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Zusammenfassung:
In the field of chemical reaction engineering intensive research is devoted to development of new processes in order to improve selectivity and yields of intermediate products. Important example reactions are catalytic partial oxidations (CPO) or selective hydrogenations [1]. For such reactions improved integral reactor performance can be achieved by optimised stage-wise dosing of one or several reactants [2]. Adjusted dosing profiles can be realised e.g. by feeding reactants separately through permeable reactor walls, e.g. through tubular membranes. Additionally, various types of stage-wise temperature profiles in combination with characteristic dosing and/or residence time profiles can be realised. The concept allows to improve significantly the selectivity and yield with respect to an intermediate product.
This contribution intense 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 partial oxidations 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 1-D and more detailed 2-D models have been unsed 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. Furhter, improved performance could be achieved by combining an optimised stage-wise temperature profile with a stage-wise dosing of one or several of the reactants, respectively.
Literature:
[1] Dixon, A.G., Int. J. Chem. React. Eng., Vol.1, 2003, R6
[2] Hamel C., Thomas S, Schädlich K., Seidel-Morgenstern A., Chem. Eng. Sci., 58, 2003, 4483
