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Abstract:
The generation of synthesis gas from methane is currently performed by conventional steam reforming or by partial oxidation (POX) in fixed-bed reactors using nickel or noble metal based catalysts. These catalysts offer the possibility to reach high yields at temperatures around 900°C [1]. In the last years several new reactor concepts were suggested to intensify the heat exchange, e.g. auto thermal reformers, catalytic coated wall reactors, fluidised bed or membrane reactors [2].
Improved POX of methane is currently the most promising direction for better generation of synthesis gas, in particular if oxygen is fed in a distributed manner separated from air using O2-selective mixed conducting membranes. The industrial applicability of this concept depends on the availability of suitable selective tubular membranes characterised by thin walls to intensify the mass and heat transfer. In addition to the desired slightly exothermic partial oxidation in such reactors also the highly exothermic total oxidation can take place. The related heat generation can be exploited by coupling with conventional steam reforming to realise a potential auto thermal operating mode.
Based on experimental results characterising a new type of mixed conducting hollow fibre membranes (BaCoxFeyZrzO3-δ,BCFZ, produced by spinning [3]) the operating behaviour of a membrane reactor integrating the process steps air separation, selective O2 transfer in the BCFZ and oxidation of methane with simultaneous steam reforming was evaluated. The coupling of the mass balances of the synthesis gas side (shell side) and the air providing side (tube side) and the description of mass transfer in the O2-selective Perowskite hollow fibres was analysed using reduced reactor models. The estimation and validation of mass transfer parameters for characterisation of the membrane was based on systematic experiments [4]. A comparison between the results of mass transfer experiments and simulations is given in figure 1. A sufficient agreement concerning the influence of temperature (a) and oxygen partial pressure (b) on the oxygen flux in the membrane was obtained. The results demonstrate the potential of the fibres used to enrich air streams in oxygen. The derived equations describing the mass transport in the membrane as well as kinetic approaches for the rates of oxidation and steam reforming of methane coming from the literature [4,5,6] were implemented in a detailed two dimensional pseudo homogeneous reactor model. Comparing with predictions of the reduced model this model was found necessary for a detailed analysis of the pronounced concentration, temperature and velocity fields.
In figure 2 are illustrated configurations of the membrane reactor with a catalyst bed (a) and a catalyst coated hollow fibre (b). The fibre separates the reactor in air and synthesis gas side. Additionally, the complex temperature fields are depicted for the considered configurations predicted with the reduced 1D (c) and detailed 2D models (d, e). The obtained hot spot is located directly on the membrane, which leads under unfavourable operation conditions to an inactivation or melting of the hollow fibre. It will be shown, that the essential temperature effects can be described only using the more detailed two dimensional reactor model.
Acknowledgements : The authors gratefully acknowledge the financial support of the German BMBF for project 03C0343A under the auspices of ConNeCat.
Literature:
[1] Bouwmeester H.J.M., Dense ceramic membranes for methane conversion, Catalysis Today, 82, 2003
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[4] Wang H., Werth S., Schiestel T., Caro J., Perovskite Hollow Fiber Membranes for the Production of O2-Enriched Air, Angew. Chem. Int. Ed. 44, 6906, 2005
[5] Hou K., Hughes R., The kinetics of methane steam reforming over a Ni/α-Al2O3 catalyst, CEJ, 82, 2001
[6] Xu J.,Froment G., Methane steam reforming, Methanation and Water-Gas Shift:1. Intrinsic Kinetics
AIChE Journal, 35, 1989
[7] Smet C.R.H., de Croon M.H.J.M., Berger R.J., Marin G.B., Schouten J.C., An experimental reactor to study the intrinsic kinetics of catalytic partial oxidation of methane in the presence of heat-transport limitation, Appl.Catal. A: General, 187, 1999
