydrocarbon emissions to 4 g/mile, CO to 40 g/mile, and NO to 0.4 g/mile. (The mixed units are those in several Federal Clean Air Acts, which mandate these maximum emissions.) In California these standards are even more stringent, and lower levels are mandated to be required in several years throughout 

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Some catalyst supports rely on a relatively low surface area stmctural member coated with a layer of a higher surface area support material. The automotive catalytic converter monolith support is an example of this technology. In this appHcation, a central core of multichanneled, low surface area, extmded ceramic about 10 cm in diameter is coated with high surface area partially hydrated alumina onto which are deposited small amounts of precious metals as the active catalytic species. 

Automotive Catalytic Converter Catalysts. California environmental legislation in the early 1960s stimulated the development of automobile engines with reduced emissions by the mid-1960s, led to enactment of the Federal Clean Air Act of 1970, and resulted in a new industry, the design and manufacture of the automotive catalytic converter (50). Between 1974 and 1989, exhaust hydrocarbons were reduced by 87% and nitrogen oxides by 24%. 

Catalytic reactors have worked to the benefit of the chemical and petroleum industries for many decades, under the watchful eyes of plant engineers and batteries of monitor and control instruments. The automotive catalytic converter will be the first mass produced catalytic reactor placed directly in the hands of the public, who can provide little more than benign neglect. 

Many elements of a mathematical model of the catalytic converter are available in the classical chemical reactor engineering literature. There are also many novel features in the automotive catalytic converter that need further analysis or even new formulations the transient analysis of catalytic beds, the shallow pellet bed, the monolith and the stacked and rolled screens, the negative order kinetics of CO oxidation over platinum,... 

This is an extremely important reaction to which we wiU refer throughout this book. It is responsible for all NO, formation in the atmosphere (the brown color of the air over large cities) as well as nitric acid and acid rain. This reaction only occurs in high-temperature combustion processes and in lightning bolts, and it occurs in automobile engines by free-radical chain reaction steps, which will be the subject of Chapter 10. It is removed from the automobile exhaust in the automotive catalytic converter, which wiU be considered in Chapter 7. 

Figure 7-16 A highly simplified sketch of an automohile engine and catalytic converter with typical gas compositions indicated before and after the automotive catalytic converter. The catalytic converter is a tube wall reactor in which a noble-metal-impregnated wash coat on an extruded ceramic monolith creates surface on which reactions occur.

There are a number of examples of tube waU reactors, the most important being the automotive catalytic converter (ACC), which was described in the previous section. These reactors are made by coating an extruded ceramic monolith with noble metals supported on a thin wash coat of y-alumina. This reactor is used to oxidize hydrocarbons and CO to CO2 and H2O and also reduce NO to N2. The rates of these reactions are very fast after warmup, and the effectiveness factor within the porous wash coat is therefore very smaU. The reactions are also eternal mass transfer limited within the monohth after warmup. We wUl consider three limiting cases of this reactor, surface reaction limiting, external mass transfer limiting, and wash coat diffusion limiting. In each case we wiU assume a first-order irreversible reaction. 

Some of the important reactions in contemporary technology involve NO, which is a designation of N2O, NO, and NO2, and was one of the first examples in this book. The formation of these molecules in combustion processes is a major source of air pollution, and the catalytic oxidation of NH3 to NO on R surfaces is used to produce nitric acid, a major industrial chemical. The decomposition of NO, to N2 is a major process in the automotive catalytic converter. 

NO must be decomposed by bimolecular reactions to keep the product oxygen from blocking the surface sites on which NO must adsorb. In the automotive catalytic converter the NO in fact reacts with CO, which is another pollutant reduced in combustion. The reactions may be written as... 

This reaction is sufficiently fast that it proceeds at low enough temperature to be useful to eliminate NO in the automotive catalytic converter. We assumed that the surface reaction step is... 

Considerable research and engineering are being devoted to improving the performance of the automotive catalytic converter to reduce emissions that occur during startup. 

The most important reactor by far in twentieth century technolo is the fluidized catalytic cracker. It processes more chemicals than any other reactor (except the automotive catalytic converter), the products it creates are the raw materials for most of chemical technology, and this reactor is undoubtedly the largest and most complex piece of equipment in our business. Yet it is very possible that a student can receive a B.S. degree in chemical engineering without ever hearing of it... 

In spite of the commercial successes of these two millisecond reactors, few processes other than NH3 oxidation (a superoxidation) and HCN synthesis (oxidative dehydrogenation or ammoxidation) have been carried out on a large scale. In the automotive catalytic converter, contact times over noble metals on wash coated extended ceramics are used with contact times of -0.1 sec with temperatures of 400°C. 

The catalyst support plays an integral part in the performance of the automotive catalytic converter system. Although the precious metal catalyst is the primary contributor to the conversion efficiency, the substrate role is to properly distribute and support the washcoat and precious metal catalyst in order to utilize the properties of these components most fully. By optimizing the substrate properties as well as the precious metal and washcoat systems, optimum benefit from the catalyst system can be obtained. 

Hayes RE, Fadic A, MmbagaJ, Najafi A CFD modeUing of the automotive catalytic converter, Catal Today 188 94—105, 2012. 

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