Modeling the Effect of Substrate Cell Shape on Conversion in Monolith Catalysts

Mass transfer limitations from the bulk gas phase to the surface of the catalyst as well as mass transfer limitations within the washcoat itself have important effects on conversion in washcoated monolith catalysts. These factors depend upon the shape of the channel as well as the loading of washcoat material. This paper outlines a method to describe the washcoat distribution profile for different channel shapes and washcoat loadings. This allows for prediction of effectiveness factors and bulk mass transfer coefficients as a function of cell geometry and washcoat loading for the oxidation of propane. It was found that differences in the diffusion limitations within the washcoat control conversion in the catalyst more than differences in bulk mass transfer rates when comparing different cell shapes. The results show that optimum washcoat loadings exist for the geometry of each cell, and that these optimum loadings are a function of catalyst temperature. These optimum loadings vary from ∼12% for hexagonal channels at 1300 K to over 50% for triangular channels at 800 K. In general, hexagonal channels and square channels show similar performance at high temperature and washcoat loadings above 20%, while triangular channels show poorer performance under all conditions. In addition, it was found that slab approximations for effectiveness factors provide reasonable values for the effectiveness factor for square and hexagonal channels at washcoat loadings above 20%.