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The article discusses a research on the capacity of monoliths to serve as catalysts for reverse-flow combustors, published on March 22, 2010 in wileyonlinelibrary.com. In this study, modeling and experimental validation of the performance of a bench-scale monolithic reverse-flow reactor (RFR) for methane combustion was conducted. Simulation of the behavior of industrial-scale reactors and the comparison of particulate and monolithic RFRs are also discussed.

Abstract: This work is focused on the application of reverse flow reactors to the combustion of lean mixtures of aliphatic and aromatic hydrocarbons in air. For this purpose, hexane and toluene were chosen as model compounds. The combustion of binary mixtures of these compounds (up to 500ppmV total hydrocarbon concentration) over a commercial catalyst in reverse flow reactors has been studied both experimentally, in a bench-scale unit, and by simulations, using a heterogeneous mono-dimensional dynamic model, good correspondence being observed between both approaches. As general trend, it was observed that the behaviour of the reactor is determined mainly by the combustion enthalpies and reactivities of toluene and hexane. Hence, increasing total concentration and increasing fraction of toluene (the most reactive compound) lead to more stable operation. Regarding the kinetic inhibition effects, in the conditions studied no influence on the reactor performance was observed, probably because the hydrocarbons combust in different reactor zones. This behaviour can be extended to the combustion of aromatic and – alkanes, characterised by their relatively low concentrations (determined by their vapour pressure) and high reaction rates. [Copyright &y& Elsevier]

Abstract: In this work, the performance of a simple logic-based controller for a reverse flow reactor (RFR) has been tested experimentally. The controller is a hybrid system using the inside reactor temperature as the controlled variable, and the switch of the flow direction as the manipulated variable. Three different control logic rules (depending on the point selected to measure the inside reactor temperature) have been compared for the catalytic combustion of methane in a bench-scale RFR unit. A procedure for tuning the controllers was established. The controller that measures the temperature in the middle of the reactor showed the best performance, as it provided complete methane conversion with high capacity to overcome both low and high feed concentration disturbances. [Copyright &y& Elsevier]

Abstract: The aim of this paper is to study the combustion of medium concentration methane–air mixtures (in the range 5000–10000ppmV, very interesting and usual in real emissions, but not widely studied in the literature) in unsteady-state reactors; the results obtained in a reverse-flow reactor (RFR) and in a network of two (RN2) and of three reactors (RN3) will be compared. The influence of the main operating parameters, namely the switching time and the methane concentration in the feed, is investigated by means of numerical simulation, using an experimentally validated model. The performance of the three configurations is compared in terms of their stability intervals (values of switching time that allow for stable autothermal operation) and temperature profiles inside the reactor. The results obtained suggest that the RFR is the most appropriate choice for this application because of the wider stability interval, as well as because of the lower temperature variations inside the catalytic bed. [Copyright &y& Elsevier]

Abstract: The performance of bulk chromium oxide is compared with that of a Mn commercial catalyst for the deep oxidation of trichloroethene (1000–2500ppmv, 55h−1 space velocity) in air, in dry and wet (20000ppm of H2O) conditions, in terms of activity, selectivity and stability. Chromium oxide was found to be more active (on a catalyst weight basis), however its activity decreases continuously with time on stream. The presence of water increases its stability, the Mn catalyst showing the opposite behaviour. The effect of water on the Cr catalyst can be explained according to the Deacon equilibrium, as the presence of water tends to decrease the Cl2 concentration, assumed to be responsible of the catalyst deactivation. Regarding to the selectivity, the Mn catalyst yields C2Cl4, CCl4 and CHCl3 as organochlorinated by-products, whereas chromium oxide produces only trace amounts of CCl4. Simple power-law kinetics expressions (first-order for Mn and zero-order for Cr) provide fairly good fits for the evolution of the conversion with the temperature. Furthermore, the kinetic behaviour of chromium oxide can be represented with a Langmuir–Hinshelwood model taking into account the chlorine inhibitory effect. [Copyright &y& Elsevier]

Abstract: The scope of this work is to compare systematically the performance of particle beds and monolithic beds in catalytic reverse flow reactors used for combustion of lean methane/air mixtures, using alumina-supported palladium as catalyst. Different values of gas surface velocity (0.1–0.3m/s), particle diameter (3–6mm, for particle bed), cell density (200–400 cpsi, for structured bed) and catalyst/inert ratio (0.4–1) were used for the simulation of the combustion of 3500ppm methane in both kinds of reverse flow reactor. An unsteady one-dimensional heterogeneous model has been developed and solved using a MATLAB code. The model, physical parameters and transport properties used had been experimentally validated in a previous work, operating with a particle bed reverse flow reactor. Results obtained indicate that the reverse flow reactor is more stable when the catalyst particle beds are use, although the difference with the monolith bed decreases as surface velocity increases. In contrast, pressure drops in the bed are higher for the particle bed. [Copyright &y& Elsevier]