(360bi) Mass Transfer through Vapor-Liquid Interfaces of Binary Mixtures studied by Non-Stationary Molecular Dynamics Simulations

In separation processes not only thermodynamic bulk but also interfacial properties play a crucial role. In classical theory, a vapor-liquid interface is a two-dimensional object. In reality it is a region in which properties change smoothly over a few nanometers from its liquid bulk to its gas bulk value. Properties of that interfacial region can be predicted using theoretical methods, for example with molecular dynamics simulations (MD) or density gradient theory (DGT). Unexpected effects are observed in that transition region for many mixtures. While the total density changes monotonously from the bulk vapor to the bulk liquid phase, this does not hold for the molarities of the components. The molarity of the low-boiling component can have a distinct maximum at the interface, which is called enrichment [1-3]. That maximum would be an insurmountable obstacle to mass transfer according to the Fickian theory and, hence, is suspected to influence for example fluid separation processes like absorption or distillation.

To study the influence of the enrichment on the mass transfer, a new non-stationary molecular simulation method was developed in the present work, in which the penetration of a low-boiling component 2 into a liquid phase that initially only contains the pure solvent 1 is studied using a multi-ensemble technique. Five model mixtures (exhibiting different phase behavior and interfacial properties) were investigated. The results provide new insights in the physical processes involved in gas-liquid mass transfer. In particular, they indicate that the enrichment at the interface hampers the mass transfer. Furthermore, it was observed, that the penetrating
component may be repelled from the interface in early stages of the mass transfer. The results from this work complement and confirm the findings obtained earlier by our group in stationary NEMD simulations of mass transfer [4].

References

[1] S. Stephan, H. Hasse, Phys. Rev. E 101 (2020) 012802

[2] S. Stephan et al., J. Chem. Phys. 150 (2019) 174704

[3] S. Stephan, H. Hasse, Int. Rev. Phys. Chem. 39, 3 (2020) 319-349

[4] S. Stephan et al., Mol. Phys. 119, 3 (2021) e1810798