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Abstract
Meso-scale mathematical model of local reaction and transport processes in a porous, supported heterogeneous catalyst with bimodal pore-size distribution is presented. The model takes into account individual reaction steps on active sites (microkinetics), diffusion of reactants in macro-pores and meso-/micro-pores (molecular and Knudsen-diffusion), and heat generation and transport. The processes are modelled within a three-dimensional domain ( ≈ 10 × 10 × 10 μm3) of computer-reconstructed porous catalyst. The methodology is demonstrated on CO oxidation on Pt/γ-Al2O3. Several 3D porous structures are digitally reconstructed by the methods of particle packing and Gaussian-correlated random fields from typical electron-microscopy images of the catalyst. Typical dependences of overall reaction rate and effectiveness factor on the temperature and properties of the porous catalyst structure are evaluated. Relative importance of diffusion in macro- and meso-pores under varying temperature is demonstrated. Local optimum of effectiveness factor is found for the mixing ratio of catalyst support particles with two different sizes. The resulting temperature gradients over the studied section of catalyst are very small (approximately 0.2 K for the CO concentrations in the order of 1% mol). The results represent the local situation on the meso-scale, which can be interpreted as one discretisation point in the full-scale model of the reactor.
Acknowledgements
The work has been supported by the grant 104/06/P301 of the Czech Grant Agency and the project MSM 6046137306 of the Czech Ministry of Education.
