Comparison of reverse flow and counter-current reactors in the case of selective catalytic reduction of NOx
Abstract The paper is focused on the comparison between a reverse flow reactor (RFR) and a counter-current reactor (CCR) in terms of the performances that may be achieved in these kinds of reactors when selective catalytic reduction (SCR) of NO x with ammonia is carried out. In order to investigate the performances of both reactor configurations, a mathematical model based on heat and mass balances for gas and solid phase was used. The possibility of achieving auto-thermal operation, even when a gas with a low temperature is going to be fed to the reactor was investigated. The temperature profiles, the reactants conversion, as well as the response to disturbances in the feeding flow have been used as criteria for comparison. Numerical simulations have shown that the CCR model is able to provide the same results as the RFR model in terms of temperature profiles when certain values of heat transfer coefficient, switching time, flow conditions and reactor geometry are taken into consideration. As a consequence of such similar thermal behaviour, the CCR model could be applied as a limiting case of RFR operation when the fast switching of the flow direction conditions is taken into account. Nevertheless, this analogy provides a simple basis for short-cut calculations since the steady-state profile of a CCR can be computed much easier than the periodic steady-state of a RFR.
Resonance Response of Reverse Flow Reactors: A Numerical Simulation
The effect of periodic time-varying inlet parameters on reverse flow reactor behavior is simulated using a one-dimensional heterogeneous model with methane as the model reactant. When the flow-reversal cycle is an odd multiple of the inlet concentration variation cycle, a resonance response occurs. Such reactor behavior will occur with various values of superficial inlet velocity, mean inlet methane concentration, flow-reversal time and inert-to-catalyst ratios during this simulation work. A synchronization of the two cycles will cause a sharp increase in the variation of maximum temperature, which may impair reactor stability. Simulation results reveal that the resonance response is attributed to the breakdown of temporal and spatial symmetry in the temperature field of reactor. When a resonance response occurs, adjusting the flow-reversal time is a feasible method to maintain autothermal operation of the reactor. Different from commonly used control strategies both increasing and decreasing flow-reversa...
Einfluß von Stoff- und Wärmetransport bei Reaktionen gasförmig/fest am Beispiel katalytischer Brennzonen in adiabatischer Kontaktschicht†
Kurze Reaktionszonen kann man mit exothermen Gasreaktionen in kornigen Katalysatorschichten bei adiabatischer Betriebsweise erhalten. Bei niedriger Eingangstemperatur des Frischgases nehmen sie die Eigenschaften wandernder Brennzonen an. Der Mechanismus der Zonenwanderung und ihr Zusammenhang mit dem Gasdurchsatz kann im Anschlus an die altere Warmetheorie der Flammenfortpflanzung verstanden werden. Die Analogie zum Verhalten von Flammen fuhrt zur Konstruktion eines katalytischen Brenners mit kalottenformiger, stationarer Brennzone, dessen Brennflache sich dem Gasdurchsatz jeweils anpast. Anwendungsmoglichkeiten bestehen unter anderem bei der katalytischen Nachverbrennung von Abgasen.