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Synthesis gas; Perowskite hollow fibres; membrane reactors; modelling
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 at temperatures around 900°C. 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 [1].
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) 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 estimation and validation of mass transfer parameters for characterisation of the membrane was based on systematic experiments [2]. 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 [3,4,5] were implemented in a detailed two dimensional pseudo homogeneous reactor model. Comparing with predictions of a reduced 1-D model this model was found necessary for a detailed analysis of the pronounced concentration, temperature and velocity fields. The fibre membrane is placed in the reactor housing and separates the reactor in air providing side (tube) and synthesis gas side (shell). The following arrangements of the used catalyst in the membrane reactor were investigated: a) catalyst bed, b) catalyst coated hollow fibre c) structured catalyst bed, d) hollow fibre and catalyst separated by an additionally Al2O3 membrane. Depending on the catalyst arrangement the obtained hot spot can be 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 and the performance of the investigated concept can be described only using a more detailed two dimensional reactor model.
Literature:
[1] Stitt E.H., Multifunctional Reactors? Up to a Point Lord Copper, Trans IChemE, Part
A, Chem. Eng. Res. and Des., 82 (A2), 2004
[2] C. Hamel, A. Seidel-Morgenstern, T. Schiestel, S. Werth, C. Tablet, J. Caro, H.
Wang, Experimental and modelling study on dense perovskite hollow fiber membranes
for production of O2-enriched air, AIChE J., 52 (2006) 3118
[3] Hou K., Hughes R., The kinetics of methane steam reforming over a Ni/a-Al2O3
catalyst, CEJ, 82, 2001
[4] Xu J.,Froment G., Methane steam reforming, Methanation and Water-Gas Shift:1.
Intrinsic Kinetics, AIChE J.l, 35, 1989
[5] 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:,187, 1999
