Dynamic modeling of aqueous electrolyte systems in Modelica

Modeling transient aqueous electrolyte systems is increasingly gaining attention for designing and optimizing processes where the equilibrium composition becomes relevant. This work revisits appropriate formulations to cope with embedding equilibrium conditions in transient systems. We follow the equation-oriented approach of writing balance equations in terms of reaction invariants and replacing the embedded Gibbs free energy minimization problem with a reformulation of the Karush-Kuhn-Tucker conditions to yield a system of differential-algebraic equations. We provide the open-source Modelica package ElectrolyteMedia for the modeling of transient aqueous electrolyte systems under consideration of chemical equilibrium in combination with detailed thermodynamic model equations for gas, liquid, and solid phases. With tailored initialization algorithms, we can integrate customized models using numerical solvers provided by a Modelica simulation environment, e.g., Dymola. We show simulation results of multiple case studies ranging from the simulation of titration experiments to complex unit models.