Rao et al.

     Reactor Design




The catalytic converter (or catcon) in your car is, actually, a very important element of the carís exhaust system because it removes harmful nitrogen oxides (NOX, x=1,2,3) and carbon monoxide (CO) from the combustion residues before they are released into the environment. It is called a catalytic converter because it converts CO into ubiquitous CO2 and NOX into N2and O2 through chemical reactions on a solid catalyst. A catalyst is a chemical compound that helps the reaction to occur faster by reducing the activation energy barrier of the reaction. It is not consumed during the reaction. In the case of the catcon, the catalyst is a solid platinum (Pt) or palladium (Pd)surface unto which reactants from the gaseous phase adsorb and react. This process of a reaction being assisted by a catalyst that is in a phase different from the reactantsí phase is known as heterogeneous catalysis. (Learn more about catalysts at this website.)

Catalytic Converter - Deconstructed

The illustration above shows the catalytic converterís major components. The main components are two honeycomb-like monoliths covered with a thin layer of Pt/Rh (first monolith) and Pd/Rh (second). They provide the surface area where the reaction occurs. As mentioned before, the oxidation reactions occur on the surface of a metal catalyst. Since the reaction depends on a surface on which species can adsorb and react, the grater the surface area provided, the greater the conversion will be. In other words, the reaction rate and conversion are directly proportional to the catalystís surface area.

The role of the honeycomb-like structure of the monoliths is to increase the exposed surface area covered by the catalyst layer. As the number of channels in the monolith increases, the surface area increases. As shown, the monolithís surface is covered by a washcoat of a rather amorphous silicate material that increases surface area by adding rugosity to the surface.

For more about catalyst surface area you may visit this website.

Further details of its design can be illustrated by the following video:

Chemical Reactions in a TWC

As seen in the video, the most obvious reactions occurring inside the catcon are these:

Yet, there are at least 15 reactions occurring at once in that small section of your vehicle. These can be broken down into four basic categories: oxidation, water-gas and steam reformation, nitrous oxide reduction, and oxygen storage. You can find more about these in the kinetics section.

Material and Energy Balances

Chemical reaction rates also depend on temperature as shown by the Arrhenius equation. Therefore, we must also consider the energy (heat) transfer across our reactor, because a catalytic converter is, in fact, a reactor. Furthermore, each channel of the monoliths serves as a mini reactor since the reactions occur in the channels. There are three main heat transfer mechanisms in this system:

     1) Heat conduction along the monolith

     2) Convection heat transfer from the gaseous species to the monolith walls.

     3) Heat generation by the chemical reactions (These oxidation reactions are exothermic)

The magnitude of each kind of heat transfer rate will depend on the reactorís parameters such as the thermal conductivity of the materials from which the reactor is made. Also, reaction rates are a function of the reactants concentration so it is crucial to take into consideration their transport and consumption along the reactor. This will also depend on reactor parameters like diffusivity, which determines how fast species can move across a material.