A Boudart Number for the Assessment of Irreducible Pellet-Scale Mass Transfer Limitations: Application to Oxidative Coupling of Methane
2021
Vandewalle, Laurien A. | Van Geem, Kevin M. | Marin, Guy B. | Bos, René
The traditional concept of the Thiele or Weisz modulus can fail to assess the relevance of pellet-scale transport limitations in the case of heterogeneously catalyzed gas-phase reactions such as oxidative coupling of methane (OCM). Irreducible transport limitations occur when surface-produced gas-phase intermediates (e.g., radicals) react fast in the pores of catalyst pellets, causing strong pellet-scale concentration profiles that directly affect the product distribution. To assess the effect of the pellet diameter on pellet-scale gradients of reactants and intermediates during OCM, a pellet-scale model is used in combination with detailed kinetic models for the gas (57 species, 317 reactions) and surface chemistry (11 species, 26 reactions). On the basis of a preliminary analysis using a skeleton reaction mechanism a so-called Boudart number, ΦBₒᵤ, is proposed for assessment of the potential relevance of transport limitations during this type of heterogeneously catalyzed gas-phase reactions. The effect of pellet-scale gradients on product yields and selectivities is evaluated using a rigorous 1D heterogeneous reactor-scale model in combination with the above-mentioned detailed kinetic models for the gas and surface chemistry. Pellet-scale diffusion limitations are found to have a significantly negative effect on C₂ selectivities during OCM with the applied kinetics. When the Weisz-Prater criterion is satisfied for reactants and products, pellet-scale gradients of the intermediates are responsible for this drop in selectivity. For larger pellet diameters, the drop in selectivity is due to diffusion limitations of the C₂ products and the corresponding increased importance of secondary total oxidation reactions.
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