Analysis of single and multi-stage membrane reactors for the oxidation of short-chain alkanes -: Simulation study and pilot scale experiments

This contribution intends to provide insight into various aspects of multi-stage dosing concepts based on an experimental and model based analysis. For this aim the oxidative dehydrogenation (ODH) of ethane to ethylene on a VOx/Al2O3 catalyst was considered as a model reaction. Additionally, first results of the industrially more important ODH of propane will be given as an outlook. For the experimental study, a pilot scale set-up has been constructed realising a single-stage packed-bed membrane reactor as well as a three-stage membrane reactor cascade. The inner/outer diameter of the asymmetric alumina membranes used was 21/35 mm. A comparison with a conventional fixed-bed reactor as a reference concept was feasible using totally vitrified membranes operating in the same equipment in a co-feed mode. Reduced simple 1D and more detailed 2D models have been developed to identify optimal operation parameters and to describe the concentration and temperature profiles, respectively. Based on a preliminary theoretical analysis, a large set of experimental studies was carried out in a temperature range between 520/630 C (ethane) and 350/500 C (propane). The molar O-2/CnHm ratio was varied between 0.5 and 8. In the three-stage membrane reactor cascade different dosing profiles could be realised, e.g. increasing, uniform and decreasing profiles. Due to the separated and distributed feeding of the reactants, the resulting concentration and residence time profiles and the corresponding product spectra are different in membrane reactors compared to fixed-bed reactors. The analysis performed reveals for the investigated operation range a higher ethylene/propylene selectivity and a simultaneously higher conversion in membrane reactors. In case of low oxygen concentrations the selectivity of the desired product ethylene can be increased significantly compared to the conventional fixed-bed reactor. The developed detailed 2D models allowed a good mathematical description of the exothermal reactions taking place in the membrane reactor exploiting a distributed oxygen dosing over the porous wall. (C) 2008 Published by Elsevier B.V. on behalf of The institution of Chemical Engineers.