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.
CFD simulations, experimental validation and parametric studies for the catalytic reduction of NO by hydrogen in a fixed bed reactor
A 3D reactor model was developed for the reduction of NO by hydrogen over a Pd(1 wt%)/Al2O3 catalyst. The reactor model built in COMSOL Multiphysics CFD software is based on momentum conservation equations corrected by the Brinkman equation for the catalytic bed zone and on mass balance equations which consist of diffusion–convection equations along and across the reactor. The kinetic model used in the proposed 3D reactor model is an adaptation of a previously reported reaction mechanism, and validated according to our experimental observations which showed that reduction of NO by H2 on the Pd(1 wt%)/Al2O3 catalyst leads to the formation of N2 and N2O only, without formation of ammonia. The results of numerical simulations performed with the proposed mathematical model are in good agreement with the experimental results obtained in the temperature range of 50–350 °C, proving the validity of the employed kinetic model. Moreover, simulation results evidenced that already at 100 °C or above, total NO conversion, with corresponding maximum N2 yield around 85% could be attained with half of the catalytic bed length employed in the experimental tests. The developed reactor model was further used to carry out parametric studies involving the reaction temperature (T), GHSV, NO concentration in the feed (CinNO), or the reactants ratio (NO/H2), in order to investigate their influence upon the catalytic performance of the reactor. These studies emphasize the possibility of using less catalyst volumes without affecting the catalytic performances, and showed that total NO conversion can be achieved in the temperature range of 100–150 °C irrespective of the employed GHSVs, the concentration of NO in the feed or the reactants ratio. Experimental runs performed with half of the catalyst volume confirmed the conclusions obtained by simulations. The 3D reactor model has important application potential for the reactor and associated control system design.
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