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A concept of catalytic reversed flow converter (RFC) is discussed for three emerging areas of emission control after automotive engines. The first potential application concerns with oxidation of methane and CO emitted from dual fueled, natural gas/diesel engines (DDF). The behavior of RFC is studied under typical transient conditions of DDF engine exhaust using mathematical modeling and experiments with converter prototype. It was shown that the RFC with a single catalyst monolith can be well suited for this application. The second potential application is aimed at NOx emission control from diesel engines. An RFC comprising a catalyst bed flanked by two beds of inert material is discussed based on a lumped kinetics of NOx reduction over a noble metal lean-NOx catalyst. Contrary to a traditional converter, the RFC retains high efficiency of NOx reduction for long time after typical excursion of NOx concentration and temperature of engine exhaust gas. Finally, an idea of RFC combining catalyst and adsorbent is suggested for emission control during cold start of automotive engines.

The process of sulphur dioxide oxidation over vanadium oxide catalyst in a fixed-bed reactor with periodic gas flow reversal was analyzed on the basis of a mathematical model. The mathematical description of unsteady-state processes in the reactor includes the dynamic model of SO 2 oxidation in the melt of active component of catalyst developed recently by Balzhinimaev and Ivanov. It describes both the main catalytic reaction steps and side processes involving changes in the catalyst state. Estimation of time scales of various unsteady-state processes in the catalyst and fixed-bed reactor allowed to include only essential dynamic elements of the model. The moving profiles of the temperature, gas phase composition and catalyst phase intermediate concentrations are calculated and analyzed. The effects of various factors governing the catalyst state dynamics are elucidated for performance parameters, such as cycle-average SO 2 conversion and maximum temperature in the catalyst bed. Results obtained provide a background for an explanation of the discrepancies between the predictions for the process performance using a steady-state kinetic model and experimental results obtained from pilot and industrial units.

The sections in this article are Introduction Dynamic Kinetic Model General Approaches to Reactor Modeling Analysis and Optimization of Cyclic Processes Inlet Composition and Temperature Modulation Partial Oxidation in Fluidized-Bed and Riser Reactors Dynamic Phenomena in a Fixed-Bed Reactor Forced Unsteady-State Processes in Fixed-Bed Reactors Reverse-Flow Operation Migration of a Reaction Zone through a Reactor Network Inlet Temperature Modulation Flow Modulation in Three-Phase Reactors Periodically Operated Adsorptive Reactors Chromatographic Reactors Flow-Reversal Adsorptive Reactors Pressure-Swing Reactors NOx Storage Reduction Systems Stabilization of Unstable Steady States Keywords: unsteady-state reactors; reactor modeling; inlet compositions; temperature modulation; flow modulation; reverse flow reactors; periodic flow reversal; chromatographic reactors; pressure-swing reactors

Publisher Summary The regenerative catalytic oxidizer (RCO) technology has evolved significantly as an efficient method to control volatile organic compounds. The RCO technology favors using oxide catalysts because it allows for easy compensation of lower activity by an increase in the catalyst amount or temperature. Heat transfer and accumulation properties of ceramic packing material strongly influence RCO performance. The rate of heat transfer affects the temperature gradients along the bed length that, in turn, determine the volume of material required to preheat the gas to the temperature of catalytic or thermal oxidation. During the catalyst operation, it gradually becomes less active and must be eventually replaced. The factors affecting the catalyst lifetime include high temperature, catalytic poisons, and masking agents. Compounds of halogens and sulfur are the most common catalyst poisons. Temperature control, poison tolerant catalysts, and gas-flow pretreatment are used to reduce the impact of catalyst deactivation. This chapter also presents the behavior of an RCO when the catalyst deactivates, and proposes strategies ensuring the required performance during the entire catalyst lifetime.

The paper discusses the development of catalytic converter for aftertreatment of exhaust gas from diesel engines powered with natural gas. The converter, operated with periodical reversals of the flow, ensures destruction of CO and hydrocarbons, including methane. Both computer simulation and engine testing results are presented.

Publisher Summary Interaction of the dynamic properties of a catalyst, a micro-scale physic-chemical system, and the dynamic properties of the macro-scale reactor creates an opportunity to improve the performance of catalytic processes using forced unsteady-state operation. Forced dynamic operation makes it possible to generate spatio-temporal patterns of temperature, composition and catalyst states that cannot be attained under steady-state operation. Probably, a potential for increase of efficiency of partial oxidation processes due to FUSC is tremendous. Frequently observed small effects apparently result from current research philosophy: trial-and-error experimentation with different forced periodic regimes while searching for drastic improvement. Money should be spent for elucidation of the process mechanism, developing models appropriate for numerical simulation, search for optimums using computer simulation, then testing the results in the lab or pilot unit.