Heat-integrated reactor concepts for catalytic reforming and automotive exhaust purification

Optimal solutions in environmental catalysis require a well-coordinated development of catalysts and of process design. This contribution is devoted to energy integrated design concepts for fuel reforming and for automotive exhaust purification. The examples presented demonstrate the importance of an innovative process design for optimal utilization of existing catalysts and show the potential of future developments.

New concepts for steam reforming through the efficient coupling of the endothermic reforming reaction with an exothermic combustion reaction are discussed in the first part. These concepts have been implemented for methanol steam reforming in a counter-current reactor with distributed side feed of burner gas and for methane steam reforming in a modular reactor with a co-current reaction section for the endothermic and the combustion reaction and attached counter-current heat exchangers. Both applications employ the so-called folded sheet reactor design, which ensures an excellent heat transfer between the reforming and combustion channels and efficient heat recovery.

A similar design solution is introduced for the apparently different case of automotive exhaust purification. The proposed concept aims at decoupling exhaust after-treatment from engine control. Its main component is a counter-current heat exchanger with integrated purification stages for HC-oxidation, NO_X storage and reduction and soot filtering. A small catalytic burner at the hot end of the heat exchanger provides both heat and oxidizing or reducing agents on demand. A new soot filter design allows for safe soot filter regeneration.     

Experimental results concerning the role of Pt,Rh, Ba,Ce and Al2O3 on NO x -storage catalyst behaviour

NO _x -storage catalysts (NSC) with varied washcoat compositions have been investigated experimentally under lean and rich conditions. Besides the fact that Ba and Rh are essential for NO _x -storage and -reduction, it was observed that Ba accelerates the NO-reduction and decreases NO-oxidation kinetics. It also turned out to be the promoting species regarding water gas shift reaction. The results revealed kinetic inhibition effects by CO, C₃H₆ and NO, being less pronounced with Ba in the washcoat. It is further shown that the cyclic NO_x-conversion of the NSC is mainly determined by the processes in the regeneration phase.     

Electrically Heated Oxide Ceramic Tubes for High Temperature Reactions

Endothermic high temperature reactions are usually carried out in metal tubes heated by gas burners. Electrical heating allows substantial reduction of CO₂ emissions. We propose the usage of a composite tube, where a thin metallic layer is embedded between an inner and outer ceramic layer. While monolithic ceramics suffer from brittleness and low tolerance to thermal stress, only the inner layer is made from monolithic ceramics, while the outer layer is made of fiber reinforced oxide ceramics. In first tests the hybrid ceramic tube was electrically heated to 1250 °C with a maximum heat release of 85 kW m⁻².     

A new simulation model for NO x storage catalyst dynamics

Topics in Catalysis - The NO x storage concept has been studied experimentally and by two differently detailed numerical simulation models. The first detailed model simulates the concentration...     

Macro‐ and Microkinetic Simulation of Diesel Oxidation Catalyst: Effect of Aging,Noble Metal Loading and Platinum Oxidation

In automotive exhaust aftertreatment simulation, both macro‐ and microkinetic models are commonly used. In this contribution both models are applied for the simulation of diesel oxidation catalysts (Pt/γ‐Al₂O₃) with different catalyst loading and degree of thermal aging. The study proves that the structure insensitive kinetics of the considered catalysts can be described with the same rate equations only by scaling the rate constants of the different reaction steps with the catalytically active Pt surface, which is accessible by CO adsorption or light‐off measurements. In addition, NO oxidation is strongly influenced by a reversible, slow transformation of Pt into Pt‐oxide.     

A New Simulation Model for NOx Storage Catalyst Dynamics

The NO _x storage concept has been studied experimentally and by two differently detailed numerical simulation models. The first detailed model simulates the concentration fronts of the solid components in the barium particles. It shows the slow, diffusion-hindered formation of dense nitrate layers around barium nanoparticles during NO _x storage and their rapid break-up during regeneration. Based upon this knowledge a new simplified model was developed which is able to describe well the storage and regeneration and to explain the main chemical and physical processes in the NO _x storage catalyst.     

Towards a realistic simulation model for NOx-storage catalyst dynamics

A model of a Pt/Rh/Ce/Ba/Al₂O₃–NO_x-storage catalyst has been developed. It considers the influence of typical exhaust components on the NO-oxidation and NO_x-reduction reactions under realistic flow conditions over a wide temperature range. The NO_x-storage dynamics are limited by the diffusion of the reactants into the Ba-particle through an increasingly thick, dense nitrate layer. The regeneration reaction instead is fast since the nitrate shell breaks up. By calculation of the nitrate front position and the nitrate fraction in the Ba-particle, it is possible to describe the complex dynamic behaviour of the NSC.     

Heat-Integrated Concepts for Automotive Exhaust Purification

Novel heat-integrated concepts for efficient automotive exhaust gas purification are presented. The general design consists of a counter-current heat exchanger coupled with purification devices. Transient and steady state experiments were carried out both in lab scale and on an engine test bench. The results show effective heat recovery at acceptable pressure drop. Comparative simulation studies show the advantages of the system under drive cycle conditions.