A theory for fast-igniting catalytic converters
- Univ. of Notre Dame, IN (United States). Dept. of Chemical Engineering
Using asymptotic expansion and numerical analysis, the authors demonstrate how the step-response ignition time of an automobile catalytic converter depends on the ratio of the reaction rate to the interphase heat-transfer rate, as measured by a key Damkoehler parameter X and the degree of monolith subcooling n. In the region of low reaction rate at small X, the normalized ignition time t scaled by the homogeneous ignition time t{sup {infinity}} from the inlet gas temperature is (t/t{sup {infinity}}) = 1 + 2X{sup 1/2}{vert_bar}ln(X{sup 1/2}/2n){vert_bar}{sup 1/2}, and the ignition takes place at a thermal front deep in the monolith. At large X when the reaction rate is high, ignition occurs at the leading edge of the monolith with (t/t{sup {infinity}}) = 2.50 + X(ln n {minus} 0.34). The delay in ignition time with increasing X is due to a Taylor-Aris dispersion mechanism induced by interphase heat transfer. Although the small-X ignition mechanism is faster, its downstream ignition location leads to a very slow upstream propagation of the thermal front that follows ignition. An optimal converter system that ignites in 13 s, 25% of the current value in a standard step-response test, is then designed by placing a small igniter, which ignites by the small-X mechanism, upstream to preheat the current converter which then ignites by the large-X mechanism. The length of the igniter is kept small by bypassing 2/3 of the exhaust since, from the authors` theory, t{sup {infinity}} is independent of the gas velocity.
- OSTI ID:
- 106221
- Journal Information:
- AIChE Journal, Vol. 41, Issue 8; Other Information: PBD: Aug 1995
- Country of Publication:
- United States
- Language:
- English
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