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BIRS Workshop Lecture Videos

Using look-up tables as scale bridges in multiscale modelling of catalytic reactors Hayes, Bob


Talk: Regular Abstract: Structured reactors offer many opportunities for advanced computational modelling. The physical and chemical phenomena that are important in the reactor happen at several length scales and thus represents a multi-scale problem. As an illustration, consider the washcoated monolith reactor, a classical structure comprised of (usually) thousands of parallel channels. The smallest scale that might be considered is the molecular scale, where the various molecules interact with active sites to effect the reaction. It is certainly common to eliminate this scale through the use of global kinetic expressions, however, it is often necessary to use more detailed mechanisms, either because the global approach lacks accuracy or a more detailed description of the product distribution is desired. The second scale would be the washcoat, which has a thickness that varies between 10 and 150 microns. Structurally, it is a porous medium composed of pores that range from 10 nanometres in radius and larger. One may wish to model the actual microstucture, or to treat the washcoat as a continuum. Thus we have either one or two additional scales to consider that are orders of magnitude different. A typical monolith reactor channel has a dimension of the order of 1 mm, which is again two orders of magnitude different from the washcoat. Each channel has a spatially dependent distribution of species, temperature and velocity. Heat transfer occurs between channels, and the velocity distribution is not usually uniform. Finally, at the reactor scale, the dimensions are of the orders of many centimetres, which represents again an orders of magnitude scale shift. To incorporate information at all scales into a complete model requires a clever computing strategy and an appropriate selection of scale bridges, otherwise the solution will be extremely expensive from a computational point of view. Linking the different scales at the simplest level requires the passage of information among the various scales using a system of sub-models for each scale. We have recently had a good measure of success in enhancing computational efficiency using pre-computed data. The look-up table may contain the results of different sub-models, and thus acts as a scale bridge. Essentially, the look-up table can be considered to be a table of values computed at discreet points over a specified range of operating conditions. Intermediate values are normally computed using spline interpolation.

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