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Free keywords:
Membrane reactors; pilot plant; selectivity enhancement; ethane; propane
Abstract:
This contribution intends to provide a deeper 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 and propane to propylene on a VOx/Al2O3 catalyst were considered as model reactions. For the experimental study, a pilot scale set-up has been constructed with a single stage packed bed membrane reactor and a three stage cascade. The inner/outer membrane diameter was 21/35 mm. Asymmetric alumina and sinter metal membranes were investigated. For comparison with a conventional fixed-bed reactor operation was feasible using the co-feed mode. Reduced simple 1D and more detailed 2D models have been used 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 O2/CnHm ratio was varied between 0.5 and 8. In the three stage membrane reactor different dosing profiles could be realised, e.g. increasing (10-30-60), uniform (33-33-33) and decreasing (60-30-10) 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 simultaneously a 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 allow a good mathematical description of the exothermal reactions taking place in the membrane reactor. The obtained results for the ODH of propane are similar even though the increase of the propylene selectivity is not so distinctive compared to ethylene.
