Experimental study of bimolecular reaction kinetics in porous media

The representation of multispecies chemi cal transformations into models for porous media flow and transport is investigated. Since the rate of transformation 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 correlation of the reactant concentrations. To examine this experimentally, a bimolecular reaction (A + B --> Product) in a saturated porous media flow in a column was studied using a spectrophotometer. The second-order rate constant of the reaction was independently determined in well-mixed batch tests. In the reactive transport experiments, the porous medium column was initially saturated with one reactant, the other reactant was injected at one longitudinal end of the column, and the effluent concentrations were measured. It is documented that a reactive transport model that does not account for the pore-scale segregation of reactants can substantially overpredict the product concentration.