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Local Peclet number in small sized fixed beds of catalytic particles

Over the last decades, the size of fixed bed reactors for catalyst screening has been drastically reduced up to a point where the size of a single catalyst pellet is nowadays of the same order as the reactor's diameter. This size reduction raises questions about the representativeness of such small reactors in building models used for extrapolating the catalyst performance to industrial scale reactors of several meters in diameter, in particular regarding uncertainties due to defects in the catalyst packing. Here, we demonstrate that the use of Direct Numerical Simulation and OpenFOAM predicts accurately the Pe numbers of a small sized reactor, a global feature quite costly to measure experimentally. Moreover, we have access for the first time to local Pe evolution in relation with local packing structures.

As many other works on fixed bed simulation [1,2,3,4,5], our workflow is based on coupling a Discrete Element Method package, namely Grains3D [6], and a single phase steady-state solver derived from simpleFoam augmented with an implementation of the moments of age distribution of Liu [7]. Meshing is done with the utility snappyHexMesh.

Local Peclet numbers are calculated from integrals of the first and second moment of age over cutting planes perpendicular to the fixed bed main axis [7]. The CFD results are in excellent agreement with global experimental measurements based on residence time distributions. In the quest to quantify effects of randomness, we observe that catalyst particles packing structures can locally induce staircases on local Peclet numbers, through fluid preferential paths. The workflow is currently being extended to multi-region heat and mass transfer simulations.

[1] Freund et al. (2005). Detailed simulation of transport processes in fixed-beds. Industrial & engineering chemistry research, 44(16) [2] Augier et al. (2010). Numerical simulations of transfer and transport properties inside packed beds of spherical particles. Chemical Engineering Science, 65(3) [3] Boccardo et al. (2015). Validation of a novel open-source work-flow for the simulation of packed-bed reactors. Chemical Engineering Journal, 279 [4] Partopour and Dixon (2017). An integrated workflow for resolved-particle packed bed models with complex particle shapes. Powder technology, 322 [5] Pozzobon et al. (2018). Hydrodynamics of a packed bed of non-spherical polydisperse particles: A fully virtual approach validated by experiments. Chemical Engineering Journal, 354 [6] Wachs et al. (2012). Grains3D, a flexible DEM approach for particles of arbitrary convex shape - Part I: Numerical model and validations. Powder Technology, 224 [7] Liu et al. (2010). Spatial distributions of mean age and higher moments in steady continuous flows. AIChE journal, 56