In this work we construct multi-scale models of continuous microreactors with catalytic walls as an alternative to the conventional packed-bed configuration. The bulk (gaseous) phase is treated macroscopically using Computational Fluid Dynamics (CFD), including transport phenomena and gas-phase reactions, while the catalytic activity is modeled at the micro/mesoscopic level using kinetic Monte Carlo (kMC) simulations including effects such as adsorption, desorption surface reactions and surface diffusion. A number of kMC lattices depending on the discretisation of the computational domain are used to effectively represent the reactive surfaces. Massively parallel computations both for the gas-phase using SANDIA's code for reaction transport processes (codename: MPSALSA) [2] and for the surface are used to accelerate the tedious computational tasks.
The efficient coupling of time and length scales is investigated through coarse-graining methodologies and comparisons with fully macroscopic mean field approaches are made in order to assess the window of parameters where the multi-scale approach becomes important. The effect of both vertical interactions (between the macroscopic gas-phase and the microscopic surface) and lateral interactions between neighbouring kMC lattices is studied with the use of efficient numerical techniques. The relative importance of these interactions is evaluated for a wide range of conditions, through sensitivity studies. The methodology is demonstrated through the use of two illustrative reaction systems with well-known kinetics: CO oxidation [3] and the simultaneous NO reduction with CO oxidation [4].
References
1. Ehrfeld W, Hessel V, Lowe H, Microreactors: New Technology for Modern Chemistry, Wiley-VCH (2000).
2. Shadid J, Hutchinson S, Hennigan G, Moffat H, Devine K, Salinger AG, Parallel Computing 23, (1997), 1307-1325.
3. Zhdanov VP and Kasemo B 20 (1994) 111-189.
4. Fink Th, Dath JP, Imbihl R, Ertl G, J. of Chem. Phys. 95 (1991) 2109-2126.