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Abstract

Unsteady-state operation modes based on feed composition modulation have been suggested to improve the yield in selective oxidation of n-butane to maleic anhydride (MA). In electrochemical membrane reactors (EMR) the oxygen feed rate can be directly controlled due to the faradaic coupling to cell current. Therefore, the application of forced periodic operation strategies is easily possible. Recently, we have shown experimentally that n-butane oxidation in an EMR under periodic and steady-state operation is feasible. In this contribution a model-based analysis of both operation strategies is presented. A dynamic (1D+1D) – model of the catalytic layer coupled to a 1D model of the gas channel has been derived which allows to take radial and axial variations of concentration into account. A comparitive analysis of steady-state operation at EMR-mode and Co-feed (CR)-mode revealed that (similar to porous catalytic membrane reactors (CMR)) the converse radial and axial oxygen concentration profiles lead to significant performance differences. Only under conditions of high oxygen conversion the EMR yield might exceed that of the CR. These findings are in qualitative agreement with experimental results. Parametric sensitivity studies on process performance were carried out and the results were subsequently used to systematically derive periodic process strategies. A hysteresis effect of oxygen loading of the two catalytic sites was found which is the more pronounced the more the oxygen storage capacities of both sites differ. Two types of periodic operation schemes were considered: (i) a periodic switch between butane feed and oxygen pumping and (ii) periodic variation of the oxygen feed at constant hydrocarbon feed. Optimal periodic parameters (cycle split, amplitude, and period) were determined for both schemes and a possible improvement in time-average yield compared to steady-state operation is predicted in both cases. The second strategy diminishes the risk of local (and possibly irreversible) deep reduction of the catalyst and might be favourable in cases where big concentration amplitudes have to be avoided, although the total yield is smaller compared to the first strategy.