Mixing in compact porous reactors

Porous reactors, such as fixed-beds, are often used to geometrically support a large reactive interface for the conversion of molecules continuously injected in a spatially compact device, where the ratio between transverse and longitudinal system sizes is minimized. We show, via pore-scale numerical simulations, that, in such compact systems, solute mixing is dominated by stretching-mediated random overlapping of concentration elements, occurring transversally to the direction of the main flow velocity U. Compared to a full three-dimensional random overlapping scenario, mixing dynamics is slower, characterized by a temporal decay of concentration variance σ_² ∼ (U/d t)^-1/2, with d the catalyst particle size. Our analysis indicates that such a mixing mechanism regulates the homogenization of reaction at the reactive fluid-solid interface. We discuss the implications for packed bed column experiments and heat releaser devices.