Efficient algorithm for modeling transport in porous media with mass exchange between mobile fluid and reactive stationary media

We compare two approaches to numerically solve the mathematical model of reactive mass transport in porous media with exchange between the mobile fluid and the stationary medium. The first approach, named the “monolithic algorithm,” is the approach in which a standard finite-difference discretization of the governing transport equations yields a single system of equations to be solved at each time step. The second approach, named the “system-splitting algorithm,” is here applied for the first time to the problem of transport with mass exchange. The system-splitting algorithm (SSA) solves two separate systems of equations at each time step: one for transport in the mobile fluid, and one for uptake and reaction in the stationary medium. The two systems are coupled by a boundary condition at the mobile– immobile interface, and are solved iteratively. Because the SSA involves the solution of two smaller systems compared to that of the monolithic algorithm, the computation time may be greatly reduced if the iterative method converges rapidly. Thus, the main objective of this paper is to determine the conditions under which the SSA is superior to the monolithic algorithm (MA) in terms of computation time. We found that the SSA is superior under all the conditions that we tested, typically requiring only 0.3–50% of the computation time required by the MA. The two methods are indistinguishable in terms of accuracy. Further advantages to the SSA are that it employs a modular code that can easily be modified to accommodate different mathematical representations of the physical phenomena (e.g., different models for reaction kinetics within the stationary medium), and that each module of the code can employ a different numerical algorithm to optimize the solution.