Bifurcation features of mixtures containing CO and hydrocarbons in DOC.

Rama Krishna  Dadi, University of Houston

A monolith reactor model with radial gradients in the washcoat is used to study the ignition and extinction behavior of pure CO, C2H6 and C3H6 and of their mixtures. The ignition temperature of the individual species oxidation increases upon an increase in feed concentration of CO, C3H6 and decreases upon an increase in feed concentration of C2H6. This is because CO, C3H6 have negative order kinetics while C2H6 has positive order kinetics. We determine steady-state hysteresis behavior during co-oxidation of CO and hydrocarbons (C3H6, C2H6). A single S shaped bifurcation diagram is obtained during the co-oxidation of all feed compositions of CO and C3H6. This is because activation energies of CO and C3H6 are close to each other. The conversion of propylene at the ignition temperature is much lower than CO conversion. The presence of CO in the feed inhibits the oxidation of propylene during co-oxidation of CO and C3H6. This behavior is apparent by comparing the bifurcation set of pure C3H6 with the bifurcation set of a feed containing both CO and C3H6.  In contrast to CO- C3H6 system the ignition temperatures of pure CO and pure C2H6 are much different. CO ignites at a much lower temperature than C2H6. A double S shaped bifurcation diagram is obtained when hysteresis exists for both CO and C2H6 during the co-oxidation. The oxidation of CO present in the feed increases the solid temperature and this reduces the C2H6 ignition temperature. The separated ignition of CO and C2H6 are transformed to a simultaneous ignition when the CO concentration in the feed is sufficiently high. We determine the boundary in the parameter space of feed compositions and other operating conditions that separates the simultaneous and separate ignition. The application of this result for the design of DOCs for treating exhaust gases at lower inlet temperatures is discussed.

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