Finite-time mixing and reaction in packed-bed reactors

We investigate mixing dynamics in porous media at finite times, using pore-scale lattice-Boltzmann simulations combined with Lagrangian particle tracking. We compute fluid deformation in randomly packed beds based on the moving Protean frame approach introduced by Lester et al. (2018 J. Fluid Mech. 855, 770-803). From the extracted Lagrangian kinematics, we construct a mixing model based on lamellar aggregation that well predicts the Eulerian scalar fields obtained from simulations. Our results reveal an early-time mixing regime dominated by shear-driven fluid deformation, where solute mixing arises from the random overlap of diffusive concentration elements. In this regime, mixing proceeds slowly and follows a temporal decay of concentration variance, 𝜎2𝑐 ∝𝑃𝑒-𝛼/(2⁢𝛼+1)⁢𝑡-1/2, where 𝑃𝑒 is the Péclet number and 𝛼 the exponent characterising shear deformation. This dynamic arises when the Péclet number is small relative to the ratio between the exponential-mixing and shear-deformation time scales. This analysis also demonstrates that shear-induced mixing governs the homogenisation of early-stage reactions at the fluid-solid interface in finite-size random packed beds, typically operating at moderate Péclet numbers 𝑃𝑒 =𝑂⁡(102).