On the dynamic behavior of automotive catalysts

doi:10.1016/S0920-5861(97)00033-3

Catalysis Today

Volume 38, Issue 1, 5 October 1997, Pages 3-12

https://doi.org/10.1016/S0920-5861(97)00033-3 Get rights and content

Abstract

Automotive catalytic converters show highly transient operation due to fluctuating exhaust gas conditions in real application. Local ignition of the reactions with very steep temperature gradients as well as moving reaction fronts can occur in the monolithic catalyst. Theoretical investigations are presented based upon detailed simulation studies with a onedimensional two-phase model for the catalytic converter. The simulation of the heat-up of the automotive catalyst during the cold-start shows a highly transient behavior for different cold-start concepts. Also, hot spot phenomena can be observed in the simulation under specific exhaust gas conditions.

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VDI Fortschrittsberichte Reihe 12: Verkehrstechnik/Fahrzeugtechnik (239)

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There have been performing a lot of numerical studies related to catalytic converter. A single channel model (Oh and Cavendish, 1982; Kirchner and Eigenberger,1997) was the first numerical model which was developed to investigate the transient response of heat and mass transfer with chemical reaction within the monolith. In recent, a single channel model accounting for the species diffusion inside the washcoat using the effectiveness factor was developed (Santos and Costa, 2009).
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Optical frequency domain reflectometry measurements of spatio-temporal temperature inside catalytic reactors: Applied to study wrong-way behavior
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These experiments are, to the best of our knowledge, the first experimental report of the wrong-way behavior following an increase in the feed flow rate to a heterogeneous catalytic reactor. Our experimental observations verified earlier numerical simulation predictions of the wrong-way behavior as a result of flow rate acceleration [28,29]. The amplitude of the transient temperature rise following a decrease in the feed temperature was much larger (37 °C) than that caused by an increase in the feed flow rate (9 °C).
An Optical Backscatter Reflectometer (OBR) with optical frequency domain reflectometry (OFDR) enables measurement of the spatial temporal temperature inside catalytic reactors with unparalleled combined spatial and temporal resolution. It was used to measure the transient temperature inside a single channel of a washcoated monolith reactor in which the Pt-catalyzed oxidation of ammonia was carried out. Measurements of the monolith temperature response to either a rapid step decrease in the feed temperature or an increase of the flow rate were used to test the capability of the OFDR technique and to determine their impact on the transient temperature rise of the monolithic catalyst. The rapid step increase in the flow rate can generate a transient peak temperature of the solid phase higher than that at the steady state. This transient temperature rise was smaller than that caused by a sudden decrease of the feed temperature.
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Dynamic simulation of the smart catalytic converter, proposed by Daimler AG, is presented. The smart catalytic converter combines NOx storage, on-board ammonia production and selective catalytic reduction (SCR) and functions in a dual-mode operation, alternating between lean burn and rich burn. It relies on intrinsic dynamic operation and synchronization of all units and its development demands a reliable dynamic simulator. A platform capable of simulating the dynamic behavior of multiple-unit aftertreatment system was developed based on COMSOL package. Predictive kinetic models were developed for NOx storage unit that includes ammonia formation function and for NH₃-SCR unit. Using these kinetic models, two-unit smart catalytic converter was simulated on the developed simulator. The results of the simulator were validated using two-unit experimental data. The simulator was also employed to control and optimize the performance of smart catalytic converter. It was shown that the simulator is vital for optimization of lean and rich periods in order to ensure stable lean–rich cycles.
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In practical applications, monolithic catalytic converters are operated at non-isothermal conditions. In this case, the active metal distribution along the length of the converter may influence its performance. Indeed, better conversions can be achieved by controlling the distribution of the same quantity of active material. In this study, we used a one-dimensional catalyst model to predict the transient thermal and conversion characteristics of a dual monolithic catalytic converter with Platinum/Rhodium (Pt/Rh) catalysts. The optimal design of a longitudinal noble metal distribution of a fixed amount of catalyst is investigated to obtain the best performance of a dual monolithic catalytic converter. A micro-genetic algorithm with considering heat transfer, mass transfer, and chemical reactions in the monolith during the FTP-75 cycle was used. The optimal design for the optimal axial distribution of the catalyst was determined by solving multi-objective optimization problems to minimize both the CO cumulative emissions during the FTP-75 cycle, and the difference between the integral value of a catalyst distribution function over the monolith volume and total catalytic surface area over the total monolith volume.
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Catalytic converters are the most effective means of reducing pollutant emissions from internal combustion engines under normal operating conditions. But the future emission requirements cannot be met by three way catalysts (TWC) as they cannot effectively remove hydrocarbon (HC) and carbon monoxide (CO) emissions from the outlet of internal combustion engines in the cold-start phase. Therefore, significant efforts have been put in improving the cold-start behavior of catalytic converters. In the experimental study, to improve cold-start performance of catalytic converter for HC and CO, a burner heated catalyst (BHC) has been tested in a four stroke, spark ignition engine. The modeling of catalytic converter performance of the engine during cold start is a difficult task. It involves complicated heat transfer and processes and chemical reactions at both the catalytic converter and exhaust pipe.
In this study, to overcome these difficulties, an artificial neural network (ANN) is used for prediction of catalyst temperature, HC emissions and CO emissions. The training data for ANN is obtained from experimental measurements. In comparison of performance analysis of ANN, the deviation coefficients of standard and heated catalyst temperature, standard and heated catalyst HC emissions, and standard and heated catalyst CO emissions for the test conditions are less than 4.925%, 1.602%, 4.798%, 4.926%, 4.82% and 4.938%, respectively. The statistical coefficient of multiple determinations for the investigated cases is about 0.9984–0.9997. The degree of accuracy is acceptable in predicting the parameters of the system. So, it can be concluded that ANN provides a feasible method in predicting the system parameters.
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This paper describes the evolutions of the distributions of temperature and concentration in the monolith during the cold-start period and the effects of the flow distributions in the monolith on the performance of a catalytic converter using numerical simulation. The effects of the flow distributions on the light-off, warm-up performance and the temporal variation of the solid temperature were studied through numerical simulation. The following conclusions were obtained from the predicted light-off curves and the steady conversion efficiency: (1) The light-off time increases and the total outlet conversion efficiency reduces as the flow uniformity index decreases, which is in good agreement with experimental results. (2) The conversion efficiency decreases due to the reduction of residence time caused by the increase of the velocity as the radius is reduced. The analyses on the distribution of temperature in the monolith show that the temperatures of the solid in the area near the front face of the substrate rise as the radius increases, and the radial gradient of the temperature of the solid becomes greater as the flow uniformity index decreases. An experiment on the flow distribution in a catalytic converter with a honeycomb spherical arc was conducted. Experimental results show that the flow distribution in the monolith can be improved greatly by installing the honeycomb spherical arc in the tapered inlet header with a penalty of a relatively small increase of pressure drop.

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On the dynamic behavior of automotive catalysts

Abstract

Chem. Eng. Sci.

Chem. Eng. Sci.

Comp. Chem. Eng.

Chem. Ing. Tech.

Chem. Ing. Tech.

SAE Technical Paper Series No. 900503

SAE Technical Paper Series No. 940467

VDI Fortschrittsberichte Reihe 12: Verkehrstechnik/Fahrzeugtechnik (239)