Experimental study of bimolecular reaction kinetics in porous media

The representation of multispecies chemi cal transformations into models fo r porous media flow and transport is investigated. Since the rate of transf ormation is dependent on the actual concentrations of the reactants inside the pore-spaces (rather than the averaged concentration that is amenable to a porous continuum model) such models may show a lesser or greater rate of transformation than is actually occurring, depending on the pore-scale cor relation of the reactant concentrations. To examine this experimentally, a bimolecular reaction (A + B --> Product) in a saturated porous media flow i n a column was studied using a spectrophotometer. The second-order rate con stant of the reaction was independently determined in well-mixed batch test s. In the reactive transport experiments, the porous medium column was init ially saturated with one reactant, the other reactant was injected at one l ongitudinal end of the column, and the effluent concentrations were measure d. It is documented that a reactive transport model that does not account f or the pore-scale segregation of reactants can substantially overpredict th e product concentration.