Multiplicities in thermodynamic activity coefficients

Thermodynamic activity coefficients play an essential role in the description of phase equilibria, which are fundamental to the simulation and optimization of all separation processes. Although it is known that multiplicities with respect to binary interaction parameters can occur, no systematic study of these multiplicities has been undertaken yet. In this contribution, their rigorous pointwise classification is presented, valid for the entire range of binary compositions. Furthermore, implications not only for the non-random two-lqiuid (NRTL)-parameterization to capture temperature-dependence are discussed, but also for extrapolation and regression tasks. The findings are illustrated for vapor-liquid equilibria of different binary mixtures.