464314 Lean-Rich Switching over a Modified Three-Way Catalyst:  Experiments and Modeling

Monday, November 14, 2016: 3:45 PM
Franciscan A (Hilton San Francisco Union Square)
Mengmeng Li, Sam Malamis, Michael P Harold and William S. Epling, Department of Chemical and Biomolecular Engineering, University of Houston, Houston, TX

Lean-Rich Switching Over a Modified Three-Way Catalyst:  Experiments and Modeling

Mengmeng Li, Sam Malamis, Michael P. Harold*, William Epling**

Department of Chemical and Biomolecular Engineering, University of Houston, Houston, TX 77204

* mharold@uh.edu, ** wsepling@central.uh.edu

 

The most recent CAFE standard for automobile requires an average fuel economy of 54.5 miles per gallon by 2025 [1]. At the same time, more stringent emission rules enacted by the EPA for 2017-18 will require an 80% reduction in non-methane organic gases (NMOG) plus NOx from current Tier 2 Bin 5 levels [2]. Gasoline vehicles that operate under both lean-burn and stoichiometric combustion hold promise since lean-burn combustion is more fuel efficient than conventional stoichiometric combustion. However, elimination of NOx (NO+NO2) by the traditional three-way catalyst (TWC) is not possible under lean conditions. Advanced NOx reduction strategies must be applied to exploit the more efficient lean-burn engine combustion such as the three-way catalyst with a NOx storage function (TWNSC). The TWNSC stores NOx storage during lean operation and reduces the trapped NOx during stoichiometric operation. To date, there have been only a few studies that report TWNSC.  In this study a predictive TWNSC model is developed to predict TWNSC performance and in so doing provide operational insight and optimization.

A model of a catalytic monolith channel was developed that incorporates a TWNSC kinetic model into a low-dimensional model formulation [3–5]. The convection-diffusion-reaction equations are averaged in the transverse direction which replaces the transverse gradient term with an algebraic term that contains an internal mass transfer coefficient. Overall heat and mass transfer coefficients are used to represent the heat and mass transfer in the transverse direction between the bulk of fluid and solid phase. The TWNSC kinetics consists of a combination of elementary steps and global reactions. Following our previous experimental study of NOx reduction by propylene over a multifunctional NOx trap catalyst [6], the hybrid model includes the kinetics of the oxidation of CO, H2, NO and hydrocarbons, of reduction of NO/NO2 by CO, H2 and representative hydrocarbons, and of storage of NOx and O2. Fig. 1 shows the reaction network including 10 main paths that described the chemistry in qualitative terms.  

Figure 1. Simplified reaction network of NOx reduction using propylene as reductant[6]

 A step-wise approach was used to develop kinetics for the oxidations of CO, H2 and propylene. First, global kinetics are fitted tuned to transient light-off experiments following Raj et al. [5]. For example, the Langmuir-Hinshelwood expression is given by

Fig. 2 shows the fitted light-off curve for CO oxidation and the comparison between experimental and model data. The model shows a reasonable fit and captures the dynamical feature of CO TPO (temperature programmed oxidation) experiment.

Figure 2. (a) Fitted light-off curve of CO oxidation (b) comparison between experimental and model-predicted conversions. [Conditions: 0.5% CO, 0.5% O; temperature ramp: 3˚C/min]  

The rate expressions for the co-oxidation of CO, H2, and C3H6 include coupling effects.  These are combined with a dual-site NOx storage model and incorporated into species balances. 

We will show how the model is effective in capturing the main trends in the lean-rich switching data.  This study advances the understanding of TWNSC catalyst and provides guidance for optimizing catalyst formulation and operation strategies.

Reference

[1]      http://www.nhtsa.gov/About+NHTSA/Press+Releases/2012/Obama+Administra

tion+Finalizes+Historic+54.5+mpg+Fuel+Efficiency+Standards.

[2]      T. Johnson, SAE Int. J. Engines. 6 (2014) 699–715.

[3]      S.Y. Joshi, M.P. Harold, V. Balakotaiah, AIChE J. 55 (2009) 1771–1783.

[4]      D. Bhatia, R.D. Clayton, M.P. Harold, V. Balakotaiah, Catal. Today. 147 (2009) 250–256.

[5]      R. Raj, M.P. Harold, V. Balakotaiah, Chem. Eng. J. 281 (2015) 322–333.

[6]      M. Li, V.G. Easterling, M.P. Harold, Catal. Today. 267 (2016) 177–191.

 


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