Hydrocarbon storage modeling for diesel oxidation catalysts

Abstract

A simple hydrocarbon (HC) storage model was proposed which represents the adsorption–desorption processes in zeolites incorporated in a diesel oxidation catalyst (DOC). Using four experiments in which the HC was stepped up and the reactor run until steady state, a Langmuir isotherm was generated that was sufficient to represent the equilibrium data, and the remaining rate constant capturing the adsorption time scale was obtained by fitting. The model thus developed was validated using reactor data which was obtained by stepping the inlet HC concentration to zero after outlet reached equilibrium. A typical DOC which contained both a storage component (zeolite) and noble metal component (for oxidation) was studied using a full scale 1D reactor model which includes the storage kinetics developed here with the oxidation kinetics developed in our previous work [ Sampara et al., 2008. Global kinetics for a commercial diesel oxidation catalyst with two exhaust hydrocarbons. Industrial & Engineering Chemistry Research 47, 311–322]. Simulations of a simplified warm-up process indicated that the zeolite storage component reduces the overall cold start HC emissions by at least a factor of two if the warm-up rate achieves 45 – 65 ∘ C min - 1 , a range commonly observed during start-up. Modeling results also showed that the HC oxidation for these reactors commonly starts at the rear end of the reactor due to reduced CO inhibition. The rates of the individual processes during cold start were analyzed in detail and compared with the rate of inlet temperature increase provided from the exhaust.