(467f) Analysis of Flow Distribution and Reactions in a Closed Coupled Diesel Oxidation Catalyst

Nishithan Balaji, Niket Kaisare, Preeti Aghalayam

Department of Chemical Engineering, Indian Institute of Technology Madras, Chennai - 600036, India. Corresponding Authors’ e-mail: nkaisare@iitm.ac.in and preeti@iitm.ac.in

Diesel Oxidation Catalysts (DOC) are used to control hydrocarbon and carbon monoxide emissions by oxidizing them catalytically to CO₂ and water. Most of the emissions during a typical drive cycle take place under cold-start conditions, when the catalyst is not hot enough for the oxidation reactions. Hence, a close-coupled convertor is being considered, wherein the DOC is mounted close to the engine, so that the catalyst reaches higher temperatures faster. Practical considerations in cc-DOC need to ensure the availability of space while determining device sizing and geometry. The flow maldistribution affects conversion of the exhaust streams and hence needs to be analyzed coherently. Heat losses from the device may result in lower temperatures at the periphery compared to the center. Due to higher flowrates and temperatures, turbulence effects have to be considered, and the reactions and heat effects could also affect the flow distribution in the device. Detailed flow velocities, concentration and temperature profiles obtained from computational fluid dynamics (CFD) simulations will enable us to understand the after-treatment system behavior, which is important in developing better design and operation strategies.

The studies reported in literature [1,2] explain the study of flow maldistribution considering the flow and heat transfer aspects alone. Often, in such flow simulations, the effect of reactions is ignored. However, reactions occurring in the reactor may strongly influence the flow dynamics, as reported previously by Agrawal et al. [3] for a regular (under-floor) convertor. Although the effect of the inlet manifold on flow maldistribution has been studied [1–3], a detailed analysis of flow maldistribution in the presence of reaction is necessary and has not yet been studied in a close-coupled converter. Furthermore, we are not aware of work in the literature that combines flow and thermal effects with realistic DOC reaction chemistry.

In this work, close coupled converters with two different geometries are chosen for the analysis. A 3D CFD model is used for our simulations, which also includes the turbulent effects modeled using the low Reynolds number k-ω SST model. A porous media approximation is used to simulate the catalytic monolith. The validity of the porous media approximation is first confirmed by carrying out the simulations in a multi-channel monolith reactor. The axial profiles of flow, temperature and concentrations at the center and the periphery of the reactor are compared for the porous media approximation and multi-channel simulation.

To delineate the roles of flow, thermal effects and reactions in the cc-DOC, CFD simulations are performed for isothermal, non-isothermal and adiabatic boundary conditions, with and without the presence of reactions. The DOC reaction kinetics were taken from Pandya et al. [4], as they are valid for temperature ranges between 350 and 700 K. Flow distribution indexes are calculated for all the cases and analyzed. The effect of flow maldistribution on the conversion of the reactants at the center and the periphery of the reactor is studied. A comparison with regular (under-floor) DOC is presented by performing these sets of simulations for the under-floor geometry as well. The entrance effect in cc-DOC, its influence on flow maldistribution and hence on the net conversion is presented. The conditions for which the flow maldistribution affects the performance of the reactor and the effects due to the temperature distributions are also studied.

References

[1] V. Chakravarthy, J.C.Conklin, C.S.Daw, and E. Azevedo, “Multi-dimensional simulations of cold- start transients in a catalytic converter under steady inflow conditions,” Applied Catalysis A: General, vol. 241, pp. 289-306, 2003.

[2] M. Badami, F. Millo, A. Zuarini, and M. Gambarotto, “CFD analysis and experimental validation of the inlet flow distribution in close coupled catalytic converters,” SAE Technical Paper Series, 2003.

[3] G. Agrawal, N. S. Kaisare, S. Pushpavanam, and K. Ramanathan, “Modeling the effect of flow mal-distribution on the performance of a three-way catalytic converter,” Chemical Engineering Science, vol. 71, pp. 310-320, 2012.

[4] A. Pandya, J.Mmbaga, R. Hayes, W.Hauptmann, and M.Votsmeier, “Global kinetic model and parameter optimization for a diesel oxidation catalyst,” Topics in Catalysis, vol. 52, no. 13, pp. 1929-1933, 2009.